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doc-src/ind-defs.tex

author | lcp |

Tue, 09 Nov 1993 16:47:38 +0100 | |

changeset 103 | 30bd42401ab2 |

child 130 | c035b6b9eafc |

permissions | -rw-r--r-- |

Initial revision

\documentstyle[11pt,a4,proof,lcp,alltt,amssymbols,draft]{article} \newif\ifCADE \CADEfalse \title{A Fixedpoint Approach to Implementing (Co-)Inductive Definitions\\ DRAFT\thanks{Research funded by the SERC (grants GR/G53279, GR/H40570) and by the ESPRIT Basic Research Action 6453 `Types'.}} \author{{\em Lawrence C. Paulson}\\ Computer Laboratory, University of Cambridge} \date{\today} \setcounter{secnumdepth}{2} \setcounter{tocdepth}{2} \def\picture #1 by #2 (#3){% pictures: width by height (name) \message{Picture #3} \vbox to #2{\hrule width #1 height 0pt depth 0pt \vfill \special{picture #3}}} \newcommand\sbs{\subseteq} \newcommand\List[1]{\lbrakk#1\rbrakk} \let\To=\Rightarrow \newcommand\Var[1]{{?\!#1}} %%%\newcommand\Pow{{\tt Pow}} \let\pow=\wp \newcommand\RepFun{{\tt RepFun}} \newcommand\pair[1]{\langle#1\rangle} \newcommand\cons{{\tt cons}} \def\succ{{\tt succ}} \newcommand\split{{\tt split}} \newcommand\fst{{\tt fst}} \newcommand\snd{{\tt snd}} \newcommand\converse{{\tt converse}} \newcommand\domain{{\tt domain}} \newcommand\range{{\tt range}} \newcommand\field{{\tt field}} \newcommand\bndmono{\hbox{\tt bnd\_mono}} \newcommand\lfp{{\tt lfp}} \newcommand\gfp{{\tt gfp}} \newcommand\id{{\tt id}} \newcommand\trans{{\tt trans}} \newcommand\wf{{\tt wf}} \newcommand\wfrec{\hbox{\tt wfrec}} \newcommand\nat{{\tt nat}} \newcommand\natcase{\hbox{\tt nat\_case}} \newcommand\transrec{{\tt transrec}} \newcommand\rank{{\tt rank}} \newcommand\univ{{\tt univ}} \newcommand\Vrec{{\tt Vrec}} \newcommand\Inl{{\tt Inl}} \newcommand\Inr{{\tt Inr}} \newcommand\case{{\tt case}} \newcommand\lst{{\tt list}} \newcommand\Nil{{\tt Nil}} \newcommand\Cons{{\tt Cons}} \newcommand\lstcase{\hbox{\tt list\_case}} \newcommand\lstrec{\hbox{\tt list\_rec}} \newcommand\length{{\tt length}} \newcommand\listn{{\tt listn}} \newcommand\acc{{\tt acc}} \newcommand\primrec{{\tt primrec}} \newcommand\SC{{\tt SC}} \newcommand\CONST{{\tt CONST}} \newcommand\PROJ{{\tt PROJ}} \newcommand\COMP{{\tt COMP}} \newcommand\PREC{{\tt PREC}} \newcommand\quniv{{\tt quniv}} \newcommand\llist{{\tt llist}} \newcommand\LNil{{\tt LNil}} \newcommand\LCons{{\tt LCons}} \newcommand\lconst{{\tt lconst}} \newcommand\lleq{{\tt lleq}} \newcommand\map{{\tt map}} \newcommand\term{{\tt term}} \newcommand\Apply{{\tt Apply}} \newcommand\termcase{{\tt term\_case}} \newcommand\rev{{\tt rev}} \newcommand\reflect{{\tt reflect}} \newcommand\tree{{\tt tree}} \newcommand\forest{{\tt forest}} \newcommand\Part{{\tt Part}} \newcommand\TF{{\tt tree\_forest}} \newcommand\Tcons{{\tt Tcons}} \newcommand\Fcons{{\tt Fcons}} \newcommand\Fnil{{\tt Fnil}} \newcommand\TFcase{\hbox{\tt TF\_case}} \newcommand\Fin{{\tt Fin}} \newcommand\QInl{{\tt QInl}} \newcommand\QInr{{\tt QInr}} \newcommand\qsplit{{\tt qsplit}} \newcommand\qcase{{\tt qcase}} \newcommand\Con{{\tt Con}} \newcommand\data{{\tt data}} \sloppy \binperiod %%%treat . like a binary operator \begin{document} \pagestyle{empty} \begin{titlepage} \maketitle \begin{abstract} Several theorem provers provide commands for formalizing recursive datatypes and/or inductively defined sets. This paper presents a new approach, based on fixedpoint definitions. It is unusually general: it admits all monotone inductive definitions. It is conceptually simple, which has allowed the easy implementation of mutual recursion and other conveniences. It also handles co-inductive definitions: simply replace the least fixedpoint by a greatest fixedpoint. This represents the first automated support for co-inductive definitions. Examples include lists of $n$ elements, the accessible part of a relation and the set of primitive recursive functions. One example of a co-inductive definition is bisimulations for lazy lists. \ifCADE\else Recursive datatypes are examined in detail, as well as one example of a ``co-datatype'': lazy lists. The appendices are simple user's manuals for this Isabelle/ZF package.\fi The method has been implemented in Isabelle's ZF set theory. It should be applicable to any logic in which the Knaster-Tarski Theorem can be proved. The paper briefly describes a method of formalizing non-well-founded data structures in standard ZF set theory. \end{abstract} % \begin{center} Copyright \copyright{} \number\year{} by Lawrence C. Paulson \end{center} \thispagestyle{empty} \end{titlepage} \tableofcontents \cleardoublepage \pagenumbering{arabic}\pagestyle{headings}\DRAFT \section{Introduction} Several theorem provers provide commands for formalizing recursive data structures, like lists and trees. Examples include Boyer and Moore's shell principle~\cite{bm79} and Melham's recursive type package for the HOL system~\cite{melham89}. Such data structures are called {\bf datatypes} below, by analogy with {\tt datatype} definitions in Standard~ML\@. A datatype is but one example of a {\bf inductive definition}. This specifies the least set closed under given rules~\cite{aczel77}. The collection of theorems in a logic is inductively defined. A structural operational semantics~\cite{hennessy90} is an inductive definition of a reduction or evaluation relation on programs. A few theorem provers provide commands for formalizing inductive definitions; these include Coq~\cite{paulin92} and again the HOL system~\cite{camilleri92}. The dual notion is that of a {\bf co-inductive definition}. This specifies the greatest set closed under given rules. Important examples include using bisimulation relations to formalize equivalence of processes~\cite{milner89} or lazy functional programs~\cite{abramsky90}. Other examples include lazy lists and other infinite data structures; these are called {\bf co-datatypes} below. Most existing implementations of datatype and inductive definitions accept an artifically narrow class of inputs, and are not easily extended. The shell principle and Coq's inductive definition rules are built into the underlying logic. Melham's packages derive datatypes and inductive definitions from specialized constructions in higher-order logic. This paper describes a package based on a fixedpoint approach. Least fixedpoints yield inductive definitions; greatest fixedpoints yield co-inductive definitions. The package is uniquely powerful: \begin{itemize} \item It accepts the largest natural class of inductive definitions, namely all monotone inductive definitions. \item It accepts a wide class of datatype definitions. \item It handles co-inductive and co-datatype definitions. Most of the discussion below applies equally to inductive and co-inductive definitions, and most of the code is shared. To my knowledge, this is the only package supporting co-inductive definitions. \item Definitions may be mutually recursive. \end{itemize} The package is implemented in Isabelle~\cite{isabelle-intro}, using ZF set theory \cite{paulson-set-I,paulson-set-II}. However, the fixedpoint approach is independent of Isabelle. The recursion equations are specified as introduction rules for the mutually recursive sets. The package transforms these rules into a mapping over sets, and attempts to prove that the mapping is monotonic and well-typed. If successful, the package makes fixedpoint definitions and proves the introduction, elimination and (co-)induction rules. The package consists of several Standard ML functors~\cite{paulson91}; it accepts its argument and returns its result as ML structures. Most datatype packages equip the new datatype with some means of expressing recursive functions. This is the main thing lacking from my package. Its fixedpoint operators define recursive sets, not recursive functions. But the Isabelle/ZF theory provides well-founded recursion and other logical tools to simplify this task~\cite{paulson-set-II}. \S2 briefly introduces the least and greatest fixedpoint operators. \S3 discusses the form of introduction rules, mutual recursion and other points common to inductive and co-inductive definitions. \S4 discusses induction and co-induction rules separately. \S5 presents several examples, including a co-inductive definition. \S6 describes datatype definitions, while \S7 draws brief conclusions. \ifCADE\else The appendices are simple user's manuals for this Isabelle/ZF package.