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="&libraryCabalLocation;/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 list</literal> command:</para>

<screen>
$ ghc-pkg list
/usr/lib/ghc-6.12.1/package.conf.d:
    Cabal-1.7.4
    array-0.2.0.1
    base-3.0.3.0
    base-4.2.0.0
    bin-package-db-0.0.0.0
    binary-0.5.0.1
    bytestring-0.9.1.4
    containers-0.2.0.1
    directory-1.0.0.2
    (dph-base-0.4.0)
    (dph-par-0.4.0)
    (dph-prim-interface-0.4.0)
    (dph-prim-par-0.4.0)
    (dph-prim-seq-0.4.0)
    (dph-seq-0.4.0)
    extensible-exceptions-0.1.1.0
    ffi-1.0
    filepath-1.1.0.1
    (ghc-6.12.1)
    ghc-prim-0.1.0.0
    haskeline-0.6.2
    haskell98-1.0.1.0
    hpc-0.5.0.2
    integer-gmp-0.1.0.0
    mtl-1.1.0.2
    old-locale-1.0.0.1
    old-time-1.0.0.1
    pretty-1.0.1.0
    process-1.0.1.1
    random-1.0.0.1
    rts-1.0
    syb-0.1.0.0
    template-haskell-2.4.0.0
    terminfo-0.3.1
    time-1.1.4
    unix-2.3.1.0
    utf8-string-0.3.4
</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>), or possibly in blue if your
      terminal supports colour, in the output of <literal>ghc-pkg
      list</literal>.  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>
      Note: if you're using Cabal, then the exposed or hidden status
      of a package is irrelevant: the available packages are instead
      determined by the dependencies listed in
      your <literal>.cabal</literal> specification.  The
      exposed/hidden status of packages is only relevant when
      using <literal>ghc</literal> or <literal>ghci</literal>
      directly.
    </para>

    <para>Similar to a package's hidden status is a package's trusted
      status. A package can be either trusted or not trusted (distrusted).
      By default packages are distrusted. This property of a package only
      plays a role when compiling code using GHC's Safe Haskell feature
      (see <xref linkend="safe-haskell"/>) with the
      <option>-fpackage-trust</option> flag enabled.
    </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>--make</option> mode
            and <option>--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>-package-id <replaceable>P</replaceable></option>
          <indexterm><primary><option>-package-id</option></primary></indexterm>
        </term>
        <listitem>
          <para>
            Exposes a package like <option>-package</option>, but the
            package is named by its installed package ID rather than by name.  This is a
            more robust way to name packages, and can be used to
            select packages that would otherwise be shadowed.  Cabal
            passes <option>-package-id</option> flags to GHC.
          </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>-no-auto-link-packages</option>
        <indexterm><primary><option>-no-auto-link-packages</option></primary>
          </indexterm></term>
        <listitem>
          <para>By default, GHC will automatically link in the
            <literal>base</literal> and <literal>rts</literal> packages.
            This flag disables that behaviour.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-this-package-key</option> <replaceable>foo</replaceable>
        <indexterm><primary><option>-this-package-key</option></primary>
          </indexterm></term>
        <listitem>
          <para>Tells GHC the the module being compiled forms part of
            package key <replaceable>foo</replaceable>; internally, these
            keys are used to determine type equality and linker symbols.
            If this flag is omitted (a very common case) then the
            default package <literal>main</literal> is assumed.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-trust</option> <replaceable>P</replaceable>
        <indexterm><primary><option>-trust</option></primary>
          </indexterm></term>
        <listitem>
          <para>This option causes the install package <replaceable>P
            </replaceable> to be both exposed and trusted by GHC. This
            command functions in the in a very similar way to the <option>
            -package</option> command but in addition sets the selected
            packaged to be trusted by GHC, regardless of the contents of
            the package database. (see <xref linkend="safe-haskell"/>).
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-distrust</option> <replaceable>P</replaceable>
        <indexterm><primary><option>-distrust</option></primary>
          </indexterm></term>
        <listitem>
          <para>This option causes the install package <replaceable>P
            </replaceable> to be both exposed and distrusted by GHC. This
            command functions in the in a very similar way to the <option>
            -package</option> command but in addition sets the selected
            packaged to be distrusted by GHC, regardless of the contents of
            the package database. (see <xref linkend="safe-haskell"/>).
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-distrust-all</option>
        <indexterm><primary><option>-distrust-all</option></primary>
          </indexterm></term>
        <listitem>
          <para>Ignore the trusted flag on installed packages, and distrust
            them by default. If you use this flag and Safe Haskell then any
            packages you require to be trusted (including <literal>base
            </literal>) need to be explicitly trusted using <option>-trust
            </option> options. This option does not change the exposed/hidden
            status of a package, so it isn't equivalent to applying <option>
            -distrust</option> to all packages on the system. (see
            <xref linkend="safe-haskell"/>).
          </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>-this-package-key</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
</programlisting>
</para>

