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>--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>
    <indexterm><primary>help options</primary></indexterm>

    <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 --make</command>
          </cmdsynopsis>
          <indexterm><primary>make mode</primary></indexterm>
          <indexterm><primary><option>--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>--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>--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>--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>--supported-extensions</option></primary><primary><option>--supported-languages</option></primary></indexterm>
        </term>
        <listitem>
          <para>Print the supported language extensions.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <cmdsynopsis>
            <command>ghc --show-options</command>
          </cmdsynopsis>
          <indexterm><primary><option>--show-options</option></primary></indexterm>
        </term>
        <listitem>
          <para>Print the supported command line options. This flag can be used for autocompletion in a shell.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <cmdsynopsis>
            <command>ghc --info</command>
          </cmdsynopsis>
          <indexterm><primary><option>--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>--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>--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>--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>--make</option></title>
      <indexterm><primary><option>--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 --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
      --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
          --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>
        <listitem>
          <para>Using the <literal>-j</literal> flag, you can compile
          modules in parallel. Specify <literal>-jN</literal> to
          compile <replaceable>N</replaceable> jobs in parallel.</para>
        </listitem>
      </itemizedlist>

      <para>Any of the command-line options described in the rest of
      this chapter can be used with
      <option>--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 what
      backend code generator is used. See <xref linkend="code-generators"/>
      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>Verbosity options</title>

    <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>--make</option> or
                <option>--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>--fprint-explicit-foralls, -fprint-explicit-kinds</option>
          <indexterm><primary><option>-fprint-explicit-foralls</option></primary></indexterm>
          <indexterm><primary><option>-fprint-explicit-kinds</option></primary></indexterm>
        </term>
        <listitem>
          <para>These two flags control the way in which GHC displays types, in error messages and in GHCi.
          Using <option>-fprint-explicit-foralls</option> makes GHC print explicit <literal>forall</literal>
          quantification at the top level of a type; normally this is suppressed.  For example, in GHCi:
<screen>
ghci> let f x = x
ghci> :t f
f :: a -> a
ghci> :set -fprint-explicit-foralls
ghci> :t f
f :: forall a. a -> a
</screen>
However, regardless of the flag setting, the quantifiers are printed under these circumstances:
<itemizedlist>
<listitem><para>For nested <literal>foralls</literal>, e.g.
<screen>
ghci> :t GHC.ST.runST
GHC.ST.runST :: (forall s. GHC.ST.ST s a) -> a
</screen>
</para></listitem>
<listitem><para>If any of the quantified type variables has a kind
that mentions a kind variable, e.g.
<screen>
ghci> :i Data.Coerce.coerce
coerce ::
  forall (k :: BOX) (a :: k) (b :: k). Coercible a b => a -> b
     -- Defined in GHC.Prim
</screen>
</para></listitem>
</itemizedlist>
          </para>
          <para>
         Using <option>-fprint-explicit-kinds</option> makes GHC print kind arguments
         in types, which are normally suppressed.  This can be important when you are using kind polymorphism.
         For example:
<screen>
ghci> :set -XPolyKinds
ghci> data T a = MkT
ghci> :t MkT
MkT :: forall (k :: BOX) (a :: k). T a
ghci> :set -fprint-explicit-foralls
ghci> :t MkT
MkT :: forall (k :: BOX) (a :: k). T k a
</screen>
         </para>
        </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-amp</option>,
    <option>-fwarn-deprecated-flags</option>,
    <option>-fwarn-unrecognised-pragmas</option>,
    <option>-fwarn-pointless-pragmas</option>,
    <option>-fwarn-duplicate-constraints</option>,
    <option>-fwarn-duplicate-exports</option>,
    <option>-fwarn-overflowed-literals</option>,
    <option>-fwarn-empty-enumerations</option>,
    <option>-fwarn-missing-fields</option>,
    <option>-fwarn-missing-methods</option>,
    <option>-fwarn-wrong-do-bind</option>,
    <option>-fwarn-unsupported-calling-conventions</option>,
    <option>-fwarn-dodgy-foreign-imports</option>,
    <option>-fwarn-inline-rule-shadowing</option>,
    <option>-fwarn-unsupported-llvm-version</option>, and
    <option>-fwarn-context-quantification</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-auto-orphans</option>,
            <option>-fwarn-implicit-prelude</option>,
            <option>-fwarn-missing-local-sigs</option>,
            <option>-fwarn-missing-import-lists</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-typed-holes</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-typed-holes</option></primary>
          </indexterm>
          <indexterm><primary>warnings</primary></indexterm>
            <para>When the compiler encounters an unbound local
            variable prefixed with <literal>_</literal>, or encounters
            the literal <literal>_</literal> on the right-hand side of
            an expression, the error message for the unbound term
            includes the type it needs to type check. It works
            particularly well with <link
            linkend="defer-type-errors">deferred type errors</link>.
            See <xref linkend="typed-holes"/></para>

