1 <chapter id="introduction-GHC">
2 <title>Introduction to GHC</title>
4 <para>This is a guide to using the Glasgow Haskell Compiler (GHC):
5 an interactive and batch compilation system for the <ulink
6 url="http://www.haskell.org/">Haskell 98</ulink>
9 <para>GHC has two main components: an interactive Haskell
10 interpreter (also known as GHCi), described in <xref
11 linkend="ghci">, and a batch compiler, described throughout <xref
12 linkend="using-ghc">. In fact, GHC consists of a single program
13 which is just run with different options to provide either the
14 interactive or the batch system.</para>
16 <para>The batch compiler can be used alongside GHCi: compiled
17 modules can be loaded into an interactive session and used in the
18 same way as interpreted code, and in fact when using GHCi most of
19 the library code will be pre-compiled. This means you get the best
20 of both worlds: fast pre-compiled library code, and fast compile
21 turnaround for the parts of your program being actively
24 <para>GHC supports numerous language extensions, including
25 concurrency, a foreign function interface, exceptions, type system
26 extensions such as multi-parameter type classes, local universal and
27 existential quantification, functional dependencies, scoped type
28 variables and explicit unboxed types. These are all described in
29 <xref linkend="ghc-language-features">.</para>
31 <para>GHC has a comprehensive optimiser, so when you want to Really
32 Go For It (and you've got time to spare) GHC can produce pretty fast
33 code. Alternatively, the default option is to compile as fast as
34 possible while not making too much effort to optimise the generated
35 code (although GHC probably isn't what you'd describe as a fast
38 <para>GHC's profiling system supports “cost centre
39 stacks”: a way of seeing the profile of a Haskell program in a
40 call-graph like structure. See <xref linkend="profiling"> for more
43 <para>GHC comes with a large collection of libraries, with
44 everything from parser combinators to networking. These are all
45 described in <xref linkend="book-hslibs">.</para>
47 <Sect1 id="mailing-lists-GHC">
48 <Title>Meta-information: Web sites, mailing lists, etc.</Title>
50 <IndexTerm><Primary>mailing lists, Glasgow Haskell</Primary></IndexTerm>
51 <IndexTerm><Primary>Glasgow Haskell mailing lists</Primary></IndexTerm>
53 <para>On the World-Wide Web, there are several URLs of likely
58 <para><ulink URL="http://www.haskell.org/" >Haskell home
63 <para><ulink URL="http://www.haskell.org/ghc/">GHC home
69 URL="http://www.cs.nott.ac.uk/Department/Staff/mpj/faq.html">comp.lang.functional
75 <para>We run the following mailing lists about Glasgow Haskell.
76 We encourage you to join, as you feel is appropriate.</para>
80 <term>glasgow-haskell-users:</term>
82 <para>This list is for GHC users to chat among
87 <term>list email address:</term>
89 <para><email>glasgow-haskell-users@haskell.org</email></para>
94 <term>subscribe at:</term>
97 url="http://www.haskell.org/mailman/listinfo/glasgow-haskell-users"><literal>http://www.haskell.org/mailman/listinfo/glasgow-haskell-users</literal></ulink>.</para>
102 <term>admin email address:</term>
104 <para><email>glasgow-haskell-users-admin@haskell.org</email></para>
109 <term>list archives:</term>
112 url="http://www.haskell.org/pipermail/glasgow-haskell-users/"><literal>http://www.haskell.org/pipermail/glasgow-haskell-users/</literal></ulink></para>
120 <term>glasgow-haskell-bugs:</term>
122 <para>Send bug reports for GHC to this address! The sad and
123 lonely people who subscribe to this list will muse upon
124 what's wrong and what you might do about it.</para>
128 <term>list email address:</term>
130 <para><email>glasgow-haskell-bugs@haskell.org</email></para>
135 <term>subscribe at:</term>
138 url="http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs"><literal>http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs</literal></ulink>.</para>
143 <term>admin email address:</term>
145 <para><email>glasgow-haskell-bugs-admin@haskell.org</email></para>
150 <term>list archives:</term>
153 url="http://www.haskell.org/pipermail/glasgow-haskell-bugs/"><literal>http://www.haskell.org/pipermail/glasgow-haskell-bugs/</literal></ulink></para>
161 <term>cvs-ghc:</term>
163 <para>The hardcore GHC developers hang out here. This list
164 also gets commit message from the CVS repository. There are
165 several other similar lists for other parts of the CVS
166 repository (eg. <literal>cvs-hslibs</literal>,
167 <literal>cvs-happy</literal>, <literal>cvs-hdirect</literal>
172 <term>list email address:</term>
174 <para><email>cvs-ghc@haskell.org</email></para>
179 <term>subscribe at:</term>
182 url="http://www.haskell.org/mailman/listinfo/cvs-ghc"><literal>http://www.haskell.org/mailman/listinfo/cvs-ghc</literal></ulink>.</para>
187 <term>admin email address:</term>
189 <para><email>cvs-ghc-admin@haskell.org</email></para>
194 <term>list archives:</term>
197 url="http://www.haskell.org/pipermail/cvs-ghc/"><literal>http://www.haskell.org/pipermail/cvs-ghc/</literal></ulink></para>
