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Standard Library

Below is the API for the OCaml standard library. It's directly copied over from the OCaml Manual, formatted to the Reason syntax and styled accordingly. The API docs are work-in-progress; we'll be polishing these gradually!

If you're targeting JavaScript, the API docs for BuckleScript includes all of below, plus JS-specific APIs.

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Module ListLabels

module ListLabels: 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.

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 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 rev : 'a list -> 'a list
List reversal.
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
ListLabels.rev_append l1 l2 reverses l1 and concatenates it to l2. This is equivalent to ListLabels.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
ListLabels.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 ListLabels.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
ListLabels.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 ListLabels.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
ListLabels.rev_map f l gives the same result as ListLabels.rev (ListLabels.map f l), but is tail-recursive and more efficient.
val fold_left : f:('a -> 'b -> 'a) -> init:'a -> 'b list -> 'a
ListLabels.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
ListLabels.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
ListLabels.iter2 f [a1; ...; an] [b1; ...; bn] calls in turn f a1 b1; ...; f an bn. Raise Invalid_argument if the two lists have different lengths.
val map2 : f:('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list
ListLabels.map2 f [a1; ...; an] [b1; ...; bn] is [f a1 b1; ...; f an bn]. Raise Invalid_argument if the two lists have different lengths. Not tail-recursive.
val rev_map2 : f:('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list
ListLabels.rev_map2 f l1 l2 gives the same result as ListLabels.rev (ListLabels.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
ListLabels.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 have different lengths.
val fold_right2 : f:('a -> 'b -> 'c -> 'c) -> 'a list -> 'b list -> init:'c -> 'c
ListLabels.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 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 ListLabels.for_all, but for a two-argument predicate. Raise Invalid_argument if the two lists have different lengths.
val exists2 : f:('a -> 'b -> bool) -> 'a list -> 'b list -> bool
Same as ListLabels.exists, but for a two-argument predicate. Raise Invalid_argument if the two lists 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 ListLabels.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 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 ListLabels.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 assq : 'a -> ('a * 'b) list -> 'b
Same as ListLabels.assoc, but uses physical equality instead of structural equality to compare keys.
val mem_assoc : 'a -> map:('a * 'b) list -> bool
Same as ListLabels.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 ListLabels.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 ListLabels.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, Pervasives.compare is a suitable comparison function. The resulting list is sorted in increasing order. ListLabels.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 ListLabels.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 ListLabels.sort or ListLabels.stable_sort, whichever is faster on typical input.
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 containting 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).