API / JavaScript / Belt / List

List

Collection functions for manipulating list data structures.

type t('a) = list('a);
let length: t('a) => int;

Returns the length of a list.

RE
Belt.List.length([1, 2, 3]); /* 3 */
let size: t('a) => int;

See: length

let head: t('a) => option('a);

Returns Some(value) where value is the first element in the list, or None if someList is an empty list.

RE
Belt.List.head([]); /* None */ Belt.List.head([1, 2, 3]); /* Some(1) */
let headExn: t('a) => 'a;

Same as head, but raises an exception if someList is empty. Use with care.

RE
Belt.List.headExn([1, 2, 3]); /* 1 */ Belt.List.headExn([]); /* Raises an Error */
let tail: t('a) => option(t('a));

Returns None if someList is empty, otherwise it returns Some(tail) where tail is everything except the first element of someList.

RE
Belt.List.tail([1, 2, 3]); /* Some([2, 3]) */ Belt.List.tail([]); /* None */
let tailExn: t('a) => t('a);

Same as tail, but raises an exception if someList is empty. Use with care.

RE
Belt.List.tailExn([1, 2, 3]); /* [2, 3] */ Belt.List.tailExn([]); /* Raises an Error */
let add: (t('a), 'a) => t('a);

Adds value to the beginning of someList.

RE
Belt.List.add([2, 3], 1); /* [1, 2, 3] */ Belt.List.add(["World", "!"], "Hello"); /* ["Hello", "World", "!"] */
let get: (t('a), int) => option('a);

Return the nth element in someList, or None if index is larger than the length.

RE
let abc = ["A", "B", "C"]; abc->Belt.List.get(1); /* Some("B") */ abc->Belt.List.get(4); /* None */
let getExn: (t('a), int) => 'a;

Same as get, but raises an exception if index is larger than the length. Use with care.

RE
let abc = ["A", "B", "C"]; abc->Belt.List.getExn(1); /* "B" */ abc->Belt.List.getExn(4); /* Raises an Error */
RE
let make: (int, 'a) => t('a);

Returns a list of length numItems with each element filled with value v. Returns an empty list if numItems is negative.

RE
Belt.List.make(3, 1); /* [1, 1, 1] */
let makeBy: (int, int => 'a) => t('a);

Return a list of length numItems with element i initialized with f(i). Returns an empty list if numItems is negative.

RE
Belt.List.makeBy(5, i => i); /* [0, 1, 2, 3, 4] */ Belt.List.makeBy(5, i => i * i); /* [0, 1, 4, 9, 16] */
let makeByU: (int, [@bs] (int => 'a)) => t('a);

Uncurried version of makeBy

let shuffle: t('a) => t('a);

Returns a new list in random order.

RE
Belt.List.shuffle([1, 2, 3]); /* [2, 1, 3] */
let drop: (t('a), int) => option(t('a));

Return a new list, dropping the first n elements. Returns None if someList has fewer than n elements.

RE
[1, 2, 3]->Belt.List.drop(2); /* Some([3]) */ [1, 2, 3]->Belt.List.drop(3); /* Some([]) */ [1, 2, 3]->Belt.List.drop(4); /* None */
let take: (t('a), int) => option(t('a));

Returns a list with the first n elements from someList, or None if someList has fewer than n elements.

RE
[1, 2, 3]->Belt.List.take(1); /* Some([1]) */ [1, 2, 3]->Belt.List.take(2); /* Some([1, 2]) */ [1, 2, 3]->Belt.List.take(4); /* None */
let splitAt: (t('a), int) => option((list('a), list('a)));

Split the list someList at index. Returns None when the length of someList is less than index.

RE
["Hello", "World"]->Belt.List.splitAt(1); /* Some((["Hello"], ["World"])) */ [0, 1, 2, 3, 4]->Belt.List.splitAt(2); /* Some(([0, 1], [2, 3, 4])) */
let concat: (t('a), t('a)) => t('a);

Returns the list obtained by adding secondList after firstList.

RE
Belt.List.concat([1, 2, 3], [4, 5]); /* [1, 2, 3, 4, 5] */
let concatMany: array(t('a)) => t('a);

Returns the list obtained by concatenating all the lists in array a, in order.

