Agent State

Agent fields are private state. They’re initialized when the agent is summoned and can be accessed throughout the agent’s lifetime.

Declaring Fields

Agent state uses record-style field declarations:

agent Person {
    name: String
    age: Int
}

Fields must have explicit type annotations.

Initializing Fields

When summoning an agent, provide values for all fields:

let p = summon Person { name: "Alice", age: 30 };

Missing fields cause a compile error:

// Error: missing field `age` in summon
let p = summon Person { name: "Alice" };

Accessing Fields

Use self.fieldName inside the agent:

agent Greeter {
    name: String

    on start {
        print("Hello, " ++ self.name ++ "!");
        yield(0);
    }
}

Fields Are Immutable

Fields cannot be reassigned after initialization:

agent Counter {
    count: Int

    on start {
        // This won't work — fields are immutable
        // self.count = self.count + 1;

        // Use a local variable instead
        let count = self.count;
        count = count + 1;
        yield(count);
    }
}

Entry Agent Fields

The entry agent (the one in run) cannot have required fields:

// Error: entry agent cannot have required fields
agent Main {
    config: String

    on start {
        yield(0);
    }
}

run Main;  // How would we provide `config`?

Design Pattern: Configuration

Use fields to configure agent behavior:

agent Fetcher {
    url: String
    timeout: Int

    on start {
        // Use self.url and self.timeout
        yield("done");
    }
}

agent Main {
    on start {
        let f1 = summon Fetcher {
            url: "https://api.example.com/a",
            timeout: 5000
        };
        let f2 = summon Fetcher {
            url: "https://api.example.com/b",
            timeout: 3000
        };

        let r1 = try await f1;
        let r2 = try await f2;
        yield(0);
    }

    on error(e) {
        yield(1);
    }
}

run Main;