[ruby-core:75871] [Ruby trunk Bug#12466] Enumerable should yield multiple values when possible

Issue #12466 has been reported by Stefan Schテシテ殕er.

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

* 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
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Some methods in `Enumerable` and `Array` yield arrays instead of multiple values, e.g.:

    [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:

    [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:

    [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:

    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:

    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:

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



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