traverse package:stack

Map each element of a structure to an action, evaluate these actions from left to right, and collect the results. For a version that ignores the results see traverse_.

Examples

Basic usage: In the first two examples we show each evaluated action mapping to the output structure.

>>> traverse Just [1,2,3,4]
Just [1,2,3,4]

>>> traverse id [Right 1, Right 2, Right 3, Right 4]
Right [1,2,3,4]

In the next examples, we show that Nothing and Left values short circuit the created structure.

>>> traverse (const Nothing) [1,2,3,4]
Nothing

>>> traverse (\x -> if odd x then Just x else Nothing)  [1,2,3,4]
Nothing

>>> traverse id [Right 1, Right 2, Right 3, Right 4, Left 0]
Left 0

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"
"!"

Evaluates the relevant functions at each element in the structure, running the action, and builds a new structure with the same shape, using the results produced from sequencing the actions.

bitraverse f g ≡ bisequenceA . bimap f g

For a version that ignores the results, see bitraverse_.

Examples

Basic usage:

>>> bitraverse listToMaybe (find odd) (Left [])
Nothing

>>> bitraverse listToMaybe (find odd) (Left [1, 2, 3])
Just (Left 1)

>>> bitraverse listToMaybe (find odd) (Right [4, 5])
Just (Right 5)

>>> bitraverse listToMaybe (find odd) ([1, 2, 3], [4, 5])
Just (1,5)

>>> bitraverse listToMaybe (find odd) ([], [4, 5])
Nothing

Map each element of a structure using one of two actions, evaluate these actions from left to right, and ignore the results. For a version that doesn't ignore the results, see bitraverse.

Examples

Basic usage:

>>> bitraverse_ print (print . show) ("Hello", True)
"Hello"
"True"

>>> bitraverse_ print (print . show) (Right True)
"True"

>>> bitraverse_ print (print . show) (Left "Hello")
"Hello"