    {-# LANGUAGE Safe #-}

    -- |
    -- Module      :  Data.Foldable
    -- Copyright   :  Ross Paterson 2005
    -- License     :  BSD-style (see the LICENSE file in the distribution)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  stable
    -- Portability :  portable
    --
    -- Class of data structures that can be folded to a summary value.
    --

    module Data.Foldable (
        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

        -- * Overview
        -- $overview

        -- ** Expectation of efficient left-to-right iteration
        -- $chirality

        -- ** Recursive and corecursive reduction
        -- $reduction

        -- *** Strict recursive folds
        -- $strict

        -- **** List of strict functions
        -- $strictlist

        -- *** Lazy corecursive folds
        -- $lazy

        -- **** List of lazy functions
        -- $lazylist

        -- *** Short-circuit folds
        -- $shortcircuit

        -- **** List of short-circuit functions
        -- $shortlist

        -- *** Hybrid folds
        -- $hybrid

        -- ** Generative Recursion
        -- $generative

        -- ** Avoiding multi-pass folds
        -- $multipass

        -- * Defining instances
        -- $instances

        -- *** Being strict by being lazy
        -- $strictlazy

        -- * Laws
        -- $laws

        -- * Notes
        -- $notes

        -- ** Generally linear-time `elem`
        -- $linear

        -- * See also
        -- $also
        ) where

    import GHC.Internal.Data.Foldable

    -- $overview
    --
    -- #overview#
    -- The Foldable class generalises some common "Data.List" functions to
    -- structures that can be reduced to a summary value one element at a time.
    --
    -- == Left and right folds
    --
    -- #leftright#
    -- The contribution of each element to the final result is combined with an
    -- accumulator via a suitable /operator/.  The operator may be explicitly
    -- provided by the caller as with `foldr` or may be implicit as in `length`.
    -- In the case of `foldMap`, the caller provides a function mapping each
    -- element into a suitable 'Monoid', which makes it possible to merge the
    -- per-element contributions via that monoid's `mappend` function.
    --
    -- A key distinction is between left-associative and right-associative
    -- folds:
    --
    -- * In left-associative folds the accumulator is a partial fold over the
    --   elements that __precede__ the current element, and is passed to the
    --   operator as its first (left) argument.  The outermost application of the
    --   operator merges the contribution of the last element of the structure with
    --   the contributions of all its predecessors.
    --
    -- * In right-associative folds the accumulator is a partial fold over the
    --   elements that __follow__ the current element, and is passed to the
    --   operator as its second (right) argument.  The outermost application of
    --   the operator merges the contribution of the first element of the structure
    --   with the contributions of all its successors.
    --
    -- These two types of folds are typified by the left-associative strict
    -- 'foldl'' and the right-associative lazy `foldr`.
    --
    -- @
    -- 'foldl'' :: Foldable t => (b -> a -> b) -> b -> t a -> b
    -- `foldr`  :: Foldable t => (a -> b -> b) -> b -> t a -> b
    -- @
    --
    -- Example usage:
    --
    -- >>> foldl' (+) 0 [1..100]
    -- 5050
    -- >>> foldr (&&) True (repeat False)
    -- False
    --
    -- The first argument of both is an explicit /operator/ that merges the
    -- contribution of an element of the structure with a partial fold over,
    -- respectively, either the preceding or following elements of the structure.
    --
    -- The second argument of both is an initial accumulator value @z@ of type
    -- @b@.  This is the result of the fold when the structure is empty.
    -- When the structure is non-empty, this is the accumulator value merged with
    -- the first element in left-associative folds, or with the last element in
    -- right-associative folds.
    --
    -- The third and final argument is a @Foldable@ structure containing elements
    -- @(a, b, c, &#x2026;)@.
    --
    -- * __'foldl''__ takes an operator argument of the form:
    --
    --     @
    --     f :: b -- accumulated fold of the initial elements
    --       -> a -- current element
    --       -> b -- updated fold, inclusive of current element
    --     @
    --
    --     If the structure's last element is @y@, the result of the fold is:
    --
    --     @
    --     g y . &#x2026; . g c . g b . g a $ z
    --       where g element !acc = f acc element
    --     @
    --
    --     Since 'foldl'' is strict in the accumulator, this is always
    --     a [strict](#strict) reduction with no opportunity for early return or
    --     intermediate results.  The structure must be finite, since no result is
    --     returned until the last element is processed.  The advantage of
    --     strictness is space efficiency: the final result can be computed without
    --     storing a potentially deep stack of lazy intermediate results.
    --
    -- * __`foldr`__ takes an operator argument of the form:
    --
    --     @
    --     f :: a -- current element
    --       -> b -- accumulated fold of the remaining elements
    --       -> b -- updated fold, inclusive of current element
    --     @
    --
    --     the result of the fold is:
    --
    --     @f a . f b . f c . &#x2026; $ z@
    --
    --     If each call of @f@ on the current element @e@, (referenced as @(f e)@
    --     below) returns a structure in which its second argument is captured in a
    --     lazily-evaluated component, then the fold of the remaining elements is
    --     available to the caller of `foldr` as a pending computation (thunk) that
    --     is computed only when that component is evaluated.
    --
    --     Alternatively, if any of the @(f e)@ ignore their second argument, the
    --     fold stops there, with the remaining elements unused.  As a result,
    --     `foldr` is well suited to define both [corecursive](#corec)
    --     and [short-circuit](#short) reductions.
    --
    --     When the operator is always strict in its second argument, 'foldl'' is
    --     generally a better choice than `foldr`.  When `foldr` is called with a
    --     strict operator, evaluation cannot begin until the last element is
    --     reached, by which point a deep stack of pending function applications
    --     may have been built up in memory.
    --

