Difference between scalaSource and sourceDirectories in sbt - scala

Question

What is the difference between scalaSource and sourceDirectories? I have a non-standard directory structure where source code lives in src/,
This line works in build.sbt:
scalaSource in Compile := file("src/")
but not
sourceDirectories in Compile := Seq(file("src/"))

Answer 1

Sources come in many kinds: Scala and Java, managed and unmanaged. sourceDirectories in Compile combines all of them, but you wouldn't normally want to set it directly; normally you'd set the more specific setting that applies to the particular kind of source you're trying to tell sbt the location of.
Note that baseDirectory.value / "src" would be more correct than file("src") (it works in more scenarios: subprojects, external projects, etc).

Answer 2

I can't tell you the whole picture, someone who worked on sbt may see this post later.
Till then, here is how I reason about this things: inspect
> inspect actual compile:scalaSource
[info] Description:
[info] Default Scala source directory.
[info] Dependencies:
[info] compile:sourceDirectory
[info] Reverse dependencies:
[info] compile:unmanagedSourceDirectories
[info] Delegates:
[info] compile:scalaSource
> inspect actual compile:sourceDirectories
[info] Description:
[info] List of all source directories, both managed and unmanaged.
[info] Dependencies:
[info] compile:unmanagedSourceDirectories
[info] compile:managedSourceDirectories
[info] Delegates:
[info] compile:sourceDirectories
[info] *:sourceDirectories
[info] {.}/compile:sourceDirectories
Now this is how I interpret this:
sourceDirectories are ... well ... completely informal ...
Lets see how this is relevant to something like compile:
> inspect compile
[info] Dependencies:
[info] compile:compile::compileInputs <----
[info] compile:compile::streams
> inspect compile:compile::compileInputs
[info] Dependencies:
...
[info] compile:compile::sources <----
...
> inspect compile:compile::sources
[info] Provided by:
[info] {file:<build-uri>}<project-id>/compile:sources
[info] Delegates:
[info] compile:compile::sources
[info] compile:sources <----
This task is delegated, we can see where we came from in Reverse dependencies with inspect actual, regular inspect woudn't show them.
> inspect actual compile:sources
[info] Description:
[info] All sources, both managed and unmanaged.
[info] Reverse dependencies:
[info] compile:doc
[info] compile:compile::compileInputs
[info] Dependencies:
[info] compile:unmanagedSources <----
[info] compile:managedSources
> inspect compile:unmanagedSources
[info] Dependencies:
[info] compile:unmanagedSourceDirectories
...
> inspect compile:unmanagedSourceDirectories
[info] Dependencies:
[info] compile:javaSource
[info] compile:scalaSource <----
[info] Reverse dependencies:
[info] compile:sourceDirectories
[info] compile:unmanagedSources
Hope this helps.