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 archive 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> and load that. The
124 <literal>ghc-pkg</literal> tool can automatically build the
125 GHCi version of each library, see <xref
126 linkend="package-management">. To build these libraries by
127 hand from the <literal>.a</literal> archive, it is possible
128 to use GNU <command>ld</command> as follows:</para>
130 <screen>ld -r --whole-archive -o HSfoo.o libHSfoo.a</screen>
134 <para>GHC does not maintain detailed cross-package
135 dependency information. It does remember which modules in
136 other packages the current module depends on, but not which
137 things within those imported things.</para>
141 <para>To compile a module which is to be part of a new package,
142 use the <literal>-package-name</literal> option:</para>
146 <term><option>-package-name <replaceable>foo</replaceable></option></term>
147 <indexterm><primary><literal>-package-name</literal></primary>
148 <secondary>option</secondary></indexterm>
150 <para>This option is added to the command line when
151 compiling a module that is destined to be part of package
152 <literal>foo</literal>. If this flag is omitted then the
153 default package <literal>Main</literal> is assumed.</para>
158 <para>Failure to use the <literal>-package-name</literal> option
159 when compiling a package will result in disaster on Windows, but
160 is relatively harmless on Unix at the moment (it will just cause
161 a few extra dependencies in some interface files). However,
162 bear in mind that we might add support for Unix shared libraries
163 at some point in the future.</para>
165 <para>It is worth noting that on Windows, when each package
166 is built as a DLL, since a reference to a DLL costs an extra
167 indirection, intra-package references are cheaper than
168 inter-package references. Of course, this applies to the
169 <filename>Main</filename> package as well.</para>
172 <sect2 id="package-management">
173 <title>Package management</title>
174 <indexterm><primary>packages</primary>
175 <secondary>management</secondary></indexterm>
177 <para>The <literal>ghc-pkg</literal> tool allows packages to be
178 added or removed from a package configuration file. By default,
179 the system-wide configuration file is used, but alternatively
180 packages can be added or removed from a user-specified
181 configuration file using the <option>--config-file</option>
182 option. An empty package configuration file consists of the
183 string <quote><literal>[]</literal></quote>.</para>
185 <para>The <literal>ghc-pkg</literal> program accepts the
186 following options:</para>
190 <term><option>--add-package</option></term>
191 <term><option>-a</option></term>
192 <indexterm><primary><option>--add-package</option></primary>
195 <para>Reads a package specification (see below) on stdin,
196 and adds it to the database of installed packages. The
197 package specification must be a package that isn't already
203 <term><option>--auto-ghci-libs</option></term>
204 <term><option>-g</option></term>
205 <indexterm><primary><option>--auto-ghci-libs</option></primary>
208 <para>Automatically generate the GHCi
209 <filename>.o</filename> version of each
210 <filename>.a</filename> Haskell library, using GNU ld (if
211 that is available). Without this option,
212 <literal>ghc-pkg</literal> will warn if GHCi versions of
213 any Haskell libraries in the package don't exist.</para>
215 <para>GHCi <literal>.o</literal> libraries don't
216 necessarily have to live in the same directory as the
217 corresponding <literal>.a</literal> library. However,
218 this option will cause the GHCi library to be created in
219 the same directory as the <literal>.a</literal>
225 <term><option>--config-file <replaceable>file</replaceable></option></term>
226 <term><option>-f <replaceable>file</replaceable></option></term>
227 <indexterm><primary><option>--config-file</option></primary>
230 <para>Use <replaceable>file</replaceable> instead of the
231 default package configuration file. This, in conjunction
232 with GHC's <option>-package-conf</option> option, allows
233 a user to have a local set of packages in addition to the
234 system-wide installed set.</para>
239 <term><option>--list-packages</option></term>
240 <term><option>-l</option></term>
241 <indexterm><primary><option>--list-packages</option></primary></indexterm>
243 <para>This option displays the list of currently installed
247 $ ghc-pkg --list-packages
248 gmp, rts, std, lang, concurrent, data, net, posix, text, util
251 <para>Note that your GHC installation might have a
252 slightly different set of packages installed.</para>
254 <para>The <literal>gmp</literal> and
255 <literal>rts</literal> packages are always present, and
256 represent the multi-precision integer and runtime system
257 libraries respectively. The <literal>std</literal>
258 package contains the Haskell prelude and standard
259 libraries. The rest of the packages are optional
265 <term><option>--remove-package <replaceable>foo</replaceable></option></term>
266 <term><option>-r <replaceable>foo</replaceable></option></term>
267 <indexterm><primary><option>--delete-package</option></primary>
270 <para>Removes the specified package from the installed
271 configuration.</para>
276 <para>When modifying the configuration file
277 <replaceable>file</replaceable>, a copy of the original file is
278 saved in <replaceable>file</replaceable><literal>.old</literal>,
279 so in an emergency you can always restore the old settings by
280 copying the old file back again.</para>
282 <para>A package specification looks like this:</para>
287 import_dirs = ["/usr/local/lib/imports/mypkg"],
289 library_dirs = ["/usr/local/lib"],
290 hs_libraries = ["HSmypkg" ],
291 extra_libraries = ["HSmypkg_cbits"],
293 c_includes = ["HsMyPkg.h"],
294 package_deps = ["text", "data"],
297 extra_ld_opts = ["-lmy_clib"]
301 <para>Components of a package specification may be specified in
302 any order, and are:</para>
306 <term><literal>name</literal></term>
307 <indexterm><primary><literal>name</literal></primary>
308 <secondary>package specification</secondary></indexterm>
310 <para>The package's name, for use with
311 the <literal>-package</literal> flag and as listed in the
312 <literal>--list-packages</literal> list.
