a simple Scala CLI parsing library
Latest commit 55b81b8 Aug 4, 2016 @Rogach Rogach Released 2.0.1



A simple command-line arguments parsing library for Scala, written in spirit of Ruby's Trollop. Cross-built for Scala 2.10, 2.11 and latest 2.12 milestone.

Scallop supports:

  • flag, single-value and multiple value options
  • POSIX-style short option names (-a) with grouping (-abc)
  • GNU-style long option names (--opt, --opt=value)
  • unnamed integer options, like GNU tail (-42)
  • Property arguments (-Dkey=value, -D key1=value key2=value)
  • Non-string types of options and properties values (with extendable converters)
  • Powerful matching on trailing args
  • Subcommands

For more info and information on usage, you can look into the project wiki or consult the API docs.

Also, I wrote a blog post and another one about Scallop.


Add following to your build.sbt:

libraryDependencies += "org.rogach" %% "scallop" % "2.0.1"

Migration from 1.x versions: opt[File] does not check that file exists anymore. Instead, use validateFileExists helper inside your configuration.

Migration from 0.x versions: due to deprecation of DelayedInit trait, automatic verification of configuration objects no longer works - now you must call .verify() on your config explicitly before retrieving options. Also, if you use subcommands, you will have to add them to parent config explicitly by calling .addSubcommand() method.

Quick example

import org.rogach.scallop._;

object Conf extends ScallopConf(List("-c","3","-E","fruit=apple","7.2")) {
  // all options that are applicable to builder (like description, default, etc)
  // are applicable here as well
  val count:ScallopOption[Int] = opt[Int]("count", descr = "count the trees", required = true)
                .map(1+) // also here work all standard Option methods -
                         // evaluation is deferred to after option construction
  val properties = props[String]('E')
  // types (:ScallopOption[Double]) can be omitted, here just for clarity
  val size:ScallopOption[Double] = trailArg[Double](required = false)
// that's it. Completely type-safe and convenient.
assert(Conf.count() == 4)
assert(Conf.properties("fruit") == Some("apple"))
assert(Conf.size.toOption == Some(7.2))
// passing into other functions
def someInternalFunc(conf:Conf.type) {
  assert(conf.count() == 4)

For more examples, you can look at Scallop's test suite.

Fancy things

Scallop supports quite powerful matching on trailing arguments. For example:

object Conf extends ScallopConf(
       List("-Ekey1=value1", "key2=value2", "key3=value3",
            "first", "1","2","3","second","4","5","6")) {
  val props = props[String]('E')
  val firstListName = trailArg[String]()
  val firstList = trailArg[List[Int]]()
  val secondListName = trailArg[String]()
  val secondList = trailArg[List[Double]]()
Conf.props("key1") should equal (Some("value1"))
Conf.firstListName() should equal ("first")
Conf.secondListName() should equal ("second")
Conf.firstList() should equal (List(1,2,3))
Conf.secondList() should equal (List[Double](4,5,6))

In this case, Scallop's backtracking parser is clever enough to distinguish the boundaries of the arguments lists.

Also, Scallop supports parsing of subcommands. Not only subcommands, but nested subcommands!

object Conf extends ScallopConf(Seq("sub1", "sub2", "sub3", "sub4", "win!")) {
  val sub1 = new Subcommand("sub1") {
    val sub2 = new Subcommand("sub2") {
      val sub3 = new Subcommand("sub3") {
        val sub4 = new Subcommand("sub4") {
          val opts = trailArg[List[String]]()
Conf.subcommands should equal (List(Conf.sub1, Conf.sub1.sub2, Conf.sub1.sub2.sub3, Conf.sub1.sub2.sub3.sub4))
Conf.sub1.sub2.sub3.sub4.opts() should equal (List("win!"))


... and the whole Scala community for help and explanations.


Scallop is distributed under MIT license.