在Haskell中包含具有默认实现的类型类

Question

考虑以下定义:

class Foo a where
    foo :: a -> Int

class Bar a where
    bar :: a -> [Int]

现在,我怎么说在Haskell中"每个Foo都是a Bar,bar默认定义为bar x = [foo x]"？

(无论我尝试什么,编译器都给我"非法实例声明"或"约束不小于实例头")

顺便说一句,我可以通过其他方式定义我的Foo和Bar类,如果这会有所帮助.

Answer 1

class Foo a where
    foo :: a -> Int

-- 'a' belongs to 'Bar' only if it belongs to 'Foo' also
class Foo a => Bar a where
    bar :: a -> [Int]
    bar x = [foo x] -- yes, you can specify default implementation

instance Foo Char where
    foo _ = 0

-- instance with default 'bar' implementation
instance Bar Char