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>If the environment variable <literal>GHC_PACKAGE_PATH</literal> is
 473       set, and its value does not end in a separator (<literal>:</literal> on
 474       Unix, <literal>;</literal> on Windows), then the last database is
 475       considered to be the global database, and will be modified by default by
 476       <literal>ghc-pkg</literal>.  The intention here is that
 477       <literal>GHC_PACKAGE_PATH</literal> can be used to create a virtual
 478       package environment into which Cabal packages can be installed without
 479       setting anything other than <literal>GHC_PACKAGE_PATH</literal>.</para>
 480
 481     <para>The <literal>ghc-pkg</literal> program may be run in the ways listed
 482       below.  Where a package name is required, the package can be named in
 483       full including the version number
 484       (e.g. <literal>network-1.0</literal>), or without the version number.
 485       Naming a package without the version number matches all versions of the
 486       package; the specified action will be applied to all the matching
 487       packages.  A package specifier that matches all version of the package
 488       can also be written <replaceable>pkg</replaceable><literal>-*</literal>,
 489       to make it clearer that multiple packages are being matched.</para>
 490
 491     <variablelist>
 492       <varlistentry>
 493         <term><literal>ghc-pkg register <replaceable>file</replaceable></literal></term>
 494         <listitem>
 495           <para>Reads a package specification from
 496             <replaceable>file</replaceable> (which may be &ldquo;<literal>-</literal>&rdquo;
 497             to indicate standard input),
 498             and adds it to the database of installed packages.  The syntax of
 499             <replaceable>file</replaceable> is given in <xref
 500               linkend="installed-pkg-info" />.</para>
 501
 502           <para>The package specification must be a package that isn't already
 503             installed.</para>
 504         </listitem>
 505       </varlistentry>
 506
 507       <varlistentry>
 508         <term><literal>ghc-pkg update <replaceable>file</replaceable></literal></term>
 509         <listitem>
 510           <para>The same as <literal>register</literal>, except that if a
 511             package of the same name is already installed, it is
 512             replaced by the new one.</para>
 513         </listitem>
 514       </varlistentry>
 515
 516       <varlistentry>
 517         <term><literal>ghc-pkg unregister <replaceable>P</replaceable></literal></term>
 518         <listitem>
 519           <para>Remove the specified package from the database.</para>
 520         </listitem>
 521       </varlistentry>
 522
 523       <varlistentry>
 524         <term><literal>ghc-pkg expose <replaceable>P</replaceable></literal></term>
 525         <listitem>
 526           <para>Sets the <literal>exposed</literal> flag for package
 527             <replaceable>P</replaceable> to <literal>True</literal>.</para>
 528         </listitem>
 529       </varlistentry>
 530
 531       <varlistentry>
 532         <term><literal>ghc-pkg hide <replaceable>P</replaceable></literal></term>
 533         <listitem>
 534           <para>Sets the <literal>exposed</literal> flag for package
 535             <replaceable>P</replaceable> to <literal>False</literal>.</para>
 536         </listitem>
 537       </varlistentry>
 538
 539       <varlistentry>
 540         <term><literal>ghc-pkg list [<replaceable>P</replaceable>] [<option>--simple-output</option>]</literal></term>
 541         <listitem>
 542           <para>This option displays the currently installed
 543             packages, for each of the databases known to
 544             <literal>ghc-pkg</literal>.  That includes the global database, the
 545             user's local database, and any further files specified using the
 546             <option>-f</option> option on the command line.</para>
 547
 548           <para>Hidden packages (those for which the <literal>exposed</literal>
 549             flag is <literal>False</literal>) are shown in parentheses in the
 550             list of packages.</para>
 551
 552           <para>If an optional package identifier <replaceable>P</replaceable>
 553             is given, then only packages matching that identifier are
 554             shown.</para>
 555
 556           <para>If the option <option>--simple-output</option> is given, then
 557             the packages are listed on a single line separated by spaces, and
 558             the database names are not included.  This is intended to make it
 559             easier to parse the output of <literal>ghc-pkg list</literal> using
 560             a script.</para>
 561         </listitem>
 562       </varlistentry>
 563
 564       <varlistentry>
 565         <term><literal>ghc-pkg latest <replaceable>P</replaceable></literal></term>
 566         <listitem>
 567           <para>Prints the latest available version of package
 568             <replaceable>P</replaceable>.</para>
 569         </listitem>
 570       </varlistentry>
 571
 572       <varlistentry>
 573         <term><literal>ghc-pkg describe <replaceable>P</replaceable></literal></term>
