Antlr Frustrations

I just spent a frustrating morning fighting to get Suneido's query grammar to work with Antlr.

I kept getting "no viable alternative" and "no start rule (no rule can obviously be followed by EOF)"

I searched on the web but didn't find any answers.

I made more or less random changes to my grammar and if I cut out the more complex stuff it would work but that wasn't much help.

I started trying small examples, like:

expr : m (('+'|'-') m)* ;
m : ID
| '(' expr ')' ;
ID : ('a'..'z')+ ;
WS : (' '|'\t'|'\r'|'\n')+ {skip();} ;

which is pretty much straight out of the book but exhibited the same problems.

I started to wonder if something was messed up in my copy of Antlr or something.

Eventually, I found a post that explained it. The problem is that Antlr identifies "start" rules (ones that can end with EOF) by looking for rules that are not used anywhere else in the grammar. When you have recursion on the "start" rule (expr in this case), it doesn't work. The above example could be fixed by adding:

prog : expr ;

It makes sense once you know the problem, but it certainly wasn't obvious at first. And I don't recall the book mentioning it anywhere. It seems like an obvious "gotcha". I can't be the only person to get caught by this. It would have been nice if the error message said something like "make sure your start rule is not used in another rule".

Or else just make it work. Obviously, the rule you start with should be endable by EOF, whether or not it's used elsewhere. That's probably a little tricky to handle, but compared to the complexity of Antlr it's nothing.

This kind of frustration is one of the downsides of using someone else's tools and libraries.

Various other stupid mistakes on my part added to the frustration, but eventually I got things starting to work.