Am So., 9. Okt. 2022 um 11:55 Uhr schrieb Marc Nieper-Wißkirchen
<marc.nie...@gmail.com>:
>
> John Cowan suggested that SRFI 226 should include a version of Common
> Lisp's unwind-protect suitable for Scheme.
>
> In [1], I replied after some discussion with the following proposal
> for a definition of a Scheme unwind-protect:
>
> (define-syntax unwind-protect
> (syntax-rules ()
> ((unwind-protect protected-form cleanup-form)
> (call-with-continuation-barrier
> (lambda ()
> (dynamic-wind
> (lambda () (values))
> (lambda () protected-form)
> (lambda () cleanup-form)))))))
I am currently adding unwind-protect to the spec. The actual
definition should be
(define-syntax unwind-protect
(syntax-rules ()
((unwind-protect protected-form cleanup-form ...)
(lambda ()
(dynamic-wind
(lambda () (values))
(lambda () (call-with-continuation-barrier (lambda ()
protected-form))))
(lambda () (values) cleanup-form ...))))))
because there is no reason to protect the cleanup-forms from
call/cc-havoc. The reason is that these forms are not protected
otherwise as well.
Marc
