import Data.Tuple
import Data.Maybe
import Data.List

data MyDataType = Foo
     | Bar
     | Baz
     deriving (Enum,Eq,Show)

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


$ ghc loo.hs
[1 of 1] Compiling Main             ( loo.hs, loo.o )

loo.hs:8:16: error:
    Duplicate instance declarations:
      instance Enum MyDataType -- Defined at loo.hs:8:16
      instance Enum MyDataType -- Defined at loo.hs:10:10
  |
8 |      deriving (Enum,Eq,Show)
  |                ^^^^
[haroldo@semiosis haskell]$