Haskell Example

summer.hs

module Assign where

import Data.IORef

mkSummer :: IO (Int -> Int -> IO Int)
mkSummer = do
  ref <- newIORef 0
  return $ \x y -> do
    val <- readIORef ref

    let ret = val + x + y

    writeIORef ref ret

    return ret

Here is a purer version of the idea using StateT (I'm mainly using the Transformer to print the intermediate results).

module Foo where

import Control.Monad (void)
import Control.Monad.Trans.State
import Control.Monad.IO.Class (liftIO)

sum' :: Monad m => Int -> Int -> StateT Int m Int
sum' x y = do
  c <- get

  let res = c + x + y

  put res

  return res

main :: IO ()
main = void $ flip runStateT 0 $ do
  sum' 5 5 >>= liftIO . print
  sum' 5 5 >>= liftIO . print