haskell - 我如何概括 Maybe 和 Either 的违约行为？

Question

在 Haskell 中，如果我有两个这样的函数:

defEither ∷   Either l r → r → r
defEither  eith defVal = either (const defVal) id eith

和

defMaybe ∷   Maybe a → a → a
defMaybe m d = fromMaybe d m

如何编写一个类型类(或类似效果的东西)，以便我可以在 Either 和 Maybe 之间概括“可默认”的概念？

类似的东西

class Defaultable ???? where
  def ∷  a b → b → b

Answer 1

事实证明，为 Either 创建实例的语法让我感到困惑。

这是我完成的:

class Defaultable a where
  def ∷  a b → b → b

instance Defaultable (Either m) where
  def e d = either (const d) id e

instance Defaultable Maybe where
   def m d = fromMaybe d m

<小时/>

还有一些测试

def (Just 1) 2
>> 1

def Nothing 2
>> 2

def (Right 2) 5
>> 2

def (Left 3) 5
>> 5

def (Left "Arrghh") 5
>> 5