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 Build Definition An sbt build definition can contain files ending in .sbt, located in the
base directory of a project, and files ending in .scala, located in the
project/ subdirectory of 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 more complex build definitions. See
.scala build definition (later in Getting Started) for more
on .scala files.
After examining a project and processing build definition files, sbt ends 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 with immutable data structures and values, 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 the 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 Settings 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.
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 := "2.10.3"
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 vals, lazy vals, and defs. Top-level objects 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 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, a single-parameter
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
You can’t write a build.sbt like this:
// will NOT compile, no blank lines
name := "hello"
version := "1.0"
scalaVersion := "2.10.3"
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.
There are three flavors of key:
SettingKey[T]: a key for a value computed once (the value is
computed 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 with side effects.
InputKey[T]: a key for a task that has command line arguments as
input. Check out Input Tasks for more details.
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 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.
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.
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"
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 kinds of setting, coming up soon.
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.
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 definition.)
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 kinds of setting. 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.
