WWDC Notes

Embrace Swift type inference

Show Apple's description.
Swift uses type inference to help you write clean, concise code without compromising type safety. We’ll show you how the compiler seeks out clues in your code to solve the type inference puzzle. Discover what happens when the compiler can't come to a solution, and find out how Xcode 12 integrates error tracking to help you understand and fix mistakes at compile time.

Topics covered:

  1. Leveraging type inference
  2. How type inference works in the compiler
  3. Using Swift and Xcode to fix compiler errors

Review: What is Type Inference?

We can omit type annotations and other verbose details in our source code if the compiler can figure out those details from context.

If we write:

let x = ""

Then the compiler infers x to be of type String because the value is a string literal. This can be explicitly written in one of two ways:

let x: String = ""
// or
let x = "" as String

This is a small example, but type inference can get very powerful in large projects. For example, SwiftUI projects are composed of small, reusable views.

1. Leveraging type inference at the call site.

The sample project here is Fruta, a smoothie app. We want to add a search feature so that users can search for smoothies.

This is how the current SmoothieList view is implemented. It maps an array of Smoothie to a list of SmoothieRowView.

import SwiftUI

struct SmoothieList: View {
  var smoothies: [Smoothie]

  var body: some View {
    List(smoothies) { smoothie in
      SmoothieRowView(smoothie: smoothie)

To add search functionality, we need to filter the array of Smoothie using a String that the user searched for. We add that variable to SmoothieList:

@State var searchPhrase = ""

We also need to use a custom list view called FilteredList, which only shows objects that match a certain condition. So, we replace:

List(smoothies) { smoothie in ... }


  filterBy: \.title
  isIncluded: { title in title.hasSubstring(searchPhrase) }
) { smoothie in ... }

In this case, smoothies will only be included if their title field has the search phrase.

This call to the FilteredList initializer leans heavily on type inference. Let's look at the declaration of FilteredList and its initializer.

FilteredList is a general-purpose view, so it should work with any type of object. This is where generics come in handy.

import SwiftUI

public struct FilteredList<Element, FilterKey, RowContent>: View {
  public init() { ... }

  public var body: some View { ... }

Element, FilterKey, and RowContent are all placeholders to be replaced with actual types at the call site. These actual types (formally called "concrete types") are either specified at the call site or inferred by the compiler. In this case, Element is a placeholder for the array element type, FilterKey is a placeholder for the property on which to filter, and RowContent is a placeholder for the type of view to show in each row of the list.

Now, let's use these in the initializer. Any time you can use a generic in the initializer, you should do so, because that means less verbosity at the call site.

public init(
  _ data: [Element],
  filterBy key: KeyPath<Element, FilterKey>,
  isIncluded: @escaping (FilterKey) -> Bool,  // 1
  @ViewBuilder rowContent: @escaping (Element) -> RowContent  // 2
  1. isIncluded is marked as @escaping because it will need to be stored in a property, so it must exist outside the lifetime of the initializer.
  2. This is a function to map an element to a view. It's also escaping because it needs to be stored.

Also, the @ViewBuilder enables SwiftUI DSL syntax. We can define several child views by listing them out in the body of the closure. The ViewBuilder will collect the child views into a tuple for the parent to work with.

Here's an image of the definition and call site of FilteredList to show how the call site leans on type inference. Notice how clean the call site is. There are no explicit type annotations, but the compiler still has all the information it needs.

If all the argument types were explicitly specified in the code, the call site would look like this:

FilteredList<Element, FilterKey, RowContent>(
  smoothies as [Element],
  filterBy: \Element.title as KeyPath<Element, FilterKey>,
  isIncluded: { (title: FilterKey) -> Bool in title.hasSubstring(searchPhrase) },
) { (smoothie: Element) -> RowContent in
  SmoothieRowView(smoothie: smoothie)

Type inference helps us write source code faster because we don't need to explicitly specify all these types in our code.

2. How type inference works in the compiler

Think of type inference like a puzzle. The inference algorithm "fills in" the puzzle using clues from the source code. Filling in just one piece can also uncover more clues about the remaining pieces.

We're going to type-infer the call site of FilteredList just like the compiler.

We start with Element. The call site has:

smoothies as [Element]

We know smoothies is of type [Smoothie], so that means that Element = Smoothie. Let's do a find + replace on the verbose version of the call site:

FilteredList<Smoothie, FilterKey, RowContent>(
  smoothies as [Smoothie],
  filterBy: \Smoothie.title as KeyPath<Smoothie, FilterKey>,
  isIncluded: { (title: FilterKey) -> Bool in title.hasSubstring(searchPhrase) },
) { (smoothie: Smoothie) -> RowContent in
  SmoothieRowView(smoothie: smoothie)

Now, filling in a concrete type for Element gave a clue about the concrete type of FilterKey because now we know that KeyPath literal is referring to Smoothie.title. We know that Smoothie.title is a String, therefore FilterKey is also a String. Let's do another find + replace.

