From: "David A. Black" <dblack@...>
Date: 2010-07-22T19:11:25+09:00
Subject: Re: Enumerators and generators

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,
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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