Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!decwrl!labrea!russell!evan
From: evan@russell.STANFORD.EDU (Evan Kirshenbaum)
Newsgroups: comp.lang.scheme
Subject: Re: !
Message-ID: <2273@russell.STANFORD.EDU>
Date: 10 Feb 88 00:05:19 GMT
References: <323426.880208.JAR@AI.AI.MIT.EDU> <8802090251.AA01964@RAVEN.LCS.MIT.EDU>
Reply-To: evan@russell.UUCP (Evan Kirshenbaum)
Organization: Center for the Study of Language and Information, Stanford U.
Lines: 40

In article <8802090251.AA01964@RAVEN.LCS.MIT.EDU> bard@THEORY.LCS.MIT.EDU (Bard Bloom) writes:
]
]   Date: Mon,  8 Feb 88 17:44:38 EST
]   From: Alan@AI.AI.MIT.EDU
]   Sender: JAR@AI.AI.MIT.EDU
]
]   (define (make-cell)
]     (call-with-current-continuation
]       (lambda (return-from-make-cell)
]	 (letrec ((state
]		    (call-with-current-continuation
]		      (lambda (return-new-state)
]			(return-from-make-cell
]			  (lambda (op)
]			    (case op
]			      ((set)
]			       (lambda (value)
]				 (call-with-current-continuation
]				   (lambda (return-from-access)
]				     (return-new-state
]				       (list value return-from-access))))))
]			      ((get) (car state)))))))))
]	   ((cadr state) 'done)))))
]
]
]
]What the hey is this?


A side-effect-free value cell (I think).  Cute.  For those who take
the trouble to walk through it, I think that last call should be to
(caddr state) [the return-from-access continuation].

Now can you do it without consing?

---
Evan Kirshenbaum
Stanford University
  evan@CSLI.STANFORD.EDU
  ...!ucbvax!russell.stanford.edu!evan