design-patterns

name	design-patterns
description	Agents should invoke this skill when choosing patterns, designing traits/interfaces/components, deciding abstraction boundaries, evaluating dependency injection/callbacks, or comparing implementation approaches in Rust, TypeScript/React, or Django/Python.

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

Approach	Dispatch	Binary Size	Flexibility
impl Trait (generics)	Static (monomorphized)	Larger (one copy per type)	Known at compile time
dyn Trait (trait objects)	Dynamic (vtable)	Smaller (one copy)	Extensible at runtime

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

Pattern	When	Crate
thiserror	Library errors — structured, specific variants	thiserror
anyhow	Application errors — context-rich, propagate quickly	anyhow
Custom enum	When you need exhaustive matching by callers	std only

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

Approach	Pros	Cons
Django signals	Decoupled, works across apps	Hidden control flow, hard to debug, ordering issues
Explicit calls	Visible, debuggable, testable	Coupling between modules

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)

Anti-Pattern	Problem	Fix
Premature abstraction	Abstracting before seeing the pattern repeat	Wait for 3 instances before extracting
Inheritance over composition	Deep inheritance hierarchies, fragile base class	Prefer composition (traits, hooks, mixins)
God object	One class/module that does everything	Split by responsibility (SRP)
Shotgun surgery	One change requires editing many files	Group related code together
Feature envy	A function uses another class's data more than its own	Move the function to the class it envies
Speculative generality	Building for use cases that may never come	YAGNI — build for now, refactor when needed
Golden hammer	Using the same pattern/tool for everything	Choose the right tool for the job

---

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