ghc/docs/users_guide/using.sgml

   1 <chapter id="using-ghc">
   2   <title>Using GHC</title>
   3
   4   <indexterm><primary>GHC, using</primary></indexterm>
   5   <indexterm><primary>using GHC</primary></indexterm>
   6
   7     <para>GHC can work in one of three &ldquo;modes&rdquo;:</para>
   8
   9     <variablelist>
  10       <varlistentry>
  11         <term><cmdsynopsis><command>ghc</command>
  12             <arg choice=plain>--interactive</arg>
  13           </cmdsynopsis></term>
  14         <indexterm><primary>interactive mode</primary>
  15         </indexterm>
  16         <indexterm><primary>ghci</primary>
  17         </indexterm>
  18         <listitem>
  19           <para>Interactive mode, which is also available as
  20           <command>ghci</command>.  Interactive mode is described in
  21           more detail in <xref linkend="ghci">.</para>
  22         </listitem>
  23       </varlistentry>
  24
  25       <varlistentry>
  26         <term><cmdsynopsis><command>ghc</command>
  27             <arg choice=plain>--make</arg>
  28           </cmdsynopsis></term>
  29         <indexterm><primary>make mode</primary>
  30         </indexterm>
  31         <indexterm><primary><option>--make</option></primary>
  32         </indexterm>
  33         <listitem>
  34           <para>In this mode, GHC will build a multi-module Haskell
  35           program automatically, figuring out dependencies for itself.
  36           If you have a straightforward Haskell program, this is
  37           likely to be much easier, and faster, than using
  38           <command>make</command>.</para>
  39         </listitem>
  40       </varlistentry>
  41
  42       <varlistentry>
  43         <term><cmdsynopsis>
  44             <command>ghc</command>
  45             <group>
  46               <arg>-E</arg>
  47               <arg>-C</arg>
  48               <arg>-S</arg>
  49               <arg>-c</arg>
  50             </group>
  51           </cmdsynopsis></term>
  52         <indexterm><primary><option>-E</option></primary>
  53         <indexterm><primary><option>-C</option></primary>
  54         <indexterm><primary><option>-S</option></primary>
  55         <indexterm><primary><option>-c</option></primary>
  56         </indexterm>
  57         <listitem>
  58           <para>This is the traditional batch-compiler mode, in which
  59           GHC can compile source files one at a time, or link objects
  60           together into an executable.</para>
  61         </listitem>
  62       </varlistentry>
  63     </variablelist>
  64
  65   <sect1>
  66     <title>Options overview</title>
  67
  68     <para>GHC's behaviour is controlled by
  69     <firstterm>options</firstterm>, which for historical reasons are
  70     also sometimes referred to as command-line flags or arguments.
  71     Options can be specified in three ways:</para>
  72
  73     <sect2>
  74       <title>Command-line arguments</title>
  75
  76       <indexterm><primary>structure, command-line</primary></indexterm>
  77       <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
  78       <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
  79
  80       <para>An invocation of GHC takes the following form:</para>
  81
  82 <Screen>
  83 ghc [argument...]
  84 </Screen>
  85
  86       <para>Command-line arguments are either options or file names.</para>
  87
  88       <para>Command-line options begin with <literal>-</literal>.
  89       They may <emphasis>not</emphasis> be grouped:
  90       <option>-vO</option> is different from <option>-v -O</option>.
  91       Options need not precede filenames: e.g., <literal>ghc *.o -o
  92       foo</literal>.  All options are processed and then applied to
  93       all files; you cannot, for example, invoke <literal>ghc -c -O1
  94       Foo.hs -O2 Bar.hs</literal> to apply different optimisation
  95       levels to the files <filename>Foo.hs</filename> and
  96       <filename>Bar.hs</filename>.</para>
  97     </sect2>
  98
  99     <Sect2 id="source-file-options">
 100       <title>Command line options in source files</title>
 101
 102       <indexterm><primary>source-file options</primary></indexterm>
 103
 104       <para>Sometimes it is useful to make the connection between a
 105       source file and the command-line options it requires quite
 106       tight. For instance, if a Haskell source file uses GHC
 107       extensions, it will always need to be compiled with the
 108       <option>-fglasgow-exts</option> option.  Rather than maintaining
 109       the list of per-file options in a <filename>Makefile</filename>,
 110       it is possible to do this directly in the source file using the
 111       <literal>OPTIONS</literal> pragma <indexterm><primary>OPTIONS
 112       pragma</primary></indexterm>:</para>
 113
 114 <ProgramListing>
 115 {-# OPTIONS -fglasgow-exts #-}
 116 module X where
 117 ...
