Very looking forward for this

Rebased, added tests and made sure the docs are accurate. I'd say let's have this!

lib.fileset.fileFilter (file:
  lib.elem file.ext [ "nix" "hs" ]
) ./.

What about tar.gz?
How about:

lib.fileset.fileFilter (file:
  file.hasExt "hs" || file.hasExt "nix"
) ./.

lib.fileset.fileFilter (file:
  ! file.hasExt "tar.gz"
) ./.

Not bad, here's another idea though:

lib.fileset.fileFilter (file:
  file.fullExt != "tar.gz"
) ./.

and

lib.fileset.fileFilter (file:
  file.lastExt != "gz"
) ./.

I do think there's some benefit to getting the extension value directly, e.g. you could do

lib.fileset.fileFilter (file:
  if file.ext == null then
    true
  else
    {
      nix = false;
      c = true;
      h = false;
    }.${file.lastExt}
    or (throw "Don't know what to do with file extension ${file.lastExt}")
) ./.

Also there wouldn't be a problem to have multiple of these. Their names make them pretty obvious.

But then you have to get into what full means. What if you have

foo-master.tar.gz
foo-1.2.tar.gz

Is that a foo-1 man page, archived and compressed?

Determining which part of the file name is extension is up to the caller, not up to fileFilter. It can not have the domain knowledge, so it must not try.

Fair enough! I guess with hasExt, you can even do

lib.fileset.fileFilter (file:
  lib.any file.hasExt [ "hs" "nix" ]
) ./.

which is pretty short

Is it a file system tree?

OfBorg is sluggish today.

Is it a file system tree?

Nice.

OfBorg is sluggish today.

Only today? 😅

Successfully created backport PR for release-23.11:

• [Backport release-23.11] lib.fileset.fileFilter: Predicate attribute for file extension #269520