[ghc-hetmet.git] / docs / users_guide / using.xml

<?xml version="1.0" encoding="iso-8859-1"?>
<chapter id="using-ghc">
  <title>Using GHC</title>

  <indexterm><primary>GHC, using</primary></indexterm>
  <indexterm><primary>using GHC</primary></indexterm>

  <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 uses GHC
      extensions, it will always need to be compiled with the
      <option>-fglasgow-exts</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 -fglasgow-exts #-}
module X where
...
</programlisting>
      
      <para><literal>OPTIONS_GHC</literal> pragmas are only looked for at
      the top of your source files, upto the first
      (non-literate,non-empty) line not containing
      <literal>OPTIONS_GHC</literal>. Multiple <literal>OPTIONS_GHC</literal>
      pragmas are recognised.  Do not put comments before, or on the same line
        as, the <literal>OPTIONS_GHC</literal> pragma.</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 prepended to the
      command-line options, so you <emphasis>do</emphasis> have the
      ability to override OPTIONS_GHC settings via 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-too</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 either
    <firstterm>static</firstterm> or <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 be only 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>GHC_OPTIONS</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>
  </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>.s</filename></term>
        <listitem>
          <para>An assembly-language source file, usually produced by
          the compiler.</para>
        </listitem>
      </varlistentry>

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

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

  </sect1>

  <sect1 id="modes">
    <title>Modes of operation</title>

    <para>GHC's behaviour is firstly controlled by a mode flag.  Only
    one of these flags may be given, but it does not necessarily need
    to be the first option on the command-line.  The available modes
    are:</para>

    <variablelist>
      <varlistentry>
        <term>
          <cmdsynopsis><command>ghc</command>
            <arg choice='plain'>&ndash;&ndash;interactive</arg>
          </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</command>
            <arg choice='plain'>&ndash;&ndash;make</arg>
          </cmdsynopsis>
          <indexterm><primary>make mode</primary></indexterm>
          <indexterm><primary><option>&ndash;&ndash;make</option></primary></indexterm>
        </term>
        <listitem>
          <para>In this mode, GHC will build a multi-module Haskell
          program automatically, figuring out dependencies for itself.
          If you have a straightforward Haskell program, this is
          likely to be much easier, and faster, than using
          <command>make</command>.  Make mode is described in <xref
          linkend="make-mode"/>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <cmdsynopsis><command>ghc</command>
            <arg choice='plain'>&ndash;e</arg> <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</command>
            <group>
              <arg>-E</arg>
              <arg>-C</arg>
              <arg>-S</arg>
              <arg>-c</arg>
            </group>
          </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</command>
            <arg choice='plain'>&ndash;M</arg>
          </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="sec-makefile-dependencies"/>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <cmdsynopsis>
            <command>ghc</command>
            <arg choice='plain'>&ndash;&ndash;mk-dll</arg>
          </cmdsynopsis>
          <indexterm><primary>dependency-generation mode</primary></indexterm>
        </term>
        <listitem>
          <para>DLL-creation mode (Windows only).  See <xref
          linkend="win32-dlls-create"/>.</para>
        </listitem>
      </varlistentry>
    </variablelist>

    <sect2 id="make-mode">
      <title>Using <command>ghc</command> <option>&ndash;&ndash;make</option></title>
      <indexterm><primary><option>&ndash;&ndash;make</option></primary></indexterm>
      <indexterm><primary>separate compilation</primary></indexterm>
      
      <para>When given the <option>&ndash;&ndash;make</option> option,
      GHC will build a multi-module Haskell program by following
      dependencies from a single root module (usually
      <literal>Main</literal>).  For example, if your
      <literal>Main</literal> module is in a file called
      <filename>Main.hs</filename>, you could compile and link the
      program like this:</para>

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

      <para>The command line may contain any number of source file
      names or module names; GHC will figure out all the modules in
      the program by following the imports from these initial modules.
      It will then attempt to compile each module which is out of
      date, and finally if there is a <literal>Main</literal> module,
      the program will also be linked into an executable.</para>

      <para>The main advantages to using <literal>ghc
      &ndash;&ndash;make</literal> over traditional
      <literal>Makefile</literal>s are:</para>

