This module provides a rapid prototyping suite for GHCi that can be used standalone or integrated into editors. You can hot-reload individual running components as you make changes to their code. It is designed to shorten the development cycle during the development of long-running programs like servers, web applications and interactive user interfaces.

It can also be used in the context of batch-style programs: Keep resources that are expensive to create in memory and reuse them across module reloads instead of reloading/recomputing them after every code change.

Technically this package is a safe and convenient wrapper around foreign-store.

Read the "Safety and securty" section before using this module!

While working on a project you may want to have your code running in the background and restart parts of it as you make changes. The premise of this introduction is that you already have such a project, for example a web application, and that you use a persistent GHCi session (either standalone or built into your editor).

To use this library in your project create a module conventionally named DevelMain that exports an action conventionally named update:

module DevelMain (update) where

import Rapid

update :: IO ()
update =
    rapid 0 $ \r ->
        -- We'll list our components here shortly.
        pure ()

The idea is that within a GHCi session you run this update action whenever you want to reload your project during development. In the simplest case, like in a web application, your project consists of a single HTTP server thread that is just restarted each time you reload. Here is an example using the Snap Framework:

import qualified Data.Text as T
import Rapid
import Snap.Core
import Snap.Http.Server

update =
    rapid 0 $ \r ->
        restart r "webserver" $
            quickHttpServe (writeText (T.pack "Hello world!"))

Once you run update in a GHCi session, a server is started (port 8000) that keeps running in the background, even when you reload modules. The REPL is fully responsive, so you can continue working. When you want to apply the changes you have made, you reload the DevelMain module and run update again. To see this in action, change the text string in the example, reload the module and then run update. Also observe that nothing is changed until you actually run update.

When you want to stop a running background thread, replace restart within the update action by stop and run update. The action given to stop is actually ignored. It only takes the action argument for your convenience.

You can run multiple threads at the same time and also have threads that are not restarted during a reload, but are only started and then kept running:

import MyProject.MyDatabase
import MyProject.MyBackgroundWorker
import MyProject.MyWebServer
import Rapid

update =
    rapid 0 $ \r -> do
        start r "database" myDatabase
        start r "worker" myBackgroundWorker
        restart r "webserver" myWebServer

Usually you would put restart in front of the component that you are currently working on, while using start with all others.

Note that even though you are working on the code in MyProject.MyWebServer you are always reloading the DevelMain module. There is nothing wrong with loading and reloading other modules, but only this module gives you access to your update action.

If you need your background threads to communicate with each other, for example by using concurrency primitives, some additional support is required. You cannot just create a TVar within your update action. It would be a different one for every invocation, so threads that are restarted would not communicate with already running threads, because they would use a fresh TVar, while the old threads would still use the old one.

To solve this, you need to wrap your newTVar action with createRef. The TVar created this way will survive reloads in the same way as background threads do. In particular, if there is already one from an older invocation of update, it will be reused:

import Control.Concurrent.STM
import Control.Monad
import Rapid

update =
    rapid 0 $ \r -> do
        mv1 <- createRef r "var1" newEmptyTMVarIO
        mv2 <- createRef r "var2" newEmptyTMVarIO

        start r "producer" $
            mapM_ (atomically . putTMVar mv1) [0 :: Integer ..]

        restart r "consumer" $
            forever . atomically $ do
                x <- takeTMVar mv1
                putTMVar mv2 (x, "blah")

        -- For debugging the update action:
        replicateM_ 3 $
            atomically (takeTMVar mv2) >>= print

You can now change the string "blah" in the consumer thread and then run update. You will notice that the numbers in the left component of the tuples keep increasing even after a reload, while the string in the right component changes. That means the producer thread was not restarted, but the consumer thread was. Yet the restarted consumer thread still refers to the same TVar as before, so it still receives from the producer.

Mutable references as introduced in the previous section can also be used to shorten the development cycle in the case when an expensive resource has to be created. As an example imagine that you need to parse a huge file into a data structure. You can keep the result of that in memory across reloads. Example with parsing JSON:

import Control.Exception
import Data.Aeson
import qualified Data.ByteString as B

update =
    rapid 0 $ \r ->
        value <- createRef r "file" $
            B.readFile "blah.json" >>=
            either (throwIO . userError) pure . eitherDecode

        -- You can now reuse 'value' across reloads.

If you want to recreate the value at some point, you can just change createRef to writeRef and then run update. Keep in mind to change it back createRef afterward. Use deleteRef to remove values you no longer need, so they can be garbage-collected.

In general a Cabal project should not have this library as a build-time dependency. However, in certain environments (like Nix-based development) it may be beneficial to include it in the .cabal file regardless. A simple solution is to add a flag:

flag Devel
    default: False
    description: Enable development dependencies
    manual: True

library
    build-depends:
        base >= 4.8 && < 5,
        
    if flag(devel)
        build-depends: rapid
    

Now you can configure your project with -fdevel during development and have this module available.

This library integrates well with haskell-interactive-mode, particularly with its somewhat hidden haskell-process-reload-devel-main function.

This function finds your DevelMain module by looking for a buffer named DevelMain.hs, loads or reloads it in your current project's interactive session and then runs update. Assuming that you are already using haskell-interactive-mode all you need to do to use it is to keep your DevelMain module open in a buffer and type M-x haskell-process-reload-devel-main RET when you want to hot-reload. You may want to bind it to a key:

(define-key haskell-mode-map (kbd "C-c m") 'haskell-process-reload-devel-main)

Since you will likely always reload the current module before running update, you can save a few keystrokes by defining a small function that does both and bind that one to a key instead:

(defun my-haskell-run-devel ()
  "Reloads the current module and then hot-reloads code via DevelMain.update."
  (interactive)
  (haskell-process-load-file)
  (haskell-process-reload-devel-main))

(define-key haskell-mode-map (kbd "C-c m") 'my-haskell-run-devel)

It's easy to crash your GHCi session with this library. In order to prevent that, you must follow these rules:

• Do not change your service name type (the type argument of Rapid, i.e. the second argument to restart, start and stop) within a session. The simplest way to do that is to resist the temptation to define a custom name type, and just use strings instead. If you do change the name type, you should restart GHCi.
• Be careful with mutable variable created with createRef: If the value type changes (e.g. constructors or fields were changed), the variable must be recreated, for example by using writeRef once. This most likely entails restarting all threads that were using the variable. Again the safest option is to just restart GHCi.
• If any package in the current environment changes (especially this library itself), for example by updating a package via cabal or stack, the update action is likely to crash or go wrong in subtle ways due to binary incompatibility. If packages change, restart GHCi.
• This library is a development tool! Do not even think of using it to hot-reload in a productive environment! There are much safer and more appropriate ways to hot-reload code in production, for example by using a plugin system.

The reason for this unsafety is that the underlying foreign-store library is itself very unsafe in nature and requires that we maintain binary compatibility. This library hides most of that unsafety, but still requires that you follow the rules above.

Please take the last rule seriously and never ever use this library in production! If something goes wrong during a reload, we do not get a convenient run-time exception; we get a memory violation, which can cause anything from a segfault to a remotely exploitable security hole.