Meaning of the word yield

问题

Currently I'm reading "The Well-Grounded Rubyist" by David A. Black, and I stuck at 10.9 chapter (Enumerators and the next dimension of enumerability). My question is about yield method. What is the meaning of the word yield in Ruby context? My native language is Russian, and Google Translate shows me a bunch of translation variants, that are confusing me. There are some of them: give, bring, surrender (give up), produce, agree, comply and many others.

UPD: please, pay attention to the fact, that I'm trying to understand the meaning of the Enumerator::Yielder#yield method, but not yield keyword itself.

UPD_2: I've found interesting article about Enumerators: "Lazy Enumerators in Ruby".

回答1:

The word yield doesn't really have any special meaning in the context of Ruby. It means the same thing as it means in every other programming language, or in programming and computer science in general.

It is usually used when some kind of execution context surrenders control flow to a different execution context. For example, in Unix, there is a sched_yield function which a thread can use to give up the CPU to another thread (or process). With coroutines, the term yield is generally used to transfer control from one coroutine to another. In C#, there is a yield keyword, which is used by an iterator method to give up control to the iterating method.

And in fact, this last usage is exactly identical to the usage of the Enumerator::Yielder#yield method in Ruby, which you were asking about. Calling this method will suspend the enumerator and give up control to the enumerating method.

Example:

fibs = Enumerator.new do |y|
  a, b = 0, 1
  y.yield a
  loop do
    y.yield b
    a, b = b, a + b
  end
end

puts fibs.next #  0
puts fibs.next #  1
puts fibs.next #  1
puts fibs.next #  2
puts fibs.next #  3
puts fibs.next #  5
puts fibs.next #  8
puts fibs.next # 13
puts fibs.next # 21

As you see, there is an infinite loop. Obviously, if this loop just ran on its own, it wouldn't be of much use. But since every time it hits the yield method, it gives up control until it is called again, this will produce the Fibonacci numbers one by one, essentially representing an infinitely long list of all Fibonacci numbers.

There is another method, Fiber.yield, which serves a similar purpose. (In fact, I already described it above, because Fiber is just Ruby's name for coroutines.) Inside a Fiber, you call Fiber.yield to give up control back to the execution context that originally gave control to you.

Lastly, there is the yield keyword, which is used inside a method body to give up control to the block that was passed into the method.

Note that, at least in the Enumerator case (i.e. the first example), you can additionally interpret yield as to produce, since the Enumerator produces a new value, every time it calls yield.

回答2:

In the context of yielding in an Enumerator, the meaning is closest to "bring forth." The Enumerator calls the yield method of its yielder object, which "brings forth" any value passed to it.

give_me = Enumerator.new do |yielder|
  (1..5).each do |n|
    yielder.yield n
  end
end

5.times do
  p give_me.next
end

This results in:
1
2
3
4
5

In the case of yielding to a block, the meaning is closest to "surrender." The method with the yield statement surrenders execution to whatever block you passed to that method.

def wage_war
  p "What should I do?"
  yield
end

wage_war { p "Surrender!" }

回答3:

You may find it amusing to read what Programming Ruby 1.9 has to say about the yield keyword:

Programming-language buffs will be pleased to know that the keyword yield was chosen to echo the yield function in Liskov’s language CLU, a language that is more than thirty years old and yet contains features that still haven’t been widely exploited by the CLU-less.

回答4:

@Prostosuper, the related definition I like best is this one:

concede, cede, grant (give over; surrender or relinquish to the physical control of another)

In @Jamie Forrest's example, when wage_war is called, "What should I do?" is printed first, then flow control is yielded (conceded, ceded, granted, given over) to the block that wage_war was called with, resulting in "Surrender!" being printed. After that block is complete, flow control resumes in wage_war. If there were another statement after the yield, it would be executed when flow control resumed in wage_war after "Surrender!" was printed.

EDIT: @Prostosuper asked about yield as it pertains to Enumerators, not blocks, and my example discusses its use in blocks. A SO question (with answers) about Enumerator::Yielder#yield is here. The sense of the definition still applies.

回答5:

yield, in the ruby context "brings" :) the block passed as a parameter to your method.

def my_method
    p "I have to say something:"
    yield
end

my_method do
   p "hello world!"
end

prints

I have to say something:
hello world

the code p "hello world" is executed when my_method reaches the yield

来源：https://stackoverflow.com/questions/5069626/meaning-of-the-word-yield

标签

ruby

point

yield

enumerable