docs/users_guide/packages.xml

   1 <?xml version="1.0" encoding="iso-8859-1"?>
   2   <sect1 id="packages">
   3  <title>
   4 Packages
   5  </title>
   6   <indexterm><primary>packages</primary></indexterm>
   7
   8   <para>A package is a library of Haskell modules known to the compiler.  GHC
   9     comes with several packages: see the accompanying
  10     <ulink url="../libraries/index.html">library documentation</ulink>.</para>
  11
  12   <para>Using a package couldn't be simpler: if you're using
  13     <option>--make</option> or GHCi, then most of the installed packages will be
  14     automatically available to your program without any further options.  The
  15     exceptions to this rule are covered below in <xref
  16       linkend="using-packages" />.</para>
  17
  18   <para>Building your own packages is also quite straightforward: we provide
  19     the <ulink url="http://www.haskell.org/cabal/">Cabal</ulink> infrastructure which
  20     automates the process of configuring, building, installing and distributing
  21     a package.  All you need to do is write a simple configuration file, put a
  22     few files in the right places, and you have a package.  See the
  23     <ulink url="../Cabal/index.html">Cabal documentation</ulink>
  24     for details, and also the Cabal libraries (<ulink url="../libraries/Cabal/Distribution-Simple.html">Distribution.Simple</ulink>,
  25     for example).</para>
  26
  27   <sect2 id="using-packages">
  28   <title>Using Packages
  29   </title>
  30     <indexterm><primary>packages</primary>
  31       <secondary>using</secondary></indexterm>
  32
  33     <para>GHC only knows about packages that are <emphasis>installed</emphasis>.</para>
  34
  35     <para>An installed package is either <emphasis>exposed</emphasis> or <emphasis>hidden</emphasis>
  36       by default.  Command-line flags, described below, allow you to expose a hidden package
  37       or hide an exposed one.
  38       Only modules from exposed packages may be imported by your Haskell code; if
  39       you try to import a module from a hidden package, GHC will emit an error
  40       message.</para>
  41
  42     <para>The GHC command line options that control packages are:</para>
  43
  44     <variablelist>
  45       <varlistentry>
  46         <term>
  47           <option>-package <replaceable>P</replaceable></option>
  48           <indexterm><primary><option>-package</option></primary></indexterm>
  49         </term>
  50         <listitem>
  51           <para>This option causes the installed package <replaceable>P</replaceable> to be
  52             exposed.  The package <replaceable>P</replaceable> can be specified
  53             in full with its version number
  54             (e.g. <literal>network-1.0</literal>) or the version number can be
  55             omitted if there is only one version of the package
  56             installed.</para>
  57
  58           <para>If there are multiple versions of <replaceable>P</replaceable>
  59             installed, then all other versions will become hidden.</para>
  60
  61           <para>The <option>-package <replaceable>P</replaceable></option>
  62             option also causes package <replaceable>P</replaceable> to be
  63             linked into the resulting executable.  In
  64             <option>&ndash;&ndash;make</option> mode and GHCi, the compiler
  65             normally determines which packages are required by the current
  66             Haskell modules, and links only those.  In batch mode however, the
  67             dependency information isn't available, and explicit
  68             <option>-package</option> options must be given when linking.</para>
  69
  70           <para>For example, to link a program consisting of objects
  71             <filename>Foo.o</filename> and <filename>Main.o</filename>, where
  72             we made use of the <literal>network</literal> package, we need to
  73             give GHC the <literal>-package</literal> flag thus:
  74
  75 <screen>$ ghc -o myprog Foo.o Main.o -package network</screen>
  76
  77             The same flag is necessary even if we compiled the modules from
  78             source, because GHC still reckons it's in batch mode:
  79
  80 <screen>$ ghc -o myprog Foo.hs Main.hs -package network</screen>
  81
  82             In <literal>--make</literal> and <literal>--interactive</literal>
  83             modes (<xref linkend="modes" />), however, GHC figures out the
  84             packages required for linking without further assistance.</para>
  85
  86           <para>The one other time you might need to use
  87             <option>-package</option> to force linking a package is when the
  88             package does not contain any Haskell modules (it might contain a C
  89             library only, for example).  In that case, GHC
  90             will never discover a dependency on it, so it has to be mentioned
  91             explicitly.</para>
  92         </listitem>
  93       </varlistentry>
  94
  95       <varlistentry>
  96         <term><option>-hide-all-packages</option>
  97         <indexterm><primary><option>-hide-package</option></primary>
  98           </indexterm></term>
  99         <listitem>
 100           <para>Ignore the exposed flag on installed packages, and hide them
 101             all by default.  If you use
 102             this flag, then any packages you require (including
 103             <literal>base</literal>) need to be explicitly exposed using
 104             <option>-package</option> options.</para>
 105
 106           <para>This is a good way to insulate your program from
 107             differences in the globally exposed packages, and being
 108             explicit about package dependencies is a Good Thing.
 109             Cabal always passes the
 110             <option>-hide-all-packages</option> flag to GHC, for
 111             exactly this reason.</para>
 112         </listitem>
 113       </varlistentry>
 114
 115       <varlistentry>
 116         <term><option>-hide-package</option> <replaceable>P</replaceable>
 117         <indexterm><primary><option>-hide-package</option></primary>
 118           </indexterm></term>
 119         <listitem>
 120           <para>This option does the opposite of <option>-package</option>: it
 121             causes the specified package to be <firstterm>hidden</firstterm>,
 122             which means that none of its modules will be available for import
 123             by Haskell <literal>import</literal> directives.</para>
 124
 125           <para>Note that the package might still end up being linked into the
 126             final program, if it is a dependency (direct or indirect) of
 127             another exposed package.</para>
 128         </listitem>
 129       </varlistentry>
 130
 131       <varlistentry>
 132         <term><option>-ignore-package</option> <replaceable>P</replaceable>
 133         <indexterm><primary><option>-ignore-package</option></primary>
 134           </indexterm></term>
 135         <listitem>
 136           <para>Causes the compiler to behave as if package
 137             <replaceable>P</replaceable>, and any packages that depend on
 138             <literal>P</literal>, are not installed at all.</para>
 139
 140           <para>Saying <literal>-ignore-package P</literal> is the same as
 141             giving <literal>-hide-package</literal> flags for
 142             <literal>P</literal> and all the packages that depend on
 143             <literal>P</literal>.  Sometimes we don't know ahead of time which
 144             packages will be installed that depend on <literal>P</literal>,
 145             which is when the <literal>-ignore-package</literal> flag can be
 146             useful.</para>
 147         </listitem>
 148       </varlistentry>
 149
 150       <varlistentry>
 151         <term><option>-package-name</option> <replaceable>foo</replaceable>
 152         <indexterm><primary><option>-package-name</option></primary>
 153           </indexterm></term>
 154         <listitem>
 155           <para>Tells GHC the the module being compiled forms part of
 156             package <replaceable>foo</replaceable>.
