Theory Completion

(*  Title:      HOL/HOLCF/Completion.thy
    Author:     Brian Huffman

section ‹Defining algebraic domains by ideal completion›

theory Completion
imports Cfun

subsection ‹Ideals over a preorder›

locale preorder =
  fixes r :: "'a::type  'a  bool" (infix "" 50)
  assumes r_refl: "x  x"
  assumes r_trans: "x  y; y  z  x  z"

  ideal :: "'a set  bool" where
  "ideal A = ((x. x  A)  (xA. yA. zA. x  z  y  z) 
    (x y. x  y  y  A  x  A))"

lemma idealI:
  assumes "x. x  A"
  assumes "x y. x  A; y  A  zA. x  z  y  z"
  assumes "x y. x  y; y  A  x  A"
  shows "ideal A"
unfolding ideal_def using assms by fast

lemma idealD1:
  "ideal A  x. x  A"
unfolding ideal_def by fast

lemma idealD2:
  "ideal A; x  A; y  A  zA. x  z  y  z"
unfolding ideal_def by fast

lemma idealD3:
  "ideal A; x  y; y  A  x  A"
unfolding ideal_def by fast

lemma ideal_principal: "ideal {x. x  z}"
  apply (rule idealI)
    apply (rule exI [where x = z])
    apply (fast intro: r_refl)
   apply (rule bexI [where x = z], fast)
   apply (fast intro: r_refl)
  apply (fast intro: r_trans)

lemma ex_ideal: "A. A  {A. ideal A}"
by (fast intro: ideal_principal)

text ‹The set of ideals is a cpo›

lemma ideal_UN:
  fixes A :: "nat  'a set"
  assumes ideal_A: "i. ideal (A i)"
  assumes chain_A: "i j. i  j  A i  A j"
  shows "ideal (i. A i)"
  apply (rule idealI)
  using idealD1 [OF ideal_A] apply fast
   apply (clarify)
  subgoal for i j
    apply (drule subsetD [OF chain_A [OF max.cobounded1]])
    apply (drule subsetD [OF chain_A [OF max.cobounded2]])
    apply (drule (1) idealD2 [OF ideal_A])
    apply blast
  apply clarify
  apply (drule (1) idealD3 [OF ideal_A])
  apply fast

lemma typedef_ideal_po:
  fixes Abs :: "'a set  'b::below"
  assumes type: "type_definition Rep Abs {S. ideal S}"
  assumes below: "x y. x  y  Rep x  Rep y"
  shows "OFCLASS('b, po_class)"
 apply (intro_classes, unfold below)
   apply (rule subset_refl)
  apply (erule (1) subset_trans)
 apply (rule type_definition.Rep_inject [OF type, THEN iffD1])
 apply (erule (1) subset_antisym)

  fixes Abs :: "'a set  'b::po"
  assumes type: "type_definition Rep Abs {S. ideal S}"
  assumes below: "x y. x  y  Rep x  Rep y"
  assumes S: "chain S"
  shows typedef_ideal_lub: "range S <<| Abs (i. Rep (S i))"
    and typedef_ideal_rep_lub: "Rep (i. S i) = (i. Rep (S i))"
proof -
  have 1: "ideal (i. Rep (S i))"
    apply (rule ideal_UN)
     apply (rule type_definition.Rep [OF type, unfolded mem_Collect_eq])
    apply (subst below [symmetric])
    apply (erule chain_mono [OF S])
  hence 2: "Rep (Abs (i. Rep (S i))) = (i. Rep (S i))"
    by (simp add: type_definition.Abs_inverse [OF type])
  show 3: "range S <<| Abs (i. Rep (S i))"
    apply (rule is_lubI)
     apply (rule is_ubI)
     apply (simp add: below 2, fast)
    apply (simp add: below 2 is_ub_def, fast)
  hence 4: "(i. S i) = Abs (i. Rep (S i))"
    by (rule lub_eqI)
  show 5: "Rep (i. S i) = (i. Rep (S i))"
    by (simp add: 4 2)

