How to define a task which needs to run other tasks sequentially

[domagojlatecki]

Mill version: 0.12.5

Since 0.12.0 update, Mill has switched to parallel task execution by default. This has broken my build, so I intitally switched to --jobs=1.

Now I am trying to re-write my task definitions so that they work sequentially where that is required. For example, I have a custom compile tasks which is supposed to first run flywayMigrate, then generate code, and finally compile the module. Until now, this is how I defined that compile task:

override def compile = Task(persistent = true) {
  flywayMigrate()()
  generateJooq()()
  super.compile()
}

This is now arbitrartily failing most of the time, because the code is not generated before super.compile() is run.

I have tried rewriting this using Task.sequence:

override def compile = Task(persistent = true) {
  Task.sequence(
    Seq(
      flywayMigrate().map(_ => None),
      generateJooq().map(_ => None),
      super.compile.map(Some(_))
    )
  ).apply().last.get
}

But the result is the same.

Is there any way to specify in the build logic that some tasks need to preceed other ones like in the case above?

[lihaoyi]

The basic approach is that dependent tasks need to call each other.

So def generateJooq should call flywayMigrate(), and def compile should call generateJooq() (perhaps indirectly through def generatedSources)

See the docs for examples of how generating sources is meant to work. From your description you aren't doing it the normal way, but it should be easy to follow the normal pattern

This will ensure the tasks get run in the right order despite parallelism/caching

[domagojlatecki]

Thank you for the answer, I will try that.

However, I have one more situation where ordering is important to me:

// inside build.mill
def cc(ev: mill.eval.Evaluator) = Task.Command(exclusive = true) {
  Task.sequence(Seq(clean(ev).map(_ => ()), compile().map(_ => ())))()
}

This is a top-level command I defined as an alias to "clean + compile all modules", so that I can run mill cc. Here clean should always run first. What approach should I use for this?

[lihaoyi]

You can do something like

def cc(ev: mill.eval.Evaluator) = Task.Command(exclusive = true) {
  mill.main.RunScript.evaluateNamed(ev, Agg(clean(ev)))
  mill.main.RunScript.evaluateNamed(ev, Agg(compile))
}

[domagojlatecki]

That seems to work, thanks again!