mainargs 0.2.3

MainArgs is a small, dependency-free library for command line argument parsing in Scala.

MainArgs is used for command-line parsing of the Ammonite Scala REPL and for user-defined @main methods in its scripts, as well as for command-line parsing for the Mill Build Tool and for user-defined T.commands.

• Usage
  • Parsing Main Method Parameters
    • runOrExit
    • runOrThrow
    • runEither
    • runRaw
  • Multiple Main Methods
  • Parsing Case Class Parameters
  • Re-using Argument Sets
  • Option or Sequence parameters
  • Annotations
    • @main
    • @arg
  • Customization
  • Custom Argument Parsers
  • Handlings Leftover Arguments
    • Varargs Parameters
• Prior Art
  • Ammonite & Mill
  • Case App
  • Scopt
• Changelog
  • 0.2.3
  • 0.2.2
  • 0.2.1
  • 0.1.7
  • 0.1.4

Usage

ivy"com.lihaoyi::mainargs:0.2.3"

Parsing Main Method Parameters

You can parse command line arguments and use them to call a main method via ParserForMethods(...):

package testhello
import mainargs.{main, arg, ParserForMethods, Flag}

object Main{
  @main
  def run(@arg(short = 'f', doc = "String to print repeatedly")
          foo: String,
          @arg(name = "my-num", doc = "How many times to print string")
          myNum: Int = 2,
          @arg(doc = "Example flag, can be passed without any value to become true")
          bool: Flag) = {
    println(foo * myNum + " " + bool.value)
  }
  def main(args: Array[String]): Unit = ParserForMethods(this).runOrExit(args)
}

$ ./mill example.hello -f hello
hellohello false

$ ./mill example.hello -f hello --my-num 3
hellohellohello false

$ ./mill example.hello -f hello --my-num 3 --bool
hellohellohello true

$ ./mill example.hello --wrong-flag
Missing argument: --foo <str>
Unknown argument: "--wrong-flag"
Expected Signature: run
  -f --foo <str>  String to print repeatedly
  --my-num <int>  How many times to print string
  --bool          Example flag

Setting default values for the method arguments makes them optional, with the default value being used if an explicit value was not passed in from the command-line arguments list.

After calling ParserForMethods(...) on the object containing your @main methods, you can call the following methods to perform the argument parsing and dispatch:

runOrExit

Runs the given main method if argument parsing succeeds, otherwise prints out the help text to standard error and calls System.exit(1) to exit the proess

runOrThrow

Runs the given main method if argument parsing succeeds, otherwise throws an exception with the help text

runEither

Runs the given main method if argument parsing succeeds, returning Right(v: Any) containing the return value of the main method if it succeeds, or Left(s: String) containing the error message if it fails.

runRaw

Runs the given main method if argument parsing succeeds, returning mainargs.Result.Success(v: Any) containing the return value of the main method if it succeeds, or mainargs.Result.Error if it fails. This gives you the greatest flexibility to handle the error cases with custom logic, e.g. if you do not like the default CLI error reporting and would like to write your own.

Multiple Main Methods

Programs with multiple entrypoints are supported by annotating multiple defs with @main. Each entrypoint can have their own set of arguments:

package testhello2
import mainargs.{main, arg, ParserForMethods, Flag}

object Main{
  @main
  def foo(@arg(short = 'f', doc = "String to print repeatedly")
          foo: String,
          @arg(name = "my-num", doc = "How many times to print string")
          myNum: Int = 2,
          @arg(doc = "Example flag")
          bool: Flag) = {
    println(foo * myNum + " " + bool.value)
  }
  @main
  def bar(i: Int,
          @arg(doc = "Pass in a custom `s` to override it")
          s: String  = "lols") = {
    println(s * i)
  }
  def main(args: Array[String]): Unit = ParserForMethods(this).runOrExit(args)
}

$ ./mill example.hello2
Need to specify a sub command: foo, bar

$ ./mill example.hello2 foo -f hello
hellohello false

$ ./mill example.hello2 bar -i 10
lolslolslolslolslolslolslolslolslolslols

Parsing Case Class Parameters

If you want to construct a configuration object instead of directly calling a method, you can do so via ParserForClass[T] and `constructOrExit:

package testclass
import mainargs.{main, arg, ParserForClass, Flag}

object Main{
  @main
  case class Config(@arg(short = 'f', doc = "String to print repeatedly")
                    foo: String,
                    @arg(name = "my-num", doc = "How many times to print string")
                    myNum: Int = 2,
                    @arg(doc = "Example flag")
                    bool: Flag)
  def main(args: Array[String]): Unit = {
    val config = ParserForClass[Config].constructOrExit(args)
    println(config)
  }
}

$ ./mill example.caseclass --foo "hello"
Config(hello,2,Flag(false))

$ ./mill example.caseclass
Missing argument: --foo <str>
Expected Signature: apply
  -f --foo <str>  String to print repeatedly
  --my-num <int>  How many times to print string
  --bool          Example flag

ParserForClass[T] also provides corresponding constructOrThrow, constructEither, or constructRaw methods for you to handle the error cases in whichever style you prefer.

