interface Guard<T> {
    (val: unknown): val is T;
}

type ObjectGuard<T> = {
    [key in keyof T]: Guard<T[key]>;
};

function isObject(val: unknown): val is Record<string, unknown> {
    return val !== undefined && val !== null && typeof val === 'object' && !Array.isArray(val);
}

function isObjectOfShape<T>(
    value: unknown,
    shape: ObjectGuard<T>,
): value is T {
    if (!isObject(value)) {
        return false;
    }
    let validShape = true;

    for (const key in shape) {
        const guard = shape[key];
        if (guard && !guard(value[key])) {
            return false;
        }
    }

    return validShape;
}

function createObjectGuard<T>(guard: ObjectGuard<T>) {
    return (val: unknown): val is T => isObjectOfShape(val, guard);
}

function asLiteral<T extends (string | boolean | number)[]>(...literals: T) {
    return (val: unknown): val is T[number] => {
        return literals.includes(val as T[number]);
    };
}

// See type of `isWorking` - should include the type key as a union of strings
const isWorking = createObjectGuard({
//    ^?
    type: asLiteral('these', 'should', 'be', 'a', 'union'),
});