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# $Id: README,v 1.2 1998/04/29 06:34:46 jake Exp $

		       ------------------------
		       Perl 5 Byacc Patches 0.6
		       ------------------------

This is a set of patches to perl-byacc1.8.2 which cause it to generate
a Perl 5 class implementing a parser (instead of a file full of global
data and procedures). It's nice if you want to be squeaky clean, and
very nice if you have a need for multiple parsers or multiple
instances of a parser in one program.

			  How to install it
			  -----------------

1. Get perl-byacc1.8.2 and unpack it. See

     http://www.perl.com/CPAN/src/misc/perl-byacc1.8.2.tar.gz

2. Patch byacc with

     cd perl-byacc1.8.2
     patch < ../perl5-byacc-patches-0.6/patch

3. Compile perl-byacc1.8.2 as usual.


			    How to use it
			    -------------

You invoke byacc a little differently:

  1. -P now takes a package name argument, and the output file is
     written as {package name}.pm.

  2. You can pass a -5 argument to get a somewhat faster parser under
     Perl 5.005 (using 'my Dog $spot' and 'use fields'). By the way,
     parsers generated with these patches are thread-safe (all package
     variables are static data), although of course you will break
     that if you do something non-thread-safe in your lexer or
     actions.

You must write your parser a little differently:

  1. Your &yylex and &yyerror procedures can (optionally) take an
     extra argument, which is passed from &yyparse (see below).

  2. Your &yylex procedure no longer side-effects $yylval; instead,
     it must return a list containing the token and the value.

  3. Your top-level rule can assign a value to $$, and this value will
     be returned as the value of &yyparse (see below).

And you must write the code that uses the parser differently too:

  1. Use 'use Parser' (or whatever package you put in -P)
     instead of 'require "y.tab.pl"'.

  2. Create a new parser object with

     $p = Parser->new($yylex, $yyerror, $yydebug);

     The first two arguments are references to your &yylex and
     &yyerror procedures, respectively. The third is true or false to
     enable or disable debugging output.

     Although you may wind up sticking your &yylex and &yyerror in the
     parser package in most cases, this is more composable if you want
     to do something weird.

  3. Start a parse by calling

     $val = $p->yyparse($s);

     Here $s is an optional "stream" argument which represents the
     state of the text you're scanning. Depending on what you want to
     do, this could be a filehandle reference, a string, some object
     you cooked up, or whatever.

     The argument is passed down to &yylex and &yyerror. &yylex should
     use it to get characters, and &yyerror should examine it to
     generate a meaningful message.

  4. $p->yyparse returns the value that your top-level rule assigned
     to $$, instead of true or false to indicate success or
     failure. This obviates the need to side-effect global variables
     to return values from the parser. You should use exceptions to
     handle error conditions.


			       Example
			       -------

I've include two examples: an ultra-simple calculator and a random
text generator. Type 'make' to build the parsers.


			       Copying
			       -------

This code is in the public domain--use it and redistribute it as you
like. I would appreciate acknowledgement of some kind (or at the very
least an email) if you use it in something that is widely used or
distributed.


			  Bugs and feedback
			  -----------------

Send them to Jake Donham ([email protected]). I'd love to hear from you
if you find these patches useful.