RE
Belt.List.concatMany([|[1, 2, 3], [], [3], [4]|]); /* [1, 2, 3, 3, 4] */
let reverseConcat: (t('a), t('a)) => t('a);

Equivalent to writing: concat(reverse(firstList), secondList)

RE
Belt.List.reverseConcat([1, 2], [3, 4]); /* [2, 1, 3, 4] */
let flatten: t(t('a)) => t('a);

Return the list obtained by concatenating all the lists in list ls, in order.

RE
Belt.List.flatten([[1, 2, 3], [], [3], [4]]); /* [1, 2, 3, 3, 4] */
let map: (t('a), 'a => 'b) => t('b);

Returns a new list with f applied to each element of someList.

RE
[1, 2]->Belt.List.map(x => x + 1); /* [3, 4] */
let mapU: (t('a), [@bs] ('a => 'b)) => t('b);

Uncurried version of map.

let zip: (t('a), t('b)) => t(('a, 'b));

Returns a list of pairs from the two lists with the length of the shorter list.

RE
Belt.List.zip([1, 2], [3, 4, 5]); /* [(1, 3), (2, 4)] */
let zipBy: (t('a), t('b), ('a, 'b) => 'c) => t('c);

See: zip

Equivalent to:

zip(firstList, secondList) |> List.map(((x, y)) => f(x, y));
RE
Belt.List.zipBy([1, 2, 3], [4, 5], (a, b) => 2 * a + b); /* [6, 9] */
let zipByU: (t('a), t('b), [@bs] (('a, 'b) => 'c)) => t('c);

Uncurried version of zipBy.

let mapWithIndex: (t('a), (int, 'a) => 'b) => t('b);

Applies f to each element of someList. Function f takes two arguments: the index starting from 0 and the element from someList, in that order.

RE
[1, 2, 3]->Belt.List.mapWithIndex((index, x) => index + x); /* [1, 3, 5] */
let mapWithIndexU: (t('a), [@bs] ((int, 'a) => 'b)) => t('b);

Uncurried version of mapWithIndex.

let fromArray: array('a) => t('a);

Converts the given array to a list.

RE
Belt.List.fromArray([|1, 2, 3|]); /* [1, 2, 3] */
let toArray: t('a) => array('a);

Converts the given list to an array.

RE
Belt.List.toArray([1, 2, 3]); /* [|1, 2, 3|] */
let reverse: t('a) => t('a);

Returns a new list whose elements are those of someList in reversed order.

RE
Belt.List.reverse([1, 2, 3]); /* [3, 2, 1] */
let mapReverse: (t('a), 'a => 'b) => t('b);

Equivalent to:

map(someList, f)->reverse;
RE
[3, 4, 5]->Belt.List.mapReverse(x => x * x); /* [25, 16, 9] */
let mapReverseU: (t('a), [@bs] ('a => 'b)) => t('b);

Uncurried version of mapReverse.

let forEach: (t('a), 'a => 'b) => unit;

Call f on each element of someList from the beginning to end. f returns unit, so no new array is created. Use forEach when you are primarily concerned with repetitively creating side effects.

RE
Belt.List.forEach(["a", "b", "c"], x => Js.log("Item: " ++ x)); /* prints: Item: a Item: b Item: c */
let forEachU: (t('a), [@bs] ('a => 'b)) => unit;

Uncurried version of forEach.

let forEachWithIndex: (t('a), (int, 'a) => 'b) => unit;

Call f on each element of someList from beginning to end. Function f takes two arguments: the index starting from 0 and the element from someList. f returns unit.

RE
Belt.List.forEachWithIndex(["a", "b", "c"], (index, x) => Js.log("Item " ++ string_of_int(index) ++ " is " ++ x)); /* prints: Item 0 is a Item 1 is b Item 2 is cc */
let forEachWithIndexU: (t('a), [@bs] ((int, 'a) => 'b)) => unit;

Uncurried version of forEachWithIndex.

let reduce: (t('a), 'b, ('b, 'a) => 'b) => 'b;

Applies f to each element of someList from beginning to end. Function f has two parameters: the item from the list and an “accumulator”, which starts with a value of initialValue. reduce returns the final value of the accumulator.

