path: root/docs/users_guide/using.xml

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<chapter id="using-ghc">
  <title>Using GHC</title>

  <indexterm><primary>GHC, using</primary></indexterm>
  <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
    <firstterm>options</firstterm>, which for historical reasons are
    also sometimes referred to as command-line flags or arguments.
    Options can be specified in three ways:</para>

    <sect2>
      <title>Command-line arguments</title>

      <indexterm><primary>structure, command-line</primary></indexterm>
      <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
      <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>

      <para>An invocation of GHC takes the following form:</para>

<screen>
ghc [argument...]
</screen>

      <para>Command-line arguments are either options or file names.</para>

      <para>Command-line options begin with <literal>-</literal>.
      They may <emphasis>not</emphasis> be grouped:
      <option>-vO</option> is different from <option>-v -O</option>.
      Options need not precede filenames: e.g., <literal>ghc *.o -o
      foo</literal>.  All options are processed and then applied to
      all files; you cannot, for example, invoke <literal>ghc -c -O1
      Foo.hs -O2 Bar.hs</literal> to apply different optimisation
      levels to the files <filename>Foo.hs</filename> and
      <filename>Bar.hs</filename>.</para>
    </sect2>

    <sect2 id="source-file-options">
      <title>Command line options in source files</title>

      <indexterm><primary>source-file options</primary></indexterm>

      <para>Sometimes it is useful to make the connection between a
      source file and the command-line options it requires quite
      tight. For instance, if a Haskell source file deliberately
	uses name shadowing, it should be compiled with  the
      <option>-fno-warn-name-shadowing</option> option.  Rather than maintaining
      the list of per-file options in a <filename>Makefile</filename>,
      it is possible to do this directly in the source file using the
      <literal>OPTIONS_GHC</literal> pragma <indexterm><primary>OPTIONS_GHC
      pragma</primary></indexterm>:</para>

<programlisting>
{-# OPTIONS_GHC -fno-warn-name-shadowing #-}
module X where
...
</programlisting>

      <para><literal>OPTIONS_GHC</literal> is a <emphasis>file-header pragma</emphasis>
      (see <xref linkend="pragmas"/>).</para>

      <para>Only <emphasis>dynamic</emphasis> flags can be used in an <literal>OPTIONS_GHC</literal> pragma
      (see <xref linkend="static-dynamic-flags"/>).</para>

      <para>Note that your command shell does not
      get to the source file options, they are just included literally
      in the array of command-line arguments the compiler
      maintains internally, so you'll be desperately disappointed if
      you try to glob etc. inside <literal>OPTIONS_GHC</literal>.</para>

      <para>NOTE: the contents of OPTIONS_GHC are appended to the
      command-line options, so options given in the source file
      override those given on the command-line.</para>

      <para>It is not recommended to move all the contents of your
      Makefiles into your source files, but in some circumstances, the
      <literal>OPTIONS_GHC</literal> pragma is the Right Thing. (If you
      use <option>-keep-hc-file</option> and have OPTION flags in
      your module, the OPTIONS_GHC will get put into the generated .hc
      file).</para>
    </sect2>

    <sect2>
      <title>Setting options in GHCi</title>

      <para>Options may also be modified from within GHCi, using the
      <literal>:set</literal> command.  See <xref linkend="ghci-set"/>
      for more details.</para>
    </sect2>
  </sect1>

  <sect1 id="static-dynamic-flags">
    <title>Static, Dynamic, and Mode options</title>
    <indexterm><primary>static</primary><secondary>options</secondary>
    </indexterm>
    <indexterm><primary>dynamic</primary><secondary>options</secondary>
    </indexterm>
    <indexterm><primary>mode</primary><secondary>options</secondary>
    </indexterm>

    <para>Each of GHC's command line options is classified as
    <firstterm>static</firstterm>, <firstterm>dynamic</firstterm> or
      <firstterm>mode</firstterm>:</para>

    <variablelist>
      <varlistentry>
	<term>Mode flags</term>
	<listitem>
	  <para>For example, <option>&ndash;&ndash;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>
      </varlistentry>
      <varlistentry>
	<term>Dynamic Flags</term>
	<listitem>
	  <para>Most non-mode flags fall into this category.  A dynamic flag
	    may be used on the command line, in a
	    <literal>OPTIONS_GHC</literal> pragma in a source file, or set
	    using <literal>:set</literal> in GHCi.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>Static Flags</term>
	<listitem>
	  <para>A few flags are "static", which means they can only be used on
	    the command-line, and remain in force over the entire GHC/GHCi
	    run.</para>
	</listitem>
      </varlistentry>
    </variablelist>

    <para>The flag reference tables (<xref
    linkend="flag-reference"/>) lists the status of each flag.</para>

    <para>There are a few flags that are static except that they can
    also be used with GHCi's <literal>:set</literal> command; these
    are listed as &ldquo;static/<literal>:set</literal>&rdquo; in the
    table.</para>
  </sect1>

  <sect1 id="file-suffixes">
    <title>Meaningful file suffixes</title>

    <indexterm><primary>suffixes, file</primary></indexterm>
    <indexterm><primary>file suffixes for GHC</primary></indexterm>

    <para>File names with &ldquo;meaningful&rdquo; suffixes (e.g.,
    <filename>.lhs</filename> or <filename>.o</filename>) cause the
    &ldquo;right thing&rdquo; to happen to those files.</para>

    <variablelist>

      <varlistentry>
	<term><filename>.hs</filename></term>
	<listitem>
	  <para>A Haskell module.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <filename>.lhs</filename>
          <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
	</term>
	<listitem>
	  <para>A &ldquo;literate Haskell&rdquo; module.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><filename>.hi</filename></term>
	<listitem>
	  <para>A Haskell interface file, probably
	  compiler-generated.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><filename>.hc</filename></term>
	<listitem>
	  <para>Intermediate C file produced by the Haskell
	  compiler.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><filename>.c</filename></term>
	<listitem>
	  <para>A C&nbsp;file not produced by the Haskell
	  compiler.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><filename>.ll</filename></term>
	<listitem>
	  <para>An llvm-intermediate-language source file, usually
          produced by the compiler.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><filename>.bc</filename></term>
	<listitem>
	  <para>An llvm-intermediate-language bitcode file, usually
          produced by the compiler.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><filename>.s</filename></term>
	<listitem>
	  <para>An assembly-language source file, usually produced by
          the compiler.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><filename>.o</filename></term>
	<listitem>
	  <para>An object file, produced by an assembler.</para>
	</listitem>
      </varlistentry>
    </variablelist>

    <para>Files with other suffixes (or without suffixes) are passed
    straight to the linker.</para>

