  <sect1 id="packages">
    <title>Packages</title>
    <indexterm><primary>packages</primary></indexterm>

    <para>Packages are collections of libraries, conveniently grouped
    together as a single entity.  The package system is flexible: a
    package may consist of Haskell code, foreign language code (eg. C
    libraries), or a mixture of the two.  A package is a good way to
    group together related Haskell modules, and is essential if you
    intend to make the modules into a Windows DLL (see below).</para>

    <para>Because packages can contain both Haskell and C libraries, they
    are also a good way to provide convenient access to a Haskell
    layer over a C library.</para>

    <para>GHC comes with several packages (see the accompanying
    library documentation), and packages can be added to or removed
    from an existing GHC installation, using the supplied
    <literal>ghc-pkg</literal><indexterm><primary><literal>ghc-pkg</literal></primary>
    </indexterm> tool, described in <xref
    linkend="package-management">.</para>

    <sect2 id="using-packages">
      <title>Using a package</title>
      <indexterm><primary>packages</primary>
	<secondary>using</secondary></indexterm>
      
      <para>To use a package, add the <literal>-package</literal> flag
      to the GHC command line:</para>

      <variablelist>
	<varlistentry>
	  <term><option>-package <replaceable>lib</replaceable></option></term>
	  <indexterm><primary>-package <replaceable>lib</replaceable> option</primary></indexterm>
	  <listitem>
	    <para>This option brings into scope all the modules from
	    package <literal><replaceable>lib</replaceable></literal> (they still have to
	    be imported in your Haskell source, however).  It also
	    causes the relevant libraries to be linked when linking is
	    being done.</para>
	  </listitem>
	</varlistentry>
      </variablelist>

      <para>Some packages depend on other packages, for example the
      <literal>text</literal> package makes use of some of the modules
      in the <literal>lang</literal> package.  The package system
      takes care of all these dependencies, so that when you say
      <literal>-package text</literal> on the command line, you
      automatically get <literal>-package lang</literal> too.</para>
    </sect2>

    <sect2 id="using-local-packages">
      <title>Maintaining a local set of packages</title>
      
      <para>When GHC starts up, it automatically reads the default set
      of packages from a configuration file, normally named
      <filename>package.conf</filename> in your GHC installation
      directory.</para>

      <para>You can load in additional package configuration files
      using the <option>-package-conf</option> option:</para>

      <variablelist>
	<varlistentry>
	  <term><option>-package-conf <replaceable>file</replaceable></option></term>
	  <indexterm><primary><option>-package-conf <replaceable>file</replaceable></option></primary>
	  </indexterm>
	  <listitem>
	    <para>Read in the package configuration file
	    <replaceable>file</replaceable> in addition to the system
	    default file.  This allows the user to have a local set of
	    packages in addition to the system-wide ones.</para>
	  </listitem>
	</varlistentry>
      </variablelist>

      <para>To create your own package configuration file, just create
      a new file and put the string
      <quote><literal>[]</literal></quote> in it.  Packages can be
      added to the new configuration file using the
      <literal>ghc-pkg</literal> tool, described in <xref
      linkend="package-management">.</para>
    </sect2>

    <sect2 id="building-packages">
      <title>Building a package from Haskell source</title>
      <indexterm><primary>packages</primary>
	<secondary>building</secondary></indexterm>

      <para>It takes some special considerations to build a new
      package:</para>

      <itemizedlist>
	<listitem>
	  <para>A package may contain several Haskell modules. A
          package may span many directories, or many packages may
          exist in a single directory. Packages may not be mutually
          recursive.</para>
	</listitem>

	<listitem>
	  <para>A package has a name
  	  (e.g. <filename>base</filename>)</para>
	</listitem>

