From: Mark Probert <probertm@...>
Date: 2002-09-24T23:11:33+09:00
Subject: Re: Lexing ...

At 05:39 AM 9/24/2002 +0900, Phil wrote:
> >
> >Given the recent discussion on lexer/parsers for Ruby, I thought that I
> >would do something I have been meaning to do for a while, port Coco/R to
> >ruby.
> >
>
>Cool.  I also don't like having a seperate lexer and parse (like using lex
>and yacc) so this sounds interesting.
>
>Can we take a look at what you've got so far?

There is not too much to show at the moment.  I scan a ruby-ised version
of the attributed grammar into token.  That is the -easy- part :-).  I think
that in a few weeks I will have something "real" to share.  There is a lot
of fine-print in working out the parser...

FWIW, a simple Coco/R grammar looks like (this is the expr example from
the Dragon book):

--------< expr.atg >----------------

$C              # generate the driver
COMPILER Expr
# This is a simple expression calculator

#----------------- Scanner Specifications ----------------------

CHARACTERS
     letter = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz".
     digit  = "0123456789".
     tab    = CHR(9).
     eol    = CHR(10).

TOKENS
     ident   =  letter {letter|digit}.
     number  =  digit  { digit }.

IGNORE eol+tab

COMMENTS FROM "--" TO eol

#--------------------Parser Specification -------------------

PRODUCTIONS
     Expr = StatSeq .

     StatSeq = SYNC { Stat ";" SYNC} .

     Stat =
        Expression<n>            (. puts "#{n}" .)
       .

     Expression<n> =
        Term<a>
        {   "+" Term<b>          (. a += b .)
          | "-" Term<b>          (. a -= b .)
        }
        .

     Term<n> =
        Factor<a>
        {   '*' Factor<b>        (. a *= b .)
          | '/' Factor<b>        (. a /= b .)
        }
        .

     Factor<n> =                 (. VAR sign =  1 .)
       [ "-"                     (.     sign = -1 .)
       ]
       (   Number<n>
         | "(" Expression<n> ")"
       )                         (. n *= sign .)
       .

    Number<n>
         =
         number                  (. n = scanner.get_string.to_i .)
         .

    Ident<s>
         =
         ident                   (. n = scanner.get_string .)
         .

END Expr.