nixpkgs/doc/language-support.xml

<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xml:id="chap-language-support">

<title>Support for specific programming languages</title>

<para>The <link linkend="chap-stdenv">standard build
environment</link> makes it easy to build typical Autotools-based
packages with very little code.  Any other kind of package can be
accomodated by overriding the appropriate phases of
<literal>stdenv</literal>.  However, there are specialised functions
in Nixpkgs to easily build packages for other programming languages,
such as Perl or Haskell.  These are described in this chapter.</para>


<section xml:id="sec-language-perl"><title>Perl</title>

<para>Nixpkgs provides a function <varname>buildPerlPackage</varname>,
a generic package builder function for any Perl package that has a
standard <varname>Makefile.PL</varname>.  It’s implemented in <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/perl-modules/generic"><filename>pkgs/development/perl-modules/generic</filename></link>.</para>

<para>Perl packages from CPAN are defined in <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/perl-packages.nix"><filename>pkgs/top-level/perl-packages.nix</filename></link>,
rather than <filename>pkgs/all-packages.nix</filename>.  Most Perl
packages are so straight-forward to build that they are defined here
directly, rather than having a separate function for each package
called from <filename>perl-packages.nix</filename>.  However, more
complicated packages should be put in a separate file, typically in
<filename>pkgs/development/perl-modules</filename>.  Here is an
example of the former:

<programlisting>
ClassC3 = buildPerlPackage rec {
  name = "Class-C3-0.21";
  src = fetchurl {
    url = "mirror://cpan/authors/id/F/FL/FLORA/${name}.tar.gz";
    sha256 = "1bl8z095y4js66pwxnm7s853pi9czala4sqc743fdlnk27kq94gz";
  };
};
</programlisting>

Note the use of <literal>mirror://cpan/</literal>, and the
<literal>${name}</literal> in the URL definition to ensure that the
name attribute is consistent with the source that we’re actually
downloading.  Perl packages are made available in
<filename>all-packages.nix</filename> through the variable
<varname>perlPackages</varname>.  For instance, if you have a package
that needs <varname>ClassC3</varname>, you would typically write

<programlisting>
foo = import ../path/to/foo.nix {
  inherit stdenv fetchurl ...;
  inherit (perlPackages) ClassC3;
};
</programlisting>

in <filename>all-packages.nix</filename>.  You can test building a
Perl package as follows:

<screen>
$ nix-build -A perlPackages.ClassC3
</screen>

<varname>buildPerlPackage</varname> adds <literal>perl-</literal> to
the start of the name attribute, so the package above is actually
called <literal>perl-Class-C3-0.21</literal>.  So to install it, you
can say:

<screen>
$ nix-env -i perl-Class-C3
</screen>

(Of course you can also install using the attribute name:
<literal>nix-env -i -A perlPackages.ClassC3</literal>.)</para>

<para>So what does <varname>buildPerlPackage</varname> do?  It does
the following:

<orderedlist>

  <listitem><para>In the configure phase, it calls <literal>perl
  Makefile.PL</literal> to generate a Makefile.  You can set the
  variable <varname>makeMakerFlags</varname> to pass flags to
  <filename>Makefile.PL</filename></para></listitem>

  <listitem><para>It adds the contents of the <envar>PERL5LIB</envar>
  environment variable to <literal>#! .../bin/perl</literal> line of
  Perl scripts as <literal>-I<replaceable>dir</replaceable></literal>
  flags.  This ensures that a script can find its
  dependencies.</para></listitem>

  <listitem><para>In the fixup phase, it writes the propagated build
  inputs (<varname>propagatedBuildInputs</varname>) to the file
  <filename>$out/nix-support/propagated-user-env-packages</filename>.
  <command>nix-env</command> recursively installs all packages listed
  in this file when you install a package that has it.  This ensures
  that a Perl package can find its dependencies.</para></listitem>

</orderedlist>

</para>

<para><varname>buildPerlPackage</varname> is built on top of
<varname>stdenv</varname>, so everything can be customised in the
usual way.  For instance, the <literal>BerkeleyDB</literal> module has
a <varname>preConfigure</varname> hook to generate a configuration
file used by <filename>Makefile.PL</filename>:

<programlisting>
{ buildPerlPackage, fetchurl, db }:

buildPerlPackage rec {
  name = "BerkeleyDB-0.36";

  src = fetchurl {
    url = "mirror://cpan/authors/id/P/PM/PMQS/${name}.tar.gz";
    sha256 = "07xf50riarb60l1h6m2dqmql8q5dij619712fsgw7ach04d8g3z1";
  };

  preConfigure = ''
    echo "LIB = ${db}/lib" > config.in
    echo "INCLUDE = ${db}/include" >> config.in
  '';
}
</programlisting>

