Array#lazy_slice: create a lazy Enumerable from an array range slice.

[tokland]

Does an extensions like this make sense to be included?

require 'enumerable/lazy'

class Array
  def lazy_slice(range)
    Enumerator.new do |yielder|
      range.each do |index|
        yielder << self[index]
      end
    end.lazy
  end
end

big_array = (0..10000).to_a # fake array, it won't generally be a range
p big_array.lazy_slice(9995..10000).map { |x| 2*x }.to_a
#=> [19990, 19992, 19994, 19996, 19998, 20000]

In this example I am trying to highlight that the currently available big_array.lazy.drop(9995).take(6) wouldn't be efficient.

[yhara]

It does make sense when the range is large enough. When the range is small, you can write just some_big_array.slice(range).

But we can't add lazy_slice to Enumerable::Lazy because an Enumerable class does not always have
the indexing method '[]' (For example, the IO class includes Enumerable but does not have IO#[].)

By the way, Enumerable#lazy is accepted as Ruby 2.0 feature. I've updated the readme.

[tokland]

It does make sense when the range is large enough. When the range is small, you can write just some_big_array.slice(range).

Yes, well, no laziness then :-)

But we can't add lazy_slice to Enumerable::Lazy because an Enumerable class does not always have
the indexing method '[]'

Of course, slices on real lazy objects must do a drop+take. Just that's Array#lazy_slice.

By the way, Enumerable#lazy is accepted as Ruby 2.0 feature. I've updated the readme.

I know, I am very excited about it :-)

FYI, I added this method in my fork: https://github.com/tokland/enumerable-lazy

Regards.