[antlr-interest] Tokens

[David-Sarah Hopwood]

Kevin J. Cummings wrote:
> On 11/27/2009 06:05 PM, Ronald Sok wrote:
>> Kevin J. Cummings wrote:
>>> You are close.  What you have here is keywords as opposed to reserved
>>> words.  When implementing the former, you will need to do something like
>>> (at least this is what I do using ANTLR 2.7.7):
>>>
>>> id : ID
>>>    | k:keyword
>>>       { #k->setType(ID); }
>>>       // This changes the token type of a keyword to an ID
>>>    ;
>>>
>>> keyword
>>>    : APPLE | PEAR | ORANGE
>>>    ;
>>>
>>> someName
>>>    :     'Name:' id NEWLINE
>>>    ;
>>>
>>> You could reduce the number of productions by folding, but the principal
>>> of changing the token type of keywords is what is important here.  And
>>> you may have to find out how to do this with ANTLR 3.x.
>>
>> Ok, I tried to change this into ANTLR 3 syntax, but ran into the fact that
>> the result of keyword is a subtype of ParserRuleReturnScope, which
> 
> Sorry, my bad, should be KEYWORD and done in the lexer, not the parser!

That won't work because either:
 - KEYWORD is before APPLE, PEAR and ORANGE, in which case it always takes
   priority and the type of a Token will never be APPLE, PEAR or ORANGE;

 - or, KEYWORD is after APPLE, PEAR and ORANGE, in which case those rules
   take priority and the type of a Token will never initially be KEYWORD.
   You could override it, but if you do that in a lexer rule then you
   don't have enough context to determine what it should be (and '$type ='
   can't be used in a parser rule).

As I said in my other followup, it's usually not necessary to change the
type (but you can do so using the code given in that post if you want).

-- 
David-Sarah Hopwood  ⚥  http://davidsarah.livejournal.com

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