ruby.rb

# From http://combinators.info/#recursive-combinators-in-idiomatic-ruby

def lambda_with_recursive_callback
  lambda { |x| x.call(x) }.call(
    lambda do |myself|
      lambda do |arg|
        yield(arg, myself.call(myself))
      end
    end
  )
end

sum_of_nested_list = lambda_with_recursive_callback do |arg, recurse|
  arg.kind_of?(Numeric) ? arg : arg.map { |item| recurse.call(item) }.injec\
t(&:+)
end

sum_of_nested_list.call([1, [[2,3], [[[4]]]]])
#=> 10