Ключевое слово yield

def greet
  puts "Before block"
  yield
  puts "After block"
end

greet do
  puts "Hi from the block!"
end
# Before block
# Hi from the block!
# After block

def with_each(items)
  items.each { |item| yield item }
end

with_each([10, 20, 30]) { |n| puts n }
# 10
# 20
# 30

def maybe
  yield if block_given?
end

maybe                       # ничего не происходит
maybe { puts "called!" }    # вызвано!