Asynchronous error handling



In a previous article, I talked about error handling in Swift using throw. But what happens when you deal with asynchronous workflows, where throw can’t really fit?

What’s wrong with throw and async?

As a reminder, a function which can fail can use throw in the following way:

// Define an error type and a throwing function
enum ComputationError: ErrorType { case DivisionByZero }
func inverse(x: Float) throws -> Float {
  guard x != 0 else { throw ComputationError.DivisionByZero }
  return 1.0/x
}
// Call it
do {
  let y = try inverse(5.0)
} catch {
  print("Whoops: \(error)")
}

But what happens when the function is asynchronous and will only return a result later, e.g. with a completion block?

func fetchUser(completion: User? /* throws */ -> Void) /* throws */ {
  let url = 
  NSURLSession.sharedSession().dataTaskWithURL(url) { (data, response, error) -> Void in
//    if let error = error { throw error } // No can do, fetchUser can't "throw asynchronously"
    let user = data.map { User(fromData: $0) }
    completion(user)
  }.resume()
}
// Call it:
fetchUser() { (user: User?) in
  /* do something */
}

How can you throw in case where the request failed?

  • It doesn’t make sense to make the fetchUser function itself to throw, because this function returns immediately and the network error would only happen later. So, when the error occurs, it would be too late to throw an error as the result of the fetchUser function call itself.
  • Maybe you can mark the completion as throws? But the code doing the call to that completion(user) method is inside the fetchUser, it’s not at call site. So the one receiving and forced to handle the error would be the code inside fetchUser itself, not the call site. So that doesn’t solve it either. 😢

Hacking it

One way of working around this limitation is to make the completion not return a User? to the caller directly, but a throwing function Void throws -> User instead, itself returning a User (let’s call this a UserBuilder). This way, we can use throw again.

Then when the completion returns the userBuilder func, we’ll then need to call try userBuilder() to access the User… or have it throw on error.

enum UserError: ErrorType { case NoData, ParsingError }
struct User {
  init(fromData: NSData) throws { /* … */ }
  /* … */
}

typealias UserBuilder = Void throws -> User
func fetchUser(completion: UserBuilder -> Void) {
  let url = 
  NSURLSession.sharedSession().dataTaskWithURL(url) { (data, response, error) -> Void in
    completion({ UserBuilder in
      if let error = error { throw error }
      guard let data = data else { throw UserError.NoData }
      return try User(fromData: data)
    })
  }.resume()
}

fetchUser { (userBuilder: UserBuilder) in
  do {
    let user = try userBuilder()
  } catch {
    print("Async error while fetching User: \(error)")
  }
}

This way, the completion does not directly return a User but a function returning a User… or throwing. And then you have your error handling again.

But, let’s be honest, that’s not the prettiest and most readable solution, having to return a Void throws -> User instead of returning a User? directly. So what are our other alternatives?

Introducing Result

Back in Swift 1.0, when throw didn’t exist yet, people started to handle errors in a functional way. As Swift borrowed a lot of features from the functional programming world, it was logical that people used the Result pattern in Swift to better handle errors. Here is what Result looks like1:

enum Result<T> {
  case Success(T)
  case Failure(ErrorType)
}

The Result type is actually quite simple: it represents either a success — which has an associated value representing the successful result — or a failure — with an associated error. Perfect to represent operations which can potentially fail.

Then how do we use it? Just build either a Result.Success or a Result.Failure and call completion with the resulting Result2:

func fetchUser(completion: Result<User> -> Void) {
  let url = 
  NSURLSession.sharedSession().dataTaskWithURL(url) { (data, response, error) -> Void in
    if let error = error {
      return completion( Result.Failure(error) )
    }
    guard let data = data else {
      return completion( Result.Failure(UserError.NoData) )
    }
    do {
      let user = try User(fromData: data)
      completion( Result.Success(user) )
    } catch {
      completion( Result.Failure(error) )
    }
  }.resume()
}

Remember monads?

The nice thing about Result is it can be turned into a Monad. Remember Monads? That simply means that we can add the high-order map and flatMap methods on Result, which will take a closure f: T->U or f: T->Result<U> and return a Result<U>.

If the original Result was a .Success(let t) then we apply the closure to that t and return the resulting f(t). If it was a .Failure then we simply pass the error along:

extension Result {
  func map<U>(f: T->U) -> Result<U> {
    switch self {
    case .Success(let t): return .Success(f(t))
    case .Failure(let err): return .Failure(err)
    }
  }
  func flatMap<U>(f: T->Result<U>) -> Result<U> {
    switch self {
    case .Success(let t): return f(t)
    case .Failure(let err): return .Failure(err)
    }
  }
}

For more information I invite you to re-read my article about Monads, but long story short, this allows us to do stuff like this:

func readFile(file: String) -> Result<NSData> {  }
func toJSON(data: NSData) -> Result<NSDictionary> {  }
func extractUserDict(dict: NSDictionary) -> Result<NSDictionary> {  }
func buildUser(userDict: NSDictionary) -> Result<User> {  }

let userResult = readFile("me.json")
  .flatMap(toJSON)
  .flatMap(extractUserDict)
  .flatMap(buildUser)

What is cool in that expression is that if one of the method (say toJSON for example) fails and return a .Failure, then the failure will be propagated to the end without even trying to apply the extractUserDict and buildUser methods.

This somehow allows the error to “take a shortcut”: exactly like with do…catch, you can process all your error cases together in one point at the end of the chain instead of handling errors at each intermediate stage. Isn’t that cool?

From Result to throw and back

Problem is, Result is not built in the Swift standard library, and a lot of functions use throw to report synchronous errors anyway. Like in practice, to build a User from a NSDictionary we might have a init(dict: NSDictionary) throws constructor instead of a NSDictionary -> Result<User> function.

