[ghc-hetmet.git] / ghc / docs / users_guide / packages.xml

<?xml version="1.0" encoding="iso-8859-1"?>
  <sect1 id="packages">
 <title>
Packages
 </title>
  <indexterm><primary>packages</primary></indexterm>
  
  <para>A package is a library of Haskell modules known to the compiler.  GHC
    comes with several packages: see the accompanying 
    <ulink url="../libraries/index.html">library documentation</ulink>.</para>

  <para>Using a package couldn't be simpler: if you're using
    <option>--make</option> or GHCi, then most of the installed packages will be
    automatically available to your program without any further options.  The
    exceptions to this rule are covered below in <xref
      linkend="using-packages" />.</para>

  <para>Building your own packages is also quite straightforward: we provide
    the <ulink url="http://www.haskell.org/cabal/">Cabal</ulink> infrastructure which
    automates the process of configuring, building, installing and distributing
    a package.  All you need to do is write a simple configuration file, put a
    few files in the right places, and you have a package.  See the
    <ulink url="../Cabal/index.html">Cabal documentation</ulink>
    for details, and also the Cabal libraries (<ulink url="../libraries/Cabal/Distribution.Simple.html">Distribution.Simple</ulink>,
    for example).</para>

  <sect2 id="using-packages">
  <title>Using Packages
  </title>
    <indexterm><primary>packages</primary>
      <secondary>using</secondary></indexterm>
    
    <para>To see which packages are installed, use the
      <literal>ghc-pkg</literal> command:</para>

<screen>
$ ghc-pkg list
/usr/lib/ghc-6.4/package.conf:
    base-1.0, haskell98-1.0, template-haskell-1.0, mtl-1.0, unix-1.0,
    Cabal-1.0, haskell-src-1.0, parsec-1.0, network-1.0,
    QuickCheck-1.0, HUnit-1.1, fgl-1.0, X11-1.1, HGL-3.1, OpenGL-2.0,
    GLUT-2.0, stm-1.0, readline-1.0, (lang-1.0), (concurrent-1.0),
    (posix-1.0), (util-1.0), (data-1.0), (text-1.0), (net-1.0),
    (hssource-1.0), rts-1.0
      </screen>

    <para>Packages are either exposed or hidden.  Only
      modules from exposed packages may be imported by your Haskell code; if
      you try to import a module from a hidden package, GHC will emit an error
      message.</para>

    <para>Each package has an exposed flag, which says whether it is exposed by
      default or not.  Packages hidden by default are listed in
      parentheses (eg. <literal>(lang-1.0)</literal>) in the output from
      <literal>ghc-pkg list</literal>.  To expose a package which is hidden by
      default, use the <option>-package</option>
      flag (see below).</para>

   <para>To see which modules are exposed by a package:</para>

<screen>
$ ghc-pkg field network exposed-modules
exposed-modules: Network.BSD,
                 Network.CGI,
                 Network.Socket,
                 Network.URI,
                 Network
</screen>

    <para>In general, packages containing hierarchical modules are usually
      exposed by default.  However, it is possible for two packages to contain
      the same module: in this case, only one of the packages should be
      exposed.  It is an error to import a module that belongs to more than one
      exposed package.</para>

    <para>The GHC command line options that control packages are:</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-package <replaceable>P</replaceable></option>
          <indexterm><primary><option>-package</option></primary></indexterm>
        </term>
        <listitem>
          <para>This option causes package <replaceable>P</replaceable> to be
            exposed.  The package <replaceable>P</replaceable> can be specified
            in full with its version number
            (e.g. <literal>network-1.0</literal>) or the version number can be
            omitted if there is only one version of the package
            installed.</para>

          <para>If there are multiple versions of <replaceable>P</replaceable>
            installed, then all other versions will become hidden.</para>

          <para>The <option>-package <replaceable>P</replaceable></option>
            option also causes package <replaceable>P</replaceable> to be
            linked into the resulting executable.  In
            <option>&ndash;&ndash;make</option> mode and GHCi, the compiler 
            normally determines which packages are required by the current
            Haskell modules, and links only those.  In batch mode however, the
            dependency information isn't available, and explicit
            <option>-package</option> options must be given when linking.</para>

