lean2/library/data/subtype.lean
Leonardo de Moura 9412e604c8 refactor(library/data): cleanup datatypes
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
2014-09-04 22:31:52 -07:00

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-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
-- Released under Apache 2.0 license as described in the file LICENSE.
-- Author: Leonardo de Moura, Jeremy Avigad
import logic.classes.inhabited logic.core.eq logic.classes.decidable
open decidable
inductive subtype {A : Type} (P : A → Prop) : Type :=
tag : Πx : A, P x → subtype P
notation `{` binders `,` r:(scoped P, subtype P) `}` := r
namespace subtype
section
parameter {A : Type}
parameter {P : A → Prop}
-- TODO: make this a coercion?
definition elt_of (a : {x, P x}) : A := rec (λ x y, x) a
theorem has_property (a : {x, P x}) : P (elt_of a) := rec (λ x y, y) a
theorem elt_of_tag (a : A) (H : P a) : elt_of (tag a H) = a := rfl
theorem destruct [protected] {Q : {x, P x} → Prop} (a : {x, P x})
(H : ∀(x : A) (H1 : P x), Q (tag x H1)) : Q a :=
rec H a
theorem tag_irrelevant {a : A} (H1 H2 : P a) : tag a H1 = tag a H2 :=
rfl
theorem tag_elt_of (a : subtype P) : ∀(H : P (elt_of a)), tag (elt_of a) H = a :=
destruct a (take (x : A) (H1 : P x) (H2 : P x), rfl)
theorem tag_eq {a1 a2 : A} {H1 : P a1} {H2 : P a2} (H3 : a1 = a2) : tag a1 H1 = tag a2 H2 :=
eq.subst H3 (take H2, tag_irrelevant H1 H2) H2
theorem equal [protected] {a1 a2 : {x, P x}} : ∀(H : elt_of a1 = elt_of a2), a1 = a2 :=
destruct a1 (take x1 H1, destruct a2 (take x2 H2 H, tag_eq H))
theorem is_inhabited [protected] [instance] {a : A} (H : P a) : inhabited {x, P x} :=
inhabited.mk (tag a H)
theorem has_decidable_eq [protected] [instance] (a1 a2 : {x, P x})
(H : decidable (elt_of a1 = elt_of a2)) : decidable (a1 = a2) :=
have H1 : (a1 = a2) ↔ (elt_of a1 = elt_of a2), from
iff.intro (assume H, eq.subst H rfl) (assume H, equal H),
decidable_iff_equiv _ (iff.symm H1)
end
end subtype