[Haskell-beginners] "Cyclic" instance of Enum

[Patrick LeBoutillier]

Hi,

I've created an instance of Enum that repeats the same elements when
it gets to the end:


module Music.Note.Name where


data Name = C | D | E | F | G | A | B
            deriving (Show, Eq)

instance Enum Name where
  toEnum 0 = C
  toEnum 1 = D
  toEnum 2 = E
  toEnum 3 = F
  toEnum 4 = G
  toEnum 5 = A
  toEnum 6 = B
  toEnum i = toEnum $ i `mod` 7

  fromEnum C = 0
  fromEnum D = 1
  fromEnum E = 2
  fromEnum F = 3
  fromEnum G = 4
  fromEnum A = 5
  fromEnum B = 6

  enumFromTo x y = map toEnum [a .. b']
    where a = fromEnum x
          b = fromEnum y
          b' = if a <= b then b else b + 7

  enumFromThen x1 x2 = error "enumFromThen not supported for Music.Note.Name"
  enumFromThenTo x1 x2 y = error "enumFromThenTo not supported for
Music.Note.Name"


Does this violate any implicit rules that an Enum instance should
follow? Is there a better way to do this?


Thanks,

Patrick

-- 
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Patrick LeBoutillier
Rosemère, Québec, Canada