</para>

<para>Dependencies on other Perl packages can be specified in the
<varname>buildInputs</varname> and
<varname>propagatedBuildInputs</varname> attributes.  If something is
exclusively a build-time dependency, use
<varname>buildInputs</varname>; if it’s (also) a runtime dependency,
use <varname>propagatedBuildInputs</varname>.  For instance, this
builds a Perl module that has runtime dependencies on a bunch of other
modules:

<programlisting>
ClassC3Componentised = buildPerlPackage rec {
  name = "Class-C3-Componentised-1.0004";
  src = fetchurl {
    url = "mirror://cpan/authors/id/A/AS/ASH/${name}.tar.gz";
    sha256 = "0xql73jkcdbq4q9m0b0rnca6nrlvf5hyzy8is0crdk65bynvs8q1";
  };
  propagatedBuildInputs = [
    ClassC3 ClassInspector TestException MROCompat
  ];
};
</programlisting>

</para>

<section xml:id="ssec-generation-from-CPAN"><title>Generation from CPAN</title>

<para>Nix expressions for Perl packages can be generated (almost)
automatically from CPAN.  This is done by the program
<command>nix-generate-from-cpan</command>, which can be installed
as follows:</para>

<screen>
$ nix-env -i nix-generate-from-cpan
</screen>

<para>This program takes a Perl module name, looks it up on CPAN,
fetches and unpacks the corresponding package, and prints a Nix
expression on standard output.  For example:

<screen>
$ nix-generate-from-cpan XML::Simple
  XMLSimple = buildPerlPackage {
    name = "XML-Simple-2.20";
    src = fetchurl {
      url = mirror://cpan/authors/id/G/GR/GRANTM/XML-Simple-2.20.tar.gz;
      sha256 = "5cff13d0802792da1eb45895ce1be461903d98ec97c9c953bc8406af7294434a";
    };
    propagatedBuildInputs = [ XMLNamespaceSupport XMLSAX XMLSAXExpat ];
    meta = {
      description = "Easily read/write XML (esp config files)";
      license = "perl";
    };
  };
</screen>

The output can be pasted into
<filename>pkgs/top-level/perl-packages.nix</filename> or wherever else
you need it.</para>

</section>

</section>


<section xml:id="sec-python"><title>Python</title>

<para>
  Currently supported interpreters are <varname>python26</varname>, <varname>python27</varname>,
  <varname>python32</varname>, <varname>python33</varname>, <varname>python34</varname>
  and <varname>pypy</varname>.
</para>

<para>
  <varname>python</varname> is an alias of <varname>python27</varname> and <varname>python3</varname> is an alias of <varname>python34</varname>.
</para>

<para>
  <varname>python26</varname> and <varname>python27</varname> do not include modules that require
  external dependencies (to reduce dependency bloat). Following modules need to be added as
  <varname>buildInput</varname> explicitly:
</para>

<itemizedlist>
  <listitem><para><varname>python.modules.bsddb</varname></para></listitem>
  <listitem><para><varname>python.modules.curses</varname></para></listitem>
  <listitem><para><varname>python.modules.curses_panel</varname></para></listitem>
  <listitem><para><varname>python.modules.crypt</varname></para></listitem>
  <listitem><para><varname>python.modules.gdbm</varname></para></listitem>
  <listitem><para><varname>python.modules.sqlite3</varname></para></listitem>
  <listitem><para><varname>python.modules.tkinter</varname></para></listitem>
  <listitem><para><varname>python.modules.readline</varname></para></listitem>
</itemizedlist>

<para>For convenience <varname>python27Full</varname> and <varname>python26Full</varname>
are provided with all modules included.</para>

<para>
  Python packages that
  use <link xlink:href="http://pypi.python.org/pypi/setuptools/"><literal>setuptools</literal></link> or <literal>distutils</literal>,
  can be built using the <varname>buildPythonPackage</varname> function as documented below.
</para>

<para>
 All packages depending on any Python interpreter get appended <varname>$out/${python.libPrefix}/site-packages</varname>
 to <literal>$PYTHONPATH</literal> if such directory exists.
</para>

<variablelist>
  <title>
     Useful attributes on interpreters packages:
  </title>

