Copyright | Ross Paterson 2005 |
---|---|
License | BSD-style (see the LICENSE file in the distribution) |
Maintainer | libraries@haskell.org |
Stability | stable |
Portability | portable |
Safe Haskell | Safe |
Language | Haskell2010 |
Class of data structures that can be folded to a summary value.
Synopsis
- class Foldable (t :: Type -> Type) where
- fold :: Monoid m => t m -> m
- foldMap :: Monoid m => (a -> m) -> t a -> m
- foldMap' :: Monoid m => (a -> m) -> t a -> m
- foldr :: (a -> b -> b) -> b -> t a -> b
- foldr' :: (a -> b -> b) -> b -> t a -> b
- foldl :: (b -> a -> b) -> b -> t a -> b
- foldl' :: (b -> a -> b) -> b -> t a -> b
- foldr1 :: (a -> a -> a) -> t a -> a
- foldl1 :: (a -> a -> a) -> t a -> a
- toList :: t a -> [a]
- null :: t a -> Bool
- length :: t a -> Int
- elem :: Eq a => a -> t a -> Bool
- maximum :: Ord a => t a -> a
- minimum :: Ord a => t a -> a
- sum :: Num a => t a -> a
- product :: Num a => t a -> a
- foldrM :: (Foldable t, Monad m) => (a -> b -> m b) -> b -> t a -> m b
- foldlM :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b
- 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 ()
- asum :: (Foldable t, Alternative f) => t (f a) -> f a
- 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 ()
- msum :: (Foldable t, MonadPlus m) => t (m a) -> m a
- concat :: Foldable t => t [a] -> [a]
- concatMap :: Foldable t => (a -> [b]) -> t a -> [b]
- and :: Foldable t => t Bool -> Bool
- or :: Foldable t => t Bool -> Bool
- any :: Foldable t => (a -> Bool) -> t a -> Bool
- all :: Foldable t => (a -> Bool) -> t a -> Bool
- maximumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
- minimumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
- notElem :: (Foldable t, Eq a) => a -> t a -> Bool
- find :: Foldable t => (a -> Bool) -> t a -> Maybe a
Documentation
class Foldable (t :: Type -> Type) where Source #
The Foldable class represents data structures that can be reduced to a summary value one element at a time. Strict left-associative folds are a good fit for space-efficient reduction, while lazy right-associative folds are a good fit for corecursive iteration, or for folds that short-circuit after processing an initial subsequence of the structure's elements.
Instances can be derived automatically by enabling the DeriveFoldable
extension. For example, a derived instance for a binary tree might be:
{-# LANGUAGE DeriveFoldable #-} data Tree a = Empty | Leaf a | Node (Tree a) a (Tree a) deriving Foldable
A more detailed description can be found in the Overview section of Data.Foldable.
For the class laws see the Laws section of Data.Foldable.
fold :: Monoid m => t m -> m Source #
Given a structure with elements whose type is a Monoid
, combine them
via the monoid's (
operator. This fold is right-associative and
lazy in the accumulator. When you need a strict left-associative fold,
use <>
)foldMap'
instead, with id
as the map.
Examples
Basic usage:
>>>
fold [[1, 2, 3], [4, 5], [6], []]
[1,2,3,4,5,6]
>>>
fold $ Node (Leaf (Sum 1)) (Sum 3) (Leaf (Sum 5))
Sum {getSum = 9}
Folds of unbounded structures do not terminate when the monoid's
(
operator is strict:<>
)
>>>
fold (repeat Nothing)
* Hangs forever *
Lazy corecursive folds of unbounded structures are fine:
>>>
take 12 $ fold $ map (\i -> [i..i+2]) [0..]
[0,1,2,1,2,3,2,3,4,3,4,5]>>>
sum $ take 4000000 $ fold $ map (\i -> [i..i+2]) [0..]
2666668666666
foldMap :: Monoid m => (a -> m) -> t a -> m Source #
Map each element of the structure into a monoid, and combine the
results with (
. This fold is right-associative and lazy in the
accumulator. For strict left-associative folds consider <>
)foldMap'
instead.
Examples
Basic usage:
>>>
foldMap Sum [1, 3, 5]
Sum {getSum = 9}
>>>
foldMap Product [1, 3, 5]
Product {getProduct = 15}
>>>
foldMap (replicate 3) [1, 2, 3]
[1,1,1,2,2,2,3,3,3]
When a Monoid's (
is lazy in its second argument, <>
)foldMap
can
return a result even from an unbounded structure. For example, lazy
accumulation enables Data.ByteString.Builder to efficiently serialise
large data structures and produce the output incrementally:
>>>
import qualified Data.ByteString.Lazy as L
>>>
import qualified Data.ByteString.Builder as B
>>>
let bld :: Int -> B.Builder; bld i = B.intDec i <> B.word8 0x20
>>>
let lbs = B.toLazyByteString $ foldMap bld [0..]
>>>
L.take 64 lbs
"0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24"
foldMap' :: Monoid m => (a -> m) -> t a -> m Source #
A left-associative variant of foldMap
that is strict in the
accumulator. Use this method for strict reduction when partial
results are merged via (
.<>
)
Examples
Define a Monoid
over finite bit strings under xor
. Use it to
strictly compute the xor
of a list of Int
values.
>>>
:set -XGeneralizedNewtypeDeriving
>>>
import Data.Bits (Bits, FiniteBits, xor, zeroBits)
>>>
import Data.Foldable (foldMap')
>>>
import Numeric (showHex)
>>>
>>>
newtype X a = X a deriving (Eq, Bounded, Enum, Bits, FiniteBits)
>>>
instance Bits a => Semigroup (X a) where X a <> X b = X (a `xor` b)
>>>
instance Bits a => Monoid (X a) where mempty = X zeroBits
>>>
>>>
let bits :: [Int]; bits = [0xcafe, 0xfeed, 0xdeaf, 0xbeef, 0x5411]
>>>
(\ (X a) -> showString "0x" . showHex a $ "") $ foldMap' X bits
"0x42"
@since base-4.13.0.0
foldr :: (a -> b -> b) -> b -> t a -> b Source #
Right-associative fold of a structure, lazy in the accumulator.
In the case of lists, foldr
, when applied to a binary operator, a
starting value (typically the right-identity of the operator), and a
list, reduces the list using the binary operator, from right to left:
foldr f z [x1, x2, ..., xn] == x1 `f` (x2 `f` ... (xn `f` z)...)
Note that since the head of the resulting expression is produced by an
application of the operator to the first element of the list, given an
operator lazy in its right argument, foldr
can produce a terminating
expression from an unbounded list.
For a general Foldable
structure this should be semantically identical
to,
foldr f z =foldr
f z .toList
Examples
Basic usage:
>>>
foldr (||) False [False, True, False]
True
>>>
foldr (||) False []
False
>>>
foldr (\c acc -> acc ++ [c]) "foo" ['a', 'b', 'c', 'd']
"foodcba"
Infinite structures
⚠️ Applying foldr
to infinite structures usually doesn't terminate.
It may still terminate under one of the following conditions:
- the folding function is short-circuiting
- the folding function is lazy on its second argument
Short-circuiting
(
short-circuits on ||
)True
values, so the following terminates
because there is a True
value finitely far from the left side:
>>>
foldr (||) False (True : repeat False)
True
But the following doesn't terminate:
>>>
foldr (||) False (repeat False ++ [True])
* Hangs forever *
Laziness in the second argument
Applying foldr
to infinite structures terminates when the operator is
lazy in its second argument (the initial accumulator is never used in
this case, and so could be left undefined
, but []
is more clear):
>>>
take 5 $ foldr (\i acc -> i : fmap (+3) acc) [] (repeat 1)
[1,4,7,10,13]
foldr' :: (a -> b -> b) -> b -> t a -> b Source #
foldr'
is a variant of foldr
that performs strict reduction from
right to left, i.e. starting with the right-most element. The input
structure must be finite, otherwise foldr'
runs out of space
(diverges).
If you want a strict right fold in constant space, you need a structure
that supports faster than O(n) access to the right-most element, such
as Seq
from the containers
package.
This method does not run in constant space for structures such as lists
that don't support efficient right-to-left iteration and so require
O(n) space to perform right-to-left reduction. Use of this method
with such a structure is a hint that the chosen structure may be a poor
fit for the task at hand. If the order in which the elements are
combined is not important, use foldl'
instead.
@since base-4.6.0.0
foldl :: (b -> a -> b) -> b -> t a -> b Source #
Left-associative fold of a structure, lazy in the accumulator. This is rarely what you want, but can work well for structures with efficient right-to-left sequencing and an operator that is lazy in its left argument.
