    {-# LANGUAGE CPP #-}
    module Distribution.Utils.NubList
        ( NubList    -- opaque
        , toNubList  -- smart construtor
        , fromNubList
        , overNubList

        , NubListR
        , toNubListR
        , fromNubListR
        , overNubListR
        ) where

    import Distribution.Compat.Binary
    #if __GLASGOW_HASKELL__ < 710
    import Data.Monoid
    #endif

    import Distribution.Simple.Utils (ordNub, listUnion, ordNubRight, listUnionRight)

    import qualified Text.Read as R

    -- | NubList : A de-duplicated list that maintains the original order.
    newtype NubList a =
        NubList { fromNubList :: [a] }
        deriving Eq

    -- NubList assumes that nub retains the list order while removing duplicate
    -- elements (keeping the first occurence). Documentation for "Data.List.nub"
    -- does not specifically state that ordering is maintained so we will add a test
    -- for that to the test suite.

    -- | Smart constructor for the NubList type.
    toNubList :: Ord a => [a] -> NubList a
    toNubList list = NubList $ ordNub list

    -- | Lift a function over lists to a function over NubLists.
    overNubList :: Ord a => ([a] -> [a]) -> NubList a -> NubList a
    overNubList f (NubList list) = toNubList . f $ list

    -- | Monoid operations on NubLists.
    -- For a valid Monoid instance we need to satistfy the required monoid laws;
    -- identity, associativity and closure.
    --
    -- Identity : by inspection:
    --      mempty `mappend` NubList xs == NubList xs `mappend` mempty
    --
    -- Associativity : by inspection:
    --      (NubList xs `mappend` NubList ys) `mappend` NubList zs
    --      == NubList xs `mappend` (NubList ys `mappend` NubList zs)
    --
    -- Closure : appending two lists of type a and removing duplicates obviously
    -- does not change the type.

    instance Ord a => Monoid (NubList a) where
        mempty = NubList []
        mappend (NubList xs) (NubList ys) = NubList $ xs `listUnion` ys

    instance Show a => Show (NubList a) where
        show (NubList list) = show list

    instance (Ord a, Read a) => Read (NubList a) where
        readPrec = readNubList toNubList

    -- | Helper used by NubList/NubListR's Read instances.
    readNubList :: (Ord a, Read a) => ([a] -> l a) -> R.ReadPrec (l a)
    readNubList toList = R.parens . R.prec 10 $ fmap toList R.readPrec

    -- | Binary instance for 'NubList a' is the same as for '[a]'. For 'put', we
    -- just pull off constructor and put the list. For 'get', we get the list and
    -- make a 'NubList' out of it using 'toNubList'.
    instance (Ord a, Binary a) => Binary (NubList a) where
        put (NubList l) = put l
        get = fmap toNubList get

    -- | NubListR : A right-biased version of 'NubList'. That is @toNubListR
    -- ["-XNoFoo", "-XFoo", "-XNoFoo"]@ will result in @["-XFoo", "-XNoFoo"]@,
    -- unlike the normal 'NubList', which is left-biased. Built on top of
    -- 'ordNubRight' and 'listUnionRight'.
    newtype NubListR a =
        NubListR { fromNubListR :: [a] }
        deriving Eq

    -- | Smart constructor for the NubListR type.
    toNubListR :: Ord a => [a] -> NubListR a
    toNubListR list = NubListR $ ordNubRight list

    -- | Lift a function over lists to a function over NubListRs.
    overNubListR :: Ord a => ([a] -> [a]) -> NubListR a -> NubListR a
    overNubListR f (NubListR list) = toNubListR . f $ list

    instance Ord a => Monoid (NubListR a) where
      mempty = NubListR []
      mappend (NubListR xs) (NubListR ys) = NubListR $ xs `listUnionRight` ys

    instance Show a => Show (NubListR a) where
      show (NubListR list) = show list

    instance (Ord a, Read a) => Read (NubListR a) where
        readPrec = readNubList toNubListR