I’d Tap That

Ruby 1.9 added a new method tap that is useful for cleaning up code. Here’s the Ruby doc:

tap{|x|…} → obj:

Yields x to the block, and then returns x. The primary purpose of this method is to “tap into” a method chain, in order to perform operations on intermediate results within the chain.

Effectively tap could be coded in pure Ruby like this:

class Object
  def tap(value)
    yield(value)
    value
  end
end

Knowing what tap does, here are some clever ways to use it.

As a With Statement

Jamis Buck demonstrated the Rails version of tap – returning – back in 2006. Since the two methods are equivalent, his example still applies:

def create_book
  book = Book.new
  book.title = "Trafalgar: The Nelson Touch"
  book.author = "David Howarth"
  @db.add(book)
  book
end

This can be simplified to the following:

def create_book
  Book.new.tap do |book|
    book.title = "Trafalgar: The Nelson Touch"
    book.author = "David Howarth"
    @db.add(book)
  end
end

As an Accumulator

I like to use tap when building up collections of data, so instead of writing this:

data = []
(1..3).each do |value|
  data << value
end
data

I can write the much clearer tap version:

[].tap do |data|
  (1..3).each do |value|
    data << value
  end
end

As an Indentor

gsinclair showed a way to use tap for code structuring:

def test_isosceles_given_base_and_height
  triangle(:ABC, :isosceles, :base => 5, :height :=> 9).tap do |t|
    assert_point_equal p(2.5,9), t.apex
    assert_point_equal p(0,0),   @register[:A]
    assert_point_equal p(5,0),   @register[:B]
    assert_point_equal p(2.5,9), @register[:C]
    assert_equal       t,        @register.retrieve(:triangle, :ABC)
  end
end

While I don’t agree that the unit test comes out looking cleaner than its counterpart, I do see the power of this type of nesting as a way to clean up code.

If you’ve discovered any other clever usages of tap leave it in the comments.