Module String

module String: sig .. end

Strings.

A string s of length n is an indexable and immutable sequence of n bytes. For historical reasons these bytes are referred to as characters.

The semantics of string functions is defined in terms of indices and positions. These are depicted and described as follows.

/* positions  0   1   2   3   4    n-1    n
 *            +---+---+---+---+     +-----+
 *   indices  | 0 | 1 | 2 | 3 | ... | n-1 |
 *            +---+---+---+---+     +-----+ */
  • An index i of s is an integer in the range [0;n-1]. It represents the ith byte (character) of s which can be accessed using the constant time string indexing operator s.[i].
  • A position i of s is an integer in the range [0;n]. It represents either the point at the beginning of the string, or the point between two indices, or the point at the end of the string. The ith byte index is between position i and i+1.

Two integers start and len are said to define a valid substring of s if len >= 0 and start, start+len are positions of s.

Unicode text. Strings being arbitrary sequences of bytes, they can hold any kind of textual encoding. However the recommended encoding for storing Unicode text in OCaml strings is UTF-8. This is the encoding used by Unicode escapes in string literals. For example the string "\u{1F42B}" is the UTF-8 encoding of the Unicode character U+1F42B.

Past mutability. OCaml strings used to be modifiable in place, for instance via the String.set and String.blit functions. This use is nowadays only possible when the compiler is put in "unsafe-string" mode by giving the -unsafe-string command-line option. This compatibility mode makes the types string and bytes (see Bytes.t) interchangeable so that functions expecting byte sequences can also accept strings as arguments and modify them.

The distinction between bytes and string was introduced in OCaml 4.02, and the "unsafe-string" compatibility mode was the default until OCaml 4.05. Starting with 4.06, the compatibility mode is opt-in; we intend to remove the option in the future.

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


Strings

type t = string;

The type for strings.

let make: (int, char) => string;

make n c is a string of length n with each index holding the character c.

let init: (int, int => char) => string;

init n f is a string of length n with index i holding the character f i (called in increasing index order).

let length: string => int;

length s is the length (number of bytes/characters) of s.

let get: (string, int) => char;

get s i is the character at index i in s. This is the same as writing s.[i].

Concatenating

Note. The (^) binary operator concatenates two strings.

let concat: (string, list(string)) => string;

concat sep ss concatenates the list of strings ss, inserting the separator string sep between each.

Predicates and comparisons

let equal: (t, t) => bool;

equal s0 s1 is true if and only if s0 and s1 are character-wise equal.

let compare: (t, t) => int;

compare s0 s1 sorts s0 and s1 in lexicographical order. compare behaves like compare on strings but may be more efficient.

let contains_from: (string, int, char) => bool;

contains_from s start c is true if and only if c appears in s after position start.

let rcontains_from: (string, int, char) => bool;

rcontains_from s stop c is true if and only if c appears in s before position stop+1.

let contains: (string, char) => bool;

contains s c is String.contains_from s 0 c.

Extracting substrings

let sub: (string, int, int) => string;

sub s pos len is a string of length len, containing the substring of s that starts at position pos and has length len.

let split_on_char: (char, string) => list(string);

split_on_char sep s is the list of all (possibly empty) substrings of s that are delimited by the character sep.

The function's result is specified by the following invariants:

  • The list is not empty.
  • Concatenating its elements using sep as a separator returns a string equal to the input (concat (make 1 sep) (split_on_char sep s) = s).
  • No string in the result contains the sep character.

Transforming

let map: (char => char, string) => string;

map f s is the string resulting from applying f to all the characters of s in increasing order.

let mapi: ((int, char) => char, string) => string;

mapi f s is like String.map but the index of the character is also passed to f.

let trim: string => string;

trim s is s without leading and trailing whitespace. Whitespace characters are: ' ', '\x0C' (form feed), '\n', '\r', and '\t'.

let escaped: string => string;

escaped s is s with special characters represented by escape sequences, following the lexical conventions of OCaml.

All characters outside the US-ASCII printable range [0x20;0x7E] are escaped, as well as backslash (0x2F) and double-quote (0x22).

The function Scanf.unescaped is a left inverse of escaped, i.e. Scanf.unescaped (escaped s) = s for any string s (unless escaped s fails).

let uppercase_ascii: string => string;

uppercase_ascii s is s with all lowercase letters translated to uppercase, using the US-ASCII character set.

let lowercase_ascii: string => string;

lowercase_ascii s is s with all uppercase letters translated to lowercase, using the US-ASCII character set.

let capitalize_ascii: string => string;

capitalize_ascii s is s with the first character set to uppercase, using the US-ASCII character set.

let uncapitalize_ascii: string => string;

uncapitalize_ascii s is s with the first character set to lowercase, using the US-ASCII character set.

Traversing

let iter: (char => unit, string) => unit;

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.[length s - 1]; ().

let iteri: ((int, char) => unit, string) => unit;

iteri is like String.iter, but the function is also given the corresponding character index.

Searching

let index_from: (string, int, char) => int;

index_from s i c is the index of the first occurrence of c in s after position i.

let index_from_opt: (string, int, char) => option(int);

index_from_opt s i c is the index of the first occurrence of c in s after position i (if any).

let rindex_from: (string, int, char) => int;

rindex_from s i c is the index of the last occurrence of c in s before position i+1.

let rindex_from_opt: (string, int, char) => option(int);

rindex_from_opt s i c is the index of the last occurrence of c in s before position i+1 (if any).

let index: (string, char) => int;

index s c is String.index_from s 0 c.

let index_opt: (string, char) => option(int);

index_opt s c is String.index_from_opt s 0 c.

let rindex: (string, char) => int;

rindex s c is String.rindex_from s (length s - 1) c.

let rindex_opt: (string, char) => option(int);

rindex_opt s c is String.rindex_from_opt s (length s - 1) c.

Converting

let to_seq: t => Seq.t(char);

to_seq s is a sequence made of the string's characters in increasing order. In "unsafe-string" mode, modifications of the string during iteration will be reflected in the iterator.

let to_seqi: t => Seq.t((int, char));

to_seqi s is like String.to_seq but also tuples the corresponding index.

let of_seq: Seq.t(char) => t;

of_seq s is a string made of the sequence's characters.

Deprecated functions

let create: int => bytes;
Deprecated.This is a deprecated alias of Bytes.create/BytesLabels.create.

create n returns a fresh byte sequence of length n. The sequence is uninitialized and contains arbitrary bytes.

let set: (bytes, int, char) => unit;
Deprecated.This is a deprecated alias of Bytes.set/BytesLabels.set.

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 set s n c.

let blit: (string, int, bytes, int, int) => unit;

blit src src_pos dst dst_pos len copies len bytes from the string src, starting at index src_pos, to byte sequence dst, starting at character number dst_pos.

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 fill: (bytes, int, int, char) => unit;
Deprecated.This is a deprecated alias of Bytes.fill/BytesLabels.fill.

fill s pos len c modifies byte sequence s in place, replacing len bytes by c, starting at pos.

let uppercase: string => string;
Deprecated.Functions operating on Latin-1 character set are deprecated.

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;
Deprecated.Functions operating on Latin-1 character set are deprecated.

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;
Deprecated.Functions operating on Latin-1 character set are deprecated.

Return a copy of the argument, with the first character set to uppercase, using the ISO Latin-1 (8859-1) character set..

let uncapitalize: string => string;
Deprecated.Functions operating on Latin-1 character set are deprecated.

Return a copy of the argument, with the first character set to lowercase, using the ISO Latin-1 (8859-1) character set.