Issue #18262 has been updated by Dan0042 (Daniel DeLorme).


+1
Since a lazy enumerator is produced for both #select and #reject, it would make sense for #partition as well.

----------------------------------------
Feature #18262: Enumerator::Lazy#partition
https://bugs.ruby-lang.org/issues/18262#change-94257

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
----------------------------------------
(Part of my set of proposals about making `.lazy` more useful/popular.)

Currently:
```ruby
file = File.open('very-large-file.txt')
lines_with_errors, lines_without_errors = file.lazy.partition { _1.start_with?('E:') }
lines_with_errors.class
# => Array, all file is read by this moment
```
This might be not very practical performance-wise and memory-wise.

I am thinking that maybe returning a pair of lazy enumerators might be a good addition to `Enumerator::Lazy`

Naive prototype:

```ruby
class Enumerator::Lazy
  def partition(&block)
    buffer1 = []
    buffer2 = []
    source = self

    [
      Enumerator.new { |y|
        loop do
          if buffer1.empty?
            begin
              item = source.next
              if block.call(item)
                y.yield(item)
              else
                buffer2.push(item)
              end
            rescue StopIteration
              break
            end
          else
            y.yield buffer1.shift
          end
        end
      }.lazy,
      Enumerator.new { |y|
        loop do
          if buffer2.empty?
            begin
              item = source.next
              if !block.call(item)
                y.yield(item)
              else
                buffer1.push(item)
              end
            rescue StopIteration
              break
            end
          else
            y.yield buffer2.shift
          end
        end
      }.lazy
    ]
  end
end
```
Testing it:
```ruby
Enumerator.produce(1) { |i| puts "processing #{i}"; i + 1 }.lazy
  .take(30)
  .partition(&:odd?)
  .then { |odd, even|
    p odd.first(3), even.first(3)
  }
# Prints:
# processing 1
# processing 2
# processing 3
# processing 4
# processing 5
# [1, 3, 5]
# [2, 4, 6]
```
As you might notice by the "processing" log, it only fetched the amount of entries that was required by produced enumerators.

The **drawback** would be―as my prototype implementation shows―the need of internal "buffering" (I don't think it is possible to implement lazy partition without it), but it still might be worth a shot?



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