maeda@MCC.COM (Christopher Maeda) (10/03/88)
Date: 3 Oct 88 07:08:04 GMT
From: pierce@locus.ucla.edu
** Subject: unwind-protect
** Message-ID: <10180@eddie.MIT.EDU>
** Date: 3 Oct 88 03:41:57 GMT
** Sender: uucp@eddie.MIT.EDU
** Reply-To: jbs@fenchurch.MIT.EDU (Jeff Siegal)
** Organization: MIT EE/CS Computer Facilities, Cambridge, MA
** Lines: 5
**
** How can one do something like UNWIND-PROTECT in Scheme? Is the
** concept completely ill-defined in the presence of first-class
** continuations, or is there an idiom which "sometimes works?"
**
** Jeff Siegal
Kent Dybvig's Chez Scheme has what seems to be a much more powerful construct
called "dynamic-wind". A brief sketch of its uses is given in Dybvig's
"The Scheme Programming Language".
-- Brad Pierce
Hmm. How would unwind-protect be expressed with just call/cc?will@uoregon.uoregon.edu (William Clinger) (10/04/88)
>Hmm. How would unwind-protect be expressed with just call/cc?
UNWIND-PROTECT is trivial to implement in terms of DYNAMIC-WIND, which can
be implemented in terms of CALL-WITH-CURRENT-CONTINUATION provided the
Scheme system permits CALL-WITH-CURRENT-CONTINUATION to be redefined,
uses close-coded calls to CALL-WITH-CURRENT-CONTINUATION for all captures
of a continuation, and contains no pre-captured continuations.
An implementation appears in Haynes and Friedman, Embedding Continuations
in Procedural Objects, TOPLAS 9, 4 (October 1987), pages 582-598.
Peace,
William Clinger