{-# LANGUAGE EmptyDataDecls #-} module Main (main) where import Unsafe.Coerce data Anything newtype Key x = Key Int deriving Eq type Dict = [(Key Anything, Anything)] put :: Key x -> x -> Dict -> Dict put k' v' = raw (unsafeCoerce k') (unsafeCoerce v') where raw k0 v0 [] = [(k0,v0)] raw k0 v0 ((k,v):zs) | k == k0 = (k0 ,v0) : zs | otherwise = (k ,v ) : raw k0 v0 zs get :: Key x -> Dict -> Maybe x get k' zs = unsafeCoerce (raw (unsafeCoerce k') zs) where raw k0 [] = Nothing raw k0 ((k,v):zs) | k == k0 = Just v | otherwise = raw k0 zs main = do let k1 = Key 1 :: Key Int let k2 = Key 2 :: Key Double let k3 = Key 3 :: Key String let k4 = Key 4 :: Key Bool let d0 = [] let d1 = put k1 123 d0 let d2 = put k2 123 d1 let d3 = put k3 "123" d2 let d4 = put k4 True d3 print (get k1 d4) print (get k2 d4) print (get k3 d4) print (get k4 d4) Unsafe coerce, anyone? This particular example appears to run without incident, but the GHC docs suggest that this is very... well, unsafe. (In particular, the docs claim this will fall over on function types.) I presume there's some less-evil way of doing this?