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]’

tcfail042.hs:17:18:
    Could not deduce (Num a) arising from a use of ‘foo’
    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 expression: foo xs
    In an equation for ‘bar’:
        bar (x : xs)
          = foo xs
          where
              u = x == x
              v = show x
    In the instance declaration for ‘Bar [a]’