[Haskell-cafe] GUI programming

[Artyom Shalkhakov]

Hello Victor,

2010/2/2, Victor Nazarov <asviraspossible at gmail.com>:
> 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.

Sure, why not?

> 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.

All this process calculus stuff reminds me of Fudgets. Maybe this
approach is more pragmatic at the moment: even more so, I think it's
theoretical underpinnings are appealing as well. Who said that all
programming should be reduced to pure functions? :-)

As a side note, there's a book "How to Design Worlds" which discusses
interactive purely functional programming (using games as an example).
While it is only tangentially related to GUI programming, I wonder if
their approach can be adapted for use in GUIs.

Cheers,
Artyom Shalkhakov

> 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.
>
> Do you see this approach viable. There are steel some details to emerge:
> * How to wait for several events
> * How to handle IO exceptions
>
> Here is the code:
> {-# LANGUAGE ExistentialQuantification #-}
> module Main where
>
> 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
>
>
> --
> Victor Nazarov
>