          <para>For example, to link a program consisting of objects
            <filename>Foo.o</filename> and <filename>Main.o</filename>, where
            we made use of the <literal>network</literal> package, we need to
            give GHC the <literal>-package</literal> flag thus:  

<screen>$ ghc -o myprog Foo.o Main.o -package network</screen>

            The same flag is necessary even if we compiled the modules from
            source, because GHC still reckons it's in batch mode: 

<screen>$ ghc -o myprog Foo.hs Main.hs -package network</screen>

            In <literal>--make</literal> and <literal>--interactive</literal>
            modes (<xref linkend="modes" />), however, GHC figures out the
            packages required for linking without further assistance.</para>

          <para>The one other time you might need to use
            <option>-package</option> to force linking a package is when the
            package does not contain any Haskell modules (it might contain a C
            library only, for example).  In that case, GHC
            will never discover a dependency on it, so it has to be mentioned
            explicitly.</para>
        </listitem>
      </varlistentry>
      
      <varlistentry>
        <term><option>-hide-all-packages</option>
        <indexterm><primary><option>-hide-package</option></primary>
          </indexterm></term>
        <listitem>
          <para>Ignore the exposed flag on installed packages, and hide them
            all by default.  If you use
            this flag, then any packages you require (including
            <literal>base</literal>) need to be explicitly exposed using
            <option>-package</option> options.</para>

          <para>This is a good way to insulate your program from differences
            in the globally exposed packages, and being explicit about package
            dependencies is a Good Thing.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-hide-package</option> <replaceable>P</replaceable>
        <indexterm><primary><option>-hide-package</option></primary>
          </indexterm></term>
        <listitem>
          <para>This option does the opposite of <option>-package</option>: it
            causes the specified package to be <firstterm>hidden</firstterm>,
            which means that none of its modules will be available for import
            by Haskell <literal>import</literal> directives.</para>

          <para>Note that the package might still end up being linked into the
            final program, if it is a dependency (direct or indirect) of
            another exposed package.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-ignore-package</option> <replaceable>P</replaceable>
        <indexterm><primary><option>-ignore-package</option></primary>
          </indexterm></term>
        <listitem>
          <para>Causes the compiler to behave as if package
            <replaceable>P</replaceable>, and any packages that depend on
            <literal>P</literal>, are not installed at all.</para>

          <para>Saying <literal>-ignore-package P</literal> is the same as
            giving <literal>-hide-package</literal> flags for
            <literal>P</literal> and all the packages that depend on
            <literal>P</literal>.  Sometimes we don't know ahead of time which
            packages will be installed that depend on <literal>P</literal>,
            which is when the <literal>-ignore-package</literal> flag can be
            useful.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </sect2>

  <sect2 id="package-overlaps">
    <title>The module overlap restriction</title>

    <para>The module names in a Haskell program must be distinct.
      This doesn't sound like a severe restriction, but in a Haskell program
      using multiple packages with interdependencies, difficulties can start to
      arise.  You should be aware of what the module overlap
      restriction means, and how to avoid it.</para>

    <para>GHC knows which packages are <emphasis>in use</emphasis> by your
      program: a package is in use if you imported something from it, or if it
      is a dependency of some other package in use.  There must be no conflicts
      between the packages in use; a conflict is when two packages contain
      a module with the same name.  If
      GHC detects a conflict, it will issue a message stating which packages
      are in conflict, and which modules are overlapping.</para>

    <para>For example, a conflict might arise if you use two packages, say P
      and Q, which respectively depend on two different versions of another
      package, say <literal>R-1.0</literal> and <literal>R-2.0</literal>.  The
      two versions of <literal>R</literal> are likely to contain at least some
      of the same modules, so this situation would be a conflict.</para>
  </sect2>

  <sect2 id="package-databases">
    <title>Package Databases</title>
      
    <para>A package database is a file, normally called
      <literal>package.conf</literal> which contains descriptions of installed
      packages.  GHC usually knows about two package databases:</para>

