Explicitly specify the `@main`-method to run

[lefou]

Currently, mainargs changes it's parsing behavior depending on the fact whether there are more that one @main-annotated methods.

1. If there is only one @main-method, the method name must not be part of the parsed arguments.
2. If there is more than on @main-method, the first argument must be the method name.

There is no way, to use the various run methods of mainargs.Parser in a generic way. E.g. with an unknown number of @main-methods.

[lefou]

The workaround is to add a second "hidden" @main-method.

package build

import mill.*

object foo extends Module {

  def cmd1(args: String*) = Task.Command(exclusive = true) {
    mainargs.Parser(this).runOrThrow(
      args = "do-cmd-1" +: args,
      allowPositional = true
    )
  }

  @mainargs.main 
  def doCmd1(verbose: mainargs.Flag = mainargs.Flag(), name: String): Unit = {
    println(s"Hello ${name}!")
    if(verbose.value) println("Nice to meet you.")
  }

  @mainargs.main
  private def hidden() = ()

}

[lefou]

Instead, we want something like this:

package build

import mill.*

object foo extends Module {

  def cmd1(args: String*) = Task.Command(exclusive = true) {
    mainargs.Parser(this).runOrThrow(
      args = args,
      allowPositional = true,
      mainMethod = Some("doCmd1")
    )
  }

  @mainargs.main 
  def doCmd1(verbose: mainargs.Flag = mainargs.Flag(), name: String): Unit = {
    println(s"Hello ${name}!")
    if(verbose.value) println("Nice to meet you.")
  }

}

Some version not using a String-ly typed method name would be even better.