-- Uses a record where the type of the fields are inferred

{-# LANGUAGE GADTs, MagicHash #-}
module RecordTest where

import Data.Monoid
import Data.IntMap(IntMap)
import qualified Data.IntMap as IntMap
import qualified GHC.Prim as Prim


-- Represents a field of the record
data Field a = Field !Int
(k1) = Field 1  -- k1 :: Field Bool
(k2) = Field 2  -- k2 :: Field String

-- Record with a list of values for each field
data Record = Record !(IntMap [Prim.Any])

single :: Field a -> a -> Record
single (Field k) = Record . IntMap.singleton k . return . Prim.unsafeCoerce#

index :: Field a -> Record -> [a]
index (Field k) (Record mp) = map Prim.unsafeCoerce# $ IntMap.findWithDefault [] k mp

instance Monoid Record where
  mempty = Record IntMap.empty
  (Record mp1) `mappend` (Record mp2) = Record mp3 where
    mp3 = IntMap.unionWith (++) mp1 mp2

-- Some two traversals
trav1 :: [Int] -> Record
trav1 = foldr add mempty where
  add x r | even x = single k1 True `mappend` r
          | odd x  = single k2 (show x) `mappend` r

trav2 :: Record -> [Int] -> ([Bool], [String])
trav2 r = foldr add ([],[]) where
  add _ (b1,s1) = (b1 ++ index k1 r, s1 ++ index k2 r)

-- Test
list :: [Int]
list = [1..4]

t :: ([Bool], [String])
t = trav2 (trav1 list) list