How do I run multiple versions of Emacs with the same .emacs.d

Question

I alternatively run 24.5 and 25.0.50. Unfortunately, packages installed and compiled from 25.0.50 don't work with Emacs 24.5 (cl-struct-define is a common culprit).

One trivial solution is to not byte-compile anything. That makes things pretty slow, though.

How can I use compiled packages (for performance) while using the same .emacs.d for both 24.5 and 25.0.50? I'd also be ok with 24.5 ignoring the compiled files.

Answer 1

Put the byte-compiled files for different Emacs versions in different directories. Make your load-path conditional on the Emacs version you are running, so that you load the appropriate byte-compiled libraries.

Answer 2

It is perfectly possible to have packages.el put byte-compiled code into versioned subdirectories of the elpa package. This gives a structure like this:

┌ elpa
└┬ the-package-date.time
 ├─ the-package.el
 ├┬ 23.4
 │└─ the-package.elc
 ├┬ 26.3
 │└─ the-package.elc
 └┬ 27.1
  └─ the-package.elc

The load path is constructed in such a fashion, that the versioned directory appears before the source directory, e.g.:

'(
  "/home/../.emacs.d/elpa/the-package-date.time/27.1"
  "/home/../.emacs.d/elpa/the-package-date.time" 
  ;; ...
 )

To accomplish all of this, I have just written the package package-compat, which uses mostly adviced functions to provide that feature for Emacs 23 to Emacs 27.

The package mechanism is just used for distribution in this case, since the code must be loaded before any packages are activated and/or installed.

Either add "https://almost-un.org/selpa/packages" to the variable package-archives

(add-to-list 'package-archives ("almost-un.org" . "https://almost-un.org/selpa/packages/") t)

and download the package with (package-install 'package-compat) or download the tar(1) archive from the above page.

Then follow the instructions in any of the Emacs lisp files.