A demo of using GHCi as a persistent development environment
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A demo of using GHCi as a persistent development environment. This example code uses a Yesod/Warp web server as a demo because it's simple to setup but a very common real-world use-case.

Associated blog post is here.

This project is runnable by simply loading DevelMain in GHCi. You can compile it with Cabal, but that's not the point of this demo.

The idea is that you have two main modules: Main and DevelMain. The former for production, the latter for development. DevelMain starts your server and manages updates to the running code.

Tip: remember to set flags if you want to avoid reloading all modules.

GHCi's role

GHCi's job here is to run the warp process in a separate thread and using the foreign-store library, to retain a reference to the IORef containing the Application (i.e. the web handler).

main =
  do c <- newChan
     app <- toWaiApp (Piggies c)
     ref <- newIORef app
     tid <- forkIO
                defaultSettings { settingsPort = 1990 }
                (\req -> do handler <- readIORef ref
                            handler req))
     _ <- newStore c
     _ <- newStore ref
     return ()

Warp's role

When the user updates the IORef Application, Warp will now server the new code by virtue of the fact it reads from the IORef every time:

    defaultSettings { settingsPort = 1990 }
    (\req -> do handler <- readIORef ref
                handler req))

IDE's role

Emacs, or your-editor-of-choice, can have some keybinding for "update code". This will run the following in the GHCi process:

:l DevelMain.hs

If that results in an "OK" and not a compile error, it can then proceed to run


Which will auto-start the server, or, if already running, update the existing code running live. That update code is like this:

update =
  do m <- lookupStore 1
     case m of
       Nothing -> main
       Just store ->
         do ref <- readStore store
            c <- readStore (Store 0)
            app <- toWaiApp (Piggies c)
            writeIORef ref app
            writeChan c ()

Emacs's haskell-mode already supports this, so you just need to setup a keybinding, e.g.

(define-key haskell-mode-map (kbd "<f12>") 'haskell-process-reload-devel-main)

Browser's role

An optional role for the browser is to auto-refresh the page, which is particularly handy if you are working on some visual aspect of the site and can't be bothered to switch to your browser, hit refresh and switch back. That is done by a trivial restart script:


And the route is:

getReloadR =
  do reload <- fmap appReload getYesod
     dupChan reload >>= readChan

This simply clones the channel (cloning in effect creates a broadcast to multiple readers, in this case; tabs in your browser) and then waits for a new event. As soon as a new event comes in the web handler returns and the script will refresh the page. Remember that earlier we read the channel from the foreign store and then write unit onto it:

c <- readStore (Store 0)
app <- toWaiApp (Piggies c)
writeIORef ref app
writeChan c ()

Safer way

Sometimes your server will not be happy about having its things updated in-place and will probably segfault or give a funny GHC error. In these cases, it's safer just to kill the thread and restart the thread:

-- | Development version to be run inside GHCi.

module DevelMain where

import Application (getApplicationDev)

import Control.Exception (finally)
import Control.Concurrent
import Data.IORef
import Foreign.Store
import Network.Wai.Handler.Warp

-- | Start or restart the server.
update :: IO ()
update = do
    mtidStore <- lookupStore tid_1
    case mtidStore of
      Nothing -> do
          done <- newEmptyMVar
          _done_0 <- newStore done
          tid <- start done
          tidRef <- newIORef tid
          _tid_1 <- newStore tidRef
          return ()
      Just tidStore -> do
          tidRef <- readStore tidStore
          tid <- readIORef tidRef
          done <- readStore (Store done_0)
          killThread tid
          takeMVar done
          newTid <- start done
          writeIORef tidRef newTid
  where tid_1 = 1
        done_0 = 0

-- | Start the server in a separate thread.
start :: MVar () -- ^ Written to when the thread is killed.
      -> IO ThreadId
start done = do
    (port,app) <- getApplicationDev
    forkIO (finally (runSettings (setPort port defaultSettings) app)
                    (putMVar done ()))