A new Swift video series exploring functional programming and more.
#35 • Monday Oct 29, 2018 • Subscriber-only

Advanced Protocol Witnesses: Part 1

Now that we know it’s possible to replace protocols with concrete datatypes, and now that we’ve seen how that opens up new ways to compose things that were previously hidden from us, let’s go a little deeper. We will show how to improve the ergonomics of writing Swift in this way, and show what Swift’s powerful conditional conformance feature is represented by just plain functions.

#35 • Monday Oct 29, 2018 • Subscriber-only

Advanced Protocol Witnesses: Part 1

Now that we know it’s possible to replace protocols with concrete datatypes, and now that we’ve seen how that opens up new ways to compose things that were previously hidden from us, let’s go a little deeper. We will show how to improve the ergonomics of writing Swift in this way, and show what Swift’s powerful conditional conformance feature is represented by just plain functions.


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Exercises

  1. Currently in Swift (as of 4.2) there is no way to extend tuples to conform to protocols. Tuples are what is known as “non-nominal”, which means they behave differently from the types that you can define. For example, one cannot make tuples Equatable by implementing extension (A, B): Equatable where A: Equatable, B: Equatable. To get around this Swift implements overloads of == for tuples, but they aren’t truly equatable, i.e. you cannot pass a tuple of equatable values to a function wanting an equatable value.

    However, protocol witnesses have no such problem! Demonstrate this by implementing the function pair: (Combining<A>, Combining<B>) -> Combining<(A, B)>. This function allows you to construct a combining witness for a tuple given two combining witnesses for each component of the tuple.

  2. Functions in Swift are also “non-nominal” types, which means you cannot extend them to conform to protocols. However, again, protocol witnesses have no such problem! Demonstrate this by implementing the function pointwise: (Combining<B>) -> Combining<(A) -> B>. This allows you to construct a combining witness for a function given a combining witnesss for the type you are mapping into. There is exactly one way to implement this function.

  3. Protocols in Swift can have “associated types”, which are types specified in the body of a protocol but aren’t determined until a type conforms to the protocol. How does this translate to an explicit datatype to represent the protocol?

  4. Translate the RawRepresentable protocol into an explicit datatype struct RawRepresenting. You will need to use the previous exercise to do this.

  5. Protocols can inherit from other protocols, for example the Comparable protocol inherits from the Equatable protocol. How does this translate to an explicit datatype to represent the protocol?

  6. Translate the Comparable protocol into an explicit datatype struct Comparing. You will need to use the previous exercise to do this.


References

  • 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

    Gabriel 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 Gabriel Gonzalez lays down the case for scrapping type classes and just using simple datatypes.

  • 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.

  • Contravariance

    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.

Chapters
Introduction
00:05
Renaming contramap
01:25
Witness ergonomics
09:52
Conditional conformance
19:25
Nested conditional conformance
30:54
Till next time…
34:53
Credits

Hosted by Brandon Williams and Stephen Celis. Recorded in Brooklyn, NY.