-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Functor.Plus
-- Copyright   :  (C) 2011 Edward Kmett
-- License     :  BSD-style (see the file LICENSE)
--
-- Maintainer  :  Edward Kmett <ekmett@gmail.com>
-- Stability   :  provisional
-- Portability :  portable
--
----------------------------------------------------------------------------
module Data.Functor.Plus
  ( Plus(..)
  , module Data.Functor.Alt
  ) where

import Control.Applicative hiding (some, many)
import Control.Arrow
-- import Control.Exception
import Control.Monad
import Control.Monad.Trans.Identity
-- import Control.Monad.Trans.Cont
import Control.Monad.Trans.Error
import Control.Monad.Trans.List
import Control.Monad.Trans.Maybe
import Control.Monad.Trans.Reader
import qualified Control.Monad.Trans.RWS.Strict as Strict
import qualified Control.Monad.Trans.State.Strict as Strict
import qualified Control.Monad.Trans.Writer.Strict as Strict
import qualified Control.Monad.Trans.RWS.Lazy as Lazy
import qualified Control.Monad.Trans.State.Lazy as Lazy
import qualified Control.Monad.Trans.Writer.Lazy as Lazy
import Data.Functor.Apply
import Data.Functor.Alt
import Data.Functor.Bind
import qualified Data.IntMap as IntMap
import Data.IntMap (IntMap)
import Data.Semigroup
import Data.Sequence (Seq)
import qualified Data.Map as Map
import Data.Map (Map)
import Prelude hiding (id, (.))

-- | Laws:
-- 
-- > zero <!> m = m
-- > m <!> zero = m
--
-- If extended to an 'Alternative' then 'zero' should equal 'empty'.

class Alt f => Plus f where
  zero :: f a 

instance Plus IO where
  zero = error "zero"

instance Plus [] where
  zero = []

instance Plus Maybe where
  zero = Nothing

instance Plus Option where
  zero = empty

instance MonadPlus m => Plus (WrappedMonad m) where
  zero = empty

instance ArrowPlus a => Plus (WrappedArrow a b) where
  zero = empty

instance Ord k => Plus (Map k) where
  zero = Map.empty

instance Plus IntMap where
  zero = IntMap.empty

instance Plus Seq where
  zero = mempty

instance Alternative f => Plus (WrappedApplicative f) where
  zero = empty

instance Plus f => Plus (IdentityT f) where
  zero = IdentityT zero

instance Plus f => Plus (ReaderT e f) where
  zero = ReaderT $ \_ -> zero

instance (Bind f, Monad f) => Plus (MaybeT f) where
  zero = MaybeT $ return zero
  
instance (Bind f, Monad f, Error e) => Plus (ErrorT e f) where
  zero = ErrorT $ return $ Left noMsg

instance (Apply f, Applicative f) => Plus (ListT f) where
  zero = ListT $ pure []

instance (Plus f) => Plus (Strict.StateT e f) where
  zero = Strict.StateT $ \_ -> zero
  
instance (Plus f) => Plus (Lazy.StateT e f) where
  zero = Lazy.StateT $ \_ -> zero

instance Plus f => Plus (Strict.WriterT w f) where
  zero = Strict.WriterT zero
  
instance Plus f => Plus (Lazy.WriterT w f) where
  zero = Lazy.WriterT zero
  
instance Plus f => Plus (Strict.RWST r w s f) where
  zero = Strict.RWST $ \_ _ -> zero 

instance Plus f => Plus (Lazy.RWST r w s f) where
  zero = Lazy.RWST $ \_ _ -> zero