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


      <varlistentry>
        <term><option>-fdefer-type-errors</option>:</term>
        <listitem>
          <indexterm><primary><option>-fdefer-type-errors</option></primary>
          </indexterm>
          <indexterm><primary>warnings</primary></indexterm>
            <para>Defer as many type errors as possible until runtime.  
            At compile time you get a warning (instead of an error).  At 
            runtime, if you use a value that depends on a type error, you 
            get a runtime error; but you can run any type-correct parts of your code 
            just fine.  See <xref linkend="defer-type-errors"/></para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-fhelpful-errors</option>:</term>
        <listitem>
          <indexterm><primary><option>-fhelpful-errors</option></primary>
          </indexterm>
          <indexterm><primary>warnings</primary></indexterm>
            <para>When a name or package is not found in scope, make
            suggestions for the name or package you might have meant instead.</para>
          <para>This option is on by default.</para>
        </listitem>
      </varlistentry>

      <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-pointless-pragmas</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-pointless-pragmas</option></primary>
          </indexterm>
          <indexterm><primary>warnings</primary></indexterm>
          <indexterm><primary>pragmas</primary></indexterm>
          <para>Causes a warning to be emitted when GHC detects that a
          module contains a pragma that has no effect.</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-amp</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-amp</option></primary>
          </indexterm>
          <indexterm><primary>amp</primary></indexterm>
          <indexterm><primary>applicative-monad proposal</primary></indexterm>
          <para>Causes a warning to be emitted when a definition
          is in conflict with the AMP (Applicative-Monad proosal),
          namely:
          1. Instance of Monad without Applicative;
          2. Instance of MonadPlus without Alternative;
          3. Custom definitions of join/pure/&lt;*&gt;</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-unsupported-calling-conventions</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-unsupported-calling-conventions</option></primary>
          </indexterm>
          <para>Causes a warning to be emitted for foreign declarations
          that use unsupported calling conventions. In particular,
          if the <literal>stdcall</literal> calling convention is used
          on an architecture other than i386 then it will be treated
          as <literal>ccall</literal>.</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 in the following cases:</para>
          <itemizedlist>
            <listitem>
              <para>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>
            <listitem>
              <para>When an <literal>import</literal> statement hides an
                entity that is not exported.</para>
            </listitem>
          </itemizedlist>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-fwarn-overflowed-literals</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-overflowed-literals</option></primary>
          </indexterm>
          <para>
              Causes a warning to be emitted if a literal will overflow,
              e.g. <literal>300 :: Word8</literal>.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-fwarn-empty-enumerations</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-empty-enumerations</option></primary>
          </indexterm>
          <para>
              Causes a warning to be emitted if an enumeration is
              empty, e.g. <literal>[5 .. 3]</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>This flag is a no-op, and will be removed in GHC 7.10.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-fwarn-duplicate-constraints</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-duplicate-constraints</option></primary></indexterm>
          <indexterm><primary>duplicate constraints, warning</primary></indexterm>

          <para>Have the compiler warn about duplicate constraints in a type signature. For
          example
          <programlisting>
             f :: (Eq a, Show a, Eq a) => a -> a
          </programlisting>
          The warning will indicate the duplicated <literal>Eq a</literal> constraint.
          </para>

          <para>This option is on by default.</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>
          <para>The MINIMAL pragma can be used to change which combination of methods will be required for instances of a particular class. See <xref linkend="minimal-pragma"/>.</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, -fwarn-auto-orphans</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
          <indexterm><primary><option>-fwarn-auto-orphans</option></primary></indexterm>
          <indexterm><primary>orphan instances, warning</primary></indexterm>
          <indexterm><primary>orphan rules, warning</primary></indexterm>

