src/HOL/Library/While_Combinator.thy
author wenzelm
Wed Sep 12 13:42:28 2012 +0200 (2012-09-12)
changeset 49322 fbb320d02420
parent 46365 547d1a1dcaf6
child 50008 eb7f574d0303
permissions -rw-r--r--
tuned headers;
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(*  Title:      HOL/Library/While_Combinator.thy
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    Author:     Tobias Nipkow
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    Author:     Alexander Krauss
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    Copyright   2000 TU Muenchen
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*)
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header {* A general ``while'' combinator *}
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theory While_Combinator
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imports Main
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begin
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subsection {* Partial version *}
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definition while_option :: "('a \<Rightarrow> bool) \<Rightarrow> ('a \<Rightarrow> 'a) \<Rightarrow> 'a \<Rightarrow> 'a option" where
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"while_option b c s = (if (\<exists>k. ~ b ((c ^^ k) s))
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   then Some ((c ^^ (LEAST k. ~ b ((c ^^ k) s))) s)
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   else None)"
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theorem while_option_unfold[code]:
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"while_option b c s = (if b s then while_option b c (c s) else Some s)"
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proof cases
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  assume "b s"
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  show ?thesis
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  proof (cases "\<exists>k. ~ b ((c ^^ k) s)")
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    case True
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    then obtain k where 1: "~ b ((c ^^ k) s)" ..
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    with `b s` obtain l where "k = Suc l" by (cases k) auto
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    with 1 have "~ b ((c ^^ l) (c s))" by (auto simp: funpow_swap1)
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    then have 2: "\<exists>l. ~ b ((c ^^ l) (c s))" ..
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    from 1
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    have "(LEAST k. ~ b ((c ^^ k) s)) = Suc (LEAST l. ~ b ((c ^^ Suc l) s))"
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      by (rule Least_Suc) (simp add: `b s`)
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    also have "... = Suc (LEAST l. ~ b ((c ^^ l) (c s)))"
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      by (simp add: funpow_swap1)
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    finally
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    show ?thesis 
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      using True 2 `b s` by (simp add: funpow_swap1 while_option_def)
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  next
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    case False
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    then have "~ (\<exists>l. ~ b ((c ^^ Suc l) s))" by blast
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    then have "~ (\<exists>l. ~ b ((c ^^ l) (c s)))"
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      by (simp add: funpow_swap1)
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    with False  `b s` show ?thesis by (simp add: while_option_def)
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  qed
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next
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  assume [simp]: "~ b s"
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  have least: "(LEAST k. ~ b ((c ^^ k) s)) = 0"
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    by (rule Least_equality) auto
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  moreover 
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  have "\<exists>k. ~ b ((c ^^ k) s)" by (rule exI[of _ "0::nat"]) auto
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  ultimately show ?thesis unfolding while_option_def by auto 
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qed
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lemma while_option_stop2:
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 "while_option b c s = Some t \<Longrightarrow> EX k. t = (c^^k) s \<and> \<not> b t"
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apply(simp add: while_option_def split: if_splits)
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by (metis (lifting) LeastI_ex)
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lemma while_option_stop: "while_option b c s = Some t \<Longrightarrow> ~ b t"
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by(metis while_option_stop2)
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theorem while_option_rule:
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assumes step: "!!s. P s ==> b s ==> P (c s)"
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and result: "while_option b c s = Some t"
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and init: "P s"
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shows "P t"
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proof -
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  def k == "LEAST k. ~ b ((c ^^ k) s)"
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  from assms have t: "t = (c ^^ k) s"
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    by (simp add: while_option_def k_def split: if_splits)    
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  have 1: "ALL i<k. b ((c ^^ i) s)"
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    by (auto simp: k_def dest: not_less_Least)
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  { fix i assume "i <= k" then have "P ((c ^^ i) s)"
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      by (induct i) (auto simp: init step 1) }
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  thus "P t" by (auto simp: t)
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qed
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subsection {* Total version *}
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definition while :: "('a \<Rightarrow> bool) \<Rightarrow> ('a \<Rightarrow> 'a) \<Rightarrow> 'a \<Rightarrow> 'a"
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where "while b c s = the (while_option b c s)"
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lemma while_unfold:
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  "while b c s = (if b s then while b c (c s) else s)"
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unfolding while_def by (subst while_option_unfold) simp
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lemma def_while_unfold:
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  assumes fdef: "f == while test do"
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  shows "f x = (if test x then f(do x) else x)"
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unfolding fdef by (fact while_unfold)
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text {*
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 The proof rule for @{term while}, where @{term P} is the invariant.
