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
   9     compiler.  GHC comes with several packages: see the accompanying
  10     <ulink url="../libraries/index.html">library
  11     documentation</ulink>.  More packages to install can be obtained
  12     from <ulink
  13     url="http://hackage.haskell.org/packages/hackage.html">HackageDB</ulink>.</para>
  14
  15   <para>Using a package couldn't be simpler: if you're using
  16     <option>--make</option> or GHCi, then most of the installed packages will be
  17     automatically available to your program without any further options.  The
  18     exceptions to this rule are covered below in <xref
  19       linkend="using-packages" />.</para>
  20
  21   <para>Building your own packages is also quite straightforward: we provide
  22     the <ulink url="http://www.haskell.org/cabal/">Cabal</ulink> infrastructure which
  23     automates the process of configuring, building, installing and distributing
  24     a package.  All you need to do is write a simple configuration file, put a
  25     few files in the right places, and you have a package.  See the
  26     <ulink url="../Cabal/index.html">Cabal documentation</ulink>
  27     for details, and also the Cabal libraries (<ulink url="../libraries/Cabal/Distribution-Simple.html">Distribution.Simple</ulink>,
  28     for example).</para>
  29
  30   <sect2 id="using-packages">
  31   <title>Using Packages
  32   </title>
  33     <indexterm><primary>packages</primary>
  34       <secondary>using</secondary></indexterm>
  35
  36     <para>GHC only knows about packages that are
  37       <emphasis>installed</emphasis>. To see which packages are installed, use
  38       the <literal>ghc-pkg</literal> command:</para>
  39
  40 <screen>
  41 $ ghc-pkg list
  42 /usr/lib/ghc-6.4/package.conf:
  43     base-1.0, haskell98-1.0, template-haskell-1.0, mtl-1.0, unix-1.0,
  44     Cabal-1.0, haskell-src-1.0, parsec-1.0, network-1.0,
  45     QuickCheck-1.0, HUnit-1.1, fgl-1.0, X11-1.1, HGL-3.1, OpenGL-2.0,
  46     GLUT-2.0, stm-1.0, readline-1.0, (lang-1.0), (concurrent-1.0),
  47     (posix-1.0), (util-1.0), (data-1.0), (text-1.0), (net-1.0),
  48     (hssource-1.0), rts-1.0
  49       </screen>
  50
  51     <para>An installed package is either <emphasis>exposed</emphasis> or <emphasis>hidden</emphasis>
  52       by default.      Packages hidden by default are listed in
  53       parentheses (eg. <literal>(lang-1.0)</literal>) in the output above.  Command-line flags, described below, allow you to expose a hidden package
  54       or hide an exposed one.
  55       Only modules from exposed packages may be imported by your Haskell code; if
  56       you try to import a module from a hidden package, GHC will emit an error
  57       message.</para>
  58
  59     <para>To see which modules are provided by a package use the
  60       <literal>ghc-pkg</literal> command (see <xref linkend="package-management"/>):</para>
  61
  62 <screen>
  63 $ ghc-pkg field network exposed-modules
  64 exposed-modules: Network.BSD,
  65                  Network.CGI,
  66                  Network.Socket,
  67                  Network.URI,
  68                  Network
  69 </screen>
  70
  71     <para>The GHC command line options that control packages are:</para>
  72
  73     <variablelist>
  74       <varlistentry>
  75         <term>
  76           <option>-package <replaceable>P</replaceable></option>
  77           <indexterm><primary><option>-package</option></primary></indexterm>
  78         </term>
  79         <listitem>
  80           <para>This option causes the installed
  81             package <replaceable>P</replaceable> to be exposed.  The
  82             package <replaceable>P</replaceable> can be specified in
  83             full with its version number
  84             (e.g. <literal>network-1.0</literal>) or the version
  85             number can be omitted if there is only one version of the
  86             package installed. If there are multiple versions
  87             of <replaceable>P</replaceable> installed, then all other
  88             versions will become hidden.</para>
  89
  90           <para>The <option>-package <replaceable>P</replaceable></option>
  91             option also causes package <replaceable>P</replaceable> to
  92             be linked into the resulting executable or shared
  93             object. Whether a packages' library is linked statically
  94             or dynamically is controlled by the flag
  95             pair <option>-static</option>/<option>-dynamic</option>.</para>
  96
  97           <para>In <option>&ndash;&ndash;make</option> mode
  98             and <option>&ndash;&ndash;interactive</option> mode (see
  99             <xref linkend="modes" />), the compiler normally
 100             determines which packages are required by the current
 101             Haskell modules, and links only those.  In batch mode
 102             however, the dependency information isn't available, and
 103             explicit
 104             <option>-package</option> options must be given when linking. The one other time you might need to use
 105             <option>-package</option> to force linking a package is
 106             when the package does not contain any Haskell modules (it
 107             might contain a C library only, for example).  In that
 108             case, GHC will never discover a dependency on it, so it
 109             has to be mentioned explicitly.</para>
 110
 111           <para>For example, to link a program consisting of objects
 112             <filename>Foo.o</filename> and <filename>Main.o</filename>, where
 113             we made use of the <literal>network</literal> package, we need to
 114             give GHC the <literal>-package</literal> flag thus:
 115
 116 <screen>$ ghc -o myprog Foo.o Main.o -package network</screen>
 117
 118             The same flag is necessary even if we compiled the modules from
 119             source, because GHC still reckons it's in batch mode:
 120
 121 <screen>$ ghc -o myprog Foo.hs Main.hs -package network</screen></para>
 122         </listitem>
 123       </varlistentry>
 124
 125       <varlistentry>
 126         <term><option>-hide-all-packages</option>
 127         <indexterm><primary><option>-hide-package</option></primary>
 128           </indexterm></term>
 129         <listitem>
 130           <para>Ignore the exposed flag on installed packages, and hide them
 131             all by default.  If you use
 132             this flag, then any packages you require (including
 133             <literal>base</literal>) need to be explicitly exposed using
 134             <option>-package</option> options.</para>
 135
 136           <para>This is a good way to insulate your program from
 137             differences in the globally exposed packages, and being
 138             explicit about package dependencies is a Good Thing.
