select with default for non-blocking channel operations

Checking whether a channel has a value or is done without blocking the current goroutine requires a non-blocking receive.

Use select with a default case:

// Non-blocking receive
select {
case v := <-ch:
    fmt.Println("received", v)
default:
    fmt.Println("no value ready")
}

// Non-blocking send
select {
case ch <- value:
    fmt.Println("sent")
default:
    fmt.Println("channel full or no receiver")
}

// Poll done channel without blocking
func isContextDone(ctx context.Context) bool {
    select {
    case <-ctx.Done():
        return true
    default:
        return false
    }
}

// Timeout with select
select {
case result := <-work:
    handle(result)
case <-time.After(5 * time.Second):
    log.Println("timeout")
}

select evaluates all cases simultaneously. A default case makes the entire select non-blocking — if no channel is ready, default executes immediately.