 118 </ProgramListing>
 119
 120       <para><literal>OPTIONS</literal> pragmas are only looked for at
 121       the top of your source files, upto the first
 122       (non-literate,non-empty) line not containing
 123       <literal>OPTIONS</literal>. Multiple <literal>OPTIONS</literal>
 124       pragmas are recognised. Note that your command shell does not
 125       get to the source file options, they are just included literally
 126       in the array of command-line arguments the compiler driver
 127       maintains internally, so you'll be desperately disappointed if
 128       you try to glob etc. inside <literal>OPTIONS</literal>.</para>
 129
 130       <para>NOTE: the contents of OPTIONS are prepended to the
 131       command-line options, so you <emphasis>do</emphasis> have the
 132       ability to override OPTIONS settings via the command
 133       line.</para>
 134
 135       <para>It is not recommended to move all the contents of your
 136       Makefiles into your source files, but in some circumstances, the
 137       <literal>OPTIONS</literal> pragma is the Right Thing. (If you
 138       use <option>-keep-hc-file-too</option> and have OPTION flags in
 139       your module, the OPTIONS will get put into the generated .hc
 140       file).</para>
 141     </sect2>
 142
 143     <sect2>
 144       <title>Setting options in GHCi</title>
 145
 146       <para>Options may also be modified from within GHCi, using the
 147       <literal>:set</literal> command.  See <xref linkend="ghci-set">
 148       for more details.</para>
 149     </sect2>
 150   </sect1>
 151
 152   <sect1 id="static-dynamic-flags">
 153     <title>Static vs. Dynamic options</title>
 154     <indexterm><primary>static</primary><secondary>options</secondary>
 155     </indexterm>
 156     <indexterm><primary>dynamic</primary><secondary>options</secondary>
 157     </indexterm>
 158
 159     <para>Each of GHC's command line options is classified as either
 160     <firstterm>static</firstterm> or <firstterm>dynamic</firstterm>.
 161     A static flag may only be specified on the command line, whereas a
 162     dynamic flag may also be given in an <literal>OPTIONS</literal>
 163     pragma in a source file or set from the GHCi command-line with
 164     <literal>:set</literal>.</para>
 165
 166     <para>As a rule of thumb, all the language options are dynamic, as
 167     are the warning options and the debugging options.  The rest are
 168     static, with the notable exceptions of <option>-v</option>,
 169     <option>-cpp</option>, <option>-fasm</option>,
 170     <option>-fvia-C</option>, and <option>-#include</option>.
 171
 172     The flag reference tables (<xref linkend="flag-reference">) lists
 173     the status of each flag.</para>
 174   </sect1>
 175
 176   <sect1 id="file-suffixes">
 177     <title>Meaningful file suffixes</title>
 178
 179     <indexterm><primary>suffixes, file</primary></indexterm>
 180     <indexterm><primary>file suffixes for GHC</primary></indexterm>
 181
 182     <para>File names with &ldquo;meaningful&rdquo; suffixes (e.g.,
 183     <filename>.lhs</filename> or <filename>.o</filename>) cause the
 184     &ldquo;right thing&rdquo; to happen to those files.</para>
 185
 186     <variablelist>
 187
 188       <varlistentry>
 189         <term><filename>.lhs</filename></term>
 190         <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
 191         <listitem>
 192           <para>A &ldquo;literate Haskell&rdquo; module.</para>
 193         </listitem>
 194       </varlistentry>
 195
 196       <varlistentry>
 197         <term><filename>.hs</filename></term>
 198         <listitem>
 199           <para>A not-so-literate Haskell module.</para>
 200         </listitem>
 201       </varlistentry>
 202
 203       <varlistentry>
 204         <term><filename>.hi</filename></term>
 205         <listitem>
 206           <para>A Haskell interface file, probably
 207           compiler-generated.</para>
 208         </listitem>
 209       </varlistentry>
 210
 211       <varlistentry>
 212         <term><filename>.hc</filename></term>
 213         <listitem>
 214           <para>Intermediate C file produced by the Haskell
 215           compiler.</para>
 216         </listitem>
 217       </varlistentry>
 218
 219       <varlistentry>
 220         <term><filename>.c</filename></term>
 221         <listitem>
 222           <para>A C&nbsp;file not produced by the Haskell
 223           compiler.</para>
 224         </listitem>
 225       </varlistentry>
 226
 227       <varlistentry>
 228         <term><filename>.s</filename></term>
 229         <listitem>
 230           <para>An assembly-language source file, usually produced by
 231           the compiler.</para>
 232         </listitem>
 233       </varlistentry>
 234
 235       <varlistentry>
 236         <term><filename>.o</filename></term>
 237         <listitem>
 238           <para>An object file, produced by an assembler.</para>
 239         </listitem>
 240       </varlistentry>
 241     </variablelist>
 242
 243     <para>Files with other suffixes (or without suffixes) are passed
 244     straight to the linker.</para>
 245
 246   </sect1>
 247
 248   <sect1 id="options-help">
 249     <title>Help and verbosity options</title>
 250
 251     <IndexTerm><Primary>help options</Primary></IndexTerm>
 252     <IndexTerm><Primary>verbosity options</Primary></IndexTerm>
 253
 254     <variablelist>
 255       <varlistentry>
 256         <term><literal>-help</literal></term>
 257         <term><literal>-?</literal></term>
 258         <indexterm><primary><literal>-?</literal></primary></indexterm>
 259         <indexterm><primary><literal>-help</literal></primary></indexterm>
 260         <listitem>
 261           <para>Cause GHC to spew a long usage message to standard