In the case of lists, foldl
, when applied to a binary operator, a
starting value (typically the left-identity of the operator), and a
list, reduces the list using the binary operator, from left to right:
foldl f z [x1, x2, ..., xn] == (...((z `f` x1) `f` x2) `f`...) `f` xn
Note that to produce the outermost application of the operator the
entire input list must be traversed. Like all left-associative folds,
foldl
will diverge if given an infinite list.
If you want an efficient strict left-fold, you probably want to use
foldl'
instead of foldl
. The reason for this is that the latter
does not force the inner results (e.g. z `f` x1
in the above
example) before applying them to the operator (e.g. to (`f` x2)
).
This results in a thunk chain O(n) elements long, which then must be
evaluated from the outside-in.
For a general Foldable
structure this should be semantically identical
to:
foldl f z =foldl
f z .toList
Examples
The first example is a strict fold, which in practice is best performed
with foldl'
.
>>>
foldl (+) 42 [1,2,3,4]
52
Though the result below is lazy, the input is reversed before prepending it to the initial accumulator, so corecursion begins only after traversing the entire input string.
>>>
foldl (\acc c -> c : acc) "abcd" "efgh"
"hgfeabcd"
A left fold of a structure that is infinite on the right cannot terminate, even when for any finite input the fold just returns the initial accumulator:
>>>
foldl (\a _ -> a) 0 $ repeat 1
* Hangs forever *
WARNING: When it comes to lists, you always want to use either foldl'
or foldr
instead.
foldl' :: (b -> a -> b) -> b -> t a -> b Source #
Left-associative fold of a structure but with strict application of the operator.
This ensures that each step of the fold is forced to Weak Head Normal
Form before being applied, avoiding the collection of thunks that would
otherwise occur. This is often what you want to strictly reduce a
finite structure to a single strict result (e.g. sum
).
For a general Foldable
structure this should be semantically identical
to,
foldl' f z =foldl'
f z .toList
@since base-4.6.0.0
foldr1 :: (a -> a -> a) -> t a -> a Source #
A variant of foldr
that has no base case,
and thus may only be applied to non-empty structures.
This function is non-total and will raise a runtime exception if the structure happens to be empty.
Examples
Basic usage:
>>>
foldr1 (+) [1..4]
10
>>>
foldr1 (+) []
Exception: Prelude.foldr1: empty list
>>>
foldr1 (+) Nothing
*** Exception: foldr1: empty structure
>>>
foldr1 (-) [1..4]
-2
>>>
foldr1 (&&) [True, False, True, True]
False
>>>
foldr1 (||) [False, False, True, True]
True
>>>
foldr1 (+) [1..]
* Hangs forever *
foldl1 :: (a -> a -> a) -> t a -> a Source #
A variant of foldl
that has no base case,
and thus may only be applied to non-empty structures.
This function is non-total and will raise a runtime exception if the structure happens to be empty.
foldl1
f =foldl1
f .toList
Examples
Basic usage:
>>>
foldl1 (+) [1..4]
10
>>>
foldl1 (+) []
*** Exception: Prelude.foldl1: empty list
>>>
foldl1 (+) Nothing
*** Exception: foldl1: empty structure
>>>
foldl1 (-) [1..4]
-8
>>>
foldl1 (&&) [True, False, True, True]
False
>>>
foldl1 (||) [False, False, True, True]
True
>>>
foldl1 (+) [1..]
* Hangs forever *
List of elements of a structure, from left to right. If the entire list is intended to be reduced via a fold, just fold the structure directly bypassing the list.
Examples
Basic usage:
>>>
toList Nothing
[]
>>>
toList (Just 42)
[42]
>>>
toList (Left "foo")
[]
>>>
toList (Node (Leaf 5) 17 (Node Empty 12 (Leaf 8)))
[5,17,12,8]
For lists, toList
is the identity:
>>>
toList [1, 2, 3]
[1,2,3]
@since base-4.8.0.0
Test whether the structure is empty. The default implementation is Left-associative and lazy in both the initial element and the accumulator. Thus optimised for structures where the first element can be accessed in constant time. Structures where this is not the case should have a non-default implementation.
Examples
Basic usage:
>>>
null []
True
>>>
null [1]
False
null
is expected to terminate even for infinite structures.
The default implementation terminates provided the structure
is bounded on the left (there is a leftmost element).
>>>
null [1..]
False
@since base-4.8.0.0
Returns the size/length of a finite structure as an Int
. The
default implementation just counts elements starting with the leftmost.
Instances for structures that can compute the element count faster
than via element-by-element counting, should provide a specialised
implementation.
Examples
Basic usage:
>>>
length []
0
>>>
length ['a', 'b', 'c']
3>>>
length [1..]
* Hangs forever *
@since base-4.8.0.0
elem :: Eq a => a -> t a -> Bool infix 4 Source #
Does the element occur in the structure?
Note: elem
is often used in infix form.
Examples
Basic usage:
>>>
3 `elem` []
False
>>>
3 `elem` [1,2]
False
>>>
3 `elem` [1,2,3,4,5]
True
For infinite structures, the default implementation of elem
terminates if the sought-after value exists at a finite distance
from the left side of the structure:
>>>
3 `elem` [1..]
True
>>>
3 `elem` ([4..] ++ [3])
* Hangs forever *
@since base-4.8.0.0
maximum :: Ord a => t a -> a Source #
The largest element of a non-empty structure.
This function is non-total and will raise a runtime exception if the structure happens to be empty. A structure that supports random access and maintains its elements in order should provide a specialised implementation to return the maximum in faster than linear time.
Examples
Basic usage:
>>>
maximum [1..10]
10
>>>
maximum []
*** Exception: Prelude.maximum: empty list
>>>
maximum Nothing
*** Exception: maximum: empty structure
WARNING: This function is partial for possibly-empty structures like lists.
@since base-4.8.0.0
minimum :: Ord a => t a -> a Source #
The least element of a non-empty structure.
This function is non-total and will raise a runtime exception if the structure happens to be empty. A structure that supports random access and maintains its elements in order should provide a specialised implementation to return the minimum in faster than linear time.
Examples
Basic usage:
>>>
minimum [1..10]
1
>>>
minimum []
*** Exception: Prelude.minimum: empty list
>>>
minimum Nothing
*** Exception: minimum: empty structure
WARNING: This function is partial for possibly-empty structures like lists.
@since base-4.8.0.0
sum :: Num a => t a -> a Source #
The sum
function computes the sum of the numbers of a structure.
Examples
Basic usage:
>>>
sum []
0
>>>
sum [42]
42
>>>
sum [1..10]
55
>>>
sum [4.1, 2.0, 1.7]
7.8
>>>
sum [1..]
* Hangs forever *
@since base-4.8.0.0
product :: Num a => t a -> a Source #
The product
function computes the product of the numbers of a
structure.
Examples
Basic usage:
>>>
product []
1
>>>
product [42]
42
>>>
product [1..10]
3628800
>>>
product [4.1, 2.0, 1.7]
13.939999999999998
>>>
product [1..]
