X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Control%2FArrow.hs;h=2710be6e9c469e3b05da06b1013ad64e4eac610b;hb=7c0b04fd273621130062418bb764809c79488dd2;hp=fd1552a05741af4613370ac71bf11730ee13e4e2;hpb=dc3a9adce050b1db078c939473c7e293721f5c2c;p=haskell-directory.git

diff --git a/Control/Arrow.hs b/Control/Arrow.hs
index fd1552a..2710be6 100644
--- a/Control/Arrow.hs
+++ b/Control/Arrow.hs
@@ -22,7 +22,10 @@ module Control.Arrow (
 		-- * Arrows
 		Arrow(..), Kleisli(..),
 		-- ** Derived combinators
-		returnA, (<<<),
+		returnA,
+		(^>>), (>>^),
+		-- ** Right-to-left variants
+		(<<<), (<<^), (^<<),
 		-- * Monoid operations
 		ArrowZero(..), ArrowPlus(..),
 		-- * Conditionals
@@ -43,8 +46,8 @@ infixr 3 ***
 infixr 3 &&&
 infixr 2 +++
 infixr 2 |||
-infixr 1 >>>
-infixr 1 <<<
+infixr 1 >>>, ^>>, >>^
+infixr 1 <<<, ^<<, <<^
 
 -- | The basic arrow class.
 --   Any instance must define either 'arr' or 'pure' (which are synonyms),
@@ -93,6 +96,23 @@ class Arrow a where
 	(&&&) :: a b c -> a b c' -> a b (c,c')
 	f &&& g = arr (\b -> (b,b)) >>> f *** g
 
+{-# RULES
+"compose/arr"	forall f g .
+		arr f >>> arr g = arr (f >>> g)
+"first/arr"	forall f .
+		first (arr f) = arr (first f)
+"second/arr"	forall f .
+		second (arr f) = arr (second f)
+"product/arr"	forall f g .
+		arr f *** arr g = arr (f *** g)
+"fanout/arr"	forall f g .
+		arr f &&& arr g = arr (f &&& g)
+"compose/first"	forall f g .
+		first f >>> first g = first (f >>> g)
+"compose/second" forall f g .
+		second f >>> second g = second (f >>> g)
+ #-}
+
 -- Ordinary functions are arrows.
 
 instance Arrow (->) where
@@ -100,11 +120,13 @@ instance Arrow (->) where
 	f >>> g = g . f
 	first f = f *** id
 	second f = id *** f
-	(f *** g) ~(x,y) = (f x, g y)
+--	(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)
+newtype Kleisli m a b = Kleisli { runKleisli :: a -> m b }
 
 instance Monad m => Arrow (Kleisli m) where
 	arr f = Kleisli (return . f)
@@ -117,11 +139,26 @@ instance Monad m => Arrow (Kleisli m) where
 returnA :: Arrow a => a b b
 returnA = arr id
 
--- | Right-to-left composition, for a better fit with arrow notation.
+-- | Precomposition with a pure function.
+(^>>) :: Arrow a => (b -> c) -> a c d -> a b d
+f ^>> a = arr f >>> a
 
+-- | Postcomposition with a pure function.
+(>>^) :: Arrow a => a b c -> (c -> d) -> a b d
+a >>^ f = a >>> arr f
+
+-- | 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
 
+-- | Precomposition with a pure function (right-to-left variant).
+(<<^) :: Arrow a => a c d -> (b -> c) -> a b d
+a <<^ f = a <<< arr f
+
+-- | Postcomposition with a pure function (right-to-left variant).
+(^<<) :: Arrow a => (c -> d) -> a b c -> a b d
+f ^<< a = arr f <<< a
+
 class Arrow a => ArrowZero a where
 	zeroArrow :: a b c
 
@@ -173,6 +210,21 @@ class Arrow a => ArrowChoice a where
 			where	untag (Left x) = x
 				untag (Right y) = y
 
+{-# RULES
+"left/arr"	forall f .
+		left (arr f) = arr (left f)
+"right/arr"	forall f .
+		right (arr f) = arr (right f)
+"sum/arr"	forall f g .
+		arr f +++ arr g = arr (f +++ g)
+"fanin/arr"	forall f g .
+		arr f ||| arr g = arr (f ||| g)
+"compose/left"	forall f g .
+		left f >>> left g = left (f >>> g)
+"compose/right"	forall f g .
+		right f >>> right g = right (f >>> g)
+ #-}
+
 instance ArrowChoice (->) where
 	left f = f +++ id
 	right f = id +++ f