src/HOL/Library/While_Combinator.thy
 author wenzelm Mon Dec 28 17:43:30 2015 +0100 (2015-12-28) changeset 61952 546958347e05 parent 61424 c3658c18b7bc child 63040 eb4ddd18d635 permissions -rw-r--r--
prefer symbols for "Union", "Inter";
```     1 (*  Title:      HOL/Library/While_Combinator.thy
```
```     2     Author:     Tobias Nipkow
```
```     3     Author:     Alexander Krauss
```
```     4 *)
```
```     5
```
```     6 section \<open>A general ``while'' combinator\<close>
```
```     7
```
```     8 theory While_Combinator
```
```     9 imports Main
```
```    10 begin
```
```    11
```
```    12 subsection \<open>Partial version\<close>
```
```    13
```
```    14 definition while_option :: "('a \<Rightarrow> bool) \<Rightarrow> ('a \<Rightarrow> 'a) \<Rightarrow> 'a \<Rightarrow> 'a option" where
```
```    15 "while_option b c s = (if (\<exists>k. ~ b ((c ^^ k) s))
```
```    16    then Some ((c ^^ (LEAST k. ~ b ((c ^^ k) s))) s)
```
```    17    else None)"
```
```    18
```
```    19 theorem while_option_unfold[code]:
```
```    20 "while_option b c s = (if b s then while_option b c (c s) else Some s)"
```
```    21 proof cases
```
```    22   assume "b s"
```
```    23   show ?thesis
```
```    24   proof (cases "\<exists>k. ~ b ((c ^^ k) s)")
```
```    25     case True
```
```    26     then obtain k where 1: "~ b ((c ^^ k) s)" ..
```
```    27     with \<open>b s\<close> obtain l where "k = Suc l" by (cases k) auto
```
```    28     with 1 have "~ b ((c ^^ l) (c s))" by (auto simp: funpow_swap1)
```
```    29     then have 2: "\<exists>l. ~ b ((c ^^ l) (c s))" ..
```
```    30     from 1
```
```    31     have "(LEAST k. ~ b ((c ^^ k) s)) = Suc (LEAST l. ~ b ((c ^^ Suc l) s))"
```
```    32       by (rule Least_Suc) (simp add: \<open>b s\<close>)
```
```    33     also have "... = Suc (LEAST l. ~ b ((c ^^ l) (c s)))"
```
```    34       by (simp add: funpow_swap1)
```
```    35     finally
```
```    36     show ?thesis
```
```    37       using True 2 \<open>b s\<close> by (simp add: funpow_swap1 while_option_def)
```
```    38   next
```
```    39     case False
```
```    40     then have "~ (\<exists>l. ~ b ((c ^^ Suc l) s))" by blast
```
```    41     then have "~ (\<exists>l. ~ b ((c ^^ l) (c s)))"
```
```    42       by (simp add: funpow_swap1)
```
```    43     with False  \<open>b s\<close> show ?thesis by (simp add: while_option_def)
```
```    44   qed
```
```    45 next
```
```    46   assume [simp]: "~ b s"
```
```    47   have least: "(LEAST k. ~ b ((c ^^ k) s)) = 0"
```
```    48     by (rule Least_equality) auto
```
```    49   moreover
```
```    50   have "\<exists>k. ~ b ((c ^^ k) s)" by (rule exI[of _ "0::nat"]) auto
```
```    51   ultimately show ?thesis unfolding while_option_def by auto
```
```    52 qed
```
```    53
```
```    54 lemma while_option_stop2:
```
```    55  "while_option b c s = Some t \<Longrightarrow> EX k. t = (c^^k) s \<and> \<not> b t"
```
```    56 apply(simp add: while_option_def split: if_splits)
```
```    57 by (metis (lifting) LeastI_ex)
```
```    58
```
```    59 lemma while_option_stop: "while_option b c s = Some t \<Longrightarrow> ~ b t"
```
```    60 by(metis while_option_stop2)
```
```    61
```
```    62 theorem while_option_rule:
```
```    63 assumes step: "!!s. P s ==> b s ==> P (c s)"
```
```    64 and result: "while_option b c s = Some t"
```
```    65 and init: "P s"
```
```    66 shows "P t"
```
```    67 proof -
```
```    68   def k == "LEAST k. ~ b ((c ^^ k) s)"
```
```    69   from assms have t: "t = (c ^^ k) s"
```
