Haskell-book/15/orphan-instance/Listy.hs


1
2
3
4
5
6
7
8
9
10

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'