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Re: Exceptions that pass continuations

From: Taylan Ulrich B.
Subject: Re: Exceptions that pass continuations
Date: Fri, 19 Jul 2013 16:12:40 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (berkeley-unix)

I agree with Thien on that all resource-management should be delegated
to the garbage collector via smob types.

However, I found this supply-demand pattern quite neat (could have other
uses perhaps), was kind of bored, with a little free time on my hands,
and enjoy an occasional exercise in delimited continuations, so here's a
cleaned-up version of the code using prompts. :D  But see the note at
the end.

(define prompt-tag (make-prompt-tag "supply"))

(define-syntax supply
  (syntax-rules ()
    ((_ (((action-name . action-args) abody abody* ...) ...) body body* ...)
     (let ((actions (alist->hash-table
                     `((,action-name . ,(lambda action-args
                                          abody abody* ...))
        (lambda () body body* ...)
        (lambda (cont demanded-action . args)
          (let ((action (hash-table-ref/default actions demanded-action #f)))
            (if action
                (cont (apply action args))
                (apply throw 'demand-not-supplied demanded-action args)))))))))

(define (demand action-name . args)
  (apply abort-to-prompt prompt-tag action-name args))

;;; It composes neatly:
(supply ((('foo . bar) bar)) (demand 'foo 0 1 2)) ;=> (0 1 2)

Now there's something funny to notice here.  I was uncomfortable forcing
the action-names to be literals, because then one couldn't rename them
and thus different modules would get name-clashes; so I don't
automatically quote the action-name, and would expect modules to export
the "names" of the actions they demand as variables holding unique
objects (e.g. a uniquely allocated cons cell), so those variables can be
renamed and the action they refer to is still unique.  And at that
point, we pretty much re-implemented parameters!  In a much worse way
than their Guile-native implementation of course, and forcing their
values to be procedures.

Long story short, use parameters for this pattern. :)


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