 157             If this flag is omitted (a very common case) then the
 158             default package <literal>main</literal> is assumed.</para>
 159             <para>Note: the argument to <option>-package-name</option>
 160             should be the full package identifier for the package,
 161             that is it should include the version number.  For example:
 162             <literal>-package mypkg-1.2</literal>.</para>
 163         </listitem>
 164       </varlistentry>
 165     </variablelist>
 166
 167
 168     <para>To see which packages are installed, use the
 169       <literal>ghc-pkg</literal> command:</para>
 170
 171 <screen>
 172 $ ghc-pkg list
 173 /usr/lib/ghc-6.4/package.conf:
 174     base-1.0, haskell98-1.0, template-haskell-1.0, mtl-1.0, unix-1.0,
 175     Cabal-1.0, haskell-src-1.0, parsec-1.0, network-1.0,
 176     QuickCheck-1.0, HUnit-1.1, fgl-1.0, X11-1.1, HGL-3.1, OpenGL-2.0,
 177     GLUT-2.0, stm-1.0, readline-1.0, (lang-1.0), (concurrent-1.0),
 178     (posix-1.0), (util-1.0), (data-1.0), (text-1.0), (net-1.0),
 179     (hssource-1.0), rts-1.0
 180       </screen>
 181
 182     <para>Packages hidden by default are listed in
 183       parentheses (eg. <literal>(lang-1.0)</literal>) in the output above.
 184       To expose a package which is hidden by
 185       default, use the <option>-package</option>
 186       flag (see above).</para>
 187
 188     <para>When a package is exposed, it makes available for import the <emphasis>exposed modules</emphasis>
 189     of the package.  To see which modules are exposed by a package use the
 190       <literal>ghc-pkg</literal> command (see <xref linkend="package-management"/>):</para>
 191
 192 <screen>
 193 $ ghc-pkg field network exposed-modules
 194 exposed-modules: Network.BSD,
 195                  Network.CGI,
 196                  Network.Socket,
 197                  Network.URI,
 198                  Network
 199 </screen>
 200
 201     <para>In general, packages containing hierarchical modules are usually
 202       exposed by default.  However, it is possible for two packages to contain
 203       the same module: in this case, only one of the packages should be
 204       exposed.  It is an error to import a module that belongs to more than one
 205       exposed package.</para>
 206
 207   </sect2>
 208
 209   <sect2 id="package-main">
 210     <title>The main package</title>
 211
 212   <para>Every complete Haskell program must define <literal>main</literal> in
 213    module <literal>Main</literal>
 214    in package <literal>main</literal>.   (Omitting the <option>-package-name</option> flag compiles
 215    code for package <literal>main</literal>.) Failure to do so leads to a somewhat obscure
 216    link-time error of the form:
 217 <programlisting>
 218 /usr/bin/ld: Undefined symbols:
 219 _ZCMain_main_closure
 220 ___stginit_ZCMain
 221 </programlisting>
 222 </para>
 223
 224   </sect2>
 225
 226   <sect2 id="package-overlaps">
 227     <title>Consequences of packages</title>
 228
 229     <para>It is possible that by using packages you might end up with
 230     a program that contains two modules with the same name: perhaps
 231     you used a package P that has a <emphasis>hidden</emphasis> module
 232     M, and there is also a module M in your program.  Or perhaps the
 233     dependencies of packages that you used contain some overlapping
 234     modules.  Perhaps the program even contains multiple versions of a
 235     certain package, due to dependencies from other packages.</para>
 236
 237     <para>None of these scenarios gives rise to an error on its
 238     own<footnote><para>it used to in GHC 6.4, but not since
 239     6.6</para></footnote>, but they may have some interesting
 240     consequences.  For instance, if you have a type
 241     <literal>M.T</literal> from version 1 of package
 242     <literal>P</literal>, then this is <emphasis>not</emphasis> the
 243     same as the type <literal>M.T</literal> from version 2 of package
 244     <literal>P</literal>, and GHC will report an error if you try to
 245     use one where the other is expected.</para>
 246
 247     <para>Formally speaking, in Haskell 98, an entity (function, type
 248     or class) in a program is uniquely identified by the pair of the
 249     module name in which it is defined and its name.  In GHC, an
 250     entity is uniquely defined by a triple: package, module, and
 251     name.</para>
 252   </sect2>
 253
 254   <sect2 id="package-databases">
 255     <title>Package Databases</title>
 256
 257     <para>A package database is a file, normally called
 258       <literal>package.conf</literal> which contains descriptions of installed
 259       packages.  GHC usually knows about two package databases:</para>
 260
 261     <itemizedlist>
 262       <listitem>
 263         <para>The global package database, which comes with your GHC
 264           installation.</para>
 265       </listitem>
 266       <listitem>
 267         <para>A package database private to each user.  On Unix
 268           systems this will be
 269           <filename>$HOME/.ghc/<replaceable>arch</replaceable>-<replaceable>os</replaceable>-<replaceable>version</replaceable>/package.conf</filename>, and on
 270           Windows it will be something like
 271           <filename>C:\Documents&nbsp;And&nbsp;Settings\<replaceable>user</replaceable>\ghc</filename>.
