Rspec how to determine if a let block has been defined?

In Rspec, I want to take advantage of using super() to call a defined let block if it exists or set a new value if it hasn't, I want to use this within a shared_example group but I just can't find how to do this.

I've tried checking if @some_let exists, I've tried checking if a super method of :some_let is owned by the kernel or not and none of them provide anything useful; I can't access instance_methods or instance_method because Rspec won't let me and searching the internet for a method hasn't revealed an answer.

I want to be able to do something like this:

shared_examples 'a shared example' do
  let(:some_let) { let_exists?(:some_let) ? super() : some_new_value }
end

is there a method like let_exists? or something to that effect?

Solution

Assuming that you call let before including the shared examples, this would work:

shared_examples 'a shared example' do
  let(:some) { 'fallback value' } unless method_defined?(:some)
end

describe 'with let' do
  let(:some) { 'explicit value' }
  include_examples 'a shared example'

  it { expect(some).to eq('explicit value') }
end

describe 'without let' do
  include_examples 'a shared example'

  it { expect(some).to eq('fallback value') }
end

method_defined? checks if a method called some has already been defined in the current context. If not, the method is defined to provide a default value.

Another (usually easier) approach is to always define a default value and to provide the explicit value after including the shared examples: (thus overwriting the default value)

shared_examples 'a shared example' do
  let(:some) { 'default value' }
end

describe 'with let' do
  include_examples 'a shared example'   # <- order is
  let(:some) { 'explicit value' }       #    important

  it { expect(some).to eq('explicit value') }
end

describe 'without let' do
  include_examples 'a shared example'

  it { expect(some).to eq('default value') }
end