tcfail042.hs:15:10:
    Could not deduce (Num a)
      arising from the superclasses of an instance declaration
    from the context (Eq a, Show a)
      bound by the instance declaration at tcfail042.hs:15:10-34
    Possible fix:
      add (Num a) to the context of the instance declaration
    In the instance declaration for ‛Bar [a]’