generics - 如何在 "where"子句中给出表达式泛型类型？

Question

(请原谅这个可怕的人为的例子)

我想要做的是在 where 子句中指定类型:

somemap :: (a -> b) -> [a] -> [b]
somemap f xs = ys
  where
    some = take 5 xs :: [a]
    ys = map f some :: [b]

但这会导致错误:

*Main> :load file.hs 
[1 of 1] Compiling Main             ( file.hs, interpreted )

fil.hs:15:18:
    Couldn't match expected type `a1' against inferred type `a'
      `a1' is a rigid type variable bound by
           an expression type signature at file.hs:15:25
      `a' is a rigid type variable bound by
          the type signature for `somemap' at file.hs:12:12
      Expected type: [a1]
      Inferred type: [a]
    In the second argument of `take', namely `xs'
    In the expression: take 5 xs :: [a]

file.hs:16:13:
    Couldn't match expected type `b1' against inferred type `b'
      `b1' is a rigid type variable bound by
           an expression type signature at file.hs:16:24
      `b' is a rigid type variable bound by
          the type signature for `somemap' at file.hs:12:17
    In the first argument of `map', namely `f'
    In the expression: map f some :: [b]
    In the definition of `ys': ys = map f some :: [b]
Failed, modules loaded: none.

而如果我只是指定具体类型，则替换为 Int对于 a和 Bool对于 b ， 没问题:

somemap :: (Int -> Bool) -> [Int] -> [Bool]
somemap f xs = ys
  where
    some = take 5 xs :: [Int]
    ys = map f some :: [Bool]

所以我的问题是:如何在 where 子句中指定泛型类型和类型约束？

Answer 1

里面where子句，类型变量 a和 b是新的类型变量；类型变量没有作用域，所以每个类型签名都有新的供应，就像它们是在顶层定义的一样。

如果您打开 ScopedTypeVariables扩展名(将 {-# LANGUAGE ScopedTypeVariables #-} 放在文件顶部)，然后更改 somemap的类型声明为:

somemap :: forall a b. (a -> b) -> [a] -> [b]

然后 where您指定的子句定义将正常工作。我认为 forall s 仅用于向后兼容，因此在 where 中重用类型变量的代码多态值的子句不会中断。

如果您不想使用扩展，另一种方法是定义丑陋的辅助函数来统一类型，例如 asTypeOf .