[Haskell-beginners] Defining of subset of constructors as valid values

Hi all I have the following code : data CmdTiming = RAS Time | CAS Time | PCH Time | ACT Time deriving (Ord,Show,Eq) data Command = RD_CMD {bank :: Int, col :: Int } | WR_CMD {bank :: Int, col :: Int} | ACT_CMD {bank :: Int, row :: Int} | PCH_CMD {bank :: Int} deriving Show I'd like to have something like : RD_CMD {bank :: Int, col :: Int,timing :: RAS Time } to limit possible values to RAS Time . The reason I use constructors is differentiate whether current value is applicable to some function , getting CmdTiming as parameter or not . I am not sure this is valid definition since RAS Time is not type . Regards, Lev