Experiment

This ppx explores one design direction to improve the Unicode support provided by the OCaml compiler. See the rationale.

ppx_utf8_lit depends on Uutf and Uunf

Note, ppx_utf8_lit is not formally released, it is an experiment.

# With opam 1.2 or later
opam pin add ppx_utf8_lit http://erratique.ch/repos/ppx_utf8_lit.git

The ppx understands the following attributes on string literals and patterns:

• "Révolte"[@u] checks for UTF-8 validity and puts the string in NFC.
• "Révolte"[@u.nfc] checks for UTF-8 validity and puts the string in NFC.
• "Révolte"[@u.nfd] checks for UTF-8 validity and puts the string in NFD.
• "Révolte"[@u.nfkd] checks for UTF-8 validity and puts the string in NFKD.
• "Révolte"[@u.nfkc] checks for UTF-8 validity and puts the string in NFKC.

To compile a source with the ppx:

ocamfind ocamlc -package ppx_utf8_lit src.ml

Rather than introducing a new Unicode string datastructure and associated new literal and pattern notations ppx_utf8_lit tries to improve the situation for libraries and programs that adopted the idea of interpreting current OCaml strings (which are fundamentally sequences of bytes) as UTF-8 encoded text. It does so by using the attribute mechanism introduced in OCaml 4.02.

The advantage of interpreting current OCaml strings as UTF-8 encoded text are:

1. No new notation or primitive type is introduced. This means that the surface syntax of the language doesn't change and that other subsystems remain untouched (e.g. format specifiers). Besides nothing needs to change in the compiler itself except at the parsing phase.

2. It plays exceptionally well with the current OCaml system as it exists (for example with Printf, Format and IO primitives).

3. Since latin1 identifiers in source code have been deprecated in OCaml 4.01, if a source is only using US-ASCII identifiers it can be UTF-8 encoded which allows to directly write UTF-8 string literals and patterns.

However there are two problems with these UTF-8 literals and patterns:

1. The compiler sees them as sequences of bytes, hence they cannot be trusted as being valid UTF-8 in your program (e.g. if your editor has bugs in its UTF-8 encoder). The only way to make sure the encoding will be correct is to escape the UTF-8 encoding which is not particularly readable (e.g. "R\xC3\xA9volte" vs "Révolte") .

2. You don't get any guarantee on the Unicode normal form (if any) in which the literals and patterns occur. They are subject to what your editor decided to choose. Which is problematic for testing equality (see here for a quick recall on why Unicode normalization is essential for testing equality). This means that you have to convert to a normal form manually and explicitely escape the UTF-8 which is neither convenient nor readable. (e.g. "Révolte" in NFD would be "Re\xCC\x81volte")

In order to alleviate this, we introduce 5 annotations on string literals and patterns. Any string sporting such an annotation will be checked for UTF-8 validity with compilation failing if that is not the case. Besides each of the annotation will guarantee the string is converted to one of the four Unicode normal form.

• "Révolte"[@u] checks for UTF-8 validity and puts the string in NFC.
• "Révolte"[@u.nfc] checks for UTF-8 validity and puts the string in NFC.
• "Révolte"[@u.nfd] checks for UTF-8 validity and puts the string in NFD.
• "Révolte"[@u.nfkd] checks for UTF-8 validity and puts the string in NFKD.
• "Révolte"[@u.nfkc] checks for UTF-8 validity and puts the string in NFKC.

The reason for using NFC for the [@u] notation is that this is the normalization recommended by the w3c for the web. I have no strong opinion about that though (thought about a filename friendly normalization form but according to this there's no cross-platform consensus – and it some sense it should be the task of the FS APIs to normalize whatever we feed them with).

This means that now, if I you make sure that the strings you input are in a given normal form (using e.g. Uutf and Uunf) you can safely pattern match on them. For example:

let is_fr_revolt s = match s (* assuming [s] is in NFC form *) with
| "Révolte"[@u] -> true
| _ -> false

let () =
  assert (is_fr_revolt ("Révolte"[@u]));
  assert (not (is_fr_revolt ("Révolte"[@u.nfd]));
  ()