learning-haskell/15-monoid-semigroup/orphan-instance/Listy.hs

module Listy where

newtype Listy a = Listy [a] deriving (Eq, Show)

instance Monoid (Listy a) where
    mempty = Listy []
    mappend (Listy l) (Listy l') =
        Listy $ mappend l l'