смотрите какая классная штуковина:

{-# LANGUAGE GADTs #-}
import Data.Dependent.Map as D
import Data.GADT.Compare

import GHC.Generics

data Hint a where
  HintPort   :: Hint Int
  HintHost   :: Hint String
  HintTeapot :: Hint Int

instance GEq Hint where
  HintPort `geq` HintPort = Just Refl
  HintHost `geq` HintHost = Just Refl
  HintTeapot `geq` HintTeapot = Just Refl
  _ `geq` _ = Nothing

instance GCompare Hint where
  HintPort `gcompare` HintPort = GEQ
  HintHost `gcompare` HintHost = GEQ
  HintTeapot `gcompare` HintTeapot = GEQ
  HintPort `gcompare` HintHost = GGT
  HintPort `gcompare` HintTeapot = GGT
  HintHost `gcompare` HintTeapot = GGT
  a `gcompare` b = case b `gcompare` a of
                     GEQ -> GEQ
		     GGT -> GLT
		     GLT -> GGT

fibs = 1:1:zipWith (+) fibs (tail fibs)

test :: DMap Hint
test = D.insert HintTeapot 7 $ D.insert HintPort 99 $ D.insert HintPort 7 D.empty

и потом:

*Main> test ! HintPort
99
*Main> test ! HintTeapot
7
*Main> test ! HintHost
"*** Exception: DMap.find: element not in the map
*Main> HintHost `D.lookup` test
Nothing


ещё бы инстансы автоматом генерить (хотя это наверняка в либе есть)