RE
[1, 2, 3, 4]->Belt.List.reduce(0, (+)); /* 10 */ /* same as */ [1, 2, 3, 4]->Belt.List.reduce(0, (acc, item) => acc + item); /* 10 */
let reduceU: (t('a), 'b, [@bs] (('b, 'a) => 'b)) => 'b;

Uncurried version of reduce.

let reduceWithIndex: (t('a), 'b, ('b, 'a, int) => 'b) => 'b;

Applies f to each element of someList from beginning to end. Function f has three parameters: the item from the list and an “accumulator”, which starts with a value of initialValue and the index of each element. reduceWithIndex returns the final value of the accumulator.

RE
[1, 2, 3, 4]->Belt.List.reduceWithIndex(0, (acc, item, index) => acc + item + index); /* 16 */
let reduceWithIndexU: (t('a), 'b, [@bs] (('b, 'a, int) => 'b)) => 'b;

Uncurried version of reduceWithIndex.

let reduceReverse: (t('a), 'b, ('b, 'a) => 'b) => 'b;

Works like reduce, except that function f is applied to each item of someList from the last back to the first.

RE
[1, 2, 3, 4]->Belt.List.reduceReverse(0, (+)); /* 10 */ [1, 2, 3, 4]->Belt.List.reduceReverse(10, (-)); /* 0 */ [1, 2, 3, 4]->Belt.List.reduceReverse([], Belt.List.add); /* [1, 2, 3, 4] */
let reduceReverseU: (t('a), 'b, [@bs] (('b, 'a) => 'b)) => 'b;

Uncurried version of reduceReverse.

let mapReverse2: (t('a), t('b), ('a, 'b) => 'c) => t('c);

Equivalent to: zipBy(xs, ys, f)->reverse

RE
Belt.List.mapReverse2([1, 2, 3], [1, 2], (+)); /* [4, 2] */
let mapReverse2U: (t('a), t('b), [@bs] (('a, 'b) => 'c)) => t('c);

Uncurried version of mapReverse2.

let forEach2: (t('a), t('b), ('a, 'b) => 'c) => unit;

Stops at the length of the shorter list.

RE
Belt.List.forEach2(["Z", "Y"], ["A", "B", "C"], (x, y) => Js.log2(x, y)); /* prints: "Z" "A" "Y" "B" */
let forEach2U: (t('a), t('b), (.'a, 'b) => 'c) => unit;

Uncurried version of forEach2.

let reduce2: (t('b), t('c), 'a, ('a, 'b, 'c) => 'a) => 'a;

Applies f to each element of firstList and secondList from beginning to end. Stops with the shorter list. Function f has three parameters: an “accumulator” which starts with a value of initialValue, an item from firstList, and an item from secondList. reduce2 returns the final value of the accumulator.

RE
Belt.List.reduce2([1, 2, 3], [4, 5], 0, (acc, x, y) => acc + x * x + y); /* 0 + (1 * 1 + 4) + (2 * 2 + 5) */
let reduce2U: (t('b), t('c), 'a, (.'a, 'b, 'c) => 'a) => 'a;

Uncurried version of reduce2.

let reduceReverse2: (t('a), t('b), 'c, ('c, 'a, 'b) => 'c) => 'c;

Applies f to each element of firstList and secondList from end to beginning. Stops with the shorter list. Function f has three parameters: an “accumulator” which starts with a value of init, an item from firstList, and an item from secondList. reduce2 returns the final value of the accumulator.

RE
Belt.List.reduceReverse2([1, 2, 3], [4, 5], 0, (acc, x, y) => acc + x * x + y); /* + (1 * 1 + 4) + (2 * 2 + 5) */
let reduceReverse2U: (t('a), t('b), 'c, (.'c, 'a, 'b) => 'c) => 'c;

Uncurried version of reduceReverse2.

let every: (t('a), 'a => bool) => bool;

Returns true if all elements satisfy pred, where pred is a predicate: a function taking an element and returning a bool.

