Single-assignment cell and Lazy types enabling one-time initialization with direct references; unsynchronized and synchronized variants provide safe concurrent lazy initialization and ergonomic API for global/static data.
This is a Kotlin Multiplatform line-by-line transliteration port of matklad/once_cell.
Original Project: This port is based on matklad/once_cell. All design credit and project intent belong to the upstream authors; this repository is a faithful port to Kotlin Multiplatform with no behavioural changes intended.
This is an in-progress port. The goal is feature parity with the upstream Rust crate while providing a native Kotlin Multiplatform API. Every Kotlin file carries a // port-lint: source <path> header naming its upstream Rust counterpart so the AST-distance tool can track provenance.
The text below is reproduced and lightly edited from
https://github.com/matklad/once_cell. It is the upstream project's own description and remains under the upstream authors' authorship; links have been rewritten to absolute upstream URLs so they continue to resolve from this repository.
once_cell provides two new cell-like types, unsync::OnceCell and sync::OnceCell. OnceCell
might store arbitrary non-Copy types, can be assigned to at most once and provide direct access
to the stored contents. In a nutshell, API looks roughly like this:
impl OnceCell<T> {
fn new() -> OnceCell<T> { ... }
fn set(&self, value: T) -> Result<(), T> { ... }
fn get(&self) -> Option<&T> { ... }
}Note that, like with RefCell and Mutex, the set method requires only a shared reference.
Because of the single assignment restriction get can return an &T instead of Ref<T>
or MutexGuard<T>.
once_cell also has a Lazy<T> type, build on top of OnceCell which provides the same API as
the lazy_static! macro, but without using any macros:
use std::{sync::Mutex, collections::HashMap};
use once_cell::sync::Lazy;
static GLOBAL_DATA: Lazy<Mutex<HashMap<i32, String>>> = Lazy::new(|| {
let mut m = HashMap::new();
m.insert(13, "Spica".to_string());
m.insert(74, "Hoyten".to_string());
Mutex::new(m)
});
fn main() {
println!("{:?}", GLOBAL_DATA.lock().unwrap());
}More patterns and use-cases are in the docs!
Parts of once_cell API are included into std as of Rust 1.70.0.
dependencies {
implementation("io.github.kotlinmania:once-cell-kotlin:0.1.1")
}./gradlew build
./gradlew testSee AGENTS.md and CLAUDE.md for translator discipline, port-lint header convention, and Rust → Kotlin idiom mapping.
This Kotlin port is distributed under the same MIT license as the upstream matklad/once_cell. See LICENSE (and any sibling LICENSE-* / NOTICE files mirrored from upstream) for the full text.
Original work copyrighted by the once_cell authors.
Kotlin port: Copyright (c) 2026 Sydney Renee and The Solace Project.
Thanks to the matklad/once_cell maintainers and contributors for the original Rust implementation. This port reproduces their work in Kotlin Multiplatform; bug reports about upstream design or behavior should go to the upstream repository.
This is a Kotlin Multiplatform line-by-line transliteration port of matklad/once_cell.
Original Project: This port is based on matklad/once_cell. All design credit and project intent belong to the upstream authors; this repository is a faithful port to Kotlin Multiplatform with no behavioural changes intended.
This is an in-progress port. The goal is feature parity with the upstream Rust crate while providing a native Kotlin Multiplatform API. Every Kotlin file carries a // port-lint: source <path> header naming its upstream Rust counterpart so the AST-distance tool can track provenance.
The text below is reproduced and lightly edited from
https://github.com/matklad/once_cell. It is the upstream project's own description and remains under the upstream authors' authorship; links have been rewritten to absolute upstream URLs so they continue to resolve from this repository.
once_cell provides two new cell-like types, unsync::OnceCell and sync::OnceCell. OnceCell
might store arbitrary non-Copy types, can be assigned to at most once and provide direct access
to the stored contents. In a nutshell, API looks roughly like this:
impl OnceCell<T> {
fn new() -> OnceCell<T> { ... }
fn set(&self, value: T) -> Result<(), T> { ... }
fn get(&self) -> Option<&T> { ... }
}Note that, like with RefCell and Mutex, the set method requires only a shared reference.
Because of the single assignment restriction get can return an &T instead of Ref<T>
or MutexGuard<T>.
once_cell also has a Lazy<T> type, build on top of OnceCell which provides the same API as
the lazy_static! macro, but without using any macros:
use std::{sync::Mutex, collections::HashMap};
use once_cell::sync::Lazy;
static GLOBAL_DATA: Lazy<Mutex<HashMap<i32, String>>> = Lazy::new(|| {
let mut m = HashMap::new();
m.insert(13, "Spica".to_string());
m.insert(74, "Hoyten".to_string());
Mutex::new(m)
});
fn main() {
println!("{:?}", GLOBAL_DATA.lock().unwrap());
}More patterns and use-cases are in the docs!
Parts of once_cell API are included into std as of Rust 1.70.0.
dependencies {
implementation("io.github.kotlinmania:once-cell-kotlin:0.1.1")
}./gradlew build
./gradlew testSee AGENTS.md and CLAUDE.md for translator discipline, port-lint header convention, and Rust → Kotlin idiom mapping.
This Kotlin port is distributed under the same MIT license as the upstream matklad/once_cell. See LICENSE (and any sibling LICENSE-* / NOTICE files mirrored from upstream) for the full text.
Original work copyrighted by the once_cell authors.
Kotlin port: Copyright (c) 2026 Sydney Renee and The Solace Project.
Thanks to the matklad/once_cell maintainers and contributors for the original Rust implementation. This port reproduces their work in Kotlin Multiplatform; bug reports about upstream design or behavior should go to the upstream repository.