 272           The <literal>ghc-pkg</literal> tool knows where this file should be
 273           located, and will create it if it doesn't exist (see <xref linkend="package-management" />).</para>
 274       </listitem>
 275     </itemizedlist>
 276
 277     <para>When GHC starts up, it reads the contents of these two package
 278       databases, and builds up a list of the packages it knows about.  You can
 279       see GHC's package table by running GHC with the <option>-v</option>
 280       flag.</para>
 281
 282     <para>Package databases may overlap: for example, packages in the user
 283       database will override those of the same name in the global
 284       database.</para>
 285
 286     <para>You can control the loading of package databases using the following
 287       GHC options:</para>
 288
 289     <variablelist>
 290       <varlistentry>
 291         <term>
 292           <option>-package-conf <replaceable>file</replaceable></option>
 293           <indexterm><primary><option>-package-conf</option></primary></indexterm>
 294         </term>
 295         <listitem>
 296           <para>Read in the package configuration file
 297             <replaceable>file</replaceable> in addition to the system
 298             default file and the user's local file.  Packages in additional
 299             files read this way will override those in the global and user
 300             databases.</para>
 301         </listitem>
 302       </varlistentry>
 303
 304       <varlistentry>
 305         <term><option>-no-user-package-conf</option>
 306           <indexterm><primary><option>-no-user-package-conf</option></primary>
 307           </indexterm>
 308         </term>
 309         <listitem>
 310           <para>Prevent loading of the user's local package database.</para>
 311         </listitem>
 312       </varlistentry>
 313     </variablelist>
 314
 315     <para>To create a new package database, just create
 316       a new file and put the string
 317       <quote><literal>[]</literal></quote> in it.  Packages can be
 318       added to the file using the
 319       <literal>ghc-pkg</literal> tool, described in <xref
 320       linkend="package-management"/>.</para>
 321
 322     <sect3 id="ghc-package-path">
 323       <title>The <literal>GHC_PACKAGE_PATH</literal> environment variable</title>
 324       <indexterm><primary>Environment variable</primary><secondary><literal>GHC_PACKAGE_PATH</literal></secondary>
 325       </indexterm>
 326       <indexterm><primary><literal>GHC_PACKAGE_PATH</literal></primary></indexterm>
 327       <para>The <literal>GHC_PACKAGE_PATH</literal> environment variable may be
 328         set to a <literal>:</literal>-separated (<literal>;</literal>-separated
 329         on Windows) list of files containing package databases.  This list of
 330         package databases is used by GHC and ghc-pkg, with earlier databases in
 331         the list overriding later ones.  This order was chosen to match the
 332         behaviour of the <literal>PATH</literal> environment variable; think of
 333         it as a list of package databases that are searched left-to-right for
 334         packages.</para>
 335
 336       <para>If <literal>GHC_PACKAGE_PATH</literal> ends in a separator, then
 337         the default user and system package databases are appended, in that
 338         order. e.g. to augment the usual set of packages with a database of
 339         your own, you could say (on Unix):
 340 <screen>
 341 $ export GHC_PACKAGE_PATH=$HOME/.my-ghc-packages.conf:</screen>
 342         (use <literal>;</literal> instead of <literal>:</literal> on
 343         Windows).</para>
 344
 345       <para>To check whether your <literal>GHC_PACKAGE_PATH</literal> setting
 346         is doing the right thing, <literal>ghc-pkg list</literal> will list all
 347         the databases in use, in the reverse order they are searched.</para>
 348
 349     </sect3>
 350   </sect2>
 351
 352   <sect2 id="building-packages">
 353     <title>Building a package from Haskell source</title>
 354     <indexterm><primary>packages</primary>
 355       <secondary>building</secondary></indexterm>
 356
 357     <para>We don't recommend building packages the hard way.  Instead, use the
 358       <ulink url="../Cabal/index.html">Cabal</ulink> infrastructure
 359       if possible.  If your package is particularly complicated or requires a
 360       lot of configuration, then you might have to fall back to the low-level
 361       mechanisms, so a few hints for those brave souls follow.</para>
 362
 363     <itemizedlist>
 364       <listitem>
 365         <para>You need to build an "installed package info" file for
 366           passing to <literal>ghc-pkg</literal> when installing your
 367           package.  The contents of this file are described in <xref
 368             linkend="installed-pkg-info" />.</para>
 369       </listitem>
 370
 371       <listitem>
 372         <para>The Haskell code in a package may be built into one or
 373           more archive libraries
 374           (e.g. <filename>libHSfoo.a</filename>), or a single DLL on
 375           Windows (e.g. <filename>HSfoo.dll</filename>).  The
 376           restriction to a single DLL on Windows is because the
 377           package system is used to tell the compiler when it should
 378           make an inter-DLL call rather than an intra-DLL call
 379           (inter-DLL calls require an extra
 380           indirection). <emphasis>Building packages as DLLs doesn't
 381           work at the moment; see <xref linkend="win32-dlls-create"/>
 382           for the gory details.</emphasis>
 383           </para>
 384
 385         <para>Building a static library is done by using the
 386           <literal>ar</literal> tool, like so:</para>
 387
 388 <screen>ar cqs libHSfoo.a A.o B.o C.o ...</screen>
 389
 390         <para>where <filename>A.o</filename>,
 391           <filename>B.o</filename> and so on are the compiled Haskell
 392           modules, and <filename>libHSfoo.a</filename> is the library
 393           you wish to create.  The syntax may differ slightly on your
 394           system, so check the documentation if you run into
 395           difficulties.</para>
 396
 397         <para>Versions of the Haskell libraries for use with GHCi
 398           may also be included: GHCi cannot load <literal>.a</literal>
 399           files directly, instead it will look for an object file
 400           called <filename>HSfoo.o</filename> and load that.  On some
 401           systems, the <literal>ghc-pkg</literal> tool can
 402           automatically build the GHCi version of each library, see
 403           <xref linkend="package-management"/>.  To build these
 404           libraries by hand from the <literal>.a</literal> archive, it
 405           is possible to use GNU <command>ld</command> as
 406           follows:</para>
 407
 408 <screen>ld -r &ndash;&ndash;whole-archive -o HSfoo.o libHSfoo.a</screen>
 409
 410         <para>(replace
 411           <literal>&ndash;&ndash;--whole-archive</literal> with
 412           <literal>&ndash;all_load</literal> on MacOS X)</para>
 413
 414           <para>GHC does not maintain detailed cross-package
 415           dependency information.  It does remember which modules in
 416           other packages the current module depends on, but not which
 417           things within those imported things.</para>
 418       </listitem>
 419     </itemizedlist>
 420
 421       <para>To compile a module which is to be part of a new package,
 422       use the <literal>-package-name</literal> option (<xref linkend="using-packages"/>).