 574         <listitem>
 575           <para>Emit the full description of the specified package.  The
 576             description is in the form of an
 577             <literal>InstalledPackageInfo</literal>, the same as the input file
 578             format for <literal>ghc-pkg register</literal>.  See <xref
 579               linkend="installed-pkg-info" /> for details.</para>
 580         </listitem>
 581       </varlistentry>
 582
 583       <varlistentry>
 584         <term><literal>ghc-pkg field <replaceable>P</replaceable> <replaceable>field</replaceable></literal></term>
 585         <listitem>
 586           <para>Show just a single field of the installed package description
 587             for <literal>P</literal>.</para>
 588         </listitem>
 589       </varlistentry>
 590     </variablelist>
 591
 592     <para>Additionally, the following flags are accepted by
 593       <literal>ghc-pkg</literal>:</para>
 594
 595     <variablelist>
 596       <varlistentry>
 597         <term>
 598           <option>&ndash;&ndash;auto-ghci-libs</option><indexterm><primary><option>&ndash;&ndash;auto-ghci-libs</option></primary>
 599           </indexterm>
 600         </term>
 601         <listitem>
 602           <para>Automatically generate the GHCi
 603             <filename>.o</filename> version of each
 604             <filename>.a</filename> Haskell library, using GNU ld (if
 605             that is available).  Without this option,
 606             <literal>ghc-pkg</literal> will warn if GHCi versions of
 607             any Haskell libraries in the package don't exist.</para>
 608
 609             <para>GHCi <literal>.o</literal> libraries don't
 610             necessarily have to live in the same directory as the
 611             corresponding <literal>.a</literal> library.  However,
 612             this option will cause the GHCi library to be created in
 613             the same directory as the <literal>.a</literal>
 614             library.</para>
 615         </listitem>
 616       </varlistentry>
 617
 618       <varlistentry>
 619         <term>
 620           <option>-f</option> <replaceable>file</replaceable>
 621           <indexterm><primary><option>-f</option></primary>
 622           </indexterm>
 623         </term>
 624         <term>
 625           <option>-package-conf</option> <replaceable>file</replaceable>
 626           <indexterm><primary><option>-package-conf</option></primary>
 627           </indexterm>
 628         </term>
 629         <listitem>
 630           <para>Adds <replaceable>file</replaceable> to the stack of package
 631             databases.  Additionally, <replaceable>file</replaceable> will
 632             also be the database modified by a <literal>register</literal>,
 633             <literal>unregister</literal>, <literal>expose</literal> or
 634             <literal>hide</literal> command, unless it is overridden by a later
 635             <option>--package-conf</option>, <option>--user</option> or
 636             <option>--global</option> option.</para>
 637         </listitem>
 638       </varlistentry>
 639
 640       <varlistentry>
 641         <term>
 642           <option>&ndash;&ndash;force</option>
 643           <indexterm><primary>
 644               <option>&ndash;&ndash;force</option>
 645             </primary></indexterm>
 646         </term>
 647         <listitem>
 648           <para>Causes <literal>ghc-pkg</literal> to ignore missing
 649             dependencies, directories and libraries when registering a package,
 650             and just go ahead and add it anyway.  This might be useful if your
 651             package installation system needs to add the package to
 652             GHC before building and installing the files.</para>
 653         </listitem>
 654       </varlistentry>
 655
 656       <varlistentry>
 657         <term>
 658           <option>&ndash;&ndash;global</option><indexterm><primary><option>&ndash;&ndash;global</option></primary>
 659           </indexterm>
 660         </term>
 661         <listitem>
 662           <para>Operate on the global package database (this is the default).
 663             This flag affects the <literal>register</literal>,
 664             <literal>update</literal>, <literal>unregister</literal>,
 665             <literal>expose</literal>, and <literal>hide</literal>
 666             commands.</para>
 667         </listitem>
 668       </varlistentry>
 669
 670       <varlistentry>
 671         <term>
 672           <option>&ndash;&ndash;help</option><indexterm><primary><option>&ndash;&ndash;help</option></primary>
 673           </indexterm>
 674         </term>
 675         <term>
 676           <option>-?</option><indexterm><primary><option>-?</option></primary>
 677           </indexterm>
 678         </term>
 679         <listitem>
 680           <para>Outputs the command-line syntax.</para>
 681         </listitem>
 682       </varlistentry>
 683
 684       <varlistentry>
 685         <term>
 686           <option>&ndash;&ndash;user</option><indexterm><primary><option>&ndash;&ndash;user</option></primary>
 687           </indexterm>
 688         </term>
 689         <listitem>
 690           <para>Operate on the current user's local package database.
