doc-src/ind-defs.tex
author lcp
Tue, 12 Jul 1994 18:05:03 +0200
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new cardinal arithmetic developments
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\documentstyle[a4,proof,iman,extra,times]{llncs}
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\newif\ifCADE
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\CADEtrue
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\title{A Fixedpoint Approach to Implementing\\ 
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  (Co)Inductive Definitions\thanks{J. Grundy and S. Thompson made detailed
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    comments; the referees were also helpful.  Research funded by
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    SERC grants GR/G53279, GR/H40570 and by the ESPRIT Project 6453
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    `Types'.}}
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\author{Lawrence C. Paulson\\{\tt lcp@cl.cam.ac.uk}}
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\institute{Computer Laboratory, University of Cambridge, England}
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\date{\today} 
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\setcounter{secnumdepth}{2} \setcounter{tocdepth}{2}
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\newcommand\sbs{\subseteq}
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\let\To=\Rightarrow
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\newcommand\pow{{\cal P}}
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%%%\let\pow=\wp
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\newcommand\RepFun{\hbox{\tt RepFun}}
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\newcommand\cons{\hbox{\tt cons}}
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\def\succ{\hbox{\tt succ}}
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\newcommand\split{\hbox{\tt split}}
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\newcommand\fst{\hbox{\tt fst}}
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\newcommand\snd{\hbox{\tt snd}}
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\newcommand\converse{\hbox{\tt converse}}
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\newcommand\domain{\hbox{\tt domain}}
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\newcommand\range{\hbox{\tt range}}
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\newcommand\field{\hbox{\tt field}}
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\newcommand\lfp{\hbox{\tt lfp}}
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\newcommand\gfp{\hbox{\tt gfp}}
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\newcommand\id{\hbox{\tt id}}
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\newcommand\trans{\hbox{\tt trans}}
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\newcommand\wf{\hbox{\tt wf}}
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\newcommand\nat{\hbox{\tt nat}}
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\newcommand\rank{\hbox{\tt rank}}
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\newcommand\univ{\hbox{\tt univ}}
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\newcommand\Vrec{\hbox{\tt Vrec}}
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\newcommand\Inl{\hbox{\tt Inl}}
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\newcommand\Inr{\hbox{\tt Inr}}
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\newcommand\case{\hbox{\tt case}}
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\newcommand\lst{\hbox{\tt list}}
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\newcommand\Nil{\hbox{\tt Nil}}
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\newcommand\Cons{\hbox{\tt Cons}}
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\newcommand\lstcase{\hbox{\tt list\_case}}
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\newcommand\lstrec{\hbox{\tt list\_rec}}
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\newcommand\length{\hbox{\tt length}}
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\newcommand\listn{\hbox{\tt listn}}
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\newcommand\acc{\hbox{\tt acc}}
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\newcommand\primrec{\hbox{\tt primrec}}
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\newcommand\SC{\hbox{\tt SC}}
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\newcommand\CONST{\hbox{\tt CONST}}
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\newcommand\PROJ{\hbox{\tt PROJ}}
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\newcommand\COMP{\hbox{\tt COMP}}
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\newcommand\PREC{\hbox{\tt PREC}}
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\newcommand\quniv{\hbox{\tt quniv}}
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\newcommand\llist{\hbox{\tt llist}}
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\newcommand\LNil{\hbox{\tt LNil}}
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\newcommand\LCons{\hbox{\tt LCons}}
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\newcommand\lconst{\hbox{\tt lconst}}
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\newcommand\lleq{\hbox{\tt lleq}}
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\newcommand\map{\hbox{\tt map}}
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\newcommand\term{\hbox{\tt term}}
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\newcommand\Apply{\hbox{\tt Apply}}
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\newcommand\termcase{\hbox{\tt term\_case}}
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\newcommand\rev{\hbox{\tt rev}}
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\newcommand\reflect{\hbox{\tt reflect}}
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\newcommand\tree{\hbox{\tt tree}}
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\newcommand\forest{\hbox{\tt forest}}
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\newcommand\Part{\hbox{\tt Part}}
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\newcommand\TF{\hbox{\tt tree\_forest}}
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\newcommand\Tcons{\hbox{\tt Tcons}}
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\newcommand\Fcons{\hbox{\tt Fcons}}
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\newcommand\Fnil{\hbox{\tt Fnil}}
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\newcommand\TFcase{\hbox{\tt TF\_case}}
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\newcommand\Fin{\hbox{\tt Fin}}
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\newcommand\QInl{\hbox{\tt QInl}}
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\newcommand\QInr{\hbox{\tt QInr}}
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\newcommand\qsplit{\hbox{\tt qsplit}}
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\newcommand\qcase{\hbox{\tt qcase}}
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\newcommand\Con{\hbox{\tt Con}}
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\newcommand\data{\hbox{\tt data}}
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\binperiod     %%%treat . like a binary operator
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\begin{document}
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%CADE%\pagestyle{empty}
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%CADE%\begin{titlepage}
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\maketitle 
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\begin{abstract}
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  This paper presents a fixedpoint approach to inductive definitions.
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  Instead of using a syntactic test such as `strictly positive,' the
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  approach lets definitions involve any operators that have been proved
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  monotone.  It is conceptually simple, which has allowed the easy
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  implementation of mutual recursion and other conveniences.  It also
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  handles coinductive definitions: simply replace the least fixedpoint by a
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  greatest fixedpoint.  This represents the first automated support for
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  coinductive definitions.
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  The method has been implemented in Isabelle's formalization of ZF set
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  theory.  It should be applicable to any logic in which the Knaster-Tarski
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  Theorem can be proved.  Examples include lists of $n$ elements, the
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  accessible part of a relation and the set of primitive recursive
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  functions.  One example of a coinductive definition is bisimulations for
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  lazy lists.  \ifCADE\else Recursive datatypes are examined in detail, as
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  well as one example of a {\bf codatatype}: lazy lists.  The appendices
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  are simple user's manuals for this Isabelle/ZF package.\fi
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\end{abstract}
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%
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%CADE%\bigskip\centerline{Copyright \copyright{} \number\year{} by Lawrence C. Paulson}
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%CADE%\thispagestyle{empty} 
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%CADE%\end{titlepage}
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%CADE%\tableofcontents\cleardoublepage\pagestyle{headings}
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\section{Introduction}
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Several theorem provers provide commands for formalizing recursive data
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structures, like lists and trees.  Examples include Boyer and Moore's shell
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principle~\cite{bm79} and Melham's recursive type package for the HOL
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system~\cite{melham89}.  Such data structures are called {\bf datatypes}
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below, by analogy with {\tt datatype} definitions in Standard~ML\@.
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A datatype is but one example of an {\bf inductive definition}.  This
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specifies the least set closed under given rules~\cite{aczel77}.  The
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collection of theorems in a logic is inductively defined.  A structural
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operational semantics~\cite{hennessy90} is an inductive definition of a
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reduction or evaluation relation on programs.  A few theorem provers
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provide commands for formalizing inductive definitions; these include
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Coq~\cite{paulin92} and again the HOL system~\cite{camilleri92}.
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The dual notion is that of a {\bf coinductive definition}.  This specifies
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the greatest set closed under given rules.  Important examples include
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using bisimulation relations to formalize equivalence of
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processes~\cite{milner89} or lazy functional programs~\cite{abramsky90}.
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Other examples include lazy lists and other infinite data structures; these
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are called {\bf codatatypes} below.
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Not all inductive definitions are meaningful.  {\bf Monotone} inductive
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definitions are a large, well-behaved class.  Monotonicity can be enforced
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by syntactic conditions such as `strictly positive,' but this could lead to
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monotone definitions being rejected on the grounds of their syntactic form.
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More flexible is to formalize monotonicity within the logic and allow users
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to prove it.
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This paper describes a package based on a fixedpoint approach.  Least
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fixedpoints yield inductive definitions; greatest fixedpoints yield
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coinductive definitions.  The package has several advantages:
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\begin{itemize}
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\item It allows reference to any operators that have been proved monotone.
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  Thus it accepts all provably monotone inductive definitions, including
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  iterated definitions.
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\item It accepts a wide class of datatype definitions, though at present
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  restricted to finite branching.
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\item It handles coinductive and codatatype definitions.  Most of
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  the discussion below applies equally to inductive and coinductive
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  definitions, and most of the code is shared.  To my knowledge, this is
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  the only package supporting coinductive definitions.
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\item Definitions may be mutually recursive.
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\end{itemize}
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The package is implemented in Isabelle~\cite{isabelle-intro}, using ZF set
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theory \cite{paulson-set-I,paulson-set-II}.  However, the fixedpoint
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approach is independent of Isabelle.  The recursion equations are specified
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as introduction rules for the mutually recursive sets.  The package
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transforms these rules into a mapping over sets, and attempts to prove that
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the mapping is monotonic and well-typed.  If successful, the package
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makes fixedpoint definitions and proves the introduction, elimination and
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(co)induction rules.  The package consists of several Standard ML
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functors~\cite{paulson91}; it accepts its argument and returns its result
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as ML structures.\footnote{This use of ML modules is not essential; the
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  package could also be implemented as a function on records.}
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Most datatype packages equip the new datatype with some means of expressing
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recursive functions.  This is the main omission from my package.  Its
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fixedpoint operators define only recursive sets.  To define recursive
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functions, the Isabelle/ZF theory provides well-founded recursion and other
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logical tools~\cite{paulson-set-II}.
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{\bf Outline.} Section~2 introduces the least and greatest fixedpoint
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operators.  Section~3 discusses the form of introduction rules, mutual
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recursion and other points common to inductive and coinductive definitions.
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Section~4 discusses induction and coinduction rules separately.  Section~5
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presents several examples, including a coinductive definition.  Section~6
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describes datatype definitions.  Section~7 presents related work.
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Section~8 draws brief conclusions.  \ifCADE\else The appendices are simple
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user's manuals for this Isabelle/ZF package.\fi
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Most of the definitions and theorems shown below have been generated by the
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package.  I have renamed some variables to improve readability.
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\section{Fixedpoint operators}
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In set theory, the least and greatest fixedpoint operators are defined as
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follows:
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\begin{eqnarray*}
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   \lfp(D,h)  & \equiv & \inter\{X\sbs D. h(X)\sbs X\} \\
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   \gfp(D,h)  & \equiv & \union\{X\sbs D. X\sbs h(X)\}
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\end{eqnarray*}   
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Let $D$ be a set.  Say that $h$ is {\bf bounded by}~$D$ if $h(D)\sbs D$, and
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{\bf monotone below~$D$} if
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$h(A)\sbs h(B)$ for all $A$ and $B$ such that $A\sbs B\sbs D$.  If $h$ is
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bounded by~$D$ and monotone then both operators yield fixedpoints:
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\begin{eqnarray*}
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   \lfp(D,h)  & = & h(\lfp(D,h)) \\
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   \gfp(D,h)  & = & h(\gfp(D,h)) 
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\end{eqnarray*}   
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These equations are instances of the Knaster-Tarski Theorem, which states
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that every monotonic function over a complete lattice has a
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fixedpoint~\cite{davey&priestley}.  It is obvious from their definitions
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that  $\lfp$ must be the least fixedpoint, and $\gfp$ the greatest.
