GHC 拒绝该计划
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}
import GHC.TypeLits
data Foo = Foo
data Bar = Bar
data (:::) :: * -> * -> * where
(:=) :: sy -> t -> sy ::: t
data Rec :: [*] -> * where
RNil :: Rec '[]
(:&) :: (sy ::: ty) -> Rec xs -> Rec ((sy ::: ty) ': xs)
infix 3 :=
infixr 2 :&
baz :: Num ty => Rec [Foo ::: String, Bar ::: ty]
baz = Foo := "foo" :& Bar := 1 :& RNil
--bang :: (String, Integer)
bang = case baz of
(Foo := a :& Bar := b :& RNil) -> (a, b)
和
Rec2.hs:25:44:
Couldn't match type ‛t’ with ‛(String, Integer)’
‛t’ is untouchable
inside the constraints (xs1 ~ '[] *)
bound by a pattern with constructor
RNil :: Rec ('[] *),
in a case alternative
at Rec2.hs:25:35-38
‛t’ is a rigid type variable bound by
the inferred type of bang :: t at Rec2.hs:24:1
Expected type: t
Actual type: (ty, ty1)
Relevant bindings include bang :: t (bound at Rec2.hs:24:1)
In the expression: (a, b)
In a case alternative: (Foo := a :& Bar := b :& RNil) -> (a, b)
In the expression:
case baz of { (Foo := a :& Bar := b :& RNil) -> (a, b) }
,使用类型注释可以正常工作。我在网上找到的所有关于不可触碰的类型变量和 GADT 的答案都断言类型推断是不可能的,或者至少是难以处理的,但在这种情况下,GHC 显然掌握了类型(String, Integer)
,它只是拒绝统一。