\fi Most of the definitions and theorems shown below have been generated by the package. I have renamed some variables to improve readability. \section{Fixedpoint operators} In set theory, the least and greatest fixedpoint operators are defined as follows: \begin{eqnarray*} \lfp(D,h) & \equiv & \inter\{X\sbs D. h(X)\sbs X\} \\ \gfp(D,h) & \equiv & \union\{X\sbs D. X\sbs h(X)\} \end{eqnarray*} Say that $h$ is {\bf bounded by}~$D$ if $h(D)\sbs D$, and {\bf monotone} if $h(A)\sbs h(B)$ for all $A$ and $B$ such that $A\sbs B\sbs D$. If $h$ is bounded by~$D$ and monotone then both operators yield fixedpoints: \begin{eqnarray*} \lfp(D,h) & = & h(\lfp(D,h)) \\ \gfp(D,h) & = & h(\gfp(D,h)) \end{eqnarray*} These equations are instances of the Knaster-Tarski theorem, which states that every monotonic function over a complete lattice has a fixedpoint~\cite{davey&priestley}. It is obvious from their definitions that $\lfp$ must be the least fixedpoint, and $\gfp$ the greatest. This fixedpoint theory is simple. The Knaster-Tarski theorem is easy to prove. Showing monotonicity of~$h$ is trivial, in typical cases. We must also exhibit a bounding set~$D$ for~$h$. Sometimes this is trivial, as when a set of ``theorems'' is (co-)inductively defined over some previously existing set of ``formulae.'' But defining the bounding set for (co-)datatype definitions requires some effort; see~\S\ref{data-sec} below. \section{Elements of an inductive or co-inductive definition}\label{basic-sec} Consider a (co-)inductive definition of the sets $R_1$, \ldots,~$R_n$, in mutual recursion. They will be constructed from previously existing sets $D_1$, \ldots,~$D_n$, respectively, which are called their {\bf domains}. The construction yields not $R_i\sbs D_i$ but $R_i\sbs D_1+\cdots+D_n$, where $R_i$ is the image of~$D_i$ under an injection~\cite[\S4.5]{paulson-set-II}. The definition may involve arbitrary parameters $\vec{p}=p_1$, \ldots,~$p_k$. Each recursive set then has the form $R_i(\vec{p})$. The parameters must be identical every time they occur within a definition. This would appear to be a serious restriction compared with other systems such as Coq~\cite{paulin92}. For instance, we cannot define the lists of $n$ elements as the set $\listn(A,n)$ using rules where the parameter~$n$ varies. \S\ref{listn-sec} describes how to express this definition using the package. To avoid clutter below, the recursive sets are shown as simply $R_i$ instead of $R_i(\vec{p})$. \subsection{The form of the introduction rules}\label{intro-sec} The body of the definition consists of the desired introduction rules, specified as strings. The conclusion of each rule must have the form $t\in R_i$, where $t$ is any term. Premises typically have the same form, but they can have the more general form $t\in M(R_i)$ or express arbitrary side-conditions. The premise $t\in M(R_i)$ is permitted if $M$ is a monotonic operator on sets, satisfying the rule \[ \infer{M(A)\sbs M(B)}{A\sbs B} \] The inductive definition package must be supplied monotonicity rules for all such premises. Because any monotonic $M$ may appear in premises, the criteria for an acceptable definition is semantic rather than syntactic. A suitable choice of~$M$ and~$t$ can express a lot. The powerset operator $\pow$ is monotone, and the premise $t\in\pow(R)$ expresses $t\sbs R$; see \S\ref{acc-sec} for an example. The `list of' operator is monotone, and the premise $t\in\lst(R)$ avoids having to encode the effect of~$\lst(R)$ using mutual recursion; see \S\ref{primrec-sec} and also my earlier paper~\cite[\S4.4]{paulson-set-II}. Introduction rules may also contain {\bf side-conditions}. These are premises consisting of arbitrary formulae not mentioning the recursive sets. Side-conditions typically involve type-checking. One example is the premise $a\in A$ in the following rule from the definition of lists: \[ \infer{\Cons(a,l)\in\lst(A)}{a\in A & l\in\lst(A)} \] \subsection{The fixedpoint definitions} The package translates the list of desired introduction rules into a fixedpoint definition. Consider, as a running example, the finite set operator $\Fin(A)$: the set of all finite subsets of~$A$. It can be defined as the least set closed under the rules \[ \emptyset\in\Fin(A) \qquad \infer{\{a\}\un b\in\Fin(A)}{a\in A & b\in\Fin(A)} \] The domain for a (co-)inductive definition must be some existing set closed under the rules. A suitable domain for $\Fin(A)$ is $\pow(A)$, the set of all subsets of~$A$. The package generates the definition \begin{eqnarray*} \Fin(A) & \equiv & \lfp(\pow(A), \; \begin{array}[t]{r@{\,}l} \lambda X. \{z\in\pow(A). & z=\emptyset \disj{} \\ &(\exists a\,b. z=\{a\}\un b\conj a\in A\conj b\in X)\}) \end{array} \end{eqnarray*} The contribution of each rule to the definition of $\Fin(A)$ should be obvious. A co-inductive definition is similar but uses $\gfp$ instead of~$\lfp$. The package must prove that the fixedpoint operator is applied to a monotonic function. If the introduction rules have the form described above, and if the package is supplied a monotonicity theorem for every $t\in M(R_i)$ premise, then this proof is trivial.\footnote{Due to the presence of logical connectives in the fixedpoint's body, the monotonicity proof requires some unusual rules. These state that the connectives $\conj$, $\disj$ and $\exists$ are monotonic with respect to the partial ordering on unary predicates given by $P\sqsubseteq Q$ if and only if $\forall x.P(x)\imp Q(x)$.} The result structure returns the definitions of the recursive sets as a theorem list called {\tt defs}. It also returns, as the theorem {\tt unfold}, a fixedpoint equation such as \begin{eqnarray*} \Fin(A) & = & \begin{array}[t]{r@{\,}l} \{z\in\pow(A). & z=\emptyset \disj{} \\ &(\exists a\,b. z=\{a\}\un b\conj a\in A\conj b\in \Fin(A))\} \end{array} \end{eqnarray*} It also returns, as the theorem {\tt dom\_subset}, an inclusion such as $\Fin(A)\sbs\pow(A)$. \subsection{Mutual recursion} \label{mutual-sec} In a mutually recursive definition, the domain for the fixedoint construction is the disjoint sum of the domain~$D_i$ of each~$R_i$, for $i=1$, \ldots,~$n$. The package uses the injections of the binary disjoint sum, typically $\Inl$ and~$\Inr$, to express injections $h_{1,n}$, \ldots, $h_{n,n}$ for the $n$-ary disjoint sum $D_1+\cdots+D_n$. As discussed elsewhere \cite[\S4.5]{paulson-set-II}, Isabelle/ZF defines the operator $\Part$ to support mutual recursion. The set $\Part(A,h)$ contains those elements of~$A$ having the form~$h(z)$: \begin{eqnarray*} \Part(A,h) & \equiv & \{x\in A. \exists z. x=h(z)\}. \end{eqnarray*} For mutually recursive sets $R_1$, \ldots,~$R_n$ with $n>1$, the package makes $n+1$ definitions. The first defines a set $R$ using a fixedpoint operator. The remaining $n$ definitions have the form \begin{eqnarray*} R_i & \equiv & \Part(R,h_{i,n}), \qquad i=1,\ldots, n. \end{eqnarray*} It follows that $R=R_1\un\cdots\un R_n$, where the $R_i$ are pairwise disjoint. \subsection{Proving the introduction rules} The uesr supplies the package with the desired form of the introduction rules. Once it has derived the theorem {\tt unfold}, it attempts to prove the introduction rules. From the user's point of view, this is the trickiest stage; the proofs often fail. The task is to show that the domain $D_1+\cdots+D_n$ of the combined set $R_1\un\cdots\un R_n$ is closed under all the introduction rules. This essentially involves replacing each~$R_i$ by $D_1+\cdots+D_n$ in each of the introduction rules and attempting to prove the result. Consider the $\Fin(A)$ example. After substituting $\pow(A)$ for $\Fin(A)$ in the rules, the package must prove \[ \emptyset\in\pow(A) \qquad \infer{\{a\}\un b\in\pow(A)}{a\in A & b\in\pow(A)} \] Such proofs can be regarded as type-checking the definition. The user supplies the package with type-checking rules to apply. Usually these are general purpose rules from the ZF theory. They could however be rules specifically proved for a particular inductive definition; sometimes this is the easiest way to get the definition through! The package returns the introduction rules as the theorem list {\tt intrs}. \subsection{The elimination rule} The elimination