Event Loop

The event loop drives async callbacks to completion. In most programs, it runs automatically — you rarely need to call it directly.

value.then(callback)

Registers a callback to fire when a value is ready. This is the primary way to attach async callbacks.

set_workers(4)

let p = http_get("https://api.example.com/users")
p.then(fn(data) {
    println(data)
})

Because the event loop drains automatically when main() returns, no explicit event_loop() call is needed for this pattern.

Auto-drain at main() exit

When main() finishes, the runtime automatically drains any pending callbacks before the program exits. This mirrors how Node.js keeps the process alive for pending async work.

fn main() {
    set_workers(4)

    http_get("https://api.example.com/users").then(fn(users) {
        println(users)
    })

    http_get("https://api.example.com/posts").then(fn(posts) {
        println(posts)
    })

    // Both callbacks fire automatically — no event_loop() needed
}

Chained callbacks

Callbacks registered inside a callback are also drained — the runtime keeps going until the queue is completely empty.

fn main() {
    set_workers(2)

    sleep(0.01).then(fn(v) {
        sleep(0.01).then(fn(v2) {
            println("both done")
        })
    })
}

on_ready(value, callback)

on_ready is the older function-based form of .then(). Both are equivalent.

set_workers(2)
on_ready(sleep(0.5), fn(v) {
    println("done")
})

event_loop([timeout_secs])

Explicitly runs the event loop until all pending callbacks have fired. Use this when you need callbacks to complete before the next line of code (rather than at main() exit).

set_workers(4)

http_get("https://api.example.com/data").then(fn(data) {
    await write_file("/tmp/result.txt", data)
})

event_loop()   // wait for the write to finish

let content = await read_file("/tmp/result.txt")
println(content)

Accepts an optional timeout in seconds:

event_loop(5.0)   // give up after 5 seconds

.then() vs await

  await .then()
Style Pull — caller waits for result Push — callback receives result
Result Returned directly Passed to callback
Use when You need the result on the next line Fire-and-forget, or dispatching many I/O tasks 
// await — sequential
let users = await http_get("https://api.example.com/users")
let posts = await http_get("https://api.example.com/posts")

// .then() — concurrent (both start immediately)
http_get("https://api.example.com/users").then(fn(u) { process_users(u) })
http_get("https://api.example.com/posts").then(fn(p) { process_posts(p) })

Queue controls

These are operational knobs, not part of normal programs. See Configuration for details.

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