 139             Cabal always passes the
 140             <option>-hide-all-packages</option> flag to GHC, for
 141             exactly this reason.</para>
 142         </listitem>
 143       </varlistentry>
 144
 145       <varlistentry>
 146         <term><option>-hide-package</option> <replaceable>P</replaceable>
 147         <indexterm><primary><option>-hide-package</option></primary>
 148           </indexterm></term>
 149         <listitem>
 150           <para>This option does the opposite of <option>-package</option>: it
 151             causes the specified package to be <firstterm>hidden</firstterm>,
 152             which means that none of its modules will be available for import
 153             by Haskell <literal>import</literal> directives.</para>
 154
 155           <para>Note that the package might still end up being linked into the
 156             final program, if it is a dependency (direct or indirect) of
 157             another exposed package.</para>
 158         </listitem>
 159       </varlistentry>
 160
 161       <varlistentry>
 162         <term><option>-ignore-package</option> <replaceable>P</replaceable>
 163         <indexterm><primary><option>-ignore-package</option></primary>
 164           </indexterm></term>
 165         <listitem>
 166           <para>Causes the compiler to behave as if package
 167             <replaceable>P</replaceable>, and any packages that depend on
 168             <literal>P</literal>, are not installed at all.</para>
 169
 170           <para>Saying <literal>-ignore-package P</literal> is the same as
 171             giving <literal>-hide-package</literal> flags for
 172             <literal>P</literal> and all the packages that depend on
 173             <literal>P</literal>.  Sometimes we don't know ahead of time which
 174             packages will be installed that depend on <literal>P</literal>,
 175             which is when the <literal>-ignore-package</literal> flag can be
 176             useful.</para>
 177         </listitem>
 178       </varlistentry>
 179
 180       <varlistentry>
 181         <term><option>-package-name</option> <replaceable>foo</replaceable>
 182         <indexterm><primary><option>-package-name</option></primary>
 183           </indexterm></term>
 184         <listitem>
 185           <para>Tells GHC the the module being compiled forms part of
 186             package <replaceable>foo</replaceable>.
 187             If this flag is omitted (a very common case) then the
 188             default package <literal>main</literal> is assumed.</para>
 189             <para>Note: the argument to <option>-package-name</option>
 190             should be the full package identifier for the package,
 191             that is it should include the version number.  For example:
 192             <literal>-package mypkg-1.2</literal>.</para>
 193         </listitem>
 194       </varlistentry>
 195     </variablelist>
 196   </sect2>
 197
 198   <sect2 id="package-main">
 199     <title>The main package</title>
 200
 201   <para>Every complete Haskell program must define <literal>main</literal> in
 202    module <literal>Main</literal>
 203    in package <literal>main</literal>.   (Omitting the <option>-package-name</option> flag compiles
 204    code for package <literal>main</literal>.) Failure to do so leads to a somewhat obscure
 205    link-time error of the form:
 206 <programlisting>
 207 /usr/bin/ld: Undefined symbols:
 208 _ZCMain_main_closure
 209 ___stginit_ZCMain
 210 </programlisting>
 211 </para>
 212
 213   </sect2>
 214
 215   <sect2 id="package-overlaps">
 216     <title>Consequences of packages</title>
 217
 218     <para>It is possible that by using packages you might end up with
 219     a program that contains two modules with the same name: perhaps
 220     you used a package P that has a <emphasis>hidden</emphasis> module
 221     M, and there is also a module M in your program.  Or perhaps the
 222     dependencies of packages that you used contain some overlapping
 223     modules.  Perhaps the program even contains multiple versions of a
 224     certain package, due to dependencies from other packages.</para>
 225
 226     <para>None of these scenarios gives rise to an error on its
 227     own<footnote><para>it used to in GHC 6.4, but not since
 228     6.6</para></footnote>, but they may have some interesting
 229     consequences.  For instance, if you have a type
 230     <literal>M.T</literal> from version 1 of package
 231     <literal>P</literal>, then this is <emphasis>not</emphasis> the
 232     same as the type <literal>M.T</literal> from version 2 of package
 233     <literal>P</literal>, and GHC will report an error if you try to
 234     use one where the other is expected.</para>
 235
 236     <para>Formally speaking, in Haskell 98, an entity (function, type
 237     or class) in a program is uniquely identified by the pair of the
 238     module name in which it is defined and its name.  In GHC, an
 239     entity is uniquely defined by a triple: package, module, and
 240     name.</para>
 241   </sect2>
 242
 243   <sect2 id="package-databases">
 244     <title>Package Databases</title>
 245
 246     <para>A package database is a file, normally called
 247       <literal>package.conf</literal> which contains descriptions of installed
 248       packages.  GHC usually knows about two package databases:</para>
 249
 250     <itemizedlist>
 251       <listitem>
 252         <para>The global package database, which comes with your GHC
 253           installation.</para>
 254       </listitem>
 255       <listitem>
 256         <para>A package database private to each user.  On Unix
 257           systems this will be
 258           <filename>$HOME/.ghc/<replaceable>arch</replaceable>-<replaceable>os</replaceable>-<replaceable>version</replaceable>/package.conf</filename>, and on
 259           Windows it will be something like
 260           <filename>C:\Documents&nbsp;And&nbsp;Settings\<replaceable>user</replaceable>\ghc</filename>.
 261           The <literal>ghc-pkg</literal> tool knows where this file should be
 262           located, and will create it if it doesn't exist (see <xref linkend="package-management" />).</para>
 263       </listitem>
 264     </itemizedlist>
 265
 266     <para>When GHC starts up, it reads the contents of these two package
 267       databases, and builds up a list of the packages it knows about.  You can
 268       see GHC's package table by running GHC with the <option>-v</option>
 269       flag.</para>
 270
 271     <para>Package databases may overlap: for example, packages in the user
 272       database will override those of the same name in the global
 273       database.</para>
 274
 275     <para>You can control the loading of package databases using the following
 276       GHC options:</para>
 277
 278     <variablelist>
 279       <varlistentry>
 280         <term>
 281           <option>-package-conf <replaceable>file</replaceable></option>
 282           <indexterm><primary><option>-package-conf</option></primary></indexterm>
 283         </term>
 284         <listitem>
 285           <para>Read in the package configuration file
 286             <replaceable>file</replaceable> in addition to the system
 287             default file and the user's local file.  Packages in additional
 288             files read this way will override those in the global and user
 289             databases.</para>
 290         </listitem>
 291       </varlistentry>
 292
 293       <varlistentry>
 294         <term><option>-no-user-package-conf</option>
