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(*<*)
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theory simplification = Main:;
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(*>*)
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text{*
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Once we have succeeded in proving all termination conditions, the recursion
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equations become simplification rules, just as with
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\isacommand{primrec}. In most cases this works fine, but there is a subtle
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problem that must be mentioned: simplification may not
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terminate because of automatic splitting of @{text if}.
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Let us look at an example:
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*}
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consts gcd :: "nat\<times>nat \<Rightarrow> nat";
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recdef gcd "measure (\<lambda>(m,n).n)"
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"gcd (m, n) = (if n=0 then m else gcd(n, m mod n))";
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text{*\noindent
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According to the measure function, the second argument should decrease with
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each recursive call. The resulting termination condition
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@{term[display]"n ~= 0 ==> m mod n < n"}
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is provded automatically because it is already present as a lemma in the
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arithmetic library. Thus the recursion equation becomes a simplification
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rule. Of course the equation is nonterminating if we are allowed to unfold
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the recursive call inside the @{text else} branch, which is why programming
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languages and our simplifier don't do that. Unfortunately the simplifier does
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something else which leads to the same problem: it splits @{text if}s if the
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condition simplifies to neither @{term True} nor @{term False}. For
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example, simplification reduces
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@{term[display]"gcd(m,n) = k"}
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in one step to
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@{term[display]"(if n=0 then m else gcd(n, m mod n)) = k"}
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where the condition cannot be reduced further, and splitting leads to
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@{term[display]"(n=0 --> m=k) & (n ~= 0 --> gcd(n, m mod n)=k)"}
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Since the recursive call @{term"gcd(n, m mod n)"} is no longer protected by
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an @{text if}, it is unfolded again, which leads to an infinite chain of
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simplification steps. Fortunately, this problem can be avoided in many
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different ways.
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The most radical solution is to disable the offending
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@{thm[source]split_if} as shown in the section on case splits in
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\S\ref{sec:Simplification}. However, we do not recommend this because it
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means you will often have to invoke the rule explicitly when @{text if} is
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involved.
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If possible, the definition should be given by pattern matching on the left
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rather than @{text if} on the right. In the case of @{term gcd} the
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following alternative definition suggests itself:
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*}
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consts gcd1 :: "nat\<times>nat \<Rightarrow> nat";
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recdef gcd1 "measure (\<lambda>(m,n).n)"
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"gcd1 (m, 0) = m"
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"gcd1 (m, n) = gcd1(n, m mod n)";
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text{*\noindent
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Note that the order of equations is important and hides the side condition
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@{prop"n ~= 0"}. Unfortunately, in general the case distinction
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may not be expressible by pattern matching.
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A very simple alternative is to replace @{text if} by @{text case}, which
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is also available for @{typ bool} but is not split automatically:
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*}
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consts gcd2 :: "nat\<times>nat \<Rightarrow> nat";
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recdef gcd2 "measure (\<lambda>(m,n).n)"
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"gcd2(m,n) = (case n=0 of True \<Rightarrow> m | False \<Rightarrow> gcd2(n,m mod n))";
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text{*\noindent
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In fact, this is probably the neatest solution next to pattern matching.
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A final alternative is to replace the offending simplification rules by
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derived conditional ones. For @{term gcd} it means we have to prove
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*}
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lemma [simp]: "gcd (m, 0) = m";
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apply(simp);
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done
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lemma [simp]: "n \<noteq> 0 \<Longrightarrow> gcd(m, n) = gcd(n, m mod n)";
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apply(simp);
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done
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text{*\noindent
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after which we can disable the original simplification rule:
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*}
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declare gcd.simps [simp del]
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(*<*)
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end
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(*>*)
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