sbt/notes/0.13.8/sequential-task.markdown

Fixes with compatibility implications

Improvements

Bug fixes

Sequential tasks

sbt 0.13.8 adds a new Def.sequential function to run tasks under semi-sequential semantics. Here's an example usage:

lazy val root = project.
  settings(
    testFile := target.value / "test.txt",
    sideEffect0 := {
      val t = testFile.value
      IO.append(t, "0")
      t
    },
    sideEffect1 := {
      val t = testFile.value
      IO.append(t, "1")
      t
    },
    foo := Def.sequential(compile in Compile, sideEffect0, sideEffect1, test in Test).value
  )

Normally sbt's task engine will reorder tasks based on the dependencies among the tasks, and run as many tasks in parallel (See Custom settings and tasks for more details on this). Def.sequential instead tries to run the tasks in the specified order. However, the task engine will still deduplicate tasks. For instance, when foo is executed, it will only compile once, even though test in Test depends on compile. #1817/#1001 by @eed3si9n