How to Write A DSL in Scala

DSL – domain specific language. DSLs are often touted as the silver bullet to let “users write their own business rules”. They’re even more often used when they shouldn’t be, and just end up complicating things. But they are fun. And used correctly, they can be a really powerful, highly expressive tool.

A well-known example, and one of my personal favorites, is of course the ScalaTest spec/assertion dsl. It lets you write highly expressive assertions like:

myBooleanResult should be (true)

And, when you see that for the first time, you might ask

  1. How the heck does that work?
  2. Isn’t that just myBoolean == true ?

To which I’d answer:

  1. Scala supports infix notation
  2. Not even close.

Packing a Punch

There’s a multitude of reasons why should be is far superior to ==. Right now I’ll hit the highlights and get on to teaching you to write you own, similar, dsl.

The combination of should be creates a layering of classes that allow us to compare myBooleanResult and true, but get natural language success and error messages, i.e.:

SUCCESS: myBooleanResult was true
FAILURE: myBooleanResult should have been true

instead of:

SUCCESS: myBooleanResult == true
FAILURE: myBooleanResult != true

This is just one (trivial) example of why ScalaTest is so great, but it should give you an idea of how dsl’s look and feel in Scala.

What’s In a DSL

Let’s start from the abstract. If we want to write code that looks like natural sentences, let’s start by examining natural sentence structure.

Harriet visits Grandma.

A mundane enough sentence, wouldn’t you say? Let’s break it down.

We have:

  1. “Harriet” – our subject
  2. “visits” – our verb
  3. “Grandma” – our direct object

So that means we’d need an API that cpatures those concepts. In regular old code it might look like:

class Subject () { def visits(obj: DirectObject): Unit = ??? }
class Visitable extends DirectObject
val Harriet = new Subject
val Grandma = new Visitable

Harriet.visits(Grandma)

Which is all good and well but there’s just something special about writing:

Harriet visits Grandma

A generalized/stubbed API might look like this:

package dsl

object Abstract {

  class SubjectWord {
    def verb(obj: DirectObject): Any = ???
  }

  class DirectObject

  val mySubject = new SubjectWord()

  val myObject = new DirectObject()

  mySubject verb myObject
}

Serving It Up

Let’s try creating a dsl to do something (semi-) useful!

We’ll write an API that lets us simulate a catering business or restaurant.

It will have:

  1. Guests (people) organized into parties
  2. Guest’s food preferences
  3. Servers who bring food to parties of guests

Servers will serve food on a First-Come-First-Serve basis to anyone with preference for that food (I didn’t say it was a competent catering business!)

Let’s start with defining our “service” interface, i.e. the business logic… aptly named Server in our domain!

This server will:

  1. be assigned to a party (group of people)
  2. serve food to the party as it becomes ready, based on the rules we described above.

We’ll take a stab at writing our Server with our ideal dsl, then we’ll go about implementing it!

  class Server(party: Seq[Person]) {
    def serves(food: Food): Unit =
      party.collectFirst {
        case person if person hasPreferenceFor food => person
      } match {
        case Some(person) => person gets food
      }
  }

We see:

  1. We’ll need a way to check someone’s preference (hasPreferenceFor)
  2. We’ll need a way for a person to receive food (gets)

Let’s start on our person implementation:

  class Person() {
    def hasPreferenceFor(food: Food): Boolean = ???

    def gets(food: Food): Unit = ()
  }

Simple enough? Just two 1-Arity methods that we can call with infix notation.

But how do we implement hasPreferenceFor? With more dsl of course!

  class Person() {
    private var preference: Option[Food] = None

    def prefers(food: Food) =
      preference = Some(food)

    def hasPreferenceFor(food: Food): Boolean =
      preference.contains(food)
    ...
  }

We add some state (preference) to our Person, which starts out as None (i.e. prefers no food). We have a prefers method that will set the Person’s preference (in Java, it might be called setPreference). And hasPreferenceFor simply checks equality of the incoming food against the Person’s preference, and when preference == None they will not prefer any incoming food.

What do our food(s) look like? They can be as simple or complex as your implementation needs them to be. For this tutorial, we’ll just use a couple of case objects:

  trait Food

  case object Pizza extends Food
  case object Salad extends Food

And that’s it! Now we can use our dsl!

val John = new Person()
val Sarah = new Person()

val johnAndSarah = Seq(John, Sarah)

val Martin = new Server(johnAndSarah)

John prefers Salad
Sarah prefers Pizza

Martin serves Pizza // Sara gets Pizza!
Martin serves Salad // John gets Salad!

John prefers None

Martin serves Salad // No one gets Salad!

No . or () to clutter our code! Now you’re ready to write readable, natural code.

If you’d like to check out/extend the source code from this post, you can find it here: https://github.com/tylermk04/how-to-dsl

Exercises

Some exercises that are left to the reader:

  1. How to add indirect objects?
  2. In a group of many people, servers would be serving the same person over and over again! How can you improve the Server so that once a person has received food, the people further down the line get served once that food comes around again?
  3. Furthermore from (2) – can you make it so people eventually finish eating their food? And after they finish, they can receive a second helping of their food-of-preference.
  4. Once (3) is done, the Server will definitely want to keep track of who had what, and in what quantities! Can you itemize a bill?

Can you do all 4 in “dsl-style”? Feel free to send Pull Requests with your implementations!

If you're working on projects like this, I'd love to hear from you! If you have any questions or thoughts about this article, please reach out! Or, if you have a topic you'd like me to write about next, let me know!
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