 691             This flag affects the <literal>register</literal>,
 692             <literal>update</literal>, <literal>unregister</literal>,
 693             <literal>expose</literal>, and <literal>hide</literal>
 694             commands.</para>
 695         </listitem>
 696       </varlistentry>
 697
 698       <varlistentry>
 699         <term>
 700           <option>-V</option><indexterm><primary><option>-V</option></primary>
 701           </indexterm>
 702         </term>
 703         <term>
 704           <option>&ndash;&ndash;version</option><indexterm><primary><option>&ndash;&ndash;version</option></primary>
 705           </indexterm>
 706         </term>
 707         <listitem>
 708           <para>Output the <literal>ghc-pkg</literal> version number.</para>
 709         </listitem>
 710       </varlistentry>
 711     </variablelist>
 712
 713     <para>When modifying the package database
 714       <replaceable>file</replaceable>, a copy of the original file is
 715       saved in <replaceable>file</replaceable><literal>.old</literal>,
 716       so in an emergency you can always restore the old settings by
 717       copying the old file back again.</para>
 718
 719   </sect2>
 720
 721   <sect2 id="installed-pkg-info">
 722     <title>
 723       <literal>InstalledPackageInfo</literal>: a package specification
 724     </title>
 725
 726     <para>A package specification is a Haskell record; in particular, it is the
 727       record <ulink
 728         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>
 729
 730     <para>An <literal>InstalledPackageInfo</literal> has a human
 731       readable/writable syntax.  The functions
 732       <literal>parseInstalledPackageInfo</literal> and
 733       <literal>showInstalledPackageInfo</literal> read and write this syntax
 734       respectively.  Here's an example of the
 735       <literal>InstalledPackageInfo</literal> for the <literal>unix</literal> package:</para>
 736
 737 <screen>
 738 $ ghc-pkg describe unix
 739 name: unix
 740 version: 1.0
 741 license: BSD3
 742 copyright:
 743 maintainer: libraries@haskell.org
 744 stability:
 745 homepage:
 746 package-url:
 747 description:
 748 category:
 749 author:
 750 exposed: True
 751 exposed-modules: System.Posix,
 752                  System.Posix.DynamicLinker.Module,
 753                  System.Posix.DynamicLinker.Prim,
 754                  System.Posix.Directory,
 755                  System.Posix.DynamicLinker,
 756                  System.Posix.Env,
 757                  System.Posix.Error,
 758                  System.Posix.Files,
 759                  System.Posix.IO,
 760                  System.Posix.Process,
 761                  System.Posix.Resource,
 762                  System.Posix.Temp,
 763                  System.Posix.Terminal,
 764                  System.Posix.Time,
 765                  System.Posix.Unistd,
 766                  System.Posix.User,
 767                  System.Posix.Signals.Exts
 768 import-dirs: /usr/lib/ghc-6.4/libraries/unix
 769 library-dirs: /usr/lib/ghc-6.4/libraries/unix
 770 hs-libraries: HSunix
 771 extra-libraries: HSunix_cbits, dl
 772 include-dirs: /usr/lib/ghc-6.4/libraries/unix/include
 773 includes: HsUnix.h
 774 depends: base-1.0
 775 </screen>
 776
 777     <para>The full <ulink url="../Cabal/index.html">Cabal documentation</ulink>
 778       is still in preparation (at time of writing), so in the meantime
 779       here is a brief description of the syntax of this file:</para>
 780
 781     <para>A package description consists of a number of field/value pairs.  A
 782       field starts with the field name in the left-hand column followed by a
 783       &ldquo;<literal>:</literal>&rdquo;, and the value continues until the next line that begins in the
 784       left-hand column, or the end of file.</para>
 785
 786     <para>The syntax of the value depends on the field.   The various field
 787       types are:</para>
 788
 789     <variablelist>
 790       <varlistentry>
 791         <term>freeform</term>
 792         <listitem>
 793           <para>Any arbitrary string, no interpretation or parsing is
 794             done.</para>
 795         </listitem>
 796       </varlistentry>
 797       <varlistentry>
 798         <term>string</term>
 799         <listitem>
 800           <para>A sequence of non-space characters, or a sequence of arbitrary
 801             characters surrounded by quotes <literal>"...."</literal>.</para>
 802         </listitem>
 803       </varlistentry>
 804       <varlistentry>
 805         <term>string list</term>
 806         <listitem>
 807           <para>A sequence of strings, separated by commas.  The sequence may
 808             be empty.</para>
 809         </listitem>
 810       </varlistentry>
 811     </variablelist>
 812
 813     <para>In addition, there are some fields with special syntax (e.g. package
 814       names, version, dependencies).</para>
 815
 816     <para>The allowed fields, with their types, are:</para>
 817
