[Nix-dev] warning if trying to install a package but it fails an assert.

[Andres Loeh]

At the risk of sounding annoying:

I don't want an error message. I want NixOS to automatically
rebuild svn with perlBindings enabled. An error message is like
manual tracking of dependencies: "It doesn't work. First do this
and try again." I'm reminded of situations with other package
managers where I sometimes spent several minutes going through
cycles of calling the install command, fixing some config setting
because of an error message, calling again, fixing again, ...
It's possible to specify what we need, there's nothing preventing
us from building it, so let's build it.

On the other hand, I'm not at all convinced why we should suddenly
be afraid of having multiple versions of a package at the same time.
Just how is this situation different from me having installed one
version of gcc and a package I want to build needing another and
therefore pulling it in as well? NixOS solves this problem by making
it easy. This is at the cost of needing potentially more disk space,
but so what?

As others have suggested, we can try to keep the number of different
versions small in practice by choosing sensible defaults. If that
doesn't work and it really becomes an issue, let's address it once
it's there. I'm somewhat surprised that we're willing to sacrifice
the very thing that makes NixOS positively unique so easily ...

Cheers,
  Andres