doc-src/Ref/theories.tex

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\chapter{Theories, Terms and Types} \label{theories}
\index{theories|(}

\section{The theory loader}\label{sec:more-theories}
\index{theories!reading}\index{files!reading}

Isabelle's theory loader manages dependencies of the internal graph of theory
nodes (the \emph{theory database}) and the external view of the file system.
See \S\ref{sec:intro-theories} for its most basic commands, such as
\texttt{use_thy}.  There are a few more operations available.

\begin{ttbox}
use_thy_only    : string -> unit
update_thy_only : string -> unit
touch_thy       : string -> unit
remove_thy      : string -> unit
delete_tmpfiles : bool ref \hfill\textbf{initially true}
\end{ttbox}

\begin{ttdescription}
\item[\ttindexbold{use_thy_only} "$name$";] is similar to \texttt{use_thy},
  but processes the actual theory file $name$\texttt{.thy} only, ignoring
  $name$\texttt{.ML}.  This might be useful in replaying proof scripts by hand
  from the very beginning, starting with the fresh theory.
  
\item[\ttindexbold{update_thy_only} "$name$";] is similar to
  \texttt{update_thy}, but processes the actual theory file
  $name$\texttt{.thy} only, ignoring $name$\texttt{.ML}.

\item[\ttindexbold{touch_thy} "$name$";] marks theory node $name$ of the
  internal graph as outdated.  While the theory remains usable, subsequent
  operations such as \texttt{use_thy} may cause a reload.
  
\item[\ttindexbold{remove_thy} "$name$";] deletes theory node $name$,
  including \emph{all} of its descendants.  Beware!  This is a quick way to
  dispose a large number of theories at once.  Note that {\ML} bindings to
  theorems etc.\ of removed theories may still persist.
  
\end{ttdescription}

\medskip Theory and {\ML} files are located by skimming through the
directories listed in Isabelle's internal load path, which merely contains the
current directory ``\texttt{.}'' by default.  The load path may be accessed by
the following operations.

\begin{ttbox}
show_path: unit -> string list
add_path: string -> unit
del_path: string -> unit
reset_path: unit -> unit
with_path: string -> ('a -> 'b) -> 'a -> 'b
no_document: ('a -> 'b) -> 'a -> 'b
\end{ttbox}

\begin{ttdescription}
\item[\ttindexbold{show_path}();] displays the load path components in
  canonical string representation (which is always according to Unix rules).
  
\item[\ttindexbold{add_path} "$dir$";] adds component $dir$ to the beginning
  of the load path.
  
\item[\ttindexbold{del_path} "$dir$";] removes any occurrences of component
  $dir$ from the load path.
  
\item[\ttindexbold{reset_path}();] resets the load path to ``\texttt{.}''
  (current directory) only.
  
\item[\ttindexbold{with_path} "$dir$" $f$ $x$;] temporarily adds component
  $dir$ to the beginning of the load path while executing $(f~x)$.
  
\item[\ttindexbold{no_document} $f$ $x$;] temporarily disables {\LaTeX}
  document generation while executing $(f~x)$.

\end{ttdescription}

Furthermore, in operations referring indirectly to some file (e.g.\ 
\texttt{use_dir}) the argument may be prefixed by a directory that will be
temporarily appended to the load path, too.


\section{Basic operations on theories}\label{BasicOperationsOnTheories}

\subsection{*Theory inclusion}
\begin{ttbox}
subthy      : theory * theory -> bool
eq_thy      : theory * theory -> bool
transfer    : theory -> thm -> thm
transfer_sg : Sign.sg -> thm -> thm
\end{ttbox}

Inclusion and equality of theories is determined by unique
identification stamps that are created when declaring new components.
Theorems contain a reference to the theory (actually to its signature)
they have been derived in.  Transferring theorems to super theories
has no logical significance, but may affect some operations in subtle
ways (e.g.\ implicit merges of signatures when applying rules, or
pretty printing of theorems).

\begin{ttdescription}

\item[\ttindexbold{subthy} ($thy@1$, $thy@2$)] determines if $thy@1$
  is included in $thy@2$ wrt.\ identification stamps.

