Reactive Programming

Reactive programming is modeled beautifully using concurrent streaming in streamly. It involves generation of streams of events, merging concurrent streams and processing events concurrently. Streamly provides native high-level facilities to do all this easily.

We will illustrate the basics below using a few trivial examples.

Acid Rain Game

Objective of the Game

The game starts with a certain measure of health of the player. As time passes the health of the player keeps on deteriorating because acid rain is going on. If the health reaches 0 the player dies and the game is over. If the player types “potion” on the CLI, the health is improved, the game continues if the player keeps typing potion rapidly enough. If the player types “harm” instead the health of the player deteriorates and the player dies sooner. If the player types “quit” then the game ends.

Importing Required Modules

Let’s first import the required modules from streamly and base.

{-# LANGUAGE FlexibleContexts #-}
import Control.Monad.IO.Class (MonadIO(liftIO))
import Control.Monad.State (MonadState, get, modify)
import Data.Function ((&))
import Streamly.Data.Stream.Prelude (MonadAsync, Stream)

import qualified Streamly.Data.Fold as Fold
import qualified Streamly.Data.Stream.Prelude as Stream

Events

The possible events in the system are represented by the Event data type. Acid rain generates the Harm event, typing “potion” on the CLI generates the Heal event, typing “harm” generates the Harm event, and typing “quit” generates the Quit event. Harm and Heal events have an integer associated which represents the degree of harm or healing.

data Event = Quit | Harm Int | Heal Int deriving (Eq, Show)

This application has two independent and concurrent sources of event streams, acidRain and userAction.

Acid Rain Stream

Now let’s simulate acid rain. The acidRain function below generates a stream of Harm 1 events, one event is generated per second.

acidRain :: MonadAsync m => Stream m Event
acidRain = Stream.parRepeatM (Stream.constRate 1) (return $ Harm 1)

User Event Stream

The second stream is the stream of events generated by the user by typing commands on the CI. The userAction function reads the standard input, interprets the command typed and generates the appropriate event. It keeps doing this forever, this is an infinite stream.

userAction :: MonadAsync m => Stream m Event
userAction = Stream.repeatM $ liftIO askUser

    where

    askUser = do
        command <- getLine
        case command of
            "potion" -> return (Heal 10)
            "harm"   -> return (Harm 10)
            "quit"   -> return Quit
            _        -> putStrLn "Type potion or harm or quit" >> askUser

Combined Stream

Now let’s combine the streams generated by acid rain and the stream generated by the CLI. Both the streams should be generated concurrently, therefore, we use the parList function to combine them, this function combines a list of streams concurrently. We use the eager True option to ensure that both the streams are evaluated as soon as possible.

parallel :: MonadAsync m => [Stream m a] -> Stream m a
parallel = Stream.parList (Stream.eager True)

eventStream :: Stream m Event
eventStream = parallel [userAction, acidRain]

Process Health Events

The runEvents function below maintains the health of the player as an integer value in the State monad. It maps the processEvents function on the event stream. The Harm or Heal events decrement or increment the player health value appropriately. If we encounter a Quit event the function returns Done otherwise it returns Continue. The resulting stream is a stream of Result values.

data Result = Continue | Done

runEvents :: (MonadAsync m, MonadState Int m) => Stream m Result
runEvents = Stream.mapM processEvents eventStream

    where

    processEvents event =
        case event of
            Harm n -> modify (\h -> h - n) >> return Continue
            Heal n -> modify (\h -> h + n) >> return Continue
            Quit -> return Done

Check the Player Status

The runEvents function above returns a stream of results after processing the health events. The stream consists of results indicating whether the game should continue or end, the State monad supplies the current health of the player.

Now we map the getStatus function on the Result stream. If we encounter a Done in the result stream then it means the user has quit the game, so we return GameOver, if the health of the player is 0 or less then the player has died and we return GameOver, otherwise we return Alive.

data Status = Alive | GameOver deriving Eq

getStatus :: (MonadAsync m, MonadState Int m) => Result -> m Status
getStatus result =
    case result of
        Done  -> liftIO $ putStrLn "You quit!" >> return GameOver
        Check -> do
            h <- get
            liftIO
                $ if (h <= 0)
                  then putStrLn "You die!" >> return GameOver
                  else putStrLn ("Health = " <> show h) >> return Alive

Tying it all Together

We start with the Result stream using the runEvents function. Then we map the getStatus function on this stream and turn it into a Status stream. Then we run the State monad using runStateT, supplying the initial health to be 60, the resulting stream is a tuple of (health, status) in the IO monad. We then discard the health and just keep the status, resulting in a Status stream. We then fold this Status stream using the takeEndBy fold, this fold terminates as soon as a GameOver value is encountered in the stream.

main :: IO ()
main = do
    putStrLn "Your health is deteriorating due to acid rain,\\
             \ type \"potion\" or \"quit\""
    runEvents                        -- Stream (StateT Int IO) Result
        & Stream.mapM getStatus      -- Stream (StateT Int IO) Status
        & Stream.runStateT (pure 60) -- Stream IO (Int, Status)
        & fmap snd                   -- Stream IO Status
        & Stream.fold (Fold.takeEndBy (== GameOver) Fold.drain) -- IO ()
    return ()

Complete Working Example

You can find a complete working source of this example in the streamly-examples repo as AcidRain.hs. The idea of this game example has been taken from Gabriella Gonzalez’s pipes-concurrency package.

Circling Square

For a simple graphical example where we generate an animation by rendering a graphics frame periodically, see the SDL based circling square example adapted from Yampa in CirclingSquare.hs.