How do defer, panic, and recover work together in Go?

defer schedules a function call to run when the surrounding function returns — regardless of how it returns (normally, via error, or via panic). It is Go's idiomatic resource-cleanup mechanism. panic stops normal execution; recover catches it inside a deferred function.

// defer — executes when surrounding function returns
func processFile(path string) error {
    f, err := os.Open(path)
    if err != nil { return err }
    defer f.Close()  // ALWAYS runs, even if function returns early
    // process file...
    return nil
}

// Multiple defers: LIFO order (last-in, first-out)
func demo() {
    defer fmt.Println("3rd")  // runs first (LIFO)
    defer fmt.Println("2nd")
    defer fmt.Println("1st")  // runs last? NO — runs first!
    // Output order: 1st, 2nd, 3rd  — wait, no:
    // ACTUAL order: 3rd → 2nd → 1st  (LIFO!)
}

// Defer argument evaluation: immediate, not deferred!
x := 5
defer fmt.Println(x)  // prints 5 — x evaluated NOW
x = 10                // too late to affect the defer

// panic — signals an unrecoverable error
func mustPositive(n int) int {
    if n <= 0 { panic(fmt.Sprintf("expected positive, got %d", n)) }
    return n
}

// recover — catches a panic; ONLY useful inside defer
func safeDiv(a, b int) (result int, err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("recovered: %v", r)
        }
    }()
    return a / b, nil  // panics if b == 0
}

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