(* Title: HOL/Partial_Function.thy
Author: Alexander Krauss, TU Muenchen
*)
section \<open>Partial Function Definitions\<close>
theory Partial_Function
imports Complete_Partial_Order Option
keywords "partial_function" :: thy_defn
begin
named_theorems partial_function_mono "monotonicity rules for partial function definitions"
ML_file \<open>Tools/Function/partial_function.ML\<close>
lemma (in ccpo) in_chain_finite:
assumes "Complete_Partial_Order.chain (\) A" "finite A" "A \ {}"
shows "\A \ A"
using assms(2,1,3)
proof induction
case empty thus ?case by simp
next
case (insert x A)
note chain = \<open>Complete_Partial_Order.chain (\<le>) (insert x A)\<close>
show ?case
proof(cases "A = {}")
case True thus ?thesis by simp
next
case False
from chain have chain': "Complete_Partial_Order.chain (\) A"
by(rule chain_subset) blast
hence "\A \ A" using False by(rule insert.IH)
show ?thesis
proof(cases "x \ \A")
case True
have "\(insert x A) \ \A" using chain
by(rule ccpo_Sup_least)(auto simp add: True intro: ccpo_Sup_upper[OF chain'])
hence "\(insert x A) = \A"
by(rule antisym)(blast intro: ccpo_Sup_upper[OF chain] ccpo_Sup_least[OF chain'])
with \<open>\<Squnion>A \<in> A\<close> show ?thesis by simp
next
case False
with chainD[OF chain, of x "\A"] \\A \ A\
have "\(insert x A) = x"
by(auto intro: antisym ccpo_Sup_least[OF chain] order_trans[OF ccpo_Sup_upper[OF chain']] ccpo_Sup_upper[OF chain])
thus ?thesis by simp
qed
qed
qed
lemma (in ccpo) admissible_chfin:
"(\S. Complete_Partial_Order.chain (\) S \ finite S)
\<Longrightarrow> ccpo.admissible Sup (\<le>) P"
using in_chain_finite by (blast intro: ccpo.admissibleI)
subsection \<open>Axiomatic setup\<close>
text \<open>This techical locale constains the requirements for function
definitions with ccpo fixed points.\<close>
definition "fun_ord ord f g \ (\x. ord (f x) (g x))"
definition "fun_lub L A = (\x. L {y. \f\A. y = f x})"
definition "img_ord f ord = (\x y. ord (f x) (f y))"
definition "img_lub f g Lub = (\A. g (Lub (f ` A)))"
lemma chain_fun:
assumes A: "chain (fun_ord ord) A"
shows "chain ord {y. \f\A. y = f a}" (is "chain ord ?C")
proof (rule chainI)
fix x y assume "x \ ?C" "y \ ?C"
then obtain f g where fg: "f \ A" "g \ A"
and [simp]: "x = f a" "y = g a" by blast
from chainD[OF A fg]
show "ord x y \ ord y x" unfolding fun_ord_def by auto
qed
lemma call_mono[partial_function_mono]: "monotone (fun_ord ord) ord (\f. f t)"
by (rule monotoneI) (auto simp: fun_ord_def)
lemma let_mono[partial_function_mono]:
"(\x. monotone orda ordb (\f. b f x))
\<Longrightarrow> monotone orda ordb (\<lambda>f. Let t (b f))"
by (simp add: Let_def)
lemma if_mono[partial_function_mono]: "monotone orda ordb F
\<Longrightarrow> monotone orda ordb G
\<Longrightarrow> monotone orda ordb (\<lambda>f. if c then F f else G f)"
unfolding monotone_def by simp
definition "mk_less R = (\x y. R x y \ \ R y x)"
locale partial_function_definitions =
fixes leq :: "'a \ 'a \ bool"
fixes lub :: "'a set \ 'a"
assumes leq_refl: "leq x x"
assumes leq_trans: "leq x y \ leq y z \ leq x z"
assumes leq_antisym: "leq x y \ leq y x \ x = y"
assumes lub_upper: "chain leq A \ x \ A \ leq x (lub A)"
assumes lub_least: "chain leq A \ (\x. x \ A \ leq x z) \ leq (lub A) z"
lemma partial_function_lift:
assumes "partial_function_definitions ord lb"
shows "partial_function_definitions (fun_ord ord) (fun_lub lb)" (is "partial_function_definitions ?ordf ?lubf")