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This fixedpoint theory is simple.  The Knaster-Tarski Theorem is easy to
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prove.  Showing monotonicity of~$h$ is trivial, in typical cases.  We must
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also exhibit a bounding set~$D$ for~$h$.  Frequently this is trivial, as
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when a set of `theorems' is (co)inductively defined over some previously
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existing set of `formulae.'  Isabelle/ZF provides a suitable bounding set
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for finitely branching (co)datatype definitions; see~\S\ref{univ-sec}
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below.  Bounding sets are also called {\bf domains}.
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The powerset operator is monotone, but by Cantor's Theorem there is no
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set~$A$ such that $A=\pow(A)$.  We cannot put $A=\lfp(D,\pow)$ because
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there is no suitable domain~$D$.  But \S\ref{acc-sec} demonstrates
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that~$\pow$ is still useful in inductive definitions.
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\section{Elements of an inductive or coinductive definition}\label{basic-sec}
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Consider a (co)inductive definition of the sets $R_1$, \ldots,~$R_n$, in
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mutual recursion.  They will be constructed from domains $D_1$,
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\ldots,~$D_n$, respectively.  The construction yields not $R_i\sbs D_i$ but
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$R_i\sbs D_1+\cdots+D_n$, where $R_i$ is contained in the image of~$D_i$
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under an injection.  Reasons for this are discussed
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elsewhere~\cite[\S4.5]{paulson-set-II}.
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The definition may involve arbitrary parameters $\vec{p}=p_1$,
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\ldots,~$p_k$.  Each recursive set then has the form $R_i(\vec{p})$.  The
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parameters must be identical every time they occur within a definition.  This
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would appear to be a serious restriction compared with other systems such as
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Coq~\cite{paulin92}.  For instance, we cannot define the lists of
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$n$ elements as the set $\listn(A,n)$ using rules where the parameter~$n$
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varies.  Section~\ref{listn-sec} describes how to express this set using the
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inductive definition package.
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To avoid clutter below, the recursive sets are shown as simply $R_i$
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instead of $R_i(\vec{p})$.
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\subsection{The form of the introduction rules}\label{intro-sec}
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The body of the definition consists of the desired introduction rules,
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specified as strings.  The conclusion of each rule must have the form $t\in
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R_i$, where $t$ is any term.  Premises typically have the same form, but
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they can have the more general form $t\in M(R_i)$ or express arbitrary
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side-conditions.
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The premise $t\in M(R_i)$ is permitted if $M$ is a monotonic operator on
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sets, satisfying the rule 
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\[ \infer{M(A)\sbs M(B)}{A\sbs B} \]
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The user must supply the package with monotonicity rules for all such premises.
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The ability to introduce new monotone operators makes the approach
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flexible.  A suitable choice of~$M$ and~$t$ can express a lot.  The
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powerset operator $\pow$ is monotone, and the premise $t\in\pow(R)$
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expresses $t\sbs R$; see \S\ref{acc-sec} for an example.  The `list of'
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operator is monotone, as is easily proved by induction.  The premise
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$t\in\lst(R)$ avoids having to encode the effect of~$\lst(R)$ using mutual
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recursion; see \S\ref{primrec-sec} and also my earlier
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paper~\cite[\S4.4]{paulson-set-II}.
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Introduction rules may also contain {\bf side-conditions}.  These are
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premises consisting of arbitrary formulae not mentioning the recursive
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sets. Side-conditions typically involve type-checking.  One example is the
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premise $a\in A$ in the following rule from the definition of lists:
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\[ \infer{\Cons(a,l)\in\lst(A)}{a\in A & l\in\lst(A)} \]
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\subsection{The fixedpoint definitions}
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The package translates the list of desired introduction rules into a fixedpoint
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definition.  Consider, as a running example, the finite powerset operator
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$\Fin(A)$: the set of all finite subsets of~$A$.  It can be
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defined as the least set closed under the rules
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\[  \emptyset\in\Fin(A)  \qquad 
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    \infer{\{a\}\un b\in\Fin(A)}{a\in A & b\in\Fin(A)} 
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\]
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The domain in a (co)inductive definition must be some existing set closed
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under the rules.  A suitable domain for $\Fin(A)$ is $\pow(A)$, the set of all
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subsets of~$A$.  The package generates the definition
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\begin{eqnarray*}
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  \Fin(A) & \equiv &  \lfp(\pow(A), \;
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  \begin{array}[t]{r@{\,}l}
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      \lambda X. \{z\in\pow(A). & z=\emptyset \disj{} \\
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                  &(\exists a\,b. z=\{a\}\un b\conj a\in A\conj b\in X)\})
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  \end{array}
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\end{eqnarray*} 
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The contribution of each rule to the definition of $\Fin(A)$ should be
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obvious.  A coinductive definition is similar but uses $\gfp$ instead
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of~$\lfp$.
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The package must prove that the fixedpoint operator is applied to a
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monotonic function.  If the introduction rules have the form described
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above, and if the package is supplied a monotonicity theorem for every
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$t\in M(R_i)$ premise, then this proof is trivial.\footnote{Due to the
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  presence of logical connectives in the fixedpoint's body, the
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  monotonicity proof requires some unusual rules.  These state that the
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  connectives $\conj$, $\disj$ and $\exists$ preserve monotonicity with respect
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  to the partial ordering on unary predicates given by $P\sqsubseteq Q$ if and
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  only if $\forall x.P(x)\imp Q(x)$.}
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The package returns its result as an ML structure, which consists of named
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components; we may regard it as a record.  The result structure contains
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the definitions of the recursive sets as a theorem list called {\tt defs}.
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It also contains, as the theorem {\tt unfold}, a fixedpoint equation such
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as
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\begin{eqnarray*}
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  \Fin(A) & = &
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  \begin{array}[t]{r@{\,}l}
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     \{z\in\pow(A). & z=\emptyset \disj{} \\
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             &(\exists a\,b. z=\{a\}\un b\conj a\in A\conj b\in \Fin(A))\}
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  \end{array}
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\end{eqnarray*}
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It also contains, as the theorem {\tt dom\_subset}, an inclusion such as 
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$\Fin(A)\sbs\pow(A)$.
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\subsection{Mutual recursion} \label{mutual-sec}
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In a mutually recursive definition, the domain of the fixedpoint construction
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is the disjoint sum of the domain~$D_i$ of each~$R_i$, for $i=1$,
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\ldots,~$n$.  The package uses the injections of the
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binary disjoint sum, typically $\Inl$ and~$\Inr$, to express injections
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$h_{1n}$, \ldots, $h_{nn}$ for the $n$-ary disjoint sum $D_1+\cdots+D_n$.
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As discussed elsewhere \cite[\S4.5]{paulson-set-II}, Isabelle/ZF defines the
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operator $\Part$ to support mutual recursion.  The set $\Part(A,h)$
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contains those elements of~$A$ having the form~$h(z)$:
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\begin{eqnarray*}
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   \Part(A,h)  & \equiv & \{x\in A. \exists z. x=h(z)\}.
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\end{eqnarray*}   
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For mutually recursive sets $R_1$, \ldots,~$R_n$ with
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$n>1$, the package makes $n+1$ definitions.  The first defines a set $R$ using
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a fixedpoint operator. The remaining $n$ definitions have the form
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\begin{eqnarray*}
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  R_i & \equiv & \Part(R,h_{in}), \qquad i=1,\ldots, n.
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\end{eqnarray*} 
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It follows that $R=R_1\un\cdots\un R_n$, where the $R_i$ are pairwise disjoint.
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\subsection{Proving the introduction rules}
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The user supplies the package with the desired form of the introduction
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rules.  Once it has derived the theorem {\tt unfold}, it attempts
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to prove those rules.  From the user's point of view, this is the
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trickiest stage; the proofs often fail.  The task is to show that the domain 
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$D_1+\cdots+D_n$ of the combined set $R_1\un\cdots\un R_n$ is
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closed under all the introduction rules.  This essentially involves replacing
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each~$R_i$ by $D_1+\cdots+D_n$ in each of the introduction rules and
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attempting to prove the result.
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Consider the $\Fin(A)$ example.  After substituting $\pow(A)$ for $\Fin(A)$
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in the rules, the package must prove
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\[  \emptyset\in\pow(A)  \qquad 
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    \infer{\{a\}\un b\in\pow(A)}{a\in A & b\in\pow(A)} 
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\]
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Such proofs can be regarded as type-checking the definition.  The user
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supplies the package with type-checking rules to apply.  Usually these are
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general purpose rules from the ZF theory.  They could however be rules
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specifically proved for a particular inductive definition; sometimes this is
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the easiest way to get the definition through!
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The result structure contains the introduction rules as the theorem list {\tt
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intrs}.
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\subsection{The case analysis rule}
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The elimination rule, called {\tt elim}, performs case analysis.  There is one
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case for each introduction rule.  The elimination rule
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for $\Fin(A)$ is
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\[ \infer{Q}{x\in\Fin(A) & \infer*{Q}{[x=\emptyset]}
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                 & \infer*{Q}{[x=\{a\}\un b & a\in A &b\in\Fin(A)]_{a,b}} }
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\]
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The subscripted variables $a$ and~$b$ above the third premise are
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eigenvariables, subject to the usual `not free in \ldots' proviso.
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The rule states that if $x\in\Fin(A)$ then either $x=\emptyset$ or else
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$x=\{a\}\un b$ for some $a\in A$ and $b\in\Fin(A)$; it is a simple consequence
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of {\tt unfold}.
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The package also returns a function for generating simplified instances of
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the case analysis rule.  It works for datatypes and for inductive
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definitions involving datatypes, such as an inductively defined relation
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between lists.  It instantiates {\tt elim} with a user-supplied term then
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simplifies the cases using freeness of the underlying datatype.  The
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simplified rules perform `rule inversion' on the inductive definition.
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Section~\S\ref{mkcases} presents an example.
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\section{Induction and coinduction rules}
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Here we must consider inductive and coinductive definitions separately.
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For an inductive definition, the package returns an induction rule derived
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directly from the properties of least fixedpoints, as well as a modified
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rule for mutual recursion and inductively defined relations.  For a
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coinductive definition, the package returns a basic coinduction rule.
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\subsection{The basic induction rule}\label{basic-ind-sec}
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The basic rule, called {\tt induct}, is appropriate in most situations.
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For inductive definitions, it is strong rule induction~\cite{camilleri92}; for
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datatype definitions (see below), it is just structural induction.  
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The induction rule for an inductively defined set~$R$ has the following form.
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The major premise is $x\in R$.  There is a minor premise for each
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introduction rule:
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\begin{itemize}
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\item If the introduction rule concludes $t\in R_i$, then the minor premise
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is~$P(t)$.
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\item The minor premise's eigenvariables are precisely the introduction
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rule's free variables that are not parameters of~$R$.  For instance, the
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eigenvariables in the $\Fin(A)$ rule below are $a$ and $b$, but not~$A$.
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\item If the introduction rule has a premise $t\in R_i$, then the minor
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premise discharges the assumption $t\in R_i$ and the induction
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hypothesis~$P(t)$.  If the introduction rule has a premise $t\in M(R_i)$
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then the minor premise discharges the single assumption
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\[ t\in M(\{z\in R_i. P(z)\}). \] 
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Because $M$ is monotonic, this assumption implies $t\in M(R_i)$.  The
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occurrence of $P$ gives the effect of an induction hypothesis, which may be
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exploited by appealing to properties of~$M$.
