函数可以是Haskell中类的实例吗

Question

class A a
instance A (Int -> Float)

doesn't works and

class B b
instance B Int

works If functions in Haskell are taken as the first classes and (Int -> Float)is undoubtly a type although it is not a (*) concrete type, why can't functions be instance of classes

Answer 1

Yes, the function type is not very special in Haskell:

module Test where

instance Show (a -> b) where
    show _ = "(function)"

As pointed out by shk in his answer, if you want to fix the type of the domain or the range, you will need an extension like FlexibleInstances – but that is not related to the function type and will be required for other type constructor applications like Maybe Int as well.

Answer 2

With FlexibleInstances extension you can do that:

{-# LANGUAGE FlexibleInstances #-}

module TestFlexibleInstances where

class A a

instance A (Int -> Float)