2 <title>Packages</title>
3 <indexterm><primary>packages</primary></indexterm>
5 <para>Packages are collections of libraries, conveniently grouped
6 together as a single entity. The package system is flexible: a
7 package may consist of Haskell code, foreign language code (eg. C
8 libraries), or a mixture of the two. A package is a good way to
9 group together related Haskell modules, and is essential if you
10 intend to make the modules into a Windows DLL (see below).</para>
12 <para>Because packages can contain both Haskell and C libraries, they
13 are also a good way to provide convenient access to a Haskell
14 layer over a C library.</para>
16 <para>GHC comes with several packages (see <xref
17 linkend="book-hslibs">), and packages can be added to or removed
18 from an existing GHC installation, using the supplied
19 <literal>ghc-pkg</literal><indexterm><primary><literal>ghc-pkg</literal></primary>
20 </indexterm> tool, described in <xref
21 linkend="package-management">.</para>
23 <sect2 id="using-packages">
24 <title>Using a package</title>
25 <indexterm><primary>packages</primary>
26 <secondary>using</secondary></indexterm>
28 <para>To use a package, add the <literal>-package</literal> flag
29 to the GHC command line:</para>
33 <term><option>-package <replaceable>lib</replaceable></option></term>
34 <indexterm><primary>-package <replaceable>lib</replaceable> option</primary></indexterm>
36 <para>This option brings into scope all the modules from
37 package <literal><replaceable>lib</replaceable></literal> (they still have to
38 be imported in your Haskell source, however). It also
39 causes the relevant libraries to be linked when linking is
45 <para>Some packages depend on other packages, for example the
46 <literal>text</literal> package makes use of some of the modules
47 in the <literal>lang</literal> package. The package system
48 takes care of all these dependencies, so that when you say
49 <literal>-package text</literal> on the command line, you
50 automatically get <literal>-package lang</literal> too.</para>
53 <sect2 id="using-local-packages">
54 <title>Maintaining a local set of packages</title>
56 <para>When GHC starts up, it automatically reads the default set
57 of packages from a configuration file, normally named
58 <filename>package.conf</filename> in your GHC installation
61 <para>You can load in additional package configuration files
62 using the <option>-package-conf</option> option:</para>
66 <term><option>-package-conf <replaceable>file</replaceable></option></term>
67 <indexterm><primary><option>-package-conf <replaceable>file</replaceable></option></primary>
70 <para>Read in the package configuration file
71 <replaceable>file</replaceable> in addition to the system
72 default file. This allows the user to have a local set of
73 packages in addition to the system-wide ones.</para>
78 <para>To create your own package configuration file, just create
79 a new file and put the string
80 <quote><literal>[]</literal></quote> in it. Packages can be
81 added to the new configuration file using the
82 <literal>ghc-pkg</literal> tool, described in <xref
83 linkend="package-management">.</para>
86 <sect2 id="building-packages">
87 <title>Building a package from Haskell source</title>
88 <indexterm><primary>packages</primary>
89 <secondary>building</secondary></indexterm>
91 <para>It takes some special considerations to build a new
96 <para>A package may contain several Haskell modules. A
97 package may span many directories, or many packages may
98 exist in a single directory. Packages may not be mutually
103 <para>A package has a name
104 (e.g. <filename>std</filename>)</para>
108 <para>The Haskell code in a package may be built into one or
109 more Unix libraries (e.g. <filename>libHSfoo.a</filename>),
110 or a single DLL on Windows
111 (e.g. <filename>HSfoo.dll</filename>). The restriction to a
112 single DLL on Windows is that the package system is used to
113 tell the compiler when it should make an inter-DLL call
114 rather than an intra-DLL call (inter-DLL calls require an
115 extra indirection). <emphasis>Building packages as DLLs
116 doesn't work at the moment; see <XRef
117 LinkEnd="win32-dlls-create"> for the gory details.</emphasis>
120 <para>Versions of the Haskell libraries for use with GHCi
121 may also be included: GHCi cannot load <literal>.a</literal>
122 files directly, instead it will look for an object file
123 called <filename>HSfoo.o</filename> (the object suffix
124 varies between platforms, as usual) and load that. An
125 object file can be built from a <literal>.a</literal>
126 archive as follows (using GNU <command>ld</command> on
130 ld -r --whole-archive -o HSfoo.o libHSfoo.a
135 <para>GHC does not maintain detailed cross-package
136 dependency information. It does remember which modules in
137 other packages the current module depends on, but not which