 818     <variablelist>
 819       <varlistentry>
 820         <term>
 821           <literal>name</literal>
 822           <indexterm><primary><literal>name</literal></primary><secondary>package specification</secondary></indexterm>
 823         </term>
 824         <listitem>
 825           <para>The package's name (without the version).</para>
 826         </listitem>
 827       </varlistentry>
 828
 829       <varlistentry>
 830         <term>
 831           <literal>version</literal>
 832           <indexterm><primary><literal>version</literal></primary><secondary>package specification</secondary></indexterm>
 833         </term>
 834         <listitem>
 835           <para>The package's version, usually in the form
 836             <literal>A.B</literal> (any number of components are allowed).</para>
 837         </listitem>
 838       </varlistentry>
 839
 840       <varlistentry>
 841         <term>
 842           <literal>license</literal>
 843           <indexterm><primary><literal>auto</literal></primary><secondary>package specification</secondary></indexterm>
 844         </term>
 845         <listitem>
 846           <para>(string) The type of license under which this package is distributed.
 847             This field is a value of the <ulink
 848         url="../libraries/Cabal/Distribution-License.html#t:License"><literal>License</literal></ulink> type.</para>
 849         </listitem>
 850       </varlistentry>
 851
 852         <varlistentry>
 853           <term>
 854             <literal>license-file</literal>
 855             <indexterm><primary><literal>license-file</literal></primary><secondary>package specification</secondary></indexterm>
 856           </term>
 857           <listitem>
 858             <para>(optional string) The name of a file giving detailed license
 859             information for this package.</para>
 860           </listitem>
 861         </varlistentry>
 862
 863         <varlistentry>
 864           <term>
 865             <literal>copyright</literal>
 866             <indexterm><primary><literal>copyright</literal></primary><secondary>package specification</secondary></indexterm>
 867           </term>
 868           <listitem>
 869             <para>(optional freeform) The copyright string.</para>
 870           </listitem>
 871         </varlistentry>
 872
 873         <varlistentry>
 874           <term>
 875             <literal>maintainer</literal>
 876             <indexterm><primary><literal>maintainer</literal></primary><secondary>package specification</secondary></indexterm>
 877           </term>
 878           <listitem>
 879             <para>(optinoal freeform) The email address of the package's maintainer.</para>
 880           </listitem>
 881         </varlistentry>
 882
 883         <varlistentry>
 884           <term>
 885             <literal>stability</literal>
 886             <indexterm><primary><literal>stability</literal></primary><secondary>package specification</secondary></indexterm>
 887           </term>
 888           <listitem>
 889             <para>(optional freeform) A string describing the stability of the package
 890             (eg. stable, provisional or experimental).</para>
 891           </listitem>
 892         </varlistentry>
 893
 894         <varlistentry>
 895           <term>
 896             <literal>homepage</literal>
 897             <indexterm><primary><literal>homepage</literal></primary><secondary>package specification</secondary></indexterm>
 898           </term>
 899           <listitem>
 900             <para>(optional freeform) URL of the package's home page.</para>
 901           </listitem>
 902         </varlistentry>
 903
 904       <varlistentry>
 905         <term>
 906             <literal>package-url</literal>
 907             <indexterm><primary><literal>package-url</literal></primary><secondary>package specification</secondary></indexterm>
 908           </term>
 909           <listitem>
 910             <para>(optional freeform) URL of a downloadable distribution for this
 911             package.  The distribution should be a Cabal package.</para>
 912           </listitem>
 913         </varlistentry>
 914
 915         <varlistentry>
 916           <term>
 917             <literal>description</literal>
 918             <indexterm><primary><literal>description</literal></primary><secondary>package specification</secondary></indexterm>
 919           </term>
 920           <listitem>
 921             <para>(optional freeform) Description of the package.</para>
 922           </listitem>
 923         </varlistentry>
 924
 925       <varlistentry>
 926           <term>
 927             <literal>category</literal>
 928             <indexterm><primary><literal>category</literal></primary><secondary>package specification</secondary></indexterm>
 929           </term>
 930           <listitem>
 931             <para>(optinoal freeform) Which category the package belongs to.  This field
 932             is for use in conjunction with a future centralised package