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\end{itemize}
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The induction rule for $\Fin(A)$ resembles the elimination rule shown above,
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but includes an induction hypothesis:
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\[ \infer{P(x)}{x\in\Fin(A) & P(\emptyset)
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        & \infer*{P(\{a\}\un b)}{[a\in A & b\in\Fin(A) & P(b)]_{a,b}} }
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\] 
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Stronger induction rules often suggest themselves.  We can derive a rule
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for $\Fin(A)$ whose third premise discharges the extra assumption $a\not\in
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b$.  The Isabelle/ZF theory defines the {\bf rank} of a
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set~\cite[\S3.4]{paulson-set-II}, which supports well-founded induction and
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recursion over datatypes.  The package proves a rule for mutual induction
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   431
and inductive relations.
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\subsection{Mutual induction}
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The mutual induction rule is called {\tt
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mutual\_induct}.  It differs from the basic rule in several respects:
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\begin{itemize}
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\item Instead of a single predicate~$P$, it uses $n$ predicates $P_1$,
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\ldots,~$P_n$: one for each recursive set.
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\item There is no major premise such as $x\in R_i$.  Instead, the conclusion
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refers to all the recursive sets:
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\[ (\forall z.z\in R_1\imp P_1(z))\conj\cdots\conj
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   (\forall z.z\in R_n\imp P_n(z))
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\]
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Proving the premises establishes $P_i(z)$ for $z\in R_i$ and $i=1$,
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\ldots,~$n$.
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\item If the domain of some $R_i$ is the Cartesian product
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$A_1\times\cdots\times A_m$, then the corresponding predicate $P_i$ takes $m$
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arguments and the corresponding conjunct of the conclusion is
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\[ (\forall z_1\ldots z_m.\pair{z_1,\ldots,z_m}\in R_i\imp P_i(z_1,\ldots,z_m))
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\]
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\end{itemize}
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The last point above simplifies reasoning about inductively defined
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   455
relations.  It eliminates the need to express properties of $z_1$,
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\ldots,~$z_m$ as properties of the tuple $\pair{z_1,\ldots,z_m}$.
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\subsection{Coinduction}\label{coind-sec}
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A coinductive definition yields a primitive coinduction rule, with no
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refinements such as those for the induction rules.  (Experience may suggest
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refinements later.)  Consider the codatatype of lazy lists as an example.  For
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   462
suitable definitions of $\LNil$ and $\LCons$, lazy lists may be defined as the
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   463
greatest fixedpoint satisfying the rules
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\[  \LNil\in\llist(A)  \qquad 
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    \infer[(-)]{\LCons(a,l)\in\llist(A)}{a\in A & l\in\llist(A)}
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\]
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The $(-)$ tag stresses that this is a coinductive definition.  A suitable
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   468
domain for $\llist(A)$ is $\quniv(A)$, a set closed under variant forms of
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   469
sum and product for representing infinite data structures
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(see~\S\ref{univ-sec}).  Coinductive definitions use these variant sums and
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products.
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diff changeset
   472
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The package derives an {\tt unfold} theorem similar to that for $\Fin(A)$. 
355
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Then it proves the theorem {\tt coinduct}, which expresses that $\llist(A)$
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is the greatest solution to this equation contained in $\quniv(A)$:
130
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\[ \infer{x\in\llist(A)}{x\in X & X\sbs \quniv(A) &
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    \infer*{z=\LNil\disj \bigl(\exists a\,l.\,
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            z=\LCons(a,l) \conj a\in A \conj l\in X\un\llist(A) \bigr)}
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           {[z\in X]_z}}
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parents: 181
diff changeset
   480
%     \begin{array}[t]{@{}l}
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   481
%       z=\LCons(a,l) \conj a\in A \conj{}\\
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   482
%       l\in X\un\llist(A) \bigr)
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   483
%     \end{array}  }{[z\in X]_z}}
103
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\]
130
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   485
This rule complements the introduction rules; it provides a means of showing
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   486
$x\in\llist(A)$ when $x$ is infinite.  For instance, if $x=\LCons(0,x)$ then
355
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lcp
parents: 181
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   487
applying the rule with $X=\{x\}$ proves $x\in\llist(\nat)$.  (Here $\nat$
77150178beb2 post-CRC corrections
lcp
parents: 181
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   488
is the set of natural numbers.)
130
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parents: 103
diff changeset
   489
103
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   490
Having $X\un\llist(A)$ instead of simply $X$ in the third premise above
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parents:
diff changeset
   491
represents a slight strengthening of the greatest fixedpoint property.  I
130
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   492
discuss several forms of coinduction rules elsewhere~\cite{paulson-coind}.
103
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diff changeset
   493
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diff changeset
   494
130
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   495
\section{Examples of inductive and coinductive definitions}\label{ind-eg-sec}
455
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   496
This section presents several examples: the finite powerset operator,
103
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   497
lists of $n$ elements, bisimulations on lazy lists, the well-founded part
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diff changeset
   498
of a relation, and the primitive recursive functions.
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diff changeset
   499
455
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parents: 355
diff changeset
   500
\subsection{The finite powerset operator}
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diff changeset
   501
This operator has been discussed extensively above.  Here
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   502
is the corresponding ML invocation (note that $\cons(a,b)$ abbreviates
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parents:
diff changeset
   503
$\{a\}\un b$ in Isabelle/ZF):
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diff changeset
   504
\begin{ttbox}
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   505
structure Fin = Inductive_Fun
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parents: 181
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   506
 (val thy        = Arith.thy addconsts [(["Fin"],"i=>i")]
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parents: 181
diff changeset
   507
  val rec_doms   = [("Fin","Pow(A)")]
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parents: 181
diff changeset
   508
  val sintrs     = ["0 : Fin(A)",
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lcp
parents: 181
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   509
                    "[| a: A;  b: Fin(A) |] ==> cons(a,b) : Fin(A)"]
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diff changeset
   510
  val monos      = []
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lcp
parents: 181
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   511
  val con_defs   = []
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   512
  val type_intrs = [empty_subsetI, cons_subsetI, PowI]
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   513
  val type_elims = [make_elim PowD]);
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diff changeset
   514
\end{ttbox}
355
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parents: 181
diff changeset
   515
We apply the functor {\tt Inductive\_Fun} to a structure describing the
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lcp
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   516
desired inductive definition.  The parent theory~{\tt thy} is obtained from
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lcp
parents: 181
diff changeset
   517
{\tt Arith.thy} by adding the unary function symbol~$\Fin$.  Its domain is
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lcp
parents: 181
diff changeset
   518
specified as $\pow(A)$, where $A$ is the parameter appearing in the
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lcp
parents: 181
diff changeset
   519
introduction rules.  For type-checking, the structure supplies introduction
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lcp
parents: 181
diff changeset
   520
rules:
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   521
\[ \emptyset\sbs A              \qquad
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lcp
parents:
diff changeset
   522
   \infer{\{a\}\un B\sbs C}{a\in C & B\sbs C}
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lcp
parents:
diff changeset
   523
\]
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lcp
parents:
diff changeset
   524
A further introduction rule and an elimination rule express the two
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   525
directions of the equivalence $A\in\pow(B)\bimp A\sbs B$.  Type-checking
355
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lcp
parents: 181
diff changeset
   526
involves mostly introduction rules.  
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lcp
parents: 181
diff changeset
   527
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   528
ML is Isabelle's top level, so such functor invocations can take place at
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   529
any time.  The result structure is declared with the name~{\tt Fin}; we can
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lcp
parents: 181
diff changeset
   530
refer to the $\Fin(A)$ introduction rules as {\tt Fin.intrs}, the induction
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lcp
parents: 181
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   531
rule as {\tt Fin.induct} and so forth.  There are plans to integrate the
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lcp
parents: 181
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   532
package better into Isabelle so that users can place inductive definitions
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lcp
parents: 181
diff changeset
   533
in Isabelle theory files instead of applying functors.
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lcp
parents: 181
diff changeset
   534
103
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diff changeset
   535
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parents:
diff changeset
   536
\subsection{Lists of $n$ elements}\label{listn-sec}
179
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lcp
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   537
This has become a standard example of an inductive definition.  Following
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diff changeset
   538
Paulin-Mohring~\cite{paulin92}, we could attempt to define a new datatype
ceb948cefb93 minor corrections
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diff changeset
   539
$\listn(A,n)$, for lists of length~$n$, as an $n$-indexed family of sets.
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diff changeset
   540
But her introduction rules
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   541
\[ \hbox{\tt Niln}\in\listn(A,0)  \qquad
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   542
   \infer{\hbox{\tt Consn}(n,a,l)\in\listn(A,\succ(n))}
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parents:
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   543
         {n\in\nat & a\in A & l\in\listn(A,n)}
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lcp
parents:
diff changeset
   544
\]
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lcp
parents:
diff changeset
   545
are not acceptable to the inductive definition package:
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lcp
parents:
diff changeset
   546
$\listn$ occurs with three different parameter lists in the definition.
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parents:
diff changeset
   547
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   548
\begin{figure}
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   549
\begin{ttbox}
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structure ListN = Inductive_Fun
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   551
 (val thy        = ListFn.thy addconsts [(["listn"],"i=>i")]
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   552
  val rec_doms   = [("listn", "nat*list(A)")]
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lcp
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   553
  val sintrs     = 
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   554
        ["<0,Nil>: listn(A)",
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         "[| a: A;  <n,l>: listn(A) |] ==> <succ(n), Cons(a,l)>: listn(A)"]
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   556
  val monos      = []
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   557
  val con_defs   = []
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   558
  val type_intrs = nat_typechecks @ List.intrs @ [SigmaI]
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   559
  val type_elims = [SigmaE2]);
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diff changeset
   560
\end{ttbox}
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   561
\hrule
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   562
\caption{Defining lists of $n$ elements} \label{listn-fig}
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parents:
diff changeset
   563
\end{figure} 
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parents:
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   564
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   565
The Isabelle/ZF version of this example suggests a general treatment of
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   566
varying parameters.  Here, we use the existing datatype definition of
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   567
$\lst(A)$, with constructors $\Nil$ and~$\Cons$.  Then incorporate the
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   568
parameter~$n$ into the inductive set itself, defining $\listn(A)$ as a
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parents: 181
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   569
relation.  It consists of pairs $\pair{n,l}$ such that $n\in\nat$
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parents: 181
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   570
and~$l\in\lst(A)$ and $l$ has length~$n$.  In fact, $\listn(A)$ is the
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   571
converse of the length function on~$\lst(A)$.  The Isabelle/ZF introduction
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diff changeset
   572
rules are
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parents:
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   573
\[ \pair{0,\Nil}\in\listn(A)  \qquad
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lcp
parents:
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   574
   \infer{\pair{\succ(n),\Cons(a,l)}\in\listn(A)}
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lcp
parents:
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   575
         {a\in A & \pair{n,l}\in\listn(A)}
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lcp
parents:
diff changeset
   576
\]
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lcp
parents:
diff changeset
   577
Figure~\ref{listn-fig} presents the ML invocation.  A theory of lists,
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lcp
parents:
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   578
extended with a declaration of $\listn$, is the parent theory.  The domain
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lcp
parents:
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   579
is specified as $\nat\times\lst(A)$.  The type-checking rules include those
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lcp
parents:
diff changeset
   580
for 0, $\succ$, $\Nil$ and $\Cons$.  Because $\listn(A)$ is a set of pairs,
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lcp
parents:
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   581
type-checking also requires introduction and elimination rules to express
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lcp
parents:
diff changeset
   582
both directions of the equivalence $\pair{a,b}\in A\times B \bimp a\in A
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lcp
parents:
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   583
\conj b\in B$. 
