Implementing Ruby's Tap Method

What is the Tap method?

Ruby’s tap method allows you to “tap into” a method chain, modify an object and receive that same object as the result. An Engine Yard blog recently spoke about five ruby methods you should be using in which tap was one of them.

Let’s create our own Tap method

I got to thinking that unless I’m seriously missing something, it can’t be too much work to create my own tap method. I wasn’t wrong. I attempted to create it and named it tapme to give it a unique name, and it ended up being only 2 lines of code.

class Object
  def tapme
    yield(self)
    self
  end
end

I then wanted to see if I could check out how Ruby actually implemented tap in its source, and it turns out that it is basically the same.

rb_obj_tap(VALUE obj)
{
  rb_yield(obj);
  return obj;
}

Using our tapme method

I wanted to come up with a quick little, somewhat contrived demo of our tapme method in action. I think Llamas and Alpacas are pretty hilarious, so I created an Alpaca object, with name, spouse, and marital status attributes, and created a method which would marry 2 alpacas, using the tapme method as part of a method chain which prints out a string representation of our alpaca’s marital status.

class Alpaca

  attr_accessor :name, :spouse, :marital_status

  def initialize(name)
    self.name = name
    self.marital_status = :single
  end

  def marry(partner, update_partner = true)
    tapme { |me|
      me.marital_status = :married
      me.spouse         = partner
      partner.marry(me, false) if update_partner
    }
  end

  def marital_string
    case marital_status
    when :single
      "#{name} is single."
    when :married
      "#{name} is married to #{spouse.name}."
    when :divorced
      "#{name} is divorced."
    else
      "#{name}'s marital status is unknown."
    end
  end

end

spitty  = Alpaca.new("Spitty")
huacaya = Alpaca.new("Huacaya")

puts spitty.marry(huacaya).marital_string

Calling our code produces this result:

ruby tapme.rb
Spitty is married to Huacaya.

And there we go! We we’re able to re-implement Ruby’s tap method. I wonder what other Ruby methods are this easy to implement?