A practical use for guard statements

[ahmedk92]

Maybe it's written already somewhere, but I only took notice of this just now. I originally looked at guard statements as just sugar for inverted if statements that only look good, but don't protect you from bugs. I had the following bug in a PR:

func foo(completionHandler: @escaping (Result<...>) -> Void) {
  bar { values in
    if things.isEmpty {
      completion(.failure(.someError))
    }
    completion(.success(values))
  }
}

A colleague pointed out that I may be missing a return after calling the completion handler in the body of the if statement above, unless I intend to call the completion handler with success right after calling it with failure which seems unlikely.

This was an eye-opener for me. Had I used a guard statement, I would have been forced to insert a return statement; and thus saving me from a bug.

func foo(completionHandler: @escaping (Result<...>) -> Void) {
  bar { values in
    guard !things.isEmpty else {
      completion(.failure(.someError))
      return
    }
    completion(.success(values))
  }
}

To be honest, I always used guards to "sanitize" my code path, keeping failures and exception handling in the else block of the guard(s) in that code, but when the code after the guard was just a single line I tended to turn it to an if for no clear reason, but no more 😉