simple scala command line options parsing
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scarg is a little command line argument parsing library.


Create a configuration map and an ArgumentParser and customise it with the options you need. To simplify life, use the ConfigMap for your configuration map to receive some nice power-ups like value type conversion. (but you can also use a simple Map)

// we want to store three values, a boolean and two strings
class Configuration(m: ValueMap) extends ConfigMap(m) {
  val verbose = ("verbose", false).as[Boolean]
  val outfile = ("outfile", "-").as[String]
  val infile = ("infile", "").as[String]

// our argument parser which uses a factory to create our Configuration
case class SimpleParser() extends ArgumentParser(new Configuration(_))
                             with DefaultHelpViewer {
  override val programName = Some("SimpleExample") // set the program name for the help text

  // define our expected arguments
  ! "-v" | "--verbose"   |% "active verbose output"            |> "verbose"
  ! "-o" |^ "OUT" |* "-" |% "output filename, default: stdout" |> 'outfile
  ("-" >>> 50)
  + "infile"             |% "input filename"                   |> 'infile

SimpleParser().parse(args) match {
  case Right(c) =>
    println("verbose: " + c.verbose)
    println("outfile: " + c.outfile)
    println(" infile: " + c.infile)
  case Left(xs) =>
    // arguments are bad, usage message will have been displayed automagically

The above generates the following usage text:

usage: SimpleExample [options] infile

  -v, --verbose   active verbose output
  -o OUT          output filename, default: stdout
  infile          input filename


Use sbt to build scarg.

$ sbt
$ > update
$ > test

To run some of the examples, switch to the scarg-examples subproject:

$ > project scarg-examples
$ > run
$ > run -v -o outfile.txt infile.txt

Using scarg with SBT

add the repository and and the dependency (the two val's in the example) to your project configuration.

import sbt._
class MyProject(info: ProjectInfo) extends DefaultProject(info) {
  val scargRepo = "scarg-repo" at ""
  val scarg = "de.downgra" % "scarg_2.8.1" % "1.0.0-SNAPSHOT"

if you don't need the examples, only use the scarg-core dependency.

val scarg = "de.downgra" % "scarg-core_2.8.1" % "1.0.0-SNAPSHOT"

Using scarg with Maven

add the following dependencies and repositories to your pom.xml.


    <name>scarg repo</name>

if you don't need the examples, only use the scarg-core dependency.



Simply extend the class ArgumentParser and provide a factory which will transform a ValueMap into your own configuration mapping. The ConfigMap can be used for this task.

The ValueMap has the type Map[String, List[String]], ergo maps a key (which is a string) to a list of string values (if parameters are given multiple times).


The ConfigMap provide some data converters which can be used to quickly create a container with your arguments with the correct type.

class MyConfig(m: ValueMap) extends ConfigMap(m) {
  val verbose = ("verbose", false).as[Boolean]
  val outfile = ("outfile", "-").as[String]
  val infile = ("infile", "").as[String]

If you define val's, type conversion errors will be generated within the parsing process. Lazy val's or def's will throw exceptions on the first access.

Argument Parser

The ArgumentParser provides a nice dsl to create the argument mappings. Two types of arguments can be specified. Options (like -f, --bar) or positionals (like the input filename on the last position).

You can also specify separators to separates the usage text.


+ "required"   |% "some description"    |> "key"
+ "required"   |% "some description"    |*> "key"
~ "optional"   |% "blah blah"           |> 'key
~ "optional"   |% "blah blah"           |*> 'key

The + define a required positional argument, which can not be omited.

A ~ defines a optional positional argument, which can be omited.

There can never be a required argument after an optional one. This will produce an BadArgumentOrderException Exception.

Descriptions denoted with a |% are optional and can be omited.

The key denoted with a |> is required and set the name under which the parsed value is inserted into the map of parsed values.

A key denoted with a |*> marks repeated positionals (e.g. unlimited number of input files).

A key can be a String or a Symbol.

Alternate syntax:
                       key("key", repeated = false)

                       key('key, true)


! "-f" | "--foo" |^ "valueName" |* "defaultValue" |% "description" |> 'key

Options are defined by using a starting !. After that, there can be any number of additional names using a |. At least there is one, sometimes more names for that option.

An option can be a flag or it can have a value. Flags are things like -f, --verbose and so on. To define such flags, omit the value name |^.

If you define a value name myValue, the following variant are allowed: -f myValue, -f=myValue and -f:myValue. The delimiter (: and =) can be changed by overriding the member optionDelimiters.

If you don't define a default value with |*, the option must be specified. The default value can be any type which is convertable to a string with the toString method.

Descriptions denoted with a |% are optional and can be omited.

The key denoted with a |> is required and can be a String or a Symbol. It's used as key for the map of parsed values.

Alternate syntax:


("---------------------" >>>)
("-" >>> 60)

the first form will produce exactly the given string, while in the second form the given string will be multiplicated n times. (like "/" * 60)

("=====================" >>>>)
("=" >>>> 60)

these behave like the operators above, but will add a newline at the start and the end.

beware, those annoying parentheses are needed.

Alternate syntax:
separator("-", 60)

separator("====================", multiLine = true)
separator("=", 60, true)

Help Viewer

The generation of the help and usage text can be specified by providing an implementation of the HelpViewer trait. A default implementation is provided with the DefaultHelpViewer trait, which can be mixed into the ArgumentParser class. If you don't want output, you can use the SilentHelpViewer or create an own by extending the HelpViewer trait.

The DefaultHelpViewer can be customized by overriding it's val's like INDENT, USAGE_HEADER, UNKNOWN_ARGUMENT, ... in your parser implementation. Also the default output to stderr can be changed by overwriting the output(s: String) method.

Extending ConfigMap

Imagine you want to get key/value pairs in your ConfigMap via -v key=value arguments.

The only thing you must do is to provide a new type class instance of the trait Reader[T]. In this example it's the implicit object KeyValueReader.

case class KeyValuePair(key: String, value: String)

class KeyValueConfig(m: ValueMap) extends ConfigMap(m) {
  // type class to read an KeyValuePair
  implicit object KeyValueReader extends Reader[KeyValuePair] {
    def read(value: String): KeyValuePair = value.indexOf('=') match {
        case n: Int if n >= 0 =>        
          val p = value.splitAt(n)        
          KeyValuePair(p._1, p._2)        
        case _ => throw new IllegalArgumentException("Expected a key=value pair")

  val pair = ("pair", KeyValuePair("", "")).as[KeyValuePair]

case class KeyValueParser() extends ArgumentParser(new KeyValueConfig(_)) with DefaultHelpViewer {
  override val programName = Some("KeyValueExample")
  ! "-k" |^ "Key=Value" |% "a key=value pair" |> config.set("pair")

I think you got the idea.

Copyright and license

scarg is copyright © 2010 Rico Schiekel and released under the WTFPL. See the accompanying license file for details.


  • nothing ;)