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SKILL.md
npx skills add actionbook/rust-skills --skill m05-type-driven
CRITICAL: Use for type-driven design. Triggers: type state, PhantomData, newtype, marker trait, builder pattern, make invalid states unrepresentable, compile-time validation, sealed trait, ZST, 类型状态, 新类型模式, 类型驱动设计
npx skills add actionbook/rust-skills --skill m05-type-driven
Layer 1: Language Mechanics
How can the type system prevent invalid states?
Before reaching for runtime checks:
| Pattern | Don't Just Say | Ask Instead |
|---|---|---|
| Primitive obsession | "It's just a string" | What does this value represent? |
| Boolean flags | "Add an is_valid flag" | Can states be types? |
| Optional everywhere | "Check for None" | Is absence really possible? |
| Validation at runtime | "Return Err if invalid" | Can we validate at construction? |
Before adding runtime validation:
Can the type encode the constraint?
When is validation possible?
Who needs to know the invariant?
When type design is unclear:
| Situation | Trace To | Question |
|---|---|---|
| What types to create | m09-domain | What's the domain model? |
| State machine design | m09-domain | What are valid transitions? |
| Marker trait usage | m04-zero-cost | Static or dynamic dispatch? |
From design to implementation:
| 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 {} } |
| 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 |
| Zero-sized type marker | ZST struct |
| 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() |
| When | See |
|---|---|
| Domain modeling | m09-domain |
| Trait design | m04-zero-cost |
| Error handling in constructors | m06-error-handling |
| Anti-patterns | m15-anti-pattern |
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> { ... }
}
impl Connection<Connected> {
fn authenticate(self) -> Connection<Authenticated> { ... }
}