Finalization

A type that claims the fin capability runs custom cleanup when its owner is released. The cleanup is a compiler-owned receiver contract: a pro named finalize that takes the value by value and returns non.

typ File()(fin): rec = { descriptor: int };

pro (File)finalize(): non = {
    close(self.descriptor);
    return;
};

The finalizer is a pro because releasing a foreign resource is an effect. A fin type cannot also claim copy: a value with cleanup is not trivially duplicable.

Scope-exit cleanup

The finalizer runs automatically when the owning scope exits, exactly once per value:

fun[] main(): int = {
    var handle: File = { descriptor = 3 };
    return handle.descriptor;
    // handle.finalize() runs here, as the scope exits
};

When a value is moved, responsibility for finalization moves with it: the new owner finalizes it, and the original binding does not.

Early finalization

[fin]value runs the finalizer immediately and invalidates the source. The value is finalized exactly once, so scope-exit cleanup does not run a second time:

var handle: File = { descriptor = 3 };
observe(handle.descriptor);
[fin]handle;
// handle is now consumed; no second finalize at scope exit