path: root/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>.  More packages to install can be obtained
    from <ulink
    url="http://hackage.haskell.org/packages/hackage.html">HackageDB</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>GHC only knows about packages that are
      <emphasis>installed</emphasis>. 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>An installed package is either <emphasis>exposed</emphasis> or <emphasis>hidden</emphasis>
      by default.      Packages hidden by default are listed in
      parentheses (eg. <literal>(lang-1.0)</literal>) in the output above.  Command-line flags, described below, allow you to expose a hidden package
      or hide an exposed one.
      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>To see which modules are provided by a package use the
      <literal>ghc-pkg</literal> command (see <xref linkend="package-management"/>):</para>
    
<screen>
$ ghc-pkg field network exposed-modules
exposed-modules: Network.BSD,
                 Network.CGI,
                 Network.Socket,
                 Network.URI,
                 Network
</screen>

    <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 the installed
	    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. 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 or shared
	    object. Whether a packages' library is linked statically
	    or dynamically is controlled by the flag
	    pair <option>-static</option>/<option>-dynamic</option>.</para>

	  <para>In <option>&ndash;&ndash;make</option> mode
	    and <option>&ndash;&ndash;interactive</option> mode (see
	    <xref linkend="modes" />), 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. 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>

	  <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></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.
	    Cabal always passes the
	    <option>-hide-all-packages</option> flag to GHC, for
	    exactly this reason.</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>

      <varlistentry>
	<term><option>-package-name</option> <replaceable>foo</replaceable>
	<indexterm><primary><option>-package-name</option></primary>
	  </indexterm></term>
	<listitem>
	  <para>Tells GHC the the module being compiled forms part of
	    package <replaceable>foo</replaceable>.
	    If this flag is omitted (a very common case) then the
	    default package <literal>main</literal> is assumed.</para>
            <para>Note: the argument to <option>-package-name</option>
            should be the full package identifier for the package,
            that is it should include the version number.  For example:
            <literal>-package mypkg-1.2</literal>.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </sect2>

  <sect2 id="package-main">
    <title>The main package</title>

  <para>Every complete Haskell program must define <literal>main</literal> in 
   module <literal>Main</literal> 
   in package <literal>main</literal>.   (Omitting the <option>-package-name</option> flag compiles
   code for package <literal>main</literal>.) Failure to do so leads to a somewhat obscure
   link-time error of the form:
<programlisting>
/usr/bin/ld: Undefined symbols:
_ZCMain_main_closure
___stginit_ZCMain
</programlisting>
</para>

  </sect2>

  <sect2 id="package-overlaps">
    <title>Consequences of packages</title>

    <para>It is possible that by using packages you might end up with
    a program that contains two modules with the same name: perhaps
    you used a package P that has a <emphasis>hidden</emphasis> module
    M, and there is also a module M in your program.  Or perhaps the
    dependencies of packages that you used contain some overlapping
    modules.  Perhaps the program even contains multiple versions of a
    certain package, due to dependencies from other packages.</para>

    <para>None of these scenarios gives rise to an error on its
    own<footnote><para>it used to in GHC 6.4, but not since
    6.6</para></footnote>, but they may have some interesting
    consequences.  For instance, if you have a type
    <literal>M.T</literal> from version 1 of package
    <literal>P</literal>, then this is <emphasis>not</emphasis> the
    same as the type <literal>M.T</literal> from version 2 of package
    <literal>P</literal>, and GHC will report an error if you try to
    use one where the other is expected.</para>

    <para>Formally speaking, in Haskell 98, an entity (function, type
    or class) in a program is uniquely identified by the pair of the
    module name in which it is defined and its name.  In GHC, an
    entity is uniquely defined by a triple: package, module, and
    name.</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 databases 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>

    <sect3 id="ghc-package-path">
      <title>The <literal>GHC_PACKAGE_PATH</literal> environment variable</title>
      <indexterm><primary>Environment variable</primary><secondary><literal>GHC_PACKAGE_PATH</literal></secondary>
      </indexterm>
      <indexterm><primary><literal>GHC_PACKAGE_PATH</literal></primary></indexterm>
      <para>The <literal>GHC_PACKAGE_PATH</literal> environment variable may be
	set to a <literal>:</literal>-separated (<literal>;</literal>-separated
	on Windows) list of files containing package databases.  This list of
	package databases is used by GHC and ghc-pkg, with earlier databases in
	the list overriding later ones.  This order was chosen to match the
	behaviour of the <literal>PATH</literal> environment variable; think of
	it as a list of package databases that are searched left-to-right for
	packages.</para>