    <itemizedlist>
      <listitem>
        <para>The global package database, which comes with your GHC
          installation.</para>
      </listitem>
      <listitem>
        <para>A package database private to each user.  On Unix
          systems this will be
          <filename>$HOME/.ghc/<replaceable>arch</replaceable>-<replaceable>os</replaceable>-<replaceable>version</replaceable>/package.conf</filename>, and on
          Windows it will be something like
          <filename>C:\Documents&nbsp;And&nbsp;Settings\<replaceable>user</replaceable>\ghc</filename>.
          The <literal>ghc-pkg</literal> tool knows where this file should be
          located, and will create it if it doesn't exist (see <xref
            linkend="package-management" />).</para>
      </listitem>
    </itemizedlist>

    <para>When GHC starts up, it reads the contents of these two package
      databases, and builds up a list of the packages it knows about.  You can
      see GHC's package table by running GHC with the <option>-v</option>
      flag.</para> 

    <para>Package databases may overlap: for example, packages in the user
      database will override those of the same name in the global
      database.</para> 

    <para>You can control the loading of package databses using the following
      GHC options:</para> 

    <variablelist>
      <varlistentry>
        <term>
          <option>-package-conf <replaceable>file</replaceable></option>
          <indexterm><primary><option>-package-conf</option></primary></indexterm>
        </term>
        <listitem>
          <para>Read in the package configuration file
            <replaceable>file</replaceable> in addition to the system
            default file and the user's local file.  Packages in additional
            files read this way will override those in the global and user
            databases.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-no-user-package-conf</option>
          <indexterm><primary><option>-no-user-package-conf</option></primary>
          </indexterm>
        </term>
        <listitem>
          <para>Prevent loading of the user's local package database.</para>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>To create a new package database, just create
      a new file and put the string
      <quote><literal>[]</literal></quote> in it.  Packages can be
      added to the 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>We don't recommend building packages the hard way.  Instead, use the
      <ulink url="../Cabal/index.html">Cabal</ulink> infrastructure
      if possible.  If your package is particularly complicated or requires a
      lot of configuration, then you might have to fall back to the low-level
      mechanisms, so a few hints for those brave souls follow.</para>
    
    <itemizedlist>
      <listitem>
        <para>You need to build an "installed package info" file for
          passing to <literal>ghc-pkg</literal> when installing your
          package.  The contents of this file are described in <xref
            linkend="installed-pkg-info" />.</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>

        <para>(replace
          <literal>&ndash;&ndash;--whole-archive</literal> with
          <literal>&ndash;all_load</literal> on MacOS X)</para>
        
          <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>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 (the <literal>ghc-pkg</literal> command)</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 database.  By default,
      the system-wide package database is modified, but alternatively
      the user's local package database or another specified
      file can be used.</para>

    <para>Commands which only inspect the database (<literal>list</literal>,
      <literal>describe</literal>, <literal>field</literal>) will take into
      account the user's local package database too, unless the
      <literal>--global</literal> option is given.  This matches the behaviour
      of GHC, which automatically reads the user's local database if it is
      available.</para>

    <para>The <literal>ghc-pkg</literal> program may be run in the ways listed
      below.  Where a package name is required, the package can be named in
      full including the version number 
      (e.g. <literal>network-1.0</literal>), or without the version number.
      Naming a package without the version number matches all versions of the
      package; the specified action will be applied to all the matching
      packages.  A package specifier that matches all version of the package
      can also be written <replaceable>pkg</replaceable><literal>-*</literal>,
      to make it clearer that multiple packages are being matched.</para>

    <variablelist>
      <varlistentry>
        <term><literal>ghc-pkg register <replaceable>file</replaceable></literal></term>
        <listitem>
          <para>Reads a package specification from
            <replaceable>file</replaceable> (which may be &ldquo;<literal>-</literal>&rdquo;
            to indicate standard input),
            and adds it to the database of installed packages.  The syntax of
            <replaceable>file</replaceable> is given in <xref
              linkend="installed-pkg-info" />.</para>

          <para>The package specification must be a package that isn't already
            installed.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>ghc-pkg update <replaceable>file</replaceable></literal></term>
        <listitem>
          <para>The same as <literal>register</literal>, except that if a
            package of the same name is already installed, it is
            replaced by the new one.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>ghc-pkg unregister <replaceable>P</replaceable></literal></term>
        <listitem>
          <para>Remove the specified package from the database.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>ghc-pkg expose <replaceable>P</replaceable></literal></term>
        <listitem>
          <para>Sets the <literal>exposed</literal> flag for package
            <replaceable>P</replaceable> to <literal>True</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>ghc-pkg hide <replaceable>P</replaceable></literal></term>
        <listitem>
          <para>Sets the <literal>exposed</literal> flag for package
            <replaceable>P</replaceable> to <literal>False</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>ghc-pkg list [<replaceable>P</replaceable>] [<option>--simple-output</option>]</literal></term>
        <listitem>
          <para>This option displays the currently installed
            packages, for each of the databases known to
            <literal>ghc-pkg</literal>.  That includes the global database, the
            user's local database (if <option>--user</option> is given), and
            any further files specified using the <option>-f</option> option on
            the command line.</para>

