Xref: utzoo comp.object:514 comp.lang.c++:5690
Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!usc!apple!apple.com!chewy
From: chewy@apple.com (Paul Snively)
Newsgroups: comp.object,comp.lang.c++
Subject: Re: Re^2: Continuations
Message-ID: <5473@internal.Apple.COM>
Date: 29 Nov 89 00:04:28 GMT
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
Lines: 37
References:<2664@bingvaxu.cc.binghamton.edu> <9624@pyr.gatech.EDU> <1623@odin.SGI.COM> <1989Nov28.183816.15252@odi.com> <9964@june.cs.washington.edu> <34657@cornell.UUCP>

In article <34657@cornell.UUCP> murthy@alsvid.cs.cornell.edu (Chet Murthy) 
writes:
> >I just had to reply when I saw this.  C and C++ are definitely 
"lexically
> >scoped" (I would prefer to call it statically scoped).
> 
> Yes, they are, but they don't provide support for closures as
> first-class objects (the funarg problem).  So they don't provide
> "full" support for lexical scoping.  Scheme/ML do.
> --chet--

I think this is why Scheme lends itself so readily to the implementation 
of a wide variety of "object-oriented" models: Scheme makes no a priori 
distinction between code and data (that is, functions, or closures if you 
wish to be more precise, can be passed as arguments, returned as values, 
stored in other data structures, etc. etc. etc.)

Of particular interest from an object-oriented standpoint, I think, is the 
fact that with full upward continuations and closures, you can VERY easily 
write a function that takes a list of arguments and returns the value of 
some arbitrary argument, and when you invoke another function (say fail), 
backtracking is performed to the last decision point, and one of the OTHER 
arguments' values is returned, etc. until there are no more arguments.  
This non-deterministic programming system is particularly useful for 
implementing delegating object systems, and can be implemented trivially 
in Scheme ("trivially" meaning in about one and a half 8 1/2" x 11" pages 
printed on a LaserWriter in 10-point Courier).  It's also worth noting 
that this non-deterministic example, using continuations and closures, 
ships with MacScheme 2.0 or later (in case you've got a Mac and are 
curious about it).

__________________________________________________________________________
Just because I work for Apple Computer, Inc. doesn't mean that they 
believe what I believe or vice-versa.
__________________________________________________________________________
C++ -- The language in which only friends can access your private members.
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