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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.

Module String

module String: sig .. end
String operations.

A string is an immutable data structure that contains a fixed-length sequence of (single-byte) characters. Each character can be accessed in constant time through its index.

Given a string s of length l, we can access each of the l characters of s via its index in the sequence. Indexes start at 0, and we will call an index valid in s if it falls within the range [0...l-1] (inclusive). A position is the point between two characters or at the beginning or end of the string. We call a position valid in s if it falls within the range [0...l] (inclusive). Note that the character at index n is between positions n and n+1.

Two parameters start and len are said to designate a valid substring of s if len >= 0 and start and start+len are valid positions in s.

OCaml strings used to be modifiable in place, for instance via the String.set and String.blit functions described below. This usage is deprecated and only possible when the compiler is put in "unsafe-string" mode by giving the -unsafe-string command-line option (which is currently the default for reasons of backward compatibility). This is done by making the types string and bytes (see module Bytes) interchangeable so that functions expecting byte sequences can also accept strings as arguments and modify them.

All new code should avoid this feature and be compiled with the -safe-string command-line option to enforce the separation between the types string and bytes.


let length: string => int;
Return the length (number of characters) of the given string.
let get: (string, int) => char;
String.get s n returns the character at index n in string s. You can also write s.[n] instead of String.get s n.

Raise Invalid_argument if n not a valid index in s.

let set: (bytes, int, char) => unit;
Deprecated.This is a deprecated alias of Bytes.set.
String.set s n c modifies byte sequence s in place, replacing the byte at index n with c. You can also write s.[n] <- c instead of String.set s n c.

Raise Invalid_argument if n is not a valid index in s.

let create: int => bytes;
Deprecated.This is a deprecated alias of Bytes.create.
String.create n returns a fresh byte sequence of length n. The sequence is uninitialized and contains arbitrary bytes.

Raise Invalid_argument if n < 0 or n > Sys.max_string_length.

let make: (int, char) => string;
String.make n c returns a fresh string of length n, filled with the character c.

Raise Invalid_argument if n < 0 or n > Sys.max_string_length.

let init: (int, int => char) => string;
String.init n f returns a string of length n, with character i initialized to the result of f i (called in increasing index order).

Raise Invalid_argument if n < 0 or n > Sys.max_string_length.
Since 4.02.0

let copy: string => string;
Deprecated.Because strings are immutable, it doesn't make much sense to make identical copies of them.
Return a copy of the given string.
let sub: (string, int, int) => string;
String.sub s start len returns a fresh string of length len, containing the substring of s that starts at position start and has length len.

Raise Invalid_argument if start and len do not designate a valid substring of s.

let fill: (bytes, int, int, char) => unit;
Deprecated.This is a deprecated alias of Bytes.fill.
String.fill s start len c modifies byte sequence s in place, replacing len bytes with c, starting at start.

Raise Invalid_argument if start and len do not designate a valid range of s.

let blit: (string, int, bytes, int, int) => unit;
let concat: (string, list(string)) => string;
String.concat sep sl concatenates the list of strings sl, inserting the separator string sep between each.

Raise Invalid_argument if the result is longer than Sys.max_string_length bytes.

let iter: (char => unit, string) => unit;
String.iter f s applies function f in turn to all the characters of s. It is equivalent to f s.[0]; f s.[1]; ...; f s.[String.length s - 1]; ().
let iteri: ((int, char) => unit, string) => unit;
Same as String.iter, but the function is applied to the index of the element as first argument (counting from 0), and the character itself as second argument.
Since 4.00.0
let map: (char => char, string) => string;
String.map f s applies function f in turn to all the characters of s (in increasing index order) and stores the results in a new string that is returned.
Since 4.00.0
let mapi: ((int, char) => char, string) => string;
String.mapi f s calls f with each character of s and its index (in increasing index order) and stores the results in a new string that is returned.
Since 4.02.0
let trim: string => string;
Return a copy of the argument, without leading and trailing whitespace. The characters regarded as whitespace are: ' ', '\012', '\n', '\r', and '\t'. If there is neither leading nor trailing whitespace character in the argument, return the original string itself, not a copy.
Since 4.00.0
let escaped: string => string;
Return a copy of the argument, with special characters represented by escape sequences, following the lexical conventions of OCaml. If there is no special character in the argument, return the original string itself, not a copy. Its inverse function is Scanf.unescaped.

Raise Invalid_argument if the result is longer than Sys.max_string_length bytes.

let index: (string, char) => int;
String.index s c returns the index of the first occurrence of character c in string s.

Raise Not_found if c does not occur in s.

let rindex: (string, char) => int;
String.rindex s c returns the index of the last occurrence of character c in string s.

Raise Not_found if c does not occur in s.

let index_from: (string, int, char) => int;
String.index_from s i c returns the index of the first occurrence of character c in string s after position i. String.index s c is equivalent to String.index_from s 0 c.

Raise Invalid_argument if i is not a valid position in s. Raise Not_found if c does not occur in s after position i.

let rindex_from: (string, int, char) => int;
String.rindex_from s i c returns the index of the last occurrence of character c in string s before position i+1. String.rindex s c is equivalent to String.rindex_from s (String.length s - 1) c.

Raise Invalid_argument if i+1 is not a valid position in s. Raise Not_found if c does not occur in s before position i+1.

let contains: (string, char) => bool;
String.contains s c tests if character c appears in the string s.
let contains_from: (string, int, char) => bool;
String.contains_from s start c tests if character c appears in s after position start. String.contains s c is equivalent to String.contains_from s 0 c.

Raise Invalid_argument if start is not a valid position in s.

let rcontains_from: (string, int, char) => bool;
String.rcontains_from s stop c tests if character c appears in s before position stop+1.

Raise Invalid_argument if stop < 0 or stop+1 is not a valid position in s.

let uppercase: string => string;
Return a copy of the argument, with all lowercase letters translated to uppercase, including accented letters of the ISO Latin-1 (8859-1) character set.
let lowercase: string => string;
Return a copy of the argument, with all uppercase letters translated to lowercase, including accented letters of the ISO Latin-1 (8859-1) character set.
let capitalize: string => string;
Return a copy of the argument, with the first character set to uppercase.
let uncapitalize: string => string;
Return a copy of the argument, with the first character set to lowercase.
type t = string;
An alias for the type of strings.
let compare: (t, t) => int;
The comparison function for strings, with the same specification as Pervasives.compare. Along with the type t, this function compare allows the module String to be passed as argument to the functors Set.Make and Map.Make.