feat(library): add 'decidable_eq' class

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

library/data/bool.lean

@@ -138,8 +138,9 @@ namespace bool
   theorem is_inhabited [protected] [instance] : inhabited bool :=
   inhabited.mk ff
 
-  theorem has_decidable_eq [protected] [instance] (a b : bool) : decidable (a = b) :=
+  theorem has_decidable_eq [protected] [instance] : decidable_eq bool :=
+  decidable_eq.intro (λ (a b : bool),
     rec_on a
       (rec_on b (inl rfl) (inr ff_ne_tt))
-    (rec_on b (inr (ne.symm ff_ne_tt)) (inl rfl))
+      (rec_on b (inr (ne.symm ff_ne_tt)) (inl rfl)))
 end bool

library/data/int/basic.lean

@@ -205,8 +205,8 @@ exists_intro (pr1 (rep a))
 
 definition of_nat (n : ℕ) : ℤ := psub (pair n 0)
 
-theorem int_eq_decidable [instance] (a b : ℤ) : decidable (a = b) := _
--- subtype_eq_decidable _ _ (prod_eq_decidable _ _ _ _)
+theorem has_decidable_eq [instance] [protected] : decidable_eq ℤ :=
+decidable_eq.intro (λ (a b : ℤ), _)
 
 opaque_hint (hiding int)
 coercion of_nat

library/data/list/basic.lean

@@ -236,7 +236,7 @@ induction_on l
           from H3 ▸ rfl,
         exists_intro _ (exists_intro _ H4)))
 
-theorem mem_is_decidable [instance] {H : Π (x y : T), decidable (x = y)} {x : T} {l : list T} : decidable (mem x l) :=
+theorem mem_is_decidable [instance] {H : decidable_eq T} {x : T} {l : list T} : decidable (mem x l) :=
 rec_on l
   (decidable.inr (iff.false_elim (@mem_nil x)))
   (λ (h : T) (l : list T) (iH : decidable (mem x l)),
@@ -264,15 +264,15 @@ rec_on l
 -- Find
 -- ----
 
-definition find (x : T) {H : Π (x y : T), decidable (x = y)} : list T → nat :=
+definition find {H : decidable_eq T} (x : T) : list T → nat :=
 rec 0 (fun y l b, if x = y then 0 else succ b)
 
-theorem find_nil {f : T} {H : Π (x y : T), decidable (x = y)} : find f nil = 0
+theorem find_nil {H : decidable_eq T} {f : T} : find f nil = 0
 
-theorem find_cons {x y : T} {l : list T} {H : Π (x y : T), decidable (x = y)} :
+theorem find_cons {H : decidable_eq T} {x y : T} {l : list T} :
     find x (cons y l) = if x = y then 0 else succ (find x l)
 
-theorem not_mem_find {l : list T} {x : T} {H : Π (x y : T), decidable (x = y)} :
+theorem not_mem_find {H : decidable_eq T} {l : list T} {x : T} :
      ¬mem x l → find x l = length l :=
 rec_on l
    (assume P₁ : ¬mem x nil, rfl)

library/data/nat/basic.lean

@@ -105,7 +105,8 @@ induction_on n
     absurd H ne)
   (take k IH H, IH (succ_inj H))
 
-theorem decidable_eq [instance] (n m : ℕ) : decidable (n = m) :=
+theorem has_decidable_eq [instance] [protected] : decidable_eq ℕ :=
+decidable_eq.intro (λ (n m : ℕ),
 have general : ∀n, decidable (n = m), from
   rec_on m
     (take n,
@@ -123,7 +124,7 @@ have general : ∀n, decidable (n = m), from
               have H1 : succ n' ≠ succ m', from
                 assume Heq, absurd (succ_inj Heq) Hne,
               inr H1))),
-general n
+general n)
 
 theorem two_step_induction_on {P : ℕ → Prop} (a : ℕ) (H1 : P 0) (H2 : P 1)
     (H3 : ∀ (n : ℕ) (IH1 : P n) (IH2 : P (succ n)), P (succ (succ n))) : P a :=

library/data/option.lean

@@ -38,13 +38,13 @@ namespace option
   theorem is_inhabited [protected] [instance] (A : Type) : inhabited (option A) :=
   inhabited.mk none
 
-  theorem has_decidable_eq [protected] [instance] {A : Type} {H : ∀a₁ a₂ : A, decidable (a₁ = a₂)}
-          (o₁ o₂ : option A) : decidable (o₁ = o₂) :=
+  theorem has_decidable_eq [protected] [instance] {A : Type} (H : decidable_eq A) : decidable_eq (option A) :=
+  decidable_eq.intro (λ (o₁ o₂ : option A),
     rec_on o₁
       (rec_on o₂ (inl rfl) (take a₂, (inr (none_ne_some a₂))))
       (take a₁ : A, rec_on o₂
         (inr (ne.symm (none_ne_some a₁)))
         (take a₂ : A, decidable.rec_on (H a₁ a₂)
           (assume Heq : a₁ = a₂, inl (Heq ▸ rfl))
-        (assume Hne : a₁ ≠ a₂, inr (assume Hn : some a₁ = some a₂, absurd (equal Hn) Hne))))
+          (assume Hne : a₁ ≠ a₂, inr (assume Hn : some a₁ = some a₂, absurd (equal Hn) Hne)))))
 end option

library/data/prod.lean

@@ -48,12 +48,12 @@ section
   theorem is_inhabited [protected] [instance] (H1 : inhabited A) (H2 : inhabited B) : inhabited (prod A B) :=
   inhabited.destruct H1 (λa, inhabited.destruct H2 (λb, inhabited.mk (pair a b)))
 
