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Project: RSpec Expectations 3.3

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
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)

  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)
I run rspec composed_negated_expression_spec.rb
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 result to have changed to an array excluding 5, but is now [2, 3, 4, 5, 6, 7, 8, 9, 10]

Last published about 2 years ago by myronmarston.