Using runTask(...mainClass...) inside inputTask with command line args, := vs <<= or? - sbt

Question

I want to define a task which runs MyMainClass defined in the same module where task is defined, passing task command line to it:
$ sbt
> project myModule
> myKey2 someArgument
...compiles MyMainClass
...runs MyMainClass.main("someArgument")
Without command line args, this works:
val myKey1 = taskKey[Unit]("myKey1")
lazy val myModule = project.settings(
myKey1 <<= runTask(Compile, "MyMainClass", "mode1"),
myKey1 <<= myKey1.dependsOn(compile in Compile)
)
But I could not make it with command line args. Trying to use Def.spaceDelimited().parsed with taskKey gives me compilation error explicitly saying that I must use inputKey instead; trying to use <<= with inputKey does not compile either; this compiles but does not work:
val myKey2 = inputKey[Unit]("myKey2")
lazy val myModule = project.settings(
...
myKey2 := runTask(
Compile, "MyMainClass", "mode2",
{
val args = Def.spaceDelimited().parsed.head)
// This line is executed, but MyMainClass.main() is not:
System.err.println("***** args=" + args)
args.head
}
),
myKey2 <<= myKey2.dependsOn(compile in Compile)
)
Tried SBT 0.13.7 and 0.13.9. Please help. Thanks. :)
UPD. Or maybe I'm doing this completely wrong (deprecated) way? I could not find SBT 0.13 docs mention <<= at all.

Answer 1

Rewritten in new style (:= instead of <<=).
This worked:
myKey1 := {
// Works without this line, but kept it for clarity and just in case:
val _ = (compile in Compile).value
runTask(Compile, "MyMainClass1", "mode1").value
},
myKey2 := {
val _ = (compile in Compile).value
runInputTask(Compile, "MyMainClass", "mode2").evaluated
}
BTW directly accessing .value in procedural style feels much conceptually simpler than old ways I used to use (I guess that was before SBT has been rewritten using macros).