What happens when you send to, receive from, or close a nil or closed channel?

This table is essential interview knowledge. Mistakes here produce panics and deadlocks that are notoriously difficult to debug.

// Nil channel: always blocks (send or receive)
var ch chan int
// ch <- 1    // deadlock
// <-ch       // deadlock
// close(ch)  // PANIC: close of nil channel

// Closed channel: reads drain buffered data then return zero
done := make(chan struct{})
close(done)
v, ok := <-done
fmt.Println(v, ok)      // {} false
// done <- struct{}{} // PANIC: send on closed channel

// Comma-ok idiom: detect channel closure
ch2 := make(chan int, 2)
ch2 <- 1; ch2 <- 2
close(ch2)
for {
    v, ok := <-ch2
    if !ok { break }   // channel closed and drained
    fmt.Println(v)
}

// Idiomatic: range handles closure automatically
ch3 := make(chan int)
go func() { ch3 <- 1; ch3 <- 2; close(ch3) }()
for v := range ch3 { fmt.Println(v) } // exits on close

// GOLDEN RULE: only the SENDER should close a channel
// A receiver cannot know when the sender is done

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