ocaml-base-compiler
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package ocaml-base-compiler
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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.

type 'a t = 'a list =
  1. | []
  2. | :: 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 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 -> 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 4.03.0
val hd : 'a list -> 'a

Return the first element of the given list.

  • raises Failure

    if the list is empty.

val tl : 'a list -> 'a list

Return the given list without its first element.

  • raises Failure

    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.

  • raises Failure

    if the list is too short.

  • raises Invalid_argument

    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.

  • raises Invalid_argument

    if n is negative.

  • since 4.05
val rev : 'a list -> 'a list

List reversal.

val init : int -> (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

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

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

List.rev_append l1 l2 reverses l1 and concatenates it to 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

An alias for concat.

Iterators

val iter : ('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 : (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 : ('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 : (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. Not tail-recursive.

  • since 4.00.0
val rev_map : ('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 : ('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 concat_map : ('a -> 'b list) -> 'a list -> 'b list

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

  • since 4.10.0
val fold_left_map : ('a -> 'b -> 'a * 'c) -> 'a -> 'b list -> 'a * 'c list

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

  • since 4.11.0
val fold_left : ('a -> 'b -> 'a) -> 'a -> 'b list -> 'a

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

val fold_right : ('a -> 'b -> 'b) -> 'a list -> '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 : ('a -> 'b -> unit) -> 'a list -> 'b list -> unit

List.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.

val map2 : ('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list

List.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.

val rev_map2 : ('a -> 'b -> 'c) -> 'a list ->