-----------------------------------------------------------------------------
-- |
-- Module      :  Control.Arrow
-- Copyright   :  (c) Ross Paterson 2002
-- License     :  BSD-style (see the LICENSE file in the distribution)
--
-- Maintainer  :  ross@soi.city.ac.uk
-- Stability   :  experimental
-- Portability :  portable
--
-- Basic arrow definitions, based on
--	/Generalising Monads to Arrows/, by John Hughes,
--	/Science of Computer Programming/ 37, pp67-111, May 2000.
-- plus a couple of definitions ('returnA' and 'loop') from
--	/A New Notation for Arrows/, by Ross Paterson, in /ICFP 2001/,
--	Firenze, Italy, pp229-240.
-- See these papers for the equations these combinators are expected to
-- satisfy.  These papers and more information on arrows can be found at
-- <http://www.haskell.org/arrows/>.

module Control.Arrow (
		-- * Arrows
		Arrow(..), Kleisli(..),
		-- ** Derived combinators
		returnA, (<<<),
		-- * Monoid operations
		ArrowZero(..), ArrowPlus(..),
		-- * Conditionals
		ArrowChoice(..),
		-- * Arrow application
		ArrowApply(..), ArrowMonad(..), leftApp,
		-- * Feedback
		ArrowLoop(..)
	) where

import Prelude

import Control.Monad
import Control.Monad.Fix

infixr 5 <+>
infixr 3 ***
infixr 3 &&&
infixr 2 +++
infixr 2 |||
infixr 1 >>>
infixr 1 <<<

-- | The basic arrow class.
--   Any instance must define either 'arr' or 'pure' (which are synonyms),
--   as well as '>>>' and 'first'.  The other combinators have sensible
--   default definitions, which may be overridden for efficiency.

class Arrow a where

	-- | Lift a function to an arrow: you must define either this
	--   or 'pure'.
	arr :: (b -> c) -> a b c
	arr = pure

	-- | A synonym for 'arr': you must define one or other of them.
	pure :: (b -> c) -> a b c
	pure = arr

	-- | Left-to-right composition of arrows.
	(>>>) :: a b c -> a c d -> a b d

	-- | Send the first component of the input through the argument
	--   arrow, and copy the rest unchanged to the output.
	first :: a b c -> a (b,d) (c,d)

	-- | A mirror image of 'first'.
	--
	--   The default definition may be overridden with a more efficient
	--   version if desired.
	second :: a b c -> a (d,b) (d,c)
	second f = arr swap >>> first f >>> arr swap
			where	swap ~(x,y) = (y,x)

	-- | Split the input between the two argument arrows and combine
	--   their output.  Note that this is in general not a functor.
	--
	--   The default definition may be overridden with a more efficient
	--   version if desired.
	(***) :: a b c -> a b' c' -> a (b,b') (c,c')
	f *** g = first f >>> second g

	-- | Fanout: send the input to both argument arrows and combine
	--   their output.
	--
	--   The default definition may be overridden with a more efficient
	--   version if desired.
	(&&&) :: a b c -> a b c' -> a b (c,c')
	f &&& g = arr (\b -> (b,b)) >>> f *** g

-- Ordinary functions are arrows.

instance Arrow (->) where
	arr f = f
	f >>> g = g . f
	first f = f *** id
	second f = id *** f
--	(f *** g) ~(x,y) = (f x, g y)
--	sorry, although the above defn is fully H'98, nhc98 can't parse it.
	(***) f g ~(x,y) = (f x, g y)

-- | Kleisli arrows of a monad.

newtype Kleisli m a b = Kleisli (a -> m b)

instance Monad m => Arrow (Kleisli m) where
	arr f = Kleisli (return . f)
	Kleisli f >>> Kleisli g = Kleisli (\b -> f b >>= g)
	first (Kleisli f) = Kleisli (\ ~(b,d) -> f b >>= \c -> return (c,d))
	second (Kleisli f) = Kleisli (\ ~(d,b) -> f b >>= \c -> return (d,c))

-- | The identity arrow, which plays the role of 'return' in arrow notation.

returnA :: Arrow a => a b b
returnA = arr id

-- | Right-to-left composition, for a better fit with arrow notation.