```    70     by (simp add: while_option_def k_def split: if_splits)
```
```    71   have 1: "ALL i<k. b ((c ^^ i) s)"
```
```    72     by (auto simp: k_def dest: not_less_Least)
```
```    73
```
```    74   { fix i assume "i <= k" then have "P ((c ^^ i) s)"
```
```    75       by (induct i) (auto simp: init step 1) }
```
```    76   thus "P t" by (auto simp: t)
```
```    77 qed
```
```    78
```
```    79 lemma funpow_commute:
```
```    80   "\<lbrakk>\<forall>k' < k. f (c ((c^^k') s)) = c' (f ((c^^k') s))\<rbrakk> \<Longrightarrow> f ((c^^k) s) = (c'^^k) (f s)"
```
```    81 by (induct k arbitrary: s) auto
```
```    82
```
```    83 lemma while_option_commute_invariant:
```
```    84 assumes Invariant: "\<And>s. P s \<Longrightarrow> b s \<Longrightarrow> P (c s)"
```
```    85 assumes TestCommute: "\<And>s. P s \<Longrightarrow> b s = b' (f s)"
```
```    86 assumes BodyCommute: "\<And>s. P s \<Longrightarrow> b s \<Longrightarrow> f (c s) = c' (f s)"
```
```    87 assumes Initial: "P s"
```
```    88 shows "map_option f (while_option b c s) = while_option b' c' (f s)"
```
```    89 unfolding while_option_def
```
```    90 proof (rule trans[OF if_distrib if_cong], safe, unfold option.inject)
```
```    91   fix k
```
```    92   assume "\<not> b ((c ^^ k) s)"
```
```    93   with Initial show "\<exists>k. \<not> b' ((c' ^^ k) (f s))"
```
```    94   proof (induction k arbitrary: s)
```
```    95     case 0 thus ?case by (auto simp: TestCommute intro: exI[of _ 0])
```
```    96   next
```
```    97     case (Suc k) thus ?case
```
```    98     proof (cases "b s")
```
```    99       assume "b s"
```
```   100       with Suc.IH[of "c s"] Suc.prems show ?thesis
```
```   101         by (metis BodyCommute Invariant comp_apply funpow.simps(2) funpow_swap1)
```
```   102     next
```
```   103       assume "\<not> b s"
```
```   104       with Suc show ?thesis by (auto simp: TestCommute intro: exI [of _ 0])
```
```   105     qed
```
```   106   qed
```
```   107 next
```
```   108   fix k
```
```   109   assume "\<not> b' ((c' ^^ k) (f s))"
```
```   110   with Initial show "\<exists>k. \<not> b ((c ^^ k) s)"
```
```   111   proof (induction k arbitrary: s)
```
```   112     case 0 thus ?case by (auto simp: TestCommute intro: exI[of _ 0])
```
```   113   next
```
```   114     case (Suc k) thus ?case
```
```   115     proof (cases "b s")
```
```   116        assume "b s"
```
```   117       with Suc.IH[of "c s"] Suc.prems show ?thesis
```
```   118         by (metis BodyCommute Invariant comp_apply funpow.simps(2) funpow_swap1)
```
```   119     next
```
```   120       assume "\<not> b s"
```
```   121       with Suc show ?thesis by (auto simp: TestCommute intro: exI [of _ 0])
```
```   122     qed
```
```   123   qed
```
```   124 next
```
```   125   fix k
```
```   126   assume k: "\<not> b' ((c' ^^ k) (f s))"
```
```   127   have *: "(LEAST k. \<not> b' ((c' ^^ k) (f s))) = (LEAST k. \<not> b ((c ^^ k) s))"
```
```   128           (is "?k' = ?k")
```
```   129   proof (cases ?k')
```
```   130     case 0
```
```   131     have "\<not> b' ((c' ^^ 0) (f s))"
```
```   132       unfolding 0[symmetric] by (rule LeastI[of _ k]) (rule k)
```
```   133     hence "\<not> b s" by (auto simp: TestCommute Initial)
```
```   134     hence "?k = 0" by (intro Least_equality) auto
```
```   135     with 0 show ?thesis by auto
```
```   136   next
```
```   137     case (Suc k')
```
```   138     have "\<not> b' ((c' ^^ Suc k') (f s))"
```
```   139       unfolding Suc[symmetric] by (rule LeastI) (rule k)
```
```   140     moreover
```
```   141     { fix k assume "k \<le> k'"
