Xref: utzoo comp.object:524 comp.lang.c++:5708
Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!odi!dlw
From: dlw@odi.com (Dan Weinreb)
Newsgroups: comp.object,comp.lang.c++
Subject: Re: Continuations
Message-ID: <1989Nov29.225826.19483@odi.com>
Date: 29 Nov 89 22:58:26 GMT
References: <2664@bingvaxu.cc.binghamton.edu> <9624@pyr.gatech.EDU> <1623@odin.SGI.COM> <1989Nov28.183816.15252@odi.com> <9964@june.cs.washington.edu>
Reply-To: dlw@odi.com
Organization: Object Design, Inc.
Lines: 45
In-Reply-To: peterd@cs.washington.edu's message of 28 Nov 89 20:11:10 GMT

In article <9964@june.cs.washington.edu> peterd@cs.washington.edu (Peter C. Damron) writes:

   In article <1989Nov28.183816.15252@odi.com> dlw@odi.com writes:
   >In article <1623@odin.SGI.COM> shap@delrey.sgi.com (Jonathan Shapiro) writes:
   >
   >   I do not believe that it is feasible to add continuations to C++ for
   >   any number of reasons, but I would be interested to hear the reactions
   >   in this community regarding their utility in object-oriented programming.
   >
   >They are just as useful in object-oriented programming as in straight
   >Lisp or Scheme.

   How can you make this claim?

Easy, I just press the keys, and...  Seriously, I have used
continuations in an object-oriented programming language (Lisp with
Flavors), for years, as have several of my former colleagues.  So I
feel pretty qualified to claim that they're useful.  Do you have any
more specific rebuttal than amazement that I could even say such a
thing?

   >However, continuations in the Scheme style are only
   >useful if full support is provided for lexical scoping.  C and C++
   >have no lexical scoping whatsoever.

   I just had to reply when I saw this.  C and C++ are definitely "lexically
   scoped" (I would prefer to call it statically scoped).

In a sense, C and C++ are lexically scoped, but only in a trivial
sense, since there are no internal procedures and no block structure.
So, you are technically right, and I should revise my statement to say
that continuations are only useful if the continuation can reference
variables of lexically enclosing blocks, and classic
"continuation-passing style" in the Sussman and Steele sense requires
further that they should be able to refer to such variables even if
the enclosing block is no longer being executed (what is traditionally
known in the Lisp/Scheme community as "supporting upward funargs").
Few languages support upward funargs; Common Lisp and Scheme are two
that do.  Many languages support downward funargs; Algol-60, Pascal,
and PL/I come to mind.  In C, you can pass a function around as an
argument, but there is no "lexical link" allowing it to reference
variables in its parent block; in fact, there are no parent blocks in C.
(It can refer to static and extern, but that's not the same thing.)

Dan Weinreb		Object Design, Inc.		dlw@odi.com


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