  </sect1>

  <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.
    </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>
	<term>
	  <cmdsynopsis><command>ghc --interactive</command>
	  </cmdsynopsis>
          <indexterm><primary>interactive mode</primary></indexterm>
          <indexterm><primary>ghci</primary></indexterm>
	</term>
	<listitem>
	  <para>Interactive mode, which is also available as
	  <command>ghci</command>.  Interactive mode is described in
	  more detail in <xref linkend="ghci"/>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <cmdsynopsis><command>ghc &ndash;&ndash;make</command>
	  </cmdsynopsis>
          <indexterm><primary>make mode</primary></indexterm>
          <indexterm><primary><option>&ndash;&ndash;make</option></primary></indexterm>
	</term>
	<listitem>
	  <para>In this mode, GHC will build a multi-module Haskell
	  program automatically, figuring out dependencies for itself.
	  If you have a straightforward Haskell program, this is
	  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>&ndash;&ndash;make</option> option can be omitted.
          </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <cmdsynopsis><command>ghc -e</command>
	     <arg choice='plain'><replaceable>expr</replaceable></arg>
          </cmdsynopsis>
          <indexterm><primary>eval mode</primary></indexterm>
	</term>
	<listitem>
	  <para>Expression-evaluation mode.  This is very similar to
	  interactive mode, except that there is a single expression
	  to evaluate (<replaceable>expr</replaceable>) which is given
	  on the command line.  See <xref linkend="eval-mode"/> for
	  more details.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <cmdsynopsis>
	    <command>ghc -E</command>
	    <command>ghc -c</command>
	    <command>ghc -S</command>
	    <command>ghc -c</command>
	  </cmdsynopsis>
	  <indexterm><primary><option>-E</option></primary></indexterm>
	  <indexterm><primary><option>-C</option></primary></indexterm>
	  <indexterm><primary><option>-S</option></primary></indexterm>
	  <indexterm><primary><option>-c</option></primary></indexterm>
        </term>
	<listitem>
	  <para>This is the traditional batch-compiler mode, in which
	  GHC can compile source files one at a time, or link objects
	  together into an executable.  This mode also applies if
	  there is no other mode flag specified on the command line,
	  in which case it means that the specified files should be
	  compiled and then linked to form a program. See <xref
	  linkend="options-order"/>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <cmdsynopsis>
            <command>ghc -M</command>
          </cmdsynopsis>
          <indexterm><primary>dependency-generation mode</primary></indexterm>
        </term>
	<listitem>
	  <para>Dependency-generation mode.  In this mode, GHC can be
	  used to generate dependency information suitable for use in
	  a <literal>Makefile</literal>.  See <xref
	  linkend="makefile-dependencies"/>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <cmdsynopsis>
            <command>ghc --mk-dll</command>
          </cmdsynopsis>
	  <indexterm><primary>DLL-creation mode</primary></indexterm>
        </term>
	<listitem>
	  <para>DLL-creation mode (Windows only).  See <xref
	  linkend="win32-dlls-create"/>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <cmdsynopsis>
          <command>ghc --help</command> <command>ghc -?</command>
	    </cmdsynopsis>
          <indexterm><primary><option>&ndash;&ndash;help</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Cause GHC to spew a long usage message to standard
          output and then exit.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <cmdsynopsis>
            <command>ghc --show-iface <replaceable>file</replaceable></command>
          </cmdsynopsis>
          <indexterm><primary><option>&ndash;&ndash;--show-iface</option></primary></indexterm>
        </term>
	<listitem>
	      <para>Read the interface in
	      <replaceable>file</replaceable> and dump it as text to
	      <literal>stdout</literal>. For example <literal>ghc --show-iface M.hi</literal>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <cmdsynopsis>
            <command>ghc --supported-extensions</command>
            <command>ghc --supported-languages</command>
          </cmdsynopsis>
          <indexterm><primary><option>&ndash;&ndash;supported-extensions</option></primary><primary><option>&ndash;&ndash;supported-languages</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Print the supported language extensions.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <cmdsynopsis>
            <command>ghc --info</command>
          </cmdsynopsis>
          <indexterm><primary><option>&ndash;&ndash;info</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Print information about the compiler.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <cmdsynopsis>
            <command>ghc --version</command>
            <command>ghc -V</command>
          </cmdsynopsis>
          <indexterm><primary><option>-V</option></primary></indexterm>
          <indexterm><primary><option>&ndash;&ndash;version</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Print a one-line string including GHC's version number.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <cmdsynopsis>
            <command>ghc --numeric-version</command>
          </cmdsynopsis>
          <indexterm><primary><option>&ndash;&ndash;numeric-version</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Print GHC's numeric version number only.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <cmdsynopsis>
            <command>ghc --print-libdir</command>
          </cmdsynopsis>
          <indexterm><primary><option>&ndash;&ndash;print-libdir</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Print the path to GHC's library directory.  This is
	  the top of the directory tree containing GHC's libraries,
	  interfaces, and include files (usually something like
	  <literal>/usr/local/lib/ghc-5.04</literal> on Unix).  This
	  is the value of
	  <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
      in the package configuration file
      (see <xref linkend="packages"/>).</para>
	</listitem>
      </varlistentry>

    </variablelist>

    <sect2 id="make-mode">
      <title>Using <command>ghc</command> <option>&ndash;&ndash;make</option></title>
      <indexterm><primary><option>&ndash;&ndash;make</option></primary></indexterm>
      <indexterm><primary>separate compilation</primary></indexterm>

      <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
      <filename>Main.hs</filename>, you could compile and link the
      program like this:</para>

<screen>
ghc &ndash;&ndash;make Main.hs
</screen>

      <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
      &ndash;&ndash;make</literal> over traditional
      <literal>Makefile</literal>s are:</para>

      <itemizedlist>
	<listitem>
	  <para>GHC doesn't have to be restarted for each compilation,
	  which means it can cache information between compilations.
	  Compiling a multi-module program with <literal>ghc
	  &ndash;&ndash;make</literal> can be up to twice as fast as
	  running <literal>ghc</literal> individually on each source
	  file.</para>
	</listitem>
	<listitem>
	  <para>You don't have to write a <literal>Makefile</literal>.</para>
          <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
	</listitem>
	<listitem>
	  <para>GHC re-calculates the dependencies each time it is
	  invoked, so the dependencies never get out of sync with the
	  source.</para>
	</listitem>
      </itemizedlist>

      <para>Any of the command-line options described in the rest of
      this chapter can be used with
      <option>&ndash;&ndash;make</option>, but note that any options
      you give on the command line will apply to all the source files
      compiled, so if you want any options to apply to a single source
      file only, you'll need to use an <literal>OPTIONS_GHC</literal>
      pragma (see <xref linkend="source-file-options"/>).</para>

      <para>If the program needs to be linked with additional objects
      (say, some auxiliary C code), then the object files can be
      given on the command line and GHC will include them when linking
      the executable.</para>

      <para>Note that GHC can only follow dependencies if it has the
      source file available, so if your program includes a module for
      which there is no source file, even if you have an object and an
      interface file for the module, then GHC will complain.  The
      exception to this rule is for package modules, which may or may
      not have source files.</para>

      <para>The source files for the program don't all need to be in
      the same directory; the <option>-i</option> option can be used
      to add directories to the search path (see <xref
      linkend="search-path"/>).</para>
    </sect2>

    <sect2 id="eval-mode">
      <title>Expression evaluation mode</title>

      <para>This mode is very similar to interactive mode, except that
      there is a single expression to evaluate which is specified on
      the command line as an argument to the <option>-e</option>
      option:</para>

<screen>
ghc -e <replaceable>expr</replaceable>
</screen>

      <para>Haskell source files may be named on the command line, and
      they will be loaded exactly as in interactive mode.  The
      expression is evaluated in the context of the loaded
      modules.</para>

