On Golang's `defer` Keyword

Golang has a defer keyword, which postpones the execution of a function to after the surrounding function returns. Functions are stacked so that the last deferred function is run first.

Let’s check out an example of this in practice.

func writeLogs(c chan []string) {
  defer stats.Duration("write_logs_fn", time.Now()) // order: 4

  file, err := os.Create("request.log", filename)
  if err != nil {
  defer file.Close() // order: 3

  writer := csv.NewWriter(file)
  defer writer.Flush() // order: 2

  for line := range c {
    err := writer.Write(line)
    if err != nil {
    defer stats.Increment("wrote_line") // order: 1

At first glance, you may ask–

How is this different from the finally keyword in other languages?

From Java Docs,

finally is useful for more than just exception handling — it allows the programmer to avoid having cleanup code accidentally bypassed by a return, continue, or break. Putting cleanup code in a finally block is always a good practice, even when no exceptions are anticipated.

Well, let’s compare defer to the following Java example using finally.

static void writeLogs(List<List<String>> lines) throws Exception {
  Date start = new Date();
  FileWriter writer = null;
  int lineCount = 0;

  try {
    writer = new FileWriter("request.log");
    // non-buffered input
    for (int i = 0; i < lines.size(); i++) {
      // naive CSV writer
      for (value : lines[i]) {
  } finally {
    if (writer != null) {

    stats.increment("wrote_line", lineCount);

Off the bat, you can identify a handful of “gotchas”:

Alike most technical debt, I’ve found that each of these “gotchas” becomes more apparent as the complexity of your function grows.


Though unconventional, Go’s defer is pure brilliance. It disguises a generally complex control flow with a simple, universal one that both helps readability and keeps things declarative. It’s far superior to traditional teardown via finally, and you should almost always prefer it to rolling out your own control flow.

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