 295           <indexterm><primary><option>-no-user-package-conf</option></primary>
 296           </indexterm>
 297         </term>
 298         <listitem>
 299           <para>Prevent loading of the user's local package database.</para>
 300         </listitem>
 301       </varlistentry>
 302     </variablelist>
 303
 304     <para>To create a new package database, just create
 305       a new file and put the string
 306       <quote><literal>[]</literal></quote> in it.  Packages can be
 307       added to the file using the
 308       <literal>ghc-pkg</literal> tool, described in <xref
 309       linkend="package-management"/>.</para>
 310
 311     <sect3 id="ghc-package-path">
 312       <title>The <literal>GHC_PACKAGE_PATH</literal> environment variable</title>
 313       <indexterm><primary>Environment variable</primary><secondary><literal>GHC_PACKAGE_PATH</literal></secondary>
 314       </indexterm>
 315       <indexterm><primary><literal>GHC_PACKAGE_PATH</literal></primary></indexterm>
 316       <para>The <literal>GHC_PACKAGE_PATH</literal> environment variable may be
 317         set to a <literal>:</literal>-separated (<literal>;</literal>-separated
 318         on Windows) list of files containing package databases.  This list of
 319         package databases is used by GHC and ghc-pkg, with earlier databases in
 320         the list overriding later ones.  This order was chosen to match the
 321         behaviour of the <literal>PATH</literal> environment variable; think of
 322         it as a list of package databases that are searched left-to-right for
 323         packages.</para>
 324
 325       <para>If <literal>GHC_PACKAGE_PATH</literal> ends in a separator, then
 326         the default user and system package databases are appended, in that
 327         order. e.g. to augment the usual set of packages with a database of
 328         your own, you could say (on Unix):
 329 <screen>
 330 $ export GHC_PACKAGE_PATH=$HOME/.my-ghc-packages.conf:</screen>
 331         (use <literal>;</literal> instead of <literal>:</literal> on
 332         Windows).</para>
 333
 334       <para>To check whether your <literal>GHC_PACKAGE_PATH</literal> setting
 335         is doing the right thing, <literal>ghc-pkg list</literal> will list all
 336         the databases in use, in the reverse order they are searched.</para>
 337
 338     </sect3>
 339   </sect2>
 340
 341   <sect2 id="building-packages">
 342     <title>Building a package from Haskell source</title>
 343     <indexterm><primary>packages</primary>
 344       <secondary>building</secondary></indexterm>
 345
 346     <para>We don't recommend building packages the hard way.  Instead, use the
 347       <ulink url="../Cabal/index.html">Cabal</ulink> infrastructure
 348       if possible.  If your package is particularly complicated or requires a
 349       lot of configuration, then you might have to fall back to the low-level
 350       mechanisms, so a few hints for those brave souls follow.</para>
 351
 352     <para>You need to build an "installed package info" file for
 353       passing to <literal>ghc-pkg</literal> when installing your
 354       package.  The contents of this file are described in
 355       <xref linkend="installed-pkg-info" />.</para>
 356
 357     <para>The Haskell code in a package may be built into one or more
 358       archive libraries (e.g. <filename>libHSfoo.a</filename>), or a
 359       single shared object
 360       (e.g. <filename>libHSfoo.dll/.so/.dylib</filename>).  The
 361       restriction to a single shared object is because the package
 362       system is used to tell the compiler when it should make an
 363       inter-shared-object call rather than an intra-shared-object-call
 364       call (inter-shared-object calls require an extra
 365       indirection).</para>
 366     <itemizedlist>
 367       <listitem><para>Building a static library is done by using the
 368           <literal>ar</literal> tool, like so:</para>
 369
 370 <screen>ar cqs libHSfoo-1.0.a A.o B.o C.o ...</screen>
 371
 372           <para>where <filename>A.o</filename>,
 373             <filename>B.o</filename> and so on are the compiled Haskell
 374             modules, and <filename>libHSfoo.a</filename> is the library you
 375             wish to create.  The syntax may differ slightly on your system,
 376             so check the documentation if you run into difficulties.</para>
 377       </listitem>
 378       <listitem>
 379         <para>Versions of the Haskell libraries for use with GHCi may also
 380           abe included: GHCi cannot load <literal>.a</literal> files
 381           directly, instead it will look for an object file
 382           called <filename>HSfoo.o</filename> and load that.  On some
 383           systems, the <literal>ghc-pkg</literal> tool can automatically
 384           build the GHCi version of each library, see
 385           <xref linkend="package-management"/>.  To build these libraries
 386           by hand from the <literal>.a</literal> archive, it is possible
 387           to use GNU <command>ld</command> as follows:</para>
 388
 389 <screen>ld -r &ndash;&ndash;whole-archive -o HSfoo.o libHSfoo.a</screen>
 390
 391         <para>(replace
 392           <literal>&ndash;&ndash;whole-archive</literal> with
 393           <literal>&ndash;all_load</literal> on MacOS X)</para>
 394       </listitem>
 395       <listitem>
 396         <para>When building the package as shared object, GHC wraps
 397           out the underlying linker so that the user gets a common
 398           interface to all shared object variants that are supported
 399           by GHC (DLLs, ELF DSOs, and Mac OS dylibs). The shared
 400           object must be named in specific way for two reasons: (1)
 401           the name must contain the GHC compiler version, so that two
 402           library variants don't collide that are compiled by
 403           different versions of GHC and that therefore are most likely
 404           incompatible with respect to calling conventions, (2) it
 405           must be different from the static name otherwise we would
 406           not be able to control the linker as precisely as necessary
 407           to make
 408           the <option>-static</option>/<option>-dynamic</option> flags
 409           work, see <xref linkend="options-linker" />.</para>
 410
 411 <screen>ghc -shared libHSfoo-1.0-ghc<replaceable>GHCVersion</replaceable>.so A.o B.o C.o</screen>
 412         <para>Using GHC's version number in the shared object name
 413           allows different library versions compiled by different GHC
 414           versions to be installed in standard system locations,
 415           e.g. under *nix /usr/lib. To obtain the version number of
 416           GHC invoke <literal>ghc --numeric-version</literal> and use
 417           its output in place
 418           of <replaceable>GHCVersion</replaceable>. See also
 419           <xref linkend="options-codegen" /> on how object files must
 420           be prepared for shared object linking.</para>
 421       </listitem>
 422     </itemizedlist>
 423
 424      <para>GHC does not maintain detailed cross-package dependency
 425        information.  It does remember which modules in other packages
 426        the current module depends on, but not which things within
 427        those imported things.</para>
 428
 429      <para>To compile a module which is to be part of a new package,
 430       use the <literal>-package-name</literal> option (<xref linkend="using-packages"/>).
