[Nix-dev] Guarantee Consistent Builds and Obsolete overrideScope

[Peter Simons]

Hi Felipe,

 > In the future it may be possible for one to use two versions of the
 > same package at the same time using Backpack. Should this catch on,
 > the non-breakable improvement will become a bit inflexible in some
 > cases.

yes, that is a good point. Fortunately, build expression that expect a
transitive closure of all dependencies still allow for building binaries
with "inconsistent" library versions -- it just takes a conscious effort
to pull it off. For example:

  foo = { binary }: mkDerivation { haskellLibraryDepends = [binary]; ... };

  bar = { binary }: mkDerivation { haskellLibraryDepends = [binary]; ... };

  foobar = { binary, foo, bar }:
    let foo' = foo { inherit binary; };
        bar' = bar { inherit binary; };
    in
    mkDerivation {
      haskellLibraryDepends = [foo' bar'];
      ...
    };

Now, we can use 'foobar' as follows:

  foobar {
    binary = null;
    foo = { binary }: self.foo { self.binary_1; };
    bar = { binary }: self.bar { self.binary_2; };
  };

'foo' and 'bar' simply ignore the argument passed to them. It's not
pretty, and obviously one wouldn't want to do this kind of thing on a
regular basis. The point is, however, that *if* we wanted to run a build
with inconsistent dependencies, then we can. The transitive dependency
closure doesn't prevent that from happening -- it just prevents that
from happening accidentally. :-)

Best regards,
Peter