Haskel-有两个争论的数据实例

Question

如何声明此数据类型的Functor实例：

data Productish a b = Productish a b

我已经尝试过：

instance (Functor a, Functor b) => Productish a b where
    fmap f (Productish a b) = Productish (f a) (f b)

但是编译器显示错误：

 error: ‘fmap’ is not a (visible) method of class ‘Productish’

Answer 1

首先，您的语法错误。要在Functor上定义Productish实例，您需要执行instance Functor (Productish a b) where …。