-  theorem has_decidable_eq [protected] [instance] (u v : A × B) (H1 : decidable (pr1 u = pr1 v))
-      (H2 : decidable (pr2 u = pr2 v)) : decidable (u = v) :=
+  theorem has_decidable_eq [protected] [instance] (H1 : decidable_eq A) (H2 : decidable_eq B) : decidable_eq (A × B) :=
+  decidable_eq.intro (λ (u v : A × B),
     have H3 : u = v ↔ (pr1 u = pr1 v) ∧ (pr2 u = pr2 v), from
       iff.intro
         (assume H, H ▸ and.intro rfl rfl)
         (assume H, and.elim H (assume H4 H5, equal H4 H5)),
-    decidable_iff_equiv _ (iff.symm H3)
+    decidable_iff_equiv _ (iff.symm H3))
 end
 end prod

library/data/subtype.lean

@@ -41,10 +41,10 @@ section
   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) :=
+  theorem has_decidable_eq [protected] [instance] (H : decidable_eq A) : decidable_eq {x, P x} :=
+  decidable_eq.intro (λ (a1 a2 : {x, P x}),
     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)
+    decidable_iff_equiv _ (iff.symm H1))
 end
 end subtype

library/data/sum.lean

@@ -55,13 +55,16 @@ namespace sum
   theorem is_inhabited_right [protected] [instance] {A B : Type} (H : inhabited B) : inhabited (A ⊎ B) :=
   inhabited.mk (inr A (default B))
 
-  theorem has_eq_decidable [protected] [instance] {A B : Type} (s1 s2 : A ⊎ B)
-    (H1 : ∀a1 a2 : A, decidable (inl B a1 = inl B a2))
-    (H2 : ∀b1 b2 : B, decidable (inr A b1 = inr A b2)) : decidable (s1 = s2) :=
+  theorem has_eq_decidable [protected] [instance] {A B : Type} (H1 : decidable_eq A) (H2 : decidable_eq B) :
+       decidable_eq (A ⊎ B) :=
+  decidable_eq.intro (λ (s1 s2 : A ⊎ B),
     rec_on s1
       (take a1, show decidable (inl B a1 = s2), from
         rec_on s2
-        (take a2, show decidable (inl B a1 = inl B a2), from H1 a1 a2)
+          (take a2, show decidable (inl B a1 = inl B a2), from
+            decidable.rec_on (H1 a1 a2)
+              (assume Heq : a1 = a2, decidable.inl (Heq ▸ rfl))
+              (assume Hne : a1 ≠ a2, decidable.inr (mt inl_inj Hne)))
           (take b2,
             have H3 : (inl B a1 = inr A b2) ↔ false,
               from iff.intro inl_neq_inr (assume H4, false_elim H4),
@@ -72,5 +75,8 @@ namespace sum
             have H3 : (inr A b1 = inl B a2) ↔ false,
               from iff.intro (assume H4, inl_neq_inr (H4⁻¹)) (assume H4, false_elim H4),
             show decidable (inr A b1 = inl B a2), from decidable_iff_equiv _ (iff.symm H3))
-        (take b2, show decidable (inr A b1 = inr A b2), from H2 b1 b2))
+          (take b2, show decidable (inr A b1 = inr A b2), from
+            decidable.rec_on (H2 b1 b2)
+              (assume Heq : b1 = b2, decidable.inl (Heq ▸ rfl))
+              (assume Hne : b1 ≠ b2, decidable.inr (mt inr_inj Hne)))))
 end sum

library/data/unit.lean

@@ -18,6 +18,6 @@ namespace unit
   theorem is_inhabited [protected] [instance] : inhabited unit :=
   inhabited.mk ⋆
 
-  theorem has_decidable_eq [protected] [instance] (a b : unit) : decidable (a = b) :=
-  inl (equal a b)
+  theorem has_decidable_eq [protected] [instance] : decidable_eq unit :=
+  decidable_eq.intro (λ (a b : unit), inl (equal a b))
 end unit

library/logic/classes/decidable.lean

@@ -95,3 +95,11 @@ theorem decidable_eq_equiv {a b : Prop} (Ha : decidable a) (H : a = b) : decidab
 decidable_iff_equiv Ha (eq_to_iff H)
 
 end decidable
+
+inductive decidable_eq [class] (A : Type) : Type :=
+intro : (Π x y : A, decidable (x = y)) → decidable_eq A
+
+theorem decidable_eq_to_decidable [instance] {A : Type} (H : decidable_eq A) (x y : A) : decidable (x = y) :=
+decidable_eq.rec (λ H, H) H x y
+
+coercion decidable_eq_to_decidable : decidable_eq

tests/lean/run/occurs_check_bug1.lean

@@ -10,7 +10,7 @@ variable gcd_aux : ℕ × ℕ → ℕ
 
 definition gcd (x y : ℕ) : ℕ := gcd_aux (pair x y)
 
-theorem gcd_def (x y : ℕ) : gcd x y = @ite (y = 0) (decidable_eq (pr2 (pair x y)) 0) nat x (gcd y (x mod y)) :=
+theorem gcd_def (x y : ℕ) : gcd x y = @ite (y = 0) (nat.has_decidable_eq (pr2 (pair x y)) 0) nat x (gcd y (x mod y)) :=
 sorry
 
 theorem gcd_succ (m n : ℕ) : gcd m (succ n) = gcd (succ n) (m mod succ n) :=