 933             distribution framework, tentatively titled Hackage.</para>
 934           </listitem>
 935         </varlistentry>
 936
 937         <varlistentry>
 938           <term>
 939             <literal>author</literal>
 940             <indexterm><primary><literal>author</literal></primary><secondary>package specification</secondary></indexterm>
 941           </term>
 942           <listitem>
 943             <para>(optional freeform) Author of the package.</para>
 944           </listitem>
 945         </varlistentry>
 946
 947         <varlistentry>
 948           <term>
 949             <literal>exposed</literal>
 950             <indexterm><primary><literal>exposed</literal></primary><secondary>package specification</secondary></indexterm>
 951           </term>
 952           <listitem>
 953             <para>(bool) Whether the package is exposed or not.</para>
 954           </listitem>
 955         </varlistentry>
 956
 957         <varlistentry>
 958           <term>
 959             <literal>exposed-modules</literal>
 960             <indexterm><primary><literal>exposed-modules</literal></primary><secondary>package specification</secondary></indexterm>
 961           </term>
 962           <listitem>
 963             <para>(string list) modules exposed by this package.</para>
 964           </listitem>
 965         </varlistentry>
 966
 967         <varlistentry>
 968           <term>
 969             <literal>hidden-modules</literal>
 970             <indexterm><primary><literal>hidden-modules</literal></primary><secondary>package specification</secondary></indexterm>
 971           </term>
 972           <listitem>
 973             <para>(string list) modules provided by this package,
 974             but not exposed to the programmer.  These modules cannot be
 975             imported, but they are still subject to the overlapping constraint:
 976             no other package in the same program may provide a module of the
 977             same name.</para>
 978         </listitem>
 979         </varlistentry>
 980
 981         <varlistentry>
 982           <term>
 983             <literal>import-dirs</literal>
 984             <indexterm><primary><literal>import-dirs</literal></primary><secondary>package specification</secondary></indexterm>
 985           </term>
 986           <listitem>
 987             <para>(string list) A list of directories containing interface files
 988             (<literal>.hi</literal> files) for this package.</para>
 989
 990             <para>If the package contains profiling libraries, then
 991             the interface files for those library modules should have
 992             the suffix <literal>.p_hi</literal>.  So the package can
 993             contain both normal and profiling versions of the same
 994             library without conflict (see also
 995             <literal>library_dirs</literal> below).</para>
 996           </listitem>
 997         </varlistentry>
 998
 999         <varlistentry>
1000           <term>
1001             <literal>library-dirs</literal>
1002             <indexterm><primary><literal>library-dirs</literal></primary><secondary>package specification</secondary></indexterm>
1003           </term>
1004           <listitem>
1005             <para>(string list) A list of directories containing libraries for this
1006             package.</para>
1007           </listitem>
1008         </varlistentry>
1009
1010         <varlistentry>
1011           <term>
1012             <literal>hs-libraries</literal>
1013             <indexterm><primary><literal>hs-libraries</literal></primary><secondary>package specification</secondary></indexterm>
1014           </term>
1015           <listitem>
1016             <para>(string list) A list of libraries containing Haskell code for this
1017             package, with the <literal>.a</literal> or
1018             <literal>.dll</literal> suffix omitted.  When packages are
1019             built as libraries, the
1020             <literal>lib</literal> prefix is also omitted.</para>
1021
1022             <para>For use with GHCi, each library should have an
1023             object file too.  The name of the object file does
1024             <emphasis>not</emphasis> have a <literal>lib</literal>
1025             prefix, and has the normal object suffix for your
1026             platform.</para>
1027
1028             <para>For example, if we specify a Haskell library as
1029             <filename>HSfoo</filename> in the package spec, then the
1030             various flavours of library that GHC actually uses will be
1031             called:</para>
1032             <variablelist>
1033               <varlistentry>
1034                 <term><filename>libHSfoo.a</filename></term>
1035                 <listitem>
1036                   <para>The name of the library on Unix and Windows
1037                   (mingw) systems.  Note that we don't support
1038                   building dynamic libraries of Haskell code on Unix