    -- $chirality
    --
    -- #chirality#
    -- Foldable structures are generally expected to be efficiently iterable from
    -- left to right. Right-to-left iteration may be substantially more costly, or
    -- even impossible (as with, for example, infinite lists).  The text in the
    -- sections that follow that suggests performance differences between
    -- left-associative and right-associative folds assumes /left-handed/
    -- structures in which left-to-right iteration is cheaper than right-to-left
    -- iteration.
    --
    -- In finite structures for which right-to-left sequencing no less efficient
    -- than left-to-right sequencing, there is no inherent performance distinction
    -- between left-associative and right-associative folds.  If the structure's
    -- @Foldable@ instance takes advantage of this symmetry to also make strict
    -- right folds space-efficient and lazy left folds corecursive, one need only
    -- take care to choose either a strict or lazy method for the task at hand.
    --
    -- Foldable instances for symmetric structures should strive to provide equally
    -- performant left-associative and right-associative interfaces. The main
    -- limitations are:
    --
    -- * The lazy 'fold', 'foldMap' and 'toList' methods have no right-associative
    --   counterparts.
    -- * The strict 'foldMap'' method has no left-associative counterpart.
    --
    -- Thus, for some foldable structures 'foldr'' is just as efficient as 'foldl''
    -- for strict reduction, and 'foldl' may be just as appropriate for corecursive
    -- folds as 'foldr'.
    --
    -- Finally, in some less common structures (e.g. /snoc/ lists) right to left
    -- iterations are cheaper than left to right.  Such structures are poor
    -- candidates for a @Foldable@ instance, and are perhaps best handled via their
    -- type-specific interfaces.  If nevertheless a @Foldable@ instance is
    -- provided, the material in the sections that follow applies to these also, by
    -- replacing each method with one with the opposite associativity (when
    -- available) and switching the order of arguments in the fold's /operator/.
    --
    -- You may need to pay careful attention to strictness of the fold's /operator/
    -- when its strictness is different between its first and second argument.
    -- For example, while @('+')@ is expected to be commutative and strict in both
    -- arguments, the list concatenation operator @('++')@ is not commutative and
    -- is only strict in the initial constructor of its first argument.  The fold:
    --
    -- > myconcat xs = foldr (\a b -> a ++ b) [] xs
    --
    -- is substantially cheaper (linear in the length of the consumed portion of
    -- the final list, thus e.g. constant time/space for just the first element)
    -- than:
    --
    -- > revconcat xs = foldr (\a b -> b ++ a) [] xs
    --
    -- In which the total cost scales up with both the number of lists combined and
    -- the number of elements ultimately consumed.  A more efficient way to combine
    -- lists in reverse order, is to use:
    --
    -- > revconcat = foldr (++) [] . reverse

    --------------

    -- $reduction
    --
    -- As observed in the [above description](#leftright) of left and right folds,
    -- there are three general ways in which a structure can be reduced to a
    -- summary value:
    --
    -- * __Recursive__ reduction, which is strict in all the elements of the
    --   structure.  This produces a single final result only after processing the
    --   entire input structure, and so the input must be finite.
    --
    -- * __Corecursion__, which yields intermediate results as it encounters
    --   additional input elements.  Lazy processing of the remaining elements
    --   makes the intermediate results available even before the rest of the
    --   input is processed.  The input may be unbounded, and the caller can
    --   stop processing intermediate results early.
    --
    -- * __Short-circuit__ reduction, which examines some initial sequence of the
    --   input elements, but stops once a termination condition is met, returning a
    --   final result based only on the elements considered up to that point.  The
    --   remaining elements are not considered.  The input should generally be
    --   finite, because the termination condition might otherwise never be met.
    --
    -- Whether a fold is recursive, corecursive or short-circuiting can depend on
    -- both the method chosen to perform the fold and on the operator passed to
    -- that method (which may be implicit, as with the `mappend` method of a monoid
    -- instance).
    --
    -- There are also hybrid cases, where the method and/or operator are not well
    -- suited to the task at hand, resulting in a fold that fails to yield
    -- incremental results until the entire input is processed, or fails to
    -- strictly evaluate results as it goes, deferring all the work to the
    -- evaluation of a large final thunk.  Such cases should be avoided, either by
    -- selecting a more appropriate @Foldable@ method, or by tailoring the operator
    -- to the chosen method.
    --
    -- The distinction between these types of folds is critical, both in deciding
    -- which @Foldable@ method to use to perform the reduction efficiently, and in
    -- writing @Foldable@ instances for new structures.  Below is a more detailed
    -- overview of each type.

    --------------

    -- $strict
    -- #strict#
    --
    -- Common examples of strict recursive reduction are the various /aggregate/
    -- functions, like 'sum', 'product', 'length', as well as more complex
    -- summaries such as frequency counts.  These functions return only a single
    -- value after processing the entire input structure.  In such cases, lazy
    -- processing of the tail of the input structure is generally not only
    -- unnecessary, but also inefficient.  Thus, these and similar folds should be
    -- implemented in terms of strict left-associative @Foldable@ methods (typically
    -- 'foldl'') to perform an efficient reduction in constant space.
    --
    -- Conversely, an implementation of @Foldable@ for a new structure should
    -- ensure that 'foldl'' actually performs a strict left-associative reduction.
    --
    -- The 'foldMap'' method is a special case of 'foldl'', in which the initial
    -- accumulator is `mempty` and the operator is @mappend . f@, where @f@ maps
    -- each input element into the 'Monoid' in question.  Therefore, 'foldMap'' is
    -- an appropriate choice under essentially the same conditions as 'foldl'', and
    -- its implementation for a given @Foldable@ structure should also be a strict
    -- left-associative reduction.
    --
    -- While the examples below are not necessarily the most optimal definitions of
    -- the intended functions, they are all cases in which 'foldMap'' is far more
    -- appropriate (as well as more efficient) than the lazy `foldMap`.
    --
    -- > length  = getSum     . foldMap' (const (Sum 1))
    -- > sum     = getSum     . foldMap' Sum
    -- > product = getProduct . foldMap' Product
    --
    -- [ The actual default definitions employ coercions to optimise out
    --   'getSum' and 'getProduct'. ]

