How to declare task dependency on tasks in 0.13?

Question

In sbt 0.12, you could specify that one task depended upon another without actually using the output from the input tasks. So you were specifying purely an ordering on the tasks:

unitTask <<= Seq(stringTask, sampleTask).dependOn

There is no such example in the Tasks documentation for sbt 0.13. What's the new syntax for specifying said dependency?

Answer 1

Use the standard syntax, but ignore the results of the tasks used:

unitTask := {
  val x = stringTask.value
  val y = sampleTask.value
  ()
}

Because of a bug in scalac, you have to use dummy names, otherwise you could just use val _ = ....

Also, I prefer the more explicit way above, but it is equivalent to this shorter version because the results aren't used:

unitTask := {
  stringTask.value
  sampleTask.value
}

Answer 2

The official migration guide recommends that instead of:

a <<= a dependsOn b

define it as:

a := (a dependsOn b).value