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lcp
parents:
diff changeset
   584
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lcp
parents:
diff changeset
   585
The package returns introduction, elimination and induction rules for
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lcp
parents:
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   586
$\listn$.  The basic induction rule, {\tt ListN.induct}, is
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lcp
parents:
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   587
\[ \infer{P(x)}{x\in\listn(A) & P(\pair{0,\Nil}) &
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lcp
parents:
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   588
             \infer*{P(\pair{\succ(n),\Cons(a,l)})}
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lcp
parents:
diff changeset
   589
                {[a\in A & \pair{n,l}\in\listn(A) & P(\pair{n,l})]_{a,l,n}}}
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lcp
parents:
diff changeset
   590
\]
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lcp
parents:
diff changeset
   591
This rule requires the induction formula to be a 
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lcp
parents:
diff changeset
   592
unary property of pairs,~$P(\pair{n,l})$.  The alternative rule, {\tt
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   593
ListN.mutual\_induct}, uses a binary property instead:
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parents: 103
diff changeset
   594
\[ \infer{\forall n\,l. \pair{n,l}\in\listn(A) \imp P(n,l)}
103
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parents:
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   595
         {P(0,\Nil) &
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lcp
parents:
diff changeset
   596
          \infer*{P(\succ(n),\Cons(a,l))}
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lcp
parents:
diff changeset
   597
                {[a\in A & \pair{n,l}\in\listn(A) & P(n,l)]_{a,l,n}}}
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lcp
parents:
diff changeset
   598
\]
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lcp
parents:
diff changeset
   599
It is now a simple matter to prove theorems about $\listn(A)$, such as
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lcp
parents:
diff changeset
   600
\[ \forall l\in\lst(A). \pair{\length(l),\, l}\in\listn(A) \]
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lcp
parents:
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   601
\[ \listn(A)``\{n\} = \{l\in\lst(A). \length(l)=n\} \]
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lcp
parents: 103
diff changeset
   602
This latter result --- here $r``X$ denotes the image of $X$ under $r$
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lcp
parents:
diff changeset
   603
--- asserts that the inductive definition agrees with the obvious notion of
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   604
$n$-element list.  
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   605
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lcp
parents:
diff changeset
   606
Unlike in Coq, the definition does not declare a new datatype.  A `list of
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lcp
parents: 103
diff changeset
   607
$n$ elements' really is a list and is subject to list operators such
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   608
as append (concatenation).  For example, a trivial induction on
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lcp
parents: 103
diff changeset
   609
$\pair{m,l}\in\listn(A)$ yields
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lcp
parents:
diff changeset
   610
\[ \infer{\pair{m\mathbin{+} m,\, l@l'}\in\listn(A)}
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lcp
parents:
diff changeset
   611
         {\pair{m,l}\in\listn(A) & \pair{m',l'}\in\listn(A)} 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   612
\]
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lcp
parents:
diff changeset
   613
where $+$ here denotes addition on the natural numbers and @ denotes append.
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lcp
parents:
diff changeset
   614
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lcp
parents: 181
diff changeset
   615
\subsection{A demonstration of rule inversion}\label{mkcases}
103
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lcp
parents:
diff changeset
   616
The elimination rule, {\tt ListN.elim}, is cumbersome:
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lcp
parents:
diff changeset
   617
\[ \infer{Q}{x\in\listn(A) & 
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lcp
parents:
diff changeset
   618
          \infer*{Q}{[x = \pair{0,\Nil}]} &
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lcp
parents:
diff changeset
   619
          \infer*{Q}
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lcp
parents:
diff changeset
   620
             {\left[\begin{array}{l}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   621
               x = \pair{\succ(n),\Cons(a,l)} \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   622
               a\in A \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   623
               \pair{n,l}\in\listn(A)
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   624
               \end{array} \right]_{a,l,n}}}
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lcp
parents:
diff changeset
   625
\]
179
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lcp
parents: 130
diff changeset
   626
The ML function {\tt ListN.mk\_cases} generates simplified instances of
ceb948cefb93 minor corrections
lcp
parents: 130
diff changeset
   627
this rule.  It works by freeness reasoning on the list constructors:
ceb948cefb93 minor corrections
lcp
parents: 130
diff changeset
   628
$\Cons(a,l)$ is injective in its two arguments and differs from~$\Nil$.  If
ceb948cefb93 minor corrections
lcp
parents: 130
diff changeset
   629
$x$ is $\pair{i,\Nil}$ or $\pair{i,\Cons(a,l)}$ then {\tt ListN.mk\_cases}
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lcp
parents: 181
diff changeset
   630
deduces the corresponding form of~$i$;  this is called rule inversion.  For
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   631
example, 
103
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lcp
parents:
diff changeset
   632
\begin{ttbox}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   633
ListN.mk_cases List.con_defs "<i,Cons(a,l)> : listn(A)"
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   634
\end{ttbox}
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lcp
parents: 103
diff changeset
   635
yields a rule with only two premises:
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lcp
parents:
diff changeset
   636
\[ \infer{Q}{\pair{i, \Cons(a,l)}\in\listn(A) & 
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lcp
parents:
diff changeset
   637
          \infer*{Q}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   638
             {\left[\begin{array}{l}
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lcp
parents:
diff changeset
   639
               i = \succ(n) \\ a\in A \\ \pair{n,l}\in\listn(A)
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   640
               \end{array} \right]_{n}}}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   641
\]
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lcp
parents:
diff changeset
   642
The package also has built-in rules for freeness reasoning about $0$
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   643
and~$\succ$.  So if $x$ is $\pair{0,l}$ or $\pair{\succ(i),l}$, then {\tt
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   644
ListN.mk\_cases} can similarly deduce the corresponding form of~$l$. 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   645
355
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lcp
parents: 181
diff changeset
   646
The function {\tt mk\_cases} is also useful with datatype definitions.  The
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lcp
parents: 181
diff changeset
   647
instance from the definition of lists, namely {\tt List.mk\_cases}, can
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lcp
parents: 181
diff changeset
   648
prove the rule
103
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lcp
parents:
diff changeset
   649
\[ \infer{Q}{\Cons(a,l)\in\lst(A) & 
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lcp
parents:
diff changeset
   650
                 & \infer*{Q}{[a\in A &l\in\lst(A)]} }
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lcp
parents:
diff changeset
   651
\]
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lcp
parents: 181
diff changeset
   652
A typical use of {\tt mk\_cases} concerns inductive definitions of
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lcp
parents: 181
diff changeset
   653
evaluation relations.  Then rule inversion yields case analysis on possible
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   654
evaluations.  For example, the Isabelle/ZF theory includes a short proof
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lcp
parents: 181
diff changeset
   655
of the diamond property for parallel contraction on combinators.
103
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lcp
parents:
diff changeset
   656
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lcp
parents: 103
diff changeset
   657
\subsection{A coinductive definition: bisimulations on lazy lists}
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lcp
parents: 103
diff changeset
   658
This example anticipates the definition of the codatatype $\llist(A)$, which
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   659
consists of finite and infinite lists over~$A$.  Its constructors are $\LNil$
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   660
and
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   661
$\LCons$, satisfying the introduction rules shown in~\S\ref{coind-sec}.  
103
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lcp
parents:
diff changeset
   662
Because $\llist(A)$ is defined as a greatest fixedpoint and uses the variant
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   663
pairing and injection operators, it contains non-well-founded elements such as
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   664
solutions to $\LCons(a,l)=l$.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   665
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   666
The next step in the development of lazy lists is to define a coinduction
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   667
principle for proving equalities.  This is done by showing that the equality
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   668
relation on lazy lists is the greatest fixedpoint of some monotonic
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   669
operation.  The usual approach~\cite{pitts94} is to define some notion of 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   670
bisimulation for lazy lists, define equivalence to be the greatest
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   671
bisimulation, and finally to prove that two lazy lists are equivalent if and
130
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lcp
parents: 103
diff changeset
   672
only if they are equal.  The coinduction rule for equivalence then yields a
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   673
coinduction principle for equalities.
103
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lcp
parents:
diff changeset
   674
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   675
A binary relation $R$ on lazy lists is a {\bf bisimulation} provided $R\sbs
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   676
R^+$, where $R^+$ is the relation
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lcp
parents: 103
diff changeset
   677
\[ \{\pair{\LNil,\LNil}\} \un 
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   678
   \{\pair{\LCons(a,l),\LCons(a,l')} . a\in A \conj \pair{l,l'}\in R\}.
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   679
\]
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   680
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   681
A pair of lazy lists are {\bf equivalent} if they belong to some bisimulation. 
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lcp
parents: 103
diff changeset
   682
Equivalence can be coinductively defined as the greatest fixedpoint for the
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   683
introduction rules
130
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lcp
parents: 103
diff changeset
   684
\[  \pair{\LNil,\LNil} \in\lleq(A)  \qquad 
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   685
    \infer[(-)]{\pair{\LCons(a,l),\LCons(a,l')} \in\lleq(A)}
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   686
          {a\in A & \pair{l,l'}\in \lleq(A)}
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   687
\]
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   688
To make this coinductive definition, we invoke \verb|CoInductive_Fun|:
103
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lcp
parents:
diff changeset
   689
\begin{ttbox}
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lcp
parents: 103
diff changeset
   690
structure LList_Eq = CoInductive_Fun
355
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lcp
parents: 181
diff changeset
   691
 (val thy = LList.thy addconsts [(["lleq"],"i=>i")]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   692
  val rec_doms   = [("lleq", "llist(A) * llist(A)")]
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lcp
parents: 181
diff changeset
   693
  val sintrs     = 
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   694
       ["<LNil, LNil> : lleq(A)",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   695
        "[| a:A; <l,l'>: lleq(A) |] ==> <LCons(a,l),LCons(a,l')>: lleq(A)"]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   696
  val monos      = []
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   697
  val con_defs   = []
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   698
  val type_intrs = LList.intrs @ [SigmaI]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   699
  val type_elims = [SigmaE2]);
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   700
\end{ttbox}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   701
Again, {\tt addconsts} declares a constant for $\lleq$ in the parent theory. 
130
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lcp
parents: 103
diff changeset
   702
The domain of $\lleq(A)$ is $\llist(A)\times\llist(A)$.  The type-checking
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   703
rules include the introduction rules for lazy lists as well as rules
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   704
for both directions of the equivalence
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   705
$\pair{a,b}\in A\times B \bimp a\in A \conj b\in B$.
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   706
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   707
The package returns the introduction rules and the elimination rule, as
130
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lcp
parents: 103
diff changeset
   708
usual.  But instead of induction rules, it returns a coinduction rule.