(<<<) :: Arrow a => a c d -> a b c -> a b d
f <<< g = g >>> f

class Arrow a => ArrowZero a where
	zeroArrow :: a b c

instance MonadPlus m => ArrowZero (Kleisli m) where
	zeroArrow = Kleisli (\x -> mzero)

class ArrowZero a => ArrowPlus a where
	(<+>) :: a b c -> a b c -> a b c

instance MonadPlus m => ArrowPlus (Kleisli m) where
	Kleisli f <+> Kleisli g = Kleisli (\x -> f x `mplus` g x)

-- | Choice, for arrows that support it.  This class underlies the
--   @if@ and @case@ constructs in arrow notation.
--   Any instance must define 'left'.  The other combinators have sensible
--   default definitions, which may be overridden for efficiency.

class Arrow a => ArrowChoice a where

	-- | Feed marked inputs through the argument arrow, passing the
	--   rest through unchanged to the output.
	left :: a b c -> a (Either b d) (Either c d)

	-- | A mirror image of 'left'.
	--
	--   The default definition may be overridden with a more efficient
	--   version if desired.
	right :: a b c -> a (Either d b) (Either d c)
	right f = arr mirror >>> left f >>> arr mirror
			where	mirror (Left x) = Right x
				mirror (Right y) = Left y

	-- | Split the input between the two argument arrows, retagging
	--   and merging their outputs.
	--   Note that this is in general not a functor.
	--
	--   The default definition may be overridden with a more efficient
	--   version if desired.
	(+++) :: a b c -> a b' c' -> a (Either b b') (Either c c')
	f +++ g = left f >>> right g

	-- | Fanin: Split the input between the two argument arrows and
	--   merge their outputs.
	--
	--   The default definition may be overridden with a more efficient
	--   version if desired.
	(|||) :: a b d -> a c d -> a (Either b c) d
	f ||| g = f +++ g >>> arr untag
			where	untag (Left x) = x
				untag (Right y) = y

instance ArrowChoice (->) where
	left f = f +++ id
	right f = id +++ f
	f +++ g = (Left . f) ||| (Right . g)
	(|||) = either

instance Monad m => ArrowChoice (Kleisli m) where
	left f = f +++ arr id
	right f = arr id +++ f
	f +++ g = (f >>> arr Left) ||| (g >>> arr Right)
	Kleisli f ||| Kleisli g = Kleisli (either f g)

-- | Some arrows allow application of arrow inputs to other inputs.

class Arrow a => ArrowApply a where
	app :: a (a b c, b) c

instance ArrowApply (->) where
	app (f,x) = f x

instance Monad m => ArrowApply (Kleisli m) where
	app = Kleisli (\(Kleisli f, x) -> f x)

-- | The 'ArrowApply' class is equivalent to 'Monad': any monad gives rise
--   to a 'Kleisli' arrow, and any instance of 'ArrowApply' defines a monad.

newtype ArrowApply a => ArrowMonad a b = ArrowMonad (a () b)

instance ArrowApply a => Monad (ArrowMonad a) where
	return x = ArrowMonad (arr (\z -> x))
	ArrowMonad m >>= f = ArrowMonad (m >>>
			arr (\x -> let ArrowMonad h = f x in (h, ())) >>>
			app)

-- | Any instance of 'ArrowApply' can be made into an instance of
--   'ArrowChoice' by defining 'left' = 'leftApp'.

leftApp :: ArrowApply a => a b c -> a (Either b d) (Either c d)
leftApp f = arr ((\b -> (arr (\() -> b) >>> f >>> arr Left, ())) |||
		 (\d -> (arr (\() -> d) >>> arr Right, ()))) >>> app

-- | The 'loop' operator expresses computations in which an output value is
--   fed back as input, even though the computation occurs only once.
--   It underlies the @rec@ value recursion construct in arrow notation.

class Arrow a => ArrowLoop a where
	loop :: a (b,d) (c,d) -> a b c

instance ArrowLoop (->) where
	loop f b = let (c,d) = f (b,d) in c

instance MonadFix m => ArrowLoop (Kleisli m) where
	loop (Kleisli f) = Kleisli (liftM fst . mfix . f')
		where	f' x y = f (x, snd y)