    --------------

    -- $strictlist
    --
    -- The full list of strict recursive functions in this module is:
    --
    -- * Provided the operator is strict in its left argument:
    --
    --     @'foldl'' :: Foldable t => (b -> a -> b) -> b -> t a -> b@
    --
    -- * Provided `mappend` is strict in its left argument:
    --
    --     @'foldMap'' :: (Foldable t, Monoid m) => (a -> m) -> t a -> m@
    --
    -- * Provided the instance is correctly defined:
    --
    --     @
    --     `length`    :: Foldable t => t a -> Int
    --     `sum`       :: (Foldable t, Num a) => t a -> a
    --     `product`   :: (Foldable t, Num a) => t a -> a
    --     `maximum`   :: (Foldable t, Ord a) => t a -> a
    --     `minimum`   :: (Foldable t, Ord a) => t a -> a
    --     `maximumBy` :: Foldable t => (a -> a -> Ordering) -> t a -> a
    --     `minimumBy` :: Foldable t => (a -> a -> Ordering) -> t a -> a
    --     @

    --------------

    -- $lazy
    --
    -- #corec#
    -- Common examples of lazy corecursive reduction are functions that map and
    -- flatten a structure to a lazy stream of result values, i.e.  an iterator
    -- over the transformed input elements.  In such cases, it is important to
    -- choose a @Foldable@ method that is lazy in the tail of the structure, such
    -- as `foldr` (or `foldMap`, if the result @Monoid@ has a lazy `mappend` as
    -- with e.g. ByteString Builders).
    --
    -- Conversely, an implementation of `foldr` for a structure that can
    -- accommodate a large (and possibly unbounded) number of elements is expected
    -- to be lazy in the tail of the input, allowing operators that are lazy in the
    -- accumulator to yield intermediate results incrementally.  Such folds are
    -- right-associative, with the tail of the stream returned as a lazily
    -- evaluated component of the result (an element of a tuple or some other
    -- non-strict constructor, e.g. the @(:)@ constructor for lists).
    --
    -- The @toList@ function below lazily transforms a @Foldable@ structure to a
    -- List.  Note that this transformation may be lossy, e.g.  for a keyed
    -- container (@Map@, @HashMap@, &#x2026;) the output stream holds only the
    -- values, not the keys.  Lossless transformations to\/from lists of @(key,
    -- value)@ pairs are typically available in the modules for the specific
    -- container types.
    --
    -- > toList = foldr (:) []
    --
    -- A more complex example is concatenation of a list of lists expressed as a
    -- nested right fold (bypassing @('++')@).  We can check that the definition is
    -- indeed lazy by folding an infinite list of lists, and taking an initial
    -- segment.
    --
    -- >>> myconcat = foldr (\x z -> foldr (:) z x) []
    -- >>> take 15 $ myconcat $ map (\i -> [0..i]) [0..]
    -- [0,0,1,0,1,2,0,1,2,3,0,1,2,3,4]
    --
    -- Of course in this case another way to achieve the same result is via a
    -- list comprehension:
    --
    -- > myconcat xss = [x | xs <- xss, x <- xs]

    --------------

    -- $lazylist
    --
    -- The full list of lazy corecursive functions in this module is:
    --
    -- * Provided the reduction function is lazy in its second argument,
    --   (otherwise best to use a strict recursive reduction):
    --
    --     @
    --     `foldr`  :: Foldable t => (a -> b -> b) -> b -> t a -> b
    --     `foldr1` :: Foldable t => (a -> a -> a) -> t a -> a
    --     @
    --
    -- * Provided the 'Monoid' `mappend` is lazy in its second argument
    --   (otherwise best to use a strict recursive reduction):
    --
    --     @
    --     `fold`    :: Foldable t => Monoid m => t m -> m
    --     `foldMap` :: Foldable t => Monoid m => (a -> m) -> t a -> m
    --     @
    --
    -- * Provided the instance is correctly defined:
    --
    --     @
    --     `toList`    :: Foldable t => t a -> [a]
    --     `concat`    :: Foldable t => t [a] -> [a]
    --     `concatMap` :: Foldable t => (a -> [b]) -> t a -> [b]
    --     @

    --------------

    -- $shortcircuit
    --
    -- #short#
    -- Examples of short-circuit reduction include various boolean predicates that
    -- test whether some or all the elements of a structure satisfy a given
    -- condition.  Because these don't necessarily consume the entire list, they
    -- typically employ `foldr` with an operator that is conditionally strict in
    -- its second argument.  Once the termination condition is met the second
    -- argument (tail of the input structure) is ignored.  No result is returned
    -- until that happens.
    --
    -- The key distinguishing feature of these folds is /conditional/ strictness
    -- in the second argument, it is sometimes evaluated and sometimes not.
    --
    -- The simplest (degenerate case) of these is 'null', which determines whether
    -- a structure is empty or not.  This only needs to look at the first element,
    -- and only to the extent of whether it exists or not, and not its value.  In
    -- this case termination is guaranteed, and infinite input structures are fine.
    -- Its default definition is of course in terms of the lazy 'foldr':
    --
    -- > null = foldr (\_ _ -> False) True
    --
    -- A more general example is `any`, which applies a predicate to each input
    -- element in turn until it finds the first one for which the predicate is
    -- true, at which point it returns success.  If, in an infinite input stream
    -- the predicate is false for all the elements, `any` will not terminate,
    -- but since it runs in constant space, it typically won't run out of memory,
    -- it'll just loop forever.

