keskiviikko 16. marraskuuta 2011

UI Monad

I got this disgusted feeling when writing Android apps where I was doing
new Thread() {
  public void run() {
    // ...
when putting something on the background and
runOnUiThread(new Runnable() {
  public void run() {
  // ...
At that point I'd been doing Haskell for some time and even defined some of my own Monads. Well, I just wanted to see if I can create a Monad that would make it easy to write UI code where your actions would be run in a single thread and where you could easily do stuff on the background too and return to the UI thread with the values returned from the background tasks.. Well, I defined my own UI monad as in UIThread.hs.. With this monad, you could write code like
twitterWiz :: UI ()
twitterWiz = do
  name <- alert "what's your twitter username?"
  tweets <- background $ queryTwitter name
  blocking $ putStrLn "Got " ++ show tweets
  showTweets tweets

alert :: UI String
quearyTwitter :: IO [String]
showTweets :: [String] -> UI ()
The actions (on each line) would be un in the UI thread, each as its own "event", without blocking the thread in between. Background operations can be run using the background function that runs your IO operations using a threadpool (or such) and then runs the next UI action in the UI thread again. Theblocking function can be used to run any IO action in the UI thread. I figured the name should be shouting aloud "I'll be blocking your UI thread if you get stupid".
The UI Monad can be thought as a way of using monads as "programmable semicolons". You can write code that looks like a sequence of actions that are executed synchronously, yet the programmable (invisible) semicolons decouple your actions into the UI thread. I find at least a couple of great things about this design:
  • Minimal boilerplate: shifting between execution modes is easy using background and blocking functions
  • No way to accidentally run stuff in the wrong thread: when you're in the UI monad, your code is run in the UI thread. Background running and blocking calls need explicit background or blocking calls
Note that the UI Monad is fully functional: You can run UI actions using the inUiThread function. The only catch here is that you have to provide it with aContext which is practically a pair of functions: one to submit a IO action to be run in the UI thread and other for running an action on the background.
The simplest possible context would be
blockingContext = Context id id
This would execute everything synchronously.
The only actually working example so far is in ConsoleExample.hs:
main = do
  inUIThread blockingContext $ do
    blocking $ putStrLn "Enter your name : "
    name <- blocking $ getLine
    blocking $ putStrLn $ "Hello, " ++ name
It's really pathetic though: it's just performing IO actions in a blocking way. Anyways it demonstrates that the UI Monad actually does something...

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