attribute) about installed packages in user environments. Thus, an
operation like `nix-env -q --description' shows useful information
not only on available packages but also on installed packages.
* nix-env now passes the entire manifest as an argument to the Nix
expression of the user environment builder (not just a list of
paths), so that in particular the user environment builder has
access to the meta attributes.
* New operation `--set-flag' in nix-env to change meta info of
installed packages. This will be useful to pass per-package
policies to the user environment builder (e.g., how to resolve
collision or whether to disable a package (NIX-80)) or upgrade
policies in nix-env (e.g., that a package should be "masked", that
is, left untouched by upgrade actions). Example:
$ nix-env --set-flag enabled false ghc-6.4
single derivation specified by the argument. This is useful when we
want to have a profile for a single derivation, such as a server
configuration. Then we can just say (e.g.)
$ nix-env -p /.../server-profile -f server.nix --set -A server
We can't do queries or upgrades on such a profile, but we can do
rollbacks. The advantage over -i is that we don't have to worry
about other packages having been installed in the profile
previously; --set gets rid of them.