	<listitem>
	  <para>The Haskell code in a package may be built into one or
	  more archive libraries
	  (e.g. <filename>libHSfoo.a</filename>), or a single DLL on
	  Windows (e.g. <filename>HSfoo.dll</filename>).  The
	  restriction to a single DLL on Windows is because the
	  package system is used to tell the compiler when it should
	  make an inter-DLL call rather than an intra-DLL call
	  (inter-DLL calls require an extra
	  indirection). <emphasis>Building packages as DLLs doesn't
	  work at the moment; see <XRef LinkEnd="win32-dlls-create">
	  for the gory details.</emphasis>
	  </para>

	  <para>Building a static library is done by using the
	  <literal>ar</literal> tool, like so:</para>

<screen>ar cqs libHSfoo.a A.o B.o C.o ...</screen>

	  <para>where <filename>A.o</filename>,
	  <filename>B.o</filename> and so on are the compiled Haskell
	  modules, and <filename>libHSfoo.a</filename> is the library
	  you wish to create.  The syntax may differ slightly on your
	  system, so check the documentation if you run into
	  difficulties.</para>

	  <para>Versions of the Haskell libraries for use with GHCi
	  may also be included: GHCi cannot load <literal>.a</literal>
	  files directly, instead it will look for an object file
	  called <filename>HSfoo.o</filename> and load that.  On some
	  systems, the <literal>ghc-pkg</literal> tool can
	  automatically build the GHCi version of each library, see
	  <xref linkend="package-management">.  To build these
	  libraries by hand from the <literal>.a</literal> archive, it
	  is possible to use GNU <command>ld</command> as
	  follows:</para>

<screen>ld -r &ndash;&ndash;whole-archive -o HSfoo.o libHSfoo.a</screen>
	</listitem>

	<listitem>
	  <para>GHC does not maintain detailed cross-package
          dependency information.  It does remember which modules in
          other packages the current module depends on, but not which
          things within those imported things.</para>
	</listitem>
      </itemizedlist>

      <para>To compile a module which is to be part of a new package,
      use the <literal>-package-name</literal> option:</para>

      <variablelist>
	<varlistentry>
	  <term><option>-package-name <replaceable>foo</replaceable></option></term>
	  <indexterm><primary><literal>-package-name</literal></primary>
	    <secondary>option</secondary></indexterm>
	  <listitem>
	    <para>This option is added to the command line when
	    compiling a module that is destined to be part of package
	    <literal>foo</literal>.  If this flag is omitted then the
	    default package <literal>Main</literal> is assumed.</para>
	  </listitem>
	</varlistentry>
      </variablelist>

      <para>Failure to use the <literal>-package-name</literal> option
      when compiling a package will result in disaster on Windows, but
      is relatively harmless on Unix at the moment (it will just cause
      a few extra dependencies in some interface files).  However,
      bear in mind that we might add support for Unix shared libraries
      at some point in the future.</para>

      <para>It is worth noting that on Windows, when each package
      is built as a DLL, since a reference to a DLL costs an extra
      indirection, intra-package references are cheaper than
      inter-package references. Of course, this applies to the
      <filename>Main</filename> package as well.</para>
    </sect2>

    <sect2 id="package-management">
      <title>Package management</title>
      <indexterm><primary>packages</primary>
	<secondary>management</secondary></indexterm>
      
      <para>The <literal>ghc-pkg</literal> tool allows packages to be
      added or removed from a package configuration file.  By default,
      the system-wide configuration file is used, but alternatively
      packages can be added, updated or removed from a user-specified
      configuration file using the <option>&ndash;&ndash;config-file</option>
      option.  An empty package configuration file consists of the
      string <quote><literal>[]</literal></quote>.</para>

      <para>The <literal>ghc-pkg</literal> program accepts the
      following options:</para>

