# Data.List.Lazy

- Package
- purescript-lists
- Repository
- purescript/purescript-lists

This module defines a type of *lazy* linked lists, and associated helper
functions and type class instances.

*Note*: Depending on your use-case, you may prefer to use
`Data.Sequence`

instead, which might give better performance for certain
use cases. This module is an improvement over `Data.Array`

when working with
immutable lists of data in a purely-functional setting, but does not have
good random-access performance.

### #toUnfoldable Source

`toUnfoldable :: forall f. Unfoldable f => List ~> f`

Convert a list into any unfoldable structure.

Running time: `O(n)`

### #fromFoldable Source

`fromFoldable :: forall f. Foldable f => f ~> List`

Construct a list from a foldable structure.

Running time: `O(n)`

### #(..) Source

Operator alias for Data.List.Lazy.range *(non-associative / precedence 8)*

An infix synonym for `range`

.

### #replicateM Source

`replicateM :: forall a m. Monad m => Int -> m a -> m (List a)`

Perform a monadic action `n`

times collecting all of the results.

### #some Source

`some :: forall a f. Alternative f => Lazy (f (List a)) => f a -> f (List a)`

Attempt a computation multiple times, requiring at least one success.

The `Lazy`

constraint is used to generate the result lazily, to ensure
termination.

### #many Source

`many :: forall a f. Alternative f => Lazy (f (List a)) => f a -> f (List a)`

Attempt a computation multiple times, returning as many successful results as possible (possibly zero).

The `Lazy`

constraint is used to generate the result lazily, to ensure
termination.

### #(!!) Source

Operator alias for Data.List.Lazy.index *(left-associative / precedence 8)*

An infix synonym for `index`

.

### #elemLastIndex Source

`elemLastIndex :: forall a. Eq a => a -> List a -> Maybe Int`

Find the index of the last element equal to the specified element.

### #findLastIndex Source

`findLastIndex :: forall a. (a -> Boolean) -> List a -> Maybe Int`

Find the last index for which a predicate holds.

### #insertAt Source

`insertAt :: forall a. Int -> a -> List a -> List a`

Insert an element into a list at the specified index, returning a new
list or `Nothing`

if the index is out-of-bounds.

This function differs from the strict equivalent in that out-of-bounds arguments
result in the element being appended at the *end* of the list.

Running time: `O(n)`

### #deleteAt Source

`deleteAt :: forall a. Int -> List a -> List a`

Delete an element from a list at the specified index, returning a new
list or `Nothing`

if the index is out-of-bounds.

This function differs from the strict equivalent in that out-of-bounds arguments result in the original list being returned unchanged.

Running time: `O(n)`

### #updateAt Source

`updateAt :: forall a. Int -> a -> List a -> List a`

Update the element at the specified index, returning a new
list or `Nothing`

if the index is out-of-bounds.

This function differs from the strict equivalent in that out-of-bounds arguments result in the original list being returned unchanged.

Running time: `O(n)`

### #modifyAt Source

`modifyAt :: forall a. Int -> (a -> a) -> List a -> List a`

Update the element at the specified index by applying a function to
the current value, returning a new list or `Nothing`

if the index is
out-of-bounds.

This function differs from the strict equivalent in that out-of-bounds arguments result in the original list being returned unchanged.

Running time: `O(n)`

### #alterAt Source

`alterAt :: forall a. Int -> (a -> Maybe a) -> List a -> List a`

Update or delete the element at the specified index by applying a
function to the current value, returning a new list or `Nothing`

if the
index is out-of-bounds.

Running time: `O(n)`

### #span Source

`span :: forall a. (a -> Boolean) -> List a -> { init :: List a, rest :: List a }`

Split a list into two parts:

- the longest initial segment for which all elements satisfy the specified predicate
- the remaining elements

For example,

```
span (\n -> n % 2 == 1) (1 : 3 : 2 : 4 : 5 : Nil) == Tuple (1 : 3 : Nil) (2 : 4 : 5 : Nil)
```

Running time: `O(n)`

### #group Source

`group :: forall a. Eq a => List a -> List (NonEmptyList a)`

Group equal, consecutive elements of a list into lists.

For example,

```
group (1 : 1 : 2 : 2 : 1 : Nil) == (1 : 1 : Nil) : (2 : 2 : Nil) : (1 : Nil) : Nil
```

Running time: `O(n)`

### #groupBy Source

`groupBy :: forall a. (a -> a -> Boolean) -> List a -> List (NonEmptyList a)`

Group equal, consecutive elements of a list into lists, using the specified equivalence relation to determine equality.

Running time: `O(n)`

### #difference Source

`difference :: forall a. Eq a => List a -> List a -> List a`

Delete the first occurrence of each element in the second list from the first list.

