sbt conditional if else style configuration - scala

Question

I want to be able to switch between parallel and serial execution of scala tests using command line.
Working example with "test.par" system property:
val parallelTestOpt = Option(System.getProperty("test.par"))
testOptions in IntegrationTest += Tests.Argument(
//Configure distributor's pool size
parallelTestOpt.map(count =>s"-P$count").getOrElse("-P1")
)
lazy val root = (project in file("."))
.configs(IntegrationTest)
.settings(Defaults.itSettings,
//If suites are executed in parallel
IntegrationTest / parallelExecution := parallelTestOpt.exists(_ != "1"),
IntegrationTest / testForkedParallel := parallelTestOpt.exists(_ != "1")
)
The "problematic" part is the parallelTestOpt.map(count =>s"-P$count").getOrElse("-P1"). I don't want to provide default value "-P1" when the "test.par" property was not specified. What is the best practice to achieve that ?
Maybe the whole concept is wrong and I should do it in a different way ?

Answer 1

As an alternative approach consider separating the parallelism concern into a single-argument custom command
commands += Command.single("testpar") { (state, numOfThreads) =>
s"""set IntegrationTest / testOptions += Tests.Argument("-P$numOfThreads")"""::
"set IntegrationTest / parallelExecution := true" ::
"set IntegrationTest / testForkedParallel := true" ::
"IntegrationTest / test" :: state
}
and execute with, say, testpar 6 to run with pool of 6 threads.
Addressing the comment, for compile-time safety try
commands += Command.single("testpar") { (state, numOfThreads) =>
val extracted = Project.extract(state)
val stateWithParallel= extracted.appendWithSession(
Seq(
IntegrationTest / testOptions += Tests.Argument(s"-P$numOfThreads"),
IntegrationTest / parallelExecution := true,
IntegrationTest / testForkedParallel := true,
),
state
)
extracted.runTask(IntegrationTest / test, stateWithParallel)
state
}