          <para>These flags cause 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>
           <para>The flag <option>-fwarn-orphans</option> warns about user-written
            orphan rules or instances.  The flag <option>-fwarn-auto-orphans</option>
            warns about automatically-generated orphan rules, notably as a result of
            specialising functions, for type classes (<literal>Specialise</literal>)
            or argument values (<literal>-fspec-constr</literal>).</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.  More precisely:

          <itemizedlist>
          <listitem><para>Warn if a binding brings into scope a variable that is not used,
          except if the variable's name starts with an underscore.  The "starts-with-underscore"
          condition provides a way to selectively disable the warning.
        </para>
          <para>
          A variable is regarded as "used" if 
          <itemizedlist>
          <listitem><para>It is exported, or</para></listitem>
          <listitem><para>It appears in the right hand side of a binding that binds at 
                           least one used variable that is used</para></listitem>
          </itemizedlist>
          For example
            <programlisting>
module A (f) where
f = let (p,q) = rhs1 in t p  -- Warning about unused q
t = rhs3                     -- No warning: f is used, and hence so is t
g = h x                      -- Warning: g unused
h = rhs2                     -- Warning: h is only used in the right-hand side of another unused binding
_w = True                    -- No warning: _w starts with an underscore
            </programlisting>
          </para></listitem>

          <listitem><para>
          Warn if a pattern binding binds no variables at all, unless it is a lone, possibly-banged, wild-card pattern.
          For example:
            <programlisting>
Just _ = rhs3    -- Warning: unused pattern binding
(_, _) = rhs4    -- Warning: unused pattern binding
_  = rhs3        -- No warning: lone wild-card pattern
!_ = rhs4        -- No warning: banged wild-card pattern; behaves like seq
            </programlisting>
          The motivation for allowing lone wild-card patterns is they
          are not very different from <literal>_v = rhs3</literal>,
          which elicits no warning; and they can be useful to add a type
          constraint, e.g. <literal>_ = x::Int</literal>. A lone
          banged wild-card pattern is is useful as an alternative 
          (to <literal>seq</literal>) way to force evaluation.
        </para>
        </listitem>
          </itemizedlist>
          </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-context-quantification</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-context-quantification</option></primary></indexterm>
          <indexterm><primary>implicit context quantification, warning</primary></indexterm>
          <indexterm><primary>context, implicit quantification</primary></indexterm>

          <para>Report if a variable is quantified only due to its presence
          in a context (see <xref linkend="universal-quantification"/>). For example,
            <programlisting>
              type T a = Monad m => a -> f a
            </programlisting>
          It is recommended to write this polymorphic type as
            <programlisting>
              type T a = forall m. Monad m => a -> f a
            </programlisting>
          instead.
          </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>

      <varlistentry>
        <term><option>-fwarn-inline-rule-shadowing</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-inline-rule-shadowing</option></primary></indexterm>
          <para>Warn if a rewrite RULE might fail to fire because the function might be
                inlined before the rule has a chance to fire.  See <xref linkend="rules-inline"/>.
              </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).  A flag <option>-fwombat</option> can be negated by 
      saying <option>-fno-wombat</option>.  The flags below are off
      by default, except where noted below.  See <xref linkend="options-f-compact"/> 
      for a compact list.
     </para>

      <variablelist>
        <varlistentry>
          <term>
            <option>-favoid-vect</option>
            <indexterm><primary><option></option></primary></indexterm>
          </term>
          <listitem>
            <para>Part of <link linkend="dph">Data Parallel Haskell
            (DPH)</link>.</para>

            <para><emphasis>Off by default.</emphasis> Enable the
            <emphasis>vectorisation</emphasis> avoidance optimisation. This
            optimisation only works when used in combination with the
            <option>-fvectorise</option> transformation.</para>

