Rio is the working name for a project to create a programming language, compiler, and graphical interactive development environment.

The immediate motivation for Rio is to explore reactive evaluation, and how that might streamline UI development. To that end, a simple language with a pure functional foundation, without mutation, appears to be best suited.

However, there are a lot of other ideas I've had over the years for a programming language and development environment, and this is an opportunity to try them out as well: Compile-time evaluation, partial evaluation, specialization, reified types, gradual typing, treating static typing as assertions, integrated unit testing. Together with reactive evaluation, they reinforce and suport each other, creating more than the sum of the parts. If this works out, we'll have a language that spans the divides between dynamically-typed and statically-typed, compiled and interpreted, high-level and low-level, offering the best of each domain.

The "concepts" document contains notes on a lot of these ideas, as well as an introduction to Rio syntax and a high-level overview of the semantics.