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.

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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 { /* ... */ state: PhantomData }
struct Disconnected;
struct Connected;
struct Authenticated;

impl Connection {
    fn connect(self) -> Result> { ... }
}
impl Connection {
    fn authenticate(self, creds: &Credentials) -> Result> { ... }
}
impl Connection {
    fn query(&self, sql: &str) -> Result { ... }
}
// 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).

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TypeScript / React Patterns

Compound Components

Components that work together implicitly via shared context.

  Choose a fruit
  
    Apple
    Banana
  

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(value: T, delay: number): T { ... }
function usePagination(items: Item[], pageSize: number) { ... }
function useLocalStorage(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 =
  | { status: "idle" }
  | { status: "loading" }
  | { status: "success"; data: T }
  | { status: "error"; error: Error };

// Exhaustive switch — TypeScript ensures all cases handled
function render(state: AsyncState) {
  switch (state.status) {
    case "idle": return ;
    case "loading": return ;
    case "success": return ;
    case "error": return ;
  }
}

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)

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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.

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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 |

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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"

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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

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Arc skill — Design pattern selection and guidance

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Firstp1ck
• Source: Firstp1ck/pi-coding-agent-forge
• License: MIT

Install and usage instructions live in the source repository linked above.