package regular

to_string b - unsafely converts a byte sequence into a string.

To reason about the use of 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.

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 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 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 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.

of_string s - unsafely converts a shared string to a byte sequence that should not be mutated.

The same ownership discipline that makes to_string correct applies to 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 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 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.