    --------------

    -- $shortlist
    --
    -- The full list of short-circuit folds in this module is:
    --
    -- * Boolean predicate folds.
    --   These functions examine elements strictly until a condition is met,
    --   but then return a result ignoring the rest (lazy in the tail).  These
    --   may loop forever given an unbounded input where no elements satisfy the
    --   termination condition.
    --
    --     @
    --     `null`    :: Foldable t => t a -> Bool
    --     `elem`    :: Foldable t => Eq a => a -> t a -> Bool
    --     `notElem` :: (Foldable t, Eq a) => a -> t a -> Bool
    --     `and`     :: Foldable t => t Bool -> Bool
    --     `or`      :: Foldable t => t Bool -> Bool
    --     `find`    :: Foldable t => (a -> Bool) -> t a -> Maybe a
    --     `any`     :: Foldable t => (a -> Bool) -> t a -> Bool
    --     `all`     :: Foldable t => (a -> Bool) -> t a -> Bool
    --     @
    --
    -- * Many instances of @('<|>')@ (e.g. the 'Maybe' instance) are conditionally
    --   lazy, and use or don't use their second argument depending on the value
    --   of the first.  These are used with the folds below, which terminate as
    --   early as possible, but otherwise generally keep going.  Some instances
    --   (e.g. for List) are always strict, but the result is lazy in the tail
    --   of the output, so that `asum` for a list of lists is in fact corecursive.
    --   These folds are defined in terms of `foldr`.
    --
    --     @
    --     `asum` :: (Foldable t, Alternative f) => t (f a) -> f a
    --     `msum` :: (Foldable t, MonadPlus m) => t (m a) -> m a
    --     @
    --
    -- * Likewise, the @('*>')@ operator in some `Applicative` functors, and @('>>')@
    --   in some monads are conditionally lazy and can /short-circuit/ a chain of
    --   computations.  The below folds will terminate as early as possible, but
    --   even infinite loops can be productive here, when evaluated solely for
    --   their stream of IO side-effects.  See "Data.Traversable#effectful"
    --   for discussion of related functions.
    --
    --     @
    --     `traverse_`  :: (Foldable t, Applicative f) => (a -> f b) -> t a -> f ()
    --     `for_`       :: (Foldable t, Applicative f) => t a -> (a -> f b) -> f ()
    --     `sequenceA_` :: (Foldable t, Applicative f) => t (f a) -> f ()
    --     `mapM_`      :: (Foldable t, Monad m) => (a -> m b) -> t a -> m ()
    --     `forM_`      :: (Foldable t, Monad m) => t a -> (a -> m b) -> m ()
    --     `sequence_`  :: (Foldable t, Monad m) => t (m a) -> m ()
    --     @
    --
    -- * Finally, there's one more special case, `foldlM`:
    --
    --     @`foldlM` :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b@
    --
    --     The sequencing of monadic effects proceeds from left to right.  If at
    --     some step the bind operator @('>>=')@ short-circuits (as with, e.g.,
    --     'mzero' with a 'MonadPlus', or an exception with a 'MonadThrow', etc.),
    --     then the evaluated effects will be from an initial portion of the
    --     element sequence.
    --
    --     >>> :set -XBangPatterns
    --     >>> import Control.Monad
    --     >>> import Control.Monad.Trans.Class
    --     >>> import Control.Monad.Trans.Maybe
    --     >>> import Data.Foldable
    --     >>> let f !_ e = when (e > 3) mzero >> lift (print e)
    --     >>> runMaybeT $ foldlM f () [0..]
    --     0
    --     1
    --     2
    --     3
    --     Nothing
    --
    --     Contrast this with `foldrM`, which sequences monadic effects from right
    --     to left, and therefore diverges when folding an unbounded input
    --     structure without ever having the opportunity to short-circuit.
    --
    --     >>> let f e _ = when (e > 3) mzero >> lift (print e)
    --     >>> runMaybeT $ foldrM f () [0..]
    --     ...hangs...
    --
    --     When the structure is finite `foldrM` performs the monadic effects from
    --     right to left, possibly short-circuiting after processing a tail portion
    --     of the element sequence.
    --
    --     >>> let f e _ = when (e < 3) mzero >> lift (print e)
    --     >>> runMaybeT $ foldrM f () [0..5]
    --     5
    --     4
    --     3
    --     Nothing

    --------------

    -- $hybrid
    --
    -- The below folds, are neither strict reductions that produce a final answer
    -- in constant space, nor lazy corecursions, and so have limited applicability.
    -- They do have specialised uses, but are best avoided when in doubt.
    --
    -- @
    -- 'foldr'' :: Foldable t => (a -> b -> b) -> b -> t a -> b
    -- 'foldl'  :: Foldable t => (b -> a -> b) -> b -> t a -> b
    -- 'foldl1' :: Foldable t => (a -> a -> a) -> t a -> a
    -- 'foldrM' :: (Foldable t, Monad m) => (a -> b -> m b) -> b -> t a -> m b
    -- @
    --
    -- The lazy left-folds (used corecursively) and 'foldrM' (used to sequence
    -- actions right-to-left) can be performant in structures whose @Foldable@
    -- instances take advantage of efficient right-to-left iteration to compute
    -- lazy left folds outside-in from the rightmost element.
    --
    -- The strict 'foldr'' is the least likely to be useful, structures that
    -- support efficient sequencing /only/ right-to-left are not common.

