Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!odi!dlw
From: dlw@odi.com (Dan Weinreb)
Newsgroups: comp.object
Subject: Re: Continuations
Message-ID: <1989Dec1.232255.24662@odi.com>
Date: 1 Dec 89 23:22:55 GMT
References: <2664@bingvaxu.cc.binghamton.edu> <9624@pyr.gatech.EDU> <1623@odin.SGI.COM> <99894@ti-csl.csc.ti.com> <1989Nov29.230331.19595@odi.com> <7159@ficc.uu.net>
Reply-To: dlw@odi.com
Organization: Object Design, Inc.
Lines: 31
In-Reply-To: peter@ficc.uu.net's message of 30 Nov 89 18:22:46 GMT

In article <7159@ficc.uu.net> peter@ficc.uu.net (Peter da Silva) writes:

   In article <1989Nov29.230331.19595@odi.com> dlw@odi.com writes:
   > It depends what you mean.  It would certainly be possible to invent a
   > definition of a language that was like C, but also had the features
   > needed to do continuation-style programming.

   There is. It's called C. Almost.

   Seriously. All you need to do is have a bit of unportable code that mucks
   around in struct jmp_buf. Save the stack between here and there, do a setjmp
   to mark the end of the current stack, and then longjmp back to the saved
   spot. To continue, restore the saved stack and longjmp back to the mark.
   It would be advisable to write this code in assembly language :->.

   This would also give you the ability to add coroutines, threads, and all
   sorts of other good stuff. Let's call it ++C.

But it would not give me the ability to write programming that are in
what is traditionally (at the MIT AI lab -- OK, I'm provincial, but
that's where I was trained) referred to as continuation-passing
style".  When all the interesting papers about "continuation-passing
style" were coming out at MIT in the late '70s, they showed a lot of
very interesting techniques and things that you could do with
continuations.  But these techniques also employed funargs (both
kinds).  It's true that you could add the control structure all alone,
and call what you get "continuation-passing style", but it would not
be the style to which I had become accustomed.  Obviously my posting
was unclear because I had in mind this particular specialized meaning
of the phrase "continuation-passing style", and I didn't explain what
I meant.  Sorry about that.


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