\item[\ttindexbold{eq_thy} ($thy@1$, $thy@2$)] determines if $thy@1$
  is exactly the same as $thy@2$.

\item[\ttindexbold{transfer} $thy$ $thm$] transfers theorem $thm$ to
  theory $thy$, provided the latter includes the theory of $thm$.
  
\item[\ttindexbold{transfer_sg} $sign$ $thm$] is similar to
  \texttt{transfer}, but identifies the super theory via its
  signature.

\end{ttdescription}


\subsection{*Primitive theories}
\begin{ttbox}
ProtoPure.thy  : theory
Pure.thy       : theory
CPure.thy      : theory
\end{ttbox}
\begin{description}
\item[\ttindexbold{ProtoPure.thy}, \ttindexbold{Pure.thy},
  \ttindexbold{CPure.thy}] contain the syntax and signature of the
  meta-logic.  There are basically no axioms: meta-level inferences
  are carried out by \ML\ functions.  \texttt{Pure} and \texttt{CPure}
  just differ in their concrete syntax of prefix function application:
  $t(u@1, \ldots, u@n)$ in \texttt{Pure} vs.\ $t\,u@1,\ldots\,u@n$ in
  \texttt{CPure}.  \texttt{ProtoPure} is their common parent,
  containing no syntax for printing prefix applications at all!

%% FIXME
%\item [\ttindexbold{extend_theory} $thy$ {\tt"}$T${\tt"} $\cdots$] extends
%  the theory $thy$ with new types, constants, etc.  $T$ identifies the theory
%  internally.  When a theory is redeclared, say to change an incorrect axiom,
%  bindings to the old axiom may persist.  Isabelle ensures that the old and
%  new theories are not involved in the same proof.  Attempting to combine
%  different theories having the same name $T$ yields the fatal error
%extend_theory  : theory -> string -> \(\cdots\) -> theory
%\begin{ttbox}
%Attempt to merge different versions of theory: \(T\)
%\end{ttbox}
\end{description}

%% FIXME
%\item [\ttindexbold{extend_theory} $thy$ {\tt"}$T${\tt"}
%      ($classes$, $default$, $types$, $arities$, $consts$, $sextopt$) $rules$]
%\hfill\break   %%% include if line is just too short
%is the \ML{} equivalent of the following theory definition:
%\begin{ttbox}
%\(T\) = \(thy\) +
%classes \(c\) < \(c@1\),\(\dots\),\(c@m\)
%        \dots
%default {\(d@1,\dots,d@r\)}
%types   \(tycon@1\),\dots,\(tycon@i\) \(n\)
%        \dots
%arities \(tycon@1'\),\dots,\(tycon@j'\) :: (\(s@1\),\dots,\(s@n\))\(c\)
%        \dots
%consts  \(b@1\),\dots,\(b@k\) :: \(\tau\)
%        \dots
%rules   \(name\) \(rule\)
%        \dots
%end
%\end{ttbox}
%where
%\begin{tabular}[t]{l@{~=~}l}
%$classes$ & \tt[("$c$",["$c@1$",\dots,"$c@m$"]),\dots] \\
%$default$ & \tt["$d@1$",\dots,"$d@r$"]\\
%$types$   & \tt[([$tycon@1$,\dots,$tycon@i$], $n$),\dots] \\
%$arities$ & \tt[([$tycon'@1$,\dots,$tycon'@j$], ([$s@1$,\dots,$s@n$],$c$)),\dots]
%\\
%$consts$  & \tt[([$b@1$,\dots,$b@k$],$\tau$),\dots] \\
%$rules$   & \tt[("$name$",$rule$),\dots]
%\end{tabular}


\subsection{Inspecting a theory}\label{sec:inspct-thy}
\index{theories!inspecting|bold}
\begin{ttbox}
print_syntax        : theory -> unit
print_theory        : theory -> unit
parents_of          : theory -> theory list
ancestors_of        : theory -> theory list
sign_of             : theory -> Sign.sg
Sign.stamp_names_of : Sign.sg -> string list
\end{ttbox}
These provide means of viewing a theory's components.
\begin{ttdescription}
\item[\ttindexbold{print_syntax} $thy$] prints the syntax of $thy$
  (grammar, macros, translation functions etc., see
  page~\pageref{pg:print_syn} for more details).
  
