[antlr-interest] Problem with semantic predicates

[Thomas Woelfle]

Hi Loring,

thanks for the tip. I'll try it with version 3.1

Regards,
Thomas
> I would guess that you are using 3.0.1; somehow a critical line was commented out in that version that allows decisions with multiple sempreds (hoisted or otherwise).  Try using one of the 3.1 betas.
> --Loring
>
>
>
> ----- Original Message ----
>   
>> From: Thomas Woelfle <thomas.woelfle at interactive-objects.com>
>> To: antlr-interest at antlr.org
>> Sent: Monday, July 28, 2008 2:18:28 AM
>> Subject: [antlr-interest] Problem with semantic predicates
>>
>> Hi,
>>
>> I have to write a grammar for an existing language that allows keywords 
>> to be used as variables. I've been able to write most of the grammar 
>> using semantic predicates. But now I've been running into a problem 
>> where I don't know whether I'm using semantic predicates the wrong way 
>> or whether it is a bug in ANTLR. I have reduced the problem to the 
>> following grammar:
>>
>>
>> grammar foo;
>>
>> options
>> {
>>     ASTLabelType = CommonTree;
>>     output = AST;
>>     language = Java;
>> }
>>
>> master  
>>     :    foo
>>     |    bar
>>     |    blup
>>     ;
>>   
>> foo    :    {input.LT(1).getText().equals("FOO")}? IDENTIFIER LBRACE 
>> IDENTIFIER RBRACE
>>     ;
>>   
>> bar    :    {input.LT(1).getText().equals("BAR")}? IDENTIFIER (LBRACE 
>> IDENTIFIER RBRACE)?
>>     ;
>>   
>> blup    :    {input.LT(1).getText().equals("BLUP")}? IDENTIFIER
>>     ;
>>
>> LBRACE    :    '(';
>> RBRACE    :    ')';
>>   
>> IDENTIFIER
>>     :    'A'..'Z'*;
>>
>> The rules 'foo' and 'bar' are almost identical. The only differences are 
>> the semantic predicates and the fact that in the rule 'bar' the braced 
>> identifier is optional.
>> Evaluating a string "BAR(HI)" using this grammar starting with the rule 
>> 'master' leads to an error since the rule 'foo' is chosen instead of 
>> rule 'bar'. Having a look at the generated JAVA code shows following logic:
>>
>>
>>     // $ANTLR start master
>>     // C:\\Temp\\foo.g:10:1: master : ( foo | bar | blup );
>>     public final master_return master() throws RecognitionException {
>>         master_return retval = new master_return();
>>         retval.start = input.LT(1);
>>
>>         CommonTree root_0 = null;
>>
>>         foo_return foo1 = null;
>>
>>         bar_return bar2 = null;
>>
>>         blup_return blup3 = null;
>>
>>         try {
>>             // C:\\Temp\\foo.g:11:5: ( foo | bar | blup )
>>             int alt1=3;
>>             int LA1_0 = input.LA(1);
>>
>>             if ( (LA1_0==IDENTIFIER) ) {
>>                 int LA1_1 = input.LA(2);
>>
>>                 if ( (LA1_1==LBRACE) ) {
>>                     alt1=1;
>>                 }
>>                 else if ( (input.LT(1).getText().equals("BAR")) ) {
>>                     alt1=2;
>>                 }
>>                 else if ( (input.LT(1).getText().equals("BLUP")) ) {
>>                     alt1=3;
>>                 }
>>                 else {
>>                     NoViableAltException nvae =
>>                         new NoViableAltException("10:1: master : ( foo | 
>> bar | blup );", 1, 1, input);
>>
>>                     throw nvae;
>>                 }
>>             }
>>           ......
>>
>>
>> The first "if" checks whether the current token is an identifier. This 
>> is ok. But the next "if" statement checks whether the next token is a 
>> left brace. If true it decides for the sub rule "foo" without evaluating 
>> the semantic predicates. Is this a bug in ANTLR or am I using semantic 
>> predicates the wrong way?
>>
>> Any help appreciated.
>>
>> Regards,
>> Thomas
>>
>> -- 
>> Interactive Objects Software GmbH
>> Basler Strasse 61
>> 79100 Freiburg, Germany
>>
>> Phone:  +49 761 400 73 0
>> mailto:thomas.woelfle at interactive-objects.com
>>
>>
>> ------------------------------------------------------------------------
>>
>> Interactive Objects' Legacy Modernization Solutions 
>>
>> Get Your Applications SOA-Ready!
>>
>> See http://www.interactive-objects.com/ for more information.
>>
>> ------------------------------------------------------------------------
>>
>>
>> Interactive Objects Software GmbH - Freiburg - Geschäftsführer: Thomas Wager - 
>> AG Frbg. HRB 5810 - USt-IdNr. DE197983057
>>     
>
>
>
>       
>   


-- 
Interactive Objects Software GmbH
Basler Strasse 61
79100 Freiburg, Germany

Phone:  +49 761 400 73 0
mailto:thomas.woelfle at interactive-objects.com


------------------------------------------------------------------------

Interactive Objects' Legacy Modernization Solutions 

Get Your Applications SOA-Ready!

See http://www.interactive-objects.com/ for more information.

------------------------------------------------------------------------
 

Interactive Objects Software GmbH - Freiburg - Geschäftsführer: Thomas Wager - AG Frbg. HRB 5810 - USt-IdNr. DE197983057