RE
let isBelow10 = value => value < 10; [1, 9, 8, 2]->Belt.List.every(isBelow10); /* true */ [1, 99, 8, 2]->Belt.List.every(isBelow10); /* false */
let everyU: (t('a), (.'a) => bool) => bool;

Uncurried version of every.

let some: (t('a), 'a => bool) => bool;

Returns true if at least one of the elements in someList satisfies pred, where pred is a predicate: a function taking an element and returning a bool.

RE
let isAbove100 = value => value > 100; [101, 1, 2, 3]->Belt.List.some(isAbove100); /* true */ [1, 2, 3, 4]->Belt.List.some(isAbove100); /* false */
let someU: (t('a), (.'a => bool)) => bool;

Uncurried version of some.

let every2: (t('a), t('b), ('a, 'b) => bool) => bool;

Returns true if predicate pred(a, b) is true for all pairs of elements up to the shorter length (i.e. min(length(firstList), length(secondList)))

RE
Belt.List.every2([1, 2, 3], [0, 1], (>)); /* true */ Belt.List.every2([], [1], (a, b) => a > b); /* true */ Belt.List.every2([2, 3], [1], (a, b) => a > b); /* true */ Belt.List.every2([0, 1], [5, 0], (a, b) => a > b); /* false */
let every2U: (t('a), t('b), [@bs] (('a, 'b) => bool)) => bool;

Uncurried version of every2.

let some2: (t('a), t('b), ('a, 'b) => bool) => bool;

Returns true if predicate pred(a, b) is true for any pair of elements up to the shorter length (i.e. min(length(firstList), length(secondList)))

RE
Belt.List.some2([1, 2, 3], [0, 1], (>)); /* true */ Belt.List.some2([], [1], (a, b) => a > b); /* false */ Belt.List.some2([2, 3], [1], (a, b) => a > b); /* true */ Belt.List.some2([0, 1], [5, 0], (a, b) => a > b); /* true */
let some2U: (t('a), t('b), [@bs] (('a, 'b) => bool)) => bool;

Uncurried version of some2.

let cmpByLength: (t('a), t('a)) => int;
cmpByLength(firstList, secondList);

Compare two lists solely by length. Returns -1 if length(firstList) is less than length(secondList), 0 if length(firstList) equals length(secondList), and 1 if length(firstList) is greater than length(secondList).

RE
Belt.List.cmpByLength([1, 2], [3, 4, 5, 6]); /* (-1) */ Belt.List.cmpByLength([1, 2, 3], [4, 5, 6]); /* = 0 */ Belt.List.cmpByLength([1, 2, 3, 4], [5, 6]); /* = 1 */
let cmp: (t('a), t('a), ('a, 'a) => int) => int;

Compare elements one by one compareFn(a, b). compareFn returns a negative number if a is "less than" b, zero if a is "equal to" b, a positive number if a is "greater than" b.

The comparison returns the first non-zero result of compareFn, or zero if compareFn returns zero for all a and b.

If all items have compared equal, but firstList is exhausted first, return -1. (firstList is shorter). If all items have compared equal, but secondList is exhausted first, return 1 (firstList is longer).

RE
Belt.List.cmp([3], [3, 7], (a, b) => compare(a, b)); /* (-1) */ Belt.List.cmp([5, 3], [5], (a, b) => compare(a, b)); /* 1 */ Belt.List.cmp([1, 3, 5], [1, 4, 2], (a, b) => compare(a, b)); /* (-1) */ Belt.List.cmp([1, 3, 5], [1, 2, 3], (a, b) => compare(a, b)); /* 1 */ Belt.List.cmp([1, 3, 5], [1, 3, 5], (a, b) => compare(a, b)); /* 0 */

Please note: The total ordering of List is different from Array, for Array, we compare the length first and, only if the lengths are equal, elements one by one. For lists, we just compare elements one by one.

let cmpU: (t('a), t('a), [@bs] (('a, 'a) => int)) => int;

Uncurried version of cmp.

let eq: (t('a), t('a), ('a, 'a) => bool) => bool;

Check equality of firstList and secondList using eqElem for equality on elements, where eqElem is a function that returns true if items x and y meet some criterion for equality, false otherwise. eq false if length of firstList and secondList are not the same.

