TIL: Asserting Exceptions with Cucumber & Ruby

Proper use of Cucumber follows a strict given-when-then syntax. This posses a small problem when trying to assert an exception in a 'then' step if it is raised in a 'when' step.

For instance, suppose I have some gherkin that looks like this:

When I do something unwanted
Then an exception is raised

The problem here is that I have to do something with the exception in the 'when' step. I could catch the exception and deal with it in the 'then' like so:

When('I do something unwanted') do
  begin
    raise 'something bad'
  rescue => e
    @excepton = e
  end
end

Then('an exception is raised') do
  expect(@exception).not_to be_nil
end

That's gross. Let's do this better by deferring the action with lambdas (and by using a state variable wrapper)

When('I do something unwanted') do
  artefacts.request = -> { make_a_dangerous_call }
end

Then('an exception is raised') do
  expect { artefacts.request.call }.to raise_exception
end

Much neater.