Module `Caml.ListLabels`

type 'a t = 'a list = | ([]) | (::) of 'a * 'a list: An alias for the type of lists.

val length : 'a list -> int: Return the length (number of elements) of the given list.

val hd : 'a list -> 'a: Return the first element of the given list. Raise Failure "hd" if the list is empty.

val compare_lengths : 'a list -> 'b list -> 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 itering on the shortest list.

since: 4.05.0

val compare_length_with : 'a list -> len:int -> int

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

since: 4.05.0

val cons : 'a -> 'a list -> 'a list

cons x xs is x :: xs

since: 4.05.0

val tl : 'a list -> 'a list: Return the given list without its first element. Raise Failure "tl" if the list is empty.

val nth : 'a list -> int -> 'a: Return the n-th element of the given list. The first element (head of the list) is at position 0. Raise Failure "nth" if the list is too short. Raise Invalid_argument "List.nth" if n is negative.

val nth_opt : 'a list -> int -> 'a option

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. Raise Invalid_argument "List.nth" if n is negative.

since: 4.05

val rev : 'a list -> 'a list: List reversal.

val init : len:int -> f:(int -> 'a) -> 'a list

List.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

val append : 'a list -> 'a list -> 'a list: Catenate two lists. Same function as the infix operator @. Not tail-recursive (length of the first argument). The @ operator is not tail-recursive either.

val rev_append : 'a list -> 'a list -> 'a list: List.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.

val concat : 'a list list -> 'a list: 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).

val flatten : 'a list list -> 'a list: Same as concat. Not tail-recursive (length of the argument + length of the longest sub-list).

Iterators

val iter : f:('a -> unit) -> 'a list -> unit: List.iter f [a1; ...; an] applies function f in turn to a1; ...; an. It is equivalent to begin f a1; f a2; ...; f an; () end.

val iteri : f:(int -> 'a -> unit) -> 'a list -> 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.

since: 4.00.0

val map : f:('a -> 'b) -> 'a list -> 'b list: List.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.

val mapi : f:(int -> 'a -> 'b) -> 'a list -> 'b list

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.

since: 4.00.0

val rev_map : f:('a -> 'b) -> 'a list -> 'b list: List.rev_map f l gives the same result as List.rev (List.map f l), but is tail-recursive and more efficient.

val filter_map : f:('a -> 'b option) -> 'a list -> 'b list

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

val fold_left : f:('a -> 'b -> 'a) -> init:'a -> 'b list -> 'a: List.fold_left f a [b1; ...; bn] is f (... (f (f a b1) b2) ...) bn.

val fold_right : f:('a -> 'b -> 'b) -> 'a list -> init:'b -> 'b: List.fold_right f [a1; ...; an] b is f a1 (f a2 (... (f an b) ...)). Not tail-recursive.

Iterators on two lists

val iter2 : f:('a -> 'b -> unit) -> 'a list -> 'b list -> unit: List.iter2 f [a1; ...; an] [b1; ...; bn] calls in turn f a1 b1; ...; f an bn. Raise Invalid_argument if the two lists are determined to have different lengths.

val map2 : f:('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list: List.map2 f [a1; ...; an] [b1; ...; bn] is [f a1 b1; ...; f an bn]. Raise Invalid_argument if the two lists are determined to have different lengths. Not tail-recursive.

val rev_map2 : f:('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list: List.rev_map2 f l1 l2 gives the same result as List.rev (List.map2 f l1 l2), but is tail-recursive and more efficient.

val fold_left2 : f:('a -> 'b -> 'c -> 'a) -> init:'a -> 'b list -> 'c list -> 'a: List.fold_left2 f a [b1; ...; bn] [c1; ...; cn] is f (... (f (f a b1 c1) b2 c2) ...) bn cn. Raise Invalid_argument if the two lists are determined to have different lengths.

val fold_right2 : f:('a -> 'b -> 'c -> 'c) -> 'a list -> 'b list -> init:'c -> 'c: List.fold_right2 f [a1; ...; an] [b1; ...; bn] c is f a1 b1 (f a2 b2 (... (f an bn c) ...)). Raise Invalid_argument if the two lists are determined to have different lengths. Not tail-recursive.