Re-using Argument Sets

You can share arguments between different @main methods by defining them in a @main case class configuration object with an implicit ParserForClass[T] defined:

package testclassarg
import mainargs.{main, arg, ParserForMethods, ParserForClass, Flag}

object Main{
  @main
  case class Config(@arg(short = 'f', doc = "String to print repeatedly")
                    foo: String,
                    @arg(name = "my-num", doc = "How many times to print string")
                    myNum: Int = 2,
                    @arg(doc = "Example flag")
                    bool: Flag)
  implicit def configParser = ParserForClass[Config]

  @main
  def bar(config: Config,
          @arg(name = "extra-message")
          extraMessage: String) = {
    println(config.foo * config.myNum + " " + config.bool.value + " " + extraMessage)
  }
  @main
  def qux(config: Config,
          n: Int) = {
    println((config.foo * config.myNum + " " + config.bool.value + "\n") * n)
  }

  def main(args: Array[String]): Unit = ParserForMethods(this).runOrExit(args)
}

$ ./mill example.classarg bar --foo cow --extra-message "hello world"
cowcow false hello world

$ ./mill example.classarg qux --foo cow --n 5
cowcow false
cowcow false
cowcow false
cowcow false
cowcow false

This allows you to re-use common command-line parsing configuration without needing to duplicate it in every @main method in which it is needed. A @main def can make use of multiple @main case classes, and @main case classes can be nested arbitrarily deeply.

Option or Sequence parameters

@main method parameters can be Option[T] or Seq[T] types, representing optional parameters without defaults or repeatable parameters

package testoptseq
import mainargs.{main, arg, ParserForMethods}

object Main{
  @main
  def runOpt(opt: Option[Int]) = println(opt)

  @main
  def runSeq(seq: Seq[Int]) = println(seq)

  @main
  def runVec(seq: Vector[Int]) = println(seq)

  def main(args: Array[String]): Unit = ParserForMethods(this).runOrExit(args)
}

$ ./mill example.optseq runOpt
None

$ ./mill example.optseq runOpt --opt 123
Some(123)

$ ./mill example.optseq runSeq --seq 123 --seq 456 --seq 789
List(123, 456, 789)

Annotations

The library's annotations and methods support the following parameters to customize your usage:

@main

• name: String: lets you specify the top-level name of @main method you are defining. If multiple @main methods are provided, this name controls the sub-command name in the CLI

• doc: String: a documentation string used to provide additional information about the command. Normally printed below the command name in the help message

@arg

• name: String: lets you specify the long name of a CLI parameter, e.g. --foo. Defaults to the name of the function parameter if not given

• short: Char: lets you specify the short name of a CLI parameter, e.g. -f. If not given, theargument can only be provided via its long name

• doc: String: a documentation string used to provide additional information about the command

Customization

Apart from taking the name of the main object or config case class, ParserForMethods and ParserForClass both have methods that support a number of useful configuration values:

• allowPositional: Boolean: allows you to pass CLI arguments "positionally" without the --name of the parameter being provided, e.g. ./mill example.hello -f hello --my-num 3 --bool could be called via ./mill example.hello hello 3 --bool. Defaults to false

• allowRepeats: Boolean: allows you to pass in a flag multiple times, and using the last provided value rather than raising an error. Defaults to false

• totalWidth: Int: how wide to re-format the doc strings to when printing the help text. Defaults to 100

• printHelpOnExit: Boolean: whether or not to print the full help text when argument parsing fails. This can be convenient, but potentially very verbose if the list of arguments is long. Defaults to true

• docsOnNewLine: Boolean: whether to print argument doc-strings on a new line below the name of the argument; this may make things easier to read, but at a cost of taking up much more vertical space. Defaults to false

• autoprintHelpAndExit: Option[(Int, PrintStream)]: whether to detect --help being passed in automatically, and if so where to print the help message and what exit code to exit the process with. Defaults t, Some((0, System.out)), but can be disabled by passing in None if you want to handle help text manually (e.g. by calling .helpText on the parser object)

• customName/customNames and customDoc/customDocs: allows you to override the main method names and documentation strings at runtime. This allows you to work around limitations in the use of the @main(name = "...", doc = "...") annotation that only allows simple static strings.

