google/built_collection.dart

Dart Package: Immutable lists, sets, maps, and multimaps, which are as fast as their native mutable counterparts. Extension methods and comparators for native Dart collections.

A port of kotlin-stdlib for Dart/Flutter including immutable collections (KtList, KtMap, KtSet) and other packages

Immutable persistent collections for Kotlin

The Capsule Hash Trie Collections Library

Immutable value types, enum classes, and serialization.

Immutable persistent data collections for Javascript which increase efficiency and simplicity.

Google core libraries for Java

Immutable and Memory-Efficient Maps and Sets in Go

Code generation for immutable classes that has a simple syntax/API without compromising on the features.