lemma typedef_ideal_cpo:
  fixes Abs :: "'a set  'b::po"
  assumes type: "type_definition Rep Abs {S. ideal S}"
  assumes below: "x y. x  y  Rep x  Rep y"
  shows "OFCLASS('b, cpo_class)"
  by standard (rule exI, erule typedef_ideal_lub [OF type below])


interpretation below: preorder "below :: 'a::po  'a  bool"
apply unfold_locales
apply (rule below_refl)
apply (erule (1) below_trans)

subsection ‹Lemmas about least upper bounds›

lemma is_ub_thelub_ex: "u. S <<| u; x  S  x  lub S"
apply (erule exE, drule is_lub_lub)
apply (drule is_lubD1)
apply (erule (1) is_ubD)

lemma is_lub_thelub_ex: "u. S <<| u; S <| x  lub S  x"
by (erule exE, drule is_lub_lub, erule is_lubD2)

subsection ‹Locale for ideal completion›

hide_const (open) Filter.principal

locale ideal_completion = preorder +
  fixes principal :: "'a::type  'b::cpo"
  fixes rep :: "'b::cpo  'a::type set"
  assumes ideal_rep: "x. ideal (rep x)"
  assumes rep_lub: "Y. chain Y  rep (i. Y i) = (i. rep (Y i))"
  assumes rep_principal: "a. rep (principal a) = {b. b  a}"
  assumes belowI: "x y. rep x  rep y  x  y"
  assumes countable: "f::'a  nat. inj f"

lemma rep_mono: "x  y  rep x  rep y"
apply (frule bin_chain)
apply (drule rep_lub)
apply (simp only: lub_eqI [OF is_lub_bin_chain])
apply (rule subsetI, rule UN_I [where a=0], simp_all)

lemma below_def: "x  y  rep x  rep y"
by (rule iffI [OF rep_mono belowI])

lemma principal_below_iff_mem_rep: "principal a  x  a  rep x"
unfolding below_def rep_principal
by (auto intro: r_refl elim: idealD3 [OF ideal_rep])

lemma principal_below_iff [simp]: "principal a  principal b  a  b"
by (simp add: principal_below_iff_mem_rep rep_principal)

lemma principal_eq_iff: "principal a = principal b  a  b  b  a"
unfolding po_eq_conv [where 'a='b] principal_below_iff ..

lemma eq_iff: "x = y  rep x = rep y"
unfolding po_eq_conv below_def by auto

lemma principal_mono: "a  b  principal a  principal b"
by (simp only: principal_below_iff)

lemma ch2ch_principal [simp]:
  "i. Y i  Y (Suc i)  chain (λi. principal (Y i))"
by (simp add: chainI principal_mono)

subsubsection ‹Principal ideals approximate all elements›

lemma compact_principal [simp]: "compact (principal a)"
by (rule compactI2, simp add: principal_below_iff_mem_rep rep_lub)

