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<?xml version="1.0" encoding="iso-8859-1"?>
<chapter id="introduction-GHC">
<title>Introduction to GHC</title>
<para>This is a guide to using the Glasgow Haskell Compiler (GHC):
an interactive and batch compilation system for the <ulink
url="http://www.haskell.org/">Haskell 98</ulink>
language.</para>
<para>GHC has two main components: an interactive Haskell
interpreter (also known as GHCi), described in <xref
linkend="ghci"/>, and a batch compiler, described throughout <xref
linkend="using-ghc"/>. In fact, GHC consists of a single program
which is just run with different options to provide either the
interactive or the batch system.</para>
<para>The batch compiler can be used alongside GHCi: compiled
modules can be loaded into an interactive session and used in the
same way as interpreted code, and in fact when using GHCi most of
the library code will be pre-compiled. This means you get the best
of both worlds: fast pre-compiled library code, and fast compile
turnaround for the parts of your program being actively
developed.</para>
<para>GHC supports numerous language extensions, including
concurrency, a foreign function interface, exceptions, type system
extensions such as multi-parameter type classes, local universal and
existential quantification, functional dependencies, scoped type
variables and explicit unboxed types. These are all described in
<xref linkend="ghc-language-features"/>.</para>
<para>GHC has a comprehensive optimiser, so when you want to Really
Go For It (and you've got time to spare) GHC can produce pretty fast
code. Alternatively, the default option is to compile as fast as
possible while not making too much effort to optimise the generated
code (although GHC probably isn't what you'd describe as a fast
compiler :-).</para>
<para>GHC's profiling system supports “cost centre
stacks”: a way of seeing the profile of a Haskell program in a
call-graph like structure. See <xref linkend="profiling"/> for more
details.</para>
<para>GHC comes with a number of libraries. These are
described in separate documentation.</para>
<sect1 id="getting">
<title>Obtaining GHC</title>
<para>Go to the <ulink url="http://www.haskell.org/ghc/">GHC home
page</ulink> and follow the "download" link to download GHC
for your platform.</para>
<para>Alternatively, if you want to build GHC yourself, head on
over to the
<ulink url="http://ghc.haskell.org/trac/ghc/wiki/Building">GHC
Building Guide</ulink> to find out how to get the sources, and
build it on your system. Note that GHC itself is written in
Haskell, so you will still need to install GHC in order to
build it.</para>
</sect1>
<sect1 id="mailing-lists-GHC">
<title>Meta-information: Web sites, mailing lists, etc.</title>
<indexterm><primary>mailing lists, Glasgow Haskell</primary></indexterm>
<indexterm><primary>Glasgow Haskell mailing lists</primary></indexterm>
<para>On the World-Wide Web, there are several URLs of likely
interest:</para>
<itemizedlist>
<listitem>
<para><ulink url="http://www.haskell.org/ghc/">GHC home
page</ulink></para>
</listitem>
<listitem>
<para><ulink url="http://ghc.haskell.org/trac/ghc/">GHC
Developers Home</ulink> (developer documentation, wiki, and
bug tracker)</para>
</listitem>
</itemizedlist>
<para>We run the following mailing lists about GHC.
We encourage you to join, as you feel is appropriate.</para>
<variablelist>
<varlistentry>
<term>glasgow-haskell-users:</term>
<listitem>
<para>This list is for GHC users to chat among themselves.
If you have a specific question about GHC, please check the
<ulink
url="http://www.haskell.org/haskellwiki/GHC/FAQ">FAQ</ulink>
first.</para>
<variablelist>
<varlistentry>
<term>list email address:</term>
<listitem>
<para><email>glasgow-haskell-users@haskell.org</email></para>
</listitem>
</varlistentry>
<varlistentry>
<term>subscribe at:</term>
<listitem>
<para><ulink
url="http://www.haskell.org/mailman/listinfo/glasgow-haskell-users"><literal>http://www.haskell.org/mailman/listinfo/glasgow-haskell-users</literal></ulink>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>admin email address:</term>
<listitem>
<para><email>glasgow-haskell-users-admin@haskell.org</email></para>
</listitem>
</varlistentry>
<varlistentry>
<term>list archives:</term>
<listitem>
<para><ulink
url="http://www.haskell.org/pipermail/glasgow-haskell-users/"><literal>http://www.haskell.org/pipermail/glasgow-haskell-users/</literal></ulink></para>
</listitem>
</varlistentry>
</variablelist>
</listitem>
</varlistentry>
<varlistentry>
<term>ghc-devs:</term>
<listitem>
<para>The hardcore GHC developers hang out here.</para>
<variablelist>
<varlistentry>
<term>list email address:</term>
<listitem>
<para><email>ghc-devs@haskell.org</email></para>
</listitem>
</varlistentry>
<varlistentry>
<term>subscribe at:</term>
<listitem>
<para><ulink
url="http://www.haskell.org/mailman/listinfo/ghc-devs"><literal>http://www.haskell.org/mailman/listinfo/ghc-devs</literal></ulink>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>admin email address:</term>
<listitem>
<para><email>ghc-devs-admin@haskell.org</email></para>
</listitem>
</varlistentry>
<varlistentry>
<term>list archives:</term>
<listitem>
<para><ulink
url="http://www.haskell.org/pipermail/ghc-devs/"><literal>http://www.haskell.org/pipermail/ghc-devs/</literal></ulink></para>
</listitem>
</varlistentry>
</variablelist>
</listitem>
</varlistentry>
</variablelist>
<para>There are several other Haskell and GHC-related mailing
lists served by <literal>www.haskell.org</literal>. Go to <ulink
url="http://www.haskell.org/mailman/listinfo/"><literal>http://www.haskell.org/mailman/listinfo/</literal></ulink>
for the full list.</para>
<para>Some Haskell-related discussion also takes place in the
Usenet newsgroup <literal>comp.lang.functional</literal>.</para>
</sect1>
<sect1 id="bug-reporting">
<title>Reporting bugs in GHC</title>
<indexterm><primary>bugs</primary><secondary>reporting</secondary>
</indexterm>
<indexterm><primary>reporting bugs</primary>
</indexterm>
<para>
Glasgow Haskell is a changing system so there are sure to be
bugs in it. If you find one, please see
<ulink url="http://ghc.haskell.org/trac/ghc/wiki/ReportABug">this wiki page</ulink>
for information on how to report it.