  </sect2>

  <sect2 id="package-overlaps">
    <title>Consequences of packages for the Haskell language</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 where the details about installed packages
      are stored.  It is a directory, usually
      called <literal>package.conf.d</literal>, that contains a file
      for each package, together with a binary cache of the package
      data in the file <literal>package.cache</literal>.  Normally
      you won't need to look at or modify the contents of a package
      database directly; all management of package databases can be
      done through the <literal>ghc-pkg</literal> tool (see
      <xref linkend="package-management" />).
    </para>

    <para>
      GHC knows about two package databases in particular:
    </para>

    <itemizedlist>
      <listitem>
        <para>The global package database, which comes with your GHC
          installation,
          e.g. <filename>/usr/lib/ghc-6.12.1/package.conf.d</filename>.</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.d</filename>, and on
          Windows it will be something like
          <filename>C:\Documents&nbsp;And&nbsp;Settings\<replaceable>user</replaceable>\ghc\package.conf.d</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, and they are arranged in a stack
      structure. Packages closer to the top of the stack will override
      (<emphasis>shadow</emphasis>) those below them. By default, the stack
      contains just the global and the user's package databases, in that
      order.</para>

    <para>You can control GHC's package database stack using the following
      options:</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-package-db <replaceable>file</replaceable></option>
          <indexterm><primary><option>-package-db</option></primary></indexterm>
        </term>
        <listitem>
          <para>Add the package database <replaceable>file</replaceable> on top
              of the current stack. Packages in additional databases read this
              way will override those in the initial stack and those in
              previously specified databases.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-no-global-package-db</option>
          <indexterm><primary><option>-no-global-package-db</option></primary>
          </indexterm>
        </term>
        <listitem>
          <para>Remove the global package database from the package database
              stack.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-no-user-package-db</option>
          <indexterm><primary><option>-no-user-package-db</option></primary>
          </indexterm>
        </term>
        <listitem>
          <para>Prevent loading of the user's local package database in the
              initial stack.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-clear-package-db</option>
          <indexterm><primary><option>-clear-package-db</option></primary>
          </indexterm>
        </term>
        <listitem>
            <para>Reset the current package database stack. This option removes
                every previously specified package database (including those
                read from the <literal>GHC_PACKAGE_PATH</literal> environment
                variable) from the package database stack.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-global-package-db</option>
          <indexterm><primary><option>-global-package-db</option></primary>
          </indexterm>
        </term>
        <listitem>
          <para>Add the global package database on top of the current stack.
            This option can be used after
            <literal>-no-global-package-db</literal> to specify the position in
            the stack where the global package database should be
            loaded.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-user-package-db</option>
          <indexterm><primary><option>-user-package-db</option></primary>
          </indexterm>
        </term>
        <listitem>
          <para>Add the user's package database on top of the current stack.
            This option can be used after
            <literal>-no-user-package-db</literal> to specify the position in
            the stack where the user's package database should be
            loaded.</para>
        </listitem>
      </varlistentry>
    </variablelist>

    <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 package database stack (i.e. the user and global
        package databases, in that order) is appended. For example, 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="package-ids">
    <title>Installed package IDs, dependencies, and broken packages</title>

    <para>Each installed package has a unique identifier (the
      &ldquo;installed package ID&rdquo;), which distinguishes it from all other
      installed packages on the system.  To see the installed package IDs
      associated with each installed package, use <literal>ghc-pkg
      list -v</literal>:</para>