* Hangs forever *
@since base-4.8.0.0
Instances
Foldable Complex Source # | Since: base-4.9.0.0 |
Defined in Data.Complex fold :: Monoid m => Complex m -> m Source # foldMap :: Monoid m => (a -> m) -> Complex a -> m Source # foldMap' :: Monoid m => (a -> m) -> Complex a -> m Source # foldr :: (a -> b -> b) -> b -> Complex a -> b Source # foldr' :: (a -> b -> b) -> b -> Complex a -> b Source # foldl :: (b -> a -> b) -> b -> Complex a -> b Source # foldl' :: (b -> a -> b) -> b -> Complex a -> b Source # foldr1 :: (a -> a -> a) -> Complex a -> a Source # foldl1 :: (a -> a -> a) -> Complex a -> a Source # toList :: Complex a -> [a] Source # null :: Complex a -> Bool Source # length :: Complex a -> Int Source # elem :: Eq a => a -> Complex a -> Bool Source # maximum :: Ord a => Complex a -> a Source # minimum :: Ord a => Complex a -> a Source # | |
Foldable First Source # | Since: base-4.9.0.0 |
Defined in Data.Semigroup fold :: Monoid m => First m -> m Source # foldMap :: Monoid m => (a -> m) -> First a -> m Source # foldMap' :: Monoid m => (a -> m) -> First a -> m Source # foldr :: (a -> b -> b) -> b -> First a -> b Source # foldr' :: (a -> b -> b) -> b -> First a -> b Source # foldl :: (b -> a -> b) -> b -> First a -> b Source # foldl' :: (b -> a -> b) -> b -> First a -> b Source # foldr1 :: (a -> a -> a) -> First a -> a Source # foldl1 :: (a -> a -> a) -> First a -> a Source # toList :: First a -> [a] Source # null :: First a -> Bool Source # length :: First a -> Int Source # elem :: Eq a => a -> First a -> Bool Source # maximum :: Ord a => First a -> a Source # minimum :: Ord a => First a -> a Source # | |
Foldable Last Source # | Since: base-4.9.0.0 |
Defined in Data.Semigroup fold :: Monoid m => Last m -> m Source # foldMap :: Monoid m => (a -> m) -> Last a -> m Source # foldMap' :: Monoid m => (a -> m) -> Last a -> m Source # foldr :: (a -> b -> b) -> b -> Last a -> b Source # foldr' :: (a -> b -> b) -> b -> Last a -> b Source # foldl :: (b -> a -> b) -> b -> Last a -> b Source # foldl' :: (b -> a -> b) -> b -> Last a -> b Source # foldr1 :: (a -> a -> a) -> Last a -> a Source # foldl1 :: (a -> a -> a) -> Last a -> a Source # toList :: Last a -> [a] Source # null :: Last a -> Bool Source # length :: Last a -> Int Source # elem :: Eq a => a -> Last a -> Bool Source # maximum :: Ord a => Last a -> a Source # minimum :: Ord a => Last a -> a Source # | |
Foldable Max Source # | Since: base-4.9.0.0 |
Defined in Data.Semigroup fold :: Monoid m => Max m -> m Source # foldMap :: Monoid m => (a -> m) -> Max a -> m Source # foldMap' :: Monoid m => (a -> m) -> Max a -> m Source # foldr :: (a -> b -> b) -> b -> Max a -> b Source # foldr' :: (a -> b -> b) -> b -> Max a -> b Source # foldl :: (b -> a -> b) -> b -> Max a -> b Source # foldl' :: (b -> a -> b) -> b -> Max a -> b Source # foldr1 :: (a -> a -> a) -> Max a -> a Source # foldl1 :: (a -> a -> a) -> Max a -> a Source # toList :: Max a -> [a] Source # null :: Max a -> Bool Source # length :: Max a -> Int Source # elem :: Eq a => a -> Max a -> Bool Source # maximum :: Ord a => Max a -> a Source # minimum :: Ord a => Max a -> a Source # | |
Foldable Min Source # | Since: base-4.9.0.0 |
Defined in Data.Semigroup fold :: Monoid m => Min m -> m Source # foldMap :: Monoid m => (a -> m) -> Min a -> m Source # foldMap' :: Monoid m => (a -> m) -> Min a -> m Source # foldr :: (a -> b -> b) -> b -> Min a -> b Source # foldr' :: (a -> b -> b) -> b -> Min a -> b Source # foldl :: (b -> a -> b) -> b -> Min a -> b Source # foldl' :: (b -> a -> b) -> b -> Min a -> b Source # foldr1 :: (a -> a -> a) -> Min a -> a Source # foldl1 :: (a -> a -> a) -> Min a -> a Source # toList :: Min a -> [a] Source # null :: Min a -> Bool Source # length :: Min a -> Int Source # elem :: Eq a => a -> Min a -> Bool Source # maximum :: Ord a => Min a -> a Source # minimum :: Ord a => Min a -> a Source # | |
Foldable NonEmpty | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => NonEmpty m -> m Source # foldMap :: Monoid m => (a -> m) -> NonEmpty a -> m Source # foldMap' :: Monoid m => (a -> m) -> NonEmpty a -> m Source # foldr :: (a -> b -> b) -> b -> NonEmpty a -> b Source # foldr' :: (a -> b -> b) -> b -> NonEmpty a -> b Source # foldl :: (b -> a -> b) -> b -> NonEmpty a -> b Source # foldl' :: (b -> a -> b) -> b -> NonEmpty a -> b Source # foldr1 :: (a -> a -> a) -> NonEmpty a -> a Source # foldl1 :: (a -> a -> a) -> NonEmpty a -> a Source # toList :: NonEmpty a -> [a] Source # null :: NonEmpty a -> Bool Source # length :: NonEmpty a -> Int Source # elem :: Eq a => a -> NonEmpty a -> Bool Source # maximum :: Ord a => NonEmpty a -> a Source # minimum :: Ord a => NonEmpty a -> a Source # | |
Foldable Identity | @since base-4.8.0.0 |
Defined in GHC.Internal.Data.Functor.Identity fold :: Monoid m => Identity m -> m Source # foldMap :: Monoid m => (a -> m) -> Identity a -> m Source # foldMap' :: Monoid m => (a -> m) -> Identity a -> m Source # foldr :: (a -> b -> b) -> b -> Identity a -> b Source # foldr' :: (a -> b -> b) -> b -> Identity a -> b Source # foldl :: (b -> a -> b) -> b -> Identity a -> b Source # foldl' :: (b -> a -> b) -> b -> Identity a -> b Source # foldr1 :: (a -> a -> a) -> Identity a -> a Source # foldl1 :: (a -> a -> a) -> Identity a -> a Source # toList :: Identity a -> [a] Source # null :: Identity a -> Bool Source # length :: Identity a -> Int Source # elem :: Eq a => a -> Identity a -> Bool Source # maximum :: Ord a => Identity a -> a Source # minimum :: Ord a => Identity a -> a Source # | |
Foldable First | @since base-4.8.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => First m -> m Source # foldMap :: Monoid m => (a -> m) -> First a -> m Source # foldMap' :: Monoid m => (a -> m) -> First a -> m Source # foldr :: (a -> b -> b) -> b -> First a -> b Source # foldr' :: (a -> b -> b) -> b -> First a -> b Source # foldl :: (b -> a -> b) -> b -> First a -> b Source # foldl' :: (b -> a -> b) -> b -> First a -> b Source # foldr1 :: (a -> a -> a) -> First a -> a Source # foldl1 :: (a -> a -> a) -> First a -> a Source # toList :: First a -> [a] Source # null :: First a -> Bool Source # length :: First a -> Int Source # elem :: Eq a => a -> First a -> Bool Source # maximum :: Ord a => First a -> a Source # minimum :: Ord a => First a -> a Source # | |
Foldable Last | @since base-4.8.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Last m -> m Source # foldMap :: Monoid m => (a -> m) -> Last a -> m Source # foldMap' :: Monoid m => (a -> m) -> Last a -> m Source # foldr :: (a -> b -> b) -> b -> Last a -> b Source # foldr' :: (a -> b -> b) -> b -> Last a -> b Source # foldl :: (b -> a -> b) -> b -> Last a -> b Source # foldl' :: (b -> a -> b) -> b -> Last a -> b Source # foldr1 :: (a -> a -> a) -> Last a -> a Source # foldl1 :: (a -> a -> a) -> Last a -> a Source # toList :: Last a -> [a] Source # null :: Last a -> Bool Source # length :: Last a -> Int Source # elem :: Eq a => a -> Last a -> Bool Source # maximum :: Ord a => Last a -> a Source # minimum :: Ord a => Last a -> a Source # | |