```
```   142       hence "k < ?k'" unfolding Suc by simp
```
```   143       hence "b' ((c' ^^ k) (f s))" by (rule iffD1[OF not_not, OF not_less_Least])
```
```   144     }
```
```   145     note b' = this
```
```   146     { fix k assume "k \<le> k'"
```
```   147       hence "f ((c ^^ k) s) = (c' ^^ k) (f s)"
```
```   148       and "b ((c ^^ k) s) = b' ((c' ^^ k) (f s))"
```
```   149       and "P ((c ^^ k) s)"
```
```   150         by (induct k) (auto simp: b' assms)
```
```   151       with \<open>k \<le> k'\<close>
```
```   152       have "b ((c ^^ k) s)"
```
```   153       and "f ((c ^^ k) s) = (c' ^^ k) (f s)"
```
```   154       and "P ((c ^^ k) s)"
```
```   155         by (auto simp: b')
```
```   156     }
```
```   157     note b = this(1) and body = this(2) and inv = this(3)
```
```   158     hence k': "f ((c ^^ k') s) = (c' ^^ k') (f s)" by auto
```
```   159     ultimately show ?thesis unfolding Suc using b
```
```   160     proof (intro Least_equality[symmetric], goal_cases)
```
```   161       case 1
```
```   162       hence Test: "\<not> b' (f ((c ^^ Suc k') s))"
```
```   163         by (auto simp: BodyCommute inv b)
```
```   164       have "P ((c ^^ Suc k') s)" by (auto simp: Invariant inv b)
```
```   165       with Test show ?case by (auto simp: TestCommute)
```
```   166     next
```
```   167       case 2
```
```   168       thus ?case by (metis not_less_eq_eq)
```
```   169     qed
```
```   170   qed
```
```   171   have "f ((c ^^ ?k) s) = (c' ^^ ?k') (f s)" unfolding *
```
```   172   proof (rule funpow_commute, clarify)
```
```   173     fix k assume "k < ?k"
```
```   174     hence TestTrue: "b ((c ^^ k) s)" by (auto dest: not_less_Least)
```
```   175     from \<open>k < ?k\<close> have "P ((c ^^ k) s)"
```
```   176     proof (induct k)
```
```   177       case 0 thus ?case by (auto simp: assms)
```
```   178     next
```
```   179       case (Suc h)
```
```   180       hence "P ((c ^^ h) s)" by auto
```
```   181       with Suc show ?case
```
```   182         by (auto, metis (lifting, no_types) Invariant Suc_lessD not_less_Least)
```
```   183     qed
```
```   184     with TestTrue show "f (c ((c ^^ k) s)) = c' (f ((c ^^ k) s))"
```
```   185       by (metis BodyCommute)
```
```   186   qed
```
```   187   thus "\<exists>z. (c ^^ ?k) s = z \<and> f z = (c' ^^ ?k') (f s)" by blast
```
```   188 qed
```
```   189
```
```   190 lemma while_option_commute:
```
```   191   assumes "\<And>s. b s = b' (f s)" "\<And>s. \<lbrakk>b s\<rbrakk> \<Longrightarrow> f (c s) = c' (f s)"
```
```   192   shows "map_option f (while_option b c s) = while_option b' c' (f s)"
```
```   193 by(rule while_option_commute_invariant[where P = "\<lambda>_. True"])
```
```   194   (auto simp add: assms)
```
```   195
```
```   196 subsection \<open>Total version\<close>
```
```   197
```
```   198 definition while :: "('a \<Rightarrow> bool) \<Rightarrow> ('a \<Rightarrow> 'a) \<Rightarrow> 'a \<Rightarrow> 'a"
```
```   199 where "while b c s = the (while_option b c s)"
```
```   200
```
```   201 lemma while_unfold [code]:
```
```   202   "while b c s = (if b s then while b c (c s) else s)"
```
```   203 unfolding while_def by (subst while_option_unfold) simp
```
```   204
```
```   205 lemma def_while_unfold:
```
```   206   assumes fdef: "f == while test do"
```
```   207   shows "f x = (if test x then f(do x) else x)"
```
```   208 unfolding fdef by (fact while_unfold)
```
```   209
```
```   210
```
```   211 text \<open>
```
```   212  The proof rule for @{term while}, where @{term P} is the invariant.
