I encountered the same problem as this guy: http://www.haskell.org/pipermail/glasgow-haskell-users/2002-May/003407.html It is still present in GHC-6.2.1. Only if you turn on -fglasgow-exts the RULES pragma is parsed correctly. I think GHC should give a warning like "turn -fglasgow-exts on for RULES pragma" instead of: Media.hs:23: Variable not in scope: `forall' Media.hs:23: Variable not in scope: `f' Media.hs:23: Variable not in scope: `f' Media.hs:23: Variable not in scope: `f'