proof -
interpret partial_function_definitions ord lb by fact
show ?thesis
proof
fix x show "?ordf x x"
unfolding fun_ord_def by (auto simp: leq_refl)
next
fix x y z assume "?ordf x y" "?ordf y z"
thus "?ordf x z" unfolding fun_ord_def
by (force dest: leq_trans)
next
fix x y assume "?ordf x y" "?ordf y x"
thus "x = y" unfolding fun_ord_def
by (force intro!: dest: leq_antisym)
next
fix A f assume f: "f \ A" and A: "chain ?ordf A"
thus "?ordf f (?lubf A)"
unfolding fun_lub_def fun_ord_def
by (blast intro: lub_upper chain_fun[OF A] f)
next
fix A :: "('b \ 'a) set" and g :: "'b \ 'a"
assume A: "chain ?ordf A" and g: "\f. f \ A \ ?ordf f g"
show "?ordf (?lubf A) g" unfolding fun_lub_def fun_ord_def
by (blast intro: lub_least chain_fun[OF A] dest: g[unfolded fun_ord_def])
qed
qed
lemma ccpo: assumes "partial_function_definitions ord lb"
shows "class.ccpo lb ord (mk_less ord)"
using assms unfolding partial_function_definitions_def mk_less_def
by unfold_locales blast+
lemma partial_function_image:
assumes "partial_function_definitions ord Lub"
assumes inj: "\x y. f x = f y \ x = y"
assumes inv: "\x. f (g x) = x"
shows "partial_function_definitions (img_ord f ord) (img_lub f g Lub)"
proof -
let ?iord = "img_ord f ord"
let ?ilub = "img_lub f g Lub"
interpret partial_function_definitions ord Lub by fact
show ?thesis
proof
fix A x assume "chain ?iord A" "x \ A"
then have "chain ord (f ` A)" "f x \ f ` A"
by (auto simp: img_ord_def intro: chainI dest: chainD)
thus "?iord x (?ilub A)"
unfolding inv img_lub_def img_ord_def by (rule lub_upper)
next
fix A x assume "chain ?iord A"
and 1: "\z. z \ A \ ?iord z x"
then have "chain ord (f ` A)"
by (auto simp: img_ord_def intro: chainI dest: chainD)
thus "?iord (?ilub A) x"
unfolding inv img_lub_def img_ord_def
by (rule lub_least) (auto dest: 1[unfolded img_ord_def])
qed (auto simp: img_ord_def intro: leq_refl dest: leq_trans leq_antisym inj)
qed
context partial_function_definitions
begin
abbreviation "le_fun \ fun_ord leq"
abbreviation "lub_fun \ fun_lub lub"
abbreviation "fixp_fun \ ccpo.fixp lub_fun le_fun"
abbreviation "mono_body \ monotone le_fun leq"
abbreviation "admissible \ ccpo.admissible lub_fun le_fun"
text \<open>Interpret manually, to avoid flooding everything with facts about
orders\<close>
lemma ccpo: "class.ccpo lub_fun le_fun (mk_less le_fun)"
apply (rule ccpo)
apply (rule partial_function_lift)
apply (rule partial_function_definitions_axioms)
done
text \<open>The crucial fixed-point theorem\<close>
lemma mono_body_fixp:
"(\x. mono_body (\f. F f x)) \ fixp_fun F = F (fixp_fun F)"
by (rule ccpo.fixp_unfold[OF ccpo]) (auto simp: monotone_def fun_ord_def)
text \<open>Version with curry/uncurry combinators, to be used by package\<close>
lemma fixp_rule_uc:
fixes F :: "'c \ 'c" and
U :: "'c \ 'b \ 'a" and
C :: "('b \ 'a) \ 'c"
assumes mono: "\x. mono_body (\f. U (F (C f)) x)"
assumes eq: "f \ C (fixp_fun (\f. U (F (C f))))"
assumes inverse: "\f. C (U f) = f"
shows "f = F f"
proof -
have "f = C (fixp_fun (\f. U (F (C f))))" by (simp add: eq)
also have "... = C (U (F (C (fixp_fun (\f. U (F (C f)))))))"
by (subst mono_body_fixp[of "%f. U (F (C f))", OF mono]) (rule refl)
also have "... = F (C (fixp_fun (\f. U (F (C f)))))" by (rule inverse)
also have "... = F f" by (simp add: eq)
finally show "f = F f" .
