wenzelm@10251: (*  Title:      HOL/Library/While.thy
wenzelm@10251:     ID:         $Id$
wenzelm@10251:     Author:     Tobias Nipkow
wenzelm@10251:     Copyright   2000 TU Muenchen
wenzelm@10251: *)
wenzelm@10251: 
wenzelm@10251: header {*
wenzelm@10251:  \title{A general ``while'' combinator}
wenzelm@10251:  \author{Tobias Nipkow}
wenzelm@10251: *}
wenzelm@10251: 
wenzelm@10251: theory While_Combinator = Main:
wenzelm@10251: 
wenzelm@10251: text {*
wenzelm@10251:  We define a while-combinator @{term while} and prove: (a) an
wenzelm@10251:  unrestricted unfolding law (even if while diverges!)  (I got this
wenzelm@10251:  idea from Wolfgang Goerigk), and (b) the invariant rule for reasoning
wenzelm@10251:  about @{term while}.
wenzelm@10251: *}
wenzelm@10251: 
wenzelm@10251: consts while_aux :: "('a => bool) \<times> ('a => 'a) \<times> 'a => 'a"
wenzelm@10251: recdef while_aux
wenzelm@10251:   "same_fst (\<lambda>b. True) (\<lambda>b. same_fst (\<lambda>c. True) (\<lambda>c.
wenzelm@10251:       {(t, s).  b s \<and> c s = t \<and>
wenzelm@10251:         \<not> (\<exists>f. f 0 = s \<and> (\<forall>i. b (f i) \<and> c (f i) = f (i + 1)))}))"
wenzelm@10251:   "while_aux (b, c, s) =
wenzelm@10251:     (if (\<exists>f. f 0 = s \<and> (\<forall>i. b (f i) \<and> c (f i) = f (i + 1)))
wenzelm@10251:       then arbitrary
wenzelm@10251:       else if b s then while_aux (b, c, c s)
wenzelm@10251:       else s)"
wenzelm@10251: 
wenzelm@10774: recdef_tc while_aux_tc: while_aux
wenzelm@10774:   apply (rule wf_same_fst)
wenzelm@10774:   apply (rule wf_same_fst)
wenzelm@10774:   apply (simp add: wf_iff_no_infinite_down_chain)
wenzelm@10774:   apply blast
wenzelm@10774:   done
wenzelm@10774: 
wenzelm@10251: constdefs
wenzelm@10251:   while :: "('a => bool) => ('a => 'a) => 'a => 'a"
wenzelm@10251:   "while b c s == while_aux (b, c, s)"
wenzelm@10251: 
wenzelm@10251: lemma while_aux_unfold:
wenzelm@10251:   "while_aux (b, c, s) =
wenzelm@10251:     (if \<exists>f. f 0 = s \<and> (\<forall>i. b (f i) \<and> c (f i) = f (i + 1))
wenzelm@10251:       then arbitrary
wenzelm@10251:       else if b s then while_aux (b, c, c s)
wenzelm@10251:       else s)"
wenzelm@10251:   apply (rule while_aux_tc [THEN while_aux.simps [THEN trans]])
wenzelm@10251:    apply (simp add: same_fst_def)
wenzelm@10251:   apply (rule refl)
wenzelm@10251:   done
wenzelm@10251: 
wenzelm@10251: text {*
wenzelm@10251:  The recursion equation for @{term while}: directly executable!
wenzelm@10251: *}
wenzelm@10251: 
wenzelm@10251: theorem while_unfold:
wenzelm@10251:     "while b c s = (if b s then while b c (c s) else s)"
wenzelm@10251:   apply (unfold while_def)
wenzelm@10251:   apply (rule while_aux_unfold [THEN trans])
wenzelm@10251:   apply auto
wenzelm@10251:   apply (subst while_aux_unfold)
wenzelm@10251:   apply simp
wenzelm@10251:   apply clarify
wenzelm@10251:   apply (erule_tac x = "\<lambda>i. f (Suc i)" in allE)
wenzelm@10251:   apply blast
wenzelm@10251:   done
wenzelm@10251: 
nipkow@10984: hide const while_aux
nipkow@10984: 
wenzelm@10251: text {*
wenzelm@10251:  The proof rule for @{term while}, where @{term P} is the invariant.
