[antlr-interest] Lookahead predicates in the Lexer?

[Terence Parr]

predicates have always been native code, though, right?
Ter
On Nov 13, 2012, at 12:09 AM, Gerald Rosenberg wrote:

> Well that was what I was hoping for.  Using the v4.0b3 jar, the Lexer rule
> 
> fragment COMMA        : ','    ;
> Identifier
>    : LETTER ( LETTER | DIGIT | UNDERSCORE )* { ~COMMA }? -> popMode
>    | LETTER ( LETTER | DIGIT | UNDERSCORE )*
>    ;
> 
> generates, in relevant part,
> 
>    public void Identifier_action(RuleContext _localctx, int actionIndex) {
>        switch (actionIndex) {
>        case 0: popMode();  break;
>        }
>    }
>    public boolean Identifier_sempred(RuleContext _localctx, int predIndex) {
>        switch (predIndex) {
>        case 5: return  ~COMMA ;
>        }
>        return true;
>    }
> 
> Switching from the fragment rule to a token rule
> 
> Comma : COMMA ;
> . . . . { ~Comma }? . . . .
> 
> generates
> . . . .
>        case 5: return  ~Comma ;
> 
> As if Antlr is only considering the content of the predicate to be a native code boolean expression.
> 
> 
> On 11/12/2012 5:05 PM, Terence Parr wrote:
>> That predicate should work.  If that predicate fails, then that rule will fail and the input will not be consumed for B.
>> Ter
>> On Nov 12, 2012, at 3:29 PM, Gerald Rosenberg wrote:
>> 
>>> In Antlr4, is there a way to do a fixed lookahead in the lexer predicate
>>> without capturing the lookahead token(s)?  In v3, predicates could be
>>> used for this purpose.
>>> 
>>> csvRule : A ( Comma B )* ;
>>> 
>>> A : P Q R -> pushMode(Alphabet)
>>> 
>>> mode Alphabet;
>>> B : X Y Z { ~Comma }? -> popMode
>>>    : X Y Z ;
>>> 
>>> In v4 , the "~Comma" is presumed to be native code.
>>> 
>>> Basically, looking for a clean, workable way to not require the use of a
>>> semicolon to explicitly terminate input that matches the csvRule, yet
>>> have a representation in the lexer that can be used as the popMode trigger.
>>> 
>>> I do realize that I can write a predicate method to do a stream scan,
>>> but would prefer a non-native code solution if possible.  Also realize
>>> that, in the simplest case, csvRule could be pushed down into the
>>> Lexer.  Where A and B  are not just single terminals in the parser,
>>> other rules would have to be pushed down also, making for a bit of a mess.
>>> 
>>> Thanks,
>>> Gerald
>>> 
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>> 
> 
>