released Tue, 29 Mar 2016
Swift Version 5.0

Three tips for concise Swift using the Guard statement

Three quick examples for how you can use guard to write shorter and simpler code

This will be a shorter post compared to some of my previous ones, but I wanted to share three useful guard tips for structuring your functions in such a way that you end up with code that is more concise and also easier to understand. This is not a post about general coding styles or coding guidelines, but more about how guard can help you simplify your code.

Binding and Condition Combination

Nesting

The first example concerns the use of pattern matching in order to let bind variables into the current scope. One thing I really like about this syntax (compared to, say if let) is that it keeps a golden code path, guarding you from the all-too common skyscraper of death. Compare:

// Lots of nonsensical code to show how nested code structures look confusing
if let a = a() {
  let x = b(a)
  x.fn()
  if let u = x.nxt() {
    let ux = u.brm()
    if let uxt = ux.nxt() {
       perform(uxt)
    }
  }
}

with:

guard let a = a() else { return }
let x = b(a)
x.fn()
guard let u = x.nxt() else { return }
let ux = u.brm()
guard let uxt = ux.nxt() else { return }

perform(uxt)

Now these are awful examples of how not to structure an internal API, but they exist more to drive a point home. Guard is great because it binds the result into the current scope instead of the nested scope. In larger functions, this makes all the difference between having huge, difficult-to-grasp deeply-nested functions and clean versions which look almost like lists of commands.

Pattern Binding

The above works even better, if your input is an enum. Consider how we're handling the following usecase:

protocol NotificationListener {
  func handleNotification(notification: Notification)
}
enum Notification {
  case userLoggedIn(user: String, date: NSDate, domain: String)
  case fileUploaded(file: String, location: String, size: Int, user: String)
}

struct FileUploadHandler: NotificationListener {
  /// Implement the notification handling to move uploaded files to temporary folder
  func handleNotification(notification: Notification) {
    guard case .fileUploaded(let file, let location, _, let user) = notification
    else { return }

    if user == self.currentUser {
       self.moveFile(file, atLocation: location)
    }
  }
}

The binding in the guard case line achieves two things for us:

  1. It makes sure handleNotifications only works for fileUploaded notifications, and not for userLoggedIn notifications.
  2. It binds all the associated values of the enum into the current scope, making it easy for us to use the data.

Where Clauses

However, with the power of guard, we can even simplify the example. Lo and behold:


struct FileUploadHandler: NotificationListener {
  /// Implement the notification handling to move uploaded files to temporary folder
  func handleNotification(notification: Notification) {
    guard case .fileUploaded(let file, let location, _, let user) = notification, 
          user == self.currentUser
          else { return }
    moveFile(file, atLocation: location)
  }
}

Now, the code is even shorter as the where clause of the guard expression does the correct matching for us.

You can have multiple where clauses in your guard statement:

import Foundation

func confirmPath(pathObject: AnyObject) -> Bool {
  guard let url = pathObject as? URL,
      let components = url.pathComponents, 
      components.count > 0,
      let first = components.dropFirst().first, 
      first == "Applications",
      let last = components.last, 
      last == "MyApp.app"
  else { return false }
  print("valid folder", last)
  return true
}
print(confirmPath(pathObject: URL(fileURLWithPath: "/Applications/MyApp.app")))

As you can see here, we're combining multiple let bindings with related where clauses which makes it easy to handle all the preconditions in one bigger guard statement instead of having to break it up into multiple singular statements.

Nested Enums

The above even works for nested enums. This may sound like a far-fetched example, but I do actually have a project where I'm using a nested enum. In this example, we have a list of different items in the sidebar of an Instagram client. Those can be headlines, seperators, or folders:

enum SidebarEntry {
  case headline(String)
  case item(String)
  case seperator
}

A sidebar could be defined by an array like this:

[.headline("Global"),
 .item("Dashboard"),
 .item("Popular"),
 .seperator,
 .headline("Me"),
 .item("Pictures"),
 .seperator,
 .headline("Folders"),
 .item("Best Pics 2013"),
 .item("Wedding")
]

Here, each Item would have to have a different action: I.e. clicking "Dashboard" should do something different compared to clicking "Pictures", or the "Wedding" folder. The solution I chose was to have another, nested, enum within the Item enum:

enum Action {
  case .popular
  case .dashboard
  case .pictures
  case .folder(name: String)
}

enum SidebarEntry {
  case headline(String)
  case item(name: String, action: Action)
  case seperator
}

[.headline("Global"),
 .item(name: "Dashboard", action: .Dashboard),
 .item(name: "Popular", action: .Popular),
 .item(name: "Wedding", action: .Folder("fo-wedding")]

Now, if we want publish a folder (to the cloud) we'd like to really make sure that we were called with a folder and not a headline or a Popular item:

func publishFolder(entry: SidebarEntry)  {
  guard case .Item(_, .folder(let name)) = entry 
    else { return }
  Folders.sharedFolders().byName(name).publish()
}

This is a great way to model complex hierarchies but still be able to match even intricate, nested types.

One-Line Guard Return

This is a short one. When you end up in the else case, you may want to perform an action before you return:

guard let a = b() else {
   print("wrong action")
   return
}
// or
guard let a = b() else {
   self.completion(items: nil, error: "Could not")
   return
}

As long as your command returns void, you can actually combine these into one:

guard let a = b() else {return print("wrong action")}
// or
guard let a = b() else {
   return self.completion(items: nil, error: "Could not")
}

I find this much easier on the eyes and better to read. However, it may reduce readability in a complex project when another developer runs into this and wonders what kind of type is being returned here.

Alternatively, you can also use the semicolon in these cases1:

guard let a = b() else {
  print("argh"); return
}

try? in guards

Finally, in cases where you'd need to perform a throwable function, and you don't care about the error result, you can still happily use guard just by utilizing the try? syntax, which converts the result of your throwing call into an optional, depending on whether it worked or not:

guard let item = item,
   result = try? item.perform()
else { return print("Could not perform") }

The neat thing about this is that it allows us to combine various Swift mechanics into one safe call to make sure that our code can safely proceed.

Wrapping Up

Everything combined into one long example. This also shows how you can combine case and let in one guard.

guard let messageids = overview.headers["message-id"],
    let messageid = messageids.first,
    case .MessageId(_, let msgid) = messageid
    where msgid == self.originalMessageID
    else { return print("Unknown Message-ID:", overview) }

That's it. For more detailed information, I recommend reading my much larger articles on pattern matching and enum.

1

After leaving Objective-C behind, you'll probably have to search your keyboard to find the key for it again ;)