[antlr-interest] Specifying optional elements in a tree grammar.

[Paul McKee]

Thanks very much Jim - that's sorted it!

Regards

Paul
  ----- Original Message ----- 
  From: Jim Idle 
  Cc: antlr-interest at antlr.org 
  Sent: Wednesday, March 25, 2009 5:12 PM
  Subject: Re: [antlr-interest] Specifying optional elements in a tree grammar.


  Paul McKee wrote: 
    Hi:

    How do I go about specifying optional elements in a tree grammar?
    For example, in my AST generator, I have:

    querySpecification :
     selectClause 
     ( intoClause ) ?
     ( fromClause  )?
     ( (whereClause) =>whereClause )?
     ( (groupByClause) => groupByClause 
      ( havingClause  )?
     )? ->^(SELECTCLAUSE selectClause ^(INTOCLAUSE intoClause)? ^(FROMCLAUSE fromClause)? ^(WHERECLAUSE whereClause)?);

    Which generates fine, but in my tree walker, I have:

    querySpecification :
     ^(SELECTCLAUSE selectClause ^(INTOCLAUSE intoClause)? ^(FROMCLAUSE fromClause)? ^(WHERECLAUSE whereClause)?)
     ;

    Which causes the following:
    [16:57:30] error(100): TsqlWalker.g:284:54: syntax error: antlr: TsqlWalker.g:284:54: expecting RPAREN, found '?'
    [16:57:30] error(100): TsqlWalker.g:284:80: syntax error: antlr: TsqlWalker.g:284:80: unexpected token: ?
    [16:57:30] error(100): TsqlWalker.g:284:107: syntax error: antlr: TsqlWalker.g:284:107: expecting SEMI, found ')'

  Probaly better to put those embedded trees into separate rules  rulex? ruley? but you need:

   ^(SELECTCLAUSE selectClause (^(INTOCLAUSE intoClause))? (^(FROMCLAUSE fromClause))? (^(WHERECLAUSE whereClause))?)


  Because the ^( xxxxx) is a tree specification and needs to be enclosed to make it optional.

  Jim


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