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Module List

module List: sig .. end

List operations.

Some functions are flagged as not tail-recursive. A tail-recursive function uses constant stack space, while a non-tail-recursive function uses stack space proportional to the length of its list argument, which can be a problem with very long lists. When the function takes several list arguments, an approximate formula giving stack usage (in some unspecified constant unit) is shown in parentheses.

The above considerations can usually be ignored if your lists are not longer than about 10000 elements.

The labeled version of this module can be used as described in the StdLabels module.

type 'a t = 'a list =
| []
| (::) of 'a * 'a list

An alias for the type of lists.

let length: list('a) => int;

Return the length (number of elements) of the given list.

let compare_lengths: (list('a), list('b)) => int;

Compare the lengths of two lists. compare_lengths l1 l2 is equivalent to compare (length l1) (length l2), except that the computation stops after reaching the end of the shortest list.

let compare_length_with: (list('a), int) => int;

Compare the length of a list to an integer. compare_length_with l len is equivalent to compare (length l) len, except that the computation stops after at most len iterations on the list.

let cons: ('a, list('a)) => list('a);

cons x xs is x :: xs

let hd: list('a) => 'a;

Return the first element of the given list.

let tl: list('a) => list('a);

Return the given list without its first element.

let nth: (list('a), int) => 'a;

Return the n-th element of the given list. The first element (head of the list) is at position 0.

let nth_opt: (list('a), int) => option('a);

Return the n-th element of the given list. The first element (head of the list) is at position 0. Return None if the list is too short.

let rev: list('a) => list('a);

List reversal.

let init: (int, int => 'a) => list('a);

init len f is f 0; f 1; ...; f (len-1), evaluated left to right.

let append: (list('a), list('a)) => list('a);

Concatenate two lists. Same function as the infix operator @. Not tail-recursive (length of the first argument). The @ operator is not tail-recursive either.

let rev_append: (list('a), list('a)) => list('a);

rev_append l1 l2 reverses l1 and concatenates it with l2. This is equivalent to (List.rev l1) @ l2, but rev_append is tail-recursive and more efficient.

let concat: list(list('a)) => list('a);

Concatenate a list of lists. The elements of the argument are all concatenated together (in the same order) to give the result. Not tail-recursive (length of the argument + length of the longest sub-list).

let flatten: list(list('a)) => list('a);

Same as List.concat. Not tail-recursive (length of the argument + length of the longest sub-list).

Comparison

let equal: (('a, 'a) => bool, list('a), list('a)) => bool;

equal eq [a1; ...; an] [b1; ..; bm] holds when the two input lists have the same length, and for each pair of elements ai, bi at the same position we have eq ai bi.

Note: the eq function may be called even if the lists have different length. If you know your equality function is costly, you may want to check List.compare_lengths first.

let compare: (('a, 'a) => int, list('a), list('a)) => int;

compare cmp [a1; ...; an] [b1; ...; bm] performs a lexicographic comparison of the two input lists, using the same 'a -> 'a -> int interface as compare:

  • a1 :: l1 is smaller than a2 :: l2 (negative result) if a1 is smaller than a2, or if they are equal (0 result) and l1 is smaller than l2
  • the empty list [] is strictly smaller than non-empty lists

Note: the cmp function will be called even if the lists have different lengths.

Iterators

let iter: ('a => unit, list('a)) => unit;

iter f [a1; ...; an] applies function f in turn to a1; ...; an. It is equivalent to begin f a1; f a2; ...; f an; () end.

let iteri: ((int, 'a) => unit, list('a)) => unit;

Same as List.iter, but the function is applied to the index of the element as first argument (counting from 0), and the element itself as second argument.

let map: ('a => 'b, list('a)) => list('b);

map f [a1; ...; an] applies function f to a1, ..., an, and builds the list [f a1; ...; f an] with the results returned by f. Not tail-recursive.

let mapi: ((int, 'a) => 'b, list('a)) => list('b);

Same as List.map, but the function is applied to the index of the element as first argument (counting from 0), and the element itself as second argument. Not tail-recursive.

