03 — Select

A goroutine with one channel to wait on just receives. Once it has two things to wait for, such as two inputs, an input and a clock, or work and a cancellation, it needs select: one statement that waits on several channel operations and carries out whichever can go first. Most of Go's concurrency patterns rest on it.

The four lessons are the rules that decide what select does. It waits until a case can proceed, and default lets it not wait at all. When several cases can proceed, it picks one at random. A timer is just another channel. And a nil channel is a case that can never proceed, which makes it an off switch.

Lesson	The one thing
select waits on many channels	it blocks until one case can proceed and does only that one; default makes it never block
select chooses at random	case order is not a priority; priority is a non-blocking check of the urgent channel first
A timeout is a channel	time.After, a Timer and a Ticker are channels, so time is one more case
A nil channel disables a case	a case on a nil channel never proceeds, so setting the variable to nil switches it off

Planned

• break leaves the select, not the loop — the for-select loop, why a bare break inside a case does not end it, and the labelled break or return that does.
• Reset a timer instead of making a new one — an idle timeout that restarts on every message, with Timer.Reset and the guarantees it has had since Go 1.23.
• A select over channels known only at run time — reflect.Select, for when the number of cases is not known until the program runs.