John Sundell recently wrote an article on the topic of categorizing and naming protocols in Swift. As I was reading it, I realized I had a different take on several ideas shared in the article.

But before I go into the details, I’d like to say that I’m a big fan of John’s work. The first thing I saw from John was his conference talk on system design. Afterward, I started listening to his podcast and reading his articles regularly. I appreciate his efforts to educate the audience about the best practices of programming in Swift. And this post is not about whether he’s wrong and I’m right, or vice versa. This post is about having different perspectives on the categorization of components and naming them. And the difference in opinion on these topics can create a lot of friction in teams. There isn’t always going to be a right and wrong approach to a certain aspect of programming, but there can always be a shared convention within a team working on the same project. And it’s important to have such conventions and consistency in following them because it helps the readability of the codebase. Btw. does anyone still choose tabs over spaces? 🙊

In this particular article, as always, John brings some good points about writing generic code with protocol-oriented design and hiding details of third-party interfaces behind internal protocols. As for the protocol categorization, he suggests four categories of protocols:

  1. Action enablers
  2. Requirement definitions
  3. Type conversion
  4. Abstract interfaces

While it’s a reasonable categorization, I would prefer one that provides more straightforward guidelines on the protocols’ naming.

Protocol categorization and naming

If we look at the Swift API design guidelines, there are two bullets describing the naming of protocols.

  • Protocols that describe what something is should read as nouns (e.g. Collection).
  • Protocols that describe a capability should be named using the suffixes able, ible, or ing (e.g. Equatable, ProgressReporting).

We can take the capability category, and we can divide it into two subcategories, one where the implementer is a subject performing an action, and the other where the implementer is an object the action is performed on.

In summary, we can define three categories of protocols:

  1. the implementer is something - named with a noun
  2. the implementer is doing something - named with an adjective ending with ing
  3. something is done to the implementer - named with an adjective ending with able or ible

I haven’t yet found short and meaningful names for these categories, but please let me know in the comments if you have some suggestions.


Now let’s go through some of the examples John provided.

There is a Loadable protocol defined which unifies several types that load various objects or values.

protocol Loadable {
    associatedtype Result
    func load() throws -> Result

If we look at what this type does, it performs an action of loading. It is not being loaded itself. The Result is the thing being loaded. Thus an appropriate name for this protocol would be Loading or ResultLoading.

protocol Loading {
    associatedtype Result
    func load() throws -> Result

And if we want the result to be Loadable, we can define the protocol that way and make the result type conform to it.

protocol Loadable {
    func load() throws -> Self

extension SomeLoadableType: Loadable {
    func load() throws -> Self { ... }

But that’s probably not a good idea. A loading operation usually requires some dependencies to a database or a network interface, and the result of a loading operation is typically a data object. And data objects should only contain data and not hold references to the infrastructure components.

The next example discussed is Cachable.

protocol Cachable: Codable {
    var cacheKey: String { get }

John argues that this is not a good name because the Cachable protocol doesn’t contain any methods that would perform the caching. But the question is, why should Cachable have any methods to perform caching? The protocol describes something that can be cached and the requirement for that something is to provide a cacheKey. Cachable is a suitable name for it. If this type itself needed to perform caching actions, then a more fitting name for it would be Caching.

The following example is a more complicated one - the Swift.Collection.Sequence protocol from the Swift Standard Library.

public protocol Sequence {
    associatedtype Iterator: IteratorProtocol
    func makeIterator() -> Self.Iterator

First, let’s look at the IteratorProtocol protocol. Adding a Protocol suffix to the protocol is easy and convenient, but I have yet to encounter an example where that was the best option. I’ve found that picking from the three categories defined above is a better approach. So how can we improve the naming here?

Whenever I deal with generics, I like to suffix the type parameter name with a Type suffix. That makes it very easy to distinguish between actual types and their placeholders in the generic code, and makes the code more readable. So in the Sequence example, I would propose the following naming.

public protocol Sequence {
    associatedtype IteratorType: Iterator
    func makeIterator() -> Self.IteratorType

And following the same convention, let’s update the earlier Loading protocol.

protocol Loading {
    associatedtype ResultType
    func load() throws -> ResultType

But getting back to the Sequence example, naming the Iterator as such doesn’t sound quite right, because the Iterator doesn’t fit the is something category. Apple’s documentation summarizes the Iterator type like this:

A type that supplies the values of a sequence one at a time.

The Iterator has a next() method which provides the next item in the sequence. The implementer of this protocol is something that can be iterated. So a more appropriate name for the protocol would be Iterable. And finally the Sequence protocol would look like the following.

public protocol Sequence {
    associatedtype IterableType: Iterable
    func makeIterable() -> Self.IterableType

The next example is a ColorProvider protocol described as an interface for a container of colors.

protocol ColorProvider {
    var foregroundColor: UIColor { get }
    var backgroundColor: UIColor { get }

Again, I find using a noun here inappropriate because this type is not in the is something category, but in the is doing something category. This type provides colors, so it should be named ColorProviding. There are many examples of type names like Manager, Coordinator, and Generator, where a verb is turned into a noun. When used in protocol names, it would be more fitting to turn these verbs into adjectives - Managing, Coordinating, Generating.

Another example from John’s article is a protocol that describes something that has a title property. This one is tricky. John placed it into the type conversion category suggesting the TitleConvertible name.

protocol TitleConvertible {
    var title: String { get }

I get the line of thinking there, but since the title is not a type to which the implementer can be converted, I would look further for a good candidate for this protocol’s name. And I would even say that the fact that something has a title doesn’t justify having a separate protocol. To put it differently, it’s not useful for many different types to conform to the same generic title-requiring protocol, because it’s unlikely that there is going to be generic logic applicable to anything that has a title. But if there is something that is something and also has a title, it might be worth having a protocol for it. For example, a view that has a title could be defined as a TitledView protocol, which extends from a View protocol.

protocol TitledView: View {
    var title: { get set }


To reiterate the previous points, I would suggest using the following categorization and naming guidelines for the protocols in Swift:

  1. If the implementer is something, name the protocol with a noun, e.g. Sequence, View, Repository
  2. If the implementer is doing something, name the protocol with an adjective ending with ing, e.g. Loading, Generating, Coordinating
  3. If something is done to the implementer, name the protocol with an adjective ending with able or ible, e.g. Comparable, Codable, Cachable

If you have any thoughts, remarks, or a different viewpoint on any of the above, please let me know in the comments.

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