[ruby-core:93117] [Ruby trunk Feature#15901] Enumerator::Lazy#eager

Issue #15901 has been updated by marcandre (Marc-Andre Lafortune).


zverok (Victor Shepelev) wrote:
> > How about this style?
> 
> ```ruby
> [0, 1, 2].lazy {|e| e.map {|n| n + 1 }.map {|n| n.to_s } }
> #=> an Enumerator containing "1", "2", and "3"
> ```
> 
> I believe that it defies the whole idea of lazy enumerators. instead of lazy enums being "just enums", it requires wrapping everything

I don't see how the suggestion defies anything, changes the current `lazy` method (without block) and or "requires" anything. It simply extends it with a block form that would automatically return an eager enumerator.

My issue with `lazy{}` is that it forces the user to return a lazy enumerator from the block (otherwise it raises?, e.g. `[1].lazy{ 42 } # => RuntimeError?`). The fact that the result could be completely unrelated is for me a design smell, e.g. `[1].lazy { [1, 2, 3].lazy }`.

I'm personally +1 for `eager`. I'd also alias `Enumerator#eager` to `#itself`.


----------------------------------------
Feature #15901: Enumerator::Lazy#eager
https://bugs.ruby-lang.org/issues/15901#change-78543

* Author: knu (Akinori MUSHA)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
----------------------------------------
There are cases where you want to create and pass a normal Enumerable object to a consumer where the methods like map and select are expected to return an array, but the calculation would be so space costly without using Enumerator::Lazy because of intermediate arrays.  In such cases, you would want to chain `lazy` and calculation methods like `flat_map` and `select`, then convert the lazy enumerator back to a normal Enumerator.

However, there is no direct method that converts a lazy Enumerator to an eager one, because the` to_enum` method returns a lazy Enumerator when the receiver is a lazy Enumerator.  So, I propose this `eager` method as the missing piece for the said use case.

Here's the rdoc from the attached patch.

```C
/*
 * call-seq:
 *   lzy.eager -> enum
 *
 * Returns a non-lazy Enumerator converted from the lazy enumerator.
 *
 * This is useful where a normal Enumerable object needs to be
 * generated while lazy operation is still desired to avoid creating
 * intermediate arrays.
 *
 *   enum = huge_collection.lazy.flat_map(&:children).reject(&:disabled?).eager
 *   enum.map {|x| ...}  # an array is returned
 */
```

---Files--------------------------------
0001-Implement-Enumerator-Lazy-eager.patch (2.32 KB)


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