How and when should you use panic and recover in production Go code?

The Go convention is clear: panics are for unrecoverable programmer errors (nil pointer dereference, index out of bounds, type assertion failure). Libraries should never let panics propagate to callers — they convert panics to errors at the public API boundary.

// Library pattern: convert internal panics to errors at the boundary
func safelyParseTemplate(tmpl string) (result string, err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("template parse panicked: %v", r)
        }
    }()
    // This might panic on invalid template syntax in some implementations
    result = parseTemplate(tmpl)
    return result, nil
}

// HTTP recovery middleware — catch handler panics gracefully
func recoveryMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        defer func() {
            if err := recover(); err != nil {
                // Log the full stack trace
                buf := make([]byte, 4096)
                n := runtime.Stack(buf, false)
                log.Printf("panic in %s %s: %v\n%s",
                    r.Method, r.URL.Path, err, buf[:n])
                // Return 500 to the client
                http.Error(w, "internal server error",
                    http.StatusInternalServerError)
            }
        }()
        next.ServeHTTP(w, r)
    })
}

// When panic IS appropriate:
// 1. Programmer invariant violation that cannot be recovered
func mustPositive(n int) int {
    if n <= 0 { panic(fmt.Sprintf("expected positive, got %d", n)) }
    return n
}

// 2. Impossible type assertion that should never fail if code is correct
// 3. Test helpers: t.Fatalf is analogous — immediate stop

// When NOT to use panic:
// - File not found, network failure, validation errors → return error
// - Any expected failure condition → return error

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