This example demonstrates building a minimal editor with
hs-fltk.

> import Graphics.UI.FLTK
> import System.Exit
> import Data.IORef

We define a datatype to hold the state of the editor.
It remembers the toplevel window, the text input widget,
whether the buffer is modifier and the filepath associated
with it.

> data NS = NS Window Input Bool (Maybe FilePath)

Quitting is done with exitWith.

> quit _ = exitWith ExitSuccess

Make the editor ready for a new file resetting the buffer.

> new state _ = do (NS m i _ fp) <- readIORef state
>	           writeIORef state (NS m i False fp)
>		   set i [value := ""]

Save a file if it is modified. Asks for the filename if it is not defined.

> save state _ = readIORef state >>= s
>      where s (NS _ _ False _) = return ()
>	     s (NS m i st Nothing) = do fp <- fileChooser "Save as" "Text (*.txt)" ""
>		   	 	        if null fp then return () else s (NS m i st (Just fp))
>	     s (NS m i _ (Just fp)) = do get i value >>= writeFile fp
>				         writeIORef state (NS m i False (Just fp))

Open a new file.

> open state _ = do fp <- fileChooser "Open" "Text (*.txt)" ""
>		    if null fp
>		     then return ()
>		     else do (NS m i _ _) <- readIORef state
>			     set i [value ::= readFile fp]
>			     writeIORef state $ NS m i False $ Just fp

Create the main window and the menus.
	
> main = do w <- newWindow 0 0 640 480 [ label := "notepad" ]
>	    m <- newMenuBar 0 0 640 30 []
>	    i <- newMultiInput 0 30 640 450 [ when := wChanged ]
>	    state <- newIORef (NS w i False Nothing)
>	    set i [action := \_ -> modifyIORef state (\(NS w i _ f) -> NS w i True f)]
>	    addL m [("&File/&New", new state),
>		    ("&File/&Open",open state),
>		    ("&File/&Save",save state),
>		    ("&File/&Quit", \x -> save state x >> quit x),
>		    ("&Help/&About",\_ -> message "notepad version 0.1")]
>	    run w