snippet.rb

class Object

  def if_present?(&blk)
    if blk.arity == -1
      instance_eval(&blk)
    else
      blk.call(self)
    end
  end
  
end

str = "string".if_present? do |s|
  s.reverse!
end

puts str

str = "string".if_present? do
  reverse!
end

puts str