[Haskell-cafe] GUI programming

Mario Blažević mblazevic at stilo.com
Fri Feb 5 13:54:23 EST 2010

Victor Nazarov wrote:
> Hello,
> I've been writing some GUI application with Gtk2hs. It's an
> interpreter for lambda-calculus and combinatory logic, it's GPL and if
> you interested I can share it with cafe.
> The problem is that the GUI code has become very ugly and I'm tempted
> to rewrite it totally. I've been looking forward to the FRP stuff, but
> I've never seen a single definition of the term. Conal Eliot's
> "denotational programming" is too general to be definition. I want to
> try Grapefruit, but I got totally lost when I see arrow notation.
> I consider more lightweight and more imperative approach, something
> closer to CSP (Communicating Secuential Processes) then FRP. I've just
> crafted some sample program to illustrate my idea.
> The behaviour is a monad and it's IO monad so you can do any IO
> (Gtk2hs) programming you wish. The differences is that you don't
> attach static event handlers and tries to determine what to do
> dependent on application state. You attach and detach handlers as much
> as possible. Behaviour looks like a process that can stop execution
> and wait for some GUI event. When event arrived it continues
> execution.

	To summarize, the behaviour is a suspendable IO computation. It looks 
very much like a coroutine, in fact. I'm planning to extract the 
Control.Concurrent.Coroutine module [1] into a separate package soon. It 
implements a similar concept but generalized to transform any monad and 
any functorial suspension.


> Do you see this approach viable. There are steel some details to emerge:
> * How to wait for several events
> * How to handle IO exceptions

	I don't really know how applicable the idea is to GUI programming. 
That's not my area of expertise. I am surprised, though, that neither 
your code not your comments seem to address the issue of concurrency, as 
I expect that would be crucial in a GUI setting. Wouldn't you need 
different behaviours to run in different threads?

> Here is the code:
> {-# LANGUAGE ExistentialQuantification #-}
> ...

	I don't see the purpose of your BBind constructor. It seems to me that 
you could simply move the first three cases of runBehaviour 
implementation into your >>= and get rid of the constructor. Do you 
need that much laziness?

> import Data.IORef
> import System.Glib
> import Graphics.UI.Gtk
> import Control.Monad.Trans
> type Event obj = IO () -> IO (ConnectId obj)
> data Behaviour a =
>   forall b. BBind (Behaviour b) (b -> Behaviour a)
>   | BIO (IO a)
>   | forall obj. GObjectClass obj => BWaitEvent (Event obj) (Behaviour a)
> instance Monad Behaviour
>  where action >>= generator = BBind action generator
>        return a = BIO (return a)
> instance MonadIO Behaviour
>  where liftIO action = BIO action
> runBehaviour :: Behaviour a -> IO a
> runBehaviour (BBind (BWaitEvent event after) f) = runBehaviour
> (BWaitEvent event (after >>= f))
> runBehaviour (BBind (BIO a) f) = a >>= \x -> runBehaviour (f x)
> runBehaviour (BBind (BBind a f) g) = runBehaviour (a >>= (\x -> f x >>= g))
> runBehaviour (BIO a) = a
> runBehaviour (BWaitEvent event after) =
>  do sigIdRef <- newIORef (error "You can't access sigIdRef before
> signal is connected")
>     sigId <- event $
>       do sigId <- readIORef sigIdRef
>          signalDisconnect sigId
>          runBehaviour after
>          return ()
>     writeIORef sigIdRef sigId
>     return (error "You can't expect result from behaviour")
> waitEvent :: GObjectClass obj => Event obj -> Behaviour ()
> waitEvent event = BWaitEvent event (return ())
> main :: IO ()
> main =
>   do initGUI
>      window <- windowNew
>      onDestroy window mainQuit
>      set window [windowTitle := "Hello World"]
>      button <- buttonNew
>      let buttonB label =
>            do liftIO $ set button [buttonLabel := label]
>               waitEvent (onClicked button)
>               buttonB (label ++ "*")
>      runBehaviour (buttonB "*")
>      set window [containerChild := button]
>      widgetShowAll window
>      mainGUI
> ------------------------------------------------------------------------
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Mario Blazevic
mblazevic at stilo.com
Stilo International

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