Module Stdlib.Bytes

Byte sequence operations.

A byte sequence is a mutable data structure that contains a fixed-length sequence of bytes. Each byte can be indexed in constant time for reading or writing.

Given a byte sequence s of length l, we can access each of the l bytes 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 bytes or at the beginning or end of the sequence. We call a position valid in s if it falls within the range [0...l] (inclusive). Note that the byte at index n is between positions n and n+1.

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

Byte sequences can be modified in place, for instance via the set and blit functions described below. See also strings (module String), which are almost the same data structure, but cannot be modified in place.

Bytes are represented by the OCaml type char.

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

let length: bytes => int;

Return the length (number of bytes) of the argument.

let get: bytes => int => char;

get s n returns the byte at index n in argument s.

  • raises Invalid_argument

    if n is not a valid index in s.

let set: bytes => int => char => unit;

set s n c modifies s in place, replacing the byte at index n with c.

  • raises Invalid_argument

    if n is not a valid index in s.

let create: int => bytes;

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

let make: int => char => bytes;

make n c returns a new byte sequence of length n, filled with the byte c.

let init: int => (int => char) => bytes;

init n f returns a fresh byte sequence of length n, with character i initialized to the result of f i (in increasing index order).

let empty: bytes;

A byte sequence of size 0.

let copy: bytes => bytes;

Return a new byte sequence that contains the same bytes as the argument.

let of_string: string => bytes;

Return a new byte sequence that contains the same bytes as the given string.

let to_string: bytes => string;

Return a new string that contains the same bytes as the given byte sequence.

let sub: bytes => int => int => bytes;

sub s pos len returns a new byte sequence of length len, containing the subsequence of s that starts at position pos and has length len.

  • raises Invalid_argument

    if pos and len do not designate a valid range of s.

let sub_string: bytes => int => int => string;

Same as sub but return a string instead of a byte sequence.

let extend: bytes => int => int => bytes;

extend s left right returns a new byte sequence that contains the bytes of s, with left uninitialized bytes prepended and right uninitialized bytes appended to it. If left or right is negative, then bytes are removed (instead of appended) from the corresponding side of s.

  • since 4.05.0 in BytesLabels
let fill: bytes => int => int => char => unit;

fill s pos len c modifies s in place, replacing len characters with c, starting at pos.

  • raises Invalid_argument

    if pos and len do not designate a valid range of s.

let blit: bytes => int => bytes => int => int => unit;

blit src src_pos dst dst_pos len copies len bytes from sequence src, starting at index src_pos, to sequence dst, starting at index dst_pos. It works correctly even if src and dst are the same byte sequence, and the source and destination intervals overlap.

  • raises Invalid_argument

    if src_pos and len do not designate a valid range of src, or if dst_pos and len do not designate a valid range of dst.

let blit_string: string => int => bytes => int => int => unit;

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

  • raises Invalid_argument

    if src_pos and len do not designate a valid range of src, or if dst_pos and len do not designate a valid range of dst.

  • since 4.05.0 in BytesLabels
let concat: bytes => list(bytes) => bytes;

concat sep sl concatenates the list of byte sequences sl, inserting the separator byte sequence sep between each, and returns the result as a new byte sequence.

let cat: bytes => bytes => bytes;

cat s1 s2 concatenates s1 and s2 and returns the result as a new byte sequence.

  • since 4.05.0 in BytesLabels
let iter: (char => unit) => bytes => unit;

iter f s applies function f in turn to all the bytes of s. It is equivalent to f (get s 0); f (get s 1); ...; f (get s (length s - 1)); ().

let iteri: (int => char => unit) => bytes => unit;

Same as iter, but the function is applied to the index of the byte as first argument and the byte itself as second argument.

let map: (char => char) => bytes => bytes;

map f s applies function f in turn to all the bytes of s (in increasing index order) and stores the resulting bytes in a new sequence that is returned as the result.

let mapi: (int => char => char) => bytes => bytes;

mapi f s calls f with each character of s and its index (in increasing index order) and stores the resulting bytes in a new sequence that is returned as the result.

let fold_left: ('a => char => 'a) => 'a => bytes => 'a;

fold_left f x s computes f (... (f (f x (get s 0)) (get s 1)) ...) (get s (n-1)), where n is the length of s.

  • since 4.13.0
let fold_right: (char => 'a => 'a) => bytes => 'a => 'a;

fold_right f s x computes f (get s 0) (f (get s 1) ( ... (f (get s (n-1)) x) ...)), where n is the length of s.

  • since 4.13.0
let for_all: (char => bool) => bytes => bool;

for_all p s checks if all characters in s satisfy the predicate p.

  • since 4.13.0
let exists: (char => bool) => bytes => bool;

exists p s checks if at least one character of s satisfies the predicate p.

