feat(init/data/option): option.get

library/init/data/option/basic.lean

@@ -27,6 +27,10 @@ def is_none {α : Type u} : option α → bool
 | (some _) := ff
 | none     := tt
 
+def get {α : Type u} : Π {o : option α}, is_some o → α
+| (some x) h := x
+| none     h := false.rec _ $ bool.ff_ne_tt h
+
 def rhoare {α : Type u} : bool → α → option α
 | tt a := none
 | ff a := some a

library/init/data/option/instances.lean

@@ -34,3 +34,7 @@ lemma option.eq_of_eq_some {α : Type u} : Π {x y : option α}, (∀z, x = some
 | none     (some z) h := option.no_confusion ((h z).2 rfl)
 | (some z) none     h := option.no_confusion ((h z).1 rfl)
 | (some z) (some w) h := option.no_confusion ((h w).2 rfl) (congr_arg some)
+
+lemma option.eq_some_of_is_some {α : Type u} : Π {o : option α} (h : option.is_some o), o = some (option.get h)
+| (some x) h := rfl
+| none     h := false.rec _ $ bool.ff_ne_tt h