author Andreas Lochbihler
Thu Dec 05 09:20:32 2013 +0100 (2013-12-05)
changeset 54630 9061af4d5ebc
parent 53361 1cb7d3c0cf31
child 58889 5b7a9633cfa8
permissions -rw-r--r--
restrict admissibility to non-empty chains to allow more syntax-directed proof rules
     1 (* Title:    HOL/Complete_Partial_Order.thy
     2    Author:   Brian Huffman, Portland State University
     3    Author:   Alexander Krauss, TU Muenchen
     4 *)
     6 header {* Chain-complete partial orders and their fixpoints *}
     8 theory Complete_Partial_Order
     9 imports Product_Type
    10 begin
    12 subsection {* Monotone functions *}
    14 text {* Dictionary-passing version of @{const Orderings.mono}. *}
    16 definition monotone :: "('a \<Rightarrow> 'a \<Rightarrow> bool) \<Rightarrow> ('b \<Rightarrow> 'b \<Rightarrow> bool) \<Rightarrow> ('a \<Rightarrow> 'b) \<Rightarrow> bool"
    17 where "monotone orda ordb f \<longleftrightarrow> (\<forall>x y. orda x y \<longrightarrow> ordb (f x) (f y))"
    19 lemma monotoneI[intro?]: "(\<And>x y. orda x y \<Longrightarrow> ordb (f x) (f y))
    20  \<Longrightarrow> monotone orda ordb f"
    21 unfolding monotone_def by iprover
    23 lemma monotoneD[dest?]: "monotone orda ordb f \<Longrightarrow> orda x y \<Longrightarrow> ordb (f x) (f y)"
    24 unfolding monotone_def by iprover
    27 subsection {* Chains *}
    29 text {* A chain is a totally-ordered set. Chains are parameterized over
    30   the order for maximal flexibility, since type classes are not enough.
    31 *}
    33 definition
    34   chain :: "('a \<Rightarrow> 'a \<Rightarrow> bool) \<Rightarrow> 'a set \<Rightarrow> bool"
    35 where
    36   "chain ord S \<longleftrightarrow> (\<forall>x\<in>S. \<forall>y\<in>S. ord x y \<or> ord y x)"
    38 lemma chainI:
    39   assumes "\<And>x y. x \<in> S \<Longrightarrow> y \<in> S \<Longrightarrow> ord x y \<or> ord y x"
    40   shows "chain ord S"
    41 using assms unfolding chain_def by fast
    43 lemma chainD:
    44   assumes "chain ord S" and "x \<in> S" and "y \<in> S"
    45   shows "ord x y \<or> ord y x"
    46 using assms unfolding chain_def by fast
    48 lemma chainE:
    49   assumes "chain ord S" and "x \<in> S" and "y \<in> S"
    50   obtains "ord x y" | "ord y x"
    51 using assms unfolding chain_def by fast
    53 lemma chain_empty: "chain ord {}"
    54 by(simp add: chain_def)
    56 subsection {* Chain-complete partial orders *}
    58 text {*
    59   A ccpo has a least upper bound for any chain.  In particular, the
    60   empty set is a chain, so every ccpo must have a bottom element.
