From f794b53658a22a1464db7168f9821c194bcf5268 Mon Sep 17 00:00:00 2001
From: Ashley Yakeley <ashley@semantic.org>
Date: Mon, 29 Oct 2007 02:25:26 +0000
Subject: [PATCH] new Control.Category, ghc ticket #1773

---
 Control/Applicative.hs |    6 ++----
 Control/Arrow.hs       |   20 ++++++++++----------
 Control/Category.hs    |   49 ++++++++++++++++++++++++++++++++++++++++++++++++
 Control/Compositor.hs  |   37 ------------------------------------
 base.cabal             |    2 +-
 5 files changed, 62 insertions(+), 52 deletions(-)
 create mode 100644 Control/Category.hs
 delete mode 100644 Control/Compositor.hs

diff --git a/Control/Applicative.hs b/Control/Applicative.hs
index cb76bcc..c54a346 100644
--- a/Control/Applicative.hs
+++ b/Control/Applicative.hs
@@ -35,11 +35,9 @@ module Control.Applicative (
 	optional, some, many
 	) where
 
-#ifdef __HADDOCK__
-import Prelude
-#endif
+import Prelude hiding (id,(.))
 
-import Control.Compositor
+import Control.Category
 import Control.Arrow
 	(Arrow(arr, (&&&)), ArrowZero(zeroArrow), ArrowPlus((<+>)))
 import Control.Monad (liftM, ap, MonadPlus(..))
diff --git a/Control/Arrow.hs b/Control/Arrow.hs
index a339adf..76c0212 100644
--- a/Control/Arrow.hs
+++ b/Control/Arrow.hs
@@ -36,11 +36,11 @@ module Control.Arrow (
 		ArrowLoop(..)
 	) where
 
-import Prelude
+import Prelude hiding (id,(.))
 
 import Control.Monad
 import Control.Monad.Fix
-import Control.Compositor
+import Control.Category
 
 infixr 5 <+>
 infixr 3 ***
@@ -55,7 +55,7 @@ infixr 1 ^<<, <<^
 --   as well as 'first'.  The other combinators have sensible
 --   default definitions, which may be overridden for efficiency.
 
-class Compositor a => Arrow a where
+class Category a => Arrow a where
 
 	-- | Lift a function to an arrow: you must define either this
 	--   or 'pure'.
@@ -96,9 +96,9 @@ class Compositor a => Arrow a where
 
 {-# RULES
 "identity"
-		arr id = identity
+		arr id = id
 "compose/arr"	forall f g .
-		arr f >>> arr g = arr (f >>> g)
+		(arr f) . (arr g) = arr (f . g)
 "first/arr"	forall f .
 		first (arr f) = arr (first f)
 "second/arr"	forall f .
@@ -108,9 +108,9 @@ class Compositor a => Arrow a where
 "fanout/arr"	forall f g .
 		arr f &&& arr g = arr (f &&& g)
 "compose/first"	forall f g .
-		first f >>> first g = first (f >>> g)
+		(first f) . (first g) = first (f . g)
 "compose/second" forall f g .
-		second f >>> second g = second (f >>> g)
+		(second f) . (second g) = second (f . g)
  #-}
 
 -- Ordinary functions are arrows.
@@ -127,9 +127,9 @@ instance Arrow (->) where
 
 newtype Kleisli m a b = Kleisli { runKleisli :: a -> m b }
 
-instance Monad m => Compositor (Kleisli m) where
-	identity = Kleisli return
-	Kleisli f >>> Kleisli g = Kleisli (\b -> f b >>= g)
+instance Monad m => Category (Kleisli m) where
+	id = Kleisli return
+	(Kleisli f) . (Kleisli g) = Kleisli (\b -> g b >>= f)
 
 instance Monad m => Arrow (Kleisli m) where
 	arr f = Kleisli (return . f)
diff --git a/Control/Category.hs b/Control/Category.hs
new file mode 100644
index 0000000..13fba06
--- /dev/null
+++ b/Control/Category.hs
@@ -0,0 +1,49 @@
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  Control.Category
+-- Copyright   :  (c) Ashley Yakeley 2007
+-- License     :  BSD-style (see the LICENSE file in the distribution)
+--
+-- Maintainer  :  ashley@semantic.org
+-- Stability   :  experimental
+-- Portability :  portable
+
+-- http://hackage.haskell.org/trac/ghc/ticket/1773
+
+module Control.Category where
+
+import Prelude hiding (id,(.))
+import qualified Prelude
+
+infixr 9 .
+infixr 1 >>>, <<<
+
+-- | A class for categories.
+--   id and (.) must form a monoid.
+class Category cat where
+	-- | the identity morphism
+	id :: cat a a
+
+	-- | morphism composition
+	(.) :: cat b c -> cat a b -> cat a c
+
+{-# RULES
+"identity/left"	forall p .
+		id . p = p
+"identity/right"	forall p .
+		p . id = p
+"association"	forall p q r .
+		(p . q) . r = p . (q . r)
+ #-}
+
+instance Category (->) where
+	id = Prelude.id
+	(.) = (Prelude..)
+
+-- | Right-to-left composition
+(<<<) :: Category cat => cat b c -> cat a b -> cat a c
+(<<<) = (.)
+
+-- | Left-to-right composition
+(>>>) :: Category cat => cat a b -> cat b c -> cat a c
+f >>> g = g . f
diff --git a/Control/Compositor.hs b/Control/Compositor.hs
deleted file mode 100644
index 7cc6381..0000000
--- a/Control/Compositor.hs
+++ /dev/null
@@ -1,37 +0,0 @@
------------------------------------------------------------------------------
--- |
--- Module      :  Control.Compositor
--- Copyright   :  (c) Ashley Yakeley 2007
--- License     :  BSD-style (see the LICENSE file in the distribution)
---
--- Maintainer  :  ashley@semantic.org
--- Stability   :  experimental
--- Portability :  portable
-
-module Control.Compositor where
-
-infixr 1 >>>, <<<
-
-class Compositor comp where
-	identity :: comp a a
-
-	-- | Left-to-right composition
-	(>>>) :: comp a b -> comp b c -> comp a c
-
-{-# RULES
-"identity/left"	forall p .
-		identity >>> p = p
-"identity/right"	forall p .
-		p >>> identity = p
-"association"	forall p q r .
-		(p >>> q) >>> r = p >>> (q >>> r)
- #-}
-
-instance Compositor (->) where
-	identity = id
-	p >>> q = q . p
-
--- | Right-to-left composition
-(<<<) :: Compositor comp => comp b c -> comp a b -> comp a c
-f <<< g = g >>> f
-
diff --git a/base.cabal b/base.cabal
index 14bcf08..376c15b 100644
--- a/base.cabal
+++ b/base.cabal
@@ -69,7 +69,7 @@ Library {
     exposed-modules:
         Control.Applicative,
         Control.Arrow,
-        Control.Compositor,
+        Control.Category,
         Control.Concurrent,
         Control.Concurrent.Chan,
         Control.Concurrent.MVar,
-- 
1.7.10.4