Patterns for Working With Associated Types

Associated Types

released Fri, 01 Mar 2019
Swift Version 5.0

Associated Types

Swift is a powerful language with a very powerful type system. Among the features that define said type system are associated types. They can be defined on a protocol to allow implementors of the protocol to specialize certain types in a generic way:

protocol Example {
  associatedtype Value
  var value: Value { get }

In the snippet above, any type that implements the Example protocol has to define the Value type. Protocols with associated types can be understood as unfinished types. Compared to regular protocols, which can be used within Swift like normal types, those protocols can only be used as a generic constraint. This means that once your type requires an associated type, using it suddenly becomes much more complicated.

The example below shows an example of finishing a type. By explicitly telling the compiler that the Value type is Int it is now able to understand ImplementExample fully.

struct ImplementExample: Example {
  typealias Value = Int

Associated types are useful for a certain kind of problems where subclassing and composition does allow you to build the right kind of abstractions. However, this is a seperate topic. The topic of this article, on the other hand, is what to do when you end up with associated types trouble.