Module

Data.List

This module defines a type of strict 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

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

Convert a list into any unfoldable structure.

Running time: O(n)

#fromFoldable

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

Construct a list from a foldable structure.

Running time: O(n)

#singleton

singleton :: forall a. a -> List a

Create a list with a single element.

Running time: O(1)

#(..)

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

An infix synonym for range.

#range

range :: Int -> Int -> List Int

Create a list containing a range of integers, including both endpoints.

#some

some :: forall f a. 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.

#someRec

someRec :: forall f a. MonadRec f => Alternative f => f a -> f (List a)

A stack-safe version of some, at the cost of a MonadRec constraint.

#many

many :: forall f a. 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.

#manyRec

manyRec :: forall f a. MonadRec f => Alternative f => f a -> f (List a)

A stack-safe version of many, at the cost of a MonadRec constraint.

#null

null :: forall a. List a -> Boolean

Test whether a list is empty.

Running time: O(1)

#length

length :: forall a. List a -> Int

Get the length of a list

Running time: O(n)

#snoc

snoc :: forall a. List a -> a -> List a

Append an element to the end of a list, creating a new list.

Running time: O(n)

#insert

insert :: forall a. Ord a => a -> List a -> List a

Insert an element into a sorted list.

Running time: O(n)

#insertBy

insertBy :: forall a. (a -> a -> Ordering) -> a -> List a -> List a

Insert an element into a sorted list, using the specified function to determine the ordering of elements.

Running time: O(n)

#head

head :: List ~> Maybe

Get the first element in a list, or Nothing if the list is empty.

Running time: O(1).

#last

last :: List ~> Maybe

Get the last element in a list, or Nothing if the list is empty.

Running time: O(n).

#tail

tail :: forall a. List a -> Maybe (List a)

Get all but the first element of a list, or Nothing if the list is empty.

Running time: O(1)

#init

init :: forall a. List a -> Maybe (List a)

Get all but the last element of a list, or Nothing if the list is empty.

Running time: O(n)

#uncons

uncons :: forall a. List a -> Maybe { head :: a, tail :: List a }

Break a list into its first element, and the remaining elements, or Nothing if the list is empty.

Running time: O(1)

#unsnoc

unsnoc :: forall a. List a -> Maybe { init :: List a, last :: a }

Break a list into its last element, and the preceding elements, or Nothing if the list is empty.

Running time: O(n)

#(!!)

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

An infix synonym for index.

#index

index :: forall a. List a -> Int -> Maybe a

Get the element at the specified index, or Nothing if the index is out-of-bounds.

Running time: O(n) where n is the required index.

#elemIndex

elemIndex :: forall a. Eq a => a -> List a -> Maybe Int

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

#elemLastIndex

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

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

#findIndex

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

Find the first index for which a predicate holds.

#findLastIndex

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

Find the last index for which a predicate holds.

#insertAt

insertAt :: forall a. Int -> a -> List a -> Maybe (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.

Running time: O(n)

#deleteAt

deleteAt :: forall a. Int -> List a -> Maybe (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.

Running time: O(n)

#updateAt

updateAt :: forall a. Int -> a -> List a -> Maybe (List a)

Update the element at the specified index, returning a new list or Nothing if the index is out-of-bounds.

Running time: O(n)

#modifyAt

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

Running time: O(n)

#alterAt

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

#reverse

reverse :: List ~> List

Reverse a list.

Running time: O(n)

#concat

concat :: forall a. List (List a) -> List a

Flatten a list of lists.

Running time: O(n), where n is the total number of elements.

#concatMap

concatMap :: forall a b. (a -> List b) -> List a -> List b

Apply a function to each element in a list, and flatten the results into a single, new list.

Running time: O(n), where n is the total number of elements.

#filter

filter :: forall a. (a -> Boolean) -> List a -> List a

Filter a list, keeping the elements which satisfy a predicate function.

Running time: O(n)

#filterM

filterM :: forall a m. Monad m => (a -> m Boolean) -> List a -> m (List a)

Filter where the predicate returns a monadic Boolean.

For example:

powerSet :: forall a. [a] -> [[a]]
powerSet = filterM (const [true, false])

#mapMaybe

mapMaybe :: forall a b. (a -> Maybe b) -> List a -> List b

Apply a function to each element in a list, keeping only the results which contain a value.

