Skip to content
Kavachकवच

Tasks

Also known as: thread async await spawn join future promise goroutine background parallel

● Advanced5 minBefore this: Functions, ErrorsSpec 9.1

spawn starts a task that runs alongside the rest of the program; .wait() gets its result. There is no async and no await.

const task = spawn work(…)
// … other work meanwhile …
const result = try task.wait()
syntax: Kavach code
fn slow_double(n: Int) -> Int {
    n * 2
}

fn main() ! Error {
    const t = spawn slow_double(21)
    print("working meanwhile")
    print(try t.wait())
}

The program ran and printed:

kavach run spawn.kav

working meanwhile42
spawn.kav: Kavach code, then what running it prints
  • Any function can be spawned, and any function can wait: no function is marked async, so there are no two kinds of functions to keep apart.
  • While a task waits, for the network, time or another task, the others run.
  • Tasks are light: a server can run one per connection, many at once.

Two things at once

use std.time

fn report(name: String, ms: Int) -> String {
    time.sleep_ms(ms)
    print("{name} done")
    name
}

fn main() ! Error {
    const slow = spawn report("slow", 200)
    const fast = spawn report("fast", 100)
    print("both started")
    print("{try slow.wait()} and {try fast.wait()} are back")
}

The program ran and printed:

kavach run turns.kav

both startedfast doneslow doneslow and fast are back
turns.kav: Kavach code, then what running it prints

The two sleeps overlap, so the program takes about 200 ms, not 300. Why it prints in exactly this order is on Taking Turns.

One page for each form

When a task fails →

Try it yourself

Spawn three tasks that sleep 300, 100 and 200 ms and print their names. In what order do they print?
Show the answer

By how long each sleeps: the 100 ms one, then 200, then 300. A task whose wait ends first becomes ready first.

Check yourself

QUIZ 1 / 1pick one
A function calls time.sleep_ms(100) inside a task. What do the program's other tasks do meanwhile?