 262           output and then exit.</para>
 263         </listitem>
 264       </varlistentry>
 265
 266       <varlistentry>
 267         <term><literal>-v</literal></term>
 268         <indexterm><primary><literal>-v</literal></primary></indexterm>
 269         <listitem>
 270           <para>The <option>-v</option> option makes GHC
 271           <emphasis>verbose</emphasis>: it reports its version number
 272           and shows (on stderr) exactly how it invokes each phase of
 273           the compilation system.  Moreover, it passes the
 274           <option>-v</option> flag to most phases; each reports its
 275           version number (and possibly some other information).</para>
 276
 277           <para>Please, oh please, use the <option>-v</option> option
 278           when reporting bugs!  Knowing that you ran the right bits in
 279           the right order is always the first thing we want to
 280           verify.</para>
 281         </listitem>
 282       </varlistentry>
 283
 284       <varlistentry>
 285         <term><literal>-v</literal><replaceable>n</replaceable></term>
 286         <indexterm><primary><option>-v</option></primary></indexterm>
 287         <listitem>
 288           <para>To provide more control over the compiler's verbosity,
 289           the <option>-v</option> flag takes an optional numeric
 290           argument.  Specifying <option>-v</option> on its own is
 291           equivalent to <option>-v3</option>, and the other levels
 292           have the following meanings:</para>
 293
 294           <variablelist>
 295             <varlistentry>
 296               <term><literal>-v0</literal></term>
 297               <listitem>
 298                 <para>Disable all non-essential messages (this is the
 299                 default).</para>
 300               </listitem>
 301             </varlistentry>
 302
 303             <varlistentry>
 304               <term><literal>-v1</literal></term>
 305               <listitem>
 306                 <para>Minimal verbosity: print one line per
 307                 compilation (this is the default when
 308                 <option>--make</option> or
 309                 <option>--interactive</option> is on).</para>
 310               </listitem>
 311             </varlistentry>
 312
 313             <varlistentry>
 314               <term><literal>-v2</literal></term>
 315               <listitem>
 316                 <para>Print the name of each compilation phase as it
 317                 is executed. (equivalent to
 318                 <option>-dshow-passes</option>).</para>
 319               </listitem>
 320             </varlistentry>
 321
 322             <varlistentry>
 323               <term><literal>-v3</literal></term>
 324               <listitem>
 325                 <para>The same as <option>-v2</option>, except that in
 326                 addition the full command line (if appropriate) for
 327                 each compilation phase is also printed.</para>
 328               </listitem>
 329             </varlistentry>
 330
 331             <varlistentry>
 332               <term><literal>-v4</literal></term>
 333               <listitem>
 334                 <para>The same as <option>-v3</option> except that the
 335                 intermediate program representation after each
 336                 compilation phase is also printed (excluding
 337                 preprocessed and C/assembly files).</para>
 338               </listitem>
 339             </varlistentry>
 340           </variablelist>
 341         </listitem>
 342       </varlistentry>
 343
 344       <varlistentry>
 345         <term><literal>--version</literal></term>
 346         <indexterm><primary><literal>--version</literal></primary></indexterm>
 347         <listitem>
 348           <para>Print a one-line string including GHC's version number.</para>
 349         </listitem>
 350       </varlistentry>
 351
 352       <varlistentry>
 353         <term><literal>--numeric-version</literal></term>
 354         <indexterm><primary><literal>--numeric-version</literal></primary></indexterm>
 355         <listitem>
 356           <para>Print GHC's numeric version number only.</para>
 357         </listitem>
 358       </varlistentry>
 359     </variablelist>
 360   </sect1>
 361
 362   <sect1 id="make-mode">
 363     <title>Using <command>ghc</command> <option>--make</option></title>
 364
 365     <indexterm><primary><option>--make</option></primary>
 366     </indexterm>
 367     <indexterm><primary>separate compilation</primary>
 368     </indexterm>
 369
 370     <para>When given the <option>--make</option> option, GHC will
 371     build a multi-module Haskell program by following dependencies
 372     from a single root module (usually <literal>Main</literal>).  For
 373     example, if your <literal>Main</literal> module is in a file
 374     called <filename>Main.hs</filename>, you could compile and link
 375     the program like this:</para>
 376
 377 <screen>
 378 ghc --make Main.hs
 379 </screen>
 380
 381     <para>The command line must contain one source file or module
 382     name; GHC will figure out all the modules in the program by
 383     following the imports from this initial module.  It will then
 384     attempt to compile each module which is out of date, and finally
 385     if the top module is <literal>Main</literal>, the program
 386     will also be linked into an executable.</para>
 387
 388     <para>The main advantages to using <literal>ghc --make</literal>
 389     over traditional <literal>Makefile</literal>s are:</para>
 390
 391     <itemizedlist>
 392       <listitem>
 393         <para>GHC doesn't have to be restarted for each compilation,
 394         which means it can cache information between compilations.
