selectively_yield_extra_context.rb

class BaseEnum
  include Enumerable
  attr_reader :ctx

  def initialize( *ctx )
    @ctx = ctx
  end

  def each( *args, &blk )
    return enum_for __callee__, *args unless block_given?
    projection *args do |obj, *ctx|
      yield_with_context obj, *ctx, &blk
    end
  end

  private

  def yield_with_context( obj, *ctx, &blk )
    fail 'Block required' unless block_given?
    args = (-1...1).include?(blk.arity) ? obj : [obj, *ctx]
    yield *args
  end
end

class Enum < BaseEnum
  def projection( *args )
    i = 0
    ctx.each do |obj|
      yield obj, i
      i += 1
    end
  end
end

e0 = Enum.new *%w[zero one two three]

Array e0

e0.each do |obj, i|
  puts '  ** %03d: %s (%s)' % [i, obj, obj.class]
end

e0.each do |obj, *args|
  puts args
end