[antlr-interest] NoSuchElementException

[Justin Murray]

Mike,

Thanks again for your input. I tried both of these suggestions, but
still no luck. With the following grammar:

----
grammar AerobasicPreprocessor;

// Parser rules
preprocess
	:	line* LAST_SOURCE_LINE_? EOF
	;

line
	:	PP_directive_
	|	SOURCE_LINE_
	;

// Lexer rules

PP_directive_
	:	'#define'
	;
            
fragment NEWLINE_
	:	'\u000D'? '\u000A'
	|	'\u0085'
	|	'\u2028'
	|	'\u2029'
	;
	
SOURCE_LINE_
	:	.* NEWLINE_
	;
	
LAST_SOURCE_LINE_
	:	.+
	;
----

I get the error:
error(201): AerobasicPreprocessor.g:31:4: The following alternatives can
never be matched: 1

Line 31 here corresponds to LAST_SOURCE_LINE_. I tried both '.*' and
'.+' here. Neither worked, but I think '.+' makes more sense (a lexer
rule shouldn't match nothing). Any other ideas?

Thanks,
- Justin

-----Original Message-----
From: Mike Lischke [mailto:mike at lischke-online.de] 
Sent: Thursday, September 06, 2012 11:18 AM
To: Justin Murray
Cc: antlr-interest at antlr.org
Subject: Re: [antlr-interest] NoSuchElementException


Justin,

> grammar Test;
> 
> // Parser rules
> preprocess
> 	:	line* EOF
> 	;
> 
> line
> 	:	PP_directive_
> 	|	SOURCE_LINE_
> 	;
> 
> // Lexer rules
> 
> PP_directive_
> 	:	'#define'
> 	;
> 
> NEWLINE_
> 	:	'\u000D'? '\u000A'
> 	|	'\u0085'
> 	|	'\u2028'
> 	|	'\u2029'
> 	;
> 	
> SOURCE_LINE_
> 	:	.* (EOF | NEWLINE_)
> 	;
> 
> ----
> 
> This one does not crash, but does give me the following error:
> 
> error(201): AerobasicPreprocessor.g:27:4: The following alternatives 
> can never be matched: 1
> 
> Line 27 corresponds to the SOURCE_LINE_ rule. This error doesn't 
> really make any sense to me.

Yes, it's sometimes difficult to actually find the source of the
error/warning, because a rule can make that trouble only in a higher
level rule, leading to ambiguities. In your case I think the EOF from
the preprocess rule is redundant and can be removed (leaving EOF in
SOURCE_LINE_).

> If I remove the EOF from the SOURCE_LINE_ rule, the grammar builds 
> successfully. However, this doesn't give me what I need, which is the 
> possibility of a line at the end of a file, without a newline. Any 
> other ideas?


If all fails define a lexer rule like:

REST: .*;

and use this so:

preprocess
	:	line* REST
	;

Mike
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