path: root/libraries/base/Data/Foldable.hs

{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE ScopedTypeVariables #-}

-----------------------------------------------------------------------------
-- |
-- 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.
--
-----------------------------------------------------------------------------

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

import Data.Bool
import Data.Either
import Data.Eq
import qualified GHC.List as List
import Data.Maybe
import Data.Monoid
import Data.Ord
import Data.Proxy

import GHC.Arr  ( Array(..), elems, numElements,
                  foldlElems, foldrElems,
                  foldlElems', foldrElems',
                  foldl1Elems, foldr1Elems)
import GHC.Base hiding ( foldr )
import GHC.Num  ( Num(..) )

infix  4 `elem`, `notElem`

-- | Data structures that can be folded.
--
-- 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 where
-- >    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.  Alternatively, one could define @foldr@:
--
-- > instance Foldable Tree where
-- >    foldr f z Empty = z
-- >    foldr f z (Leaf x) = f x z
-- >    foldr f z (Node l k r) = foldr f (f k (foldr f z r)) l
--
-- @Foldable@ instances are expected to satisfy the following laws:
--
-- > foldr f z t = appEndo (foldMap (Endo . f) t ) z
--
-- > foldl f z t = appEndo (getDual (foldMap (Dual . Endo . flip f) t)) z
--
-- > fold = foldMap id
--
-- @sum@, @product@, @maximum@, and @minimum@ should all be essentially
-- equivalent to @foldMap@ forms, such as
--
-- > sum = getSum . foldMap Sum
--
-- but may be less defined.
--
-- If the type is also a 'Functor' instance, it should satisfy
--
-- > foldMap f = fold . fmap f
--
-- which implies that
--
-- > foldMap f . fmap g = foldMap (f . g)

class Foldable t where
    {-# MINIMAL foldMap | foldr #-}

    -- | 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

    -- | Right-associative fold of a structure,
    -- but with strict application of the operator.
    foldr' :: (a -> b -> b) -> b -> t a -> b
    foldr' f z0 xs = foldl f' id xs z0
      where f' k x z = k $! f x z

    -- | Left-associative fold of a structure.
    --
    -- @'foldl' f z = 'Prelude.foldl' f z . 'toList'@
    foldl :: (b -> a -> b) -> b -> t a -> b
    foldl f z t = appEndo (getDual (foldMap (Dual . Endo . flip f) t)) z
    -- There's no point mucking around with coercions here,
    -- because flip forces us to build a new function anyway.

    -- | Left-associative fold of a structure.
    -- but with strict application of the operator.
    --
    -- @'foldl' f z = 'List.foldl'' f z . 'toList'@
    foldl' :: (b -> a -> b) -> b -> t a -> b
    foldl' f z0 xs = foldr f' id xs z0
      where f' x k z = k $! f z x

    -- | 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 m = Just (case m of
                         Nothing -> x
                         Just y  -> 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 m y = Just (case m of
                         Nothing -> y
                         Just x  -> f x y)

    -- | List of elements of a structure, from left to right.
    toList :: t a -> [a]
    {-# INLINE toList #-}
    toList t = build (\ c n -> foldr c n t)

    -- | Test whether the structure is empty. The default implementation is
    -- optimized for structures that are similar to cons-lists, because there
    -- is no general way to do better.
    null :: t a -> Bool
    null = foldr (\_ _ -> False) True

    -- | Returns the size/length of a finite structure as an 'Int'.  The
    -- default implementation is optimized for structures that are similar to
    -- cons-lists, because there is no general way to do better.
    length :: t a -> Int
    length = foldl' (\c _ -> c+1) 0

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

    -- | The largest element of a non-empty structure.
    maximum :: forall a . Ord a => t a -> a
    maximum = fromMaybe (error "maximum: empty structure") .
       getMax . foldMap (Max #. (Just :: a -> Maybe a))

    -- | The least element of a non-empty structure.
    minimum :: forall a . Ord a => t a -> a
    minimum = fromMaybe (error "minimum: empty structure") .
       getMin . foldMap (Min #. (Just :: a -> Maybe a))

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

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

-- instances for Prelude types

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

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

instance Foldable [] where
    elem    = List.elem
    foldl   = List.foldl
    foldl'  = List.foldl'
    foldl1  = List.foldl1
    foldr   = List.foldr
    foldr1  = List.foldr1
    length  = List.length
    maximum = List.maximum
    minimum = List.minimum
    null    = List.null
    product = List.product
    sum     = List.sum
    toList  = id

instance Foldable (Either a) where
    foldMap _ (Left _) = mempty
    foldMap f (Right y) = f y

    foldr _ z (Left _) = z
    foldr f z (Right y) = f y z

    length (Left _)  = 0
    length (Right _) = 1

    null             = isLeft

instance Foldable ((,) a) where
    foldMap f (_, y) = f y

    foldr f z (_, y) = f y z

instance Foldable (Array i) where
    foldr = foldrElems
    foldl = foldlElems
    foldl' = foldlElems'
    foldr' = foldrElems'
    foldl1 = foldl1Elems
    foldr1 = foldr1Elems
    toList = elems
    length = numElements
    null a = numElements a == 0

instance Foldable Proxy where
    foldMap _ _ = mempty
    {-# INLINE foldMap #-}
    fold _ = mempty
    {-# INLINE fold #-}
    foldr _ z _ = z
    {-# INLINE foldr #-}
    foldl _ z _ = z
    {-# INLINE foldl #-}
    foldl1 _ _ = error "foldl1: Proxy"
    foldr1 _ _ = error "foldr1: Proxy"
    length _   = 0
    null _     = True
    elem _ _   = False
    sum _      = 0
    product _  = 1