      <itemizedlist>
        <listitem>
          <para>GHC doesn't have to be restarted for each compilation,
          which means it can cache information between compilations.
          Compiling a multi-module program with <literal>ghc
          &ndash;&ndash;make</literal> can be up to twice as fast as
          running <literal>ghc</literal> individually on each source
          file.</para>
        </listitem>
        <listitem>
          <para>You don't have to write a <literal>Makefile</literal>.</para>
          <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
        </listitem>
        <listitem>
          <para>GHC re-calculates the dependencies each time it is
          invoked, so the dependencies never get out of sync with the
          source.</para>
        </listitem>
      </itemizedlist>
      
      <para>Any of the command-line options described in the rest of
      this chapter can be used with
      <option>&ndash;&ndash;make</option>, but note that any options
      you give on the command line will apply to all the source files
      compiled, so if you want any options to apply to a single source
      file only, you'll need to use an <literal>OPTIONS_GHC</literal>
      pragma (see <xref linkend="source-file-options"/>).</para>

      <para>If the program needs to be linked with additional objects
      (say, some auxiliary C code), then the object files can be
      given on the command line and GHC will include them when linking
      the executable.</para>
      
      <para>Note that GHC can only follow dependencies if it has the
      source file available, so if your program includes a module for
      which there is no source file, even if you have an object and an
      interface file for the module, then GHC will complain.  The
      exception to this rule is for package modules, which may or may
      not have source files.</para>

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

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

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

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

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

<screen>
ghc -e Main.main Main.hs
</screen>
      
      <para>or we can just use this mode to evaluate expressions in
      the context of the <literal>Prelude</literal>:</para>

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

    <sect2 id="options-order">
      <title>Batch compiler mode</title>
      
      <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
      given on the command line.</para>
      
      <para>The first phase to run is determined by each input-file
      suffix, and the last phase is determined by a flag.  If no
      relevant flag is present, then go all the way through linking.
      This table summarises:</para>
      
      <informaltable>
        <tgroup cols="4">
          <colspec align="left"/>
          <colspec align="left"/>
          <colspec align="left"/>
          <colspec align="left"/>
          
          <thead>
            <row>
              <entry>Phase of the compilation system</entry>
              <entry>Suffix saying &ldquo;start here&rdquo;</entry>
              <entry>Flag saying &ldquo;stop after&rdquo;</entry>
              <entry>(suffix of) output file</entry>
            </row>
          </thead>
          <tbody>
            <row>
              <entry>literate pre-processor</entry>
              <entry><literal>.lhs</literal></entry>
              <entry>-</entry>
              <entry><literal>.hs</literal></entry>
            </row>
            
            <row>
              <entry>C pre-processor (opt.) </entry>
              <entry><literal>.hs</literal> (with
              <option>-cpp</option>)</entry>
              <entry><option>-E</option></entry>
              <entry><literal>.hspp</literal></entry>
            </row>
            
            <row>
              <entry>Haskell compiler</entry>
              <entry><literal>.hs</literal></entry>
              <entry><option>-C</option>, <option>-S</option></entry>
              <entry><literal>.hc</literal>, <literal>.s</literal></entry>
            </row>
            
            <row>
              <entry>C compiler (opt.)</entry>
              <entry><literal>.hc</literal> or <literal>.c</literal></entry>
              <entry><option>-S</option></entry>
              <entry><literal>.s</literal></entry>
            </row>
            
            <row>
              <entry>assembler</entry>
              <entry><literal>.s</literal></entry>
              <entry><option>-c</option></entry>
              <entry><literal>.o</literal></entry>
            </row>
            
            <row>
              <entry>linker</entry>
              <entry><replaceable>other</replaceable></entry>
              <entry>-</entry>
              <entry><filename>a.out</filename></entry>
            </row>
          </tbody>
        </tgroup>
      </informaltable>
      
      <indexterm><primary><option>-C</option></primary></indexterm>
      <indexterm><primary><option>-E</option></primary></indexterm>
      <indexterm><primary><option>-S</option></primary></indexterm>
      <indexterm><primary><option>-c</option></primary></indexterm>
      