      <para>For example, to load and run a Haskell program containing
      a module <literal>Main</literal>, we might say</para>

<screen>
ghc -e Main.main Main.hs
</screen>

      <para>or we can just use this mode to evaluate expressions in
      the context of the <literal>Prelude</literal>:</para>

<screen>
$ ghc -e "interact (unlines.map reverse.lines)"
hello
olleh
</screen>
    </sect2>

    <sect2 id="options-order">
      <title>Batch compiler mode</title>

      <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
      given on the command line.</para>

      <para>The first phase to run is determined by each input-file
      suffix, and the last phase is determined by a flag.  If no
      relevant flag is present, then go all the way through to linking.
      This table summarises:</para>

      <informaltable>
	<tgroup cols="4">
	  <colspec align="left"/>
	  <colspec align="left"/>
	  <colspec align="left"/>
	  <colspec align="left"/>

	  <thead>
	    <row>
	      <entry>Phase of the compilation system</entry>
	      <entry>Suffix saying &ldquo;start here&rdquo;</entry>
	      <entry>Flag saying &ldquo;stop after&rdquo;</entry>
	      <entry>(suffix of) output file</entry>
	    </row>
	  </thead>
	  <tbody>
	    <row>
	      <entry>literate pre-processor</entry>
	      <entry><literal>.lhs</literal></entry>
	      <entry>-</entry>
	      <entry><literal>.hs</literal></entry>
	    </row>

	    <row>
	      <entry>C pre-processor (opt.) </entry>
	      <entry><literal>.hs</literal> (with
	      <option>-cpp</option>)</entry>
	      <entry><option>-E</option></entry>
	      <entry><literal>.hspp</literal></entry>
	    </row>

	    <row>
	      <entry>Haskell compiler</entry>
	      <entry><literal>.hs</literal></entry>
	      <entry><option>-C</option>, <option>-S</option></entry>
	      <entry><literal>.hc</literal>, <literal>.s</literal></entry>
	    </row>

	    <row>
	      <entry>C compiler (opt.)</entry>
	      <entry><literal>.hc</literal> or <literal>.c</literal></entry>
	      <entry><option>-S</option></entry>
	      <entry><literal>.s</literal></entry>
	    </row>

	    <row>
	      <entry>assembler</entry>
	      <entry><literal>.s</literal></entry>
	      <entry><option>-c</option></entry>
	      <entry><literal>.o</literal></entry>
	    </row>

	    <row>
	      <entry>linker</entry>
	      <entry><replaceable>other</replaceable></entry>
	      <entry>-</entry>
	      <entry><filename>a.out</filename></entry>
	    </row>
	  </tbody>
	</tgroup>
      </informaltable>

      <indexterm><primary><option>-C</option></primary></indexterm>
      <indexterm><primary><option>-E</option></primary></indexterm>
      <indexterm><primary><option>-S</option></primary></indexterm>
      <indexterm><primary><option>-c</option></primary></indexterm>

      <para>Thus, a common invocation would be: </para>

<screen>
ghc -c Foo.hs</screen>

      <para>to compile the Haskell source file
      <filename>Foo.hs</filename> to an object file
      <filename>Foo.o</filename>.</para>

      <para>Note: What the Haskell compiler proper produces depends on
      whether a native-code generator<indexterm><primary>native-code
      generator</primary></indexterm> is used (producing assembly
      language) or not (producing C).  See <xref
      linkend="options-codegen"/> for more details.</para>

      <para>Note: C pre-processing is optional, the
      <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
      flag turns it on.  See <xref linkend="c-pre-processor"/> for more
      details.</para>

      <para>Note: The option <option>-E</option><indexterm><primary>-E
      option</primary></indexterm> runs just the pre-processing passes
      of the compiler, dumping the result in a file.</para>

      <sect3 id="overriding-suffixes">
	<title>Overriding the default behaviour for a file</title>

	<para>As described above, the way in which a file is processed by GHC
	  depends on its suffix.  This behaviour can be overridden using the
	  <option>-x</option> option:</para>

	<variablelist>
	  <varlistentry>
	    <term><option>-x</option> <replaceable>suffix</replaceable>
	    	      <indexterm><primary><option>-x</option></primary>
	      </indexterm></term>
	      <listitem>
		<para>Causes all files following this option on the command
		  line to be processed as if they had the suffix
		  <replaceable>suffix</replaceable>.  For example, to compile a
		  Haskell module in the file <literal>M.my-hs</literal>,
		  use <literal>ghc -c -x hs M.my-hs</literal>.</para>
	      </listitem>
	  </varlistentry>
	</variablelist>
      </sect3>

    </sect2>
  </sect1>

  <sect1 id="options-help">
    <title>Help and verbosity options</title>

    <indexterm><primary>help options</primary></indexterm>
    <indexterm><primary>verbosity options</primary></indexterm>

    <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
    and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
    <variablelist>
      <varlistentry>
	<term>
          <option>-v</option>
          <indexterm><primary><option>-v</option></primary></indexterm>
        </term>
	<listitem>
	  <para>The <option>-v</option> option makes GHC
          <emphasis>verbose</emphasis>: it reports its version number
          and shows (on stderr) exactly how it invokes each phase of
          the compilation system.  Moreover, it passes the
          <option>-v</option> flag to most phases; each reports its
          version number (and possibly some other information).</para>

	  <para>Please, oh please, use the <option>-v</option> option
          when reporting bugs!  Knowing that you ran the right bits in
          the right order is always the first thing we want to
          verify.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-v</option><replaceable>n</replaceable>
          <indexterm><primary><option>-v</option></primary></indexterm>
        </term>
	<listitem>
	  <para>To provide more control over the compiler's verbosity,
	  the <option>-v</option> flag takes an optional numeric
	  argument.  Specifying <option>-v</option> on its own is
	  equivalent to <option>-v3</option>, and the other levels
	  have the following meanings:</para>

	  <variablelist>
	    <varlistentry>
	      <term><option>-v0</option></term>
	      <listitem>
		<para>Disable all non-essential messages (this is the
		default).</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term><option>-v1</option></term>
	      <listitem>
		<para>Minimal verbosity: print one line per
		compilation (this is the default when
		<option>&ndash;&ndash;make</option> or
		<option>&ndash;&ndash;interactive</option> is on).</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term><option>-v2</option></term>
	      <listitem>
		<para>Print the name of each compilation phase as it
		is executed. (equivalent to
		<option>-dshow-passes</option>).</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term><option>-v3</option></term>
	      <listitem>
		<para>The same as <option>-v2</option>, except that in
                addition the full command line (if appropriate) for
                each compilation phase is also printed.</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term><option>-v4</option></term>
	      <listitem>
		<para>The same as <option>-v3</option> except that the
		intermediate program representation after each
		compilation phase is also printed (excluding
		preprocessed and C/assembly files).</para>
	      </listitem>
	    </varlistentry>
	  </variablelist>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-ferror-spans</option>
          <indexterm><primary><option>-ferror-spans</option></primary>
	  </indexterm>
        </term>
	<listitem>
	  <para>Causes GHC to emit the full source span of the
	  syntactic entity relating to an error message.  Normally, GHC
	  emits the source location of the start of the syntactic
	  entity only.</para>

