---
name: pfw-case-paths
description: Add ergonomics and generic algorithms to Swift enums using key paths and the CasePaths library.
license: Proprietary. LICENSE has complete terms
metadata:
  short-description: Add ergonomics and generic algorithms to Swift enums.
---

# CasePaths

## IMPORTANT

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## Goal

Add ergonomics and generic algorithms to Swift enums using key paths and the [CasePaths] library. Use this skill when you want to work with enums and their associated values in succinct, expressive ways that avoid verbose pattern-matching statements, or when you want to write generic algorithms over an enum that need to embed or extract or otherwise probe an enum's cases and/or associated values.

## Quick start

1. Add the `swift-case-paths` package dependency from `1.0.0` or newer.
2. Add the `CasePaths` product to your target's dependencies.
3. `import CasePaths` as needed
4. Apply the `@CasePathable` macro to enums

    ```swift
    @CasePathable
    enum MyEnum {
      ...
    }
    ```
    
## API interface

Full API interface for when solution can't be found elsewhere:

- Core functionality: `references/interface/CasePathsCore.swiftinterface`
- Macros and deprecations: `references/interface/CasePaths.swiftinterface`

## How to add key paths to an enum:

Annotate the enum with the `@CasePathable` macro.

```swift
@CasePathable enum LoadState<Value> {
  case idle
  case loading(progress: Float)
  case loaded(Value)
  case failed(any Error)
}
```

## How to check if an enum value is a certain case

Use the `is` method with a case key path:

```swift
loadState.is(\.idle)
```

## How to modify an associated value

Use the `modify` method with a case key path:

```swift
loadState.modify(\.loading) { $0 += 0.1 }
```

## How to ergonomically access an associated value

### Step 1: Apply `@dynamicMemberLookup`

```swift
@CasePathable
@dynamicMemberLookup
enum LoadState<Value> {
  ...
}
```

### Step 2: Use dot-syntax

Use dot-syntax and the case name to access an associated value as an optional:

```swift
loadState.loading  // Optional<Float>
```

Use `compactMap` on an array of enum values to unwrap each associated value:

```swift
loadStates.compactMap(\.loaded)  // [Value]
```

## How to iterate over all case key paths of an enum

Use the `allCasePaths` static variable:

```swift
for caseKeyPath in LoadState<String>.allCasePaths {
  _: PartialCaseKeyPath<LoadState<String>> = caseKeyPath 
}
```

## How to get the case key path of an enum instance

Use the subscript defined on `allCasePaths`:

```swift
LoadState.allCasePaths[loadState]  // PartialCaseKeyPath<LoadState<String>>
```

## How to check if two enums are the same case

```swift
LoadState.allCasePaths[lhs] == LoadState.allCasePaths[rhs]
```

## How to generically embed a value in an enum with a case key path

Invoke a case key path with a value to return it embedded in the enum:

```swift
let caseKeyPath: CaseKeyPath<Root, Value>
caseKeyPath(value)  // Root
```

## How to generically extract a value from an enum with a case key path

Use the `[case:]` subscript define on case-pathable enums:

```swift
let caseKeyPath: CaseKeyPath<Root, Value>
root[case: caseKeyPath]  // Value?
```

## How to fix "inaccessible due to 'private' protection level" error

**DO NOT** define `private` case-pathable enums in a nested scope. This can produce the following error:

> Error: 'EnumType' is inaccessible due to 'private' protection level

**DO** use `fileprivate` instead to avoid this error:

```swift
struct Feature {
  @CasePathable fileprivate enum Action {
    ...
  }
}
```

## How to add case key paths to an existing type

Manually extend the type to be `CasePathable` and `CaseIterable`, and define a nested `AllCasePaths` struct with a property per case.

For example, if a `LoadState` enum is defined in a module you can't make edits to:

```swift
public enum LoadState<Value> {
  case idle
  case loading(progress: Float)
  case loaded(Value)
  case failed(any Error)
}
```

You can extend it manually:

```swift
extension LoadState: CasePathable, CaseIterable {
  public struct AllCasePaths: CasePathReflectable, Sendable {
    public subscript(root: LoadState) -> PartialCaseKeyPath<LoadState> {
      switch root {
      case .idle: return \.idle
      case .loading: return \.loading
      case .loaded: return \.loaded
      case .failed: return \.failed
      }
    }

    public var idle: AnyCasePath<LoadState, Void> {
      AnyCasePath {
        .idle
      } extract: {
        guard case .idle = $0 else { return nil }
        return
      }
    }

    public var loading: AnyCasePath<LoadState, Float> {
      AnyCasePath {
        .loading(progress: $0)
      } extract: {
        guard case .loading(let progress) = $0 else { return nil }
        return progress
      }
    }

    public var loaded: AnyCasePath<LoadState, Value> {
      AnyCasePath {
        .loaded($0)
      } extract: {
        guard case .loaded(let value) = $0 else { return nil }
        return value
      }
    }

    public var failed: AnyCasePath<LoadState, any Error> {
      AnyCasePath {
        .failed($0)
      } extract: {
        guard case .failed(let error) = $0 else { return nil }
        return error
      }
    }
  }

  public static var allCasePaths: AllCasePaths {
    AllCasePaths()
  }
}
```

## How to add dynamic member lookup for case key paths

Extend the type with a subscript:

```swift
extension Binding {
  subscript<Member>(dynamicMember keyPath: CaseKeyPath<Value, Member>) -> Binding<Member>? {
    guard let member = wrappedValue[case: keyPath]
    else { return nil }
    return Binding<Member>(
      get: { wrappedValue[case: keyPath] ?? member },
      set: { wrappedValue[case: keyPath] = $0 }
    )
  }
}
```

## How to add a "computed" case to a case-pathable enum

Given a case-pathable enum:

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

Extend the nested `AllCasePaths` type with a property to an `AnyCasePath`:

```swift
extension Authentication.AllCasePaths {
  var crypted: AnyCasePath<Authentication, String> {
    AnyCasePath { decryptedToken in
      .authenticated(accessToken: encrypt(decryptedToken))
    } extract: { authentication in
      guard
        case let .authenticated(encryptedToken) = authentication,
        let decryptedToken = decrypt(token)
      else { return nil }
      return decryptedToken
    }
  }
}
```