Module Stdlib.List

An alias for the type of lists.

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

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.

• since 4.05.0

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.

• since 4.05.0

cons x xs is x :: xs

• since 4.03.0 (4.05.0 in ListLabels)

Return the first element of the given list.

• raises Failure

  if the list is empty.

Return the given list without its first element.

• raises Failure

  if the list is empty.

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

• raises Failure

  if the list is too short.

• raises Invalid_argument

  if n is negative.

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.

• raises Invalid_argument

  if n is negative.

• since 4.05

List reversal.

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

• raises Invalid_argument

  if len < 0.

• since 4.06.0

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

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

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).

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

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 compare_lengths first.

• since 4.12.0

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

• since 4.12.0

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

Same as 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.

• since 4.00.0

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.

Same as 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.

• since 4.00.0

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

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.

• since 4.08.0

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

• since 4.10.0

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

• since 4.11.0

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

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

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

• raises Invalid_argument

  if the two lists are determined to have different lengths.

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

• raises Invalid_argument

  if the two lists are determined to have different lengths. Not tail-recursive.

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

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

• raises Invalid_argument

  if the two lists are determined to have different lengths.

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

• raises Invalid_argument

  if the two lists are determined to have different lengths. Not tail-recursive.

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.

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.

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

• raises Invalid_argument

  if the two lists are determined to have different lengths.

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

• raises Invalid_argument

  if the two lists are determined to have different lengths.

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

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

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

• raises Not_found

  if there is no value that satisfies f in the list l.

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.

• since 4.05

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.

• since 4.10.0

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.

find_all is another name for filter.

Same as 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.

• since 4.11.0

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.

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.

• since 4.12.0

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.

• raises Not_found

  if there is no value associated with a in the list l.

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.

• since 4.05

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

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

• since 4.05.0

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

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

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

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

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

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

• raises Invalid_argument

  if the two lists have different lengths. Not tail-recursive.

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, Stdlib.compare is a suitable comparison function. The resulting list is sorted in increasing order. 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.

Same as 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.

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

Same as sort, but also remove duplicates.

• since 4.02.0 (4.03.0 in ListLabels)

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).

Iterate on the list.

• since 4.07

Create a list from a sequence.

• since 4.07