module DList where

newtype DList a = DL ([a] -> [a])

fromDList :: DList a -> [a]
fromDList (DL f) = f []

singleton :: a -> DList a
singleton c = DL (c:)

empty :: DList a
empty = DL id

(+++) :: DList a -> DList a -> DList a
DL f +++  DL g = DL (f . g)