1 -----------------------------------------------------------------------------
3 -- Module : Control.Applicative
4 -- Copyright : Conor McBride and Ross Paterson 2005
5 -- License : BSD-style (see the LICENSE file in the distribution)
7 -- Maintainer : libraries@haskell.org
8 -- Stability : experimental
9 -- Portability : portable
11 -- This module describes a structure intermediate between a functor and
12 -- a monad: it provides pure expressions and sequencing, but no binding.
13 -- (Technically, a strong lax monoidal functor.) For more details, see
14 -- /Applicative Programming with Effects/,
15 -- by Conor McBride and Ross Paterson, online at
16 -- <http://www.soi.city.ac.uk/~ross/papers/Applicative.html>.
18 -- This interface was introduced for parsers by Niklas Röjemo, because
19 -- it admits more sharing than the monadic interface. The names here are
20 -- mostly based on recent parsing work by Doaitse Swierstra.
22 -- This class is also useful with instances of the
23 -- 'Data.Traversable.Traversable' class.
25 module Control.Applicative (
26 -- * Applicative functors
31 Const(..), WrappedMonad(..), WrappedArrow(..), ZipList(..),
32 -- * Utility functions
34 liftA, liftA2, liftA3,
38 import Prelude hiding (id,(.))
40 import Control.Category
42 (Arrow(arr, (&&&)), ArrowZero(zeroArrow), ArrowPlus((<+>)))
43 import Control.Monad (liftM, ap, MonadPlus(..))
44 import Control.Monad.Instances ()
45 import Control.Monad.ST (ST)
46 import qualified Control.Monad.ST.Lazy as Lazy (ST)
47 import Data.Functor ((<$>), (<$))
48 import Data.Monoid (Monoid(..))
50 #ifdef __GLASGOW_HASKELL__
51 import GHC.Conc (STM, retry, orElse)
55 infixl 4 <*>, <*, *>, <**>
57 -- | A functor with application.
59 -- Instances should satisfy the following laws:
62 -- @'pure' 'id' '<*>' v = v@
65 -- @'pure' (.) '<*>' u '<*>' v '<*>' w = u '<*>' (v '<*>' w)@
68 -- @'pure' f '<*>' 'pure' x = 'pure' (f x)@
71 -- @u '<*>' 'pure' y = 'pure' ('$' y) '<*>' u@
73 -- [/ignore left value/]
74 -- @u '*>' v = 'pure' ('const' 'id') '<*>' u '<*>' v@
76 -- [/ignore right value/]
77 -- @u '<*' v = 'pure' 'const' '<*>' u '<*>' v@
79 -- The 'Functor' instance should satisfy
82 -- 'fmap' f x = 'pure' f '<*>' x
85 -- If @f@ is also a 'Monad', define @'pure' = 'return'@ and @('<*>') = 'ap'@.
87 -- Minimal complete definition: 'pure' and '<*>'.
89 class Functor f => Applicative f where
93 -- | Sequential application.
94 (<*>) :: f (a -> b) -> f a -> f b
96 -- | Sequence actions, discarding the value of the first argument.
97 (*>) :: f a -> f b -> f b
98 (*>) = liftA2 (const id)
100 -- | Sequence actions, discarding the value of the second argument.
101 (<*) :: f a -> f b -> f a
104 -- | A monoid on applicative functors.
106 -- Minimal complete definition: 'empty' and '<|>'.
