From: Zach Dennis <zdennis@...>
Date: 2005-01-27T02:24:07+09:00
Subject: Re: When little languages grow...

To make sure myself and the rest of the list is correctly hearing you 
question. You are interested in the ruby way to write a code generator?

And you are looking for input other parsing or implemented solutions 
others may have experience with, with languges and tools such as; lisp, 
forth, yacc, bison, racc, etc.. ?

Zach




Hugh Sasse Staff Elec Eng wrote:
> I seem to have run into my parsing problem again.  Whatever I'm
> doing I usually end up having to parse non-simplistic input, and I'm
> still not happy about the apparently available solutions to this.
> So I'm wondering what other people do.
> 
> The application is immaterial at the moment, but the problem is that
> I need to do more than can be done with a simple case statement, and
> if I were to use case statements managing the problem would get too
> big.
> 
> The conventional wisdom is to use some form of parser
> generator (Yacc, Bison, Racc, Rockit,...) but I don't have
> confidence in my ability to get these working well.[1].
> I have had great difficulty in the past, certainly.
> 
> Other possibilities I have considered and tried are to lash together
> some form of Lisp [cf Greenspun's 10th rule of programming] or Forth,
> but I don't consider myself fluent in either of those languages, and
> they are not as easy a user interface for other people as Ruby would
> be.  I can get something working, but find it hard to maintain or
> improve. [2]
> 
> So the next possibility is to use something like
> 
>         input = nil
>         File.open("input.txt"){|f|
>           input = f.read
>         }
>         Thread.new(input){|source|
>           $SAFE=5
>           instance_eval source
>         }.value
> 
> or something, and actually make the commands in the language methods
> of some Ruby object.
> 
> It is often observed that it is difficult to add security to a
> system, compared to building it in from the start.  Can I do this
> and still have a good level of security?  Should I make the parser
> object (whose method's I'm using) a subclass of Nil, to limit it as
> much as possible?  I need to give people enough rope to hold their
> input together, but not enough to hang themselves (or me). I don't
> want people to be able to execute arbitrary code, or fiddle with
> objects they should not need to touch.
> 
> Is there another way to handle input flexibly that I have completely
> missed?  I've googled for things to do with little languages and 
> parsing, but have found nothing enlightening.
> 
>         Thank you,
>         Hugh
> 
> [1] I find that thinking in the manner of a shift/reduce parser is
> particularly unnatural to me.  This might just be a weakness on my
> part or may have something to do with people's difficulties in
> handling modal interfaces: it is hard to switch contexts rapidly.
> Maybe there is something I can read which will turn the problem
> around, so it becomes easy to handle?
> 
> [2] Immensely powerful and fast systems have been written in Forth,
> and Lisp is very powerful in the right hands.  I just don't have the
> experience with these to be effective, yet.
> 
> 
>