Base.Bytes
OCaml's byte sequence type, semantically similar to a char array
, but taking less space in memory.
A byte sequence is a mutable data structure that contains a fixed-length sequence of bytes (of type char
). Each byte can be indexed in constant time for reading or writing.
include Sexplib0.Sexpable.S with type t := t
val t_of_sexp : Sexplib0.Sexp.t -> t
val sexp_of_t : t -> Sexplib0.Sexp.t
val t_sexp_grammar : t Sexplib0.Sexp_grammar.t
include Comparable.S with type t := t
include Comparisons.S with type t := t
compare t1 t2
returns 0 if t1
is equal to t2
, a negative integer if t1
is less than t2
, and a positive integer if t1
is greater than t2
.
ascending
is identical to compare
. descending x y = ascending y x
. These are intended to be mnemonic when used like List.sort ~compare:ascending
and List.sort
~cmp:descending
, since they cause the list to be sorted in ascending or descending order, respectively.
clamp_exn t ~min ~max
returns t'
, the closest value to t
such that between t' ~low:min ~high:max
is true.
Raises if not (min <= max)
.
val clamp : t -> min:t -> max:t -> t Or_error.t
include Comparator.S with type t := t
val comparator : (t, comparator_witness) Comparator.comparator
Note that pp
allocates in order to preserve the state of the byte sequence it was initially called with.
include Pretty_printer.S with type t := t
val pp : Formatter.t -> t -> unit
include Invariant.S with type t := t
val invariant : t -> unit
module To_string : sig ... end
module From_string : Blit.S_distinct with type src := string and type dst := t
val create : int -> t
create len
returns a newly-allocated and uninitialized byte sequence of length len
. No guarantees are made about the contents of the return value.
val create_local : int -> t
create_local
is like create
, but returns a stack-allocated Bytes.t
.
val make : int -> char -> t
make len c
returns a newly-allocated byte sequence of length len
filled with the byte c
.
map f t
applies function f
to every byte, in order, and builds the byte sequence with the results returned by f
.
Like map
, but passes each character's index to f
along with the char.
val init : int -> f:(int -> char) -> t
init len ~f
returns a newly-allocated byte sequence of length len
with index i
in the sequence being initialized with the result of f i
.
val of_char_list : char list -> t
of_char_list l
returns a newly-allocated byte sequence where each byte in the sequence corresponds to the byte in l
at the same index.
val length : t -> int
length t
returns the number of bytes in t
.
val get : t -> int -> char
get t i
returns the i
th byte of t
.
val unsafe_get : t -> int -> char
val set : t -> int -> char -> unit
set t i c
sets the i
th byte of t
to c
.
val unsafe_set : t -> int -> char -> unit
val unsafe_get_int64 : t -> int -> int64
val unsafe_set_int64 : t -> int -> int64 -> unit
val unsafe_get_int32 : t -> int -> int32
val unsafe_set_int32 : t -> int -> int32 -> unit
val unsafe_get_int16 : t -> int -> int
val unsafe_set_int16 : t -> int -> int -> unit
val fill : t -> pos:int -> len:int -> char -> unit
fill t ~pos ~len c
modifies t
in place, replacing all the bytes from pos
to pos + len
with c
.
val tr : target:char -> replacement:char -> t -> unit
tr ~target ~replacement t
modifies t
in place, replacing every instance of target
in s
with replacement
.
tr_multi ~target ~replacement
returns an in-place function that replaces every instance of a character in target
with the corresponding character in replacement
.
If replacement
is shorter than target
, it is lengthened by repeating its last character. Empty replacement
is illegal unless target
also is.
If target
contains multiple copies of the same character, the last corresponding replacement
character is used. Note that character ranges are not supported, so ~target:"a-z"
means the literal characters 'a'
, '-'
, and 'z'
.
val to_list : t -> char list
to_list t
returns the bytes in t
as a list of chars.
val to_array : t -> char array
to_array t
returns the bytes in t
as an array of chars.
val fold : t -> init:'acc -> f:('acc -> char -> 'acc) -> 'acc
fold a ~f ~init:b
is f a1 (f a2 (...))
val foldi : t -> init:'acc -> f:(int -> 'acc -> char -> 'acc) -> 'acc
foldi
works similarly to fold
, but also passes the index of each character to f
.
val contains : ?pos:int -> ?len:int -> t -> char -> bool
contains ?pos ?len t c
returns true
iff c
appears in t
between pos
and pos + len
.
Maximum length of a byte sequence, which is architecture-dependent. Attempting to create a Bytes
larger than this will raise an exception.
This section describes unsafe, low-level conversion functions between bytes
and string
. They might 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 : no_mutation_while_string_reachable:t -> 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 is linear: passing the data to another piece of code means giving up ownership (we cannot access 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 (the same mode of) ownership of the returned string. There are two valid use-cases that respect this ownership discipline:
The first is 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 ~no_mutation_while_string_reachable: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.
The second is 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 ~no_mutation_while_string_reachable: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.
val unsafe_of_string_promise_no_mutation : string -> t
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_promise_no_mutation
, however unique ownership of string values is extremely difficult to reason about correctly in practice. As such, one 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)
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.