rule, called {\tt elim}, is derived in a straightforward manner. Applying the rule performs a case analysis, with one case for each introduction rule. Continuing our example, the elimination rule for $\Fin(A)$ is \[ \infer{Q}{x\in\Fin(A) & \infer*{Q}{[x=\emptyset]} & \infer*{Q}{[x=\{a\}\un b & a\in A &b\in\Fin(A)]_{a,b}} } \] The package also returns a function {\tt mk\_cases}, for generating simplified instances of the elimination rule. However, {\tt mk\_cases} only works for datatypes and for inductive definitions involving datatypes, such as an inductively defined relation between lists. It instantiates {\tt elim} with a user-supplied term, then simplifies the cases using the freeness of the underlying datatype. \section{Induction and co-induction rules} Here we must consider inductive and co-inductive definitions separately. For an inductive definition, the package returns an induction rule derived directly from the properties of least fixedpoints, as well as a modified rule for mutual recursion and inductively defined relations. For a co-inductive definition, the package returns a basic co-induction rule. \subsection{The basic induction rule}\label{basic-ind-sec} The basic rule, called simply {\tt induct}, is appropriate in most situations. For inductive definitions, it is strong rule induction~\cite{camilleri92}; for datatype definitions (see below), it is just structural induction. The induction rule for an inductively defined set~$R$ has the following form. The major premise is $x\in R$. There is a minor premise for each introduction rule: \begin{itemize} \item If the introduction rule concludes $t\in R_i$, then the minor premise is~$P(t)$. \item The minor premise's eigenvariables are precisely the introduction rule's free variables that are not parameters of~$R$ --- for instance, $A$ is not an eigenvariable in the $\Fin(A)$ rule below. \item If the introduction rule has a premise $t\in R_i$, then the minor premise discharges the assumption $t\in R_i$ and the induction hypothesis~$P(t)$. If the introduction rule has a premise $t\in M(R_i)$ then the minor premise discharges the single assumption \[ t\in M(\{z\in R_i. P(z)\}). \] Because $M$ is monotonic, this assumption implies $t\in M(R_i)$. The occurrence of $P$ gives the effect of an induction hypothesis, which may be exploited by appealing to properties of~$M$. \end{itemize} The rule for $\Fin(A)$ is \[ \infer{P(x)}{x\in\Fin(A) & P(\emptyset) & \infer*{P(\{a\}\un b)}{[a\in A & b\in\Fin(A) & P(b)]_{a,b}} } \] Stronger induction rules often suggest themselves. In the case of $\Fin(A)$, the Isabelle/ZF theory proceeds to derive a rule whose third premise discharges the extra assumption $a\not\in b$. Most special induction rules must be proved manually, but the package proves a rule for mutual induction and inductive relations. \subsection{Mutual induction} The mutual induction rule is called {\tt mutual\_induct}. It differs from the basic rule in several respects: \begin{itemize} \item Instead of a single predicate~$P$, it uses $n$ predicates $P_1$, \ldots,~$P_n$: one for each recursive set. \item There is no major premise such as $x\in R_i$. Instead, the conclusion refers to all the recursive sets: \[ (\forall z.z\in R_1\imp P_1(z))\conj\cdots\conj (\forall z.z\in R_n\imp P_n(z)) \] Proving the premises simultaneously establishes $P_i(z)$ for $z\in R_i$ and $i=1$, \ldots,~$n$. \item If the domain of some $R_i$ is the Cartesian product $A_1\times\cdots\times A_m$, then the corresponding predicate $P_i$ takes $m$ arguments and the corresponding conjunct of the conclusion is \[ (\forall z_1\ldots z_m.\pair{z_1,\ldots,z_m}\in R_i\imp P_i(z_1,\ldots,z_m)) \] \end{itemize} The last point above simplifies reasoning about inductively defined relations. It eliminates the need to express properties of $z_1$, \ldots,~$z_m$ as properties of the tuple $\pair{z_1,\ldots,z_m}$. \subsection{Co-induction}\label{co-ind-sec} A co-inductive definition yields a primitive co-induction rule, with no refinements such as those for the induction rules. (Experience may suggest refinements later.) Consider the co-datatype of lazy lists as an example. For suitable definitions of $\LNil$ and $\LCons$, lazy lists may be defined as the greatest fixedpoint satisfying the rules \[ \LNil\in\llist(A) \qquad \infer[(-)]{\LCons(a,l)\in\llist(A)}{a\in A & l\in\llist(A)} \] The $(-)$ tag stresses that this is a co-inductive definition. A suitable domain for $\llist(A)$ is $\quniv(A)$, a set closed under variant forms of sum and product for representing infinite data structures (\S\ref{data-sec}). Co-inductive definitions use these variant sums and products. The package derives an {\tt unfold} theorem similar to that for $\Fin(A)$. Then it proves the theorem {\tt co\_induct}, which expresses that $\llist(A)$ is the greatest solution to this equation contained in $\quniv(A)$: \[ \infer{a\in\llist(A)}{a\in X & X\sbs \quniv(A) & \infer*{z=\LNil\disj \bigl(\exists a\,l.\, \begin{array}[t]{@{}l} z=\LCons(a,l) \conj a\in A \conj{}\\ l\in X\un\llist(A) \bigr) \end{array} }{[z\in X]_z}} \] Having $X\un\llist(A)$ instead of simply $X$ in the third premise above represents a slight strengthening of the greatest fixedpoint property. I discuss several forms of co-induction rules elsewhere~\cite{paulson-coind}. \section{Examples of inductive and co-inductive definitions}\label{ind-eg-sec} This section presents several examples: the finite set operator, lists of $n$ elements, bisimulations on lazy lists, the well-founded part of a relation, and the primitive recursive functions. \subsection{The finite set operator} The definition of finite sets has been discussed extensively above. Here is the corresponding ML invocation (note that $\cons(a,b)$ abbreviates $\{a\}\un b$ in Isabelle/ZF): \begin{ttbox} structure Fin = Inductive_Fun (val thy = Arith.thy addconsts [(["Fin"],"i=>i")]; val rec_doms = [("Fin","Pow(A)")]; val sintrs = ["0 : Fin(A)", "[| a: A; b: Fin(A) |] ==> cons(a,b) : Fin(A)"]; val monos = []; val con_defs = []; val type_intrs = [empty_subsetI, cons_subsetI, PowI] val type_elims = [make_elim PowD]); \end{ttbox} The parent theory is obtained from {\tt Arith.thy} by adding the unary function symbol~$\Fin$. Its domain is specified as $\pow(A)$, where $A$ is the parameter appearing in the introduction rules. For type-checking, the package supplies the introduction rules: \[ \emptyset\sbs A \qquad \infer{\{a\}\un B\sbs C}{a\in C & B\sbs C} \] A further introduction rule and an elimination rule express the two directions of the equivalence $A\in\pow(B)\bimp A\sbs B$. Type-checking involves mostly introduction rules. When the package returns, we can refer to the $\Fin(A)$ introduction rules as {\tt Fin.intrs}, the induction rule as {\tt Fin.induct}, and so forth. \subsection{Lists of $n$ elements}\label{listn-sec} This has become a standard example in the literature. Following Paulin-Mohring~\cite{paulin92}, we could attempt to define a new datatype $\listn(A,n)$, for lists of length~$n$, as an $n$-indexed family of sets. But her introduction rules \[ {\tt Niln}\in\listn(A,0) \qquad \infer{{\tt Consn}(n,a,l)\in\listn(A,\succ(n))} {n\in\nat & a\in A & l\in\listn(A,n)} \] are not acceptable to the inductive definition package: $\listn$ occurs with three different parameter lists in the definition. \begin{figure} \begin{small} \begin{verbatim} structure ListN = Inductive_Fun (val thy = ListFn.thy addconsts [(["listn"],"i=>i")]; val rec_doms = [("listn", "nat*list(A)")]; val sintrs = ["<0,Nil> : listn(A)", "[| a: A; <n,l> : listn(A) |] ==> <succ(n), Cons(a,l)> : listn(A)"]; val monos = []; val con_defs = []; val type_intrs = nat_typechecks@List.intrs@[SigmaI] val type_elims = [SigmaE2]); \end{verbatim} \end{small} \hrule \caption{Defining lists of $n$ elements} \label{listn-fig} \end{figure} There is an obvious way of handling this particular example, which may suggest a general approach to varying parameters. Here, we can take advantage of an existing datatype definition of $\lst(A)$, with constructors $\Nil$ and~$\Cons$. Then incorporate the number~$n$ into the inductive set itself, defining $\listn(A)$ as a relation. It consists of pairs $\pair{n,l}$ such that $n\in\nat$ and~$l\in\lst(A)$ and $l$ has length~$n$. In fact, $\listn(A)$ turns out to be the converse of