[Nix-dev] Introducing Nixpkgs Overlays

[Domen Kožar]

+1 from me, we need to plan out the details such as deprecation timeline
(involving dates and docs).

I'll review the PR.

On Tue, Dec 27, 2016 at 8:43 PM, Benno Fünfstück <
benno.fuenfstueck at gmail.com> wrote:

> 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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