bug#20255: 'search-paths' should respect both user and system profile.

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hi Leo,

[...]

> I think there is a solution, that works not only for the case of
> disabling this unwanted feature, but also to add in support for
> multiple profiles, i.e. if the user has more than just their .guix-
> profile to load.
>
> If we made this feature opt-in in that a user would first have to write
> their profiles to $HOME/.config/guix/default-profiles or a similarly
> named file in $HOME/.config/guix, we could simply not run the command
> if the file doesn't exist, and if it exists run it using the profiles
> in there.
>
> Most users will likely have
>
> /home/myself/.guix-profile
> /run/current-system/profile
>
> in it, but you could also have
>
> /home/myself/.guix-extra-profiles/emacs
> /home/myself/.guix-extra-profiles/hundreds-of-npm-packages
> /home/myself/.guix-extra-profiles/rusty-rust
> /home/myself/.guix-profile
> /run/current-system/profile
>
> Of course, having to type out /home/myself is somewhat weird, and the
> last two lines are a bit of boilerplate, that one might want to avoid. 
> We could alternatively make it so that an empty file means "use
> $HOME/.guix-profile and /run/current-system/profile", such that those
> are always sourced no matter what.  WDYT?

I like this later idea (does what a user would reasonably expect by
default, which is to consider both the system and the user profile by
default).  A user specifying that file manually could then override the
default behavior to have it source extra profiles, or none at all.

Some small difference I'd propose:

An non-existing $HOME/.config/guix/default-profiles file would be
equivalent to an existing file containing:

- $HOME/.guix-profile
- /run/current-system/profile

as the default behavior.

Now we'd need to look at how feasible it is to implement such a feature.

Thanks for this clever suggestion!  It seems we may be able to find a
good middle ground :-).

Maxim