I am pleased to present my command line tool that I hacked together on a Saturday morning, the “existentialannotator,” which can be found on github. As the name suggests, this tool performs a specific function: scanning your Swift files, identifying all declared protocols, and annotating all existential types with any. This feature will prove invaluable with the upcoming release of Swift 6. To get started, you have two options: installing it via Homebrew or obtaining the source code directly from GitHub and building it yourself. Once installed, simply navigate to your working directory and execute the command existentialannotator . to let the tool do its job.


Concept of existential types in Swift has not been heavily discussed topic until recently when it was introduced as part of the Swift Evolution process on GitHub. Essentially, an existential type represents the existence of any specific type without specifying that particular type explicitly. This means that certain code, like the example below, will not compile in Swift 6:

protocol Vehicle {
  var batteryPercentage: Float { get }

struct HumanDriver {
  let vehicle: Vehicle

  func drive() {
    // Drive the vehicle

The compilation fails because let vehicle: Vehicle utilizes an existential type without being explicitly annotated with the new keyword any.

Understanding Existential Types

In Swift, an existential type provides a way to handle values of different types in a unified manner, abstracting their specific type information. In the example above, we used the protocol Vehicle instead of a concrete type, demonstrating the essence of an existential type.

What’s New in Swift 6?

In Swift 6, every existential type must be marked with any. Failure to do so will result in a compilation error. Consequently, the code above let vehicle: Vehicle would now require the notation let vehicle: any Vehicle. This is where my tool, the Existential Annotator, comes in handy, particularly when dealing with large codebases.

Final Remarks

Although I put this tool together on Saturday morning, it is not flawless. There are numerous potential performance improvements that could be implemented. Nonetheless, I consider this tool complete, given its limited lifespan. As we move past the initial release of Swift 6, this tool will likely lose its relevance. Nevertheless, if you believe it could benefit from enhancements, please feel free to submit a pull request.