138 things within those imported things.</para>
142 <para>To compile a module which is to be part of a new package,
143 use the <literal>-package-name</literal> option:</para>
147 <term><option>-package-name <replaceable>foo</replaceable></option></term>
148 <indexterm><primary><literal>-package-name</literal></primary>
149 <secondary>option</secondary></indexterm>
151 <para>This option is added to the command line when
152 compiling a module that is destined to be part of package
153 <literal>foo</literal>. If this flag is omitted then the
154 default package <literal>Main</literal> is assumed.</para>
159 <para>Failure to use the <literal>-package-name</literal> option
160 when compiling a package will result in disaster on Windows, but
161 is relatively harmless on Unix at the moment (it will just cause
162 a few extra dependencies in some interface files). However,
163 bear in mind that we might add support for Unix shared libraries
164 at some point in the future.</para>
166 <para>It is worth noting that on Windows, when each package
167 is built as a DLL, since a reference to a DLL costs an extra
168 indirection, intra-package references are cheaper than
169 inter-package references. Of course, this applies to the
170 <filename>Main</filename> package as well.</para>
173 <sect2 id="package-management">
174 <title>Package management</title>
175 <indexterm><primary>packages</primary>
176 <secondary>management</secondary></indexterm>
178 <para>The <literal>ghc-pkg</literal> tool allows packages to be
179 added or removed from a package configuration file. By default,
180 the system-wide configuration file is used, but alternatively
181 packages can be added or removed from a user-specified
182 configuration file using the <option>--config-file</option>
183 option. An empty package configuration file consists of the
184 string <quote><literal>[]</literal></quote>.</para>
186 <para>The <literal>ghc-pkg</literal> program accepts the
187 following options:</para>
191 <term><option>--add-package</option></term>
192 <term><option>-a</option></term>
193 <indexterm><primary><option>--add-package</option></primary>
196 <para>Reads a package specification (see below) on stdin,
197 and adds it to the database of installed packages. The
198 package specification must be a package that isn't already
204 <term><option>--config-file <replaceable>file</replaceable></option></term>
205 <term><option>-f <replaceable>file</replaceable></option></term>
206 <indexterm><primary><option>--config-file</option></primary>
209 <para>Use <replaceable>file</replaceable> instead of the
210 default package configuration file. This, in conjunction
211 with GHC's <option>-package-conf</option> option, allows
212 a user to have a local set of packages in addition to the
213 system-wide installed set.</para>
218 <term><option>--list-packages</option></term>
219 <term><option>-l</option></term>
220 <indexterm><primary><option>--list-packages</option></primary></indexterm>
222 <para>This option displays the list of currently installed
226 $ ghc-pkg --list-packages
227 gmp, rts, std, lang, concurrent, data, net, posix, text, util
230 <para>Note that your GHC installation might have a
231 slightly different set of packages installed.</para>
233 <para>The <literal>gmp</literal> and
234 <literal>rts</literal> packages are always present, and
235 represent the multi-precision integer and runtime system
236 libraries respectively. The <literal>std</literal>
237 package contains the Haskell prelude and standard
238 libraries. The rest of the packages are optional
244 <term><option>--remove-package <replaceable>foo</replaceable></option></term>
245 <term><option>-r <replaceable>foo</replaceable></option></term>
246 <indexterm><primary><option>--delete-package</option></primary>
249 <para>Removes the specified package from the installed
250 configuration.</para>
255 <para>When modifying the configuration file
256 <replaceable>file</replaceable>, a copy of the original file is
257 saved in <replaceable>file</replaceable><literal>.old</literal>,
258 so in an emergency you can always restore the old settings by
259 copying the old file back again.</para>
261 <para>A package specification looks like this:</para>
266 import_dirs = ["/usr/local/lib/imports/mypkg"],
268 library_dirs = ["/usr/local/lib"],
269 hs_libraries = ["HSmypkg" ],
270 extra_libraries = ["HSmypkg_cbits"],
272 c_includes = ["HsMyPkg.h"],
273 package_deps = ["text", "data"],
276 extra_ld_opts = ["-lmy_clib"]
280 <para>Components of a package specification may be specified in
281 any order, and are:</para>
285 <term><literal>name</literal></term>
286 <indexterm><primary><literal>name</literal></primary>
287 <secondary>package specification</secondary></indexterm>
289 <para>The package's name, for use with
290 the <literal>-package</literal> flag and as listed in the
291 <literal>--list-packages</literal> list.
