* Channels. These allow you to stay current with an evolving set of

Nix expressions.

  To subscribe to a channel (needs to be done only once):

    nix-channel --add \
      http://catamaran.labs.cs.uu.nl/dist/nix/channels/nixpkgs-unstable

  This just adds the given URL to ~/.nix-channels (which can also be
  edited manually).

  To update from all channels:

    nix-channel --update

  This fetches the latest expressions and pulls cache manifests.  The
  default Nix expression (~/.nix-defexpr) is made to point to the
  conjunction of the expressions downloaded from all channels.

  So to update all installed derivations in the current user
  environment:

    nix-channel --update
    nix-env --upgrade '*'

  If you are really courageous, you can put this in a cronjob or
  something.

  You can subscribe to multiple channels.  It is not entirely clear
  what happens when there are name clashes between derivations from
  different channels.  From nix-env/main.cc it appears that the one
  with the lowest (highest?) hash will be used, which is pretty
  meaningless.

corepkgs/fetchurl/default.nix

@@ -1,8 +1,8 @@
-{system, url, md5}: derivation {
+{system, url, md5}:
+
+derivation {
   name = baseNameOf (toString url);
-  system = system;
   builder = ./builder.sh;
-  url = url;
-  md5 = md5;
   id = md5;
+  inherit system url md5;
 }