doc-src/TutorialI/Recdef/document/Nested2.tex
author nipkow
Mon Aug 28 10:16:58 2000 +0200 (2000-08-28)
changeset 9690 50f22b1b136a
child 9698 f0740137a65d
permissions -rw-r--r--
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\begin{isabelle}%
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%
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\begin{isamarkuptext}%
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\noindent
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The termintion condition is easily proved by induction:%
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\end{isamarkuptext}%
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\isacommand{lemma}\ [simp]:\ {"}t\ {\isasymin}\ set\ ts\ {\isasymlongrightarrow}\ size\ t\ <\ Suc(term\_size\ ts){"}\isanewline
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\isacommand{by}(induct\_tac\ ts,\ auto)%
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\begin{isamarkuptext}%
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\noindent
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By making this theorem a simplification rule, \isacommand{recdef}
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applies it automatically and the above definition of \isa{trev}
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succeeds now. As a reward for our effort, we can now prove the desired
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lemma directly. The key is the fact that we no longer need the verbose
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induction schema for type \isa{term} but the simpler one arising from
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\isa{trev}:%
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\end{isamarkuptext}%
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\isacommand{lemmas}\ [cong]\ =\ map\_cong\isanewline
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\isacommand{lemma}\ {"}trev(trev\ t)\ =\ t{"}\isanewline
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\isacommand{apply}(induct\_tac\ t\ rule:trev.induct)%
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\begin{isamarkuptxt}%
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\noindent
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This leaves us with a trivial base case \isa{trev\ (trev\ (Var\ \mbox{x}))\ =\ Var\ \mbox{x}} and the step case
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\begin{quote}
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\begin{isabelle}%
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{\isasymforall}\mbox{t}.\ \mbox{t}\ {\isasymin}\ set\ \mbox{ts}\ {\isasymlongrightarrow}\ trev\ (trev\ \mbox{t})\ =\ \mbox{t}\ {\isasymLongrightarrow}\isanewline
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trev\ (trev\ (App\ \mbox{f}\ \mbox{ts}))\ =\ App\ \mbox{f}\ \mbox{ts}
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\end{isabelle}%
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\end{quote}
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both of which are solved by simplification:%
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\end{isamarkuptxt}%
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\isacommand{by}(simp\_all\ del:map\_compose\ add:sym[OF\ map\_compose]\ rev\_map)%
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\begin{isamarkuptext}%
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\noindent
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If this surprises you, see Datatype/Nested2......
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The above definition of \isa{trev} is superior to the one in \S\ref{sec:nested-datatype}
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because it brings \isa{rev} into play, about which already know a lot, in particular
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\isa{rev\ (rev\ \mbox{xs})\ =\ \mbox{xs}}.
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Thus this proof is a good example of an important principle:
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\begin{quote}
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\emph{Chose your definitions carefully\\
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because they determine the complexity of your proofs.}
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\end{quote}
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Let us now return to the question of how \isacommand{recdef} can come up with sensible termination
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conditions in the presence of higher-order functions like \isa{map}. For a start, if nothing
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were known about \isa{map}, \isa{map\ trev\ \mbox{ts}} might apply \isa{trev} to arbitrary terms,
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and thus \isacommand{recdef} would try to prove the unprovable
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\isa{size\ \mbox{t}\ <\ Suc\ (term\_size\ \mbox{ts})}, without any assumption about \isa{t}.
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Therefore \isacommand{recdef} has been supplied with the congruence theorem \isa{map\_cong}: 
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\begin{quote}
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\begin{isabelle}%
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{\isasymlbrakk}\mbox{xs}\ =\ \mbox{ys};\ {\isasymAnd}\mbox{x}.\ \mbox{x}\ {\isasymin}\ set\ \mbox{ys}\ {\isasymLongrightarrow}\ \mbox{f}\ \mbox{x}\ =\ \mbox{g}\ \mbox{x}{\isasymrbrakk}\isanewline
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{\isasymLongrightarrow}\ map\ \mbox{f}\ \mbox{xs}\ =\ map\ \mbox{g}\ \mbox{ys}
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\end{isabelle}%
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\end{quote}
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Its second premise expresses (indirectly) that the second argument of \isa{map} is only applied
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to elements of its third argument. Congruence rules for other higher-order functions on lists would
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look very similar but have not been proved yet because they were never needed.
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If you get into a situation where you need to supply \isacommand{recdef} with new congruence
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rules, you can either append the line
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\begin{ttbox}
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congs <congruence rules>
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\end{ttbox}
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to the specific occurrence of \isacommand{recdef} or declare them globally:
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\begin{ttbox}
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lemmas [????????] = <congruence rules>
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\end{ttbox}
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Note that \isacommand{recdef} feeds on exactly the same \emph{kind} of
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congruence rules as the simplifier (\S\ref{sec:simp-cong}) but that
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declaring a congruence rule for the simplifier does not make it
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available to \isacommand{recdef}, and vice versa. This is intentional.%
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\end{isamarkuptext}%
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\end{isabelle}%
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%%% Local Variables:
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%%% mode: latex
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%%% TeX-master: "root"
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%%% End: