Tasks and channels

RCE programs can run several tasks that talk over channels.

fn producer(out: chan i64, n: i64)
    for i in 0..n
        out <- i
    out.close()

fn main()
    let c = chan[i64](8)
    spawn producer(c, 10)
    var total = 0
    loop
        match <- c
            some(v) -> total += v
            none -> break
    emit total

• chan[i64](8) makes a channel that holds up to 8 values of type i64.
• spawn producer(c, 10) starts producer as a new task.
• out <- i sends i on the channel. <- c receives. When the channel is closed and empty, receiving gives none.

$ rce run channels.rce
emit 45
exit status: 0

Why the result is always the same

Tasks in RCE are scheduled in ticks. In each tick, every task that can move takes one communication step, in an order fixed by the tasks' ids. The ids are computed from the program itself, not from a clock or a counter. So the whole run is a function of the program and its inputs, and running it again gives the same result. When no task can move, the run ends with the outcome Deadlock instead of hanging.

The specification also defines select (wait on several channels) and actor (a state with its own mailbox). See section 6.8 of the specification.

Current limits

The reference interpreter runs tasks and channels as shown here. The Rust, Python and JavaScript emitters of chapter 15 do not translate them yet: in emitted code, channel operations trap.

What you learned

• spawn, chan[T](n), send with <- and receive with <- c.
• Scheduling is fixed by the program, so concurrent runs repeat exactly.