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From: shap@delrey.sgi.com (Jonathan Shapiro)
Newsgroups: comp.object,comp.lang.c++
Subject: Continuations
Message-ID: <1623@odin.SGI.COM>
Date: 27 Nov 89 16:34:59 GMT
References: <2664@bingvaxu.cc.binghamton.edu> <9624@pyr.gatech.EDU>
Sender: news@odin.SGI.COM
Organization: Silicon Graphics, Inc., Mountain View, CA
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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.

For those of you who are not already conversant with continuations,
the idea comes from Scheme (though it has earlier antecedents), and
looks like this:

   (call-with-current-continuation
     (lambda (continuation)
       (body...)))

the call-with-current-continuation form potentially returns multiple
times, because the continuation can be saved to a global variable.  A
subsequent invocation can be done with:

   (continuation value-producing-form)

which causes a return from the call-with-current-continuation with
that value.

Logically, the continuation captures the future state of the program
at the point at which it was captured.  That is, it captures a pc, a
set of registers, and a stack.  Note that this implies garbage
collection of stack frames, because a live frame in an enclosing
context can be shared between a continuation and subsequent code.

Continuations have seen use as error handlers, because they look like
function invocations.  One can set up an exception handling routine,
capture it with a continuation, and use the continuation as a
"longjump" type mechanism to get out of a bad state.

Continuations are different from longjump in that they capture the
stack as well as the registers.

A detailed description can be found in the Revised Revised... Report
on scheme, which can be obtained via anonymous FTP from
zurich.ai.mit.edu.

Jonathan Shapiro
Silicon Graphics, Inc.


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