Defer Order

defer schedules a call to run when the surrounding function returns, no matter which return it takes or whether it panics on the way out:

func read() {
	f := open("data.txt")
	defer close(f)
	// ... close(f) runs here, whatever happens above
}

Two rules explain everything defer does.

Deferred calls run last-in, first-out. They go on a stack, so the last one you defer is the first one to run. That is what you want: you release things in the reverse of the order you acquired them.

The arguments are evaluated at the defer statement, not when the call runs. defer fmt.Println(n) snapshots n immediately. A deferred closure that reads n in its body is the opposite: it sees whatever n holds at return time.

Because deferred functions run after the return value has been set, a closure over a named result can still change it, and the caller sees the change.

Task

Fill in the five functions in main.go.

1. LIFO(labels []string) (order []string) defers one appending call per label and returns the order they ran in, so a, b, c comes back as c, b, a.
2. Steps() (log []string) logs enter, defers cleanup-1 then cleanup-2, logs work, and returns [enter work cleanup-2 cleanup-1].
3. CapturedValue() (recorded int) passes n to the deferred function as an argument before changing n, and returns what was recorded.
4. CapturedVariable() (recorded int) is the same, except the deferred function closes over n instead.
5. DoubleResult(n int) (result int) returns n doubled, with the doubling done by a deferred function.

Hints

• These functions all have named results. A bare return sets them to what they currently hold and then runs the defers, which can still change them.
• defer f() calls f now and defers whatever it returned. To defer a block of work, defer a function literal: defer func() { ... }(), trailing () included.
• LIFO needs no reversing logic and no counter. Defer inside the loop and let the stack do it.
• The difference between 3 and 4 is one line of syntax: func(seen int) { ... }(n) freezes n, func() { ... }() does not.
• DoubleResult cannot double before the return; the test would still pass, but the point is that the deferred call edits result after it is set.