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Re: Compiled code in Emacs-26 will fail in Emacs-25 if use pcase [was: A

From: Kaushal Modi
Subject: Re: Compiled code in Emacs-26 will fail in Emacs-25 if use pcase [was: Add new bytecode op `switch' for implementing branch tables.]
Date: Thu, 23 Feb 2017 14:33:42 +0000

I have this in my emacs config for a while now.

It works pretty well. It also allows you to use an older emacs version without have to mess up the compiled elpa dir of the current version. The only side-effect is that when switching major versions the package updates/installs will happen independently.

(setq package-user-dir (format "%selpa_%s/"
                               user-emacs-directory emacs-major-version))

On Thu, Feb 23, 2017 at 8:42 AM Stefan Monnier <address@hidden> wrote:
> I) Compile under master branch.
> II) Load the resultant .elc with Emacs-25

As explained by Richard, this is usually considered acceptable.
We don't really try to provide forward compatibility of byte-code files
between major versions.

This said, occasionally we try to reduce the pain a little: e.g. in
Emacs-25 a similar incompatibility was introduced for the compilation of
catch&condition-case, but the bytecodes were introduced a bit earlier.
IOW the incompatibility was fundamentally introduced in 24.4 (IIRC), but
the use of the this new feature was only enabled in Emacs-25.1, so files
compiled with 25.1 will usually work in 24.5 as well.



Kaushal Modi

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