Instances.hs

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, UndecidableInstances, OverloadedStrings #-}
{-# OPTIONS_GHC -fno-warn-orphans #-}

-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Monoid.Instances
-- Copyright   :  (c) Edward Kmett 2009
-- License     :  BSD-style
-- Maintainer  :  libraries@haskell.org
-- Stability   :  experimental
-- Portability :  portable
--
-- A collection of orphan instance declarations for Monoids that should
-- eventually be pushed back down to the source packages.
--
-- Every package that uses these instances includes this package internally.
--
-- Includes:
--
-- * 'Monoid' instances for the 'Monad' transformers from the mtl package
--
-- * A 'Monoid' instance for the 'ParsecT' 'Monad' from parsec-3.
--
-- * 'IsString' instances for tuples of 'IsString' for overloaded string support.
--
-- * A 'Monoid' instance for the 'FingerTree' in the fingertree package 
--
-- * 'Monoid' instances for 'Int', 'Integer', and 'Ratio' using @(+,0)@
--
-- This module is automatically included everywhere this functionality is required
-- within this package. You should only have to import this module yourself if you 
-- want these instances for your own purposes.
-----------------------------------------------------------------------------

module Data.Monoid.Instances where

import Control.Monad.Reader

import qualified Control.Monad.RWS.Lazy as LRWS
import qualified Control.Monad.RWS.Strict as SRWS

import qualified Control.Monad.State.Lazy as LState
import qualified Control.Monad.State.Strict as SState

import Control.Monad.Writer
import qualified Control.Monad.Writer.Strict as SWriter

import Data.String

import Data.Ratio

import Data.FingerTree

import Text.Parsec.Prim

-- orphan Monoid instances for Monad Transformers
instance (MonadPlus m, Monoid w) => Monoid (SWriter.WriterT w m n) where
    mempty = mzero
    mappend = mplus

instance (MonadPlus m, Monoid w) => Monoid (WriterT w m n) where
    mempty = mzero
    mappend = mplus

instance (MonadPlus m, Monoid w) => Monoid (SRWS.RWST r w s m n) where 
    mempty = mzero
    mappend = mplus

instance (MonadPlus m, Monoid w) => Monoid (LRWS.RWST r w s m n) where 
    mempty = mzero
    mappend = mplus

instance MonadPlus m => Monoid (ReaderT e m n) where
    mempty = mzero
    mappend = mplus

instance MonadPlus m => Monoid (SState.StateT s m n) where
    mempty = mzero
    mappend = mplus

instance MonadPlus m => Monoid (LState.StateT s m n) where
    mempty = mzero
    mappend = mplus

-- orphan, which should be in Data.FingerTree
instance Measured v a => Monoid (FingerTree v a) where
    mempty = empty
    mappend = (><)

-- orphan, which should be in Parsec
instance Stream s m t => Monoid (ParsecT s u m a) where
    mempty = mzero
    a `mappend` b = try a <|> b

-- orphan, perhaps should be in Data.String
instance (IsString a, IsString b) => IsString (a,b) where
    fromString a = (fromString a, fromString a)

instance (IsString a, IsString b, IsString c) => IsString (a,b,c) where
    fromString a = (fromString a, fromString a, fromString a)

instance (IsString a, IsString b, IsString c, IsString d) => IsString (a,b,c,d) where
    fromString a = (fromString a, fromString a, fromString a, fromString a)

instance (IsString a, IsString b, IsString c, IsString d, IsString e) => IsString (a,b,c,d,e) where
    fromString a = (fromString a, fromString a, fromString a, fromString a, fromString a)

instance Monoid Int where
    mempty = 0
    mappend = (+)

instance Monoid Integer where
    mempty = 0
    mappend = (+)

instance Integral m => Monoid (Ratio m) where
    mempty = 0
    mappend = (+)