1039                   systems.</para>
1040                 </listitem>
1041               </varlistentry>
1042               <varlistentry>
1043                 <term><filename>HSfoo.dll</filename></term>
1044                 <listitem>
1045                   <para>The name of the dynamic library on Windows
1046                   systems (optional).</para>
1047                 </listitem>
1048               </varlistentry>
1049               <varlistentry>
1050                 <term><filename>HSfoo.o</filename></term>
1051                 <term><filename>HSfoo.obj</filename></term>
1052                 <listitem>
1053                   <para>The object version of the library used by
1054                   GHCi.</para>
1055                 </listitem>
1056               </varlistentry>
1057             </variablelist>
1058           </listitem>
1059         </varlistentry>
1060
1061         <varlistentry>
1062           <term>
1063             <literal>extra-libraries</literal>
1064             <indexterm><primary><literal>extra-libraries</literal></primary><secondary>package specification</secondary></indexterm>
1065           </term>
1066           <listitem>
1067             <para>(string list) A list of extra libraries for this package.  The
1068             difference between <literal>hs-libraries</literal> and
1069             <literal>extra-libraries</literal> is that
1070             <literal>hs-libraries</literal> normally have several
1071             versions, to support profiling, parallel and other build
1072             options.  The various versions are given different
1073             suffixes to distinguish them, for example the profiling
1074             version of the standard prelude library is named
1075             <filename>libHSbase_p.a</filename>, with the
1076             <literal>_p</literal> indicating that this is a profiling
1077             version.  The suffix is added automatically by GHC for
1078             <literal>hs-libraries</literal> only, no suffix is added
1079             for libraries in
1080             <literal>extra-libraries</literal>.</para>
1081
1082             <para>The libraries listed in
1083             <literal>extra-libraries</literal> may be any libraries
1084             supported by your system's linker, including dynamic
1085             libraries (<literal>.so</literal> on Unix,
1086             <literal>.DLL</literal> on Windows).</para>
1087
1088             <para>Also, <literal>extra-libraries</literal> are placed
1089             on the linker command line after the
1090             <literal>hs-libraries</literal> for the same package.  If
1091             your package has dependencies in the other direction (i.e.
1092             <literal>extra-libraries</literal> depends on
1093             <literal>hs-libraries</literal>), and the libraries are
1094             static, you might need to make two separate
1095             packages.</para>
1096           </listitem>
1097         </varlistentry>
1098
1099         <varlistentry>
1100           <term>
1101             <literal>include-dirs</literal>
1102             <indexterm><primary><literal>include-dirs</literal></primary><secondary>package specification</secondary></indexterm>
1103           </term>
1104           <listitem>
1105             <para>(string list) A list of directories containing C includes for this
1106             package.</para>
1107           </listitem>
1108         </varlistentry>
1109
1110         <varlistentry>
1111           <term>
1112            <literal>includes</literal>
1113            <indexterm><primary><literal>includes</literal></primary><secondary>package specification</secondary></indexterm>
1114           </term>
1115           <listitem>
1116             <para>(string list) A list of files to include for via-C compilations
1117             using this package.  Typically the include file(s) will
1118             contain function prototypes for any C functions used in
1119             the package, in case they end up being called as a result
1120             of Haskell functions from the package being
1121             inlined.</para>
1122           </listitem>
1123         </varlistentry>
1124
1125         <varlistentry>
1126           <term>
1127             <literal>depends</literal>
1128             <indexterm><primary><literal>depends</literal></primary><secondary>package specification</secondary></indexterm>
1129           </term>
1130           <listitem>
1131             <para>(package name list) Packages on which this package depends.  This field contains
1132             packages with explicit versions are required, except that when
1133             submitting a package to <literal>ghc-pkg register</literal>, the
1134             versions will be filled in if they are unambiguous.</para>
1135           </listitem>
1136         </varlistentry>
1137
1138         <varlistentry>
1139           <term>
1140             <literal>hugs-options</literal>
1141             <indexterm><primary><literal>hugs-options</literal></primary><secondary>package specification</secondary></indexterm>
