Defer

defer schedules a function to run after ms milliseconds, without blocking the caller. It returns a future you can await for the result (or error).

std.async.defer(fn, ms[, args...]) # → future

Basic usage

var f = std.async.defer(fn(name) {
  return "delayed, " + name
}, 100, "scrii")
print(std.async.await(f))        # delayed, scrii (after ~100 ms)

Differences from spawn + sleep:

Use defer when you want the start delayed (timers, debounced work). Use spawn when you want it running now.

Arguments

Args are deep-copied at defer time, like spawn:

var f = std.async.defer(fn(a, b) { return a + b }, 50, 10, 20)
print(std.async.await(f)) # 30

Example — delayed log

print("now:", std.time.now())
var f = std.async.defer(fn() {
  print("later:", std.time.now())
  return "done"
}, 200)
print(std.async.await(f)) # done

Cancellation

Neither defer nor one-shot futures are cancellable: await waits for
completion, it does not stop anything. The only cancellation mechanism in
the system is a repeating timer’s handle — t.cancel() prevents further
ticks (see std.time). To abandon one-shot work,
structure it so the result is simply ignored (see Race).

See also

• Spawn & Await — immediate concurrency
• Sleep — cooperative delay
• Race — first-to-finish among defers/spawns
• Errors — defer errors are stored on the future like spawn