[antlr-interest] simple grammar question

[Michael Roberts]

yep, that fixed it.  Everything else worked fine.

Thanks for the clear explanation - now I understand antlr fragments :)

MR

On Fri, Mar 2, 2012 at 6:55 PM, John B. Brodie <jbb at acm.org> wrote:

>  Greetings!
>
> On 03/02/2012 09:37 PM, Michael Roberts wrote:
>
> Hi folks.
>
> I'm in the process of learning antlr (I just ordered Terrance's books), but
> there something I am missing here.  The below compiles fine using
> AntlrWorks, but when I feed it:
>
> foo/aa/bb
>
> I get a a MissingTokenException (see attached bitmap), after it's matched
> an identifier and a /.
>
>
> I do not use ANTLRWorks so this assessment may be bogus, but...
>
> Do you see that the "MissingTokenException" node is a child of
> "identifier"?
>
> I believe that ANTLRWorks is trying to tell you  that "identifier" was not
> found.
>
>
> In any case. Regardless of how to analyze the bitmap.
>
> Because your IdentifierPart and IdentifierStart Lexer rules are fragments,
> your Parser will never see them (that is the intent of fragment lexer
> rules; e.g. define a portion, fragment, of a token but keep it hidden from
> the parser). So your identifier rule, as written, is useless; e.g. will
> never recognize anything.
>
> I would simply (with out any deep analysis of your example) suggest that
> you turn your `identifier` Parser rule into a Lexer rule by capitalizing it
> (identifier ==> Identifier).
>
>
> As an aside, I would avoid the backtrack=true; option until and unless you
> **really** understand what that will do to your parser's performance (but
> that is, perhaps, just my own personal bias ;-).
>
> Hope this helps...
>    -jbb
>
>  grammar JLG2;
>
> options {
>    backtrack=true;
>    memoize=true;
> }
>
> @header {
> package org.veve.reflect.interpreter.output;
> }
>
> @lexer::header {
> package org.veve.reflect.interpreter.output;
> }
>
> compilationUnit : relativePath;
>
> identifier : IdentifierStart IdentifierPart* ;
> relativePath : identifier (SLASH identifier)* ;
>
> SLASH : '/';
>
> fragment IdentifierPart
>     :
>     'a'..'z' | 'A'..'Z' | '_' | '0'..'9'
>     ;
>
> fragment IdentifierStart
>     :
>     'a'..'z' | 'A'..'Z' | '_'
>     ;
>
> My expectation there was that identifier would match an alphanumeric
> string, and that the rule for relativePath would let me string such
> identifiers together with / in between.  However, I'm clearly missing
> something obvious.  Thanks in advance.
>
> MR
>
>
>
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