    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Foldable
    -- Copyright   :  Ross Paterson 2005
    -- License     :  BSD-style (see the LICENSE file in the distribution)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  portable
    --
    -- Class of data structures that can be folded to a summary value.
    --
    -- Many of these functions generalize "Prelude", "Control.Monad" and
    -- "Data.List" functions of the same names from lists to any 'Foldable'
    -- functor.  To avoid ambiguity, either import those modules hiding
    -- these names or qualify uses of these function names with an alias
    -- for this module.

    module Data.Foldable (
            -- * Folds
            Foldable(..),
            -- ** Special biased folds
            foldr',
            foldl',
            foldrM,
            foldlM,
            -- ** Folding actions
            -- *** Applicative actions
            traverse_,
            for_,
            sequenceA_,
            asum,
            -- *** Monadic actions
            mapM_,
            forM_,
            sequence_,
            msum,
            -- ** Specialized folds
            toList,
            concat,
            concatMap,
            and,
            or,
            any,
            all,
            sum,
            product,
            maximum,
            maximumBy,
            minimum,
            minimumBy,
            -- ** Searches
            elem,
            notElem,
            find
            ) where

    import Prelude hiding (foldl, foldr, foldl1, foldr1, mapM_, sequence_,
                    elem, notElem, concat, concatMap, and, or, any, all,
                    sum, product, maximum, minimum)
    import qualified Prelude (foldl, foldr, foldl1, foldr1)
    import Control.Applicative
    import Control.Monad (MonadPlus(..))
    import Data.Maybe (fromMaybe, listToMaybe)
    import Data.Monoid

    #ifdef __NHC__
    import Control.Arrow (ArrowZero(..)) -- work around nhc98 typechecker problem
    #endif

    #ifdef __GLASGOW_HASKELL__
    import GHC.Exts (build)
    #endif

    -- | Data structures that can be folded.
    --
    -- Minimal complete definition: 'foldMap' or 'foldr'.
    --
    -- For example, given a data type
    --
    -- > data Tree a = Empty | Leaf a | Node (Tree a) a (Tree a)
    --
    -- a suitable instance would be
    --
    -- > instance Foldable Tree
    -- >    foldMap f Empty = mempty
    -- >    foldMap f (Leaf x) = f x
    -- >    foldMap f (Node l k r) = foldMap f l `mappend` f k `mappend` foldMap f r
    --
    -- This is suitable even for abstract types, as the monoid is assumed
    -- to satisfy the monoid laws.
    --
    class Foldable t where
            -- | Combine the elements of a structure using a monoid.
            fold :: Monoid m => t m -> m
            fold = foldMap id

            -- | Map each element of the structure to a monoid,
            -- and combine the results.
            foldMap :: Monoid m => (a -> m) -> t a -> m
            foldMap f = foldr (mappend . f) mempty

            -- | Right-associative fold of a structure.
            --
            -- @'foldr' f z = 'Prelude.foldr' f z . 'toList'@
            foldr :: (a -> b -> b) -> b -> t a -> b
            foldr f z t = appEndo (foldMap (Endo . f) t) z

            -- | Left-associative fold of a structure.
            --
            -- @'foldl' f z = 'Prelude.foldl' f z . 'toList'@
            foldl :: (a -> b -> a) -> a -> t b -> a
            foldl f z t = appEndo (getDual (foldMap (Dual . Endo . flip f) t)) z

            -- | A variant of 'foldr' that has no base case,
            -- and thus may only be applied to non-empty structures.
            --
            -- @'foldr1' f = 'Prelude.foldr1' f . 'toList'@
            foldr1 :: (a -> a -> a) -> t a -> a
            foldr1 f xs = fromMaybe (error "foldr1: empty structure")
                            (foldr mf Nothing xs)
              where mf x Nothing = Just x
                    mf x (Just y) = Just (f x y)

            -- | A variant of 'foldl' that has no base case,
            -- and thus may only be applied to non-empty structures.
            --
            -- @'foldl1' f = 'Prelude.foldl1' f . 'toList'@
            foldl1 :: (a -> a -> a) -> t a -> a
            foldl1 f xs = fromMaybe (error "foldl1: empty structure")
                            (foldl mf Nothing xs)
              where mf Nothing y = Just y
                    mf (Just x) y = Just (f x y)

    -- instances for Prelude types

    instance Foldable Maybe where
            foldr f z Nothing = z
            foldr f z (Just x) = f x z

            foldl f z Nothing = z
            foldl f z (Just x) = f z x

    instance Foldable [] where
            foldr = Prelude.foldr
            foldl = Prelude.foldl
            foldr1 = Prelude.foldr1
            foldl1 = Prelude.foldl1

    -- | Fold over the elements of a structure,
    -- associating to the right, but strictly.
    foldr' :: Foldable t => (a -> b -> b) -> b -> t a -> b
    foldr' f z xs = foldl f' id xs z
      where f' k x z = k $! f x z

    -- | Monadic fold over the elements of a structure,
    -- associating to the right, i.e. from right to left.
    foldrM :: (Foldable t, Monad m) => (a -> b -> m b) -> b -> t a -> m b
    foldrM f z xs = foldl f' return xs z
      where f' k x z = f x z >>= k

    -- | Fold over the elements of a structure,
    -- associating to the left, but strictly.
    foldl' :: Foldable t => (a -> b -> a) -> a -> t b -> a
    foldl' f z xs = foldr f' id xs z
      where f' x k z = k $! f z x

