Alex Ozun: Type-Driven Design with Swift (SwiftCraft 2024)

Type-Driven Design with Swift by Alex Ozun from this year’s SwiftCraft is a recommended watch.

Alex covers concepts like replacing the notion of validation user input (with true/false results to represent its validity) with the notion of parsing – transforming unprocessed values into processed ones that uphold certain guarantees, like matching a string pattern. For example for email addresses.

The biggest idea tying all of the things Alex covers together is iteratively using Swift’s strong type system to sketch and then build a domain that ensures certain classes of mistakes are impossible to make. Because if you would try to do them, the compiler would complain.

I also put into my Zettelkasten a couple of concepts that didn’t have a home before, like

  • Black hole arguments to express requirements in types;
  • the idea that only compilation errors prevent faults in production;
  • “Type, Define, Refine” flow to give iterative type-writing a name;
  • that we can achieve type unions in Swift via enums;
  • the idea of “Parse, Don’t Validate”.

Model-View-Presenter, Form Validation, and a Unidirectional State Machine

As an exercise, I wrote a simple iOS scene transition from a login form (root view controller) to a success scene. I was experimenting with the Model-View-Presenter approach where view and presenter know each other, but communicate through protocols, so you program to an interface, not the implementation. I ended up using a simple state machine to represent the view’s state, and employed form validation that is represented in the types. I like how the result reads, so here’s a rundown of the exercise:

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