Using Spacemacs with several Emacs versions

Since Spacemacs officially supports at least two major versions of Emacs, as a collaborator I have to switch between Emacs versions time to time. And since installed packages don’t usually work between different major version of Emacs, I came up with a hack allowing me to seamlessly switch Emacs versions.

Update on [2016-10-11 Tue]: thanks to bmag Spacemacs has an option to set packages directory based on Emacs version. Just set dotspacemacs-elpa-subdirectory to emacs-version and be happy about it. This option is available starting with Spacemacs v0.200.

Usually when I need to switch Emacs version, it means that I have to totally nuke ~/.emacs.d/elpa directory and wait for several minutes in order to install 300+ packages. This is not critical, but there is room for improvement. Thanks to bmag now it is possible to easily (without any dirty hacks) to change directory where all packages are installed. I use separate directories for every Emacs version. So packages are downloaded only the first time I use specific version. But for the next time everything will be ready for use.

;; setup package-user-dir to allow seamless switch between emacs versions
(setq package-user-dir (file-name-as-directory
                        (concat user-emacs-directory "elpa/" emacs-version)))

Add this expression to dotspacemacs/user-init function in your .spacemacs and make it easier to switch between versions.