Module

Data.TraversableWithIndex

#TraversableWithIndex

class TraversableWithIndex :: Type -> (Type -> Type) -> Constraintclass (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

Instances

#traverseWithIndexDefault

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

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

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

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

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

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

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

Re-exports from Data.Traversable.Accum

#Accum

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

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