SimpleFail16.hs:10:12:
    Couldn't match expected type `p0 a0' with actual type `F ()'
    The type variables `p0', `a0' are ambiguous
    Possible cause: the monomorphism restriction applied to the following:
      bar :: p0 a0 (bound at SimpleFail16.hs:10:1)
    Probable fix: give these definition(s) an explicit type signature
                  or use -XNoMonomorphismRestriction
    In the first argument of `foo', namely `(undefined :: F ())'
    In the expression: foo (undefined :: F ())
    In an equation for `bar': bar = foo (undefined :: F ())