defer 2

package main

func bar() (r int) {
	defer func() {
		r += 4
		if recover() != nil {
			r += 8
		}
	}()
	
	var f func()
	defer f()
	f = func() {
		r += 2
	}

	return 1
}

func main() {
	println(bar())
}

Choices:

• 1
• 7
• 12
• 13

Answer: 13

Run it on Go play.

Key points:

• Line 12: when the call f() is pushed to the defer call queue, f is evaluated as nil. Calling a nil function produces a panic. But the panic doesn't happen at the push time. Instead, it will panic at the execution time later during the exiting phase of the bar() call.
• Line 13-15: the assignment to f has not any impacts on the execution effect. In other words, this assignment is actually a no-op.
• Line 4-9: a defer call could modify the named return result of its containing function (here the bar function).

• slice: 1, 2, 3
• map: 1
• channel: 1
• loop: 1, 2, 3, 4
• switch: 1
• defer: 1, 2
• panic/recover: 1, 2
• reflect: 1, 2
• const: 1, 2, 3
• operator: 1, 2, 3
• function call: 1
• scope: 1, 2
• nil: 1
• embedding: 1

(more are coming ...)