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Overlays: Difference between revisions

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Add an example for overriding Perl packages, since the other languages have examples of special cases. This also helps make a definitive version that can be updated (going forward) for people coming from search engines too, rather than forum posts...etc. from years ago.
 
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As you can see, <tt>final</tt> is the same for every stage, but <tt>prev</tt> comes from only the stage before. So when you define an attribute <tt>foo</tt> in the set to override it, within that overlay <tt>final.foo</tt> will be its version, and <tt>prev.foo</tt> will be the non-overriden version. This is why you see patterns like <tt>foo = prev.foo.override { ... }</tt>.
As you can see, <tt>final</tt> is the same for every stage, but <tt>prev</tt> comes from only the stage before. So when you define an attribute <tt>foo</tt> in the set to override it, within that overlay <tt>final.foo</tt> will be its version, and <tt>prev.foo</tt> will be the non-overriden version. This is why you see patterns like <tt>foo = prev.foo.override { ... }</tt>.


The names <tt>final</tt> and <tt>prev</tt> might remind you of inheritance in object-oriented languages. In fact, overlays are exactly the same thing as subclasses, with regards to overriding and calling methods. This data flow is also how objects know which method to call. This is probably how the two arguments got their names, too.
The alternate names <tt>self</tt> and <tt>super</tt> might remind you of inheritance in object-oriented languages. In fact, overlays are exactly the same thing as subclasses, with regards to overriding and calling methods. This data flow is also how objects know which method to call. This is probably how the two arguments got those names, too.


== Data flow of overlays (alternative explanation) ==
== Data flow of overlays (alternative explanation) ==
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       # specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
       # specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
       # got:    sha256-173gxk0ymiw94glyjzjizp8bv8g72gwkjhacigd1an09jshdrjb4
       # got:    sha256-173gxk0ymiw94glyjzjizp8bv8g72gwkjhacigd1an09jshdrjb4
       hash = "173gxk0ymiw94glyjzjizp8bv8g72gwkjhacigd1an09jshdrjb4";
       hash = "sha256-173gxk0ymiw94glyjzjizp8bv8g72gwkjhacigd1an09jshdrjb4";
     };
     };
   });
   });
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=== Overriding a package inside an extensible attribute set ===
=== Overriding a package inside an extensible attribute set ===


Here is an example of adding plugins to `vimPlugins`.
Here is an example of adding plugins to <code>vimPlugins</code>.
<syntaxhighlight lang="nix">
<syntaxhighlight lang="nix">
final: prev: {
final: prev: {
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       };
       };
     };
     };
</syntaxhighlight>
=== Perl Package overlays ===
Perl packages require some extra care prevent the error <code>undefined variable 'perl'</code>. This example turns off tests for the <code>example</code> package:
<syntaxhighlight lang="nix">
  final: prev: {
    perlPackages = prev.perlPackages // {
      example = prev.perlPackages.example.overrideAttrs (attrs:
        { doChecks = false; }
      )
    };
  };
</syntaxhighlight>
</syntaxhighlight>


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final: prev:
final: prev:
# Within the overlay we use a recursive set, though I think we can use `final` as well.
# Within the overlay we use a recursive set, though I think we can use `final` as well.
rec {
{
   # nix-shell -p python.pkgs.my_stuff
   # nix-shell -p python.pkgs.my_stuff
   python = prev.python.override {
   python = prev.python.override {
     # Careful, we're using a different final and prev here!
     # Careful, we're using a different final and prev here!
     packageOverrides = final: prev: {
     packageOverrides = pyfinal: pyprev: {
       my_stuff = prev.buildPythonPackage rec {
       my_stuff = pyprev.buildPythonPackage rec {
         pname = "pyaes";
         pname = "pyaes";
         version = "1.6.0";
         version = "1.6.0";
         src = prev.fetchPypi {
         src = pyprev.fetchPypi {
           inherit pname version;
           inherit pname version;
           hash = "0bp9bjqy1n6ij1zb86wz9lqa1dhla8qr1d7w2kxyn7jbj56sbmcw";
           hash = "0bp9bjqy1n6ij1zb86wz9lqa1dhla8qr1d7w2kxyn7jbj56sbmcw";
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   };
   };
   # nix-shell -p pythonPackages.my_stuff
   # nix-shell -p pythonPackages.my_stuff
   pythonPackages = python.pkgs;
   pythonPackages = final.python.pkgs;


   # nix-shell -p my_stuff
   # nix-shell -p my_stuff
   my_stuff = pythonPackages.buildPythonPackage rec {
   my_stuff = final.pythonPackages.buildPythonPackage rec {
     pname = "pyaes";
     pname = "pyaes";
     version = "1.6.0";
     version = "1.6.0";
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* [https://nixos.org/nixpkgs/manual/#chap-overlays Overlays  in nixpkgs manual]
* [https://nixos.org/nixpkgs/manual/#chap-overlays Overlays  in nixpkgs manual]
* [https://nixcademy.com/posts/mastering-nixpkgs-overlays-techniques-and-best-practice/ Blog post "Mastering Nixpkgs Overlays: Techniques and Best Practice"]
* [https://blog.flyingcircus.io/2017/11/07/nixos-the-dos-and-donts-of-nixpkgs-overlays/ Blog post "The DOs and DON’Ts of nixpkgs overlays"]
* [https://blog.flyingcircus.io/2017/11/07/nixos-the-dos-and-donts-of-nixpkgs-overlays/ Blog post "The DOs and DON’Ts of nixpkgs overlays"]
* [https://www.youtube.com/watch?v=s2fkgkN55vk&list=PLgknCdxP89ReD6gxl755B6G_CI65z4J2e Nixpkgs Overlays – A place for all excluded packages] - Talk by Nicolas B. Pierron at NixCon 2017
* [https://www.youtube.com/watch?v=s2fkgkN55vk&list=PLgknCdxP89ReD6gxl755B6G_CI65z4J2e Nixpkgs Overlays – A place for all excluded packages] - Talk by Nicolas B. Pierron at NixCon 2017
* [[Nixpkgs/Patching Nixpkgs]]


==== References ====
==== References ====
Retrieved from "https://wiki.nixos.org/wiki/Overlays"