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

Alexander V Vershilov alexander.vershilov at gmail.com
Fri May 8 21:54:41 UTC 2015


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


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