Simulating key-value database using StateT

statet_map_io.hs

import Control.Monad.State
import qualified Data.Map as Map

type MyMap = Map.Map Int Char

runDB :: StateT MyMap IO ()
runDB = do
    add 1 'a'
    add 2 'b'
    x <- find 1
    y <- find 2
    z <- find 3
    io $ print x
    io $ print y
    io $ print z -- shows Nothing
    return ()

-- find the value of the key if exists
find :: Int -> StateT MyMap IO (Maybe Char)
find k = do
    m <- get
    io $ print ("getting from disk: " ++ show k)
    return (Map.lookup k m)

-- add some key/value pair to a map
add :: Int -> Char -> StateT MyMap IO ()
add k v = do
    m <- get
    put $ Map.insert k v m
    io $ print ("writing to disk: " ++ show k)
    return ()

io :: IO a -> StateT MyMap IO a
io = liftIO

main :: IO ()
main = runStateT runDB (Map.empty) >> return ()