Custom concat (++) operator in haskell

Question

Is it possible to define my own ++ operator for a custom data type in Haskell?

I have:

data MyType = MyType [String]

and I would like to define my own concatenation operator as:

instance ? MyType where
    (MyType x) ++ (MyType y) = MyType (x ++ y)

I can't seem to find the name of the instance class anywhere.

Answer 1

If you don't insist on calling the operator (++) ,

import Data.Monoid

instance Monoid MyType where
    (MyType x) `mappend` (MyType y) = MyType (x ++ y)
    mempty = MyType []

Then you can use

(<>) :: Monoid m => m -> m -> m

which is an alias for mappend (I thought it was already a type class member, but it isn't :/). Lists hava a Monoid instance where mappend is (++) , so that would do what you desire. The Monoid instance also gives you

mconcat :: Monoid m => [m] -> m

which you can use to concatenate a list of MyType s.

Answer 2

easiest it would be to do

import Prelude hiding ((++))
import qualified Prelude as P

data MyType = MyType [String]

class PlusAble a where
    infixr 5 ++
    (++) :: a -> a -> a

instance PlusAble MyType where
    (MyType x) ++ (MyType y) = MyType (x P.++ y)

-- EDIT:
instance PlusAble [a] where
    x ++ y = x P.++ y

Answer 3

(++) operator does not belong to any type class. You can easily check this:

$ ghci
Prelude> :info (++)
(++) :: [a] -> [a] -> [a]   -- Defined in `GHC.Base'
infixr 5 ++

So, it's just simple function defined in GHC.Base module. You can hide it and define your own one:

import Prelude hiding ((++))
import qualified Prelude -- to get hidden (++) as Prelude.(++)

-- your brand new (++)
infixr 5 ++
(MyType x) ++ (MyType y) = MyType (x Prelude.++ y)