Add optional positional parameter #55

Overview

Hello. 👋 I was wondering if it'd be possible to add optional positional parameter support since it's awkward to use a control parameter (i.e. actual || fallback) in the body of an #on method when an optional positional parameter could be provided via the block arguments.

Steps to Recreate

Here's a snippet for illustration purposes:

require "optparse"

parser = OptionParser.new do |instance|
  instance.on("--demo_a [TEXT]") { |value| puts value || "fallback" }
  instance.on("--demo_b [TEXT]") { |value = "fallback"| puts value }
end

parser.parse ["--demo_a"]        # "fallback"
parser.parse %w[--demo_a hello]  # "hello"

parser.parse ["--demo_b"]        # "" <- 😢
parser.parse %w[--demo_b hello]  # "hello"

As per the official option parser documentation, the --demo_a implementation is the recommended way to solve this problem but I'd really like to support the --demo_b implementation since it would clean up the code and is more intuitive/idiomatic.

Environment

• Ruby: ruby 3.2.2 (2023-03-30 revision e51014f9c0) +YJIT [arm64-darwin22.4.0]
• Option Parser: 0.3.1