Advanced Protocol Witnesses: Part 2

Episode #36 • Nov 5, 2018 • Subscriber-Only

We complete our dictionary for translating Swift protocol concepts into concrete datatypes and functions. This includes protocol inheritance, protocol extensions, default implementations and protocols with associated types. Along the way we will also show how concrete types can express things that are currently impossible with Swift protocols.

Advanced Protocol Witnesses: Part 2
Tuple witnesses
Function witnesses
Protocol inheritance
Protocol extensions
Associated types
What’s the point?

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It’s amazing that we can recover conditional conformance with just plain functions. It seems like such a complex feature, and it definitely is from the perspective of protocols and type systems, but at the value level it is so simple.

A complicated compiler feature can be reframed as function composition. Still, we have that compiler feature and we can use it, even if it took four years to get there. And we used it to great benefit with our Tagged and NonEmpty types. Still, there are plenty of things that you can’t do these days with protocols, but you can do them with witnesses.

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  1. Currently Swift does not allow protocol methods to contain arguments with default values. For example, the following protocol is not representable in Swift:

    protocol Service {
      func fetchUser(id: Int, cache: Bool = false) -> User?
    // 🛑 Default argument not permitted in a protocol method

    Show how this can be done with a concrete data type representation of Service.

  2. Currently Swift does not allow protocols to extend other protocols, even if you provide all of the extensions requirements. For example, we cannot extend Numeric to be combinable even though it is easy to implement the requirement:

    extension Numeric: Combinable {
      func combine(with other: Self) -> Self {
        return self + other
    // 🛑 Extension of protocol 'Numeric' cannot have an inheritance clause

    Show how this can be done with a concrete data type representation of Service.


Protocol-Oriented Programming in Swift

Apple • Tuesday Jun 16, 2015

Apple’s eponymous WWDC talk on protocol-oriented programming:

At the heart of Swift’s design are two incredibly powerful ideas: protocol-oriented programming and first class value semantics. Each of these concepts benefit predictability, performance, and productivity, but together they can change the way we think about programming. Find out how you can apply these ideas to improve the code you write.

Modern Swift API Design

Apple • Wednesday Jan 2, 2019

As of WWDC 2019, Apple no longer recommends that we “start with a protocol” when designing our APIs. A more balanced approach is discussed instead, including trying out concrete data types. Fast forward to 12:58 for the discussion.

Every programming language has a set of conventions that people come to expect. Learn about the patterns that are common to Swift API design, with examples from new APIs like SwiftUI, Combine, and RealityKit. Whether you’re developing an app as part of a team, or you’re publishing a library for others to use, find out how to use new features of Swift to ensure clarity and correct use of your APIs.


We use the term pullback for the strange, unintuitive backwards composition that seems to show up often in programming. The term comes from a very precise concept in mathematics. Here is the Wikipedia entry:

In mathematics, a pullback is either of two different, but related processes: precomposition and fibre-product. Its “dual” is a pushforward.

Protocols with Associated Types

Alexis Gallagher • Tuesday Dec 15, 2015

This talk by Alexis Gallagher shows why protocols with associated types are so complicated, and tries to understand why Swift chose to go with that design instead of other alternatives.

Protocol Oriented Programming is Not a Silver Bullet

Chris Eidhof • Thursday Nov 24, 2016

An old article detailing many of the pitfalls of Swift protocols, and how often you can simplify your code by just using concrete datatypes and values. Chris walks the reader through an example of some networking API library code, and shows how abstracting the library with protocols does not give us any tangible benefits, but does increase the complexity of the code.

Value-Oriented Programming

Matt Diephouse • Sunday Jul 29, 2018

Matt gives another account of protocol-oriented programming gone awry, this time by breaking down the famous WWDC talk where a shape library is designed using protocols. By rewriting the library without protocols Matt ends up with something that can be tested without mocks, can be inspected at runtime, and is more flexible in general.

Scrap your type classes

Gabriella Gonzalez • Wednesday May 2, 2012

Haskell’s notion of protocols are called “type classes,” and the designers of Swift have often stated that Swift’s protocols took a lot of inspiration from Haskell. This means that Haskellers run into a lot of the same problems we do when writing abstractions with type classes. In this article Gabriella Gonzalez lays down the case for scrapping type classes and just using simple datatypes.

Haskell Antipattern: Existential Typeclass

Luke Palmer • Sunday Jan 24, 2010

A Haskell article that demonstrates a pattern in the Haskell community, and why it might be an anti-pattern. In a nutshell, the pattern is for libraries to express their functionality with typeclasses (i.e. protocols) and provide Any* wrappers around the protocol for when you do not want to refer to a particular instance of that protocol. The alternative is to replace the typeclass with a simple concrete data type. Sound familiar?

Some news about contramap

Brandon Williams • Monday Oct 29, 2018

A few months after releasing our episode on Contravariance we decided to rename this fundamental operation. The new name is more friendly, has a long history in mathematics, and provides some nice intuitions when dealing with such a counterintuitive idea.


Julie Moronuki & Chris Martin

This article describes the ideas of contravariance using the Haskell language. In many ways exploring functional programming concepts in Haskell is “easier” because the syntax is sparse and allows you to focus on just the core ideas.

Protocol Witnesses: App Builders 2019

Brandon Williams • Friday May 3, 2019

Brandon gave a talk about “protocol witnesses” at the 2019 App Builders conference. The basics of scraping protocols is covered as well as some interesting examples of where this technique really shines when applied to snapshot testing and animations.

Protocol-oriented programming is strongly recommended in the Swift community, and Apple has given a lot of guidance on how to use it in your everyday code. However, there has not been a lot of attention on when it is not appropriate, and what to do in that case. We will explore this idea, and show that there is a completely straightforward and mechanical way to translate any protocol into a concrete datatype. Once you do this you can still write your code much like you would with protocols, but all of the complexity inherit in protocols go away. Even more amazing, a new type of composition appears that is difficult to see when dealing with only protocols. We will also demo a real life, open source library that was originally written in the protocol-oriented way, but after running into many problems with the protocols, it was rewritten entirely in this witness-oriented way. The outcome was really surprising, and really powerful.

Some news about contramap

Brandon Williams • Monday Oct 29, 2018

A few months after releasing our episode on Contravariance we decided to rename this fundamental operation. The new name is more friendly, has a long history in mathematics, and provides some nice intuitions when dealing with such a counterintuitive idea.