 431       Failure to use the <literal>-package-name</literal> option
 432       when compiling a package will probably result in disaster, but
 433       you will only discover later when you attempt to import modules
 434       from the package.  At this point GHC will complain that the
 435       package name it was expecting the module to come from is not the
 436       same as the package name stored in the <literal>.hi</literal>
 437       file.</para>
 438
 439     <para>It is worth noting with shared objects, when each package
 440       is built as a single shared object file, since a reference to a shared object costs an extra
 441       indirection, intra-package references are cheaper than
 442       inter-package references. Of course, this applies to the
 443       <filename>main</filename> package as well.</para>
 444     </sect2>
 445
 446   <sect2 id="package-management">
 447     <title>Package management (the <literal>ghc-pkg</literal> command)</title>
 448     <indexterm><primary>packages</primary>
 449       <secondary>management</secondary></indexterm>
 450
 451     <para>The <literal>ghc-pkg</literal> tool allows packages to be
 452       added or removed from a package database.  By default,
 453       the system-wide package database is modified, but alternatively
 454       the user's local package database or another specified
 455       file can be used.</para>
 456
 457     <para>To see what package databases are in use, say
 458       <literal>ghc-pkg&nbsp;list</literal>.  The stack of databases that
 459       <literal>ghc-pkg</literal> knows about can be modified using the
 460       <literal>GHC_PACKAGE_PATH</literal> environment variable (see <xref
 461         linkend="ghc-package-path" />, and using
 462         <literal>--package-conf</literal> options on the
 463         <literal>ghc-pkg</literal> command line.</para>
 464
 465     <para>When asked to modify a database, <literal>ghc-pkg</literal> modifies
 466       the global database by default.  Specifying <option>--user</option>
 467       causes it to act on the user database, or <option>--package-conf</option>
 468       can be used to act on another database entirely.  When multiple of these
 469       options are given, the rightmost one is used as the database to act
 470       upon.</para>
 471
 472    <para>Commands that query the package database (list, latest,
 473      describe, field) operate on the list of databases specified by
 474      the flags <option>--user</option>, <option>--global</option>, and
 475      <option>--package-conf</option>.  If none of these flags are
 476      given, the default is <option>--global</option>
 477      <option>--user</option>.</para>
 478
 479     <para>If the environment variable <literal>GHC_PACKAGE_PATH</literal> is
 480       set, and its value does not end in a separator (<literal>:</literal> on
 481       Unix, <literal>;</literal> on Windows), then the last database is
 482       considered to be the global database, and will be modified by default by
 483       <literal>ghc-pkg</literal>.  The intention here is that
 484       <literal>GHC_PACKAGE_PATH</literal> can be used to create a virtual
 485       package environment into which Cabal packages can be installed without
 486       setting anything other than <literal>GHC_PACKAGE_PATH</literal>.</para>
 487
 488     <para>The <literal>ghc-pkg</literal> program may be run in the ways listed
 489       below.  Where a package name is required, the package can be named in
 490       full including the version number
 491       (e.g. <literal>network-1.0</literal>), or without the version number.
 492       Naming a package without the version number matches all versions of the
 493       package; the specified action will be applied to all the matching
 494       packages.  A package specifier that matches all version of the package
 495       can also be written <replaceable>pkg</replaceable><literal>-*</literal>,
 496       to make it clearer that multiple packages are being matched.</para>
 497
 498     <variablelist>
 499       <varlistentry>
 500         <term><literal>ghc-pkg register <replaceable>file</replaceable></literal></term>
 501         <listitem>
 502           <para>Reads a package specification from
 503             <replaceable>file</replaceable> (which may be &ldquo;<literal>-</literal>&rdquo;
 504             to indicate standard input),
 505             and adds it to the database of installed packages.  The syntax of
 506             <replaceable>file</replaceable> is given in <xref
 507               linkend="installed-pkg-info" />.</para>
 508
 509           <para>The package specification must be a package that isn't already
 510             installed.</para>
 511         </listitem>
 512       </varlistentry>
 513
 514       <varlistentry>
 515         <term><literal>ghc-pkg update <replaceable>file</replaceable></literal></term>
 516         <listitem>
 517           <para>The same as <literal>register</literal>, except that if a
 518             package of the same name is already installed, it is
 519             replaced by the new one.</para>
 520         </listitem>
 521       </varlistentry>
 522
 523       <varlistentry>
 524         <term><literal>ghc-pkg unregister <replaceable>P</replaceable></literal></term>
 525         <listitem>
 526           <para>Remove the specified package from the database.</para>
 527         </listitem>
 528       </varlistentry>
 529
 530       <varlistentry>
 531         <term><literal>ghc-pkg expose <replaceable>P</replaceable></literal></term>
 532         <listitem>
 533           <para>Sets the <literal>exposed</literal> flag for package
 534             <replaceable>P</replaceable> to <literal>True</literal>.</para>
 535         </listitem>
 536       </varlistentry>
 537
 538       <varlistentry>
 539         <term><literal>ghc-pkg check</literal></term>
 540         <listitem>
 541           <para>Check consistency of dependencies in the package
 542           database, and report packages that have missing
 543           dependencies.</para>
 544         </listitem>
 545       </varlistentry>
 546
 547       <varlistentry>
 548         <term><literal>ghc-pkg hide <replaceable>P</replaceable></literal></term>
 549         <listitem>
 550           <para>Sets the <literal>exposed</literal> flag for package
 551             <replaceable>P</replaceable> to <literal>False</literal>.</para>
 552         </listitem>
 553       </varlistentry>
 554
 555       <varlistentry>
 556         <term><literal>ghc-pkg list [<replaceable>P</replaceable>] [<option>--simple-output</option>]</literal></term>
 557         <listitem>
 558           <para>This option displays the currently installed
 559             packages, for each of the databases known to
 560             <literal>ghc-pkg</literal>.  That includes the global database, the
 561             user's local database, and any further files specified using the
 562             <option>-f</option> option on the command line.</para>
 563
 564           <para>Hidden packages (those for which the <literal>exposed</literal>
 565             flag is <literal>False</literal>) are shown in parentheses in the
 566             list of packages.</para>
 567
 568           <para>If an optional package identifier <replaceable>P</replaceable>
 569             is given, then only packages matching that identifier are
 570             shown.</para>
 571
 572           <para>If the option <option>--simple-output</option> is given, then
 573             the packages are listed on a single line separated by spaces, and
 574             the database names are not included.  This is intended to make it
 575             easier to parse the output of <literal>ghc-pkg list</literal> using
 576             a script.</para>
 577         </listitem>
