doc-src/TutorialI/Ifexpr/document/Ifexpr.tex

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\begin{isabelle}%
%
\begin{isamarkuptext}%
\subsubsection{How can we model boolean expressions?}

We want to represent boolean expressions built up from variables and
constants by negation and conjunction. The following datatype serves exactly
that purpose:%
\end{isamarkuptext}%
\isacommand{datatype}\ boolex\ =\ Const\ bool\ |\ Var\ nat\ |\ Neg\ boolex\isanewline
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ |\ And\ boolex\ boolex%
\begin{isamarkuptext}%
\noindent
The two constants are represented by \isa{Const\ True} and
\isa{Const\ False}. Variables are represented by terms of the form
\isa{Var\ n}, where \isa{n} is a natural number (type \isa{nat}).
For example, the formula $P@0 \land \neg P@1$ is represented by the term
\isa{And\ (Var\ 0)\ (Neg\ (Var\ 1))}.

\subsubsection{What is the value of a boolean expression?}

The value of a boolean expression depends on the value of its variables.
Hence the function \isa{value} takes an additional parameter, an {\em
  environment} of type \isa{nat\ {\isasymRightarrow}\ bool}, which maps variables to
their values:%
\end{isamarkuptext}%
\isacommand{consts}\ value\ ::\ {"}boolex\ {\isasymRightarrow}\ (nat\ {\isasymRightarrow}\ bool)\ {\isasymRightarrow}\ bool{"}\isanewline
\isacommand{primrec}\isanewline
{"}value\ (Const\ b)\ env\ =\ b{"}\isanewline
{"}value\ (Var\ x)\ \ \ env\ =\ env\ x{"}\isanewline
{"}value\ (Neg\ b)\ \ \ env\ =\ ({\isasymnot}\ value\ b\ env){"}\isanewline
{"}value\ (And\ b\ c)\ env\ =\ (value\ b\ env\ {\isasymand}\ value\ c\ env){"}%
\begin{isamarkuptext}%
\noindent
\subsubsection{If-expressions}

An alternative and often more efficient (because in a certain sense
canonical) representation are so-called \emph{If-expressions} built up
from constants (\isa{CIF}), variables (\isa{VIF}) and conditionals
(\isa{IF}):%
\end{isamarkuptext}%
\isacommand{datatype}\ ifex\ =\ CIF\ bool\ |\ VIF\ nat\ |\ IF\ ifex\ ifex\ ifex%
\begin{isamarkuptext}%
\noindent
The evaluation if If-expressions proceeds as for \isa{boolex}:%
\end{isamarkuptext}%
\isacommand{consts}\ valif\ ::\ {"}ifex\ {\isasymRightarrow}\ (nat\ {\isasymRightarrow}\ bool)\ {\isasymRightarrow}\ bool{"}\isanewline
\isacommand{primrec}\isanewline
{"}valif\ (CIF\ b)\ \ \ \ env\ =\ b{"}\isanewline
{"}valif\ (VIF\ x)\ \ \ \ env\ =\ env\ x{"}\isanewline
{"}valif\ (IF\ b\ t\ e)\ env\ =\ (if\ valif\ b\ env\ then\ valif\ t\ env\isanewline
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ else\ valif\ e\ env){"}%
\begin{isamarkuptext}%
\subsubsection{Transformation into and of If-expressions}

The type \isa{boolex} is close to the customary representation of logical
formulae, whereas \isa{ifex} is designed for efficiency. It is easy to
translate from \isa{boolex} into \isa{ifex}:%
\end{isamarkuptext}%
\isacommand{consts}\ bool2if\ ::\ {"}boolex\ {\isasymRightarrow}\ ifex{"}\isanewline
\isacommand{primrec}\isanewline
{"}bool2if\ (Const\ b)\ =\ CIF\ b{"}\isanewline
{"}bool2if\ (Var\ x)\ \ \ =\ VIF\ x{"}\isanewline
{"}bool2if\ (Neg\ b)\ \ \ =\ IF\ (bool2if\ b)\ (CIF\ False)\ (CIF\ True){"}\isanewline
{"}bool2if\ (And\ b\ c)\ =\ IF\ (bool2if\ b)\ (bool2if\ c)\ (CIF\ False){"}%
\begin{isamarkuptext}%
\noindent
At last, we have something we can verify: that \isa{bool2if} preserves the
value of its argument:%
\end{isamarkuptext}%
\isacommand{lemma}\ {"}valif\ (bool2if\ b)\ env\ =\ value\ b\ env{"}%
\begin{isamarkuptxt}%
\noindent
The proof is canonical:%
\end{isamarkuptxt}%
\isacommand{apply}(induct\_tac\ b)\isanewline
\isacommand{by}(auto)%
\begin{isamarkuptext}%
\noindent
In fact, all proofs in this case study look exactly like this. Hence we do
not show them below.

