<indexterm><primary>using GHC</primary></indexterm>
<sect1>
+ <title>Getting started: compiling programs</title>
+
+ <para>
+ In this chapter you'll find a complete reference to the GHC
+ command-line syntax, including all 400+ flags. It's a large and
+ complex system, and there are lots of details, so it can be
+ quite hard to figure out how to get started. With that in mind,
+ this introductory section provides a quick introduction to the
+ basic usage of GHC for compiling a Haskell program, before the
+ following sections dive into the full syntax.
+ </para>
+
+ <para>
+ Let's create a Hello World program, and compile and run it.
+ First, create a file <filename>hello.hs</filename> containing
+ the Haskell code:
+ </para>
+
+<programlisting>
+main = putStrLn "Hello, World!"
+</programlisting>
+
+ <para>To compile the program, use GHC like this:</para>
+
+<screen>
+$ ghc hello.hs</screen>
+
+ <para>(where <literal>$</literal> represents the prompt: don't
+ type it). GHC will compile the source
+ file <filename>hello.hs</filename>, producing
+ an <firstterm>object
+ file</firstterm> <filename>hello.o</filename> and
+ an <firstterm>interface
+ file</firstterm> <filename>hello.hi</filename>, and then it
+ will link the object file to the libraries that come with GHC
+ to produce an executable called <filename>hello</filename> on
+ Unix/Linux/Mac, or <filename>hello.exe</filename> on
+ Windows.</para>
+
+ <para>
+ By default GHC will be very quiet about what it is doing, only
+ printing error messages. If you want to see in more detail
+ what's going on behind the scenes, add <option>-v</option> to
+ the command line.
+ </para>
+
+ <para>
+ Then we can run the program like this:
+ </para>
+
+<screen>
+$ ./hello
+Hello World!</screen>
+
+ <para>
+ If your program contains multiple modules, then you only need to
+ tell GHC the name of the source file containing
+ the <filename>Main</filename> module, and GHC will examine
+ the <literal>import</literal> declarations to find the other
+ modules that make up the program and find their source files.
+ This means that, with the exception of
+ the <literal>Main</literal> module, every source file should be
+ named after the module name that it contains (with dots replaced
+ by directory separators). For example, the
+ module <literal>Data.Person</literal> would be in the
+ file <filename>Data/Person.hs</filename> on Unix/Linux/Mac,
+ or <filename>Data\Person.hs</filename> on Windows.
+ </para>
+ </sect1>
+
+ <sect1>
<title>Options overview</title>
<para>GHC's behaviour is controlled by
<varlistentry>
<term>Mode flags</term>
<listitem>
- <para>For example, <option>--make</option> or <option>-E</option>.
+ <para>For example, <option>––make</option> or <option>-E</option>.
There may only be a single mode flag on the command line. The
available modes are listed in <xref linkend="modes"/>.</para>
</listitem>
<sect1 id="modes">
<title>Modes of operation</title>
- <para>GHC's behaviour is firstly controlled by a mode flag. Only
- one of these flags may be given, but it does not necessarily need
- to be the first option on the command-line. The available modes
- are:</para>
+ <para>
+ GHC's behaviour is firstly controlled by a mode flag. Only one
+ of these flags may be given, but it does not necessarily need to
+ be the first option on the command-line.
+ </para>
+
+ <para>
+ If no mode flag is present, then GHC will enter make mode
+ (<xref linkend="make-mode" />) if there are any Haskell source
+ files given on the command line, or else it will link the
+ objects named on the command line to produce an executable.
+ </para>
+
+ <para>The available mode flags are:</para>
<variablelist>
<varlistentry>
<varlistentry>
<term>
- <cmdsynopsis><command>ghc --make</command>
+ <cmdsynopsis><command>ghc ––make</command>
</cmdsynopsis>
<indexterm><primary>make mode</primary></indexterm>
<indexterm><primary><option>––make</option></primary></indexterm>
likely to be much easier, and faster, than using
<command>make</command>. Make mode is described in <xref
linkend="make-mode"/>.</para>
+
+ <para>
+ This mode is the default if there are any Haskell
+ source files mentioned on the command line, and in this case
+ the <option>––make</option> option can be omitted.
+ </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<cmdsynopsis>
+ <command>ghc --supported-extensions</command>
<command>ghc --supported-languages</command>
</cmdsynopsis>
- <indexterm><primary><option>––supported-languages</option></primary></indexterm>
+ <indexterm><primary><option>––supported-extensions</option></primary><primary><option>––supported-languages</option></primary></indexterm>
</term>
<listitem>
<para>Print the supported language extensions.</para>
<indexterm><primary><option>––make</option></primary></indexterm>
<indexterm><primary>separate compilation</primary></indexterm>
- <para>When given the <option>––make</option> option,
- GHC will build a multi-module Haskell program by following
+ <para>In this mode, GHC will build a multi-module Haskell program by following
dependencies from one or more root modules (usually just
<literal>Main</literal>). For example, if your
<literal>Main</literal> module is in a file called
ghc ––make Main.hs
</screen>
- <para>The command line may contain any number of source file
- names or module names; GHC will figure out all the modules in
- the program by following the imports from these initial modules.
