import Data.List

-- Given three data types...
data A = A deriving (Show)
data B = B deriving (Show)
data C = C deriving (Show)

-- and a unifying type that covers them all...
data Covered
  = CoveredA A
  | CoveredB B
  | CoveredC C


-- what's the best way to write this function?
extract :: [Covered] -> ([A], [B], [C])
-- my best guess follows.
extract covereds = (as, bs, cs) where
  (coveredAs, others) = partition isA covereds
  (coveredBs, coveredCs) = partition isB others
  isA (CoveredA _) = True
  isA _ = False
  isB (CoveredB _) = True
  isB _ = False
  as = map (\(CoveredA a) -> a) coveredAs
  bs = map (\(CoveredB b) -> b) coveredBs
  cs = map (\(CoveredC c) -> c) coveredCs


testData = [CoveredC C, CoveredA A, CoveredB B, CoveredC C, CoveredB B, CoveredA A]
main = print $ extract testData