            <para>While vectorisation of code using DPH is often a big win, it
            can also produce worse results for some kinds of code. This
            optimisation modifies the vectorisation transformation to try to
            determine if a function would be better of unvectorised and if
            so, do just that.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fcase-merge</option>
            <indexterm><primary><option></option></primary></indexterm>
          </term>
          <listitem>
            <para><emphasis>On by default.</emphasis> 
            Merge immediately-nested case expressions that scrutinse the same variable. Example
<programlisting>
  case x of
     Red -> e1
     _   -> case x of 
              Blue -> e2
              Green -> e3
==>
  case x of
     Red -> e1
     Blue -> e2
     Green -> e2
</programlisting>
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fcse</option>
            <indexterm><primary><option>-fcse</option></primary></indexterm>
          </term>
          <listitem>
            <para><emphasis>On by default.</emphasis>.  Enables the common-sub-expression 
            elimination optimisation.
            Switching this off can be useful if you have some <literal>unsafePerformIO</literal>
            expressions that you don't want commoned-up.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fdicts-cheap</option>
            <indexterm><primary><option></option></primary></indexterm>
          </term>
          <listitem>
            <para>A very experimental flag that makes dictionary-valued
            expressions seem cheap to the optimiser.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fdo-lambda-eta-expansion</option>
            <indexterm><primary><option></option></primary></indexterm>
          </term>
          <listitem>
            <para><emphasis>On by default.</emphasis>
            Eta-expand let-bindings to increase their arity.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fdo-eta-reduction</option>
            <indexterm><primary><option></option></primary></indexterm>
          </term>
          <listitem>
            <para><emphasis>On by default.</emphasis>
            Eta-reduce lambda expressions, if doing so gets rid of a whole
            group of lambdas.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-feager-blackholing</option>
            <indexterm><primary><option></option></primary></indexterm>
          </term>
          <listitem>
            <para>Usually GHC black-holes a thunk only when it switches
            threads. This flag makes it do so as soon as the thunk is
            entered. See <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/">
              Haskell on a shared-memory multiprocessor</ulink>.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fexcess-precision</option>
            <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
          </term>
          <listitem>
            <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>

            <para>
              Note that the 32-bit x86 native code generator only
              supports excess-precision mode, so neither
              <option>-fexcess-precision</option> nor
              <option>-fno-excess-precision</option> has any effect.
              This is a known bug, see <xref linkend="bugs-ghc" />.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fexpose-all-unfoldings</option>
            <indexterm><primary><option></option></primary></indexterm>
          </term>
          <listitem>
            <para>An experimental flag to expose all unfoldings, even for very
            large or recursive functions. This allows for all functions to be
            inlined while usually GHC would avoid inlining larger functions.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-ffloat-in</option>
            <indexterm><primary><option></option></primary></indexterm>
          </term>
          <listitem>
            <para><emphasis>On by default.</emphasis>
            Float let-bindings inwards, nearer their binding site.  See
            <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">
              Let-floating: moving bindings to give faster programs (ICFP'96)</ulink>.
            </para>

            <para>This optimisation moves let bindings closer to their use
            site. The benefit here is that this may avoid unnecessary
            allocation if the branch the let is now on is never executed. It
            also enables other optimisation passes to work more effectively
            as they have more information locally.
            </para>

