sbt using different Scala versions in a multi project build - scala

Question

I have a multi project which uses two versions of Scala. The Build.scala looks like this:
import sbt._
import Keys._
object Build extends sbt.Build {
lazy val root: Project = Project(
id = "scalacolliderugens",
base = file("."),
aggregate = Seq(gen, core)
)
lazy val gen = Project(...)
lazy val core = Project(...)
val ugenGenerator = TaskKey[Seq[File]]("ugen-generate", "Generate UGen class files")
def runUGenGenerator(source: File, cp: Seq[File]): Seq[File] = Nil // TODO
}
Where I have a pure source generating project using Scala 2.9.2:
lazy val gen = Project(
id = "scalacolliderugens-gen",
base = file("gen"),
settings = Project.defaultSettings ++ Seq(
scalaVersion := "2.9.2", // !!!
libraryDependencies ++= Seq(
"org.scala-refactoring" % "org.scala-refactoring_2.9.1" % "0.4.1"
),
libraryDependencies <+= scalaVersion { sv =>
"org.scala-lang" % "scala-compiler" % sv
}
)
)
And the actual project which incorporates the generated sources, compiling against Scala 2.10:
lazy val core = Project(
id = "scalacolliderugens-core",
base = file("core"),
settings = Project.defaultSettings ++ Seq(
scalaVersion := "2.10.0", // !!!
sourceGenerators in Compile <+= (ugenGenerator in Compile),
ugenGenerator in Compile <<=
(scalaSource in Compile, dependencyClasspath in Runtime in gen) map {
(src, cp) => runUGenGenerator(src, cp.files)
}
)
).dependsOn(gen)
When I compile this, I get sbt warnings:
[warn] Binary version (2.9.2) for dependency org.scala-lang#scala-library;2.9.2
[warn] in de.sciss#scalacolliderugens_2.10;1.0.1 differs from Scala binary \
version in project (2.10).
[info] Resolving org.scala-lang#scala-library;2.9.2 ...
[info] Done updating.
[warn] Binary version (2.9.2) for dependency org.scala-lang#scala-library;2.9.2
[warn] in de.sciss#scalacolliderugens-gen_2.10;1.0.1 differs from Scala binary \
version in project (2.10).
[warn] Binary version (2.9.2) for dependency org.scala-lang#scala-compiler;2.9.2
[warn] in de.sciss#scalacolliderugens-gen_2.10;1.0.1 differs from Scala binary \
version in project (2.10).
Should I be worried? Am I doing something bad here?

Answer 1

Add the following setting to the gen project.
scalaBinaryVersion:= CrossVersion.binaryScalaVersion("2.9.2")

Answer 2

OK...
I have multi-project SBT projects and they generally have SBT settings that are partly shared by all and partly specific to each sub-project. In my case the common settings include the Scala version to use.
You could structure you build specification the same way and put the Scala version setting in the sub-project-specific settings. If necessary, you could stratify the settings further if there are some settings common to all and some common to subsets of the sub-projects and other specific to each individual sub-project.

Answer 3

You cannot mix bytecode from different minor versions of Scala. I.e., code compiled by any 2.9.x series compiler is not compatible with that emitted by a 2.10.x (or 2.8.x) compiler.