      <variablelist>
	<varlistentry>
	  <term><option>&ndash;&ndash;add-package</option></term>
	  <term><option>-a</option></term>
	  <indexterm><primary><option>&ndash;&ndash;add-package</option></primary></indexterm>
	  <listitem>
	    <para>Reads package specification from the input (see below),
	    and adds it to the database of installed packages.  The
	    package specification must be a package that isn't already
	    installed.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>&ndash;&ndash;input-file=<replaceable>file</replaceable></option></term>
	  <term><option>-i <replaceable>file</replaceable></option></term>
	  <indexterm><primary><option>&ndash;&ndash;input-file</option></primary></indexterm>
	  <listitem>
	    <para>Read new package specifications from file
	    <replaceable>file</replaceable>. If a value of
	    <filename>"-"</filename> is given, standard input is used.
	    If no <option>-i</option> is present on the command-line,
	    an input file of <filename>"-"</filename> is assumed.
	    </para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>&ndash;&ndash;auto-ghci-libs</option></term>
	  <term><option>-g</option></term>
	  <indexterm><primary><option>&ndash;&ndash;auto-ghci-libs</option></primary>
	      </indexterm>
	  <listitem>
	    <para>Automatically generate the GHCi
	    <filename>.o</filename> version of each
	    <filename>.a</filename> Haskell library, using GNU ld (if
	    that is available).  Without this option,
	    <literal>ghc-pkg</literal> will warn if GHCi versions of
	    any Haskell libraries in the package don't exist.</para>
	    
	    <para>GHCi <literal>.o</literal> libraries don't
	    necessarily have to live in the same directory as the
	    corresponding <literal>.a</literal> library.  However,
	    this option will cause the GHCi library to be created in
	    the same directory as the <literal>.a</literal>
	    library.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>&ndash;&ndash;config-file <replaceable>file</replaceable></option></term>
	  <term><option>-f <replaceable>file</replaceable></option></term>
	  <indexterm><primary><option>&ndash;&ndash;config-file</option></primary>
	      </indexterm>
	  <listitem>
	    <para>Use <replaceable>file</replaceable> instead of the
	    default package configuration file.  This, in conjunction
	    with GHC's <option>-package-conf</option> option, allows
	    a user to have a local set of packages in addition to the
	    system-wide installed set.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>&ndash;&ndash;list-packages</option></term>
	  <term><option>-l</option></term>
	  <indexterm><primary><option>&ndash;&ndash;list-packages</option></primary></indexterm>
	  <listitem>
	    <para>This option displays the list of currently installed
	    packages.</para>

<screen>
  $ ghc-pkg &ndash;&ndash;list-packages
  gmp, rts, std, lang, concurrent, data, net, posix, text, util
</screen>

	    <para>Note that your GHC installation might have a
            slightly different set of packages installed.</para>

	    <para>The <literal>gmp</literal> and
            <literal>rts</literal> packages are always present, and
            represent the multi-precision integer and runtime system
            libraries respectively.  The <literal>std</literal>
            package contains the Haskell prelude and standard
            libraries.  The rest of the packages are optional
            libraries.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>&ndash;&ndash;remove-package <replaceable>foo</replaceable></option></term>
	  <term><option>-r <replaceable>foo</replaceable></option></term>
	  <indexterm><primary><option>&ndash;&ndash;delete-package</option></primary>
	      </indexterm>
	  <listitem>
	    <para>Removes the specified package from the installed
	    configuration.</para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><option>&ndash;&ndash;update-package</option></term>
	  <term><option>-u</option></term>
	  <indexterm><primary><option>&ndash;&ndash;update-package</option></primary></indexterm>
	  <listitem>
	    <para>Reads package specification from the input, and
	    adds it to the database of installed packages. If a package
	    with the same name is already installed, its configuration
	    data is replaced with the new information. If the package
	    doesn't already exist, it's added.
	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><option>&ndash;&ndash;force</option></term>
	  <indexterm><primary><option>&ndash;&ndash;force</option></primary></indexterm>
	  <listitem>
	    <para>Causes <literal>ghc-pkg</literal> to ignore missing
	    directories and libraries when adding a package, and just
	    go ahead and add it anyway.  This might be useful if your
	    package installation system needs to add the package to
	    GHC before building and installing the files.</para>
	  </listitem>
	</varlistentry>
      </variablelist>