-- We don't export Max and Min because, as Edward Kmett pointed out to me,
-- there are two reasonable ways to define them. One way is to use Maybe, as we
-- do here; the other way is to impose a Bounded constraint on the Monoid
-- instance. We may eventually want to add both versions, but we don't want to
-- trample on anyone's toes by imposing Max = MaxMaybe.

newtype Max a = Max {getMax :: Maybe a}
newtype Min a = Min {getMin :: Maybe a}

instance Ord a => Monoid (Max a) where
  mempty = Max Nothing

  {-# INLINE mappend #-}
  m `mappend` Max Nothing = m
  Max Nothing `mappend` n = n
  (Max m@(Just x)) `mappend` (Max n@(Just y))
    | x >= y    = Max m
    | otherwise = Max n

instance Ord a => Monoid (Min a) where
  mempty = Min Nothing

  {-# INLINE mappend #-}
  m `mappend` Min Nothing = m
  Min Nothing `mappend` n = n
  (Min m@(Just x)) `mappend` (Min n@(Just y))
    | x <= y    = Min m
    | otherwise = Min n

-- | 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 z0 xs = foldl f' return xs z0
  where f' k x z = f x z >>= k

-- | Monadic fold over the elements of a structure,
-- associating to the left, i.e. from left to right.
foldlM :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b
foldlM f z0 xs = foldr f' return xs z0
  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. For a version
-- that doesn't ignore the results see 'Data.Traversable.traverse'.
traverse_ :: (Foldable t, Applicative f) => (a -> f b) -> t a -> f ()
traverse_ f = foldr ((*>) . f) (pure ())

-- | 'for_' is 'traverse_' with its arguments flipped. For a version
-- that doesn't ignore the results see 'Data.Traversable.for'.
--
-- >>> for_ [1..4] print
-- 1
-- 2
-- 3
-- 4
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. For a
-- version that doesn't ignore the results see
-- 'Data.Traversable.mapM'.
--
-- As of base 4.8.0.0, 'mapM_' is just 'traverse_', specialized to
-- 'Monad'.
mapM_ :: (Foldable t, Monad m) => (a -> m b) -> t a -> m ()
mapM_ f= foldr ((>>) . f) (return ())

-- | 'forM_' is 'mapM_' with its arguments flipped. For a version that
-- doesn't ignore the results see 'Data.Traversable.forM'.
--
-- As of base 4.8.0.0, 'forM_' is just 'for_', specialized to 'Monad'.
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. For a version that doesn't ignore the results
-- see 'Data.Traversable.sequenceA'.
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. For a version that doesn't ignore the
-- results see 'Data.Traversable.sequence'.
--
-- As of base 4.8.0.0, 'sequence_' is just 'sequenceA_', specialized
-- to 'Monad'.
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'.
-- As of base 4.8.0.0, 'msum' is just 'asum', specialized to 'MonadPlus'.
msum :: (Foldable t, MonadPlus m) => t (m a) -> m a
{-# INLINE msum #-}
msum = asum

-- | The concatenation of all the elements of a container of lists.
concat :: Foldable t => t [a] -> [a]
concat xs = build (\c n -> foldr (\x y -> foldr c y x) n xs)
{-# INLINE concat #-}

-- | Map a function over all the elements of a container and concatenate
-- the resulting lists.
concatMap :: Foldable t => (a -> [b]) -> t a -> [b]
concatMap f xs = build (\c n -> foldr (\x b -> foldr c b (f x)) n xs)
{-# INLINE concatMap #-}

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

-- | '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 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 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

-- | '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 = getFirst . foldMap (\ x -> First (if p x then Just x else Nothing))

-- See Note [Function coercion]
(#.) :: Coercible b c => (b -> c) -> (a -> b) -> (a -> c)
(#.) _f = coerce
{-# INLINE (#.) #-}

{-
Note [Function coercion]
~~~~~~~~~~~~~~~~~~~~~~~~

Several functions here use (#.) instead of (.) to avoid potential efficiency
problems relating to #7542. The problem, in a nutshell:

If N is a newtype constructor, then N x will always have the same
representation as x (something similar applies for a newtype deconstructor).
However, if f is a function,

N . f = \x -> N (f x)

This looks almost the same as f, but the eta expansion lifts it--the lhs could
be _|_, but the rhs never is. This can lead to very inefficient code.  Thus we
steal a technique from Shachaf and Edward Kmett and adapt it to the current
(rather clean) setting. Instead of using  N . f,  we use  N .## f, which is
just

coerce f `asTypeOf` (N . f)

That is, we just *pretend* that f has the right type, and thanks to the safety
of coerce, the type checker guarantees that nothing really goes wrong. We still
have to be a bit careful, though: remember that #. completely ignores the
*value* of its left operand.
-}