213 lines
7.5 KiB
XML
213 lines
7.5 KiB
XML
<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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xml:id="sec-overrides">
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<title>Overriding</title>
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<para>
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Sometimes one wants to override parts of <literal>nixpkgs</literal>, e.g.
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derivation attributes, the results of derivations.
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</para>
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<para>
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These functions are used to make changes to packages, returning only single
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packages. <link xlink:href="#chap-overlays">Overlays</link>, on the other
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hand, can be used to combine the overridden packages across the entire
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package set of Nixpkgs.
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</para>
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<section xml:id="sec-pkg-override">
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<title><pkg>.override</title>
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<para>
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The function <varname>override</varname> is usually available for all the
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derivations in the nixpkgs expression (<varname>pkgs</varname>).
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</para>
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<para>
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It is used to override the arguments passed to a function.
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</para>
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<para>
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Example usages:
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<programlisting>pkgs.foo.override { arg1 = val1; arg2 = val2; ... }</programlisting>
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<!-- TODO: move below programlisting to a new section about extending and overlays
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and reference it
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-->
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<programlisting>
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import pkgs.path { overlays = [ (self: super: {
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foo = super.foo.override { barSupport = true ; };
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})]};
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</programlisting>
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<programlisting>
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mypkg = pkgs.callPackage ./mypkg.nix {
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mydep = pkgs.mydep.override { ... };
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}
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</programlisting>
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</para>
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<para>
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In the first example, <varname>pkgs.foo</varname> is the result of a
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function call with some default arguments, usually a derivation. Using
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<varname>pkgs.foo.override</varname> will call the same function with the
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given new arguments.
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</para>
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</section>
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<section xml:id="sec-pkg-overrideAttrs">
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<title><pkg>.overrideAttrs</title>
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<para>
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The function <varname>overrideAttrs</varname> allows overriding the
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attribute set passed to a <varname>stdenv.mkDerivation</varname> call,
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producing a new derivation based on the original one. This function is
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available on all derivations produced by the
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<varname>stdenv.mkDerivation</varname> function, which is most packages in
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the nixpkgs expression <varname>pkgs</varname>.
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</para>
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<para>
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Example usage:
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<programlisting>
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helloWithDebug = pkgs.hello.overrideAttrs (oldAttrs: rec {
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separateDebugInfo = true;
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});
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</programlisting>
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</para>
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<para>
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In the above example, the <varname>separateDebugInfo</varname> attribute is
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overridden to be true, thus building debug info for
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<varname>helloWithDebug</varname>, while all other attributes will be
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retained from the original <varname>hello</varname> package.
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</para>
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<para>
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The argument <varname>oldAttrs</varname> is conventionally used to refer to
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the attr set originally passed to <varname>stdenv.mkDerivation</varname>.
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</para>
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<note>
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<para>
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Note that <varname>separateDebugInfo</varname> is processed only by the
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<varname>stdenv.mkDerivation</varname> function, not the generated, raw Nix
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derivation. Thus, using <varname>overrideDerivation</varname> will not work
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in this case, as it overrides only the attributes of the final derivation.
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It is for this reason that <varname>overrideAttrs</varname> should be
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preferred in (almost) all cases to <varname>overrideDerivation</varname>,
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i.e. to allow using <varname>stdenv.mkDerivation</varname> to process input
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arguments, as well as the fact that it is easier to use (you can use the
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same attribute names you see in your Nix code, instead of the ones
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generated (e.g. <varname>buildInputs</varname> vs
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<varname>nativeBuildInputs</varname>), and it involves less typing).
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</para>
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</note>
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</section>
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<section xml:id="sec-pkg-overrideDerivation">
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<title><pkg>.overrideDerivation</title>
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<warning>
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<para>
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You should prefer <varname>overrideAttrs</varname> in almost all cases, see
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its documentation for the reasons why.
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<varname>overrideDerivation</varname> is not deprecated and will continue
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to work, but is less nice to use and does not have as many abilities as
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<varname>overrideAttrs</varname>.
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</para>
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</warning>
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<warning>
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<para>
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Do not use this function in Nixpkgs as it evaluates a Derivation before
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modifying it, which breaks package abstraction and removes error-checking
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of function arguments. In addition, this evaluation-per-function
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application incurs a performance penalty, which can become a problem if
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many overrides are used. It is only intended for ad-hoc customisation, such
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as in <filename>~/.config/nixpkgs/config.nix</filename>.
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</para>
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</warning>
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<para>
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The function <varname>overrideDerivation</varname> creates a new derivation
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based on an existing one by overriding the original's attributes with the
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attribute set produced by the specified function. This function is available
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on all derivations defined using the <varname>makeOverridable</varname>
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function. Most standard derivation-producing functions, such as
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<varname>stdenv.mkDerivation</varname>, are defined using this function,
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which means most packages in the nixpkgs expression,
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<varname>pkgs</varname>, have this function.
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</para>
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<para>
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Example usage:
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<programlisting>
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mySed = pkgs.gnused.overrideDerivation (oldAttrs: {
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name = "sed-4.2.2-pre";
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src = fetchurl {
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url = ftp://alpha.gnu.org/gnu/sed/sed-4.2.2-pre.tar.bz2;
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sha256 = "11nq06d131y4wmf3drm0yk502d2xc6n5qy82cg88rb9nqd2lj41k";
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};
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patches = [];
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});
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</programlisting>
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</para>
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<para>
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In the above example, the <varname>name</varname>, <varname>src</varname>,
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and <varname>patches</varname> of the derivation will be overridden, while
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all other attributes will be retained from the original derivation.
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</para>
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<para>
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The argument <varname>oldAttrs</varname> is used to refer to the attribute
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set of the original derivation.
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</para>
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<note>
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<para>
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A package's attributes are evaluated *before* being modified by the
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<varname>overrideDerivation</varname> function. For example, the
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<varname>name</varname> attribute reference in <varname>url =
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"mirror://gnu/hello/${name}.tar.gz";</varname> is filled-in *before* the
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<varname>overrideDerivation</varname> function modifies the attribute set.
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This means that overriding the <varname>name</varname> attribute, in this
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example, *will not* change the value of the <varname>url</varname>
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attribute. Instead, we need to override both the <varname>name</varname>
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*and* <varname>url</varname> attributes.
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</para>
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</note>
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</section>
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<section xml:id="sec-lib-makeOverridable">
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<title>lib.makeOverridable</title>
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<para>
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The function <varname>lib.makeOverridable</varname> is used to make the
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result of a function easily customizable. This utility only makes sense for
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functions that accept an argument set and return an attribute set.
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</para>
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<para>
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Example usage:
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<programlisting>
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f = { a, b }: { result = a+b; };
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c = lib.makeOverridable f { a = 1; b = 2; };
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</programlisting>
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</para>
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<para>
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The variable <varname>c</varname> is the value of the <varname>f</varname>
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function applied with some default arguments. Hence the value of
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<varname>c.result</varname> is <literal>3</literal>, in this example.
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</para>
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<para>
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The variable <varname>c</varname> however also has some additional
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functions, like <link linkend="sec-pkg-override">c.override</link> which can
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be used to override the default arguments. In this example the value of
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<varname>(c.override { a = 4; }).result</varname> is 6.
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</para>
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</section>
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</section>
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