Promisery

A JavaScript-inspired, ergonomic, and composable Promise type for Rust, supporting background work, chaining, and error handling with Result.

Features

• ECMAScript 5-style promises with Rust's type safety
• Chaining with .then, .map, and .map_err
• Combinators: Promise::all, Promise::race
• Panic-safe: panics in promise tasks are detected and reported
• Fully documented with tested examples
• Timeout and deadline support: Wait with [wait_timeout] or [wait_deadline]
• Safe and unsafe waiting: Choose between [wait] (safe, double Result) and [wait_nopanic] (unsafe, panics on background panic)

Installation

Add to your Cargo.toml:

promisery = "2.0"

Example

use promisery::Promise;

let p = Promise::<_, ()>::new(|| Ok(2))
    .then(|res| res.map(|v| v * 10))
    .then(|res| res.map(|v| v + 5));
assert_eq!(p.wait(), Ok(Ok(25)));

API Highlights

• Chaining:
  • .then for full control over result and error
  • .map for value transformation
  • .map_err for error transformation
• Combinators:
  • Promise::all waits for all promises (returns Result<Result<Vec<T>, E>, PromisePanic>)
  • Promise::race resolves/rejects with the first to finish (returns Result<Result<T, E>, PromisePanic>)
• Waiting:
  • .wait() — safe, never panics, returns a double Result
  • .wait_nopanic() — unsafe, panics if the background task panicked
  • .wait_timeout(duration) — wait with a timeout
  • .wait_deadline(instant) — wait until a deadline
• Panic Handling:
  • Panics in promise tasks are detected and reported via PromisePanic

Why Promisery?

• Lightweight, no async runtime required
• Familiar API for those coming from JavaScript
• Integrates with Rust's Result-based error handling
• Does not spawn a new thread for each action that produces a new promise (see the documentation for each method for more details)

License

MIT OR Apache-2.0

See docs.rs/promisery for full documentation, API details, and more examples, including usage of wait_nopanic, wait_timeout, and wait_deadline.