Question

我正在尝试定义以下类＆amp;实例

class Adder a where  
    plus :: a -> a -> a  

instance Adder Num where  
    plus x y = x + y

但我收到此错误

Expecting one more argument to ‘Num’
The first argument of ‘Adder’ should have kind ‘*’,
  but ‘Num’ has kind ‘* -> Constraint’
In the instance declaration for ‘Adder Num’
Failed, modules loaded: none.

后来我想定义

instance Adder String where  
    plus x y = x + y

Answer 1

如果您希望任何Num实例的类型成为Adder的实例，您可以实现以下目标：

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}

class Adder a where
    plus :: a -> a -> a

instance Num a => Adder a where   
    plus = (+)

要添加String个实例，您还需要一个语言扩展

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE OverlappingInstances #-}

class Adder a where
    plus :: a -> a -> a

instance Num a => Adder a where    
    plus = (+)

instance Adder String where
    plus = (++)

实例定义的约束错误

1 个答案: