Issue #12648 has been updated by Tsuyoshi Sawada.


When the number of arguments passed is less then the sort keys, then sort should be ascended or descended at a higher array level.

```ruby
[3, 1, 2, 0].sort_by{|e| [e % 2, e]}        # => [0, 2, 1, 3]
[3, 1, 2, 0].sort_by(false){|e| [e % 2, e]} # => [0, 2, 1, 3]
[3, 1, 2, 0].sort_by(true){|e| [e % 2, e]}  # => [3, 1, 2, 0]
```

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Feature #12648: `Enumerable#sort_by` with descending option
https://bugs.ruby-lang.org/issues/12648#change-59880

* Author: Tsuyoshi Sawada
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
I would like to pass an optional argument to `Enumerable#sort_by` or `Enumerable#sort_by!` to allow descending sort. When the sort key is singular, this could be done by passing a single optinal boolean variable that represents ascending when `false` (default) and descending when `true`:

```ruby
[3, 1, 2].sort_by(&:itself)        # => [1, 2, 3]
[3, 1, 2].sort_by(false, &:itself) # => [1, 2, 3]
[3, 1, 2].sort_by(true, &:itself)  # => [3, 2, 1]
```

When there are multiple sort keys, corresponding numbers of arguments should be passed:

```ruby
[3, 1, 2, 0].sort_by{|e| [e % 2, e]}               # => [0, 2, 1, 3]
[3, 1, 2, 0].sort_by(false, false){|e| [e % 2, e]} # => [0, 2, 1, 3]
[3, 1, 2, 0].sort_by(false, true){|e| [e % 2, e]}  # => [2, 0, 3, 1]
[3, 1, 2, 0].sort_by(true, false){|e| [e % 2, e]}  # => [1, 3, 0, 2]
[3, 1, 2, 0].sort_by(true, true){|e| [e % 2, e]}   # => [3, 1, 2, 0]
```




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