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\begin{isabellebody}%
\def\isabellecontext{Product}%
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\isamarkupheader{Syntactic classes%
}
\isamarkuptrue%
\isacommand{theory}\ Product\ {\isacharequal}\ Main{\isacharcolon}\isamarkupfalse%
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\begin{isamarkuptext}%
\medskip\noindent There is still a feature of Isabelle's type system
left that we have not yet discussed. When declaring polymorphic
constants \isa{c\ {\isasymColon}\ {\isasymsigma}}, the type variables occurring in \isa{{\isasymsigma}}
may be constrained by type classes (or even general sorts) in an
arbitrary way. Note that by default, in Isabelle/HOL the
declaration \isa{{\isasymodot}\ {\isasymColon}\ {\isacharprime}a\ {\isasymRightarrow}\ {\isacharprime}a\ {\isasymRightarrow}\ {\isacharprime}a} is actually an abbreviation
for \isa{{\isasymodot}\ {\isasymColon}\ {\isacharprime}a{\isasymColon}type\ {\isasymRightarrow}\ {\isacharprime}a\ {\isasymRightarrow}\ {\isacharprime}a} Since class \isa{type} is the
universal class of HOL, this is not really a constraint at all.
The \isa{product} class below provides a less degenerate example of
syntactic type classes.%
\end{isamarkuptext}%
\isamarkuptrue%
\isacommand{axclass}\isanewline
\ \ product\ {\isasymsubseteq}\ type\isanewline
\isamarkupfalse%
\isacommand{consts}\isanewline
\ \ product\ {\isacharcolon}{\isacharcolon}\ {\isachardoublequote}{\isacharprime}a{\isasymColon}product\ {\isasymRightarrow}\ {\isacharprime}a\ {\isasymRightarrow}\ {\isacharprime}a{\isachardoublequote}\ \ \ \ {\isacharparenleft}\isakeyword{infixl}\ {\isachardoublequote}{\isasymodot}{\isachardoublequote}\ {\isadigit{7}}{\isadigit{0}}{\isacharparenright}\isamarkupfalse%
%
\begin{isamarkuptext}%
Here class \isa{product} is defined as subclass of \isa{type}
without any additional axioms. This effects in logical equivalence
of \isa{product} and \isa{type}, as is reflected by the trivial
introduction rule generated for this definition.
\medskip So what is the difference of declaring \isa{{\isasymodot}\ {\isasymColon}\ {\isacharprime}a{\isasymColon}product\ {\isasymRightarrow}\ {\isacharprime}a\ {\isasymRightarrow}\ {\isacharprime}a} vs.\ declaring \isa{{\isasymodot}\ {\isasymColon}\ {\isacharprime}a{\isasymColon}type\ {\isasymRightarrow}\ {\isacharprime}a\ {\isasymRightarrow}\ {\isacharprime}a} anyway? In this particular case where \isa{product\ {\isasymequiv}\ type}, it should be obvious that both declarations are the same
from the logic's point of view. It even makes the most sense to
remove sort constraints from constant declarations, as far as the
purely logical meaning is concerned \cite{Wenzel:1997:TPHOL}.
On the other hand there are syntactic differences, of course.
Constants \isa{{\isasymodot}} on some type \isa{{\isasymtau}} are rejected by the
type-checker, unless the arity \isa{{\isasymtau}\ {\isasymColon}\ product} is part of the
type signature. In our example, this arity may be always added when
required by means of an $\INSTANCE$ with the default proof $\DDOT$.
\medskip Thus, we may observe the following discipline of using
syntactic classes. Overloaded polymorphic constants have their type
arguments restricted to an associated (logically trivial) class
\isa{c}. Only immediately before \emph{specifying} these
constants on a certain type \isa{{\isasymtau}} do we instantiate \isa{{\isasymtau}\ {\isasymColon}\ c}.
This is done for class \isa{product} and type \isa{bool} as
follows.%
\end{isamarkuptext}%
\isamarkuptrue%
\isacommand{instance}\ bool\ {\isacharcolon}{\isacharcolon}\ product\ \isamarkupfalse%
\isacommand{{\isachardot}{\isachardot}}\isanewline
\isamarkupfalse%
\isacommand{defs}\ {\isacharparenleft}\isakeyword{overloaded}{\isacharparenright}\isanewline
\ \ product{\isacharunderscore}bool{\isacharunderscore}def{\isacharcolon}\ {\isachardoublequote}x\ {\isasymodot}\ y\ {\isasymequiv}\ x\ {\isasymand}\ y{\isachardoublequote}\isamarkupfalse%
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\begin{isamarkuptext}%
The definition \isa{prod{\isacharunderscore}bool{\isacharunderscore}def} becomes syntactically
well-formed only after the arity \isa{bool\ {\isasymColon}\ product} is made
known to the type checker.
\medskip It is very important to see that above $\DEFS$ are not
directly connected with $\INSTANCE$ at all! We were just following
our convention to specify \isa{{\isasymodot}} on \isa{bool} after having
instantiated \isa{bool\ {\isasymColon}\ product}. Isabelle does not require
these definitions, which is in contrast to programming languages like
Haskell \cite{haskell-report}.
\medskip While Isabelle type classes and those of Haskell are almost
the same as far as type-checking and type inference are concerned,
there are important semantic differences. Haskell classes require
their instances to \emph{provide operations} of certain \emph{names}.
Therefore, its \texttt{instance} has a \texttt{where} part that tells
the system what these ``member functions'' should be.
This style of \texttt{instance} would not make much sense in
Isabelle's meta-logic, because there is no internal notion of
``providing operations'' or even ``names of functions''.%
\end{isamarkuptext}%
\isamarkuptrue%
\isacommand{end}\isanewline
\isamarkupfalse%
\end{isabellebody}%
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