RSpec::Matchers.define :act_like_a_good_foo do
  match do
    # subject is implicit in example
    subject.has_bar?.should == true
    subject.should be_true  # predicate matchers defined within RSpec::Matchers
    subject.should have_bar
  end
end

describe Foo do
  it { should act_like_a_good_foo }
end

class MyAssertions
  include RSpec::Matchers

  def assert_everything_is_ok
    @foo = Foo.new
    @foo.has_bar?.should == true  # works!
    @foo.has_bar?.should be_true  # works now :)
    @foo.should have_bar          # works now :)
  end
end