List scanning

val for_all : f:('a -> bool) -> 'a list -> bool: for_all p [a1; ...; an] checks if all elements of the list satisfy the predicate p. That is, it returns (p a1) && (p a2) && ... && (p an).

val exists : f:('a -> bool) -> 'a list -> bool: exists p [a1; ...; an] checks if at least one element of the list satisfies the predicate p. That is, it returns (p a1) || (p a2) || ... || (p an).

val for_all2 : f:('a -> 'b -> bool) -> 'a list -> 'b list -> bool: Same as List.for_all, but for a two-argument predicate. Raise Invalid_argument if the two lists are determined to have different lengths.

val exists2 : f:('a -> 'b -> bool) -> 'a list -> 'b list -> bool: Same as List.exists, but for a two-argument predicate. Raise Invalid_argument if the two lists are determined to have different lengths.

val mem : 'a -> set:'a list -> bool: mem a l is true if and only if a is equal to an element of l.

val memq : 'a -> set:'a list -> bool: Same as List.mem, but uses physical equality instead of structural equality to compare list elements.

List searching

val find : f:('a -> bool) -> 'a list -> 'a: find p l returns the first element of the list l that satisfies the predicate p. Raise Not_found if there is no value that satisfies p in the list l.

val find_opt : f:('a -> bool) -> 'a list -> 'a option

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

since: 4.05

val filter : f:('a -> bool) -> 'a list -> 'a list: filter p l returns all the elements of the list l that satisfy the predicate p. The order of the elements in the input list is preserved.

val find_all : f:('a -> bool) -> 'a list -> 'a list: find_all is another name for List.filter.

val partition : f:('a -> bool) -> 'a list -> 'a list * 'a list: partition p l returns a pair of lists (l1, l2), where l1 is the list of all the elements of l that satisfy the predicate p, and l2 is the list of all the elements of l that do not satisfy p. The order of the elements in the input list is preserved.

Association lists

val assoc : 'a -> ('a * 'b) list -> '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. Raise Not_found if there is no value associated with a in the list l.

val assoc_opt : 'a -> ('a * 'b) list -> 'b option

assoc_opt 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. Returns None if there is no value associated with a in the list l.

since: 4.05

val assq : 'a -> ('a * 'b) list -> 'b: Same as List.assoc, but uses physical equality instead of structural equality to compare keys.

val assq_opt : 'a -> ('a * 'b) list -> 'b option

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

since: 4.05.0

val mem_assoc : 'a -> map:('a * 'b) list -> bool: Same as List.assoc, but simply return true if a binding exists, and false if no bindings exist for the given key.

val mem_assq : 'a -> map:('a * 'b) list -> bool: Same as List.mem_assoc, but uses physical equality instead of structural equality to compare keys.

val remove_assoc : 'a -> ('a * 'b) list -> ('a * 'b) list: remove_assoc a l returns the list of pairs l without the first pair with key a, if any. Not tail-recursive.

val remove_assq : 'a -> ('a * 'b) list -> ('a * 'b) list: Same as List.remove_assoc, but uses physical equality instead of structural equality to compare keys. Not tail-recursive.

Lists of pairs

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

val combine : 'a list -> 'b list -> ('a * 'b) list: Transform a pair of lists into a list of pairs: combine [a1; ...; an] [b1; ...; bn] is [(a1,b1); ...; (an,bn)]. Raise Invalid_argument if the two lists have different lengths. Not tail-recursive.

Sorting

val sort : cmp:('a -> 'a -> int) -> 'a list -> 'a list

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. 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.

val stable_sort : cmp:('a -> 'a -> int) -> 'a list -> 'a list

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.

val fast_sort : cmp:('a -> 'a -> int) -> 'a list -> 'a list: Same as List.sort or List.stable_sort, whichever is faster on typical input.

val sort_uniq : cmp:('a -> 'a -> int) -> 'a list -> 'a list

Same as List.sort, but also remove duplicates.

since: 4.03.0

val merge : cmp:('a -> 'a -> int) -> 'a list -> 'a list -> 'a list: 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

val to_seq : 'a list -> 'a Seq.t

Iterate on the list

since: 4.07

val of_seq : 'a Seq.t -> 'a list

Create a list from the iterator

since: 4.07