  • since 4.13.0
let trim: bytes => bytes;

Return a copy of the argument, without leading and trailing whitespace. The bytes regarded as whitespace are the ASCII characters ' ', '\012', '\n', '\r', and '\t'.

let escaped: bytes => bytes;

Return a copy of the argument, with special characters represented by escape sequences, following the lexical conventions of OCaml. All characters outside the ASCII printable range (32..126) are escaped, as well as backslash and double-quote.

let index: bytes => char => int;

index s c returns the index of the first occurrence of byte c in s.

  • raises Not_found

    if c does not occur in s.

let index_opt: bytes => char => option(int);

index_opt s c returns the index of the first occurrence of byte c in s or None if c does not occur in s.

  • since 4.05
let rindex: bytes => char => int;

rindex s c returns the index of the last occurrence of byte c in s.

  • raises Not_found

    if c does not occur in s.

let rindex_opt: bytes => char => option(int);

rindex_opt s c returns the index of the last occurrence of byte c in s or None if c does not occur in s.

  • since 4.05
let index_from: bytes => int => char => int;

index_from s i c returns the index of the first occurrence of byte c in s after position i. index s c is equivalent to index_from s 0 c.

  • raises Invalid_argument

    if i is not a valid position in s.

  • raises Not_found

    if c does not occur in s after position i.

let index_from_opt: bytes => int => char => option(int);

index_from_opt s i c returns the index of the first occurrence of byte c in s after position i or None if c does not occur in s after position i. index_opt s c is equivalent to index_from_opt s 0 c.

  • raises Invalid_argument

    if i is not a valid position in s.

  • since 4.05
let rindex_from: bytes => int => char => int;

rindex_from s i c returns the index of the last occurrence of byte c in s before position i+1. rindex s c is equivalent to rindex_from s (length s - 1) c.

  • raises Invalid_argument

    if i+1 is not a valid position in s.

  • raises Not_found

    if c does not occur in s before position i+1.

let rindex_from_opt: bytes => int => char => option(int);

rindex_from_opt s i c returns the index of the last occurrence of byte c in s before position i+1 or None if c does not occur in s before position i+1. rindex_opt s c is equivalent to rindex_from s (length s - 1) c.

  • raises Invalid_argument

    if i+1 is not a valid position in s.

  • since 4.05
let contains: bytes => char => bool;

contains s c tests if byte c appears in s.

let contains_from: bytes => int => char => bool;

contains_from s start c tests if byte c appears in s after position start. contains s c is equivalent to contains_from s 0 c.

  • raises Invalid_argument

    if start is not a valid position in s.

let rcontains_from: bytes => int => char => bool;

rcontains_from s stop c tests if byte c appears in s before position stop+1.

  • raises Invalid_argument

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

let uppercase: bytes => bytes;

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.

  • deprecated

    Functions operating on Latin-1 character set are deprecated.

let lowercase: bytes => bytes;

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.

  • deprecated

    Functions operating on Latin-1 character set are deprecated.

let capitalize: bytes => bytes;

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

  • deprecated

    Functions operating on Latin-1 character set are deprecated.

let uncapitalize: bytes => bytes;

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

  • deprecated

    Functions operating on Latin-1 character set are deprecated.

let uppercase_ascii: bytes => bytes;

Return a copy of the argument, with all lowercase letters translated to uppercase, using the US-ASCII character set.

  • since 4.03.0 (4.05.0 in BytesLabels)
let lowercase_ascii: bytes => bytes;

Return a copy of the argument, with all uppercase letters translated to lowercase, using the US-ASCII character set.

  • since 4.03.0 (4.05.0 in BytesLabels)
let capitalize_ascii: bytes => bytes;

Return a copy of the argument, with the first character set to uppercase, using the US-ASCII character set.

  • since 4.03.0 (4.05.0 in BytesLabels)
let uncapitalize_ascii: bytes => bytes;

Return a copy of the argument, with the first character set to lowercase, using the US-ASCII character set.

  • since 4.03.0 (4.05.0 in BytesLabels)
type t = bytes;

An alias for the type of byte sequences.

let compare: t => t => int;

The comparison function for byte sequences, with the same specification as Stdlib.compare. Along with the type t, this function compare allows the module Bytes to be passed as argument to the functors Set.Make and Map.Make.

let equal: t => t => bool;

The equality function for byte sequences.

  • since 4.03.0 (4.05.0 in BytesLabels)
let starts_with: prefix:bytes => bytes => bool;

starts_with ~prefix s is true if and only if s starts with prefix.

  • since 4.13.0
let ends_with: suffix:bytes => bytes => bool;

ends_with suffix s is true if and only if s ends with suffix.