            <para>This optimisation isn't always beneficial though (so GHC
            applies some heuristics to decide when to apply it). The details
            get complicated but a simple example is that it is often beneficial
            to move let bindings outwards so that multiple let bindings can be
            grouped into a larger single let binding, effectively batching
            their allocation and helping the garbage collector and allocator.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-ffull-laziness</option>
            <indexterm><primary><option>-ffull-laziness</option></primary></indexterm>
          </term>
          <listitem>
            <para><emphasis>On by default.</emphasis>
            Run the full laziness optimisation (also known as let-floating),
            which floats let-bindings outside enclosing lambdas, in the hope
            they will be thereby be computed less often.  See
            <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">Let-floating:
              moving bindings to give faster programs (ICFP'96)</ulink>.
            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>-ffun-to-thunk</option>
            <indexterm><primary><option>-ffun-to-thunk</option></primary></indexterm>
          </term>
          <listitem>
            <para>Worker-wrapper removes unused arguments, but usually we do
            not remove them all, lest it turn a function closure into a thunk,
            thereby perhaps creating a space leak and/or disrupting inlining.
            This flag allows worker/wrapper to remove <emphasis>all</emphasis>
            value lambdas.  Off by default.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fignore-asserts</option>
            <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
          </term>
          <listitem>
            <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>-flate-dmd-anal</option>
            <indexterm><primary><option>-flate-dmd-anal</option></primary></indexterm>
          </term>
          <listitem>
            <para><emphasis>Off by default.</emphasis>Run demand analysis
            again, at the end of the simplification pipeline.  We found some opportunities
            for discovering strictness that were not visible earlier; and optimisations like
            <literal>-fspec-constr</literal> can create functions with unused arguments which
            are eliminated by late demand analysis.  Improvements are modest, but so is the
            cost.  See notes on the <ulink href="http://ghc.haskell.org/trac/ghc/wiki/LateDmd">Trac wiki page</ulink>.
            </para>
            </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fliberate-case</option>
            <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
          </term>
          <listitem>
            <para><emphasis>Off by default, but enabled by -O2.</emphasis> 
            Turn on the liberate-case transformation.  This unrolls recursive
            function once in its own RHS, to avoid repeated case analysis of
            free variables.  It's a bit like the call-pattern specialiser
            (<option>-fspec-constr</option>) but for free variables rather than
            arguments.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fliberate-case-threshold=N</option>
            <indexterm><primary><option>-fliberate-case-threshold</option></primary></indexterm>
          </term>
          <listitem>
            <para>Set the size threshold for the liberate-case transformation.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fmax-relevant-bindings=N</option>
            <indexterm><primary><option>-fmax-relevant-bindings</option></primary></indexterm>
          </term>
          <listitem>
            <para>The type checker sometimes displays a fragment of the type environment
                  in error messages, but only up to some maximum number, set by this flag.
                  The default is 6.  Turning it off with <option>-fno-max-relevant-bindings</option> 
                   gives an unlimited number. Syntactically top-level bindings are also 
                   usually excluded (since they may be numerous), but 
                   <option>-fno-max-relevant-bindings</option> includes them too.
            </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>-fomit-yields</option>
            <indexterm><primary><option>-fomit-yields</option></primary></indexterm>
          </term>
          <listitem>
              <para><emphasis>On by default.</emphasis>  Tells GHC to omit
            heap checks when no allocation is being performed.  While this improves
            binary sizes by about 5%, it also means that threads run in
            tight non-allocating loops will not get preempted in a timely
            fashion.  If it is important to always be able to interrupt such
            threads, you should turn this optimization off.  Consider also
            recompiling all libraries with this optimization turned off, if you
            need to guarantee interruptibility.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fpedantic-bottoms</option>
            <indexterm><primary><option>-fpedantic-bottoms</option></primary></indexterm>
          </term>
          <listitem>
            <para>Make GHC be more precise about its treatment of bottom (but see also
            <option>-fno-state-hack</option>). In particular, stop GHC
            eta-expanding through a case expression, which is good for
            performance, but bad if you are using <literal>seq</literal> on
            partial applications.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fregs-graph</option>
            <indexterm><primary><option></option></primary></indexterm>
          </term>
          <listitem>
            <para><emphasis>Off by default, but enabled by -O2. Only applies in
              combination with the native code generator.</emphasis>
            Use the graph colouring register allocator for register allocation
            in the native code generator. By default, GHC uses a simpler,
            faster linear register allocator. The downside being that the
            linear register allocator usually generates worse code.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fregs-iterative</option>
            <indexterm><primary><option></option></primary></indexterm>
          </term>
          <listitem>
            <para><emphasis>Off by default, only applies in combination with
              the native code generator.</emphasis>
            Use the iterative coalescing graph colouring register allocator for
            register allocation in the native code generator. This is the same
            register allocator as the <option>-freg-graph</option> one but also
            enables iterative coalescing during register allocation.
            </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>-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 (aka inlining) 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
            <option>-funfolding-use-threshold</option> is more useful.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-funfolding-use-threshold=<replaceable>n</replaceable></option>
            <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
            <indexterm><primary>inlining, controlling</primary></indexterm>
            <indexterm><primary>unfolding, controlling</primary></indexterm>
          </term>
          <listitem>
            <para>(Default: 8) This is the magic cut-off figure for unfolding
            (aka inlining): 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>

            <para>The difference between this and
            <option>-funfolding-creation-threshold</option> is that this one
            determines if a function definition will be inlined <emphasis>at
              a call site</emphasis>. The other option determines if a
            function definition will be kept around at all for potential
            inlining.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fvectorise</option>
            <indexterm><primary><option></option></primary></indexterm>
          </term>
          <listitem>
            <para>Part of <link linkend="dph">Data Parallel Haskell
            (DPH)</link>.</para>

