Noah Lavine <noah.b.lav...@gmail.com> writes:
> On Thu, Mar 1, 2012 at 7:00 PM, 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? It is fine if it fails with an exception if one
>> still tries calling the continuation after it has already returned.
>
> If I understand correctly, you only need to call the continuation
> once. Is that right?
>
> In that case, I think you could use either catch and throw or prompts.
Neither of which are hygienic and also less straightforward. It is not
like I did not start my posting by giving the catch/throw-based example
and saying what I disliked about it.
--
David Kastrup
