How can I gracefully drop support for older emacsen in my elisp package?

Question

I maintain a fairly well-used emacs package (ido-ubiquitous), and in the next version I plan to drop support for Emacs 23 and below. People who use Emacs 23 and below will be able to continue using the current version of my package.

However, I don't want to have Emacs 23 users upgrading via ELPA or git or something else and ending up with the new version that isn't compatible with their emacs. Is there a generally accepted way of handling this gracefully? Do I have any choice short of renaming the new version to "ido-ubiquitous-ng" or something?

Answer 1

ELPA/package.el

To prevent updates via package.el, add the special dependency (emacs "24.1") to the Package-Requires list. See Library Headers in the Emacs Lisp Manual, in the description of the Package-Requires: header:

[…] The package code automatically defines a package named ‘emacs’ with the version number of the currently running Emacs. This can be used to require a minimal version of Emacs for a package.

The package.el which is distributed independently for Emacs 23 and below does not provide this special package. Thus, any attempt to install your package on Emacs 23 will fail with a message complaining about “emacs” being unavailable for installation, leaving the old, compatible version in place.

However, when using this, be prepared to handle complaints from users of Emacs 24. Many users apparently do not delete their old package.el when upgrading to Emacs 24. Thus the old package.el overrides the new built-in one, leading to spurious errors on installation.

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ELGet

I do not know Elget. Probably ask its author for help in this matter.

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Git submodules, Tarballs and other legacy methods

I do not think, that you can really prevent updates, if users install your package in a legacy way (e.g. Git submodules, distribution packages, etc.). You can only complain after your package was updated, which is arguably too late, because the incompatible code is now already there.

You may choose to add an explicit version check, with a detailed error. I consider this superfluous, though. If you really go for Emacs 24, you will be using incompatible functions, so your package won't load successfully, whether you explicitly prevent it or not. So save yourself of superfluous code :)

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TL;DR (+ personal experience)

First of all, please do not rename your package. Few users can follow the news on each and every installed package. Thus many users will not immediately realize that the package was renamed, and continue to use an outdated version without notice or warning. Effectively, you would sort of punish Emacs 24 users of your package.

Add the special dependency to prevent accidental updates via package.el. Add prominent documentation, that your package requires Emacs 24, like in the first section of your Github Readme. Then, let the matter rest. Anything else is likely more hassle that it is worth.

In my personal experience, Emacs users are not stupid (well, at least the majority isn't). They read documentation. They understand documentation.

Users of Emacs 23 know that their Emacs is outdated. Many of them expect incompatibilities and breakage. If the package suddenly breaks for them, they will seek advice on Github, realize that the package is not available for Emacs 23 anymore, and either go back to the last working release, or (hopefully) upgrade their Emacs.