Re: The load path

From: Rob Browning <rlb@defaultvalue.org>
Cc: Marius Vollmer <marius.vollmer@uni-dortmund.de>,
	guile-devel <guile-devel@gnu.org>, Greg Troxel <gdt@ir.bbn.com>
Subject: Re: The load path
Date: Sun, 07 Nov 2004 15:16:50 -0600	[thread overview]
Message-ID: <87hdo1e4vx.fsf@trouble.defaultvalue.org> (raw)
In-Reply-To: <418E6D90.5060705@ossau.uklinux.net> (Neil Jerram's message of "Sun, 07 Nov 2004 18:46:40 +0000")

Neil Jerram <neil@ossau.uklinux.net> writes:

> Agreed. I'd just note that versioning also has to cope with the case
> where an add-on package A also provides an interface that further
> packages {B, C, ...} may use, so there is also a requirement for
> handling multiple installed versions of A, and for {B, C, ...} to be
> able to get the version that they need.
>
> Unfortunately, I have no idea at this point how to solve such a
> problem!

One of the simplest ways I can think of to handle this (athough it's
not all that pretty) is for packages to just put something like a
"soname" in the module name.  i.e. (use-modules (foo-1)).  Then when
the API changes incompatibly, the author just changes the module name
to foo-2.  This is already the best way to solve the dynamic linking
problem, given the currently available dynamic linking facilities like
libltdl.  i.e. (dynamic-link "libfoo-2").

As far as use-modules is concerned, I can also imagine adding
something fancier like (use-modules (foo #:version 4)), which might
have more sophisticated semantics (like ld.so or something else).

There's a further complication if you want to try to handle problems
created when only some of the packages on a system have been upgraded
and there are multiple levels of dependencies,

   A -> B -> D                (-> means "depends on") 
   A -> C -> D

but the currently installed version of B depends on a different
version of D than the currently installed version of C.  This is also
a problem in the shared library arena, and as far as I know the
primary solution right now is to just say "don't do that".  A
distribution like Debian tries to make sure that the transition period
where such situations exists is a short as possible and only happens
in unstable (and maybe testing).

In any case, I suspect all of this will be the subject of some future
discussion.

-- 
Rob Browning
rlb @defaultvalue.org and @debian.org; previously @cs.utexas.edu
GPG starting 2002-11-03 = 14DD 432F AE39 534D B592  F9A0 25C8 D377 8C7E 73A4

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