<section xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         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 rec {
    name = "XML-Simple-2.22";
    src = fetchurl {
      url = "mirror://cpan/authors/id/G/GR/GRANTM/${name}.tar.gz";
      sha256 = "b9450ef22ea9644ae5d6ada086dc4300fa105be050a2030ebd4efd28c198eb49";
    };
    propagatedBuildInputs = [ XMLNamespaceSupport XMLSAX XMLSAXExpat ];
    meta = {
      description = "An API for simple XML files";
      license = with stdenv.lib.licenses; [ artistic1 gpl1Plus ];
    };
  };
</screen>

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

</section>

</section>