Foldable Down | @since base-4.12.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Down m -> m Source # foldMap :: Monoid m => (a -> m) -> Down a -> m Source # foldMap' :: Monoid m => (a -> m) -> Down a -> m Source # foldr :: (a -> b -> b) -> b -> Down a -> b Source # foldr' :: (a -> b -> b) -> b -> Down a -> b Source # foldl :: (b -> a -> b) -> b -> Down a -> b Source # foldl' :: (b -> a -> b) -> b -> Down a -> b Source # foldr1 :: (a -> a -> a) -> Down a -> a Source # foldl1 :: (a -> a -> a) -> Down a -> a Source # toList :: Down a -> [a] Source # null :: Down a -> Bool Source # length :: Down a -> Int Source # elem :: Eq a => a -> Down a -> Bool Source # maximum :: Ord a => Down a -> a Source # minimum :: Ord a => Down a -> a Source # | |
Foldable Dual | @since base-4.8.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Dual m -> m Source # foldMap :: Monoid m => (a -> m) -> Dual a -> m Source # foldMap' :: Monoid m => (a -> m) -> Dual a -> m Source # foldr :: (a -> b -> b) -> b -> Dual a -> b Source # foldr' :: (a -> b -> b) -> b -> Dual a -> b Source # foldl :: (b -> a -> b) -> b -> Dual a -> b Source # foldl' :: (b -> a -> b) -> b -> Dual a -> b Source # foldr1 :: (a -> a -> a) -> Dual a -> a Source # foldl1 :: (a -> a -> a) -> Dual a -> a Source # toList :: Dual a -> [a] Source # null :: Dual a -> Bool Source # length :: Dual a -> Int Source # elem :: Eq a => a -> Dual a -> Bool Source # maximum :: Ord a => Dual a -> a Source # minimum :: Ord a => Dual a -> a Source # | |
Foldable Product | @since base-4.8.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Product m -> m Source # foldMap :: Monoid m => (a -> m) -> Product a -> m Source # foldMap' :: Monoid m => (a -> m) -> Product a -> m Source # foldr :: (a -> b -> b) -> b -> Product a -> b Source # foldr' :: (a -> b -> b) -> b -> Product a -> b Source # foldl :: (b -> a -> b) -> b -> Product a -> b Source # foldl' :: (b -> a -> b) -> b -> Product a -> b Source # foldr1 :: (a -> a -> a) -> Product a -> a Source # foldl1 :: (a -> a -> a) -> Product a -> a Source # toList :: Product a -> [a] Source # null :: Product a -> Bool Source # length :: Product a -> Int Source # elem :: Eq a => a -> Product a -> Bool Source # maximum :: Ord a => Product a -> a Source # minimum :: Ord a => Product a -> a Source # | |
Foldable Sum | @since base-4.8.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Sum m -> m Source # foldMap :: Monoid m => (a -> m) -> Sum a -> m Source # foldMap' :: Monoid m => (a -> m) -> Sum a -> m Source # foldr :: (a -> b -> b) -> b -> Sum a -> b Source # foldr' :: (a -> b -> b) -> b -> Sum a -> b Source # foldl :: (b -> a -> b) -> b -> Sum a -> b Source # foldl' :: (b -> a -> b) -> b -> Sum a -> b Source # foldr1 :: (a -> a -> a) -> Sum a -> a Source # foldl1 :: (a -> a -> a) -> Sum a -> a Source # toList :: Sum a -> [a] Source # null :: Sum a -> Bool Source # length :: Sum a -> Int Source # elem :: Eq a => a -> Sum a -> Bool Source # maximum :: Ord a => Sum a -> a Source # minimum :: Ord a => Sum a -> a Source # | |
Foldable ZipList | @since base-4.9.0.0 |
Defined in GHC.Internal.Functor.ZipList fold :: Monoid m => ZipList m -> m Source # foldMap :: Monoid m => (a -> m) -> ZipList a -> m Source # foldMap' :: Monoid m => (a -> m) -> ZipList a -> m Source # foldr :: (a -> b -> b) -> b -> ZipList a -> b Source # foldr' :: (a -> b -> b) -> b -> ZipList a -> b Source # foldl :: (b -> a -> b) -> b -> ZipList a -> b Source # foldl' :: (b -> a -> b) -> b -> ZipList a -> b Source # foldr1 :: (a -> a -> a) -> ZipList a -> a Source # foldl1 :: (a -> a -> a) -> ZipList a -> a Source # toList :: ZipList a -> [a] Source # null :: ZipList a -> Bool Source # length :: ZipList a -> Int Source # elem :: Eq a => a -> ZipList a -> Bool Source # maximum :: Ord a => ZipList a -> a Source # minimum :: Ord a => ZipList a -> a Source # | |
Foldable Par1 | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Par1 m -> m Source # foldMap :: Monoid m => (a -> m) -> Par1 a -> m Source # foldMap' :: Monoid m => (a -> m) -> Par1 a -> m Source # foldr :: (a -> b -> b) -> b -> Par1 a -> b Source # foldr' :: (a -> b -> b) -> b -> Par1 a -> b Source # foldl :: (b -> a -> b) -> b -> Par1 a -> b Source # foldl' :: (b -> a -> b) -> b -> Par1 a -> b Source # foldr1 :: (a -> a -> a) -> Par1 a -> a Source # foldl1 :: (a -> a -> a) -> Par1 a -> a Source # toList :: Par1 a -> [a] Source # null :: Par1 a -> Bool Source # length :: Par1 a -> Int Source # elem :: Eq a => a -> Par1 a -> Bool Source # maximum :: Ord a => Par1 a -> a Source # minimum :: Ord a => Par1 a -> a Source # | |
Foldable Maybe | @since base-2.01 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Maybe m -> m Source # foldMap :: Monoid m => (a -> m) -> Maybe a -> m Source # foldMap' :: Monoid m => (a -> m) -> Maybe a -> m Source # foldr :: (a -> b -> b) -> b -> Maybe a -> b Source # foldr' :: (a -> b -> b) -> b -> Maybe a -> b Source # foldl :: (b -> a -> b) -> b -> Maybe a -> b Source # foldl' :: (b -> a -> b) -> b -> Maybe a -> b Source # foldr1 :: (a -> a -> a) -> Maybe a -> a Source # foldl1 :: (a -> a -> a) -> Maybe a -> a Source # toList :: Maybe a -> [a] Source # null :: Maybe a -> Bool Source # length :: Maybe a -> Int Source # elem :: Eq a => a -> Maybe a -> Bool Source # maximum :: Ord a => Maybe a -> a Source # minimum :: Ord a => Maybe a -> a Source # | |
Foldable Solo | @since base-4.15 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Solo m -> m Source # foldMap :: Monoid m => (a -> m) -> Solo a -> m Source # foldMap' :: Monoid m => (a -> m) -> Solo a -> m Source # foldr :: (a -> b -> b) -> b -> Solo a -> b Source # foldr' :: (a -> b -> b) -> b -> Solo a -> b Source # foldl :: (b -> a -> b) -> b -> Solo a -> b Source # foldl' :: (b -> a -> b) -> b -> Solo a -> b Source # foldr1 :: (a -> a -> a) -> Solo a -> a Source # foldl1 :: (a -> a -> a) -> Solo a -> a Source # toList :: Solo a -> [a] Source # null :: Solo a -> Bool Source # length :: Solo a -> Int Source # elem :: Eq a => a -> Solo a -> Bool Source # maximum :: Ord a => Solo a -> a Source # minimum :: Ord a => Solo a -> a Source # | |
Foldable [] | @since base-2.01 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => [m] -> m Source # foldMap :: Monoid m => (a -> m) -> [a] -> m Source # foldMap' :: Monoid m => (a -> m) -> [a] -> m Source # foldr :: (a -> b -> b) -> b -> [a] -> b Source # foldr' :: (a -> b -> b) -> b -> [a] -> b Source # foldl :: (b -> a -> b) -> b -> [a] -> b Source # foldl' :: (b -> a -> b) -> b -> [a] -> b Source # foldr1 :: (a -> a -> a) -> [a] -> a Source # foldl1 :: (a -> a -> a) -> [a] -> a Source # elem :: Eq a => a -> [a] -> Bool Source # maximum :: Ord a => [a] -> a Source # minimum :: Ord a => [a] -> a Source # | |
Foldable (Arg a) Source # | Since: base-4.9.0.0 |
Defined in Data.Semigroup fold :: Monoid m => Arg a m -> m Source # foldMap :: Monoid m => (a0 -> m) -> Arg a a0 -> m Source # foldMap' :: Monoid m => (a0 -> m) -> Arg a a0 -> m Source # foldr :: (a0 -> b -> b) -> b -> Arg a a0 -> b Source # foldr' :: (a0 -> b -> b) -> b -> Arg a a0 -> b Source # foldl :: (b -> a0 -> b) -> b -> Arg a a0 -> b Source # foldl' :: (b -> a0 -> b) -> b -> Arg a a0 -> b Source # foldr1 :: (a0 -> a0 -> a0) -> Arg a a0 -> a0 Source # foldl1 :: (a0 -> a0 -> a0) -> Arg a a0 -> a0 Source # toList :: Arg a a0 -> [a0] Source # null :: Arg a a0 -> Bool Source # length :: Arg a a0 -> Int Source # elem :: Eq a0 => a0 -> Arg a a0 -> Bool Source # maximum :: Ord a0 => Arg a a0 -> a0 Source # minimum :: Ord a0 => Arg a a0 -> a0 Source # | |
