[antlr-interest] recursive rule invocations

[Aaron Leiby]

Everyone's favorite error!

I'm playing with a subset of the javascript language, but am having
difficulties building a grammar in ANTLR for it.  I've pared down one of my
core problems to the following subset:

expression
 : Name (invocation)* // this is the simplest definition that exhibits the
problem
 ;
// an expressionStatement is an assignment or invocation - used in for
loops, etc.
expressionStatement
 : (assignmentExpression)=> assignmentExpression
 | Name ( refinement* invocation )+
 ;
// this is giving me the problem
assignmentExpression
 : leftHandSideExpression ( '+=' | '-=' | '=' )
  ( leftHandSideExpression '=' )* expression
 ;
// l-vals must not end with an invocation
leftHandSideExpression
 : Name ( invocation* refinement )*
 ;
refinement
 : '.' Name
 ;
invocation // normally an optional expression list
 : '(' expression ')'
 ;
 // remaining rule references are stubs similar to Name here
Name
 : 'name'
 ;

The error I get is:
[fatal] rule assignmentExpression has non-LL(*) decision due to recursive
rule invocations reachable from alts 1,2.  Resolve by left-factoring or
using syntactic predicates or using backtrack=true option.

There's only one alternative (which I fail to see how you can only have one
if it's an alternative - why does it say "alts 1,2" in the error?) and it
shows up as a little red bar before the 'expression' rule, between repeat
and skip arrows.  This does not help at all.  Can someone explain how to
interpret this?  All the examples of left factoring I've seen tend to
be quite simple and obvious.  I'm finding it much more difficult to apply
here.

I'm sure I'm going about this the wrong way (starting with the language spec
and working backward instead of starting with the top rule and decorating
the grammar one feature at a time), but I still figured it'd be worth
understanding how to deal with these situations when they inevitably pop up.

I started with Crockford's fancy railroad diagrams, which have implied
precedence.  I feel like that's been lost in the translation to ANTLR (or
maybe just lost sight of).
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