One-liner syntax

RSpec supports a one-liner syntax for setting an expectation on the subject. RSpec will give the examples a doc string that is auto- generated from the matcher used in the example. This is designed specifically to help avoid duplication in situations where the doc string and the matcher used in the example mirror each other exactly. When used excessively, it can produce documentation output that does not read well or contribute to understanding the object you are describing.

This comes in two flavors:

• is_expected is defined simply as expect(subject) and is designed for when you are using rspec-expectations with its newer expect-based syntax.
• should was designed back when rspec-expectations only had a should-based syntax. However, it continues to be available and work even if the :should syntax is disabled (since that merely removes Object#should but this is RSpec::Core::ExampleGroup#should).

Notes:

• This feature is only available when using rspec-expectations.
• Examples defined using this one-liner syntax cannot be directly selected from the command line using the --example option.
• The one-liner syntax only works with non-block expectations (e.g. expect(obj).to eq, etc) and it cannot be used with block expectations (e.g. expect { object }).

Implicit subject

Given a file named "example_spec.rb" with:

RSpec.describe Array do
  describe "when first created" do
    # Rather than:
    # it "should be empty" do
    #   subject.should be_empty
    # end

    it { should be_empty }
    # or
    it { is_expected.to be_empty }
  end
end

When I run rspec example_spec.rb --format doc

Then the examples should all pass

And the output should contain:

Array
  when first created
    is expected to be empty
    is expected to be empty

Explicit subject

Given a file named "example_spec.rb" with:

RSpec.describe Array do
  describe "with 3 items" do
    subject { [1,2,3] }
    it { should_not be_empty }
    # or
    it { is_expected.not_to be_empty }
  end
end

When I run rspec example_spec.rb --format doc

Then the examples should all pass

And the output should contain:

Array
  with 3 items
    is expected not to be empty
    is expected not to be empty