Xref: utzoo comp.object:554 comp.lang.c++:5737
Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!wuarchive!decwrl!shelby!neon!max
From: max@Neon.Stanford.EDU (Max Hailperin)
Newsgroups: comp.object,comp.lang.c++
Subject: Re: Continuations
Message-ID: <1989Dec1.205817.5178@Neon.Stanford.EDU>
Date: 1 Dec 89 20:58:17 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> <1989Nov29.225826.19483@odi.com> <10008@june.cs.washington.edu> <21989@brunix.UUCP>
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Reply-To: mxh@sumex-aim.Stanford.EDU (Max Hailperin)
Organization: Computer Science Department, Stanford University
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In article <21989@brunix.UUCP> twl@boojum.UUCP (Ted "Theodore" (W) Leung)
writes:
>In article <10008@june.cs.washington.edu> peterd@june.cs.washington.edu (Peter
>C. Damron) writes:
>>4. C and C++ have first class functions.  
>>   ... no closure required ...
>It seems like it would be useful to make a distinction between "first
>class" functions which require their implementation via a closure, and
>those which do not.

As an asside to this discussion, it appears to me that closures can be
emulated relatively smoothely in C++.  The two key features that support
this are:
 1) The procedure-call operator can be overloaded, so that you can define
    a class of closure objects which can be called as if they were procedures.
    The constructor (lambda equivalent) can copy the closed-over variables
    into slots with the same name, so that then the procedure-call member
    function can use them.
 2) The notion of a reference type (explicit aliasing) allows the closures
    to share mutable variables that are in a common scope.

Of course, this is still a bit messy, but nothing compared with
emuating closures in C.  Also, there are other language features that
interact poorly with a higher-order style, limiting the utility.

I haven't actually used this technique (I'm lucky enough to have a
real lisp, instead of having to make C++ pretend it's one).  I'd be
very interested to hear from anyone who has.


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