Custom Rspec matcher for checking class for constant, with optional class type.

example.rb

class ExampleClass
  PI = 3.14159265359
end

example_spec.rb

require 'spec_helper'

describe ExampleClass do

  its(:class) { should have_constant(:PI, Float) }
end

have_constant.rb

# spec/support/matchers/have_constant.rb
RSpec::Matchers.define :have_constant do |const,klass|
  match do |owner|
    const_defined = owner.const_defined?(const)
    klass_match = owner.const_get(const).class == klass unless klass.nil?
    klass.nil? ? const_defined : (const_defined && klass_match)
  end

  failure_message_for_should do |actual|
    msg = "constant #{expected[0]} not defined in #{actual}"
    msg += " as a #{expected[1]}" unless expected[1].nil?
    msg
  end
 
  failure_message_for_should_not do |actual|
    msg = "constant #{expected[0]} is defined in #{actual}"
    msg += " as a #{expected[1]}" unless expected[1].nil?
    msg
  end

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

spec_helper.rb

# spec/spec_helper.rb
Dir[File.dirname(__FILE__) + "/support/**/*.rb"].each {|f| require f}