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>Getting started: compiling programs</title>

    <para>
      In this chapter you'll find a complete reference to the GHC
      command-line syntax, including all 400+ flags.  It's a large and
      complex system, and there are lots of details, so it can be
      quite hard to figure out how to get started.  With that in mind,
      this introductory section provides a quick introduction to the
      basic usage of GHC for compiling a Haskell program, before the
      following sections dive into the full syntax.
    </para>

    <para>
      Let's create a Hello World program, and compile and run it.
      First, create a file <filename>hello.hs</filename> containing
      the Haskell code:
    </para>

<programlisting>
main = putStrLn "Hello, World!"
</programlisting>

    <para>To compile the program, use GHC like this:</para>

<screen>
$ ghc hello.hs
</screen>

     <para>(where <literal>$</literal> represents the prompt: don't
       type it).  GHC will compile the source
       file <filename>hello.hs</filename>, producing
       an <firstterm>object
       file</firstterm> <filename>hello.o</filename> and
       an <firstterm>interface
       file</firstterm> <filename>hello.hi</filename>, and then it
       will link the object file to the libraries that come with GHC
       to produce an executable called <filename>hello</filename> on
       Unix/Linux/Mac, or <filename>hello.exe</filename> on
       Windows.</para>

    <para>
      By default GHC will be very quiet about what it is doing, only
      printing error messages.  If you want to see in more detail
      what's going on behind the scenes, add <option>-v</option> to
      the command line.
    </para>

    <para>
      Then we can run the program like this:
    </para>

<screen>
$ ./hello
Hello World!
</screen>

    <para>
      If your program contains multiple modules, then you only need to
      tell GHC the name of the source file containing
      the <filename>Main</filename> module, and GHC will examine
      the <literal>import</literal> declarations to find the other
      modules that make up the program and find their source files.
      This means that, with the exception of
      the <literal>Main</literal> module, every source file should be
      named after the module name that it contains (with dots replaced
      by directory separators).  For example, the
      module <literal>Data.Person</literal> would be in the
      file <filename>Data/Person.hs</filename> on Unix/Linux/Mac,
      or <filename>Data\Person.hs</filename> on Windows.
    </para>
  </sect1>

  <sect1>
    <title>Options overview</title>

    <para>GHC's behaviour is controlled by
    <firstterm>options</firstterm>, which for historical reasons are
    also sometimes referred to as command-line flags or arguments.
    Options can be specified in three ways:</para>

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    <variablelist>

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

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

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

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

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

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

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

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

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

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

  </sect1>

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

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

    <para>
      If no mode flag is present, then GHC will enter make mode
      (<xref linkend="make-mode" />) if there are any Haskell source
      files given on the command line, or else it will link the
      objects named on the command line to produce an executable.
    </para>

    <para>The available mode flags are:</para>

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

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

          <para>
            This mode is the default if there are any Haskell
            source files mentioned on the command line, and in this case
            the <option>&ndash;&ndash;make</option> option can be omitted.
          </para>
        </listitem>
      </varlistentry>

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

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

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

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

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

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

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

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

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

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

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

    </variablelist>

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

      <para>In this mode, GHC will build a multi-module Haskell program by following
      dependencies from one or more root modules (usually just
      <literal>Main</literal>).  For example, if your
      <literal>Main</literal> module is in a file called
      <filename>Main.hs</filename>, you could compile and link the
      program like this:</para>

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

      <para>
        In fact, GHC enters make mode automatically if there are any
        Haskell source files on the command line and no other mode is
        specified, so in this case we could just type
      </para>

<screen>
ghc Main.hs
</screen>

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

      <para>Note: What the Haskell compiler proper produces depends on 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>Help and verbosity options</title>

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

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

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

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

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

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

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

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

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

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

          <para>For example:</para>

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

          <para>becomes:</para>

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

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

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

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

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

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

  &separate;

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

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


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

    <variablelist>

      <varlistentry>
        <term><option>-W</option>:</term>
        <listitem>
          <indexterm><primary>-W option</primary></indexterm>
          <para>Provides the standard warnings plus
          <option>-fwarn-incomplete-patterns</option>,
          <option>-fwarn-dodgy-exports</option>,
          <option>-fwarn-dodgy-imports</option>,
          <option>-fwarn-unused-matches</option>,
          <option>-fwarn-unused-imports</option>, and
          <option>-fwarn-unused-binds</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-Wall</option>:</term>
        <listitem>
          <indexterm><primary><option>-Wall</option></primary></indexterm>
          <para>Turns on all warning options that indicate potentially
          suspicious code.  The warnings that are
          <emphasis>not</emphasis> enabled by <option>-Wall</option>
          are
            <option>-fwarn-tabs</option>,
            <option>-fwarn-incomplete-uni-patterns</option>,
            <option>-fwarn-incomplete-record-updates</option>,
            <option>-fwarn-monomorphism-restriction</option>,
            <option>-fwarn-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>-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.</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-warnings-deprecations</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
          </indexterm>
          <indexterm><primary>warnings</primary></indexterm>
          <indexterm><primary>deprecations</primary></indexterm>
          <para>Causes a warning to be emitted when a
          module, function or type with a WARNING or DEPRECATED pragma
      is used. See <xref linkend="warning-deprecated-pragma"/> for more
      details on the pragmas.</para>

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

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

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

      <varlistentry>
        <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
          </indexterm>
          <para>Causes a warning to be emitted for foreign imports of
          the following form:</para>

<programlisting>
foreign import "f" f :: FunPtr t
</programlisting>

          <para>on the grounds that it probably should be</para>

<programlisting>
foreign import "&amp;f" f :: FunPtr t
</programlisting>

          <para>The first form declares that `f` is a (pure) C
          function that takes no arguments and returns a pointer to a
          C function with type `t`, whereas the second form declares
          that `f` itself is a C function with type `t`.  The first
          declaration is usually a mistake, and one that is hard to
          debug because it results in a crash, hence this
          warning.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-fwarn-dodgy-exports</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-dodgy-exports</option></primary>
          </indexterm>
          <para>Causes a warning to be emitted when a datatype
      <literal>T</literal> is exported
      with all constructors, i.e. <literal>T(..)</literal>, but is it
      just a type synonym.</para>
          <para>Also causes a warning to be emitted when a module is
      re-exported, but that module exports nothing.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-fwarn-dodgy-imports</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
          </indexterm>
          <para>Causes a warning to be emitted when a datatype
      <literal>T</literal> is imported
      with all constructors, i.e. <literal>T(..)</literal>, but has been
      exported abstractly, i.e. <literal>T</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-fwarn-lazy-unlifted-bindings</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-lazy-unlifted-bindings</option></primary>
          </indexterm>
          <para>Causes a warning to be emitted when an unlifted type
      is bound in a way that looks lazy, e.g.
      <literal>where (I# x) = ...</literal>. Use
      <literal>where !(I# x) = ...</literal> instead. This will be an
      error, rather than a warning, in GHC 7.2.
      </para>
        </listitem>
      </varlistentry>

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

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

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

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

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

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