sleep

Builtin index · Concurrency

sleep(duration)

Input: duration · Output: no output

What it is

duration: a time quantity in any time unit (sleep(2 s), sleep(250 ms), sleep(10 us)), or a bare number, which is milliseconds (sleep(50) is 50 ms; fractions honoured). Pauses the calling thread for that long, first flushing printed output. A non-time unit (sleep(3 V)), a negative or a non-finite duration is an error. *Units accepted since v0.4.4.* Returns none.

Overloads

One call shape — this function is not overloaded.

Examples

From the documentation, and run by the build -- the output and the figure below are what the engine actually produced. Simplest first.

n = 0
watch file("data.csv") do
    n = n + 1
end

wait until n >= 1
    sleep(200)          # real wall-clock pause between checks

    pump_watches()
end until

from tour

square(x) := x * x

function run_listener()
    listen_pool(9321, allow=("square"))
end function

w = spawn("run_listener")     # the "remote" side, on its own thread here

sleep(0.3)                    # give it time to bind before connecting


pool workers with cpu=1, remote=("127.0.0.1:9321")
end pool

q = queue()
for i in 0 to 4
    q.push("square", i)
end for

results = run q on workers    # some local, some "remote" — one ordered List

print(results)                # (0, 1, 4, 9, 16)

from concurrency

How it works

sleep(ms) (§ wait until / sleep, scoped 2026-08-26) — a REAL OS sleep (std::thread::sleep), not a simulated- timeline wait like every/after/at's run_for virtual clock. This is the one piece that makes wait until COND ... end until actually usable instead of busy-spinning a CPU core at 100%: wait until is plain sugar for a while loop (see qu-syntax's wait_until_stmt), and a while loop re-checking a condition with no delay between checks has no way to yield the CPU without this. Blocks only the calling thread — doesn't touch pool/run's worker threads, produces no value, and consults no RNG state, so it doesn't interact with seed(n)/order-stable-reduction determinism at all; it's real wall-clock time, which this codebase already accepts as inherently non-reproducible elsewhere (tic/toc).

From the note above this builtin in the interpreter, where the reasoning was written down as it was built.