sleep

(sleep seconds)

Blocks for seconds — any non-negative real, so sub-second waits are written as 0.5 — and returns nil. A zero or negative duration returns immediately. Durations are rounded to whole milliseconds, the resolution every backend shares with get-internal-real-time.

The interpreter and the JVM park the thread. --component waits on the real host timer (wasi:clocks), forcing it through the module scheduler: the wait costs no CPU and other pending tasks still progress. WASM Preview 1 is the one backend that busy-waits, looping on the clock until the deadline — its imports include a clock but no timer to wait on, so burning the interval is the only way to let it pass. The wait is honest there, but it costs a core and blocks the instance for its duration. A --no-wasi module signals instead of waiting: it imports no timer, and its clock only moves when its host writes it, so no interval can elapse while the call is running (see the clock and randomness guide).

(let ((start (get-internal-real-time)))
  (sleep 0.5)
  (print (>= (- (get-internal-real-time) start) 500)))