 578       </varlistentry>
 579
 580       <varlistentry>
 581         <term><literal>ghc-pkg find-module <replaceable>M</replaceable> [<option>--simple-output</option>]</literal></term>
 582         <listitem>
 583     <para>This option lists registered packages exposing module
 584       <replaceable>M</replaceable>. Examples:</para>
 585 <screen>
 586 $ ghc-pkg find-module Var
 587 c:/fptools/validate/ghc/driver/package.conf.inplace:
 588     (ghc-6.9.20080428)
 589
 590 $ ghc-pkg find-module Data.Sequence
 591 c:/fptools/validate/ghc/driver/package.conf.inplace:
 592     containers-0.1
 593 </screen>
 594   <para>Otherwise, it behaves like <literal>ghc-pkg list</literal>,
 595   including options.</para>
 596         </listitem>
 597       </varlistentry>
 598
 599
 600       <varlistentry>
 601         <term><literal>ghc-pkg latest <replaceable>P</replaceable></literal></term>
 602         <listitem>
 603           <para>Prints the latest available version of package
 604             <replaceable>P</replaceable>.</para>
 605         </listitem>
 606       </varlistentry>
 607
 608       <varlistentry>
 609         <term><literal>ghc-pkg describe <replaceable>P</replaceable></literal></term>
 610         <listitem>
 611           <para>Emit the full description of the specified package.  The
 612             description is in the form of an
 613             <literal>InstalledPackageInfo</literal>, the same as the input file
 614             format for <literal>ghc-pkg register</literal>.  See <xref
 615               linkend="installed-pkg-info" /> for details.</para>
 616         </listitem>
 617       </varlistentry>
 618
 619       <varlistentry>
 620         <term><literal>ghc-pkg field <replaceable>P</replaceable> <replaceable>field</replaceable>[,<replaceable>field</replaceable>]*</literal></term>
 621         <listitem>
 622           <para>Show just a single field of the installed package description
 623       for <literal>P</literal>. Multiple fields can be selected by separating
 624       them with commas</para>
 625         </listitem>
 626       </varlistentry>
 627     </variablelist>
 628
 629     <para>
 630       Substring matching is supported for <replaceable>M</replaceable> in
 631       <literal>find-module</literal> and for <replaceable>P</replaceable> in
 632       <literal>list</literal>, <literal>describe</literal>, and
 633       <literal>field</literal>, where a <literal>'*'</literal> indicates open
 634       substring ends (<literal>prefix*</literal>, <literal>*suffix</literal>,
 635       <literal>*infix*</literal>). Examples (output omitted):
 636     </para>
 637     <screen>
 638     -- list all regex-related packages
 639     ghc-pkg list '*regex*' --ignore-case
 640     -- list all string-related packages
 641     ghc-pkg list '*string*' --ignore-case
 642     -- list OpenGL-related packages
 643     ghc-pkg list '*gl*' --ignore-case
 644     -- list packages exporting modules in the Data hierarchy
 645     ghc-pkg find-module 'Data.*'
 646     -- list packages exporting Monad modules
 647     ghc-pkg find-module '*Monad*'
 648     -- list names and maintainers for all packages
 649     ghc-pkg field '*' name,maintainer
 650     -- list location of haddock htmls for all packages
 651     ghc-pkg field '*' haddock-html
 652     -- dump the whole database
 653     ghc-pkg describe '*'
 654     </screen>
 655
 656     <para>Additionally, the following flags are accepted by
 657       <literal>ghc-pkg</literal>:</para>
 658
 659     <variablelist>
 660       <varlistentry>
 661         <term>
 662           <option>&ndash;&ndash;auto-ghci-libs</option><indexterm><primary><option>&ndash;&ndash;auto-ghci-libs</option></primary>
 663           </indexterm>
 664         </term>
 665         <listitem>
 666           <para>Automatically generate the GHCi
 667             <filename>.o</filename> version of each
 668             <filename>.a</filename> Haskell library, using GNU ld (if
 669             that is available).  Without this option,
 670             <literal>ghc-pkg</literal> will warn if GHCi versions of
 671             any Haskell libraries in the package don't exist.</para>
 672
 673             <para>GHCi <literal>.o</literal> libraries don't
 674             necessarily have to live in the same directory as the
 675             corresponding <literal>.a</literal> library.  However,
 676             this option will cause the GHCi library to be created in
 677             the same directory as the <literal>.a</literal>
 678             library.</para>
 679         </listitem>
 680       </varlistentry>
 681
 682       <varlistentry>
 683         <term>
 684           <option>-f</option> <replaceable>file</replaceable>
 685           <indexterm><primary><option>-f</option></primary>
 686           </indexterm>
 687         </term>
 688         <term>
 689           <option>-package-conf</option> <replaceable>file</replaceable>
 690           <indexterm><primary><option>-package-conf</option></primary>
 691           </indexterm>
 692         </term>
 693         <listitem>
 694           <para>Adds <replaceable>file</replaceable> to the stack of package
 695             databases.  Additionally, <replaceable>file</replaceable> will
 696             also be the database modified by a <literal>register</literal>,
 697             <literal>unregister</literal>, <literal>expose</literal> or
 698             <literal>hide</literal> command, unless it is overridden by a later
 699             <option>--package-conf</option>, <option>--user</option> or
 700             <option>--global</option> option.</para>
 701         </listitem>
 702       </varlistentry>
 703
 704       <varlistentry>
 705         <term>
 706           <option>&ndash;&ndash;force</option>
 707           <indexterm><primary>
 708               <option>&ndash;&ndash;force</option>
 709             </primary></indexterm>
 710         </term>
 711         <listitem>
 712           <para>Causes <literal>ghc-pkg</literal> to ignore missing
 713             dependencies, directories and libraries when registering a package,
 714             and just go ahead and add it anyway.  This might be useful if your
 715             package installation system needs to add the package to
 716             GHC before building and installing the files.</para>
 717         </listitem>
 718       </varlistentry>
 719
 720       <varlistentry>
 721         <term>
 722           <option>&ndash;&ndash;global</option><indexterm><primary><option>&ndash;&ndash;global</option></primary>
 723           </indexterm>
 724         </term>
 725         <listitem>
 726           <para>Operate on the global package database (this is the default).
 727             This flag affects the <literal>register</literal>,
 728             <literal>update</literal>, <literal>unregister</literal>,
 729             <literal>expose</literal>, and <literal>hide</literal>
 730             commands.</para>
 731         </listitem>
 732       </varlistentry>
 733
 734       <varlistentry>
 735         <term>
 736           <option>&ndash;&ndash;help</option><indexterm><primary><option>&ndash;&ndash;help</option></primary>
 737           </indexterm>
 738         </term>
 739         <term>
 740           <option>-?</option><indexterm><primary><option>-?</option></primary>
 741           </indexterm>
 742         </term>
 743         <listitem>
 744           <para>Outputs the command-line syntax.</para>
 745         </listitem>
 746       </varlistentry>
 747
 748       <varlistentry>
 749         <term>
 750           <option>&ndash;&ndash;user</option><indexterm><primary><option>&ndash;&ndash;user</option></primary>
 751           </indexterm>
 752         </term>
 753         <listitem>
 754           <para>Operate on the current user's local package database.
