rchillyard / args   1.0.3

Apache License 2.0 GitHub

This library is to make it easy to use and verify command line arguments in Scala programs

Scala versions: 2.13 2.12 2.11 2.10


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This library is to make it easy to use and verify command line arguments in Scala programs


Args provides a mechanism for parsing, validating, and processing a set of command line arguments. Option processing is typically achieved via a map of options to functions. The format for the arguments follows POSIX standards, which allow for a set of options followed by a set of operands (positional arguments). Either (or both) the sets of options/arguments may be empty. By convention, options come first, and have single-character names, while operands are positional and follow the last option, if any. An option may be required or optional; An option may be required to have a value or not. Options may be grouped together in one argument, with only one "-" preceding them. Options that take optional values may be part of a single String (without intervening space); required arguments must be in separate arguments. More information about the syntax of command line arguments is to be found here: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html

Args will always parse and process a set of command line options. Validation is performed if the application programmer provides a syntax template (known as the synopsis).

There is additional support for non-POSIX styles of command line arguments, but in this case, there is no means of validating the command line.

Whether your application extends the App interface, or creates its own main program, the command-line arguments will be available as Strings in an Array[String] usually called args.


The main class used is a case class: Args[X] which is defined thus:

case class Args[X](xas: Seq[Arg[X]]) extends Iterable[Arg[X]\] {
    // ...

Args defines a sequence of Arg[X] elements. The order follows the order as parsed, but the order of the options (but not the operands) is immaterial.

The class Arg[X] is defined thus:

case class Arg[X](name: Option[String], value: Option[X]) extends Ordered[Arg[X]] {
    // ...

An Arg is a single command line argument and can have a name, a value, or both. The operands (positional arguments) which come at the end of the command line have values but no name. Optional arguments ("options") which come first have a name but may or may not have a value.

The underlying type X is the type of the (optional) value. When an Arg results directly from parsing the command line, then X is always String. But a map method is defined which allows an Arg[X] to be transformed into an Arg[Y].

Parsing and Processing

An example of parsing with validation is the following:

val args = Array("-f", "argFilename", "operand")
val say: Try[Args[String]] = Args.parse(args, Some("-f filename"))

This will create an Args[String] with two Arg elements: one corresponding to f:filename and one corresponding to operand. In this case, filename is a required argument to the f option, and the f option is itself required. Note that, currently at least, there is no way to validate that the required number of operands is present.

There is another form of parse which takes only the args parameter. In this case, there will be no validation.

val args = Array("-f", "argFilename", "operand")
val say: Try[Args[String]] = Args.parse(args)

There is another method for parsing which doesn't require POSIX-style and so cannot be validated:

val args = Array("-f", "argFilename", "operand")
val say: Try[Args[String]] = Args.parseSimple(args)

Once parsed and validated, an Args object can be processed by invoking the process method with a map of "name"->function pairs. The signature of the process method is:

def process(fm: Map[String, Option[X] => Unit]): Try[Seq[X]]

The args are processed as side-effects (!!) but any operands are returned in the result. If any exceptions are thrown by the functions, the result will be a Failure and not a Success.

An alternative to invoking process is to split the Args up into options and operands using the options and operands methods:

def options: Map[String, Option[X]]
def operands(s: Synopsis): Map[String, X]

The result of invoking the options method is a map of String->Option[X] pairs. Each String is the name of an option (according to the synopsis) and the Option[X] value is its actual value read from the command line (if there is an argument to the option).

The result of invoking the operands method is a map of String->X pairs. Each String is the name of an operand (according to the synopsis) and the X value is its actual value read from the command line.

For non-posix-style parsing, there is an additional signature of operands:

def operands: Seq[X]

This result is simply the list of the operands as given on the command line.

Please see the ArgsSpec class for more examples of invoking the various methods available.

More on (posix) validation

The following strings are valid synopsis/command line pairs/tuples (separated by "<-->"):

-f filename <--> -f README.md
-f[ filename]  <--> -f README.md <--> -f
-[f filename] <--> -f README.md <--> 
-[f[ filename]] <--> -f README.md <--> -fREADME.md <--> -f <--> 
-xf filename <--> -xf README.md <--> -x -f README.md
-x operand1 [operand2] <--> -x 1 <--> <--> -x 1 2
operand1 [operand2] <--> 1 <--> <--> 1 2

Square brackets make the option or its parameter optional. Options can be combined in the synopsis (and in the command line) but, in the command line, an option which is optional must be the last of any group.

Class Arg: method signatures

def isOption: Boolean
def hasValue: Boolean
def isOptional(s: Synopsis): Maybe
def byName(w: String): Option[Arg[X]]
def map[Y](f: X => Y): Arg[Y]
def flatMap[Y](f: X => Arg[Y]): Arg[Y]
def mapMap[Y](f: X => Option[Y]): Arg[Y]
def as[Y: Derivable]: Arg[Y]
def toY[Y: Derivable]: Try[Y]
def process(fm: Map[String, Option[X] => Unit]): Try[Option[X]]
def compare(that: Arg[X]): Int
lazy val asOption: Option[(String, Option[X])]
lazy val operand: Option[X]

Object Arg: method signatures

def apply(w: String): Arg[String]
def apply(w: String, v: String): Arg[String]

Class Args: method signatures

def :+(xa: Arg[X]): Args[X]
def +:(xa: Arg[X]): Args[X]
def ++(xq: Args[X]): Args[X]
def validate(w: String): Try[Args[X]]
def validate(sy: Try[Synopsis]): Args[X]
def validate(s: Synopsis): Boolean
def mapMap[Y](f: X => Y): Args[Y]
def map[Y](f: Arg[X] => Arg[Y]): Args[Y]
def flatMap[Y](f: Arg[X] => Args[Y]): Args[Y]
def as[Y: Derivable]: Args[Y]
def operands(s: Synopsis): Map[String, X]
def getArg(w: String): Option[Arg[X]]
def getArgValueAs[Y: Derivable](w: String): Option[Y]
def getArgValue(w: String): Option[X]
def isDefined(w: String): Boolean
def process(fm: Map[String, Option[X] => Unit]): Try[Seq[X]]
def matchAndShift(f: PartialFunction[Arg[X], Unit]): Args[X]
def matchAndShiftOrElse(f: PartialFunction[Arg[X], Unit])(default: => Args[X]): Args[X]
lazy val options: Map[String, Option[X]]
lazy val operands: Seq[X]

Additionally, many methods of Iterable[Arg[X]] are also included where it makes sense, such as:

def iterator: Iterator[Arg[X]]

Object Args:

def parseSimple(args: Array[String]): Try[Args[String]]
def parse(args: Array[String], synopsis: Option[String] = None): Try[Args[String]]
def create(args: Arg[String]*): Args[String]
def make(args: Seq[String]): Args[String]

Trait Derivable:

Derivable[T] is a type class which defines the following methods:

def deriveFrom[X](x: X): T
def deriveFromOpt[X](x: X): Option[T]

There are implicitly instance of Derivable of type Derivable[T] with methods defined for String parameters. The currently supported [T] types are Boolean, Int, Double, File, URL.


The current version is 1.0.3 Differences from V1.0.2:

  • added deriveFromOpt method to Derivable; added more Derivable objects;
  • added concatenation methods for Args;
  • added mapMap methods to Arg and Args;
  • a few other minor changes to name/signature;

Differences from V1.0.1: more usage of Try (mostly internal).