          <para>Hidden packages (those for which the <literal>exposed</literal>
            flag is <literal>False</literal>) are shown in parentheses in the
            list of packages.</para>

          <para>If an optional package identifier <replaceable>P</replaceable>
            is given, then only packages matching that identifier are
            shown.</para>
          
          <para>If the option <option>--simple-output</option> is given, then
            the packages are listed on a single line separated by spaces, and
            the database names are not included.  This is intended to make it
            easier to parse the output of <literal>ghc-pkg list</literal> using
            a script.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>ghc-pkg latest <replaceable>P</replaceable></literal></term>
        <listitem>
          <para>Prints the latest available version of package
            <replaceable>P</replaceable>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>ghc-pkg describe <replaceable>P</replaceable></literal></term>
        <listitem>
          <para>Emit the full description of the specified package.  The
            description is in the form of an
            <literal>InstalledPackageInfo</literal>, the same as the input file
            format for <literal>ghc-pkg register</literal>.  See <xref
              linkend="installed-pkg-info" /> for details.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>ghc-pkg field <replaceable>P</replaceable> <replaceable>field</replaceable></literal></term>
        <listitem>
          <para>Show just a single field of the installed package description
            for <literal>P</literal>.</para>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>Additionally, the following flags are accepted by
      <literal>ghc-pkg</literal>:</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>&ndash;&ndash;auto-ghci-libs</option><indexterm><primary><option>&ndash;&ndash;user</option></primary>
          </indexterm>
        </term>
        <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>-f</option> <replaceable>file</replaceable>
          <indexterm><primary><option>-f</option></primary>
          </indexterm>
        </term>
        <term>
          <option>-package-conf</option> <replaceable>file</replaceable>
          <indexterm><primary><option>-package-conf</option></primary>
          </indexterm>
        </term>
        <listitem>
          <para>Operate on the package database in
            <replaceable>file</replaceable>.</para>

          <para>When multiple <option>-f</option> options are given, or
            <option>-f</option> is used in conjunction with
            <option>--user</option> or <option>--global</option>, the last
            one on the command-line is the one that will be modified.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>&ndash;&ndash;force</option>
          <indexterm><primary>
              <option>&ndash;&ndash;force</option>
            </primary></indexterm>
        </term>
        <listitem>
          <para>Causes <literal>ghc-pkg</literal> to ignore missing
            dependencies, directories and libraries when registering 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>

      <varlistentry>
        <term>
          <option>&ndash;&ndash;global</option><indexterm><primary><option>&ndash;&ndash;user</option></primary>
          </indexterm>
        </term>
        <listitem>
          <para>Operate on the global package database (this is the default).
            This flag affects the <literal>register</literal>,
            <literal>update</literal>, <literal>unregister</literal>,
            <literal>expose</literal>, and <literal>hide</literal>
            commands.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>&ndash;&ndash;help</option><indexterm><primary><option>&ndash;&ndash;user</option></primary>
          </indexterm>
        </term>
        <term>
          <option>-?</option><indexterm><primary><option>-?</option></primary>
          </indexterm>
        </term>
        <listitem>
          <para>Outputs the command-line syntax.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>&ndash;&ndash;user</option><indexterm><primary><option>&ndash;&ndash;user</option></primary>
          </indexterm>
        </term>
        <listitem>
          <para>Operate on the current user's local package database.
            This flag affects the <literal>register</literal>,
            <literal>update</literal>, <literal>unregister</literal>,
            <literal>expose</literal>, and <literal>hide</literal>
            commands.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-V</option><indexterm><primary><option>-V</option></primary>
          </indexterm>
        </term>
        <term>
          <option>&ndash;&ndash;version</option><indexterm><primary><option>&ndash;&ndash;user</option></primary>
          </indexterm>
        </term>
        <listitem>
          <para>Output the <literal>ghc-pkg</literal> version number.</para>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>When modifying the package database
      <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>

