haskell - 使用类型同义词的函数定义是 "less polymorphic than expected"

Question

给定这种类型的同义词:

type Synonym a b = (a, b)

此代码在 GHCi 中不起作用:

ghci> let myFirst (f, s) = f :: Synonym a b -> a

<interactive>:1:21:
    Inferred type is less polymorphic than expected
      Quantified type variable `b' is mentioned in the environment:
        f :: Synonym a b -> a (bound at <interactive>:1:13)
      Quantified type variable `a' is mentioned in the environment:
        f :: Synonym a b -> a (bound at <interactive>:1:13)
    In the expression: f :: Synonym a b -> a
    In the definition of `myFirst':
        myFirst (f, s) = f :: Synonym a b -> a

但这确实:

ghci> let myFirst = fst :: Synonym a b -> a
-- no problem --

只有当我直接将其输入 GHCi 时才会发生这种情况；当我将它们放入文件中并 :load 它们时，这两个定义都有效。

这里有什么问题吗？我多次遇到这个问题，但一直不明白为什么。

附:我尝试了 :set -XNoMonomorphismRestriction，但这并没有什么区别。

Answer 1

Haskell 试图将类型签名与 f 匹配，而不是与 myFirst 匹配，但它不起作用(但我无法给出更多解释，其他人？)。即 Haskell 将其视为:

let myFirst (f,s) = (f :: Synonym a b -> a)

您可以通过提供单独的签名来解决此问题

let myFirst :: Synonym a b -> a; myFirst (f,s) = f

甚至使用 lambda(这本质上等同于 myFirst = fst 定义)

let myFirst = (\(f,s) -> f) :: Synonym a b -> a

(请注意，即使没有类型同义词，此操作也会失败:let myFirst (f,s) = f::(a,b) -> a 也不起作用。)