ocamlyacc and error messages

ocamlyacc and error messages

[David Mentr'e]

Hello/Bonjour,

[[ Français (résumé) ]]

Comment donner le numéro de ligne et le point d'échec dans une grammaire
d'un parser généré par ocamlyacc ?

[[ English ]]

What is the simplest way to report an error in a parser generated by
ocamlyacc ?

I would like to report :

 1. line number in parsed file

 2. failing rule number in grammar 

The point (1) is essential for me. Right know, I have a 'Fatal error:
uncaught exception Parsing.Parse_error' not very meaningful. :) I would
prefer have a message like 'parse error on line N in source file,
stucked in grammar rule foo:'.

I've looked at FAQs, mailing-list archive, ocaml sources and ocamlyacc
documentation but did not found very useful info :

 . nothing in faqs
 . mailing-list : an old message related to caml (not *o*caml)
    http://caml.inria.fr/caml-list/0061.html
 . ocaml parser in too complicated (rich error handling)
 . ocamlyacc doc is minimal


Thanks for any pointer,
david

d.
-- 
 David.Mentre@irisa.fr -- http://www.irisa.fr/prive/dmentre/
 Opinions expressed here are only mine.

Re: ocamlyacc and error messages

[Jean-Christophe Filliatre]

> Comment donner le numéro de ligne et le point d'échec dans une grammaire
> d'un parser généré par ocamlyacc ?
> 
> What is the simplest way to report an error in a parser generated by
> ocamlyacc ?

The module Parsing of the ocaml standard library provides two functions 

======================================================================
val symbol_start : unit -> int
val symbol_end : unit -> int
======================================================================

to  get the  start and  end  positions of  the left-hand  side of  the
grammar rule that matched  (with a ocamlyacc grammar). Those positions
are characters  count from  the beginning of  the file. You  can store
those informations in your syntax  tree, together with the name of the
rule if you want.

Then, if you want to indicate  the corresponding line in the file when
an error occurs, one possible solution is to re-scan the file from the
beginning, counting  newline characters until the  desired position is
found. It  is the solution  used by the  ocaml parser. Have a  look in
ocaml sources (directory parsing/) for more informations.

Another solution is to count the  lines in your lexer, in a reference,
and to store that information in your syntax tree when building it.

-- 
Jean-Christophe FILLIATRE
  mailto:Jean-Christophe.Filliatre@lri.fr
  http://www.lri.fr/~filliatr