<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>
<prompt>$ </prompt>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>
<prompt>$ </prompt>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. (This can cause this shebang line to become too long for Darwin to handle; see the note below.)
    </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.out}/lib" > config.in
    echo "INCLUDE = ${db.dev}/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>

 <para>
  On Darwin, if a script has too many <literal>-I<replaceable>dir</replaceable></literal> flags in its first line (its “shebang line”), it will not run. This can be worked around by calling the <literal>shortenPerlShebang</literal> function from the <literal>postInstall</literal> phase:
<programlisting>
{ stdenv, buildPerlPackage, fetchurl, shortenPerlShebang }:

ImageExifTool = buildPerlPackage {
  pname = "Image-ExifTool";
  version = "11.50";

  src = fetchurl {
    url = "https://www.sno.phy.queensu.ca/~phil/exiftool/Image-ExifTool-11.50.tar.gz";
    sha256 = "0d8v48y94z8maxkmw1rv7v9m0jg2dc8xbp581njb6yhr7abwqdv3";
  };

  buildInputs = stdenv.lib.optional stdenv.isDarwin shortenPerlShebang;
  postInstall = stdenv.lib.optional stdenv.isDarwin ''
    shortenPerlShebang $out/bin/exiftool
  '';
};
</programlisting>
  This will remove the <literal>-I</literal> flags from the shebang line, rewrite them in the <literal>use lib</literal> form, and put them on the next line instead. This function can be given any number of Perl scripts as arguments; it will modify them in-place.
 </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>
<prompt>$ </prompt>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>
<prompt>$ </prompt>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 xml:id="ssec-perl-cross-compilation">
  <title>Cross-compiling modules</title>

  <para>
   Nixpkgs has experimental support for cross-compiling Perl modules. In many cases, it will just work out of the box, even for modules with native extensions. Sometimes, however, the Makefile.PL for a module may (indirectly) import a native module. In that case, you will need to make a stub for that module that will satisfy the Makefile.PL and install it into <filename>lib/perl5/site_perl/cross_perl/${perl.version}</filename>. See the <varname>postInstall</varname> for <varname>DBI</varname> for an example.
  </para>
 </section>
</section>
