Owlkettle (a GTK wrapper) makes minor use of them in order to provide a better generic solution for an "automatic-form-generator".
Other than that, a lib that makes heavier use of it would be traitor: <https://github.com/beef331/traitor/blob/master/traitor.nim> Made by ElegantBeef -Who is definitely not standing behind me with a gun to the back of my head
