Command returning withought result or with exception

[Sailsman63]

Partly inspired by mill's own build script, I'm attempting to move my basic "Install Local" and "Launch" commands available on the top level, not encased in any module.

The following compiles just fine (ie, ammonite and the mill runtime accept it as a valid script - no syntax errors or similar) but it does not work the way I anticipated

def installLocal() = T.command {
  os.makeDir(T.workspace / "LocalInstall")
  os.copy.into(Suite(localPlatform()).stage().path, T.workspace / "LocalInstall", replaceExisting = true, createFolders=true, mergeFolders = true)
}

def localPlatform() = T.command {
  osName match {
    case x if osName.toLowerCase().contains("linux") => "linux"
    case x if osName.toLowerCase().contains("mac") | osName.contains("osx") => "mac"
    case x if osName.toLowerCase().contains("windows") => "win"
  }
}

• If there are no existing mill runtimes floating about, the script returns with exit_code 0, but does not trigger any of the other tasks.
• If there is an existing mill runtime still active, I instead get some variation on a runtime exception, attached below. The stacktrace varies from run to run, but always has the ThreadDeath line.
• Note that localPlatform() is returning sane, sensible results when called separately
• The cross-module Suite is building as expected if called directly as mill Suite(linux).stage or similar

runtime exception folded for brevity

Exception in thread "MillServerActionRunner" java.lang.reflect.InvocationTargetException
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at mill.define.Module$Internal.$anonfun$reflect$4(Module.scala:70)
	at scala.collection.ArrayOps$WithFilter.map(ArrayOps.scala:90)
	at mill.define.Module$Internal.reflect(Module.scala:62)
	at mill.define.Module$Internal.reflectSingle(Module.scala:75)
	at mill.main.ResolveTasks$.endResolveLabel(Resolve.scala:203)
	at mill.main.Resolve.resolve(Resolve.scala:417)
	at mill.main.RunScript$.$anonfun$resolveTasks$7(RunScript.scala:249)
	at scala.util.Either.map(Either.scala:382)
	at mill.main.RunScript$.$anonfun$resolveTasks$6(RunScript.scala:233)
	at scala.collection.immutable.List.map(List.scala:246)
	at scala.collection.immutable.List.map(List.scala:79)
	at mill.main.RunScript$.$anonfun$resolveTasks$5(RunScript.scala:232)
	at scala.util.Either.flatMap(Either.scala:352)
	at mill.main.RunScript$.resolveTasks(RunScript.scala:229)
	at mill.main.RunScript$.$anonfun$resolveTasks$2(RunScript.scala:202)
	at scala.collection.immutable.List.map(List.scala:246)
	at scala.collection.immutable.List.map(List.scala:79)
	at mill.main.RunScript$.$anonfun$resolveTasks$1(RunScript.scala:201)
	at scala.util.Either.flatMap(Either.scala:352)
	at mill.main.RunScript$.resolveTasks(RunScript.scala:200)
	at mill.main.RunScript$.evaluateTasks(RunScript.scala:312)
	at mill.main.RunScript$.$anonfun$runScript$8(RunScript.scala:105)
Exception: java.lang.ThreadDeath thrown from the UncaughtExceptionHandler in thread "MillServerActionRunner"
	at mill.main.RunScript$.$anonfun$runScript$8(RunScript.scala:105)

.

What have I broken, or how do I troubleshoot this further?

[Sailsman63]

Further noodling about has shown that whatever is going on is in is in the call to Suite(localPlatform()). When I pull that out onto its own line, I get this:

1 targets failed
installLocal java.util.NoSuchElementException: key not found: List(localPlatform)
    scala.collection.immutable.Map$Map4.apply(Map.scala:517)
    mill.define.Cross.apply(Cross.scala:86)
    ammonite.$file.build.$anonfun$installLocal$2(build.sc:208)
    scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)

EDIT: and it turns out that localPlatform() is of type Command[String] instead of String. See #1239 for other variants. In my case, I can drop the T.command part of the definition, as it's not really needed except for troubleshooting.

[lefou]

In your OP you're calling localPlatform(), which results in Result[String] whearas you're interested in it's outcome, so you need to call localPlatform()(). Then, you will get a String and Mill will known, that localPlatform command is a dependency and will execute it before.

[lefou]

There rule of thumb is: whenever you need the result of a Task/Target/Command, you add a (). So it's always one parenthesis pair more then on the definition site.

[Sailsman63]

This isn't working with current main (61698d6).

I still get

Target#apply() can only be used with a T{...} block
  val srcpath = Suite(localPlatform()()).stage().path
                                     ^

[lefou]

Oh, Suite is a cross module? That wasn't clear. Yeah, then the error makes sense and it won't work. That's because we need to know the involved modules and tasks before we start evaluation, but you try to get the platform only after the evaluation started.

[Sailsman63]

Okay. Would you approach this in a different way, or am I just better off doing the platform detection as a simple def with none of the target stuff?