text ‹Construct a chain whose lub is the same as a given ideal›

lemma obtain_principal_chain:
  obtains Y where "i. Y i  Y (Suc i)" and "x = (i. principal (Y i))"
proof -
  obtain count :: "'a  nat" where inj: "inj count"
    using countable ..
  define enum where "enum i = (THE a. count a = i)" for i
  have enum_count [simp]: "x. enum (count x) = x"
    unfolding enum_def by (simp add: inj_eq [OF inj])
  define a where "a = (LEAST i. enum i  rep x)"
  define b where "b i = (LEAST j. enum j  rep x  ¬ enum j  enum i)" for i
  define c where "c i j = (LEAST k. enum k  rep x  enum i  enum k  enum j  enum k)" for i j
  define P where "P i  (j. enum j  rep x  ¬ enum j  enum i)" for i
  define X where "X = rec_nat a (λn i. if P i then c i (b i) else i)"
  have X_0: "X 0 = a" unfolding X_def by simp
  have X_Suc: "n. X (Suc n) = (if P (X n) then c (X n) (b (X n)) else X n)"
    unfolding X_def by simp
  have a_mem: "enum a  rep x"
    unfolding a_def
    apply (rule LeastI_ex)
    apply (insert ideal_rep [of x])
    apply (drule idealD1)
    apply (clarify)
    subgoal for a by (rule exI [where x="count a"]) simp
  have b: "i. P i  enum i  rep x
     enum (b i)  rep x  ¬ enum (b i)  enum i"
    unfolding P_def b_def by (erule LeastI2_ex, simp)
  have c: "i j. enum i  rep x  enum j  rep x
     enum (c i j)  rep x  enum i  enum (c i j)  enum j  enum (c i j)"
    unfolding c_def
    apply (drule (1) idealD2 [OF ideal_rep], clarify)
    subgoal for … z by (rule LeastI2 [where a="count z"], simp, simp)
  have X_mem: "enum (X n)  rep x" for n
  proof (induct n)
    case 0
    then show ?case by (simp add: X_0 a_mem)
    case (Suc n)
    with b c show ?case by (auto simp: X_Suc)
  have X_chain: "n. enum (X n)  enum (X (Suc n))"
    apply (clarsimp simp add: X_Suc r_refl)
    apply (simp add: b c X_mem)
  have less_b: "n i. n < b i  enum n  rep x  enum n  enum i"
    unfolding b_def by (drule not_less_Least, simp)
  have X_covers: "kn. enum k  rep x  enum k  enum (X n)" for n
  proof (induct n)
    case 0
    then show ?case
      apply (clarsimp simp add: X_0 a_def)
      apply (drule Least_le [where k=0], simp add: r_refl)
    case (Suc n)
    then show ?case
      apply clarsimp
      apply (erule le_SucE)
       apply (rule r_trans [OF _ X_chain], simp)
      apply (cases "P (X n)", simp add: X_Suc)
       apply (rule linorder_cases [where x="b (X n)" and y="Suc n"])
         apply (simp only: less_Suc_eq_le)
         apply (drule spec, drule (1) mp, simp add: b X_mem)
        apply (simp add: c X_mem)
       apply (drule (1) less_b)
       apply (erule r_trans)
       apply (simp add: b c X_mem)
      apply (simp add: X_Suc)
      apply (simp add: P_def)
  have 1: "i. enum (X i)  enum (X (Suc i))"
    by (simp add: X_chain)
  have "x = (n. principal (enum (X n)))"
    apply (simp add: eq_iff rep_lub 1 rep_principal)
    apply auto
    subgoal for a
      apply (subgoal_tac "i. a = enum i", erule exE)
       apply (rule_tac x=i in exI, simp add: X_covers)
      apply (rule_tac x="count a" in exI, simp)
      apply (erule idealD3 [OF ideal_rep])
      apply (rule X_mem)
  with 1 show ?thesis ..

lemma principal_induct:
  assumes adm: "adm P"
  assumes P: "a. P (principal a)"
  shows "P x"
apply (rule obtain_principal_chain [of x])
apply (simp add: admD [OF adm] P)

lemma compact_imp_principal: "compact x  a. x = principal a"
apply (rule obtain_principal_chain [of x])
apply (drule adm_compact_neq [OF _ cont_id])
apply (subgoal_tac "chain (λi. principal (Y i))")
apply (drule (2) admD2, fast, simp)

subsection ‹Defining functions in terms of basis elements›

  extension :: "('a::type  'c::cpo)  'b  'c" where
  "extension = (λf. (Λ x. lub (f ` rep x)))"

lemma extension_lemma:
  fixes f :: "'a::type  'c::cpo"
  assumes f_mono: "a b. a  b  f a  f b"
  shows "u. f ` rep x <<| u"
proof -
  obtain Y where Y: "i. Y i  Y (Suc i)"
  and x: "x = (i. principal (Y i))"
    by (rule obtain_principal_chain [of x])
  have chain: "chain (λi. f (Y i))"
    by (rule chainI, simp add: f_mono Y)
  have rep_x: "rep x = (n. {a. a  Y n})"
    by (simp add: x rep_lub Y rep_principal)
  have "f ` rep x <<| (n. f (Y n))"
    apply (rule is_lubI)
     apply (rule ub_imageI)
    subgoal for a
      apply (clarsimp simp add: rep_x)
      apply (drule f_mono)
      apply (erule below_lub [OF chain])
    apply (rule lub_below [OF chain])
    subgoal for … n
      apply (drule ub_imageD [where x="Y n"])
       apply (simp add: rep_x, fast intro: r_refl)
      apply assumption
  then show ?thesis ..

