Design Patterns

Design Patterns

Pattern recommendations tailored to the user's tech stacks. Every pattern recommendation includes when to use it, when to avoid it, and the trade-offs.

Quick Start

Choose a Pattern

1. Identify the problem: What design challenge are you solving?
2. Consider the context: What language? What scale? How often will this change?
3. Evaluate options: 2-3 candidate patterns with trade-offs
4. Recommend: The best fit for this specific context (not the "best pattern in general")

The golden rule: Patterns are tools, not goals. Use the simplest pattern that solves the problem. If no pattern fits, a plain function or struct is fine.

Rust Patterns

Newtype Pattern

Wrap a primitive to add type safety and domain meaning.

struct UserId(u64);
struct OrderId(u64);

// Now the compiler prevents mixing up UserId and OrderId
fn get_order(user: UserId, order: OrderId) -> Order { ... }

When to use: When two values have the same underlying type but different meanings. When to skip: One-off uses where a type alias suffices.

Builder Pattern

Construct complex objects step by step.

let config = ServerConfig::builder()
    .host("localhost")
    .port(8080)
    .max_connections(100)
    .build()?;

When to use: Structs with many optional fields, complex construction logic. When to skip: Structs with 1-3 required fields — just use new().

Typestate Pattern

Encode state transitions in the type system so invalid states are unrepresentable.

struct Connection<S: State> { /* ... */ state: PhantomData<S> }
struct Disconnected;
struct Connected;
struct Authenticated;

impl Connection<Disconnected> {
    fn connect(self) -> Result<Connection<Connected>> { ... }
}
impl Connection<Connected> {
    fn authenticate(self, creds: &Credentials) -> Result<Connection<Authenticated>> { ... }
}
impl Connection<Authenticated> {
    fn query(&self, sql: &str) -> Result<Rows> { ... }
}
// Can't call query() on a Disconnected connection — compile error

When to use: Protocols with clear state transitions (connections, workflows, parsing stages). When to skip: Simple on/off states — a boolean or enum is clearer.

Trait Objects vs Generics

Use generics when: Performance matters, types are known at compile time, you want zero-cost abstraction. Use trait objects when: You need heterogeneous collections, plugin systems, or the set of types is open-ended.

Error Handling Patterns

Rule of thumb: Libraries use thiserror (callers need to match). Applications use anyhow (callers need context).

TypeScript / React Patterns

Compound Components

Components that work together implicitly via shared context.

<Select>
  <Select.Trigger>Choose a fruit</Select.Trigger>
  <Select.Options>
    <Select.Option value="apple">Apple</Select.Option>
    <Select.Option value="banana">Banana</Select.Option>
  </Select.Options>
</Select>

When to use: Complex UI components with multiple related parts (menus, accordions, tabs). When to skip: Simple components with 1-2 elements.

Custom Hook Composition

Extract and compose behavior into reusable hooks.

function useDebounce<T>(value: T, delay: number): T { ... }
function usePagination(items: Item[], pageSize: number) { ... }
function useLocalStorage<T>(key: string, initial: T) { ... }

When to use: Shared stateful logic across components, complex state management within a component. When to skip: One-off logic that doesn't repeat.

Discriminated Unions

Type-safe state modeling using TypeScript's union types.

type AsyncState<T> =
  | { status: "idle" }
  | { status: "loading" }
  | { status: "success"; data: T }
  | { status: "error"; error: Error };

// Exhaustive switch — TypeScript ensures all cases handled
function render(state: AsyncState<User>) {
  switch (state.status) {
    case "idle": return <Placeholder />;
    case "loading": return <Spinner />;
    case "success": return <UserCard user={state.data} />;
    case "error": return <ErrorBanner error={state.error} />;
  }
}

When to use: State machines, API response states, form states, anything with distinct modes. When to skip: Boolean flags are fine for simple on/off states.

State Management Decision Tree

1. UI-local state? -> useState / useReducer
2. Shared between siblings? -> Lift state to common parent
3. Shared across distant components? -> React Context
4. Complex state logic? -> useReducer + Context
5. Server state (API data)? -> TanStack Query / SWR
6. Global app state? -> Zustand / Jotai (prefer over Redux for new projects)

Django / Python Patterns

Service Layer

Separate business logic from views and models.

# services/order_service.py
class OrderService:
    @staticmethod
    def create_order(user: User, items: list[CartItem]) -> Order:
        """Business logic lives here, not in the view."""
        order = Order.objects.create(user=user, total=calculate_total(items))
        OrderItem.objects.bulk_create([...])
        send_confirmation_email(user, order)
        return order

When to use: Business logic that involves multiple models, external calls, or complex validation. When to skip: Simple CRUD with no business rules beyond Django's built-in validation.

Repository Pattern (QuerySet Abstraction)

Custom managers and querysets to encapsulate query logic.

class PublishedManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().filter(status="published")

class Article(models.Model):
    objects = models.Manager()  # Default
    published = PublishedManager()  # Article.published.all()

When to use: Complex or reusable query logic, domain-specific filtering. When to skip: One-off queries or simple filters.

Signals vs Explicit Calls

Rule of thumb: Use signals for truly cross-cutting concerns (audit logging, cache invalidation). Use explicit calls for business-critical logic where you need to understand and test the flow.

Anti-Patterns (All Stacks)

Pattern Selection Guide

When asked "which pattern should I use?":

1. Describe the problem (not the pattern you think fits)
2. Identify constraints (performance, extensibility, team size, timeline)
3. Present 2-3 options with trade-offs
4. Recommend one with clear justification
5. Note when to revisit — "If X changes, reconsider Y"

Integration

• Data: Pattern decisions logged to MEMORY.md (Patterns in Use section)
• Cross-reference: architecture-review skill for structural assessment, refactoring-advisor for migration planning
• Anti-patterns: When anti-patterns are detected, link to refactoring-advisor for remediation planning

Arc skill — Design pattern selection and guidance