Collect a repeated option

You want --tag beta --tag eu to give you both values rather than the last one.

Use .multiple()

.multiple() changes the property's type to List<T> and tells the parser to accumulate instead of replace:

import org.draftcode.argot.Arguments

class TagArgs : Arguments(programName = "tag", description = "Apply tags to a build.") {
    val tag: List<String> by option("--tag", "-t", help = "Tag to apply").multiple()
    val env: List<String> by option("--env", "-e", help = "Environment").multiple().required()
}

$ tag --tag beta -t eu --env staging
tags=beta,eu envs=staging

Values arrive in the order they were supplied, and short and long names are interchangeable — -t eu and --tag eu land in the same list.

Absent means empty, not null

A .multiple() property is never null. Supply the option zero times and you get an empty list, so you can iterate without a null check:

for (tag in args.tag) apply(tag)

That is why .multiple() on its own is not the same as "optional". If you need at least one value, say so:

val env: List<String> by option("--env", "-e", help = "Environment").multiple().required()

Now omitting --env entirely is a parse error, while --env prod is fine.

Convert before you accumulate

Order matters: refine the type first, then make it repeatable. .int() describes what one value is, .multiple() describes how many there are.

class PortArgs : Arguments(programName = "expose", description = "Expose ports.") {
    val port: List<Int> by option("--port", "-p", help = "Port to expose").int().multiple()
}

$ expose -p 80 -p 443     # args.port == listOf(80, 443)

Every value goes through the converter, so -p 80 -p https fails on the second one with the usual message rather than silently producing a partial list.

In the annotation style

Declare the parameter as a List<T> and the processor infers the same behaviour:

@Command(name = "tag", description = "Apply tags to a build.")
data class TagArgs(
    @Option(names = ["--tag", "-t"], help = "Tag to apply") val tag: List<String>,
)

See also

• Accept one of a fixed set of values — repeatable enums work the same way.
• OptionListBuilder for the exact signatures.