src/HOL/Library/Quotient.thy
author wenzelm
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tuned;
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(*  Title:      HOL/Library/Quotient.thy
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    ID:         $Id$
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    Author:     Gertrud Bauer and Markus Wenzel, TU Muenchen
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*)
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header {*
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  \title{Quotients}
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  \author{Gertrud Bauer and Markus Wenzel}
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*}
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theory Quotient = Main:
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text {*
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 We introduce the notion of quotient types over equivalence relations
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 via axiomatic type classes.
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*}
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subsection {* Equivalence relations and quotient types *}
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text {*
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 \medskip Type class @{text equiv} models equivalence relations @{text
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 "\<sim> :: 'a => 'a => bool"}.
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*}
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axclass eqv < "term"
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consts
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  eqv :: "('a::eqv) => 'a => bool"    (infixl "\<sim>" 50)
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axclass equiv < eqv
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  equiv_refl [intro]: "x \<sim> x"
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  equiv_trans [trans]: "x \<sim> y ==> y \<sim> z ==> x \<sim> z"
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  equiv_sym [elim?]: "x \<sim> y ==> y \<sim> x"
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text {*
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 \medskip The quotient type @{text "'a quot"} consists of all
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 \emph{equivalence classes} over elements of the base type @{typ 'a}.
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*}
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typedef 'a quot = "{{x. a \<sim> x}| a::'a::eqv. True}"
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  by blast
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lemma quotI [intro]: "{x. a \<sim> x} \<in> quot"
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  by (unfold quot_def) blast
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lemma quotE [elim]: "R \<in> quot ==> (!!a. R = {x. a \<sim> x} ==> C) ==> C"
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  by (unfold quot_def) blast
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text {*
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 \medskip Abstracted equivalence classes are the canonical
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 representation of elements of a quotient type.
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*}
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constdefs
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  equivalence_class :: "'a::equiv => 'a quot"    ("\<lfloor>_\<rfloor>")
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  "\<lfloor>a\<rfloor> == Abs_quot {x. a \<sim> x}"
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theorem quot_exhaust: "\<exists>a. A = \<lfloor>a\<rfloor>"
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proof (cases A)
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  fix R assume R: "A = Abs_quot R"
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  assume "R \<in> quot" hence "\<exists>a. R = {x. a \<sim> x}" by blast
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  with R have "\<exists>a. A = Abs_quot {x. a \<sim> x}" by blast
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  thus ?thesis by (unfold equivalence_class_def)
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qed
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lemma quot_cases [cases type: quot]: "(!!a. A = \<lfloor>a\<rfloor> ==> C) ==> C"
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  by (insert quot_exhaust) blast
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subsection {* Equality on quotients *}
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text {*
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 Equality of canonical quotient elements coincides with the original
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 relation.
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*}
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theorem equivalence_class_eq [iff?]: "(\<lfloor>a\<rfloor> = \<lfloor>b\<rfloor>) = (a \<sim> b)"
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proof
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  assume eq: "\<lfloor>a\<rfloor> = \<lfloor>b\<rfloor>"
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  show "a \<sim> b"
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  proof -
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    from eq have "{x. a \<sim> x} = {x. b \<sim> x}"
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      by (simp only: equivalence_class_def Abs_quot_inject quotI)
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    moreover have "a \<sim> a" ..
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    ultimately have "a \<in> {x. b \<sim> x}" by blast
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    hence "b \<sim> a" by blast
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    thus ?thesis ..
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  qed
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next
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  assume ab: "a \<sim> b"
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  show "\<lfloor>a\<rfloor> = \<lfloor>b\<rfloor>"
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  proof -
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    have "{x. a \<sim> x} = {x. b \<sim> x}"
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    proof (rule Collect_cong)
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      fix x show "(a \<sim> x) = (b \<sim> x)"
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      proof
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        from ab have "b \<sim> a" ..
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        also assume "a \<sim> x"
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        finally show "b \<sim> x" .
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      next
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        note ab
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        also assume "b \<sim> x"
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        finally show "a \<sim> x" .
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      qed
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    qed
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    thus ?thesis by (simp only: equivalence_class_def)
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  qed
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qed
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subsection {* Picking representing elements *}
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constdefs
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  pick :: "'a::equiv quot => 'a"
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  "pick A == SOME a. A = \<lfloor>a\<rfloor>"
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theorem pick_equiv [intro]: "pick \<lfloor>a\<rfloor> \<sim> a"
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proof (unfold pick_def)
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  show "(SOME x. \<lfloor>a\<rfloor> = \<lfloor>x\<rfloor>) \<sim> a"
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  proof (rule someI2)
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    show "\<lfloor>a\<rfloor> = \<lfloor>a\<rfloor>" ..
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    fix x assume "\<lfloor>a\<rfloor> = \<lfloor>x\<rfloor>"
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    hence "a \<sim> x" .. thus "x \<sim> a" ..
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  qed
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qed
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theorem pick_inverse: "\<lfloor>pick A\<rfloor> = A"
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proof (cases A)
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  fix a assume a: "A = \<lfloor>a\<rfloor>"
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  hence "pick A \<sim> a" by (simp only: pick_equiv)
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  hence "\<lfloor>pick A\<rfloor> = \<lfloor>a\<rfloor>" ..
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  with a show ?thesis by simp
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qed
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text {*
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 \medskip The following rules support canonical function definitions
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 on quotient types.
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*}
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theorem cong_definition1:
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  "(!!X. f X == g (pick X)) ==>
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    (!!x x'. x \<sim> x' ==> g x = g x') ==>
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    f \<lfloor>a\<rfloor> = g a"
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proof -
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  assume cong: "!!x x'. x \<sim> x' ==> g x = g x'"
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  assume "!!X. f X == g (pick X)"
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  hence "f \<lfloor>a\<rfloor> = g (pick \<lfloor>a\<rfloor>)" by (simp only:)
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  also have "\<dots> = g a"
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  proof (rule cong)
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    show "pick \<lfloor>a\<rfloor> \<sim> a" ..
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  qed
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  finally show ?thesis .
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qed
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theorem cong_definition2:
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  "(!!X Y. f X Y == g (pick X) (pick Y)) ==>
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    (!!x x' y y'. x \<sim> x' ==> y \<sim> y' ==> g x y = g x' y') ==>
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    f \<lfloor>a\<rfloor> \<lfloor>b\<rfloor> = g a b"
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proof -
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  assume cong: "!!x x' y y'. x \<sim> x' ==> y \<sim> y' ==> g x y = g x' y'"
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  assume "!!X Y. f X Y == g (pick X) (pick Y)"
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  hence "f \<lfloor>a\<rfloor> \<lfloor>b\<rfloor> = g (pick \<lfloor>a\<rfloor>) (pick \<lfloor>b\<rfloor>)" by (simp only:)
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  also have "\<dots> = g a b"
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  proof (rule cong)
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    show "pick \<lfloor>a\<rfloor> \<sim> a" ..
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    show "pick \<lfloor>b\<rfloor> \<sim> b" ..
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  qed
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  finally show ?thesis .
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qed
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wenzelm
parents: 10278
diff changeset
   169
10250
ca93fe25a84b Quotient types;
wenzelm
parents:
diff changeset
   170
end