    <screen>
$ ghc-pkg list -v
using cache: /usr/lib/ghc-6.12.1/package.conf.d/package.cache
/usr/lib/ghc-6.12.1/package.conf.d
   Cabal-1.7.4 (Cabal-1.7.4-48f5247e06853af93593883240e11238)
   array-0.2.0.1 (array-0.2.0.1-9cbf76a576b6ee9c1f880cf171a0928d)
   base-3.0.3.0 (base-3.0.3.0-6cbb157b9ae852096266e113b8fac4a2)
   base-4.2.0.0 (base-4.2.0.0-247bb20cde37c3ef4093ee124e04bc1c)
   ...
</screen>

    <para>
      The string in parentheses after the package name is the installed package
      ID: it normally begins with the package name and version, and
      ends in a hash string derived from the compiled package.
      Dependencies between packages are expressed in terms of installed package
      IDs, rather than just packages and versions.  For example, take
      a look at the dependencies of the <literal>haskell98</literal>
      package:
    </para>

    <screen>
$ ghc-pkg field haskell98 depends
depends: array-0.2.0.1-9cbf76a576b6ee9c1f880cf171a0928d
         base-4.2.0.0-247bb20cde37c3ef4093ee124e04bc1c
         directory-1.0.0.2-f51711bc872c35ce4a453aa19c799008
         old-locale-1.0.0.1-d17c9777c8ee53a0d459734e27f2b8e9
         old-time-1.0.0.1-1c0d8ea38056e5087ef1e75cb0d139d1
         process-1.0.1.1-d8fc6d3baf44678a29b9d59ca0ad5780
         random-1.0.0.1-423d08c90f004795fd10e60384ce6561
</screen>

    <para>
      The purpose of the installed package ID is to detect problems caused by
      re-installing a package without also recompiling the packages
      that depend on it.  Recompiling dependencies is necessary,
      because the newly compiled package may have a different ABI
      (Application Binary Interface) than the previous version, even
      if both packages were built from the same source code using the
      same compiler.  With installed package IDs, a recompiled
      package will have a different installed package ID from the previous
      version, so packages that depended on the previous version are
      now orphaned - one of their dependencies is not satisfied.
      Packages that are broken in this way are shown in
      the <literal>ghc-pkg list</literal> output either in red (if
      possible) or otherwise surrounded by braces.  In the following
      example, we have recompiled and reinstalled
      the <literal>filepath</literal> package, and this has caused
      various dependencies including <literal>Cabal</literal> to
      break:</para>

    <screen>
$ ghc-pkg list
WARNING: there are broken packages.  Run 'ghc-pkg check' for more details.
/usr/lib/ghc-6.12.1/package.conf.d:
    {Cabal-1.7.4}
    array-0.2.0.1
    base-3.0.3.0
    ... etc ...
</screen>

    <para>
      Additionally, <literal>ghc-pkg list</literal> reminds you that
      there are broken packages and suggests <literal>ghc-pkg
      check</literal>, which displays more information about the
      nature of the failure:
    </para>

    <screen>
$ ghc-pkg check
There are problems in package ghc-6.12.1:
  dependency "filepath-1.1.0.1-87511764eb0af2bce4db05e702750e63" doesn't exist
There are problems in package haskeline-0.6.2:
  dependency "filepath-1.1.0.1-87511764eb0af2bce4db05e702750e63" doesn't exist
There are problems in package Cabal-1.7.4:
  dependency "filepath-1.1.0.1-87511764eb0af2bce4db05e702750e63" doesn't exist
There are problems in package process-1.0.1.1:
  dependency "filepath-1.1.0.1-87511764eb0af2bce4db05e702750e63" doesn't exist
There are problems in package directory-1.0.0.2:
  dependency "filepath-1.1.0.1-87511764eb0af2bce4db05e702750e63" doesn't exist

The following packages are broken, either because they have a problem
listed above, or because they depend on a broken package.
ghc-6.12.1
haskeline-0.6.2
Cabal-1.7.4
process-1.0.1.1
directory-1.0.0.2
bin-package-db-0.0.0.0
hpc-0.5.0.2
haskell98-1.0.1.0
</screen>