How can we mix both those worlds? Easy: let’s extend Result just for that3!

extension Result {
  // Return the value if it's a .Success or throw the error if it's a .Failure
  func resolve() throws -> T {
    switch self {
    case Result.Success(let value): return value
    case Result.Failure(let error): throw error
    }
  }

  // Construct a .Success if the expression returns a value or a .Failure if it throws
  init(@noescape _ throwingExpr: Void throws -> T) {
    do {
      let value = try throwingExpr()
      self = Result.Success(value)
    } catch {
      self = Result.Failure(error)
    }
  }
}

And now we can easily convert our throwing initializer to a closure returning a Result:

func buildUser(userDict: NSDictionary) -> Result<User> {
  // Here we build a `Result` by calling the `init` using a throwing trailing closure
  return Result { try User(dictionary: userDict) }
}

And, if we wrap NSURLSession into a function asynchronously returning a Result, we can balance between the two worlds however we like. For example:

func fetch(url: NSURL, completion: Result<NSData> -> Void) {
  NSURLSession.sharedSession().dataTaskWithURL(url) { (data, response, error) -> Void in
    completion(Result {
      if let error = error { throw error }
      guard let data = data else { throw UserError.NoData }
      return data
    })
  }.resume()
}

This also calls the completion block by passing a Result object built from a throwing closure4.

Now we can chain it all using flatMap, and/or go back in the do…catch world, depending on our needs:

fetch(someURL) { (resultData: Result<NSData>) in
  let resultUser = resultData
    .flatMap(toJSON)
    .flatMap(extractUserDict)
    .flatMap(buildUser)

  // Then if we want to go back in the do/try/catch world for the rest of the code:
  do {
    let user = try resultUser.resolve()
    updateUI(user: user)
  } catch {
    print("Error: \(error)")
  }
}

I Promise, this is the Future

Result is cool, but given they are mainly useful in asynchronous functions (since we already have throw for synchronous function), why not make it so it also manage the asynchronicity?

Well actually, there is a type for that™. It’s called Promise (and sometimes Future, the two terms are very alike).

Promise is a type which kinda brings both the Result type (which can succeed or fail) and the asynchronicity together. A Promise<T> can either fulfill with a successful value of type T when that value becomes available later (hence the async aspect of it), or be rejected if an error occurred.

A Promise is also a monad. But usually instead of calling the monadic functions by the usual name map and flatMap, both are called then by convention:

class Promise<T> {
  // the monadic equivalent of "map", which is usually called "then" in the Promise type
  func then(f: T->U) -> Promise<U>
  // the monadic equivalent of "flatMap", which is usually called "then" too
  func then(f: T->Promise<U>) -> Promise<U>
}

And errors are unwrapped using .error or .recover. At call site, it can mainly be used the same way you’d use a Result. They are both monads after all:

fetch(someURL) // returns a `Promise<NSData>`
  .then(toJSON) // assuming toJSON is now a `NSData -> Promise<NSDictionary>`
  .then(extractUserDict) // assuming extractUserDict is now a `NSDictionary -> Promise<NSDictionary>`
  .then(buildUser) // assuming buildUser is now a `NSDictionary -> Promise<User>`
  .then {
    updateUI(user: user)
  }
  .error { err in
    print("Error: \(err)")
  }

See how smooth and readable this looks? It’s all about a stream of small processing steps chained together nicely. And it does all the heavy lifting of handling both the asynchronicity and the errors for you. If an error happens during the process, e.g. in extractUserDict, it jumps directly into the error block. Like with a do…catch. Or like with Result.

The implementation of fetch to use a Promise — instead of a completion block and a Result — would probably look something like this:

func fetch(url: NSURL) -> Promise<NSData> {
  // PromiseKit has a convenience `init` which turns a (T?, NSError?) closure into a `Promise`
  return Promise { resolve in
    NSURLSession.sharedSession().dataTaskWithURL(url) { (data, _, error) -> Void in
      resolve(data, error)
    }).resume()
  }
}

This fetch method will return immediately, so there is no completionBlock necessary. But it will return a Promise object, which will only execute the closure — given via the then — when the promise is fulfilled and the data (asynchronously) arrives later.

Observe and be Reactive

Promises are cool, but there is another concept which allows for a stream of small processing steps, handling asynchronicity, as well as handling and propagating errors whenever and wherever they occur during that stream.

This other concept is called Reactive Programming. Some of you might already know ReactiveCocoa (RAC in short), or RxSwift. Even if it’s sharing some concepts with Promises (asynchronicity, error propagation, …), that’s the next level beyond Futures & Promises: Rx allows multiple values to be emitted during time (instead of just one return value), and is a lot richer with tons of other features.

But that’s a whole new subject, which is gonna be an exploration for another day!

  1. This is only one possible implementation for Result. E.g. other implementations might allow to also specify a more specific error type. 

  2. Here I use a little trick by calling return completion(…) in this code instead of calling completion(…) then return to exit the function scope. This works because completion returns a Void and fetchUser returns a Void too (returns nothing), and because return Void is equivalent to just return. It’s a matter of taste, but I think it’s nice to be able to write this as a one-liner. 

  3. In this code, the @noescape keyword means that the throwingExpr closure is guaranteed to be used directly in the scope of the init function before the init function returns — in contrast of being stored in a property and used later. This allows the compiler not to force you in using self. or [weak self] at the call site when passing the closure, and be ensured retain cycles are avoided. 

  4. Take a pause for a second there. See how this code looks really like the one we wrote using UserBuilder at the beginning of that article? Feels like we were on the right path 😉