 395         Compiling a muli-module program with <literal>ghc
 396         --make</literal> can be up to twice as fast as running
 397         <literal>ghc</literal> individually on each source
 398         file.</para>
 399       </listitem>
 400       <listitem>
 401         <para>You don't have to write a
 402         <literal>Makefile</literal>.</para>
 403       </listitem>
 404       <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary>
 405       </indexterm>
 406       <listitem>
 407         <para>GHC re-calculates the dependencies each time it is
 408         invoked, so the dependencies never get out of sync with the
 409         source.</para>
 410       </listitem>
 411     </itemizedlist>
 412
 413     <para>Any of the command-line options described in the rest of
 414     this chapter can be used with <option>--make</option>, but note
 415     that any options you give on the command line will apply to all
 416     the source files compiled, so if you want any options to apply to
 417     a single source file only, you'll need to use an
 418     <literal>OPTIONS</literal> pragma (see <xref
 419     linkend="source-file-options">).</para>
 420
 421     <para>If the program needs to be linked with additional objects
 422     (say, some auxilliary C code), these can be specified on the
 423     command line as usual.</para>
 424
 425     <para>Note that GHC can only follow dependencies if it has the
 426     source file available, so if your program includes a module for
 427     which there is no source file, even if you have an object and an
 428     interface file for the module, then GHC will complain.  The
 429     exception to this rule is for package modules, which may or may
 430     not have source files.</para>
 431   </sect1>
 432
 433   <Sect1 id="options-order">
 434     <title>GHC without <option>--make</option></title>
 435
 436     <para>Without <option>--make</option>, GHC will compile one or
 437     more source files given on the command line.</para>
 438
 439     <para>The first phase to run is determined by each input-file
 440     suffix, and the last phase is determined by a flag.  If no
 441     relevant flag is present, then go all the way through linking.
 442     This table summarises:</para>
 443
 444     <informaltable>
 445       <tgroup cols="4">
 446         <colspec align="left">
 447         <colspec align="left">
 448         <colspec align="left">
 449         <colspec align="left">
 450
 451         <thead>
 452           <row>
 453             <entry>Phase of the compilation system</entry>
 454             <entry>Suffix saying &ldquo;start here&rdquo;</entry>
 455             <entry>Flag saying &ldquo;stop after&rdquo;</entry>
 456             <entry>(suffix of) output file</entry>
 457           </row>
 458         </thead>
 459         <tbody>
 460           <row>
 461             <entry>literate pre-processor</entry>
 462             <entry><literal>.lhs</literal></entry>
 463             <entry>-</entry>
 464             <entry><literal>.hs</literal></entry>
 465           </row>
 466
 467           <row>
 468             <entry>C pre-processor (opt.)
 469            </entry>
 470             <entry><literal>.hs</literal> (with
 471             <option>-cpp</option>)</entry>
 472             <entry><option>-E</option></entry>
 473             <entry><literal>.hspp</literal></entry>
 474           </row>
 475
 476           <row>
 477             <entry>Haskell compiler</entry>
 478             <entry><literal>.hs</literal></entry>
 479             <entry><option>-C</option>, <option>-S</option></entry>
 480             <entry><literal>.hc</literal>, <literal>.s</literal></entry>
 481           </row>
 482
 483           <row>
 484             <entry>C compiler (opt.)</entry>
 485             <entry><literal>.hc</literal> or <literal>.c</literal></entry>
 486             <entry><option>-S</option></entry>
 487             <entry><literal>.s</literal></entry>
 488           </row>
 489
 490           <row>
 491             <entry>assembler</entry>
 492             <entry><literal>.s</literal></entry>
 493             <entry><option>-c</option></entry>
 494             <entry><literal>.o</literal></entry>
 495           </row>
 496
 497           <row>
 498             <entry>linker</entry>
 499             <entry><replaceable>other</replaceable></entry>
 500             <entry>-</entry>
 501             <entry><filename>a.out</filename></entry>
 502           </row>
 503         </tbody>
 504       </tgroup>
 505     </informaltable>
 506
 507     <indexterm><primary><option>-C</option></primary></indexterm>
 508     <indexterm><primary><option>-E</option></primary></indexterm>
 509     <indexterm><primary><option>-S</option></primary></indexterm>
 510     <indexterm><primary><option>-c</option></primary></indexterm>
 511
 512     <para>Thus, a common invocation would be: <literal>ghc -c
 513     Foo.hs</literal></para>
 514
 515     <para>Note: What the Haskell compiler proper produces depends on
 516     whether a native-code generator<indexterm><primary>native-code
 517     generator</primary></indexterm> is used (producing assembly
 518     language) or not (producing C).  See <xref
 519     linkend="options-codegen"> for more details.</para>
 520
 521     <para>Note: C pre-processing is optional, the
 522     <option>-ccp</option><indexterm><primary><option>-cpp</option></primary>
 523       </indexterm>flag turns it on.  See <xref
 524     linkend="c-pre-processor"> for more details.</para>
 525
 526     <para>Note: The option <option>-E</option><IndexTerm><Primary>-E
 527     option</Primary></IndexTerm> runs just the pre-processing passes
