content: {-# LANGUAGE Trustworthy #-}
  {-# LANGUAGE AutoDeriveTypeable #-}
  {-# LANGUAGE DeriveGeneric #-}
  {-# LANGUAGE GeneralizedNewtypeDeriving #-}
  {-# LANGUAGE NoImplicitPrelude #-}
  {-# LANGUAGE ScopedTypeVariables #-}
  
  -----------------------------------------------------------------------------
  -- |
  -- 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 R&#xF6;jemo, 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
  -- <http://www.soi.city.ac.uk/~ross/papers/Applicative.html Applicative Programming with Effects>,
  -- by Conor McBride and Ross Paterson.
  
  module Control.Applicative (
      -- * Applicative functors
      Applicative(..),
      -- * Alternatives
      Alternative(..),
      -- * Instances
      Const(..), WrappedMonad(..), WrappedArrow(..), ZipList(..),
      -- * Utility functions
      (<$>), (<$), (<**>),
      liftA, liftA3,
      optional,
      ) 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(..))
  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)
  
  newtype WrappedMonad m a = WrapMonad { unwrapMonad :: m a }
                           deriving (Generic, Generic1, Monad)
  
  -- | @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, Generic1)
  
  -- | @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, Eq, Ord, Read, Functor
                             , Foldable, Generic, Generic1)
  -- 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.
  optional :: Alternative f => f a -> f (Maybe a)
  optional v = Just <$> v <|> pure Nothing
  