How to make Monoid instance for my type?

Question

I have the following type:

data SomeType = Var String
    deriving (Eq,Show)

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

Also, I have a function:

fun1 :: TypeA -> TypeA -> TypeA

I can use this function for mappend .

I don't understand, how to implement Monoid interface in my case.

Answer 1

If you already have fun1 , just add an instance:

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

Probably, you want mempty = TypeA [] .