 528     of the compiler, dumping the result in a file.  Note that this
 529     differs from the previous behaviour of dumping the file to
 530     standard output.</para>
 531   </sect1>
 532
 533   <sect1 id="options-output">
 534     <title>Re-directing the compilation output(s)</title>
 535
 536     <indexterm><primary>output-directing options</primary></indexterm>
 537     <indexterm><primary>redirecting compilation output</primary></indexterm>
 538
 539
 540     <variablelist>
 541       <varlistentry>
 542         <term><literal>-o</literal></term>
 543         <indexterm><primary><literal>-o</literal></primary></indexterm>
 544         <listitem>
 545           <para>GHC's compiled output normally goes into a
 546           <filename>.hc</filename>, <filename>.o</filename>, etc.,
 547           file, depending on the last-run compilation phase.  The
 548           option <option>-o foo</option><IndexTerm><Primary>-o
 549           option</Primary></IndexTerm> re-directs the output of that
 550           last-run phase to file <filename>foo</filename>.</para>
 551
 552           <para>Note: this &ldquo;feature&rdquo; can be
 553           counterintuitive: <command>ghc -C -o foo.o foo.hs</command>
 554           will put the intermediate C code in the file
 555           <filename>foo.o</filename>, name notwithstanding!</para>
 556         </listitem>
 557       </varlistentry>
 558
 559       <varlistentry>
 560         <term><literal>-odir</literal></term>
 561         <indexterm><primary><literal>-odir</literal></primary></indexterm>
 562         <listitem>
 563           <para>The <option>-o</option> option isn't of much use if
 564           you have <emphasis>several</emphasis> input files&hellip;
 565           Non-interface output files are normally put in the same
 566           directory as their corresponding input file came from.  You
 567           may specify that they be put in another directory using the
 568           <option>-odir &lt;dir&gt;</option><IndexTerm><Primary>-odir
 569           &lt;dir&gt; option</Primary></IndexTerm> (the &ldquo;Oh,
 570           dear&rdquo; option).  For example:</para>
 571
 572 <Screen>
 573 % ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
 574 </Screen>
 575
 576           <para>The output files, <filename>Foo.o</filename>,
 577           <filename>Bar.o</filename>, and
 578           <filename>Bumble.o</filename> would be put into a
 579           subdirectory named after the architecture of the executing
 580           machine (<filename>sun4</filename>,
 581           <filename>mips</filename>, etc).  The directory must already
 582           exist; it won't be created.</para>
 583
 584           <para>Note that the <option>-odir</option> option does
 585           <emphasis>not</emphasis> affect where the interface files
 586           are put.  In the above example, they would still be put in
 587           <filename>parse/Foo.hi</filename>,
 588           <filename>parse/Bar.hi</filename>, and
 589           <filename>gurgle/Bumble.hi</filename>.</para>
 590         </listitem>
 591       </varlistentry>
 592
 593       <varlistentry>
 594         <term><option>-ohi</option>  <replaceable>file</replaceable></term>
 595         <indexterm><primary><option>-ohi</option></primary>
 596         </indexterm>
 597         <listitem>
 598           <para>The interface output may be directed to another file
 599           <filename>bar2/Wurble.iface</filename> with the option
 600           <option>-ohi bar2/Wurble.iface</option> (not
 601           recommended).</para>
 602
 603           <para>WARNING: if you redirect the interface file somewhere
 604           that GHC can't find it, then the recompilation checker may
 605           get confused (at the least, you won't get any recompilation
 606           avoidance).  We recommend using a combination of
 607           <option>-hidir</option> and <option>-hisuf</option> options
 608           instead, if possible.</para>
 609
 610           <para>To avoid generating an interface at all, you could use
 611           this option to redirect the interface into the bit bucket:
 612           <literal>-ohi /dev/null</literal>, for example.</para>
 613         </listitem>
 614       </varlistentry>
 615
 616       <varlistentry>
 617         <term><option>-hidir</option>  <replaceable>directory</replaceable></term>
 618         <indexterm><primary><option>-hidir</option></primary>
 619         </indexterm>
 620         <listitem>
 621           <para>Redirects all generated interface files into
 622           <replaceable>directory</replaceable>, instead of the default
 623           which is to place the interface file in the same directory
 624           as the source file.</para>
 625         </listitem>
 626       </varlistentry>
 627
 628       <varlistentry>
 629         <term><option>-osuf</option> <replaceable>suffix</replaceable></term>
 630         <term><option>-hisuf</option> <replaceable>suffix</replaceable></term>
 631         <term><option>-hcsuf</option> <replaceable>suffix</replaceable></term>
 632         <indexterm><primary><option>-osuf</option></primary></indexterm>
 633         <indexterm><primary><option>-hisuf</option></primary></indexterm>
 634         <indexterm><primary><option>-hcsuf</option></primary></indexterm>
 635         <listitem>
 636           <para>EXOTICA: The <option>-osuf</option>
 637           <replaceable>suffix</replaceable> will change the
 638           <literal>.o</literal> file suffix for object files to
 639           whatever you specify.  We use this when compiling libraries,
 640           so that objects for the profiling versions of the libraries
