> - A more general alternative would be a user-supplied filter procedure.
> But that again gets into the territory of higher-level frameworks IMHO.
>
> - Common Lisp used to have a 'with-directory-iterator' function in some
> third-party library. So instead of returning a list of names, it
> returned an iterable. For filtering, this could be the best option. A
> directory _can_ in principle have 10k files or even more.
In fact, Chibi has a directory-fold procedure:
<https://github.com/ashinn/chibi-scheme/blob/master/lib/chibi/filesystem.scm#L23>.
It might be worth having that in the SRFI. For some applications, you
want to collect the filenames into a hashtable instead of a list, so a
list is superfluous.

The only thing is, the implementation is going to make FFI calls to
readdir() or getdirentries() in a loop. Going back and forth between the
FFI and the user-supplied fold function might be about as costly as just
returning a list/vector of everything and filtering it after the fact.