CODEWITHSUNDEEP
haskellmonoids
David
David

Reputation: 674

How to make Monoid instance for my type?

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.

Upvotes: 1

Views: 296

Answers (1)

chi
chi

Reputation: 116174

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 [].

Upvotes: 1

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