 641           don't clobber the normal ones.</para>
 642
 643           <para>Similarly, the <option>-hisuf</option>
 644           <replaceable>suffix</replaceable> will change the
 645           <literal>.hi</literal> file suffix for non-system interface
 646           files (see <XRef LinkEnd="hi-options">).</para>
 647
 648           <para>Finally, the option <option>-hcsuf</option>
 649           <replaceable>suffix</replaceable> will change the
 650           <literal>.hc</literal> file suffix for compiler-generated
 651           intermediate C files.</para>
 652
 653           <para>The <option>-hisuf</option>/<option>-osuf</option>
 654           game is useful if you want to compile a program with both
 655           GHC and HBC (say) in the same directory.  Let HBC use the
 656           standard <filename>.hi</filename>/<filename>.o</filename>
 657           suffixes; add <option>-hisuf g&lowbar;hi -osuf
 658           g&lowbar;o</option> to your <command>make</command> rule for
 659           GHC compiling&hellip;</para>
 660         </listitem>
 661       </varlistentry>
 662     </variablelist>
 663
 664     <sect2 id="keeping-intermediates">
 665       <title>Keeping Intermediate Files</title>
 666       <indexterm><primary>intermediate files, saving</primary>
 667       </indexterm>
 668       <indexterm><primary><literal>.hc</literal> files, saving</primary>
 669       </indexterm>
 670       <indexterm><primary><literal>.s</literal> files, saving</primary>
 671       </indexterm>
 672
 673
 674       <para>The following options are useful for keeping certain
 675       intermediate files around, when normally GHC would throw these
 676       away after compilation:</para>
 677
 678       <variablelist>
 679         <varlistentry>
 680           <term><literal>-keep-hc-files</literal></term>
 681           <indexterm>
 682             <primary><literal>-keep-hc-files</literal></primary>
 683           </indexterm>
 684           <listitem>
 685             <para>Keep intermediate <literal>.hc</literal> files when
 686             doing <literal>.hs</literal>-to-<literal>.o</literal>
 687             compilations via C (NOTE: <literal>.hc</literal> files
 688             aren't generated when using the native code generator, you
 689             may need to use <literal>-fvia-C</literal> to force them
 690             to be produced).</para>
 691           </listitem>
 692         </varlistentry>
 693
 694         <varlistentry>
 695           <term><literal>-keep-s-files</literal></term>
 696           <indexterm>
 697             <primary><literal>-keep-s-files</literal></primary>
 698           </indexterm>
 699           <listitem>
 700             <para>Keep intermediate <literal>.s</literal> files.</para>
 701           </listitem>
 702         </varlistentry>
 703
 704         <varlistentry>
 705           <term><literal>-keep-raw-s-files</literal></term>
 706           <indexterm>
 707             <primary><literal>-keep-raw-s-files</literal></primary>
 708           </indexterm>
 709           <listitem>
 710             <para>Keep intermediate <literal>.raw-s</literal> files.
 711             These are the direct output from the C compiler, before
 712             GHC does &ldquo;assembly mangling&rdquo; to produce the
 713             <literal>.s</literal> file.  Again, these are not produced
 714             when using the native code generator.</para>
 715           </listitem>
 716         </varlistentry>
 717
 718         <varlistentry>
 719           <term><literal>-keep-tmp-files</literal></term>
 720           <indexterm>
 721             <primary><literal>-keep-tmp-files</literal></primary>
 722           </indexterm>
 723           <indexterm>
 724             <primary>temporary files</primary>
 725             <secondary>keeping</secondary>
 726           </indexterm>
 727           <listitem>
 728             <para>Instructs the GHC driver not to delete any of its
 729             temporary files, which it normally keeps in
 730             <literal>/tmp</literal> (or possibly elsewhere; see <xref
 731             linkend="temp-files">).  Running GHC with
 732             <literal>-v</literal> will show you what temporary files
 733             were generated along the way.</para>
 734           </listitem>
 735         </varlistentry>
 736       </variablelist>
 737     </sect2>
 738
 739     <sect2 id="temp-files">
 740       <title>Redirecting temporary files</title>
 741
 742       <indexterm>
 743         <primary>temporary files</primary>
 744         <secondary>redirecting</secondary>
 745       </indexterm>
 746
 747       <variablelist>
 748         <varlistentry>
 749           <term><literal>-tmpdir</literal></term>
 750           <indexterm><primary><literal>-tmpdir</literal></primary></indexterm>
 751           <listitem>
 752             <para>If you have trouble because of running out of space
 753             in <filename>/tmp</filename> (or wherever your
 754             installation thinks temporary files should go), you may
 755             use the <option>-tmpdir
 756             &lt;dir&gt;</option><IndexTerm><Primary>-tmpdir
 757             &lt;dir&gt; option</Primary></IndexTerm> option to specify
 758             an alternate directory.  For example, <option>-tmpdir
 759             .</option> says to put temporary files in the current
 760             working directory.</para>
 761
 762             <para>Alternatively, use your <Constant>TMPDIR</Constant>
 763             environment variable.<IndexTerm><Primary>TMPDIR
 764             environment variable</Primary></IndexTerm> Set it to the
 765             name of the directory where temporary files should be put.
