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<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