title: ANTLR ambiguity in DeCaf - professor unsure where error is
body: <p>I'm working on a project for school with converting a BNF form Decaf spec into a context-free grammar and building it in ANTLR. I've been working on it for a few weeks and been going to the professor when I've become stuck, but I finally ran into something that he says should not be causing an error. Here's the isolated part of my grammar, expr is the starting point. Before I do that I have one question.</p>
  
  <p>Does it matter if my lexer rules appear before my parser rules in my grammar, or if they're mixed in intermittently through my grammar file?</p>
  
  <pre><code>calloutarg:         expr | STRING;
  expr:  multexpr ((PLUS|MINUS) multexpr)* ;
  multexpr : atom ((MULT|DIVISION) atom)*
  ;
  
  atom : OPENPAR expr CLOSEPAR | ID ((OPENBRACKET expr CLOSEBRACKET)? | OPENPAR ((expr (COMMA)* )+)? CLOSEPAR)|
  CALLOUT OPENPAR STRING (COMMA (calloutarg)+ COMMA)? CLOSEPAR | constant;
  constant: INT | CHAR | boolconstant;
  boolconstant: TRUE|FALSE;
  </code></pre>
  
  <p>The ugly formatting is because part of his advice for debugging was to take individual rules and break them down where the ambiguity is to see where the errors are starting. In this case, it's saying the problem is in the long ID portion, that OPENBRACKET and OPENPAR are the cause. If you have any ideas at all, I am deeply appreciative. Thank you, and sorry for how nasty the formatting is on the code I posted.</p>