 755             This flag affects the <literal>register</literal>,
 756             <literal>update</literal>, <literal>unregister</literal>,
 757             <literal>expose</literal>, and <literal>hide</literal>
 758             commands.</para>
 759         </listitem>
 760       </varlistentry>
 761
 762       <varlistentry>
 763         <term>
 764           <option>-V</option><indexterm><primary><option>-V</option></primary>
 765           </indexterm>
 766         </term>
 767         <term>
 768           <option>&ndash;&ndash;version</option><indexterm><primary><option>&ndash;&ndash;version</option></primary>
 769           </indexterm>
 770         </term>
 771         <listitem>
 772           <para>Output the <literal>ghc-pkg</literal> version number.</para>
 773         </listitem>
 774       </varlistentry>
 775     </variablelist>
 776
 777     <para>When modifying the package database
 778       <replaceable>file</replaceable>, a copy of the original file is
 779       saved in <replaceable>file</replaceable><literal>.old</literal>,
 780       so in an emergency you can always restore the old settings by
 781       copying the old file back again.</para>
 782
 783   </sect2>
 784
 785   <sect2 id="installed-pkg-info">
 786     <title>
 787       <literal>InstalledPackageInfo</literal>: a package specification
 788     </title>
 789
 790     <para>A package specification is a Haskell record; in particular, it is the
 791       record <ulink
 792         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>
 793
 794     <para>An <literal>InstalledPackageInfo</literal> has a human
 795       readable/writable syntax.  The functions
 796       <literal>parseInstalledPackageInfo</literal> and
 797       <literal>showInstalledPackageInfo</literal> read and write this syntax
 798       respectively.  Here's an example of the
 799       <literal>InstalledPackageInfo</literal> for the <literal>unix</literal> package:</para>
 800
 801 <screen>
 802 $ ghc-pkg describe unix
 803 name: unix
 804 version: 1.0
 805 license: BSD3
 806 copyright:
 807 maintainer: libraries@haskell.org
 808 stability:
 809 homepage:
 810 package-url:
 811 description:
 812 category:
 813 author:
 814 exposed: True
 815 exposed-modules: System.Posix,
 816                  System.Posix.DynamicLinker.Module,
 817                  System.Posix.DynamicLinker.Prim,
 818                  System.Posix.Directory,
 819                  System.Posix.DynamicLinker,
 820                  System.Posix.Env,
 821                  System.Posix.Error,
 822                  System.Posix.Files,
 823                  System.Posix.IO,
 824                  System.Posix.Process,
 825                  System.Posix.Resource,
 826                  System.Posix.Temp,
 827                  System.Posix.Terminal,
 828                  System.Posix.Time,
 829                  System.Posix.Unistd,
 830                  System.Posix.User,
 831                  System.Posix.Signals.Exts
 832 import-dirs: /usr/lib/ghc-6.4/libraries/unix
 833 library-dirs: /usr/lib/ghc-6.4/libraries/unix
 834 hs-libraries: HSunix
 835 extra-libraries: HSunix_cbits, dl
 836 include-dirs: /usr/lib/ghc-6.4/libraries/unix/include
 837 includes: HsUnix.h
 838 depends: base-1.0
 839 </screen>
 840
 841     <para>The full <ulink url="../Cabal/index.html">Cabal documentation</ulink>
 842       is still in preparation (at time of writing), so in the meantime
 843       here is a brief description of the syntax of this file:</para>
 844
 845     <para>A package description consists of a number of field/value pairs.  A
 846       field starts with the field name in the left-hand column followed by a
 847       &ldquo;<literal>:</literal>&rdquo;, and the value continues until the next line that begins in the
 848       left-hand column, or the end of file.</para>
 849
 850     <para>The syntax of the value depends on the field.   The various field
 851       types are:</para>
 852
 853     <variablelist>
 854       <varlistentry>
 855         <term>freeform</term>
 856         <listitem>
 857           <para>Any arbitrary string, no interpretation or parsing is
 858             done.</para>
 859         </listitem>
 860       </varlistentry>
 861       <varlistentry>
 862         <term>string</term>
 863         <listitem>
 864           <para>A sequence of non-space characters, or a sequence of arbitrary
 865             characters surrounded by quotes <literal>"...."</literal>.</para>
 866         </listitem>
 867       </varlistentry>
 868       <varlistentry>
 869         <term>string list</term>
 870         <listitem>
 871           <para>A sequence of strings, separated by commas.  The sequence may
 872             be empty.</para>
 873         </listitem>
 874       </varlistentry>
 875     </variablelist>
 876
 877     <para>In addition, there are some fields with special syntax (e.g. package
 878       names, version, dependencies).</para>
 879
 880     <para>The allowed fields, with their types, are:</para>
 881
 882     <variablelist>
 883       <varlistentry>
 884         <term>
 885           <literal>name</literal>
 886           <indexterm><primary><literal>name</literal></primary><secondary>package specification</secondary></indexterm>
 887         </term>
 888         <listitem>
 889           <para>The package's name (without the version).</para>
 890         </listitem>
 891       </varlistentry>
 892
 893       <varlistentry>
 894         <term>
 895           <literal>version</literal>
 896           <indexterm><primary><literal>version</literal></primary><secondary>package specification</secondary></indexterm>
 897         </term>
 898         <listitem>
 899           <para>The package's version, usually in the form
 900             <literal>A.B</literal> (any number of components are allowed).</para>
 901         </listitem>
 902       </varlistentry>
 903
 904       <varlistentry>
 905         <term>
 906           <literal>license</literal>
 907           <indexterm><primary><literal>auto</literal></primary><secondary>package specification</secondary></indexterm>
 908         </term>
 909         <listitem>
 910           <para>(string) The type of license under which this package is distributed.
