-----------------------------------------------------------------------------
-- |
-- Module      :  Distribution.Compat.DList
-- Copyright   :  (c) Ben Gamari 2015-2019
-- License     :  BSD3
--
-- Maintainer  :  cabal-dev@haskell.org
-- Stability   :  experimental
-- Portability :  portable
--
-- A very simple difference list.
module Distribution.Compat.DList (
    DList,
    runDList,
    empty,
    singleton,
    fromList,
    toList,
    snoc,
) where

import Prelude ()
import Distribution.Compat.Prelude hiding (toList, empty)

-- | Difference list.
newtype DList a = DList ([a] -> [a])

runDList :: DList a -> [a]
runDList :: forall a. DList a -> [a]
runDList (DList [a] -> [a]
run) = [a] -> [a]
run []

-- | Make 'DList' with containing single element.
singleton :: a -> DList a
singleton :: forall a. a -> DList a
singleton a
a = ([a] -> [a]) -> DList a
forall a. ([a] -> [a]) -> DList a
DList (a
aa -> [a] -> [a]
forall a. a -> [a] -> [a]
:)

-- | @since 3.4.0.0
empty :: DList a
empty :: forall a. DList a
empty = ([a] -> [a]) -> DList a
forall a. ([a] -> [a]) -> DList a
DList [a] -> [a]
forall a. a -> a
id

fromList :: [a] -> DList a
fromList :: forall a. [a] -> DList a
fromList [a]
as = ([a] -> [a]) -> DList a
forall a. ([a] -> [a]) -> DList a
DList ([a]
as [a] -> [a] -> [a]
forall a. [a] -> [a] -> [a]
++)

toList :: DList a -> [a]
toList :: forall a. DList a -> [a]
toList = DList a -> [a]
forall a. DList a -> [a]
runDList

snoc :: DList a -> a -> DList a
snoc :: forall a. DList a -> a -> DList a
snoc DList a
xs a
x = DList a
xs DList a -> DList a -> DList a
forall a. Semigroup a => a -> a -> a
<> a -> DList a
forall a. a -> DList a
singleton a
x

instance Monoid (DList a) where
  mempty :: DList a
mempty  = DList a
forall a. DList a
empty
  mappend :: DList a -> DList a -> DList a
mappend = DList a -> DList a -> DList a
forall a. Semigroup a => a -> a -> a
(<>)

instance Semigroup (DList a) where
  DList [a] -> [a]
a <> :: DList a -> DList a -> DList a
<> DList [a] -> [a]
b = ([a] -> [a]) -> DList a
forall a. ([a] -> [a]) -> DList a
DList ([a] -> [a]
a ([a] -> [a]) -> ([a] -> [a]) -> [a] -> [a]
forall b c a. (b -> c) -> (a -> b) -> a -> c
. [a] -> [a]
b)