Law of Demeter for Swift (Strict Review Mode)

Purpose

Enforce Law of Demeter (LoD) in Swift code with a strict-by-default review posture:

Flag deep structural access (a.b.c, a?.b?.c)
Flag chained domain lookups across model/service boundaries
Flag async/actor traversal chains after await
Prefer owner-level, intent-focused APIs
Preserve Swift API naming style (no Java-style getX() suggestions)

Core rule: Ask for what you need. Do not traverse internal structure to reach strangers.

Strict Review Mode (Default Behavior)

In this strict version, assume a likely LoD violation when all of the following are true:

The code is in domain/app/business logic (not a boundary adapter or fluent DSL)
A caller traverses 2+ domain hops to obtain a value or trigger behavior
The traversal exposes another type’s internal structure or storage shape
The caller could reasonably ask an owning type/service for the same result

Examples that should be flagged in strict mode

company.employee(for: id)?.address.city
order.customer.paymentMethod.last4
session.user.profile.preferences.theme
try await accountService.currentAccount().owner.notificationSettings.marketingEmailsEnabled

What Counts as a “Stranger” (Swift Edition)

Inside a method or computed property body, safe collaborators are generally:

self
parameters
locals created in the method
direct stored properties / direct collaborators

A likely LoD violation occurs when code reaches through one of those collaborators to a nested collaborator that the caller does not own.

Ask yourself

Am I using a direct collaborator?
Or am I using a collaborator only as a path to reach a stranger?

Swift-Specific Requirements (Naming + Design)

1) Preserve Swift API style at the call site

When proposing a refactor:

cheap read-only data → property
- employee.city
- order.paymentLast4
action / async / throws / work → method
- company.city(for:)
- accountService.marketingEmailsEnabled()

Do not suggest Java-style names like:

getCity()
getPostalCode()
getMarketingEmailsEnabled()

2) Dot count is a trigger, not proof

A long chain is a signal. In strict mode, review it, then classify it:

Structural reach-through → flag
Intentional fluent API / stdlib pipeline → usually allow

3) Concurrency does not exempt LoD

Swift async/await and actor code is still subject to LoD. If await is followed by traversal through returned internals, treat it as a likely design smell.

Aggressive Detection Heuristics (Use in Reviews)

Flag or strongly scrutinize code matching these patterns:

Pattern A: Deep property traversal

a.b.c
a.b.c.d
a?.b?.c
a?.b?.c?.d

Pattern B: Domain call + traversal

service.fetchX().y.z
repo.load().nested.value
store.state.user.profile

Pattern C: Async call + traversal (high-priority smell)

try await service.currentSession().user.profile.preferences
await actor.snapshot().nested.value

Pattern D: Temporary drilling variables

let profile = user.profile
let address = profile.address
return address.postalCode

Pattern E: Repeated chain access across files

If the same chain or a similar chain appears in multiple locations, escalate severity and suggest centralizing the API.

Review Severity Levels (Strict)

Use these levels when reporting:

Domain hop definition: Count domain hops as the number of transitions (dots) between domain segments.
For example, order.customer.address.city has 3 domain hops (order → customer → address → city).
Treat 2+ domain hops as a likely LoD violation; the severity depends on context as described below.

High severity

Async/actor traversal chain after await
2+ domain hops in app/business logic
Public API exposing nested structure
Repeated chain smell in multiple call sites

Medium severity

2-hop domain traversal in internal code
UI/view model reaches into nested domain types
Tests needing deep stubs because of traversal

Low severity / review note

Borderline chain in boundary mapping code
One-off read in local adapter code (still worth watching)

What Not to Flag (False-Positive Guardrails)

Even in strict mode, do not auto-flag these unless they also expose domain internals improperly:

1) Standard library pipelines

let ids = orders
    .filter(\.isOpen)
    .map(\.id)
    .sorted()

2) Common string/value transformations

let normalized = input
    .trimmingCharacters(in: .whitespacesAndNewlines)
    .lowercased()

3) Intentional fluent APIs / builders / DSLs

If chaining is the designed public abstraction, it may be fine.

4) DTO/adapter mapping code at boundaries

Some structural traversal is expected when decoding / mapping external data. Keep it localized and do not leak it throughout the app.

In strict mode, boundary code is reviewed, not automatically exempted.

Refactoring Strategy (Strict, Minimal, Swift-Idiomatic)

When you flag a violation, propose the smallest safe refactor that improves design.