 423       Failure to use the <literal>-package-name</literal> option
 424       when compiling a package will probably result in disaster, but
 425       you will only discover later when you attempt to import modules
 426       from the package.  At this point GHC will complain that the
 427       package name it was expecting the module to come from is not the
 428       same as the package name stored in the <literal>.hi</literal>
 429       file.</para>
 430
 431     <para>It is worth noting that on Windows, when each package
 432       is built as a DLL, since a reference to a DLL costs an extra
 433       indirection, intra-package references are cheaper than
 434       inter-package references. Of course, this applies to the
 435       <filename>main</filename> package as well.</para>
 436     </sect2>
 437
 438   <sect2 id="package-management">
 439     <title>Package management (the <literal>ghc-pkg</literal> command)</title>
 440     <indexterm><primary>packages</primary>
 441       <secondary>management</secondary></indexterm>
 442
 443     <para>The <literal>ghc-pkg</literal> tool allows packages to be
 444       added or removed from a package database.  By default,
 445       the system-wide package database is modified, but alternatively
 446       the user's local package database or another specified
 447       file can be used.</para>
 448
 449     <para>To see what package databases are in use, say
 450       <literal>ghc-pkg&nbsp;list</literal>.  The stack of databases that
 451       <literal>ghc-pkg</literal> knows about can be modified using the
 452       <literal>GHC_PACKAGE_PATH</literal> environment variable (see <xref
 453         linkend="ghc-package-path" />, and using
 454         <literal>--package-conf</literal> options on the
 455         <literal>ghc-pkg</literal> command line.</para>
 456
 457     <para>When asked to modify a database, <literal>ghc-pkg</literal> modifies
 458       the global database by default.  Specifying <option>--user</option>
 459       causes it to act on the user database, or <option>--package-conf</option>
 460       can be used to act on another database entirely.  When multiple of these
 461       options are given, the rightmost one is used as the database to act
 462       upon.</para>
 463
 464     <para>If the environment variable <literal>GHC_PACKAGE_PATH</literal> is
 465       set, and its value does not end in a separator (<literal>:</literal> on
 466       Unix, <literal>;</literal> on Windows), then the last database is
 467       considered to be the global database, and will be modified by default by
 468       <literal>ghc-pkg</literal>.  The intention here is that
 469       <literal>GHC_PACKAGE_PATH</literal> can be used to create a virtual
 470       package environment into which Cabal packages can be installed without
 471       setting anything other than <literal>GHC_PACKAGE_PATH</literal>.</para>
 472
 473     <para>The <literal>ghc-pkg</literal> program may be run in the ways listed
 474       below.  Where a package name is required, the package can be named in
 475       full including the version number
 476       (e.g. <literal>network-1.0</literal>), or without the version number.
 477       Naming a package without the version number matches all versions of the
 478       package; the specified action will be applied to all the matching
 479       packages.  A package specifier that matches all version of the package
 480       can also be written <replaceable>pkg</replaceable><literal>-*</literal>,
 481       to make it clearer that multiple packages are being matched.</para>
 482
 483     <variablelist>
 484       <varlistentry>
 485         <term><literal>ghc-pkg register <replaceable>file</replaceable></literal></term>
 486         <listitem>
 487           <para>Reads a package specification from
 488             <replaceable>file</replaceable> (which may be &ldquo;<literal>-</literal>&rdquo;
 489             to indicate standard input),
 490             and adds it to the database of installed packages.  The syntax of
 491             <replaceable>file</replaceable> is given in <xref
 492               linkend="installed-pkg-info" />.</para>
 493
 494           <para>The package specification must be a package that isn't already
 495             installed.</para>
 496         </listitem>
 497       </varlistentry>
 498
 499       <varlistentry>
 500         <term><literal>ghc-pkg update <replaceable>file</replaceable></literal></term>
 501         <listitem>
 502           <para>The same as <literal>register</literal>, except that if a
 503             package of the same name is already installed, it is
 504             replaced by the new one.</para>
 505         </listitem>
 506       </varlistentry>
 507
 508       <varlistentry>
 509         <term><literal>ghc-pkg unregister <replaceable>P</replaceable></literal></term>
 510         <listitem>
 511           <para>Remove the specified package from the database.</para>
 512         </listitem>
 513       </varlistentry>
 514
 515       <varlistentry>
 516         <term><literal>ghc-pkg expose <replaceable>P</replaceable></literal></term>
 517         <listitem>
 518           <para>Sets the <literal>exposed</literal> flag for package
 519             <replaceable>P</replaceable> to <literal>True</literal>.</para>
 520         </listitem>
 521       </varlistentry>
 522
 523       <varlistentry>
 524         <term><literal>ghc-pkg hide <replaceable>P</replaceable></literal></term>
 525         <listitem>
 526           <para>Sets the <literal>exposed</literal> flag for package
 527             <replaceable>P</replaceable> to <literal>False</literal>.</para>
 528         </listitem>
 529       </varlistentry>
 530
 531       <varlistentry>
 532         <term><literal>ghc-pkg list [<replaceable>P</replaceable>] [<option>--simple-output</option>]</literal></term>
 533         <listitem>
 534           <para>This option displays the currently installed
 535             packages, for each of the databases known to
 536             <literal>ghc-pkg</literal>.  That includes the global database, the
 537             user's local database, and any further files specified using the
 538             <option>-f</option> option on the command line.</para>
 539
 540           <para>Hidden packages (those for which the <literal>exposed</literal>
 541             flag is <literal>False</literal>) are shown in parentheses in the
 542             list of packages.</para>
 543
 544           <para>If an optional package identifier <replaceable>P</replaceable>
 545             is given, then only packages matching that identifier are
 546             shown.</para>
 547
 548           <para>If the option <option>--simple-output</option> is given, then
 549             the packages are listed on a single line separated by spaces, and
 550             the database names are not included.  This is intended to make it
 551             easier to parse the output of <literal>ghc-pkg list</literal> using
 552             a script.</para>
 553         </listitem>
 554       </varlistentry>
 555
 556       <varlistentry>
 557         <term><literal>ghc-pkg latest <replaceable>P</replaceable></literal></term>
 558         <listitem>
 559           <para>Prints the latest available version of package
 560             <replaceable>P</replaceable>.</para>
 561         </listitem>
 562       </varlistentry>
 563
 564       <varlistentry>
 565         <term><literal>ghc-pkg describe <replaceable>P</replaceable></literal></term>
 566         <listitem>
 567           <para>Emit the full description of the specified package.  The