297 <term><literal>import_dirs</literal></term>
298 <indexterm><primary><literal>import_dirs</literal></primary>
299 <secondary>package specification</secondary></indexterm>
301 <para>A list of directories containing interface files
302 (<literal>.hi</literal> files) for this package.</para>
307 <term><literal>source_dirs</literal></term>
308 <indexterm><primary><literal>source_dirs</literal></primary>
309 <secondary>package specification</secondary></indexterm>
311 <para>A list of directories containing Haskell source
312 files for this package. This field isn't used by GHC, but
313 could potentially be used by an all-interpreted system
319 <term><literal>library_dirs</literal></term>
320 <indexterm><primary><literal>library_dirs</literal></primary>
321 <secondary>package specification</secondary></indexterm>
323 <para>A list of directories containing libraries for this
329 <term><literal>hs_libraries</literal></term>
330 <indexterm><primary><literal>hs_libraries</literal></primary>
331 <secondary>package specification</secondary></indexterm>
333 <para>A list of libraries containing Haskell code for this
334 package, with the <literal>.a</literal> or
335 <literal>.dll</literal> suffix omitted. When packages are
336 built as libraries, the
337 <literal>lib</literal> prefix is also omitted.</para>
339 <para>For use with GHCi, each library should have an
340 object file too. The name of the object file does
341 <emphasis>not</emphasis> have a <literal>lib</literal>
342 prefix, and has the normal object suffix for your
345 <para>For example, if we specify a Haskell library as
346 <filename>HSfoo</filename> in the package spec, then the
347 various flavours of library that GHC actually uses will be
351 <term><filename>libHSfoo.a</filename></term>
353 <para>The name of the library on Unix
358 <term><filename>HSfoo.dll</filename></term>
360 <para>The name of the dynamic library on Windows
365 <term><filename>HSfoo.o</filename></term>
366 <term><filename>HSfoo.obj</filename></term>
368 <para>The object version of the library used by
378 <term><literal>extra_libraries</literal></term>
379 <indexterm><primary><literal>extra_libraries</literal></primary>
380 <secondary>package specification</secondary></indexterm>
382 <para>A list of extra libraries for this package. The
383 difference between <literal>hs_libraries</literal> and
384 <literal>extra_libraries</literal> is that
385 <literal>hs_libraries</literal> normally have several
386 versions, to support profiling, parallel and other build
387 options. The various versions are given different
388 suffixes to distinguish them, for example the profiling
389 version of the standard prelude library is named
390 <filename>libHSstd_p.a</filename>, with the
391 <literal>_p</literal> indicating that this is a profiling
392 version. The suffix is added automatically by GHC for
393 <literal>hs_libraries</literal> only, no suffix is added
395 <literal>extra_libraries</literal>.</para>
397 <para>Also, <literal>extra_libraries</literal> are placed
398 on the linker command line after the
399 <literal>hs_libraries</literal> for the same package. If
400 your package has dependencies in the other direction (i.e.
401 <literal>extra_libraries</literal> depends on
402 <literal>hs_libraries</literal>), and the libraries are
403 static, you might need to make two separate
409 <term><literal>include_dirs</literal></term>
410 <indexterm><primary><literal>include_dirs</literal></primary>
411 <secondary>package specification</secondary></indexterm>
413 <para>A list of directories containing C includes for this
414 package (maybe the empty list).</para>
419 <term><literal>c_includes</literal></term>
420 <indexterm><primary><literal>c_includes</literal></primary>
421 <secondary>package specification</secondary></indexterm>
423 <para>A list of files to include for via-C compilations
424 using this package. Typically this include file will
425 contain function prototypes for any C functions used in
426 the package, in case they end up being called as a result
427 of Haskell functions from the package being
433 <term><literal>package_deps</literal></term>
434 <indexterm><primary><literal>package_deps</literal></primary>
435 <secondary>package specification</secondary></indexterm>
437 <para>A list of packages which this package depends
443 <term><literal>extra_ghc_opts</literal></term>
444 <indexterm><primary><literal>extra_ghc_opts</literal></primary>
445 <secondary>package specification</secondary></indexterm>
447 <para>Extra arguments to be added to the GHC command line
448 when this package is being used.</para>
453 <term><literal>extra_cc_opts</literal></term>
454 <indexterm><primary><literal>extra_cc_opts</literal></primary>
455 <secondary>package specification</secondary></indexterm>
457 <para>Extra arguments to be added to the gcc command line
458 when this package is being used (only for via-C
459 compilations).</para>
464 <term><literal>extra_ld_opts</literal></term>
465 <indexterm><primary><literal>extra_ld_opts</literal></primary>
466 <secondary>package specification</secondary></indexterm>
468 <para>Extra arguments to be added to the gcc command line
469 (for linking) when this package is being used.</para>
474 <para>For examples of more package specifications, take a look
475 at the <literal>package.conf</literal> in your GHC
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