- It will then attempt to compile each module which is out of
- date, and finally, if there is a <literal>Main</literal> module,
- the program will also be linked into an executable.</para>
+ <para>
+ In fact, GHC enters make mode automatically if there are any
+ Haskell source files on the command line and no other mode is
+ specified, so in this case we could just type
+ </para>
+
+<screen>
+ghc Main.hs
+</screen>
+
+ <para>Any number of source file names or module names may be
+ specified; GHC will figure out all the modules in the program by
+ following the imports from these initial modules. It will then
+ attempt to compile each module which is out of date, and
+ finally, if there is a <literal>Main</literal> module, the
+ program will also be linked into an executable.</para>
<para>The main advantages to using <literal>ghc
––make</literal> over traditional
<option>-fwarn-duplicate-exports</option>,
<option>-fwarn-missing-fields</option>,
<option>-fwarn-missing-methods</option>,
- <option>-fwarn-lazy-unlifted-bindings</option>, and
+ <option>-fwarn-lazy-unlifted-bindings</option>,
+ <option>-fwarn-wrong-do-bind</option>, and
<option>-fwarn-dodgy-foreign-imports</option>. The following
flags are
simple ways to select standard “packages” of warnings:
<indexterm><primary>-W option</primary></indexterm>
<para>Provides the standard warnings plus
<option>-fwarn-incomplete-patterns</option>,
+ <option>-fwarn-dodgy-exports</option>,
<option>-fwarn-dodgy-imports</option>,
<option>-fwarn-unused-matches</option>,
<option>-fwarn-unused-imports</option>, and
<option>-fwarn-simple-patterns</option>,
<option>-fwarn-tabs</option>,
<option>-fwarn-incomplete-record-updates</option>,
- <option>-fwarn-monomorphism-restriction</option>, and
+ <option>-fwarn-monomorphism-restriction</option>,
+ <option>-fwarn-unused-do-bind</option>, and
<option>-fwarn-implicit-prelude</option>.</para>
</listitem>
</varlistentry>
</varlistentry>
<varlistentry>
+ <term><option>-fwarn-dodgy-exports</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-dodgy-exports</option></primary>
+ </indexterm>
+ <para>Causes a warning to be emitted when a datatype
+ <literal>T</literal> is exported
+ with all constructors, i.e. <literal>T(..)</literal>, but is it
+ just a type synonym.</para>
+ <para>Also causes a warning to be emitted when a module is
+ re-exported, but that module exports nothing.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
<term><option>-fwarn-dodgy-imports</option>:</term>
<listitem>
<indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
is bound in a way that looks lazy, e.g.
<literal>where (I# x) = ...</literal>. Use
<literal>where !(I# x) = ...</literal> instead. This will be an
- error, rather than a warning, in GHC 6.14.
+ error, rather than a warning, in GHC 7.0.
</para>
</listitem>
</varlistentry>
</listitem>
</varlistentry>
+ <varlistentry>
+ <term><option>-fwarn-unused-do-bind</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-unused-do-bind</option></primary></indexterm>
+ <indexterm><primary>unused do binding, warning</primary></indexterm>
+ <indexterm><primary>do binding, unused</primary></indexterm>
+
+ <para>Report expressions occuring in <literal>do</literal> and <literal>mdo</literal> blocks
+ that appear to silently throw information away.
+ For instance <literal>do { mapM popInt xs ; return 10 }</literal> would report
+ the first statement in the <literal>do</literal> block as suspicious,
+ as it has the type <literal>StackM [Int]</literal> and not <literal>StackM ()</literal>, but that
+ <literal>[Int]</literal> value is not bound to anything. The warning is suppressed by
+ explicitly mentioning in the source code that your program is throwing something away:
+ <programlisting>
+ do { _ <- mapM popInt xs ; return 10 }
+ </programlisting>
+ Of course, in this particular situation you can do even better:
+ <programlisting>
+ do { mapM_ popInt xs ; return 10 }
+ </programlisting>
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>-fwarn-wrong-do-bind</option>:</term>
+ <listitem>
+ <indexterm><primary><option>-fwarn-wrong-do-bind</option></primary></indexterm>
+ <indexterm><primary>apparently erroneous do binding, warning</primary></indexterm>
+ <indexterm><primary>do binding, apparently erroneous</primary></indexterm>
+
+ <para>Report expressions occuring in <literal>do</literal> and <literal>mdo</literal> blocks
+ that appear to lack a binding.
+ For instance <literal>do { return (popInt 10) ; return 10 }</literal> would report
+ the first statement in the <literal>do</literal> block as suspicious,
+ as it has the type <literal>StackM (StackM Int)</literal> (which consists of two nested applications
+ of the same monad constructor), but which is not then "unpacked" by binding the result.
