[ghc-hetmet.git] / 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 library, GHC can be used to
          perform the link step. This hides some of the details
          out the underlying linker and provides 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>Commands that query the package database (list, latest,
     describe, field) operate on the list of databases specified by
     the flags <option>--user</option>, <option>--global</option>, and
     <option>--package-conf</option>.  If none of these flags are
     given, the default is <option>--global</option>
     <option>--user</option>.</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 check</literal></term>
        <listitem>
          <para>Check consistency of dependencies in the package
          database, and report packages that have missing
          dependencies.</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 find-module <replaceable>M</replaceable> [<option>--simple-output</option>]</literal></term>
        <listitem>
    <para>This option lists registered packages exposing module 
      <replaceable>M</replaceable>. Examples:</para>
<screen>
$ ghc-pkg find-module Var
c:/fptools/validate/ghc/driver/package.conf.inplace:
    (ghc-6.9.20080428)

$ ghc-pkg find-module Data.Sequence
c:/fptools/validate/ghc/driver/package.conf.inplace:
    containers-0.1
</screen>
  <para>Otherwise, it behaves like <literal>ghc-pkg list</literal>,
  including options.</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>

          <para>If the pattern matches multiple packages, the
            description for each package is emitted, separated by the
            string <literal>---</literal> on a line by itself.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>ghc-pkg field <replaceable>P</replaceable> <replaceable>field</replaceable>[,<replaceable>field</replaceable>]*</literal></term>
        <listitem>
          <para>Show just a single field of the installed package description
      for <literal>P</literal>. Multiple fields can be selected by separating 
      them with commas</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>ghc-pkg dump</literal></term>
        <listitem>
          <para>Emit the full description of every package, in the
            form of an <literal>InstalledPackageInfo</literal>.
            Multiple package descriptions are separated by the
            string <literal>---</literal> on a line by itself.</para>

          <para>This is almost the same as <literal>ghc-pkg describe '*'</literal>, except that <literal>ghc-pkg dump</literal>
            is intended for use by tools that parse the results, so
            for example where <literal>ghc-pkg describe '*'</literal>
            will emit an error if it can't find any packages that
            match the pattern, <literal>ghc-pkg dump</literal> will
            simply emit nothing.</para>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>
      Substring matching is supported for <replaceable>M</replaceable> in
      <literal>find-module</literal> and for <replaceable>P</replaceable> in
      <literal>list</literal>, <literal>describe</literal>, and
      <literal>field</literal>, where a <literal>'*'</literal> indicates open
      substring ends (<literal>prefix*</literal>, <literal>*suffix</literal>,
      <literal>*infix*</literal>). Examples (output omitted):
    </para>
    <screen>
    -- list all regex-related packages
    ghc-pkg list '*regex*' --ignore-case
    -- list all string-related packages
    ghc-pkg list '*string*' --ignore-case
    -- list OpenGL-related packages
    ghc-pkg list '*gl*' --ignore-case
    -- list packages exporting modules in the Data hierarchy
    ghc-pkg find-module 'Data.*'
    -- list packages exporting Monad modules
    ghc-pkg find-module '*Monad*'
    -- list names and maintainers for all packages
    ghc-pkg field '*' name,maintainer
    -- list location of haddock htmls for all packages
    ghc-pkg field '*' haddock-html
    -- dump the whole database
    ghc-pkg describe '*'
    </screen>

    <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 overridden 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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