wenzelm@10251: *}
wenzelm@10251: 
nipkow@10653: theorem while_rule_lemma[rule_format]:
nipkow@10984:   "[| !!s. P s ==> b s ==> P (c s);
nipkow@10984:       !!s. P s ==> \<not> b s ==> Q s;
nipkow@10984:       wf {(t, s). P s \<and> b s \<and> t = c s} |] ==>
wenzelm@10251:     P s --> Q (while b c s)"
wenzelm@10251: proof -
wenzelm@10251:   case antecedent
wenzelm@10251:   assume wf: "wf {(t, s). P s \<and> b s \<and> t = c s}"
wenzelm@10251:   show ?thesis
wenzelm@10251:     apply (induct s rule: wf [THEN wf_induct])
wenzelm@10251:     apply simp
wenzelm@10251:     apply clarify
wenzelm@10251:     apply (subst while_unfold)
wenzelm@10251:     apply (simp add: antecedent)
wenzelm@10251:     done
wenzelm@10251: qed
wenzelm@10251: 
nipkow@10653: theorem while_rule:
nipkow@10984:   "[| P s;
nipkow@10984:       !!s. [| P s; b s  |] ==> P (c s);
nipkow@10984:       !!s. [| P s; \<not> b s  |] ==> Q s;
wenzelm@10997:       wf r;
nipkow@10984:       !!s. [| P s; b s  |] ==> (c s, s) \<in> r |] ==>
nipkow@10984:    Q (while b c s)"
nipkow@10653: apply (rule while_rule_lemma)
nipkow@10653: prefer 4 apply assumption
nipkow@10653: apply blast
nipkow@10653: apply blast
nipkow@10653: apply(erule wf_subset)
nipkow@10653: apply blast
nipkow@10653: done
nipkow@10653: 
nipkow@10984: text {*
nipkow@10984:  \medskip An application: computation of the @{term lfp} on finite
nipkow@10984:  sets via iteration.
nipkow@10984: *}
nipkow@10984: 
nipkow@10984: theorem lfp_conv_while:
nipkow@10984:   "[| mono f; finite U; f U = U |] ==>
nipkow@10984:     lfp f = fst (while (\<lambda>(A, fA). A \<noteq> fA) (\<lambda>(A, fA). (fA, f fA)) ({}, f {}))"
nipkow@10984: apply (rule_tac P = "\<lambda>(A, B). (A \<subseteq> U \<and> B = f A \<and> A \<subseteq> B \<and> B \<subseteq> lfp f)" and
nipkow@10984:                 r = "((Pow U <*> UNIV) <*> (Pow U <*> UNIV)) \<inter>
nipkow@10984:                      inv_image finite_psubset (op - U o fst)" in while_rule)
nipkow@10984:    apply (subst lfp_unfold)
nipkow@10984:     apply assumption
nipkow@10984:    apply (simp add: monoD)
nipkow@10984:   apply (subst lfp_unfold)
nipkow@10984:    apply assumption
nipkow@10984:   apply clarsimp
nipkow@10984:   apply (blast dest: monoD)
nipkow@10984:  apply (fastsimp intro!: lfp_lowerbound)
nipkow@10984:  apply (blast intro: wf_finite_psubset Int_lower2 [THEN [2] wf_subset])
nipkow@10984: apply (clarsimp simp add: inv_image_def finite_psubset_def order_less_le)
nipkow@10984: apply (blast intro!: finite_Diff dest: monoD)
nipkow@10984: done
nipkow@10984: 
nipkow@10984: 
nipkow@10984: text {*
wenzelm@10997:  An example of using the @{term while} combinator.\footnote{It is safe
wenzelm@10997:  to keep the example here, since there is no effect on the current
wenzelm@10997:  theory.}
nipkow@10984: *}
nipkow@10984: 
nipkow@10984: theorem "P (lfp (\<lambda>N::int set. {#0} \<union> {(n + #2) mod #6 | n. n \<in> N})) =
nipkow@10984:     P {#0, #4, #2}"
wenzelm@10997: proof -
wenzelm@10997:   have aux: "!!f A B. {f n | n. A n \<or> B n} = {f n | n. A n} \<union> {f n | n. B n}"
nipkow@10984:     apply blast
wenzelm@10997:     done
wenzelm@10997:   show ?thesis
wenzelm@10997:     apply (subst lfp_conv_while [where ?U = "{#0, #1, #2, #3, #4, #5}"])
wenzelm@10997:        apply (rule monoI)
wenzelm@10997:       apply blast
wenzelm@10997:      apply simp
wenzelm@10997:     apply (simp add: aux set_eq_subset)
wenzelm@10997:     txt {* The fixpoint computation is performed purely by rewriting: *}
wenzelm@10997:     apply (simp add: while_unfold aux set_eq_subset del: subset_empty)
wenzelm@10997:     done
wenzelm@10997: qed
wenzelm@10251: 
wenzelm@10251: end