 911             This field is a value of the <ulink
 912         url="../libraries/Cabal/Distribution-License.html#t:License"><literal>License</literal></ulink> type.</para>
 913         </listitem>
 914       </varlistentry>
 915
 916         <varlistentry>
 917           <term>
 918             <literal>license-file</literal>
 919             <indexterm><primary><literal>license-file</literal></primary><secondary>package specification</secondary></indexterm>
 920           </term>
 921           <listitem>
 922             <para>(optional string) The name of a file giving detailed license
 923             information for this package.</para>
 924           </listitem>
 925         </varlistentry>
 926
 927         <varlistentry>
 928           <term>
 929             <literal>copyright</literal>
 930             <indexterm><primary><literal>copyright</literal></primary><secondary>package specification</secondary></indexterm>
 931           </term>
 932           <listitem>
 933             <para>(optional freeform) The copyright string.</para>
 934           </listitem>
 935         </varlistentry>
 936
 937         <varlistentry>
 938           <term>
 939             <literal>maintainer</literal>
 940             <indexterm><primary><literal>maintainer</literal></primary><secondary>package specification</secondary></indexterm>
 941           </term>
 942           <listitem>
 943             <para>(optinoal freeform) The email address of the package's maintainer.</para>
 944           </listitem>
 945         </varlistentry>
 946
 947         <varlistentry>
 948           <term>
 949             <literal>stability</literal>
 950             <indexterm><primary><literal>stability</literal></primary><secondary>package specification</secondary></indexterm>
 951           </term>
 952           <listitem>
 953             <para>(optional freeform) A string describing the stability of the package
 954             (eg. stable, provisional or experimental).</para>
 955           </listitem>
 956         </varlistentry>
 957
 958         <varlistentry>
 959           <term>
 960             <literal>homepage</literal>
 961             <indexterm><primary><literal>homepage</literal></primary><secondary>package specification</secondary></indexterm>
 962           </term>
 963           <listitem>
 964             <para>(optional freeform) URL of the package's home page.</para>
 965           </listitem>
 966         </varlistentry>
 967
 968       <varlistentry>
 969         <term>
 970             <literal>package-url</literal>
 971             <indexterm><primary><literal>package-url</literal></primary><secondary>package specification</secondary></indexterm>
 972           </term>
 973           <listitem>
 974             <para>(optional freeform) URL of a downloadable distribution for this
 975             package.  The distribution should be a Cabal package.</para>
 976           </listitem>
 977         </varlistentry>
 978
 979         <varlistentry>
 980           <term>
 981             <literal>description</literal>
 982             <indexterm><primary><literal>description</literal></primary><secondary>package specification</secondary></indexterm>
 983           </term>
 984           <listitem>
 985             <para>(optional freeform) Description of the package.</para>
 986           </listitem>
 987         </varlistentry>
 988
 989       <varlistentry>
 990           <term>
 991             <literal>category</literal>
 992             <indexterm><primary><literal>category</literal></primary><secondary>package specification</secondary></indexterm>
 993           </term>
 994           <listitem>
 995             <para>(optinoal freeform) Which category the package belongs to.  This field
 996             is for use in conjunction with a future centralised package
 997             distribution framework, tentatively titled Hackage.</para>
 998           </listitem>
 999         </varlistentry>
1000
1001         <varlistentry>
1002           <term>
1003             <literal>author</literal>
1004             <indexterm><primary><literal>author</literal></primary><secondary>package specification</secondary></indexterm>
1005           </term>
1006           <listitem>
1007             <para>(optional freeform) Author of the package.</para>
1008           </listitem>
1009         </varlistentry>
1010
1011         <varlistentry>
1012           <term>
1013             <literal>exposed</literal>
1014             <indexterm><primary><literal>exposed</literal></primary><secondary>package specification</secondary></indexterm>
1015           </term>
1016           <listitem>
1017             <para>(bool) Whether the package is exposed or not.</para>
1018           </listitem>
1019         </varlistentry>
1020
1021         <varlistentry>
1022           <term>
1023             <literal>exposed-modules</literal>
1024             <indexterm><primary><literal>exposed-modules</literal></primary><secondary>package specification</secondary></indexterm>
1025           </term>
1026           <listitem>
1027             <para>(string list) modules exposed by this package.</para>
1028           </listitem>
1029         </varlistentry>
1030
1031         <varlistentry>
1032           <term>
1033             <literal>hidden-modules</literal>
1034             <indexterm><primary><literal>hidden-modules</literal></primary><secondary>package specification</secondary></indexterm>
1035           </term>
1036           <listitem>
1037             <para>(string list) modules provided by this package,
1038             but not exposed to the programmer.  These modules cannot be
1039             imported, but they are still subject to the overlapping constraint:
1040             no other package in the same program may provide a module of the
1041             same name.</para>
1042         </listitem>
1043         </varlistentry>
1044
1045         <varlistentry>
1046           <term>
1047             <literal>import-dirs</literal>
1048             <indexterm><primary><literal>import-dirs</literal></primary><secondary>package specification</secondary></indexterm>
1049           </term>
1050           <listitem>
1051             <para>(string list) A list of directories containing interface files
1052             (<literal>.hi</literal> files) for this package.</para>
1053
1054             <para>If the package contains profiling libraries, then
1055             the interface files for those library modules should have
1056             the suffix <literal>.p_hi</literal>.  So the package can
1057             contain both normal and profiling versions of the same
1058             library without conflict (see also
1059             <literal>library_dirs</literal> below).</para>
1060           </listitem>
1061         </varlistentry>
1062
1063         <varlistentry>
1064           <term>
1065             <literal>library-dirs</literal>
1066             <indexterm><primary><literal>library-dirs</literal></primary><secondary>package specification</secondary></indexterm>
1067           </term>
1068           <listitem>
1069             <para>(string list) A list of directories containing libraries for this
1070             package.</para>
1071           </listitem>
1072         </varlistentry>
1073
1074         <varlistentry>
1075           <term>
1076             <literal>hs-libraries</literal>
1077             <indexterm><primary><literal>hs-libraries</literal></primary><secondary>package specification</secondary></indexterm>
1078           </term>
1079           <listitem>
1080             <para>(string list) A list of libraries containing Haskell code for this
1081             package, with the <literal>.a</literal> or
1082             <literal>.dll</literal> suffix omitted.  When packages are
1083             built as libraries, the
1084             <literal>lib</literal> prefix is also omitted.</para>
1085
1086             <para>For use with GHCi, each library should have an
1087             object file too.  The name of the object file does
1088             <emphasis>not</emphasis> have a <literal>lib</literal>
1089             prefix, and has the normal object suffix for your
1090             platform.</para>
1091
1092             <para>For example, if we specify a Haskell library as
1093             <filename>HSfoo</filename> in the package spec, then the
1094             various flavours of library that GHC actually uses will be
1095             called:</para>
1096             <variablelist>
1097               <varlistentry>
1098                 <term><filename>libHSfoo.a</filename></term>
1099                 <listitem>
1100                   <para>The name of the library on Unix and Windows
1101                   (mingw) systems.  Note that we don't support
1102                   building dynamic libraries of Haskell code on Unix
1103                   systems.</para>
1104                 </listitem>
1105               </varlistentry>
1106               <varlistentry>
1107                 <term><filename>HSfoo.dll</filename></term>
1108                 <listitem>
1109                   <para>The name of the dynamic library on Windows
1110                   systems (optional).</para>
1111                 </listitem>
1112               </varlistentry>
1113               <varlistentry>
1114                 <term><filename>HSfoo.o</filename></term>
1115                 <term><filename>HSfoo.obj</filename></term>
1116                 <listitem>
1117                   <para>The object version of the library used by
1118                   GHCi.</para>
1119                 </listitem>
1120               </varlistentry>
1121             </variablelist>
1122           </listitem>
1123         </varlistentry>
1124
1125         <varlistentry>
1126           <term>
1127             <literal>extra-libraries</literal>
1128             <indexterm><primary><literal>extra-libraries</literal></primary><secondary>package specification</secondary></indexterm>
1129           </term>
1130           <listitem>
1131             <para>(string list) A list of extra libraries for this package.  The
1132             difference between <literal>hs-libraries</literal> and
1133             <literal>extra-libraries</literal> is that
1134             <literal>hs-libraries</literal> normally have several
1135             versions, to support profiling, parallel and other build
1136             options.  The various versions are given different
1137             suffixes to distinguish them, for example the profiling
1138             version of the standard prelude library is named
1139             <filename>libHSbase_p.a</filename>, with the
1140             <literal>_p</literal> indicating that this is a profiling
1141             version.  The suffix is added automatically by GHC for
1142             <literal>hs-libraries</literal> only, no suffix is added
1143             for libraries in
1144             <literal>extra-libraries</literal>.</para>
1145
1146             <para>The libraries listed in
1147             <literal>extra-libraries</literal> may be any libraries
1148             supported by your system's linker, including dynamic
1149             libraries (<literal>.so</literal> on Unix,
1150             <literal>.DLL</literal> on Windows).</para>
1151
1152             <para>Also, <literal>extra-libraries</literal> are placed
1153             on the linker command line after the
1154             <literal>hs-libraries</literal> for the same package.  If
1155             your package has dependencies in the other direction (i.e.