    61 *}
    63 class ccpo = order + Sup +
    64   assumes ccpo_Sup_upper: "\<lbrakk>chain (op \<le>) A; x \<in> A\<rbrakk> \<Longrightarrow> x \<le> Sup A"
    65   assumes ccpo_Sup_least: "\<lbrakk>chain (op \<le>) A; \<And>x. x \<in> A \<Longrightarrow> x \<le> z\<rbrakk> \<Longrightarrow> Sup A \<le> z"
    66 begin
    68 subsection {* Transfinite iteration of a function *}
    70 inductive_set iterates :: "('a \<Rightarrow> 'a) \<Rightarrow> 'a set"
    71 for f :: "'a \<Rightarrow> 'a"
    72 where
    73   step: "x \<in> iterates f \<Longrightarrow> f x \<in> iterates f"
    74 | Sup: "chain (op \<le>) M \<Longrightarrow> \<forall>x\<in>M. x \<in> iterates f \<Longrightarrow> Sup M \<in> iterates f"
    76 lemma iterates_le_f:
    77   "x \<in> iterates f \<Longrightarrow> monotone (op \<le>) (op \<le>) f \<Longrightarrow> x \<le> f x"
    78 by (induct x rule: iterates.induct)
    79   (force dest: monotoneD intro!: ccpo_Sup_upper ccpo_Sup_least)+
    81 lemma chain_iterates:
    82   assumes f: "monotone (op \<le>) (op \<le>) f"
    83   shows "chain (op \<le>) (iterates f)" (is "chain _ ?C")
    84 proof (rule chainI)
    85   fix x y assume "x \<in> ?C" "y \<in> ?C"
    86   then show "x \<le> y \<or> y \<le> x"
    87   proof (induct x arbitrary: y rule: iterates.induct)
    88     fix x y assume y: "y \<in> ?C"
    89     and IH: "\<And>z. z \<in> ?C \<Longrightarrow> x \<le> z \<or> z \<le> x"
    90     from y show "f x \<le> y \<or> y \<le> f x"
    91     proof (induct y rule: iterates.induct)
    92       case (step y) with IH f show ?case by (auto dest: monotoneD)
    93     next
    94       case (Sup M)
    95       then have chM: "chain (op \<le>) M"
    96         and IH': "\<And>z. z \<in> M \<Longrightarrow> f x \<le> z \<or> z \<le> f x" by auto
    97       show "f x \<le> Sup M \<or> Sup M \<le> f x"
    98       proof (cases "\<exists>z\<in>M. f x \<le> z")
    99         case True then have "f x \<le> Sup M"
   100           apply rule
   101           apply (erule order_trans)
   102           by (rule ccpo_Sup_upper[OF chM])
   103         thus ?thesis ..
   104       next
   105         case False with IH'
   106         show ?thesis by (auto intro: ccpo_Sup_least[OF chM])
   107       qed
   108     qed
   109   next
   110     case (Sup M y)
   111     show ?case
   112     proof (cases "\<exists>x\<in>M. y \<le> x")
   113       case True then have "y \<le> Sup M"
   114         apply rule
   115         apply (erule order_trans)
   116         by (rule ccpo_Sup_upper[OF Sup(1)])
   117       thus ?thesis ..
   118     next
   119       case False with Sup
   120       show ?thesis by (auto intro: ccpo_Sup_least)
   121     qed
   122   qed
   123 qed
   125 lemma bot_in_iterates: "Sup {} \<in> iterates f"
   126 by(auto intro: iterates.Sup simp add: chain_empty)
   128 subsection {* Fixpoint combinator *}
   130 definition
   131   fixp :: "('a \<Rightarrow> 'a) \<Rightarrow> 'a"
   132 where
   133   "fixp f = Sup (iterates f)"
   135 lemma iterates_fixp:
   136   assumes f: "monotone (op \<le>) (op \<le>) f" shows "fixp f \<in> iterates f"
   137 unfolding fixp_def
   138 by (simp add: iterates.Sup chain_iterates f)
   140 lemma fixp_unfold:
   141   assumes f: "monotone (op \<le>) (op \<le>) f"
   142   shows "fixp f = f (fixp f)"
   143 proof (rule antisym)
   144   show "fixp f \<le> f (fixp f)"
   145     by (intro iterates_le_f iterates_fixp f)
   146   have "f (fixp f) \<le> Sup (iterates f)"
   147     by (intro ccpo_Sup_upper chain_iterates f iterates.step iterates_fixp)
   148   thus "f (fixp f) \<le> fixp f"
   149     unfolding fixp_def .
