Argument Types

In the guide, we specified the type of an option’s argument like so:

import com.monovore.decline._
import java.nio.file.Path

val path = Opts.option[Path]("input", "Path to the input file.")
// path: Opts[Path] = Opts(--input <path>)

This does two different things for us:

  • It specifies a parsing function – when the user passes a string as an argument, decline will try and interpret it as a path and report an error if it can’t.
  • It specifies a default ‘metavar’ – the <path> text you can see in the output above. This helps the user understand what sort of input your program expects in that position.

This information is provided by the com.monovore.decline.Argument type class. decline provides instances for many commonly used standard-library types: strings, numbers, paths, URIs…

java.time support

decline has built-in support for the java.time library introduced in Java 8, including argument instances for Duration, ZonedDateTime, ZoneId, Instant, and others. You’ll need to pull these in with an explicit import:

import java.time._
import com.monovore.decline.time._

val fromDate = Opts.option[LocalDate]("fromDate", help = "Local date from where start looking at data")
// fromDate: Opts[LocalDate] = Opts(--fromDate <iso-local-date>)
val timeout = Opts.option[Duration]("timeout", help = "Operation timeout")
// timeout: Opts[Duration] = Opts(--timeout <iso-duration>)

By default, this parses using the standard ISO 8601 formats. If you’d like to use a custom time format, decline also provides Argument builders that take a java.time.format.DateTimeFormatter. For example, you can define a custom parse for a LocalDate by calling localDateWithFormatter:

import java.time.format.DateTimeFormatter
import com.monovore.decline.time.localDateWithFormatter

val myDateArg: Argument[LocalDate] = localDateWithFormatter(
// myDateArg: Argument[LocalDate] = Argument(<local-date>)

In general, any date or time type should have a xWithFormatter method available.

refined support

decline has support for refined types via the decline-refined module. Refined types add an extra layer of safety by decorating standard types with predicates that get validated automatically at compile time. While command line arguments can’t be validated at compile time, refined argument types’ runtime validation can still prevent the introduction of invalid values by the user.

To make use of decline-refined, add the following to your build.sbt:

libraryDependencies += "com.monovore" %% "decline-refined" % "2.1.0"

As an example, let’s define a simple refined type and use it as a command-line argument.

import eu.timepit.refined.api.Refined
import eu.timepit.refined.numeric.Positive
import com.monovore.decline.refined._

type PosInt = Int Refined Positive

val lines = Command("lines", "Parse a positive number of lines.") {
// lines: Command[PosInt] = com.monovore.decline.Command@3652c1e7

We can see that positive numbers will parse correctly, but anything zero or below will fail:

// res0: Either[Help, PosInt] = Right(10)
// res1: Either[Help, PosInt] = Left(Predicate failed: (0 > 0).
// Usage: lines <count>
// Parse a positive number of lines.
// Options and flags:
//     --help
//         Display this help text.)

enumeratum Support

NB: as of version 2.1 and the move to Scala 3, enumeratum support has been dropped. If you’re still using enumeratum for Scala 2, you may wish to stick with either an older version or reimplement… implementing Argument for EnumEntry is typically straightforward.

decline also supports enumeratum via the decline-enumeratum module. Enumeratum provides a powerful Scala-idiomatic and Java-friendly implementation of enums.

To make use of the enumeratum support, add the following to your build.sbt:

libraryDependencies += "com.monovore" %% "decline-enumeratum" % "2.1.0"

As an example, we’ll define a plain enumeration as required by enumeratum, and use it as a command-line argument:

import _root_.enumeratum._
import com.monovore.decline.enumeratum._

sealed trait Color extends EnumEntry with EnumEntry.Lowercase

object Color extends Enum[Color] {
  case object Red extends Color
  case object Green extends Color
  case object Blue extends Color
  val values = findValues

val color = Command("color", "Return the chosen color.") {

This parser should successfully read in red, green, or blue, and fail on anything else. (NB: parsers are case sensitive!)




enumeratum also supports value enums, which are enumerations that are based on a value different than the actual enum value name. Here’s the same enum type as before, but backed by an integer:

import _root_.enumeratum.values._
import com.monovore.decline.enumeratum._

sealed abstract class IntColor(val value: Int) extends IntEnumEntry

object IntColor extends IntEnum[IntColor] {
  case object Red extends IntColor(0)
  case object Green extends IntColor(1)
  case object Blue extends IntColor(2)

  val values = findValues

val intColor = Command("int-color", "Shows the chosen color") {

Value parsers expect the underlying enum value. Our new IntEnum parser will fail on anything but 0, 1, or 2.




Defining Your Own

In some cases, you’ll want to take a command-line argument that doesn’t quite map to some provided type. Say you have the following key-value config type:

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

You can define an option that collects a list of configs, by specifying a custom metavar and adding additional validation and parsing logic:


Opts.option[String]("config", "Specify an additional config.", metavar = "key:value")
  .mapValidated { string =>
    string.split(":", 2) match {
      case Array(key, value) => Validated.valid(Config(key, value))
      case _ => Validated.invalidNel(s"Invalid key:value pair: $string")
// res2: Opts[Config] = Opts(--config <key:value>)

For most cases, this works perfectly well! For larger applications, though – where many different options, subcommands or programs might want to use this same basic config type – doing this sort of thing each time is verbose and error-prone.

It’s easy enough to bundle the metavar and parsing logic together in an Argument instance:

implicit val configArgument: Argument[Config] = new Argument[Config] {

  def read(string: String) = {
    string.split(":", 2) match {
      case Array(key, value) => Validated.valid(Config(key, value))
      case _ => Validated.invalidNel(s"Invalid key:value pair: $string")

  def defaultMetavar = "key:value"
// configArgument: Argument[Config] = Argument(<key:value>)

…and then defining new options that take configs becomes trivial:

Opts.option[Config]("config", "Specify an additional config.")
// res3: Opts[Config] = Opts(--config <key:value>)

Missing Instances

In a few cases, decline has intentionally not defined an Argument instance for a particular type – since there are better ways to achieve the same effect. Some examples:

  • Boolean: supporting Boolean arguments like Opts.option[Boolean]("verbose", ???) would lead to command-line usage like my-command --verbose true… but users of other POSIX-ish command line tools would expect my-command --verbose. You can get that more idiomatic style with Opts.flag("verbose", ???).orFalse; consider using that instead!
  •, these types are mostly superseded by better alternatives (java.nio.file.Path and, respectively), and they support easy conversions to the older types to interoperate with existing code.
  • List[A]: you might expect to be able to define a Opts.option[List[String]](...) to parse a comma-separated list of strings, like --exclude foo,bar. This ends up a little bit tricky in the general case: either you can’t parse strings that contain commas, or you need some “escaping” mechanism, neither of which is particularly pleasant or idiomatic for users. Instead, consider using the plural methods like Opts.options or Opts.arguments to accumulate a list, like --exclude foo --exclude bar. (This is also easier to use programatically!)