the length function on~$\lst(A)$. The Isabelle/ZF introduction rules are \[ \pair{0,\Nil}\in\listn(A) \qquad \infer{\pair{\succ(n),\Cons(a,l)}\in\listn(A)} {a\in A & \pair{n,l}\in\listn(A)} \] Figure~\ref{listn-fig} presents the ML invocation. A theory of lists, extended with a declaration of $\listn$, is the parent theory. The domain is specified as $\nat\times\lst(A)$. The type-checking rules include those for 0, $\succ$, $\Nil$ and $\Cons$. Because $\listn(A)$ is a set of pairs, type-checking also requires introduction and elimination rules to express both directions of the equivalence $\pair{a,b}\in A\times B \bimp a\in A \conj b\in B$. The package returns introduction, elimination and induction rules for $\listn$. The basic induction rule, {\tt ListN.induct}, is \[ \infer{P(x)}{x\in\listn(A) & P(\pair{0,\Nil}) & \infer*{P(\pair{\succ(n),\Cons(a,l)})} {[a\in A & \pair{n,l}\in\listn(A) & P(\pair{n,l})]_{a,l,n}}} \] This rule requires the induction formula to be a unary property of pairs,~$P(\pair{n,l})$. The alternative rule, {\tt ListN.mutual\_induct}, uses a binary property instead: \[ \infer{\forall n\,l. \pair{n,l}\in\listn(A) \imp P(\pair{n,l})} {P(0,\Nil) & \infer*{P(\succ(n),\Cons(a,l))} {[a\in A & \pair{n,l}\in\listn(A) & P(n,l)]_{a,l,n}}} \] It is now a simple matter to prove theorems about $\listn(A)$, such as \[ \forall l\in\lst(A). \pair{\length(l),\, l}\in\listn(A) \] \[ \listn(A)``\{n\} = \{l\in\lst(A). \length(l)=n\} \] This latter result --- here $r``A$ denotes the image of $A$ under $r$ --- asserts that the inductive definition agrees with the obvious notion of $n$-element list. Unlike in Coq, the definition does not declare a new datatype. A `list of $n$ elements' really is a list, and is subject to list operators such as append. For example, a trivial induction yields \[ \infer{\pair{m\mathbin{+} m,\, l@l'}\in\listn(A)} {\pair{m,l}\in\listn(A) & \pair{m',l'}\in\listn(A)} \] where $+$ here denotes addition on the natural numbers and @ denotes append. \ifCADE\typeout{****Omitting mk_cases from CADE version!} \else \subsection{A demonstration of {\tt mk\_cases}} The elimination rule, {\tt ListN.elim}, is cumbersome: \[ \infer{Q}{x\in\listn(A) & \infer*{Q}{[x = \pair{0,\Nil}]} & \infer*{Q} {\left[\begin{array}{l} x = \pair{\succ(n),\Cons(a,l)} \\ a\in A \\ \pair{n,l}\in\listn(A) \end{array} \right]_{a,l,n}}} \] The function {\tt ListN.mk\_cases} generates simplified instances of this rule. It works by freeness reasoning on the list constructors. If $x$ is $\pair{i,\Nil}$ or $\pair{i,\Cons(a,l)}$ then {\tt ListN.mk\_cases} deduces the corresponding form of~$i$. For example, \begin{ttbox} ListN.mk_cases List.con_defs "<i,Cons(a,l)> : listn(A)" \end{ttbox} yields the rule \[ \infer{Q}{\pair{i, \Cons(a,l)}\in\listn(A) & \infer*{Q} {\left[\begin{array}{l} i = \succ(n) \\ a\in A \\ \pair{n,l}\in\listn(A) \end{array} \right]_{n}}} \] The package also has built-in rules for freeness reasoning about $0$ and~$\succ$. So if $x$ is $\pair{0,l}$ or $\pair{\succ(i),l}$, then {\tt ListN.mk\_cases} can similarly deduce the corresponding form of~$l$. The function {\tt mk\_cases} is also useful with datatype definitions themselves. The version from the definition of lists, namely {\tt List.mk\_cases}, can prove the rule \[ \infer{Q}{\Cons(a,l)\in\lst(A) & & \infer*{Q}{[a\in A &l\in\lst(A)]} } \] The most important uses of {\tt mk\_cases} concern inductive definitions of evaluation relations. Then {\tt mk\_cases} supports the kind of backward inference typical of hand proofs, for example to prove that the evaluation relation is functional. \fi %CADE \subsection{A co-inductive definition: bisimulations on lazy lists} This example anticipates the definition of the co-datatype $\llist(A)$, which consists of lazy lists over~$A$. Its constructors are $\LNil$ and $\LCons$, satisfying the introduction rules shown in~\S\ref{co-ind-sec}. Because $\llist(A)$ is defined as a greatest fixedpoint and uses the variant pairing and injection operators, it contains non-well-founded elements such as solutions to $\LCons(a,l)=l$. The next step in the development of lazy lists is to define a co-induction principle for proving equalities. This is done by showing that the equality relation on lazy lists is the greatest fixedpoint of some monotonic operation. The usual approach~\cite{pitts94} is to define some notion of bisimulation for lazy lists, define equivalence to be the greatest bisimulation, and finally to prove that two lazy lists are equivalent if and only if they are equal. The co-induction rule for equivalence then yields a co-induction principle for equalities. A binary relation $R$ on lazy lists is a {\bf bisimulation} provided $R\sbs R^+$, where $R^+$ is the relation \[ \{\pair{\LNil;\LNil}\} \un \{\pair{\LCons(a,l);\LCons(a,l')} . a\in A \conj \pair{l;l'}\in R\}. \] Variant pairs are used, $\pair{l;l'}$ instead of $\pair{l,l'}$, because this is a co-inductive definition. A pair of lazy lists are {\bf equivalent} if they belong to some bisimulation. Equivalence can be co-inductively defined as the greatest fixedpoint for the introduction rules \[ \pair{\LNil;\LNil} \in\lleq(A) \qquad \infer[(-)]{\pair{\LCons(a,l);\LCons(a,l')} \in\lleq(A)} {a\in A & \pair{l;l'}\in \lleq(A)} \] To make this co-inductive definition, we invoke \verb|Co_Inductive_Fun|: \begin{ttbox} structure LList_Eq = Co_Inductive_Fun (val thy = LList.thy addconsts [(["lleq"],"i=>i")]; val rec_doms = [("lleq", "llist(A) <*> llist(A)")]; val sintrs = ["<LNil; LNil> : lleq(A)", "[| a:A; <l;l'>: lleq(A) |] ==> <LCons(a,l); LCons(a,l')>: lleq(A)"]; val monos = []; val con_defs = []; val type_intrs = LList.intrs@[QSigmaI]; val type_elims = [QSigmaE2]); \end{ttbox} Again, {\tt addconsts} declares a constant for $\lleq$ in the parent theory. The domain of $\lleq(A)$ is $\llist(A)\otimes\llist(A)$, where $\otimes$ denotes the variant Cartesian product. The type-checking rules include the introduction rules for lazy lists as well as rules expressinve both definitions of the equivalence \[ \pair{a;b}\in A\otimes B \bimp a\in A \conj b\in B. \] The package returns the introduction rules and the elimination rule, as usual. But instead of induction rules, it returns a co-induction rule. The rule is too big to display in the usual notation; its conclusion is $a\in\lleq(A)$ and its premises are $a\in X$, $X\sbs \llist(A)\otimes\llist(A)$ and \[ \infer*{z=\pair{\LNil;\LNil}\disj \bigl(\exists a\,l\,l'.\, \begin{array}[t]{@{}l} z=\pair{\LCons(a,l);\LCons(a,l')} \conj a\in A \conj{}\\ \pair{l;l'}\in X\un\lleq(A) \bigr) \end{array} }{[z\in X]_z} \] Thus if $a\in X$, where $X$ is a bisimulation contained in the domain of $\lleq(A)$, then $a\in\lleq(A)$. It is easy to show that $\lleq(A)$ is reflexive: the equality relation is a bisimulation. And $\lleq(A)$ is symmetric: its converse is a bisimulation. But showing that $\lleq(A)$ coincides with the equality relation takes considerable work. \subsection{The accessible part of a relation}\label{acc-sec} Let $\prec$ be a binary relation on~$D$; in short, $(\prec)\sbs D\times D$. The {\bf accessible} or {\bf well-founded} part of~$\prec$, written $\acc(\prec)$, is essentially that subset of~$D$ for which $\prec$ admits no infinite decreasing chains~\cite{aczel77}. Formally, $\acc(\prec)$ is inductively defined to be the least set that contains $a$ if it contains all $\prec$-predecessors of~$a$, for $a\in D$. Thus we need an introduction rule of the form %%%%\[ \infer{a\in\acc(\prec)}{\infer*{y\in\acc(\prec)}{[y\prec a]_y}} \] \[ \infer{a\in\acc(\prec)}{\forall y.y\prec a\imp y\in\acc(\prec)} \] Paulin-Mohring treats this example in Coq~\cite{paulin92}, but it causes difficulties for other systems. Its premise does not conform to the structure of introduction rules for HOL's inductive definition package~\cite{camilleri92}. It is also unacceptable to Isabelle package (\S\ref{intro-sec}), but fortunately can be transformed into one of the form $t\in M(R)$. The powerset operator is monotonic, and $t\in\pow(R)$ is equivalent to $t\sbs R$. This in turn is equivalent to $\forall y\in t. y\in R$. To express $\forall y.y\prec a\imp y\in\acc(\prec)$ we need only find a term~$t$ such that $y\in t$ if and only if $y\prec a$. A suitable $t$ is the inverse image of~$\{a\}$ under~$\prec$. The ML invocation