    {-# LANGUAGE DeriveDataTypeable #-}
    {-# LANGUAGE DeriveGeneric #-}
    {-# LANGUAGE PolyKinds #-}
    {-# LANGUAGE Safe #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Functor.Sum
    -- Copyright   :  (c) Ross Paterson 2014
    -- License     :  BSD-style (see the file LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  portable
    --
    -- Sums, lifted to functors.
    --
    -- @since 4.9.0.0
    -----------------------------------------------------------------------------

    module Data.Functor.Sum (
        Sum(..),
      ) where

    import Data.Data (Data)
    import Data.Foldable (Foldable(foldMap))
    import Data.Functor.Classes
    import Data.Monoid (mappend)
    import Data.Traversable (Traversable(traverse))
    import GHC.Generics (Generic, Generic1)

    -- | Lifted sum of functors.
    data Sum f g a = InL (f a) | InR (g a)
      deriving (Data, Generic, Generic1)

    instance (Eq1 f, Eq1 g) => Eq1 (Sum f g) where
        liftEq eq (InL x1) (InL x2) = liftEq eq x1 x2
        liftEq _ (InL _) (InR _) = False
        liftEq _ (InR _) (InL _) = False
        liftEq eq (InR y1) (InR y2) = liftEq eq y1 y2

    instance (Ord1 f, Ord1 g) => Ord1 (Sum f g) where
        liftCompare comp (InL x1) (InL x2) = liftCompare comp x1 x2
        liftCompare _ (InL _) (InR _) = LT
        liftCompare _ (InR _) (InL _) = GT
        liftCompare comp (InR y1) (InR y2) = liftCompare comp y1 y2

    instance (Read1 f, Read1 g) => Read1 (Sum f g) where
        liftReadsPrec rp rl = readsData $
            readsUnaryWith (liftReadsPrec rp rl) "InL" InL `mappend`
            readsUnaryWith (liftReadsPrec rp rl) "InR" InR

    instance (Show1 f, Show1 g) => Show1 (Sum f g) where
        liftShowsPrec sp sl d (InL x) =
            showsUnaryWith (liftShowsPrec sp sl) "InL" d x
        liftShowsPrec sp sl d (InR y) =
            showsUnaryWith (liftShowsPrec sp sl) "InR" d y

    instance (Eq1 f, Eq1 g, Eq a) => Eq (Sum f g a) where
        (==) = eq1
    instance (Ord1 f, Ord1 g, Ord a) => Ord (Sum f g a) where
        compare = compare1
    instance (Read1 f, Read1 g, Read a) => Read (Sum f g a) where
        readsPrec = readsPrec1
    instance (Show1 f, Show1 g, Show a) => Show (Sum f g a) where
        showsPrec = showsPrec1

    instance (Functor f, Functor g) => Functor (Sum f g) where
        fmap f (InL x) = InL (fmap f x)
        fmap f (InR y) = InR (fmap f y)

    instance (Foldable f, Foldable g) => Foldable (Sum f g) where
        foldMap f (InL x) = foldMap f x
        foldMap f (InR y) = foldMap f y

    instance (Traversable f, Traversable g) => Traversable (Sum f g) where
        traverse f (InL x) = InL <$> traverse f x
        traverse f (InR y) = InR <$> traverse f y