1142           </term>
1143           <listitem>
1144             <para>(string list) Options to pass to Hugs for this package.</para>
1145           </listitem>
1146         </varlistentry>
1147
1148         <varlistentry>
1149           <term>
1150             <literal>cc-options</literal>
1151             <indexterm><primary><literal>cc-options</literal></primary><secondary>package specification</secondary></indexterm>
1152           </term>
1153           <listitem>
1154             <para>(string list) Extra arguments to be added to the gcc command line
1155             when this package is being used (only for via-C
1156             compilations).</para>
1157           </listitem>
1158         </varlistentry>
1159
1160         <varlistentry>
1161           <term>
1162             <literal>ld-options</literal>
1163             <indexterm><primary><literal>ld-options</literal></primary><secondary>package specification</secondary></indexterm>
1164           </term>
1165           <listitem>
1166             <para>(string list) Extra arguments to be added to the
1167             <command>gcc</command> command line (for linking) when
1168             this package is being used.</para>
1169           </listitem>
1170         </varlistentry>
1171
1172         <varlistentry>
1173           <term>
1174             <literal>framework-dirs</literal>
1175             <indexterm><primary><literal>framework-dirs</literal></primary><secondary>package specification</secondary></indexterm>
1176           </term>
1177           <listitem>
1178             <para>(string list) On Darwin/MacOS X, a list of directories containing
1179             frameworks for this package. This corresponds to the
1180             <option>-framework-path</option> option. It is ignored on all other
1181             platforms.</para>
1182           </listitem>
1183         </varlistentry>
1184
1185         <varlistentry>
1186           <term>
1187             <literal>frameworks</literal>
1188             <indexterm><primary><literal>frameworks</literal></primary><secondary>package specification</secondary></indexterm>
1189           </term>
1190           <listitem>
1191             <para>(string list) On Darwin/MacOS X, a list of frameworks to link to. This
1192             corresponds to the <option>-framework</option> option. Take a look
1193             at Apple's developer documentation to find out what frameworks
1194             actually are. This entry is ignored on all other platforms.</para>
1195           </listitem>
1196         </varlistentry>
1197
1198         <varlistentry>
1199           <term>
1200             <literal>haddock-interfaces</literal>
1201             <indexterm><primary><literal>haddock-interfaces</literal></primary><secondary>package specification</secondary></indexterm>
1202           </term>
1203           <listitem>
1204             <para>(string list) A list of filenames containing <ulink
1205               url="http://www.haskell.org/haddock/">Haddock</ulink> interface
1206             files (<literal>.haddock</literal> files) for this package.</para>
1207           </listitem>
1208         </varlistentry>
1209
1210         <varlistentry>
1211           <term>
1212             <literal>haddock-html</literal>
1213             <indexterm><primary><literal>haddock-html</literal></primary><secondary>package specification</secondary></indexterm>
1214           </term>
1215           <listitem>
1216             <para>(optional string) The directory containing the Haddock-generated HTML
1217             for this package.</para>
1218           </listitem>
1219         </varlistentry>
1220       </variablelist>
1221
1222 <!--  This isn't true any more.  I'm not sure if we still need it -SDM
1223       <para>
1224       The <literal>ghc-pkg</literal> tool performs expansion of
1225       environment variables occurring in input package specifications.
1226       So, if the <literal>mypkg</literal> was added to the package
1227       database as follows:
1228       </para>
1229 <screen>
1230   $ installdir=/usr/local/lib ghc-pkg -a &lt; mypkg.pkg
1231 </screen>
1232
1233       <para>
1234       The occurrence of <literal>${installdir}</literal> is replaced
1235       with <literal>/usr/local/lib</literal> in the package data that
1236       is added for <literal>mypkg</literal>.
1237       </para>
1238
1239       <para>
1240       This feature enables the distribution of package specification
1241       files that can be easily configured when installing.
1242       </para>
1243
1244       <para>For examples of more package specifications, take a look
1245       at the <literal>package.conf</literal> in your GHC
1246       installation.</para>
1247
1248 -->
1249
1250     </sect2>
1251   </sect1>
1252
1253 <!-- Emacs stuff:
1254      ;;; Local Variables: ***
1255      ;;; mode: xml ***
1256      ;;; sgml-parent-document: ("users_guide.xml" "book" "chapter" "sect1") ***
1257      ;;; End: ***
1258  -->