Tappety Tap Tap Tap Tap - Object#tap in Ruby

Last updated 18 June 2015

There are a number of features that were added to Ruby in versions 1.9 or later that don’t seem to be in wide use for some reason. One that I have a fondness for is the tap method.

The tap method was added to the Object class in Ruby 1.9. All it does is yield self to a block and then return self.

Here’s a simple example:

Object.new.tap do |object|
  puts object
# returns the object

Doesn’t that seem useful? Not really? Okay. Well, according to the docs the method is meant to “tap into” a method chain, in the same way you’d tap into a rich vein of silver. Let’s see if we can find the silver here.

Imagine you need to create an object, call one of its methods, and then return it.

Hat.new(type: :bowler).tap do |hat|

We’re getting closer.

What if you need to construct a string in a complicated way?

"".tap do |string|
  string << "Hello " if hello
  string << "Sir " if should_sir?
  string << "I'm writing to you"
  string << " about #{about}" if about
  string << "."

That’s so much better than the string = ""; string << "X" if something alternative that ends with string at the end to make sure you return it. tap takes care of that for you.

The biggest benefit I’ve found of using tap is that it makes it easy to create a variable with limited scope. This can be very useful in Rails views, especially if you use decorators to add view-related methods to an object.

Take this haml:

- decorated_post = PostDecorator.new(@post)
%h1= decorated_post.decorated_title
%p= decorated_post.decorated_body

That’s all fine, but if you are in a situation where you want to use two different decorators in the same view, you might have something like this:

- readably_decorated_post = ReadablePostDecorator.new(@post)
%h1= readably_decorated_post.decorated_title
%p= readably_decorated_post.decorated_body

- sortably_decorated_post = SortablePostDecorator.new(@post)
= link_to sortably_decorated_post.next_post

It can be hard to keep track of where a variable might be used and how. Also, those variable names are long to differentiate from themselves from each other. Try this instead:

- ReadablePostDecorator.new(@post).tap do |decorated_post|
  %h1= decorated_post.decorated_title
  %p= decorated_post.decorated_body

- SortablePostDecorator.new(@post).tap do |decorated_post|
  = link_to decorated_post.next_post

I find that more readable and you have a good idea of the start and end of the scope of the variables.

I hope this encourages you to bring out Object#tap for a little spin the next chance you get.