below follows this approach. Here $r$ is~$\prec$ and $\field(r)$ refers to~$D$, the domain of $\acc(r)$. Finally $r^{-}``\{a\}$ denotes the inverse image of~$\{a\}$ under~$r$. The package is supplied the theorem {\tt Pow\_mono}, which asserts that $\pow$ is monotonic. \begin{ttbox} structure Acc = Inductive_Fun (val thy = WF.thy addconsts [(["acc"],"i=>i")]; val rec_doms = [("acc", "field(r)")]; val sintrs = ["[| r-``\{a\} : Pow(acc(r)); a : field(r) |] ==> a : acc(r)"]; val monos = [Pow_mono]; val con_defs = []; val type_intrs = []; val type_elims = []); \end{ttbox} The Isabelle theory proceeds to prove facts about $\acc(\prec)$. For instance, $\prec$ is well-founded if and only if its field is contained in $\acc(\prec)$. As mentioned in~\S\ref{basic-ind-sec}, a premise of the form $t\in M(R)$ gives rise to an unusual induction hypothesis. Let us examine the induction rule, {\tt Acc.induct}: \[ \infer{P(x)}{x\in\acc(r) & \infer*{P(a)}{[r^{-}``\{a\}\in\pow(\{z\in\acc(r).P(z)\}) & a\in\field(r)]_a}} \] The strange induction hypothesis is equivalent to $\forall y. \pair{y,a}\in r\imp y\in\acc(r)\conj P(y)$. Therefore the rule expresses well-founded induction on the accessible part of~$\prec$. The use of inverse image is not essential. The Isabelle package can accept introduction rules with arbitrary premises of the form $\forall \vec{y}.P(\vec{y})\imp f(\vec{y})\in R$. The premise can be expressed equivalently as \[ \{z\in D. P(\vec{y}) \conj z=f(\vec{y})\} \] provided $f(\vec{y})\in D$ for all $\vec{y}$ such that~$P(\vec{y})$. The following section demonstrates another use of the premise $t\in M(R)$, where $M=\lst$. \subsection{The primitive recursive functions}\label{primrec-sec} The primitive recursive functions are traditionally defined inductively, as a subset of the functions over the natural numbers. One difficulty is that functions of all arities are taken together, but this is easily circumvented by regarding them as functions on lists. Another difficulty, the notion of composition, is less easily circumvented. Here is a more precise definition. Letting $\vec{x}$ abbreviate $x_0,\ldots,x_{n-1}$, we can write lists such as $[\vec{x}]$, $[y+1,\vec{x}]$, etc. A function is {\bf primitive recursive} if it belongs to the least set of functions in $\lst(\nat)\to\nat$ containing \begin{itemize} \item The {\bf successor} function $\SC$, such that $\SC[y,\vec{x}]=y+1$. \item All {\bf constant} functions $\CONST(k)$, such that $\CONST(k)[\vec{x}]=k$. \item All {\bf projection} functions $\PROJ(i)$, such that $\PROJ(i)[\vec{x}]=x_i$ if $0\leq i<n$. \item All {\bf compositions} $\COMP(g,[f_0,\ldots,f_{m-1}])$, where $g$ and $f_0$, \ldots, $f_{m-1}$ are primitive recursive, such that \begin{eqnarray*} \COMP(g,[f_0,\ldots,f_{m-1}])[\vec{x}] & = & g[f_0[\vec{x}],\ldots,f_{m-1}[\vec{x}]]. \end{eqnarray*} \item All {\bf recursions} $\PREC(f,g)$, where $f$ and $g$ are primitive recursive, such that \begin{eqnarray*} \PREC(f,g)[0,\vec{x}] & = & f[\vec{x}] \\ \PREC(f,g)[y+1,\vec{x}] & = & g[\PREC(f,g)[y,\vec{x}],\, y,\, \vec{x}]. \end{eqnarray*} \end{itemize} Composition is awkward because it combines not two functions, as is usual, but $m+1$ functions. In her proof that Ackermann's function is not primitive recursive, Nora Szasz was unable to formalize this definition directly~\cite{szasz93}. So she generalized primitive recursion to tuple-valued functions. This modified the inductive definition such that each operation on primitive recursive functions combined just two functions. \begin{figure} \begin{ttbox} structure Primrec = Inductive_Fun (val thy = Primrec0.thy; val rec_doms = [("primrec", "list(nat)->nat")]; val ext = None val sintrs = ["SC : primrec", "k: nat ==> CONST(k) : primrec", "i: nat ==> PROJ(i) : primrec", "[| g: primrec; fs: list(primrec) |] ==> COMP(g,fs): primrec", "[| f: primrec; g: primrec |] ==> PREC(f,g): primrec"]; val monos = [list_mono]; val con_defs = [SC_def,CONST_def,PROJ_def,COMP_def,PREC_def]; val type_intrs = pr0_typechecks val type_elims = []); \end{ttbox} \hrule \caption{Inductive definition of the primitive recursive functions} \label{primrec-fig} \end{figure} \def\fs{{\it fs}} Szasz was using ALF, but Coq and HOL would also have problems accepting this definition. Isabelle's package accepts it easily since $[f_0,\ldots,f_{m-1}]$ is a list of primitive recursive functions and $\lst$ is monotonic. There are five introduction rules, one for each of the five forms of primitive recursive function. Note the one for $\COMP$: \[ \infer{\COMP(g,\fs)\in\primrec}{g\in\primrec & \fs\in\lst(\primrec)} \] The induction rule for $\primrec$ has one case for each introduction rule. Due to the use of $\lst$ as a monotone operator, the composition case has an unusual induction hypothesis: \[ \infer*{P(\COMP(g,\fs))} {[g\in\primrec & \fs\in\lst(\{z\in\primrec.P(x)\})]_{\fs,g}} \] The hypothesis states that $\fs$ is a list of primitive recursive functions satisfying the induction formula. Proving the $\COMP$ case typically requires structural induction on lists, yielding two subcases: either $\fs=\Nil$ or else $\fs=\Cons(f,\fs')$, where $f\in\primrec$, $P(f)$, and $\fs'$ is another list of primitive recursive functions satisfying~$P$. Figure~\ref{primrec-fig} presents the ML invocation. Theory {\tt Primrec0.thy} defines the constants $\SC$, etc.; their definitions consist of routine list programming and are omitted. The Isabelle theory goes on to formalize Ackermann's function and prove that it is not primitive recursive, using the induction rule {\tt Primrec.induct}. The proof follows Szasz's excellent account. ALF and Coq treat inductive definitions as datatypes, with a new constructor for each introduction rule. This forced Szasz to define a small programming language for the primitive recursive functions, and then define their semantics. But the Isabelle/ZF formulation defines the primitive recursive functions directly as a subset of the function set $\lst(\nat)\to\nat$. This saves a step and conforms better to mathematical tradition. \section{Datatypes and co-datatypes}\label{data-sec} A (co-)datatype definition is a (co-)inductive definition with automatically defined constructors and case analysis operator. The package proves that the case operator inverts the constructors, and can also prove freeness theorems involving any pair of constructors. \subsection{Constructors and their domain} Conceptually, our two forms of definition are distinct: a (co-)inductive definition selects a subset of an existing set, but a (co-)datatype definition creates a new set. But the package reduces the latter to the former. A set having strong closure properties must serve as the domain of the (co-)inductive definition. Constructing this set requires some theoretical effort. Consider first datatypes and then co-datatypes. Isabelle/ZF defines the standard notion of Cartesian product $A\times B$, containing ordered pairs $\pair{a,b}$. Now the $m$-tuple $\pair{x_1,\ldots\,x_m}$ is the empty set~$\emptyset$ if $m=0$, simply $x_1$ if $m=1$, and $\pair{x_1,\pair{x_2,\ldots\,x_m}}$ if $m\geq2$. Isabelle/ZF also defines the disjoint sum $A+B$, containing injections $\Inl(a)\equiv\pair{0,a}$ and $\Inr(b)\equiv\pair{1,b}$. A datatype constructor $\Con(x_1,\ldots\,x_m)$ is defined to be $h(\pair{x_1,\ldots\,x_m})$, where $h$ is composed of $\Inl$ and~$\Inr$. In a mutually recursive definition, all constructors for the set~$R_i$ have the outer form~$h_{i,n}$, where $h_{i,n}$ is the injection described in~\S\ref{mutual-sec}. Further nested injections ensure that the constructors for~$R_i$ are pairwise distinct. Isabelle/ZF defines the set $\univ(A)$, which contains~$A$ and furthermore contains $\pair{a,b}$, $\Inl(a)$ and $\Inr(b)$ for $a$, $b\in\univ(A)$. In a typical datatype definition with set parameters $A_1$, \ldots, $A_k$, a suitable domain for all the recursive sets is $\univ(A_1\un\cdots\un A_k)$. This solves the problem for datatypes~\cite[\S4.2]{paulson-set-II}. The standard pairs and injections can only yield well-founded constructions. This eases the (manual!) definition of recursive functions over datatypes. But they are unsuitable for co-datatypes, which typically contain non-well-founded objects. To support co-datatypes, Isabelle/ZF defines a variant notion of ordered pair, written~$\pair{a;b}$. It also defines the corresponding variant notion of Cartesian product $A\otimes B$, variant injections $\QInl(a)$ and~$\QInr(b)$, and variant disjoint sum $A\oplus B$. Finally it defines the set $\quniv(A)$, which contains~$A$ and furthermore contains $\pair{a;b}$, $\QInl(a)$ and $\QInr(b)$ for $a$, $b\in\quniv(A)$. In a typical co-datatype definition with set parameters $A_1$, \ldots, $A_k$, a suitable domain is $\quniv(A_1\un\cdots\un A_k)$. This approach is an alternative to adopting an Anti-Foundation Axiom~\cite{aczel88}.