  <varlistentry>
    <term><varname>libPrefix</varname></term>
    <listitem><para>
        Name of the folder in <literal>${python}/lib/</literal> for corresponding interpreter.
    </para></listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>interpreter</varname></term>
    <listitem><para>
        Alias for <literal>${python}/bin/${executable}.</literal>
    </para></listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>buildEnv</varname></term>
    <listitem><para>
        Function to build python interpreter environments with extra packages bundled together.
        See <xref linkend="ssec-python-build-env" /> for usage and documentation.
    </para></listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>sitePackages</varname></term>
    <listitem><para>
      Alias for <literal>lib/${libPrefix}/site-packages</literal>.
    </para></listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>executable</varname></term>
    <listitem><para>
      Name of the interpreter executable, ie <literal>python3.4</literal>.
    </para></listitem>
  </varlistentry>

</variablelist>
<section xml:id="ssec-build-python-package"><title><varname>buildPythonPackage</varname> function</title>

  <para>
  The function is implemented in <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/python-modules/generic/default.nix">
  <filename>pkgs/development/python-modules/generic/default.nix</filename></link>.
  Example usage:

<programlisting language="nix">
twisted = buildPythonPackage {
  name = "twisted-8.1.0";

  src = pkgs.fetchurl {
    url = http://tmrc.mit.edu/mirror/twisted/Twisted/8.1/Twisted-8.1.0.tar.bz2;
    sha256 = "0q25zbr4xzknaghha72mq57kh53qw1bf8csgp63pm9sfi72qhirl";
  };

  propagatedBuildInputs = [ self.ZopeInterface ];

  meta = {
    homepage = http://twistedmatrix.com/;
    description = "Twisted, an event-driven networking engine written in Python";
    license = stdenv.lib.licenses.mit;
  };
};
</programlisting>

  Most of Python packages that use <varname>buildPythonPackage</varname> are defined
  in <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/python-packages.nix"><filename>pkgs/top-level/python-packages.nix</filename></link>
  and generated for each python interpreter separately into attribute sets <varname>python26Packages</varname>,
  <varname>python27Packages</varname>, <varname>python32Packages</varname>, <varname>python33Packages</varname>,
  <varname>python34Packages</varname> and <varname>pypyPackages</varname>.
  </para>

  <para>
    <function>buildPythonPackage</function> mainly does four things:

    <orderedlist>
      <listitem><para>
        In the <varname>configurePhase</varname>, it patches
        <literal>setup.py</literal> to always include setuptools before
        distutils for monkeypatching machinery to take place.
      </para></listitem>

      <listitem><para>
        In the <varname>buildPhase</varname>, it calls
        <literal>${python.interpreter} setup.py build ...</literal>
      </para></listitem>

      <listitem><para>
        In the <varname>installPhase</varname>, it calls
        <literal>${python.interpreter} setup.py install ...</literal>
      </para></listitem>

      <listitem><para>
        In the <varname>postFixup</varname> phase, <literal>wrapPythonPrograms</literal>
        bash function is called to wrap all programs in <filename>$out/bin/*</filename>
        directory to include <literal>$PYTHONPATH</literal> and <literal>$PATH</literal>
        environment variables.
      </para></listitem>
    </orderedlist>
  </para>

  <para>By default <varname>doCheck = true</varname> is set and tests are run with
  <literal>${python.interpreter} setup.py test</literal> command in <varname>checkPhase</varname>.</para>

  <para>
    As in Perl, dependencies on other Python packages can be specified in the
    <varname>buildInputs</varname> and
    <varname>propagatedBuildInputs</varname> attributes.  If something is
    exclusively a build-time dependency, use
    <varname>buildInputs</varname>; if it’s (also) a runtime dependency,
    use <varname>propagatedBuildInputs</varname>.
  </para>

  <para>
    By default <varname>meta.platforms</varname> is set to the same value
    as the interpreter unless overriden otherwise.
  </para>

  <variablelist>
    <title>
      <varname>buildPythonPackage</varname> parameters
      (all parameters from <varname>mkDerivation</varname> function are still supported)
    </title>

    <varlistentry>
      <term><varname>namePrefix</varname></term>
      <listitem><para>
        Prepended text to <varname>${name}</varname> parameter.
        Defaults to <literal>"python3.3-"</literal> for Python 3.3, etc. Set it to
        <literal>""</literal>
        if you're packaging an application or a command line tool.
      </para></listitem>
    </varlistentry>

    <varlistentry>
      <term><varname>disabled</varname></term>
      <listitem><para>
        If <varname>true</varname>, package is not build for
        particular python interpreter version. Grep around
        <filename>pkgs/top-level/python-packages.nix</filename>
        for examples.
      </para></listitem>
    </varlistentry>