	  <para>For example:</para>

<screen>test.hs:3:6: parse error on input `where'</screen>

	  <para>becomes:</para>

<screen>test296.hs:3:6-10: parse error on input `where'</screen>

	  <para>And multi-line spans are possible too:</para>

<screen>test.hs:(5,4)-(6,7):
    Conflicting definitions for `a'
    Bound at: test.hs:5:4
              test.hs:6:7
    In the binding group for: a, b, a</screen>

	  <para>Note that line numbers start counting at one, but
	  column numbers start at zero.  This choice was made to
	  follow existing convention (i.e. this is how Emacs does
	  it).</para>
	</listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-H</option><replaceable>size</replaceable>
        <indexterm><primary><option>-H</option></primary></indexterm>
        </term>
        <listitem>
          <para>Set the minimum size of the heap to
          <replaceable>size</replaceable>.
          This option is equivalent to
          <literal>+RTS&nbsp;-H<replaceable>size</replaceable></literal>,
          see <xref linkend="rts-options-gc" />.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-Rghc-timing</option>
        <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
        </term>
        <listitem>
          <para>Prints a one-line summary of timing statistics for the
          GHC run.  This option is equivalent to
          <literal>+RTS&nbsp;-tstderr</literal>, see <xref
          linkend="rts-options-gc" />.
          </para>
        </listitem>
      </varlistentry>
    </variablelist>
  </sect1>

  &separate;

  <sect1 id="options-sanity">
    <title>Warnings and sanity-checking</title>

    <indexterm><primary>sanity-checking options</primary></indexterm>
    <indexterm><primary>warnings</primary></indexterm>


    <para>GHC has a number of options that select which types of
    non-fatal error messages, otherwise known as warnings, can be
    generated during compilation.  By default, you get a standard set
    of warnings which are generally likely to indicate bugs in your
    program.  These are:
    <option>-fwarn-overlapping-patterns</option>,
    <option>-fwarn-warnings-deprecations</option>,
    <option>-fwarn-deprecated-flags</option>,
    <option>-fwarn-duplicate-exports</option>,
    <option>-fwarn-missing-fields</option>,
    <option>-fwarn-missing-methods</option>,
    <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 &ldquo;packages&rdquo; of warnings:
    </para>

    <variablelist>

      <varlistentry>
	<term><option>-W</option>:</term>
	<listitem>
	  <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-unused-binds</option>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-Wall</option>:</term>
	<listitem>
	  <indexterm><primary><option>-Wall</option></primary></indexterm>
	  <para>Turns on all warning options that indicate potentially
	  suspicious code.  The warnings that are
	  <emphasis>not</emphasis> enabled by <option>-Wall</option>
	  are
            <option>-fwarn-tabs</option>,
            <option>-fwarn-incomplete-uni-patterns</option>,
            <option>-fwarn-incomplete-record-updates</option>,
            <option>-fwarn-monomorphism-restriction</option>,
            <option>-fwarn-unrecognised-pragmas</option>,
            <option>-fwarn-auto-orphans</option>,
            <option>-fwarn-implicit-prelude</option>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-w</option>:</term>
	<listitem>
	  <indexterm><primary><option>-w</option></primary></indexterm>
	  <para>Turns off all warnings, including the standard ones and
      those that <literal>-Wall</literal> doesn't enable.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-Werror</option>:</term>
	<listitem>
	  <indexterm><primary><option>-Werror</option></primary></indexterm>
	  <para>Makes any warning into a fatal error. Useful so that you don't
	    miss warnings when doing batch compilation. </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-Wwarn</option>:</term>
	<listitem>
	  <indexterm><primary><option>-Wwarn</option></primary></indexterm>
	  <para>Warnings are treated only as warnings, not as errors. This is
	    the default, but can be useful to negate a
        <option>-Werror</option> flag.</para>
	</listitem>
      </varlistentry>

    </variablelist>

    <para>The full set of warning options is described below.  To turn
    off any warning, simply give the corresponding
    <option>-fno-warn-...</option> option on the command line.</para>

    <variablelist>

      <varlistentry>
	<term><option>-fwarn-unrecognised-pragmas</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-unrecognised-pragmas</option></primary>
	  </indexterm>
	  <indexterm><primary>warnings</primary></indexterm>
	  <indexterm><primary>pragmas</primary></indexterm>
	  <para>Causes a warning to be emitted when a
	  pragma that GHC doesn't recognise is used. As well as pragmas
      that GHC itself uses, GHC also recognises pragmas known to be used
      by other tools, e.g. <literal>OPTIONS_HUGS</literal> and
      <literal>DERIVE</literal>.</para>

	  <para>This option is on by default.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-warnings-deprecations</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
	  </indexterm>
	  <indexterm><primary>warnings</primary></indexterm>
	  <indexterm><primary>deprecations</primary></indexterm>
	  <para>Causes a warning to be emitted when a
	  module, function or type with a WARNING or DEPRECATED pragma
      is used. See <xref linkend="warning-deprecated-pragma"/> for more
      details on the pragmas.</para>

	  <para>This option is on by default.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-deprecated-flags</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
	  </indexterm>
	  <indexterm><primary>deprecated-flags</primary></indexterm>
	  <para>Causes a warning to be emitted when a deprecated
	  commandline flag is used.</para>

	  <para>This option is on by default.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-dodgy-foreign-imports</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
	  </indexterm>
	  <para>Causes a warning to be emitted for foreign imports of
	  the following form:</para>
<programlisting>
foreign import "f" f :: FunPtr t
</programlisting>
          <para>on the grounds that it probably should be</para>
<programlisting>
foreign import "&amp;f" f :: FunPtr t
</programlisting>
          <para>The first form declares that `f` is a (pure) C
          function that takes no arguments and returns a pointer to a
          C function with type `t`, whereas the second form declares
          that `f` itself is a C function with type `t`.  The first
          declaration is usually a mistake, and one that is hard to
          debug because it results in a crash, hence this
          warning.</para>
	</listitem>
      </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>
	  </indexterm>
	  <para>Causes a warning to be emitted when a datatype
      <literal>T</literal> is imported
      with all constructors, i.e. <literal>T(..)</literal>, but has been
      exported abstractly, i.e. <literal>T</literal>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-lazy-unlifted-bindings</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-lazy-unlifted-bindings</option></primary>
	  </indexterm>
	  <para>Causes a warning to be emitted when an unlifted type
      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 7.2.
      </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-duplicate-exports</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
	  <indexterm><primary>duplicate exports, warning</primary></indexterm>
	  <indexterm><primary>export lists, duplicates</primary></indexterm>

	  <para>Have the compiler warn about duplicate entries in
          export lists. This is useful information if you maintain
          large export lists, and want to avoid the continued export
          of a definition after you've deleted (one) mention of it in
          the export list.</para>

	  <para>This option is on by default.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-hi-shadowing</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
	  <indexterm><primary>shadowing</primary>
	    <secondary>interface files</secondary></indexterm>

