Persistent vs Immutable Collections

You may have heard about a helpful new collections library added in Kotlinx, specifically the kotlinx.collections.immutable library. In that library there are Persistent and Immutable versions of common collection classes like List. So which version should you use? Does it matter?

Persistent and Immutable #

A quick overview of terms is the best place to start before we cover guidelines. A Persistent collection is one that can only be modified by making updates to the underlying data structures pointers which now link the unaffected parts of the collection with new updated parts. On the other hand, an Immutable collection is one that cannot be changed under any circumstance. Once the collection exists, its contents are fixed. In most cases, including in the Kotlinx Immutable Collections library, Immutable collections inherit from Persistent collections.

Aside - Readonly Collections #

A brief aside, because you may be thinking something like "Hey! I thought Kotlin collections already were immutable? I know I can't make updates to a List, only a MutableList!". While this is technically correct, the default collections are only read only, that is to say the object cannot be modified, but says nothing about the underlying data structure implementation and how changes will affect that structure. So while Immutable collections are also read only, Persistent collections are not.

Conclusion #

In conclusion, what should you use? For general use, the built in read only base collections should cover all your use cases, there is additional overhead to copy and maintain an Immutable or Persistent collection as compared to the base collections. But if you need persistence, such as in the example of Compose rendering, you just need to ask if you want mutability on the collection. If yes, reach for Persistent collections, if no, reach for Immutable collections. Until next time, thanks!

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