(* Title: HOL/Partial_Function.thy
Author: Alexander Krauss, TU Muenchen
*)
header {* Partial Function Definitions *}
theory Partial_Function
imports Complete_Partial_Order Option
uses
"Tools/Function/function_lib.ML"
"Tools/Function/partial_function.ML"
begin
setup Partial_Function.setup
subsection {* Axiomatic setup *}
text {* This techical locale constains the requirements for function
definitions with ccpo fixed points. *}
definition "fun_ord ord f g \<longleftrightarrow> (\<forall>x. ord (f x) (g x))"
definition "fun_lub L A = (\<lambda>x. L {y. \<exists>f\<in>A. y = f x})"
definition "img_ord f ord = (\<lambda>x y. ord (f x) (f y))"
definition "img_lub f g Lub = (\<lambda>A. g (Lub (f ` A)))"
lemma call_mono[partial_function_mono]: "monotone (fun_ord ord) ord (\<lambda>f. f t)"
by (rule monotoneI) (auto simp: fun_ord_def)
lemma let_mono[partial_function_mono]:
"(\<And>x. monotone orda ordb (\<lambda>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 = (\<lambda>x y. R x y \<and> \<not> R y x)"
locale partial_function_definitions =
fixes leq :: "'a \<Rightarrow> 'a \<Rightarrow> bool"
fixes lub :: "'a set \<Rightarrow> 'a"
assumes leq_refl: "leq x x"
assumes leq_trans: "leq x y \<Longrightarrow> leq y z \<Longrightarrow> leq x z"
assumes leq_antisym: "leq x y \<Longrightarrow> leq y x \<Longrightarrow> x = y"
assumes lub_upper: "chain leq A \<Longrightarrow> x \<in> A \<Longrightarrow> leq x (lub A)"
assumes lub_least: "chain leq A \<Longrightarrow> (\<And>x. x \<in> A \<Longrightarrow> leq x z) \<Longrightarrow> 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
{ fix A a assume A: "chain ?ordf A"
have "chain ord {y. \<exists>f\<in>A. y = f a}" (is "chain ord ?C")
proof (rule chainI)
fix x y assume "x \<in> ?C" "y \<in> ?C"
then obtain f g where fg: "f \<in> A" "g \<in> A"
and [simp]: "x = f a" "y = g a" by blast
from chainD[OF A fg]
show "ord x y \<or> ord y x" unfolding fun_ord_def by auto
qed }
note chain_fun = this
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!: ext dest: leq_antisym)
next
fix A f assume f: "f \<in> 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 \<Rightarrow> 'a) set" and g :: "'b \<Rightarrow> 'a"
assume A: "chain ?ordf A" and g: "\<And>f. f \<in> A \<Longrightarrow> ?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 ord (mk_less ord) lb"
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: "\<And>x y. f x = f y \<Longrightarrow> x = y"
assumes inv: "\<And>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 \<in> A"
then have "chain ord (f ` A)" "f x \<in> 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: "\<And>z. z \<in> A \<Longrightarrow> ?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 \<equiv> fun_ord leq"
abbreviation "lub_fun \<equiv> fun_lub lub"
abbreviation "fixp_fun == ccpo.fixp le_fun lub_fun"
abbreviation "mono_body \<equiv> monotone le_fun leq"
text {* Interpret manually, to avoid flooding everything with facts about
orders *}
lemma ccpo: "class.ccpo le_fun (mk_less le_fun) lub_fun"
apply (rule ccpo)
apply (rule partial_function_lift)
apply (rule partial_function_definitions_axioms)
done
text {* The crucial fixed-point theorem *}
lemma mono_body_fixp:
"(\<And>x. mono_body (\<lambda>f. F f x)) \<Longrightarrow> fixp_fun F = F (fixp_fun F)"
by (rule ccpo.fixp_unfold[OF ccpo]) (auto simp: monotone_def fun_ord_def)
text {* Version with curry/uncurry combinators, to be used by package *}
lemma fixp_rule_uc:
fixes F :: "'c \<Rightarrow> 'c" and
U :: "'c \<Rightarrow> 'b \<Rightarrow> 'a" and
C :: "('b \<Rightarrow> 'a) \<Rightarrow> 'c"
assumes mono: "\<And>x. mono_body (\<lambda>f. U (F (C f)) x)"
assumes eq: "f \<equiv> C (fixp_fun (\<lambda>f. U (F (C f))))"
assumes inverse: "\<And>f. C (U f) = f"
shows "f = F f"
proof -
have "f = C (fixp_fun (\<lambda>f. U (F (C f))))" by (simp add: eq)
also have "... = C (U (F (C (fixp_fun (\<lambda>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 (\<lambda>f. U (F (C f)))))" by (rule inverse)
also have "... = F f" by (simp add: eq)
finally show "f = F f" .
qed
text {* Rules for @{term mono_body}: *}
lemma const_mono[partial_function_mono]: "monotone ord leq (\<lambda>f. c)"
by (rule monotoneI) (rule leq_refl)
end
subsection {* Flat interpretation: tailrec and option *}
definition
"flat_ord b x y \<longleftrightarrow> x = b \<or> x = y"
definition
"flat_lub b A = (if A \<subseteq> {b} then b else (THE x. x \<in> 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 \<in> 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 \<noteq> 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: "\<And>x. x \<in> A \<Longrightarrow> flat_ord b x z"
show "flat_ord b (flat_lub b A) z"
proof cases
assume "A \<subseteq> {b}"
thus ?thesis
by (auto simp: flat_lub_def flat_ord_def)
next
assume nb: "\<not> A \<subseteq> {b}"
then obtain y where y: "y \<in> A" "y \<noteq> 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)
interpretation tailrec!:
partial_function_definitions "flat_ord undefined" "flat_lub undefined"
by (rule flat_interpretation)
interpretation option!:
partial_function_definitions "flat_ord None" "flat_lub None"
by (rule flat_interpretation)
declaration {* Partial_Function.init "tailrec" @{term tailrec.fixp_fun}
@{term tailrec.mono_body} @{thm tailrec.fixp_rule_uc} *}
declaration {* Partial_Function.init "option" @{term option.fixp_fun}
@{term option.mono_body} @{thm option.fixp_rule_uc} *}
abbreviation "option_ord \<equiv> flat_ord None"
abbreviation "mono_option \<equiv> monotone (fun_ord option_ord) option_ord"
lemma bind_mono[partial_function_mono]:
assumes mf: "mono_option B" and mg: "\<And>y. mono_option (\<lambda>f. C y f)"
shows "mono_option (\<lambda>f. Option.bind (B f) (\<lambda>y. C y f))"
proof (rule monotoneI)
fix f g :: "'a \<Rightarrow> '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) (\<lambda>y. C y f)) (Option.bind (B g) (\<lambda>y. C y f))"
unfolding flat_ord_def by auto
also from mg
have "\<And>y'. option_ord (C y' f) (C y' g)"
by (rule monotoneD) (rule fg)
then have "option_ord (Option.bind (B g) (\<lambda>y'. C y' f)) (Option.bind (B g) (\<lambda>y'. C y' g))"
unfolding flat_ord_def by (cases "B g") auto
finally (option.leq_trans)
show "option_ord (Option.bind (B f) (\<lambda>y. C y f)) (Option.bind (B g) (\<lambda>y'. C y' g))" .
qed
hide_const (open) chain
end