    <para>
      To fix the problem, you need to recompile the broken packages
      against the new dependencies.  The easiest way to do this is to
      use <literal>cabal-install</literal>, or download the packages
      from <ulink
    url="http://hackage.haskell.org/packages/hackage.html">HackageDB</ulink>
      and build and install them as normal.</para>

    <para>Be careful not to recompile any packages that GHC itself
      depends on, as this may render the <literal>ghc</literal>
      package itself broken, and <literal>ghc</literal> cannot be
      simply recompiled.  The only way to recover from this would be
      to re-install GHC.</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 is for querying and
      modifying package databases.  To see what package databases are
      in use, use
      <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-db</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-db</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, dot) operate on the list of databases specified by
     the flags <option>--user</option>, <option>--global</option>, and
     <option>--package-db</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.  To match
      against the installed package ID instead of just package name and version,
      pass the <option>--ipid</option> flag.</para>

    <variablelist>
      <varlistentry>
        <term><literal>ghc-pkg init <replaceable>path</replaceable></literal></term>
        <listitem>
          <para>Creates a new, empty, package database
            at <replaceable>path</replaceable>, which must not already
            exist.</para>
        </listitem>
      </varlistentry>

      <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 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 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 trust <replaceable>P</replaceable></literal></term>
        <listitem>
          <para>Sets the <literal>trusted</literal> flag for package
            <replaceable>P</replaceable> to <literal>True</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>ghc-pkg distrust <replaceable>P</replaceable></literal></term>
        <listitem>
          <para>Sets the <literal>trusted</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 dot</literal></term>
        <listitem>
          <para>
            Generate a graph of the package dependencies in a form
            suitable for input for the <ulink url="http://www.graphviz.org/">graphviz</ulink> tools.  For example,
            to generate a PDF of the dependency graph:</para>
<screen>
ghc-pkg dot | tred | dot -Tpdf >pkgs.pdf
</screen>
        </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>

      <varlistentry>
        <term><literal>ghc-pkg recache</literal></term>
        <listitem>
          <para>
            Re-creates the binary cache
            file <filename>package.cache</filename> for the selected
            database.  This may be necessary if the cache has somehow
            become out-of-sync with the contents of the database
            (<literal>ghc-pkg</literal> will warn you if this might be
            the case).</para>

          <para>
            The other time when <literal>ghc-pkg recache</literal> is
            useful is for registering packages manually: it is
            possible to register a package by simply putting the
            appropriate file in the package database directory and
            invoking <literal>ghc-pkg recache</literal> to update the
            cache.  This method of registering packages may be more
            convenient for automated packaging systems.
          </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>-f</option> <replaceable>file</replaceable>
          <indexterm><primary><option>-f</option></primary>
          </indexterm>
        </term>
        <term>
          <option>-package-db</option> <replaceable>file</replaceable>
          <indexterm><primary><option>-package-db</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-db</option>, <option>--user</option> or
            <option>--global</option> option.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--force</option>
          <indexterm><primary>
              <option>--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>--global</option><indexterm><primary><option>--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>--help</option><indexterm><primary><option>--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>--user</option><indexterm><primary><option>--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><optional><replaceable>n</replaceable></optional><indexterm><primary><option>-v</option></primary><secondary>ghc-pkg
              option</secondary></indexterm>
        </term>
        <term>
          <option>--verbose</option><optional>=<replaceable>n</replaceable></optional><indexterm><primary><option>--verbose</option></primary><secondary>ghc-pkg option</secondary></indexterm>
        </term>
        <listitem>
          <para>
            Control verbosity.  Verbosity levels range from 0-2, where
            the default is 1, and <option>-v</option> alone selects
            level 2.
          </para>
        </listitem>
      </varlistentry>

