data structure for fast merge-insert

custom_hashset.hs

import qualified Data.HashSet as HS
import Data.Hashable

-- 2014-07-25 20:27:29
-- TODO custom HashSet with special ``insert`` ``fromListWith`` ...

data Test = A Int | B Double | C Char | D
    deriving (Show)

instance Eq Test where
    A _ == A _ = True
    B _ == B _ = True
    C _ == C _ = True
    D == D     = True
    _ == _     = False

instance Hashable Test where
    hash v =
        case v of
            A _ -> 1
            B _ -> 2
            C _ -> 3
            D   -> 4

t :: HS.HashSet Test
t = HS.fromList
    [ A 1
    , B 2.5
    , B 3
    , C 'H'
    , D
    ]