            <para><emphasis>Off by default.</emphasis> Enable the
            <emphasis>vectorisation</emphasis> optimisation transformation. This
            optimisation transforms the nested data parallelism code of programs
            using DPH into flat data parallelism. Flat data parallel programs
            should have better load balancing, enable SIMD parallelism and
            friendlier cache behaviour.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fspec-constr</option>
            <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
          </term>
          <listitem>
            <para><emphasis>Off by default, but enabled by -O2.</emphasis>
            Turn on call-pattern specialisation; see
            <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/spec-constr/index.htm">
              Call-pattern specialisation for Haskell programs</ulink>.
            </para>

            <para>This optimisation specializes recursive functions according to
            their argument "shapes". This is best explained by example so
            consider:
<programlisting>
last :: [a] -> a
last [] = error "last"
last (x : []) = x
last (x : xs) = last xs
</programlisting>
            In this code, once we pass the initial check for an empty list we
            know that in the recursive case this pattern match is redundant. As
            such <option>-fspec-constr</option> will transform the above code
            to:
<programlisting>
last :: [a] -> a
last []       = error "last"
last (x : xs) = last' x xs
    where
      last' x []       = x
      last' x (y : ys) = last' y ys
</programlisting>
            </para>

            <para>As well avoid unnecessary pattern matching it also helps avoid
            unnecessary allocation. This applies when a argument is strict in
            the recursive call to itself but not on the initial entry. As
            strict recursive branch of the function is created similar to the
            above example.
            </para>

            <para>It is also possible for library writers to instruct
            GHC to perform call-pattern specialisation extremely
            aggressively. This is necessary for some highly optimized
            libraries, where we may want to specialize regardless of
            the number of specialisations, or the size of the code. As
            an example, consider a simplified use-case from the
            <literal>vector</literal> library:</para>
<programlisting>
import GHC.Types (SPEC(..))

foldl :: (a -> b -> a) -> a -> Stream b -> a
{-# INLINE foldl #-}
foldl f z (Stream step s _) = foldl_loop SPEC z s
  where
    foldl_loop !sPEC z s = case step s of
                            Yield x s' -> foldl_loop sPEC (f z x) s'
                            Skip       -> foldl_loop sPEC z s'
                            Done       -> z
</programlisting>

            <para>Here, after GHC inlines the body of
            <literal>foldl</literal> to a call site, it will perform
            call-pattern specialization very aggressively on
            <literal>foldl_loop</literal> due to the use of
            <literal>SPEC</literal> in the argument of the loop
            body. <literal>SPEC</literal> from
            <literal>GHC.Types</literal> is specifically recognized by
            the compiler.</para>

            <para>(NB: it is extremely important you use
            <literal>seq</literal> or a bang pattern on the
            <literal>SPEC</literal> argument!)</para>

            <para>In particular, after inlining this will
            expose <literal>f</literal> to the loop body directly,
            allowing heavy specialisation over the recursive
            cases.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fspecialise</option>
            <indexterm><primary><option>-fspecialise</option></primary></indexterm>
          </term>
          <listitem>
            <para><emphasis>On by default.</emphasis>
            Specialise each type-class-overloaded function defined in this
            module for the types at which it is called in this module.  Also
            specialise imported functions that have an INLINABLE pragma
            (<xref linkend="inlinable-pragma"/>) for the types at which they
            are called in this module.
            </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, which turns a
            recursive function into a non-recursive one with a local
            recursive loop. See Chapter 7 of
            <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/santos-thesis.ps.gz">
              Andre Santos's PhD thesis</ulink>
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-fstrictness</option>
            <indexterm><primary><option></option></primary></indexterm>
          </term>
          <listitem>
            <para> <emphasis>On by default.</emphasis>.  
            Switch on the strictness analyser. There is a very old paper about GHC's 
            strictness analyser, <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/simple-strictnes-analyser.ps.gz">
              Measuring the effectiveness of a simple strictness analyser</ulink>,
            but the current one is quite a bit different.
            </para>

