[ruby-core:76436] [Ruby trunk Bug#12466] Enumerable should yield multiple values when possible

Issue #12466 has been updated by Nobuyoshi Nakada.

Description updated

IIRC, these methods yielded multiple values once, but reverted due to the backward compatibilities.

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Bug #12466: Enumerable should yield multiple values when possible
https://bugs.ruby-lang.org/issues/12466#change-59682

* Author: Stefan Schテシテ殕er
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: 2.3.1
* Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
----------------------------------------
Some methods in `Enumerable` and `Array` yield arrays instead of multiple values, e.g.:

```ruby
[1, 2, 3, 4].each_cons(2).peek_values #=> [[1, 2]]
```

If `each_cons` would yield multiple values (i.e. `yield 1, 2` instead of `yield [1, 2]`), we could write:

```ruby
[1, 2, 3, 4].each_cons(2).map(&:quo) #=> [(1/2), (1/3), (1/4)]
```

But currently, this results in an exception and we have to provide a block to achieve the above:

```ruby
[1, 2, 3, 4].each_cons(2).map { |a, b| a.quo(b) } #=> [(1/2), (2/3), (3/4)]
```

No big deal, but it looks a bit cumbersome (in Ruby terms) and I don't see how the current behavior is preferable.

Another example is `reverse_each` which cripples yielded values:

```ruby
def each_two_values
  return enum_for(__method__) unless block_given?
  yield 1, 2
  yield 2, 3
  yield 3, 4
end

each_cons_values.map(&:quo) #=> [(1/2), (2/3), (3/4)]

each_cons_values.reverse_each.map(&:quo) #=> NoMethodError: undefined method `quo' for [3, 4]:Array
```

Of course, I can easily provide my own implementation:

```ruby
module Enumerable
  def my_reverse_each
    return enum_for(__method__) unless block_given?
    map { |*values| values }.reverse.each { |values| yield *values }
  end
end

each_cons_values.my_reverse_each.map(&:quo) #=> [(3/4), (2/3), (1/2)]
```

But shouldn't this be the default behavior?

If an array is actually needed (instead of multiple values), there's `Enumerable#each_entry` which performs the conversion.

BTW, other methods _do_ yield multiple values:

```ruby
%w(a b c).each.with_index(1).peek_values #=> ["a", 1]
```




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