 766             GCC and other programs will honour the
 767             <Constant>TMPDIR</Constant> variable as well.</para>
 768
 769             <para>Even better idea: Set the
 770             <Constant>DEFAULT_TMPDIR</Constant> make variable when
 771             building GHC, and never worry about
 772             <Constant>TMPDIR</Constant> again. (see the build
 773             documentation).</para>
 774           </listitem>
 775         </varlistentry>
 776       </variablelist>
 777     </sect2>
 778
 779   </sect1>
 780
 781   <sect1 id="options-sanity">
 782     <title>Warnings and sanity-checking</title>
 783
 784     <indexterm><primary>sanity-checking options</primary></indexterm>
 785     <indexterm><primary>warnings</primary></indexterm>
 786
 787
 788     <para>GHC has a number of options that select which types of
 789     non-fatal error messages, otherwise known as warnings, can be
 790     generated during compilation.  By default, you get a standard set
 791     of warnings which are generally likely to indicate bugs in your
 792     program.  These are:
 793     <option>-fwarn-overlpapping-patterns</option>,
 794     <option>-fwarn-deprecations</option>,
 795     <option>-fwarn-duplicate-exports</option>,
 796     <option>-fwarn-missing-fields</option>, and
 797     <option>-fwarn-missing-methods</option>.  The following flags are
 798     simple ways to select standard &ldquo;packages&rdquo; of warnings:
 799     </para>
 800
 801     <VariableList>
 802
 803       <varlistentry>
 804         <term><option>-W</option>:</term>
 805         <listitem>
 806           <IndexTerm><Primary>-W option</Primary></IndexTerm>
 807           <para>Provides the standard warnings plus
 808           <option>-fwarn-incomplete-patterns</option>,
 809           <option>-fwarn-unused-matches</option>,
 810           <option>-fwarn-unused-imports</option>,
 811           <option>-fwarn-misc</option>, and
 812           <option>-fwarn-unused-binds</option>.</para>
 813         </listitem>
 814       </varlistentry>
 815
 816       <varlistentry>
 817         <term><option>-w</option>:</term>
 818         <listitem>
 819           <IndexTerm><Primary><option>-w</option></Primary></IndexTerm>
 820           <para>Turns off all warnings, including the standard ones.</para>
 821         </listitem>
 822       </varlistentry>
 823
 824       <varlistentry>
 825         <term><option>-Wall</option>:</term>
 826         <listitem>
 827           <indexterm><primary><option>-Wall</option></primary></indexterm>
 828           <para>Turns on all warning options.</para>
 829         </listitem>
 830       </varlistentry>
 831
 832     </variablelist>
 833
 834     <para>The full set of warning options is described below.  To turn
 835     off any warning, simply give the corresponding
 836     <option>-fno-warn-...</option> option on the command line.</para>
 837
 838     <variablelist>
 839
 840       <varlistentry>
 841         <term><option>-fwarn-deprecations</option>:</term>
 842         <listitem>
 843           <indexterm><primary><option>-fwarn-deprecations</option></primary>
 844           </indexterm>
 845           <indexterm><primary>deprecations</primary></indexterm>
 846           <para>Causes a warning to be emitted when a deprecated
 847           function or type is used.  Entities can be marked as
 848           deprecated using a pragma, see <xref
 849           linkend="deprecated-pragma">.</para>
 850         </listitem>
 851       </varlistentry>
 852
 853       <varlistentry>
 854         <term><option>-fwarn-duplicate-exports</option>:</term>
 855         <listitem>
 856           <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
 857           <indexterm><primary>duplicate exports, warning</primary></indexterm>
 858           <indexterm><primary>export lists, duplicates</primary></indexterm>
 859
 860           <para>Have the compiler warn about duplicate entries in
 861           export lists. This is useful information if you maintain
 862           large export lists, and want to avoid the continued export
 863           of a definition after you've deleted (one) mention of it in
 864           the export list.</para>
 865
 866           <para>This option is on by default.</para>
 867         </listitem>
 868       </varlistentry>
 869
 870       <varlistentry>
 871         <term><option>-fwarn-hi-shadowing</option>:</term>
 872         <listitem>
 873           <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
 874           <indexterm><primary>shadowing</primary>
 875             <secondary>interface files</secondary></indexterm>
 876
 877           <para>Causes the compiler to emit a warning when a module or
 878           interface file in the current directory is shadowing one
 879           with the same module name in a library or other
 880           directory.</para>
 881         </listitem>
 882       </varlistentry>
 883
 884       <varlistentry>
 885         <term><option>-fwarn-incomplete-patterns</option>:</term>
 886         <listitem>
 887           <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
 888           <indexterm><primary>incomplete patterns, warning</primary></indexterm>
 889           <indexterm><primary>patterns, incomplete</primary></indexterm>
 890
 891           <para>Similarly for incomplete patterns, the function
 892           <function>g</function> below will fail when applied to
 893           non-empty lists, so the compiler will emit a warning about
 894           this when <option>-fwarn-incomplete-patterns</option> is
 895           enabled.</para>
 896
 897 <programlisting>
 898 g [] = 2
 899 </programlisting>
 900
 901           <para>This option isn't enabled be default because it can be
 902           a bit noisy, and it doesn't always indicate a bug in the
 903           program.  However, it's generally considered good practice
 904           to cover all the cases in your functions.</para>
 905         </listitem>
 906       </varlistentry>
 907
 908       <varlistentry>
 909         <term><option>-fwarn-misc</option>:</term>
 910         <indexterm><primary><option>-fwarn-misc</option></primary></indexterm>
 911         <listitem>
 912           <para>Turns on warnings for various harmless but untidy
 913           things.  This currently includes: importing a type with
 914           <literal>(..)</literal> when the export is abstract, and
 915           listing duplicate class assertions in a qualified type.</para>
 916         </listitem>
 917       </varlistentry>
 918
 919       <varlistentry>
 920         <term><option>-fwarn-missing-fields</option>:</term>
 921         <listitem>
 922           <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
 923           <indexterm><primary>missing fields, warning</primary></indexterm>
 924           <indexterm><primary>fields, missing</primary></indexterm>
 925
 926           <para>This option is on by default, and warns you whenever
 927           the construction of a labelled field constructor isn't