Custom Argument Parsers

If you want to parse arguments into types that are not provided by the library, you can do so by defining an implicit TokensReader[T] for that type:

package testcustom
import mainargs.{main, arg, ParserForMethods, TokensReader}

object Main{
  implicit object PathRead extends TokensReader[os.Path](
    "path",
    strs => Right(os.Path(strs.head, os.pwd))
  )
  @main
  def run(from: os.Path, to: os.Path) = {
    println("from: " + from)
    println("to:   " + to)
  }

  def main(args: Array[String]): Unit = ParserForMethods(this).runOrExit(args)
}

$ ./mill example.custom --from mainargs --to out
from: /Users/lihaoyi/Github/mainargs/mainargs
to:   /Users/lihaoyi/Github/mainargs/out

In this example, we define an implicit PathRead to teach MainArgs how to parse os.Paths from the OS-Lib library. ArgReader requires the following fields:

class ArgReader[T](val shortName: String, // what to print in <...> in the help text
                   val read: Seq[String] => Either[String, T],
                   val alwaysRepeatable: Boolean = false, // used to allow Seq[T]-like parsers
                   val allowEmpty: Boolean = false) // used to allow Option[T]-like parsers

Note that read takes all tokens that were passed to a particular parameter. Normally this is a Seq of length 1, but if allowEmpty is true it could be an empty Seq, and if alwaysRepeatable is true then it could be arbitrarily long.

The allowRepeats parameter can also result in multiple tokens being passed to your ArgReader; for ArgReaders that do not expect that, the convention is to simply pick the last token in the list. There is no need to raise an error on duplicates, as you can simply disable allowRepeats if you want the parser to raise an error when a parameter is provided more than once.

Handlings Leftover Arguments

You can use the special Leftover[T] type to store any tokens that are not consumed by other parsers:

package testvararg
import mainargs.{main, arg, ParserForMethods, Leftover}

object Main{
  @main
  def run(foo: String,
          myNum: Int = 2,
          rest: Leftover[String]) = {
    println(foo * myNum + " " + rest.value)
  }

  def main(args: Array[String]): Unit = ParserForMethods(this).runOrExit(args)
}

$ ./mill example.vararg --foo bar i am cow
barbar List(i, am, cow)

This also works with ParserForClass:

package testvararg2
import mainargs.{main, arg, ParserForClass, Leftover}

object Main{
  @main
  case class Config(foo: String,
                    myNum: Int = 2,
                    rest: Leftover[String])

  def main(args: Array[String]): Unit = {
    val config = ParserForClass[Config].constructOrExit(args)
    println(config)
  }
}

$ ./mill example.vararg2 --foo bar i am cow
Config(bar,2,Leftover(List(i, am, cow)))

You can also pass in a different type to Leftover, e.g. Leftover[Int] or Leftover[Boolean], if you want to specify that leftover tokens all parse to a particular type. Any tokens that do not conform to that type will result in an argument parsing error.

Varargs Parameters

You can also use * "varargs" to define a parameter that takes in the remainder of the tokens passed to the CLI:

package testvararg
import mainargs.{main, arg, ParserForMethods, Leftover}

object Main{
  @main
  def run(foo: String,
          myNum: Int,
          rest: String*) = {
    println(foo * myNum + " " + rest.value)
  }

  def main(args: Array[String]): Unit = ParserForMethods(this).runOrExit(args)
}

Note that this has a limitation that you cannot then assign default values to the other parameters of the function, and hence using Leftover[T] is preferable for those cases.

Prior Art

Ammonite & Mill

MainArgs grew out of the user-defined @main method feature supported by Ammonite Scala Scripts:

• http://ammonite.io/#ScriptArguments

This implementation was largely copy-pasted into the Mill build tool, to use for its user-defined T.commands. A parallel implementation was used to parse command-line parameters for Ammonite and Mill themselves.

Now all four implementations have been unified in the MainArgs library, which both Ammonite and Mill rely heavily upon. MainArgs also provides some additional features, such as making it easy to define short versions of flags like -c via the short = '...' parameter, or re-naming the command line flags via name = "...".

Case App

MainArgs' support for parsing Scala case classes was inspired by Alex Archambault's case-app library:

• https://github.com/alexarchambault/case-app

MainArgs has the following differentiators over case-app:

• Support for directly dispatching to @main method(s), rather than only parsing into case classes
• A dependency-free implementation, without pulling in the heavyweight Shapeless library.

Scopt

MainArgs takes a lot of inspiration from the old Scala Scopt library:

• https://github.com/scopt/scopt

Unlike Scopt, MainArgs lets you call @main methods or instantiate case classes directly, without needing to separately define a case class and parser. This makes it usable with much less boilerplate than Scopt: a single method annotated with @main is all you need to turn your program into a command-line friendly tool.

Changelog

0.2.3

• Support Scala 3 #18

0.2.2

• Fix hygiene of macros #12
• Allow special characters in method names and argument names #13

0.2.1

• Scala-Native 0.4.0 support

0.1.7

• Add support for positional=true flag in mainargs.arg, to specify a specific argument can only be passed positionally regardless of whether allowPositional is enabled for the entire parser

• Allow - and -- to be passed as argument values without being treated as flags

0.1.4

• First release