Adrian Neumann wrote:
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
Define them as deriving Enum. Now on top of this define bitValue :: MyType -> Int bitValue m = 2^(fromEnum m) and, if you need it, the converse. Or even something neater which splits a number into an array of constructors corresponding to bitnums manyValues :: [MyType] -> Int manyValues = foldr (.|.) . map bitValue -- you need Data.Bits for the bitwise or, .|. fromInt :: Int -> [MyType] fromInt i = filter (\n -> i .&. (bitValue n) > 0) [One..Eight] Of course, I'm taking the liberty of guessing here that you're modelling binary-flags-as-bitfields Jules