  • since 4.13.0

Unsafe conversions (for advanced users)

This section describes unsafe, low-level conversion functions between bytes and string. They do not copy the internal data; used improperly, they can break the immutability invariant on strings provided by the -safe-string option. They are available for expert library authors, but for most purposes you should use the always-correct to_string and of_string instead.

let unsafe_to_string: bytes => string;

Unsafely convert a byte sequence into a string.

To reason about the use of unsafe_to_string, it is convenient to consider an "ownership" discipline. A piece of code that manipulates some data "owns" it; there are several disjoint ownership modes, including:

  • Unique ownership: the data may be accessed and mutated
  • Shared ownership: the data has several owners, that may only access it, not mutate it.

Unique ownership is linear: passing the data to another piece of code means giving up ownership (we cannot write the data again). A unique owner may decide to make the data shared (giving up mutation rights on it), but shared data may not become uniquely-owned again.

unsafe_to_string s can only be used when the caller owns the byte sequence s -- either uniquely or as shared immutable data. The caller gives up ownership of s, and gains ownership of the returned string.

There are two valid use-cases that respect this ownership discipline:

1. Creating a string by initializing and mutating a byte sequence that is never changed after initialization is performed.

let string_init len f : string =
  let s = Bytes.create len in
  for i = 0 to len - 1 do Bytes.set s i (f i) done;
  Bytes.unsafe_to_string s

This function is safe because the byte sequence s will never be accessed or mutated after unsafe_to_string is called. The string_init code gives up ownership of s, and returns the ownership of the resulting string to its caller.

Note that it would be unsafe if s was passed as an additional parameter to the function f as it could escape this way and be mutated in the future -- string_init would give up ownership of s to pass it to f, and could not call unsafe_to_string safely.

We have provided the String.init, String.map and String.mapi functions to cover most cases of building new strings. You should prefer those over to_string or unsafe_to_string whenever applicable.

2. Temporarily giving ownership of a byte sequence to a function that expects a uniquely owned string and returns ownership back, so that we can mutate the sequence again after the call ended.

let bytes_length (s : bytes) =
  String.length (Bytes.unsafe_to_string s)

In this use-case, we do not promise that s will never be mutated after the call to bytes_length s. The String.length function temporarily borrows unique ownership of the byte sequence (and sees it as a string), but returns this ownership back to the caller, which may assume that s is still a valid byte sequence after the call. Note that this is only correct because we know that String.length does not capture its argument -- it could escape by a side-channel such as a memoization combinator.

The caller may not mutate s while the string is borrowed (it has temporarily given up ownership). This affects concurrent programs, but also higher-order functions: if String.length returned a closure to be called later, s should not be mutated until this closure is fully applied and returns ownership.

let unsafe_of_string: string => bytes;

Unsafely convert a shared string to a byte sequence that should not be mutated.

The same ownership discipline that makes unsafe_to_string correct applies to unsafe_of_string: you may use it if you were the owner of the string value, and you will own the return bytes in the same mode.

In practice, unique ownership of string values is extremely difficult to reason about correctly. You should always assume strings are shared, never uniquely owned.

For example, string literals are implicitly shared by the compiler, so you never uniquely own them.

let incorrect = Bytes.unsafe_of_string "hello"
let s = Bytes.of_string "hello"

The first declaration is incorrect, because the string literal "hello" could be shared by the compiler with other parts of the program, and mutating incorrect is a bug. You must always use the second version, which performs a copy and is thus correct.

Assuming unique ownership of strings that are not string literals, but are (partly) built from string literals, is also incorrect. For example, mutating unsafe_of_string ("foo" ^ s) could mutate the shared string "foo" -- assuming a rope-like representation of strings. More generally, functions operating on strings will assume shared ownership, they do not preserve unique ownership. It is thus incorrect to assume unique ownership of the result of unsafe_of_string.

The only case we have reasonable confidence is safe is if the produced bytes is shared -- used as an immutable byte sequence. This is possibly useful for incremental migration of low-level programs that manipulate immutable sequences of bytes (for example Marshal.from_bytes) and previously used the string type for this purpose.

let split_on_char: char => bytes => list(bytes);

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

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

  • The list is not empty.
  • Concatenating its elements using sep as a separator returns a byte sequence equal to the input (Bytes.concat (Bytes.make 1 sep) (Bytes.split_on_char sep s) = s).
  • No byte sequence in the result contains the sep character.
  • since 4.13.0

Iterators

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

Iterate on the string, in increasing index order. Modifications of the string during iteration will be reflected in the sequence.

  • since 4.07
let to_seqi: t => Seq.t((int, char));

Iterate on the string, in increasing order, yielding indices along chars

  • since 4.07
let of_seq: Seq.t(char) => t;

Create a string from the generator

  • since 4.07