\footnote{No reference is available. Variant pairs are defined by $\pair{a;b}\equiv a+b \equiv (\{0\}\times a) \un (\{1\}\times b)$, where $\times$ is the Cartesian product for standard ordered pairs. Now $\pair{a;b}$ is injective and monotonic in its two arguments. Non-well-founded constructions, such as infinite lists, are constructed as least fixedpoints; the bounding set typically has the form $\univ(a_1\un\cdots\un a_k)$, where $a_1$, \ldots, $a_k$ are specified elements of the construction.} \subsection{The case analysis operator} The (co-)datatype package automatically defines a case analysis operator, called {\tt$R$\_case}. A mutually recursive definition still has only one operator, called {\tt$R_1$\_\ldots\_$R_n$\_case}. The case operator is analogous to those for products and sums. Datatype definitions employ standard products and sums, whose operators are $\split$ and $\case$ and satisfy the equations \begin{eqnarray*} \split(f,\pair{x,y}) & = & f(x,y) \\ \case(f,g,\Inl(x)) & = & f(x) \\ \case(f,g,\Inr(y)) & = & g(y) \end{eqnarray*} Suppose the datatype has $k$ constructors $\Con_1$, \ldots,~$\Con_k$. Then its case operator takes $k+1$ arguments and satisfies an equation for each constructor: \begin{eqnarray*} R\hbox{\_case}(f_1,\ldots,f_k, {\tt Con}_i(\vec{x})) & = & f_i(\vec{x}), \qquad i = 1, \ldots, k \end{eqnarray*} Note that if $f$ and $g$ have meta-type $i\To i$ then so do $\split(f)$ and $\case(f,g)$. This works because $\split$ and $\case$ operate on their last argument. They are easily combined to make complex case analysis operators. Here are two examples: \begin{itemize} \item $\split(\lambda x.\split(f(x)))$ performs case analysis for $A\times (B\times C)$, as is easily verified: \begin{eqnarray*} \split(\lambda x.\split(f(x)), \pair{a,b,c}) & = & (\lambda x.\split(f(x))(a,\pair{b,c}) \\ & = & \split(f(a), \pair{b,c}) \\ & = & f(a,b,c) \end{eqnarray*} \item $\case(f,\case(g,h))$ performs case analysis for $A+(B+C)$; let us verify one of the three equations: \begin{eqnarray*} \case(f,\case(g,h), \Inr(\Inl(b))) & = & \case(g,h,\Inl(b)) \\ & = & g(b) \end{eqnarray*} \end{itemize} Co-datatype definitions are treated in precisely the same way. They express case operators using those for the variant products and sums, namely $\qsplit$ and~$\qcase$. \ifCADE The package has processed all the datatypes discussed in my earlier paper~\cite{paulson-set-II} and the co-datatype of lazy lists. Space limitations preclude discussing these examples here, but they are distributed with Isabelle. \typeout{****Omitting datatype examples from CADE version!} \else To see how constructors and the case analysis operator are defined, let us examine some examples. These include lists and trees/forests, which I have discussed extensively in another paper~\cite{paulson-set-II}. \begin{figure} \begin{ttbox} structure List = Datatype_Fun (val thy = Univ.thy; val rec_specs = [("list", "univ(A)", [(["Nil"], "i"), (["Cons"], "[i,i]=>i")])]; val rec_styp = "i=>i"; val ext = None val sintrs = ["Nil : list(A)", "[| a: A; l: list(A) |] ==> Cons(a,l) : list(A)"]; val monos = []; val type_intrs = datatype_intrs val type_elims = datatype_elims); \end{ttbox} \hrule \caption{Defining the datatype of lists} \label{list-fig} \medskip \begin{ttbox} structure LList = Co_Datatype_Fun (val thy = QUniv.thy; val rec_specs = [("llist", "quniv(A)", [(["LNil"], "i"), (["LCons"], "[i,i]=>i")])]; val rec_styp = "i=>i"; val ext = None val sintrs = ["LNil : llist(A)", "[| a: A; l: llist(A) |] ==> LCons(a,l) : llist(A)"]; val monos = []; val type_intrs = co_datatype_intrs val type_elims = co_datatype_elims); \end{ttbox} \hrule \caption{Defining the co-datatype of lazy lists} \label{llist-fig} \end{figure} \subsection{Example: lists and lazy lists} Figures \ref{list-fig} and~\ref{llist-fig} present the ML definitions of lists and lazy lists, respectively. They highlight the (many) similarities and (few) differences between datatype and co-datatype definitions. Each form of list has two constructors, one for the empty list and one for adding an element to a list. Each takes a parameter, defining the set of lists over a given set~$A$. Each uses the appropriate domain from a Isabelle/ZF theory: \begin{itemize} \item $\lst(A)$ specifies domain $\univ(A)$ and parent theory {\tt Univ.thy}. \item $\llist(A)$ specifies domain $\quniv(A)$ and parent theory {\tt QUniv.thy}. \end{itemize} Since $\lst(A)$ is a datatype, it enjoys a structural rule, {\tt List.induct}: \[ \infer{P(x)}{x\in\lst(A) & P(\Nil) & \infer*{P(\Cons(a,l))}{[a\in A & l\in\lst(A) & P(l)]_{a,l}} } \] Induction and freeness yield the law $l\not=\Cons(a,l)$. To strengthen this, Isabelle/ZF defines the rank of a set and proves that the standard pairs and injections have greater rank than their components. An immediate consequence, which justifies structural recursion on lists \cite[\S4.3]{paulson-set-II}, is \[ \rank(l) < \rank(\Cons(a,l)). \] Since $\llist(A)$ is a co-datatype, it has no induction rule. Instead it has the co-induction rule shown in \S\ref{co-ind-sec}. Since variant pairs and injections are monotonic and need not have greater rank than their components, fixedpoint operators can create cyclic constructions. For example, the definition \begin{eqnarray*} \lconst(a) & \equiv & \lfp(\univ(a), \lambda l. \LCons(a,l)) \end{eqnarray*} yields $\lconst(a) = \LCons(a,\lconst(a))$. \medskip It may be instructive to examine the definitions of the constructors and case operator for $\lst(A)$. The definitions for $\llist(A)$ are similar. The list constructors are defined as follows: \begin{eqnarray*} \Nil & = & \Inl(\emptyset) \\ \Cons(a,l) & = & \Inr(\pair{a,l}) \end{eqnarray*} The operator $\lstcase$ performs case analysis on these two alternatives: \begin{eqnarray*} \lstcase(c,h) & \equiv & \case(\lambda u.c, \split(h)) \end{eqnarray*} Let us verify the two equations: \begin{eqnarray*} \lstcase(c, h, \Nil) & = & \case(\lambda u.c, \split(h), \Inl(\emptyset)) \\ & = & (\lambda u.c)(\emptyset) \\ & = & c.\\[1ex] \lstcase(c, h, \Cons(x,y)) & = & \case(\lambda u.c, \split(h), \Inr(\pair{x,y})) \\ & = & \split(h, \pair{x,y}) \\ & = & h(x,y). \end{eqnarray*} \begin{figure} \begin{small} \begin{verbatim} structure TF = Datatype_Fun (val thy = Univ.thy; val rec_specs = [("tree", "univ(A)", [(["Tcons"], "[i,i]=>i")]), ("forest", "univ(A)", [(["Fnil"], "i"), (["Fcons"], "[i,i]=>i")])]; val rec_styp = "i=>i"; val ext = None val sintrs = ["[| a:A; f: forest(A) |] ==> Tcons(a,f) : tree(A)", "Fnil : forest(A)", "[| t: tree(A); f: forest(A) |] ==> Fcons(t,f) : forest(A)"]; val monos = []; val type_intrs = datatype_intrs val type_elims = datatype_elims); \end{verbatim} \end{small} \hrule \caption{Defining the datatype of trees and forests} \label{tf-fig} \end{figure} \subsection{Example: mutual recursion} In the mutually recursive trees/forests~\cite[\S4.5]{paulson-set-II}, trees have the one constructor $\Tcons$, while forests have the two constructors $\Fnil$ and~$\Fcons$. Figure~\ref{tf-fig} presents the ML definition. It has much in common with that of $\lst(A)$, including its use of $\univ(A)$ for the domain and {\tt Univ.thy} for the parent theory. The three introduction rules define the mutual recursion. The distinguishing feature of this example is its two induction rules. The basic induction rule is called {\tt TF.induct}: \[ \infer{P(x)}{x\in\TF(A) & \infer*{P(\Tcons(a,f))} {\left[\begin{array}{l} a\in A \\ f\in\forest(A) \\ P(f) \end{array} \right]_{a,f}} & P(\Fnil) & \infer*{P(\Fcons(a,l))} {\left[\begin{array}{l} t\in\tree(A) \\ P(t) \\ f\in\forest(A) \\ P(f) \end{array} \right]_{t,f}} } \] This rule establishes a single predicate for $\TF(A)$, the union of the recursive sets. Although such reasoning is