Give a better error message when we try to print a value of unknown type

  Stopped at ../Test3.hs:(1,0)-(2,30)
  _result :: [a]
  [../Test3.hs:(1,0)-(2,30)] *Main> _result

  <interactive>:1:0:
      Ambiguous type variable `a' in the constraint:
        `Show a' arising from a use of `print' at <interactive>:1:0-6
      Cannot resolve unkonwn runtime types: a
      Use :print or :force to determine these types