小编Dmi*_*sky的帖子

是否有一些原因导致此代码未编译:

type family Foo a b :: Bool where
    Foo a b = a == b

foo :: Foo a b ~ True => Proxy a -> Proxy b
foo _ = Proxy

bar :: Proxy a -> Proxy a
bar = foo

有错误:

Couldn't match type ‘a == a’ with ‘'True’
Expected type: 'True
  Actual type: Foo a a

但如果我将类型族定义更改为

type family Foo a b :: Bool where
    Foo a a = True
    Foo a b = False

编译得好吗？ …