Foldable (Array i) | @since base-4.8.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Array i m -> m Source # foldMap :: Monoid m => (a -> m) -> Array i a -> m Source # foldMap' :: Monoid m => (a -> m) -> Array i a -> m Source # foldr :: (a -> b -> b) -> b -> Array i a -> b Source # foldr' :: (a -> b -> b) -> b -> Array i a -> b Source # foldl :: (b -> a -> b) -> b -> Array i a -> b Source # foldl' :: (b -> a -> b) -> b -> Array i a -> b Source # foldr1 :: (a -> a -> a) -> Array i a -> a Source # foldl1 :: (a -> a -> a) -> Array i a -> a Source # toList :: Array i a -> [a] Source # null :: Array i a -> Bool Source # length :: Array i a -> Int Source # elem :: Eq a => a -> Array i a -> Bool Source # maximum :: Ord a => Array i a -> a Source # minimum :: Ord a => Array i a -> a Source # | |
Foldable (Either a) | @since base-4.7.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Either a m -> m Source # foldMap :: Monoid m => (a0 -> m) -> Either a a0 -> m Source # foldMap' :: Monoid m => (a0 -> m) -> Either a a0 -> m Source # foldr :: (a0 -> b -> b) -> b -> Either a a0 -> b Source # foldr' :: (a0 -> b -> b) -> b -> Either a a0 -> b Source # foldl :: (b -> a0 -> b) -> b -> Either a a0 -> b Source # foldl' :: (b -> a0 -> b) -> b -> Either a a0 -> b Source # foldr1 :: (a0 -> a0 -> a0) -> Either a a0 -> a0 Source # foldl1 :: (a0 -> a0 -> a0) -> Either a a0 -> a0 Source # toList :: Either a a0 -> [a0] Source # null :: Either a a0 -> Bool Source # length :: Either a a0 -> Int Source # elem :: Eq a0 => a0 -> Either a a0 -> Bool Source # maximum :: Ord a0 => Either a a0 -> a0 Source # minimum :: Ord a0 => Either a a0 -> a0 Source # | |
Foldable (Proxy :: Type -> Type) | @since base-4.7.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Proxy m -> m Source # foldMap :: Monoid m => (a -> m) -> Proxy a -> m Source # foldMap' :: Monoid m => (a -> m) -> Proxy a -> m Source # foldr :: (a -> b -> b) -> b -> Proxy a -> b Source # foldr' :: (a -> b -> b) -> b -> Proxy a -> b Source # foldl :: (b -> a -> b) -> b -> Proxy a -> b Source # foldl' :: (b -> a -> b) -> b -> Proxy a -> b Source # foldr1 :: (a -> a -> a) -> Proxy a -> a Source # foldl1 :: (a -> a -> a) -> Proxy a -> a Source # toList :: Proxy a -> [a] Source # null :: Proxy a -> Bool Source # length :: Proxy a -> Int Source # elem :: Eq a => a -> Proxy a -> Bool Source # maximum :: Ord a => Proxy a -> a Source # minimum :: Ord a => Proxy a -> a Source # | |
Foldable (U1 :: Type -> Type) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => U1 m -> m Source # foldMap :: Monoid m => (a -> m) -> U1 a -> m Source # foldMap' :: Monoid m => (a -> m) -> U1 a -> m Source # foldr :: (a -> b -> b) -> b -> U1 a -> b Source # foldr' :: (a -> b -> b) -> b -> U1 a -> b Source # foldl :: (b -> a -> b) -> b -> U1 a -> b Source # foldl' :: (b -> a -> b) -> b -> U1 a -> b Source # foldr1 :: (a -> a -> a) -> U1 a -> a Source # foldl1 :: (a -> a -> a) -> U1 a -> a Source # toList :: U1 a -> [a] Source # length :: U1 a -> Int Source # elem :: Eq a => a -> U1 a -> Bool Source # maximum :: Ord a => U1 a -> a Source # minimum :: Ord a => U1 a -> a Source # | |
Foldable (UAddr :: Type -> Type) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => UAddr m -> m Source # foldMap :: Monoid m => (a -> m) -> UAddr a -> m Source # foldMap' :: Monoid m => (a -> m) -> UAddr a -> m Source # foldr :: (a -> b -> b) -> b -> UAddr a -> b Source # foldr' :: (a -> b -> b) -> b -> UAddr a -> b Source # foldl :: (b -> a -> b) -> b -> UAddr a -> b Source # foldl' :: (b -> a -> b) -> b -> UAddr a -> b Source # foldr1 :: (a -> a -> a) -> UAddr a -> a Source # foldl1 :: (a -> a -> a) -> UAddr a -> a Source # toList :: UAddr a -> [a] Source # null :: UAddr a -> Bool Source # length :: UAddr a -> Int Source # elem :: Eq a => a -> UAddr a -> Bool Source # maximum :: Ord a => UAddr a -> a Source # minimum :: Ord a => UAddr a -> a Source # | |
Foldable (UChar :: Type -> Type) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => UChar m -> m Source # foldMap :: Monoid m => (a -> m) -> UChar a -> m Source # foldMap' :: Monoid m => (a -> m) -> UChar a -> m Source # foldr :: (a -> b -> b) -> b -> UChar a -> b Source # foldr' :: (a -> b -> b) -> b -> UChar a -> b Source # foldl :: (b -> a -> b) -> b -> UChar a -> b Source # foldl' :: (b -> a -> b) -> b -> UChar a -> b Source # foldr1 :: (a -> a -> a) -> UChar a -> a Source # foldl1 :: (a -> a -> a) -> UChar a -> a Source # toList :: UChar a -> [a] Source # null :: UChar a -> Bool Source # length :: UChar a -> Int Source # elem :: Eq a => a -> UChar a -> Bool Source # maximum :: Ord a => UChar a -> a Source # minimum :: Ord a => UChar a -> a Source # | |
Foldable (UDouble :: Type -> Type) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => UDouble m -> m Source # foldMap :: Monoid m => (a -> m) -> UDouble a -> m Source # foldMap' :: Monoid m => (a -> m) -> UDouble a -> m Source # foldr :: (a -> b -> b) -> b -> UDouble a -> b Source # foldr' :: (a -> b -> b) -> b -> UDouble a -> b Source # foldl :: (b -> a -> b) -> b -> UDouble a -> b Source # foldl' :: (b -> a -> b) -> b -> UDouble a -> b Source # foldr1 :: (a -> a -> a) -> UDouble a -> a Source # foldl1 :: (a -> a -> a) -> UDouble a -> a Source # toList :: UDouble a -> [a] Source # null :: UDouble a -> Bool Source # length :: UDouble a -> Int Source # elem :: Eq a => a -> UDouble a -> Bool Source # maximum :: Ord a => UDouble a -> a Source # minimum :: Ord a => UDouble a -> a Source # | |
Foldable (UFloat :: Type -> Type) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => UFloat m -> m Source # foldMap :: Monoid m => (a -> m) -> UFloat a -> m Source # foldMap' :: Monoid m => (a -> m) -> UFloat a -> m Source # foldr :: (a -> b -> b) -> b -> UFloat a -> b Source # foldr' :: (a -> b -> b) -> b -> UFloat a -> b Source # foldl :: (b -> a -> b) -> b -> UFloat a -> b Source # foldl' :: (b -> a -> b) -> b -> UFloat a -> b Source # foldr1 :: (a -> a -> a) -> UFloat a -> a Source # foldl1 :: (a -> a -> a) -> UFloat a -> a Source # toList :: UFloat a -> [a] Source # null :: UFloat a -> Bool Source # length :: UFloat a -> Int Source # elem :: Eq a => a -> UFloat a -> Bool Source # maximum :: Ord a => UFloat a -> a Source # minimum :: Ord a => UFloat a -> a Source # | |
Foldable (UInt :: Type -> Type) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => UInt m -> m Source # foldMap :: Monoid m => (a -> m) -> UInt a -> m Source # foldMap' :: Monoid m => (a -> m) -> UInt a -> m Source # foldr :: (a -> b -> b) -> b -> UInt a -> b Source # foldr' :: (a -> b -> b) -> b -> UInt a -> b Source # foldl :: (b -> a -> b) -> b -> UInt a -> b Source # foldl' :: (b -> a -> b) -> b -> UInt a -> b Source # foldr1 :: (a -> a -> a) -> UInt a -> a Source # foldl1 :: (a -> a -> a) -> UInt a -> a Source # toList :: UInt a -> [a] Source # null :: UInt a -> Bool Source # length :: UInt a -> Int Source # elem :: Eq a => a -> UInt a -> Bool Source # maximum :: Ord a => UInt a -> a Source # minimum :: Ord a => UInt a -> a Source # | |
