# The Many Faces of Zip: Part 1

Episode #23 • Jul 23, 2018 • Subscriber-Only

The zip function comes with the Swift standard library, but its utility goes far beyond what we can see there. Turns out, zip generalizes a function that we are all familiar with, and it can unify many seemingly disparate concepts. Today we begin a multipart journey into exploring the power behind zip.

###### Collection

Map, Zip, Flat‑Map › Zip

The Many Faces of Zip: Part 1
Introduction
00:05
The standard library zip
01:23
Defining our own zip
10:20
Zip as a generalization of map
15:03
Zip on other types
20:32
What’s the point?
27:35

### Introduction

On this series we’ve done a number of “deep dives” to really analyze a topic. Things like exploring the map function to see that it’s a very universal idea and we should be comfortable defining it on our own types. And things like “contravariance”, which is kind of unintuitive at first but is really quite handy.

This often helps us see something we think we’re familiar with in a new light, or helps us see something that is very unintuitive but become comfortable with it because we develop ways to concisely manage it and transform it.

We’re doing another one of those today, and this time it’s the zip function. You’ve probably used zip a few times in your every day coding, and you probably thought it was pretty handy. But what we want to show people is that it’s a generalization of something you are already familiar with, but you may have never thought of it like this before. And once you see this generalization it really helps unify a bunch of disparate ideas.

Let’s begin by exploring the zip that comes with Swift’s standard library.

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### Exercises

1. In this episode we came across closures of the form { (\$0, \$1.0, \$1.1) } a few times in order to unpack a tuple of the form (A, (B, C)) to (A, B, C). Create a few overloaded functions named unpack to automate this.

2. Define zip4, zip5, zip4(with:) and zip5(with:) on arrays and optionals. Bonus: learn how to use Apple’s gyb tool to generate higher-arity overloads.

3. Do you think zip2 can be seen as a kind of associative infix operator? For example, is it true that zip(xs, zip(ys, zs)) == zip(zip(xs, ys), zs)? If it’s not strictly true, can you define an equivalence between them?

4. Define unzip2 on arrays, which does the opposite of zip2: ([(A, B)]) -> ([A], [B]). Can you think of any applications of this function?

5. It turns out, that unlike the map function, zip2 is not uniquely defined. A single type can have multiple, completely different zip2 functions. Can you find another zip2 on arrays that is different from the one we defined? How does it differ from our zip2 and how could it be useful?

6. Define zip2 on the result type: (Result<A, E>, Result<B, E>) -> Result<(A, B), E>. Is there more than one possible implementation? Also define zip3, zip2(with:) and zip3(with:).

Is there anything that seems wrong or “off” about your implementation? If so, it will be improved in the next episode 😃.

7. In previous episodes we’ve considered the type that simply wraps a function, and let’s define it as struct Func<R, A> { let apply: (R) -> A }. Show that this type supports a zip2 function on the A type parameter. Also define zip3, zip2(with:) and zip3(with:).

8. The nested type [A]? = Optional<Array<A>> is composed of two containers, each of which has their own zip2 function. Can you define zip2 on this nested container that somehow involves each of the zip2’s on the container types?