Issue #10475 has been updated by Nobuyoshi Nakada. I agree that this is a documentation issue. But afraid that the negative value is worth to mention. ---------------------------------------- Bug #10475: Array#flatten should not accept a nil argument https://bugs.ruby-lang.org/issues/10475#change-50072 * Author: Dov Murik * Status: Open * Priority: Normal * Assignee: Yukihiro Matsumoto * Category: core * Target version: * ruby -v: ruby 2.1.4p265 (2014-10-27 revision 48166) [x86_64-linux] * Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN ---------------------------------------- Currently `Array#flatten` accepts an explicit `nil` argument which has the same meaning as `-1` (or no arguemnt) - meaning endless recursive flattening. I'd expect `flatten` to accept an integer argument only (or not at all, with a default of `-1`). Moreover, the behaviour of `arr.flatten(-1)` is not described in the method's documentation. **Current behaviour** (Ruby 2.1.4): ~~~ 2.1.4 :001 > arr = [1, [2, [3, [4, 5]]]] => [1, [2, [3, [4, 5]]]] 2.1.4 :002 > arr.flatten => [1, 2, 3, 4, 5] 2.1.4 :003 > arr.flatten(-1) => [1, 2, 3, 4, 5] 2.1.4 :004 > arr.flatten(nil) => [1, 2, 3, 4, 5] ~~~ **Expected behaviour**: * The last call (`arr.flatten(nil)`) should raise ArgumentError exception. * The -1 (as "indefinite") should be documented. Note that this suggestion will break applications/gems that rely on the fact that `arr.flatten(nil)` is identical to `arr.flatten`. I'd like to hear your opinions on this. If accepted, I'm willing to try and write a patch. -- https://bugs.ruby-lang.org/