Issue #16995 has been updated by zverok (Victor Shepelev). Status changed from Open to Closed Implemented in Ruby 3.0: ```ruby Set[1] <=> Set[1, 2] # => -1 [Set[1], Set[1, 2]].sort # => [#<Set: {1}>, #<Set: {1, 2}>] ``` ---------------------------------------- Feature #16995: Sets: <=> should be specialized https://bugs.ruby-lang.org/issues/16995#change-100927 * Author: marcandre (Marc-Andre Lafortune) * Status: Closed * Priority: Normal ---------------------------------------- This is quite minor, but `Set#<=>` should be refined. Reminder: `Set` defines `<`, `>`, etc. as inclusion, but does not have a corresponding `<=>`: ```ruby Set[1] < Set[1, 2] # => true Set[1] <=> Set[1, 2] # => nil, should be -1 Set[1] <=> Set[2] # => nil, ok, not orderable ``` The official stated reason for `Set` to *not* implement is that some sets are not comparable. That is exactly what `nil` result type is for IMO. Sets are partically ordered and `<=>` should reflect that. https://en.wikipedia.org/wiki/Partially_ordered_set ```ruby Set[1] < Set[1, 2] # => true [Set[1], Set[1, 2]].sort # => ArgumentError, should be [Set[1], Set[1, 2]] [Set[1], Set[2]].sort # => ArgumentError, ok, can't be ordered ``` This is *exactly the same* idea as `Class`, which correctly refines `<=>`: ```ruby Array < Enumerable # => true Array <=> Enumerable # => -1, ok [Array, Enumerable].sort # => [Array, Enumerable] [Array, String].sort # => ArgumentError (comparison of Class with Class failed), ok ``` -- https://bugs.ruby-lang.org/