From: "nobu (Nobuyoshi Nakada)" <noreply@...>
Date: 2022-08-25T05:35:18+00:00
Subject: [ruby-core:109677] [Ruby master Bug#18971] Enumerator::Lazy.take(0) leaks first element into next operation

Issue #18971 has been updated by nobu (Nobuyoshi Nakada).


Although not so elegant, I was thinking to add "precheck" to enumerators.
https://github.com/nobu/ruby/tree/lazy_take0

----------------------------------------
Bug #18971: Enumerator::Lazy.take(0) leaks first element into next operation
https://bugs.ruby-lang.org/issues/18971#change-98901

* Author: Voileexperiments (Library Voile)
* Status: Open
* Priority: Normal
* ruby -v: 3.0.0p0 (2020-12-25 revision 95aff21468)
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
These results with lazy enumerators are as expected:

``` ruby
(2..10).take(0).to_a # => []
(2..10).take(0).map(:&itself).to_a # => []
(2..10).lazy.take(0).to_a # => []
```

However, once another operation is added after `take(0)`, if first element will pass through directly it will leak through:

``` ruby
(2..10).lazy.take(0).map(&:itself).to_a # => [2]
(2..10).lazy.take(0).select(&:even?).to_a # => [2]
(2..10).lazy.take(0).select(&:odd?).to_a # => []
(2..10).lazy.take(0).reject(&:even?).to_a # => []
(2..10).lazy.take(0).reject(&:odd?).to_a # => [2]
(2..10).lazy.take(0).take(1).to_a # => [2]
(2..10).lazy.take(0).take(0).take(1).to_a # => [2]
(2..10).lazy.take(0).drop(0).to_a # => [2]
(2..10).lazy.take(0).find_all {|_| true}.to_a # => [2]
(2..10).lazy.take(0).zip((12..20)).to_a # => [[2, 12]]
(2..10).lazy.take(0).uniq.to_a # => [2]
(2..10).lazy.take(0).sort.to_a # => []
(2..2).lazy.take(0).sort.to_a # => []
```

Non lazy versions all return `[]` as expected.

In 3.1.0 All of them behave as expected as well:

``` ruby
(2..10).lazy.take(0).map(&:itself).to_a # => []
(2..10).lazy.take(0).select(&:even?).to_a # => []
(2..10).lazy.take(0).select(&:odd?).to_a # => []
(2..10).lazy.take(0).reject(&:even?).to_a # => []
(2..10).lazy.take(0).reject(&:odd?).to_a # => []
(2..10).lazy.take(0).take(1).to_a # => []
(2..10).lazy.take(0).take(0).take(1).to_a # => []
(2..10).lazy.take(0).drop(0).to_a # => []
(2..10).lazy.take(0).find_all {|_| true}.to_a # => []
(2..10).lazy.take(0).zip((12..20)).to_a # => []
(2..10).lazy.take(0).uniq.to_a # => []
(2..10).lazy.take(0).sort.to_a # => []
(2..2).lazy.take(0).sort.to_a # => []
```



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