    <varlistentry>
      <term><varname>setupPyInstallFlags</varname></term>
      <listitem><para>
        List of flags passed to <command>setup.py install</command> command.
      </para></listitem>
    </varlistentry>

    <varlistentry>
      <term><varname>setupPyBuildFlags</varname></term>
      <listitem><para>
        List of flags passed to <command>setup.py build</command> command.
      </para></listitem>
    </varlistentry>

    <varlistentry>
      <term><varname>pythonPath</varname></term>
      <listitem><para>
        List of packages to be added into <literal>$PYTHONPATH</literal>.
        Packages in <varname>pythonPath</varname> are not propagated into user environment
        (contrary to <varname>propagatedBuildInputs</varname>).
      </para></listitem>
    </varlistentry>

    <varlistentry>
      <term><varname>preShellHook</varname></term>
      <listitem><para>
        Hook to execute commands before <varname>shellHook</varname>.
      </para></listitem>
    </varlistentry>

    <varlistentry>
      <term><varname>postShellHook</varname></term>
      <listitem><para>
        Hook to execute commands after <varname>shellHook</varname>.
      </para></listitem>
    </varlistentry>

    <varlistentry>
      <term><varname>distutilsExtraCfg</varname></term>
      <listitem><para>
        Extra lines passed to <varname>[easy_install]</varname> section of
        <filename>distutils.cfg</filename> (acts as global setup.cfg
        configuration).
      </para></listitem>
    </varlistentry>

    <varlistentry>
      <term><varname>makeWrapperArgs</varname></term>
      <listitem><para>
        A list of strings. Arguments to be passed to
        <varname>makeWrapper</varname>, which wraps generated binaries. By
        default, the arguments to <varname>makeWrapper</varname> set
        <varname>PATH</varname> and <varname>PYTHONPATH</varname> environment
        variables before calling the binary. Additional arguments here can
        allow a developer to set environment variables which will be
        available when the binary is run. For example,
        <varname>makeWrapperArgs = ["--set FOO BAR" "--set BAZ QUX"]</varname>.
      </para></listitem>
    </varlistentry>

  </variablelist>

</section>

<section xml:id="ssec-python-build-env"><title><function>python.buildEnv</function> function</title>
  <para>
    Create Python environments using low-level <function>pkgs.buildEnv</function> function. Example <filename>default.nix</filename>:

<programlisting language="nix">
<![CDATA[with import <nixpkgs> {};

python.buildEnv.override {
  extraLibs = [ pkgs.pythonPackages.pyramid ];
  ignoreCollisions = true;
}]]>
</programlisting>

    Running <command>nix-build</command> will create
    <filename>/nix/store/cf1xhjwzmdki7fasgr4kz6di72ykicl5-python-2.7.8-env</filename>
    with wrapped binaries in <filename>bin/</filename>.
  </para>

  <para>
  You can also use <varname>env</varname> attribute to create local
  environments with needed packages installed (somewhat comparable to
  <literal>virtualenv</literal>). For example, with the following
  <filename>shell.nix</filename>:

<programlisting language="nix">
<![CDATA[with import <nixpkgs> {};

(python3.buildEnv.override {
  extraLibs = with python3Packages;
    [ numpy
      requests
    ];
}).env]]>
</programlisting>

  Running <command>nix-shell</command> will drop you into a shell where
  <command>python</command> will have specified packages in its path.
  </para>

  <variablelist>
    <title>
      <function>python.buildEnv</function> arguments
    </title>

    <varlistentry>
      <term><varname>extraLibs</varname></term>
      <listitem><para>
        List of packages installed inside the environment.
      </para></listitem>
    </varlistentry>

    <varlistentry>
      <term><varname>postBuild</varname></term>
      <listitem><para>
        Shell command executed after the build of environment.
      </para></listitem>
    </varlistentry>

    <varlistentry>
      <term><varname>ignoreCollisions</varname></term>
      <listitem><para>
         Ignore file collisions inside the environment (default is <varname>false</varname>).
      </para></listitem>
    </varlistentry>
  </variablelist>
</section>

<section xml:id="ssec-python-tools"><title>Tools</title>

<para>Packages inside nixpkgs are written by hand. However many tools
exist in community to help save time. No tool is preferred at the moment.
</para>

<itemizedlist>

  <listitem><para>
    <link xlink:href="https://github.com/proger/python2nix">python2nix</link>
    by Vladimir Kirillov
  </para></listitem>

