Re: Anyone got an LALR(2) parser-generator ?

>"John H. Lindsay" <lindsay-j%rmc.ca@trsvr> wrote:
> I'm working with a language definition that needs an LALR(2)
> parser-generator for a couple of good reasons;
>


Sometimes you can use the scanner lookahead to help with this kind of problem,
for example:


Scanner:
id - Letter Letter*


Parser:
S -> Xid "=" Xid "."
XId -> id | id "." id


In LALR(1) you will have a Shift reduce conflict with ".". But it is
LALR(2).


Using the scanner look ahead, the grammar can be rewrite to:


Scanner:
id - Letter Letter*
struct_id = Letter Letter* LOOKAHEAD("." Letter Letter*)


Parser:
S -> Xid "=" Xid "."
XId -> id | struct_id "." id


And this new LALR(1) grammar doesn't have shift/reduce conflict.


The struct_id token will only be recognice if ("." id) is after the first id.
for the input:


      a = a.x.


      | | |
      | | |-- id
      | |---- struct_id
      |-------- id


In lex the Lookahead operator is "/", many other scanner/parser geneator have
this operator also.


Maybe you can use somthing like this to help you use a LALR(1) grammar for
your language.


Best Regards,


Frankie Arzu
Computer Science Depatment,
Universidad del Valle de Guatemala
E-mail: farzu@uvg.edu.gt
                sunguat!assist!frankie@sun.com




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