\item[\ttindexbold{print_theory} $thy$] prints the logical parts of
  $thy$, excluding the syntax.
  
\item[\ttindexbold{parents_of} $thy$] returns the direct ancestors
  of~$thy$.
  
\item[\ttindexbold{ancestors_of} $thy$] returns all ancestors of~$thy$
  (not including $thy$ itself).
  
\item[\ttindexbold{sign_of} $thy$] returns the signature associated
  with~$thy$.  It is useful with functions like {\tt
    read_instantiate_sg}, which take a signature as an argument.
  
\item[\ttindexbold{Sign.stamp_names_of} $sg$]\index{signatures}
  returns the names of the identification \rmindex{stamps} of ax
  signature.  These coincide with the names of its full ancestry
  including that of $sg$ itself.

\end{ttdescription}


\section{Terms}\label{sec:terms}
\index{terms|bold}
Terms belong to the \ML\ type \mltydx{term}, which is a concrete datatype
with six constructors:
\begin{ttbox}
type indexname = string * int;
infix 9 $;
datatype term = Const of string * typ
              | Free  of string * typ
              | Var   of indexname * typ
              | Bound of int
              | Abs   of string * typ * term
              | op $  of term * term;
\end{ttbox}
\begin{ttdescription}
\item[\ttindexbold{Const} ($a$, $T$)] \index{constants|bold}
  is the \textbf{constant} with name~$a$ and type~$T$.  Constants include
  connectives like $\land$ and $\forall$ as well as constants like~0
  and~$Suc$.  Other constants may be required to define a logic's concrete
  syntax.

\item[\ttindexbold{Free} ($a$, $T$)] \index{variables!free|bold}
  is the \textbf{free variable} with name~$a$ and type~$T$.

\item[\ttindexbold{Var} ($v$, $T$)] \index{unknowns|bold}
  is the \textbf{scheme variable} with indexname~$v$ and type~$T$.  An
  \mltydx{indexname} is a string paired with a non-negative index, or
  subscript; a term's scheme variables can be systematically renamed by
  incrementing their subscripts.  Scheme variables are essentially free
  variables, but may be instantiated during unification.

\item[\ttindexbold{Bound} $i$] \index{variables!bound|bold}
  is the \textbf{bound variable} with de Bruijn index~$i$, which counts the
  number of lambdas, starting from zero, between a variable's occurrence
  and its binding.  The representation prevents capture of variables.  For
  more information see de Bruijn \cite{debruijn72} or
  Paulson~\cite[page~376]{paulson-ml2}.

\item[\ttindexbold{Abs} ($a$, $T$, $u$)]
  \index{lambda abs@$\lambda$-abstractions|bold}
  is the $\lambda$-\textbf{abstraction} with body~$u$, and whose bound
  variable has name~$a$ and type~$T$.  The name is used only for parsing
  and printing; it has no logical significance.

\item[$t$ \$ $u$] \index{$@{\tt\$}|bold} \index{function applications|bold}
is the \textbf{application} of~$t$ to~$u$.
\end{ttdescription}
Application is written as an infix operator to aid readability.  Here is an
\ML\ pattern to recognize FOL formulae of the form~$A\imp B$, binding the
subformulae to~$A$ and~$B$:
\begin{ttbox}
Const("Trueprop",_) $ (Const("op -->",_) $ A $ B)
\end{ttbox}


\section{*Variable binding}
\begin{ttbox}
loose_bnos     : term -> int list
incr_boundvars : int -> term -> term
abstract_over  : term*term -> term
variant_abs    : string * typ * term -> string * term
aconv          : term * term -> bool\hfill\textbf{infix}
\end{ttbox}
These functions are all concerned with the de Bruijn representation of
bound variables.
\begin{ttdescription}
\item[\ttindexbold{loose_bnos} $t$]
  returns the list of all dangling bound variable references.  In
  particular, \texttt{Bound~0} is loose unless it is enclosed in an
  abstraction.  Similarly \texttt{Bound~1} is loose unless it is enclosed in
  at least two abstractions; if enclosed in just one, the list will contain
  the number 0.  A well-formed term does not contain any loose variables.

