#map #once #lazy

no-std once_map

Single assignment and lazy maps

8 releases

0.4.5 Mar 8, 2023
0.4.4 Feb 15, 2023
0.4.2 Dec 22, 2022
0.4.1 Nov 23, 2022
0.1.1 Jun 27, 2022

#588 in Rust patterns

24 downloads per month


1.5K SLoC


Crates.io Docs.rs Minimum rustc version

This crate provides OnceMap, a type of HashMap where entries can be written with a shared reference, but can be written ony once. This is similar to once_cell, but with a map. This enables to reference values inside the map for the lifetime of the map, without the need of further locks.

This makes this type perfect for implementation of caches. A type LazyMap is provided for such cases.

This crate provides such a map heavily optimized for concurrent use, but also a single-threaded version.


let map = OnceMap::new();

// All these are `&str` pointing directly in the map.
// Note that we don't need a mutable reference, so we can have several of
// them at the same time.
let roses = map.insert(String::from("rose"), |_| String::from("red"));
let violets = map.insert(String::from("violets"), |_| String::from("blue"));
let sugar = map.insert(String::from("sugar"), |_| String::from("sweet"));

assert_eq!(roses, "red");
assert_eq!(violets, "blue");
assert_eq!(sugar, "sweet");

// The closure is never run here, because we already have a value for "rose"
let roses = map.insert(String::from("rose"), |_| String::from("green"));
// The old value did not change
assert_eq!(roses, "red");


~79K SLoC