Rex is a notation system which takes the uniform syntax of Lisp s-expression syntax, which underlies it's powerful metaprogramming features, but extends that with significantly more expressive notation. The result is that languages can have notations that have many of the syntactic nicities found in languages like Haskell, Python, and Hoon, while still having an extremely simple parser and a totally regular notation system which can be arbitrarily transformed using macros.
The documentation and code will eventually be hosted here, but you can see the current placeholder documentation and code on Microsoft Github.