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121 lines
3.6 KiB
XML
121 lines
3.6 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xml:id="chap-functions">
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<title>Functions reference</title>
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<para>
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The nixpkgs repository has several utility functions to manipulate Nix expressions.
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</para>
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<section xml:id="sec-pkgs-overridePackages">
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<title>pkgs.overridePackages</title>
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<para>
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This function inside the nixpkgs expression (<varname>pkgs</varname>)
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can be used to override the set of packages itself.
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</para>
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<para>
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Warning: this function is expensive and must not be used from within
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the nixpkgs repository.
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</para>
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<para>
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Example usage:
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<programlisting>let
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pkgs = import <nixpkgs> {};
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newpkgs = pkgs.overridePackages (self: super: {
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foo = super.foo.override { ... };
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};
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in ...</programlisting>
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</para>
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<para>
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The resulting <varname>newpkgs</varname> will have the new <varname>foo</varname>
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expression, and all other expressions depending on <varname>foo</varname> will also
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use the new <varname>foo</varname> expression.
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</para>
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<para>
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The behavior of this function is similar to <link
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linkend="sec-modify-via-packageOverrides">config.packageOverrides</link>.
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</para>
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<para>
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The <varname>self</varname> parameter refers to the final package set with the
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applied overrides. Using this parameter may lead to infinite recursion if not
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used consciously.
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</para>
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<para>
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The <varname>super</varname> parameter refers to the old package set.
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It's equivalent to <varname>pkgs</varname> in the above example.
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</para>
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</section>
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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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<programlisting>pkgs.overridePackages (self: super: {
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foo = super.foo.override { barSupport = true ; };
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})</programlisting>
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<programlisting>mypkg = pkgs.callPackage ./mypkg.nix {
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mydep = pkgs.mydep.override { ... };
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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 function call
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with some default arguments, usually a derivation.
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Using <varname>pkgs.foo.override</varname> will call the same function with
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the given new arguments.
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</para>
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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 make the result
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of a function easily customizable. This utility only makes sense for functions
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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>f = { a, b }: { result = a+b; }
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c = lib.makeOverridable f { a = 1; b = 2; }</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> function
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applied with some default arguments. Hence the value of <varname>c.result</varname>
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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 functions, like
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<link linkend="sec-pkg-override">c.override</link> which can be used to
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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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</chapter>
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