···

       
42
       
42
       
 
       
elements.)</para>

     

       
43
       
43
       
 
       


     

       
44
       
44
       
 
       
<para>Even greater customisation is possible using the function

     

       
45
       
45
       
-
       
<varname>overrideDerivation</varname>.  While the

     

       
45
       
45
       
+
       
<varname>overrideAttrs</varname>.  While the

     

       
46
       
46
       
 
       
<varname>override</varname> mechanism above overrides the arguments of

     

       
47
       
47
       
-
       
a package function, <varname>overrideDerivation</varname> allows

     

       
48
       
48
       
-
       
changing the <emphasis>result</emphasis> of the function.  This

     

       
47
       
47
       
+
       
a package function, <varname>overrideAttrs</varname> allows

     

       
48
       
48
       
+
       
changing the <emphasis>attributes</emphasis> of the function.  This

     

       
49
       
49
       
 
       
permits changing any aspect of the package, such as the source code.

     

       
50
       
50
       
 
       
For instance, if you want to override the source code of Emacs, you

     

       
51
       
51
       
 
       
can say:

     

       
52
       
52
       
 
       


     

       
53
       
53
       
 
       
<programlisting>

     

       
54
       
54
       
-
       
environment.systemPackages =

     

       
55
       
55
       
-
       
  [ (pkgs.lib.overrideDerivation pkgs.emacs (attrs: {

     

       
56
       
56
       
-
       
      name = "emacs-25.0-pre";

     

       
57
       
57
       
-
       
      src = /path/to/my/emacs/tree;

     

       
58
       
58
       
-
       
    }))

     

       
59
       
59
       
-
       
  ];

     

       
54
       
54
       
+
       
environment.systemPackages = [

     

       
55
       
55
       
+
       
  (pkgs.emacs.overrideAttrs (oldAttrs: {

     

       
56
       
56
       
+
       
    name = "emacs-25.0-pre";

     

       
57
       
57
       
+
       
    src = /path/to/my/emacs/tree;

     

       
58
       
58
       
+
       
  }))

     

       
59
       
59
       
+
       
];

     

       
60
       
60
       
 
       
</programlisting>

     

       
61
       
61
       
 
       


     

       
62
       
62
       
-
       
Here, <varname>overrideDerivation</varname> takes the Nix derivation

     

       
62
       
62
       
+
       
Here, <varname>overrideAttrs</varname> takes the Nix derivation

     

       
63
       
63
       
 
       
specified by <varname>pkgs.emacs</varname> and produces a new

     

       
64
       
64
       
 
       
derivation in which the original’s <literal>name</literal> and

     

       
65
       
65
       
 
       
<literal>src</literal> attribute have been replaced by the given

     

       
66
       
66
       
-
       
values.  The original attributes are accessible via

     

       
67
       
67
       
-
       
<varname>attrs</varname>.</para>

     

       
66
       
66
       
+
       
values by re-calling <literal>stdenv.mkDerivation</literal>.

     

       
67
       
67
       
+
       
The original attributes are accessible via the function argument,

     

       
68
       
68
       
+
       
which is conventionally named <varname>oldAttrs</varname>.</para>

     

       
68
       
69
       
 
       


     

       
69
       
70
       
 
       
<para>The overrides shown above are not global.  They do not affect

     

       
70
       
71
       
 
       
the original package; other packages in Nixpkgs continue to depend on

     

···

       
17
       
17
       
 
       
        where tools such as <command>gdb</command> can find them.

     

       
18
       
18
       
 
       
        If you need debug symbols for a package that doesn't

     

       
19
       
19
       
 
       
        provide them by default, you can enable them as follows:

     

       
20
       
20
       
-
       
        <!-- FIXME: ugly, see #10721 -->

     

       
21
       
20
       
 
       
        <programlisting>

     

       
22
       
21
       
 
       
        nixpkgs.config.packageOverrides = pkgs: {

     

       
23
       
23
       
-
       
          hello = pkgs.lib.overrideDerivation pkgs.hello (attrs: {

     

       
24
       
24
       
-
       
            outputs = attrs.outputs or ["out"] ++ ["debug"];

     

       
25
       
25
       
-
       
            buildInputs = attrs.buildInputs ++ [&lt;nixpkgs/pkgs/build-support/setup-hooks/separate-debug-info.sh>];

     

       
22
       
22
       
+
       
          hello = pkgs.hello.overrideAttrs (oldAttrs: {

     

       
23
       
23
       
+
       
            separateDebugInfo = true;

     

       
26
       
24
       
 
       
          });

     

       
27
       
25
       
 
       
        };

     

       
28
       
26
       
 
       
        </programlisting>

     

···

       
356
       
356
       
 
       
        <programlisting><![CDATA[

     

       
357
       
357
       
 
       
{ pkgs ? import <nixpkgs> {} }:

     

       
358
       
358
       
 
       
let

     

       
359
       
359
       
-
       
  myEmacs = pkgs.lib.overrideDerivation (pkgs.emacs.override {

     

       
359
       
359
       
+
       
  myEmacs = (pkgs.emacs.override {

     

       
360
       
360
       
 
       
    # Use gtk3 instead of the default gtk2

     

       
361
       
361
       
 
       
    withGTK3 = true;

     

       
362
       
362
       
 
       
    withGTK2 = false;

     

       
363
       
363
       
-
       
  }) (attrs: {

     

       
363
       
363
       
+
       
  }).overrideAttrs (attrs: {

     

       
364
       
364
       
 
       
    # I don't want emacs.desktop file because I only use

     

       
365
       
365
       
 
       
    # emacsclient.

     

       
366
       
366
       
-
       
    postInstall = attrs.postInstall + ''

     

       
366
       
366
       
+
       
    postInstall = (attrs.postInstall or "") + ''

     

       
367
       
367
       
 
       
      rm $out/share/applications/emacs.desktop

     

       
368
       
368
       
 
       
    '';

     

       
369
       
369
       
 
       
  });