Orphan Instances

Typeclasses have unique pairings of the class and the instance for a particular type.

An orphan instance is a big deal. It’s when you have an instance for a datatype and a typeclass, but the instance is not in the same module as said datatype or typeclass.

Why is this important? Well if you don’t own the typeclass/datatype you can newtype it! Even when you think you want an orphan instance so that you can have multiple instances for the same type, you still actually want to use newtype. Here’s an example of what this means:

-- Listy.hs
module Listy where

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

-- ListyInstances.hs
module ListyInstances where

import Data.Monoid
import Listy

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