1156             <literal>extra-libraries</literal> depends on
1157             <literal>hs-libraries</literal>), and the libraries are
1158             static, you might need to make two separate
1159             packages.</para>
1160           </listitem>
1161         </varlistentry>
1162
1163         <varlistentry>
1164           <term>
1165             <literal>include-dirs</literal>
1166             <indexterm><primary><literal>include-dirs</literal></primary><secondary>package specification</secondary></indexterm>
1167           </term>
1168           <listitem>
1169             <para>(string list) A list of directories containing C includes for this
1170             package.</para>
1171           </listitem>
1172         </varlistentry>
1173
1174         <varlistentry>
1175           <term>
1176            <literal>includes</literal>
1177            <indexterm><primary><literal>includes</literal></primary><secondary>package specification</secondary></indexterm>
1178           </term>
1179           <listitem>
1180             <para>(string list) A list of files to include for via-C compilations
1181             using this package.  Typically the include file(s) will
1182             contain function prototypes for any C functions used in
1183             the package, in case they end up being called as a result
1184             of Haskell functions from the package being
1185             inlined.</para>
1186           </listitem>
1187         </varlistentry>
1188
1189         <varlistentry>
1190           <term>
1191             <literal>depends</literal>
1192             <indexterm><primary><literal>depends</literal></primary><secondary>package specification</secondary></indexterm>
1193           </term>
1194           <listitem>
1195             <para>(package name list) Packages on which this package depends.  This field contains
1196             packages with explicit versions are required, except that when
1197             submitting a package to <literal>ghc-pkg register</literal>, the
1198             versions will be filled in if they are unambiguous.</para>
1199           </listitem>
1200         </varlistentry>
1201
1202         <varlistentry>
1203           <term>
1204             <literal>hugs-options</literal>
1205             <indexterm><primary><literal>hugs-options</literal></primary><secondary>package specification</secondary></indexterm>
1206           </term>
1207           <listitem>
1208             <para>(string list) Options to pass to Hugs for this package.</para>
1209           </listitem>
1210         </varlistentry>
1211
1212         <varlistentry>
1213           <term>
1214             <literal>cc-options</literal>
1215             <indexterm><primary><literal>cc-options</literal></primary><secondary>package specification</secondary></indexterm>
1216           </term>
1217           <listitem>
1218             <para>(string list) Extra arguments to be added to the gcc command line
1219             when this package is being used (only for via-C
1220             compilations).</para>
1221           </listitem>
1222         </varlistentry>
1223
1224         <varlistentry>
1225           <term>
1226             <literal>ld-options</literal>
1227             <indexterm><primary><literal>ld-options</literal></primary><secondary>package specification</secondary></indexterm>
1228           </term>
1229           <listitem>
1230             <para>(string list) Extra arguments to be added to the
1231             <command>gcc</command> command line (for linking) when
1232             this package is being used.</para>
1233           </listitem>
1234         </varlistentry>
1235
1236         <varlistentry>
1237           <term>
1238             <literal>framework-dirs</literal>
1239             <indexterm><primary><literal>framework-dirs</literal></primary><secondary>package specification</secondary></indexterm>
1240           </term>
1241           <listitem>
1242             <para>(string list) On Darwin/MacOS X, a list of directories containing
1243             frameworks for this package. This corresponds to the
1244             <option>-framework-path</option> option. It is ignored on all other
1245             platforms.</para>
1246           </listitem>
1247         </varlistentry>
1248
1249         <varlistentry>
1250           <term>
1251             <literal>frameworks</literal>
1252             <indexterm><primary><literal>frameworks</literal></primary><secondary>package specification</secondary></indexterm>
1253           </term>
1254           <listitem>
1255             <para>(string list) On Darwin/MacOS X, a list of frameworks to link to. This
1256             corresponds to the <option>-framework</option> option. Take a look
1257             at Apple's developer documentation to find out what frameworks
1258             actually are. This entry is ignored on all other platforms.</para>
1259           </listitem>
1260         </varlistentry>
1261
1262         <varlistentry>
1263           <term>
1264             <literal>haddock-interfaces</literal>
1265             <indexterm><primary><literal>haddock-interfaces</literal></primary><secondary>package specification</secondary></indexterm>
1266           </term>
1267           <listitem>
1268             <para>(string list) A list of filenames containing <ulink
1269               url="http://www.haskell.org/haddock/">Haddock</ulink> interface
1270             files (<literal>.haddock</literal> files) for this package.</para>
1271           </listitem>
1272         </varlistentry>
1273
1274         <varlistentry>
1275           <term>
1276             <literal>haddock-html</literal>
1277             <indexterm><primary><literal>haddock-html</literal></primary><secondary>package specification</secondary></indexterm>
1278           </term>
1279           <listitem>
1280             <para>(optional string) The directory containing the Haddock-generated HTML
1281             for this package.</para>
1282           </listitem>
1283         </varlistentry>
1284       </variablelist>
1285
1286 <!--  This isn't true any more.  I'm not sure if we still need it -SDM
1287       <para>
1288       The <literal>ghc-pkg</literal> tool performs expansion of
1289       environment variables occurring in input package specifications.
1290       So, if the <literal>mypkg</literal> was added to the package
1291       database as follows:
1292       </para>
1293 <screen>
1294   $ installdir=/usr/local/lib ghc-pkg -a &lt; mypkg.pkg
1295 </screen>
1296
1297       <para>
1298       The occurrence of <literal>${installdir}</literal> is replaced
1299       with <literal>/usr/local/lib</literal> in the package data that
1300       is added for <literal>mypkg</literal>.
1301       </para>
1302
1303       <para>
1304       This feature enables the distribution of package specification
1305       files that can be easily configured when installing.
1306       </para>
1307
1308       <para>For examples of more package specifications, take a look
1309       at the <literal>package.conf</literal> in your GHC
1310       installation.</para>
1311
1312 -->
1313
1314     </sect2>
1315   </sect1>
1316
1317 <!-- Emacs stuff:
1318      ;;; Local Variables: ***
1319      ;;; mode: xml ***
1320      ;;; sgml-parent-document: ("users_guide.xml" "book" "chapter" "sect1") ***
1321      ;;; End: ***
1322  -->