Something

owen@bast:~/Code/Haskell$ cat Format.hs
module Format where
    import Express
    import Text.Printf

format Express.X   = "X"
owen@bast:~/Code/Haskell$ hugs Deriv.hs Format.hs
__   __ __  __  ____   ___      _________________________________________
||   || ||  || ||  || ||__      Hugs 98: Based on the Haskell 98 standard
||___|| ||__|| ||__||  __||     Copyright (c) 1994-2005
||---||         ___||           World Wide Web: http://haskell.org/hugs
||   ||                         Report bugs to: hugs-bugs@haskell.org
||   || Version: 20050308       _________________________________________

Haskell 98 mode: Restart with command line option -98 to enable extensions

ERROR "Format.hs":5 - Syntax error in input (unexpected symbol "format")