103
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lcp
parents:
diff changeset
   709
The rule is too big to display in the usual notation; its conclusion is
130
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lcp
parents: 103
diff changeset
   710
$x\in\lleq(A)$ and its premises are $x\in X$, 
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   711
${X\sbs\llist(A)\times\llist(A)}$ and
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   712
\[ \infer*{z=\pair{\LNil,\LNil}\disj \bigl(\exists a\,l\,l'.\,
355
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lcp
parents: 181
diff changeset
   713
      z=\pair{\LCons(a,l),\LCons(a,l')} \conj 
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   714
      a\in A \conj\pair{l,l'}\in X\un\lleq(A) \bigr)
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lcp
parents: 181
diff changeset
   715
%     \begin{array}[t]{@{}l}
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lcp
parents: 181
diff changeset
   716
%       z=\pair{\LCons(a,l),\LCons(a,l')} \conj a\in A \conj{}\\
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lcp
parents: 181
diff changeset
   717
%       \pair{l,l'}\in X\un\lleq(A) \bigr)
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lcp
parents: 181
diff changeset
   718
%     \end{array}  
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   719
    }{[z\in X]_z}
103
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lcp
parents:
diff changeset
   720
\]
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parents: 103
diff changeset
   721
Thus if $x\in X$, where $X$ is a bisimulation contained in the
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   722
domain of $\lleq(A)$, then $x\in\lleq(A)$.  It is easy to show that
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   723
$\lleq(A)$ is reflexive: the equality relation is a bisimulation.  And
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   724
$\lleq(A)$ is symmetric: its converse is a bisimulation.  But showing that
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   725
$\lleq(A)$ coincides with the equality relation takes some work.
103
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lcp
parents:
diff changeset
   726
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   727
\subsection{The accessible part of a relation}\label{acc-sec}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   728
Let $\prec$ be a binary relation on~$D$; in short, $(\prec)\sbs D\times D$.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   729
The {\bf accessible} or {\bf well-founded} part of~$\prec$, written
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   730
$\acc(\prec)$, is essentially that subset of~$D$ for which $\prec$ admits
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   731
no infinite decreasing chains~\cite{aczel77}.  Formally, $\acc(\prec)$ is
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   732
inductively defined to be the least set that contains $a$ if it contains
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   733
all $\prec$-predecessors of~$a$, for $a\in D$.  Thus we need an
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   734
introduction rule of the form 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   735
\[ \infer{a\in\acc(\prec)}{\forall y.y\prec a\imp y\in\acc(\prec)} \]
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   736
Paulin-Mohring treats this example in Coq~\cite{paulin92}, but it causes
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   737
difficulties for other systems.  Its premise does not conform to 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   738
the structure of introduction rules for HOL's inductive definition
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   739
package~\cite{camilleri92}.  It is also unacceptable to Isabelle package
130
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lcp
parents: 103
diff changeset
   740
(\S\ref{intro-sec}), but fortunately can be transformed into the acceptable
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   741
form $t\in M(R)$.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   742
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   743
The powerset operator is monotonic, and $t\in\pow(R)$ is equivalent to
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   744
$t\sbs R$.  This in turn is equivalent to $\forall y\in t. y\in R$.  To
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   745
express $\forall y.y\prec a\imp y\in\acc(\prec)$ we need only find a
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   746
term~$t$ such that $y\in t$ if and only if $y\prec a$.  A suitable $t$ is
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   747
the inverse image of~$\{a\}$ under~$\prec$.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   748
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   749
The ML invocation below follows this approach.  Here $r$ is~$\prec$ and
130
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parents: 103
diff changeset
   750
$\field(r)$ refers to~$D$, the domain of $\acc(r)$.  (The field of a
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   751
relation is the union of its domain and range.)  Finally
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   752
$r^{-}``\{a\}$ denotes the inverse image of~$\{a\}$ under~$r$.  The package is
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   753
supplied the theorem {\tt Pow\_mono}, which asserts that $\pow$ is monotonic.
103
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lcp
parents:
diff changeset
   754
\begin{ttbox}
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lcp
parents:
diff changeset
   755
structure Acc = Inductive_Fun
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lcp
parents: 181
diff changeset
   756
 (val thy        = WF.thy addconsts [(["acc"],"i=>i")]
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lcp
parents: 181
diff changeset
   757
  val rec_doms   = [("acc", "field(r)")]
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lcp
parents: 181
diff changeset
   758
  val sintrs     = ["[| r-``\{a\}:\,Pow(acc(r)); a:\,field(r) |] ==> a:\,acc(r)"]
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lcp
parents: 181
diff changeset
   759
  val monos      = [Pow_mono]
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lcp
parents: 181
diff changeset
   760
  val con_defs   = []
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   761
  val type_intrs = []
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   762
  val type_elims = []);
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   763
\end{ttbox}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   764
The Isabelle theory proceeds to prove facts about $\acc(\prec)$.  For
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   765
instance, $\prec$ is well-founded if and only if its field is contained in
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   766
$\acc(\prec)$.  
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   767
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   768
As mentioned in~\S\ref{basic-ind-sec}, a premise of the form $t\in M(R)$
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   769
gives rise to an unusual induction hypothesis.  Let us examine the
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   770
induction rule, {\tt Acc.induct}:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   771
\[ \infer{P(x)}{x\in\acc(r) &
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   772
     \infer*{P(a)}{[r^{-}``\{a\}\in\pow(\{z\in\acc(r).P(z)\}) & 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   773
                   a\in\field(r)]_a}}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   774
\]
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   775
The strange induction hypothesis is equivalent to
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   776
$\forall y. \pair{y,a}\in r\imp y\in\acc(r)\conj P(y)$.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   777
Therefore the rule expresses well-founded induction on the accessible part
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   778
of~$\prec$.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   779
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   780
The use of inverse image is not essential.  The Isabelle package can accept
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   781
introduction rules with arbitrary premises of the form $\forall
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   782
\vec{y}.P(\vec{y})\imp f(\vec{y})\in R$.  The premise can be expressed
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   783
equivalently as 
130
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lcp
parents: 103
diff changeset
   784
\[ \{z\in D. P(\vec{y}) \conj z=f(\vec{y})\} \in \pow(R) \] 
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   785
provided $f(\vec{y})\in D$ for all $\vec{y}$ such that~$P(\vec{y})$.  The
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   786
following section demonstrates another use of the premise $t\in M(R)$,
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   787
where $M=\lst$. 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   788
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   789
\subsection{The primitive recursive functions}\label{primrec-sec}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   790
The primitive recursive functions are traditionally defined inductively, as
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   791
a subset of the functions over the natural numbers.  One difficulty is that
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   792
functions of all arities are taken together, but this is easily
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   793
circumvented by regarding them as functions on lists.  Another difficulty,
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   794
the notion of composition, is less easily circumvented.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   795
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   796
Here is a more precise definition.  Letting $\vec{x}$ abbreviate
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   797
$x_0,\ldots,x_{n-1}$, we can write lists such as $[\vec{x}]$,
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   798
$[y+1,\vec{x}]$, etc.  A function is {\bf primitive recursive} if it
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   799
belongs to the least set of functions in $\lst(\nat)\to\nat$ containing
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   800
\begin{itemize}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   801
\item The {\bf successor} function $\SC$, such that $\SC[y,\vec{x}]=y+1$.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   802
\item All {\bf constant} functions $\CONST(k)$, such that
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   803
  $\CONST(k)[\vec{x}]=k$. 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   804
\item All {\bf projection} functions $\PROJ(i)$, such that
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   805
  $\PROJ(i)[\vec{x}]=x_i$ if $0\leq i<n$. 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   806
\item All {\bf compositions} $\COMP(g,[f_0,\ldots,f_{m-1}])$, 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   807
where $g$ and $f_0$, \ldots, $f_{m-1}$ are primitive recursive,
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   808
such that
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   809
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   810
  \COMP(g,[f_0,\ldots,f_{m-1}])[\vec{x}] & = & 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   811
  g[f_0[\vec{x}],\ldots,f_{m-1}[\vec{x}]].
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   812
\end{eqnarray*} 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   813
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   814
\item All {\bf recursions} $\PREC(f,g)$, where $f$ and $g$ are primitive
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   815
  recursive, such that
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   816
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   817
  \PREC(f,g)[0,\vec{x}] & = & f[\vec{x}] \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   818
  \PREC(f,g)[y+1,\vec{x}] & = & g[\PREC(f,g)[y,\vec{x}],\, y,\, \vec{x}].
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   819
\end{eqnarray*} 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   820
\end{itemize}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   821
Composition is awkward because it combines not two functions, as is usual,
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   822
but $m+1$ functions.  In her proof that Ackermann's function is not
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   823
primitive recursive, Nora Szasz was unable to formalize this definition
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   824
directly~\cite{szasz93}.  So she generalized primitive recursion to
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   825
tuple-valued functions.  This modified the inductive definition such that
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   826
each operation on primitive recursive functions combined just two functions.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   827
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   828
\begin{figure}
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   829
\begin{ttbox}
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   830
structure Primrec = Inductive_Fun
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   831
 (val thy        = Primrec0.thy
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   832
  val rec_doms   = [("primrec", "list(nat)->nat")]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   833
  val sintrs     = 
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   834
        ["SC : primrec",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   835
         "k: nat ==> CONST(k) : primrec",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   836
         "i: nat ==> PROJ(i) : primrec",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   837
         "[| g: primrec; fs: list(primrec) |] ==> COMP(g,fs): primrec",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   838
         "[| f: primrec; g: primrec |] ==> PREC(f,g): primrec"]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   839
  val monos      = [list_mono]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   840
  val con_defs   = [SC_def,CONST_def,PROJ_def,COMP_def,PREC_def]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   841
  val type_intrs = pr0_typechecks
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   842
  val type_elims = []);
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   843
\end{ttbox}
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   844
\hrule
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   845
\caption{Inductive definition of the primitive recursive functions} 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   846
\label{primrec-fig}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   847
\end{figure}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   848
\def\fs{{\it fs}} 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   849
Szasz was using ALF, but Coq and HOL would also have problems accepting
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   850
this definition.  Isabelle's package accepts it easily since
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   851
$[f_0,\ldots,f_{m-1}]$ is a list of primitive recursive functions and
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   852
$\lst$ is monotonic.  There are five introduction rules, one for each of
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   853
the five forms of primitive recursive function.  Let us examine the one for
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   854
$\COMP$: 
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   855
\[ \infer{\COMP(g,\fs)\in\primrec}{g\in\primrec & \fs\in\lst(\primrec)} \]
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   856
The induction rule for $\primrec$ has one case for each introduction rule.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   857
Due to the use of $\lst$ as a monotone operator, the composition case has
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   858
an unusual induction hypothesis:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   859
 \[ \infer*{P(\COMP(g,\fs))}
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   860
          {[g\in\primrec & \fs\in\lst(\{z\in\primrec.P(z)\})]_{\fs,g}} \]
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   861
The hypothesis states that $\fs$ is a list of primitive recursive functions
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   862
satisfying the induction formula.  Proving the $\COMP$ case typically requires
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   863
structural induction on lists, yielding two subcases: either $\fs=\Nil$ or
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   864
else $\fs=\Cons(f,\fs')$, where $f\in\primrec$, $P(f)$, and $\fs'$ is
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   865
another list of primitive recursive functions satisfying~$P$.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   866
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   867
Figure~\ref{primrec-fig} presents the ML invocation.  Theory {\tt
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   868
  Primrec0.thy} defines the constants $\SC$, $\CONST$, etc.  These are not
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   869
constructors of a new datatype, but functions over lists of numbers.  Their
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   870
definitions, which are omitted, consist of routine list programming.  In
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   871
Isabelle/ZF, the primitive recursive functions are defined as a subset of
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   872
the function set $\lst(\nat)\to\nat$.
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   873
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   874
The Isabelle theory goes on to formalize Ackermann's function and prove
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   875
that it is not primitive recursive, using the induction rule {\tt
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   876
  Primrec.induct}.  The proof follows Szasz's excellent account.
