tcfail171.hs:9:10:
    No instance for (PrintfArg a) arising from a use of `printf'
    Possible fix:
      add (PrintfArg a) to the context of
        the type signature for phex :: a -> b
    In the expression: printf "0x%x" x
    In an equation for `phex': phex x = printf "0x%x" x