More interesting is the transformation of If-expressions into a normal form
where the first argument of \isa{IF} cannot be another \isa{IF} but
must be a constant or variable. Such a normal form can be computed by
repeatedly replacing a subterm of the form \isa{IF\ (IF\ b\ x\ y)\ z\ u} by
\isa{IF\ b\ (IF\ x\ z\ u)\ (IF\ y\ z\ u)}, which has the same value. The following
primitive recursive functions perform this task:%
\end{isamarkuptext}%
\isacommand{consts}\ normif\ ::\ {"}ifex\ {\isasymRightarrow}\ ifex\ {\isasymRightarrow}\ ifex\ {\isasymRightarrow}\ ifex{"}\isanewline
\isacommand{primrec}\isanewline
{"}normif\ (CIF\ b)\ \ \ \ t\ e\ =\ IF\ (CIF\ b)\ t\ e{"}\isanewline
{"}normif\ (VIF\ x)\ \ \ \ t\ e\ =\ IF\ (VIF\ x)\ t\ e{"}\isanewline
{"}normif\ (IF\ b\ t\ e)\ u\ f\ =\ normif\ b\ (normif\ t\ u\ f)\ (normif\ e\ u\ f){"}\isanewline
\isanewline
\isacommand{consts}\ norm\ ::\ {"}ifex\ {\isasymRightarrow}\ ifex{"}\isanewline
\isacommand{primrec}\isanewline
{"}norm\ (CIF\ b)\ \ \ \ =\ CIF\ b{"}\isanewline
{"}norm\ (VIF\ x)\ \ \ \ =\ VIF\ x{"}\isanewline
{"}norm\ (IF\ b\ t\ e)\ =\ normif\ b\ (norm\ t)\ (norm\ e){"}%
\begin{isamarkuptext}%
\noindent
Their interplay is a bit tricky, and we leave it to the reader to develop an
intuitive understanding. Fortunately, Isabelle can help us to verify that the
transformation preserves the value of the expression:%
\end{isamarkuptext}%
\isacommand{theorem}\ {"}valif\ (norm\ b)\ env\ =\ valif\ b\ env{"}%
\begin{isamarkuptext}%
\noindent
The proof is canonical, provided we first show the following simplification
lemma (which also helps to understand what \isa{normif} does):%
\end{isamarkuptext}%
\isacommand{lemma}\ [simp]:\isanewline
\ \ {"}{\isasymforall}t\ e.\ valif\ (normif\ b\ t\ e)\ env\ =\ valif\ (IF\ b\ t\ e)\ env{"}%
\begin{isamarkuptext}%
\noindent
Note that the lemma does not have a name, but is implicitly used in the proof
of the theorem shown above because of the \isa{[simp]} attribute.

But how can we be sure that \isa{norm} really produces a normal form in
the above sense? We define a function that tests If-expressions for normality%
\end{isamarkuptext}%
\isacommand{consts}\ normal\ ::\ {"}ifex\ {\isasymRightarrow}\ bool{"}\isanewline
\isacommand{primrec}\isanewline
{"}normal(CIF\ b)\ =\ True{"}\isanewline
{"}normal(VIF\ x)\ =\ True{"}\isanewline
{"}normal(IF\ b\ t\ e)\ =\ (normal\ t\ {\isasymand}\ normal\ e\ {\isasymand}\isanewline
\ \ \ \ \ (case\ b\ of\ CIF\ b\ {\isasymRightarrow}\ True\ |\ VIF\ x\ {\isasymRightarrow}\ True\ |\ IF\ x\ y\ z\ {\isasymRightarrow}\ False)){"}%
\begin{isamarkuptext}%
\noindent
and prove \isa{normal(norm b)}. Of course, this requires a lemma about
normality of \isa{normif}:%
\end{isamarkuptext}%
\isacommand{lemma}[simp]:\ {"}{\isasymforall}t\ e.\ normal(normif\ b\ t\ e)\ =\ (normal\ t\ {\isasymand}\ normal\ e){"}\end{isabelle}%
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