 568             description is in the form of an
 569             <literal>InstalledPackageInfo</literal>, the same as the input file
 570             format for <literal>ghc-pkg register</literal>.  See <xref
 571               linkend="installed-pkg-info" /> for details.</para>
 572         </listitem>
 573       </varlistentry>
 574
 575       <varlistentry>
 576         <term><literal>ghc-pkg field <replaceable>P</replaceable> <replaceable>field</replaceable></literal></term>
 577         <listitem>
 578           <para>Show just a single field of the installed package description
 579             for <literal>P</literal>.</para>
 580         </listitem>
 581       </varlistentry>
 582     </variablelist>
 583
 584     <para>Additionally, the following flags are accepted by
 585       <literal>ghc-pkg</literal>:</para>
 586
 587     <variablelist>
 588       <varlistentry>
 589         <term>
 590           <option>&ndash;&ndash;auto-ghci-libs</option><indexterm><primary><option>&ndash;&ndash;auto-ghci-libs</option></primary>
 591           </indexterm>
 592         </term>
 593         <listitem>
 594           <para>Automatically generate the GHCi
 595             <filename>.o</filename> version of each
 596             <filename>.a</filename> Haskell library, using GNU ld (if
 597             that is available).  Without this option,
 598             <literal>ghc-pkg</literal> will warn if GHCi versions of
 599             any Haskell libraries in the package don't exist.</para>
 600
 601             <para>GHCi <literal>.o</literal> libraries don't
 602             necessarily have to live in the same directory as the
 603             corresponding <literal>.a</literal> library.  However,
 604             this option will cause the GHCi library to be created in
 605             the same directory as the <literal>.a</literal>
 606             library.</para>
 607         </listitem>
 608       </varlistentry>
 609
 610       <varlistentry>
 611         <term>
 612           <option>-f</option> <replaceable>file</replaceable>
 613           <indexterm><primary><option>-f</option></primary>
 614           </indexterm>
 615         </term>
 616         <term>
 617           <option>-package-conf</option> <replaceable>file</replaceable>
 618           <indexterm><primary><option>-package-conf</option></primary>
 619           </indexterm>
 620         </term>
 621         <listitem>
 622           <para>Adds <replaceable>file</replaceable> to the stack of package
 623             databases.  Additionally, <replaceable>file</replaceable> will
 624             also be the database modified by a <literal>register</literal>,
 625             <literal>unregister</literal>, <literal>expose</literal> or
 626             <literal>hide</literal> command, unless it is overriden by a later
 627             <option>--package-conf</option>, <option>--user</option> or
 628             <option>--global</option> option.</para>
 629         </listitem>
 630       </varlistentry>
 631
 632       <varlistentry>
 633         <term>
 634           <option>&ndash;&ndash;force</option>
 635           <indexterm><primary>
 636               <option>&ndash;&ndash;force</option>
 637             </primary></indexterm>
 638         </term>
 639         <listitem>
 640           <para>Causes <literal>ghc-pkg</literal> to ignore missing
 641             dependencies, directories and libraries when registering a package,
 642             and just go ahead and add it anyway.  This might be useful if your
 643             package installation system needs to add the package to
 644             GHC before building and installing the files.</para>
 645         </listitem>
 646       </varlistentry>
 647
 648       <varlistentry>
 649         <term>
 650           <option>&ndash;&ndash;global</option><indexterm><primary><option>&ndash;&ndash;global</option></primary>
 651           </indexterm>
 652         </term>
 653         <listitem>
 654           <para>Operate on the global package database (this is the default).
 655             This flag affects the <literal>register</literal>,
 656             <literal>update</literal>, <literal>unregister</literal>,
 657             <literal>expose</literal>, and <literal>hide</literal>
 658             commands.</para>
 659         </listitem>
 660       </varlistentry>
 661
 662       <varlistentry>
 663         <term>
 664           <option>&ndash;&ndash;help</option><indexterm><primary><option>&ndash;&ndash;help</option></primary>
 665           </indexterm>
 666         </term>
 667         <term>
 668           <option>-?</option><indexterm><primary><option>-?</option></primary>
 669           </indexterm>
 670         </term>
 671         <listitem>
 672           <para>Outputs the command-line syntax.</para>
 673         </listitem>
 674       </varlistentry>
 675
 676       <varlistentry>
 677         <term>
 678           <option>&ndash;&ndash;user</option><indexterm><primary><option>&ndash;&ndash;user</option></primary>
 679           </indexterm>
 680         </term>
 681         <listitem>
 682           <para>Operate on the current user's local package database.
 683             This flag affects the <literal>register</literal>,
 684             <literal>update</literal>, <literal>unregister</literal>,
 685             <literal>expose</literal>, and <literal>hide</literal>
 686             commands.</para>
 687         </listitem>
 688       </varlistentry>
 689
 690       <varlistentry>
 691         <term>
 692           <option>-V</option><indexterm><primary><option>-V</option></primary>
 693           </indexterm>
 694         </term>
 695         <term>
 696           <option>&ndash;&ndash;version</option><indexterm><primary><option>&ndash;&ndash;version</option></primary>
 697           </indexterm>
 698         </term>
 699         <listitem>
 700           <para>Output the <literal>ghc-pkg</literal> version number.</para>
 701         </listitem>
 702       </varlistentry>
 703     </variablelist>
 704
 705     <para>When modifying the package database
 706       <replaceable>file</replaceable>, a copy of the original file is
 707       saved in <replaceable>file</replaceable><literal>.old</literal>,
 708       so in an emergency you can always restore the old settings by
 709       copying the old file back again.</para>
 710
 711   </sect2>
 712
 713   <sect2 id="installed-pkg-info">
 714     <title>
 715       <literal>InstalledPackageInfo</literal>: a package specification
 716     </title>
 717
 718     <para>A package specification is a Haskell record; in particular, it is the
 719       record <ulink
 720         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>
 721
 722     <para>An <literal>InstalledPackageInfo</literal> has a human
 723       readable/writable syntax.  The functions
 724       <literal>parseInstalledPackageInfo</literal> and
 725       <literal>showInstalledPackageInfo</literal> read and write this syntax
 726       respectively.  Here's an example of the
 727       <literal>InstalledPackageInfo</literal> for the <literal>unix</literal> package:</para>
 728
 729 <screen>
 730 $ ghc-pkg describe unix
 731 name: unix
 732 version: 1.0
 733 license: BSD3
 734 copyright:
 735 maintainer: libraries@haskell.org
 736 stability:
 737 homepage:
 738 package-url:
 739 description:
 740 category:
 741 author:
 742 exposed: True
 743 exposed-modules: System.Posix,
 744                  System.Posix.DynamicLinker.Module,
 745                  System.Posix.DynamicLinker.Prim,
 746                  System.Posix.Directory,
 747                  System.Posix.DynamicLinker,
 748                  System.Posix.Env,
 749                  System.Posix.Error,
 750                  System.Posix.Files,
 751                  System.Posix.IO,
 752                  System.Posix.Process,
 753                  System.Posix.Resource,
 754                  System.Posix.Temp,
 755                  System.Posix.Terminal,
 756                  System.Posix.Time,
 757                  System.Posix.Unistd,
 758                  System.Posix.User,
 759                  System.Posix.Signals.Exts
 760 import-dirs: /usr/lib/ghc-6.4/libraries/unix
 761 library-dirs: /usr/lib/ghc-6.4/libraries/unix
 762 hs-libraries: HSunix
 763 extra-libraries: HSunix_cbits, dl
 764 include-dirs: /usr/lib/ghc-6.4/libraries/unix/include
 765 includes: HsUnix.h
 766 depends: base-1.0
 767 </screen>
 768
 769     <para>The full <ulink url="../Cabal/index.html">Cabal documentation</ulink>
 770       is still in preparation (at time of writing), so in the meantime
 771       here is a brief description of the syntax of this file:</para>
 772
 773     <para>A package description consists of a number of field/value pairs.  A
 774       field starts with the field name in the left-hand column followed by a
 775       &ldquo;<literal>:</literal>&rdquo;, and the value continues until the next line that begins in the
 776       left-hand column, or the end of file.</para>
 777
 778     <para>The syntax of the value depends on the field.   The various field
 779       types are:</para>
 780
 781     <variablelist>
 782       <varlistentry>
 783         <term>freeform</term>
 784         <listitem>
 785           <para>Any arbitrary string, no interpretation or parsing is
 786             done.</para>
 787         </listitem>
 788       </varlistentry>
 789       <varlistentry>
 790         <term>string</term>
 791         <listitem>
 792           <para>A sequence of non-space characters, or a sequence of arbitrary
 793             characters surrounded by quotes <literal>"...."</literal>.</para>
 794         </listitem>
 795       </varlistentry>
 796       <varlistentry>
 797         <term>string list</term>
 798         <listitem>
 799           <para>A sequence of strings, separated by commas.  The sequence may
 800             be empty.</para>
 801         </listitem>
 802       </varlistentry>
 803     </variablelist>
 804
 805     <para>In addition, there are some fields with special syntax (e.g. package
 806       names, version, dependencies).</para>
 807
 808     <para>The allowed fields, with their types, are:</para>
 809
 810     <variablelist>
 811       <varlistentry>
 812         <term>
 813           <literal>name</literal>
 814           <indexterm><primary><literal>name</literal></primary><secondary>package specification</secondary></indexterm>
 815         </term>
 816         <listitem>
 817           <para>The package's name (without the version).</para>
 818         </listitem>
 819       </varlistentry>
 820
 821       <varlistentry>
 822         <term>
 823           <literal>version</literal>
 824           <indexterm><primary><literal>version</literal></primary><secondary>package specification</secondary></indexterm>
 825         </term>
 826         <listitem>
 827           <para>The package's version, usually in the form
 828             <literal>A.B</literal> (any number of components are allowed).</para>
 829         </listitem>
 830       </varlistentry>
 831
 832       <varlistentry>
 833         <term>
 834           <literal>license</literal>
 835           <indexterm><primary><literal>auto</literal></primary><secondary>package specification</secondary></indexterm>
 836         </term>
 837         <listitem>
 838           <para>(string) The type of license under which this package is distributed.
 839             This field is a value of the <ulink
 840         url="../libraries/Cabal/Distribution-License.html#t:License"><literal>License</literal></ulink> type.</para>
 841         </listitem>
 842       </varlistentry>
 843
 844         <varlistentry>
 845           <term>
 846             <literal>license-file</literal>
 847             <indexterm><primary><literal>license-file</literal></primary><secondary>package specification</secondary></indexterm>
 848           </term>
 849           <listitem>
 850             <para>(optional string) The name of a file giving detailed license
 851             information for this package.</para>
 852           </listitem>
 853         </varlistentry>
 854
 855         <varlistentry>
 856           <term>
 857             <literal>copyright</literal>
 858             <indexterm><primary><literal>copyright</literal></primary><secondary>package specification</secondary></indexterm>
 859           </term>
 860           <listitem>
 861             <para>(optional freeform) The copyright string.</para>
 862           </listitem>
 863         </varlistentry>
 864
 865         <varlistentry>
 866           <term>
 867             <literal>maintainer</literal>
 868             <indexterm><primary><literal>maintainer</literal></primary><secondary>package specification</secondary></indexterm>
 869           </term>
 870           <listitem>
 871             <para>(optinoal freeform) The email address of the package's maintainer.</para>
 872           </listitem>
 873         </varlistentry>
 874
 875         <varlistentry>
 876           <term>
 877             <literal>stability</literal>
 878             <indexterm><primary><literal>stability</literal></primary><secondary>package specification</secondary></indexterm>
 879           </term>
 880           <listitem>
 881             <para>(optional freeform) A string describing the stability of the package
 882             (eg. stable, provisional or experimental).</para>
 883           </listitem>
 884         </varlistentry>
 885
 886         <varlistentry>
 887           <term>
 888             <literal>homepage</literal>
 889             <indexterm><primary><literal>homepage</literal></primary><secondary>package specification</secondary></indexterm>
 890           </term>
 891           <listitem>
 892             <para>(optional freeform) URL of the package's home page.</para>
 893           </listitem>
 894         </varlistentry>
 895
 896       <varlistentry>
 897         <term>
 898             <literal>package-url</literal>
 899             <indexterm><primary><literal>package-url</literal></primary><secondary>package specification</secondary></indexterm>
 900           </term>
 901           <listitem>
 902             <para>(optional freeform) URL of a downloadable distribution for this
 903             package.  The distribution should be a Cabal package.</para>
 904           </listitem>
 905         </varlistentry>
 906
 907         <varlistentry>
 908           <term>
 909             <literal>description</literal>
 910             <indexterm><primary><literal>description</literal></primary><secondary>package specification</secondary></indexterm>
 911           </term>
 912           <listitem>
 913             <para>(optional freeform) Description of the package.</para>
 914           </listitem>
 915         </varlistentry>
 916
 917       <varlistentry>
 918           <term>
 919             <literal>category</literal>
 920             <indexterm><primary><literal>category</literal></primary><secondary>package specification</secondary></indexterm>
 921           </term>
 922           <listitem>
 923             <para>(optinoal freeform) Which category the package belongs to.  This field
 924             is for use in conjunction with a future centralised package
 925             distribution framework, tentatively titled Hackage.</para>
