Build definition basics

This page discusses the build.sbt build definition.

What is a build definition?

A build definition is defined in build.sbt, and it consists of a set of projects (of type Project). Because the term project can be ambiguous, we often call it a subproject in this guide.

For instance, in build.sbt you define the subproject located in the current directory like this:

scalaVersion := "3.3.3"
name := "Hello"

or more explicitly:

lazy val root = (project in file("."))
  .settings(
    scalaVersion := "3.3.3",
    name := "Hello",
  )

Each subproject is configured by key-value pairs. For example, one key is name and it maps to a string value, the name of your subproject. The key-value pairs are listed under the .settings(...) method.

build.sbt DSL

build.sbt defines subprojects using a DSL called build.sbt DSL, which is based on Scala. Initially you can use build.sbt DSL, like a YAML file, declaring just scalaVersion and libraryDependencies, but it can supports more features to keep the build definition organized as the build grows larger.

Typed setting expression

Let's take a closer look at the build.sbt DSL:

organization  :=         "com.example"
^^^^^^^^^^^^  ^^^^^^^^   ^^^^^^^^^^^^^
key           operator   (setting/task) body

Each entry is called a setting expression. Some among them are also called task expressions. We will see more on the difference later in this page.

A setting expression consists of three parts:

1. Left-hand side is a key.
2. Operator, which in this case is :=
3. Right-hand side is called the body, or the setting/task body.

On the left-hand side, name, version, and scalaVersion are keys. A key is an instance of SettingKey[A], TaskKey[A], or InputKey[A] where A is the expected value type.

Because key name is typed to SettingKey[String], the := operator on name is also typed specifically to String. If you use the wrong value type, the build definition will not compile:

name := 42 // will not compile

vals and lazy vals

To avoid repeating the same information, like the version number for a library, build.sbt may be interspersed with vals, lazy vals, and defs.

val toolkitV = "0.2.0"
val toolkit = "org.scala-lang" %% "toolkit" % toolkitV
val toolkitTest = "org.scala-lang" %% "toolkit-test" % toolkitV

scalaVersion := "3.3.3"
libraryDependencies += toolkit
libraryDependencies += (toolkitTest % Test)

In the above, val defines a variable, which are initialized from the top to bottom. This means that toolkitV must be defined before it is referenced.

Here's a bad example:

// bad example
val toolkit = "org.scala-lang" %% "toolkit" % toolkitV // uninitialized reference!
val toolkitTest = "org.scala-lang" %% "toolkit-test" % toolkitV // uninitialized reference!
val toolkitV = "0.2.0"

sbt will fail to load with java.lang.ExceptionInInitializerError cased by a NullPointerException if your build.sbt contains an uninitialized forward reference. One way to let the compiler fix this is to define the variables as lazy:

lazy val toolkit = "org.scala-lang" %% "toolkit" % toolkitV
lazy val toolkitTest = "org.scala-lang" %% "toolkit-test" % toolkitV
lazy val toolkitV = "0.2.0"

Some frown upon gratuitous lazy vals, but Scala 3 lazy vals are efficient, and we think it makes the build definition more robust for copy-pasting.