Re: use of lookaheads

[Etienne Gagnon <etienne.gagnon@uqam.ca>]

Hi Pieter,

Lookaheads are not implemented.  One of the reasons is that the 
algorithm proposed in the Dragon book (which I used to implement the 
LALR(1) calculations) is broken; another is that it is fairly easy to
simply recognize the full regular expression, and push back the extra
characters into the pushback reader, using a custom lexer (see my thesis 
for an example of custom lexer).

In other words, you could write:

in = 'in';
variable = var 'in';

then in the filter() method of you custome lexer, push back 'in', then 
change the TVariable token's text to 
token.setText(token.getText().substring(....))

Have fun!

Etienne

Pieter Verheyden wrote:
> Hi,
>
> I wanted to use a lookahead for variables in my language. As a variable
> is always followed by the keyword 'in', I defined the variable token as
> follow:
>
>     in = 'in';
>     variable = var / 'in';
>
> where "in" is the token for the in-keyword, and "var" is defined in the
> Helpers section as:
>
>     var = letter (letter | digit )*;
>
> With both versions, i.e. SableCC2 and SableCC3, I get the following
> error:
>
>     java.lang.RuntimeException: [146,17] Look ahead not yet supported.
>
> What does this error mean? In other words, what am I possibly doing
> wrong?
>
>
> Thanks in advance,
> Pieter.


--
Etienne M. Gagnon, Ph.D.             http://www.info.uqam.ca/~egagnon/
SableVM:                                       http://www.sablevm.org/
SableCC:                                       http://www.sablecc.org/