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   877
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   878
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   879
\section{Datatypes and codatatypes}\label{data-sec}
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   880
A (co)datatype definition is a (co)inductive definition with automatically
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   881
defined constructors and a case analysis operator.  The package proves that
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   882
the case operator inverts the constructors and can prove freeness theorems
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   883
involving any pair of constructors.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   884
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   885
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   886
\subsection{Constructors and their domain}\label{univ-sec}
355
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lcp
parents: 181
diff changeset
   887
Conceptually, our two forms of definition are distinct.  A (co)inductive
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   888
definition selects a subset of an existing set; a (co)datatype definition
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   889
creates a new set.  But the package reduces the latter to the former.  A
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   890
set having strong closure properties must serve as the domain of the
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   891
(co)inductive definition.  Constructing this set requires some theoretical
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   892
effort, which must be done anyway to show that (co)datatypes exist.  It is
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   893
not obvious that standard set theory is suitable for defining codatatypes.
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   894
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   895
Isabelle/ZF defines the standard notion of Cartesian product $A\times B$,
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   896
containing ordered pairs $\pair{a,b}$.  Now the $m$-tuple
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   897
$\pair{x_1,\ldots,x_m}$ is the empty set~$\emptyset$ if $m=0$, simply
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   898
$x_1$ if $m=1$ and $\pair{x_1,\pair{x_2,\ldots,x_m}}$ if $m\geq2$.
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   899
Isabelle/ZF also defines the disjoint sum $A+B$, containing injections
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   900
$\Inl(a)\equiv\pair{0,a}$ and $\Inr(b)\equiv\pair{1,b}$.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   901
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   902
A datatype constructor $\Con(x_1,\ldots,x_m)$ is defined to be
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   903
$h(\pair{x_1,\ldots,x_m})$, where $h$ is composed of $\Inl$ and~$\Inr$.
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   904
In a mutually recursive definition, all constructors for the set~$R_i$ have
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   905
the outer form~$h_{in}$, where $h_{in}$ is the injection described
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   906
in~\S\ref{mutual-sec}.  Further nested injections ensure that the
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   907
constructors for~$R_i$ are pairwise distinct.  
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   908
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   909
Isabelle/ZF defines the set $\univ(A)$, which contains~$A$ and
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   910
furthermore contains $\pair{a,b}$, $\Inl(a)$ and $\Inr(b)$ for $a$,
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   911
$b\in\univ(A)$.  In a typical datatype definition with set parameters
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   912
$A_1$, \ldots, $A_k$, a suitable domain for all the recursive sets is
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   913
$\univ(A_1\un\cdots\un A_k)$.  This solves the problem for
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   914
datatypes~\cite[\S4.2]{paulson-set-II}.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   915
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   916
The standard pairs and injections can only yield well-founded
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   917
constructions.  This eases the (manual!) definition of recursive functions
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   918
over datatypes.  But they are unsuitable for codatatypes, which typically
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   919
contain non-well-founded objects.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   920
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   921
To support codatatypes, Isabelle/ZF defines a variant notion of ordered
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   922
pair, written~$\pair{a;b}$.  It also defines the corresponding variant
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   923
notion of Cartesian product $A\otimes B$, variant injections $\QInl(a)$
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   924
and~$\QInr(b)$ and variant disjoint sum $A\oplus B$.  Finally it defines
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   925
the set $\quniv(A)$, which contains~$A$ and furthermore contains
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   926
$\pair{a;b}$, $\QInl(a)$ and $\QInr(b)$ for $a$, $b\in\quniv(A)$.  In a
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   927
typical codatatype definition with set parameters $A_1$, \ldots, $A_k$, a
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   928
suitable domain is $\quniv(A_1\un\cdots\un A_k)$.  This approach using
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   929
standard ZF set theory~\cite{paulson-final} is an alternative to adopting
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   930
Aczel's Anti-Foundation Axiom~\cite{aczel88}.
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   931
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   932
\subsection{The case analysis operator}
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   933
The (co)datatype package automatically defines a case analysis operator,
179
ceb948cefb93 minor corrections
lcp
parents: 130
diff changeset
   934
called {\tt$R$\_case}.  A mutually recursive definition still has only one
ceb948cefb93 minor corrections
lcp
parents: 130
diff changeset
   935
operator, whose name combines those of the recursive sets: it is called
ceb948cefb93 minor corrections
lcp
parents: 130
diff changeset
   936
{\tt$R_1$\_\ldots\_$R_n$\_case}.  The case operator is analogous to those
ceb948cefb93 minor corrections
lcp
parents: 130
diff changeset
   937
for products and sums.
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   938
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   939
Datatype definitions employ standard products and sums, whose operators are
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   940
$\split$ and $\case$ and satisfy the equations
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   941
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   942
  \split(f,\pair{x,y})  & = &  f(x,y) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   943
  \case(f,g,\Inl(x))    & = &  f(x)   \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   944
  \case(f,g,\Inr(y))    & = &  g(y)
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   945
\end{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   946
Suppose the datatype has $k$ constructors $\Con_1$, \ldots,~$\Con_k$.  Then
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   947
its case operator takes $k+1$ arguments and satisfies an equation for each
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   948
constructor:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   949
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   950
  R\hbox{\_case}(f_1,\ldots,f_k, {\tt Con}_i(\vec{x})) & = & f_i(\vec{x}),
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   951
    \qquad i = 1, \ldots, k
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   952
\end{eqnarray*}
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   953
The case operator's definition takes advantage of Isabelle's representation
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   954
of syntax in the typed $\lambda$-calculus; it could readily be adapted to a
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   955
theorem prover for higher-order logic.  If $f$ and~$g$ have meta-type
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   956
$i\To i$ then so do $\split(f)$ and
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   957
$\case(f,g)$.  This works because $\split$ and $\case$ operate on their last
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   958
argument.  They are easily combined to make complex case analysis
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   959
operators.  Here are two examples:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   960
\begin{itemize}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   961
\item $\split(\lambda x.\split(f(x)))$ performs case analysis for
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   962
$A\times (B\times C)$, as is easily verified:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   963
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   964
  \split(\lambda x.\split(f(x)), \pair{a,b,c}) 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   965
    & = & (\lambda x.\split(f(x))(a,\pair{b,c}) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   966
    & = & \split(f(a), \pair{b,c}) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   967
    & = & f(a,b,c)
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   968
\end{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   969
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   970
\item $\case(f,\case(g,h))$ performs case analysis for $A+(B+C)$; let us
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   971
verify one of the three equations:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   972
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   973
  \case(f,\case(g,h), \Inr(\Inl(b))) 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   974
    & = & \case(g,h,\Inl(b)) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   975
    & = & g(b)
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   976
\end{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   977
\end{itemize}
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
   978
Codatatype definitions are treated in precisely the same way.  They express
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   979
case operators using those for the variant products and sums, namely
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   980
$\qsplit$ and~$\qcase$.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   981
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   982
\medskip
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   983
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   984
\ifCADE The package has processed all the datatypes discussed in
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   985
my earlier paper~\cite{paulson-set-II} and the codatatype of lazy lists.
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   986
Space limitations preclude discussing these examples here, but they are
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   987
distributed with Isabelle.  \typeout{****Omitting datatype examples from
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   988
  CADE version!} \else
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   989
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   990
To see how constructors and the case analysis operator are defined, let us
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   991
examine some examples.  These include lists and trees/forests, which I have
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   992
discussed extensively in another paper~\cite{paulson-set-II}.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   993
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   994
\begin{figure}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   995
\begin{ttbox} 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
   996
structure List = Datatype_Fun
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   997
 (val thy        = Univ.thy
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   998
  val rec_specs  = [("list", "univ(A)",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
   999
                      [(["Nil"],    "i"), 
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1000
                       (["Cons"],   "[i,i]=>i")])]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1001
  val rec_styp   = "i=>i"
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1002
  val ext        = None
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1003
  val sintrs     = ["Nil : list(A)",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1004
                    "[| a: A;  l: list(A) |] ==> Cons(a,l) : list(A)"]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1005
  val monos      = []
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1006
  val type_intrs = datatype_intrs
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1007
  val type_elims = datatype_elims);
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1008
\end{ttbox}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1009
\hrule
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1010
\caption{Defining the datatype of lists} \label{list-fig}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1011
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1012
\medskip
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1013
\begin{ttbox}
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1014
structure LList = CoDatatype_Fun
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1015
 (val thy        = QUniv.thy
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1016
  val rec_specs  = [("llist", "quniv(A)",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1017
                      [(["LNil"],   "i"), 
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1018
                       (["LCons"],  "[i,i]=>i")])]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1019
  val rec_styp   = "i=>i"
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1020
  val ext        = None
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1021
  val sintrs     = ["LNil : llist(A)",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1022
                    "[| a: A;  l: llist(A) |] ==> LCons(a,l) : llist(A)"]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1023
  val monos      = []
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1024
  val type_intrs = codatatype_intrs
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1025
  val type_elims = codatatype_elims);
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1026
\end{ttbox}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1027
\hrule
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1028
\caption{Defining the codatatype of lazy lists} \label{llist-fig}
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1029
\end{figure}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1030
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1031
\subsection{Example: lists and lazy lists}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1032
Figures \ref{list-fig} and~\ref{llist-fig} present the ML definitions of
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1033
lists and lazy lists, respectively.  They highlight the (many) similarities
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1034
and (few) differences between datatype and codatatype definitions.
