{-# LANGUAGE Trustworthy #-} {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE GeneralizedNewtypeDeriving #-} {-# LANGUAGE NoImplicitPrelude #-} ----------------------------------------------------------------------------- -- | -- Module : Control.Applicative -- Copyright : Conor McBride and Ross Paterson 2005 -- License : BSD-style (see the LICENSE file in the distribution) -- -- Maintainer : libraries@haskell.org -- Stability : experimental -- Portability : portable -- -- This module describes a structure intermediate between a functor and -- a monad (technically, a strong lax monoidal functor). Compared with -- monads, this interface lacks the full power of the binding operation -- '>>=', but -- -- * it has more instances. -- -- * it is sufficient for many uses, e.g. context-free parsing, or the -- 'Data.Traversable.Traversable' class. -- -- * instances can perform analysis of computations before they are -- executed, and thus produce shared optimizations. -- -- This interface was introduced for parsers by Niklas Röjemo, because -- it admits more sharing than the monadic interface. The names here are -- mostly based on parsing work by Doaitse Swierstra. -- -- For more details, see -- , -- by Conor McBride and Ross Paterson. module Control.Applicative ( -- * Applicative functors Applicative(..), -- * Alternatives Alternative(..), -- * Instances Const(..), WrappedMonad(..), WrappedArrow(..), ZipList(..), -- * Utility functions (<$>), (<$), (<**>), liftA, liftA3, optional, asum, ) where import Control.Category hiding ((.), id) import Control.Arrow import Data.Maybe import Data.Tuple import Data.Eq import Data.Ord import Data.Foldable (Foldable(..), asum) import Data.Functor ((<$>)) import Data.Functor.Const (Const(..)) import GHC.Base import GHC.Generics import GHC.List (repeat, zipWith, drop) import GHC.Read (Read) import GHC.Show (Show) -- $setup -- >>> import Prelude newtype WrappedMonad m a = WrapMonad { unwrapMonad :: m a } deriving ( Generic -- ^ @since 4.7.0.0 , Generic1 -- ^ @since 4.7.0.0 , Monad -- ^ @since 4.7.0.0 ) -- | @since 2.01 instance Monad m => Functor (WrappedMonad m) where fmap f (WrapMonad v) = WrapMonad (liftM f v) -- | @since 2.01 instance Monad m => Applicative (WrappedMonad m) where pure = WrapMonad . pure WrapMonad f <*> WrapMonad v = WrapMonad (f `ap` v) liftA2 f (WrapMonad x) (WrapMonad y) = WrapMonad (liftM2 f x y) -- | @since 2.01 instance MonadPlus m => Alternative (WrappedMonad m) where empty = WrapMonad mzero WrapMonad u <|> WrapMonad v = WrapMonad (u `mplus` v) newtype WrappedArrow a b c = WrapArrow { unwrapArrow :: a b c } deriving ( Generic -- ^ @since 4.7.0.0 , Generic1 -- ^ @since 4.7.0.0 ) -- | @since 2.01 instance Arrow a => Functor (WrappedArrow a b) where fmap f (WrapArrow a) = WrapArrow (a >>> arr f) -- | @since 2.01 instance Arrow a => Applicative (WrappedArrow a b) where pure x = WrapArrow (arr (const x)) liftA2 f (WrapArrow u) (WrapArrow v) = WrapArrow (u &&& v >>> arr (uncurry f)) -- | @since 2.01 instance (ArrowZero a, ArrowPlus a) => Alternative (WrappedArrow a b) where empty = WrapArrow zeroArrow WrapArrow u <|> WrapArrow v = WrapArrow (u <+> v) -- | Lists, but with an 'Applicative' functor based on zipping. newtype ZipList a = ZipList { getZipList :: [a] } deriving ( Show -- ^ @since 4.7.0.0 , Eq -- ^ @since 4.7.0.0 , Ord -- ^ @since 4.7.0.0 , Read -- ^ @since 4.7.0.0 , Functor -- ^ @since 2.01 , Foldable -- ^ @since 4.9.0.0 , Generic -- ^ @since 4.7.0.0 , Generic1 -- ^ @since 4.7.0.0 ) -- See Data.Traversable for Traversable instance due to import loops -- | -- > f <$> ZipList xs1 <*> ... <*> ZipList xsN -- > = ZipList (zipWithN f xs1 ... xsN) -- -- where @zipWithN@ refers to the @zipWith@ function of the appropriate arity -- (@zipWith@, @zipWith3@, @zipWith4@, ...). For example: -- -- > (\a b c -> stimes c [a, b]) <$> ZipList "abcd" <*> ZipList "567" <*> ZipList [1..] -- > = ZipList (zipWith3 (\a b c -> stimes c [a, b]) "abcd" "567" [1..]) -- > = ZipList {getZipList = ["a5","b6b6","c7c7c7"]} -- -- @since 2.01 instance Applicative ZipList where pure x = ZipList (repeat x) liftA2 f (ZipList xs) (ZipList ys) = ZipList (zipWith f xs ys) -- | @since 4.11.0.0 instance Alternative ZipList where empty = ZipList [] ZipList xs <|> ZipList ys = ZipList (xs ++ drop (length xs) ys) -- extra functions -- | One or none. -- -- It is useful for modelling any computation that is allowed to fail. -- -- ==== __Examples__ -- -- Using the 'Alternative' instance of "Control.Monad.Except", the following functions: -- -- >>> import Control.Monad.Except -- -- >>> canFail = throwError "it failed" :: Except String Int -- >>> final = return 42 :: Except String Int -- -- Can be combined by allowing the first function to fail: -- -- >>> runExcept $ canFail *> final -- Left "it failed" -- >>> runExcept $ optional canFail *> final -- Right 42 optional :: Alternative f => f a -> f (Maybe a) optional v = Just <$> v <|> pure Nothing