If you're interested in the details of how pyre handles generics, unions, etc., you can take a look at the end-to-end typechecking tests, which provide lots of examples: https://github.com/facebook/pyre-check/blob/master/analysis/...

Thanks! I'll dig into it later, as I've been looking for a tool like his, and it so happens that the details are interesting to me :) (I built my own namedtuple-style library for sum types, go figure). It didn't seem like the easiest reading though - that could make some potential users bounce... Are you working on docs that expose these features in a friendlier way?

edit: Also, was I correct in that it's basically "The stuff that's specified in the PEP and works in mypy should work"?