      <para>If <literal>GHC_PACKAGE_PATH</literal> ends in a separator, then
	the default user and system package databases are appended, in that
	order. e.g. to augment the usual set of packages with a database of
	your own, you could say (on Unix):
<screen>
$ export GHC_PACKAGE_PATH=$HOME/.my-ghc-packages.conf:</screen>
	(use <literal>;</literal> instead of <literal>:</literal> on
	Windows).</para>

      <para>To check whether your <literal>GHC_PACKAGE_PATH</literal> setting
	is doing the right thing, <literal>ghc-pkg list</literal> will list all
	the databases in use, in the reverse order they are searched.</para>
      
    </sect3>
  </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>
    
    <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>

    <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 shared object
      (e.g. <filename>libHSfoo.dll/.so/.dylib</filename>).  The
      restriction to a single shared object is because the package
      system is used to tell the compiler when it should make an
      inter-shared-object call rather than an intra-shared-object-call
      call (inter-shared-object calls require an extra
      indirection).</para>
    <itemizedlist>
      <listitem><para>Building a static library is done by using the
	  <literal>ar</literal> tool, like so:</para>

<screen>ar cqs libHSfoo-1.0.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>
      </listitem>
      <listitem>
	<para>Versions of the Haskell libraries for use with GHCi may also
	  abe 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>
      </listitem>
      <listitem>
	<para>When building the package as shared object, GHC wraps
	  out the underlying linker so that the user gets a common
	  interface to all shared object variants that are supported
	  by GHC (DLLs, ELF DSOs, and Mac OS dylibs). The shared
	  object must be named in specific way for two reasons: (1)
	  the name must contain the GHC compiler version, so that two
	  library variants don't collide that are compiled by
	  different versions of GHC and that therefore are most likely
	  incompatible with respect to calling conventions, (2) it
	  must be different from the static name otherwise we would
	  not be able to control the linker as precisely as necessary
	  to make
	  the <option>-static</option>/<option>-dynamic</option> flags
	  work, see <xref linkend="options-linker" />.</para>

<screen>ghc -shared libHSfoo-1.0-ghc<replaceable>GHCVersion</replaceable>.so A.o B.o C.o</screen>
	<para>Using GHC's version number in the shared object name
	  allows different library versions compiled by different GHC
	  versions to be installed in standard system locations,
	  e.g. under *nix /usr/lib. To obtain the version number of
	  GHC invoke <literal>ghc --numeric-version</literal> and use
	  its output in place
	  of <replaceable>GHCVersion</replaceable>. See also
	  <xref linkend="options-codegen" /> on how object files must
	  be prepared for shared object linking.</para>
      </listitem>
    </itemizedlist>

     <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>
    
     <para>To compile a module which is to be part of a new package,
      use the <literal>-package-name</literal> option (<xref linkend="using-packages"/>).
      Failure to use the <literal>-package-name</literal> option
      when compiling a package will probably result in disaster, but
      you will only discover later when you attempt to import modules
      from the package.  At this point GHC will complain that the
      package name it was expecting the module to come from is not the
      same as the package name stored in the <literal>.hi</literal>
      file.</para>

    <para>It is worth noting with shared objects, when each package
      is built as a single shared object file, since a reference to a shared object 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>To see what package databases are in use, say
      <literal>ghc-pkg&nbsp;list</literal>.  The stack of databases that
      <literal>ghc-pkg</literal> knows about can be modified using the
      <literal>GHC_PACKAGE_PATH</literal> environment variable (see <xref
	linkend="ghc-package-path" />, and using
	<literal>--package-conf</literal> options on the
	<literal>ghc-pkg</literal> command line.</para>

    <para>When asked to modify a database, <literal>ghc-pkg</literal> modifies
      the global database by default.  Specifying <option>--user</option>
      causes it to act on the user database, or <option>--package-conf</option>
      can be used to act on another database entirely.  When multiple of these
      options are given, the rightmost one is used as the database to act
      upon.</para>

    <para>If the environment variable <literal>GHC_PACKAGE_PATH</literal> is
      set, and its value does not end in a separator (<literal>:</literal> on
      Unix, <literal>;</literal> on Windows), then the last database is
      considered to be the global database, and will be modified by default by
      <literal>ghc-pkg</literal>.  The intention here is that
      <literal>GHC_PACKAGE_PATH</literal> can be used to create a virtual
      package environment into which Cabal packages can be installed without
      setting anything other than <literal>GHC_PACKAGE_PATH</literal>.</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, 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;auto-ghci-libs</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>Adds <replaceable>file</replaceable> to the stack of package
	    databases.  Additionally, <replaceable>file</replaceable> will
	    also be the database modified by a <literal>register</literal>,
	    <literal>unregister</literal>, <literal>expose</literal> or
	    <literal>hide</literal> command, unless it is overriden by a later
	    <option>--package-conf</option>, <option>--user</option> or
	    <option>--global</option> option.</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;global</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;help</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;version</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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