    --------------

    -- $instances
    --
    -- #instances#
    -- For many structures reasonably efficient @Foldable@ instances can be derived
    -- automatically, by enabling the @DeriveFoldable@ GHC extension.  When this
    -- works, it is generally not necessary to define a custom instance by hand.
    -- Though in some cases one may be able to get slightly faster hand-tuned code,
    -- care is required to avoid producing slower code, or code that is not
    -- sufficiently lazy, strict or /lawful/.
    --
    -- The hand-crafted instances can get away with only defining one of 'foldr' or
    -- 'foldMap'.  All the other methods have default definitions in terms of one
    -- of these.  The default definitions have the expected strictness and the
    -- expected asymptotic runtime and space costs, modulo small constant factors.
    -- If you choose to hand-tune, benchmarking is advised to see whether you're
    -- doing better than the default derived implementations, plus careful tests to
    -- ensure that the custom methods are correct.
    --
    -- Below we construct a @Foldable@ instance for a data type representing a
    -- (finite) binary tree with depth-first traversal.
    --
    -- > data Tree a = Empty | Leaf a | Node (Tree a) a (Tree a)
    --
    -- a suitable instance would be:
    --
    -- > 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
    --
    -- The 'Node' case is a right fold of the left subtree whose initial
    -- value is a right fold of the rest of the tree.
    --
    -- For example, when @f@ is @(':')@, all three cases return an immediate value,
    -- respectively @z@ or a /cons cell/ holding @x@ or @l@, with the remainder the
    -- structure, if any, encapsulated in a lazy thunk.  This meets the expected
    -- efficient [corecursive](#corec) behaviour of 'foldr'.
    --
    -- Alternatively, one could define @foldMap@:
    --
    -- > instance Foldable Tree where
    -- >    foldMap f Empty = mempty
    -- >    foldMap f (Leaf x) = f x
    -- >    foldMap f (Node l k r) = foldMap f l <> f k <> foldMap f r
    --
    -- And indeed some efficiency may be gained by directly defining both,
    -- avoiding some indirection in the default definitions that express
    -- one in terms of the other.  If you implement just one, likely 'foldr'
    -- is the better choice.
    --
    -- A binary tree typically (when balanced, or randomly biased) provides equally
    -- efficient access to its left and right subtrees.  This makes it possible to
    -- define a `foldl` optimised for [corecursive](#corec) folds with operators
    -- that are lazy in their first (left) argument.
    --
    -- > 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
    -- >    --
    -- >    foldMap f Empty = mempty
    -- >    foldMap f (Leaf x) = f x
    -- >    foldMap f (Node l k r) = foldMap f l <> f k <> foldMap f r
    -- >    --
    -- >    foldl f z Empty = z
    -- >    foldl f z (Leaf x) = f z x
    -- >    foldl f z (Node l k r) = foldl f (f (foldl f z l) k) r
    --
    -- Now left-to-right and right-to-left iteration over the structure
    -- elements are equally efficient (note the mirror-order output when
    -- using `foldl`):
    --
    -- >>> foldr (\e acc -> e : acc) [] (Node (Leaf 1) 2 (Leaf 3))
    -- [1,2,3]
    -- >>> foldl (\acc e -> e : acc) [] (Node (Leaf 1) 2 (Leaf 3))
    -- [3,2,1]
    --
    -- We can carry this further, and define more non-default methods...
    --
    -- The structure definition actually admits trees that are unbounded on either
    -- or both sides.  The only fold that can plausibly terminate for a tree
    -- unbounded on both left and right is `null`, when defined as shown below.
    -- The default definition in terms of `foldr` diverges if the tree is unbounded
    -- on the left.  Here we define a variant that avoids travelling down the tree
    -- to find the leftmost element and just examines the root node.
    --
    -- >    null Empty = True
    -- >    null _     = False
    --
    -- This is a sound choice also for finite trees.
    --
    -- In practice, unbounded trees are quite uncommon, and can barely be said to
    -- be @Foldable@.  They would typically employ breadth first traversal, and
    -- would support only corecursive and short-circuit folds (diverge under strict
    -- reduction).
    --
    -- Returning to simpler instances, defined just in terms of `foldr`, it is
    -- somewhat surprising that a fairly efficient /default/ implementation of the
    -- strict 'foldl'' is defined in terms of lazy `foldr` when only the latter is
    -- explicitly provided by the instance.  It may be instructive to take a look
    -- at how this works.

    --------------

    -- $strictlazy
    --
    -- #strictlazy#
    --
    -- Sometimes, it is useful for the result of applying 'foldr' to be a
    -- /function/.  This is done by mapping the structure elements to functions
    -- with the same argument and result types.  The per-element functions are then
    -- composed to give the final result.
    --
    -- For example, we can /flip/ the strict left fold 'foldl'' by writing:
    --
    -- > foldl' f z xs = flippedFoldl' f xs z
    --
    -- with the function 'flippedFoldl'' defined as below, with 'seq' used to
    -- ensure the strictness in the accumulator:
    --
    -- > flippedFoldl' f [] z = z
    -- > flippedFoldl' f (x : xs) z = z `seq` flippedFoldl' f xs (f z x)
    --
    -- Rewriting to use lambdas, this is:
    --
    -- > flippedFoldl' f [] = \ b -> b
    -- > flippedFoldl' f (x : xs) = \ b -> b `seq` r (f b x)
    -- >     where r = flippedFoldl' f xs
    --
    -- The above has the form of a right fold, enabling a rewrite to:
    --
    -- > flippedFoldl' f = \ xs -> foldr f' id xs
    -- >     where f' x r = \ b -> b `seq` r (f b x)
    --
    -- We can now unflip this to get 'foldl'':
    --
    -- > foldl' f z = \ xs -> foldr f' id xs z
    -- >           -- \ xs -> flippedFoldl' f xs z
    -- >   where f' x r = \ b -> b `seq` r (f b x)
    --
    -- The function __@foldr f' id xs@__ applied to @z@ is built corecursively, and
    -- its terms are applied to an eagerly evaluated accumulator before further
    -- terms are applied to the result.  As required, this runs in constant space,
    -- and can be optimised to an efficient loop.
    --
    -- (The actual definition of 'foldl'' labels the lambdas in the definition of
    -- __@f'@__ above as /oneShot/, which enables further optimisations).

