Motivation

Building a poll-based UI event loop (xsofy #56) that multiplexes a keyboard source and a cosmetic clock source and must never block while idle. This is mandatory under single-threaded Go-wasm, where a parked <! (backed by Atomics.wait) freezes the entire runtime, not just the goroutine. The loop needs to drain a producer-fed channel once per frame if and only if a value is immediately ready.

What's missing

async exposes only blocking takes (<!/<!!). There is no non-blocking variant and no alts!. Registered channel fns today are just chan, >!, <!, >!!, <!! (pkg/rt/lang.go:5969-5973).

Everything else the use case needs already exists, which is why only this one primitive is requested:

• Cancellable channel ops — <!/>! already select on vm.CurrentContext().Done() (pkg/rt/lang.go:3446, :3472), so a producer goroutine parked on a put/take is released by scope-close!'s cancel+drain.
• Scoped lifecycle — with-scope (pkg/rt/core/core.lg:2091), scope-open/scope-close!/scope-live (pkg/rt/lang.go:5974-5976), *scope-drain-timeout-ms* (core.lg:2089).

So a ctx-aware, drainable producer is writable today; the only gap is the consumer's non-blocking peek.

Requested

1. (poll! ch) — non-blocking take. Returns a value if one is immediately available, else a distinct empty signal (so a nil value can be distinguished from "nothing ready" — e.g. return a sentinel, or [val ok]). Go semantics:

   select {
   case v, ok := <-ch:
       // ok==false => closed
   default:
       // nothing ready
   }

2. (nice-to-have) (alts! [ch…] :default v) — select over several channels with a non-blocking default; generalizes poll! to multiple upstreams.
3. (symmetry, optional) (offer! ch v) — non-blocking put.

Notes

• poll! alone unblocks the use case; alts! is the general form.
• Keep ctx-awareness for consistency, though a :default branch means the non-blocking path never parks anyway.