{-# 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

-- $setup
-- >>> import Prelude
-- >>> import qualified Data.List as List

-- $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 (List.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) []
-- >>> List.take 15 $ myconcat $ List.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>.