sometimes useful \cite[\S4.5]{paulson-set-II}, a proper mutual induction rule should establish separate predicates for $\tree(A)$ and $\forest(A)$. The package calls this rule {\tt TF.mutual\_induct}. Observe the usage of $P$ and $Q$ in the induction hypotheses: \[ \infer{(\forall z. z\in\tree(A)\imp P(z)) \conj (\forall z. z\in\forest(A)\imp Q(z))} {\infer*{P(\Tcons(a,f))} {\left[\begin{array}{l} a\in A \\ f\in\forest(A) \\ Q(f) \end{array} \right]_{a,f}} & Q(\Fnil) & \infer*{Q(\Fcons(a,l))} {\left[\begin{array}{l} t\in\tree(A) \\ P(t) \\ f\in\forest(A) \\ Q(f) \end{array} \right]_{t,f}} } \] As mentioned above, the package does not define a structural recursion operator. I have described elsewhere how this is done \cite[\S4.5]{paulson-set-II}. Both forest constructors have the form $\Inr(\cdots)$, while the tree constructor has the form $\Inl(\cdots)$. This pattern would hold regardless of how many tree or forest constructors there were. \begin{eqnarray*} \Tcons(a,l) & = & \Inl(\pair{a,l}) \\ \Fnil & = & \Inr(\Inl(\emptyset)) \\ \Fcons(a,l) & = & \Inr(\Inr(\pair{a,l})) \end{eqnarray*} There is only one case operator; it works on the union of the trees and forests: \begin{eqnarray*} {\tt tree\_forest\_case}(f,c,g) & \equiv & \case(\split(f),\, \case(\lambda u.c, \split(g))) \end{eqnarray*} \begin{figure} \begin{small} \begin{verbatim} structure Data = Datatype_Fun (val thy = Univ.thy; val rec_specs = [("data", "univ(A Un B)", [(["Con0"], "i"), (["Con1"], "i=>i"), (["Con2"], "[i,i]=>i"), (["Con3"], "[i,i,i]=>i")])]; val rec_styp = "[i,i]=>i"; val ext = None val sintrs = ["Con0 : data(A,B)", "[| a: A |] ==> Con1(a) : data(A,B)", "[| a: A; b: B |] ==> Con2(a,b) : data(A,B)", "[| a: A; b: B; d: data(A,B) |] ==> Con3(a,b,d) : data(A,B)"]; val monos = []; val type_intrs = datatype_intrs val type_elims = datatype_elims); \end{verbatim} \end{small} \hrule \caption{Defining the four-constructor sample datatype} \label{data-fig} \end{figure} \subsection{A four-constructor datatype} Finally let us consider a fairly general datatype. It has four constructors $\Con_0$, $\Con_1$\ $\Con_2$ and $\Con_3$, with the corresponding arities. Figure~\ref{data-fig} presents the ML definition. Because this datatype has two set parameters, $A$ and~$B$, it specifies $\univ(A\un B)$ as its domain. The structural induction rule has four minor premises, one per constructor: \[ \infer{P(x)}{x\in\data(A,B) & P(\Con_0) & \infer*{P(\Con_1(a))}{[a\in A]_a} & \infer*{P(\Con_2(a,b))} {\left[\begin{array}{l} a\in A \\ b\in B \end{array} \right]_{a,b}} & \infer*{P(\Con_3(a,b,d))} {\left[\begin{array}{l} a\in A \\ b\in B \\ d\in\data(A,B) \\ P(d) \end{array} \right]_{a,b,d}} } \] The constructor definitions are \begin{eqnarray*} \Con_0 & = & \Inl(\Inl(\emptyset)) \\ \Con_1(a) & = & \Inl(\Inr(a)) \\ \Con_2(a,b) & = & \Inr(\Inl(\pair{a,b})) \\ \Con_3(a,b,c) & = & \Inr(\Inr(\pair{a,b,c})). \end{eqnarray*} The case operator is \begin{eqnarray*} {\tt data\_case}(f_0,f_1,f_2,f_3) & \equiv & \case(\begin{array}[t]{@{}l} \case(\lambda u.f_0,\; f_1),\, \\ \case(\split(f_2),\; \split(\lambda v.\split(f_3(v)))) ) \end{array} \end{eqnarray*} This may look cryptic, but the case equations are trivial to verify. In the constructor definitions, the injections are balanced. A more naive approach is to define $\Con_3(a,b,c)$ as $\Inr(\Inr(\Inr(\pair{a,b,c})))$; instead, each constructor has two injections. The difference here is small. But the ZF examples include a 60-element enumeration type, where each constructor has 5 or~6 injections. The naive approach would require 1 to~59 injections; the definitions would be quadratic in size. It is like the difference between the binary and unary numeral systems. The package returns the constructor and case operator definitions as the theorem list \verb|con_defs|. The head of this list defines the case operator and the tail defines the constructors. The package returns the case equations, such as \begin{eqnarray*} {\tt data\_case}(f_0,f_1,f_2,f_3,\Con_3(a,b,c)) & = & f_3(a,b,c), \end{eqnarray*} as the theorem list \verb|case_eqns|. There is one equation per constructor. \subsection{Proving freeness theorems} There are two kinds of freeness theorems: \begin{itemize} \item {\bf injectiveness} theorems, such as \[ \Con_2(a,b) = \Con_2(a',b') \bimp a=a' \conj b=b' \] \item {\bf distinctness} theorems, such as \[ \Con_1(a) \not= \Con_2(a',b') \] \end{itemize} Since the number of such theorems is quadratic in the number of constructors, the package does not attempt to prove them all. Instead it returns tools for proving desired theorems --- either explicitly or `on the fly' during simplification or classical reasoning. The theorem list \verb|free_iffs| enables the simplifier to perform freeness reasoning. This works by incremental unfolding of constructors that appear in equations. The theorem list contains logical equivalences such as \begin{eqnarray*} \Con_0=c & \bimp & c=\Inl(\Inl(\emptyset)) \\ \Con_1(a)=c & \bimp & c=\Inl(\Inr(a)) \\ & \vdots & \\ \Inl(a)=\Inl(b) & \bimp & a=b \\ \Inl(a)=\Inr(b) & \bimp & \bot \\ \pair{a,b} = \pair{a',b'} & \bimp & a=a' \conj b=b' \end{eqnarray*} For example, these rewrite $\Con_1(a)=\Con_1(b)$ to $a=b$ in four steps. The theorem list \verb|free_SEs| enables the classical reasoner to perform similar replacements. It consists of elimination rules to replace $\Con_0=c$ by $c=\Inl(\Inl(\emptyset))$, and so forth, in the assumptions. Such incremental unfolding combines freeness reasoning with other proof steps. It has the unfortunate side-effect of unfolding definitions of constructors in contexts such as $\exists x.\Con_1(a)=x$, where they should be left alone. Calling the Isabelle tactic {\tt fold\_tac con\_defs} restores the defined constants. \fi %CADE \section{Conclusions and future work} The fixedpoint approach makes it easy to implement a uniquely powerful package for inductive and co-inductive definitions. It is efficient: it processes most definitions in seconds and even a 60-constructor datatype requires only two minutes. It is also simple: the package consists of under 1100 lines (35K bytes) of Standard ML code. The first working version took under a week to code. The approach is not restricted to set theory. It should be suitable for any logic that has some notion of set and the Knaster-Tarski Theorem. Indeed, Melham's inductive definition package for the HOL system~\cite{camilleri92} implicitly proves this theorem. Datatype and co-datatype definitions furthermore require a particular set closed under a suitable notion of ordered pair. I intend to use the Isabelle/ZF package as the basis for a higher-order logic one, using Isabelle/HOL\@. The necessary theory is already mechanizeds~\cite{paulson-coind}. HOL represents sets by unary predicates; defining the corresponding types may cause complication. \bibliographystyle{plain} \bibliography{atp,theory,funprog,isabelle} %%%%%\doendnotes \ifCADE\typeout{****Omitting appendices from CADE version!} \else \newpage \appendix \section{Inductive and co-inductive definitions: users guide} The ML functors \verb|Inductive_Fun| and \verb|Co_Inductive_Fun| build inductive and co-inductive definitions, respectively. This section describes how to invoke them. \subsection{The result structure} Many of the result structure's components have been discussed in~\S\ref{basic-sec}; others are self-explanatory. \begin{description} \item[\tt thy] is the new theory containing the recursive sets. \item[\tt defs] is the list of definitions of the recursive sets. \item[\tt bnd\_mono] is a monotonicity theorem for the fixedpoint operator. \item[\tt unfold] is a fixedpoint equation for the recursive set (the union of the recursive sets, in the case of mutual recursion). \item[\tt dom\_subset] is a theorem stating inclusion in the domain. \item[\tt intrs] is the list of introduction rules, now proved as theorems, for the recursive sets. \item[\tt elim] is the elimination rule. \item[\tt mk\_cases] is a function to create simplified instances of {\tt elim}, using freeness reasoning on some underlying datatype. \end{description} For an inductive definition, the result structure contains two induction rules, {\tt induct} and \verb|mutual_induct|. For a co-inductive definition, it contains the rule \verb|co_induct|. \begin{figure} \begin{ttbox} sig