Running time: O(n)

#catMaybes

catMaybes :: forall a. List (Maybe a) -> List a

Filter a list of optional values, keeping only the elements which contain a value.

#mapWithIndex

mapWithIndex :: forall a b. (Int -> a -> b) -> List a -> List b

Apply a function to each element and its index in a list starting at 0.

Deprecated. Use Data.FunctorWithIndex instead.

#sort

sort :: forall a. Ord a => List a -> List a

Sort the elements of an list in increasing order.

#sortBy

sortBy :: forall a. (a -> a -> Ordering) -> List a -> List a

Sort the elements of a list in increasing order, where elements are compared using the specified ordering.

#Pattern

newtype Pattern a

A newtype used in cases where there is a list to be matched.

Constructors

Instances

#stripPrefix

stripPrefix :: forall a. Eq a => Pattern a -> List a -> Maybe (List a)

If the list starts with the given prefix, return the portion of the list left after removing it, as a Just value. Otherwise, return Nothing.

  • stripPrefix (Pattern (1:Nil)) (1:2:Nil) == Just (2:Nil)
  • stripPrefix (Pattern Nil) (1:Nil) == Just (1:Nil)
  • stripPrefix (Pattern (2:Nil)) (1:Nil) == Nothing

Running time: O(n) where n is the number of elements to strip.

#slice

slice :: Int -> Int -> List ~> List

Extract a sublist by a start and end index.

#take

take :: forall a. Int -> List a -> List a

Take the specified number of elements from the front of a list.

Running time: O(n) where n is the number of elements to take.

#takeEnd

takeEnd :: forall a. Int -> List a -> List a

Take the specified number of elements from the end of a list.

Running time: O(2n - m) where n is the number of elements in list and m is number of elements to take.

#takeWhile

takeWhile :: forall a. (a -> Boolean) -> List a -> List a

Take those elements from the front of a list which match a predicate.

Running time (worst case): O(n)

#drop

drop :: forall a. Int -> List a -> List a

Drop the specified number of elements from the front of a list.

Running time: O(n) where n is the number of elements to drop.

#dropEnd

dropEnd :: forall a. Int -> List a -> List a

Drop the specified number of elements from the end of a list.

Running time: O(2n - m) where n is the number of elements in list and m is number of elements to drop.

#dropWhile

dropWhile :: forall a. (a -> Boolean) -> List a -> List a

Drop those elements from the front of a list which match a predicate.

Running time (worst case): O(n)

#span

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

Split a list into two parts:

  1. the longest initial segment for which all elements satisfy the specified predicate
  2. the remaining elements

For example,

span (\n -> n % 2 == 1) (1 : 3 : 2 : 4 : 5 : Nil) == { init: (1 : 3 : Nil), rest: (2 : 4 : 5 : Nil) }

Running time: O(n)

#group

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) ==
  (NonEmptyList (NonEmpty 1 (1 : Nil))) : (NonEmptyList (NonEmpty 2 (2 : Nil))) : (NonEmptyList (NonEmpty 1 Nil)) : Nil

Running time: O(n)

#groupAll

groupAll :: forall a. Ord a => List a -> List (NonEmptyList a)

Group equal elements of a list into lists.

For example,

groupAll (1 : 1 : 2 : 2 : 1 : Nil) ==
  (NonEmptyList (NonEmpty 1 (1 : 1 : Nil))) : (NonEmptyList (NonEmpty 2 (2 : Nil))) : Nil

#group'

group' :: forall a. Warn (Text "\'group\'\' is deprecated, use groupAll instead") => Ord a => List a -> List (NonEmptyList a)

Deprecated previous name of groupAll.

#groupBy

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.

For example,

groupBy (\a b -> odd a && odd b) (1 : 3 : 2 : 4 : 3 : 3 : Nil) ==
  (NonEmptyList (NonEmpty 1 (3 : Nil))) : (NonEmptyList (NonEmpty 2 Nil)) : (NonEmptyList (NonEmpty 4 Nil)) : (NonEmptyList (NonEmpty 3 (3 : Nil))) : Nil

Running time: O(n)

#groupAllBy

groupAllBy :: forall a. Ord a => (a -> a -> Boolean) -> List a -> List (NonEmptyList a)

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

For example,

groupAllBy (\a b -> odd a && odd b) (1 : 3 : 2 : 4 : 3 : 3 : Nil) ==
   (NonEmptyList (NonEmpty 1 Nil)) : (NonEmptyList (NonEmpty 2 Nil)) : (NonEmptyList (NonEmpty 3 (3 : 3 : Nil))) : (NonEmptyList (NonEmpty 4 Nil)) : Nil

#partition

partition :: forall a. (a -> Boolean) -> List a -> { no :: List a, yes :: List a }

Returns a lists of elements which do and do not satisfy a predicate.

