Swift: Idiomatic Efficiency Reference

When to Use

• Use this skill when the task matches this description: Language-specific super-code guidelines for swift.

Table of Contents

1. Optionals
2. Collections & Functional Transforms
3. Value vs Reference Types
4. Error Handling
5. Concurrency
6. Protocol-Oriented Design
7. Anti-patterns specific to Swift

1. Optionals {#optionals}

// ❌ Force unwrap
let name = user.name!

// ✅ — guard or if-let
guard let name = user.name else { return }

// ❌ Nested if-let pyramid
if let user = fetchUser() {
    if let address = user.address {
        if let city = address.city {
            display(city)
        }
    }
}

// ✅ — chained optional binding
if let city = fetchUser()?.address?.city {
    display(city)
}
// or guard-let for early exit
guard let city = fetchUser()?.address?.city else { return }
display(city)

// ❌ Ternary for default
let name = user.name != nil ? user.name! : "Unknown"

// ✅
let name = user.name ?? "Unknown"

// ❌ Optional map when if-let is clearer for side effects
user.name.map { display($0) }

// ✅ — map for transforms, if-let for side effects
let upper = user.name.map { $0.uppercased() }
if let name = user.name { display(name) }

2. Collections & Functional Transforms {#collections}

// ❌ Imperative filter + map
var result: [String] = []
for item in items {
    if item.isActive { result.append(item.name.uppercased()) }
}

// ✅
let result = items
    .filter(\.isActive)
    .map { $0.name.uppercased() }

// ❌ Manual dictionary construction
var dict: [String: User] = [:]
for user in users { dict[user.id] = user }

// ✅
let dict = Dictionary(uniqueKeysWithValues: users.map { ($0.id, $0) })
// or with possible duplicates:
let dict = Dictionary(grouping: users, by: \.department)

// ❌ Checking isEmpty then accessing first
if !items.isEmpty { process(items[0]) }

// ✅
if let first = items.first { process(first) }

// ❌ Index-based loop
for i in 0..<items.count { process(items[i]) }

// ✅
for item in items { process(item) }
// with index:
for (i, item) in items.enumerated() { process(i, item) }

Use key paths (\.isActive) as closure shorthand where supported.

3. Value vs Reference Types {#value-types}

// ❌ Class for plain data (reference semantics where value semantics suffice)
class Point {
    var x: Double
    var y: Double
    init(x: Double, y: Double) { self.x = x; self.y = y }
}

// ✅
struct Point { var x, y: Double }

// ❌ Large struct copied repeatedly (performance hit)
struct HugeData { var buffer: [UInt8] /* thousands of elements */ }
func process(_ data: HugeData) { ... } // copies entire buffer

// ✅ — use class or pass inout for mutation
func process(_ data: inout HugeData) { ... }
// or use copy-on-write wrapper for large value types

Default to struct. Use class when you need identity, inheritance, or reference semantics.

4. Error Handling {#errors}

// ❌ Using optionals to mask errors
func parse(_ input: String) -> Data? { ... } // caller doesn't know why it failed

// ✅
func parse(_ input: String) throws -> Data { ... }

// ❌ try! in production code
let data = try! JSONDecoder().decode(User.self, from: jsonData)

// ✅
do {
    let data = try JSONDecoder().decode(User.self, from: jsonData)
} catch {
    logger.error("decode failed: \(error)")
    throw AppError.decodingFailed(underlying: error)
}

// ❌ Generic Error type
enum AppError: Error { case generic(String) }

// ✅ — specific, actionable error cases
enum AppError: Error {
    case networkUnreachable
    case invalidInput(field: String, reason: String)
    case unauthorized
}

// ❌ Catching all errors and ignoring
do { try riskyOperation() } catch { }

// ✅
do {
    try riskyOperation()
} catch let error as NetworkError {
    handleNetworkError(error)
} catch {
    throw error // rethrow unknown
}

5. Concurrency {#concurrency}

// ❌ Callback-based async (pyramid of doom)
fetchUser { user in
    fetchPosts(for: user) { posts in
        fetchComments(for: posts.first!) { comments in
            display(comments)
        }
    }
}

// ✅ (Swift 5.5+)
let user = try await fetchUser()
let posts = try await fetchPosts(for: user)

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