    --------------

    -- $generative
    --
    -- #generative#
    -- So far, we have not discussed /generative recursion/.  Unlike recursive
    -- reduction or corecursion, instead of processing a sequence of elements
    -- already in memory, generative recursion involves producing a possibly
    -- unbounded sequence of values from an initial seed value.  The canonical
    -- example of this is 'Data.List.unfoldr' for Lists, with variants available
    -- for Vectors and various other structures.
    --
    -- A key issue with lists, when used generatively as /iterators/, rather than as
    -- poor-man's containers (see [[1\]](#uselistsnot)), is that such iterators
    -- tend to consume memory when used more than once.  A single traversal of a
    -- list-as-iterator will run in constant space, but as soon as the list is
    -- retained for reuse, its entire element sequence is stored in memory, and the
    -- second traversal reads the copy, rather than regenerates the elements.  It
    -- is sometimes better to recompute the elements rather than memoise the list.
    --
    -- Memoisation happens because the built-in Haskell list __@[]@__ is
    -- represented as __data__, either empty or a /cons-cell/ holding the first
    -- element and the tail of the list.  The @Foldable@ class enables a variant
    -- representation of iterators as /functions/, which take an operator and a
    -- starting accumulator and output a summary result.
    --
    -- The [@fmlist@](https://hackage.haskell.org/package/fmlist) package takes
    -- this approach, by representing a list via its `foldMap` action.
    --
    -- Below we implement an analogous data structure using a representation
    -- based on `foldr`.  This is an example of /Church encoding/
    -- (named after Alonzo Church, inventor of the lambda calculus).
    --
    -- > {-# LANGUAGE RankNTypes #-}
    -- > newtype FRList a = FR { unFR :: forall b. (a -> b -> b) -> b -> b }
    --
    -- The __@unFR@__ field of this type is essentially its `foldr` method
    -- with the list as its first rather than last argument.  Thus we
    -- immediately get a @Foldable@ instance (and a 'toList' function
    -- mapping an __@FRList@__ to a regular list).
    --
    -- > instance Foldable FRList where
    -- >     foldr f z l = unFR l f z
    -- >     -- With older versions of @base@, also define sum, product, ...
    -- >     -- to ensure use of the strict 'foldl''.
    -- >     -- sum = foldl' (+) 0
    -- >     -- ...
    --
    -- We can convert a regular list to an __@FRList@__ with:
    --
    -- > fromList :: [a] -> FRList a
    -- > fromList as = FRList $ \ f z -> foldr f z as
    --
    -- However, reuse of an __@FRList@__ obtained in this way will typically
    -- memoise the underlying element sequence.  Instead, we can define
    -- __@FRList@__ terms directly:
    --
    -- > -- | Immediately return the initial accumulator
    -- > nil :: FRList a
    -- > nil = FRList $ \ _ z -> z
    -- > {-# INLINE nil #-}
    --
    -- > -- | Fold the tail to use as an accumulator with the new initial element
    -- > cons :: a -> FRList a -> FRList a
    -- > cons a l = FRList $ \ f z -> f a (unFR l f z)
    -- > {-# INLINE cons #-}
    --
    -- More crucially, we can also directly define the key building block for
    -- generative recursion:
    --
    -- > -- | Generative recursion, dual to `foldr`.
    -- > unfoldr :: (s -> Maybe (a, s)) -> s -> FRList a
    -- > unfoldr g s0 = FR generate
    -- >   where generate f z = loop s0
    -- >           where loop s | Just (a, t) <- g s = f a (loop t)
    -- >                        | otherwise = z
    -- > {-# INLINE unfoldr #-}
    --
    -- Which can, for example, be specialised to number ranges:
    --
    -- > -- | Generate a range of consecutive integral values.
    -- > range :: (Ord a, Integral a) => a -> a -> FRList a
    -- > range lo hi =
    -- >     unfoldr (\s -> if s > hi then Nothing else Just (s, s+1)) lo
    -- > {-# INLINE range #-}
    --
    -- The program below, when compiled with optimisation:
    --
    -- > main :: IO ()
    -- > main = do
    -- >     let r :: FRList Int
    -- >         r = range 1 10000000
    -- >      in print (sum r, length r)
    --
    -- produces the expected output with no noticeable garbage-collection, despite
    -- reuse of the __@FRList@__ term __@r@__.
    --
    -- > (50000005000000,10000000)
    -- >     52,120 bytes allocated in the heap
    -- >      3,320 bytes copied during GC
    -- >     44,376 bytes maximum residency (1 sample(s))
    -- >     25,256 bytes maximum slop
    -- >          3 MiB total memory in use (0 MB lost due to fragmentation)
    --
    -- The Weak Head Normal Form of an __@FRList@__ is a lambda abstraction not a
    -- data value, and reuse does not lead to memoisation.  Reuse of the iterator
    -- above is somewhat contrived, when computing multiple folds over a common
    -- list, you should generally traverse a  list only [once](#multipass).  The
    -- goal is to demonstrate that the separate computations of the 'sum' and
    -- 'length' run efficiently in constant space, despite reuse.  This would not
    -- be the case with the list @[1..10000000]@.
    --
    -- This is, however, an artificially simple reduction.  More typically, there
    -- are likely to be some allocations in the inner loop, but the temporary
    -- storage used will be garbage-collected as needed, and overall memory
    -- utilisation will remain modest and will not scale with the size of the list.
    --
    -- If we go back to built-in lists (i.e. __@[]@__), but avoid reuse by
    -- performing reduction in a single pass, as below:
    --
    -- > data PairS a b = P !a !b -- We define a strict pair datatype
    -- >
    -- > main :: IO ()
    -- > main = do
    -- >     let l :: [Int]
    -- >         l = [1..10000000]
    -- >      in print $ average l
    -- >   where
    -- >     sumlen :: PairS Int Int -> Int -> PairS Int Int
    -- >     sumlen (P s l) a = P (s + a) (l + 1)
    -- >
    -- >     average is =
    -- >         let (P s l) = foldl' sumlen (P 0 0) is
    -- >          in (fromIntegral s :: Double) / fromIntegral l
    --
    -- the result is again obtained in constant space:
    --
    -- > 5000000.5
    -- >          102,176 bytes allocated in the heap
    -- >            3,320 bytes copied during GC
    -- >           44,376 bytes maximum residency (1 sample(s))
    -- >           25,256 bytes maximum slop
    -- >                3 MiB total memory in use (0 MB lost due to fragmentation)
    --
    -- (and, in fact, faster than with __@FRList@__ by a small factor).
    --
    -- The __@[]@__ list structure works as an efficient iterator when used
    -- just once.  When space-leaks via list reuse are not a concern, and/or
    -- memoisation is actually desirable, the regular list implementation is
    -- likely to be faster.  This is not a suggestion to replace all your uses of
    -- __@[]@__ with a generative alternative.
    --
    -- The __@FRList@__ type could be further extended with instances of 'Functor',
    -- 'Applicative', 'Monad', 'Alternative', etc., and could then provide a
    -- fully-featured list type, optimised for reuse without space-leaks.  If,
    -- however, all that's required is space-efficient, re-use friendly iteration,
    -- less is perhaps more, and just @Foldable@ may be sufficient.

