Question

I am trying to use the new immutable OptionParser in the Scala scopt 2.0.1 library. Since OptionParser takes a generic type and the help method already defines an action that returns Unit, I am getting a compile-time error:

case class Config(directory: String = null)

val parser = new OptionParser[Config]() {
  def options = Seq(
    opt("d", "directory", "directory containing the files to be processed") {
      (value: String,  config: Config) => config.copy(directory = value)
    },
    help("?", "help", "Show a usage message and exit"))
}

error: type mismatch;
[INFO]  found   : scopt.generic.FlagOptionDefinition[Nothing]
[INFO]  required: scopt.generic.OptionDefinition[Config]
[INFO] Note: Nothing <: Config, but class OptionDefinition is invariant in type C.

How can I include a “help” option?

Answer 1

First of all, there seems to be an error in the library, where one of the overloaded methods of opt takes a type parameter C which it shouldn’t — at least from what I can tell. It should just take C from the class. Anyway, although you use that call, I guess Scala still correctly infers that this C is the same as the class’s C (Config).

The problem seems to be that help is completely useless — it gives you FlagOptionDefinition[Nothing] because its action: => C implementation is {this.showUsage; exit}.

I think that the OptionParser class needs fixing…

You could write your own help method that enforces the C type parameter:

def help2(shortopt: String, longopt: String, description: String) =
  new FlagOptionDefinition[C](Some(shortopt), longopt, description, 
    { this.showUsage; exit })