A convenience method. In our Rails model tests, we often had:

class ThingTest < ActiveSupport::TestCase
  def new_thing(attributes = {})
    ...

    thing = Thing.new(attributes)
    thing.valid?  # Allows us to use errors method on newly-created things.
    thing
  end

  def some_test
    assert_equal ["an error message"], new_thing(:bogus => "attributes").errors[:base]
  end

  ...
end

Well, this part is ugly:

thing = Thing.new(attributes)
thing.valid?  # Allows us to use errors method on newly-created things.
thing

We would like instead to say:

Thing.new(attributes).after_sending {valid?}

This is similar to the standard method Object#tap, but has a better name and better syntax.

We also support this syntax, but are not sure we like it:

Thing.new(attributes).after_sending.valid?