Data.TraversableWithIndex

class (FunctorWithIndex i t, FoldableWithIndex i t, Traversable t) <= TraversableWithIndex i t | t -> i where

A Traversable with an additional index.
A TraversableWithIndex instance must be compatible with its Traversable instance

traverse f = traverseWithIndex (const f)

with its FoldableWithIndex instance

foldMapWithIndex f = unwrap <<< traverseWithIndex (\i -> Const <<< f i)

and with its FunctorWithIndex instance

mapWithIndex f = unwrap <<< traverseWithIndex (\i -> Identity <<< f i)

A default implementation is provided by traverseWithIndexDefault.

Members

• traverseWithIndex :: forall a b m. Applicative m => (i -> a -> m b) -> t a -> m (t b)

Instances

• TraversableWithIndex Int Array
• TraversableWithIndex Unit Maybe
• TraversableWithIndex Unit First
• TraversableWithIndex Unit Last
• TraversableWithIndex Unit Additive
• TraversableWithIndex Unit Dual
• TraversableWithIndex Unit Conj
• TraversableWithIndex Unit Disj
• TraversableWithIndex Unit Multiplicative

traverseWithIndexDefault :: forall i t a b m. TraversableWithIndex i t => Applicative m => (i -> a -> m b) -> t a -> m (t b)

A default implementation of traverseWithIndex using sequence and mapWithIndex.

forWithIndex :: forall i a b m t. Applicative m => TraversableWithIndex i t => t a -> (i -> a -> m b) -> m (t b)

A version of traverseWithIndex with its arguments flipped.

This can be useful when running an action written using do notation for every element in a data structure:

For example:

for [1, 2, 3] \i x -> do
  logShow i
  pure (x * x)

scanlWithIndex :: forall i a b f. TraversableWithIndex i f => (i -> b -> a -> b) -> b -> f a -> f b

Fold a data structure from the left with access to the indices, keeping all intermediate results instead of only the final result. Note that the initial value does not appear in the result (unlike Haskell's Prelude.scanl).

scanlWithIndex (\i y x -> i + y + x) 0 [1, 2, 3] = [1, 4, 9]

mapAccumLWithIndex :: forall i a b s f. TraversableWithIndex i f => (i -> s -> a -> Accum s b) -> s -> f a -> Accum s (f b)

Fold a data structure from the left with access to the indices, keeping all intermediate results instead of only the final result.

Unlike scanlWithIndex, mapAccumLWithIndex allows the type of accumulator to differ from the element type of the final data structure.

scanrWithIndex :: forall i a b f. TraversableWithIndex i f => (i -> a -> b -> b) -> b -> f a -> f b

Fold a data structure from the right with access to the indices, keeping all intermediate results instead of only the final result. Note that the initial value does not appear in the result (unlike Haskell's Prelude.scanr).

scanrWithIndex (\i x y -> i + x + y) 0 [1, 2, 3] = [9, 8, 5]

mapAccumRWithIndex :: forall i a b s f. TraversableWithIndex i f => (i -> s -> a -> Accum s b) -> s -> f a -> Accum s (f b)

Fold a data structure from the right with access to the indices, keeping all intermediate results instead of only the final result.

Unlike scanrWithIndex, imapAccumRWithIndex allows the type of accumulator to differ from the element type of the final data structure.

traverseDefault :: forall i t a b m. TraversableWithIndex i t => Applicative m => (a -> m b) -> t a -> m (t b)

A default implementation of traverse in terms of traverseWithIndex

type Accum s a = { accum :: s, value :: a }