Guides type-driven design in Rust. Use when encoding invariants in types, applying newtype pattern, implementing type state machines, using PhantomData, or making invalid states unrepresentable.
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Layer 1: Language Mechanics
How can the type system prevent invalid states?
Before reaching for runtime checks:
Before adding runtime validation:
Can the type encode the constraint?
When is validation possible?
Result with clear errorWho needs to know the invariant?
| Pattern | Purpose | Example |
|---|---|---|
| Newtype | Type safety | struct UserId(u64); |
| Type State | State machine | Connection<Connected> |
| PhantomData | Variance/lifetime | PhantomData<&'a T> |
| Marker Trait | Capability flag | trait Validated {} |
| Builder | Gradual construction | Builder::new().name("x").build() |
| Sealed Trait | Prevent external impl | mod private { pub trait Sealed {} } |
struct Email(String); // Not just any string
impl Email {
pub fn new(s: &str) -> Result<Self, ValidationError> {
// Validate once, trust forever
validate_email(s)?;
Ok(Self(s.to_string()))
}
}
struct Connection<State>(TcpStream, PhantomData<State>);
struct Disconnected;
struct Connected;
struct Authenticated;
impl Connection<Disconnected> {
fn connect(self) -> Connection<Connected> { todo!() }
}
impl Connection<Connected> {
fn authenticate(self) -> Connection<Authenticated> { todo!() }
}
// compile error if you call authenticate() on Disconnected
| Need | Pattern |
|---|---|
| Type safety for primitives | Newtype |
| Compile-time state validation | Type State |
| Lifetime/variance markers | PhantomData |
| Capability flags | Marker Trait |
| Gradual construction | Builder |
| Closed set of impls | Sealed Trait |
| Anti-Pattern | Why Bad | Better |
|---|---|---|
| Boolean flags for states | Runtime errors | Type state |
| String for semantic types | No type safety | Newtype |
Option for uninitialized | Unclear invariant | Builder |
| Public fields with invariants | Invariant violation | Private + validated new() |
zero-cost-abstractions — trait design for newtypeserror-handling — validation errors in constructorsanti-patterns — primitive obsession and boolean blindnessNo reviews yet. Tell others what you thought.
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