using threepenny-gui/reactive in client/server programming

Question

I am trying to figure out how to use Haskell threepenny-gui with its reactive functionality to write a program that lets

• the user select an item from a listBox
• send the selection to an external server
• get back a list of results from the server
• populate the listBox with the results
• repeat

It seems I will need to use Handler, newEvent and register to do the above. If someone could point me to some existing code that does something like the above that would be great.

The closest I have found is GameThing.hs in the threepenny-gui samples directory (but it doesn't use register).

UPDATE:

I am asking whether I should be using Handler, newEvent and register --- and, if so, some clarification or example of those functions.

Also, to be clear, the big picture is:

browser  --> threepenny-gui (on localhost) --> backend server (anywhere on network)
         <--                               <--

In other words, I need to do some IO (based on user selection) then display the results of that IO.

UPDATE: here is my solution (based on @Taldykin's response): https://github.com/haroldcarr/rdf-triple-browser/tree/master/haskell/src

Answer 1

Here is a piece of code. I will add description a bit later.

{-# LANGUAGE LambdaCase #-}
import Graphics.UI.Threepenny as UI


main :: IO ()
main = do
  (evFillList, doFillList) <- newEvent
  initialList <- valuesSupply ""
  behFillList <- stepper initialList evFillList

  startGUI defaultConfig $ \win -> do
    list <- ul
    sel <- listBox
        behFillList
        (pure Nothing)
        (pure $ \it -> UI.span # set text it)

    getBody win #+ [grid [[element list, element sel]]]
    setFocus $ getElement sel

    on selectionChange (getElement sel) $ \case
      Nothing -> return ()
      Just ix -> do
        items <- currentValue behFillList
        let it = items !! ix
        liftIO $ valuesSupply it >>= doFillList
        element list #+ [li # set html it]
        setFocus $ getElement sel


valuesSupply :: String -> IO [String]
valuesSupply x = return [x ++ show i | i <- [0..9]]