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Social Swift Style Survey #1

So, Xcode 6 beta 7 came out today, and with it, lots of changes to the Swift APIs for iOS and OS X. Mostly, it’s the ongoing removal of the ! character from parameters and return types. The bang is an implicitly-unwrapped optional, meaning it’s an optional — a variable that may or may not have a value at all — but since having to check lots of variables against nil and unwrap them would be burdensome, you’re allowed to treat it like a normal non-optional variable. Only downside is that your app crashes if you don’t unwrap and it turns out to be nil.

In the early Xcode 6 betas, nearly every parameter in Apple APIs was a bang, since they had to be optionals (nils were commonplace in Objective-C), but Swift adoption would have been hindered if we’d spent June through August unwrapping them.

For the last few betas, Apple engineers have been doing yeoman’s work, going through all these APIs and identifying what must be an optional, and what doesn’t need to be. So, the bangs are disappearing, replaced by either ? for honest-to-goodness optionals that should be tested against nil, or empty space, indicating the variable will always have a value.

If you’re writing a lot of Swift code — say, for a beginner’s book with 30+ versions of your sample code app — you’re used to each new build breaking your code, and forcing you to do through and make some pretty simple edits to deal with API changes as the bangs disappear.

Swift compiler errors caused by removal of implicitly-unwrapped optionals in Xcode betas

In some cases, it’s making me do unwraps where I’d previously been playing fast and loose with the bang-type. And that brings me to a question. There are two ways to do unwraps, and I’m not exactly in love with either of them.

Here are two examples. In beta 7, all the UIViewController properties for finding parent containers (navigationController, splitViewController, tabBarController) have all become full-on optionals, so a reference to self.splitViewController.delegate no longer works. One way to deal with this is to explicitly test for nil, and then use the bang operator to unwrap inside the if block.

if self.splitViewController != nil {
    self.splitViewController!.delegate = self

This is OK, except that when you have a non-trivial number of lines in the block, all the bangs seem dirty. We can avoid this with option #2: casting to a non-optional type in the if:

if let validSplitViewController = self.splitViewController {
    validSplitViewController.delegate = self

Fewer bangs, but now I’m burdened with coming up with a name for the non-optional version of the variable. And good variable names are hard to come up with. Heck, what’s even a good convention for this? validFoo? unwrappedFoo? safeFoo? It feels like I’m polluting my code with more variables to keep track of.

Since I’m not pair-programming or working with a lot of people producing Swift code, I’m not sure which is becoming the preferred idiom, and I don’t want to inadvertently write a book of my own idiosyncratic coding style.

Next best thing… a poll! Let me know what you think, and I’ll ack back at some point with the results.

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Thanks in advance for your votes, comments, and pre-orders.

RESULTS UPDATE (Sept. 5): After a couple days and 38 votes, results are:

  • 26 votes for Use if let / if var to unwrap to a new variable
  • 5 votes for Test against nil, unwrap with !
  • 7 write-ins, almost all for an optional-chaining alternative I failed to mention: self.splitViewController?.delegate = self