stateful-closure.hs

-- Stateful closure in Haskell

import Data.IORef

makeCounter :: IO (Int -> IO Int)
makeCounter = do
    state <- newIORef 0
    return $ \i -> do
        modifyIORef state (+i)
        readIORef state

main = do
    closure <- makeCounter
    print =<< closure 1
    print =<< closure 2