318 <term><literal>import_dirs</literal></term>
319 <indexterm><primary><literal>import_dirs</literal></primary>
320 <secondary>package specification</secondary></indexterm>
322 <para>A list of directories containing interface files
323 (<literal>.hi</literal> files) for this package.</para>
328 <term><literal>source_dirs</literal></term>
329 <indexterm><primary><literal>source_dirs</literal></primary>
330 <secondary>package specification</secondary></indexterm>
332 <para>A list of directories containing Haskell source
333 files for this package. This field isn't used by GHC, but
334 could potentially be used by an all-interpreted system
340 <term><literal>library_dirs</literal></term>
341 <indexterm><primary><literal>library_dirs</literal></primary>
342 <secondary>package specification</secondary></indexterm>
344 <para>A list of directories containing libraries for this
350 <term><literal>hs_libraries</literal></term>
351 <indexterm><primary><literal>hs_libraries</literal></primary>
352 <secondary>package specification</secondary></indexterm>
354 <para>A list of libraries containing Haskell code for this
355 package, with the <literal>.a</literal> or
356 <literal>.dll</literal> suffix omitted. When packages are
357 built as libraries, the
358 <literal>lib</literal> prefix is also omitted.</para>
360 <para>For use with GHCi, each library should have an
361 object file too. The name of the object file does
362 <emphasis>not</emphasis> have a <literal>lib</literal>
363 prefix, and has the normal object suffix for your
366 <para>For example, if we specify a Haskell library as
367 <filename>HSfoo</filename> in the package spec, then the
368 various flavours of library that GHC actually uses will be
372 <term><filename>libHSfoo.a</filename></term>
374 <para>The name of the library on Unix
379 <term><filename>HSfoo.dll</filename></term>
381 <para>The name of the dynamic library on Windows
386 <term><filename>HSfoo.o</filename></term>
387 <term><filename>HSfoo.obj</filename></term>
389 <para>The object version of the library used by
399 <term><literal>extra_libraries</literal></term>
400 <indexterm><primary><literal>extra_libraries</literal></primary>
401 <secondary>package specification</secondary></indexterm>
403 <para>A list of extra libraries for this package. The
404 difference between <literal>hs_libraries</literal> and
405 <literal>extra_libraries</literal> is that
406 <literal>hs_libraries</literal> normally have several
407 versions, to support profiling, parallel and other build
408 options. The various versions are given different
409 suffixes to distinguish them, for example the profiling
410 version of the standard prelude library is named
411 <filename>libHSstd_p.a</filename>, with the
412 <literal>_p</literal> indicating that this is a profiling
413 version. The suffix is added automatically by GHC for
414 <literal>hs_libraries</literal> only, no suffix is added
416 <literal>extra_libraries</literal>.</para>
418 <para>Also, <literal>extra_libraries</literal> are placed
419 on the linker command line after the
420 <literal>hs_libraries</literal> for the same package. If
421 your package has dependencies in the other direction (i.e.
422 <literal>extra_libraries</literal> depends on
423 <literal>hs_libraries</literal>), and the libraries are
424 static, you might need to make two separate
430 <term><literal>include_dirs</literal></term>
431 <indexterm><primary><literal>include_dirs</literal></primary>
432 <secondary>package specification</secondary></indexterm>
434 <para>A list of directories containing C includes for this
435 package (maybe the empty list).</para>
440 <term><literal>c_includes</literal></term>
441 <indexterm><primary><literal>c_includes</literal></primary>
442 <secondary>package specification</secondary></indexterm>
444 <para>A list of files to include for via-C compilations
445 using this package. Typically this include file will
446 contain function prototypes for any C functions used in
447 the package, in case they end up being called as a result
448 of Haskell functions from the package being
454 <term><literal>package_deps</literal></term>
455 <indexterm><primary><literal>package_deps</literal></primary>
456 <secondary>package specification</secondary></indexterm>
458 <para>A list of packages which this package depends
464 <term><literal>extra_ghc_opts</literal></term>
465 <indexterm><primary><literal>extra_ghc_opts</literal></primary>
466 <secondary>package specification</secondary></indexterm>
468 <para>Extra arguments to be added to the GHC command line
469 when this package is being used.</para>
474 <term><literal>extra_cc_opts</literal></term>
475 <indexterm><primary><literal>extra_cc_opts</literal></primary>
476 <secondary>package specification</secondary></indexterm>
478 <para>Extra arguments to be added to the gcc command line
479 when this package is being used (only for via-C
480 compilations).</para>
485 <term><literal>extra_ld_opts</literal></term>
486 <indexterm><primary><literal>extra_ld_opts</literal></primary>
487 <secondary>package specification</secondary></indexterm>
489 <para>Extra arguments to be added to the gcc command line
490 (for linking) when this package is being used.</para>
495 <para>For examples of more package specifications, take a look
496 at the <literal>package.conf</literal> in your GHC
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