Detect Optional Keyword Arguments in Ruby

detect.rb

NOT_PASSED = Object.new
def foo(bar: NOT_PASSED)
  if bar.equal?(NOT_PASSED)
    ...
  end
end

round2.rb

# For context, this is a validator. Here's what I was trying to make work initially:

module Validations
  class Acceptance
    NOT_PASSED = Object.new
    def self.valid?(value, accepts: NOT_PASSED)
      if accepts.equal?(NOT_PASSED)
        OtherValidator.valid?(value)
      else
        value == accepts
      end
    end
  end
end

Acceptance.valid?(true)
# => true
Acceptance.valid?(nil)
# => false
Acceptance.valid?("no", accepts: "yes")
# => false

# BUT, you could do this instead (sort of resembling what @piisalie was suggesting on twitter):

module Validations
  class Acceptance
    def self.valid?(value, accepts: OtherValidator)
      if accepts.respond_to?(:valid?)
        accepts.valid?(value)
      else
        value == accepts
      end
    end
  end
end

Alright, your update is closer to what I was thinking but I would probably go one step further using a generic validator like:

class GenericValidator
  def self.acceptable(value)
    new(value)
  end

  def self.valid?(value)
    true
  end

  def initialize(value)
    @value = value
  end

  def valid?(value)
    @value == value
  end
end

module Validations
  class Acceptance
    def self.valid?(value, accepts: GenericValidator)
      accepts.valid?(value) && !!value
    end
  end
end

Validations::Acceptance.valid?("no", accepts: GenericValidator.acceptable("yes"))
# => false
Validations::Acceptance.valid?("yes", accepts: GenericValidator.acceptable("yes"))
# => true
Validations::Acceptance.valid?(nil)
# => false
Validations::Acceptance.valid?(false)
# => false

The idea being, we should always pass a Validator as the accepts option, and that validator knows how to tell if it is actually valid through the generic valid? interface. In this way the Acceptance class doesn't have to have any knowledge about what validators exist and what to do with them.