Running time: `O(n^2)`

### #intersectBy Source

`intersectBy :: forall a. (a -> a -> Boolean) -> List a -> List a -> List a`

Calculate the intersection of two lists, using the specified function to determine equality of elements.

Running time: `O(n^2)`

### #zipWith Source

`zipWith :: forall c b a. (a -> b -> c) -> List a -> List b -> List c`

Apply a function to pairs of elements at the same positions in two lists, collecting the results in a new list.

If one list is longer, elements will be discarded from the longer list.

For example

```
zipWith (*) (1 : 2 : 3 : Nil) (4 : 5 : 6 : 7 Nil) == 4 : 10 : 18 : Nil
```

Running time: `O(min(m, n))`

### #zipWithA Source

`zipWithA :: forall c b a m. Applicative m => (a -> b -> m c) -> List a -> List b -> m (List c)`

A generalization of `zipWith`

which accumulates results in some `Applicative`

functor.

### #transpose Source

`transpose :: forall a. List (List a) -> List (List a)`

The 'transpose' function transposes the rows and columns of its argument. For example,

```
transpose ((1:2:3:nil) : (4:5:6:nil) : nil) ==
((1:4:nil) : (2:5:nil) : (3:6:nil) : nil)
```

If some of the rows are shorter than the following rows, their elements are skipped:

```
transpose ((10:11:nil) : (20:nil) : nil : (30:31:32:nil) : nil) ==
((10:20:30:nil) : (11:31:nil) : (32:nil) : nil)
```

## Re-exports from **Data.**Foldable

### #foldMap

`foldMap :: forall f m a. Foldable f => Monoid m => (a -> m) -> f a -> m`

### #foldl

`foldl :: forall f b a. Foldable f => (b -> a -> b) -> b -> f a -> b`

### #foldr

`foldr :: forall f b a. Foldable f => (a -> b -> b) -> b -> f a -> b`

### #notElem

`notElem :: forall f a. Foldable f => Eq a => a -> f a -> Boolean`

Test whether a value is not an element of a data structure.

### #intercalate

`intercalate :: forall m f. Foldable f => Monoid m => m -> f m -> m`

Fold a data structure, accumulating values in some `Monoid`

,
combining adjacent elements using the specified separator.

### #fold

`fold :: forall m f. Foldable f => Monoid m => f m -> m`

Fold a data structure, accumulating values in some `Monoid`

.

### #findMap

`findMap :: forall f b a. Foldable f => (a -> Maybe b) -> f a -> Maybe b`

Try to find an element in a data structure which satisfies a predicate mapping.

### #find

`find :: forall f a. Foldable f => (a -> Boolean) -> f a -> Maybe a`

Try to find an element in a data structure which satisfies a predicate.

### #elem

`elem :: forall f a. Foldable f => Eq a => a -> f a -> Boolean`

Test whether a value is an element of a data structure.

### #any

`any :: forall f b a. Foldable f => HeytingAlgebra b => (a -> b) -> f a -> b`

`any f`

is the same as `or <<< map f`

; map a function over the structure,
and then get the disjunction of the results.

### #all

`all :: forall f b a. Foldable f => HeytingAlgebra b => (a -> b) -> f a -> b`

`all f`

is the same as `and <<< map f`

; map a function over the structure,
and then get the conjunction of the results.

## Re-exports from **Data.**List.Lazy.Types

### #List Source

`newtype List a`

A lazy linked list.

#### Constructors

#### Instances

`Newtype (List a) _`

`(Show a) => Show (List a)`

`(Eq a) => Eq (List a)`

`Eq1 List`

`(Ord a) => Ord (List a)`

`Ord1 List`

`Lazy (List a)`

`Semigroup (List a)`

`Monoid (List a)`

`Functor List`

`Foldable List`

`Unfoldable List`

`Traversable List`

`Apply List`

`Applicative List`

`Bind List`

`Monad List`

`Alt List`

`Plus List`

`Alternative List`

`MonadZero List`

`MonadPlus List`

`Extend List`

### #(:) Source

Operator alias for Data.List.Lazy.Types.cons *(right-associative / precedence 6)*

An infix alias for `cons`

; attaches an element to the front of
a list.

Running time: `O(1)`

## Re-exports from **Data.**Traversable

### #scanr

`scanr :: forall f b a. Traversable f => (a -> b -> b) -> b -> f a -> f b`

Fold a data structure from the right, 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`

).

```
scanr (+) 0 [1,2,3] = [1,3,6]
scanr (flip (-)) 10 [1,2,3] = [4,5,7]
```

### #scanl

`scanl :: forall f b a. Traversable f => (b -> a -> b) -> b -> f a -> f b`

Fold a data structure from the left, 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`

).

```
scanl (+) 0 [1,2,3] = [1,3,6]
scanl (-) 10 [1,2,3] = [9,7,4]
```