<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
xml:id="chap-functions">
<title>Functions reference</title>
<para>
The nixpkgs repository has several utility functions to manipulate Nix
expressions.
</para>
<section xml:id="sec-overrides">
<title>Overriding</title>
<para>
Sometimes one wants to override parts of <literal>nixpkgs</literal>, e.g.
derivation attributes, the results of derivations or even the whole package
set.
</para>
<section xml:id="sec-pkg-override">
<title><pkg>.override</title>
<para>
The function <varname>override</varname> is usually available for all the
derivations in the nixpkgs expression (<varname>pkgs</varname>).
</para>
<para>
It is used to override the arguments passed to a function.
</para>
<para>
Example usages:
<programlisting>pkgs.foo.override { arg1 = val1; arg2 = val2; ... }</programlisting>
<programlisting>
import pkgs.path { overlays = [ (self: super: {
foo = super.foo.override { barSupport = true ; };
})]};
</programlisting>
<programlisting>
mypkg = pkgs.callPackage ./mypkg.nix {
mydep = pkgs.mydep.override { ... };
}
</programlisting>
</para>
<para>
In the first example, <varname>pkgs.foo</varname> is the result of a
function call with some default arguments, usually a derivation. Using
<varname>pkgs.foo.override</varname> will call the same function with the
given new arguments.
</para>
</section>
<section xml:id="sec-pkg-overrideAttrs">
<title><pkg>.overrideAttrs</title>
<para>
The function <varname>overrideAttrs</varname> allows overriding the
attribute set passed to a <varname>stdenv.mkDerivation</varname> call,
producing a new derivation based on the original one. This function is
available on all derivations produced by the
<varname>stdenv.mkDerivation</varname> function, which is most packages in
the nixpkgs expression <varname>pkgs</varname>.
</para>
<para>
Example usage:
<programlisting>
helloWithDebug = pkgs.hello.overrideAttrs (oldAttrs: rec {
separateDebugInfo = true;
});
</programlisting>
</para>
<para>
In the above example, the <varname>separateDebugInfo</varname> attribute is
overridden to be true, thus building debug info for
<varname>helloWithDebug</varname>, while all other attributes will be
retained from the original <varname>hello</varname> package.
</para>
<para>
The argument <varname>oldAttrs</varname> is conventionally used to refer to
the attr set originally passed to <varname>stdenv.mkDerivation</varname>.
</para>
<note>
<para>
Note that <varname>separateDebugInfo</varname> is processed only by the
<varname>stdenv.mkDerivation</varname> function, not the generated, raw
Nix derivation. Thus, using <varname>overrideDerivation</varname> will not
work in this case, as it overrides only the attributes of the final
derivation. It is for this reason that <varname>overrideAttrs</varname>
should be preferred in (almost) all cases to
<varname>overrideDerivation</varname>, i.e. to allow using
<varname>sdenv.mkDerivation</varname> to process input arguments, as well
as the fact that it is easier to use (you can use the same attribute names
you see in your Nix code, instead of the ones generated (e.g.
<varname>buildInputs</varname> vs <varname>nativeBuildInputs</varname>,
and involves less typing.
</para>
</note>
</section>
<section xml:id="sec-pkg-overrideDerivation">
<title><pkg>.overrideDerivation</title>
<warning>
<para>
You should prefer <varname>overrideAttrs</varname> in almost all cases,
see its documentation for the reasons why.
<varname>overrideDerivation</varname> is not deprecated and will continue
to work, but is less nice to use and does not have as many abilities as
<varname>overrideAttrs</varname>.
</para>
</warning>
<warning>
<para>
Do not use this function in Nixpkgs as it evaluates a Derivation before
modifying it, which breaks package abstraction and removes error-checking
of function arguments. In addition, this evaluation-per-function
application incurs a performance penalty, which can become a problem if
many overrides are used. It is only intended for ad-hoc customisation,
such as in <filename>~/.config/nixpkgs/config.nix</filename>.
</para>
</warning>
<para>
The<