Scope delegation (.value lookup) 

This page describes scope delegation. It assumes you’ve read and understood the previous pages, build definition and scopes.

Now that we’ve covered all the details of scoping, we can explain the .value lookup in detail. It’s ok to skip this section if this is your first time reading this page.

Because the term Global is used for both a scope component *, and as shorthand for the scope (Global, Global, Global), in this page we will use the symbol * when we mean it as the scope component.

To summarize what we’ve learned so far:

Now let’s suppose we have the following build definition:

lazy val foo = settingKey[Int]("")
lazy val bar = settingKey[Int]("")

lazy val projX = (project in file("x"))
  .settings(
    foo := {
      (bar in Test).value + 1
    },
    bar in Compile := 1
  )

Inside of foo’s setting body a dependency on the scoped key (bar in Test) is declared. However, despite bar in Test being undefined in projX, sbt is still able to resolve (bar in Test) to another scoped key, resulting in foo initialized as 2.

sbt has a well-defined fallback search path called scope delegation. This feature allows you to set a value once in a more general scope, allowing multiple more-specific scopes to inherit the value.

Scope delegation rules 

Here are the rules for scope delegation:

We will look at each rule in the rest of this page.

Rule 1: Scope axis precedence 

In other words, given two scopes candidates, if one has more specific value on the subproject axis, it will always win regardless of the configuration or the task scoping. Similarly, if subprojects are the same, one with more specific configuration value will always win regardless of the task scoping. We will see more rules to define more specific.

Rule 2: The task axis delegation 

Here we have a concrete rule for how sbt will generate delegate scopes given a key. Remember, we are trying to show the search path given an arbitrary (xxx in yyy).value.

Exercise A: Given the following build definition:

lazy val projA = (project in file("a"))
  .settings(
    name := {
      "foo-" + (scalaVersion in packageBin).value
    },
    scalaVersion := "2.11.11"
  )

What is the value of name in projA (projA/name in sbt shell)?

  1. "foo-2.11.11"
  2. "foo-2.12.3"
  3. something else?

The answer is "foo-2.11.11". Inside of .settings(...), scalaVersion is automatically scoped to (projA, *, *), so scalaVersion in packageBin becomes scalaVersion in (projA, *, packageBin). That particular scoped key is undefined. By using Rule 2, sbt will substitute the task axis to * as (projA, *, *) (or proj/scalaVersion in shell). That scoped key is defined to be "2.11.11".

Rule 3: The configuration axis search path 

The example for that is projX that we saw earlier:

lazy val foo = settingKey[Int]("")
lazy val bar = settingKey[Int]("")

lazy val projX = (project in file("x"))
  .settings(
    foo := {
      (bar in Test).value + 1
    },
    bar in Compile := 1
  )

If we write out the full scope again, it’s (projX, Test, *). Also recall that Test extends Runtime, and Runtime extends Compile.

(bar in Test) is undefined, but due to Rule 3 sbt will look for bar scoped in (projX, Test, *), (projX, Runtime, *), and then (projX, Compile, *). The last one is found, which is bar in Compile.

Rule 4: The subproject axis search path 

Exercise B: Given the following build definition:

organization in ThisBuild := "com.example"

lazy val projB = (project in file("b"))
  .settings(
    name := "abc-" + organization.value,
    organization := "org.tempuri"
  )

What is the value of name in projB (projB/name in shell)?

  1. "abc-com.example"
  2. "abc-org.tempuri"
  3. something else?

The answer is abc-org.tempuri. So based on Rule 4, the first search path is organization scoped to (projB, *, *), which is defined in projB as "org.tempuri". This has higher precedence than the build-level setting organization in ThisBuild.

Scope axis precedence, again 

Exercise C: Given the following build definition:

scalaVersion in (ThisBuild, packageBin) := "2.12.2"

lazy val projC = (project in file("c"))
  .settings(
    name := {
      "foo-" + (scalaVersion in packageBin).value
    },
    scalaVersion := "2.11.11"
  )

What is value of name in projC?

  1. "foo-2.12.2"
  2. "foo-2.11.11"
  3. something else?

The answer is foo-2.11.11. scalaVersion scoped to (projC, *, packageBin) is undefined. Rule 2 finds (projC, *, *). Rule 4 finds (ThisBuild, *, packageBin). In this case Rule 1 dictates that more specific value on the subproject axis wins, which is (projC, *, *) that is defined to "2.11.11".

Exercise D: Given the following build definition:

scalacOptions in ThisBuild += "-Ywarn-unused-import"

lazy val projD = (project in file("d"))
  .settings(
    test := {
      println((scalacOptions in (Compile, console)).value)
    },
    scalacOptions in console -= "-Ywarn-unused-import",
    scalacOptions in Compile := scalacOptions.value // added by sbt
  )

What would you see if you ran projD/test?

  1. List()
  2. List(-Ywarn-unused-import)
  3. something else?

The answer is List(-Ywarn-unused-import). Rule 2 finds (projD, Compile, *), Rule 3 finds (projD, *, console), and Rule 4 finds (ThisBuild, *, *). Rule 1 selects (projD, Compile, *) because it has the subproject axis projD, and the configuration axis has higher precedence over the task axis.