      <para>When modifying the configuration file
      <replaceable>file</replaceable>, a copy of the original file is
      saved in <replaceable>file</replaceable><literal>.old</literal>,
      so in an emergency you can always restore the old settings by
      copying the old file back again.</para>

      <para>A package specification looks like this:</para>

<screen>
  Package {
     name            = "mypkg",
     import_dirs     = ["${installdir}/imports/mypkg"],
     source_dirs     = [],
     library_dirs    = ["${installdir}"],
     hs_libraries    = ["HSmypkg" ],
     extra_libraries = ["HSmypkg_cbits"],
     include_dirs    = [],
     c_includes      = ["HsMyPkg.h"],
     package_deps    = ["text", "data"],
     extra_ghc_opts  = [],
     extra_cc_opts   = [],
     extra_ld_opts   = ["-lmy_clib"]
  }
</screen>

      <para>Components of a package specification may be specified in
      any order, and are:</para>

      <variablelist>
	<varlistentry>
	  <term><literal>name</literal></term>
	  <indexterm><primary><literal>name</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
            <para>The package's name, for use with
            the <literal>-package</literal> flag and as listed in the
            <literal>&ndash;&ndash;list-packages</literal> list. 
            </para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>import_dirs</literal></term>
	  <indexterm><primary><literal>import_dirs</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
	    <para>A list of directories containing interface files
	    (<literal>.hi</literal> files) for this package.</para>

	    <para>If the package contains profiling libraries, then
	    the interface files for those library modules should have
	    the suffix <literal>.p_hi</literal>.  So the package can
	    contain both normal and profiling versions of the same
	    library without conflict (see also
	    <literal>library_dirs</literal> below).</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>source_dirs</literal></term>
	  <indexterm><primary><literal>source_dirs</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
	    <para>A list of directories containing Haskell source
	    files for this package.  This field isn't used by GHC, but
	    could potentially be used by an all-interpreted system
	    like Hugs.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>library_dirs</literal></term>
	  <indexterm><primary><literal>library_dirs</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
	    <para>A list of directories containing libraries for this
	    package.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>hs_libraries</literal></term>
	  <indexterm><primary><literal>hs_libraries</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
	    <para>A list of libraries containing Haskell code for this
	    package, with the <literal>.a</literal> or
	    <literal>.dll</literal> suffix omitted.  When packages are
	    built as libraries, the
	    <literal>lib</literal> prefix is also omitted.</para>

	    <para>For use with GHCi, each library should have an
	    object file too.  The name of the object file does
	    <emphasis>not</emphasis> have a <literal>lib</literal>
	    prefix, and has the normal object suffix for your
	    platform.</para>

	    <para>For example, if we specify a Haskell library as
	    <filename>HSfoo</filename> in the package spec, then the
	    various flavours of library that GHC actually uses will be
	    called:</para>
	    <variablelist>
	      <varlistentry>
		<term><filename>libHSfoo.a</filename></term>
		<listitem>
		  <para>The name of the library on Unix and Windows
		  (mingw) systems.  Note that we don't support
		  building dynamic libraries of Haskell code on Unix
		  systems.</para>
		</listitem>
	      </varlistentry>
	      <varlistentry>
		<term><filename>HSfoo.dll</filename></term>
		<listitem>
		  <para>The name of the dynamic library on Windows
		  systems (optional).</para>
		</listitem>
	      </varlistentry>
	      <varlistentry>
		<term><filename>HSfoo.o</filename></term>
		<term><filename>HSfoo.obj</filename></term>
		<listitem>
		  <para>The object version of the library used by
		  GHCi.</para>
		</listitem>
	      </varlistentry>
	    </variablelist>

