How to reference a custom SBT Setting in sub-projects

Question

Somewhat similar to this question, how can reference a custom setting in a sub project.

In build.sbt:

import sbt.Keys._

val finagleVersion = settingKey[String]("Defines the Finagle version")

val defaultSettings = Defaults.coreDefaultSettings ++ Seq(
  finagleVersion in ThisBuild := "6.20.0",
  organization in ThisBuild := "my.package",      
  scalaVersion in ThisBuild := "2.10.4",
  version in ThisBuild := "0.1-SNAPSHOT"
)

lazy val root = project.in(file(".")).aggregate(thrift).settings(
  publishArtifact in (Compile, packageBin) := false,
  publishArtifact in (Compile, packageDoc) := false,
  publishArtifact in (Compile, packageSrc) := false
)

lazy val thrift = project.in(file("thrift"))

In thrift/build.sbt:

name := "thrift"

// doesn't work
libraryDependencies ++= Seq(
  "com.twitter" %% "finagle-thriftmux" % (finagleVersion in LocalRootProject).value
)

Answer 1

.sbt files cannot see the definitions (e.g., vals) in other .sbt files, even if they are part of the same build.

However, all .sbt files in a build can see/import the content of project/*.scala files. So you'll have to declare your val finagleVersion in a .scala file:

project/CustomKeys.scala:

import sbt._
import Keys._

object CustomKeys {
  val finagleVersion = settingKey[String]("Defines the Finagle version")
}

Now, in your .sbt files, just

import CustomKeys._

and you're good to go.