Next, scalacOptions in Compile refers to scalacOptions.value, we next need to find a delegate for (projD, *, *). Rule 4 finds (ThisBuild, *, *) and thus it resolves to List(-Ywarn-unused-import).

Inspect command lists the delegates 

You might want to look up quickly what is going on. This is where inspect can be used.

Hello> inspect projD/compile:console::scalacOptions
[info] Task: scala.collection.Seq[java.lang.String]
[info] Description:
[info]  Options for the Scala compiler.
[info] Provided by:
[info]  {file:/Users/xxxx/}projD/compile:scalacOptions
[info] Defined at:
[info]  /Users/xxxx/build.sbt:47
[info] Reverse dependencies:
[info]  projD/compile:console
[info]  projD/*:test
[info] Delegates:
[info]  projD/compile:console::scalacOptions
[info]  projD/compile:scalacOptions
[info]  projD/*:console::scalacOptions
[info]  projD/*:scalacOptions
[info]  {.}/compile:console::scalacOptions
[info]  {.}/compile:scalacOptions
[info]  {.}/*:console::scalacOptions
[info]  {.}/*:scalacOptions
[info]  */compile:console::scalacOptions
[info]  */compile:scalacOptions
[info]  */*:console::scalacOptions
[info]  */*:scalacOptions
....

Note how “Provided by” shows that projD/compile:console::scalacOptions is provided by projD/compile:scalacOptions. Also under “Delegates”, all of the possible delegate candidates listed in the order of precedence!

.value lookup vs dynamic dispatch 

Note that scope delegation feels similar to class inheritance in an object-oriented language, but there’s a difference. In an OO language like Scala if there’s a method named drawShape on a trait Shape, its subclasses can override the behavior even when drawShape is used by other methods in the Shape trait, which is called dynamic dispatch.

In sbt, however, scope delegation can delegate a scope to a more general scope, like a project-level setting to a build-level settings, but that build-level setting cannot refer to the project-level setting.

Exercise E: Given the following build definition:

lazy val root = (project in file("."))
  .settings(
    inThisBuild(List(
      organization := "com.example",
      scalaVersion := "2.12.2",
      version      := scalaVersion.value + "_0.1.0"
    )),
    name := "Hello"
  )

lazy val projE = (project in file("e"))
  .settings(
    scalaVersion := "2.11.11"
  )

What will projE/version return?

  1. "2.12.2_0.1.0"
  2. "2.11.11_0.1.0"
  3. something else?

The answer is 2.12.2_0.1.0. projD/version delegates to version in ThisBuild, which depends on scalaVersion in ThisBuild. Because of this reason, build level setting should be limited mostly to simple value assignments.

Exercise F: Given the following build definition:

scalacOptions in ThisBuild += "-D0"
scalacOptions += "-D1"

lazy val projF = (project in file("f"))
  .settings(
    scalacOptions in compile += "-D2",
    scalacOptions in Compile += "-D3",
    scalacOptions in (Compile, compile) += "-D4",
    test := {
      println("bippy" + (scalacOptions in (Compile, compile)).value.mkString)
    }
  )

What will projF/test show?

  1. "bippy-D4"
  2. "bippy-D2-D4"
  3. "bippy-D0-D3-D4"
  4. something else?

The answer is "bippy-D0-D3-D4". This is a variation of an exercise originally created by Paul Phillips.

It’s a great demonstration of all the rules because someKey += "x" expands to

someKey += {
  val old = someKey.value
  old :+ "x"
}

Retrieving the old value would cause delegation, and due to Rule 5, it will go to another scoped key. Let’s get rid of += first, and annotate the delegates for old values:

scalacOptions in ThisBuild := {
  // scalacOptions in Global <- Rule 4
  val old = (scalacOptions in ThisBuild).value
  old :+ "-D0"
}

scalacOptions := {
  // scalacOptions in ThisBuild <- Rule 4
  val old = scalacOptions.value
  old :+ "-D1"
}

lazy val projF = (project in file("f"))
  .settings(
    scalacOptions in compile := {
      // scalacOptions in ThisBuild <- Rules 2 and 4
      val old = (scalacOptions in compile).value
      old :+ "-D2"
    },
    scalacOptions in Compile := {
      // scalacOptions in ThisBuild <- Rules 3 and 4
      val old = (scalacOptions in Compile).value
      old :+ "-D3"
    },
    scalacOptions in (Compile, compile) := {
      // scalacOptions in (projF, Compile) <- Rules 1 and 2
      val old = (scalacOptions in (Compile, compile)).value
      old :+ "-D4"
    },
    test := {
      println("bippy" + (scalacOptions in (Compile, compile)).value.mkString)
    }
  )

This becomes:

scalacOptions in ThisBuild := {
  Nil :+ "-D0"
}

scalacOptions := {
  List("-D0") :+ "-D1"
}

lazy val projF = (project in file("f"))
  .settings(
    scalacOptions in compile := List("-D0") :+ "-D2",
    scalacOptions in Compile := List("-D0") :+ "-D3",
    scalacOptions in (Compile, compile) := List("-D0", "-D3") :+ "-D4",
    test := {
      println("bippy" + (scalacOptions in (Compile, compile)).value.mkString)
    }
  )

Contents

sbt Reference Manual
    1. Scope delegation (.value lookup)