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

      <varlistentry>
        <term>
          <option>--ipid</option>
          <indexterm><primary>
              <option>--ipid</option>
            </primary></indexterm>
        </term>
        <listitem>
          <para>Causes <literal>ghc-pkg</literal> to interpret arguments
          as installed package IDs (e.g., an identifier like
          <literal>unix-2.3.1.0-de7803f1a8cd88d2161b29b083c94240
          </literal>).  This is useful if providing just the package
          name and version are ambiguous (in old versions of GHC, this
          was guaranteed to be unique, but this invariant no longer
          necessarily holds).</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </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>To load a package <literal>foo</literal>, GHCi can load
          its <literal>libHSfoo.a</literal> library directly, but it
          can also load a package in the form of a
          single <literal>HSfoo.o</literal> file that has been
          pre-linked.  Loading the <literal>.o</literal> file is
          slightly quicker, but at the expense of having another copy
          of the compiled package.  The rule of thumb is that if the
          modules of the package were compiled
          with <option>-split-objs</option> then building
          the <literal>HSfoo.o</literal> is worthwhile because it
          saves time when loading the package into GHCi.
          Without <option>-split-objs</option>, there is not much
          difference in load time between the <literal>.o</literal>
          and <literal>.a</literal> libraries, so it is better to save
          the disk space and only keep the <literal>.a</literal>
          around.  In a GHC distribution we
          provide <literal>.o</literal> files for most packages except
          the GHC package itself.
        </para>

        <para>The <literal>HSfoo.o</literal> file is built by Cabal
          automatically;
          use <option>--disable-library-for-ghci</option> to disable
          it.  To build one manually, the following
          GNU <command>ld</command> command can be used:</para>

<screen>ld -r --whole-archive -o HSfoo.o libHSfoo.a</screen>

        <para>(replace
          <literal>--whole-archive</literal> with
          <literal>-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>To compile a module which is to be part of a new package,
      use the <literal>-this-package-key</literal> option (<xref linkend="using-packages"/>).
      Failure to use the <literal>-this-package-key</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="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="&libraryCabalLocation;/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: 2.3.1.0
id: unix-2.3.1.0-de7803f1a8cd88d2161b29b083c94240
license: BSD3
copyright:
maintainer: libraries@haskell.org
stability:
homepage:
package-url:
description: This package gives you access to the set of operating system
             services standardised by POSIX 1003.1b (or the IEEE Portable
             Operating System Interface for Computing Environments -
             IEEE Std. 1003.1).
             .
             The package is not supported under Windows (except under Cygwin).
category: System
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.Process.Internals System.Posix.Resource
                 System.Posix.Temp System.Posix.Terminal System.Posix.Time
                 System.Posix.Unistd System.Posix.User System.Posix.Signals
                 System.Posix.Signals.Exts System.Posix.Semaphore
                 System.Posix.SharedMem
hidden-modules:
trusted: False
import-dirs: /usr/lib/ghc-6.12.1/unix-2.3.1.0
library-dirs: /usr/lib/ghc-6.12.1/unix-2.3.1.0
hs-libraries: HSunix-2.3.1.0
extra-libraries: rt util dl
extra-ghci-libraries:
include-dirs: /usr/lib/ghc-6.12.1/unix-2.3.1.0/include
includes: HsUnix.h execvpe.h
depends: base-4.2.0.0-247bb20cde37c3ef4093ee124e04bc1c
hugs-options:
cc-options:
ld-options:
framework-dirs:
frameworks:
haddock-interfaces: /usr/share/doc/ghc/html/libraries/unix/unix.haddock
haddock-html: /usr/share/doc/ghc/html/libraries/unix
</screen>

    <para>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>id</literal>
          <indexterm><primary><literal>id</literal></primary><secondary>package specification</secondary></indexterm>
        </term>
        <listitem>
          <para>The installed package ID.  It is up to you to choose a suitable
          one.</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="&libraryCabalLocation;/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>(optional 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>(optional 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>reexported-modules</literal>
            <indexterm><primary><literal>reexported-modules</literal></primary><secondary>reexport specification</secondary></indexterm>
          </term>
          <listitem>
            <para>Modules reexported by this package.  This list takes
            the form of <literal>pkg:OldName as NewName
            (A@orig-pkg-0.1-HASH)</literal>: the first portion of the
            string is the user-written reexport specification (possibly
            omitting the package qualifier and the renaming), while the
            parenthetical is the original package which exposed the
            module under are particular name.  Reexported modules have
            a relaxed overlap constraint: it's permissible for two
            packages to reexport the same module as the same name if the
            reexported moduleis identical.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <literal>trusted</literal>
            <indexterm><primary><literal>trusted</literal></primary><secondary>package specification</secondary></indexterm>
          </term>
          <listitem>
            <para>(bool) Whether the package is trusted or not.</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 id list) Packages on which this package
            depends.</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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