[Caml-list] META file standards for ppx extensions

[Dario Teixeira <dario.teixeira@nleyten.com>]

Hi,

I think we need to standardise the naming and structure of META
files associated with ppx extensions.  Case in point: I was hoping
to start using sedlex instead of ulex, because the latter has been
deprecated for a while now and all the cool kids are switching to
the former.  However, sedlex currently ships with a broken META
file that severely restricts its usefulness [1].  Therefore, in
order to avoid similar problems with other ppx extensions and to
foster some uniformity in the ecosystem, here's what I suggest:

a) Suppose you have a ppx extension called 'foobar' that relies
    on a runtime library foobar.cma (and native code counterparts,
    which I'll omit from now on).  The top-level declarations in
    foobar's META should reference foobar.cma, but should *not*
    include a ppx declaration.  The ppx declaration should be
    part of a 'ppx' sub-package.

b) If the ppx extension 'foobar' does not have a runtime library,
    then the top-level declarations in the META file should consist
    only of generic version and description information.  The actual
    ppx declaration should be part of a 'ppx' sub-package, as in a).

There is obviously an alternative approach where the ppx invocations
are located at the top-level and any potential runtime libraries
are moved into a 'lib' sub-package.  I prefer the approach outlined
above, however, as it makes ppx invocations explicit and follows the
'syntax' sub-package tradition from Camlp4's glory days.

Thoughts or suggestions?

Best regards,
Dario Teixeira

[1] https://github.com/alainfrisch/sedlex/issues/22