{-# language
  TypeApplications,
  ScopedTypeVariables
#-}

module Data.DifferenceList where

import Data.Coerce (coerce)
import Data.Foldable (Foldable (toList))

newtype DifferenceList a = DifferenceList ([a] -> [a])

instance Semigroup (DifferenceList a) where
  (<>) =
    coerce @(([a] -> [a]) -> ([a] -> [a]) -> [a] -> [a]) -- ignore
      (.)

instance Monoid (DifferenceList a) where
  mempty =
    coerce @([a] -> [a]) -- ignore
      id

instance Foldable DifferenceList where
  toList (DifferenceList differenceList) = differenceList []
  foldr f initial = foldr f initial . toList

singleton :: a -> DifferenceList a
singleton = DifferenceList . (:)