Define negated matcher

You can use RSpec::Matchers.define_negated_matcher to define a negated version of an existing matcher. This is particularly useful in composed matcher expressions.

Composed negated matcher expression

Given a file named "composed_negated_expression_spec.rb" with:

RSpec::Matchers.define_negated_matcher :an_array_excluding, :include

RSpec.describe "A negated matcher" do
  let(:list) { 1.upto(10).to_a }

  it "can be used in a composed matcher expression" do
    expect { list.delete(5) }.to change { list }.to(an_array_excluding 5)
  end

  it "provides a good failure message based on the name" do
    # deliberate failure
    expect { list.delete(1) }.to change { list }.to(an_array_excluding 5)
  end
end

When I run rspec composed_negated_expression_spec.rb

Then the output should contain all of these:

| 2 examples, 1 failure | | 1) A negated matcher provides a good failure message based on the name | | Failure/Error: expect { list.delete(1) }.to change { list }.to(an_array_excluding 5) | | expected list to have changed to an array excluding 5, but is now [2, 3, 4, 5, 6, 7, 8, 9, 10] |