content: {-# LANGUAGE Trustworthy #-}
  {-# LANGUAGE DeriveGeneric #-}
  {-# LANGUAGE GeneralizedNewtypeDeriving #-}
  {-# LANGUAGE NoImplicitPrelude #-}
  {-# LANGUAGE KindSignatures #-}
  {-# LANGUAGE StandaloneDeriving #-}
  {-# LANGUAGE DeriveDataTypeable #-}
  
  -----------------------------------------------------------------------------
  -- |
  -- 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   :  stable
  -- 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,
      asum,
      thenA,
      ) where
  
  import GHC.Internal.Control.Category hiding ((.), id)
  import GHC.Internal.Control.Arrow
  import GHC.Internal.Data.Maybe
  import GHC.Internal.Data.Tuple
  import GHC.Internal.Data.Foldable (asum)
  import GHC.Internal.Data.Functor ((<$>))
  import GHC.Internal.Data.Functor.Const (Const(..))
  import GHC.Internal.Data.Typeable (Typeable)
  import GHC.Internal.Data.Data (Data)
  
  import GHC.Internal.Base
  import GHC.Internal.Functor.ZipList (ZipList(..))
  import GHC.Generics
  
  -- $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)
  
  -- | @since 4.14.0.0
  deriving instance (Typeable (m :: Type -> Type), Typeable a, Data (m a))
           => Data (WrappedMonad m a)
  
  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)
  
  -- | @since 4.14.0.0
  deriving instance (Typeable (a :: Type -> Type -> Type), Typeable b, Typeable c,
                     Data (a b c))
           => Data (WrappedArrow a b c)
  
  -- 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
  