On Fri, Mar 2, 2012 at 8:00 AM, David Kastrup <dak@gnu.org> wrote:

>
> Hi,
>
> I am just meddling around with coding and have come up with the
> following:
>
> (define-public (find-child music predicate)
>  "Find the first node in @var{music} that satisfies @var{predicate}."
>  (catch 'music-found
>         (lambda ()
>           (fold-some-music predicate
>                            (lambda (music . _) (throw 'music-found music))
>                            #f music))
>         (lambda (key music) music)))
>
> Now the problem with that is that it is unhygienic.  If fold-some-music
> were to use music-found signals, or if the predicate did, things would
> be awkward.  One would need to work with a uniquely generated symbol.
> It turns out that the above can be expressed much clearer and cleaner as
>
> (define-public (find-child music predicate)
>  "Find the first node in @var{music} that satisfies @var{predicate}."
>  (call-with-current-continuation
>   (lambda (music-found)
>     (fold-some-music predicate
>                      (lambda (music . _) (music-found music))
>                      #f music))))
>
> at least if I did not make some thinko here.  It is basically the same
> code and stack-upwards-only, but hygienic.  Nothing can call the
> continuation but what is inside.  Well, of course fold-some-music could
> save the closure calling music-found for later.  But it doesn't.
>
> Is there a way to get a call-with-current-continuation that does not
> create a stack copy?


IIRC, the stack copying in Guile's continuation implementation is
inevitable. Though it's inefficient, the consideration is to cooperate with
other languages such as C.


> It is fine if it fails with an exception if one
> still tries calling the continuation after it has already returned.
>
> --
> David Kastrup
>
>
>