Issue #10475 has been updated by Jared Beck. `flatten` also accepts `Float` values, apparently using `to_i` (aka. `truncate`) to convert to `Integer`. irb(main):008:0> arr.flatten(0.9) => [1, [2, [3, [4, 5]]]] irb(main):009:0> arr.flatten(1.1) => [1, 2, [3, [4, 5]]] irb(main):011:0> arr.flatten(-0.9) => [1, [2, [3, [4, 5]]]] irb(main):012:0> arr.flatten(-1.1) => [1, 2, 3, 4, 5] Given that `to_i` (or something like it) is used on `Float` values, it's odd that `nil` is treated as `-1`, rather than `0`. ---------------------------------------- Bug #10475: Array#flatten should not accept a nil argument https://bugs.ruby-lang.org/issues/10475#change-49984 * Author: Dov Murik * Status: Open * Priority: Normal * Assignee: * 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/