Feature: Return value contracts

  Background:
    Given a file named "foo.rb" with:
    """ruby
    class SessionController
      def initialize(authentication_service)
        @authentication_service = authentication_service
      end

      def create(params)
        @authentication_service.authenticate(params[:login], params[:password])
        :render_welcome
      rescue
        :render_error
      end
    end

    class WrongPassword < StandardError
    end

    class AuthenticationService
      def initialize(user_db)
        @user_db = user_db
      end

      def authenticate(login, password)
        unless @user_db[login] == Digest::SHA1.hexdigest(password)
          raise WrongPassword
        end
      end
    end
    """
    And a spec file named "session_controller_spec.rb" with:
    """ruby
    describe SessionController do
      fake(:authentication_service)
      let(:controller) { SessionController.new(authentication_service) }

      it "logs in the user with valid data" do
        stub(authentication_service).authenticate('foo', 'bar')

        controller.create(login: 'foo', password: 'bar').should == :render_welcome
      end

      it "fails with invalid data" do
        stub(authentication_service).authenticate('baz', 'bar') { raise WrongPassword }

        controller.create(login: 'baz', password: 'bar').should == :render_error
      end
    end
    """

  Scenario: Fails when returned values differ
    Then spec file with following content should fail:
    """ruby
    describe AuthenticationService do
      verify_contract(:authentication_service)

      it "logs in the user" do
        service = AuthenticationService.new('foo' => Digest::SHA1.hexdigest('bar'))
        expect {
          service.authenticate('foo', 'bar')
        }.not_to raise_error
      end
    end
    """

  Scenario: Passes when interfaces match
    Then spec file with following content should pass:
    """ruby
    describe AuthenticationService do
      verify_contract(:authentication_service)

      it "logs in the user" do
        service = AuthenticationService.new('foo' => Digest::SHA1.hexdigest('bar'))
        expect {
          service.authenticate('foo', 'bar')
        }.not_to raise_error
      end

      it "raises WrongPassword with incorrect credentials" do
        service = AuthenticationService.new('foo' => Digest::SHA1.hexdigest('bar'))
        expect {
          service.authenticate('baz', 'bar')
        }.to raise_error(WrongPassword)
      end
    end
    """