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Question about ANTLR and recursions [message #555844]

Sun, 29 August 2010 13:55

Michael Frey

Messages: 48
Registered: August 2010

Member

Hi,

I'm not sure but my ANTLR issue might be an recursion issue. So far, I've specified a language which consists upon variables, statements, functions, loops and conditions. While a block in my language could contain a variable, statement, loop or condition I've specified a block expression:

BlockExpression:
  Variable | Statement | Loop | ConditionalExpression;

The conditional expression itself is:

ConditionalExpression:
  IfElseCondition | SwitchCaseCondition;
	
IfElseCondition:
  'if' '(' condition=Or ')' '{' 
    ifAction+=BlockExpression		
  '}' 'else' '{'
    elseAction+=BlockExpression
  '}';

SwitchCaseCondition:
  'switch' '(' condition=Or ')' '{' 
    case+=(CaseStatement)* 'default' ':' default=Or
  '}';

CaseStatement:
  'case' condition=Or ':' action=Or;

and I assume that the following part is the problem

...
  ifAction+=BlockExpression		
...
  elseAction+=BlockExpression

And that's the error I get while I generate the artifacts

warning(205): ../de.example.flow.ui/src-gen/de/hs_rm/cs/vs/dsm/ui/contentassist/antlr/internal/InternalFlow.g:1:8: ANTLR could not analyze this decision in rule Tokens; often this is because of recursive rule references visible from the left edge of alternatives.  ANTLR will re-analyze the decision with a fixed lookahead of k=1.  Consider using "options {k=1;}" for that decision and possibly adding a syntactic predicate.
warning(209): ../de.example.flow.ui/src-gen/de/hs_rm/cs/vs/dsm/ui/contentassist/antlr/internal/InternalFlow.g:10186:1: Multiple token rules can match input such as "'^'": RULE_ID, RULE_ANY_OTHER
As a result, tokens(s) RULE_ANY_OTHER were disabled for that input
warning(209): ../de.example.flow.ui/src-gen/de/hs_rm/cs/vs/dsm/ui/contentassist/antlr/internal/InternalFlow.g:10186:1: Multiple token rules can match input such as "'b'": T51, RULE_ID, RULE_ANY_OTHER
As a result, tokens(s) RULE_ID,RULE_ANY_OTHER were disabled for that input
...

Probably that's a total no go in xtext? Somebody has an idea for a workaround? Thanks in advance!

Regards,
Michael

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Re: Question about ANTLR and recursions [message #555857 is a reply to message #555844]

Sun, 29 August 2010 17:07

Henrik Lindberg

Messages: 2509
Registered: July 2009

Senior Member

You have to show the entire grammar as the part we are seeing does not
tell enough about what is going on.

Besides other issues, your BlockExpression is non repeating - you
probably want:
ifAction+=BlockExpression*

or
ifAction = BlockExpression

and make BlockExpression repeating
BlockExpression: expressions += (Variable | Statement | Loop |
ConditionalExpression)*;

Implementing an expression based language is a bit tricky - take a look
at the arithmetic example, and xbase. You can also look at the eclipse
b3 project which has a rich grammar.

Regards
- henrik

On 8/29/10 3:55 PM, Michael Frey wrote:
> Hi,
>
> I'm not sure but my ANTLR issue might be an recursion issue. So far,
> I've specified a language which consists upon variables, statements,
> functions, loops and conditions. While a block in my language could
> contain a variable, statement, loop or condition I've specified a block
> expression:
>
>
> BlockExpression:
> Variable | Statement | Loop | ConditionalExpression;
>
>
> The conditional expression itself is:
>
>
> ConditionalExpression:
> IfElseCondition | SwitchCaseCondition;
>
> IfElseCondition:
> 'if' '(' condition=Or ')' '{' ifAction+=BlockExpression
> '}' 'else' '{'
> elseAction+=BlockExpression
> '}';
>
> SwitchCaseCondition:
> 'switch' '(' condition=Or ')' '{' case+=(CaseStatement)* 'default' ':'
> default=Or
> '}';
>
> CaseStatement:
> 'case' condition=Or ':' action=Or;
>
>
> and I assume that the following part is the problem
>
>
> ...
> ifAction+=BlockExpression
> ...
> elseAction+=BlockExpression
>
>
> And that's the error I get while I generate the artifacts
>
>
> warning(205):
> ../de.example.flow.ui/src-gen/de/hs_rm/cs/vs/dsm/ui/contenta ssist/antlr/internal/InternalFlow.g:1:8:
> ANTLR could not analyze this decision in rule Tokens; often this is
> because of recursive rule references visible from the left edge of
> alternatives. ANTLR will re-analyze the decision with a fixed lookahead
> of k=1. Consider using "options {k=1;}" for that decision and possibly
> adding a syntactic predicate.
> warning(209):
> ../de.example.flow.ui/src-gen/de/hs_rm/cs/vs/dsm/ui/contenta ssist/antlr/internal/InternalFlow.g:10186:1:
> Multiple token rules can match input such as "'^'": RULE_ID, RULE_ANY_OTHER
> As a result, tokens(s) RULE_ANY_OTHER were disabled for that input
> warning(209):
> ../de.example.flow.ui/src-gen/de/hs_rm/cs/vs/dsm/ui/contenta ssist/antlr/internal/InternalFlow.g:10186:1:
> Multiple token rules can match input such as "'b'": T51, RULE_ID,
> RULE_ANY_OTHER
> As a result, tokens(s) RULE_ID,RULE_ANY_OTHER were disabled for that input
> ...
>
>
> Probably that's a total no go in xtext? Somebody has an idea for a
> workaround? Thanks in advance!
>
> Regards,
> Michael

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