rspec_example.rb

require 'rubygems'
require 'rspec'

shared_examples_for :a_foo do |supported_features|
  it 'responds to #to_s' do
    subject.should respond_to(:to_s)
  end

  it 'responds to #bar' do
    subject.should respond_to(:bar)
  end if supported_features.include?(:bar)

  it 'responds to #bazz' do
    subject.should respond_to(:bazz)
  end if supported_features.include?(:bazz)
end

class Foo1
  def bar; 'bar!'; end
end

class Foo2
  def bar; 'bar!'; end
  def bazz; 'bazz!'; end
end

describe Foo1 do
  it_should_behave_like :a_foo, [:bar]
end

describe Foo2 do
  it_should_behave_like :a_foo, [:bar, :bazz]
end

ruby 1.8.6 result

ruby-1.8.6-p399 ➜  code  ruby rspec_example.rb 


Finished in 0.00008 seconds
0 examples, 0 failures
rspec_example.rb:11:in `include?': wrong argument type Symbol (expected Module) (TypeError)
        from rspec_example.rb:11
        from /Users/mmarston/.rvm/gems/ruby-1.8.6-p399/gems/rspec-core-2.0.0.beta.19/lib/rspec/core/extensions/module_eval_with_args.rb:45:in `class_eval'
        from /Users/mmarston/.rvm/gems/ruby-1.8.6-p399/gems/rspec-core-2.0.0.beta.19/lib/rspec/core/extensions/module_eval_with_args.rb:45:in `extract_static_instance_method_defs_from'
        from /Users/mmarston/.rvm/gems/ruby-1.8.6-p399/gems/rspec-core-2.0.0.beta.19/lib/rspec/core/extensions/module_eval_with_args.rb:36:in `initialize'
        from /Users/mmarston/.rvm/gems/ruby-1.8.6-p399/gems/rspec-core-2.0.0.beta.19/lib/rspec/core/extensions/module_eval_with_args.rb:36:in `new'
        from /Users/mmarston/.rvm/gems/ruby-1.8.6-p399/gems/rspec-core-2.0.0.beta.19/lib/rspec/core/extensions/module_eval_with_args.rb:36:in `extract_static_instance_method_defs_from'
        from /Users/mmarston/.rvm/gems/ruby-1.8.6-p399/gems/rspec-core-2.0.0.beta.19/lib/rspec/core/extensions/module_eval_with_args.rb:27:in `module_eval_with_args'
        from /Users/mmarston/.rvm/gems/ruby-1.8.6-p399/gems/rspec-core-2.0.0.beta.19/lib/rspec/core/example_group.rb:70:in `it_should_behave_like'
         ... 6 levels...
        from /Users/mmarston/.rvm/gems/ruby-1.8.6-p399/gems/rspec-core-2.0.0.beta.19/lib/rspec/core/example_group.rb:129:in `subclass'
        from /Users/mmarston/.rvm/gems/ruby-1.8.6-p399/gems/rspec-core-2.0.0.beta.19/lib/rspec/core/example_group.rb:116:in `describe'
        from /Users/mmarston/.rvm/gems/ruby-1.8.6-p399/gems/rspec-core-2.0.0.beta.19/lib/rspec/core/extensions/object.rb:7:in `describe'
        from rspec_example.rb:27

ruby 1.8.7 result

ruby-1.8.7-p299 ➜  code  ruby rspec_example.rb
.....

Finished in 0.00335 seconds
5 examples, 0 failures