[swift-evolution] Introduction of OrderedSet

[Maik Koslowski]

Hello,

in the past there have been a few requests for an OrderedSet implementation in Swift. In the proposal https://github.com/apple/swift-evolution/blob/master/proposals/0069-swift-mutability-for-foundation.md was mentioned that the OrderedSet will be considered for the feature.

However, since then there were a few discussions on OrderedSet but it doesn’t get much attention and there wasn’t any comment about it from the swift team.

I want to bring up some points, why an OrderedSet is needed in the base library.

1. CoreData is probably the most obvious place where people would use an ordered set. Especially when working with large amounts of data, presorting can save a lot of time and battery life. If a bridgeable ordered set was part of the standard library we could use a ordered set in swift without having to use the NSOrderedSet from objective c. Which would be pretty nice in my opinion. Even when using a NSOrderedSet we couldn’t have a generic version of it.

2. A shared datamodel between App and Server. One main advantage of having web servers written in Swift is that we can share code between the server and the app. For servers performance does matter a lot, since they are usually working with much more data than apps. Databases are represented as sets and fetching sorted data from the database can be represented as an ordered set. However, since we don’t have ordered sets we have to choose either a normal set or an array. Sets don’t have an order and arrays can contain the same object multiple times, which makes them both a less suitable choice.

3. Swift has the potential to be used for education. There is a lot of support, for example the playground app on iPad. When it comes to the theory behind data structures and algorithms or to the theory of computation a defined order plays an important role.

The biggest issue is that we always have to copy data from a set into an array to have it in a sorted order with losing the safety of uniqueness. Which is not suitable for a safe and performance oriented programming language at all.

Last but not least, it fits in the goals of Swift 4 stage 2 and an ordered set can be found in other popular programming languages, too.

What do you think?

Best regards,

Maik

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