PropLang

A design for a GUI library which is more like Haskell and less like C. To be written over Gtk2Hs.

Link: http://www.cse.unsw.edu.au/~chak/haskell/ports/

Thoughts by

Neil Mitchell, Duncan Coutts

The Var concept

This is the low level stuff, on which the library will be built

data Var a = ...
data Notify = ...

get :: Var a -> IO a
set :: Var a -> a -> IO ()
addNotify :: Var a -> (a -> IO ()) -> IO Notify
remNotify :: Notify -> IO ()

newVar :: a -> IO (Var a)

A concrete implementation of Var

One thing to avoid, global state that needs a specific init function

data Var a = Var {value :: IORef a,
                  notifyId :: IORef Integer,
                  notifys :: IORef [(Integer, a -> IO ())],
                  source :: IORef (Maybe Notify)}

data Notify = Notify {notifyId :: Integer, notifys :: IORef [(Integer, a -> IO ())]}

newVar :: a -> IO (Var a)
newVar x = do v <- newIORef x
              n <- newIORef []
              c <- newIORef 0
              a <- newIORef Nothing
              return $ Var v c n a

get :: Var a -> IO a
get var = readIORef (value var)

set :: Var a -> a -> IO ()
set var x = do n <- readIORef (notifys var)
               writeIORef (value var) x
               mapM_ (\(a,b) -> b x) n

addNotify :: Var a -> (a -> IO ()) -> IO Notify
addNotify var f = do n <- readIORef (notifys var)
                     c <- readIORef (notifyId var)
                     writeIORef (notifyId var) (c+1)
                     writeIORef (notifys var) ((c, f) : n)
                     return $ Notify c (notifys var)

remNotify :: Notify -> IO ()
remNotify notify = do n <- readIORef (notifys notify)
                      writeIORef (notifys notify) (filter (\x -> fst x /= notifyId notify) n)

Object layering

textBox~text -< "test"
filename <- newVar Nothing
addNotify filename hatCover
lbl~text =< with filename $ \x ->
                 case x of
                     Nothing -> "Select a file"
                     Just x -> "Loaded: " ++ x

where

(~) :: GtkObject -> GtkProp -> Var a -- ignoring lots of details here
(-<) :: Var a -> a -> IO ()
(=<) :: Var a -> Action a b -> IO ()

with :: Var a -> (a -> b) -> Action a b

Implementation of Object Layering

data Action a b = Action (Var a) (a -> b)

with :: Var a -> (a -> b) -> Action a b
with var f = Action var f

(-<) :: Var a -> a -> IO ()
(-<) var val = set var val

(=<) :: Var a -> Action a -> IO ()
(=<) dest (Act src f) = do srcOld <- readIORef (source dest)
                           when (isJust srcOld) $ remNotify (fromJust srcOld)
                           note <- addNotify src (\x -> set dest (f x))
                           writeIORef (source dest) (Just note)

Implementation of Gtk Layering

class TextProvider a where
    text :: a -> Var String

instance TextProvider TextView where
    text x = ...

txt :: TextView

(!) :: a -> (a -> b) -> b
object ! prop = prop object

txt!text :: Var String