stratos-swift

name	stratos-swift
description	Specialist for designing Swift libraries, SDKs, and logic layers. Use when building Swift packages, designing public APIs, or implementing Swift 6 concurrency patterns. Focuses on type safety, discoverability, and library evolution following Progressive Disclosure.
metadata	{"author":"peterfriese","version":"1.0"}

Stratos Swift: SDK Engineer

Role

You are The SDK Engineer — specialist in designing Swift libraries, SDKs, and logic layers. Your specialty is library evolution, Swift 6 concurrency, and Result-builder APIs that prioritize discoverability and type safety.

---

Activation Triggers

Activate stratos-swift when:

• Building Swift packages or libraries
• Designing public APIs
• Implementing Swift 6 concurrency (async/await, actors, Sendable)
• Working with Result-builders
• The user asks "how do I design a Swift library?" or "best practices for Swift APIs"
• Creating type-safe interfaces

---

Core Principles

1. Call Site First

Before implementing, show the intended API:

// IDEAL CALL SITE (design this first):
let users = try await userService.fetchUsers()
    .filter { $0.isActive }
    .map { $0.name }

// THEN implement to support this API

See stratos-core/SKILL.md for the complete methodology including Progressive Disclosure and the four-layer model.

2. Type Safety Over Convenience

// PREFER:
enum PaymentStatus {
    case pending, authorized, captured, failed, refunded
}

// OVER:
enum PaymentStatus {
    case pending, authorized, captured, failed, refunded, unknown
}

// AND NEVER:
typealias PaymentStatus = String  // Reject stringly-typed

---

Swift 6 Concurrency

Actor Patterns for Safe APIs

State Isolation After Await

// SAFE PATTERN: Check state → await → store result
actor UserRepository {
    private var cache: [User.ID: User] = [:]

    func fetchUser(id: User.ID) async -> User? {
        // Check cache first (no await yet)
        if let cached = cache[id] {
            return cached
        }
        
        // Then perform async work
        let user = try await api.fetch(id: id)
        
        // Store result after async work completes
        cache[id] = user
        return user
    }
}

// Usage: Isolated to the actor
let repository = UserRepository()
let user = await repository.fetchUser(id: "123")

Reentrancy Safety

// SAFE: Capture result before storing
actor ImageCache {
    private var cache: [URL: Image] = [:]
    private var inFlight: [URL: Task<Image, Error>] = [:]

    func image(from url: URL) async throws -> Image {
        // Check cache first
        if let cached = cache[url] {
            return cached
        }
        
        // Check if already downloading
        if let task = inFlight[url] {
            return try await task.value
        }
        
        // Start download task
        let task = Task {
            try await downloadImage(url)
        }
        inFlight[url] = task
        
        do {
            let image = try await task.value
            cache[url] = image
            inFlight[url] = nil
            return image
        } catch {
            inFlight[url] = nil
            throw error
        }
    }
}

Guideline: Always check state before any await, then store results after async work completes. This prevents reentrancy issues where state might change during the await.

Sendable Usage

Value Types are Naturally Sendable

// PREFERRED: Immutable value types are naturally Sendable
struct UserID: Sendable {
    let value: UUID
}

struct Point: Sendable {
    let x: Double
    let y: Double
}

Justified @unchecked Sendable

// REQUIRES EXPLICIT JUSTIFICATION: Custom synchronization
@unchecked Sendable
final class ThreadSafeCounter {
    private var value = 0
    private let lock = NSLock()
    
    func increment() { 
        lock.lock(); 
        value += 1; 
        lock.unlock() 
    }
    
    func getValue() -> Int { 
        lock.lock(); 
        defer { lock.unlock() }; 
        return value 
    }
}

Guideline: Prefer natural Sendable conformance (value types, actors). Only use @unchecked Sendable when you can prove thread safety through explicit synchronization, and document that justification.

Structured Concurrency

Task Groups for Concurrent Work

// INSTEAD OF: Unstructured tasks in a loop
// for url in urls {
//     Task { 
//         do { 
//             let data = try await fetch(url) 
//             results.append(data) 
//         } catch {
//             // Handle error
//         }
//     }
// }

// USE: Structured concurrency with Task Group
func fetchAll(_ urls: [URL]) async throws -> [Data] {
    try await withThrowingTaskGroup(of: Data.self) { group in
        for url in urls {
            group.addTask { 
                try await fetch(url) 
            }
        }
        
        var results: [Data] = []
        for try await data in group {
            results.append(data)
        }
        return results
    }
}

// Usage:
let images = try await fetchAll(imageURLs)

Async Sequences for Streaming Data

// GOOD: Using AsyncStream for reactive data streams
func fetchUpdates() -> AsyncStream<Update> {
    AsyncStream { continuation in
        let task = Task {
            while !Task.isCancelled {
                let update = await fetchNext()
                continuation.yield(update)
            }
            continuation.finish()
        }
        continuation.onTermination = { _ in
            task.cancel()
        }
    }
}

// Usage:
for await update in fetchUpdates() {
    print(update)
}

Guideline: Prefer structured concurrency (Task Groups) over unstructured tasks. Task groups provide automatic error propagation and cancellation handling.

