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.

Side note: you *probably* should make `Sometype` a newtype as well. — dfeuer, May 13 '15 at 18:36

chi · Accepted Answer · 2015-05-13T18:22:31.170

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 [].

edited May 13 '15 at 18:22

answered May 13 '15 at 17:30

chi

2

@David You must also verify that `fun1` is associative for it to be a correct Monoid instance. – Rein Henrichs May 13 '15 at 20:29

1 Answers1