  <listitem><para>
    <link xlink:href="https://github.com/garbas/pypi2nix">pypi2nix</link>
    by Rok Garbas
  </para></listitem>

  <listitem><para>
    <link xlink:href="https://github.com/offlinehacker/pypi2nix">pypi2nix</link>
    by Jaka Hudoklin
  </para></listitem>

</itemizedlist>

</section>

<section xml:id="ssec-python-development"><title>Development</title>

  <para>
    To develop Python packages <function>buildPythonPackage</function> has
    additional logic inside <varname>shellPhase</varname> to run
    <command>${python.interpreter} setup.py develop</command> for the package.
  </para>

  <warning><para><varname>shellPhase</varname> is executed only if <filename>setup.py</filename>
  exists.</para></warning>

  <para>
    Given a <filename>default.nix</filename>:

<programlisting language="nix">
<![CDATA[with import <nixpkgs> {};

buildPythonPackage {
  name = "myproject";

  buildInputs = with pkgs.pythonPackages; [ pyramid ];

  src = ./.;
}]]>
</programlisting>

    Running <command>nix-shell</command> with no arguments should give you
    the environment in which the package would be build with
    <command>nix-build</command>.
  </para>

  <para>
    Shortcut to setup environments with C headers/libraries and python packages:

    <programlisting language="bash">$ nix-shell -p pythonPackages.pyramid zlib libjpeg git</programlisting>
  </para>

  <note><para>
    There is a boolean value <varname>lib.inNixShell</varname> set to
    <varname>true</varname> if nix-shell is invoked.
  </para></note>

</section>

<section xml:id="ssec-python-faq"><title>FAQ</title>

<variablelist>

  <varlistentry>
    <term>How to solve circular dependencies?</term>
    <listitem><para>
      If you have packages <varname>A</varname> and <varname>B</varname> that
      depend on each other, when packaging <varname>B</varname> override package
      <varname>A</varname> not to depend on <varname>B</varname> as input
      (and also the other way around).
    </para></listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>install_data / data_files</varname> problems resulting into <literal>error: could not create '/nix/store/6l1bvljpy8gazlsw2aw9skwwp4pmvyxw-python-2.7.8/etc': Permission denied</literal></term>
    <listitem><para>
      <link xlink:href="https://bitbucket.org/pypa/setuptools/issue/130/install_data-doesnt-respect-prefix">
      Known bug in setuptools <varname>install_data</varname> does not respect --prefix</link>. Example of
      such package using the feature is <filename>pkgs/tools/X11/xpra/default.nix</filename>. As workaround
      install it as an extra <varname>preInstall</varname> step:

      <programlisting>${python.interpreter} setup.py install_data --install-dir=$out --root=$out
sed -i '/ = data_files/d' setup.py</programlisting>
    </para></listitem>
  </varlistentry>

  <varlistentry>
    <term>Rationale of non-existent global site-packages</term>
    <listitem><para>
      There is no need to have global site-packages in Nix. Each package has isolated
      dependency tree and installing any python package will only populate <varname>$PATH</varname>
      inside user environment. See <xref linkend="ssec-python-build-env" /> to create self-contained
      interpreter with a set of packages.
    </para></listitem>
  </varlistentry>

</variablelist>

</section>


<section xml:id="ssec-python-contrib"><title>Contributing guidelines</title>
<para>
  Following rules are desired to be respected:
</para>

<itemizedlist>

  <listitem><para>
    Make sure package builds for all python interpreters. Use <varname>disabled</varname> argument to
    <function>buildPythonPackage</function> to set unsupported interpreters.
  </para></listitem>

  <listitem><para>
    If tests need to be disabled for a package, make sure you leave a comment about reasoning.
  </para></listitem>

  <listitem><para>
    Packages in <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/python-packages.nix"><filename>pkgs/top-level/python-packages.nix</filename></link>
    are sorted quasi-alphabetically to avoid merge conflicts.
  </para></listitem>

</itemizedlist>

</section>

</section>


<section xml:id="sec-language-ruby"><title>Ruby</title>
  <para>There currently is support to bundle applications that are packaged as Ruby gems. The utility "bundix" allows you to write a <filename>Gemfile</filename>, let bundler create a <filename>Gemfile.lock</filename>, and then convert
  this into a nix expression that contains all Gem dependencies automatically.</para>