```
```   213 \<close>
```
```   214
```
```   215 theorem while_rule_lemma:
```
```   216   assumes invariant: "!!s. P s ==> b s ==> P (c s)"
```
```   217     and terminate: "!!s. P s ==> \<not> b s ==> Q s"
```
```   218     and wf: "wf {(t, s). P s \<and> b s \<and> t = c s}"
```
```   219   shows "P s \<Longrightarrow> Q (while b c s)"
```
```   220   using wf
```
```   221   apply (induct s)
```
```   222   apply simp
```
```   223   apply (subst while_unfold)
```
```   224   apply (simp add: invariant terminate)
```
```   225   done
```
```   226
```
```   227 theorem while_rule:
```
```   228   "[| P s;
```
```   229       !!s. [| P s; b s  |] ==> P (c s);
```
```   230       !!s. [| P s; \<not> b s  |] ==> Q s;
```
```   231       wf r;
```
```   232       !!s. [| P s; b s  |] ==> (c s, s) \<in> r |] ==>
```
```   233    Q (while b c s)"
```
```   234   apply (rule while_rule_lemma)
```
```   235      prefer 4 apply assumption
```
```   236     apply blast
```
```   237    apply blast
```
```   238   apply (erule wf_subset)
```
```   239   apply blast
```
```   240   done
```
```   241
```
```   242 text\<open>Proving termination:\<close>
```
```   243
```
```   244 theorem wf_while_option_Some:
```
```   245   assumes "wf {(t, s). (P s \<and> b s) \<and> t = c s}"
```
```   246   and "!!s. P s \<Longrightarrow> b s \<Longrightarrow> P(c s)" and "P s"
```
```   247   shows "EX t. while_option b c s = Some t"
```
```   248 using assms(1,3)
```
```   249 proof (induction s)
```
```   250   case less thus ?case using assms(2)
```
```   251     by (subst while_option_unfold) simp
```
```   252 qed
```
```   253
```
```   254 lemma wf_rel_while_option_Some:
```
```   255 assumes wf: "wf R"
```
```   256 assumes smaller: "\<And>s. P s \<and> b s \<Longrightarrow> (c s, s) \<in> R"
```
```   257 assumes inv: "\<And>s. P s \<and> b s \<Longrightarrow> P(c s)"
```
```   258 assumes init: "P s"
```
```   259 shows "\<exists>t. while_option b c s = Some t"
```
```   260 proof -
```
```   261  from smaller have "{(t,s). P s \<and> b s \<and> t = c s} \<subseteq> R" by auto
```
```   262  with wf have "wf {(t,s). P s \<and> b s \<and> t = c s}" by (auto simp: wf_subset)
```
```   263  with inv init show ?thesis by (auto simp: wf_while_option_Some)
```
```   264 qed
```
```   265
```
```   266 theorem measure_while_option_Some: fixes f :: "'s \<Rightarrow> nat"
```
```   267 shows "(!!s. P s \<Longrightarrow> b s \<Longrightarrow> P(c s) \<and> f(c s) < f s)
```
```   268   \<Longrightarrow> P s \<Longrightarrow> EX t. while_option b c s = Some t"
```
```   269 by(blast intro: wf_while_option_Some[OF wf_if_measure, of P b f])
```
```   270
```
```   271 text\<open>Kleene iteration starting from the empty set and assuming some finite
```
```   272 bounding set:\<close>
```
```   273
```
```   274 lemma while_option_finite_subset_Some: fixes C :: "'a set"
```
```   275   assumes "mono f" and "!!X. X \<subseteq> C \<Longrightarrow> f X \<subseteq> C" and "finite C"
```
```   276   shows "\<exists>P. while_option (\<lambda>A. f A \<noteq> A) f {} = Some P"
```
```   277 proof(rule measure_while_option_Some[where
```
```   278     f= "%A::'a set. card C - card A" and P= "%A. A \<subseteq> C \<and> A \<subseteq> f A" and s= "{}"])
```
```   279   fix A assume A: "A \<subseteq> C \<and> A \<subseteq> f A" "f A \<noteq> A"
```
