Building Type-Safe DSLs

Create internal Domain Specific Languages that catch logic errors at compile time.

In this lesson, you’ll learn how to leverage Rust’s type system to create internal Domain Specific Languages (DSLs) that turn logic errors into compilation errors.

The "State Machine" Approach to DSLs

When building a DSL in Rust, we don't just want a series of method calls; we want a grammar that enforces valid sequences. We can achieve this by using Phantom Types to track the state of a "builder" object. By ensuring that methods only accept a self in a specific state, we prevent invalid operations before the code even runs.

Consider a simplified DSL for an HTTP request builder:

struct Request<State> {
    url: String,
    _marker: std::marker::PhantomData<State>,
}

// Marker types to represent our DSL stages
struct Empty;
struct WithUrl;

impl Request<Empty> {
    fn new() -> Request<Empty> {
        Request { url: String::new(), _marker: std::marker::PhantomData }
    }

    fn set_url(self, url: &str) -> Request<WithUrl> {
        Request { url: url.to_string(), _marker: std::marker::PhantomData }
    }
}

impl Request<WithUrl> {
    fn send(self) {
        println!("Sending request to {}", self.url);
    }
}

Why This Matters

In the code above, if a user attempts to call .send() on a Request<Empty>, the compiler will throw an error. Because send is only defined on Request<WithUrl>, the API itself acts as the documentation and the gatekeeper.

By defining your DSL as a sequence of state transitions, you move logic validation from runtime checks (like if !url.is_empty()) to the type level. The user of your DSL cannot physically construct an invalid program because the type system requires them to fulfill the "grammar" of your library to reach the final state. This is the foundation of type-safe design: the language constraints prevent you from accidentally creating "impossible" states in your domain model.

Key Takeaway: By using phantom types and distinct state markers, you can design an API that forces users to follow a valid operational sequence, catching logic errors at compile time.