lemma extension_beta:
  fixes f :: "'a::type  'c::cpo"
  assumes f_mono: "a b. a  b  f a  f b"
  shows "extension fx = lub (f ` rep x)"
unfolding extension_def
proof (rule beta_cfun)
  have lub: "x. u. f ` rep x <<| u"
    using f_mono by (rule extension_lemma)
  show cont: "cont (λx. lub (f ` rep x))"
    apply (rule contI2)
     apply (rule monofunI)
     apply (rule is_lub_thelub_ex [OF lub ub_imageI])
     apply (rule is_ub_thelub_ex [OF lub imageI])
     apply (erule (1) subsetD [OF rep_mono])
    apply (rule is_lub_thelub_ex [OF lub ub_imageI])
    apply (simp add: rep_lub, clarify)
    apply (erule rev_below_trans [OF is_ub_thelub])
    apply (erule is_ub_thelub_ex [OF lub imageI])

lemma extension_principal:
  fixes f :: "'a::type  'c::cpo"
  assumes f_mono: "a b. a  b  f a  f b"
  shows "extension f(principal a) = f a"
apply (subst extension_beta, erule f_mono)
apply (subst rep_principal)
apply (rule lub_eqI)
apply (rule is_lub_maximal)
apply (rule ub_imageI)
apply (simp add: f_mono)
apply (rule imageI)
apply (simp add: r_refl)

lemma extension_mono:
  assumes f_mono: "a b. a  b  f a  f b"
  assumes g_mono: "a b. a  b  g a  g b"
  assumes below: "a. f a  g a"
  shows "extension f  extension g"
  apply (rule cfun_belowI)
  apply (simp only: extension_beta f_mono g_mono)
  apply (rule is_lub_thelub_ex)
   apply (rule extension_lemma, erule f_mono)
  apply (rule ub_imageI)
  subgoal for x a
    apply (rule below_trans [OF below])
    apply (rule is_ub_thelub_ex)
     apply (rule extension_lemma, erule g_mono)
    apply (erule imageI)

lemma cont_extension:
  assumes f_mono: "a b x. a  b  f x a  f x b"
  assumes f_cont: "a. cont (λx. f x a)"
  shows "cont (λx. extension (λa. f x a))"
 apply (rule contI2)
  apply (rule monofunI)
  apply (rule extension_mono, erule f_mono, erule f_mono)
  apply (erule cont2monofunE [OF f_cont])
 apply (rule cfun_belowI)
 apply (rule principal_induct, simp)
 apply (simp only: contlub_cfun_fun)
 apply (simp only: extension_principal f_mono)
 apply (simp add: cont2contlubE [OF f_cont])


lemma (in preorder) typedef_ideal_completion:
  fixes Abs :: "'a set  'b::cpo"
  assumes type: "type_definition Rep Abs {S. ideal S}"
  assumes below: "x y. x  y  Rep x  Rep y"
  assumes principal: "a. principal a = Abs {b. b  a}"
  assumes countable: "f::'a  nat. inj f"
  shows "ideal_completion r principal Rep"
  interpret type_definition Rep Abs "{S. ideal S}" by fact
  fix a b :: 'a and x y :: 'b and Y :: "nat  'b"
  show "ideal (Rep x)"
    using Rep [of x] by simp
  show "chain Y  Rep (i. Y i) = (i. Rep (Y i))"
    using type below by (rule typedef_ideal_rep_lub)
  show "Rep (principal a) = {b. b  a}"
    by (simp add: principal Abs_inverse ideal_principal)
  show "Rep x  Rep y  x  y"
    by (simp only: below)
  show "f::'a  nat. inj f"
    by (rule countable)