A let method to memoize values that could be nil.

gistfile1.rb

module Let

  def let(name, &block)
    ivar = "@#{name}".to_sym
    if instance_variable_defined?(ivar)
      instance_variable_get(ivar)
    else
      instance_variable_set(ivar, instance_eval(&block))
    end
    
  end

end

class Thing
  include Let

  def result
    let(:result) { some_expensive_calculation_that_could_return_nil }
  end
end

t = Thing.new
t.result #=> calculated result that could be nil
t.result #=> cached result that could be nil