      <para>Thus, a common invocation would be: </para>

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

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

      <para>Note: C pre-processing is optional, the
      <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
      flag turns it on.  See <xref linkend="c-pre-processor"/> for more
      details.</para>
      
      <para>Note: The option <option>-E</option><indexterm><primary>-E
      option</primary></indexterm> runs just the pre-processing passes
      of the compiler, dumping the result in a file.  Note that this
      differs from the previous behaviour of dumping the file to
      standard output.</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 overriden using the
          <option>-x</option> option:</para>

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

    </sect2>
  </sect1>

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

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

    <variablelist>
      <varlistentry>
        <term>
          <option>&ndash;&ndash;help</option>
          <indexterm><primary><option>&ndash;&ndash;help</option></primary></indexterm>
        </term>
        <term>
          <option>-?</option>
          <indexterm><primary><option>-?</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>
          <option>-v</option>
          <indexterm><primary><option>-v</option></primary></indexterm>
        </term>
        <listitem>
          <para>The <option>-v</option> option makes GHC
          <emphasis>verbose</emphasis>: it reports its version number
          and shows (on stderr) exactly how it invokes each phase of
          the compilation system.  Moreover, it passes the
          <option>-v</option> flag to most phases; each reports its
          version number (and possibly some other information).</para>

          <para>Please, oh please, use the <option>-v</option> option
          when reporting bugs!  Knowing that you ran the right bits in
          the right order is always the first thing we want to
          verify.</para>
        </listitem>
      </varlistentry>
        
      <varlistentry>
        <term>
          <option>-v</option><replaceable>n</replaceable>
          <indexterm><primary><option>-v</option></primary></indexterm>
        </term>
        <listitem>
          <para>To provide more control over the compiler's verbosity,
          the <option>-v</option> flag takes an optional numeric
          argument.  Specifying <option>-v</option> on its own is
          equivalent to <option>-v3</option>, and the other levels
          have the following meanings:</para>
          
          <variablelist>
            <varlistentry>
              <term><option>-v0</option></term>
              <listitem>
                <para>Disable all non-essential messages (this is the
                default).</para>
              </listitem>
            </varlistentry>

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

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

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

            <varlistentry>
              <term><option>-v4</option></term>
              <listitem>
                <para>The same as <option>-v3</option> except that the
                intermediate program representation after each
                compilation phase is also printed (excluding
                preprocessed and C/assembly files).</para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>
      
      <varlistentry>
        <term>
          <option>-V</option>
          <indexterm><primary><option>-V</option></primary></indexterm>
        </term>
        <term>
          <option>&ndash;&ndash;version</option>
          <indexterm><primary><option>&ndash;&ndash;version</option></primary></indexterm>
        </term>
        <listitem>
          <para>Print a one-line string including GHC's version number.</para>
        </listitem>
      </varlistentry>

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

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

      <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>-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-deprecations</option>,
    <option>-fwarn-duplicate-exports</option>,
    <option>-fwarn-missing-fields</option>, and
    <option>-fwarn-missing-methods</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-unused-matches</option>,
          <option>-fwarn-unused-imports</option>, and
          <option>-fwarn-unused-binds</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.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-Wall</option>:</term>
        <listitem>
          <indexterm><primary><option>-Wall</option></primary></indexterm>
          <para>Turns on all warning options.</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>

    </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-deprecations</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-deprecations</option></primary>
          </indexterm>
          <indexterm><primary>deprecations</primary></indexterm>
          <para>Causes a warning to be emitted when a deprecated
          function or type is used.  Entities can be marked as
          deprecated using a pragma, see <xref
          linkend="deprecated-pragma"/>.</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-incomplete-patterns</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
          <indexterm><primary>incomplete patterns, warning</primary></indexterm>
          <indexterm><primary>patterns, incomplete</primary></indexterm>

          <para>Similarly for incomplete patterns, 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.</para>

<programlisting>
g [] = 2
</programlisting>

          <para>This option isn't enabled be 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.</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 be 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-methods</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
          <indexterm><primary>missing methods, warning</primary></indexterm>
          <indexterm><primary>methods, missing</primary></indexterm>

