<chapter xmlns="http://docbook.org/ns/docbook"
	 xmlns:xlink="http://www.w3.org/1999/xlink"
	 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-pkgs-overridePackages">
  <title>pkgs.overridePackages</title>

  <para>
    This function inside the nixpkgs expression (<varname>pkgs</varname>)
    can be used to override the set of packages itself.
  </para>
  <para>
    Warning: this function is expensive and must not be used from within
    the nixpkgs repository.
  </para>
  <para>
    Example usage:

    <programlisting>let
  pkgs = import &lt;nixpkgs&gt; {};
  newpkgs = pkgs.overridePackages (self: super: {
    foo = super.foo.override { ... };
  };
in ...</programlisting>
  </para>

  <para>
    The resulting <varname>newpkgs</varname> will have the new <varname>foo</varname>
    expression, and all other expressions depending on <varname>foo</varname> will also
    use the new <varname>foo</varname> expression.
  </para>

  <para>
    The behavior of this function is similar to <link 
    linkend="sec-modify-via-packageOverrides">config.packageOverrides</link>.
  </para>

  <para>
    The <varname>self</varname> parameter refers to the final package set with the
    applied overrides. Using this parameter may lead to infinite recursion if not
    used consciously.
  </para>

  <para>
    The <varname>super</varname> parameter refers to the old package set.
    It's equivalent to <varname>pkgs</varname> in the above example.
  </para>

</section>

<section xml:id="sec-pkg-override">
  <title>&lt;pkg&gt;.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>pkgs.overridePackages (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-overrideDerivation">
  <title>&lt;pkg&gt;.overrideDerivation</title>

  <warning>
    <para>Do not use this function in Nixpkgs. Because it breaks
    package abstraction and doesn’t provide error checking for
    function arguments, it is only intended for ad-hoc customisation
    (such as in <filename>~/.nixpkgs/config.nix</filename>).</para>
  </warning>

  <para>
    The function <varname>overrideDerivation</varname> is usually available for all the
    derivations in the nixpkgs expression (<varname>pkgs</varname>).
  </para> 
  <para>
    It is used to create a new derivation by overriding the attributes of
    the original derivation according to the given function.
  </para>

  <para>
    Example usage:

    <programlisting>mySed = pkgs.gnused.overrideDerivation (oldAttrs: {
  name = "sed-4.2.2-pre";
  src = fetchurl {
    url = ftp://alpha.gnu.org/gnu/sed/sed-4.2.2-pre.tar.bz2;
    sha256 = "11nq06d131y4wmf3drm0yk502d2xc6n5qy82cg88rb9nqd2lj41k";
  };
  patches = [];
});</programlisting>
  </para>

  <para>
    In the above example, the name, src and patches of the derivation
    will be overridden, while all other attributes will be retained from the
    original derivation.
  </para>

  <para>
    The argument <varname>oldAttrs</varname> is used to refer to the attribute set of
    the original derivation.
  </para>

</section>

<section xml:id="sec-lib-makeOverridable">
  <title>lib.makeOverridable</title>

  <para>
    The function <varname>lib.makeOverridable</varname> is used to make the result
    of a function easily customizable. This utility only makes sense for functions
    that accept an argument set and return an attribute set.
  </para>

  <para>
    Example usage:

    <programlisting>f = { a, b }: { result = a+b; }
c = lib.makeOverridable f { a = 1; b = 2; }</programlisting>

  </para>

  <para>
    The variable <varname>c</varname> is the value of the <varname>f</varname> function
    applied with some default arguments. Hence the value of <varname>c.result</varname>
    is <literal>3</literal>, in this example.
  </para>

  <para>
    The variable <varname>c</varname> however also has some additional functions, like
    <link linkend="sec-pkg-override">c.override</link> which can be used to
    override the default arguments. In this example the value of
    <varname>(c.override { a = 4; }).result</varname> is 6.
  </para>

</section>


<section xml:id="sec-fhs-environments">
  <title>buildFHSChrootEnv/buildFHSUserEnv</title>

