[antlr-interest] rooting a parser rule rather than a token

[Eric Mahurin]

I'm not sure why, but antlr doesn't currently allow you to make a call
to a rule from another rule an AST root.  So, you can't do something
like this:

binary_expression : primary (binary_operator^ primary)* ;

I haven't yet dove into the antlr code generation to completely fix
this issue, but you can get around this issue with these changes:


1. Instead of using "^" to root the rule, discard it using "!" and
then put in this action: {astFactory.makeASTRoot(currentAST,
returnAST);}.  For example, the above rule would become:

binary_expression : primary (
    binary_operator! {astFactory.makeASTRoot(currentAST, returnAST);}
    primary
)* ;


2. If your rule you want to root contains more than one token and it
can be at the beginning of the rule calling it, there is a bug in the
routine makeASTRoot in ASTFactory.java.  Change "currentAST.child =
currentAST.root;" to "currentAST.child = root.getFirstChild();" to fix
the problem.  I did this in a copy of ASTFactory.java in my local
antlr directory instead of changing the source.


3. This simple fix doesn't handle the case where the the rule you want
to root is a list of 2 or more tokens (i.e. has 2 or more tokens and
is unrooted).  I can't think of a case where you'd want to root such a
rule, but if you did want to do this, make these changes in
ASTFactory.java:

a. In addASTChild, change "currentAST.root.setFirstChild(child);" to:
    currentAST.child = currentAST.root;
    currentAST.advanceChildToEnd();
    currentAST.child.setFirstChild(child);

b. In makeASTRoot, change "currentAST.child = currentAST.root;" to:
    currentAST.child = root;
    currentAST.advanceChildToEnd();
    currentAST.child = currentAST.child.getFirstChild();

These changes *should* be backwards compatible.  It also allows you to
make AST's that have separate left and right/normal children pointers
to preserve token order.

Anybody want to figure out how to antlr code generation do the right
thing for using "^" with a rule instead of a token?  It's probably
trivial.

I have more AST suggestions, so expect another message...

Eric




 
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