[antlr-interest] Access to lexer warning/error messages after parsing

[Jim Idle]

On Tue, 2008-07-01 at 08:54 +0200, Raphael Reitzig wrote:

> Hi!
> 
> I second that for I am about to write something quite similar. System.err
> is no good in a user oriented GUI application.
> 
> I can think of two possibilties to integrate such behaviour in ANTLR:
> * grammar option like "warnMode", i.e. with values "console" and "collect".
> I'd like to have _one_ exceptions thrown if there ocurred any error while
> parsing.
> * possibility to set output stream for error messages via grammar option:
> @errors { System.err } (default)
> Implementation of either should be no obstacle (*guess*).


In the case of lexers, it is best to build a lexer that almost cannot
throw errors as once you lex incorrectly then there isn't much you can
do. Having rules in the lexer that catch known common mistakes and/or
catch any character that makes no sense in your lexer means that your
whole solution will be more robust. For most lexers,. just having:

BADCHAR: . {insert your error code};

As the last rule will improve things.

However, in the case of lexer, parser and tree parser it is trivial to
override the error output method and add your errors to collections/a
collection. As the standard error messages are usually of no use to a
real application (and they cannot be, there are too many things you
might wish to do on error), then you will almost certainly want to
implement your own error output anyway. Just add the message to a
collection. I do this with virtually every recognizer I write and it
takes less time than learning some new syntax and access methods for
ANTLR (which everyone will then complain about because they don't do
exactly what they had in mind. ;-)

So, the method that is called has all the information that you could
need, but YOU have to make it in to a collection, format it in a way
that makes sense for your application, and present the errors to your
users. There is no generic solution that would provide much more than a
different set of questions than there is right now. Sure, the errors
could all be collected as objects that you then iterate, but then there
is more code for people to rip out when they don't want that!

Come on guys the error messages are an afternoons coding that you can
probably reuse on related projects (if they are living in the same
environment.) I last did this in C# and if it took an hour to get it all
together I would be surprised. You only need to learn the ANTLR bit
once.

Jim
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