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.

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

ivy"com.lihaoyi::mainargs:0.7.6"

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(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 # short name
hellohello false

$ ./mill example.hello --foo hello # long name
hellohello false

$ ./mill example.hello --foo=hello # gflags-style
hellohello false

$ ./mill example.hello --foo "" # set to empty value
 false

$ ./mill example.hello --foo= # gflags-style empty value
 false

$ ./mill example.hello -f x --my-num 3 # camelCase automatically converted to kebab-case
xxx false

$ ./mill example.hello -f hello --my-num 3 --bool # flags
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:

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

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

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.

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.

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(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

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(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.

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(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.

@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)

@main method arguments that have single-character names are automatically converted to short arguments, invoked with a single - instead of double --. The short version of an argument can also be given explicitly via the @arg(short = '...'):

object Base {
  @main
  def bools(a: Flag, b: Boolean = false) = println(Seq(a.value, b, c.value))
  
  @main
  def strs(a: Flag, b: String) = println(Seq(a.value, b))
}

These can be invoked as normal, for Flags like -a or normal arguments that take a value like -b below:

$ ./mill example.short bools -a
Seq(true, false)

$ ./mill example.short bools -b true
Seq(false, true)

Multiple short arguments can be combined into one -ab call:

$ ./mill example.short bools -ab true Seq(true, true)

Short arguments can be combined with their value:

```scala
$ ./mill example.short bools -btrue
Seq(false, true)

And you can combine both multiple short arguments as well as the resulting value:

$ ./mill example.short bools -abtrue
Seq(true, true)

Note that when multiple short arguments are combined, whether via -ab true or via -abtrue, only the last short argument (in this case b) can take a value.

If an = is present in the short argument group after the first character, the short argument group is treated as a key-value pair with the remaining characters after the = passed as the value to the first short argument:

$ ./mill example.short strs -b=value
Seq(false, value)

$ ./mill example.short strs -a -b=value
Seq(true, value)

You can use -b= as a shorthand to set the value of b to an empty string:

$ ./mill example.short strs -a -b=
  Seq(true, )

If an = is present in the short argument group after subsequent character, all characters except the first are passed to the first short argument. This can be useful for concisely passing key-value pairs to a short argument:

$ ./mill example.short strs -a -bkey=value
Seq(true, key=value)

These can also be combined into a single token, with the first non-`Flag` short argument in the
token consuming the subsequent characters as a string (unless the subsequent characters start with
an `=`, which is skipped):

```scala
$ ./mill example.short strs -ab=value
Seq(true, value)

$ ./mill example.short strs -abkey=value 
Seq(true, key=value)

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

• 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. If an explicit name not passed, both the (typically) camelCase name of the Scala def as well as its kebab-case equivalents will be accepted

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

• name: String: lets you specify the long name of a CLI parameter, e.g. --foo. If an explicit name not passed, both the (typically) camelCase name of the Scala method parameter as well as its kebab-case equivalents will be accepted

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

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

• hidden: Boolean: if true this arg will not be included in the rendered help text.

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.

• sorted: Boolean: whether to sort the arguments alphabetically in the help text. Defaults to true

• nameMapper: String => Option[String]: how Scala camelCase names are mapping to CLI command and flag names. Defaults to translation to kebab-case, but you can pass in mainargs.Util.snakeCaseNameMapper for snake_case CLI names or mainargs.Util.nullNameMapper to disable mapping.

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.Simple[os.Path]{
    def shortName = "path"
    def read(strs: Seq[String]) = 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.

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.

You can see the Scaladoc for TokenReaders.Simple for other things you can override:

• mainargs.TokenReaders.Simple

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.

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.

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 = "...".

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.

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.

• add support for Scala 2 varargs in scala 3 macro #167

• Fix reporting of incomplete short args #162
• Add BigDecimal TokensReader #152

• Fix issue with binary infinite recursion in binary compatibility forwarders #156

• Add missing nameMapper argument to .runOrExit, make sort param on runEither return true for consistency with the docs #155

• Various improvements to Scala 3 macros to match Scala 2 implementation #148

• Fix detection of @main methods inherited from traits in Scala 3.x #142

• Support for Scala-Native 0.5.0
• Minimum version of Scala 3.x raised to 3.3.1

• Fix usage of ParserForClass for case classes with more than 22 parameters in Scala 2.x

• Make combine short args that fail to parse go through normal leftover-token code paths #112

• Fix stackoverflow from incorrect binary compatibility shim #107

• Automatically map camelCase Scala method and argument names to kebab-case CLI commands and flag names, with configurability by passing in custom nameMappers #101

• Allow short arguments and their values to be combined into a single token #102

• Remove unnecessary PPrint dependency

• Support GFlags-style --foo=bar syntax #98

• Fix handling of case class main method parameter default parameters and annotations in Scala 3 #88

• Remove hard-code support for mainargs.Leftover/Flag/Subparser to support alternate implementations #62. Note that this is a binary-incompatible change, and any custom mainargs.TokenReaders you may implement will need to be updated to implement the mainargs.TokenReader.Simple trait

• Fix argument parsing of flags in the presence of allowPositional=true #66

• Support sorting to args in help text and sort by default
• Various dependency updates
• This release is binary compatible with mainargs 0.3.0

• Update all dependencies to latest
• Support for Scala Native on Scala 3

• Backport of Fix usage of ParserForClass for case classes with more than 22 parameters with some default values in Scala 2.x (#123) on top of 0.2.3

• Support Scala 3 #18

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

• Scala-Native 0.4.0 support

• 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

• First release