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
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