Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!munnari.oz.au!goanna!ok
From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.scheme
Subject: Re: catch/throw in scheme - how to ??
Message-ID: <6246@goanna.cs.rmit.oz.au>
Date: 13 Jun 91 05:00:56 GMT
References: <1991Jun10.124127.17926@eua.ericsson.se> <JINX.91Jun10165336@chamarti.ai.mit.edu>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 40

In article <1991Jun10.124127.17926@eua.ericsson.se> joe@erix.ericsson.se (Joe Armstrong) writes:
> Could sombody who understands these things kindly explain how 
> to do the equivalent of 'catch' and 'throw' in scheme?? -
> can one use call/cc for such operations? - if so how?

In article <JINX.91Jun10165336@chamarti.ai.mit.edu>,
jinx@zurich.ai.mit.edu (Guillermo J. Rozas)
explained how to implement catch and throw on top of call/cc.

I suggest that it is simpler to use call-with-current-continuation directly.
For example, on p80 of Dybvig's book, we find

"Here is a more practical example, showing the use of call/cc to
provide a nonlocal exit from a loop.

    (define member
	(lambda (x ls)
	    (call/cc
		(lambda (break)
		    (do ((ls ls (cdr ls)))
			((null? ls) #f)
			(if (equal? x (car ls))
			    (break ls)))))))		"

(I have corrected two gratuitous incompatibilities with the standard.)

In general, the picture is
	(call-with-current-continuation (lambda (escape-function)
	    ;; any amount of your code goes here.
	    ;; When you want to return from the entire construct
	    ;; with value x, just call
	    (escape-function x)
	    ;; and that can go anywhere you like.
	    ))

The advantage of catch and throw is that a procedure can "throw" to
a "catch" which doesn't *lexically* enclose it.
-- 
Q:  What should I know about quicksort?   A:  That it is *slow*.
Q:  When should I use it?  A:  When you have only 256 words of main storage.