  </sect2>
  
  <sect2 id="installed-pkg-info">
    <title>
      <literal>InstalledPackageInfo</literal>: a package specification
    </title>

    <para>A package specification is a Haskell record; in particular, it is the
      record <ulink
        url="../libraries/Cabal/Distribution.InstalledPackageInfo.html#%tInstalledPackageInfo">InstalledPackageInfo</ulink> in the module Distribution.InstalledPackageInfo, which is part of the Cabal package distributed with GHC.</para>

    <para>An <literal>InstalledPackageInfo</literal> has a human
      readable/writable syntax.  The functions
      <literal>parseInstalledPackageInfo</literal> and
      <literal>showInstalledPackageInfo</literal> read and write this syntax
      respectively.  Here's an example of the
      <literal>InstalledPackageInfo</literal> for the <literal>unix</literal> package:</para>

<screen>
$ ghc-pkg describe unix
name: unix
version: 1.0
license: BSD3
copyright:
maintainer: libraries@haskell.org
stability:
homepage:
package-url:
description:
category:
author:
exposed: True
exposed-modules: System.Posix,
                 System.Posix.DynamicLinker.Module,
                 System.Posix.DynamicLinker.Prim,
                 System.Posix.Directory,
                 System.Posix.DynamicLinker,
                 System.Posix.Env,
                 System.Posix.Error,
                 System.Posix.Files,
                 System.Posix.IO,
                 System.Posix.Process,
                 System.Posix.Resource,
                 System.Posix.Temp,
                 System.Posix.Terminal,
                 System.Posix.Time,
                 System.Posix.Unistd,
                 System.Posix.User,
                 System.Posix.Signals.Exts
import-dirs: /usr/lib/ghc-6.4/libraries/unix
library-dirs: /usr/lib/ghc-6.4/libraries/unix
hs-libraries: HSunix
extra-libraries: HSunix_cbits, dl
include-dirs: /usr/lib/ghc-6.4/libraries/unix/include
includes: HsUnix.h
depends: base-1.0
</screen>

    <para>The full <ulink url="../Cabal/index.html">Cabal documentation</ulink>
      is still in preparation (at time of writing), so in the meantime
      here is a brief description of the syntax of this file:</para>

    <para>A package description consists of a number of field/value pairs.  A
      field starts with the field name in the left-hand column followed by a
      &ldquo;<literal>:</literal>&rdquo;, and the value continues until the next line that begins in the
      left-hand column, or the end of file.</para>

    <para>The syntax of the value depends on the field.   The various field
      types are:</para>

    <variablelist>
      <varlistentry>
        <term>freeform</term>
        <listitem>
          <para>Any arbitrary string, no interpretation or parsing is
            done.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>string</term>
        <listitem>
          <para>A sequence of non-space characters, or a sequence of arbitrary
            characters surrounded by quotes <literal>"...."</literal>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>string list</term>
        <listitem>
          <para>A sequence of strings, separated by commas.  The sequence may
            be empty.</para>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>In addition, there are some fields with special syntax (e.g. package
      names, version, dependencies).</para>

    <para>The allowed fields, with their types, are:</para>
        
    <variablelist>
      <varlistentry>
        <term>
          <literal>name</literal>
          <indexterm><primary><literal>name</literal></primary><secondary>package specification</secondary></indexterm>
        </term>
        <listitem>
          <para>The package's name (without the version).</para>
        </listitem>
      </varlistentry>
      
      <varlistentry>
        <term>
          <literal>version</literal>
          <indexterm><primary><literal>version</literal></primary><secondary>package specification</secondary></indexterm>
        </term>
        <listitem>
          <para>The package's version, usually in the form
            <literal>A.B</literal> (any number of components are allowed).</para>
        </listitem>
      </varlistentry>
      
      <varlistentry>
        <term>
          <literal>license</literal>
          <indexterm><primary><literal>auto</literal></primary><secondary>package specification</secondary></indexterm>
        </term>
        <listitem>
          <para>(string) The type of license under which this package is distributed.
            This field is a value of the <ulink
        url="../libraries/Cabal/Distribution.License.html#t:License"><literal>License</literal></ulink> type.</para>
        </listitem>
      </varlistentry>