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*}
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theorem while_rule_lemma:
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  assumes invariant: "!!s. P s ==> b s ==> P (c s)"
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    and terminate: "!!s. P s ==> \<not> b s ==> Q s"
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    and wf: "wf {(t, s). P s \<and> b s \<and> t = c s}"
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  shows "P s \<Longrightarrow> Q (while b c s)"
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  using wf
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  apply (induct s)
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  apply simp
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  apply (subst while_unfold)
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  apply (simp add: invariant terminate)
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  done
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theorem while_rule:
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  "[| P s;
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      !!s. [| P s; b s  |] ==> P (c s);
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      !!s. [| P s; \<not> b s  |] ==> Q s;
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      wf r;
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      !!s. [| P s; b s  |] ==> (c s, s) \<in> r |] ==>
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   Q (while b c s)"
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  apply (rule while_rule_lemma)
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     prefer 4 apply assumption
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    apply blast
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   apply blast
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  apply (erule wf_subset)
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  apply blast
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  done
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text{* Proving termination: *}
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theorem wf_while_option_Some:
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  assumes "wf {(t, s). (P s \<and> b s) \<and> t = c s}"
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  and "!!s. P s \<Longrightarrow> b s \<Longrightarrow> P(c s)" and "P s"
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  shows "EX t. while_option b c s = Some t"
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using assms(1,3)
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apply (induct s)
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using assms(2)
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apply (subst while_option_unfold)
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apply simp
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done
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theorem measure_while_option_Some: fixes f :: "'s \<Rightarrow> nat"
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shows "(!!s. P s \<Longrightarrow> b s \<Longrightarrow> P(c s) \<and> f(c s) < f s)
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  \<Longrightarrow> P s \<Longrightarrow> EX t. while_option b c s = Some t"
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by(blast intro: wf_while_option_Some[OF wf_if_measure, of P b f])
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text{* Kleene iteration starting from the empty set and assuming some finite
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bounding set: *}
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lemma while_option_finite_subset_Some: fixes C :: "'a set"
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  assumes "mono f" and "!!X. X \<subseteq> C \<Longrightarrow> f X \<subseteq> C" and "finite C"
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  shows "\<exists>P. while_option (\<lambda>A. f A \<noteq> A) f {} = Some P"
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proof(rule measure_while_option_Some[where
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    f= "%A::'a set. card C - card A" and P= "%A. A \<subseteq> C \<and> A \<subseteq> f A" and s= "{}"])
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  fix A assume A: "A \<subseteq> C \<and> A \<subseteq> f A" "f A \<noteq> A"
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  show "(f A \<subseteq> C \<and> f A \<subseteq> f (f A)) \<and> card C - card (f A) < card C - card A"
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    (is "?L \<and> ?R")
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  proof
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    show ?L by(metis A(1) assms(2) monoD[OF `mono f`])
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    show ?R by (metis A assms(2,3) card_seteq diff_less_mono2 equalityI linorder_le_less_linear rev_finite_subset)
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  qed
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qed simp
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lemma lfp_the_while_option:
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  assumes "mono f" and "!!X. X \<subseteq> C \<Longrightarrow> f X \<subseteq> C" and "finite C"
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  shows "lfp f = the(while_option (\<lambda>A. f A \<noteq> A) f {})"
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proof-
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  obtain P where "while_option (\<lambda>A. f A \<noteq> A) f {} = Some P"
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    using while_option_finite_subset_Some[OF assms] by blast
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  with while_option_stop2[OF this] lfp_Kleene_iter[OF assms(1)]
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  show ?thesis by auto
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qed
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end