  <para>For example, to package sensu, we did:</para>

<screen>
<![CDATA[$ cd pkgs/servers/monitoring
$ mkdir sensu
$ cat > Gemfile
source 'https://rubygems.org'
gem 'sensu'
$ bundler package --path /tmp/vendor/bundle
$ $(nix-build '<nixpkgs>' -A bundix)/bin/bundix
$ cat > default.nix
{ lib, bundlerEnv, ruby }:

bundlerEnv {
  name = "sensu-0.17.1";

  inherit ruby;
  gemfile = ./Gemfile;
  lockfile = ./Gemfile.lock;
  gemset = ./gemset.nix;

  meta = with lib; {
    description = "A monitoring framework that aims to be simple, malleable,
and scalable.";
    homepage    = http://sensuapp.org/;
    license     = with licenses; mit;
    maintainers = with maintainers; [ theuni ];
    platforms   = platforms.unix;
  };
}]]>
</screen>

<para>Please check in the <filename>Gemfile</filename>, <filename>Gemfile.lock</filename> and the <filename>gemset.nix</filename> so future updates can be run easily.
</para>

</section>

<section xml:id="sec-language-go"><title>Go</title>

<para>The function <varname>buildGoPackage</varname> builds
standard Go packages.
</para>

<example xml:id='ex-buildGoPackage'><title>buildGoPackage</title>
<programlisting>
net = buildGoPackage rec {
  name = "go.net-${rev}";
  goPackagePath = "golang.org/x/net"; <co xml:id='ex-buildGoPackage-1' />
  subPackages = [ "ipv4" "ipv6" ]; <co xml:id='ex-buildGoPackage-2' />
  rev = "e0403b4e005";
  src = fetchFromGitHub {
    inherit rev;
    owner = "golang";
    repo = "net";
    sha256 = "1g7cjzw4g4301a3yqpbk8n1d4s97sfby2aysl275x04g0zh8jxqp";
  };
  goPackageAliases = [ "code.google.com/p/go.net" ]; <co xml:id='ex-buildGoPackage-3' />
  propagatedBuildInputs = [ goPackages.text ]; <co xml:id='ex-buildGoPackage-4' />
  buildFlags = "--tags release"; <co xml:id='ex-buildGoPackage-5' />
  disabled = isGo13;<co xml:id='ex-buildGoPackage-6' />
};
</programlisting>
</example>

<para><xref linkend='ex-buildGoPackage'/> is an example expression using buildGoPackage,
the following arguments are of special significance to the function:

<calloutlist>

  <callout arearefs='ex-buildGoPackage-1'>
    <para>
      <varname>goPackagePath</varname> specifies the package's canonical Go import path.
    </para>
  </callout>

  <callout arearefs='ex-buildGoPackage-2'>
    <para>
      <varname>subPackages</varname> limits the builder from building child packages that
      have not been listed. If <varname>subPackages</varname> is not specified, all child
      packages will be built.
    </para>
    <para>
      In this example only <literal>code.google.com/p/go.net/ipv4</literal> and
      <literal>code.google.com/p/go.net/ipv4</literal> will be built.
    </para>
  </callout>

  <callout arearefs='ex-buildGoPackage-3'>
    <para>
      <varname>goPackageAliases</varname> is a list of alternative import paths
      that are valid for this library.
      Packages that depend on this library will automatically rename
      import paths that match any of the aliases to <literal>goPackagePath</literal>.
    </para>
    <para>
      In this example imports will be renamed from
      <literal>code.google.com/p/go.net</literal> to
      <literal>golang.org/x/net</literal> in every package that depend on the
      <literal>go.net</literal> library.
    </para>
  </callout>

  <callout arearefs='ex-buildGoPackage-4'>
    <para>
      <varname>propagatedBuildInputs</varname> is where the dependencies of a Go library are
      listed. Only libraries should list <varname>propagatedBuildInputs</varname>. If a standalone
      program is being build instead, use <varname>buildInputs</varname>. If a library's tests require
      additional dependencies that are not propagated, they should be listed in <varname>buildInputs</varname>.
    </para>
  </callout>

  <callout arearefs='ex-buildGoPackage-5'>
    <para>
      <varname>buildFlags</varname> is a list of flags passed to the go build command.
    </para>
  </callout>

  <callout arearefs='ex-buildGoPackage-6'>
    <para>
      If <varname>disabled</varname> is <literal>true</literal>,
      nix will refuse to build this package.
    </para>
    <para>
      In this example the package will not be built for go 1.3. The <literal>isGo13</literal>
      is an utility function that returns <literal>true</literal> if go used to build the
      package has version 1.3.x.
    </para>
  </callout>

</calloutlist>

</para>

<para>
Reusable Go libraries may be found in the <varname>goPackages</varname> set. You can test
build a Go package as follows:

