[Caml-list] proposal for finding, loading and composing ppx preprocessors

[Benjamin Canou <benjamin.canou@gmail.com>]

   Dear OCaml'ers,

To accompany the arrival of annotations in the forthcoming release, we 
propose to discuss and hopefully agree on a standard way of selecting 
the adequate preprocessor(s) for a given file. Gerd already introduced a 
mechanism in the last version of OCamlFind, that we'd like to extend as 
follows.

Our idea is to indicate via a pragma inside the file, how it has to be 
processed. You can see that as a machine readable way of writing (* this 
file has to be preprocessed with blah using options blah *). For this, 
we propose that build systems use a default preprocessor systematically, 
that would interpret these pragmas and find and call its fellow 
preprocessors accordingly.

The compiler already provides built-in annotations. In a way, it's a 
second predefined layer, this time for the build system.

We designed and prototyped such a tool [1], which understand annotations 
of the form [@@@ syntax "package"], to preprocess the following code 
using the ppx declared in the corresponding OCamlFind package.

The tool also defines a way to register plug-ins instead of executables 
so preprocessors can be dynlinked instead of called as processes.

Extensions could be:
  - delimiting a fragment of the code to process
  - add a way to associate a given annotation identifier to a 
preprocessor, so that only the mother preprocessor would do the tree 
traversal, specific preprocessors only seeing the nodes they are 
actually interested in.
  - other kind of pragmas for the build system ?

Again, the goal is to agree now on a standard way and avoid the mess we 
had with camlp4. What do you think ?

   Benjamin, Grégoire and Pierre @ OCamlPro.

[1] https://github.com/hnrgrgr/ppx_loader