        <varlistentry>
          <term>
            <literal>license-file</literal>
            <indexterm><primary><literal>license-file</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(optional string) The name of a file giving detailed license
            information for this package.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>copyright</literal>
            <indexterm><primary><literal>copyright</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(optional freeform) The copyright string.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>maintainer</literal>
            <indexterm><primary><literal>maintainer</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(optinoal freeform) The email address of the package's maintainer.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>stability</literal>
            <indexterm><primary><literal>stability</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(optional freeform) A string describing the stability of the package
            (eg. stable, provisional or experimental).</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>homepage</literal>
            <indexterm><primary><literal>homepage</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(optional freeform) URL of the package's home page.</para>
          </listitem>
        </varlistentry>

      <varlistentry>
        <term>
            <literal>package-url</literal>
            <indexterm><primary><literal>package-url</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(optional freeform) URL of a downloadable distribution for this
            package.  The distribution should be a Cabal package.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>description</literal>
            <indexterm><primary><literal>description</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(optional freeform) Description of the package.</para>
          </listitem>
        </varlistentry>

      <varlistentry>
          <term>
            <literal>category</literal>
            <indexterm><primary><literal>category</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(optinoal freeform) Which category the package belongs to.  This field
            is for use in conjunction with a future centralised package
            distribution framework, tentatively titled Hackage.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>author</literal>
            <indexterm><primary><literal>author</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(optional freeform) Author of the package.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>exposed</literal>
            <indexterm><primary><literal>exposed</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(bool) Whether the package is exposed or not.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>exposed-modules</literal>
            <indexterm><primary><literal>exposed-modules</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) modules exposed by this package.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>hidden-modules</literal>
            <indexterm><primary><literal>hidden-modules</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) modules provided by this package,
            but not exposed to the programmer.  These modules cannot be
            imported, but they are still subject to the overlapping constraint:
            no other package in the same program may provide a module of the
            same name.</para>
        </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>import-dirs</literal>
            <indexterm><primary><literal>import-dirs</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) 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>library-dirs</literal>
            <indexterm><primary><literal>library-dirs</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) A list of directories containing libraries for this
            package.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>hs-libraries</literal>
            <indexterm><primary><literal>hs-libraries</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) 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>
            <indexterm><primary><literal>extra-libraries</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) 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>libHSbase_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>
            <indexterm><primary><literal>include-dirs</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) A list of directories containing C includes for this
            package.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
           <literal>includes</literal>
           <indexterm><primary><literal>includes</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) A list of files to include for via-C compilations
            using this package.  Typically the include file(s) 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>depends</literal>
            <indexterm><primary><literal>depends</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(package name list) Packages on which this package depends.  This field contains
            packages with explicit versions are required, except that when
            submitting a package to <literal>ghc-pkg register</literal>, the
            versions will be filled in if they are unambiguous.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>hugs-options</literal>
            <indexterm><primary><literal>hugs-options</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) Options to pass to Hugs for this package.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>cc-options</literal>
            <indexterm><primary><literal>cc-options</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) 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>ld-options</literal>
            <indexterm><primary><literal>ld-options</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) Extra arguments to be added to the
            <command>gcc</command> command line (for linking) when
            this package is being used.</para>
          </listitem>
        </varlistentry>
        
        <varlistentry>
          <term>
            <literal>framework-dirs</literal>
            <indexterm><primary><literal>framework-dirs</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) 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>frameworks</literal>
            <indexterm><primary><literal>frameworks</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) 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>

        <varlistentry>
          <term>
            <literal>haddock-interfaces</literal>
            <indexterm><primary><literal>haddock-interfaces</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(string list) A list of filenames containing <ulink
              url="http://www.haskell.org/haddock/">Haddock</ulink> interface
            files (<literal>.haddock</literal> files) for this package.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>haddock-html</literal>
            <indexterm><primary><literal>haddock-html</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(optional string) The directory containing the Haddock-generated HTML
            for this package.</para>
          </listitem>
        </varlistentry>
      </variablelist>
      
<!--  This isn't true any more.  I'm not sure if we still need it -SDM
      <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 &lt; 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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