[Nix-dev] Introducing Nixpkgs Overlays

[Benno Fünfstück]

Hi Nicolas,

I like the idea of Nixpkgs overlays. I have not thought fully about the
whole proposal (and I need to catch up with how security updates work
first), but I think there is small mistake when you say:

As part of this modification, I intend to remove the
`pkgs.overridePackages` function, as this one can literally be
replaced by the following Nix expression:

```nix
let pkgs = import <nixpkgs> {}; in
  import pkgs.path { overlay = [ (self: super: {
    foo = super.foo.override { enableBar = true; };
    bar = import ./pkgs/bar { inherit (self) stdenv fetchurl foo; };
  }) ]; }
```


 The `overridePackages` function is useful when you want to chain multiple
functions that modify the package set, like

```nix
let
  pkgs = import <nixpkgs> {};
  pkgsA = functionA pkgs
  pkgsB = pkgsA.overridePackages (super: self: {
  });
```

In this code example. I do not need to know how functionA is implemented
internally. It seems to me that, if I instead used overlays for this, it
would be hard to achieve the same effect: I would have to manually take
care to re-apply the overrides that `functionA` put in place.

This does not mean that `overridePackages` needs to stay as-is. But perhaps
it would be possible to implement it using overlays, while still providing
the same interface to the user?


Regards,

Benno Fünfstück
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