Help understanding yield and enumerators in Ruby

Question

I would appreciate it if someone could help me understand the difference between using a Yielder in an Enumerator vs. just invoking yield in an Enumerator.

The "Well-grounded Rubyist" suggests that one doesn't "yield from the block" but doesn't explain precisely what's going on.

Thanks

Answer 1

The Enumerator::Yielder#yield method and the Enumerator::Yielder::<< method are exactly the same. In fact, they are aliases.

So, which one of those two you use, is 100% personal preference, just like Enumerable#collect and Enumerable#map or Enumerable#inject and Enumerable#reduce.

Answer 2

Well, unless I'm missing something, the method with yield simply doesn't work. Try it:

e = Enumerator.new do |y|
  y << 1
  y << 2
  y << 3
end

f = Enumerator.new do
  yield 1
  yield 2
  yield 3
end

e.each { |x| puts x }
f.each { |x| puts x }

Which produces this:

telemachus ~ $ ruby yield.rb 
1
2
3
yield.rb:13:in `block in <main>': no block given (yield) (LocalJumpError)
        from yield.rb:19:in `each'
        from yield.rb:19:in `each'
        from yield.rb:19:in `<main>

When he says (page 304) "you don't do this," he doesn't mean "it's not the best way to do it." He means, "that won't work."

Edit: You can, however, call yield explicitly this way:

e = Enumerator.new do |y|
  y.yield 1
  y.yield 2
  y.yield 3
end

If you find saying yield more explicit or clearer than <<, then do it that way.

Second edit: Looking at David's original post and Jorg's updated answer, I think that there was a confusion originally about the question. Jorg thought David was asking about the difference between Enumerator::Yielder#yield and Enumerator::Yielder::<<, but David wasn't sure what The Well Grounded Rubyist means when it says "don't write yield 1 etc." My answer applies to the question about The Well Grounded Rubyist. (When I looked this thread back over today, my answer looked odd in the light of other updates.)