[Nix-dev] nix-env and nixpkgs usability improvements

[Eelco Visser]

Hi Eelco,

Good ideas, these improvements.

> - For people who use a channel, it is hard to install packages that
>   are not pre-built by the Nixpkgs release.  They only get to see the
>   packages available in populate-cache.nix.

You discussed various issues to improve the usage of the nixpkgs
collection, e.g., mixing pre-built binary distribution and source
building. 

Another problem with channels is when building nix expressions on top of
(not within) nixpkgs. For example, the services collection is not
integrated in nixpkgs; it requires a checkout/download of the nixpkgs
expression tree. If one has also pulled the manifest from a channel that
overlaps with this version of nixpkgs then you may be lucky to get
mostly binary deployment, but this is not guaranteed. So I would rather
like to import in such an extension, the URL of the channel, so that I
can be sure that I get binary distributions where these are available.

-- Eelco