Issue #15901 has been updated by knu (Akinori MUSHA).


Suppose I want to pass the `enum` to another method that takes an Enumerable object where the passed object is expected to act eagerly.  One of the most significant use cases is when the callee uses `take_while` on a given object expecting to receive the result as an array, where a normal Enumerable object that generates a list lazily is ideal.

My understanding is that Lazy is kind of a low-level tool for implementing efficient pipelines and not normally part of a method API, so being able to convert a lazy Enumerator to an eager Enumerator helps users write efficient code while not requiring library developers to support Lazy in each method that takes an Enumerable.

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Feature #15901: Enumerator::Lazy#eager
https://bugs.ruby-lang.org/issues/15901#change-78372

* Author: knu (Akinori MUSHA)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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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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