Re: [Haskell-cafe] I'm trying to design a GUI library -- a design question

Again, sorry, just pasted wrong code, correct version should be:

{-# LANGUAGE PackageImports, MultiParamTypeClasses, FlexibleInstances, FlexibleContexts #-}

import "mtl" Control.Monad.State import "mtl" Control.Monad.Identity import qualified Data.Map as M import Data.Maybe

type Key = Int type Widget = Int -- placeholder type FocusIdx = Int

data Program = Program { widgets :: (M.Map Int Int, Int) }

class HasWidgets s where getWidgets :: s -> (M.Map Int Widget, FocusIdx) updateWidgets :: (M.Map Int Widget, FocusIdx) -> s -> s

instance HasWidgets Program where getWidgets p = widgets p updateWidgets w p = p{widgets=w}

handleKey :: (MonadState s m, HasWidgets s) => Key -> m () handleKey key = do p <- get let (widgets, focusIdx) = getWidgets p w = fromJust $ M.lookup focusIdx widgets w' = undefined -- just update the widget put $ updateWidgets (M.insert focusIdx w' widgets, focusIdx) p

test :: StateT Program Identity () test = do handleKey undefined return ()

--- Ömer Sinan Ağacan http://osa1.net 2014-03-03 11:17 GMT+02:00 Ömer Sinan Ağacan :

Ops, sorry. I misunderstand your code. Now that looks like solving my problem of updating widgets, and maybe I can use Data.Map.Map to keep widgets and update them when methods are called.

Now this works but that explicit state updating and passing is not ideal for me. I know I can always hide that kind of things behind a state monad:

...

data Program = Program { ... , widgets :: (Map Int Widget, Int) , ... }

handleKey' :: Key -> State Program () handleKey' key = do programState@Program{(widgets, focusIdx)=widgets} <- get let widget = fromJust $ lookup focusIdx widgets widget' = handlekey widget key

put programState{widgets=(M.insert focusIdx widget', focusIdx)}

and I can even create a typeclass like `HasWidgets` which provides required methods for updating widget states and that would be even more flexible:

...

{-# LANGUAGE PackageImports, MultiParamTypeClasses, FlexibleInstance #-}

import "mtl" Control.Monad.State import qualified Data.Map as M import Data.Maybe

type Key = Int type Widget = Int -- placeholder type FocusIdx = Int

data Program = Program { widgets :: (M.Map Int Int, Int) }

class HasWidgets s where getWidgets :: s -> (M.Map Int Widget, FocusIdx) updateWidgets :: (M.Map Int Widget, FocusIdx) -> s -> s

class (MonadState s m, HasWidgets s) => Widgets s m where handleKey_ :: Key -> m ()

instance HasWidgets Program where getWidgets = widgets updateWidgets w p = p{widgets=w}

instance Monad m => Widgets Program (StateT Program m) where handleKey_ key = do programState@Program{widgets=(widgets, focusIdx)} <- get let w = fromJust $ M.lookup focusIdx widgets w' = undefined -- just call handleKey method of widget `w` put programState{widgets=(M.insert focusIdx w' widgets, focusIdx)}

test :: State Program () test = do programState@Program{widgets=(widgets, focusIdx)} <- get return ()

... but is there a better way to do this? Maybe by using Lens(I'm not sure if something like that makes sense -- this just came to my mind because all I do here is to do nested record updates, which as far as I know where Lens shines) ?

--- Ömer Sinan Ağacan http://osa1.net

2014-03-03 10:10 GMT+02:00 Daniel Trstenjak :

...

...

*Main> let c = someCounter 0 *Main> draw c 0 *Main> let c' = increase someCounter *Main> draw c' 1

Sorry, the example should have been:

*Main> let c = someCounter 0 *Main> draw c 0 *Main> let c' = increase c *Main> draw c' 1

Greetings, Daniel _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe