I haven&#x27;t had an opportunity to actually use it, so take this with a grain of salt, but the optional&#x2F;gradual type system of TypeScript looks decently expressive (<a href="http:&#x2F;&#x2F;www.typescriptlang.org&#x2F;Handbook" rel="nofollow">http:&#x2F;&#x2F;www.typescriptlang.org&#x2F;Handbook</a>).