Feature #16995: Sets: <=> should be specialized

• Status: Closed
• Priority: Normal
• Assignee: -
• Target version: -

[marcandre (Marc-Andre Lafortune)]

This is quite minor, but Set#<=> should be refined.

Reminder: Set defines <, >, etc. as inclusion, but does not have a corresponding <=>:

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

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 <=>:

Array < Enumerable # => true
Array <=> Enumerable # => -1, ok
[Array, Enumerable].sort # => [Array, Enumerable]
[Array, String].sort # => ArgumentError (comparison of Class with Class failed), ok

[marcandre (Marc-Andre Lafortune)]

• Related to Feature #16989: Sets: need ♥️ added

[zverok (Victor Shepelev)]

• Status changed from Open to Closed

Implemented in Ruby 3.0:

Set[1] <=> Set[1, 2]
# => -1 
[Set[1], Set[1, 2]].sort
# => [#<Set: {1}>, #<Set: {1, 2}>]