Default Value support

[TApplencourt]

Hi,

Thanks a lot for this gem!

I wrote a little hacky class that adds support for default values. So you can do:

parser.on('-m', '--tracing-mode MODE', 'Define the category of events traced', default: 'default')

I have no idea if you can find it interesting, but I use it quite often in my project, so I share it with you. Putting the default values into the into for me to duplicate the key (one into the the on and one into the into. I lead from some silly mistake due to a typo).

If you are interested, maybe I can try to do a cleaner PR. If you are not, please feel free to close this issue.

Thanks a lot!

require 'optparse'
class OptionParserWithDefaultAndValidation < OptionParser
  def initialize
    # Dictionary to save the default values passed by `on`
    @defaults = {}
    super
  end

  def parse!(argv = default_argv, into: nil)
    # Merge the default value with the `into` dict
    into.merge!(@defaults) unless into.nil?
    # This will populate the `into` dict (overwriting our default if needed)
    super
  end

  def define(*opts, &block)
    switch = super
    @defaults[switch.switch_name] = @tmp_default unless @tmp_default.nil?
    switch
  end

  def on(*opts, default, &block)
    # Save the default, which will be used in `refine.`
    @tmp_default = default
    # Default can contain Array (or Set).
    # We cannot use the [a].flatten trick.
    # So we use the respond_to one
    #   irb(main):028:0> default = Set[-1,2]
    #   => #<Set: {-1, 2}>
    #   irb(main):029:0> [default].flatten.join(',')
    #   => "#<Set: {-1, 2}>"
    #   irb(main):030:0> default.respond_to?(:flatten)? default.flatten.join(',') : default
    #   => "-1,2"
    opts << "Default: #{default.respond_to?(:flatten) ? default.flatten.join(',') : default}" unless default.nil?
    super(*opts, &block)
  end
end

[nobu]

I think this can conflict with #55. What do you think?

[TApplencourt]

Thanks for the link! Sorry that I didn't find it before. It indeed solves the same problem.

The options: alternative improve the help message. instance.on("--demo_b [TEXT]", default: "fallback" will generate the error message:

Usage: test [options]
        --demo_b [TEXT]              Default value: fallback

I'm not sure if it's possible to do the same with #55 approach.

A corned case can be if the developer modifies the default value in the block. instance.on("--demo_b [TEXT]") { |value = 0| value+1 }. I think it makes sense to print default : 0 in the help message and not default : 1. I don't know how to implement it with #55