Progress with wai-devel

Posted on August 13, 2015

wai-devel is a development server for wai compliant haskell web applications.

Its name changed from yesod-devel (the haskell reddit community suggested this). You can find it at:

What wai-devel expects from your application

Since wai-devel is very loosely coupled to your application it expects mainly two things from your application: a host:port pair and a function, Application.develMain.

Due to it’s dependence on ide-backend it also expects you to set the environment variable GHC_PACKAGE_PATH.

Mine for example is: export GHC_PACKAGE_PATH=~/.stack/snapshots/x86_64-linux/lts-2.22/7.8.4/pkgdb:

The host:port pair is expected to be passed in as two environment variables: wai_host and wai_port for example:

Better yet, the application itself should set the environment variables as in the example code below.

wai-devel looks for a function Application.develMain I have a fork of yesod, that builds a yesod binary which generates a scaffold with this function implemented. I recommend using it to generate the scaffold with which to try out wai-devel with.

The specifics of how to set the port and host within yesod applications will obviously change. The point of this fork is to generate a scaffold that works with wai-devel out of the box.

Here is a snippet develMain function from my yesod fork.

-- | main function for use by yesod devel
develMain :: IO ()
develMain = develMainHelper' getApplicationDev

develMainHelper' :: IO (Settings, Application) -> IO ()
develMainHelper' getSettingsApp = do
    (settings, app) <- getSettingsApp

    _ <- unsetEnv "wai_port" >> setEnv "wai_port" "3001"
    _ <- unsetEnv "wai_host" >> setEnv "wai_host" ""

    let settings'  = setPort (3001 :: Port) settings
        settings'' = setHost ((read "") :: HostPreference) settings\'

    sock <- createSocket

    runSettingsSocket settings'' sock app

    where -- | Create the socket that we will use to communicate with
          -- localhost:3001 here.
          createSocket :: IO Socket
          createSocket = do

            sock <- socket AF_INET Stream defaultProtocol

            -- Tell the OS *not* to reserve the socket after your program exits.
            setSocketOption sock ReuseAddr 1

            -- Bind the socket to localhost:3000 and listen.
            -- I wonder why I can't specify localhost instead of iNADDR_ANY
            bindSocket sock (SockAddrInet 3001 iNADDR_ANY)
            listen sock 2
            return sock

During socket creation I made sure that the socket option ReuseAddr has been set to 1.
This way the operating system doesn’t hold on to the socket after the program exits. This is important for when wai-devel takes note of file changes and the development server is restarted.

Ignoring files and directories

wai-devel expects that there will be a single Main.main function. In the case of having more than one, for example with yesod, we ignore all but one. Specifically, we ignore the file app/DevelMain.hs. There is no need for app/devel.hs so it has been removed in my fork.

Moreover, wai-devel ignores files in your test/ directory.
This is because wai-devel depends on ide-backend which will attempt to build all files in the current working diretory, including your test directory. This leads to a world of hurt because the test/ directory also has a Main.main function.

Please report an issue if you would like any file ignored during builds.

Moved to stack

Since the Haskell community has moved in this direction, so has wai-devel.
wai-devel only depends on cabal in that stack and ide-backend depend on Cabal the library. Otherwise, the cabal binary is not used and hasn’t been tested to work with wai-devel.

Compatible versions of GHC

Currently wai-devel is built and tested against:

Regarding file watching

wai-devel watches for file changes on files with the following extensions:

When a change takes place wai-devel will recompile and re-run your application on localhost:3001 or display an error if any on the browser at localhost:3000

If you would want another extension added to the list of file extensions to watch for please report it as an issue.

Command line arguments

Currently wai-devel takes only these two arguments and the two are optional. If you feel the need for more arguments please report it as an issue on github.