小编are*_*enl的帖子

使用函数依赖项,我可以使用多参数类型类约束类型类中的依赖参数类型.像这样:

{-# LANGUAGE FunctionalDependencies, MultiParamTypeClasses,TypeSynonymInstances  #-}

class (Num a, Integral b) => F a b | a -> b where
  f :: a -> b

instance F Int Int where
  f = id

instance F Float Integer where
  f = truncate

一切都会很完美.

> f (1 :: Int)
1
> f (1.9 :: Float)
1

但是,如果我尝试写类似的东西

instance F Double String where
  f = show

我将得到以下编译错误:

No instance for (Integral String)
arising from the superclasses of an instance declaration
Possible fix: add …