205 <para>There are several other haskell and GHC-related mailing
206 lists served by <literal>www.haskell.org</literal>. Go to <ulink
207 url="http://www.haskell.org/mailman/listinfo/"><literal>http://www.haskell.org/mailman/listinfo/</literal></ulink>
208 for the full list.</para>
210 <para>Some Haskell-related discussion also takes place in the
211 Usenet newsgroup <literal>comp.lang.functional</literal>.</para>
215 <sect1 id="bug-reporting">
216 <title>Reporting bugs in GHC</title>
217 <indexterm><primary>bugs</primary><secondary>reporting</secondary>
219 <indexterm><primary>reporting bugs</primary>
222 <para>Glasgow Haskell is a changing system so there are sure to be
225 <para>To repot a bug, either:</para>
229 <para>Go to the <ulink
230 url="http://sourceforge.net/projects/ghc/">SoureForge GHC
231 page</ulink>, go to the <quote>bugs</quote> section, click on
232 <quote>submit</quote>, and enter your bug report. You can also
233 check the outstanding bugs here to make sure it hasn't already
234 been reported. Or:</para>
237 <para>Email your bug report to
238 <Email>glasgow-haskell-bugs@haskell.org</Email>. </para>
243 <title>What to put in a bug report</title>
244 <indexterm><primary>bug reports</primary><secondary>contents</secondary></indexterm>
246 <para>The name of the bug-reporting game is: facts, facts,
247 facts. Don't omit them because “Oh, they won't be
248 interested…”</para>
252 <para>What kind of machine are you running on, and exactly
253 what version of the operating system are you using?
254 (on a Unix system, <Command>uname -a</Command> or <Command>cat
255 /etc/motd</Command> will show the desired information.)</para>
259 <para> What version of GCC are you using? <Command>gcc
260 -v</Command> will tell you.</para>
264 <para> Run the sequence of compiles/runs that caused the
265 offending behaviour, capturing all the input/output in a
266 “script” (a UNIX command) or in an Emacs shell
267 window. We'd prefer to see the whole thing.</para>
271 <para> Be sure any Haskell compilations are run with a
272 <Option>-v</Option> (verbose) flag, so we can see exactly
273 what was run, what versions of things you have, etc.</para>
277 <para> What is the program behaviour that is wrong, in your
282 <para> If practical, please send enough source files for us
283 to duplicate the problem.</para>
287 <para> If you are a Hero and track down the problem in the
288 compilation-system sources, please send us patches relative
289 to a known released version of GHC, or whole files if you
296 <sect1 id="version-numbering">
297 <title>GHC version numbering policy</title>
298 <indexterm><primary>version, of ghc</primary></indexterm>
300 <para>As of GHC version 4.08, we have adopted the following
301 policy for numbering GHC versions:</para>
305 <term>Stable Releases</term>
307 <para>These are numbered <literal>x.yy.z</literal>, where
308 <literal>yy</literal> is <emphasis>even</emphasis>, and
309 <literal>z</literal> is the patchlevel number (the trailing
310 <literal>.z</literal> can be omitted if <literal>z</literal>
311 is zero). Patchlevels are bug-fix releases only, and never
312 change the programmer interface to any system-supplied code.
313 However, if you install a new patchlevel over an old one you
314 will need to recompile any code that was compiled against the
315 old libraries.</para>
317 <para>The value of <literal>__GLASGOW_HASKELL__</literal>
318 (see <xref linkend="c-pre-processor">) for a major release
319 <literal>x.yy.z</literal> is the integer
320 <literal>xyy</literal>.</para>
322 <primary><literal>__GLASGOW_HASKELL__</literal></primary>
328 <term>Snapshots/unstable releases</term>
330 <para>We may make snapshot releases of the current
331 development sources from time to time, and the current
332 sources are always available via the CVS repository (see the
333 GHC web site for details).</para>
335 <para>Snapshot releases are named
336 <literal>x.yy.YYYYMMDD</literal> where <literal>yy</literal>
337 is <emphasis>odd</emphasis>, and <literal>YYYYMMDD</literal>
338 is the date of the sources from which the snapshot was
339 built. In theory, you can check out the exact same sources
340 from the CVS repository using this date.</para>
342 <para>The value of <literal>__GLASGOW_HASKELL__</literal>
343 for a snapshot release is the integer
344 <literal>xyy</literal>. You should never write any
345 conditional code which tests for this value, however: since
346 interfaces change on a day-to-day basis, and we don't have
347 finer granularity in the values of
348 <literal>__GLASGOW_HASKELL__</literal>, you should only
349 conditionally compile using predicates which test whether
350 <literal>__GLASGOW_HASKELL__</literal> is equal to, later
351 than, or earlier than a given major release.</para>
353 <primary><literal>__GLASGOW_HASKELL__</literal></primary>
359 <para>The version number of your copy of GHC can be found by
360 invoking <literal>ghc</literal> with the
361 <literal>––version</literal> flag (see <xref
362 linkend="options-help">).</para>
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