108 -- 'some' and 'many' should be the least solutions of the equations:
110 -- * @some v = (:) '<$>' v '<*>' many v@
112 -- * @many v = some v '<|>' 'pure' []@
113 class Applicative f => Alternative f where
114 -- | The identity of '<|>'
116 -- | An associative binary operation
117 (<|>) :: f a -> f a -> f a
122 where many_v = some_v <|> pure []
123 some_v = (:) <$> v <*> many_v
128 where many_v = some_v <|> pure []
129 some_v = (:) <$> v <*> many_v
131 -- instances for Prelude types
133 instance Applicative Maybe where
137 instance Alternative Maybe where
140 Just x <|> _ = Just x
142 instance Applicative [] where
146 instance Alternative [] where
150 instance Applicative IO where
154 instance Applicative (ST s) where
158 instance Applicative (Lazy.ST s) where
162 #ifdef __GLASGOW_HASKELL__
163 instance Applicative STM where
167 instance Alternative STM where
172 instance Applicative ((->) a) where
174 (<*>) f g x = f x (g x)
176 instance Monoid a => Applicative ((,) a) where
178 (u, f) <*> (v, x) = (u `mappend` v, f x)
180 instance Applicative (Either e) where
182 Left e <*> _ = Left e
183 Right f <*> r = fmap f r
187 newtype Const a b = Const { getConst :: a }
189 instance Functor (Const m) where
190 fmap _ (Const v) = Const v
192 instance Monoid m => Applicative (Const m) where
193 pure _ = Const mempty
194 Const f <*> Const v = Const (f `mappend` v)
196 newtype WrappedMonad m a = WrapMonad { unwrapMonad :: m a }
198 instance Monad m => Functor (WrappedMonad m) where
199 fmap f (WrapMonad v) = WrapMonad (liftM f v)
201 instance Monad m => Applicative (WrappedMonad m) where
202 pure = WrapMonad . return
203 WrapMonad f <*> WrapMonad v = WrapMonad (f `ap` v)
205 instance MonadPlus m => Alternative (WrappedMonad m) where
206 empty = WrapMonad mzero
207 WrapMonad u <|> WrapMonad v = WrapMonad (u `mplus` v)
209 newtype WrappedArrow a b c = WrapArrow { unwrapArrow :: a b c }
211 instance Arrow a => Functor (WrappedArrow a b) where
212 fmap f (WrapArrow a) = WrapArrow (a >>> arr f)
214 instance Arrow a => Applicative (WrappedArrow a b) where
215 pure x = WrapArrow (arr (const x))
216 WrapArrow f <*> WrapArrow v = WrapArrow (f &&& v >>> arr (uncurry id))
218 instance (ArrowZero a, ArrowPlus a) => Alternative (WrappedArrow a b) where
219 empty = WrapArrow zeroArrow
220 WrapArrow u <|> WrapArrow v = WrapArrow (u <+> v)
222 -- | Lists, but with an 'Applicative' functor based on zipping, so that
224 -- @f '<$>' 'ZipList' xs1 '<*>' ... '<*>' 'ZipList' xsn = 'ZipList' (zipWithn f xs1 ... xsn)@
226 newtype ZipList a = ZipList { getZipList :: [a] }
228 instance Functor ZipList where
229 fmap f (ZipList xs) = ZipList (map f xs)
231 instance Applicative ZipList where
232 pure x = ZipList (repeat x)
233 ZipList fs <*> ZipList xs = ZipList (zipWith id fs xs)
237 -- | A variant of '<*>' with the arguments reversed.
238 (<**>) :: Applicative f => f a -> f (a -> b) -> f b
239 (<**>) = liftA2 (flip ($))
241 -- | Lift a function to actions.
242 -- This function may be used as a value for `fmap` in a `Functor` instance.
243 liftA :: Applicative f => (a -> b) -> f a -> f b
244 liftA f a = pure f <*> a
246 -- | Lift a binary function to actions.
247 liftA2 :: Applicative f => (a -> b -> c) -> f a -> f b -> f c
248 liftA2 f a b = f <$> a <*> b
250 -- | Lift a ternary function to actions.
251 liftA3 :: Applicative f => (a -> b -> c -> d) -> f a -> f b -> f c -> f d
252 liftA3 f a b c = f <$> a <*> b <*> c
255 optional :: Alternative f => f a -> f (Maybe a)
256 optional v = Just <$> v <|> pure Nothing