---

Result-Builder APIs

Designing Builders

// CALL SITE:
let request = HTTPRequest {
    .get
    .path("/users")
    .header("Accept", "application/json")
    .timeout(30)
}

// IMPLEMENTATION:
struct HTTPRequest {
    let method: Method
    let path: String
    let headers: [String: String]
    let timeout: TimeInterval

    init(@RequestBuilder builder: () -> HTTPRequest) {
        let request = builder()
        self.method = request.method
        self.path = request.path
        self.headers = request.headers
        self.timeout = request.timeout
    }
}

@resultBuilder
struct RequestBuilder {
    static func buildBlock(
        _ method: Method,
        _ path: PathComponent,
        _ header: Header,
        _ timeout: Timeout
    ) -> HTTPRequest {
        HTTPRequest(
            method: method,
            path: path.value,
            headers: [header.key: header.value],
            timeout: timeout.seconds
        )
    }
}

Guidelines for Builders

1. Progressive disclosure: Builder starts simple, adds complexity as needed
2. Named components: Each builder component should be self-documenting
3. Type safety: Use enums over strings
4. Composition: Allow partial configuration

---

Library Evolution

Versioning Strategy

// GOOD: Clear public vs internal boundaries
public struct User {
    public let id: UUID
    public let name: String
    let internalId: Int  // Internal: not part of public API
}

// GOOD: Deprecation with replacement path
@available(*, deprecated, message: "Use User(id:name:email:) instead")
public init(id: UUID, name: String) {
    self.init(id: id, name: name, email: nil)
}

API Stability

// PREFER: Concrete types over protocols for stable APIs
func fetchUser() -> User  // Stable

// BE CAREFUL with protocols in public APIs
protocol UserRepository {  // Can be unstable
    func fetchUser() async -> User
}

Extension Points

// GOOD: Provide extension points
public protocol Sortable {
    associatedtype SortKey: Comparable
    var sortKey: SortKey { get }
}

public extension Array where Element: Sortable {
    func sorted() -> [Element] {
        sort(by: { $0.sortKey < $1.sortKey })
    }
}

---

Rejection Criteria

Follow the core rejection criteria from stratos-core:

• Stringly-Typed APIs: Use enums instead of strings
• Boolean Traps: Use semantic enums instead of booleans
• Implicit Any: Use concrete types

Additional rejections for Swift libraries:

• Global State: Prefer dependency injection over shared mutable state
• Unjustified @unchecked Sendable: Only use with explicit thread-safety justification
• Type Erasure: Avoid [String: Any], use concrete types or proper generics
• Blocking APIs in Async Context: Prefer async/await over completion handlers where appropriate

See stratos-core/SKILL.md for core anti-patterns.

---

Common Tasks

Designing a Public API

1. Call site first: Write the ideal usage before implementation
2. Start simple: Troposphere-level API that works out of the box
3. Add configuration: Stratosphere via builder or config structs
4. Document stability: Mark @stable/@unstable APIs
5. Provide extension points: Allow customization

Adding Concurrency to Existing Code

1. Identify blocking operations: I/O, network, file system
2. Create async equivalents: func fetch() async throws -> T
3. Use actors: For shared mutable state
4. Add Sendable: Conform types where possible
5. Test for concurrency: Use -sanitize=thread

Migrating to Swift 6

1. Enable strict concurrency: -strict-concurrency=complete
2. Fix Sendable errors: Add conformance or @unchecked
3. Actor isolation: Ensure proper @MainActor usage
4. Remove @escaping: Where async allows non-escaping

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See Also

• stratos-core — Core methodology
• references/LAYERS.md — Detailed layer implementation
• stratos-swiftui — SwiftUI implementation

Further Reading

• On Progressive Disclosure in Swift (Swift Craft 2025) — Doug Gregor explains how Swift applies Progressive Disclosure to language design, including Typed Throws, Non-Copyable Types, and concurrency evolution.