Feature #5574: Make arrays comparable

• Status: Rejected
• Assignee: matz (Yukihiro Matsumoto)
• Target version: -

[tokland (Arnau Sanchez)]

Why are arrays not comparable? Array already defines <=>, so it seems only natural to mix-in the Comparable module.

> [1, 2] < [1, 1] 
NoMethodError: undefined method `<' for [1, 2]:Array

> Array.send(:include, Comparable)
> [1, 2] < [1, 1] 
=> false

This was already discussed in the mailing list a while ago and people agreed it would be useful:

http://www.ruby-forum.com/topic/76535

[duerst (Martin Dürst)]

I support this proposal. In a basic programming lecture, I just recently told students how to compare (<, <=, and so on) numbers and strings, and then mentioned that it would work the same way for Arrays. But then when I wanted to show this, it didn't work.

I was quite surprised, because I had sorted Arrays many times before. Then I vaguely remembered: Ah, yes, there's a <=> operator for Arrays, but if you want to use <, <=, ..., you have to include Comparable. It's the way it currently is, but it's totally unclear why, and it's difficult to remember.

[trans (Thomas Sawyer)]

Ruby Facets provides a Tuple class, which is an immutable Array that is also Comparable. I believe I may have suggested Ruby provide such a class in the past. If Array can not be Comparable for some reason, perhaps this is a reasonable alternative.

[trans (Thomas Sawyer)]

Ruby Facets provides a Tuple class, which is an immutable Array that
is also Comparable. I believe I may have suggested Ruby provide such a
class in the past. If Array can not be Comparable for some reason,
perhaps this is a reasonable alternative.

[neleai (Ondrej Bilka)]

I remember that in ruby-core long ago matz explained that array don't include comparable because they could contain elements that are not comparable.
I dont see this as problem as this is violated in most classes when comparing field that can be nil.

[shevegen (Robert A. Heiler)]

Ok so it does not yet exist because it can contain elements that are not comparable.

But in the example given with the 1, 2] < [1, 1] all elements are comparable. Ruby wants you to include Comparable.

But even when you do that, and then try to compare an invalid Array element:

[1, 2] < [1, "adc"]

You get an argument error:

ArgumentError: comparison of Array with Array failed

So I think the better way would be to include Comparable module in Array?

[mame (Yusuke Endoh)]

• Status changed from Open to Assigned
• Assignee set to matz (Yukihiro Matsumoto)

[matz (Yukihiro Matsumoto)]

• Status changed from Assigned to Rejected

Since arrays are not always compatible, I don't want to give wrong implication by making arrays comparable.
Adding <=> was a realistic compromise for me to allow array sorting.

Matz.