instance Enum MyDataType where
    fromEnum = fromJust . flip lookup table
    toEnum = fromJust . flip lookup (map swap table)
table = [(Foo, 0), (Bar, 1), (Baz, 2)]

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instance Enum MyDataType where
    fromEnum Foo = 0
    fromEnum Bar = 1
    fromEnum Bax = 2
    toEnum 0 = Foo
    toEnum 1 = Bar
    toEnum 2 = Baz