[antlr-interest] error in tree grammar syntax...

[Gavin Lambert]

At 10:27 30/12/2008, Pady Srinivasan wrote:
 >When I try something like this in my tree grammar, I get an 
error:
 >
 >statement
 > : assignment
 > | ^('if' boolExpression assignment? statement* ^('else'
 >   (statement)? (statement)? )? )
 > ;
 >
 >
 >error(100): GwfWalker.g:32:85: syntax error: antlr:
 >GwfWalker.g:32:85: expecting RPAREN, found '?'
 >error(100): GwfWalker.g:32:87: syntax error: antlr:
 >GwfWalker.g:32:87: expecting SEMI, found ')'
 >error(106): GwfWalker.g:26:4: reference to undefined rule:
 >statement

Did you try removing the parentheses around "statement"?

Also, you're going to have trouble with a construct like 
that.  Having two optional statements in series -- when only one 
statement is present in the input tree, which is it?  The first or 
the second?  You need to either put more structure in your AST or 
change the nesting so that it's obvious which one is "more 
optional" than the other, when only one of the two is present.

You'll have similar trouble with "assignment? statement*", since 
an assignment is a statement.  This is a redundancy, and those 
almost always lead to ambiguity and weird behaviour.