集合与有序集合

Question

Can you please give me an example when to use OrderedSet instead of Set ? I've run a couple of tests and even tho the immutable-js documentation says

Iteration order of a Set is undefined, however is stable

it seems element order within Set is always the same as the one in which elements were added. That's what seems to be the sole benefit of the OrderedSet structure:

A type of Set that has the additional guarantee that the iteration order of values will be the order in which they were added.

Answer 1

It coincidally does add elements at the end, but it is not guaranteed to always be that way. It might change in the next release and it does not always have to be predictable. All it promises is to be stable across multiple iterations over the same data. To be honest, I can't see any useful use case for OrderedSet. Depending on the needs, List , Map or OrderedMap or even Set are better suited.

If you manage to update a Set, it can change the order. Again, this is usually a bad choice for a data structure and you probably should rearrange your data structure, eg use OrderedMap or List instead.

The following sample shows that the order can be a bit unexpected:

 function modifySet(set) { set = set.add(0); set = set.add(1); set = set.add(2); return set.remove(0); } let unorderedSet = Immutable.Set([4,5]); unorderedSet = modifySet(unorderedSet); console.log('Set:'); for (const value of unorderedSet) { console.log(value); } let orderedSet = Immutable.OrderedSet([4,5]); orderedSet = modifySet(orderedSet); console.log('OrderedSet:'); for (const value of orderedSet) { console.log(value); }

 <script src="https://cdnjs.cloudflare.com/ajax/libs/immutable/4.0.0-rc.12/immutable.js"></script>

Since you can modify keys (,) of a set: it can reorder the elements too:

 let set = Immutable.Set([ Immutable.Map({b:1, a:true}), Immutable.Map({b:2,a:true}), Immutable.Map({b:3,a:true}) ]).map((t) => { if (t.get('b') === 2) return t.set('a', false); return t; }); console.log('2 is now at the end'); console.log(set.toJS());

 <script src="https://cdnjs.cloudflare.com/ajax/libs/immutable/4.0.0-rc.12/immutable.js"></script>

To add insult to injury, there is a bug in ImmutableJs RC12, which makes OrderedSet behave in the same way (moving the updated element to the end of the list). That one is already fixed in the (so far) unreleased upcoming 4.0 release.

Ok that was an interesting excursion, you made us (the loose group of maintainers) look again into how this rarely used structure works.