Foldable (UWord :: Type -> Type) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => UWord m -> m Source # foldMap :: Monoid m => (a -> m) -> UWord a -> m Source # foldMap' :: Monoid m => (a -> m) -> UWord a -> m Source # foldr :: (a -> b -> b) -> b -> UWord a -> b Source # foldr' :: (a -> b -> b) -> b -> UWord a -> b Source # foldl :: (b -> a -> b) -> b -> UWord a -> b Source # foldl' :: (b -> a -> b) -> b -> UWord a -> b Source # foldr1 :: (a -> a -> a) -> UWord a -> a Source # foldl1 :: (a -> a -> a) -> UWord a -> a Source # toList :: UWord a -> [a] Source # null :: UWord a -> Bool Source # length :: UWord a -> Int Source # elem :: Eq a => a -> UWord a -> Bool Source # maximum :: Ord a => UWord a -> a Source # minimum :: Ord a => UWord a -> a Source # | |
Foldable (V1 :: Type -> Type) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => V1 m -> m Source # foldMap :: Monoid m => (a -> m) -> V1 a -> m Source # foldMap' :: Monoid m => (a -> m) -> V1 a -> m Source # foldr :: (a -> b -> b) -> b -> V1 a -> b Source # foldr' :: (a -> b -> b) -> b -> V1 a -> b Source # foldl :: (b -> a -> b) -> b -> V1 a -> b Source # foldl' :: (b -> a -> b) -> b -> V1 a -> b Source # foldr1 :: (a -> a -> a) -> V1 a -> a Source # foldl1 :: (a -> a -> a) -> V1 a -> a Source # toList :: V1 a -> [a] Source # length :: V1 a -> Int Source # elem :: Eq a => a -> V1 a -> Bool Source # maximum :: Ord a => V1 a -> a Source # minimum :: Ord a => V1 a -> a Source # | |
Foldable ((,) a) | @since base-4.7.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => (a, m) -> m Source # foldMap :: Monoid m => (a0 -> m) -> (a, a0) -> m Source # foldMap' :: Monoid m => (a0 -> m) -> (a, a0) -> m Source # foldr :: (a0 -> b -> b) -> b -> (a, a0) -> b Source # foldr' :: (a0 -> b -> b) -> b -> (a, a0) -> b Source # foldl :: (b -> a0 -> b) -> b -> (a, a0) -> b Source # foldl' :: (b -> a0 -> b) -> b -> (a, a0) -> b Source # foldr1 :: (a0 -> a0 -> a0) -> (a, a0) -> a0 Source # foldl1 :: (a0 -> a0 -> a0) -> (a, a0) -> a0 Source # toList :: (a, a0) -> [a0] Source # null :: (a, a0) -> Bool Source # length :: (a, a0) -> Int Source # elem :: Eq a0 => a0 -> (a, a0) -> Bool Source # maximum :: Ord a0 => (a, a0) -> a0 Source # minimum :: Ord a0 => (a, a0) -> a0 Source # | |
Foldable (Const m :: Type -> Type) | @since base-4.7.0.0 |
Defined in GHC.Internal.Data.Functor.Const fold :: Monoid m0 => Const m m0 -> m0 Source # foldMap :: Monoid m0 => (a -> m0) -> Const m a -> m0 Source # foldMap' :: Monoid m0 => (a -> m0) -> Const m a -> m0 Source # foldr :: (a -> b -> b) -> b -> Const m a -> b Source # foldr' :: (a -> b -> b) -> b -> Const m a -> b Source # foldl :: (b -> a -> b) -> b -> Const m a -> b Source # foldl' :: (b -> a -> b) -> b -> Const m a -> b Source # foldr1 :: (a -> a -> a) -> Const m a -> a Source # foldl1 :: (a -> a -> a) -> Const m a -> a Source # toList :: Const m a -> [a] Source # null :: Const m a -> Bool Source # length :: Const m a -> Int Source # elem :: Eq a => a -> Const m a -> Bool Source # maximum :: Ord a => Const m a -> a Source # minimum :: Ord a => Const m a -> a Source # | |
Foldable f => Foldable (Ap f) | @since base-4.12.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Ap f m -> m Source # foldMap :: Monoid m => (a -> m) -> Ap f a -> m Source # foldMap' :: Monoid m => (a -> m) -> Ap f a -> m Source # foldr :: (a -> b -> b) -> b -> Ap f a -> b Source # foldr' :: (a -> b -> b) -> b -> Ap f a -> b Source # foldl :: (b -> a -> b) -> b -> Ap f a -> b Source # foldl' :: (b -> a -> b) -> b -> Ap f a -> b Source # foldr1 :: (a -> a -> a) -> Ap f a -> a Source # foldl1 :: (a -> a -> a) -> Ap f a -> a Source # toList :: Ap f a -> [a] Source # null :: Ap f a -> Bool Source # length :: Ap f a -> Int Source # elem :: Eq a => a -> Ap f a -> Bool Source # maximum :: Ord a => Ap f a -> a Source # minimum :: Ord a => Ap f a -> a Source # | |
Foldable f => Foldable (Alt f) | @since base-4.12.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Alt f m -> m Source # foldMap :: Monoid m => (a -> m) -> Alt f a -> m Source # foldMap' :: Monoid m => (a -> m) -> Alt f a -> m Source # foldr :: (a -> b -> b) -> b -> Alt f a -> b Source # foldr' :: (a -> b -> b) -> b -> Alt f a -> b Source # foldl :: (b -> a -> b) -> b -> Alt f a -> b Source # foldl' :: (b -> a -> b) -> b -> Alt f a -> b Source # foldr1 :: (a -> a -> a) -> Alt f a -> a Source # foldl1 :: (a -> a -> a) -> Alt f a -> a Source # toList :: Alt f a -> [a] Source # null :: Alt f a -> Bool Source # length :: Alt f a -> Int Source # elem :: Eq a => a -> Alt f a -> Bool Source # maximum :: Ord a => Alt f a -> a Source # minimum :: Ord a => Alt f a -> a Source # | |
Foldable f => Foldable (Rec1 f) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => Rec1 f m -> m Source # foldMap :: Monoid m => (a -> m) -> Rec1 f a -> m Source # foldMap' :: Monoid m => (a -> m) -> Rec1 f a -> m Source # foldr :: (a -> b -> b) -> b -> Rec1 f a -> b Source # foldr' :: (a -> b -> b) -> b -> Rec1 f a -> b Source # foldl :: (b -> a -> b) -> b -> Rec1 f a -> b Source # foldl' :: (b -> a -> b) -> b -> Rec1 f a -> b Source # foldr1 :: (a -> a -> a) -> Rec1 f a -> a Source # foldl1 :: (a -> a -> a) -> Rec1 f a -> a Source # toList :: Rec1 f a -> [a] Source # null :: Rec1 f a -> Bool Source # length :: Rec1 f a -> Int Source # elem :: Eq a => a -> Rec1 f a -> Bool Source # maximum :: Ord a => Rec1 f a -> a Source # minimum :: Ord a => Rec1 f a -> a Source # | |
(Foldable f, Foldable g) => Foldable (Product f g) Source # | Since: base-4.9.0.0 |
Defined in Data.Functor.Product fold :: Monoid m => Product f g m -> m Source # foldMap :: Monoid m => (a -> m) -> Product f g a -> m Source # foldMap' :: Monoid m => (a -> m) -> Product f g a -> m Source # foldr :: (a -> b -> b) -> b -> Product f g a -> b Source # foldr' :: (a -> b -> b) -> b -> Product f g a -> b Source # foldl :: (b -> a -> b) -> b -> Product f g a -> b Source # foldl' :: (b -> a -> b) -> b -> Product f g a -> b Source # foldr1 :: (a -> a -> a) -> Product f g a -> a Source # foldl1 :: (a -> a -> a) -> Product f g a -> a Source # toList :: Product f g a -> [a] Source # null :: Product f g a -> Bool Source # length :: Product f g a -> Int Source # elem :: Eq a => a -> Product f g a -> Bool Source # maximum :: Ord a => Product f g a -> a Source # minimum :: Ord a => Product f g a -> a Source # | |
(Foldable f, Foldable g) => Foldable (Sum f g) Source # | Since: base-4.9.0.0 |
Defined in Data.Functor.Sum fold :: Monoid m => Sum f g m -> m Source # foldMap :: Monoid m => (a -> m) -> Sum f g a -> m Source # foldMap' :: Monoid m => (a -> m) -> Sum f g a -> m Source # foldr :: (a -> b -> b) -> b -> Sum f g a -> b Source # foldr' :: (a -> b -> b) -> b -> Sum f g a -> b Source # foldl :: (b -> a -> b) -> b -> Sum f g a -> b Source # foldl' :: (b -> a -> b) -> b -> Sum f g a -> b Source # foldr1 :: (a -> a -> a) -> Sum f g a -> a Source # foldl1 :: (a -> a -> a) -> Sum f g a -> a Source # toList :: Sum f g a -> [a] Source # null :: Sum f g a -> Bool Source # length :: Sum f g a -> Int Source # elem :: Eq a => a -> Sum f g a -> Bool Source # maximum :: Ord a => Sum f g a -> a Source # minimum :: Ord a => Sum f g a -> a Source # | |
(Foldable f, Foldable g) => Foldable (f :*: g) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => (f :*: g) m -> m Source # foldMap :: Monoid m => (a -> m) -> (f :*: g) a -> m Source # foldMap' :: Monoid m => (a -> m) -> (f :*: g) a -> m Source # foldr :: (a -> b -> b) -> b -> (f :*: g) a -> b Source # foldr' :: (a -> b -> b) -> b -> (f :*: g) a -> b Source # foldl :: (b -> a -> b) -> b -> (f :*: g) a -> b Source # foldl' :: (b -> a -> b) -> b -> (f :*: g) a -> b Source # foldr1 :: (a -> a -> a) -> (f :*: g) a -> a Source # foldl1 :: (a -> a -> a) -> (f :*: g) a -> a Source # toList :: (f :*: g) a -> [a] Source # null :: (f :*: g) a -> Bool Source # length :: (f :*: g) a -> Int Source # elem :: Eq a => a -> (f :*: g) a -> Bool Source # maximum :: Ord a => (f :*: g) a -> a Source # minimum :: Ord a => (f :*: g) a -> a Source # | |
