Sunday, June 05, 2011

My first scala app

Well I say first, but that’s not quite the whole story. I’ve worked on existing Scala code bases but recently got the opportunity to start a new project and I got to see how the experiences on those projects and conversations with colleges influenced design decisions. I wanted to note down some of the tricks I’d picked up.


We start with the tools and first up is SBT, version 0.7.x. We depend on the IDEA plugin to generate IDEA project files. We also use the Scaliform plugin which formats code every compile within SBT and the Sources plugin which when run downloads the source Jars without having to stipulate the ‘withSources’ directive on the dependencies.

The IDEA and Sources plugin are run as processors and are not checked into the SBT project file, whereas the Scalariform plugin is. The distinction is that a developer might choose to use a different IDE or not download the sources, but we should always try and conform to the same coding practices and being lazy, having a plugin do it for you is one less thing to worry about.

Inversion of control / Dependency injection

One of the interesting developments early on was the choice of the dependency injection frameworks. Would it be Spring, Guice or something custom? Actually, we decided not to have any and just use plain old servlets through our routing framework, Scalatra. I think this has actually lead us to write better code, for instance we have taken advantage of the companion objects for classes instead of relaying on repository classes that would need to be injected.


We started with the routing framework, Scalatra which is similar to Sinatra and other simple routing frameworks. It provides a clean way of exposing your application to the web and combined with the following, enables all of the methods to be only a few very focused lines of code. We used Scalate and SSP for the template rendering which can feel a little verbose with the variable declarations at the top, but when you can pre-compile the templates you can understand why those are required.


All new projects are using MongoDB and we’ve been using the Scala driver, Casbah, for a while but this gave us an opportunity to try out Salat the case class wrapper. This is probably as close as I’ve seen ORM like features in Scala while maintaining the MongoDB query sugar. As mentioned, using companion objects instead of repository classes allowed us to do tidy things like:

case class Artist(_id: ObjectId = new ObjectId, name: String) {
def save() = Artist.saveArtist(this)

object Artist extends SalatDAO[Artist, ObjectId](...mongo config...) {
def saveArtist(artist: Artist) = update(Map(“_id” -> artist._id, artist

And use them like:

val artist = Artist(“My new favourite band”)

This meant we were using a repository like model, but hiding it behind the case class and the companion object. It makes for very neat code further upstream.


The app has to call out to other web APIs and for this we use dispatch which wraps the Apache HttpClient project with a bow making it really simple to get resources and convert them to string, XML or JSON. We were already familiar with lift-json so we would get the response as a string and use case classes to parse the JSON into. Once again, XML is almost a joy to work with in Scala compared to other languages, but lift-json still wins in simplicity so we use JSON formatted APIs where we can.

What’s next

I’ve got my eye on Lift and Akka. I’ve had a play with Akka, but I feel I’d like to understand it a little more before using it in a project. Other than that, I think I’m generally getting my head around Scala and functional programming all the more and I’m rather enjoying it. I still feel like I’m just scratching the surface, but it is my first app remember?

1 comment:

Nigel Pepper said...

Very nice! As elegant as ruby but a little less idiomatic.