   150 qed
   152 lemma fixp_lowerbound:
   153   assumes f: "monotone (op \<le>) (op \<le>) f" and z: "f z \<le> z" shows "fixp f \<le> z"
   154 unfolding fixp_def
   155 proof (rule ccpo_Sup_least[OF chain_iterates[OF f]])
   156   fix x assume "x \<in> iterates f"
   157   thus "x \<le> z"
   158   proof (induct x rule: iterates.induct)
   159     fix x assume "x \<le> z" with f have "f x \<le> f z" by (rule monotoneD)
   160     also note z finally show "f x \<le> z" .
   161   qed (auto intro: ccpo_Sup_least)
   162 qed
   164 end
   166 subsection {* Fixpoint induction *}
   168 setup {* Sign.map_naming (Name_Space.mandatory_path "ccpo") *}
   170 definition admissible :: "('a set \<Rightarrow> 'a) \<Rightarrow> ('a \<Rightarrow> 'a \<Rightarrow> bool) \<Rightarrow> ('a \<Rightarrow> bool) \<Rightarrow> bool"
   171 where "admissible lub ord P = (\<forall>A. chain ord A \<longrightarrow> (A \<noteq> {}) \<longrightarrow> (\<forall>x\<in>A. P x) \<longrightarrow> P (lub A))"
   173 lemma admissibleI:
   174   assumes "\<And>A. chain ord A \<Longrightarrow> A \<noteq> {} \<Longrightarrow> \<forall>x\<in>A. P x \<Longrightarrow> P (lub A)"
   175   shows "ccpo.admissible lub ord P"
   176 using assms unfolding ccpo.admissible_def by fast
   178 lemma admissibleD:
   179   assumes "ccpo.admissible lub ord P"
   180   assumes "chain ord A"
   181   assumes "A \<noteq> {}"
   182   assumes "\<And>x. x \<in> A \<Longrightarrow> P x"
   183   shows "P (lub A)"
   184 using assms by (auto simp: ccpo.admissible_def)
   186 setup {* Sign.map_naming Name_Space.parent_path *}
   188 lemma (in ccpo) fixp_induct:
   189   assumes adm: "ccpo.admissible Sup (op \<le>) P"
   190   assumes mono: "monotone (op \<le>) (op \<le>) f"
   191   assumes bot: "P (Sup {})"
   192   assumes step: "\<And>x. P x \<Longrightarrow> P (f x)"
   193   shows "P (fixp f)"
   194 unfolding fixp_def using adm chain_iterates[OF mono]
   195 proof (rule ccpo.admissibleD)
   196   show "iterates f \<noteq> {}" using bot_in_iterates by auto
   197   fix x assume "x \<in> iterates f"
   198   thus "P x"
   199     by (induct rule: iterates.induct)
   200       (case_tac "M = {}", auto intro: step bot ccpo.admissibleD adm)
   201 qed
   203 lemma admissible_True: "ccpo.admissible lub ord (\<lambda>x. True)"
   204 unfolding ccpo.admissible_def by simp
   206 (*lemma admissible_False: "\<not> ccpo.admissible lub ord (\<lambda>x. False)"
   207 unfolding ccpo.admissible_def chain_def by simp
   208 *)
   209 lemma admissible_const: "ccpo.admissible lub ord (\<lambda>x. t)"
   210 by(auto intro: ccpo.admissibleI)
   212 lemma admissible_conj:
   213   assumes "ccpo.admissible lub ord (\<lambda>x. P x)"
   214   assumes "ccpo.admissible lub ord (\<lambda>x. Q x)"
   215   shows "ccpo.admissible lub ord (\<lambda>x. P x \<and> Q x)"
   216 using assms unfolding ccpo.admissible_def by simp
   218 lemma admissible_all:
   219   assumes "\<And>y. ccpo.admissible lub ord (\<lambda>x. P x y)"
   220   shows "ccpo.admissible lub ord (\<lambda>x. \<forall>y. P x y)"
