From: Ricardo Panaggio <panaggio.ricardo@...>
Date: 2010-07-22T21:59:12+09:00
Subject: Re: Enumerators and generators

On Thu, Jul 22, 2010 at 07:11, David A. Black <dblack@rubypal.com> wrote:
> Hi --
>
> On Thu, 22 Jul 2010, Ricardo Panaggio wrote:
>
>> Hello Hal
>>
>> Generators "externalize" iteration, whereas Enumerator "internalize"
>> it (but this docs may have already told you).
>>
>> Generators can be used to create a "controlled stream" of objects,
>> that you can operate as you where using a remove control: go to the
>> next (Generator#next), give me the position (Generator#pos), return to
>> the beginning (Generator#rewind), ...
>
> Enumerators have #next and #rewind too, though (though not #pos). In
> 1.9, as far as I can tell, the generator.rb library has been removed,

I wasn't aware of it in 1.9. Good to now :)

> and Generator is now a class inside Enumerator. An Enumerator::Generator
> is created automatically, for the use of the enumerator, if you create
> an enumerator with a block. Also, these Generators don't have #next and
> friends; that's available via the enumerator. (I'm not sure what
> happened to #pos.)
>
> As I understand it, the main thing about generators is that "controlled
> stream" thing, where you can roll your own sense of iteration, rather
> than just piggy-backing on what some enumerable object with its own
> ideas about iteration thinks. Here's a (very contrived) 1.9 example:
>
> [dblack@ruby-versions ~]$ cat e.rb message = nil
>
> g = Enumerator::Generator.new do |yielder|
>  yielder << "Hi."
>  puts "I've been told to #{message}."
>  case message
>  when "leave"
>    yielder << "Bye."
>  when "stay"
>    yielder << "I'm still here!"
>  end
> end
>
> e = Enumerator.new(g)
>
> puts e.next
> message = "stay"
> puts e.next
>
> e.rewind
>
> puts e.next
> message = "leave"
> puts e.next
>
> [dblack@ruby-versions ~]$ ruby e.rb Hi.
> I've been told to stay.
> I'm still here!
> Hi.
> I've been told to leave.
> Bye.
>
>
> David
>
> --
> David A. Black, Senior Developer, Cyrus Innovation Inc.
>
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>
>