{-- A dummy monad transformer GUI for Gui programming
    whose functionality has been lifted to class Gui.
    
    This module is almost identical (except identifiers) 
    to Db.hs
                                                            --} 
module Gui ( Gui, gui, GUI,
	     guiExampleOp 
	   )
where

import Io
import STM
import MonadT

-- (in this example it is assumed that Gui implies Io for some reason):

class Io m => Gui m where
    gui :: GUIImpl m

-- monad transformer GUI:

newtype GUI m a = GUI (STM GUIStat m a)

fromGUI (GUI a) = a
toGUI = GUI

instance Monad m => Functor (GUI m) where 
    fmap f = toGUI . fmap f . fromGUI

instance Monad m => Monad (GUI m) where
    m >>= f = toGUI ((fromGUI m) >>= (fromGUI . f))
    return  = toGUI . return

instance MonadT GUI where 
    lift  = toGUI . lift
    start = run 0 . fromGUI

instance Io m => Io (GUI m) where
    io = lift . io

instance Io m => Gui (GUI m) where
    gui = GUIImpl (toGUI . modify)

-- state 

type GUIStat = Int

-- ADT that encapsulates monad operations:

data GUIImpl m = GUIImpl ((GUIStat -> GUIStat) -> m GUIStat)

instance Liftable GUIImpl where
    mapLift (GUIImpl m) = GUIImpl (lift . m)

-- some example class op: set state & read it & print:

guiExampleOp :: Gui m => Int -> m ()
guiExampleOp i
    = do
      modify (\_ -> i)
      result <- modify id
      putStrC (show result)
      where  (GUIImpl modify) = gui