From: Matthew Flatt <>
Subject: v299.2
Date: Mon, 8 Mar 2004 16:50:32 -0700

The "v299"-tagged code in CVS for MzScheme and MrEd is now version 299.2.

Makeshift docs:

In 299.2, a continuation is no longer tied to its creating thread.

  (define (stooges)
    (let ([k #f])
       (thread (lambda ()
                 (let/ec done
                   (let ([t (let/cc -k (set! k -k) (done))])
                     (sleep t)
                     (printf "Hellooooo,~n")
                     (sleep (- 2 t))
                     (printf "Hello!~n")
                     (printf "Shouldn’t get here!~n"))))))
      (thread (lambda () (k 0)))
      (thread (lambda () (k 0.5)))
      (thread (lambda () (k 1)))))

  (stooges) ; prints "Hello"s

Various continuation barriers remain in place, such as around the call
to an exception handler or syntax expander, and also around the start
of MzScheme’s main thread.

In particular, the main thread’s barrier prevents continuations moving
into or out of the main thread (because then other threads could become
"main"). A newly created thread, however, has no such barrier, so that
most threads can trade continuations. In other words, v299.2 really
just removes the continuation barrer that use `thread’ used to install.

To make thread-independent continuations practical, the "parameter"
construct has been redefined, much as we discussed about a year ago.
The new definition is indistinguishable from the old one, except that
(1) the body of a `parameterize’ is in tail position with respect to
the entire `parameterize’ expression, (2) a `parameterize’ expression
tends to execute much more quickly, while parameter lookup tends to be
slightly slower, (3) a `parameterize’ has the expected effect if a
continuation is captured during the `parameterize’ body and invoked in
a different thread.

Compared to Scsh [see Scheme’02]:

 * A "parameter" in MzScheme is exactly (I think) a Scsh preserved
   thread fluid, where MzScheme’s `thread’ is Scsh’s `fork-thread’
   (a.k.a `spoon’).

 * MzScheme doesn’t provide `fluid’ as a separate construct. Parameters
   are implemented in terms of "preserved thread cells" and
   continuation marks. Specifically, a continuation’s parameterization
   is designated by a continuation mark whose value is a mapping from
   parameters to thread cells. (A new `continuation-mark-set-first’
   procedure provides a fast version of `car’ composed with
   `continuation-mark-set->list’, which is used by the expansion of

 * A "preserved thread cell" is exactly the old "parameter", and plain
   "thread cells" are now available (as in Scsh). The
   `make-thread-cell’ procedure takes an optional argument to indicate
   whether the thread cell is preserved (defaults to #f).