Auto-reload threepenny-gui apps during development

17 Mar 2018

When you are developing a threepenny app, you need to perform the same dance after every change: recompile and restart the app, and then manually reload a page in a browser. This becomes tiresome very quickly. But with the help of ghcid and with small changes to your Haskell code you can completely automate this process.

All commands assume the simplest project layout with all the code in Main.hs. It’s a direct copy from threepenny-gui documentation, and you can clone my example repository which also adds all the necessary boilerplate.

Using ghcid for reloading

Almost all of the problems can be solved by using ghcid, which will give us a freshly recompiled and started application every time we’ll change the source code. Here is an example of a command that is suitable for my example:

ghcid -c 'cabal new-repl' \
      --reload=./Main.hs \
      -T Main.main \
  1. cabal new-repl can be replaced with whatever you use to get a ghci REPL (stack ghci, cabal repl, etc.)
  2. --reload switches (which can be specified multiple times) tell what files or directories should be watched for changes. You may want to monitor not only Haskell sources, but also some web assets like CSS and JavaScript files - so that reload will happen on all relevant changes
  3. This is a command to start an app that is being sent to ghci after every successful (re)compilation
  4. And we want to restart the whole ghci process when there are some changes to a cabal file, like new dependencies added or flags changed

Reloading browser page

That’s better, yet we still need to refresh our browser manually. With small changes to Haskell code we can automate this part also. Our main usually looks like this:

main = do
  appInit -- can take indeterminate amount of time
  startGUI defaultConfig setup

To achieve automatic reloading we can introduce alternative main which will be only used by ghcid (with the -T switch):

mainDevel = do
  appInit -- can take indeterminate amount of time
  forkIO $ do
      threadDelay 500000 -- small delay so startGUI can start listening
  startGUI defaultConfig setup

Now you need to come up with refreshBrowserPage for the system where you do development. Below you can see the dumbest implementation which will work only for Firefox on Linux, and only when xdotool is installed. But it’s OK, as this is development-only code that doesn’t even need to be robust or universal.

My Linux/Firefox implementation uses the fact that threepenny browser-side code tries to reload a page as soon as it looses connection to a server, which usually results in Problem loading page error (because recompiling and restarting application is not as fast as page reload). The code just searches for all windows which has Problem loading page in their title and sends reload hotkey (Ctrl-R) to each of them.

import Control.Monad (forM_)
import Control.Monad.Catch
import System.Process (readProcess, callProcess)
import Text.Read (readMaybe)

refreshBrowserPage :: IO ()
refreshBrowserPage = do
    maybeWindows :: Either SomeException String <- try
      (readProcess "xdotool"
        ["search", "--all", "--name", "Problem loading"] "")
    case maybeWindows of
      Left _ -> return ()
      Right idStr -> forM_ (lines idStr) $ \windowId ->
        case readMaybe windowId of
          Nothing -> return ()
          Just (n :: Integer) -> do
              putStrLn $ show n
              callProcess "xdotool" ["key", "--window", show n, "CTRL+R"]
              return ()

And this is all that’s needed to see your changes in browser almost immediately after you save an edited file.