On Fri, Mar 2, 2012 at 8:00 AM, David Kastrup <d...@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
>
>
>
