Custom RSpec matcher for checking class for constant, with optional class type and value

example.rb

# frozen_string_literal: true

class ExampleClass
  POPULAR_LIMIT = 12
end

example_spec.rb

# frozen_string_literal: true

require 'spec_helper'

RSpec.describe ExampleClass do
  subject { described_class }
  
  it { is_expected.to have_constant(:POPULAR_LIMIT, Integer).with_value(12) }
  it { is_expected.to define_constant(:POPULAR_LIMIT, Integer).with_value(12) }
end

have_constant.rb

# frozen_string_literal: true
#
# File: spec/support/matchers/have_constant.rb

RSpec::Matchers.define :have_constant do |const, klass|
  match do |owner|
    return owner.const_defined?(const) if klass.nil?
    return owner.const_defined?(const) && owner.const_get(const).class == klass if @value.nil?

    owner.const_defined?(const) && owner.const_get(const).class == klass &&
      owner.const_get(const) == @value
  end

  chain :with_value do |value|
    @value = value
  end

  failure_message do |actual|
    msg = +"constant #{const} not defined in #{actual}"
    msg += " as a #{klass}" unless klass.nil?
    msg += " with value #{@value}" unless @value.nil?
    msg
  end

  failure_message_when_negated do |actual|
    msg = +"constant #{const} is defined in #{actual}"
    msg += " as a #{klass}" unless klass.nil?
    msg += " with value #{@value}" unless @value.nil?
    msg
  end

  description do
    msg = +"have constant #{const}"
    msg += " defined with class #{klass}" unless klass.nil?
    msg += " and value #{@value}" unless @value.nil?
    msg
  end
end

RSpec::Matchers.alias_matcher :define_constant, :have_constant

spec_helper.rb

# frozen_string_literal: true
#
# File: spec/spec_helper.rb

Dir[File.join(__dir__, "/support/**/*.rb")].each { |f| require f }