X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Control%2FApplicative.hs;h=c38e5809d9c75b7e71b64ac96dd169e3657bbca6;hb=41e8fba828acbae1751628af50849f5352b27873;hp=2149b7a796d9590261c3443925cedb0b12577f6d;hpb=bc428b7366dab68da9eb40815e05aead9713770d;p=ghc-base.git

diff --git a/Control/Applicative.hs b/Control/Applicative.hs
index 2149b7a..c38e580 100644
--- a/Control/Applicative.hs
+++ b/Control/Applicative.hs
@@ -1,10 +1,12 @@
+{-# LANGUAGE CPP #-}
+
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Applicative
 -- 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
 --
@@ -23,24 +25,36 @@
 -- '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
+    -- * Applicative functors
+    Applicative(..),
+    -- * Alternatives
+    Alternative(..),
+    -- * Instances
+    Const(..), WrappedMonad(..), WrappedArrow(..), ZipList(..),
+    -- * Utility functions
+    (<$>), (<$), (<**>),
+    liftA, liftA2, liftA3,
+    optional,
+    ) where
+
+import Prelude hiding (id,(.))
 
-import Control.Monad (liftM, ap)
+import Control.Category
+import Control.Arrow (Arrow(arr, (&&&)), ArrowZero(zeroArrow), ArrowPlus((<+>)))
+import Control.Monad (liftM, ap, MonadPlus(..))
 import Control.Monad.Instances ()
+#ifndef __NHC__
+import Control.Monad.ST (ST)
+import qualified Control.Monad.ST.Lazy as Lazy (ST)
+#endif
+import Data.Functor ((<$>), (<$))
 import Data.Monoid (Monoid(..))
 
-infixl 4 <$>, <$
+#ifdef __GLASGOW_HASKELL__
+import GHC.Conc (STM, retry, orElse)
+#endif
+
+infixl 3 <|>
 infixl 4 <*>, <*, *>, <**>
 
 -- | A functor with application.
@@ -48,73 +62,169 @@ 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@
+--
+-- [/ignore left value/]
+--      @u '*>' v = 'pure' ('const' 'id') '<*>' u '<*>' v@
+--
+-- [/ignore right value/]
+--      @u '<*' v = 'pure' 'const' '<*>' u '<*>' v@
 --
 -- The 'Functor' instance should satisfy
 --
 -- @
---	'fmap' f x = 'pure' f '<*>' x
+--      'fmap' f x = 'pure' f '<*>' x
 -- @
 --
 -- If @f@ is also a 'Monad', define @'pure' = 'return'@ and @('<*>') = 'ap'@.
+--
+-- Minimal complete definition: 'pure' and '<*>'.
 
 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
 
-        -- | Sequential application.
-	(<*>) :: f (a -> b) -> f a -> f b
+    -- | Sequence actions, discarding the value of the first argument.
+    (*>) :: f a -> f b -> f b
+    (*>) = liftA2 (const id)
+
+    -- | Sequence actions, discarding the value of the second argument.
+    (<*) :: f a -> f b -> f a
+    (<*) = liftA2 const
+
+-- | A monoid on applicative functors.
+--
+-- Minimal complete definition: 'empty' and '<|>'.
+--
+-- 'some' and 'many' should be the least solutions of the equations:
+--
+-- * @some v = (:) '<$>' v '<*>' many v@
+--
+-- * @many v = some v '<|>' 'pure' []@
+class Applicative f => Alternative f where
+    -- | The identity of '<|>'
+    empty :: f a
+    -- | An associative binary operation
+    (<|>) :: f a -> f a -> f a
+
+    -- | One or more.
+    some :: f a -> f [a]
+    some v = some_v
+      where
+        many_v = some_v <|> pure []
+        some_v = (:) <$> v <*> many_v
+
+    -- | Zero or more.
+    many :: f a -> f [a]
+    many v = many_v
+      where
+        many_v = some_v <|> pure []
+        some_v = (:) <$> v <*> many_v
 
 -- 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
+
+#ifndef __NHC__
+instance Applicative (ST s) where
+    pure = return
+    (<*>) = ap
+
+instance Applicative (Lazy.ST s) where
+    pure = return
+    (<*>) = ap
+#endif
+
+#ifdef __GLASGOW_HASKELL__
+instance Applicative STM where
+    pure = return
+    (<*>) = ap
+
+instance Alternative STM where
+    empty = retry
+    (<|>) = orElse
+#endif
 
 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)
+
+instance Applicative (Either e) where
+    pure          = Right
+    Left  e <*> _ = Left e
+    Right f <*> r = fmap f r
 
 -- 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
 --
@@ -123,30 +233,14 @@ 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
 
--- | A synonym for 'fmap'.
-(<$>) :: Functor f => (a -> b) -> f a -> f b
-f <$> a = fmap f a
-
--- | Replace the value.
-(<$) :: Functor f => a -> f b -> f a
-(<$) = (<$>) . const
- 
--- | Sequence actions, discarding the value of the first argument.
-(*>) :: Applicative f => f a -> f b -> f b
-(*>) = liftA2 (const id)
- 
--- | Sequence actions, discarding the value of the second argument.
-(<*) :: Applicative f => f a -> f b -> f a
-(<*) = liftA2 const
- 
 -- | A variant of '<*>' with the arguments reversed.
 (<**>) :: Applicative f => f a -> f (a -> b) -> f b
 (<**>) = liftA2 (flip ($))
@@ -163,3 +257,7 @@ 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