(Foldable f, Foldable g) => Foldable (f :+: g) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => (f :+: g) m -> m Source # foldMap :: Monoid m => (a -> m) -> (f :+: g) a -> m Source # foldMap' :: Monoid m => (a -> m) -> (f :+: g) a -> m Source # foldr :: (a -> b -> b) -> b -> (f :+: g) a -> b Source # foldr' :: (a -> b -> b) -> b -> (f :+: g) a -> b Source # foldl :: (b -> a -> b) -> b -> (f :+: g) a -> b Source # foldl' :: (b -> a -> b) -> b -> (f :+: g) a -> b Source # foldr1 :: (a -> a -> a) -> (f :+: g) a -> a Source # foldl1 :: (a -> a -> a) -> (f :+: g) a -> a Source # toList :: (f :+: g) a -> [a] Source # null :: (f :+: g) a -> Bool Source # length :: (f :+: g) a -> Int Source # elem :: Eq a => a -> (f :+: g) a -> Bool Source # maximum :: Ord a => (f :+: g) a -> a Source # minimum :: Ord a => (f :+: g) a -> a Source # | |
Foldable (K1 i c :: Type -> Type) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => K1 i c m -> m Source # foldMap :: Monoid m => (a -> m) -> K1 i c a -> m Source # foldMap' :: Monoid m => (a -> m) -> K1 i c a -> m Source # foldr :: (a -> b -> b) -> b -> K1 i c a -> b Source # foldr' :: (a -> b -> b) -> b -> K1 i c a -> b Source # foldl :: (b -> a -> b) -> b -> K1 i c a -> b Source # foldl' :: (b -> a -> b) -> b -> K1 i c a -> b Source # foldr1 :: (a -> a -> a) -> K1 i c a -> a Source # foldl1 :: (a -> a -> a) -> K1 i c a -> a Source # toList :: K1 i c a -> [a] Source # null :: K1 i c a -> Bool Source # length :: K1 i c a -> Int Source # elem :: Eq a => a -> K1 i c a -> Bool Source # maximum :: Ord a => K1 i c a -> a Source # minimum :: Ord a => K1 i c a -> a Source # | |
(Foldable f, Foldable g) => Foldable (Compose f g) Source # | Since: base-4.9.0.0 |
Defined in Data.Functor.Compose fold :: Monoid m => Compose f g m -> m Source # foldMap :: Monoid m => (a -> m) -> Compose f g a -> m Source # foldMap' :: Monoid m => (a -> m) -> Compose f g a -> m Source # foldr :: (a -> b -> b) -> b -> Compose f g a -> b Source # foldr' :: (a -> b -> b) -> b -> Compose f g a -> b Source # foldl :: (b -> a -> b) -> b -> Compose f g a -> b Source # foldl' :: (b -> a -> b) -> b -> Compose f g a -> b Source # foldr1 :: (a -> a -> a) -> Compose f g a -> a Source # foldl1 :: (a -> a -> a) -> Compose f g a -> a Source # toList :: Compose f g a -> [a] Source # null :: Compose f g a -> Bool Source # length :: Compose f g a -> Int Source # elem :: Eq a => a -> Compose f g a -> Bool Source # maximum :: Ord a => Compose f g a -> a Source # minimum :: Ord a => Compose f g a -> a Source # | |
(Foldable f, Foldable g) => Foldable (f :.: g) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => (f :.: g) m -> m Source # foldMap :: Monoid m => (a -> m) -> (f :.: g) a -> m Source # foldMap' :: Monoid m => (a -> m) -> (f :.: g) a -> m Source # foldr :: (a -> b -> b) -> b -> (f :.: g) a -> b Source # foldr' :: (a -> b -> b) -> b -> (f :.: g) a -> b Source # foldl :: (b -> a -> b) -> b -> (f :.: g) a -> b Source # foldl' :: (b -> a -> b) -> b -> (f :.: g) a -> b Source # foldr1 :: (a -> a -> a) -> (f :.: g) a -> a Source # foldl1 :: (a -> a -> a) -> (f :.: g) a -> a Source # toList :: (f :.: g) a -> [a] Source # null :: (f :.: g) a -> Bool Source # length :: (f :.: g) a -> Int Source # elem :: Eq a => a -> (f :.: g) a -> Bool Source # maximum :: Ord a => (f :.: g) a -> a Source # minimum :: Ord a => (f :.: g) a -> a Source # | |
Foldable f => Foldable (M1 i c f) | @since base-4.9.0.0 |
Defined in GHC.Internal.Data.Foldable fold :: Monoid m => M1 i c f m -> m Source # foldMap :: Monoid m => (a -> m) -> M1 i c f a -> m Source # foldMap' :: Monoid m => (a -> m) -> M1 i c f a -> m Source # foldr :: (a -> b -> b) -> b -> M1 i c f a -> b Source # foldr' :: (a -> b -> b) -> b -> M1 i c f a -> b Source # foldl :: (b -> a -> b) -> b -> M1 i c f a -> b Source # foldl' :: (b -> a -> b) -> b -> M1 i c f a -> b Source # foldr1 :: (a -> a -> a) -> M1 i c f a -> a Source # foldl1 :: (a -> a -> a) -> M1 i c f a -> a Source # toList :: M1 i c f a -> [a] Source # null :: M1 i c f a -> Bool Source # length :: M1 i c f a -> Int Source # elem :: Eq a => a -> M1 i c f a -> Bool Source # maximum :: Ord a => M1 i c f a -> a Source # minimum :: Ord a => M1 i c f a -> a Source # |
Special biased folds
foldrM :: (Foldable t, Monad m) => (a -> b -> m b) -> b -> t a -> m b Source #
Right-to-left monadic fold over the elements of a structure.
Given a structure t
with elements (a, b, c, ..., x, y)
, the result of
a fold with an operator function f
is equivalent to:
foldrM f z t = do yy <- f y z xx <- f x yy ... bb <- f b cc aa <- f a bb return aa -- Just @return z@ when the structure is empty
For a Monad m
, given two functions f1 :: a -> m b
and f2 :: b -> m c
,
their Kleisli composition (f1 >=> f2) :: a -> m c
is defined by:
(f1 >=> f2) a = f1 a >>= f2
Another way of thinking about foldrM
is that it amounts to an application
to z
of a Kleisli composition:
foldrM f z t = f y >=> f x >=> ... >=> f b >=> f a $ z
The monadic effects of foldrM
are sequenced from right to left, and e.g.
folds of infinite lists will diverge.
If at some step the bind operator (
short-circuits (as with, e.g.,
>>=
)mzero
in a MonadPlus
), the evaluated effects will be from a tail of the
element sequence. If you want to evaluate the monadic effects in
left-to-right order, or perhaps be able to short-circuit after an initial
sequence of elements, you'll need to use foldlM
instead.
If the monadic effects don't short-circuit, the outermost application of
f
is to the leftmost element a
, so that, ignoring effects, the result
looks like a right fold:
a `f` (b `f` (c `f` (... (x `f` (y `f` z))))).
Examples
Basic usage:
>>>
let f i acc = do { print i ; return $ i : acc }
>>>
foldrM f [] [0..3]
3 2 1 0 [0,1,2,3]
foldlM :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b Source #
Left-to-right monadic fold over the elements of a structure.
Given a structure t
with elements (a, b, ..., w, x, y)
, the result of
a fold with an operator function f
is equivalent to:
foldlM f z t = do aa <- f z a bb <- f aa b ... xx <- f ww x yy <- f xx y return yy -- Just @return z@ when the structure is empty
For a Monad m
, given two functions f1 :: a -> m b
and f2 :: b -> m c
,
their Kleisli composition (f1 >=> f2) :: a -> m c
is defined by:
(f1 >=> f2) a = f1 a >>= f2
Another way of thinking about foldlM
is that it amounts to an application
to z
of a Kleisli composition:
foldlM f z t = flip f a >=> flip f b >=> ... >=> flip f x >=> flip f y $ z
The monadic effects of foldlM
are sequenced from left to right.
If at some step the bind operator (
short-circuits (as with, e.g.,
>>=
)mzero
in a MonadPlus
), the evaluated effects will be from an initial
segment of the element sequence. If you want to evaluate the monadic
effects in right-to-left order, or perhaps be able to short-circuit after
processing a tail of the sequence of elements, you'll need to use foldrM
instead.