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

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

          <para>If you would like GHC to check that every top-level
          function/value has a type signature, use the
          <option>-fwarn-missing-signatures</option> option.  This
          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 cyclic definition <literal>let x = ... x
          ... in</literal>.</para>

          <para>Consequently, this option does
          <emphasis>will</emphasis> complain about cyclic recursive
          definitions.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-fwarn-orphans</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
          <indexterm><primary>orphan instances, warning</primary></indexterm>
          <indexterm><primary>orphan rules, warning</primary></indexterm>
          
          <para>This option causes a warning to be emitted whenever the 
            module contains an "orphan" instance declaration or rewrite rule.
            An instance declartion 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.  Other things being equal, avoid orphan modules.</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, i.e.,</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-simple-patterns</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
          </indexterm>
          <para>Causes the compiler to warn about lambda-bound
          patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
          Normally, these aren't treated as incomplete patterns by
          <option>-fwarn-incomplete-patterns</option>.</para>
          <para>``Lambda-bound patterns'' includes all places where there is a single pattern,
            including list comprehensions and do-notation.  In these cases, a pattern-match 
            failure is quite legitimate, and triggers filtering (list comprehensions) or
            the monad <literal>fail</literal> operation (monads). For example:
            <programlisting>
              f :: [Maybe a] -> [a]
              f xs = [y | Just y &lt;- xs]
              </programlisting>
            Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
            these probably-innocent cases, which is why the flag is off by default. </para>
          <para> The <literal>deriving( Read )</literal> mechanism produces monadic code with
            pattern matches, so you will also get misleading warnings about the compiler-generated
            code.  (This is arguably a Bad Thing, but it's awkward to fix.)</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 `default default' for Haskell 1.4 caused the
          otherwise unconstrained value <constant>1</constant> to be
          given the type <literal>Int</literal>, whereas Haskell 98
          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-unused-binds</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
          <indexterm><primary>unused binds, warning</primary></indexterm>
          <indexterm><primary>binds, unused</primary></indexterm>
          <para>Report any function definitions (and local bindings)
          which are unused.  For top-level functions, the warning is
          only given if the binding is not exported.</para>
          <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
            mentioned in the right hand side of another definition that is used, or (c) the 
            function it defines begins with an underscore.  The last case provides a 
            way to suppress unused-binding warnings selectively.  </para>
          <para> Notice that a variable
            is reported as unused even if it appears in the right-hand side of another
            unused binding. </para>
        </listitem>
      </varlistentry>

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

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

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

          <para>Report all unused variables which arise from pattern
          matches, including patterns consisting of a single variable.
          For instance <literal>f x y = []</literal> would report
          <varname>x</varname> and <varname>y</varname> as unused.  The
          warning is suppressed if the variable name begins with an underscore, thus:
            <programlisting>
               f _x = True
            </programlisting>
          </para>
        </listitem>
      </varlistentry>

    </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 reaosn 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>

            <para><option>-O</option> currently also implies
            <option>-fvia-C</option>.  This may change in the
            future.</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>

        <varlistentry>
          <term>
            <option>-Ofile &lt;file&gt;</option>:
            <indexterm><primary>-Ofile &lt;file&gt; option</primary></indexterm>
            <indexterm><primary>optimising, customised</primary></indexterm>
          </term>
          <listitem>
            <para>(NOTE: not supported since GHC 4.x.  Please ask if
            you're interested in this.)</para>
            
            <para>For those who need <emphasis>absolute</emphasis>
            control over <emphasis>exactly</emphasis> what options are
            used (e.g., compiler writers, sometimes :-), a list of
            options can be put in a file and then slurped in with
            <option>-Ofile</option>.</para>

            <para>In that file, comments are of the
            <literal>&num;</literal>-to-end-of-line variety; blank
            lines and most whitespace is ignored.</para>

            <para>Please ask if you are baffled and would like an
            example of <option>-Ofile</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 -fvia-C</option> (and we go for
      lots of coffee breaks).</para>

