Immutable Collections Library for Kotlin
In the Keynote...
Immutable Collections Library for Kotlin
- One of the kotlinx libraries
- Provides "Immutable collection interfaces and implementation prototypes"
- https://github.com/Kotlin/kotlinx.collections.immutable
- Current version: 0.2
- Maybe 0.3 will be released soon...?
- 0.3 includes MPP support
- "Will be part of the standard library in the future" (from keynote)
Collections in Standard Library Are Not Immutable
• Those interfaces gives read-only APIs, but their instances can be modified
• e.g. In Kotlin/JVM, the instances are actually Java collections in java.util
• They can be casted into mutable collections
• Further reading: The Mystery of Mutable Kotlin Collections - ProAndroidDev
Collections in Standard Library Are Not Immutable
Collections in Standard Library Are Not Immutable
Collections in Standard Library Are Not Immutable
Interfaces
Two families of collections + collection builder:
- Immutable collections
- Read-only APIs, not modifiable implementations
- Persistent collections
- Immutable collections + modification operation APIs (e.g. set, add, removeAt)
- Collection Builders
- Builders to create new instances with specified mutation applied to the original collections
Interfaces: Immutable Collections
| Interface | Base Interface(s) |
|---|---|
| ImmutableCollection | Collection |
| ImmutableList | ImmutableCollection, List |
| ImmutableSet | ImmutableCollection, Set |
| ImmutableMap | Map |
Interfaces: Persistent Collections
| Interface | Base Interface(s) |
|---|---|
| PersistentCollection | ImmutableCollection |
| PersistentList | PersistentCollection, ImmutableList |
| PersistentSet | PersistentCollection, ImmutableSet |
| PersistentMap | ImmutableMap |
Interfaces: Collections Builders
| Interface | Base Interface(s) |
|---|---|
| PersistentCollection.Builder | MutableCollection |
| PersistentList.Builder | PersistentCollection.Builder, MutableList |
| PersistentSet.Builder | PersistentCollection.Builder, MutableSet |
| PersistentMap.Builder | MutableMap |
Immutable + Modification Operation?
- Modification functions return new instances to which the modification is applied
- Efficient new instance creation by "structural sharing"
- Shares common parts with the original instance
- Returns the same instance if the applied operation did not actually modify it
Implementations of Interfaces
- Performance-considered data structure
- Details: https://github.com/Kotlin/kotlinx.collections.immutable/blob/master/proposal.md#persistent-collection-implementations
Transformation Operations
- Collection transformation operations (e.g. map, filter) suitable for persistent collections is not implemented yet.
- Only operations in standard library are available, and they return collections in standard library
- Immutability will be broken
- "Improve standard library collection operations to exploit benefits of immutability of the collections on which the operation is performed."