From: "akr (Akira Tanaka)" <akr@...>
Date: 2013-10-02T11:46:40+09:00
Subject: [ruby-core:57577] [ruby-trunk - Feature #8840] Yielder#state


Issue #8840 has been updated by akr (Akira Tanaka).


marcandre (Marc-Andre Lafortune) wrote:
> 
> The proposition of `with_state` is interesting, but I personally find it leads to complex/convoluted solutions and is cumbersome to use. Note that the given implementation of `drop2` is slightly incomplete as it needs to return a lazy enumerator, so `Enumerator.new` needs to be followed by a call to `lazy`.

Would��you��explain��the��incompleteness��concretely?
I��couldn't��understand.

----------------------------------------
Feature #8840: Yielder#state
https://bugs.ruby-lang.org/issues/8840#change-42192

Author: marcandre (Marc-Andre Lafortune)
Status: Feedback
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: core
Target version: current: 2.1.0


Defining an Enumerator that require a state is currently troublesome. For example, it is not really possible to define an equivalent of Lazy#drop in Ruby without making an assumption on the implementation.

To address this, I propose that we

(a) guarantee that a new Yielder object will be given for each enumeration
(b) add a 'state' attribute to Yielder.

This way, one could implement Lazy#drop in a way similar to:

  class Enumerator::Lazy < Enumerator
    def drop(n)
      n = n.to_i
      Lazy.new(self) do |yielder, *values|
        yielder.state ||= n
        if yielder.state > 0
          yielder.state -= 1
        else
          yielder.yield(*values)
        end
      end
    end
  end

Note that (a) is currently true for Ruby MRI, JRuby and Rubinius, but it is not explicit in the documentation.


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