FilteredList<Smoothie, String, RowContent>(
  smoothies as [Smoothie],
  filterBy: \Smoothie.title as KeyPath<Smoothie, String>,
  isIncluded: { (title: String) -> Bool in title.hasSubstring(searchPhrase) },
) { (smoothie: Smoothie) -> RowContent in
  SmoothieRowView(smoothie: smoothie)

The last piece of the puzzle is RowContent, which is the return type of the trailing ViewBuilder closure. Since this closure only has 1 view in the body, the ViewBuilder will return the same type as the child view, SmoothieRowView. Find + replace again:

FilteredList<Smoothie, String, SmoothieRowView>(
  smoothies as [Smoothie],
  filterBy: \Smoothie.title as KeyPath<Smoothie, String>,
  isIncluded: { (title: String) -> Bool in title.hasSubstring(searchPhrase) },
) { (smoothie: Smoothie) -> SmoothieRowView in
  SmoothieRowView(smoothie: smoothie)

We've solved the last piece of the puzzle. This is the same strategy that the compiler uses with our code. Each step of the algorithm uncovers more clues for the next step.

However, it's possible for one of the clues to cause the compiler to fill in a concrete type that doesn't fit in with the rest of the puzzle. If one of the pieces doesn't fit and the puzzle can't be solved, there's an error in the source code.

Solving the puzzle in the presence of source code errors

Let's rewind to when the compiler found the concrete type for FilterKey:

FilteredList<Smoothie, FilterKey, RowContent>(
  smoothies as [Smoothie],
  filterBy: \Smoothie.title as KeyPath<Smoothie, FilterKey>,
  isIncluded: { (title: FilterKey) -> Bool in title.hasSubstring(searchPhrase) },
) { (smoothie: Smoothie) -> RowContent in
  SmoothieRowView(smoothie: smoothie)

In the previous step, the compiler inferred Smoothie as the KeyPath base type. It used this information to figure out the concrete type for FilterKey by looking up the type of Smoothie.title.

What if we had incorrectly passed Smoothie.isPopular: Bool instead of Smoothie.title: String? The compiler would have tried to infer the type of FilterKey to be Bool. It would continue to fill in the other FilterKey placeholders with that same incorrect type. Eventually, it would have tried to make sense of the following line of code:

isIncluded: { (title: Bool) -> Bool in title.hasSubstring(searchPhrase) }

Bool does not have a property hasSubstring and the compiler would report an error.

3. Using Swift and Xcode to fix compiler errors

The Swift compiler is designed to catch mistakes by integrating error tracking into the type inference algorithm to use later on in error messages.

Integrated error tracking

During type inference, the compiler:

  1. Records information about errors in source code
  2. Attempts to fix errors using heuristics so it can continue the type inference algorithm
  3. Provides actionable error messages based on collected information

Integrated error tracking was introduced in Swift 5.2 and Xcode 11.4. In Swift 5.3 and Xcode 12, the compiler uses this new strategy for all error messages and expressions.

Using Xcode to fix errors in Swift code

Before writing any code, open up Xcode > Behaviors > Edit Behaviors. Add a behavior to automatically show the issue navigator when the build fails:

Now, Xcode will show all the errors across the project each time it fails to build.

The current scene shows the implementation of SmoothieList and its preview. FilteredList has already been added to the project.

Before replacing List with FilteredList, we need to add a search field above the list as a TextField.

After attempting to build, there is a compiler error on the line of code that was just added.

We can expand the error by clicking on it.

The error tells us that we provided a String, but the TextField expects a different type. We made the mistake of passing the value, not the binding. The Swift compiler was able to figure out that the binding does have a compatible type, and it provided a fix-it to refer to the binding with $.

Next, we want to replace List with FilteredList as in the code examples above. We attempt to build and get another error:

The error tells us that Smoothie needs to conform to Identifiable in order to work with FilteredList. That might be confusing because Identifiable isn't written anywhere in this code. However, in the left, there is a compiler note in gray attached to the error saying "Where 'Element' = 'Smoothie'". This note is a "breadcrumb" from the compiler so we know what it was doing.

To view the note side-by-side with the error, we can close the preview canvas with the "CMD+Enter" shortcut. Then, we can hold down "Option+Shift" while clicking on the gray compiler note. This opens the Destination Chooser.

We can move our cursor over to the right and hit "Enter" to open the source of the error in a new editor on the right.

Looking at the full declaration, we can see that Element must conform to Identifiable. Because the compiler inferred Smoothie as the concrete type of Element, Smoothie must also conform to Identifiable.

All we need to do is jump to the definition of Smoothie and add a conformance to Identifiable. Smoothie already has a property called id, so it is eligible to conform.

We attempt to build again, and it succeeds this time.

Wrap up

  • SwiftUI code relies on type inference for reusable views
  • Type inference fills in incidental details using clues from source code
  • Integrated error tracking leaves breadcrumbs for error messages

To learn more about integrated error tracking, read the blog post on Swift's new diagnostic architecture.

To learn more about generics, watch WWDC 2018's video on Swift Generics.

Missing anything? Corrections? Contributions are welcome 😃


Written by


Sarthak Khillon

User-focused Software Engineer with a love for mobile platforms.