\item[\ttindexbold{incr_boundvars} $j$]
  increases a term's dangling bound variables by the offset~$j$.  This is
  required when moving a subterm into a context where it is enclosed by a
  different number of abstractions.  Bound variables with a matching
  abstraction are unaffected.

\item[\ttindexbold{abstract_over} $(v,t)$]
  forms the abstraction of~$t$ over~$v$, which may be any well-formed term.
  It replaces every occurrence of \(v\) by a \texttt{Bound} variable with the
  correct index.

\item[\ttindexbold{variant_abs} $(a,T,u)$]
  substitutes into $u$, which should be the body of an abstraction.
  It replaces each occurrence of the outermost bound variable by a free
  variable.  The free variable has type~$T$ and its name is a variant
  of~$a$ chosen to be distinct from all constants and from all variables
  free in~$u$.

\item[$t$ \ttindexbold{aconv} $u$]
  tests whether terms~$t$ and~$u$ are \(\alpha\)-convertible: identical up
  to renaming of bound variables.
\begin{itemize}
  \item
    Two constants, \texttt{Free}s, or \texttt{Var}s are \(\alpha\)-convertible
    if their names and types are equal.
    (Variables having the same name but different types are thus distinct.
    This confusing situation should be avoided!)
  \item
    Two bound variables are \(\alpha\)-convertible
    if they have the same number.
  \item
    Two abstractions are \(\alpha\)-convertible
    if their bodies are, and their bound variables have the same type.
  \item
    Two applications are \(\alpha\)-convertible
    if the corresponding subterms are.
\end{itemize}

\end{ttdescription}

\section{Certified terms}\index{terms!certified|bold}\index{signatures}
A term $t$ can be \textbf{certified} under a signature to ensure that every type
in~$t$ is well-formed and every constant in~$t$ is a type instance of a
constant declared in the signature.  The term must be well-typed and its use
of bound variables must be well-formed.  Meta-rules such as \texttt{forall_elim}
take certified terms as arguments.

Certified terms belong to the abstract type \mltydx{cterm}.
Elements of the type can only be created through the certification process.
In case of error, Isabelle raises exception~\ttindex{TERM}\@.

\subsection{Printing terms}
\index{terms!printing of}
\begin{ttbox}
     string_of_cterm :           cterm -> string
Sign.string_of_term  : Sign.sg -> term -> string
\end{ttbox}
\begin{ttdescription}
\item[\ttindexbold{string_of_cterm} $ct$]
displays $ct$ as a string.

\item[\ttindexbold{Sign.string_of_term} $sign$ $t$]
displays $t$ as a string, using the syntax of~$sign$.
\end{ttdescription}

\subsection{Making and inspecting certified terms}
\begin{ttbox}
cterm_of       : Sign.sg -> term -> cterm
read_cterm     : Sign.sg -> string * typ -> cterm
cert_axm       : Sign.sg -> string * term -> string * term
read_axm       : Sign.sg -> string * string -> string * term
rep_cterm      : cterm -> \{T:typ, t:term, sign:Sign.sg, maxidx:int\}
Sign.certify_term : Sign.sg -> term -> term * typ * int
\end{ttbox}
\begin{ttdescription}
  
\item[\ttindexbold{cterm_of} $sign$ $t$] \index{signatures} certifies
  $t$ with respect to signature~$sign$.
  
\item[\ttindexbold{read_cterm} $sign$ ($s$, $T$)] reads the string~$s$
  using the syntax of~$sign$, creating a certified term.  The term is
  checked to have type~$T$; this type also tells the parser what kind
  of phrase to parse.
  
