Data.FoldableWithIndex

class (Foldable f) <= FoldableWithIndex i f | f -> i where

A Foldable with an additional index. A FoldableWithIndex instance must be compatible with its Foldable instance

foldr f = foldrWithIndex (const f)
foldl f = foldlWithIndex (const f)
foldMap f = foldMapWithIndex (const f)

Default implementations are provided by the following functions:

• foldrWithIndexDefault
• foldlWithIndexDefault
• foldMapWithIndexDefaultR
• foldMapWithIndexDefaultL

Note: some combinations of the default implementations are unsafe to use together - causing a non-terminating mutually recursive cycle. These combinations are documented per function.

Members

• foldrWithIndex :: forall b a. (i -> a -> b -> b) -> b -> f a -> b
• foldlWithIndex :: forall b a. (i -> b -> a -> b) -> b -> f a -> b
• foldMapWithIndex :: forall m a. Monoid m => (i -> a -> m) -> f a -> m

Instances

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

foldrWithIndexDefault :: forall b a f i. FoldableWithIndex i f => (i -> a -> b -> b) -> b -> f a -> b

A default implementation of foldrWithIndex using foldMapWithIndex.

Note: when defining a FoldableWithIndex instance, this function is unsafe to use in combination with foldMapWithIndexDefaultR.

foldlWithIndexDefault :: forall b a f i. FoldableWithIndex i f => (i -> b -> a -> b) -> b -> f a -> b

A default implementation of foldlWithIndex using foldMapWithIndex.

Note: when defining a FoldableWithIndex instance, this function is unsafe to use in combination with foldMapWithIndexDefaultL.

foldMapWithIndexDefaultR :: forall m a f i. FoldableWithIndex i f => Monoid m => (i -> a -> m) -> f a -> m

A default implementation of foldMapWithIndex using foldrWithIndex.

Note: when defining a FoldableWithIndex instance, this function is unsafe to use in combination with foldrWithIndexDefault.

foldMapWithIndexDefaultL :: forall m a f i. FoldableWithIndex i f => Monoid m => (i -> a -> m) -> f a -> m

A default implementation of foldMapWithIndex using foldlWithIndex.

Note: when defining a FoldableWithIndex instance, this function is unsafe to use in combination with foldlWithIndexDefault.

foldWithIndexM :: forall b a m f i. FoldableWithIndex i f => Monad m => (i -> a -> b -> m a) -> a -> f b -> m a

Similar to 'foldlWithIndex', but the result is encapsulated in a monad.

Note: this function is not generally stack-safe, e.g., for monads which build up thunks a la Eff.

traverseWithIndex_ :: forall m f b a i. Applicative m => FoldableWithIndex i f => (i -> a -> m b) -> f a -> m Unit

Traverse a data structure with access to the index, performing some effects encoded by an Applicative functor at each value, ignoring the final result.

For example:

> traverseWithIndex_ (curry logShow) ["a", "b", "c"]
(Tuple 0 "a")
(Tuple 1 "b")
(Tuple 2 "c")

forWithIndex_ :: forall m f b a i. Applicative m => FoldableWithIndex i f => f a -> (i -> a -> m b) -> m Unit

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:

forWithIndex_ ["a", "b", "c"] \i x -> do
  logShow i
  log x

surroundMapWithIndex :: forall m a f i. FoldableWithIndex i f => Semigroup m => m -> (i -> a -> m) -> f a -> m

foldMapWithIndex but with each element surrounded by some fixed value.

For example:

> surroundMapWithIndex "*" (\i x -> show i <> x) []
= "*"

> surroundMapWithIndex "*" (\i x -> show i <> x) ["a"]
= "*0a*"

> surroundMapWithIndex "*" (\i x -> show i <> x) ["a", "b"]
= "*0a*1b*"

> surroundMapWithIndex "*" (\i x -> show i <> x) ["a", "b", "c"]
= "*0a*1b*2c*"

allWithIndex :: forall f b a i. FoldableWithIndex i f => HeytingAlgebra b => (i -> a -> b) -> f a -> b

allWithIndex f is the same as and <<< mapWithIndex f; map a function over the structure, and then get the conjunction of the results.

anyWithIndex :: forall f b a i. FoldableWithIndex i f => HeytingAlgebra b => (i -> a -> b) -> f a -> b

anyWithIndex f is the same as or <<< mapWithIndex f; map a function over the structure, and then get the disjunction of the results.

findWithIndex :: forall f a i. FoldableWithIndex i f => (i -> a -> Boolean) -> f a -> Maybe { index :: i, value :: a }

Try to find an element in a data structure which satisfies a predicate with access to the index.

foldrDefault :: forall b a f i. FoldableWithIndex i f => (a -> b -> b) -> b -> f a -> b

A default implementation of foldr using foldrWithIndex

foldlDefault :: forall b a f i. FoldableWithIndex i f => (b -> a -> b) -> b -> f a -> b

A default implementation of foldl using foldlWithIndex

foldMapDefault :: forall m a f i. FoldableWithIndex i f => Monoid m => (a -> m) -> f a -> m

A default implementation of foldMap using foldMapWithIndex