haskell - 有类型类限制时，如何在实例语句中指定类型？

Question

我试图定义一个非常简单的数据结构，假设将 Infinity 元素添加到 Num 下的任何类型。我还将它放在定义的类 NumContainer 下，它有一个方法 fromNum 使用常规 Num 构造一个 NumWithInf >。代码非常简单。

data NumWithInf a = Infinity | Finite a deriving Show

class NumContainer k where
    fromNum :: Num a => a -> k

instance Num a => NumContainer (NumWithInf a) where
    fromNum x = Finite x

但是当我运行它时，GHCI 给了我以下错误:

hw.hs:7:24:
    Could not deduce (a ~ a1)
    from the context (Num a)
      bound by the instance declaration at hw.hs:6:10-45
    or from (Num a1)
      bound by the type signature for
                 fromNum :: Num a1 => a1 -> NumWithInf a
      at hw.hs:7:5-24
      `a' is a rigid type variable bound by
          the instance declaration at hw.hs:6:10
      `a1' is a rigid type variable bound by
           the type signature for fromNum :: Num a1 => a1 -> NumWithInf a
           at hw.hs:7:5
    In the first argument of `Finite', namely `x'
    In the expression: Finite x
    In an equation for `fromNum': fromNum x = Finite x
Failed, modules loaded: none.

我知道它说 Finite x 中的 x 不一定与签名 中的 a 具有相同的类型fromNum::Num a => a -> k。我应该如何指定它？

Answer 1

一个简单的解决方案(即不求助于奇特的类型系统技巧的解决方案)是让您的类使用类型 * -> * 而不是 *。实际上，这只是意味着您将其定义为:

class NumContainer k where
    fromNum :: Num a => a -> k a -- Result type parametrised on `a` 

然后实例变为:

instance NumContainer NumWithInf where
    fromNum x = Finite x

请注意，实例中没有放置Num 约束的地方。无论如何这都是不必要的——fromNum 类型的约束已经足够好了。