+ The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away:
+ <programlisting>
+ do { _ <- return (popInt 10) ; return 10 }
+ </programlisting>
+ For almost all sensible programs this will indicate a bug, and you probably intended to write:
+ <programlisting>
+ do { popInt 10 ; return 10 }
+ </programlisting>
+ </para>
+ </listitem>
+ </varlistentry>
+
</variablelist>
<para>If you're feeling really paranoid, the
<varlistentry>
<term>
+ <option>-fno-float-in</option>
+ <indexterm><primary><option>-fno-float-in</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Turns off the float-in transformation.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <option>-fno-specialise</option>
+ <indexterm><primary><option>-fno-specialise</option></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Turns off the automatic specialisation of overloaded functions.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
<option>-fspec-constr</option>
<indexterm><primary><option>-fspec-constr</option></primary></indexterm>
</term>
</sect1>
&phases;
-
+
+ &shared_libs;
+
<sect1 id="using-concurrent">
<title>Using Concurrent Haskell</title>
<indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
special option or libraries compiled in a certain way. To get access to
the support libraries for Concurrent Haskell, just import
<ulink
- url="../libraries/base/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
+ url="&libraryBaseLocation;/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
<para>The following RTS option(s) affect the behaviour of Concurrent
Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
<para>There is no means (currently) by which this value
may vary after the program has started.</para>
+
+ <para>The current value of the <option>-N</option> option
+ is available to the Haskell program
+ via <literal>GHC.Conc.numCapabilities</literal>.</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry>
+ <term><option>-qa</option></term>
+ <indexterm><primary><option>-qa</option></primary><secondary>RTS
+ option</secondary></indexterm>
+ <listitem>
+ <para>Use the OS's affinity facilities to try to pin OS
+ threads to CPU cores. This is an experimental feature,
+ and may or may not be useful. Please let us know
+ whether it helps for you!</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
<term><option>-qm</option></term>
<indexterm><primary><option>-qm</option></primary><secondary>RTS
option</secondary></indexterm>
<para>Disable automatic migration for load balancing.
Normally the runtime will automatically try to schedule
threads across the available CPUs to make use of idle
- CPUs; this option disables that behaviour. It is probably
- only of use if you are explicitly scheduling threads onto
- CPUs with <literal>GHC.Conc.forkOnIO</literal>.</para>
+ CPUs; this option disables that behaviour. Note that
+ migration only applies to threads; sparks created
+ by <literal>par</literal> are load-balanced separately
+ by work-stealing.</para>
+
+ <para>
+ This option is probably only of use for concurrent
+ programs that explicitly schedule threads onto CPUs
+ with <literal>GHC.Conc.forkOnIO</literal>.
+ </para>
</listitem>
</varlistentry>
<varlistentry>
whether your program got faster by using more CPUs or not. If the user
time is greater than
the elapsed time, then the program used more than one CPU. You should
- also run the program without <literal>-N</literal> for comparison.</para>
-
- <para>GHC's parallelism support is new and experimental. It may make your
- program go faster, or it might slow it down - either way, we'd be
- interested to hear from you.</para>
-
- <para>One significant limitation with the current implementation is that
- the garbage collector is still single-threaded, and all execution must
- stop when GC takes place. This can be a significant bottleneck in a
- parallel program, especially if your program does a lot of GC. If this
- happens to you, then try reducing the cost of GC by tweaking the GC
- settings (<xref linkend="rts-options-gc" />): enlarging the heap or the
- allocation area size is a good start.</para>
+ also run the program without <literal>-N</literal> for
+ comparison.</para>
+
+ <para>The output of <literal>+RTS -s</literal> tells you how
+ many “sparks” were created and executed during the
+ run of the program (see <xref linkend="rts-options-gc" />), which
+ will give you an idea how well your <literal>par</literal>
+ annotations are working.</para>
+
+ <para>GHC's parallelism support has improved in 6.12.1 as a
+ result of much experimentation and tuning in the runtime
+ system. We'd still be interested to hear how well it works
+ for you, and we're also interested in collecting parallel
+ programs to add to our benchmarking suite.</para>
</sect2>
</sect1>
<variablelist>
<varlistentry>
+ <term><option>-msse2</option>:</term>
+ <listitem>
+ <para>
+ (x86 only, added in GHC 7.0.1) Use the SSE2 registers and
+ instruction set to implement floating point operations
+ when using the native code generator. This gives a
+ substantial performance improvement for floating point,
+ but the resulting compiled code will only run on
+ processors that support SSE2 (Intel Pentium 4 and later,
+ or AMD Athlon 64 and later).
+ </para>
+ <para>
+ SSE2 is unconditionally used on x86-64 platforms.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
<term><option>-monly-[32]-regs</option>:</term>
<listitem>
- <para>(iX86 machines)<indexterm><primary>-monly-N-regs
+ <para>(x86 only)<indexterm><primary>-monly-N-regs
option (iX86 only)</primary></indexterm> GHC tries to
“steal” four registers from GCC, for performance
reasons; it almost always works. However, when GCC is
<para>GHC can dump its optimized intermediate code (said to be in “Core” format)
to a file as a side-effect of compilation. Non-GHC back-end tools can read and process Core files; these files have the suffix
- <filename>.hcr</filename>. The Core format is described in <ulink url="../ext-core/core.pdf">
+ <filename>.hcr</filename>. The Core format is described in <ulink url="../../core.pdf">
<citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
and sample tools
for manipulating Core files (in Haskell) are in the GHC source distribution
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