Macro Bonanza: Case Paths

Monday November 13, 2023
Preamble

To celebrate the release of Swift macros we releasing updates to 4 of our popular libraries to greatly simplify and enhance their abilities: CasePaths, ComposableArchitecture, SwiftUINavigation, and Dependencies. Each day this week we will detail how macros have allowed us to massively simplify one of these libraries, and increase their powers.

When we first saw macros we knew they had the ability to completely transform our CasePaths library, which currently heavily depends on runtime reflection to work. We feel we have settled on a design that brings many of the powers of key paths to enums. Join us for a quick overview, and be sure to check out version 1.1 of CasePaths today!

@CasePathable

The only macro added to the CasePaths library is @CasePathable, and it can be applied to any enum:

@CasePathable
enum Destination {
  case activity(ActivityModel)
  case settings(SettingsModel)
}

That immediately gives you access to what is known as a CaseKeyPath for each case of the enum, and you can even use key path syntax to construct them:

let activityCase = \Destination.Cases.activity
// CaseKeyPath<Destination, ActivityModel>

Previously, case paths were constructed by using a custom prefix operator, /, which meant there was no type inference or autocomplete help from the compiler:

let activityCase = /Destination.activity
// CasePath<Destination, ActivityModel>

Now, case paths by themselves are not very useful, just as key paths by themselves are not very useful. Their use is only in generic algorithms that allow you to abstract over the shape of your enums.

This concept is used heavily in our Composable Architecture library, where one often uses case paths on action enums in order to isolate a child domain. For example, the Scope reducer is the fundamental unit for composing a child reducer into a parent. We can now use a more familiar syntax to do this composition:

 var body: some ReducerOf<Self> {
-  Scope(state: \.child, action: /Action.child) {
+  Scope(state: \.child, action: \.child) {
     ChildFeature()
   }
 }

Similarly, when enhancing a parent feature with the functionality of an optional feature one turns to the ifLet reducer operator, and we can again use familiar key path syntax to do this:

 Reduce { state, action in 
   …
 }
-.ifLet(\.child, action: /Action.child)
+.ifLet(\.child, action: \.child)

And when enhance a parent feature with the functionality of a collection of features, one can use the forEach operator, and again with familiar key path syntax:

 Reduce { state, action in 
   …
 }
-.forEach(\.rows, action: /Action.row(id:action:))
+.forEach(\.rows, action: \.rows)

And even better, the new key path syntax for case paths works better with Xcode autocomplete and Swift type inference.

This greatly simplifies nearly every reducer operator in the Composable Architecture, but it can also be used to simplify other libraries using case paths such as our SwiftUI Navigation library. But we will discuss that more later this week.

Expressive case checking

But even if you are not using a library out there that uses case paths, you may still have use of case paths directly in your application. It makes it possible to easily check if an enum value is of a particular case, and you can immediately access getter properties for each case of your enum.

For example, if you mark your enum with the @CasePathable macro:

@CasePathable
enum Destination {
  case activity(ActivityModel)
  case settings(SettingsModel)
}

…then you immediately get access to an is method on your enum. It allows you to quickly check the case of an enum value as an expression:

let destination: Destination = .activity(ActivityModel())

destination.is(\.activity)  // true

Typically this must be done as a statement, such as with an if case let or guard case let. But now you can do it quickly, and inline as an expression.

Case getters

Further, if you mark the Destination enum from above with both the @CasePathable macro and @dynamicMemberLookup:

@CasePathable
@dynamicMemberLookup
enum Destination {
  case activity(ActivityModel)
  case settings(SettingsModel)
}

…then you can use the name of each case as a property on the destination to attempt to extract that data from the enum:

let destination = Destination.activity(ActivityModel())
destination.activity  // ActivityModel
destination.settings  // nil

Or if you have a collection of enum values, you can use compactMap with the property to extract out all of the values matching a particular case:

let destinations: [Destination] = […]

let activityModels = destinations.compactMap(\.activity)
// [ActivityModel]

All of this comes for free with CasePaths, but you do have to opt into the functionality by applying @dynamicMemberLookup to your enum. If you only need the case paths for your enum and don’t want to clutter your type with unneeded properties, then you can use @CasePathable by itself.

This tool also helps simplify a common pattern in Composable Architecture applications, which we will show off tomorrow.

Get started today

Update your dependency on CasePaths to 1.1 today to start taking advantage of the new @CasePathable macro, and more. Tomorrow we will discuss how these new case path tools have massively improved our Composable Architecture library.

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