In ruby, why is Set unordered while Hash is guaranteed in insertion order?

Hash in ruby 2.5.1:

Hashes enumerate their values in the order that the corresponding keys were inserted.

Set in ruby 2.5.1:

Set implements a collection of unordered values with no duplicates.

I thought set was implemented as a Hash in ruby. Then why is Set unordered while Hash is guaranteed in insertion order?

UPDATE:

I understand that mathematically sets are supposed to be unordered.

An important question arises, even though the Set class is actually in insertion order, it is not documented. I think the documentation should be updated.

Set

Even in the Set#add source, we can see it's actually using Hash:

Set#add

Hash

# File set.rb, line 348

def add(o)
@hash[o] = true
self
end


Note: I don't mean to spark off discussion, but think it's important that this fact is documented somewhere. If not in the official docs, atleast on stack overflow.

1 Answer
1

Set is semantically unordered; a compliant implementation could build it atop an actually-unordered core. (I'm not sure what JRuby does, for example, though I imagine they probably use the same Hash-based implementation as CRuby.)

As you note, Set is in practice implemented using Hash -- which means it will in fact preserve insertion order... it's just not guaranteed by the specification.

If you're asking why it's defined that way, it's very possible that documentation predates guaranteed Hash ordering and has never been revised (and equally possible that it's a conscious choice). It was recently raised in https://bugs.ruby-lang.org/issues/14069, but has seen no discussion so far.

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