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1035
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1036
Each form of list has two constructors, one for the empty list and one for
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1037
adding an element to a list.  Each takes a parameter, defining the set of
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1038
lists over a given set~$A$.  Each uses the appropriate domain from a
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1039
Isabelle/ZF theory:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1040
\begin{itemize}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1041
\item $\lst(A)$ specifies domain $\univ(A)$ and parent theory {\tt Univ.thy}.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1042
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1043
\item $\llist(A)$ specifies domain $\quniv(A)$ and parent theory {\tt
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1044
QUniv.thy}.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1045
\end{itemize}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1046
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1047
Since $\lst(A)$ is a datatype, it enjoys a structural induction rule, {\tt
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1048
  List.induct}:
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1049
\[ \infer{P(x)}{x\in\lst(A) & P(\Nil)
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1050
        & \infer*{P(\Cons(a,l))}{[a\in A & l\in\lst(A) & P(l)]_{a,l}} }
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1051
\] 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1052
Induction and freeness yield the law $l\not=\Cons(a,l)$.  To strengthen this,
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1053
Isabelle/ZF defines the rank of a set and proves that the standard pairs and
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1054
injections have greater rank than their components.  An immediate consequence,
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1055
which justifies structural recursion on lists \cite[\S4.3]{paulson-set-II},
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1056
is
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1057
\[ \rank(l) < \rank(\Cons(a,l)). \]
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1058
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1059
Since $\llist(A)$ is a codatatype, it has no induction rule.  Instead it has
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1060
the coinduction rule shown in \S\ref{coind-sec}.  Since variant pairs and
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1061
injections are monotonic and need not have greater rank than their
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1062
components, fixedpoint operators can create cyclic constructions.  For
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1063
example, the definition
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1064
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1065
  \lconst(a) & \equiv & \lfp(\univ(a), \lambda l. \LCons(a,l))
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1066
\end{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1067
yields $\lconst(a) = \LCons(a,\lconst(a))$.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1068
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1069
\medskip
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1070
It may be instructive to examine the definitions of the constructors and
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1071
case operator for $\lst(A)$.  The definitions for $\llist(A)$ are similar.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1072
The list constructors are defined as follows:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1073
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1074
  \Nil       & = & \Inl(\emptyset) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1075
  \Cons(a,l) & = & \Inr(\pair{a,l})
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1076
\end{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1077
The operator $\lstcase$ performs case analysis on these two alternatives:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1078
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1079
  \lstcase(c,h) & \equiv & \case(\lambda u.c, \split(h)) 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1080
\end{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1081
Let us verify the two equations:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1082
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1083
    \lstcase(c, h, \Nil) & = & 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1084
       \case(\lambda u.c, \split(h), \Inl(\emptyset)) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1085
     & = & (\lambda u.c)(\emptyset) \\
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1086
     & = & c\\[1ex]
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1087
    \lstcase(c, h, \Cons(x,y)) & = & 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1088
       \case(\lambda u.c, \split(h), \Inr(\pair{x,y})) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1089
     & = & \split(h, \pair{x,y}) \\
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1090
     & = & h(x,y)
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1091
\end{eqnarray*} 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1092
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1093
\begin{figure}
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1094
\begin{ttbox}
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1095
structure TF = Datatype_Fun
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1096
 (val thy        = Univ.thy
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1097
  val rec_specs  = [("tree", "univ(A)",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1098
                       [(["Tcons"],  "[i,i]=>i")]),
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1099
                    ("forest", "univ(A)",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1100
                       [(["Fnil"],   "i"),
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1101
                        (["Fcons"],  "[i,i]=>i")])]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1102
  val rec_styp   = "i=>i"
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1103
  val ext        = None
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1104
  val sintrs     = 
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1105
        ["[| a:A;  f: forest(A) |] ==> Tcons(a,f) : tree(A)",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1106
         "Fnil : forest(A)",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1107
         "[| t: tree(A);  f: forest(A) |] ==> Fcons(t,f) : forest(A)"]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1108
  val monos      = []
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1109
  val type_intrs = datatype_intrs
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1110
  val type_elims = datatype_elims);
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1111
\end{ttbox}
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1112
\hrule
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1113
\caption{Defining the datatype of trees and forests} \label{tf-fig}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1114
\end{figure}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1115
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1116
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1117
\subsection{Example: mutual recursion}
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1118
In mutually recursive trees and forests~\cite[\S4.5]{paulson-set-II}, trees
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1119
have the one constructor $\Tcons$, while forests have the two constructors
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1120
$\Fnil$ and~$\Fcons$.  Figure~\ref{tf-fig} presents the ML
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1121
definition.  It has much in common with that of $\lst(A)$, including its
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1122
use of $\univ(A)$ for the domain and {\tt Univ.thy} for the parent theory.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1123
The three introduction rules define the mutual recursion.  The
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1124
distinguishing feature of this example is its two induction rules.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1125
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1126
The basic induction rule is called {\tt TF.induct}:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1127
\[ \infer{P(x)}{x\in\TF(A) & 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1128
     \infer*{P(\Tcons(a,f))}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1129
        {\left[\begin{array}{l} a\in A \\ 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1130
                                f\in\forest(A) \\ P(f)
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1131
               \end{array}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1132
         \right]_{a,f}}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1133
     & P(\Fnil)
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1134
     & \infer*{P(\Fcons(t,f))}
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1135
        {\left[\begin{array}{l} t\in\tree(A)   \\ P(t) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1136
                                f\in\forest(A) \\ P(f)
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1137
                \end{array}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1138
         \right]_{t,f}} }
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1139
\] 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1140
This rule establishes a single predicate for $\TF(A)$, the union of the
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1141
recursive sets.  
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1142
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1143
Although such reasoning is sometimes useful
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1144
\cite[\S4.5]{paulson-set-II}, a proper mutual induction rule should establish
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1145
separate predicates for $\tree(A)$ and $\forest(A)$.   The package calls this
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1146
rule {\tt TF.mutual\_induct}.  Observe the usage of $P$ and $Q$ in the
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1147
induction hypotheses:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1148
\[ \infer{(\forall z. z\in\tree(A)\imp P(z)) \conj
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1149
          (\forall z. z\in\forest(A)\imp Q(z))}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1150
     {\infer*{P(\Tcons(a,f))}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1151
        {\left[\begin{array}{l} a\in A \\ 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1152
                                f\in\forest(A) \\ Q(f)
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1153
               \end{array}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1154
         \right]_{a,f}}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1155
     & Q(\Fnil)
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1156
     & \infer*{Q(\Fcons(t,f))}
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1157
        {\left[\begin{array}{l} t\in\tree(A)   \\ P(t) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1158
                                f\in\forest(A) \\ Q(f)
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1159
                \end{array}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1160
         \right]_{t,f}} }
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1161
\] 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1162
As mentioned above, the package does not define a structural recursion
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1163
operator.  I have described elsewhere how this is done
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1164
\cite[\S4.5]{paulson-set-II}.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1165
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1166
Both forest constructors have the form $\Inr(\cdots)$,
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1167
while the tree constructor has the form $\Inl(\cdots)$.  This pattern would
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1168
hold regardless of how many tree or forest constructors there were.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1169
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1170
  \Tcons(a,l)  & = & \Inl(\pair{a,l}) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1171
  \Fnil        & = & \Inr(\Inl(\emptyset)) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1172
  \Fcons(a,l)  & = & \Inr(\Inr(\pair{a,l}))
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1173
\end{eqnarray*} 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1174
There is only one case operator; it works on the union of the trees and
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1175
forests:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1176
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1177
  {\tt tree\_forest\_case}(f,c,g) & \equiv & 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1178
    \case(\split(f),\, \case(\lambda u.c, \split(g)))
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1179
\end{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1180
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1181
\begin{figure}
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1182
\begin{ttbox}
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1183
structure Data = Datatype_Fun
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1184
 (val thy        = Univ.thy
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1185
  val rec_specs  = [("data", "univ(A Un B)",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1186
                       [(["Con0"],   "i"),
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1187
                        (["Con1"],   "i=>i"),
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1188
                        (["Con2"],   "[i,i]=>i"),
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1189
                        (["Con3"],   "[i,i,i]=>i")])]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1190
  val rec_styp   = "[i,i]=>i"
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1191
  val ext        = None
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1192
  val sintrs     = 
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1193
        ["Con0 : data(A,B)",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1194
         "[| a: A |] ==> Con1(a) : data(A,B)",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1195
         "[| a: A; b: B |] ==> Con2(a,b) : data(A,B)",
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1196
         "[| a: A; b: B;  d: data(A,B) |] ==> Con3(a,b,d) : data(A,B)"]
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1197
  val monos      = []
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1198
  val type_intrs = datatype_intrs
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1199
  val type_elims = datatype_elims);
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1200
\end{ttbox}
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1201
\hrule
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1202
\caption{Defining the four-constructor sample datatype} \label{data-fig}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1203
\end{figure}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1204
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1205
\subsection{A four-constructor datatype}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1206
Finally let us consider a fairly general datatype.  It has four
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1207
constructors $\Con_0$, \ldots, $\Con_3$, with the
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1208
corresponding arities.  Figure~\ref{data-fig} presents the ML definition. 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1209
Because this datatype has two set parameters, $A$ and~$B$, it specifies
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1210
$\univ(A\un B)$ as its domain.  The structural induction rule has four
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1211
minor premises, one per constructor:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1212
\[ \infer{P(x)}{x\in\data(A,B) & 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1213
    P(\Con_0) &
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1214
    \infer*{P(\Con_1(a))}{[a\in A]_a} &
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1215
    \infer*{P(\Con_2(a,b))}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1216
      {\left[\begin{array}{l} a\in A \\ b\in B \end{array}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1217
       \right]_{a,b}} &
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1218
    \infer*{P(\Con_3(a,b,d))}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1219
      {\left[\begin{array}{l} a\in A \\ b\in B \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1220
                              d\in\data(A,B) \\ P(d)
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1221
              \end{array}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1222
       \right]_{a,b,d}} }
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1223
\] 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1224
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1225
The constructor definitions are
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1226
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1227
  \Con_0         & = & \Inl(\Inl(\emptyset)) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1228
  \Con_1(a)      & = & \Inl(\Inr(a)) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1229
  \Con_2(a,b)    & = & \Inr(\Inl(\pair{a,b})) \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1230
  \Con_3(a,b,c)  & = & \Inr(\Inr(\pair{a,b,c})).
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1231
\end{eqnarray*} 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1232
The case operator is
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1233
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1234
  {\tt data\_case}(f_0,f_1,f_2,f_3) & \equiv & 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1235
    \case(\begin{array}[t]{@{}l}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1236
          \case(\lambda u.f_0,\; f_1),\, \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1237
          \case(\split(f_2),\; \split(\lambda v.\split(f_3(v)))) )
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1238
   \end{array} 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1239
\end{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1240
This may look cryptic, but the case equations are trivial to verify.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1241
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1242
In the constructor definitions, the injections are balanced.  A more naive
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1243
approach is to define $\Con_3(a,b,c)$ as
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1244
$\Inr(\Inr(\Inr(\pair{a,b,c})))$; instead, each constructor has two
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1245
injections.  The difference here is small.  But the ZF examples include a
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1246
60-element enumeration type, where each constructor has 5 or~6 injections.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1247
The naive approach would require 1 to~59 injections; the definitions would be
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1248
quadratic in size.  It is like the difference between the binary and unary
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1249
numeral systems. 
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1250
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1251
The result structure contains the case operator and constructor definitions as
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1252
the theorem list \verb|con_defs|. It contains the case equations, such as 
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1253
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1254
  {\tt data\_case}(f_0,f_1,f_2,f_3,\Con_3(a,b,c)) & = & f_3(a,b,c),
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1255
\end{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1256
as the theorem list \verb|case_eqns|.  There is one equation per constructor.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1257
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1258
\subsection{Proving freeness theorems}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1259
There are two kinds of freeness theorems:
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1260
\begin{itemize}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1261
\item {\bf injectiveness} theorems, such as
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1262
\[ \Con_2(a,b) = \Con_2(a',b') \bimp a=a' \conj b=b' \]
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1263
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1264
\item {\bf distinctness} theorems, such as
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1265
\[ \Con_1(a) \not= \Con_2(a',b')  \]
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1266
\end{itemize}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1267
Since the number of such theorems is quadratic in the number of constructors,
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1268
the package does not attempt to prove them all.  Instead it returns tools for
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1269
proving desired theorems --- either explicitly or `on the fly' during
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1270
simplification or classical reasoning.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1271
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1272
The theorem list \verb|free_iffs| enables the simplifier to perform freeness
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1273
reasoning.  This works by incremental unfolding of constructors that appear in
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1274
equations.  The theorem list contains logical equivalences such as
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1275
\begin{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1276
  \Con_0=c      & \bimp &  c=\Inl(\Inl(\emptyset))     \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1277
  \Con_1(a)=c   & \bimp &  c=\Inl(\Inr(a))             \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1278
                & \vdots &                             \\
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1279
  \Inl(a)=\Inl(b)   & \bimp &  a=b                     \\
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1280
  \Inl(a)=\Inr(b)   & \bimp &  {\tt False}             \\
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1281
  \pair{a,b} = \pair{a',b'} & \bimp & a=a' \conj b=b'
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1282
\end{eqnarray*}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1283
For example, these rewrite $\Con_1(a)=\Con_1(b)$ to $a=b$ in four steps.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1284
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1285
The theorem list \verb|free_SEs| enables the classical
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1286
reasoner to perform similar replacements.  It consists of elimination rules
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1287
to replace $\Con_0=c$ by $c=\Inl(\Inl(\emptyset))$ and so forth, in the
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1288
assumptions.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1289
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1290
Such incremental unfolding combines freeness reasoning with other proof
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1291
steps.  It has the unfortunate side-effect of unfolding definitions of
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1292
constructors in contexts such as $\exists x.\Con_1(a)=x$, where they should
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1293
be left alone.  Calling the Isabelle tactic {\tt fold\_tac con\_defs}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1294
restores the defined constants.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1295
\fi  %CADE
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1296
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1297
\section{Related work}\label{related}
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1298
The use of least fixedpoints to express inductive definitions seems
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1299
obvious.  Why, then, has this technique so seldom been implemented?
