Text.Printf.printf has a bug where it doesn't like negative numbers for * modifiers. I opened a ticket along with a patch at http://hackage.haskell.org/trac/ghc/ticket/7457 I feel like it shouldn't need a discussion since it's just a bug fix, but if someone wants to speak up go ahead. It will change behaviour for people using negative numbers, but since the previous behaviour was "doesn't work" I hope they won't mind :)