[antlr-interest] Actions in predicates

[Marcin Rzeźnicki]

2010/2/22 Marcin Rzeźnicki <marcin.rzeznicki at gmail.com>

>
>
> On Mon, Feb 22, 2010 at 6:21 PM, Jim Idle <jimi at temporal-wave.com> wrote:
>
>> It's not completely flawed. To do what you asked, you do not need to
>> influence the lexer, just look ahead in the off-channel at certain points
>> and change the channel of the SEMI back to the default if you wanted it to
>> appear; but as I said, that is an awkward way to do it I think.
>>
>> Jim
>>
>>
>
> My ideas was more or less that whenever I have ambigous syntax (with no
> semicolons whatsoever in the parser)  I resolve problems using syntactic
> predicates  taking "optional" parts of syntax (like semicolons) into
> consideration. If syntactic predicate hit the optional part then it would
> fail - meaning that separator divided potentially ambigous construct and it
> is time to try the next alternative. It seemed clean to me, but you've made
> me think about it again. I could use your suggestion to make semicolon
> tokens reappear but how do I do this? CommonTokenStream does not seem to
> provide any public method of inspecting non-default channels
>
>
>
I think that some clear example of this construction is needed. Let me
reiterate.
Consider following constructs (as parser sees them):

(1) super

(2) super [TYPE] method

(3) [TYPE] method

(1) calls direct super method, (2) calls method on some parent determined by
TYPE, (3) calls static method on arbitrary type
Upon seeing (2) parser cannot decide whether what it sees is really (2) or
rather (1) followed by (3). So I can use syntactic predicate binding [TYPE]
to the nearest 'super' construct, if present. Simply: ( super '[' ) =>
super_alternative. But user might have really typed :

super; [TYPE] method

, which parser did not see because of ';' optionality.
But this problem, in my opinion, is simply solved by saying somehow: see all
semicolons in this predicate - and that was my original question's subject.
If parser saw semicolons when executing predicates then it could not match (
super '[' ) so it would fail, and it in turn would lead to taking correct
path. I hope my reasoning is more or less clear to you. Thanks for any
input.




-- 
Greetings
Marcin Rzeźnicki