A monadic commandline options parser.

import scopt.OParser
val builder = OParser.builder[Config]
val parser1 = {
  import builder._
  OParser.sequence(
    programName("scopt"),
    head("scopt", "4.x"),
    opt[Int]('f', "foo")
      .action((x, c) => c.copy(foo = x))
      .text("foo is an integer property"),
    opt[File]('o', "out")
      .required()
      .valueName("<file>")
      .action((x, c) => c.copy(out = x))
      .text("out is a required file property"),
    opt[(String, Int)]("max")
      .action({ case ((k, v), c) => c.copy(libName = k, maxCount = v) })
      .validate(x =>
        if (x._2 > 0) success
        else failure("Value <max> must be >0"))
      .keyValueName("<libname>", "<max>")
      .text("maximum count for <libname>"),
    opt[Seq[File]]('j', "jars")
      .valueName("<jar1>,<jar2>...")
      .action((x, c) => c.copy(jars = x))
      .text("jars to include"),
    opt[Map[String, String]]("kwargs")
      .valueName("k1=v1,k2=v2...")
      .action((x, c) => c.copy(kwargs = x))
      .text("other arguments"),
    opt[Unit]("verbose")
      .action((_, c) => c.copy(verbose = true))
      .text("verbose is a flag"),
    opt[Unit]("debug")
      .hidden()
      .action((_, c) => c.copy(debug = true))
      .text("this option is hidden in the usage text"),
    help("help").text("prints this usage text"),
    arg[File]("<file>...")
      .unbounded()
      .optional()
      .action((x, c) => c.copy(files = c.files :+ x))
      .text("optional unbounded args"),
    note("some notes." + sys.props("line.separator")),
    cmd("update")
      .action((_, c) => c.copy(mode = "update"))
      .text("update is a command.")
      .children(
        opt[Unit]("not-keepalive")
          .abbr("nk")
          .action((_, c) => c.copy(keepalive = false))
          .text("disable keepalive"),
        opt[Boolean]("xyz")
          .action((x, c) => c.copy(xyz = x))
          .text("xyz is a boolean property"),
        opt[Unit]("debug-update")
          .hidden()
          .action((_, c) => c.copy(debug = true))
          .text("this option is hidden in the usage text"),
        checkConfig(
          c =>
            if (c.keepalive && c.xyz) failure("xyz cannot keep alive")
            else success)
      )
  )
}

// OParser.parse returns Option[Config]
OParser.parse(parser1, args, Config()) match {
  case Some(config) =>
    // do something
  case _ =>
    // arguments are bad, error message will have been displayed
}

// alternatively, use OParser.runParser returns (Option[Config], List[OEffect])
OParser.runParser(parser1, args, Config()) match {
  case (result, effects) =>
    OParser.runEffects(effects, new DefaultOEffectSetup {
      // override def displayToOut(msg: String): Unit = Console.out.println(msg)
      // override def displayToErr(msg: String): Unit = Console.err.println(msg)
      // override def reportError(msg: String): Unit = displayToErr("Error: " + msg)
      // override def reportWarning(msg: String): Unit = displayToErr("Warning: " + msg)

      // ignore terminate
      override def terminate(exitState: Either[String, Unit]): Unit = ()
    })

    result match {
      Some(config) =>
        // do something
      case _ =>
        // arguments are bad, error message will have been displayed
    }
}