qed
text \<open>Fixpoint induction rule\<close>
lemma fixp_induct_uc:
fixes F :: "'c \ 'c"
and U :: "'c \ 'b \ 'a"
and C :: "('b \ 'a) \ 'c"
and P :: "('b \ 'a) \ bool"
assumes mono: "\x. mono_body (\f. U (F (C f)) x)"
and eq: "f \ C (fixp_fun (\f. U (F (C f))))"
and inverse: "\f. U (C f) = f"
and adm: "ccpo.admissible lub_fun le_fun P"
and bot: "P (\_. lub {})"
and step: "\f. P (U f) \ P (U (F f))"
shows "P (U f)"
unfolding eq inverse
apply (rule ccpo.fixp_induct[OF ccpo adm])
apply (insert mono, auto simp: monotone_def fun_ord_def bot fun_lub_def)[2]
apply (rule_tac f5="C x" in step)
apply (simp add: inverse)
done
text \<open>Rules for \<^term>\<open>mono_body\<close>:\<close>
lemma const_mono[partial_function_mono]: "monotone ord leq (\f. c)"
by (rule monotoneI) (rule leq_refl)
end
subsection \<open>Flat interpretation: tailrec and option\<close>
definition
"flat_ord b x y \ x = b \ x = y"
definition
"flat_lub b A = (if A \ {b} then b else (THE x. x \ A - {b}))"
lemma flat_interpretation:
"partial_function_definitions (flat_ord b) (flat_lub b)"
proof
fix A x assume 1: "chain (flat_ord b) A" "x \ A"
show "flat_ord b x (flat_lub b A)"
proof cases
assume "x = b"
thus ?thesis by (simp add: flat_ord_def)
next
assume "x \ b"
with 1 have "A - {b} = {x}"
by (auto elim: chainE simp: flat_ord_def)
then have "flat_lub b A = x"
by (auto simp: flat_lub_def)
thus ?thesis by (auto simp: flat_ord_def)
qed
next
fix A z assume A: "chain (flat_ord b) A"
and z: "\x. x \ A \ flat_ord b x z"
show "flat_ord b (flat_lub b A) z"
proof cases
assume "A \ {b}"
thus ?thesis
by (auto simp: flat_lub_def flat_ord_def)
next
assume nb: "\ A \ {b}"
then obtain y where y: "y \ A" "y \ b" by auto
with A have "A - {b} = {y}"
by (auto elim: chainE simp: flat_ord_def)
with nb have "flat_lub b A = y"
by (auto simp: flat_lub_def)
with z y show ?thesis by auto
qed
qed (auto simp: flat_ord_def)
lemma flat_ordI: "(x \ a \ x = y) \ flat_ord a x y"
by(auto simp add: flat_ord_def)
lemma flat_ord_antisym: "\ flat_ord a x y; flat_ord a y x \ \ x = y"
by(auto simp add: flat_ord_def)
lemma antisymp_flat_ord: "antisymp (flat_ord a)"
by(rule antisympI)(auto dest: flat_ord_antisym)
interpretation tailrec:
partial_function_definitions "flat_ord undefined" "flat_lub undefined"
rewrites "flat_lub undefined {} \ undefined"
by (rule flat_interpretation)(simp add: flat_lub_def)
interpretation option:
partial_function_definitions "flat_ord None" "flat_lub None"
rewrites "flat_lub None {} \ None"
by (rule flat_interpretation)(simp add: flat_lub_def)
abbreviation "tailrec_ord \ flat_ord undefined"
abbreviation "mono_tailrec \ monotone (fun_ord tailrec_ord) tailrec_ord"
lemma tailrec_admissible:
"ccpo.admissible (fun_lub (flat_lub c)) (fun_ord (flat_ord c))
(\<lambda>a. \<forall>x. a x \<noteq> c \<longrightarrow> P x (a x))"
proof(intro ccpo.admissibleI strip)
fix A x
assume chain: "Complete_Partial_Order.chain (fun_ord (flat_ord c)) A"
and P [rule_format]: "\f\A. \x. f x \ c \ P x (f x)"
and defined: "fun_lub (flat_lub c) A x \ c"
from defined obtain f where f: "f \ A" "f x \ c"
by(auto simp add: fun_lub_def flat_lub_def split: if_split_asm)
hence "P x (f x)" by(rule P)