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

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

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

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

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

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

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

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

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

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

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

            <para>This option is a bit of a sledgehammer: it might
            sometimes make things worse.  Selectively unboxing fields
            by using <literal>UNPACK</literal> pragmas might be
            better.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>
            <option>-funfolding-update-in-place&lt;n&gt;</option>
            <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
          </term>
          <listitem>
            <para>Switches on an experimental "optimisation".
            Switching it on makes the compiler a little keener to
            inline a function that returns a constructor, if the
            context is that of a thunk.
<programlisting>
   x = plusInt a b
</programlisting>
            If we inlined plusInt we might get an opportunity to use
            update-in-place for the thunk 'x'.</para>
          </listitem>
        </varlistentry>

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

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


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

        <varlistentry>
          <term><option>-funfolding-use-threshold&lt;n&gt;</option>:</term>
          <listitem>
            <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
            <indexterm><primary>inlining, controlling</primary></indexterm>
            <indexterm><primary>unfolding, controlling</primary></indexterm>

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

    </sect2>
    
  </sect1>
  
  &phases;  
  
  <sect1 id="sec-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="../libraries/base/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>.  More information on Concurrent Haskell is provided in the documentation for that module.</para>

    <para>The following RTS option(s) affect the behaviour of Concurrent
      Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>

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

  <sect1 id="sec-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 desribe the language features that affect
    parallelism.</para>
    
    <sect2 id="parallel-options">
      <title>Options to enable 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" />).  Then, to run a program on multiple
        CPUs, use the RTS <option>-N</option> option:</para>
      
      <variablelist>
        <varlistentry>
          <term><option>-N<replaceable>x</replaceable></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.
              There is no means (currently) by which this value may vary after
              the program has started.</para> 
            
            <para>For example, on a dual-core machine we would probably use
              <literal>+RTS -N2 -RTS</literal>.</para>
            
            <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>
          </listitem>
        </varlistentry>
      </variablelist>
    </sect2>
      
    <sect2>
      <title>Hints for using SMP parallelism</title>

      <para>Add the <literal>-sstderr</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>GHC's parallelism support is new and experimental.  It may make your
        program go faster, or it might slow it down - either way, we'd be
        interested to hear from you.</para>
      
      <para>One significant limitation with the current implementation is that
        the garbage collector is still single-threaded, and all execution must
        stop when GC takes place.  This can be a significant bottleneck in a
        parallel program, especially if your program does a lot of GC.  If this
        happens to you, then try reducing the cost of GC by tweaking the GC
        settings (<xref linkend="rts-options-gc" />): enlarging the heap or the
        allocation area size is a good start.</para>
    </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>-monly-[32]-regs</option>:</term>
        <listitem>
          <para>(iX86 machines)<indexterm><primary>-monly-N-regs
          option (iX86 only)</primary></indexterm> GHC tries to
          &ldquo;steal&rdquo; four registers from GCC, for performance
          reasons; it almost always works.  However, when GCC is
          compiling some modules with four stolen registers, it will
          crash, probably saying:

<screen>
Foo.hc:533: fixed or forbidden register was spilled.
This may be due to a compiler bug or to impossible asm
statements or clauses.
</screen>

          Just give some registers back with
          <option>-monly-N-regs</option>.  Try `3' first, then `2'.
          If `2' doesn't work, please report the bug to us.</para>
        </listitem>
      </varlistentry>
    </variablelist>

  </sect1>

&runtime;

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

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

  <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format) 
  to a file as a side-effect of compilation. Core files, which are given the suffix
  <filename>.hcr</filename>, can be read and processed by non-GHC back-end
  tools.  The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz">
  <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>, 
  and sample tools (in Haskell)
  for manipulating Core files are available in the GHC source distribution 
  directory <literal>/fptools/ghc/utils/ext-core</literal>.  
  Note that the format of <literal>.hcr</literal> 
  files is <emphasis>different</emphasis> (though similar) to the Core output format generated 
  for debugging purposes (<xref linkend="options-debugging"/>).</para>

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

    <variablelist>

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

    </variablelist>

<para>GHC can also read in External Core files as source; just give the <literal>.hcr</literal> file on
the command line, instead of the <literal>.hs</literal> or <literal>.lhs</literal> Haskell source.
A current infelicity is that you need to give the <literal>-fglasgow-exts</literal> flag too, because
ordinary Haskell 98, when translated to External Core, uses things like rank-2 types.</para>
</sect1>

&debug;
&flags;

</chapter>

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