R is both a language and an implementation of that language. So far, when we've been talking about the R environment/platform, we've been talking about the GNU Project started by R. Ihaka and R. Gentleman at the University of Auckland in 1993, and hosted at http://www.r-project.org. Since R has no standard specification, this canonical implementation serves as R's de facto specification. If a project is able to implement this specification, and rewrite the GNU-R functionality-for-functionality and bug-for-bug-any valid R code can be run on that implementation.
Some time around 2009, various other implementations of R started to crop up. Among these were:
- Renjin (running on the Java Virtual Machine)
- pqR (which ...