From: Hugh Sasse <hgs@...>
Date: 2005-10-28T21:33:29+09:00
Subject: Re: PARSER for RUBY

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On Fri, 28 Oct 2005, puellula@gmail.com wrote:

> Hi Hugh Sasse and thank you for you message :)
> Well... I had write on my file .g this, how you told to me:
>=20
> // PARSER
> class RubyParser extends Parser;
> options { buildAST=3Dtrue; }
>=20
> program=09=09: compstmt;
> compstmt=09: stmt;
>=20
> stmt          : stmt IF expr
>                 | stmt WHILE expr
>                 | stmt UNLESS expr
>                 | ;
>=20
> expr=09: ;
>=20
> // LEXER
> class RubyLexer extends Lexer;
> IF : "if";
> WHILE : "while";
> UNLESS : "unless";
> UNTIL : "until";
>=20
>=20
> but there is left recursione now! What do you think about? You can see
> that there is left recursion!
>=20
> The output is:
>=20
> C:\Personal\Universit=E0\Materiale Esami\Teconologie dei Linguaggi di
> Programmazio
> ne + Lab>antlr provaLexerParser.g
> java -classpath "C:\antlr\275\lib\antlr.jar;" antlr.Tool
> provaLexerParser.g
> ANTLR Parser Generator   Version 2.7.5 (20050128)   1989-2005 jGuru.com
> provaLexerParser.g:8:17: infinite recursion to rule stmt from rule stmt
> provaLexerParser.g:9:17: infinite recursion to rule stmt from rule stmt
> provaLexerParser.g:10:17: infinite recursion to rule stmt from rule
> stmt
        [...]
> provaLexerParser.g:8: warning:nondeterminism between alts 1 and 4 of
> block upon
> provaLexerParser.g:8:     k=3D=3D1:IF
> provaLexerParser.g:8: warning:nondeterminism between alts 2 and 4 of
> block upon
> provaLexerParser.g:8:     k=3D=3D1:WHILE
> provaLexerParser.g:8: warning:nondeterminism between alts 3 and 4 of
> block upon
> provaLexerParser.g:8:     k=3D=3D1:UNLESS
> Exiting due to errors.

Right.  I'm not sure what to do about this. You can have=20

    do_this if condition1 unless condition2=20

in ruby.  What about:

  program=09=09: compstmt;
  compstmt=09: stmt;
 =20
  stmt          : conditionalbody IF expr
                  | conditionalbody WHILE expr
                  | conditionalbody UNLESS expr
                  | conditionalbody   // a way out=20
                  | ;
 =20
  conditionalbody : stmt
                  | simplestatement;
                  ;

  //simplestatement is a way out of the stmt definition loop
  simplestatement : returnstatement
                  ;  // other statemtents go here

  returnstatement : RETURN expr
                  ;

  expr=09: ;

 =20
  // LEXER
  class RubyLexer extends Lexer;
  IF : "if";
  WHILE : "while";
  UNLESS : "unless";
  UNTIL : "until";
  RETURN : "return";
 =20
>=20
> I think the problem finish when I know how I can write on ANTLR the
> empty string. I try to rappresent this with " ", when I write "| ;". I
> hope you undestand what I would tell to you.

Well, I'm not familiar with ANTLR.  Actually, I read about PCCTS but
not ANTLR.  I find this stuff pretty difficult to debug.
>=20
> Thank you so much for your message
> bye,
> puellula

        Good luck,
        Hugh
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