  <para>
    <function>buildFHSChrootEnv</function> and
    <function>buildFHSUserEnv</function> provide a way to build and run
    FHS-compatible lightweight sandboxes. They get their own isolated root with
    binded <filename>/nix/store</filename>, so their footprint in terms of disk
    space needed is quite small. This allows one to run software which is hard or
    unfeasible to patch for NixOS -- 3rd-party source trees with FHS assumptions,
    games distributed as tarballs, software with integrity checking and/or external
    self-updated binaries.
  </para>

  <para>
    <function>buildFHSChrootEnv</function> allows to create persistent
    environments, which can be constructed, deconstructed and entered by
    multiple users at once. A downside is that it requires
    <literal>root</literal> access for both those who create and destroy and
    those who enter it. It can be useful to create environments for daemons that
    one can enter and observe.
  </para>

  <para>
    <function>buildFHSUserEnv</function> uses Linux namespaces feature to create
    temporary lightweight environments which are destroyed after all child
    processes exit. It does not require root access, and can be useful to create
    sandboxes and wrap applications.
  </para>

  <para>
    Those functions both rely on <function>buildFHSEnv</function>, which creates
    an actual directory structure given a list of necessary packages and extra
    build commands.
    <function>buildFHSChrootEnv</function> and <function>buildFHSUserEnv</function>
    both accept those arguments which are passed to
    <function>buildFHSEnv</function>:
  </para>

  <variablelist>
    <varlistentry>
    <term><literal>name</literal></term>

    <listitem><para>Environment name.</para></listitem>
    </varlistentry>

    <varlistentry>
    <term><literal>targetPkgs</literal></term>

    <listitem><para>Packages to be installed for the main host's architecture
    (i.e. x86_64 on x86_64 installations).</para></listitem>
    </varlistentry>

    <varlistentry>
    <term><literal>multiPkgs</literal></term>

    <listitem><para>Packages to be installed for all architectures supported by
    a host (i.e. i686 and x86_64 on x86_64 installations).</para></listitem>
    </varlistentry>

    <varlistentry>
    <term><literal>extraBuildCommands</literal></term>

    <listitem><para>Additional commands to be executed for finalizing the
    directory structure.</para></listitem>
    </varlistentry>

    <varlistentry>
    <term><literal>extraBuildCommandsMulti</literal></term>

    <listitem><para>Like <literal>extraBuildCommandsMulti</literal>, but
    executed only on multilib architectures.</para></listitem>
    </varlistentry>
  </variablelist>

  <para>
    Additionally, <function>buildFHSUserEnv</function> accepts
    <literal>runScript</literal> parameter, which is a command that would be
    executed inside the sandbox and passed all the command line arguments. It
    default to <literal>bash</literal>.
  </para>
  <para>
    It also uses <literal>CHROOTENV_EXTRA_BINDS</literal> environment variable
    for binding extra directories in the sandbox to outside places. The format of
    the variable is <literal>/mnt=test-mnt:/data</literal>, where
    <literal>/mnt</literal> would be mounted as <literal>/test-mnt</literal>
    and <literal>/data</literal> would be mounted as <literal>/data</literal>.
    <literal>extraBindMounts</literal> array argument to
    <function>buildFHSUserEnv</function> function is prepended to this variable.
    Latter entries take priority if defined several times -- i.e. in case of
    <literal>/data=data1:/data=data2</literal> the actual bind path would be
    <literal>/data2</literal>.
  </para>
  <para>
    One can create a simple environment using a <literal>shell.nix</literal>
    like that:
  </para>

<programlisting><![CDATA[
{ pkgs ? import <nixpkgs> {} }:

(pkgs.buildFHSUserEnv {
  name = "simple-x11-env";
  targetPkgs = pkgs: (with pkgs;
    [ udev
      alsaLib
    ]) ++ (with pkgs.xorg;
    [ libX11
      libXcursor
      libXrandr
    ]);
  multiPkgs = pkgs: (with pkgs;
    [ udev
      alsaLib
    ]);
  runScript = "bash";
}).env
]]></programlisting>

  <para>
    Running <literal>nix-shell</literal> would then drop you into a shell with
    these libraries and binaries available. You can use this to run
    closed-source applications which expect FHS structure without hassles:
    simply change <literal>runScript</literal> to the application path,
    e.g. <filename>./bin/start.sh</filename> -- relative paths are supported.
  </para>
</section>

</chapter>
