[ruby-core:106311] [Ruby master Feature#18366] Enumerator#return_eval

Issue #18366 has been updated by shan (Shannon Skipper).


Just a thought, but another option to achieve the aims of this proposal might be to add `return_eval: true` kwargs for Enumerable methods. On the transducer front Brandon mentions, I've wished we had Enumerable kwargs to set the reducing function and an accumulator other than an Array.

For example.

``` ruby
module TransducerSelect
  refine Array do
    def select(acc: [], step: :<<, step_eval: false)
      unless block_given?
        return to_enum(__method__) { size if respond_to?(:size) }
      end

      each do
        yielded = yield _1
        step_value = step_eval ? yielded : _1
        acc.public_send(step, step_value) if yielded
      end

      acc
    end
  end
end

using TransducerSelect

["Ms. Foo", "Dr. Bar", "Baz"].select(step_eval: true){_1[/\b[A-Z]\w+\./]}
#=> ["Ms.", "Dr."]

["Ms. Foo", "Dr. Bar", "Baz"].select acc: Set.new, step: :add, step_eval: true do
  _1[/\b[A-Z]\w+\./]
end
#=> #<Set: {"Ms.", "Dr."}>

["Ms. Foo", "Dr. Bar", "Baz"].select acc: $stdout, step: :puts, step_eval: true do
  _1[/\b[A-Z]\w+\./]
end
#>> Ms.
#>> Dr.
#=> #<IO:<STDOUT>>
```


----------------------------------------
Feature #18366: Enumerator#return_eval
https://bugs.ruby-lang.org/issues/18366#change-94939

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
----------------------------------------
Some `Enumerable` methods return one or more of the receiver's elements according to the return value of a block it takes. Often, we want such evaluated value rather than the original element.

For example, suppose we want to know the character width sufficient to fit all the strings in an array:

```ruby
a = ["Hello", "my", "name", "is", "Ruby"]
```

We either have to repeat the evaluation of the block:

```ruby
a.max_by(&:length).length # => 5
```

or create a temporal array:

```ruby
a.map(&:length).max # => 5
```

both of which seem not to be optimal.

I propose to have a method `Enumerator#return_eval` that returns the evaluated value(s) of the block:

```ruby
a.max_by.return_eval(&:length) # => 5
a.min_by.return_eval(&:length) # => 2
a.minmax_by.return_eval(&:length) # => [2, 5]
["Ava Davidson", "Benjamin Anderson", "Charlie Baker"]
.sort_by.return_eval{_1.split.reverse.join(", ")}  # => ["Anderson, Benjamin", "Baker, Charlie", "Davidson, Ava"]
```




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