Re: [Caml-list] Compiling with camlp4 extensions

[Aaron Bohannon <bohannon@seas.upenn.edu>]

Thanks for the reply.  The example is helpful.  However, I should have
been more clear: I don't exactly want to write a syntax extension, per
se.  Rather, I am trying to use camlp4 to parse a non-OCaml grammar
and to generate an OCaml AST.  So the "Register.OCamlSyntaxExtension"
functor doesn't seem like it will work for me.  Instead, I tried using
"Printers.Ocaml.print_implem" in my "extension" code and everything
works fine, except for error locations.  Of course, I realize this is
because the AST is being printed and then re-parsed, but I don't know
how to prevent it from being reparsed.  I looked through all the
Camlp4 interfaces and thought that perhaps I need to use the function
"Register.register_str_item_parser".  But I couldn't make that work.
Either that's not the function I need or else I don't know how to use
it -- I can't tell which.

 - Aaron

On Fri, Jun 22, 2012 at 10:36 AM, Gabriel Scherer
<gabriel.scherer@gmail.com> wrote:
> All nodes in a Camlp4 AST are annotated with location information; the
> locations you get from the parser are correct, and it is your
> responsibility, as an extension writer, to ensure that any new nodes
> you generate also have (approximately) correct location information.
>
> If you build AST nodes "by hand", you have to provide this location
> explicitly. If you use the concrete syntax quotations, the location
> used is the value _loc present in the environment, whatever it may be.
> So to have correct locations, you have to make sure that, at every AST
> you produce through a quotation, there is a "_loc" variable in scope
> with the correct value. If you match AST pieces with quotation
> patterns (match e with <:expr< $a$ + $b$ >> -> ...), you may bind the
> location variable through the syntax "<:expr@foo<", for example:
> (match e with <:expr@_loc< $a$ + $b$ >> -> ...). Finally, if you're
> inside an EXTEND block defining a parsing rule, the idenfitier _loc is
> implicitely bound to a location corresponding to what was parsed by
> this rule.
>
> See for example the toy extension pa_refutable, that has example of
> those various things:
>  http://bluestorm.info/camlp4/pa_refutable.ml.html
>
> In some very rare cases (or if you are perfectionist), you may want to
> give to a new node a location that is not quite the location of any of
> the parsed node you're working on. You may use various functions of
> the Loc submodule of your syntax definition to forge new locations; in
> particular, Loc.merge merges two (supposed contiguous) locations.
>  http://bluestorm.info/camlp4/camlp4-doc/Sig.Loc.html
>
>
>
>
>
> On Fri, Jun 22, 2012 at 5:53 PM, Aaron Bohannon <bohannon@seas.upenn.edu> wrote:
>> Hi,
>>
>> I have been trying to use the new camlp4 to write an OCaml syntax
>> extension.  All the examples I have seen so far suggest that I use the
>> extension by passing ocamlc the "-pp" option.  But it seems that all the
>> location info for error messages gets lost when I do this unless I catch and
>> report the parse error myself within the extension.  Is there some way to
>> get ocamlc to report the parse error at the correct location automatically?
>>
>> - Aaron