sbt/src/sphinx/Getting-Started/Basic-Def.rst

=========================
`.sbt` Build Definition
=========================

This page describes sbt build definitions, including some "theory" and
the syntax of `build.sbt`. It assumes you know how to :doc:`use sbt <Running>` 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 :doc:`.scala build definition <Full-Def>` (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 :doc:`/Extending/Input-Tasks` for more.


Built-in Keys
~~~~~~~~~~~~~

The built-in keys are just fields in an object called
`Keys <../../sxr/Keys.scala.html>`_. 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 `val`\ s and `def`\ s in addition to settings.
All such definitions are evaluated before settings regardless of where they are defined in the file.
`val`\ s and `def`\ s must be separated from settings by blank lines.

.. note::

    Typically, `lazy val`\ s are used instead of `val`\ s 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 :doc:`more about settings <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 :doc:`.scala files <Full-Def>`,
the contents of any `Build` or `Plugin` objects in those files will
be imported. More on that when we get to :doc:`.scala build definitions <Full-Def>`.)


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
:doc:`more about settings </Getting-Started/More-About-Settings>`.
The `%` method is used to construct an Ivy module ID from strings,
explained in :doc:`library dependencies </Getting-Started/Library-Dependencies>`.

We'll skip over the details of library dependencies until later in the
Getting Started Guide. There's a :doc:`whole page </Getting-Started/Library-Dependencies>`
covering it later on.

Next
----

Move on to :doc:`learn about scopes </Getting-Started/Scopes>`.