Taking Turns

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")
}

both startedfast doneslow doneslow and fast are back

Step through it:

• spawn makes the new task ready; the task that spawned it keeps running.
• A task runs until it waits. Then the task that became ready first runs.
• When no task is ready, the program waits for the earliest of them.

Why it matters: a bug in a program with threads often shows up once in a hundred runs. With fixed turns, the same program does the same thing every time, so a bug that happens once happens every time, and can be found.