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1300
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1301
Most automated logics can only express inductive definitions by asserting
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1302
new axioms.  Little would be left of Boyer and Moore's logic~\cite{bm79} if
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1303
their shell principle were removed.  With ALF the situation is more
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1304
complex; earlier versions of Martin-L\"of's type theory could (using
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1305
wellordering types) express datatype definitions, but the version
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1306
underlying ALF requires new rules for each definition~\cite{dybjer91}.
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1307
With Coq the situation is subtler still; its underlying Calculus of
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1308
Constructions can express inductive definitions~\cite{huet88}, but cannot
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1309
quite handle datatype definitions~\cite{paulin92}.  It seems that
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1310
researchers tried hard to circumvent these problems before finally
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1311
extending the Calculus with rule schemes for strictly positive operators.
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1312
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1313
Higher-order logic can express inductive definitions through quantification
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1314
over unary predicates.  The following formula expresses that~$i$ belongs to the
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1315
least set containing~0 and closed under~$\succ$:
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1316
\[ \forall P. P(0)\conj (\forall x.P(x)\imp P(\succ(x))) \imp P(i) \] 
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1317
This technique can be used to prove the Knaster-Tarski Theorem, but it is
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1318
little used in the HOL system.  Melham~\cite{melham89} clearly describes
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1319
the development.  The natural numbers are defined as shown above, but lists
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1320
are defined as functions over the natural numbers.  Unlabelled
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1321
trees are defined using G\"odel numbering; a labelled tree consists of an
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1322
unlabelled tree paired with a list of labels.  Melham's datatype package
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1323
expresses the user's datatypes in terms of labelled trees.  It has been
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1324
highly successful, but a fixedpoint approach would have yielded greater
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1325
functionality with less effort.
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1326
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1327
Melham's inductive definition package~\cite{camilleri92} uses
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1328
quantification over predicates, which is implicitly a fixedpoint approach.
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1329
Instead of formalizing the notion of monotone function, it requires
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1330
definitions to consist of finitary rules, a syntactic form that excludes
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1331
many monotone inductive definitions.
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1332
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1333
The earliest use of least fixedpoints is probably Robin Milner's datatype
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1334
package for Edinburgh LCF~\cite{milner-ind}.  Brian Monahan extended this
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1335
package considerably~\cite{monahan84}, as did I in unpublished
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1336
work.\footnote{The datatype package described in my LCF
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1337
  book~\cite{paulson87} does {\it not\/} make definitions, but merely
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1338
  asserts axioms.  I justified this shortcut on grounds of efficiency:
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1339
  existing packages took tens of minutes to run.  Such an explanation would
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1340
  not do today.}
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1341
LCF is a first-order logic of domain theory; the relevant fixedpoint
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1342
theorem is not Knaster-Tarski but concerns fixedpoints of continuous
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1343
functions over domains.  LCF is too weak to express recursive predicates.
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1344
Thus it would appear that the Isabelle/ZF package is the first to be based
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1345
on the Knaster-Tarski Theorem.
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1346
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1347
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1348
\section{Conclusions and future work}
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1349
Higher-order logic and set theory are both powerful enough to express
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1350
inductive definitions.  A growing number of theorem provers implement one
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1351
of these~\cite{IMPS,saaltink-fme}.  The easiest sort of inductive
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1352
definition package to write is one that asserts new axioms, not one that
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1353
makes definitions and proves theorems about them.  But asserting axioms
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1354
could introduce unsoundness.
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1356
The fixedpoint approach makes it fairly easy to implement a package for
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1357
(co)inductive definitions that does not assert axioms.  It is efficient: it
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1358
processes most definitions in seconds and even a 60-constructor datatype
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1359
requires only two minutes.  It is also simple: the package consists of
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1360
under 1100 lines (35K bytes) of Standard ML code.  The first working
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1361
version took under a week to code.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1362
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1363
In set theory, care is required to ensure that the inductive definition
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1364
yields a set (rather than a proper class).  This problem is inherent to set
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1365
theory, whether or not the Knaster-Tarski Theorem is employed.  We must
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1366
exhibit a bounding set (called a domain above).  For inductive definitions,
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1367
this is often trivial.  For datatype definitions, I have had to formalize
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1368
much set theory.  I intend to formalize cardinal arithmetic and the
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1369
$\aleph$-sequence to handle datatype definitions that have infinite
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1370
branching.  The need for such efforts is not a drawback of the fixedpoint
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1371
approach, for the alternative is to take such definitions on faith.
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1372
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1373
The approach is not restricted to set theory.  It should be suitable for
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1374
any logic that has some notion of set and the Knaster-Tarski Theorem.  I
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1375
intend to use the Isabelle/ZF package as the basis for a higher-order logic
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1376
one, using Isabelle/HOL\@.  The necessary theory is already
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1377
mechanized~\cite{paulson-coind}.  HOL represents sets by unary predicates;
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1378
defining the corresponding types may cause complications.
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1379
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1380
355
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1381
\bibliographystyle{springer}
77150178beb2 post-CRC corrections
lcp
parents: 181
diff changeset
  1382
\bibliography{string-abbrv,atp,theory,funprog,isabelle}
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1383
%%%%%\doendnotes
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1384
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1385
\ifCADE\typeout{****Omitting appendices from CADE version!}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1386
\else
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1387
\newpage
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1388
\appendix
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1389
\section{Inductive and coinductive definitions: users guide}
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1390
The ML functors \verb|Inductive_Fun| and \verb|CoInductive_Fun| build
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1391
inductive and coinductive definitions, respectively.  This section describes
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1392
how to invoke them.  
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1393
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1394
\subsection{The result structure}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1395
Many of the result structure's components have been discussed
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1396
in~\S\ref{basic-sec}; others are self-explanatory.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1397
\begin{description}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1398
\item[\tt thy] is the new theory containing the recursive sets.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1399
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1400
\item[\tt defs] is the list of definitions of the recursive sets.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1401
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1402
\item[\tt bnd\_mono] is a monotonicity theorem for the fixedpoint operator.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1403
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1404
\item[\tt unfold] is a fixedpoint equation for the recursive set (the union of
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1405
the recursive sets, in the case of mutual recursion).
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1406
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1407
\item[\tt dom\_subset] is a theorem stating inclusion in the domain.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1408
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1409
\item[\tt intrs] is the list of introduction rules, now proved as theorems, for
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1410
the recursive sets.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1411
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1412
\item[\tt elim] is the elimination rule.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1413
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1414
\item[\tt mk\_cases] is a function to create simplified instances of {\tt
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1415
elim}, using freeness reasoning on some underlying datatype.
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1416
\end{description}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1417
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1418
For an inductive definition, the result structure contains two induction rules,
130
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1419
{\tt induct} and \verb|mutual_induct|.  For a coinductive definition, it
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1420
contains the rule \verb|coinduct|.
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1421
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1422
Figure~\ref{def-result-fig} summarizes the two result signatures,
c035b6b9eafc Many edits suggested by Grundy & Thompson
lcp
parents: 103
diff changeset
  1423
specifying the types of all these components.
103
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1424
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1425
\begin{figure}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1426
\begin{ttbox}
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1427
sig
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1428
val thy          : theory
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1429
val defs         : thm list
30bd42401ab2 Initial revision
lcp
parents:
diff changeset
  1430
val bnd_mono     : thm
30bd42401ab2 Initial revision
lcp
parents:
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  1431
val unfold       : thm
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parents:
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  1432
val dom_subset   : thm
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parents:
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  1433
val intrs        : thm list
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parents:
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  1434
val elim         : thm
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parents:
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  1435
val mk_cases     : thm list -> string -> thm
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parents:
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  1436
{\it(Inductive definitions only)} 
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parents:
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  1437
val induct       : thm
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parents:
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  1438
val mutual_induct: thm
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parents: 103
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  1439
{\it(Coinductive definitions only)}
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  1440
val coinduct    : thm
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parents:
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  1441
end
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parents:
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  1442
\end{ttbox}
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parents:
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  1443
\hrule
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parents: 103
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  1444
\caption{The result of a (co)inductive definition} \label{def-result-fig}
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parents:
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  1445
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  1446
\medskip
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parents:
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  1447
\begin{ttbox}
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parents:
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  1448
sig  
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parents:
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  1449
val thy          : theory
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parents:
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  1450
val rec_doms     : (string*string) list
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parents:
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  1451
val sintrs       : string list
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parents:
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  1452
val monos        : thm list
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parents:
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  1453
val con_defs     : thm list
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parents:
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  1454
val type_intrs   : thm list
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parents:
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  1455
val type_elims   : thm list
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lcp
parents:
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  1456
end
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parents:
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  1457
\end{ttbox}
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  1458
\hrule
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  1459
\caption{The argument of a (co)inductive definition} \label{def-arg-fig}
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  1460
\end{figure}
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  1461
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  1462
\subsection{The argument structure}
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  1463
Both \verb|Inductive_Fun| and \verb|CoInductive_Fun| take the same argument
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  1464
structure (Figure~\ref{def-arg-fig}).  Its components are as follows:
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  1465
\begin{description}
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  1466
\item[\tt thy] is the definition's parent theory, which {\it must\/}
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parents:
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  1467
declare constants for the recursive sets.
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  1468
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  1469
\item[\tt rec\_doms] is a list of pairs, associating the name of each recursive
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  1470
set with its domain.
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lcp
parents:
diff changeset
  1471
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  1472
\item[\tt sintrs] specifies the desired introduction rules as strings.
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parents:
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  1473
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  1474
\item[\tt monos] consists of monotonicity theorems for each operator applied
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parents:
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  1475
to a recursive set in the introduction rules.
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parents:
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  1476
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  1477
\item[\tt con\_defs] contains definitions of constants appearing in the
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  1478
introduction rules.  The (co)datatype package supplies the constructors'
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parents:
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  1479
definitions here.  Most direct calls of \verb|Inductive_Fun| or
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  1480
\verb|CoInductive_Fun| pass the empty list; one exception is the primitive
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parents:
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  1481
recursive functions example (\S\ref{primrec-sec}).
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parents:
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  1482
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  1483
\item[\tt type\_intrs] consists of introduction rules for type-checking the
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  1484
  definition, as discussed in~\S\ref{basic-sec}.  They are applied using
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parents:
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  1485
  depth-first search; you can trace the proof by setting
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  1486
  \verb|trace_DEPTH_FIRST := true|.
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parents:
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  1487
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  1488
\item[\tt type\_elims] consists of elimination rules for type-checking the
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  1489
definition.  They are presumed to be `safe' and are applied as much as
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parents:
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  1490
possible, prior to the {\tt type\_intrs} search.
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  1491
\end{description}
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  1492
The package has a few notable restrictions:
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  1493
\begin{itemize}
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  1494
\item The parent theory, {\tt thy}, must declare the recursive sets as
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lcp