moreover from chain f have "\f' \ A. f' x = c \ f' x = f x"
by(auto 4 4 simp add: Complete_Partial_Order.chain_def flat_ord_def fun_ord_def)
hence "fun_lub (flat_lub c) A x = f x"
using f by(auto simp add: fun_lub_def flat_lub_def)
ultimately show "P x (fun_lub (flat_lub c) A x)" by simp
qed
lemma fixp_induct_tailrec:
fixes F :: "'c \ 'c" and
U :: "'c \ 'b \ 'a" and
C :: "('b \ 'a) \ 'c" and
P :: "'b \ 'a \ bool" and
x :: "'b"
assumes mono: "\x. monotone (fun_ord (flat_ord c)) (flat_ord c) (\f. U (F (C f)) x)"
assumes eq: "f \ C (ccpo.fixp (fun_lub (flat_lub c)) (fun_ord (flat_ord c)) (\f. U (F (C f))))"
assumes inverse2: "\f. U (C f) = f"
assumes step: "\f x y. (\x y. U f x = y \ y \ c \ P x y) \ U (F f) x = y \ y \ c \ P x y"
assumes result: "U f x = y"
assumes defined: "y \ c"
shows "P x y"
proof -
have "\x y. U f x = y \ y \ c \ P x y"
by(rule partial_function_definitions.fixp_induct_uc[OF flat_interpretation, of _ U F C, OF mono eq inverse2])
(auto intro: step tailrec_admissible simp add: fun_lub_def flat_lub_def)
thus ?thesis using result defined by blast
qed
lemma admissible_image:
assumes pfun: "partial_function_definitions le lub"
assumes adm: "ccpo.admissible lub le (P \ g)"
assumes inj: "\x y. f x = f y \ x = y"
assumes inv: "\x. f (g x) = x"
shows "ccpo.admissible (img_lub f g lub) (img_ord f le) P"
proof (rule ccpo.admissibleI)
fix A assume "chain (img_ord f le) A"
then have ch': "chain le (f ` A)"
by (auto simp: img_ord_def intro: chainI dest: chainD)
assume "A \ {}"
assume P_A: "\x\A. P x"
have "(P \ g) (lub (f ` A))" using adm ch'
proof (rule ccpo.admissibleD)
fix x assume "x \ f ` A"
with P_A show "(P \ g) x" by (auto simp: inj[OF inv])
qed(simp add: \<open>A \<noteq> {}\<close>)
thus "P (img_lub f g lub A)" unfolding img_lub_def by simp
qed
lemma admissible_fun:
assumes pfun: "partial_function_definitions le lub"
assumes adm: "\x. ccpo.admissible lub le (Q x)"
shows "ccpo.admissible (fun_lub lub) (fun_ord le) (\f. \x. Q x (f x))"
proof (rule ccpo.admissibleI)
fix A :: "('b \ 'a) set"
assume Q: "\f\A. \x. Q x (f x)"
assume ch: "chain (fun_ord le) A"
assume "A \ {}"
hence non_empty: "\a. {y. \f\A. y = f a} \ {}" by auto
show "\x. Q x (fun_lub lub A x)"
unfolding fun_lub_def
by (rule allI, rule ccpo.admissibleD[OF adm chain_fun[OF ch] non_empty])
(auto simp: Q)
qed
abbreviation "option_ord \ flat_ord None"
abbreviation "mono_option \ monotone (fun_ord option_ord) option_ord"
lemma bind_mono[partial_function_mono]:
assumes mf: "mono_option B" and mg: "\y. mono_option (\f. C y f)"
shows "mono_option (\f. Option.bind (B f) (\y. C y f))"
proof (rule monotoneI)
fix f g :: "'a \ 'b option" assume fg: "fun_ord option_ord f g"
with mf
have "option_ord (B f) (B g)" by (rule monotoneD[of _ _ _ f g])
then have "option_ord (Option.bind (B f) (\y. C y f)) (Option.bind (B g) (\y. C y f))"
unfolding flat_ord_def by auto
also from mg
have "\y'. option_ord (C y' f) (C y' g)"
by (rule monotoneD) (rule fg)
then have "option_ord (Option.bind (B g) (\y'. C y' f)) (Option.bind (B g) (\y'. C y' g))"
unfolding flat_ord_def by (cases "B g") auto
finally (option.leq_trans)
show "option_ord (Option.bind (B f) (\y. C y f)) (Option.bind (B g) (\y'. C y' g))" .
