Bob Rogers <rogers-emacs@rgrjr.dyndns.org> writes:

>    From: Rupert Swarbrick <rswarbrick@gmail.com>
>    Date: Thu, 17 Sep 2009 12:50:04 +0100
>
>    . . .
>
>    One thing to bear in mind is that shared libraries on unix are probably
>    a simpler problem than emacs libraries: I can install multiple versions
>    of libfoo and the dynamic linker will select the correct one for an
>    application. Trying to install multiple versions of an emacs package
>    would result in a godawful mess, because it's not just a list of
>    functions that can be called.
>
>    I don't know how one could solve that, so I suspect that the only
>    solution is to only allow one version of a package at once . . .
>
>    Rupert
>
> This does not seem so hard.  If the "installed" location depended on the
> Emacs version, e.g. by having the version number as part of the
> directory name, then you could easily have one version of a package per
> Emacs version.  The downside is that if you want to have all X versions
> of Emacs on a given system find the "same" package, you would have to
> install that package X times.  But this seems like a small price to pay
> to be certain that each installed package version is known to work with
> that version of Emacs -- and that it has been byte-compiled by the right
> version of the byte compiler.

Ah, sorry, I don't think I was clear. When I said "package" above, I
meant elisp libraries. So, for example, maybe foo.el needs nxml version
<= n to work, but bar.el needs nxml version >= n+1. Is it possible to
install foo.el on my system?

Rupert