为什么这个函子的设置不起作用?

为了避免与Prelude冲突,我定义了自己的Functor类:

class F f where
    fm :: (a -> b) -> f a -> f b

instance F ((->) a) where
    fm g f = g . f

GHCI公司:

*Main> fm (+1) (+10) $ 100
111

... 但这不是吗?

instance F ((->) Int) where
    fm g f = g . f

我不明白-我应该限制底层集合到映射的函数吗 Int

错误消息没有帮助:

    • Illegal instance declaration for ‘F ((->) Int)’
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are *distinct type variables*,
         and each type variable appears at most once in the instance head.
         Use FlexibleInstances if you want to disable this.)
    • In the instance declaration for ‘F ((->) Int)’
  |
4 | instance F ((->) Int) where
  |          ^^^^^^^^^^^^