.sbt Build Definition

.sbt Build Definition

This page describes sbt build definitions, including some "theory" and the syntax of build.sbt. It assumes you know how to use sbt and have read the previous pages in the Getting Started Guide.

.sbt vs. .scala Definition

An sbt build definition can contain files ending in .sbt, located in the base directory, and files ending in .scala, located in the project/ subdirectory the base directory.

This page discusses .sbt files, which are suitable for most cases. The .scala files are typically used for sharing code across .sbt files and for larger build definitions. See .scala build definition (later in Getting Started) for more on .scala files.

What is a build definition?

** PLEASE READ THIS SECTION **

After examining a project and processing any build definition files, sbt will end up with an immutable map (set of key-value pairs) describing the build.

For example, one key is name and it maps to a string value, the name of your project.

Build definition files do not affect sbt's map directly.

Instead, the build definition creates a huge list of objects with type Setting[T] where T is the type of the value in the map. A Setting describes a transformation to the map, such as adding a new key-value pair or appending to an existing value. (In the spirit of functional programming, a transformation returns a new map, it does not update the old map in-place.)

In build.sbt, you might create a Setting[String] for the name of your project like this:

name := "hello"

This Setting[String] transforms the map by adding (or replacing) the name key, giving it the value "hello". The transformed map becomes sbt's new map.

To create its map, sbt first sorts the list of settings so that all changes to the same key are made together, and values that depend on other keys are processed after the keys they depend on. Then sbt walks over the sorted list of Setting and applies each one to the map in turn.

Summary: A build definition defines a list of Setting[T], where a Setting[T] is a transformation affecting sbt's map of key-value pairs and T is the type of each value.

How build.sbt defines settings

build.sbt defines a Seq[Setting[_]]; it's a list of Scala expressions, separated by blank lines, where each one becomes one element in the sequence. If you put Seq( in front of the .sbt file and ) at the end and replace the blank lines with commas, you'd be looking at the equivalent .scala code.

Here's an example:

name := "hello"

version := "1.0"

scalaVersion := "2.9.2"

A build.sbt file is a list of Setting, separated by blank lines. Each Setting is defined with a Scala expression. The expressions in build.sbt are independent of one another, and they are expressions, rather than complete Scala statements. These expressions may be interspersed with val`s, `lazy val`s, and `def`s, but top-level `object`s and classes are not allowed in `build.sbt. Those should go in the project/ directory as full Scala source files.

On the left, name, version, and scalaVersion are keys. A key is an instance of SettingKey[T], TaskKey[T], or InputKey[T] where T is the expected value type. The kinds of key are explained more below.

Keys have a method called :=, which returns a Setting[T]. You could use a Java-like syntax to call the method:

name.:=("hello")

But Scala allows name := "hello" instead (in Scala, any method can use either syntax).

The := method on key name returns a Setting, specifically a Setting[String]. String also appears in the type of name itself, which is SettingKey[String]. In this case, the returned Setting[String] is a transformation to add or replace the name key in sbt's map, giving it the value "hello".

If you use the wrong value type, the build definition will not compile:

name := 42  // will not compile

Settings are separated by blank lines

You can't write a build.sbt like this:

// will NOT work, no blank lines
name := "hello"
version := "1.0"
scalaVersion := "2.9.2"

sbt needs some kind of delimiter to tell where one expression stops and the next begins.

.sbt files contain a list of Scala expressions, not a single Scala program. These expressions have to be split up and passed to the compiler individually.

Keys

Types

There are three flavors of key:

  • SettingKey[T]: a key for a value computed once (the value is computed one time when loading the project, and kept around).
  • TaskKey[T]: a key for a value, called a task, that has to be recomputed each time, potentially creating side effects.
  • InputKey[T]: a key for a task that has command line arguments as input. The Getting Started Guide doesn't cover InputKey, but when you finish this guide, check out Input Tasks for more.

Built-in Keys

The built-in keys are just fields in an object called Keys. A build.sbt implicitly has an import sbt.Keys._, so sbt.Keys.name can be referred to as name.

Custom Keys

Custom keys may be defined with their respective creation methods: settingKey, taskKey, and inputKey. Each method expects the type of the value associated with the key as well as a description. The name of the key is taken from the val the key is assigned to. For example, to define a key for a new task called hello,

lazy val hello = taskKey[Unit]("An example task")

Here we have used the fact that an .sbt file can contain vals and defs in addition to settings. All such definitions are evaluated before settings regardless of where they are defined in the file. vals and defs must be separated from settings by blank lines.

Note

Typically, lazy vals are used instead of vals to avoid initialization order problems.

Task v. Setting keys

A TaskKey[T] is said to define a task. Tasks are operations such as compile or package. They may return Unit (Unit is Scala for void), or they may return a value related to the task, for example package is a TaskKey[File] and its value is the jar file it creates.

Each time you start a task execution, for example by typing compile at the interactive sbt prompt, sbt will re-run any tasks involved exactly once.

sbt's map describing the project can keep around a fixed string value for a setting such as name, but it has to keep around some executable code for a task such as compile -- even if that executable code eventually returns a string, it has to be re-run every time.

A given key always refers to either a task or a plain setting. That is, "taskiness" (whether to re-run each time) is a property of the key, not the value.

Defining tasks and settings

Using :=, you can assign a value to a setting and a computation to a task. For a setting, the value will be computed once at project load time. For a task, the computation will be re-run each time the task is executed.

For example, to implement the hello task from the previous section,

hello := { println("Hello!") }

We already saw an example of defining settings when we defined the project's name,

name := "hello"

Types for tasks and settings

From a type-system perspective, the Setting created from a task key is slightly different from the one created from a setting key. taskKey := 42 results in a Setting[Task[T]] while settingKey := 42 results in a Setting[T]. For most purposes this makes no difference; the task key still creates a value of type T when the task executes.

The T vs. Task[T] type difference has this implication: a setting can't depend on a task, because a setting is evaluated only once on project load and is not re-run. More on this in more about settings, coming up soon.

Keys in sbt interactive mode

In sbt's interactive mode, you can type the name of any task to execute that task. This is why typing compile runs the compile task. compile is a task key.

If you type the name of a setting key rather than a task key, the value of the setting key will be displayed. Typing a task key name executes the task but doesn't display the resulting value; to see a task's result, use show <task name> rather than plain <task name>. The convention for keys names is to use camelCase so that the command line name and the Scala identifiers are the same.

To learn more about any key, type inspect <keyname> at the sbt interactive prompt. Some of the information inspect displays won't make sense yet, but at the top it shows you the setting's value type and a brief description of the setting.

Imports in build.sbt

You can place import statements at the top of build.sbt; they need not be separated by blank lines.

There are some implied default imports, as follows:

import sbt._
import Process._
import Keys._

(In addition, if you have .scala files, the contents of any Build or Plugin objects in those files will be imported. More on that when we get to .scala build definitions.)

Adding library dependencies

To depend on third-party libraries, there are two options. The first is to drop jars in lib/ (unmanaged dependencies) and the other is to add managed dependencies, which will look like this in build.sbt:

libraryDependencies += "org.apache.derby" % "derby" % "10.4.1.3"

This is how you add a managed dependency on the Apache Derby library, version 10.4.1.3.

The libraryDependencies key involves two complexities: += rather than :=, and the % method. += appends to the key's old value rather than replacing it, this is explained in more about settings. The % method is used to construct an Ivy module ID from strings, explained in library dependencies.

We'll skip over the details of library dependencies until later in the Getting Started Guide. There's a whole page covering it later on.