require 'rspec/expectations'

RSpec::Matchers.define :include_one do |expected|
  function_passed = expected.is_a?(Proc)

  match do |actual|
    if function_passed
      actual.select {|el| expected.call(el) }.size == 1
    else
      actual.select {|el| el.eql?(expected) }.size == 1
    end
  end

  failure_message do |actual|
    "expected #{actual.inspect} to have only one #{expected}"
  end
end

RSpec::Matchers.alias_matcher :include_exactly_one, :include_one
RSpec::Matchers.alias_matcher :contain_one, :include_one