    --------------

    -- $multipass
    --
    -- #multipass#
    -- In applications where you want to compute a composite function of a
    -- structure, which requires more than one aggregate as an input, it is
    -- generally best to compute all the aggregates in a single pass, rather
    -- than to traverse the same structure repeatedly.
    --
    -- The [@foldl@](http://hackage.haskell.org/package/foldl) package implements a
    -- robust general framework for dealing with this situation.  If you choose to
    -- to do it yourself, with a bit of care, the simplest cases are not difficult
    -- to handle directly.  You just need to accumulate the individual aggregates
    -- as __strict__ components of a single data type, and then apply a final
    -- transformation to it to extract the composite result.  For example,
    -- computing an average requires computing both the 'sum' and the 'length' of a
    -- (non-empty) structure and dividing the sum by the length:
    --
    -- > import Data.Foldable (foldl')
    -- >
    -- > data PairS a b = P !a !b -- We define a strict pair datatype
    -- >
    -- > -- | Compute sum and length in a single pass, then reduce to the average.
    -- > average :: (Foldable f, Fractional a) => f a -> a
    -- > average xs =
    -- >     let sumlen (P s l) a = P (s + a) (l + 1 :: Int)
    -- >         (P s l) = foldl' sumlen (P 0 0) xs
    -- >      in s / fromIntegral l
    --
    -- The above example is somewhat contrived, some structures keep track of their
    -- length internally, and can return it in /O(1)/ time, so this particular
    -- recipe for averages is not always the most efficient.  In general, composite
    -- aggregate functions of large structures benefit from single-pass reduction.
    -- This is especially the case when reuse of a list and memoisation of its
    -- elements is thereby avoided.

    --------------

    -- $laws
    -- #laws#
    --
    -- The type constructor 'Endo' from "Data.Monoid", associates with each type
    -- __@b@__ the __@newtype@__-encapsulated type of functions mapping __@b@__ to
    -- itself.  Functions from a type to itself are called /endomorphisms/, hence
    -- the name /Endo/.  The type __@Endo b@__ is a 'Monoid' under function
    -- composition:
    --
    -- > newtype Endo b = Endo { appEndo :: b -> b }
    -- > instance Semigroup Endo b where
    -- >     Endo f <> Endo g = Endo (f . g)
    -- > instance Monoid Endo b where
    -- >     mempty = Endo id
    --
    -- For every 'Monoid' m, we also have a 'Dual' monoid __@Dual m@__ which
    -- combines elements in the opposite order:
    --
    -- > newtype Dual m = Dual { getDual :: m }
    -- > instance Semigroup m => Semigroup Dual m where
    -- >     Dual a <> Dual b = Dual (b <> a)
    -- > instance Monoid m => Monoid Dual m where
    -- >     mempty = Dual mempty
    --
    -- With the above preliminaries out of the way, 'Foldable' instances are
    -- expected to satisfy the following laws:
    --
    -- The 'foldr' method must be equivalent in value and strictness to replacing
    -- each element __@a@__ of a 'Foldable' structure with __@Endo (f a)@__,
    -- composing these via 'foldMap' and applying the result to the base case
    -- __@z@__:
    --
    -- > foldr f z t = appEndo (foldMap (Endo . f) t ) z
    --
    -- Likewise, the 'foldl' method must be equivalent in value and strictness
    -- to composing the functions __@flip f a@__ in reverse order and applying
    -- the result to the base case:
    --
    -- > foldl f z t = appEndo (getDual (foldMap (Dual . Endo . flip f) t)) z
    --
    -- When the elements of the structure are taken from a 'Monoid', the
    -- definition of 'fold' must agree with __@foldMap id@__:
    --
    -- > fold = foldMap id
    --
    -- The 'length' method must agree with a 'foldMap' mapping each element to
    -- __@Sum 1@__ (The 'Sum' type abstracts numbers as a monoid under addition).
    --
    -- > length = getSum . foldMap (Sum . const 1)
    --
    -- @sum@, @product@, @maximum@, and @minimum@ should all be essentially
    -- equivalent to @foldMap@ forms, such as
    --
    -- > sum     = getSum     . foldMap' Sum
    -- > product = getProduct . foldMap' Product
    --
    -- but are generally more efficient when defined more directly as:
    --
    -- > sum = foldl' (+) 0
    -- > product = foldl' (*) 1
    --
    -- If the 'Foldable' structure has a 'Functor' instance, then for every
    -- function __@f@__ mapping the elements into a 'Monoid', it should satisfy:
    --
    -- > foldMap f = fold . fmap f
    --
    -- which implies that
    --
    -- > foldMap f . fmap g = foldMap (f . g)
    --

