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foldl :: Vector v a => (b -> a -> b) -> b -> v a -> bfixed-vector Data.Vector.Fixed Left fold over vector
foldl :: forall (n :: PeanoNum) b a . ArityPeano n => (b -> a -> b) -> b -> ContVec n a -> bfixed-vector Data.Vector.Fixed.Cont Left fold over continuation vector.
foldl :: Foldable t => (b -> a -> b) -> b -> t a -> bmixed-types-num Numeric.MixedTypes.PreludeHiding 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 :: (r -> v -> r) -> r -> MonoidMap k v -> rmonoidmap Data.MonoidMap Folds over the values in the map using the given left-associative binary operator. Satisfies the following property:
foldl f r m == Map.foldl f r (toMap m)
foldl :: (a -> b -> a) -> a -> NonEmptyVector b -> anonempty-vector Data.Vector.NonEmpty O(n) Left monoidal fold
foldl :: (b -> Binding k p -> b) -> b -> PSQ k p -> bPSQueue Data.PSQueue Left fold over the bindings in the queue, in key order.
foldl :: (b -> Binding k p -> b) -> b -> PSQ k p -> bPSQueue Data.PSQueue.Internal Left fold over the bindings in the queue, in key order.
foldl :: Foldable t => (b -> a -> b) -> b -> t a -> bcabal-install-solver Distribution.Solver.Compat.Prelude 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 :: Foldable t => (b -> a -> b) -> b -> t a -> bincipit-base Incipit.Foldable Default to using the strict version since the lazy one is so controversial.
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linear-base Foreign.Heap Guaranteed to yield pairs in ascending key order