Feature #8639: Add Queue#each

• Status: Closed
• Assignee: ko1 (Koichi Sasada)
• Target version: 2.6

[avdi (Avdi Grimm)]

I was fiddling around with Queue the other day and realized it doesn't have an #each method. So I made one: https://github.com/ruby/ruby/pull/361

As for why, it makes for a convenient way to build consumer processes:

inq = Queue.new
outq = Queue.new
doubler = Thread.new do
  inq.each do |n|
    outq << n + n
  end
end

My PR also returns an Enumerator when no block is given, and handles the non_block argument.

I'm sure there's some good reason that this method wasn't there already, so feel free to explain.

Also, this is my first Ruby feature ticket so please let me know if I'm missing any points of protocol.

Thanks!

[rkh (Konstantin Haase)]

+1 for the feature, should have gone into CommonRuby, I think, maybe.

[avdi (Avdi Grimm)]

On Mon, Jul 15, 2013 at 11:54 AM, rkh (Konstantin Haase) [e-mail] wrote:

+1 for the feature, should have gone into CommonRuby, I think.

Is that something I can change?

[avdi (Avdi Grimm)]

P.S. Should I reply on the ticket or do these conversations eventually get
posted back to it? I'm new to all of this!

[regularfry (Alex Young)]

On Tue, 2013-07-16 at 00:44 +0900, avdi (Avdi Grimm) wrote:

Issue #8639 has been reported by avdi (Avdi Grimm).

I thought this sounded familiar. It has come up before:
http://bugs.ruby-lang.org/issues/4589

--
Alex

[avdi (Avdi Grimm)]

On Mon, Jul 15, 2013 at 4:24 PM, Alex Young [e-mail] wrote:

I thought this sounded familiar. It has come up before:
http://bugs.ruby-lang.org/issues/4589

Hey, thanks for finding that! I figured it had probably come up before.
Some notes responding to that conversation:

• First, as Alex pointed out then, IO#each is often destructive in the
  sense that it's either moving the read pointer forward or (in the case of a
  pipe or socket) losing info once it advances. So there IS a precedent.
• I feel like the semantics I've implemented in the PR are moderately sane,
  but feel free to point out anything I've missed.
• "Loop forever over the things coming out of this queue until something
  breaks the loop" seems like a common enough use case to warrant some sugar.
• Note this is NOT "loop until the queue is exhausted", unless you turn
  non_block on.
• I am explicitly not discussing the inclusion of Enumerable in this
  ticket. That's a much bigger can of worms and I haven't even begun to think
  through all the implications.
• Note, though, that with this PR if you DO want a full Enumerable over a
  queue all you have to do now is say q.each, omitting the block. So we're
  not making Queue enumerable, but we're making it easy to get at a queue
  Enumerator if you really want one. Without this PR the shortest code I've
  found to do that is:

  q = Queue.new
  eq = Enumerator.new do |y|
    loop do
      y << q.shift
    end
  end

...and this code doesn't cover all the cases that I've covered in the PR.

[avdi (Avdi Grimm)]

BTW, I feel like I've left out the biggest justification for this, which
for me is POLS. What prompted me to submit the PR was that I actually
started writing an example of using Queue#each---it just made sense for it
to exist, so much so I thought I remembered using it before---and then
realized it wasn't there. To me it felt very natural and intuitive to
expect Queue#each to exist. YMMV, obviously, but #each is such a widespread
convention in Ruby that it seems natural to look for it on a Queue class.

OK, I'll shut up now :-)

[ko1 (Koichi Sasada)]

# Please assume
# @q_buf is current buffer (array) of queue in following example.
# @q_lock is lock of this Queue.

People think Queue#each should be:

(1) Infinite loop

# iterate forever
def each
  loop{
    yield @q_buf.shift
  }
end

(2) Finite loop

(2-1) Mutable behavior

# iterate untile elements are exists
def each
  loop{
    e = nil
    @q_lock.lock{
      return if @q_buf.empty?
      e = @q_buf.shift
    }
    yield e
  }
end

(2-2) Immutable behaviour

def each(&b)
  @q_buf.dup.each(&b)
end
# only for chekcking current elements
# not for inter-thraed communication

# Please add another possible version.

Your proposal is (1). (1) is shorter version of

while e = q.pop
  break if e == :end
  ...
end

which I wrote frequently.

q.each{|e|
  break if e == :end
  ...
}

I'm weak negative because
(a) seems not so convinient
(b) #each method is for Enumerable
If we find #each method, then we think it is Enumerable.
(c) we have only few cases for infinite iteration #each methods.
(d) I can think 3 versions described above with in seconds.
(e) I want to use Queue with push' and pop' operations.
Operating with `push' and #each is not consistent.
(Of course, this is my thought. not practical reason)

BTW, I think your `non-blocking' parameter should be a switch of (1) and (2-1).

[ko1 (Koichi Sasada)]

• Assignee set to ko1 (Koichi Sasada)

[zzak (zzak _)]

• File 361.patch 361.patch added

Attaching the patch from Avdi's pull request on github, thank you!

[avdi (Avdi Grimm)]

Thanks! I'm off to a conf this week, it probably would have been a awhile
before I got around to it.

--
Avdi Grimm
http://avdi.org

I only check email twice a day. to reach me sooner, go to
http://awayfind.com/avdi

[ko1 (Koichi Sasada)]

• Category set to lib
• Status changed from Open to Feedback
• Target version set to 2.6

http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/56421
any comments?

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)

[nobu (Nobuyoshi Nakada)]

• Related to Bug #10485: NoMethodError "undefined method `initialize_copy'" when trying to execute Queue#dup added

[ko1 (Koichi Sasada)]

• Status changed from Feedback to Closed

No discussion.