    --------------

    -- $notes
    --
    -- #notes#
    -- Since 'Foldable' does not have 'Functor' as a superclass, it is possible to
    -- define 'Foldable' instances for structures that constrain their element
    -- types.  Therefore, __@Set@__ can be 'Foldable', even though sets keep their
    -- elements in ascending order.  This requires the elements to be comparable,
    -- which precludes defining a 'Functor' instance for @Set@.
    --
    -- The 'Foldable' class makes it possible to use idioms familiar from the @List@
    -- type with container structures that are better suited to the task at hand.
    -- This supports use of more appropriate 'Foldable' data types, such as @Seq@,
    -- @Set@, @NonEmpty@, etc., without requiring new idioms (see
    -- [[1\]](#uselistsnot) for when not to use lists).
    --
    -- The more general methods of the 'Foldable' class are now exported by the
    -- "Prelude" in place of the original List-specific methods (see the
    -- [FTP Proposal](https://wiki.haskell.org/Foldable_Traversable_In_Prelude)).
    -- The List-specific variants are for now still available in "GHC.OldList", but
    -- that module is intended only as a transitional aid, and may be removed in
    -- the future.
    --
    -- Surprises can arise from the @Foldable@ instance of the 2-tuple @(a,)@ which
    -- now behaves as a 1-element @Foldable@ container in its second slot.  In
    -- contexts where a specific monomorphic type is expected, and you want to be
    -- able to rely on type errors to guide refactoring, it may make sense to
    -- define and use less-polymorphic variants of some of the @Foldable@ methods.
    --
    -- Below are two examples showing a definition of a reusable less-polymorphic
    -- 'sum' and a one-off in-line specialisation of 'length':
    --
    -- > {-# LANGUAGE TypeApplications #-}
    -- >
    -- > mySum :: Num a => [a] -> a
    -- > mySum = sum
    -- >
    -- > type SlowVector a = [a]
    -- > slowLength :: SlowVector -> Int
    -- > slowLength v = length @[] v
    --
    -- In both cases, if the data type to which the function is applied changes
    -- to something other than a list, the call-site will no longer compile until
    -- appropriate changes are made.

    -- $linear
    --
    -- It is perhaps worth noting that since the __`elem`__ function in the
    -- 'Foldable' class carries only an __`Eq`__ constraint on the element type,
    -- search for the presence or absence of an element in the structure generally
    -- takes /O(n)/ time, even for ordered structures like __@Set@__ that are
    -- potentially capable of performing the search faster.  (The @member@ function
    -- of the @Set@ module carries an `Ord` constraint, and can perform the search
    -- in /O(log n)/ time).
    --
    -- An alternative to Foldable's __`elem`__ method is required in order to
    -- abstract potentially faster than linear search over general container
    -- structures.  This can be achieved by defining an additional type class (e.g.
    -- @HasMember@ below).  Instances of such a type class (that are also
    -- `Foldable') can employ the `elem` linear search as a last resort, when
    -- faster search is not supported.
    --
    -- > {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}
    -- >
    -- > import qualified Data.Set as Set
    -- >
    -- > class Eq a => HasMember t a where
    -- >     member :: a -> t a -> Bool
    -- >
    -- > instance Eq a => HasMember [] a where
    -- >     member = elem
    -- > [...]
    -- > instance Ord a => HasMember Set.Set a where
    -- >     member = Set.member
    --
    -- The above suggests that 'elem' may be a misfit in the 'Foldable' class.
    -- Alternative design ideas are solicited on GHC's bug tracker via issue
    -- [\#20421](https://gitlab.haskell.org/ghc/ghc/-/issues/20421).
    --
    -- Note that some structure-specific optimisations may of course be possible
    -- directly in the corresponding @Foldable@ instance, e.g. with @Set@ the size
    -- of the set is known in advance, without iterating to count the elements, and
    -- its `length` instance takes advantage of this to return the size directly.

    --------------

    -- $also
    --
    --  * [1] #uselistsnot# \"When You Should Use Lists in Haskell (Mostly, You Should Not)\",
    --    by Johannes Waldmann,
    --    in arxiv.org, Programming Languages (cs.PL), at
    --    <https://arxiv.org/abs/1808.08329>.
    --
    --  * [2] \"The Essence of the Iterator Pattern\",
    --    by Jeremy Gibbons and Bruno Oliveira,
    --    in /Mathematically-Structured Functional Programming/, 2006, online at
    --    <http://www.cs.ox.ac.uk/people/jeremy.gibbons/publications/#iterator>.
    --
    --  * [3] \"A tutorial on the universality and expressiveness of fold\",
    --    by Graham Hutton, J\. Functional Programming 9 (4): 355–372, July 1999,
    --    online at <http://www.cs.nott.ac.uk/~pszgmh/fold.pdf>.