[Haskell-cafe] StateT IO Action on `onKeyPress`

[Ryan Ingram]

Hi Andy.

The GTK bindings use IO for their callbacks, not any custom monad like
your WindowListT.

I suggest, instead of StateT s IO a, you use ReaderT (IORef s) IO a:

putR :: s -> ReaderT (IORef s) IO ()
putR s = do
    r <- ask
    liftIO $ writeIORef r s

getR :: ReaderT (IORef s) IO s
getR = ask >>= liftIO . readIORef

Otherwise, there are techniques to use an IORef to hold onto the state
while calling into IO (which might make callbacks), and then read it
back out and put it in the state while running your action.  But it's
simpler to just switch to ReaderT

  -- ryan

On Sun, May 3, 2009 at 8:27 AM, Andy Stewart <lazycat.manatee at gmail.com> wrote:
> Hi all,
>
> I have a function named `keymapTest` need moand state WindowListT, and
> WindowListT is `type WindowListT = StateT WindowList IO`.
>
> when i add "(\event -> keymapTest winList event >> return False)" after
> `onKeyPress` for handle key press event, i got GHC error:
>
> Manatee.hs:57:58:
>    Couldn't match expected type `IO a'
>           against inferred type `WindowListT Bool'
>    In the first argument of `(>>)', namely `keymapTest winList event'
>    In the expression: keymapTest winList event >> return False
>    In the second argument of `onKeyPress', namely
>        `(\ event -> keymapTest winList event >> return False)'
>
> So function `onKeyPress` just accept *one* IO-action?
> Have a way to fix above problem?
>
> Any help?
>
> Thanks!
>
>  -- Andy
>
> Below is source code of Manatee.hs file.
>
> ------------------------------> Manatee.hs start   <------------------------------
> module Main where
>
> import Text.Printf
> import Data.Monoid
> import Data.List
> import Data.Maybe
> import Control.Monad
> import Control.Monad.State
> import Control.Applicative
>
> import Data.IORef
>
> import Graphics.UI.Gtk hiding (Window, windowNew, get)
> import Graphics.UI.Gtk.SourceView
> import Graphics.UI.Gtk.Abstract.Widget
>
> import Manatee.Event
> import Manatee.Buffer
> import Manatee.WindowList
> import Manatee.Pane
> import Manatee.Statusbar
> import Manatee.Utils
> import Manatee.Window
>
> import qualified Data.Set as Set
> import qualified Graphics.UI.Gtk.Windows.Window as W
> import qualified Graphics.UI.Gtk.Gdk.Events as E
>
> main :: IO ()
> main = do
>  -- Init.
>  initGUI
>
>  -- Root frame.
>  rootFrame <- W.windowNew
>  rootFrame `onDestroy` mainQuit  -- quit main loop when root frame close
>
>  -- Root frame status.
>  windowFullscreen rootFrame   -- fullscreen
>
>  -- Windows list.
>  let windowsList = WindowList 0 Set.empty
>
>  evalStateT (do
>               -- Window 1
>               window1 <- windowNewWithBuffer DTop "test"
>               liftIO $ containerAdd rootFrame $ windowPaned window1
>
>               (window2, window3) <- windowSplitVertically window1
>
>               (window4, window5) <- windowSplitHorizontally window3
>
>               winList <- windowListGetList
>               liftIO $ rootFrame `onKeyPress` (\event -> keymapTest winList event >> return False)
>
>               -- Handle window content synchronous.
>               windowHandleSynchronous
>               ) windowsList
>
>  -- Loop
>  widgetShowAll rootFrame       -- display all widget
>  mainGUI
>
> keymapTest :: [Window] -> E.Event -> WindowListT Bool
> keymapTest winList event = do
>  window <- liftIO $ windowFindFocus winList
>  case window of
>    Just x -> handleKeyPress x event
>    Nothing -> return False
>
> handleKeyPress :: Window -> E.Event -> WindowListT Bool
> handleKeyPress window ev = do
>  liftIO $
>         case eventTransform ev of
>           Nothing -> return False
>           Just e -> do
>             let display = statusbarOutputSubitemSetText $ paneStatusbar $ windowPane $ window
>                 eventName = eventGetName e
>             case eventName of
>                -- Window commands.
>                "M-t" -> display "windowSplitVertically"
>                -- "M-t" -> windowSplitVertically window >> return False
>                _ -> display $ printf "%s undefined." eventName
> ------------------------------> Manatee.hs end   <------------------------------
>
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