 926           </listitem>
 927         </varlistentry>
 928
 929         <varlistentry>
 930           <term>
 931             <literal>author</literal>
 932             <indexterm><primary><literal>author</literal></primary><secondary>package specification</secondary></indexterm>
 933           </term>
 934           <listitem>
 935             <para>(optional freeform) Author of the package.</para>
 936           </listitem>
 937         </varlistentry>
 938
 939         <varlistentry>
 940           <term>
 941             <literal>exposed</literal>
 942             <indexterm><primary><literal>exposed</literal></primary><secondary>package specification</secondary></indexterm>
 943           </term>
 944           <listitem>
 945             <para>(bool) Whether the package is exposed or not.</para>
 946           </listitem>
 947         </varlistentry>
 948
 949         <varlistentry>
 950           <term>
 951             <literal>exposed-modules</literal>
 952             <indexterm><primary><literal>exposed-modules</literal></primary><secondary>package specification</secondary></indexterm>
 953           </term>
 954           <listitem>
 955             <para>(string list) modules exposed by this package.</para>
 956           </listitem>
 957         </varlistentry>
 958
 959         <varlistentry>
 960           <term>
 961             <literal>hidden-modules</literal>
 962             <indexterm><primary><literal>hidden-modules</literal></primary><secondary>package specification</secondary></indexterm>
 963           </term>
 964           <listitem>
 965             <para>(string list) modules provided by this package,
 966             but not exposed to the programmer.  These modules cannot be
 967             imported, but they are still subject to the overlapping constraint:
 968             no other package in the same program may provide a module of the
 969             same name.</para>
 970         </listitem>
 971         </varlistentry>
 972
 973         <varlistentry>
 974           <term>
 975             <literal>import-dirs</literal>
 976             <indexterm><primary><literal>import-dirs</literal></primary><secondary>package specification</secondary></indexterm>
 977           </term>
 978           <listitem>
 979             <para>(string list) A list of directories containing interface files
 980             (<literal>.hi</literal> files) for this package.</para>
 981
 982             <para>If the package contains profiling libraries, then
 983             the interface files for those library modules should have
 984             the suffix <literal>.p_hi</literal>.  So the package can
 985             contain both normal and profiling versions of the same
 986             library without conflict (see also
 987             <literal>library_dirs</literal> below).</para>
 988           </listitem>
 989         </varlistentry>
 990
 991         <varlistentry>
 992           <term>
 993             <literal>library-dirs</literal>
 994             <indexterm><primary><literal>library-dirs</literal></primary><secondary>package specification</secondary></indexterm>
 995           </term>
 996           <listitem>
 997             <para>(string list) A list of directories containing libraries for this
 998             package.</para>
 999           </listitem>
1000         </varlistentry>
1001
1002         <varlistentry>
1003           <term>
1004             <literal>hs-libraries</literal>
1005             <indexterm><primary><literal>hs-libraries</literal></primary><secondary>package specification</secondary></indexterm>
1006           </term>
1007           <listitem>
1008             <para>(string list) A list of libraries containing Haskell code for this
1009             package, with the <literal>.a</literal> or
1010             <literal>.dll</literal> suffix omitted.  When packages are
1011             built as libraries, the
1012             <literal>lib</literal> prefix is also omitted.</para>
1013
1014             <para>For use with GHCi, each library should have an
1015             object file too.  The name of the object file does
1016             <emphasis>not</emphasis> have a <literal>lib</literal>
1017             prefix, and has the normal object suffix for your
1018             platform.</para>
1019
1020             <para>For example, if we specify a Haskell library as
1021             <filename>HSfoo</filename> in the package spec, then the
1022             various flavours of library that GHC actually uses will be
1023             called:</para>
1024             <variablelist>
1025               <varlistentry>
1026                 <term><filename>libHSfoo.a</filename></term>
1027                 <listitem>
1028                   <para>The name of the library on Unix and Windows
1029                   (mingw) systems.  Note that we don't support
1030                   building dynamic libraries of Haskell code on Unix
1031                   systems.</para>
1032                 </listitem>
1033               </varlistentry>
1034               <varlistentry>
1035                 <term><filename>HSfoo.dll</filename></term>
1036                 <listitem>
1037                   <para>The name of the dynamic library on Windows
1038                   systems (optional).</para>
1039                 </listitem>
1040               </varlistentry>
1041               <varlistentry>
1042                 <term><filename>HSfoo.o</filename></term>
1043                 <term><filename>HSfoo.obj</filename></term>
1044                 <listitem>
1045                   <para>The object version of the library used by
1046                   GHCi.</para>
1047                 </listitem>
1048               </varlistentry>
1049             </variablelist>
1050           </listitem>
1051         </varlistentry>
1052
1053         <varlistentry>
1054           <term>
1055             <literal>extra-libraries</literal>
1056             <indexterm><primary><literal>extra-libraries</literal></primary><secondary>package specification</secondary></indexterm>
1057           </term>
1058           <listitem>
1059             <para>(string list) A list of extra libraries for this package.  The
1060             difference between <literal>hs-libraries</literal> and
1061             <literal>extra-libraries</literal> is that
1062             <literal>hs-libraries</literal> normally have several
1063             versions, to support profiling, parallel and other build
1064             options.  The various versions are given different
1065             suffixes to distinguish them, for example the profiling
1066             version of the standard prelude library is named
1067             <filename>libHSbase_p.a</filename>, with the
1068             <literal>_p</literal> indicating that this is a profiling
1069             version.  The suffix is added automatically by GHC for
1070             <literal>hs-libraries</literal> only, no suffix is added
1071             for libraries in
1072             <literal>extra-libraries</literal>.</para>
1073
1074             <para>The libraries listed in
1075             <literal>extra-libraries</literal> may be any libraries
1076             supported by your system's linker, including dynamic
1077             libraries (<literal>.so</literal> on Unix,
1078             <literal>.DLL</literal> on Windows).</para>
1079
1080             <para>Also, <literal>extra-libraries</literal> are placed
1081             on the linker command line after the
1082             <literal>hs-libraries</literal> for the same package.  If
1083             your package has dependencies in the other direction (i.e.
