Re: Delimited continuations

[Michael Heerdegen <michael_heerdegen@web.de>]

"John Wiegley" <johnw@gnu.org> writes:

>   ;; -*- lexical-binding: t -*-
>   
>   (defun shift (k entry)
>     (if (eq (nth 0 k) 'outer)
>         (throw (nth 1 k)
>                (funcall entry #'(lambda (val)
>                                   (funcall (nth 2 k)
>                                            (list 'inner val)))))
>       (nth 1 k)))
>   
>   (defun reset (thunk)
>     (let ((bound (make-symbol "reset--bound")))
>       (catch bound
>         (funcall thunk (list 'outer bound thunk)))))
   
This seems to be clever.

But one little disadvantage seems to be that a part of the code (the
part of the `reset' thunk that is evaluated until `shift' is reached) is
processed twice: once till the `shift' is reached (which `throw's and
hasn't access to this calculation, so that part of the calculation is
thrown away) and a second time when the delimited continuation is
called.

E.g. in

#+begin_src emacs-lisp
(reset (lambda (p)
         (+ (+ 1 2)
            (shift p (lambda (k) (funcall k 1))))))
#+end_src

(+ 1 2) is executed twice.  I think this isn't necessarily so?


BTW, I wonder what one can potentially do with this stuff.


Michael.