\item[\ttindexbold{cert_axm} $sign$ ($name$, $t$)] certifies $t$ with
  respect to $sign$ as a meta-proposition and converts all exceptions
  to an error, including the final message
\begin{ttbox}
The error(s) above occurred in axiom "\(name\)"
\end{ttbox}

\item[\ttindexbold{read_axm} $sign$ ($name$, $s$)] similar to {\tt
    cert_axm}, but first reads the string $s$ using the syntax of
  $sign$.
  
\item[\ttindexbold{rep_cterm} $ct$] decomposes $ct$ as a record
  containing its type, the term itself, its signature, and the maximum
  subscript of its unknowns.  The type and maximum subscript are
  computed during certification.
  
\item[\ttindexbold{Sign.certify_term}] is a more primitive version of
  \texttt{cterm_of}, returning the internal representation instead of
  an abstract \texttt{cterm}.

\end{ttdescription}


\section{Types}\index{types|bold}
Types belong to the \ML\ type \mltydx{typ}, which is a concrete datatype with
three constructor functions.  These correspond to type constructors, free
type variables and schematic type variables.  Types are classified by sorts,
which are lists of classes (representing an intersection).  A class is
represented by a string.
\begin{ttbox}
type class = string;
type sort  = class list;

datatype typ = Type  of string * typ list
             | TFree of string * sort
             | TVar  of indexname * sort;

infixr 5 -->;
fun S --> T = Type ("fun", [S, T]);
\end{ttbox}
\begin{ttdescription}
\item[\ttindexbold{Type} ($a$, $Ts$)] \index{type constructors|bold}
  applies the \textbf{type constructor} named~$a$ to the type operand list~$Ts$.
  Type constructors include~\tydx{fun}, the binary function space
  constructor, as well as nullary type constructors such as~\tydx{prop}.
  Other type constructors may be introduced.  In expressions, but not in
  patterns, \hbox{\tt$S$-->$T$} is a convenient shorthand for function
  types.

\item[\ttindexbold{TFree} ($a$, $s$)] \index{type variables|bold}
  is the \textbf{type variable} with name~$a$ and sort~$s$.

\item[\ttindexbold{TVar} ($v$, $s$)] \index{type unknowns|bold}
  is the \textbf{type unknown} with indexname~$v$ and sort~$s$.
  Type unknowns are essentially free type variables, but may be
  instantiated during unification.
\end{ttdescription}


\section{Certified types}
\index{types!certified|bold}
Certified types, which are analogous to certified terms, have type
\ttindexbold{ctyp}.

\subsection{Printing types}
\index{types!printing of}
\begin{ttbox}
     string_of_ctyp :           ctyp -> string
Sign.string_of_typ  : Sign.sg -> typ -> string
\end{ttbox}
\begin{ttdescription}
\item[\ttindexbold{string_of_ctyp} $cT$]
displays $cT$ as a string.

\item[\ttindexbold{Sign.string_of_typ} $sign$ $T$]
displays $T$ as a string, using the syntax of~$sign$.
\end{ttdescription}


\subsection{Making and inspecting certified types}
\begin{ttbox}
ctyp_of          : Sign.sg -> typ -> ctyp
rep_ctyp         : ctyp -> \{T: typ, sign: Sign.sg\}
Sign.certify_typ : Sign.sg -> typ -> typ
\end{ttbox}
\begin{ttdescription}
  
\item[\ttindexbold{ctyp_of} $sign$ $T$] \index{signatures} certifies
  $T$ with respect to signature~$sign$.
  
\item[\ttindexbold{rep_ctyp} $cT$] decomposes $cT$ as a record
  containing the type itself and its signature.
  
\item[\ttindexbold{Sign.certify_typ}] is a more primitive version of
  \texttt{ctyp_of}, returning the internal representation instead of
  an abstract \texttt{ctyp}.

\end{ttdescription}


\index{theories|)}


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