Re: [Caml-list] syntax extensions with ocamlfind

[Maxence Guesdon <Maxence.Guesdon@inria.fr>]

On Tue, 03 Mar 2015 10:40:14 +0100
Gerd Stolpmann <info@gerd-stolpmann.de> wrote:

> Am Montag, den 02.03.2015, 19:24 +0100 schrieb Maxence Guesdon:
> > On Mon, 2 Mar 2015 18:07:40 +0000
> > Andreas Hauptmann <andreas@ml.ignorelist.com> wrote:
> > 
> > > On Mon, 2 Mar 2015 18:48:46 +0100
> > > Maxence Guesdon wrote:
> > > 
> > > > It seems that ocamlfind only supports camlp4 for preprocessors. Am I
> > > > right ? Does anybody known how to achieve this ?
> > > 
> > > Take a look at the META file of camlp5. If i remember right, it achieves
> > > similar without native support inside ocamlfind.
> > 
> > Indded, thanks. But it seems that ocamlfind can handle only one
> > preprocessor, instead of building a command chaining the preprocessors.
> 
> Chaining isn't that easy. Remember that preprocessors cannot only output
> source code, but also parsed ASTs. But ASTs are normally not understood
> as input by the next preprocessor in the chain.

This is because "legacy" preprocessors were quite advanced. One
constraint could be that preprocessor read and output text.

> Also, preprocessors usually exist to process non-standard syntax. In a
> chain pp1|pp2, however, pp1 will most likely not understand the
> extensions understood by pp2, and instead run into a parser error. That
> makes chaining a questionable concept.

Indeed, but in my case, my preprocessor is just a lexer, mapping
<:blabla< >> quotes to extension nodes [%blabla ]. So it can read any
other syntax and output it as it is in the original file. I make this
to replace a camlp4 extension. This simple preprocessor maps the
quotations to the ppx world.

> Note that chaining works with the new-style ppx preprocessors. This is
> possible because these preprocessors are restricted to the official
> syntax (which was extended to make this useful). The ppx chaining is
> directly implemented in the compiler.
>

And that's great :)

> > When I have more than one packages defining a preprocessor, I get:
> > 
> >   ocamlfind: Several packages are selected that specify preprocessors:
> >   package camlp4 defines `camlp4', package mypkg.syntax defines
> >   `./mypp'
> > 
> [...]
> > So the preprocessor machinery seems very camlp{4,5} oriented.
> 
> Yes, it is, at least regarding the style the preprocessor is invoked.
> 
> What is imaginable is that there is some additional preprocessor driver.
> Let's call it ocamlpp. It would do the chaining for those preprocessors
> that are compatible. If you call it like
> 
> ocamlpp (command | object.cmo) ...
> 
> it runs the preprocessors on the command line in turn, either by
> executing a command or loading the object. Such a driver would fit into
> the findlib framework.
> 
> But as said, I doubt that such a driver would be very useful, as
> chaining several preprocessors is normally not possible.

I agree this is a corner case. Thanks for your response.

- Maxence