Running time: O(n)

#nub

nub :: forall a. Ord a => List a -> List a

Remove duplicate elements from a list. Keeps the first occurrence of each element in the input list, in the same order they appear in the input list.

nub 1:2:1:3:3:Nil == 1:2:3:Nil

Running time: O(n log n)

#nubBy

nubBy :: forall a. (a -> a -> Ordering) -> List a -> List a

Remove duplicate elements from a list based on the provided comparison function. Keeps the first occurrence of each element in the input list, in the same order they appear in the input list.

nubBy (compare `on` Array.length) ([1]:[2]:[3,4]:Nil) == [1]:[3,4]:Nil

Running time: O(n log n)

#nubEq

nubEq :: forall a. Eq a => List a -> List a

Remove duplicate elements from a list. Keeps the first occurrence of each element in the input list, in the same order they appear in the input list. This less efficient version of nub only requires an Eq instance.

nubEq 1:2:1:3:3:Nil == 1:2:3:Nil

Running time: O(n^2)

#nubByEq

nubByEq :: forall a. (a -> a -> Boolean) -> List a -> List a

Remove duplicate elements from a list, using the provided equivalence function. Keeps the first occurrence of each element in the input list, in the same order they appear in the input list. This less efficient version of nubBy only requires an equivalence function, rather than an ordering function.

mod3eq = eq `on` \n -> mod n 3
nubByEq mod3eq 1:3:4:5:6:Nil == 1:3:5:Nil

Running time: O(n^2)

#union

union :: forall a. Eq a => List a -> List a -> List a

Calculate the union of two lists.

Running time: O(n^2)

#unionBy

unionBy :: forall a. (a -> a -> Boolean) -> List a -> List a -> List a

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

Running time: O(n^2)

#delete

delete :: forall a. Eq a => a -> List a -> List a

Delete the first occurrence of an element from a list.

Running time: O(n)

#deleteBy

deleteBy :: forall a. (a -> a -> Boolean) -> a -> List a -> List a

Delete the first occurrence of an element from a list, using the specified function to determine equality of elements.

Running time: O(n)

#(\\)

Operator alias for Data.List.difference (non-associative / precedence 5)

#difference

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)

#intersect

intersect :: forall a. Eq a => List a -> List a -> List a

Calculate the intersection of two lists.

Running time: O(n^2)

#intersectBy

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

zipWith :: forall a b c. (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

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

A generalization of zipWith which accumulates results in some Applicative functor.

#zip

zip :: forall a b. List a -> List b -> List (Tuple a b)

Collect pairs of elements at the same positions in two lists.

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

#unzip

unzip :: forall a b. List (Tuple a b) -> Tuple (List a) (List b)

Transforms a list of pairs into a list of first components and a list of second components.

#transpose

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)

#foldM

foldM :: forall m a b. Monad m => (b -> a -> m b) -> b -> List a -> m b

Perform a fold using a monadic step function.

Re-exports from Data.Foldable

#foldMap

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

#foldl

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

#foldr

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

#notElem

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

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

#intercalate

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

Fold a data structure, accumulating values in some Monoid, combining adjacent elements using the specified separator.

For example:

> intercalate ", " ["Lorem", "ipsum", "dolor"]
= "Lorem, ipsum, dolor"

> intercalate "*" ["a", "b", "c"]
= "a*b*c"

> intercalate [1] [[2, 3], [4, 5], [6, 7]]
= [2, 3, 1, 4, 5, 1, 6, 7]

#fold

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

Fold a data structure, accumulating values in some Monoid.

#findMap

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

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

#elem

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

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

#any

any :: forall a b f. 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 a b f. 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.Types

#(:)

Operator alias for Data.List.Types.Cons (right-associative / precedence 6)

Re-exports from Data.Traversable

#scanr

scanr :: forall a b f. 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] = [6,5,3]
scanr (flip (-)) 10 [1,2,3] = [4,5,7]

#scanl

scanl :: forall a b f. 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]

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