Ah, true. Penlight helps, but you are correct it is a separate module. I think for being the target of a higher-level language it might help that you aren't married to a standard library.

I fully agree with you but just want to add that you can do it in "pure" Lua :

for file in io.popen("ls "..dir):lines() do ... end

This should never be used in real applications and a proper filesystem module should be used instead. But, I find it very useful for quick one-shot scripts I write for batch processing some files.