Preferred order of fixes

Add forwarding property (cheap, stable data)
Add owner-level method (query/action/async/throws)
Move behavior to owner
Add façade/protocol (especially across module boundaries)
Collapse async traversal behind actor/service API

Do this first (minimal change path)

Add a property or method on the most appropriate owner
Replace the call site chain
Keep behavior unchanged
Keep names Swift-idiomatic

Canonical Examples (Strict Mode)

❌ Flag: caller traverses nested structure

import Foundation

struct Address {
    let city: String
    let postalCode: String
}

struct Employee {
    let id: UUID
    let address: Address
}

final class Company {
    private let employeesByID: [UUID: Employee]

    init(employeesByID: [UUID: Employee]) {
        self.employeesByID = employeesByID
    }

    func employee(for employeeID: UUID) -> Employee? {
        employeesByID[employeeID]
    }
}

let postalCode = company.employee(for: employeeID)?.address.postalCode

✅ Prefer: owner-level query

import Foundation

struct Address {
    let city: String
    let postalCode: String
}

struct Employee {
    let id: UUID
    private let address: Address

    var city: String { address.city }
    var postalCode: String { address.postalCode }
}

final class Company {
    private let employeesByID: [UUID: Employee]

    init(employeesByID: [UUID: Employee]) {
        self.employeesByID = employeesByID
    }

    func postalCode(for employeeID: UUID) -> String? {
        employeesByID[employeeID]?.postalCode
    }

    func city(for employeeID: UUID) -> String? {
        employeesByID[employeeID]?.city
    }
}

let postalCode = company.postalCode(for: employeeID)

Swift Concurrency Example (Strict)

❌ High severity: async traversal chain

let marketingEmailsEnabled = try await accountService
    .currentAccount()
    .owner
    .notificationSettings
    .marketingEmailsEnabled

Why this is high severity:

await boundary + structural traversal
Caller learns internal account/owner/settings layout
Refactors spread across many call sites

✅ Prefer: intent-focused actor/service API

let marketingEmailsEnabled = try await accountService.marketingEmailsEnabled()

✅ Possible implementation

actor AccountService {
    private var account: Account?

    func marketingEmailsEnabled() throws -> Bool {
        guard let account else { throw AccountError.notLoaded }
        return account.marketingEmailsEnabled
    }
}

enum AccountError: Error {
    case notLoaded
}

struct Account {
    private let owner: User

    var marketingEmailsEnabled: Bool {
        owner.marketingEmailsEnabled
    }
}

struct User {
    private let notificationSettings: NotificationSettings

    var marketingEmailsEnabled: Bool {
        notificationSettings.marketingEmailsEnabled
    }
}

struct NotificationSettings {
    let marketingEmailsEnabled: Bool
}

Copilot/Codex Review Instructions (Strict Output Format)

When reviewing code, use this response pattern for each likely violation:

1) Identify the chain

Quote the exact chain
Name the owner and the stranger(s)

2) Explain the coupling

What internal structure is leaked?
Why will refactors be harder?
Why is this especially risky if async/actor-based?

3) Propose a Swift-idiomatic replacement

Prefer property for cheap data
Prefer method for async/throws/work
Avoid getX() naming

4) Show a minimal patch direction

Add owner-level property/method
Replace call site
Preserve behavior

5) Classify severity

High / Medium / Low (with a one-line reason)

Review Comment Templates (Strict)

Template: medium severity (sync chain)

LoD concern (medium): company.employee(for: id)?.address.city reaches through Employee into Address, which leaks internal structure to the caller. Consider adding company.city(for:) (or Employee.city if that’s the right ownership boundary) and calling that instead.

Template: high severity (async/actor chain)

LoD concern (high): try await accountService.currentAccount().owner.notificationSettings.marketingEmailsEnabled crosses an async boundary and then traverses nested internals. This tightly couples callers to Account/User/NotificationSettings layout. Prefer an intent-level API such as try await accountService.marketingEmailsEnabled().

Template: naming reminder

Swift API naming: If you add a replacement API, prefer Swift-style names (city, city(for:), marketingEmailsEnabled()) rather than Java-style getCity() / getMarketingEmailsEnabled().

Quick Decision Rules (Strict)

Flag immediately if

2+ domain hops (a.b.c in business logic)
async/await + traversal (await x().y.z)
public API returns structure only to force traversal by callers
repeated chain smells in multiple files

Usually allow if

stdlib pipeline chain
string/value fluent transformations
builder/DSL chain
localized DTO mapping code at a boundary

If unsure

Treat as review note and ask:

“Can this caller ask an owner for intent-level data instead?”

Anti-Regression Guidance

After refactoring one LoD violation, look for siblings:

Same chain in other files
Similar chains on the same type
Tests that still mock deep internals
Public APIs that encourage traversal

If found, propose a small follow-up refactor to centralize the new owner-level API.

Bottom Line

Strict mode favors maintainability over convenience.

In Swift code, especially with actors and async services, prefer APIs that:

express intent
hide structure
preserve Swift naming conventions
reduce refactor blast radius
keep call sites simple and resilient