Why aren't Enumerators in Ruby (2.0 +) lazy by default?
ruby, ruby-2.0
Solution
Make laziness the default would be a breaking change for any code that expects/relies on side-effects.
Any previous usage of Enumerator would need to be examined and tested. The "safest"/"sane" approach is simply to keep the non-lazy behavior as the default - while providing a Lazy alternative.
Some fundamental/previous design decisions are not so easily discarded; Enumerators are supported as far back as Ruby 1.8.7.
Problem
Why don't Ruby `Enumerator` behave like `Enumerator::Lazy` by default? Is there any case where someone would want use a non-lazy `Enumerator`? Edited: A comment to backward compatibility answers below, explaining why I'm not yet convinced: Say we had added these 'breaking' changes to Ruby 2.0.0, which is a major version, you would test your code (especially if you're going to production) thoroughly before making the switch, no? Edit#2 I suspected it has to do with efficiency (let me know if anything is wrong), so I did the following benchmark: (of course there are places where lazy is better. this is to demonstrate, potentially, why Ruby is not using lazy all the time?) ``` require 'fruity' require 'prime' compare do lazy { g = Prime::EratosthenesGenerator.new; 1000.times { g.lazy.take(100).to_a } } diligent { g = Prime::EratosthenesGenerator.new; 1000.times { g.take(100).to_a } } end ``` I get the following: (maybe due to the calling of `.lazy` method instead of using `Enumerator::Lazy` from the beginning?) ``` diligent is faster than lazy by 19.999999999999996% ± 1.0% ```