   221 using assms unfolding ccpo.admissible_def by fast
   223 lemma admissible_ball:
   224   assumes "\<And>y. y \<in> A \<Longrightarrow> ccpo.admissible lub ord (\<lambda>x. P x y)"
   225   shows "ccpo.admissible lub ord (\<lambda>x. \<forall>y\<in>A. P x y)"
   226 using assms unfolding ccpo.admissible_def by fast
   228 lemma chain_compr: "chain ord A \<Longrightarrow> chain ord {x \<in> A. P x}"
   229 unfolding chain_def by fast
   231 context ccpo begin
   233 lemma admissible_disj_lemma:
   234   assumes A: "chain (op \<le>)A"
   235   assumes P: "\<forall>x\<in>A. \<exists>y\<in>A. x \<le> y \<and> P y"
   236   shows "Sup A = Sup {x \<in> A. P x}"
   237 proof (rule antisym)
   238   have *: "chain (op \<le>) {x \<in> A. P x}"
   239     by (rule chain_compr [OF A])
   240   show "Sup A \<le> Sup {x \<in> A. P x}"
   241     apply (rule ccpo_Sup_least [OF A])
   242     apply (drule P [rule_format], clarify)
   243     apply (erule order_trans)
   244     apply (simp add: ccpo_Sup_upper [OF *])
   245     done
   246   show "Sup {x \<in> A. P x} \<le> Sup A"
   247     apply (rule ccpo_Sup_least [OF *])
   248     apply clarify
   249     apply (simp add: ccpo_Sup_upper [OF A])
   250     done
   251 qed
   253 lemma admissible_disj:
   254   fixes P Q :: "'a \<Rightarrow> bool"
   255   assumes P: "ccpo.admissible Sup (op \<le>) (\<lambda>x. P x)"
   256   assumes Q: "ccpo.admissible Sup (op \<le>) (\<lambda>x. Q x)"
   257   shows "ccpo.admissible Sup (op \<le>) (\<lambda>x. P x \<or> Q x)"
   258 proof (rule ccpo.admissibleI)
   259   fix A :: "'a set" assume A: "chain (op \<le>) A"
   260   assume "A \<noteq> {}"
   261     and "\<forall>x\<in>A. P x \<or> Q x"
   262   hence "(\<exists>x\<in>A. P x) \<and> (\<forall>x\<in>A. \<exists>y\<in>A. x \<le> y \<and> P y) \<or> (\<exists>x\<in>A. Q x) \<and> (\<forall>x\<in>A. \<exists>y\<in>A. x \<le> y \<and> Q y)"
   263     using chainD[OF A] by blast
   264   hence "(\<exists>x. x \<in> A \<and> P x) \<and> Sup A = Sup {x \<in> A. P x} \<or> (\<exists>x. x \<in> A \<and> Q x) \<and> Sup A = Sup {x \<in> A. Q x}"
   265     using admissible_disj_lemma [OF A] by blast
   266   thus "P (Sup A) \<or> Q (Sup A)"
   267     apply (rule disjE, simp_all)
   268     apply (rule disjI1, rule ccpo.admissibleD [OF P chain_compr [OF A]], simp, simp)
   269     apply (rule disjI2, rule ccpo.admissibleD [OF Q chain_compr [OF A]], simp, simp)
   270     done
   271 qed
   273 end
   275 instance complete_lattice \<subseteq> ccpo
   276   by default (fast intro: Sup_upper Sup_least)+
   278 lemma lfp_eq_fixp:
   279   assumes f: "mono f" shows "lfp f = fixp f"
   280 proof (rule antisym)
   281   from f have f': "monotone (op \<le>) (op \<le>) f"
   282     unfolding mono_def monotone_def .
   283   show "lfp f \<le> fixp f"
   284     by (rule lfp_lowerbound, subst fixp_unfold [OF f'], rule order_refl)
   285   show "fixp f \<le> lfp f"
   286     by (rule fixp_lowerbound [OF f'], subst lfp_unfold [OF f], rule order_refl)
   287 qed
   289 hide_const (open) iterates fixp
   291 end