qed
lemma flat_lub_in_chain:
assumes ch: "chain (flat_ord b) A "
assumes lub: "flat_lub b A = a"
shows "a = b \ a \ A"
proof (cases "A \ {b}")
case True
then have "flat_lub b A = b" unfolding flat_lub_def by simp
with lub show ?thesis by simp
next
case False
then obtain c where "c \ A" and "c \ b" by auto
{ fix z assume "z \ A"
from chainD[OF ch \<open>c \<in> A\<close> this] have "z = c \<or> z = b"
unfolding flat_ord_def using \<open>c \<noteq> b\<close> by auto }
with False have "A - {b} = {c}" by auto
with False have "flat_lub b A = c" by (auto simp: flat_lub_def)
with \<open>c \<in> A\<close> lub show ?thesis by simp
qed
lemma option_admissible: "option.admissible (%(f::'a \ 'b option).
(\<forall>x y. f x = Some y \<longrightarrow> P x y))"
proof (rule ccpo.admissibleI)
fix A :: "('a \ 'b option) set"
assume ch: "chain option.le_fun A"
and IH: "\f\A. \x y. f x = Some y \ P x y"
from ch have ch': "\x. chain option_ord {y. \f\A. y = f x}" by (rule chain_fun)
show "\x y. option.lub_fun A x = Some y \ P x y"
proof (intro allI impI)
fix x y assume "option.lub_fun A x = Some y"
from flat_lub_in_chain[OF ch' this[unfolded fun_lub_def]]
have "Some y \ {y. \f\A. y = f x}" by simp
then have "\f\A. f x = Some y" by auto
with IH show "P x y" by auto
qed
qed
lemma fixp_induct_option:
fixes F :: "'c \ 'c" and
U :: "'c \ 'b \ 'a option" and
C :: "('b \ 'a option) \ 'c" and
P :: "'b \ 'a \ bool"
assumes mono: "\x. mono_option (\f. U (F (C f)) x)"
assumes eq: "f \ C (ccpo.fixp (fun_lub (flat_lub None)) (fun_ord option_ord) (\f. U (F (C f))))"
assumes inverse2: "\f. U (C f) = f"
assumes step: "\f x y. (\x y. U f x = Some y \ P x y) \ U (F f) x = Some y \ P x y"
assumes defined: "U f x = Some y"
shows "P x y"
using step defined option.fixp_induct_uc[of U F C, OF mono eq inverse2 option_admissible]
unfolding fun_lub_def flat_lub_def by(auto 9 2)
declaration \<open>Partial_Function.init "tailrec" \<^term>\<open>tailrec.fixp_fun\<close>
\<^term>\<open>tailrec.mono_body\<close> @{thm tailrec.fixp_rule_uc} @{thm tailrec.fixp_induct_uc}
(SOME @{thm fixp_induct_tailrec[where c = undefined]})\<close>
declaration \<open>Partial_Function.init "option" \<^term>\<open>option.fixp_fun\<close>
\<^term>\<open>option.mono_body\<close> @{thm option.fixp_rule_uc} @{thm option.fixp_induct_uc}
(SOME @{thm fixp_induct_option})\<close>
hide_const (open) chain
end
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