[Haskell-cafe] Automatic liftIO or how to write this shorter?

[Alexander V Vershilov]

Hi, Gleb.

Assuming that you already in IO, and don't want to use lift or liftIO
to lift actions into another stack level, you can choose one of the
following:

1. Create a module with lifted operations for all operations in the
framework. Then by the cost of some boilerplate code you'll have
a framework that could be used in any MonadBaseControl.

2. Another way is to introduce concurrent primitives that will allow
you to 'log' events. Here is an incomplete sketch:

data ConfigUI = CUI { setText :: Text -> IO (),  setAnotherText ::
Text -> IO () }

defCUI = ...

newtype LogRef a = LV (IORef (Endo a))

newLog  :: IO (LogRef a)
newLog = LV <$> newIORef id

writeLog :: LogRef a -> (a -> a) -> IO ()
writeLog (LV r) f = modifyIORef r (\x -> x <> Endo f)

applyLog :: LogRef a -> a -> IO a
applyLog (LV r) f = ($) <$> fmap appEndo (readIORef r)  <*>  pure f

withLog ::  a -> (LogRef a -> IO b) -> IO (a,b)
withLog f v = newLog >>= \l -> f l >>= liftM2 (,) (applyLog lg v)

configureConfigUI = do
     (cui, a) <- withLog defCUI $ \lg -> do
                        ....
                        writeLog (\x -> x{setText = entrySet e})

There is a big window for solutions that are using mutable references
to log events in IO monad. Each with it's own pros and cons.

Hope it helps

--
Alexander

On 8 May 2015 at 14:20, Gleb Popov <6yearold at gmail.com> wrote:
> Hello haskell-cafe@
>
> I'm writing a GUI app in Haskell and bindings to the widget toolkit i'm
> using in parallel. These bindings are very simple and all its functions have
> return type (IO something).
>
> So far so good, i wrote the following code to create an config window:
>
> createConfigUI root = do
>     box <- Box.add root
>
>     -- first field
>     addToPackEnd box =<< do
>         f <- Fr.add box
>         setPartText f Nothing "E-mail"
>         setPartContent f Nothing =<< do
>             box <- Box.add box
>
>             addToPackEnd box =<< do
>                 e <- Ent.add box
>                 Ent.singleLineSet e True
>                 -- Here
>                 onEvent "changed,user" e $ do
>                      reactOnUserInput e
>                 objectShow e
>                 return e
>
>             objectShow box
>             return box
>
>         objectShow f
>         return f
>
>     -- next field
>     addToPackEnd box =<< do
>         ...
>
> Initially, i was quite satisfied with flipped bind use for creating UI
> elements and arranging them. Nested do scopes allow copypasting code without
> renaming variables and also provide some visual representation on widget
> hierarchy.
>
> But at some point i need to return some stuff from some inner do block into
> outmost. For example, at the line with comment "Here" i defined
>
> let setText t = entrySet e t
>
> and wanted to return it from whole createConfigUI action. Moreover,
> createConfigUI have much more fields, for each of them i want to do the
> same.
>
> My initial thought was to wrap everything with runWriter and just call
>
> tell setText
>
> wherever i want to gather all setter functions into a list, but i can't do
> this because i would need to put liftIO before every IO action all over the
> place.
>
> If only there was i way to turn an (IO a) into (MonadIO m => m a), it would
> be easy.
>
> Another solution is to make my bindings return (MonadIO m => m a). This
> would be equal effort of plugging liftIO's everywhere, but at least it would
> be hidden from user of bindings. I'd gone this way, but looked at gtk
> bindings first and found that (IO a) is used there.
>
> So, my questions are:
>
> 1. What would you recommend in my situation? Is it possible yield values
> from inner do blocks into outer without much hassle?
> 2. If there is nothing wrong with switching bindings from (IO a) to MonadIO
> typeclass, why not to do this for gtk, wxWidgets and nearly every FFI
> binding?
>
> Thanks in advance.
>
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-- 
Alexander