La keyword yield

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

greet do
  puts "Hi from the block!"
end
# Prima del blocco
# Ciao dal blocco!
# Dopo il blocco

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                       # no pasa nada
maybe { puts "called!" }    # called!