From: Brian Candler <b.candler@...>
Date: 2009-11-11T00:47:26+09:00
Subject: Re: RCR enumerable extra into core

Roger Pack wrote:
> we currently have
> list.map => Enumerator
> 
> so should
> list.map(:method)
> 
> best return an array or an enumerator?

The ultimate conclusion of this: all Enumerable methods should return 
other Enumerators. When you want a real array, then add .to_a to the 
end.

> (though I don't really understand 
> why enumerators are cool, apparently they've become quite common lately)

It's cool because evaluation takes place "left to right" instead of 
generating potentially huge intermediate arrays. You start getting 
answers sooner, you can write results out to disk or a socket without 
buffering, and you can deal with infinite lists, but using the same 
syntax.

e.g.
    a = (1..1/0.0).select{ |i| i % 2 == 0 }.map{ |i| i + 100 
}.take(10).to_a

Implementing this turns out to be easier than you'd think, and is 
efficient. No funky Fibers or Continuations required.

http://github.com/trans/facets/blob/master/lib/core/facets/denumerable.rb
http://github.com/trans/facets/blob/master/lib/core/facets/enumerable/defer.rb
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