[antlr-interest] Understanding lookahead

[Wincent Colaiuta]

I tried that, on both the FOO rule and at the whole-grammar level  
(not sure if that only applies to the parser though) and it didn't  
work...

Cheers,
Wincent

El 6/6/2007, a las 22:48, Oliver Zeigermann escribió:

> Maybe it could help to manually set the amount of lookahead to 7? I
> have not tried it, but this should include checks for all characters
> and only let FOO match for "bar:baz"
>
> HTH
>
> Oliver
>
> 2007/6/6, Wincent Colaiuta <win at wincent.com>:
>> I'm trying to understand how ANTLR's lookahead mechanisms work using
>> this grammar:
>>
>>    grammar Simple;
>>
>>    FOO: BAR ':' BAZ {System.out.println("FOO");};
>>    fragment BAR: 'bar' {System.out.println("BAR");};
>>    fragment BAZ: 'baz' {System.out.println("BAZ");};
>>    EVERYTHING_ELSE: . {System.out.println("EVERYTHING_ELSE");};
>>
>>    thing: .* EOF {System.out.println("done");};
>>
>> I basically wanted to explore the way lookahead works and what ANTLR
>> does when its lookahead predictions fail. For example, given the
>> following inputs:
>>
>> - "bar:baz": recognizes this as a FOO token
>> - "bar:ba": predicts FOO and complains about missing "z"
>> - "bar:b": predicts FOO and complains about missing "a"
>> - "bar:": predicts FOO and complains about missing "b"
>> - "bar": predicts FOO and complains about missing ":"
>> - "ba": predicts FOO and complains about missing "r"
>> - "b": accepts input as EVERYTHING_ELSE
>> - "...ba": accepts the periods as EVERYTHING_ELSE, then predicts FOO
>> complains about missing "r"
>>
>> This exercise was very helpful for me in seeing how ANTLR's lookahead
>> operates: basically, as soon as its seen enough input to predict the
>> presence of a particular token ("ba" is enough in this case), it
>> assumes that it really is that token, scans ahead, and raises an
>> exception if its expectations aren't met.
>>
>> So, one way to get this grammar to handle strings like "...ba"
>> without throwing exceptions is to use the filter=true option. I'm
>> curious to know, however, is there any other way?
>>
>> Cheers,
>> Wincent
>>
>>
>>