Asserting a restriction on an associated value in an enum

Question

I have an enum like...

enum Analytics {
  case event(name: String)
  case other1
  case other2
}

But I need to make sure that the name value is no longer than 40 characters.

Is there a way to have some sort of precondition on the value put into the enum? I'm not sure where this would/should exist? If this were a struct I could put a precondition or something in the init but how would I do that for an enum?

Thanks

Answer 1

In Swift, a restriction or refinement of a type can only be put in place using the type system.

enum Analytics {
  case event(name: String.LimitedTo20Count)
  case other1
  case other2
}

extension Collection {
  typealias LimitedTo20Count = ModuleName.LimitedTo20Count<Self>
}

struct LimitedTo20Count<Collection: Swift.Collection> {
  struct Error: Swift.Error { }
  
  init(wrappedValue: Collection) throws {
    guard wrappedValue.count <= 20 else {
      throw Error()
    }
    self.wrappedValue = wrappedValue
  }

  let wrappedValue: Collection
}