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Open Sourcing Case Paths

Tuesday Feb 4, 2020


Today we are open sourcing CasePaths, a library that introduces the power and ergonomics of key paths to enums!


We are excited to announce the 0.1.0 release of CasePaths, a new API that brings key path-like functionality and ergonomics to enums and unlocks the potential to write code that is generic over enums.

Motivation

Swift endows every struct and class property with a key path.

struct User {
  var id: Int
  var name: String
}

\User.id   // WritableKeyPath<User, Int>
\User.name // WritableKeyPath<User, String>

This is compiler-generated code that can be used to abstractly zoom in on part of a structure, inspect and even change it, while propagating these changes to the structure’s whole. They unlock the ability to do many things, like key-value observing and reactive bindings, dynamic member lookup, and scoping changes to the SwiftUI environment.

Unfortunately, no such structure exists for enum cases!

enum Authentication {
  case authenticated(accessToken: String)
  case unauthenticated
}

\Authentication.authenticated // 🛑

And so it’s impossible to write similar generic algorithms that can zoom in on a particular enum case.

Introducing: case paths

CasePaths intends to bridge this gap by introducing what we call “case paths.” Case paths can be constructed simply by prepending the enum type and case name with a forward slash:

import CasePaths

/Authentication.authenticated // CasePath<Authentication, String>

Case paths vs. key paths

While key paths package up the functionality of getting and setting a value on a root structure, case paths package up the functionality of extracting and embedding a value on a root enumeration.

user[keyPath: \User.id] = 42
user[keyPath: \User.id] // 42

let authentication = (/Authentication.authenticated).embed("cafebeef")
(/Authentication.authenticated).extract(from: authentication) // Optional("cafebeef")

Case path extraction can fail and return nil because the cases may not match up.

(/Authentication.authenticated).extract(from: .unauthenticated) // nil

Case paths, like key paths, compose. Where key paths use dot-syntax to dive deeper into a structure, case paths use a double-dot syntax:

\HighScore.user.name
// WritableKeyPath<HighScore, String>

/Result<Authentication, Error>..Authentication.authenticated
// CasePath<Result<Authentication, Error>, String>

Case paths, also like key paths, provide an “identity” path, which is useful for interacting with APIs that use key paths and case paths but you want to work with entire structure.

\User.self           // WritableKeyPath<User, User>
/Authentication.self // CasePath<Authentication, Authentication>

Key paths are created for every property, even computed ones, so what is the equivalent for case paths? “Computed” case paths can be created by providing custom embed and extract functions:

CasePath<Authentication, String>(
  embed: { decryptedToken in
    Authentication.authenticated(token: encrypt(decryptedToken))
  },
  extract: { authentication in
    guard
      case let .authenticated(encryptedToken) = authentication,
      let decryptedToken = decrypt(token)
      else { return nil }
    return decryptedToken
  }
)

Since Swift 5.2, key path expressions can be passed directly to methods like map. The same is true of case path expressions, which can be passed to methods like compactMap:

users.map(\User.name)
authentications.compactMap(/Authentication.authenticated)

Ergonomic associated value access

CasePaths uses Swift reflection to automatically and extract associated values from any enum in a single, short expression. This helpful utility is made available as a public module function that can be used in your own libraries and apps:

extract(case: Authentication.authenticated, from: .authenticated("cafebeef"))
// Optional("cafebeef")

Try it out today!

The official 0.1.0 release of CasePaths is on GitHub now, and we have more improvements and refinements coming soon. We hope that case paths will unlock all new kinds of interesting, expressive code! Try it today!


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