Records

A record is a value type whose fields cannot be changed after construction. Records are useful for values that should be created complete and remain stable.

Define and construct a record

record User {
    id: Int,
    name: String,
}

var user = User { id: 42, name: "Ada" };
println(user.name);

Each required field must receive a value.

Field immutability

Assigning to a record field is a compile-time error:

var user = User { id: 42, name: "Ada" };

// Does not compile:
// user.name = "Grace";

The binding itself may still be mutable. Replace the complete record when the program needs a new value:

var user = User { id: 42, name: "Ada" };
user = User { id: user.id, name: "Grace" };

Records are copied by value

record Coordinate {
    x: Int,
    y: Int,
}

var start = Coordinate { x: 2, y: 4 };
var end = start;

start and end are two record values. Managed fields such as String follow their normal ownership-aware copy semantics.

Default field values

record RequestOptions {
    retries: Int = 3,
    trace: Bool = false,
}

var defaults = RequestOptions {};
var traced = RequestOptions { retries: 3, trace: true };

Shallow immutability

Record immutability applies to its fields. If a field stores a class reference, the field cannot be redirected to another object, but that object may still have mutable state:

class Counter {
    pub value: Int;

    fun init(value: Int) {
        self.value = value;
    }
}

record Snapshot {
    counter: Counter,
}

var snapshot = Snapshot { counter: new Counter(1) };
snapshot.counter.value = 2;

Use records for configuration, identifiers, parsed results, and other value-shaped domain data. Use a struct for mutable value data.