Question

Going through the rspec docs i found these lines of code in yielding section of this page http://rubydoc.info/gems/rspec-expectations/frames

Can anyone explain what each line of code in the yielding section does step-by-step

Answer 1

What does expect { |b| 5.tap(&b) }.to yield_control do?

It is expecting that the code given in the block invokes yield statement.

The code given in the block (viz. { |b| 5.tap(&b) }) calls the ruby 1.9 tap method which has a yield statement within it’s implementation.

So the statement is effectively asserting that ruby 1.9’s tap method has a yield statement. 🙂

To better understand this statement, try the following code example:

order.rb file

class Order
end

order_spec.rb file

require 'rspec-expectations'
require './order'

describe Order do
  it "should yield regardless of yielded args" do
    expect { |b| 5.tap(&b);puts "The value of b is #{b.inspect}" }.to yield_control
  end
end 

Output from executing the spec:

$ rspec yield_spec.rb

Order
The value of b is #<RSpec::Matchers::BuiltIn::YieldProbe:0x000001008745f0 @used=true, @num_yields=1, @yielded_args=[[5]]>
  should yield regardless of yielded args

Finished in 0.01028 seconds
1 example, 0 failures

To understand the other lines in the yielding section, similarly include the expect statements in the spec file as follows:

 it "should yield with no args" do
    expect { |b| Order.yield_if_true(true, &b) }.to yield_with_no_args
  end 
      it "should yield with 5 args" do
    expect { |b| 5.tap(&b) }.to yield_with_args(5)
  end 

  it "should yield with integer args" do
    expect { |b| 5.tap(&b) }.to yield_with_args(Fixnum)
  end 

  it "should yield with string args" do        
    expect { |b| "a string".tap(&b) }.to yield_with_args(/str/)
  end 

  it "should yield 1,2,3 successively" do
    expect { |b| [1, 2, 3].each(&b) }.to yield_successive_args(1, 2, 3)
  end 

  it "should yield ([:a,1], [:b,2]) successively" do
    expect { |b| { :a => 1, :b => 2 }.each(&b) }.to yield_successive_args([:a, 1], [:b, 2]) 
  end 

Note: The yield_if_true method used in the second line seems to be removed from where it was originally defined; I got it go work by adding it into the Order class as follows:

class Order
  def self.yield_if_true(flag)
    yield if flag
  end
end