On the HHVM blog there's a recent post looking at some of the requirements around traits and interfaces in the Hack language. More specifically, they talk about type checking with traits and how interfaces can be used to help provide extra structure.

In PHP, traits are a mechanism of code reuse that, while very powerful, are also difficult to type check both efficiently and exhaustively. In this post we’ll dive more deeply into the reasons for that and see how Hack solves those problems, allowing you to use traits in a safe way without limiting their expressiveness.

They start by talking about the main problem with PHP's handling of traits (essentially copy and paste into the current class) and how they felt Hack should "just work" in allowing type checking on these "pasted" methods too. Performance limitations prevented them from handling it how they do with other variable types, so they changed things up, using a "require extends" syntax to tell the Hack engine how to allow the checking based on an interface. There's a lot more to it than this, so be sure to read the rest of the post on how they came to that conclusion.