-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
-- Released under Apache 2.0 license as described in the file LICENSE.
-- Author: Leonardo de Moura
import decidable tactic
using bit decidable tactic

definition ite (c : Bool) {H : decidable c} {A : Type} (t e : A) : A
:= rec H (assume Hc,  t) (assume Hnc, e)

notation `if` c `then` t `else` e:45 := ite c t e

theorem if_pos {c : Bool} {H : decidable c} (Hc : c) {A : Type} (t e : A) : (if c then t else e) = t
:= decidable_rec
    (assume Hc : c,    calc (@ite c (inl Hc) A t e) = t : refl _)
    (assume Hnc : ¬c,  absurd_elim (@ite c (inr Hnc) A t e = t) Hc Hnc)
    H

theorem if_neg {c : Bool} {H : decidable c} (Hnc : ¬c) {A : Type} (t e : A) : (if c then t else e) = e
:= decidable_rec
    (assume Hc : c,    absurd_elim (@ite c (inl Hc) A t e = e) Hc Hnc)
    (assume Hnc : ¬c,  calc (@ite c (inr Hnc) A t e) = e : refl _)
    H

theorem if_t_t (c : Bool) {H : decidable c} {A : Type} (t : A) : (@ite c H A t t) = t -- check why '(if c then t else t) = t' fails
:= decidable_rec
    (assume Hc  : c,  calc (@ite c (inl Hc)  A t t) = t : refl _)
    (assume Hnc : ¬c, calc (@ite c (inr Hnc) A t t) = t : refl _)
    H

theorem if_true {A : Type} (t e : A) : (if true then t else e) = t
:= if_pos trivial t e

theorem if_false {A : Type} (t e : A) : (if false then t else e) = e
:= if_neg not_false_trivial t e