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From: bevan@cs.man.ac.uk (Stephen J Bevan)
Newsgroups: comp.lang.scheme
Subject: REPOST : Exception handling - how to define it ?
Message-ID: <BEVAN.91Apr5090348@panda.cs.man.ac.uk>
Date: 5 Apr 91 08:03:48 GMT
Sender: news@cs.man.ac.uk
Distribution: comp
Organization: Department of Computer Science, University of Manchester
Lines: 40

[I originally posted this on the 29th of March, but I've a feeling it
never made it off my local site.  If it did, then I appologise for
wasting time (and money) with a question nobody seems to be able to
answer]

What is the general style used to write exceptions and their handlers
in Scheme?  By exceptions I mean facilities similar to catch/throw in
Lisp.  The ways I can see of doing it are :-

1) Pass a success and fail continuation to every function.

2) Pass multiple continuations, one for each error to be handled.

3) Call a function that is assumed to be set by the user e.g.
 
     (define (foo x y z)
       ...
       (if (some-unexpected-error)
         (unexpected-error-exception args) ...))

   where `unexpected-end-of-file-exception' as a default just aborts.
   It would be up the user to re-define this as appropriate.  I guess
   the easiest way of doing this would be via fluid-let e.g. :-

      (fluid-let ((unexpected-error-exception
                    (lambda x (do-something-sensible x))))
        (foo an-arg another-arg yet-another-arg))

   However, as fluid-let isn't part of the standard (as far as I'm
   aware), this solution isn't portable.

So some questions :-

* Are there better mechanisms that those above?
* Any opinons as to which is the best?
* Am I totally off course trying to use exceptions in Scheme?

All answers greatefully received.

Stephen J. Bevan			bevan@cs.man.ac.uk