funarray.hs

import Prelude hiding (lookup)
import Data.IORef
import Data.Maybe

type NatArray a = IORef (Int -> Maybe a)

newArray :: IO (NatArray a)
newArray = newIORef (const Nothing)

lookup :: NatArray a -> Int -> IO (Maybe a)
lookup a n = readIORef a >>= \f -> return (f n)

update :: NatArray a -> Int -> a -> IO ()
update a m v =
  readIORef a >>= \oldf ->
    writeIORef a (\n -> if n == m then Just v else oldf n)

main = do
  a <- newArray :: IO (NatArray String)
  lookup a 0 >>= print . isNothing
  update a 10 "foo"
  update a 20 "bar"
  update a 10 "baz"
  lookup a 10 >>= print . (==Just "baz")
  lookup a 20 >>= print . (==Just "bar")