如何从外部上下文中关联类型

Question

让我们考虑以下代码片段：

blah :: a -> b -> a
blah x y = ble x where
    ble :: b -> b
    ble x = x

这在 GHC 下编译得很好，这本质上意味着第三行中的

b

与第一行中的

b

不同。

我的问题很简单：有没有办法以某种方式将

ble

的类型声明与外部上下文中使用的类型（即

blah

的类型声明）相关联？

显然，这只是一个示例，而不是类型声明的实际用例。

Answer 1

这可以通过 ScopedTypeVariables 扩展来实现。您需要使用显式的 forall 将类型变量带入作用域。

blah :: forall a b. a -> b -> a
blah x y = ble x where
    ble :: b -> b
    ble x = x

尝试在启用 ScopedTypeVariables 的情况下加载此定义会给出：

foo.hs:2:16:
    Couldn't match type `a' with `b'
      `a' is a rigid type variable bound by
          the type signature for blah :: a -> b -> a at foo.hs:2:1
      `b' is a rigid type variable bound by
          the type signature for blah :: a -> b -> a at foo.hs:2:1
    In the first argument of `ble', namely `x'
    In the expression: ble x
    In an equation for `blah':
        blah x y
          = ble x
          where
              ble :: b -> b
              ble x = x

您可以看出 GHC 将两个

b

解释为同一类型，因为错误表明

a

和

b

绑定在同一行上。

Answer 2

SO是一场狗屎秀。感谢您的搭车。