RE
Belt.List.eq([1, 2, 3], [1, 2], (==)); /* false */ Belt.List.eq([1, 2], [1, 2], (==)); /* true */ Belt.List.eq([1, 2, 3], [(-1), (-2), (-3)], (a, b) => abs(a) == abs(b)); /* true */
let eqU: (t('a), t('a), [@bs] (('a, 'a) => bool)) => bool;

Uncurried version of eqU.

let has: (t('a), 'b, ('a, 'b) => bool) => bool;

Returns true if the list contains at least one element for which eqFunction(x) returns true.

RE
[1, 2, 3]->Belt.List.has(2, (==)); /* true */ [1, 2, 3]->Belt.List.has(4, (==)); /* false */ [(-1), (-2), (-3)]->Belt.List.has(2, (a, b) => abs(a) == abs(b)); /* true */
let hasU: (t('a), 'b, [@bs] (('a, 'b) => bool)) => bool;

Uncurried version of has.

let getBy: (t('a), 'a => bool) => option('a);

Returns Some(value) for the first value in someList that satisfies the predicate function pred. Returns None if no element satisfies the function.

RE
Belt.List.getBy([1, 4, 3, 2], x => x > 3); /* Some(4) */ Belt.List.getBy([1, 4, 3, 2], x => x > 4); /* None */
let getByU: (t('a), [@bs] ('a => bool)) => option('a);

Uncurried version of getBy.

let keep: (t('a), 'a => bool) => t('a);

Returns a list of all elements in someList which satisfy the predicate function pred.

RE
let isEven = x => x mod 2 == 0; Belt.List.keep([1, 2, 3, 4], isEven); /* [2, 4] */ Belt.List.keep([None, Some(2), Some(3), None], Belt.Option.isSome); /* [Some(2), Some(3)] */
let keepU: (t('a), [@bs] ('a => bool)) => t('a);

Uncurried version of keep.

let filter: (t('a), 'a => bool) => t('a);

Returns a list of all elements in someList which satisfy the predicate function pred.

RE
let isEven = x => x mod 2 == 0; Belt.List.filter([1, 2, 3, 4], isEven); /* [2, 4] */ Belt.List.filter([None, Some(2), Some(3), None], Belt.Option.isSome); /* [Some(2), Some(3)] */
let keepWithIndex: (t('a), ('a, int) => bool) => t('a);

Returns a list of all elements in someList which satisfy the predicate function pred.

RE
let isEven = x => x mod 2 == 0; Belt.List.keepWithIndex([1, 2, 3, 4], (_x, index) => isEven(index)); /* [1, 3] */
let keepWithIndexU: (t('a), [@bs] (('a, int) => bool)) => t('a);

Uncurried version of keepWithIndex.

let filterWithIndex: (t('a), ('a, int) => bool) => t('a);

Returns a list of all elements in someList which satisfy the predicate function pred.

RE
let isEven = x => x mod 2 == 0; Belt.List.filterWithIndex([1, 2, 3, 4], (_x, index) => isEven(index)); /* [1, 3] */
let keepMap: (t('a), 'a => option('b)) => t('b);

Applies f to each element of someList. If f(x) returns Some(value), then value is kept in the resulting list. If f(x) returns None, the element is not retained in the result.

RE
let isEven = x => x mod 2 == 0; [1, 2, 3, 4] ->Belt.List.keepMap(x => if (isEven(x)) { Some(x); } else { None; } ); /* [2, 4] */ [Some(1), Some(2), None]->Belt.List.keepMap(x => x); /* [1, 2] */
let keepMapU: (t('a), [@bs] ('a => option('b))) => t('b);

Uncurried version of keepMap.

let partition: (t('a), 'a => bool) => (t('a), t('a));

Creates a pair of lists; the first list consists of all elements of someList that satisfy the predicate function pred; the second list consists of all elements of someList that do not satisfy `pred.

In other words:

RE
([elementsThatSatisfies], [elementsThatDoesNotSatisfy]);
RE
Belt.List.partition([1, 2, 3, 4], x => x > 2); /* ([3, 4], [1, 2]) */
let partitionU: (t('a), [@bs] ('a => bool)) => (t('a), t('a));

Uncurried version of partition.

let unzip: t(('a, 'b)) => (t('a), t('b));

Takes a list of pairs and creates a pair of lists. The first list contains all the first items of the pairs; the second list contains all the second items.

