If you suspected "the right thing" you'd probably be right:

irb(main):001:0> require 'lazy'
irb(main):002:0> x = promise { 5 + 3 }
=> #<Lazy::Promise computation=#<Proc:0x0035788c@(irb):2>>
irb(main):003:0> p 3 * x
24
=> nil