val thy : theory val defs : thm list val bnd_mono : thm val unfold : thm val dom_subset : thm val intrs : thm list val elim : thm val mk_cases : thm list -> string -> thm {\it(Inductive definitions only)} val induct : thm val mutual_induct: thm {\it(Co-inductive definitions only)} val co_induct : thm end \end{ttbox} \hrule \caption{The result of a (co-)inductive definition} \label{def-result-fig} \end{figure} Figure~\ref{def-result-fig} summarizes the two result signatures, specifying the types of all these components. \begin{figure} \begin{ttbox} sig val thy : theory val rec_doms : (string*string) list val sintrs : string list val monos : thm list val con_defs : thm list val type_intrs : thm list val type_elims : thm list end \end{ttbox} \hrule \caption{The argument of a (co-)inductive definition} \label{def-arg-fig} \end{figure} \subsection{The argument structure} Both \verb|Inductive_Fun| and \verb|Co_Inductive_Fun| take the same argument structure (Figure~\ref{def-arg-fig}). Its components are as follows: \begin{description} \item[\tt thy] is the definition's parent theory, which {\it must\/} declare constants for the recursive sets. \item[\tt rec\_doms] is a list of pairs, associating the name of each recursive set with its domain. \item[\tt sintrs] specifies the desired introduction rules as strings. \item[\tt monos] consists of monotonicity theorems for each operator applied to a recursive set in the introduction rules. \item[\tt con\_defs] contains definitions of constants appearing in the introduction rules. The (co-)datatype package supplies the constructors' definitions here. Most direct calls of \verb|Inductive_Fun| or \verb|Co_Inductive_Fun| pass the empty list; one exception is the primitive recursive functions example (\S\ref{primrec-sec}). \item[\tt type\_intrs] consists of introduction rules for type-checking the definition, as discussed in~\S\ref{basic-sec}. They are applied using depth-first search; you can trace the proof by setting \verb|trace_DEPTH_FIRST := true|. \item[\tt type\_elims] consists of elimination rules for type-checking the definition. They are presumed to be `safe' and are applied as much as possible, prior to the {\tt type\_intrs} search. \end{description} The package has a few notable restrictions: \begin{itemize} \item The parent theory, {\tt thy}, must declare the recursive sets as constants. You can extend a theory with new constants using {\tt addconsts}, as illustrated in~\S\ref{ind-eg-sec}. If the inductive definition also requires new concrete syntax, then it is simpler to express the parent theory using a theory file. It is often convenient to define an infix syntax for relations, say $a\prec b$ for $\pair{a,b}\in R$. \item The names of the recursive sets must be identifiers, not infix operators. \item Side-conditions must not be conjunctions. However, an introduction rule may contain any number of side-conditions. \end{itemize} \section{Datatype and co-datatype definitions: users guide} The ML functors \verb|Datatype_Fun| and \verb|Co_Datatype_Fun| define datatypes and co-datatypes, invoking \verb|Datatype_Fun| and \verb|Co_Datatype_Fun| to make the underlying (co-)inductive definitions. \subsection{The result structure} The result structure extends that of (co-)inductive definitions (Figure~\ref{def-result-fig}) with several additional items: \begin{ttbox} val con_thy : theory val con_defs : thm list val case_eqns : thm list val free_iffs : thm list val free_SEs : thm list val mk_free : string -> thm \end{ttbox} Most of these have been discussed in~\S\ref{data-sec}. Here is a summary: \begin{description} \item[\tt con\_thy] is a new theory containing definitions of the (co-)datatype's constructors and case operator. It also declares the recursive sets as constants, so that it may serve as the parent theory for the (co-)inductive definition. \item[\tt con\_defs] is a list of definitions: the case operator followed by the constructors. This theorem list can be supplied to \verb|mk_cases|, for example. \item[\tt case\_eqns] is a list of equations, stating that the case operator inverts each constructor. \item[\tt free\_iffs] is a list of logical equivalences to perform freeness reasoning by rewriting. A typical application has the form \begin{ttbox} by (asm_simp_tac (ZF_ss addsimps free_iffs) 1); \end{ttbox} \item[\tt free\_SEs] is a list of `safe' elimination rules to perform freeness reasoning. It can be supplied to \verb|eresolve_tac| or to the classical reasoner: \begin{ttbox} by (fast_tac (ZF_cs addSEs free_SEs) 1); \end{ttbox} \item[\tt mk\_free] is a function to prove freeness properties, specified as strings. The theorems can be expressed in various forms, such as logical equivalences or elimination rules. \end{description} The result structure also inherits everything from the underlying (co-)inductive definition, such as the introduction rules, elimination rule, and induction/co-induction rule. \begin{figure} \begin{ttbox} sig val thy : theory val rec_specs : (string * string * (string list*string)list) list val rec_styp : string val ext : Syntax.sext option val sintrs : string list val monos : thm list val type_intrs: thm list val type_elims: thm list end \end{ttbox} \hrule \caption{The argument of a (co-)datatype definition} \label{data-arg-fig} \end{figure} \subsection{The argument structure} Both (co-)datatype functors take the same argument structure (Figure~\ref{data-arg-fig}). It does not extend that for (co-)inductive definitions, but shares several components and passes them uninterpreted to \verb|Datatype_Fun| or \verb|Co_Datatype_Fun|. The new components are as follows: \begin{description} \item[\tt thy] is the (co-)datatype's parent theory. It {\it must not\/} declare constants for the recursive sets. Recall that (co-)inductive definitions have the opposite restriction. \item[\tt rec\_specs] is a list of triples of the form ({\it recursive set\/}, {\it domain\/}, {\it constructors\/}) for each mutually recursive set. {\it Constructors\/} is a list of the form (names, type). See the discussion and examples in~\S\ref{data-sec}. \item[\tt rec\_styp] is the common meta-type of the mutually recursive sets, specified as a string. They must all have the same type because all must take the same parameters. \item[\tt ext] is an optional syntax extension, usually omitted by writing {\tt None}. You can supply mixfix syntax for the constructors by supplying \begin{ttbox} Some (Syntax.simple_sext [{\it mixfix declarations\/}]) \end{ttbox} \end{description} The choice of domain is usually simple. Isabelle/ZF defines the set $\univ(A)$, which contains~$A$ and is closed under the standard Cartesian products and disjoint sums \cite[\S4.2]{paulson-set-II}. In a typical datatype definition with set parameters $A_1$, \ldots, $A_k$, a suitable domain for all the recursive sets is $\univ(A_1\un\cdots\un A_k)$. For a co-datatype definition, the set $\quniv(A)$ contains~$A$ and is closed under the variant Cartesian products and disjoint sums; the appropropriate domain is $\quniv(A_1\un\cdots\un A_k)$. The {\tt sintrs} specify the introduction rules, which govern the recursive structure of the datatype. Introduction rules may involve monotone operators and side-conditions to express things that go beyond the usual notion of datatype. The theorem lists {\tt monos}, {\tt type\_intrs} and {\tt type\_elims} should contain precisely what is needed for the underlying (co-)inductive definition. Isabelle/ZF defines theorem lists that can be defined for the latter two components: \begin{itemize} \item {\tt datatype\_intrs} and {\tt datatype\_elims} are type-checking rules for $\univ(A)$. \item {\tt co\_datatype\_intrs} and {\tt co\_datatype\_elims} are type-checking rules for $\quniv(A)$. \end{itemize} In typical definitions, these theorem lists need not be supplemented with other theorems. The constructor definitions' right-hand sides can overlap. A simple example is the datatype for the combinators, whose constructors are \begin{eqnarray*} {\tt K} & \equiv & \Inl(\emptyset) \\ {\tt S} & \equiv & \Inr(\Inl(\emptyset)) \\ p{\tt\#}q & \equiv & \Inr(\Inl(\pair{p,q})) \end{eqnarray*} Unlike in previous versions of Isabelle, \verb|fold_tac| now ensures that the longest right-hand sides are folded first. \fi \end{document}