	  <para>Causes the compiler to emit a warning when a module or
	  interface file in the current directory is shadowing one
	  with the same module name in a library or other
	  directory.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-fwarn-identities</option>:</term>
	<listitem>
          <indexterm><primary><option>-fwarn-identities</option></primary></indexterm>
          <para>Causes the compiler to emit a warning when a Prelude numeric
            conversion converts a type T to the same type T; such calls
            are probably no-ops and can be omitted.  The functions checked for
            are: <literal>toInteger</literal>,
            <literal>toRational</literal>,
            <literal>fromIntegral</literal>,
            and <literal>realToFrac</literal>.
          </para>
	</listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-fwarn-implicit-prelude</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
          <indexterm><primary>implicit prelude, warning</primary></indexterm>
          <para>Have the compiler warn if the Prelude is implicitly
          imported.  This happens unless either the Prelude module is
          explicitly imported with an <literal>import ... Prelude ...</literal>
          line, or this implicit import is disabled (either by
          <option>-XNoImplicitPrelude</option> or a
          <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>

          <para>Note that no warning is given for syntax that implicitly
          refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
          would change whether it refers to the Prelude.
          For example, no warning is given when
          <literal>368</literal> means
          <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
          (where <literal>Prelude</literal> refers to the actual Prelude module,
          regardless of the imports of the module being compiled).</para>

          <para>This warning is off by default.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-incomplete-patterns</option>,
              <option>-fwarn-incomplete-uni-patterns</option>:
        </term>
	<listitem>
	  <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
	  <indexterm><primary><option>-fwarn-incomplete-uni-patterns</option></primary></indexterm>
	  <indexterm><primary>incomplete patterns, warning</primary></indexterm>
	  <indexterm><primary>patterns, incomplete</primary></indexterm>

          <para>The option <option>-fwarn-incomplete-patterns</option> warns
            about places where
	    a pattern-match might fail at runtime.
          The function
          <function>g</function> below will fail when applied to
          non-empty lists, so the compiler will emit a warning about
          this when <option>-fwarn-incomplete-patterns</option> is
          enabled.
<programlisting>
g [] = 2
</programlisting>
	  This option isn't enabled by default because it can be
          a bit noisy, and it doesn't always indicate a bug in the
          program.  However, it's generally considered good practice
          to cover all the cases in your functions, and it is switched
          on by <option>-W</option>.</para>

          <para>The flag <option>-fwarn-incomplete-uni-patterns</option> is
          similar, except that it
          applies only to lambda-expressions and pattern bindings, constructs
	  that only allow a single pattern:
<programlisting>
h = \[] -> 2
Just k = f y
</programlisting>
          </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-incomplete-record-updates</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
	  <indexterm><primary>incomplete record updates, warning</primary></indexterm>
	  <indexterm><primary>record updates, incomplete</primary></indexterm>

	  <para>The function
          <function>f</function> below will fail when applied to
          <literal>Bar</literal>, so the compiler will emit a warning about
          this when <option>-fwarn-incomplete-record-updates</option> is
          enabled.</para>

<programlisting>
data Foo = Foo { x :: Int }
         | Bar

f :: Foo -> Foo
f foo = foo { x = 6 }
</programlisting>

	  <para>This option isn't enabled by default because it can be
          very noisy, and it often doesn't indicate a bug in the
          program.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-fwarn-missing-fields</option>:
	  <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
	  <indexterm><primary>missing fields, warning</primary></indexterm>
	  <indexterm><primary>fields, missing</primary></indexterm>
        </term>
	<listitem>

	  <para>This option is on by default, and warns you whenever
          the construction of a labelled field constructor isn't
          complete, missing initializers for one or more fields. While
          not an error (the missing fields are initialised with
          bottoms), it is often an indication of a programmer error.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-fwarn-missing-import-lists</option>:
	  <indexterm><primary><option>-fwarn-import-lists</option></primary></indexterm>
	  <indexterm><primary>missing import lists, warning</primary></indexterm>
	  <indexterm><primary>import lists, missing</primary></indexterm>
        </term>
	<listitem>

	  <para>This flag warns if you use an unqualified
            <literal>import</literal> declaration
	    that does not explicitly list the entities brought into scope. For
	    example
      </para>
<programlisting>
module M where
  import X( f )
  import Y
  import qualified Z
  p x = f x x
</programlisting>
        <para>
          The <option>-fwarn-import-lists</option> flag will warn about the import
	  of <literal>Y</literal> but not <literal>X</literal>
	  If module <literal>Y</literal> is later changed to export (say) <literal>f</literal>,
          then the reference to <literal>f</literal> in <literal>M</literal> will become
	  ambiguous.  No warning is produced for the import of <literal>Z</literal>
	  because extending <literal>Z</literal>'s exports would be unlikely to produce
	  ambiguity in <literal>M</literal>.
        </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-missing-methods</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
	  <indexterm><primary>missing methods, warning</primary></indexterm>
	  <indexterm><primary>methods, missing</primary></indexterm>

	  <para>This option is on by default, and warns you whenever
          an instance declaration is missing one or more methods, and
          the corresponding class declaration has no default
          declaration for them.</para>
	  <para>The warning is suppressed if the method name
	  begins with an underscore.  Here's an example where this is useful:
	    <programlisting>
	      class C a where
	        _simpleFn :: a -> String
	        complexFn :: a -> a -> String
	        complexFn x y = ... _simpleFn ...
	      </programlisting>
	    The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
	    never <literal>_simpleFn</literal>; and (b)
	    instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
	    </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-missing-signatures</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
	  <indexterm><primary>type signatures, missing</primary></indexterm>

	  <para>If you would like GHC to check that every top-level
          function/value has a type signature, use the
          <option>-fwarn-missing-signatures</option> option.  As part of
	    the warning GHC also reports the inferred type.  The
          option is off by default.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-missing-local-sigs</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-missing-local-sigs</option></primary></indexterm>
	  <indexterm><primary>type signatures, missing</primary></indexterm>

	  <para>If you use the
          <option>-fwarn-missing-local-sigs</option> flag GHC will warn
          you about any polymorphic local bindings. As part of
	    the warning GHC also reports the inferred type. The
          option is off by default.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-name-shadowing</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
	  <indexterm><primary>shadowing, warning</primary></indexterm>

	  <para>This option causes a warning to be emitted whenever an
          inner-scope value has the same name as an outer-scope value,
          i.e. the inner value shadows the outer one.  This can catch
          typographical errors that turn into hard-to-find bugs, e.g.,
          in the inadvertent capture of what would be a recursive call in
          <literal>f = ... let f = id in ... f ...</literal>.</para>
          <para>The warning is suppressed for names beginning with an underscore.  For example
          <programlisting>
             f x = do { _ignore &lt;- this; _ignore &lt;- that; return (the other) }
          </programlisting>
         </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-orphans</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
	  <indexterm><primary>orphan instances, warning</primary></indexterm>
	  <indexterm><primary>orphan rules, warning</primary></indexterm>

	  <para>This option causes a warning to be emitted whenever the
	    module contains an "orphan" instance declaration or rewrite rule.
	    An instance declaration is an orphan if it appears in a module in
	    which neither the class nor the type being instanced are declared
	    in the same module.  A rule is an orphan if it is a rule for a
	    function declared in another module.  A module containing any
	  orphans is called an orphan module.</para>
	  <para>The trouble with orphans is that GHC must pro-actively read the interface
	    files for all orphan modules, just in case their instances or rules
	    play a role, whether or not the module's interface would otherwise
	    be of any use.  See <xref linkend="orphan-modules"/> for details.
            </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-fwarn-overlapping-patterns</option>:
          <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
          <indexterm><primary>overlapping patterns, warning</primary></indexterm>
          <indexterm><primary>patterns, overlapping</primary></indexterm>
        </term>
	<listitem>
	  <para>By default, the compiler will warn you if a set of
          patterns are overlapping, e.g.,</para>

