example-with-let.rb

class EmployeeTest < Test::Unit::TestCase
  let(:email) { Employee.new('[email protected]') }

  def employee_has_an_email_test
    assert_equal('[email protected]', employee.email)
  end
end

example-without-let.rb

class EmployeeTest < Test::Unit::TestCase
  def employee_has_an_email_test
    assert_equal('[email protected]', employee.email)
  end

  private

  def employee
    @employee ||= Employee.new('[email protected]')
  end
end

let.rb

class Test::Unit::TestCase
  # Syntactic sugar for defining a memoized helper method.
  def self.let(name, &block)
    ivar = "@#{name}"
    self.class_eval do
      define_method(name) do
        if instance_variable_defined?(ivar)
          instance_variable_get(ivar)
        else
          value = self.instance_eval(&block)
          instance_variable_set(ivar, value)
        end
      end
    end
  end
end