Customizable CommandParser

[larsrh]

The current ways to construct a CommandParser are fairly limited. It would be cool to have something like this:

CommandParser.fromRecord(
  "build" ->> BuildCommand ::
  "check" ->> CheckCommand ::
  HNil
)

I'm a little fuzzy on the details, though. Ideally, this should support common and command-specific options, e.g.

cli-app --config /home/lars/.local/cli-app.conf build --session HOL

Maybe something like this?

CommandParser.fromRecord[Common] { common =>
  "build" ->> BuildCommand(common) ::
  "check" ->> CheckCommand(common) ::
  HNil
}

... where the return type of *Command is some Parser[T].

[larsrh]

I just realized one could possibly build that on top of the existing primitives.

case class Common(x: Int = 3, y: String = "bla")
case class Command[T](@Recurse common: Common, @Recurse specific: T)

def parseAux[T, Aux](args: List[String])(implicit aux: Parser.Aux[T, Aux]) =
  CaseApp.parse[Command[T]](args)                                                                                                                                   

// implicit not found when trying to define this directly, needs `parseAux`
def parseCmd[T : Parser](args: List[String]) = parseAux(args)(Parser[T])                                                                   

case class Cmd1(lol: String = "foo")
parseCmd[Cmd1](List("--lol", "fooo", "-x", "3"))

And then build some implicits recursing over a record, constructing these composite parsers.

[alexarchambault]

I've been using a workaround for that too, in coursier: https://github.com/coursier/coursier/blob/b4dd43e03c478fb1d72fe49999751f9cf6e8e6be/cli/src/main/scala-2.11/coursier/cli/Coursier.scala.

I'd like to have something along the lines of your first example work too.