package 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.

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

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 reaching the end of 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 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
val cons : 'a -> 'a list -> 'a list

cons x xs is x :: xs

  • since 4.03.0 (4.05.0 in ListLabels)
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

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 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

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.

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).

Comparison

val equal : ('a -> 'a -> bool) -> 'a list -> 'a list -> 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 compare_lengths first.

  • since 4.12.0
val compare : ('a -> 'a -> int) -> 'a list -> 'a list -> int

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

Iterators

val iter : ('a -> unit) -> 'a list -> 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.

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

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
val map : ('a -> 'b) -> 'a list -> 'b 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 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

rev_map f l gives the same result as rev (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

concat_map f l gives the same result as concat (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

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

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

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

Iterators on two lists

val iter2 : ('a -> 'b -> unit) -> 'a list -> 'b list -> unit

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

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 -> 'b list -> 'c list

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

val fold_left2 : ('a -> 'b -> 'c -> 'a) -> 'a -> 'b list -> 'c list -> 'a

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.

val fold_right2 : ('a -> 'b -> 'c -> 'c) -> 'a list -> 'b list -> 'c -> 'c

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.

List scanning

val for_all : ('a -> bool) -> 'a list -> 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.

val exists : ('a -> bool) -> 'a list -> 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.

val for_all2 : ('a -> 'b -> bool) -> 'a list -> 'b list -> bool

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

  • raises Invalid_argument

    if the two lists are determined to have different lengths.

val exists2 : ('a -> 'b -> bool) -> 'a list -> 'b list -> bool

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

  • raises Invalid_argument

    if the two lists are determined to have different lengths.

val mem : 'a -> 'a list -> bool

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

val memq : 'a -> 'a list -> bool

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

List searching

val find : ('a -> bool) -> 'a list -> 'a

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.

val find_opt : ('a