[antlr-interest] Predicate hoisting pain

[Sam Barnett-Cormack]

Sam Barnett-Cormack wrote:
> Sam Harwell wrote:
>> This will fail unexpectedly with certain types of grammars, as
>> demonstrated by some grammars I've sent to the list in the past. I'll
>> find them later today and send a link.
> 
> I can't see why it'd fail horribly in this case... the problem would 
> come if os were being used in a higher rule for something entirely 
> different and the predicate got hoisted.
> 
> In this case, the predicates getting hoisted in just wasteful. It's 
> checking loads of stuff unnecessarily, because it's going to call the 
> rule the predicate is in in either case! I think the logic that needs 
> fixing is examining the branches of the choice the predicate got hoisted 
> into, and if it's hoisted into every branch drop it again. That would 
> solve my issue at least, and I can't imagine a situation where it'd be 
> dropped that it would cause a problem.
> 
> The other option is to detect predicates that use a parameter, and do 
> some substitution in the hoist so that the parameter is substituted (as 
> it's the calling rule that sets it). This might leave it with a 
> parameter (of the "outer" rule) in use. However, if you do it repeatedly 
> when hoisting through multiple layers, it'd work out in the end. As it 
> is, if you have "stacked" rules like this passing a parameter, you may 
> get inconsistent results depending on naming,

Come to think of it, the really sensible thing (from a quick think) 
would be to do both of those things. That'd make hoisting "safe" (or at 
least safer) and avoid wasteful things like has just happened.

Sam