    -- | Monadic fold over the elements of a structure,
    -- associating to the left, i.e. from left to right.
    foldlM :: (Foldable t, Monad m) => (a -> b -> m a) -> a -> t b -> m a
    foldlM f z xs = foldr f' return xs z
      where f' x k z = f z x >>= k

    -- | Map each element of a structure to an action, evaluate
    -- these actions from left to right, and ignore the results.
    traverse_ :: (Foldable t, Applicative f) => (a -> f b) -> t a -> f ()
    traverse_ f = foldr ((*>) . f) (pure ())

    -- | 'for_' is 'traverse_' with its arguments flipped.
    for_ :: (Foldable t, Applicative f) => t a -> (a -> f b) -> f ()
    {-# INLINE for_ #-}
    for_ = flip traverse_

    -- | Map each element of a structure to a monadic action, evaluate
    -- these actions from left to right, and ignore the results.
    mapM_ :: (Foldable t, Monad m) => (a -> m b) -> t a -> m ()
    mapM_ f = foldr ((>>) . f) (return ())

    -- | 'forM_' is 'mapM_' with its arguments flipped.
    forM_ :: (Foldable t, Monad m) => t a -> (a -> m b) -> m ()
    {-# INLINE forM_ #-}
    forM_ = flip mapM_

    -- | Evaluate each action in the structure from left to right,
    -- and ignore the results.
    sequenceA_ :: (Foldable t, Applicative f) => t (f a) -> f ()
    sequenceA_ = foldr (*>) (pure ())

    -- | Evaluate each monadic action in the structure from left to right,
    -- and ignore the results.
    sequence_ :: (Foldable t, Monad m) => t (m a) -> m ()
    sequence_ = foldr (>>) (return ())

    -- | The sum of a collection of actions, generalizing 'concat'.
    asum :: (Foldable t, Alternative f) => t (f a) -> f a
    {-# INLINE asum #-}
    asum = foldr (<|>) empty

    -- | The sum of a collection of actions, generalizing 'concat'.
    msum :: (Foldable t, MonadPlus m) => t (m a) -> m a
    {-# INLINE msum #-}
    msum = foldr mplus mzero

    -- These use foldr rather than foldMap to avoid repeated concatenation.

    -- | List of elements of a structure.
    toList :: Foldable t => t a -> [a]
    #ifdef __GLASGOW_HASKELL__
    toList t = build (\ c n -> foldr c n t)
    #else
    toList = foldr (:) []
    #endif

    -- | The concatenation of all the elements of a container of lists.
    concat :: Foldable t => t [a] -> [a]
    concat = fold

    -- | Map a function over all the elements of a container and concatenate
    -- the resulting lists.
    concatMap :: Foldable t => (a -> [b]) -> t a -> [b]
    concatMap = foldMap

    -- | 'and' returns the conjunction of a container of Bools.  For the
    -- result to be 'True', the container must be finite; 'False', however,
    -- results from a 'False' value finitely far from the left end.
    and :: Foldable t => t Bool -> Bool
    and = getAll . foldMap All

    -- | 'or' returns the disjunction of a container of Bools.  For the
    -- result to be 'False', the container must be finite; 'True', however,
    -- results from a 'True' value finitely far from the left end.
    or :: Foldable t => t Bool -> Bool
    or = getAny . foldMap Any

    -- | Determines whether any element of the structure satisfies the predicate.
    any :: Foldable t => (a -> Bool) -> t a -> Bool
    any p = getAny . foldMap (Any . p)

    -- | Determines whether all elements of the structure satisfy the predicate.
    all :: Foldable t => (a -> Bool) -> t a -> Bool
    all p = getAll . foldMap (All . p)

    -- | The 'sum' function computes the sum of the numbers of a structure.
    sum :: (Foldable t, Num a) => t a -> a
    sum = getSum . foldMap Sum

    -- | The 'product' function computes the product of the numbers of a structure.
    product :: (Foldable t, Num a) => t a -> a
    product = getProduct . foldMap Product

    -- | The largest element of a non-empty structure.
    maximum :: (Foldable t, Ord a) => t a -> a
    maximum = foldr1 max

    -- | The largest element of a non-empty structure with respect to the
    -- given comparison function.
    maximumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
    maximumBy cmp = foldr1 max'
      where max' x y = case cmp x y of
                            GT -> x
                            _  -> y

    -- | The least element of a non-empty structure.
    minimum :: (Foldable t, Ord a) => t a -> a
    minimum = foldr1 min

    -- | The least element of a non-empty structure with respect to the
    -- given comparison function.
    minimumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
    minimumBy cmp = foldr1 min'
      where min' x y = case cmp x y of
                            GT -> y
                            _  -> x

    -- | Does the element occur in the structure?
    elem :: (Foldable t, Eq a) => a -> t a -> Bool
    elem = any . (==)

    -- | 'notElem' is the negation of 'elem'.
    notElem :: (Foldable t, Eq a) => a -> t a -> Bool
    notElem x = not . elem x

    -- | The 'find' function takes a predicate and a structure and returns
    -- the leftmost element of the structure matching the predicate, or
    -- 'Nothing' if there is no such element.
    find :: Foldable t => (a -> Bool) -> t a -> Maybe a
    find p = listToMaybe . concatMap (\ x -> if p x then [x] else [])