[antlr-interest] What exactly does 'follow set' mean?

[Monty Zukowski]

Prashant Deva wrote:
> What exactly do you mean by a 'follow set'?

Follow set is what tokens may follow this in the stream, i.e. the 
lookahead at that point of the production.

> In the doc it says that, when antlr parsers encounter an error they
> look up tokens until they find one in the 'follow set' of the rule.

So the idea is 'hey that doesn't belong here, so I'll just skip ahead in 
the token stream until I see a token that the calling rule might match.' 
  So that when the errorneous rule returns the calling rule might be 
able to continue where it left off.

Note that in ANTLR 2, the follow set for a rule is the union of the 
lookahead following every call to the rule.  ANTLR 2 doesn't distinguish 
follow sets based on the caller.

Hey Ter, this may be able to improve in ANTLR 3 since you actually know 
the follow set specific to the calling rule.  Or maybe you figured that 
out already?  Wouldn't suprise me ;)

> So what exaclty does the parser look for when it encounters an error?
> 

Hopefully I've answered that by now.

Monty