[antlr-interest] predicate question

[Gerald Rosenberg]

At 07:27 PM 5/5/2008, Terence Parr wrote:

>On May 5, 2008, at 7:17 PM, Gerald Rosenberg wrote:
>
>>Is the question whether
>>
>>a : {p1}? {action();} {p2}? A | A ;
>>
>>should perform differently from
>>
>>a : {p1}? {action(); return true;}? {p2}? A | A ;
>>
>> From a high level the-grammar-as-a-language point of view, seems
>>like those two rules should match equivalently.
>
>p1 / p2 are evaluated out of order of normal flow, which is the
>issue.  Predicates can be hoisted into parsing decision, which
>different than the matching. the matching works...prediction won't :)
>
>Ter

I was imprecise:  "seems like those two rules should both predict and 
match equivalently."

I know there is a historical basis for treating actions as first 
class elements, but is there a language-level reason why an action is 
not logically just an unchecked validating predicate (or a predicate 
just a checked action with a bunch of nifty turbo-performance 
accelerators under the hood)? 
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