rspec

y.rb

require 'rspec/autorun'

RSpec::Matchers.define :be_one_of do |expected|
  match do |actual|
    expected.include? actual
  end
end

RSpec.describe 13 do
  it 'is in the list of allowed values' do
    expect([1,2,3]).to include 13
  end

  it 'is really in the list of allowed values' do
    expect(13).to be_one_of [1,2,3]
  end
end

# >> FF
# >> 
# >> Failures:
# >> 
# >>   1) 13 is in the list of allowed values
# >>      Failure/Error: expect([1,2,3]).to include 13
# >>        expected [1, 2, 3] to include 13
# >>      # /tmp/seeing_is_believing_temp_dir20200316-2154923-1o5c992/program.rb:11:in `block (2 levels) in <main>'
# >> 
# >>   2) 13 is really in the list of allowed values
# >>      Failure/Error: expect(13).to be_one_of [1,2,3]
# >>        expected 13 to be one of 1, 2, and 3
# >>      # /tmp/seeing_is_believing_temp_dir20200316-2154923-1o5c992/program.rb:15:in `block (2 levels) in <main>'
# >> 
# >> Finished in 0.02524 seconds (files took 0.21963 seconds to load)
# >> 2 examples, 2 failures
# >> 
# >> Failed examples:
# >> 
# >> rspec /tmp/seeing_is_believing_temp_dir20200316-2154923-1o5c992/program.rb:10 # 13 is in the list of allowed values
# >> rspec /tmp/seeing_is_believing_temp_dir20200316-2154923-1o5c992/program.rb:14 # 13 is really in the list of allowed values
# >>