X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Control%2FApplicative.hs;h=d2899162bc4179ff105e1e099341e5fee6c843f6;hb=e7a5a94532447b071406f8e7ee6f6ae626d2d754;hp=fa7837140a835fcb87d857bef72212667b3730bd;hpb=0f7f84221836acde80b6337ef2e51d6508f73f7f;p=ghc-base.git

diff --git a/Control/Applicative.hs b/Control/Applicative.hs
index fa78371..d289916 100644
--- a/Control/Applicative.hs
+++ b/Control/Applicative.hs
@@ -4,7 +4,7 @@
 -- Copyright   :  Conor McBride and Ross Paterson 2005
 -- License     :  BSD-style (see the LICENSE file in the distribution)
 --
--- Maintainer  :  ross@soi.city.ac.uk
+-- Maintainer  :  libraries@haskell.org
 -- Stability   :  experimental
 -- Portability :  portable
 --
@@ -23,23 +23,29 @@
 -- 'Data.Traversable.Traversable' class.
 
 module Control.Applicative (
-	-- * Applicative functors
-	Applicative(..),
-	-- * Instances
-	WrappedMonad(..), Const(..), ZipList(..),
-	-- * Utility functions
-	(<$>), (<$), (*>), (<*), (<**>),
-	liftA, liftA2, liftA3
-	) where
-
-#ifdef __HADDOCK__
-import Prelude
-#endif
-
-import Control.Monad (liftM, ap)
+        -- * Applicative functors
+        Applicative(..),
+        -- * Alternatives
+        Alternative(..),
+        -- * Instances
+        Const(..), WrappedMonad(..), WrappedArrow(..), ZipList(..),
+        -- * Utility functions
+        (<$>), (<$), (*>), (<*), (<**>),
+        liftA, liftA2, liftA3,
+        optional, some, many
+        ) where
+
+import Prelude hiding (id,(.))
+import qualified Prelude
+
+import Control.Category
+import Control.Arrow
+        (Arrow(arr, (&&&)), ArrowZero(zeroArrow), ArrowPlus((<+>)))
+import Control.Monad (liftM, ap, MonadPlus(..))
 import Control.Monad.Instances ()
 import Data.Monoid (Monoid(..))
 
+infixl 3 <|>
 infixl 4 <$>, <$
 infixl 4 <*>, <*, *>, <**>
 
@@ -48,67 +54,106 @@ infixl 4 <*>, <*, *>, <**>
 -- Instances should satisfy the following laws:
 --
 -- [/identity/]
---	@'pure' 'id' '<*>' v = v@
+--      @'pure' 'id' '<*>' v = v@
 --
 -- [/composition/]
---	@'pure' (.) '<*>' u '<*>' v '<*>' w = u '<*>' (v '<*>' w)@
+--      @'pure' (.) '<*>' u '<*>' v '<*>' w = u '<*>' (v '<*>' w)@
 --
 -- [/homomorphism/]
---	@'pure' f '<*>' 'pure' x = 'pure' (f x)@
+--      @'pure' f '<*>' 'pure' x = 'pure' (f x)@
 --
 -- [/interchange/]
---	@u '<*>' 'pure' y = 'pure' ('$' y) '<*>' u@
+--      @u '<*>' 'pure' y = 'pure' ('$' y) '<*>' u@
+--
+-- The 'Functor' instance should satisfy
+--
+-- @
+--      'fmap' f x = 'pure' f '<*>' x
+-- @
 --
 -- If @f@ is also a 'Monad', define @'pure' = 'return'@ and @('<*>') = 'ap'@.
 
 class Functor f => Applicative f where
-	-- | Lift a value.
-	pure :: a -> f a
+        -- | Lift a value.
+        pure :: a -> f a
 
         -- | Sequential application.
-	(<*>) :: f (a -> b) -> f a -> f b
+        (<*>) :: f (a -> b) -> f a -> f b
+
+-- | A monoid on applicative functors.
+class Applicative f => Alternative f where
+        -- | The identity of '<|>'
+        empty :: f a
+        -- | An associative binary operation
+        (<|>) :: f a -> f a -> f a
 