            <para>The strictness analyser figures out when arguments and
            variables in a function can be treated 'strictly' (that is they
            are always evaluated in the function at some point). This allow
            GHC to apply certain optimisations such as unboxing that
            otherwise don't apply as they change the semantics of the program
            when applied to lazy arguments.
            </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 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>-funbox-small-strict-fields</option>:
            <indexterm><primary><option>-funbox-small-strict-fields</option></primary></indexterm>
            <indexterm><primary>strict constructor fields</primary></indexterm>
            <indexterm><primary>constructor fields, strict</primary></indexterm>
          </term>
          <listitem>
            <para><emphasis>On by default.</emphasis>. This option
            causes all constructor fields which are marked strict
            (i.e. &ldquo;!&rdquo;) and which representation is smaller
            or equal to the size of a pointer to be unpacked, if
            possible. It is equivalent to adding an
            <literal>UNPACK</literal> pragma (see <xref
            linkend="unpack-pragma"/>) to every strict constructor
            field that fulfils the size restriction.
            </para>

            <para>For example, the constructor fields in the following
            data types
<programlisting>
data A = A !Int
data B = B !A
newtype C = C B
data D = D !C
</programlisting>
            would all be represented by a single
            <literal>Int#</literal> (see <xref linkend="primitives"/>)
            value with
            <option>-funbox-small-strict-fields</option> enabled.
            </para>

            <para>This option is less of a sledgehammer than
            <option>-funbox-strict-fields</option>: it should rarely make things
            worse. If you use <option>-funbox-small-strict-fields</option>
            to turn on unboxing by default you can disable it for certain
            constructor fields using the <literal>NOUNPACK</literal> pragma (see
            <xref linkend="nounpack-pragma"/>).</para>

            <para>
            Note that for consistency <literal>Double</literal>,
            <literal>Word64</literal>, and <literal>Int64</literal> constructor
            fields are unpacked on 32-bit platforms, even though they are
            technically larger than a pointer on those platforms.
            </para>
          </listitem>
        </varlistentry>

      </variablelist>

    </sect2>

  </sect1>

  &code-gens;

  &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>
      Optionally, the program may be linked with
      the <option>-threaded</option> option (see
      <xref linkend="options-linker" />.  This provides two benefits:

      <itemizedlist>
        <listitem>
          <para>It enables the <option>-N</option><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm> RTS option to be
            used, which allows threads to run in
            parallel<indexterm><primary>parallelism</primary></indexterm>
            on a
            multiprocessor<indexterm><primary>multiprocessor</primary></indexterm><indexterm><primary>SMP</primary></indexterm>
            or
            multicore<indexterm><primary>multicore</primary></indexterm>
            machine.  See <xref linkend="using-smp" />.</para>
        </listitem>
        <listitem>
          <para>If a thread makes a foreign call (and the call is
            not marked <literal>unsafe</literal>), then other
            Haskell threads in the program will continue to run
            while the foreign call is in progress.
            Additionally, <literal>foreign export</literal>ed
            Haskell functions may be called from multiple OS
            threads simultaneously.  See
            <xref linkend="ffi-threads" />.</para>
        </listitem>
      </itemizedlist>
    </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>There are two ways to run a program on multiple
        processors:
        call <literal>Control.Concurrent.setNumCapabilities</literal> from your
        program, or 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>The current value of the <option>-N</option> option
              is available to the Haskell program
              via <literal>Control.Concurrent.getNumCapabilities</literal>, and
              it may be changed while the program is running by
              calling <literal>Control.Concurrent.setNumCapabilities</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>Control.Concurrent.forkOn</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 <link linkend="native-code-gen">native code generator</link>.
                                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). The
                                <link linkend="llvm-code-gen">LLVM backend</link> will also use SSE2
                                if your processor supports it but detects this automatically so no
                                flag is required.
          </para>
          <para>
            SSE2 is unconditionally used on x86-64 platforms.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-msse4.2</option>:</term>
        <listitem>
          <para>
                                (x86 only, added in GHC 7.4.1) Use the SSE4.2 instruction set to
                                implement some floating point and bit operations when using the
                                <link linkend="native-code-gen">native code generator</link>. The
                                resulting compiled code will only run on processors that
                                support SSE4.2 (Intel Core i7 and later). The
                                <link linkend="llvm-code-gen">LLVM backend</link> will also use
                                SSE4.2 if your processor supports it but detects this automatically
                                so no flag is required.
          </para>
        </listitem>
      </varlistentry>

    </variablelist>

  </sect1>

&runtime;
&debug;
&flags;

</chapter>

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