Bytes(3) OCaml library Bytes(3)
NAME
Bytes - Byte sequence operations.
Module
Module Bytes
Documentation
Module Bytes
: sig end
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] (inclu-
sive). 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 .
Since 4.02.0
val length : bytes -> int
Return the length (number of bytes) of the argument.
val get : bytes -> int -> char
get s n returns the byte at index n in argument s .
Raise Invalid_argument if n not a valid index in s .
val set : bytes -> int -> char -> unit
set s n c modifies s in place, replacing the byte at index n with c .
Raise Invalid_argument if n is not a valid index in s .
val create : int -> bytes
create n returns a new 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 .
val make : int -> char -> bytes
make n c returns a new byte sequence of length n , filled with the byte
c .
Raise Invalid_argument if n < 0 or n > Sys.max_string_length .
val init : int -> (int -> char) -> bytes
Bytes.init n f returns a fresh byte sequence of length n , with charac-
ter i initialized to the result of f i (in increasing index order).
Raise Invalid_argument if n < 0 or n > Sys.max_string_length .
val empty : bytes
A byte sequence of size 0.
val copy : bytes -> bytes
Return a new byte sequence that contains the same bytes as the argu-
ment.
val of_string : string -> bytes
Return a new byte sequence that contains the same bytes as the given
string.
val to_string : bytes -> string
Return a new string that contains the same bytes as the given byte
sequence.
val sub : bytes -> int -> int -> bytes
sub s start len returns a new byte sequence of length len , containing
the subsequence of s that starts at position start and has length len .
Raise Invalid_argument if start and len do not designate a valid range
of s .
val sub_string : bytes -> int -> int -> string
Same as sub but return a string instead of a byte sequence.
val 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 .
Raise Invalid_argument if the result length is negative or longer than
Sys.max_string_length bytes.
val fill : bytes -> int -> int -> char -> unit
fill s start len c modifies s in place, replacing len characters with c
, starting at start .
Raise Invalid_argument if start and len do not designate a valid range
of s .
val blit : bytes -> int -> bytes -> int -> int -> unit
blit src srcoff dst dstoff len copies len bytes from sequence src ,
starting at index srcoff , to sequence dst , starting at index dstoff .
It works correctly even if src and dst are the same byte sequence, and
the source and destination intervals overlap.
Raise Invalid_argument if srcoff and len do not designate a valid range
of src , or if dstoff and len do not designate a valid range of dst .
val blit_string : string -> int -> bytes -> int -> int -> unit
blit src srcoff dst dstoff len copies len bytes from string src ,
starting at index srcoff , to byte sequence dst , starting at index
dstoff .
Raise Invalid_argument if srcoff and len do not designate a valid range
of src , or if dstoff and len do not designate a valid range of dst .
val concat : bytes -> bytes list -> 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.
Raise Invalid_argument if the result is longer than
Sys.max_string_length bytes.
val cat : bytes -> bytes -> bytes
cat s1 s2 concatenates s1 and s2 and returns the result as new byte
sequence.
Raise Invalid_argument if the result is longer than
Sys.max_string_length bytes.
val 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));
() .
val iteri : (int -> char -> unit) -> bytes -> unit
Same as Bytes.iter , but the function is applied to the index of the
byte as first argument and the byte itself as second argument.
val 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.
val 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.
val 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' .
val escaped : bytes -> bytes
Return a copy of the argument, with special characters represented by
escape sequences, following the lexical conventions of OCaml.
Raise Invalid_argument if the result is longer than
Sys.max_string_length bytes.
val index : bytes -> char -> int
index s c returns the index of the first occurrence of byte c in s .
Raise Not_found if c does not occur in s .
val rindex : bytes -> char -> int
rindex s c returns the index of the last occurrence of byte c in s .
Raise Not_found if c does not occur in s .
val 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 . Bytes.index s c is equivalent to Bytes.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 .
val 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
(Bytes.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 .
val contains : bytes -> char -> bool
contains s c tests if byte c appears in s .
val 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 .
Raise Invalid_argument if start is not a valid position in s .
val rcontains_from : bytes -> int -> char -> bool
rcontains_from s stop c tests if byte c appears in s before position
stop+1 .
Raise Invalid_argument if stop < 0 or stop+1 is not a valid position in
s .
val 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) char-
acter set.
val 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) char-
acter set.
val capitalize : bytes -> bytes
Return a copy of the argument, with the first byte set to uppercase.
val uncapitalize : bytes -> bytes
Return a copy of the argument, with the first byte set to lowercase.
type t = bytes
An alias for the type of byte sequences.
val compare : t -> t -> int
The comparison function for byte sequences, with the same specification
as Pervasives.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 .
=== 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 Bytes.to_string and Bytes.of_string
instead. ===
val 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 con-
sider 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 result-
ing 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 argu-
ment -- it could escape by a side-channel such as a memoization combi-
nator.
The caller may not mutate s while the string is borrowed (it has tempo-
rarily 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.
val 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 exam-
ple, mutating unsafe_of_string ("foo" ^ s) could mutate the shared
string foo -- assuming a rope-like representation of strings. More gen-
erally, 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 pre-
viously used the string type for this purpose.
OCamldoc 2014-10-18 Bytes(3)
ocaml 4.02.1 - Generated Sun Oct 19 06:45:33 CDT 2014