RE
Belt.List.unzip([(1, 2), (3, 4)]); /* ([1, 3], [2, 4]) */ Belt.List.unzip([("H", "W"), ("e", "o"), ("l", "r"), ("l", "l"), ("o", "d"), (" ", "!")]); /* (["H", "e", "l", "l", "o", " "], ["W", "o", "r", "l", "d", "!"]); */
let getAssoc: (t(('a, 'c)), 'b, ('a, 'b) => bool) => option('c);

Return the second element of a pair in someList where the first element equals k as per the predicate function eqFunction, or None if not found.

RE
[(1, "a"), (2, "b"), (3, "c")]->Belt.List.getAssoc(3, (==)); /* Some("c") */ [(9, "morning"), (15, "afternoon"), (22, "night")] ->Belt.List.getAssoc(15, (k, item) => k /* 15 */ == item /* 9, 5, 22 */); /* Some("afternoon"); */
let getAssocU: (t(('a, 'c)), 'b, [@bs] (('a, 'b) => bool)) => option('c);

Uncurried version of getAssoc.

let hasAssoc: (t(('a, 'c)), 'b, ('a, 'b) => bool) => bool;

Returns true if there is a pair in someList where the first element equals k as per the predicate function eqFunction.

RE
[(1, "a"), (2, "b"), (3, "c")]->Belt.List.hasAssoc(1, (==)); /* true */ [(9, "morning"), (15, "afternoon"), (22, "night")] ->Belt.List.hasAssoc(25, (k, item) => k /* 25 */ == item /* 9, 5, 22 */); /* false */
let hasAssocU: (t(('a, 'c)), 'b, [@bs] (('a, 'b) => bool)) => bool;

Uncurried version of hasAssoc.

let removeAssoc: (t(('a, 'c)), 'b, ('a, 'b) => bool) => t(('a, 'c));

Return a list after removing the first pair whose first value is k per the equality predicate eqFunction; if not found, return a new list identical to someList.

RE
[(1, "a"), (2, "b"), (3, "c")]->Belt.List.removeAssoc(1, (==)); /* [(2, "b"), (3, "c")] */ [(9, "morning"), (15, "afternoon"), (22, "night")] ->Belt.List.removeAssoc(9, (k, item) => k /* 9 */ == item /* 9, 5, 22 */); /* [(15, "afternoon"), (22, "night")] */
let removeAssocU: (t(('a, 'c)), 'b, [@bs] (('a, 'b) => bool)) => t(('a, 'c));

Uncurried version of removeAssoc.

let setAssoc: (t(('a, 'c)), 'a, 'c, ('a, 'a) => bool) => t(('a, 'c));

If k exists in someList by satisfying the eqFunction predicate, return a new list with the key and value replaced by the new k and v; otherwise, return a new list with the pair k, v added to the head of someList.

RE
[(1, "a"), (2, "b"), (3, "c")]->Belt.List.setAssoc(2, "x", (==)); /* [(1, "a"), (2, "x"), (3, "c")]; */ [(1, "a"), (3, "c")]->Belt.List.setAssoc(2, "b", (==)); /* [(2, "b"), (1, "a"), (3, "c")] */ [(9, "morning"), (3, "morning?!"), (22, "night")] ->Belt.List.setAssoc(15, "afternoon", (a, b) => a mod 12 == b mod 12); /* [(9, "morning"), (15, "afternoon"), (22, "night")] */

Please note

In the last example, since: 15 mod 12 equals 3 mod 12

Both the key and the value are replaced in the list.

let setAssocU: (t(('a, 'c)), 'a, 'c, [@bs] (('a, 'a) => bool)) => t(('a, 'c));

Uncurried version of setAssoc.

let sort: (t('a), ('a, 'a) => int) => t('a);

Returns a sorted list.

RE
Belt.List.sort([5, 4, 9, 3, 7], (a, b) => a - b); /* [3, 4, 5, 7, 9] */
let sortU: (t('a), [@bs] (('a, 'a) => int)) => t('a);

Uncurried version of sort.