[Nix-dev] flags / configuration etc?

[Marc Weber]

When introducing more possibilitios of configuration there is more to
test..
Do you agree this is useful nevertheless?

When having configuration flags you'll also need a way to check wether a
passed dependency is suited eg by

args: with args;
assert args.glibc.flags.nptl; stdenv.mkDerivation {
  name = "wine"; 
  ...

So using my configuration proposal I would just have added all selected
features to a attr set called flags. This won't work because nix-env
will complain about a missing outpath because it treats all sets as
derivations, right?

Ways to solve this:
  introduce some kind of type safety :
    1) ignore sets not having outPath
    2) create a new type called Derivation which is a attrSet only to
      distinguish them
    3) merge the flags with the derivation itself (then you even don't
      have to pass them to the builder)..
      This would mean that the flags and configuration stuff has to
      executed not only for dependencies (which are to be installed) but
      also for packages which are to be shown (which might slow down the
      system a little bit)

Shall I just go for 3) and do some benchmarking?

Marc