I’ve seen some posts about stan 3, but I haven’t found any information about an approximate release date. Is there one? I’m happy to know if it is even 2020…
And is there more information besides https://statmodeling.stat.columbia.edu/2019/03/13/stanc3-rewriting-the-stan-compiler/ ?
Is it going to be backward compatible? Are there going to be many changes in syntax?
the majority (if not all posts) you have seen in the last few months are on the new Stan compiler which is named stanc3. This new compiler or transpiler from Stan-to-C++ is part of the latest Cmdstan 2.22.1 (and 2.22.0) release. This compiler is intended to be backwards compatible with the existing Stan language and should accept all models that compile under the release 2.21 compiler and before, (see this list of bug fixes).
This compiler has not made its way to rstan/pystan interfaces, but you can use it using cmdstanpy and cmdstanr. Dont know the timeline for the other interfaces.
The term Stan 3, without the “c”, refers to the language changes, that wont (all) be backwards compatible, hence the change in major version. But I dont know if there is a set timeline for that, any specific plan on changes, etc.