<programlisting>
f :: String -&#62; Int
f []     = 0
f (_:xs) = 1
f "2"    = 2
</programlisting>

	  <para>where the last pattern match in <function>f</function>
          won't ever be reached, as the second pattern overlaps
          it. More often than not, redundant patterns is a programmer
          mistake/error, so this option is enabled by default.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-tabs</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
	  <indexterm><primary>tabs, warning</primary></indexterm>
	  <para>Have the compiler warn if there are tabs in your source
          file.</para>

	  <para>This warning is off by default.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-type-defaults</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
	  <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
	  <para>Have the compiler warn/inform you where in your source
          the Haskell defaulting mechanism for numeric types kicks
          in. This is useful information when converting code from a
          context that assumed one default into one with another,
          e.g., the &lsquo;default default&rsquo; for Haskell 1.4 caused the
          otherwise unconstrained value <constant>1</constant> to be
          given the type <literal>Int</literal>, whereas Haskell 98
          and later
          defaults it to <literal>Integer</literal>.  This may lead to
          differences in performance and behaviour, hence the
          usefulness of being non-silent about this.</para>

	  <para>This warning is off by default.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-monomorphism-restriction</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
	  <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
	  <para>Have the compiler warn/inform you where in your source
          the Haskell Monomorphism Restriction is applied.  If applied silently
	  the MR can give rise to unexpected behaviour, so it can be helpful
	  to have an explicit warning that it is being applied.</para>

	  <para>This warning is off by default.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-unused-binds</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
	  <indexterm><primary>unused binds, warning</primary></indexterm>
	  <indexterm><primary>binds, unused</primary></indexterm>
	  <para>Report any function definitions (and local bindings)
          which are unused.  For top-level functions, the warning is
          only given if the binding is not exported.</para>
	  <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
	    mentioned in the right hand side of another definition that is used, or (c) the
	    function it defines begins with an underscore.  The last case provides a
	    way to suppress unused-binding warnings selectively.  </para>
	  <para> Notice that a variable
	    is reported as unused even if it appears in the right-hand side of another
	    unused binding. </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-unused-imports</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
	  <indexterm><primary>unused imports, warning</primary></indexterm>
	  <indexterm><primary>imports, unused</primary></indexterm>

	  <para>Report any modules that are explicitly imported but
	  never used.  However, the form <literal>import M()</literal> is
	  never reported as an unused import, because it is a useful idiom
	  for importing instance declarations, which are anonymous in Haskell.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fwarn-unused-matches</option>:</term>
	<listitem>
	  <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
	  <indexterm><primary>unused matches, warning</primary></indexterm>
	  <indexterm><primary>matches, unused</primary></indexterm>

	  <para>Report all unused variables which arise from pattern
          matches, including patterns consisting of a single variable.
          For instance <literal>f x y = []</literal> would report
          <varname>x</varname> and <varname>y</varname> as unused.  The
          warning is suppressed if the variable name begins with an underscore, thus:
	    <programlisting>
	       f _x = True
	    </programlisting>
          </para>
	</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 occurring 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 { _ &lt;- 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 occurring 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 &quot;unpacked&quot; by binding the result.
          The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away:
	    <programlisting>
	       do { _ &lt;- 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
    <option>-dcore-lint</option>
    option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
    is a good choice.  It turns on heavyweight intra-pass
    sanity-checking within GHC.  (It checks GHC's sanity, not
    yours.)</para>

  </sect1>

  &packages;

  <sect1 id="options-optimise">
    <title>Optimisation (code improvement)</title>

    <indexterm><primary>optimisation</primary></indexterm>
    <indexterm><primary>improvement, code</primary></indexterm>

    <para>The <option>-O*</option> options specify convenient
    &ldquo;packages&rdquo; of optimisation flags; the
    <option>-f*</option> options described later on specify
    <emphasis>individual</emphasis> optimisations to be turned on/off;
    the <option>-m*</option> options specify
    <emphasis>machine-specific</emphasis> optimisations to be turned
    on/off.</para>

    <sect2 id="optimise-pkgs">
      <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>

      <para>There are <emphasis>many</emphasis> options that affect
      the quality of code produced by GHC.  Most people only have a
      general goal, something like &ldquo;Compile quickly&rdquo; or
      &ldquo;Make my program run like greased lightning.&rdquo; The
      following &ldquo;packages&rdquo; of optimisations (or lack
      thereof) should suffice.</para>

      <para>Note that higher optimisation levels cause more
      cross-module optimisation to be performed, which can have an
      impact on how much of your program needs to be recompiled when
      you change something.  This is one reason to stick to
      no-optimisation when developing code.</para>

      <variablelist>

	<varlistentry>
	  <term>
            No <option>-O*</option>-type option specified:
            <indexterm><primary>-O* not specified</primary></indexterm>
          </term>
	  <listitem>
	    <para>This is taken to mean: &ldquo;Please compile
            quickly; I'm not over-bothered about compiled-code
            quality.&rdquo; So, for example: <command>ghc -c
            Foo.hs</command></para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-O0</option>:
            <indexterm><primary><option>-O0</option></primary></indexterm>
          </term>
	  <listitem>
	    <para>Means &ldquo;turn off all optimisation&rdquo;,
	    reverting to the same settings as if no
	    <option>-O</option> options had been specified.  Saying
	    <option>-O0</option> can be useful if
	    eg. <command>make</command> has inserted a
	    <option>-O</option> on the command line already.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-O</option> or <option>-O1</option>:
            <indexterm><primary>-O option</primary></indexterm>
            <indexterm><primary>-O1 option</primary></indexterm>
            <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
          </term>
	  <listitem>
	    <para>Means: &ldquo;Generate good-quality code without
            taking too long about it.&rdquo; Thus, for example:
            <command>ghc -c -O Main.lhs</command></para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-O2</option>:
            <indexterm><primary>-O2 option</primary></indexterm>
            <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
          </term>
	  <listitem>
	    <para>Means: &ldquo;Apply every non-dangerous
            optimisation, even if it means significantly longer
            compile times.&rdquo;</para>

	    <para>The avoided &ldquo;dangerous&rdquo; optimisations
            are those that can make runtime or space
            <emphasis>worse</emphasis> if you're unlucky.  They are
            normally turned on or off individually.</para>

	    <para>At the moment, <option>-O2</option> is
            <emphasis>unlikely</emphasis> to produce better code than
            <option>-O</option>.</para>
	  </listitem>
	</varlistentry>
      </variablelist>

      <para>We don't use a <option>-O*</option> flag for day-to-day
      work.  We use <option>-O</option> to get respectable speed;
      e.g., when we want to measure something.  When we want to go for
      broke, we tend to use <option>-O2</option> (and we go for
      lots of coffee breaks).</para>

