Referring to a setting in an sbt root project from a sub project

Question

How do I refer to a baseDirectory of a root project from within sub-projects in a multi-project sbt build? Like

lazy val full: Project = Project(
  id = "full",
  base = file("."),
  ..
)

lazy val sub = Project(
  id = "sub",
  base = file("sub"),
  ..
  settings = Seq(
    javaSource in Compile := full.settings.baseDirectory / "foo" / "src"
  )
)

This attempt just gives me:

: error: ambiguous reference to overloaded definition,
both method settings in trait Project of type (ss: sbt.Def.Setting[_]*)sbt.Project
and  method settings in trait ProjectDefinition of type => Seq[sbt.Def.Setting[_]]
match expected type ?
    javaSource in Compile := full.settings.baseDirectory / "foo" / "src",
                                  ^

Answer 1

In sbt, projects are just another setting axis (along with configurations and tasks). So you can use the in operator to access the value of a setting in another project. To get the value of the baseDirectory setting key in the project full , you would write

(baseDirectory in full).value

Hence, your complete javaSource setting should be:

javaSource in Compile := (baseDirectory in full).value / "foo" / "src"

See the documentation on Scopes in sbt for the whole story.

(Note that accessing the settings method of a Project rarely does what one wants. I believe it is only relevant when write custom commands .)