tcfail171.hs:9:10:
    No instance for (PrintfType b) arising from a use of ‘printf’
    Possible fix:
      add (PrintfType b) 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