      <para>The easiest way to see what <option>-O</option> (etc.)
      &ldquo;really mean&rdquo; is to run with <option>-v</option>,
      then stand back in amazement.</para>
    </sect2>

    <sect2 id="options-f">
      <title><option>-f*</option>: platform-independent flags</title>

      <indexterm><primary>-f* options (GHC)</primary></indexterm>
      <indexterm><primary>-fno-* options (GHC)</primary></indexterm>

      <para>These flags turn on and off individual optimisations.
      They are normally set via the <option>-O</option> options
      described above, and as such, you shouldn't need to set any of
      them explicitly (indeed, doing so could lead to unexpected
      results).  However, there are one or two that may be of
      interest:</para>

      <variablelist>
	<varlistentry>
	  <term><option>-fexcess-precision</option>:</term>
	  <listitem>
	    <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
	    <para>When this option is given, intermediate floating
	    point values can have a <emphasis>greater</emphasis>
	    precision/range than the final type.  Generally this is a
	    good thing, but some programs may rely on the exact
	    precision/range of
	    <literal>Float</literal>/<literal>Double</literal> values
	    and should not use this option for their compilation.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>-fignore-asserts</option>:</term>
	  <listitem>
	    <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
	    <para>Causes GHC to ignore uses of the function
	    <literal>Exception.assert</literal> in source code (in
	    other words, rewriting <literal>Exception.assert p
	    e</literal> to <literal>e</literal> (see <xref
	    linkend="assertions"/>).  This flag is turned on by
	    <option>-O</option>.
	    </para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-fignore-interface-pragmas</option>
	    <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
          </term>
	  <listitem>
	    <para>Tells GHC to ignore all inessential information when reading interface files.
	    That is, even if <filename>M.hi</filename> contains unfolding or strictness information
	    for a function, GHC will ignore that information.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-fliberate-case</option>
            <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
          </term>
	  <listitem>
	    <para>Turn on the liberate-case transformation.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-fno-cse</option>
            <indexterm><primary><option>-fno-cse</option></primary></indexterm>
          </term>
	  <listitem>
	    <para>Turns off the common-sub-expression elimination optimisation.
	      Can be useful if you have some <literal>unsafePerformIO</literal>
	    expressions that you don't want commoned-up.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-fno-strictness</option>
            <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
          </term>
	  <listitem>
	    <para>Turns off the strictness analyser; sometimes it eats
	    too many cycles.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-fno-full-laziness</option>
            <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
          </term>
	  <listitem>
	    <para>Turns off the full laziness optimisation (also known as
	      let-floating).  Full laziness increases sharing, which can lead
	      to increased memory residency.</para>

            <para>NOTE: GHC doesn't implement complete full-laziness.
            When optimisation in on, and
            <option>-fno-full-laziness</option> is not given, some
            transformations that increase sharing are performed, such
            as extracting repeated computations from a loop.  These
            are the same transformations that a fully lazy
            implementation would do, the difference is that GHC
            doesn't consistently apply full-laziness, so don't rely on
            it.</para>
	  </listitem>
	</varlistentry>

	<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>-fno-state-hack</option>
            <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
          </term>
	  <listitem>
	    <para>Turn off the "state hack" whereby any lambda with a
	      <literal>State#</literal> token as argument is considered to be
	      single-entry, hence it is considered OK to inline things inside
	      it.  This can improve performance of IO and ST monad code, but it
	    runs the risk of reducing sharing.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-fomit-interface-pragmas</option>
	    <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
          </term>
	  <listitem>
	    <para>Tells GHC to omit all inessential information from the interface file
	      generated for the module being compiled (say M).  This means that a module
	      importing M will see only the <emphasis>types</emphasis> of the functions that M exports, but not
	      their unfoldings, strictness info, etc.  Hence, for example,
	      no function exported by M will be inlined
	      into an importing module.  The benefit is that modules that import M will
	      need to be recompiled less often (only when M's exports change their type,
	      not when they change their implementation).
	      </para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-fsimpl-tick-factor=<replaceable>n</replaceable></option>
            <indexterm><primary><option>-fsimpl-tick-factor</option></primary></indexterm>
          </term>
	  <listitem>
	    <para>GHC's optimiser can diverge if you write rewrite rules (<xref linkend="rewrite-rules"/>) 
              that don't terminate, or (less satisfactorily) if you
              code up recursion through data types
              (<xref linkend="bugs-ghc"/>).  To avoid making the compiler fall into an infinite
	      loop, the optimiser carries a "tick count" and stops inlining and applying rewrite rules
              when this count is exceeded.  The limit is set as a multiple of the program size, so 
              bigger programs get more ticks. The <option>-fsimpl-tick-factor</option> flag lets
              you change the multiplier. The default is 100; numbers larger than 100 give more ticks,
              and numbers smaller than 100 give fewer.</para>
            <para>If the tick-count expires, GHC summarises what simplifier steps it has done; 
            you can use <option>-fddump-simpl-stats</option> to generate a much more detailed list.
            Usually that identifies the loop quite accurately, because some numbers are very large.
	      </para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-fstatic-argument-transformation</option>
            <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
          </term>
	  <listitem>
	    <para>Turn on the static argument transformation.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-fspec-constr</option>
            <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
          </term>
	  <listitem>
	    <para>Turn on call-pattern specialisation.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-funbox-strict-fields</option>:
	    <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
	    <indexterm><primary>strict constructor fields</primary></indexterm>
	    <indexterm><primary>constructor fields, strict</primary></indexterm>
          </term>
	  <listitem>
	    <para>This option causes all constructor fields which are
            marked strict (i.e. &ldquo;!&rdquo;) to be unboxed or
            unpacked if possible.  It is equivalent to adding an
            <literal>UNPACK</literal> pragma to every strict
            constructor field (see <xref
            linkend="unpack-pragma"/>).</para>

	    <para>This option is a bit of a sledgehammer: it might
	    sometimes make things worse.  Selectively unboxing fields
	    by using <literal>UNPACK</literal> pragmas might be
	    better. An alternative is to use
        <option>-funbox-strict-fields</option> to turn on
        unboxing by default but disable it for certain constructor
        fields using the <literal>NOUNPACK</literal> pragma
        (see <xref linkend="nounpack-pragma"/>).
        </para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>
            <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
	    <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
	    <indexterm><primary>inlining, controlling</primary></indexterm>
	    <indexterm><primary>unfolding, controlling</primary></indexterm>
          </term>
	  <listitem>
	    <para>(Default: 45) Governs the maximum size that GHC will
            allow a function unfolding to be.   (An unfolding has a
            &ldquo;size&rdquo; that reflects the cost in terms of
            &ldquo;code bloat&rdquo; of expanding that unfolding
            at a call site. A bigger function would be assigned a
            bigger cost.) </para>

	    <para> Consequences: (a) nothing larger than this will be
	    inlined (unless it has an INLINE pragma); (b) nothing
	    larger than this will be spewed into an interface
	    file. </para>


            <para> Increasing this figure is more likely to result in longer
            compile times than faster code.  The next option is more
            useful:</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><option>-funfolding-use-threshold=<replaceable>n</replaceable></option></term>
	  <listitem>
	    <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
	    <indexterm><primary>inlining, controlling</primary></indexterm>
	    <indexterm><primary>unfolding, controlling</primary></indexterm>