<screen>
$ nix-build -A goPackages.net
</screen>

</para>

<para>
You may use Go packages installed into the active Nix profiles by adding
the following to your ~/.bashrc:

<screen>
for p in $NIX_PROFILES; do
    GOPATH="$p/share/go:$GOPATH"
done
</screen>
</para>

  <para>To extract dependency information from a Go package in automated way use <link xlink:href="https://github.com/cstrahan/go2nix">go2nix</link>.</para>
</section>


<section xml:id="sec-language-java"><title>Java</title>

<para>Ant-based Java packages are typically built from source as follows:

<programlisting>
stdenv.mkDerivation {
  name = "...";
  src = fetchurl { ... };

  buildInputs = [ jdk ant ];

  buildPhase = "ant";
}
</programlisting>

Note that <varname>jdk</varname> is an alias for the OpenJDK.</para>

<para>JAR files that are intended to be used by other packages should
be installed in <filename>$out/share/java</filename>.  The OpenJDK has
a stdenv setup hook that adds any JARs in the
<filename>share/java</filename> directories of the build inputs to the
<envar>CLASSPATH</envar> environment variable.  For instance, if the
package <literal>libfoo</literal> installs a JAR named
<filename>foo.jar</filename> in its <filename>share/java</filename>
directory, and another package declares the attribute

<programlisting>
buildInputs = [ jdk libfoo ];
</programlisting>

then <envar>CLASSPATH</envar> will be set to
<filename>/nix/store/...-libfoo/share/java/foo.jar</filename>.</para>

<para>Private JARs
should be installed in a location like
<filename>$out/share/<replaceable>package-name</replaceable></filename>.</para>

<para>If your Java package provides a program, you need to generate a
wrapper script to run it using the OpenJRE.  You can use
<literal>makeWrapper</literal> for this:

<programlisting>
buildInputs = [ makeWrapper ];

installPhase =
  ''
    mkdir -p $out/bin
    makeWrapper ${jre}/bin/java $out/bin/foo \
      --add-flags "-cp $out/share/java/foo.jar org.foo.Main"
  '';
</programlisting>

Note the use of <literal>jre</literal>, which is the part of the
OpenJDK package that contains the Java Runtime Environment.  By using
<literal>${jre}/bin/java</literal> instead of
<literal>${jdk}/bin/java</literal>, you prevent your package from
depending on the JDK at runtime.</para>

<para>It is possible to use a different Java compiler than
<command>javac</command> from the OpenJDK.  For instance, to use the
Eclipse Java Compiler:

<programlisting>
buildInputs = [ jre ant ecj ];
</programlisting>

(Note that here you don’t need the full JDK as an input, but just the
JRE.)  The ECJ has a stdenv setup hook that sets some environment
variables to cause Ant to use ECJ, but this doesn’t work with all Ant
files.  Similarly, you can use the GNU Java Compiler:

<programlisting>
buildInputs = [ gcj ant ];
</programlisting>

Here, Ant will automatically use <command>gij</command> (the GNU Java
Runtime) instead of the OpenJRE.</para>

</section>


<section xml:id="sec-language-lua"><title>Lua</title>

<para>
  Lua packages are built by the <varname>buildLuaPackage</varname> function.  This function is
  implemented
  in <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/lua-modules/generic/default.nix">
  <filename>pkgs/development/lua-modules/generic/default.nix</filename></link>
  and works similarly to <varname>buildPerlPackage</varname>. (See
  <xref linkend="sec-language-perl"/> for details.)
</para>

<para>
  Lua packages are defined
  in <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/lua-packages.nix"><filename>pkgs/top-level/lua-packages.nix</filename></link>.
  Most of them are simple. For example:

  <programlisting>
fileSystem = buildLuaPackage {
  name = "filesystem-1.6.2";
  src = fetchurl {
    url = "https://github.com/keplerproject/luafilesystem/archive/v1_6_2.tar.gz";
    sha256 = "1n8qdwa20ypbrny99vhkmx8q04zd2jjycdb5196xdhgvqzk10abz";
  };
  meta = {
    homepage = "https://github.com/keplerproject/luafilesystem";
    hydraPlatforms = stdenv.lib.platforms.linux;
    maintainers = with maintainers; [ flosse ];
  };
};
  </programlisting>
</para>