1084             <literal>extra-libraries</literal> depends on
1085             <literal>hs-libraries</literal>), and the libraries are
1086             static, you might need to make two separate
1087             packages.</para>
1088           </listitem>
1089         </varlistentry>
1090
1091         <varlistentry>
1092           <term>
1093             <literal>include-dirs</literal>
1094             <indexterm><primary><literal>include-dirs</literal></primary><secondary>package specification</secondary></indexterm>
1095           </term>
1096           <listitem>
1097             <para>(string list) A list of directories containing C includes for this
1098             package.</para>
1099           </listitem>
1100         </varlistentry>
1101
1102         <varlistentry>
1103           <term>
1104            <literal>includes</literal>
1105            <indexterm><primary><literal>includes</literal></primary><secondary>package specification</secondary></indexterm>
1106           </term>
1107           <listitem>
1108             <para>(string list) A list of files to include for via-C compilations
1109             using this package.  Typically the include file(s) will
1110             contain function prototypes for any C functions used in
1111             the package, in case they end up being called as a result
1112             of Haskell functions from the package being
1113             inlined.</para>
1114           </listitem>
1115         </varlistentry>
1116
1117         <varlistentry>
1118           <term>
1119             <literal>depends</literal>
1120             <indexterm><primary><literal>depends</literal></primary><secondary>package specification</secondary></indexterm>
1121           </term>
1122           <listitem>
1123             <para>(package name list) Packages on which this package depends.  This field contains
1124             packages with explicit versions are required, except that when
1125             submitting a package to <literal>ghc-pkg register</literal>, the
1126             versions will be filled in if they are unambiguous.</para>
1127           </listitem>
1128         </varlistentry>
1129
1130         <varlistentry>
1131           <term>
1132             <literal>hugs-options</literal>
1133             <indexterm><primary><literal>hugs-options</literal></primary><secondary>package specification</secondary></indexterm>
1134           </term>
1135           <listitem>
1136             <para>(string list) Options to pass to Hugs for this package.</para>
1137           </listitem>
1138         </varlistentry>
1139
1140         <varlistentry>
1141           <term>
1142             <literal>cc-options</literal>
1143             <indexterm><primary><literal>cc-options</literal></primary><secondary>package specification</secondary></indexterm>
1144           </term>
1145           <listitem>
1146             <para>(string list) Extra arguments to be added to the gcc command line
1147             when this package is being used (only for via-C
1148             compilations).</para>
1149           </listitem>
1150         </varlistentry>
1151
1152         <varlistentry>
1153           <term>
1154             <literal>ld-options</literal>
1155             <indexterm><primary><literal>ld-options</literal></primary><secondary>package specification</secondary></indexterm>
1156           </term>
1157           <listitem>
1158             <para>(string list) Extra arguments to be added to the
1159             <command>gcc</command> command line (for linking) when
1160             this package is being used.</para>
1161           </listitem>
1162         </varlistentry>
1163
1164         <varlistentry>
1165           <term>
1166             <literal>framework-dirs</literal>
1167             <indexterm><primary><literal>framework-dirs</literal></primary><secondary>package specification</secondary></indexterm>
1168           </term>
1169           <listitem>
1170             <para>(string list) On Darwin/MacOS X, a list of directories containing
1171             frameworks for this package. This corresponds to the
1172             <option>-framework-path</option> option. It is ignored on all other
1173             platforms.</para>
1174           </listitem>
1175         </varlistentry>
1176
1177         <varlistentry>
1178           <term>
1179             <literal>frameworks</literal>
1180             <indexterm><primary><literal>frameworks</literal></primary><secondary>package specification</secondary></indexterm>
1181           </term>
1182           <listitem>
1183             <para>(string list) On Darwin/MacOS X, a list of frameworks to link to. This
1184             corresponds to the <option>-framework</option> option. Take a look
1185             at Apple's developer documentation to find out what frameworks
1186             actually are. This entry is ignored on all other platforms.</para>
1187           </listitem>
1188         </varlistentry>
1189
1190         <varlistentry>
1191           <term>
1192             <literal>haddock-interfaces</literal>
1193             <indexterm><primary><literal>haddock-interfaces</literal></primary><secondary>package specification</secondary></indexterm>
1194           </term>
1195           <listitem>
1196             <para>(string list) A list of filenames containing <ulink
1197               url="http://www.haskell.org/haddock/">Haddock</ulink> interface
1198             files (<literal>.haddock</literal> files) for this package.</para>
1199           </listitem>
1200         </varlistentry>
1201
1202         <varlistentry>
1203           <term>
1204             <literal>haddock-html</literal>
1205             <indexterm><primary><literal>haddock-html</literal></primary><secondary>package specification</secondary></indexterm>
1206           </term>
1207           <listitem>
1208             <para>(optional string) The directory containing the Haddock-generated HTML
1209             for this package.</para>
1210           </listitem>
1211         </varlistentry>
1212       </variablelist>
1213
1214 <!--  This isn't true any more.  I'm not sure if we still need it -SDM
1215       <para>
1216       The <literal>ghc-pkg</literal> tool performs expansion of
1217       environment variables occurring in input package specifications.
1218       So, if the <literal>mypkg</literal> was added to the package
1219       database as follows:
1220       </para>
1221 <screen>
1222   $ installdir=/usr/local/lib ghc-pkg -a &lt; mypkg.pkg
1223 </screen>
1224
1225       <para>
1226       The occurrence of <literal>${installdir}</literal> is replaced
1227       with <literal>/usr/local/lib</literal> in the package data that
1228       is added for <literal>mypkg</literal>.
1229       </para>
1230
1231       <para>
1232       This feature enables the distribution of package specification
1233       files that can be easily configured when installing.
1234       </para>
1235
1236       <para>For examples of more package specifications, take a look
1237       at the <literal>package.conf</literal> in your GHC
1238       installation.</para>
1239
1240 -->
1241
1242     </sect2>
1243   </sect1>
1244
1245 <!-- Emacs stuff:
1246      ;;; Local Variables: ***
1247      ;;; mode: xml ***
1248      ;;; sgml-parent-document: ("users_guide.xml" "book" "chapter" "sect1") ***
1249      ;;; End: ***
1250  -->