Re: AMBLER, a modern, simplified "WALKER/Visitor" package.

[Marco A. <ma...@cs...>]

To: nic...@iw...
CC: clo...@li...
In-reply-to: <wsk...@or...> (message from
	Nicolas Neuss on 05 Jun 2001 15:42:01 +0200)
Subject: Re: AMBLER, a modern, simplified "WALKER/Visitor" package.
References: <200...@oc...> <wsk...@or...>
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> Date: 05 Jun 2001 15:42:01 +0200
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> Marco Antoniotti <ma...@cs...> writes:
> 
> > Hi,
> > 
> > I decided to put together the following little utility that can be
> > used to "amble" through Lisp code while doing extra things in the
> > meanwhile.
> > 
> > I decided to write it, because most of the WALKER packages around are
> > kinda oldish and I find them too tied to non-portable aspects of the
> > underlying Common Lisp implementation.
> > 
> > I'd like some comments about it.
> > 
> > Please play around with it and let me know what you think.
> > 
> > Of course, I hope to eventually put it into the CLOCC.
> > 
> > Cheers
> > 
> > Marco
> 
> Hello, Marco.
> 
> I've looked at your code hoping that the documentation would give me a
> clue what an ambler is and how to use it.  I have read:
> 
> ;;; ambler.lisp --
> ;;; Simple code ambler. Purely syntactic and without `environment'
> ;;; hacking.
> ;;; This is essentially an instantiation of the "Visitor Pattern"
> ;;; which, by means of CL multiple dispatching is readily implemented.
> ;;;
> ;;; This facility is inspied by the WALKER functionalities available
> ;;; in many implementations.  This facility does not implement all the
> ;;; features (mostly non-portable) available in many WALKER
> ;;; implementations, but at least it is (should be) very portable and
> ;;; usable.
> 
> Unfortunately, I have not yet used another Lisp than CMUCL, so that I
> am somewhat lost in what your WALKER or AMBLER can do.  Could you give
> me a small example of usage?

Suppose you want to define a nice macro which implements a new
language extension.  Or, suppose you want to define a new sublanguage
altogether.  E.g. you want a Estere\lambda (check out
www.esterel.org).

I.e you want to be able to write something like

    (defmodule abro ((a :signal :input)
                     (b :signal :input)
                     (r :signal :input)
                     (o :signal :output))
       (loop (par (await a) (await b))
             (emit o)
	     each r))

CL programs are already in AST form, but they may need further
processing.  E.g. you may need to figure out that the 'o' in the
'(emit o)' expression has been already declared.

To this end you can write a AMBLER for (or a "WALKER" form).  Suppose
you have defined a new ambler class (forget the packages for the time
being).


    (defclass esterel-ambler-context (identity-ambling-context)
       ((environments () :accessor environments))) ; a stack of alists.

Now you can write something like

    (def-form-ambler emit (emit-form esterel-ambler-context)
        (let* ((output-signal (second emit-form))
              ((declared-signal
                (find-id output-signal
                         (environments esterel-ambler-context))))
              )
          (unless declared-signal
            (error "Trying to emit an unknown signal ~S." output-signal))
          ))

And

   (def-form-ambler loop (loop-form esterel-ambler-context)
       (let ((termination (first (last (butlast loop-form)))))
          (when (eq termination 'each)
            ;; process EACH loop clause)
          (amble-expression (subseq loop-form 1
                                              (when (eq termination 'each)
                                                 (- (length loop-form) 2)))
                            esterel-ambler-context)))


I apologize for not having written a more through documentation of
this little package with more examples.  I just got frustrated with
ahvng tons of code implementing what is essentially the "Visitor"
pattern for S-exprs.  Really, there is not much more to it.  I just
thought that packaging it would be of use to other people as well.

Cheers

-- 
Marco Antoniotti ========================================================
NYU Courant Bioinformatics Group        tel. +1 - 212 - 998 3488
719 Broadway 12th Floor                 fax  +1 - 212 - 995 4122
New York, NY 10003, USA                 http://bioinformatics.cat.nyu.edu
                    "Hello New York! We'll do what we can!"
                           Bill Murray in `Ghostbusters'.