 -- instances for Prelude types
 
 instance Applicative Maybe where
-	pure = return
-	(<*>) = ap
+        pure = return
+        (<*>) = ap
+
+instance Alternative Maybe where
+        empty = Nothing
+        Nothing <|> p = p
+        Just x <|> _ = Just x
 
 instance Applicative [] where
-	pure = return
-	(<*>) = ap
+        pure = return
+        (<*>) = ap
+
+instance Alternative [] where
+        empty = []
+        (<|>) = (++)
 
 instance Applicative IO where
-	pure = return
-	(<*>) = ap
+        pure = return
+        (<*>) = ap
 
 instance Applicative ((->) a) where
-	pure = const
-	(<*>) f g x = f x (g x)
+        pure = const
+        (<*>) f g x = f x (g x)
 
 instance Monoid a => Applicative ((,) a) where
-	pure x = (mempty, x)
-	(u, f) <*> (v, x) = (u `mappend` v, f x)
+        pure x = (mempty, x)
+        (u, f) <*> (v, x) = (u `mappend` v, f x)
 
 -- new instances
 
+newtype Const a b = Const { getConst :: a }
+
+instance Functor (Const m) where
+        fmap _ (Const v) = Const v
+
+instance Monoid m => Applicative (Const m) where
+        pure _ = Const mempty
+        Const f <*> Const v = Const (f `mappend` v)
+
 newtype WrappedMonad m a = WrapMonad { unwrapMonad :: m a }
 
 instance Monad m => Functor (WrappedMonad m) where
-	fmap f (WrapMonad v) = WrapMonad (liftM f v)
+        fmap f (WrapMonad v) = WrapMonad (liftM f v)
 
 instance Monad m => Applicative (WrappedMonad m) where
-	pure = WrapMonad . return
-	WrapMonad f <*> WrapMonad v = WrapMonad (f `ap` v)
+        pure = WrapMonad . return
+        WrapMonad f <*> WrapMonad v = WrapMonad (f `ap` v)
 
-newtype Const a b = Const { getConst :: a }
+instance MonadPlus m => Alternative (WrappedMonad m) where
+        empty = WrapMonad mzero
+        WrapMonad u <|> WrapMonad v = WrapMonad (u `mplus` v)
 
-instance Functor (Const m) where
-	fmap _ (Const v) = Const v
+newtype WrappedArrow a b c = WrapArrow { unwrapArrow :: a b c }
 
-instance Monoid m => Applicative (Const m) where
-	pure _ = Const mempty
-	Const f <*> Const v = Const (f `mappend` v)
+instance Arrow a => Functor (WrappedArrow a b) where
+        fmap f (WrapArrow a) = WrapArrow (a >>> arr f)
+
+instance Arrow a => Applicative (WrappedArrow a b) where
+        pure x = WrapArrow (arr (const x))
+        WrapArrow f <*> WrapArrow v = WrapArrow (f &&& v >>> arr (uncurry id))
+
+instance (ArrowZero a, ArrowPlus a) => Alternative (WrappedArrow a b) where
+        empty = WrapArrow zeroArrow
+        WrapArrow u <|> WrapArrow v = WrapArrow (u <+> v)
 
 -- | Lists, but with an 'Applicative' functor based on zipping, so that
 --
@@ -117,11 +162,11 @@ instance Monoid m => Applicative (Const m) where
 newtype ZipList a = ZipList { getZipList :: [a] }
 
 instance Functor ZipList where
-	fmap f (ZipList xs) = ZipList (map f xs)
+        fmap f (ZipList xs) = ZipList (map f xs)
 
 instance Applicative ZipList where
-	pure x = ZipList (repeat x)
-	ZipList fs <*> ZipList xs = ZipList (zipWith id fs xs)
+        pure x = ZipList (repeat x)
+        ZipList fs <*> ZipList xs = ZipList (zipWith id fs xs)
 
 -- extra functions
 
@@ -157,3 +202,19 @@ liftA2 f a b = f <$> a <*> b
 -- | Lift a ternary function to actions.
 liftA3 :: Applicative f => (a -> b -> c -> d) -> f a -> f b -> f c -> f d
 liftA3 f a b c = f <$> a <*> b <*> c
+
+-- | One or none.
+optional :: Alternative f => f a -> f (Maybe a)
+optional v = Just <$> v <|> pure Nothing
+
+-- | One or more.
+some :: Alternative f => f a -> f [a]
+some v = some_v
+  where many_v = some_v <|> pure []
+        some_v = (:) <$> v <*> many_v
+
+-- | Zero or more.
+many :: Alternative f => f a -> f [a]
+many v = many_v
+  where many_v = some_v <|> pure []
+        some_v = (:) <$> v <*> many_v