zhanghandong/rust-skills/m05-type-driven

"Need to validate email format"
    ↑ Ask: Is this a domain value object?
    ↑ Check: m09-domain (Email as Value Object)
    ↑ Check: domain-* (validation requirements)

"Need type-safe wrapper for primitives"
    ↓ Newtype: struct UserId(u64);
 
"Need compile-time state validation"
    ↓ Type State: Connection<Connected>
 
"Need to track phantom type parameters"
    ↓ PhantomData: PhantomData<T>
 
"Need capability markers"
    ↓ Marker Trait: trait Validated {}
 
"Need gradual construction"
    ↓ Builder: Builder::new().field(x).build()

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> { ... }
}