	    <para>(Default: 8) This is the magic cut-off figure for
            unfolding: below this size, a function definition will be
            unfolded at the call-site, any bigger and it won't.  The
            size computed for a function depends on two things: the
            actual size of the expression minus any discounts that
            apply (see <option>-funfolding-con-discount</option>).</para>
	  </listitem>
	</varlistentry>
      </variablelist>

    </sect2>

  </sect1>

  &phases;

  &shared_libs;

  <sect1 id="using-concurrent">
    <title>Using Concurrent Haskell</title>
    <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>

    <para>GHC supports Concurrent Haskell by default, without requiring a
      special option or libraries compiled in a certain way.  To get access to
      the support libraries for Concurrent Haskell, just import
      <ulink
	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>

    <variablelist>
      <varlistentry>
	<term><option>-C<replaceable>s</replaceable></option></term>
	<listitem>
	  <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
	    Sets the context switch interval to <replaceable>s</replaceable>
	    seconds.  A context switch will occur at the next heap block
	    allocation after the timer expires (a heap block allocation occurs
	    every 4k of allocation).  With <option>-C0</option> or
	    <option>-C</option>, context switches will occur as often as
	    possible (at every heap block allocation).  By default, context
	    switches occur every 20ms.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </sect1>

  <sect1 id="using-smp">
    <title>Using SMP parallelism</title>
    <indexterm><primary>parallelism</primary>
    </indexterm>
    <indexterm><primary>SMP</primary>
    </indexterm>

    <para>GHC supports running Haskell programs in parallel on an SMP
      (symmetric multiprocessor).</para>

    <para>There's a fine distinction between
      <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
      parallelism is all about making your program run
      <emphasis>faster</emphasis> by making use of multiple processors
      simultaneously.  Concurrency, on the other hand, is a means of
      abstraction: it is a convenient way to structure a program that must
      respond to multiple asynchronous events.</para>

    <para>However, the two terms are certainly related.  By making use of
      multiple CPUs it is possible to run concurrent threads in parallel,
      and this is exactly what GHC's SMP parallelism support does.  But it
      is also possible to obtain performance improvements with parallelism
      on programs that do not use concurrency.  This section describes how to
      use GHC to compile and run parallel programs, in <xref
	linkend="lang-parallel" /> we describe the language features that affect
    parallelism.</para>

    <sect2 id="parallel-compile-options">
      <title>Compile-time options for SMP parallelism</title>

      <para>In order to make use of multiple CPUs, your program must be
	linked with the <option>-threaded</option> option (see <xref
	  linkend="options-linker" />).  Additionally, the following
	compiler options affect parallelism:</para>

      <variablelist>
        <varlistentry>
          <term><option>-feager-blackholing</option></term>
          <indexterm><primary><option>-feager-blackholing</option></primary></indexterm>
          <listitem>
          <para>
            Blackholing is the act of marking a thunk (lazy
            computuation) as being under evaluation.  It is useful for
            three reasons: firstly it lets us detect certain kinds of
            infinite loop (the <literal>NonTermination</literal>
            exception), secondly it avoids certain kinds of space
            leak, and thirdly it avoids repeating a computation in a
            parallel program, because we can tell when a computation
            is already in progress.</para>

          <para>
            The option <option>-feager-blackholing</option> causes
            each thunk to be blackholed as soon as evaluation begins.
            The default is "lazy blackholing", whereby thunks are only
            marked as being under evaluation when a thread is paused
            for some reason.  Lazy blackholing is typically more
            efficient (by 1-2&percnt; or so), because most thunks don't
            need to be blackholed.  However, eager blackholing can
            avoid more repeated computation in a parallel program, and
            this often turns out to be important for parallelism.
          </para>

          <para>
            We recommend compiling any code that is intended to be run
            in parallel with the <option>-feager-blackholing</option>
            flag.
          </para>
          </listitem>
        </varlistentry>
      </variablelist>
    </sect2>

    <sect2 id="parallel-options">
      <title>RTS options for SMP parallelism</title>

      <para>To run a program on multiple CPUs, use the
	RTS <option>-N</option> option:</para>

      <variablelist>
	<varlistentry>
	  <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
	  <listitem>
	    <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
	      Use <replaceable>x</replaceable> simultaneous threads when
	      running the program.  Normally <replaceable>x</replaceable>
	      should be chosen to match the number of CPU cores on the
	      machine<footnote><para>Whether hyperthreading cores should be counted or not is an
	      open question; please feel free to experiment and let us know what
	          results you find.</para></footnote>.  For example,
	      on a dual-core machine we would probably use
	      <literal>+RTS -N2 -RTS</literal>.</para>

            <para>Omitting <replaceable>x</replaceable>,
              i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
              choose the value of <replaceable>x</replaceable> itself
              based on how many processors are in your machine.</para>

            <para>Be careful when using all the processors in your
              machine: if some of your processors are in use by other
              programs, this can actually harm performance rather than
              improve it.</para>

            <para>Setting <option>-N</option> also has the effect of
              enabling the parallel garbage collector (see
              <xref linkend="rts-options-gc" />).</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>

      <para>The following options affect the way the runtime schedules
      threads on CPUs:</para>

      <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>
	  <listitem>
            <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.  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>
       </variablelist>
    </sect2>

    <sect2>
      <title>Hints for using SMP parallelism</title>

      <para>Add the <literal>-s</literal> RTS option when
	running the program to see timing stats, which will help to tell you
	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>The output of <literal>+RTS -s</literal> tells you how
        many &ldquo;sparks&rdquo; 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>

  <sect1 id="options-platform">
    <title>Platform-specific Flags</title>

    <indexterm><primary>-m* options</primary></indexterm>
    <indexterm><primary>platform-specific options</primary></indexterm>
    <indexterm><primary>machine-specific options</primary></indexterm>

    <para>Some flags only make sense for particular target
    platforms.</para>

    <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>

    </variablelist>

  </sect1>

&runtime;

<sect1 id="ext-core">
  <title>Generating and compiling External Core Files</title>

  <indexterm><primary>intermediate code generation</primary></indexterm>

  <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; 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="../../core.pdf">
  <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
  and sample tools
  for manipulating Core files (in Haskell) are available in the
  <ulink url="http://hackage.haskell.org/package/extcore">extcore package on Hackage</ulink>.  Note that the format of <literal>.hcr</literal>
  files is <emphasis>different</emphasis> from the Core output format that GHC generates
  for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>

  <para>The Core format natively supports notes which you can add to
  your source code using the <literal>CORE</literal> pragma (see <xref
  linkend="pragmas"/>).</para>

    <variablelist>

  	<varlistentry>
	  <term>
            <option>-fext-core</option>
            <indexterm><primary><option>-fext-core</option></primary></indexterm>
          </term>
	  <listitem>
	    <para>Generate <literal>.hcr</literal> files.</para>
	  </listitem>
	</varlistentry>

    </variablelist>

<para>Currently (as of version 6.8.2), GHC does not have the ability to read in External Core files as source. If you would like GHC to have this ability, please <ulink url="http://hackage.haskell.org/trac/ghc/wiki/MailingListsAndIRC">make your wishes known to the GHC Team</ulink>.</para>

</sect1>

&debug;
&flags;

</chapter>

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