```   280   show "(f A \<subseteq> C \<and> f A \<subseteq> f (f A)) \<and> card C - card (f A) < card C - card A"
```
```   281     (is "?L \<and> ?R")
```
```   282   proof
```
```   283     show ?L by(metis A(1) assms(2) monoD[OF \<open>mono f\<close>])
```
```   284     show ?R by (metis A assms(2,3) card_seteq diff_less_mono2 equalityI linorder_le_less_linear rev_finite_subset)
```
```   285   qed
```
```   286 qed simp
```
```   287
```
```   288 lemma lfp_the_while_option:
```
```   289   assumes "mono f" and "!!X. X \<subseteq> C \<Longrightarrow> f X \<subseteq> C" and "finite C"
```
```   290   shows "lfp f = the(while_option (\<lambda>A. f A \<noteq> A) f {})"
```
```   291 proof-
```
```   292   obtain P where "while_option (\<lambda>A. f A \<noteq> A) f {} = Some P"
```
```   293     using while_option_finite_subset_Some[OF assms] by blast
```
```   294   with while_option_stop2[OF this] lfp_Kleene_iter[OF assms(1)]
```
```   295   show ?thesis by auto
```
```   296 qed
```
```   297
```
```   298 lemma lfp_while:
```
```   299   assumes "mono f" and "!!X. X \<subseteq> C \<Longrightarrow> f X \<subseteq> C" and "finite C"
```
```   300   shows "lfp f = while (\<lambda>A. f A \<noteq> A) f {}"
```
```   301 unfolding while_def using assms by (rule lfp_the_while_option) blast
```
```   302
```
```   303
```
```   304 text\<open>Computing the reflexive, transitive closure by iterating a successor
```
```   305 function. Stops when an element is found that dos not satisfy the test.
```
```   306
```
```   307 More refined (and hence more efficient) versions can be found in ITP 2011 paper
```
```   308 by Nipkow (the theories are in the AFP entry Flyspeck by Nipkow)
```
```   309 and the AFP article Executable Transitive Closures by René Thiemann.\<close>
```
```   310
```
```   311 context
```
```   312 fixes p :: "'a \<Rightarrow> bool"
```
```   313 and f :: "'a \<Rightarrow> 'a list"
```
```   314 and x :: 'a
```
```   315 begin
```
```   316
```
```   317 qualified fun rtrancl_while_test :: "'a list \<times> 'a set \<Rightarrow> bool"
```
```   318 where "rtrancl_while_test (ws,_) = (ws \<noteq> [] \<and> p(hd ws))"
```
```   319
```
```   320 qualified fun rtrancl_while_step :: "'a list \<times> 'a set \<Rightarrow> 'a list \<times> 'a set"
```
```   321 where "rtrancl_while_step (ws, Z) =
```
```   322   (let x = hd ws; new = remdups (filter (\<lambda>y. y \<notin> Z) (f x))
```
```   323   in (new @ tl ws, set new \<union> Z))"
```
```   324
```
```   325 definition rtrancl_while :: "('a list * 'a set) option"
```
```   326 where "rtrancl_while = while_option rtrancl_while_test rtrancl_while_step ([x],{x})"
```
```   327
```
```   328 qualified fun rtrancl_while_invariant :: "'a list \<times> 'a set \<Rightarrow> bool"
```
```   329 where "rtrancl_while_invariant (ws, Z) =
```
```   330    (x \<in> Z \<and> set ws \<subseteq> Z \<and> distinct ws \<and> {(x,y). y \<in> set(f x)} `` (Z - set ws) \<subseteq> Z \<and>
```
```   331     Z \<subseteq> {(x,y). y \<in> set(f x)}^* `` {x} \<and> (\<forall>z\<in>Z - set ws. p z))"
```
```   332
```
```   333 qualified lemma rtrancl_while_invariant:
```
```   334   assumes inv: "rtrancl_while_invariant st" and test: "rtrancl_while_test st"
```
```   335   shows   "rtrancl_while_invariant (rtrancl_while_step st)"
```
```   336 proof (cases st)
```
```   337   fix ws Z assume st: "st = (ws, Z)"