let rev_map: ('a => 'b, list('a)) => list('b);

rev_map f l gives the same result as List.rev (List.map f l), but is tail-recursive and more efficient.

let filter_map: ('a => option('b), list('a)) => list('b);

filter_map f l applies f to every element of l, filters out the None elements and returns the list of the arguments of the Some elements.

let concat_map: ('a => list('b), list('a)) => list('b);

concat_map f l gives the same result as List.concat (List.map f l). Tail-recursive.

let fold_left_map: (('a, 'b) => ('a, 'c), 'a, list('b)) => ('a, list('c));

fold_left_map is a combination of fold_left and map that threads an accumulator through calls to f.

let fold_left: (('a, 'b) => 'a, 'a, list('b)) => 'a;

fold_left f init [b1; ...; bn] is f (... (f (f init b1) b2) ...) bn.

let fold_right: (('a, 'b) => 'b, list('a), 'b) => 'b;

fold_right f [a1; ...; an] init is f a1 (f a2 (... (f an init) ...)). Not tail-recursive.

Iterators on two lists

let iter2: (('a, 'b) => unit, list('a), list('b)) => unit;

iter2 f [a1; ...; an] [b1; ...; bn] calls in turn f a1 b1; ...; f an bn.

let map2: (('a, 'b) => 'c, list('a), list('b)) => list('c);

map2 f [a1; ...; an] [b1; ...; bn] is [f a1 b1; ...; f an bn].

let rev_map2: (('a, 'b) => 'c, list('a), list('b)) => list('c);

rev_map2 f l1 l2 gives the same result as List.rev (List.map2 f l1 l2), but is tail-recursive and more efficient.

let fold_left2: (('a, 'b, 'c) => 'a, 'a, list('b), list('c)) => 'a;

fold_left2 f init [a1; ...; an] [b1; ...; bn] is f (... (f (f init a1 b1) a2 b2) ...) an bn.

let fold_right2: (('a, 'b, 'c) => 'c, list('a), list('b), 'c) => 'c;

fold_right2 f [a1; ...; an] [b1; ...; bn] init is f a1 b1 (f a2 b2 (... (f an bn init) ...)).

List scanning

let for_all: ('a => bool, list('a)) => bool;

for_all f [a1; ...; an] checks if all elements of the list satisfy the predicate f. That is, it returns (f a1) && (f a2) && ... && (f an) for a non-empty list and true if the list is empty.

let exists: ('a => bool, list('a)) => bool;

exists f [a1; ...; an] checks if at least one element of the list satisfies the predicate f. That is, it returns (f a1) || (f a2) || ... || (f an) for a non-empty list and false if the list is empty.

let for_all2: (('a, 'b) => bool, list('a), list('b)) => bool;

Same as List.for_all, but for a two-argument predicate.

let exists2: (('a, 'b) => bool, list('a), list('b)) => bool;

Same as List.exists, but for a two-argument predicate.

let mem: ('a, list('a)) => bool;

mem a set is true if and only if a is equal to an element of set.

let memq: ('a, list('a)) => bool;

Same as List.mem, but uses physical equality instead of structural equality to compare list elements.

List searching

let find: ('a => bool, list('a)) => 'a;

find f l returns the first element of the list l that satisfies the predicate f.

let find_opt: ('a => bool, list('a)) => option('a);

find f l returns the first element of the list l that satisfies the predicate f. Returns None if there is no value that satisfies f in the list l.

let find_map: ('a => option('b), list('a)) => option('b);

find_map f l applies f to the elements of l in order, and returns the first result of the form Some v, or None if none exist.

let filter: ('a => bool, list('a)) => list('a);

filter f l returns all the elements of the list l that satisfy the predicate f. The order of the elements in the input list is preserved.

let find_all: ('a => bool, list('a)) => list('a);

find_all is another name for List.filter.

let filteri: ((int, 'a) => bool, list('a)) => list('a);

Same as List.filter, but the predicate is applied to the index of the element as first argument (counting from 0), and the element itself as second argument.

let partition: ('a => bool, list('a)) => (list('a), list('a));

partition f l returns a pair of lists (l1, l2), where l1 is the list of all the elements of l that satisfy the predicate f, and l2 is the list of all the elements of l that do not satisfy f. The order of the elements in the input list is preserved.

let partition_map:
  ('a => Either.t('b, 'c), list('a)) => (list('b), list('c));

partition_map f l returns a pair of lists (l1, l2) such that, for each element x of the input list l:

  • if f x is Left y1, then y1 is in l1, and
  • if f x is Right y2, then y2 is in l2.