If the monadic effects don't short-circuit, the outermost application of
f
is to the rightmost element y
, so that, ignoring effects, the result
looks like a left fold:
((((z `f` a) `f` b) ... `f` w) `f` x) `f` y
Examples
Basic usage:
>>>
let f a e = do { print e ; return $ e : a }
>>>
foldlM f [] [0..3]
0 1 2 3 [3,2,1,0]
Folding actions
Applicative actions
traverse_ :: (Foldable t, Applicative f) => (a -> f b) -> t a -> f () Source #
Map each element of a structure to an Applicative
action, evaluate these
actions from left to right, and ignore the results. For a version that
doesn't ignore the results see traverse
.
traverse_
is just like mapM_
, but generalised to Applicative
actions.
Examples
Basic usage:
>>>
traverse_ print ["Hello", "world", "!"]
"Hello" "world" "!"
for_ :: (Foldable t, Applicative f) => t a -> (a -> f b) -> f () Source #
for_
is traverse_
with its arguments flipped. For a version
that doesn't ignore the results see for
. This
is forM_
generalised to Applicative
actions.
for_
is just like forM_
, but generalised to Applicative
actions.
Examples
Basic usage:
>>>
for_ [1..4] print
1 2 3 4
sequenceA_ :: (Foldable t, Applicative f) => t (f a) -> f () Source #
Evaluate each action in the structure from left to right, and
ignore the results. For a version that doesn't ignore the results
see sequenceA
.
sequenceA_
is just like sequence_
, but generalised to Applicative
actions.
Examples
Basic usage:
>>>
sequenceA_ [print "Hello", print "world", print "!"]
"Hello" "world" "!"
asum :: (Foldable t, Alternative f) => t (f a) -> f a Source #
The sum of a collection of actions using (<|>)
, generalizing concat
.
asum
is just like msum
, but generalised to Alternative
.
Examples
Basic usage:
>>>
asum [Just "Hello", Nothing, Just "World"]
Just "Hello"
Monadic actions
sequence_ :: (Foldable t, Monad m) => t (m a) -> m () Source #
Evaluate each monadic action in the structure from left to right,
and ignore the results. For a version that doesn't ignore the
results see sequence
.
sequence_
is just like sequenceA_
, but specialised to monadic
actions.
Specialized folds
concat :: Foldable t => t [a] -> [a] Source #
The concatenation of all the elements of a container of lists.
Examples
Basic usage:
>>>
concat (Just [1, 2, 3])
[1,2,3]
>>>
concat (Left 42)
[]
>>>
concat [[1, 2, 3], [4, 5], [6], []]
[1,2,3,4,5,6]
concatMap :: Foldable t => (a -> [b]) -> t a -> [b] Source #
Map a function over all the elements of a container and concatenate the resulting lists.
Examples
Basic usage:
>>>
concatMap (take 3) [[1..], [10..], [100..], [1000..]]
[1,2,3,10,11,12,100,101,102,1000,1001,1002]
>>>
concatMap (take 3) (Just [1..])
[1,2,3]
and :: Foldable t => t Bool -> Bool Source #
and
returns the conjunction of a container of Bools. For the
result to be True
, the container must be finite; False
, however,
results from a False
value finitely far from the left end.
Examples
Basic usage:
>>>
and []
True
>>>
and [True]
True
>>>
and [False]
False
>>>
and [True, True, False]
False
>>>
and (False : repeat True) -- Infinite list [False,True,True,True,...
False
>>>
and (repeat True)
* Hangs forever *
or :: Foldable t => t Bool -> Bool Source #
or
returns the disjunction of a container of Bools. For the
result to be False
, the container must be finite; True
, however,
results from a True
value finitely far from the left end.
Examples
Basic usage:
>>>
or []
False
>>>
or [True]
True
>>>
or [False]
False
>>>
or [True, True, False]
True
>>>
or (True : repeat False) -- Infinite list [True,False,False,False,...
True
>>>
or (repeat False)
* Hangs forever *
any :: Foldable t => (a -> Bool) -> t a -> Bool Source #
Determines whether any element of the structure satisfies the predicate.
Examples
Basic usage:
>>>
any (> 3) []
False
>>>
any (> 3) [1,2]
False
>>>
any (> 3) [1,2,3,4,5]
True
>>>
any (> 3) [1..]
True
>>>
any (> 3) [0, -1..]
* Hangs forever *
all :: Foldable t => (a -> Bool) -> t a -> Bool Source #
Determines whether all elements of the structure satisfy the predicate.
Examples
Basic usage:
>>>
all (> 3) []
True
>>>
all (> 3) [1,2]
False
>>>
all (> 3) [1,2,3,4,5]
False
>>>
all (> 3) [1..]
False
>>>
all (> 3) [4..]
* Hangs forever *
maximumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a Source #
The largest element of a non-empty structure with respect to the given comparison function.
Examples
Basic usage:
>>>
maximumBy (compare `on` length) ["Hello", "World", "!", "Longest", "bar"]
"Longest"
WARNING: This function is partial for possibly-empty structures like lists.
minimumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a Source #
The least element of a non-empty structure with respect to the given comparison function.
Examples
Basic usage:
>>>
minimumBy (compare `on` length) ["Hello", "World", "!", "Longest", "bar"]
"!"
WARNING: This function is partial for possibly-empty structures like lists.
Searches
notElem :: (Foldable t, Eq a) => a -> t a -> Bool infix 4 Source #
notElem
is the negation of elem
.
Examples
Basic usage:
>>>
3 `notElem` []
True
>>>
3 `notElem` [1,2]
True
>>>
3 `notElem` [1,2,3,4,5]
False
For infinite structures, notElem
terminates if the value exists at a
finite distance from the left side of the structure:
>>>
3 `notElem` [1..]
False
>>>
3 `notElem` ([4..] ++ [3])
* Hangs forever *
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
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 -> bfoldr
:: 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, …)
.
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 . … . 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 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 . … $ z
If each call of
f
on the current elemente
, (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 offoldr
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 and short-circuit reductions.When the operator is always strict in its second argument,
foldl'
is generally a better choice thanfoldr
. Whenfoldr
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.
Expectation of efficient left-to-right iteration
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
andtoList
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
Recursive and corecursive reduction
As observed in the above description 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 recursive folds
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
. ]
List of strict functions
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 -> bProvided
mappend
is strict in its left argument:foldMap'
:: (Foldable t, Monoid m) => (a -> m) -> t a -> mProvided the instance is correctly defined:
length
:: Foldable t => t a -> Intsum
:: (Foldable t, Num a) => t a -> aproduct
:: (Foldable t, Num a) => t a -> amaximum
:: (Foldable t, Ord a) => t a -> aminimum
:: (Foldable t, Ord a) => t a -> amaximumBy
:: Foldable t => (a -> a -> Ordering) -> t a -> aminimumBy
:: Foldable t => (a -> a -> Ordering) -> t a -> a
Lazy corecursive folds
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
, …) 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]
List of lazy functions
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 -> bfoldr1
:: Foldable t => (a -> a -> a) -> t a -> aProvided the
Monoid
mappend
is lazy in its second argument (otherwise best to use a strict recursive reduction):fold
:: Foldable t => Monoid m => t m -> mfoldMap
:: Foldable t => Monoid m => (a -> m) -> t a -> mProvided 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]
Short-circuit folds
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.
List of short-circuit functions
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 -> Boolelem
:: Foldable t => Eq a => a -> t a -> BoolnotElem
:: (Foldable t, Eq a) => a -> t a -> Booland
:: Foldable t => t Bool -> Boolor
:: Foldable t => t Bool -> Boolfind
:: Foldable t => (a -> Bool) -> t a -> Maybe aany
:: Foldable t => (a -> Bool) -> t a -> Boolall
:: Foldable t => (a -> Bool) -> t a -> BoolMany 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 thatasum
for a list of lists is in fact corecursive. These folds are defined in terms offoldr
.asum
:: (Foldable t, Alternative f) => t (f a) -> f amsum
:: (Foldable t, MonadPlus m) => t (m a) -> m aLikewise, 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 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 bThe sequencing of monadic effects proceeds from left to right. If at some step the bind operator
(
short-circuits (as with, e.g.,>>=
)mzero
with aMonadPlus
, or an exception with aMonadThrow
, 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 NothingContrast 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 folds
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 -> bfoldl
:: Foldable t => (b -> a -> b) -> b -> t a -> bfoldl1
:: Foldable t => (a -> a -> a) -> t a -> afoldrM
:: (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.
Generative Recursion
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 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]), 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
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. 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.
Avoiding multi-pass folds
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
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.
Defining 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 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 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.
Being strict by being lazy
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).
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
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] 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).
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.
Generally linear-time elem
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.
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.
See also
- [1] "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.