	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>extra_libraries</literal></term>
	  <indexterm><primary><literal>extra_libraries</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
	    <para>A list of extra libraries for this package.  The
	    difference between <literal>hs_libraries</literal> and
	    <literal>extra_libraries</literal> is that
	    <literal>hs_libraries</literal> normally have several
	    versions, to support profiling, parallel and other build
	    options.  The various versions are given different
	    suffixes to distinguish them, for example the profiling
	    version of the standard prelude library is named
	    <filename>libHSstd_p.a</filename>, with the
	    <literal>_p</literal> indicating that this is a profiling
	    version.  The suffix is added automatically by GHC for
	    <literal>hs_libraries</literal> only, no suffix is added
	    for libraries in
	    <literal>extra_libraries</literal>.</para>

	    <para>The libraries listed in
	    <literal>extra_libraries</literal> may be any libraries
	    supported by your system's linker, including dynamic
	    libraries (<literal>.so</literal> on Unix,
	    <literal>.DLL</literal> on Windows).</para>

	    <para>Also, <literal>extra_libraries</literal> are placed
	    on the linker command line after the
	    <literal>hs_libraries</literal> for the same package.  If
	    your package has dependencies in the other direction (i.e.
	    <literal>extra_libraries</literal> depends on
	    <literal>hs_libraries</literal>), and the libraries are
	    static, you might need to make two separate
	    packages.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>include_dirs</literal></term>
	  <indexterm><primary><literal>include_dirs</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
	    <para>A list of directories containing C includes for this
	    package (maybe the empty list).</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>c_includes</literal></term>
	  <indexterm><primary><literal>c_includes</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
	    <para>A list of files to include for via-C compilations
	    using this package.  Typically this include file will
	    contain function prototypes for any C functions used in
	    the package, in case they end up being called as a result
	    of Haskell functions from the package being
	    inlined.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>package_deps</literal></term>
	  <indexterm><primary><literal>package_deps</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
	    <para>A list of packages which this package depends
	    on.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>extra_ghc_opts</literal></term>
	  <indexterm><primary><literal>extra_ghc_opts</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
	    <para>Extra arguments to be added to the GHC command line
	    when this package is being used.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>extra_cc_opts</literal></term>
	  <indexterm><primary><literal>extra_cc_opts</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
	    <para>Extra arguments to be added to the gcc command line
	    when this package is being used (only for via-C
	    compilations).</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>extra_ld_opts</literal></term>
	  <indexterm><primary><literal>extra_ld_opts</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
	    <para>Extra arguments to be added to the gcc command line
	    (for linking) when this package is being used.</para>
	  </listitem>
	</varlistentry>
	
	<varlistentry>
	  <term><literal>framework_dirs</literal></term>
	  <indexterm><primary><literal>framework_dirs</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
	    <para>On Darwin/MacOS X, a list of directories containing frameworks for this
	    package. This corresponds to the <option>-framework-path</option> option.
	    It is ignored on all other platforms.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><literal>extra_frameworks</literal></term>
	  <indexterm><primary><literal>extra_frameworks</literal></primary>
	    <secondary>package specification</secondary></indexterm>
	  <listitem>
	    <para>On Darwin/MacOS X, a list of frameworks to link to. This corresponds to the
	    <option>-framework</option> option. Take a look at Apple's developer documentation
	    to find out what frameworks actually are. This entry is ignored on all other platforms.</para>
	  </listitem>
	</varlistentry>
      </variablelist>
      
      <para>
      The <literal>ghc-pkg</literal> tool performs expansion of
      environment variables occurring in input package specifications.
      So, if the <literal>mypkg</literal> was added to the package
      database as follows:
      </para>
<screen>
  $ installdir=/usr/local/lib ghc-pkg -a < mypkg.pkg
</screen>
      
      <para>
      The occurrence of <literal>${installdir}</literal> is replaced
      with <literal>/usr/local/lib</literal> in the package data that
      is added for <literal>mypkg</literal>.
      </para>
      
      <para>
      This feature enables the distribution of package specification
      files that can be easily configured when installing.
      </para>

      <para>For examples of more package specifications, take a look
      at the <literal>package.conf</literal> in your GHC
      installation.</para>
    </sect2>
  </sect1>

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