[plt-scheme] call/cc puzzle

From: Shriram Krishnamurthi (sk at cs.brown.edu)
Date: Sat Jan 17 14:38:20 EST 2009

Just remember that putting a use of call/cc in a context changes the meaning
of that use...

On Jan 17, 2009 2:22 PM, "Jon Rafkind" <rafkind at cs.utah.edu> wrote:

Henk Boom wrote: > > 2009/1/17 Jos Koot <jos.koot at telefonica.net>: >   >> >>
Is that a quation or ju...
I think I figured out at least part of it. Hope I don't spoil it for

(let ([f (call/cc1 call/cc2)])
 (printf "f is ~a\n" f)
 (f (lambda (x) (printf "x is ~a\n" x))))

I wrote call/cc1 and call/cc2 to mean call/cc but so I could identify them
in the explanation. The result of this prints

f is #<continuation>
f is #<procedure:/home/jon/tmp/x.ss:17:5>
x is #<procedure:/home/jon/tmp/x.ss:17:5>

The flow is this: call/cc1 passes its continuation to call/cc2 which invokes
the first continuation with the second. So f is now equal to the
continuation of call/cc2 and it prints #<continuation> as expected. Now f is
invoked with the continuation of call/cc2 with (lambda (x) ...) which starts
the computation at (let ([f ...]) ...) so f is now bound to (lambda (x)
...). Printing this results in <procedure>, and then the f [which is now
(lambda (x) ...)] is applied to (lambda (x) ...) which prints #<procedure>

So at the end it does ((lambda (x) (printf "x is ~a\n" x)) (lambda (x)
(printf "x is ~a\n" x))). Maybe this is obvious to some people but it took
me at least 15 minutes to figure out.

_________________________________________________  For list-related
administrative tasks:  http://l...
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.racket-lang.org/users/archive/attachments/20090117/1ed746df/attachment.html>

Posted on the users mailing list.