[Haskell-cafe] Flags

[Donald Bruce Stewart]

Maduser:
> 
> I have started to program in Haskell.  Now I want programm something like
> flags.  It's a set of flags. It sould be possible to convert the Flag as
> Int/String (the bit combination). I have written this:
> 
> import Data.Set as Set
> 
> type Flags = Set Flag
> data Flag = Flag1 | Flag2 | Flag3
> 	deriving(Eq, Ord, Enum, Show)
> 	
> flagToInt :: Flags -> Int
> flagToInt flags = flag1 + flag2 + flag3
> 	where
> 	flag1 = if member Flag1 flags then 1 else 0
> 	flag2 = if member Flag2 flags then 2 else 0
> 	flag3 = if member Flag3 flags then 4 else 0
> 
> but this seems to me to be inelegant. Can somebody help me making it better,
> please.

Something like:

    type Flags = [Flag]

    data Flag = Flag1 | Flag2 | Flag3
            deriving (Eq, Ord, Enum, Show)

    flagToInt :: Flags -> Int
    flagToInt = sum . map ((2 ^) . fromEnum)

Perhaps?

-- Don