lean2/library/logic/if.lean

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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
----------------------------------------------------------------------------------------------------
import logic.decidable tools.tactic
open decidable tactic eq.ops
definition ite (c : Prop) [H : decidable c] {A : Type} (t e : A) : A :=
decidable.rec_on H (assume Hc, t) (assume Hnc, e)
notation `if` c `then` t:45 `else` e:45 := ite c t e
theorem if_pos {c : Prop} [H : decidable c] (Hc : c) {A : Type} {t e : A} : (if c then t else e) = t :=
decidable.rec
(assume Hc : c, eq.refl (@ite c (inl Hc) A t e))
(assume Hnc : ¬c, absurd Hc Hnc)
H
theorem if_neg {c : Prop} [H : decidable c] (Hnc : ¬c) {A : Type} {t e : A} : (if c then t else e) = e :=
decidable.rec
(assume Hc : c, absurd Hc Hnc)
(assume Hnc : ¬c, eq.refl (@ite c (inr Hnc) A t e))
H
theorem if_t_t (c : Prop) [H : decidable c] {A : Type} (t : A) : (if c then t else t) = t :=
decidable.rec
(assume Hc : c, eq.refl (@ite c (inl Hc) A t t))
(assume Hnc : ¬c, eq.refl (@ite c (inr Hnc) A t t))
H
theorem if_true {A : Type} (t e : A) : (if true then t else e) = t :=
if_pos trivial
theorem if_false {A : Type} (t e : A) : (if false then t else e) = e :=
if_neg not_false_trivial
theorem if_cond_congr {c₁ c₂ : Prop} [H₁ : decidable c₁] [H₂ : decidable c₂] (Heq : c₁ ↔ c₂) {A : Type} (t e : A)
: (if c₁ then t else e) = (if c₂ then t else e) :=
decidable.rec_on H₁
(assume Hc₁ : c₁, decidable.rec_on H₂
(assume Hc₂ : c₂, if_pos Hc₁ ⬝ (if_pos Hc₂)⁻¹)
(assume Hnc₂ : ¬c₂, absurd (iff.elim_left Heq Hc₁) Hnc₂))
(assume Hnc₁ : ¬c₁, decidable.rec_on H₂
(assume Hc₂ : c₂, absurd (iff.elim_right Heq Hc₂) Hnc₁)
(assume Hnc₂ : ¬c₂, if_neg Hnc₁ ⬝ (if_neg Hnc₂)⁻¹))
theorem if_congr_aux {c₁ c₂ : Prop} [H₁ : decidable c₁] [H₂ : decidable c₂] {A : Type} {t₁ t₂ e₁ e₂ : A}
(Hc : c₁ ↔ c₂) (Ht : t₁ = t₂) (He : e₁ = e₂) :
(if c₁ then t₁ else e₁) = (if c₂ then t₂ else e₂) :=
Ht ▸ He ▸ (if_cond_congr Hc t₁ e₁)
theorem if_congr {c₁ c₂ : Prop} [H₁ : decidable c₁] {A : Type} {t₁ t₂ e₁ e₂ : A} (Hc : c₁ ↔ c₂) (Ht : t₁ = t₂) (He : e₁ = e₂) :
(if c₁ then t₁ else e₁) = (@ite c₂ (decidable_iff_equiv H₁ Hc) A t₂ e₂) :=
have H2 [visible] : decidable c₂, from (decidable_iff_equiv H₁ Hc),
if_congr_aux Hc Ht He