[Haskell-cafe] (newbie) instance Enum MyType where, smarter way?

[Adrian Neumann]

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Hello,

I defined an enumeration datatype like this

data MyType = One | Two | Four | Eight

and want to make it an instance of the class Enum. deriving Enum won't
do what I want, as it labels the items 0,1,2,3. Is there a better way to
do this than

instance Enum MyType where
	fromEnum One = 1
	fromEnum Two = 2
	...
	toEnum 8 = Eight

Something like

instance Enum MyType where
	fromEnum One = 1
	fromEnum x = 2*pred x
	toEnum 1 = One
	toEnum x = succ (toEnum x `div` 2)

Which doesn't work because succ and pred are not (properly?) defined. Is
there a way to let deriving Enum do *some* of work (i.e. defining succ
and pred) while manually defining the other functions?
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