Introduction: Immutable collection prototypes for Kotlin
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Immutable collection interfaces and implementation prototypes for Kotlin.

For further details see the proposal.

Prototype implementation is based on pcollections (Copyright 2015 The pcollections Authors.)

What's in this library

Interfaces and implementations

This library provides interfaces for immutable persistent collections:

Interface Bases Implementations
ImmutableCollection Collection
ImmutableList ImmutableCollection, List immutableListOf
ImmutableSet ImmutableCollection, Set immutableSetOf, immutableHashSetOf
ImmutableMap Map immutableMapOf, immutableHashMapOf

The default implementations of ImmutableSet and ImmutableMap, which are returned by immutableSetOf and immutableMapOf preserve the element insertion order during iteration. This comes at expense of maintaining more complex data structures. If the order of elements doesn't matter, more efficient immutableHashSetOf and immutableHashMapOf could be used.



Converts a read-only or mutable collection to an immutable one. If the receiver is already immutable and has the required type, returns itself.

fun Iterable<T>.toImmutableList(): ImmutableList<T>
fun Iterable<T>.toImmutableSet(): ImmutableSet<T>

+ and - operators

plus and minus operators on immutable collections exploit their immutability and delegate the implementation to the collections themselves. The operation is performed with persistence in mind: the returned immutable collection may share storage with the original collection.

val newList = immutableListOf("a", "b") + "c"
// newList is also ImmutableList

Note: you need to import these operators from kotlinx.collections.immutable package in order for them to take the precedence over the ones from the standard library.

import kotlinx.collections.immutable.*


mutate extension function simplifies quite common pattern of immutable collection modification: get a builder, apply some mutating operations on it, transform it back to an immutable collection:

collection.builder().apply { some_actions_on(this) }.build()

With mutate it transforms to:

collection.mutate { some_actions_on(it) }

Using in your projects

Note that these libraries are experimental and are subject to change.

The libraries are published to kotlinx bintray repository.

These libraries require kotlin compiler version to be at least 1.1.0 and require kotlin runtime of the same version as a dependency.


Add the bintray repository to <repositories> section:


Add dependencies (you can also add other modules that you need):



Add the bintray repository:

repositories {
    maven {
        url "http://dl.bintray.com/kotlin/kotlinx"

Add the dependency:

compile 'org.jetbrains.kotlinx:kotlinx-collections-immutable:0.1'

Building from source

To initialize submodules after checkout, use the following commands:

git submodule init
git submodule update

Then you can build and install artifacts to maven local with:

gradlew build install
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