I haven't had an opportunity to actually use it, so take this with a grain of salt, but the optional/gradual type system of TypeScript looks decently expressive

I haven't had an opportunity to actually use it, so take this with a grain of salt, but the optional/gradual type system of TypeScript looks decently expressive (http://www.typescriptlang.org/Handbook http://www.typescriptlang.org/Handbook).