</para>
</sect1>
<sect1 id="version-numbering">
<title>GHC version numbering policy</title>
<indexterm><primary>version, of ghc</primary></indexterm>
<para>As of GHC version 6.8, we have adopted the following policy
for numbering GHC versions:</para>
<variablelist>
<varlistentry>
<term>Stable Releases</term>
<listitem>
<para>Stable branches are numbered <literal><replaceable>x</replaceable>.<replaceable>y</replaceable></literal>, where
<replaceable>y</replaceable> is <emphasis>even</emphasis>.
Releases on the stable branch <literal><replaceable>x</replaceable>.<replaceable>y</replaceable></literal> are numbered <literal><replaceable>x</replaceable>.<replaceable>y</replaceable>.<replaceable>z</replaceable></literal>, where
<replaceable>z</replaceable> (>= 1) is the patchlevel number.
Patchlevels are bug-fix releases only, and never
change the programmer interface to any system-supplied code.
However, if you install a new patchlevel over an old one you
will need to recompile any code that was compiled against the
old libraries.</para>
<para>The value of <literal>__GLASGOW_HASKELL__</literal>
(see <xref linkend="c-pre-processor"/>) for a major release
<literal><replaceable>x</replaceable>.<replaceable>y</replaceable>.<replaceable>z</replaceable></literal>
is the integer <replaceable>xyy</replaceable> (if
<replaceable>y</replaceable> is a single digit, then a leading zero
is added, so for example in version 6.8.2 of GHC we would have
<literal>__GLASGOW_HASKELL__==608</literal>).</para>
<indexterm>
<primary><literal>__GLASGOW_HASKELL__</literal></primary>
</indexterm>
</listitem>
</varlistentry>
<varlistentry>
<term>Stable snapshots</term>
<listitem>
<para>
We may make snapshot releases of the current stable branch <ulink
url="http://www.haskell.org/ghc/dist/stable/dist/">available for
download</ulink>, and the latest sources are available from <ulink
url="http://ghc.haskell.org/trac/ghc/wiki/Repositories">the git
repositories</ulink>.
</para>
<para>Stable snapshot releases are named
<literal><replaceable>x</replaceable>.<replaceable>y</replaceable>.<replaceable>z</replaceable>.<replaceable>YYYYMMDD</replaceable></literal>.
where <literal><replaceable>YYYYMMDD</replaceable></literal> is the date of the sources
from which the snapshot was built, and <literal><replaceable>x</replaceable>.<replaceable>y</replaceable>.<replaceable>z+1</replaceable></literal> is the next release to be made on that branch.
For example, <literal>6.8.1.20040225</literal> would be a
snapshot of the <literal>6.8</literal> branch during the development
of <literal>6.8.2</literal>.</para>
<para>The value of <literal>__GLASGOW_HASKELL__</literal>
for a snapshot release is the integer
<replaceable>xyy</replaceable>. You should never write any
conditional code which tests for this value, however: since
interfaces change on a day-to-day basis, and we don't have
finer granularity in the values of
<literal>__GLASGOW_HASKELL__</literal>, you should only
conditionally compile using predicates which test whether
<literal>__GLASGOW_HASKELL__</literal> is equal to, later
than, or earlier than a given major release.</para>
<indexterm>
<primary><literal>__GLASGOW_HASKELL__</literal></primary>
</indexterm>
</listitem>
</varlistentry>
<varlistentry>
<term>Unstable snapshots</term>
<listitem>
<para>
We may make snapshot releases of the HEAD <ulink
url="http://www.haskell.org/ghc/dist/current/dist/">available for
download</ulink>, and the latest sources are available from <ulink
url="http://ghc.haskell.org/trac/ghc/wiki/Repositories">the git
repositories</ulink>.
</para>
<para>Unstable snapshot releases are named
<literal><replaceable>x</replaceable>.<replaceable>y</replaceable>.<replaceable>YYYYMMDD</replaceable></literal>.
where <literal><replaceable>YYYYMMDD</replaceable></literal> is the date of the sources
from which the snapshot was built.
For example, <literal>6.7.20040225</literal> would be a
snapshot of the HEAD before the creation of the
<literal>6.8</literal> branch.</para>
<para>The value of <literal>__GLASGOW_HASKELL__</literal>
for a snapshot release is the integer
<replaceable>xyy</replaceable>. You should never write any
conditional code which tests for this value, however: since
interfaces change on a day-to-day basis, and we don't have
finer granularity in the values of
<literal>__GLASGOW_HASKELL__</literal>, you should only
conditionally compile using predicates which test whether
<literal>__GLASGOW_HASKELL__</literal> is equal to, later
than, or earlier than a given major release.</para>
<indexterm>
<primary><literal>__GLASGOW_HASKELL__</literal></primary>
</indexterm>
</listitem>
</varlistentry>
</variablelist>
<para>The version number of your copy of GHC can be found by
invoking <literal>ghc</literal> with the
<literal>--version</literal> flag (see <xref
linkend="options-help"/>).</para>
</sect1>
&relnotes1;
</chapter>
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