Issue #17312 has been updated by zverok (Victor Shepelev).


> But even with Enumerator the recursive aspect still represents a compatibility problem.

I am not sure about its severity, though. I mean, Universe is big and sure somewhere in it there should be a code which has an *array of enumerators* and then does `flatten` on them... But I am not sure there is much of this code in the wild. 

I believe that this situation has the similar rarity class as the situation with code which does `unless obj.respond_to?(:except)` and will be broken by newly introduced `Hash#except` method... Like, *every* change is incompatibility for somebody, as https://xkcd.com/1172/ points, but `Enumerator#flatten` seems quite innocent.

> So as long as the behavior of Array#flatten is not modified I think all this is trivial to implement:
```ruby
def flatten(...); to_a.flatten(...); end
```

Note that this ticket is a follow-up of #16987. What I interested in, is more usages for `.lazy`, and *eager* implementation of `Enumerator::Lazy#flatten` is definitely a no-go.

So, I actually *could* propose just `Enumerator::Lazy#flatten`, but it seems quite weird that lazy enumerator can be flattened, while regular one can't.


----------------------------------------
Feature #17312: New methods in Enumerable and Enumerator::Lazy: flatten, product, compact
https://bugs.ruby-lang.org/issues/17312#change-88563

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
----------------------------------------
(The offspring of #16987, which was too vague/philosophical)

I propose to add to `Enumerable` and `Enumerator::Lazy` the following methods:
* `compact`
* `product`
* `flatten`

All of them can be performed with a one-way enumerator. All of them make sense for situations other than "just an array". All of them can be used for processing large sequences, and therefore meaningful to add to `Lazy`.



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