Adding live css and js reload to Yesod

By default Yesod does live reloading when editing .hs files or Shakespeare template files. When working directly with js or css files this doesn’t happen and we’re forced to manually reload the browser at every change.

This is a simple setup to improve this behaviour.


The first step is installing browser-sync:

npm i -g browser-sync

Then we can run a server instance with:

browser-sync start --no-open --files="static" --proxy="localhost:3000" --port=4000

I’m making browser-sync watch the static folder, because that’s where all my built js and css files end up in. You might need to change the ports according to your Yesod settings and whether there’s already some other process using port 4000 (in that case browser-sync tries with 4001).

Start your server as usual (i.e. stack exec -- yesod devel) and then you can visit localhost:4000, edit some css and see the browser refresh automatically :)

Enabling CORS

Sadly, that’s not enough to make everything work. If you try to trigger any AJAX request, you will see that they probably don’t work because of CORS.

We can fix this by adding wai-cors to our project’s pacakge.yaml dependencies.

Then go to src/Application.hs, we’ll need a few simple edits there:

1) Add the import on top

import Network.Wai.Middleware.Cors

2) Create a CORS middleware

corsWare :: Middleware
corsWare = let
    appCorsResourcePolicy = 
      simpleCorsResourcePolicy { corsMethods = ["OPTIONS", "GET", "POST", "PUT", "DELETE"]
                               , corsOrigins = Just (["http://localhost:3000", "http://localhost:4000"], True)
                               , corsRequestHeaders = ["Authorization", "Content-Type", "x-requested-with", "x-xsrf-token"]
    in cors (const $ Just appCorsResourcePolicy)

3) Add the middleware to getApplicationDev, for example:

getApplicationDev :: IO (Settings, Application)
getApplicationDev = do
    settings <- getAppSettings
    foundation <- makeFoundation settings
    startDequeue foundation
    startJobSchedule foundation
    wsettings <- getDevSettings $ warpSettings foundation
    app <- makeApplication foundation
    return (wsettings, corsWare app)

AJAX request should start working again. Please be careful with CORS, as it could lead to serious security issues if enabled on a live server.

Reloading everything

My current setup includes building some js and purescript code in a subfolder and using compass to build sass files. I’ve created this simple script to manage all build processes from a single terminal:


trap killgroup SIGINT

  echo killing...
  kill 0

browser-sync start --no-open --files="static" --proxy="localhost:3000" --port=4000 &
compass watch &
cd purescript; npm run watch &

This allows me to close all three processes with a single CTRL-C and avoid having multiple terminals open.

I hope this was useful enough, see you next time!

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