Scala for Backend Engineering & Functional Programming · Lesson

Composing Domains

Build larger models.

Composing Domains is a free Scala for Backend Engineering & Functional Programming lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Scala for Backend Engineering & Functional Programming learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Building Larger Models

Real domains are assembled from smaller pieces. Composing domains means combining well-modeled ADTs, newtypes, and validated values into bigger, still-correct structures.

Small, focused types compose into larger ones.
Each piece keeps its own invariants.

case class Money(cents: Long)
case class Product(name: String, price: Money)

object Main:
  def main(args: Array[String]): Unit =
    println(Product("Book", Money(1999)))

Nesting Products

Compose product types by nesting case classes. Each level adds structure while staying readable.

case class Address(city: String, zip: String)
case class Customer(name: String, address: Address)

object Main:
  def main(args: Array[String]): Unit =
    val c = Customer("Ada", Address("London", "E1"))
    println(c.address.city)

Composing with Collections

Use List, Map, and Set to model one-to-many relationships inside your domain.

case class Item(name: String, qty: Int)
case class Order(id: Int, items: List[Item])

object Main:
  def main(args: Array[String]): Unit =
    val order = Order(1, List(Item("pen", 2), Item("ink", 1)))
    println(order.items.map(_.qty).sum)

Composing Sum Types

Larger choices combine smaller ones. An enum variant can itself contain another ADT, layering decisions.

enum Payment:
  case Card(last4: String)
  case Cash

enum Checkout:
  case Completed(payment: Payment)
  case Abandoned

object Main:
  def main(args: Array[String]): Unit =
    val c: Checkout = Checkout.Completed(Payment.Card("1234"))
    println(c)

Functions That Compose Domains

Domain operations are pure functions from input types to output types. Composing them builds workflows.

case class Cart(total: Long)
case class Receipt(total: Long, tax: Long)

object Main:
  def checkout(c: Cart): Receipt =
    Receipt(c.total, (c.total * 0.2).toLong)

  def main(args: Array[String]): Unit =
    println(checkout(Cart(1000)))

Composing Validations

When each field is built by a smart constructor, compose them with a for comprehension. The whole object is created only if every part validates.

object V:
  def name(s: String): Option[String] = if s.nonEmpty then Some(s) else None
  def age(n: Int): Option[Int] = if n >= 0 then Some(n) else None

case class Person(name: String, age: Int)

object Main:
  def make(n: String, a: Int): Option[Person] =
    for nm <- V.name(n); ag <- V.age(a) yield Person(nm, ag)

  def main(args: Array[String]): Unit =
    println(Main.make("Bob", 30))
    println(Main.make("", 30))

Folding Over Composite Data

Aggregate composed structures with fold or map plus sum. The shape of the data guides the computation.

case class Line(price: Long, qty: Int)
case class Invoice(lines: List[Line])

object Main:
  def total(inv: Invoice): Long =
    inv.lines.foldLeft(0L)((acc, l) => acc + l.price * l.qty)

  def main(args: Array[String]): Unit =
    val inv = Invoice(List(Line(100, 2), Line(50, 4)))
    println(total(inv))

Updating Immutable Models

Compose changes with copy, which returns a new value with selected fields replaced. The original stays untouched.

case class Account(id: Int, balance: Long)

object Main:
  def deposit(a: Account, amt: Long): Account =
    a.copy(balance = a.balance + amt)

  def main(args: Array[String]): Unit =
    val a = Account(1, 100)
    println(deposit(a, 50))

Modeling a State Machine

Compose sum types and transition functions to model a state machine where only legal transitions exist.

enum Light:
  case Red, Green, Yellow

object Main:
  def next(l: Light): Light = l match
    case Light.Red    => Light.Green
    case Light.Green  => Light.Yellow
    case Light.Yellow => Light.Red

  def main(args: Array[String]): Unit =
    println(next(Light.Red))
    println(next(Light.Green))

Layering Domain and Operations

Keep data types pure and put behavior in functions or methods. A composed model plus a set of transformations forms a small domain library.

case class Temp(celsius: Double):
  def toF: Double = celsius * 9 / 5 + 32

object Main:
  def main(args: Array[String]): Unit =
    val t = Temp(25.0)
    println(t.toF)

Putting It Together

A complete model composes newtypes, products, sums, collections, and validated construction. The result is hard to misuse and easy to reason about.

Build small types first.
Compose into aggregates.
Express operations as pure functions.

case class Sku(value: String)
case class Line(sku: Sku, qty: Int)
case class Basket(lines: List[Line]):
  def count: Int = lines.map(_.qty).sum

object Main:
  def main(args: Array[String]): Unit =
    val b = Basket(List(Line(Sku("A"), 2), Line(Sku("B"), 3)))
    println(b.count)

Quick Check

Test your understanding of composing domains.

Recap

You learned to compose domains.

Nest products and sums to model rich structures.
Use collections for one-to-many relationships.
Compose validations with for comprehensions.
Update immutably with copy.
Keep data pure and express operations as functions.

case class User(id: Int, name: String)
case class Post(author: User, text: String)

object Main:
  def main(args: Array[String]): Unit =
    val p = Post(User(1, "Ada"), "hello")
    println(p.copy(text = "hi"))

Frequently asked questions

Is the “Composing Domains” lesson free?

Yes — the full text of “Composing Domains” is free to read here on the web, and the Scala for Backend Engineering & Functional Programming course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Scala for Backend Engineering & Functional Programming course, upgrade to CoddyKit PRO.

What will I learn in “Composing Domains”?

Build larger models. You practise Scala for Backend Engineering & Functional Programming with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Scala for Backend Engineering & Functional Programming?

No prior experience is required. Scala for Backend Engineering & Functional Programming on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Composing Domains” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Scala for Backend Engineering & Functional Programming lesson?

Yes. Every Scala for Backend Engineering & Functional Programming lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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