Swift/Cocoa Type Dissonance

January 12, 2015

Cocoa was designed for Objective-C. Compared with Objective-C (b. 1989, like Taylor Swift), Swift is an infant. Clashes with the much older framework were inevitable. Apple has baked in special-purpose language features to ease interoperation, but fundamental disconnects remain.

Here are two cases where Swift’s type system fails to capture Cocoa’s intent. Modern type system features from other languages could help save the day in each case.

Stringly Typed Segues

Swift’s type system is much stronger (and, subjectively, much better) than Objective-C’s. As a result, Cocoa idioms that felt natural in Objective-C, where casts and checks are commonplace, are uncomfortable in Swift.

Case in point: storyboard transitions, called segues, move the application between views. Segues are associated with particular objects, which have specific types. But segues are stringly typed. The code that consumes identifier strings typically knows the associated types, but the compiler is powerless to help: the programmer has to supply all the type information.

This comes up when contacting one UIViewController from another when transitioning. The pattern invariably looks like this:

override func prepareForSegue(segue: UIStoryboardSegue,
                              sender: AnyObject?) {
  if (segue.identifier == "MySpecialSegue") {
    let myController =
      segue.destinationViewController as MyViewController

The transition code almost always needs to explicitly cast the destination controller (with as MyViewController). So the programmer needs to manually map the magic string identifier to this type. The redundancy feels natural in Objective-C, where this kind of thing happens all the time, and deeply uncomfortable in Swift.

A future Cocoa+Swift framework should make segues first class and strongly typed. A mechanism like type providers could help supply types from the storyboard file.

Storyboards and Dependency Injection

To communicate between view controllers, the standard advice is to inject dependencies on transitioning from one controller to another. In the above example, this would involve setting some fields on the destination controller inside the if. For example:

myController.parentController = self

Since the field is guaranteed to be set before the controller does anything else, you would want to declare the field as a non-optional type. Like so:

var parentController : SomeViewController

But the framework again writes checks that the type system can’t cash. Since the field is set on transition and not on controller initialization, the compiler cannot enforce what the programmer knows is true: this field will never be null when it matters. The field needs to be an implicitly unwrapped optional:

var parentController : SomeViewController!

This workaround papers over a fundamental disconnect. There is again an opportunity for Swift and Cocoa to evolve together. A notion of typestate or Rust’s simpler lifetimes, for instance, could resolve this discomfort.

More Types? Really?

Both examples call out for Apple to make Swift’s type system more powerful—but also more complicated. Languages with stronger types run the risk of offending workaday programmers with their complexity. Lattner and crew have a difficult job: to make good on their promise of safety while maintaining the ineffable mouthfeel of a “mainstream” language.