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