<para>
  Though, more complicated package should be placed in a seperate file in
  <link
  xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/lua-modules"><filename>pkgs/development/lua-modules</filename></link>.
</para>
<para>
  Lua packages accept additional parameter <varname>disabled</varname>, which defines
  the condition of disabling package from luaPackages. For example, if package has
  <varname>disabled</varname> assigned to <literal>lua.luaversion != "5.1"</literal>,
  it will not be included in any luaPackages except lua51Packages, making it
  only be built for lua 5.1.
</para>

</section>

<section xml:id="sec-language-coq"><title>Coq</title>
  <para>
    Coq libraries should be installed in
    <literal>$(out)/lib/coq/${coq.coq-version}/user-contrib/</literal>.
    Such directories are automatically added to the
    <literal>$COQPATH</literal> environment variable by the hook defined
    in the Coq derivation.
  </para>
  <para>
    Some libraries require OCaml and sometimes also Camlp5. The exact
    versions that were used to build Coq are saved in the
    <literal>coq.ocaml</literal> and <literal>coq.camlp5</literal>
    attributes.
  </para>
  <para>
    Here is a simple package example. It is a pure Coq library, thus it
    only depends on Coq. Its <literal>makefile</literal> has been
    generated using <literal>coq_makefile</literal> so we only have to
    set the <literal>$COQLIB</literal> variable at install time.
  </para>
  <programlisting>
{stdenv, fetchurl, coq}:
stdenv.mkDerivation {
  src = fetchurl {
    url = http://coq.inria.fr/pylons/contribs/files/Karatsuba/v8.4/Karatsuba.tar.gz;
    sha256 = "0ymfpv4v49k4fm63nq6gcl1hbnnxrvjjp7yzc4973n49b853c5b1";
  };

  name = "coq-karatsuba";

  buildInputs = [ coq ];

  installFlags = "COQLIB=$(out)/lib/coq/${coq.coq-version}/";
}
</programlisting>
</section>

<section xml:id="sec-language-qt"><title>Qt</title>

<para>The information in this section applies to Qt 5.5 and later.</para>

<para>Qt is an application development toolkit for C++. Although it is
not a distinct programming language, there are special considerations
for packaging Qt-based programs and libraries. A small set of tools
and conventions has grown out of these considerations.</para>

<section xml:id="ssec-qt-libraries"><title>Libraries</title>

<para>Packages that provide libraries should be listed in
<varname>qt5LibsFun</varname> so that the library is built with each
Qt version. A set of packages is provided for each version of Qt; for
example, <varname>qt5Libs</varname> always provides libraries built
with the latest version, <varname>qt55Libs</varname> provides
libraries built with Qt 5.5, and so on. To avoid version conflicts, no
top-level attributes are created for these packages.</para>

</section>

<section xml:id="ssec-qt-programs"><title>Programs</title>

<para>Application packages do not need to be built with every Qt
version. To ensure consistency between the package's dependencies,
call the package with <literal>qt5Libs.callPackage</literal> instead
of the usual <literal>callPackage</literal>. An older version may be
selected in case of incompatibility. For example, to build with Qt
5.5, call the package with
<literal>qt55Libs.callPackage</literal>.</para>

<para>Several environment variables must be set at runtime for Qt
applications to function correctly, including:</para>

<itemizedlist>
  <listitem><para><envar>QT_PLUGIN_PATH</envar></para></listitem>
  <listitem><para><envar>QML_IMPORT_PATH</envar></para></listitem>
  <listitem><para><envar>QML2_IMPORT_PATH</envar></para></listitem>
  <listitem><para><envar>XDG_DATA_DIRS</envar></para></listitem>
</itemizedlist>

<para>To ensure that these are set correctly, the program must be wrapped by
invoking <literal>wrapQtProgram <replaceable>program</replaceable></literal>
during installation (for example, during
<literal>fixupPhase</literal>). <literal>wrapQtProgram</literal>
accepts the same options as <literal>makeWrapper</literal>.
</para>

</section>

<section xml:id="ssec-qt-kde"><title>KDE</title>

<para>Many of the considerations above also apply to KDE packages,
especially the need to set the correct environment variables at
runtime. To ensure that this is done, invoke <literal>wrapKDEProgram
<replaceable>program</replaceable></literal> during
installation. <literal>wrapKDEProgram</literal> also generates a
<literal>ksycoca</literal> database so that required data and services
can be found. Like its Qt counterpart,
<literal>wrapKDEProgram</literal> accepts the same options as
<literal>makeWrapper</literal>.</para>

</section>

</section>

<!--
<section><title>Haskell</title>

<para>TODO</para>

</section>


<section><title>TeX / LaTeX</title>

<para>* Special support for building TeX documents</para>

</section>
-->


</chapter>