{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}

-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Monoid.Lexical.RunLengthEncoding
-- Copyright   :  (c) Edward Kmett 2009
-- License     :  BSD-style
-- Maintainer  :  libraries@haskell.org
-- Stability   :  experimental
-- Portability :  non-portable (MPTCs)
--
-- A simple 'Monoid' transformer that turns any monoidal 'Reducer' into a
-- a reducer that expects to be supplied both a run length @n@ with each item
-- and which efficiently exponentiates the result of 'unit' @n@ times through 
-- 'replicate'.
--
-----------------------------------------------------------------------------

module Data.Monoid.Lexical.RunLengthEncoding 
    ( module Data.Monoid.Reducer
    , RLE(RLE,getRLE) 
    ) where

import Prelude hiding (replicate)
import Data.Monoid.Reducer
import Data.Monoid.Combinators (replicate)

newtype RLE n m = RLE { getRLE :: m } 

instance (Integral n, Monoid m) => Monoid (RLE n m) where
    mempty = RLE mempty
    RLE a `mappend` RLE b = RLE (a `mappend` b)

instance (Integral n, Reducer c m) => Reducer (n,c) (RLE n m) where
    unit ~(n,c) = RLE $ replicate (unit c) n