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Re: threads & dynamic environment & continuations
> I don't think this gets you all the way home. For example, assume K
> is a continuation -- something we made with CALL/CC. This won't work:
> (k (thunk))
> That sort-of punts the current continuation and "installs" K as
> the continuation for the THUNK call. But a simple implementation of
> Scheme wouldn't even realise that K was a continuation until THUNK
> had returned -- we wanted to punt the current stack eagerly, *before*
> we leapt off to THUNK. See the problem?
Yes, what you want are the procedures
(continuation-graft cont thunk)
(continuation-return cont result)
which are defined at the end of this message. They only rely on
call-with-current-continuation. I'm hoping to write a short paper on
this for the Scheme workshop.
> But put into SRFI-18 some *generic* language saying that dynamic binding
> facilities should be part of the continuation, hence threads pick up
> a new dynamic env on a continuation throw.
> How do you like that?
(define (continuation-capture receiver)
((call-with-current-continuation ; note important extra paren!
(continuation-return cont (receiver cont))))))
(define (continuation-graft cont thunk)
(define (continuation-return cont result)
(continuation-graft (lambda () result)))
; a test
(write 1) ; written to stdout
(write 2) ; written to the file "foo"
(continuation-graft cont (lambda () (write 3))) ; written to stdout
(write 4))))) ; never written
(write 5)) ; written to stdout