The output elements are included in l1 and l2 in the same relative order as the corresponding input elements in l.

In particular, partition_map (fun x -> if f x then Left x else Right x) l is equivalent to partition f l.

Association lists

let assoc: ('a, list(('a, 'b))) => 'b;

assoc a l returns the value associated with key a in the list of pairs l. That is, assoc a [ ...; (a,b); ...] = b if (a,b) is the leftmost binding of a in list l.

let assoc_opt: ('a, list(('a, 'b))) => option('b);

assoc_opt a l returns the value associated with key a in the list of pairs l. That is, assoc_opt a [ ...; (a,b); ...] = Some b if (a,b) is the leftmost binding of a in list l. Returns None if there is no value associated with a in the list l.

let assq: ('a, list(('a, 'b))) => 'b;

Same as List.assoc, but uses physical equality instead of structural equality to compare keys.

let assq_opt: ('a, list(('a, 'b))) => option('b);

Same as List.assoc_opt, but uses physical equality instead of structural equality to compare keys.

let mem_assoc: ('a, list(('a, 'b))) => bool;

Same as List.assoc, but simply return true if a binding exists, and false if no bindings exist for the given key.

let mem_assq: ('a, list(('a, 'b))) => bool;

Same as List.mem_assoc, but uses physical equality instead of structural equality to compare keys.

let remove_assoc: ('a, list(('a, 'b))) => list(('a, 'b));

remove_assoc a l returns the list of pairs l without the first pair with key a, if any. Not tail-recursive.

let remove_assq: ('a, list(('a, 'b))) => list(('a, 'b));

Same as List.remove_assoc, but uses physical equality instead of structural equality to compare keys. Not tail-recursive.

Lists of pairs

let split: list(('a, 'b)) => (list('a), list('b));

Transform a list of pairs into a pair of lists: split [(a1,b1); ...; (an,bn)] is ([a1; ...; an], [b1; ...; bn]). Not tail-recursive.

let combine: (list('a), list('b)) => list(('a, 'b));

Transform a pair of lists into a list of pairs: combine [a1; ...; an] [b1; ...; bn] is [(a1,b1); ...; (an,bn)].

Sorting

let sort: (('a, 'a) => int, list('a)) => list('a);

Sort a list in increasing order according to a comparison function. The comparison function must return 0 if its arguments compare as equal, a positive integer if the first is greater, and a negative integer if the first is smaller (see Array.sort for a complete specification). For example, compare is a suitable comparison function. The resulting list is sorted in increasing order. List.sort is guaranteed to run in constant heap space (in addition to the size of the result list) and logarithmic stack space.

The current implementation uses Merge Sort. It runs in constant heap space and logarithmic stack space.

let stable_sort: (('a, 'a) => int, list('a)) => list('a);

Same as List.sort, but the sorting algorithm is guaranteed to be stable (i.e. elements that compare equal are kept in their original order).

The current implementation uses Merge Sort. It runs in constant heap space and logarithmic stack space.

let fast_sort: (('a, 'a) => int, list('a)) => list('a);

Same as List.sort or List.stable_sort, whichever is faster on typical input.

let sort_uniq: (('a, 'a) => int, list('a)) => list('a);

Same as List.sort, but also remove duplicates.

let merge: (('a, 'a) => int, list('a), list('a)) => list('a);

Merge two lists: Assuming that l1 and l2 are sorted according to the comparison function cmp, merge cmp l1 l2 will return a sorted list containing all the elements of l1 and l2. If several elements compare equal, the elements of l1 will be before the elements of l2. Not tail-recursive (sum of the lengths of the arguments).

Iterators

let to_seq: list('a) => Seq.t('a);

Iterate on the list.

let of_seq: Seq.t('a) => list('a);

Create a list from the iterator.