如何为我的类型制作Monoid实例？

Question

I have the following type: 我有以下类型：

data SomeType = Var String
    deriving (Eq,Show)

newtype TypeA = TypeA [(String, SomeType)]
  deriving (Eq,Show)

Also, I have a function: 另外，我有一个功能：

fun1 :: TypeA -> TypeA -> TypeA

I can use this function for mappend . 我可以将此功能用于mappend 。

I don't understand, how to implement Monoid interface in my case. 我不明白如何在我的情况下实现Monoid接口。

Answer 1

If you already have fun1 , just add an instance: 如果您已经拥有fun1 ，只需添加一个实例：

instance Monoid TypeA where
   mempty = .... -- fill this with the neutral element
   mappend = fun1

Probably, you want mempty = TypeA [] . 可能您想要mempty = TypeA [] 。