Data.Lens.Traversal

traversed :: forall t a b. Traversable t => Traversal (t a) (t b) a b

A Traversal for the elements of a Traversable functor.

over traversed negate [1, 2, 3] == [-1,-2,-3]
over traversed negate (Just 3) == Just -3

element :: forall p s t a. Wander p => Int -> Traversal s t a a -> Optic p s t a a

Combine an index and a traversal to narrow the focus to a single element. Compare to Data.Lens.Index.

set     (element 2 traversed) 8888 [0, 0, 3] == [0, 0, 8888]
preview (element 2 traversed)      [0, 0, 3] == Just 3

The resulting traversal is called an affine traversal, which means that the traversal focuses on one or zero (if the index is out of range) results.

traverseOf :: forall f s t a b. Optic (Star f) s t a b -> (a -> f b) -> s -> f t

Turn a pure profunctor Traversal into a lens-like Traversal.

sequenceOf :: forall f s t a. Optic (Star f) s t (f a) a -> s -> f t

Sequence the foci of an optic, pulling out an "effect". If you do not need the result, see sequenceOf_ for Folds.

sequenceOf traversed has the same result as Data.Traversable.sequence:

sequenceOf traversed (Just [1, 2]) == [Just 1, Just 2]
sequence             (Just [1, 2]) == [Just 1, Just 2]

An example with effects:

> array = [random, random]
> :t array
Array (Eff ... Number)

> effect = sequenceOf traversed array
> :t effect
Eff ... (Array Number)

> effect >>= logShow
[0.15556037108154985,0.28500369615270515]
unit

failover :: forall f s t a b. Alternative f => Optic (Star (Tuple (Disj Boolean))) s t a b -> (a -> b) -> s -> f t

Tries to map over a Traversal; returns empty if the traversal did not have any new focus.

elementsOf :: forall p i s t a. Wander p => IndexedTraversal i s t a a -> (i -> Boolean) -> IndexedOptic p i s t a a

Traverse elements of an IndexedTraversal whose index satisfy a predicate.

itraverseOf :: forall f i s t a b. IndexedOptic (Star f) i s t a b -> (i -> a -> f b) -> s -> f t

Turn a pure profunctor IndexedTraversal into a lens-like IndexedTraversal.

iforOf :: forall f i s t a b. IndexedOptic (Star f) i s t a b -> s -> (i -> a -> f b) -> f t

Flipped version of itraverseOf.

cloneTraversal :: forall s t a b. ATraversal s t a b -> Traversal s t a b

both :: forall r a b. Bitraversable r => Traversal (r a a) (r b b) a b

type Traversal' s a = Traversal s s a a

type Traversal s t a b = forall p. Wander p => Optic p s t a b

The specialized type Traversal s a means that the type s contains zero, one, or many values of type a. They're especially useful when working with collections like arrays, maps, and any other member of the Traversable type class. They are also a more general form of lenses and prisms. Traversals which focus on at most one element (like lenses, prisms, and their composition) are called affine traversals.