haskell - 如何在 Haskell 中正确实例化类？

Question

试图创建一个基类，我可以从中派生出不同的类型。以下有什么问题？

class (Eq a) => MyClass a 

data Alpha = Alpha
instance MyClass Alpha where
    Alpha == Alpha = True

我收到错误:

test.hs:5:10: `==' is not a (visible) method of class `MyClass'
Failed, modules loaded: none.

Answer 1

您必须明确地使 Alpha 成为 Eq 的实例。这将起作用:

data Alpha = Alpha
instance Eq Alpha where
    Alpha == Alpha = True
instance MyClass Alpha