```
```   338   with test obtain h t where "ws = h # t" "p h" by (cases ws) auto
```
```   339   with inv st show ?thesis by (auto intro: rtrancl.rtrancl_into_rtrancl)
```
```   340 qed
```
```   341
```
```   342 lemma rtrancl_while_Some: assumes "rtrancl_while = Some(ws,Z)"
```
```   343 shows "if ws = []
```
```   344        then Z = {(x,y). y \<in> set(f x)}^* `` {x} \<and> (\<forall>z\<in>Z. p z)
```
```   345        else \<not>p(hd ws) \<and> hd ws \<in> {(x,y). y \<in> set(f x)}^* `` {x}"
```
```   346 proof -
```
```   347   have "rtrancl_while_invariant ([x],{x})" by simp
```
```   348   with rtrancl_while_invariant have I: "rtrancl_while_invariant (ws,Z)"
```
```   349     by (rule while_option_rule[OF _ assms[unfolded rtrancl_while_def]])
```
```   350   { assume "ws = []"
```
```   351     hence ?thesis using I
```
```   352       by (auto simp del:Image_Collect_case_prod dest: Image_closed_trancl)
```
```   353   } moreover
```
```   354   { assume "ws \<noteq> []"
```
```   355     hence ?thesis using I while_option_stop[OF assms[unfolded rtrancl_while_def]]
```
```   356       by (simp add: subset_iff)
```
```   357   }
```
```   358   ultimately show ?thesis by simp
```
```   359 qed
```
```   360
```
```   361 lemma rtrancl_while_finite_Some:
```
```   362   assumes "finite ({(x, y). y \<in> set (f x)}\<^sup>* `` {x})" (is "finite ?Cl")
```
```   363   shows "\<exists>y. rtrancl_while = Some y"
```
```   364 proof -
```
```   365   let ?R = "(\<lambda>(_, Z). card (?Cl - Z)) <*mlex*> (\<lambda>(ws, _). length ws) <*mlex*> {}"
```
```   366   have "wf ?R" by (blast intro: wf_mlex)
```
```   367   then show ?thesis unfolding rtrancl_while_def
```
```   368   proof (rule wf_rel_while_option_Some[of ?R rtrancl_while_invariant])
```
```   369     fix st assume *: "rtrancl_while_invariant st \<and> rtrancl_while_test st"
```
```   370     hence I: "rtrancl_while_invariant (rtrancl_while_step st)"
```
```   371       by (blast intro: rtrancl_while_invariant)
```
```   372     show "(rtrancl_while_step st, st) \<in> ?R"
```
```   373     proof (cases st)
```
```   374       fix ws Z let ?ws = "fst (rtrancl_while_step st)" and ?Z = "snd (rtrancl_while_step st)"
```
```   375       assume st: "st = (ws, Z)"
```
```   376       with * obtain h t where ws: "ws = h # t" "p h" by (cases ws) auto
```
```   377       { assume "remdups (filter (\<lambda>y. y \<notin> Z) (f h)) \<noteq> []"
```
```   378         then obtain z where "z \<in> set (remdups (filter (\<lambda>y. y \<notin> Z) (f h)))" by fastforce
```
```   379         with st ws I have "Z \<subset> ?Z" "Z \<subseteq> ?Cl" "?Z \<subseteq> ?Cl" by auto
```
```   380         with assms have "card (?Cl - ?Z) < card (?Cl - Z)" by (blast intro: psubset_card_mono)
```
```   381         with st ws have ?thesis unfolding mlex_prod_def by simp
```
```   382       }
```
```   383       moreover
```
```   384       { assume "remdups (filter (\<lambda>y. y \<notin> Z) (f h)) = []"
```
```   385         with st ws have "?Z = Z" "?ws = t"  by (auto simp: filter_empty_conv)
```
```   386         with st ws have ?thesis unfolding mlex_prod_def by simp
```
```   387       }
```
```   388       ultimately show ?thesis by blast
```
```   389     qed
```
```   390   qed (simp_all add: rtrancl_while_invariant)
```
```   391 qed
```
```   392
```
```   393 end
```
```   394
```
```   395 end
```