Re: [Caml-list] Compiling with camlp4 extensions

[Gabriel Scherer <gabriel.scherer@gmail.com>]

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