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+<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 {
+    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>
+