 928           complete, missing initializers for one or more fields. While
 929           not an error (the missing fields are initialised with
 930           bottoms), it is often an indication of a programmer error.</para>
 931         </listitem>
 932       </varlistentry>
 933
 934       <varlistentry>
 935         <term><option>-fwarn-missing-methods</option>:</term>
 936         <listitem>
 937           <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
 938           <indexterm><primary>missing methods, warning</primary></indexterm>
 939           <indexterm><primary>methods, missing</primary></indexterm>
 940
 941           <para>This option is on by default, and warns you whenever
 942           an instance declaration is missing one or more methods, and
 943           the corresponding class declaration has no default
 944           declaration for them.</para>
 945         </listitem>
 946       </varlistentry>
 947
 948       <varlistentry>
 949         <term><option>-fwarn-missing-signatures</option>:</term>
 950         <listitem>
 951           <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
 952           <indexterm><primary>type signatures, missing</primary></indexterm>
 953
 954           <para>If you would like GHC to check that every top-level
 955           function/value has a type signature, use the
 956           <option>-fwarn-missing-signatures</option> option.  This
 957           option is off by default.</para>
 958         </listitem>
 959       </varlistentry>
 960
 961       <varlistentry>
 962         <term><option>-fwarn-name-shadowing</option>:</term>
 963         <listitem>
 964           <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
 965           <indexterm><primary>shadowing, warning</primary></indexterm>
 966
 967           <para>This option causes a warning to be emitted whenever an
 968           inner-scope value has the same name as an outer-scope value,
 969           i.e. the inner value shadows the outer one.  This can catch
 970           typographical errors that turn into hard-to-find bugs, e.g.,
 971           in the inadvertent cyclic definition <literal>let x = ... x
 972           ... in</literal>.</para>
 973
 974           <para>Consequently, this option does
 975           <emphasis>will</emphasis> complain about cyclic recursive
 976           definitions.</para>
 977         </listitem>
 978       </varlistentry>
 979
 980       <varlistentry>
 981         <term><option>-fwarn-overlapping-patterns</option>:</term>
 982         <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
 983         <indexterm><primary>overlapping patterns, warning</primary></indexterm>
 984         <indexterm><primary>patterns, overlapping</primary></indexterm>
 985         <listitem>
 986           <para>By default, the compiler will warn you if a set of
 987           patterns are overlapping, i.e.,</para>
 988
 989 <programlisting>
 990 f :: String -&#62; Int
 991 f []     = 0
 992 f (_:xs) = 1
 993 f "2"    = 2
 994 </programlisting>
 995
 996           <para>where the last pattern match in <Function>f</Function>
 997           won't ever be reached, as the second pattern overlaps
 998           it. More often than not, redundant patterns is a programmer
 999           mistake/error, so this option is enabled by default.</para>
1000         </listitem>
1001       </varlistentry>
1002
1003       <varlistentry>
1004         <term><option>-fwarn-simple-patterns</option>:</term>
1005         <listitem>
1006           <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1007           </indexterm>
1008           <para>Causes the compiler to warn about lambda-bound
1009           patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1010           Normally, these aren't treated as incomplete patterns by
1011           <option>-fwarn-incomplete-patterns</option>.</para>
1012         </listitem>
1013       </varlistentry>
1014
1015       <varlistentry>
1016         <term><option>-fwarn-type-defaults</option>:</term>
1017         <listitem>
1018           <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1019           <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1020           <para>Have the compiler warn/inform you where in your source
1021           the Haskell defaulting mechanism for numeric types kicks
1022           in. This is useful information when converting code from a
1023           context that assumed one default into one with another,
1024           e.g., the `default default' for Haskell 1.4 caused the
1025           otherwise unconstrained value <Constant>1</Constant> to be
1026           given the type <literal>Int</literal>, whereas Haskell 98
1027           defaults it to <literal>Integer</literal>.  This may lead to
1028           differences in performance and behaviour, hence the
1029           usefulness of being non-silent about this.</para>
1030
1031           <para>This warning is off by default.</para>
1032         </listitem>
1033       </varlistentry>
1034
1035       <varlistentry>
1036         <term><option>-fwarn-unused-binds</option>:</term>
1037         <listitem>
1038           <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1039           <indexterm><primary>unused binds, warning</primary></indexterm>
1040           <indexterm><primary>binds, unused</primary></indexterm>
1041           <para>Report any function definitions (and local bindings)
1042           which are unused.  For top-level functions, the warning is
1043           only given if the binding is not exported.</para>
1044         </listitem>
1045       </varlistentry>
1046
1047       <varlistentry>
1048         <term><option>-fwarn-unused-imports</option>:</term>
1049         <listitem>
1050           <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1051           <indexterm><primary>unused imports, warning</primary></indexterm>
1052           <indexterm><primary>imports, unused</primary></indexterm>
1053
1054           <para>Report any objects that are explicitly imported but
1055           never used.</para>
1056         </listitem>
1057       </varlistentry>
1058
1059       <varlistentry>
1060         <term><option>-fwarn-unused-matches</option>:</term>
1061         <listitem>
1062           <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1063           <indexterm><primary>unused matches, warning</primary></indexterm>
1064           <indexterm><primary>matches, unused</primary></indexterm>
1065
1066           <para>Report all unused variables which arise from pattern
1067           matches, including patterns consisting of a single variable.
1068           For instance <literal>f x y = []</literal> would report
1069           <VarName>x</VarName> and <VarName>y</VarName> as unused.  To
1070           eliminate the warning, all unused variables can be replaced
1071           with wildcards.</para>
1072         </listitem>
1073       </varlistentry>
1074
1075     </VariableList>
1076
1077     <para>If you're feeling really paranoid, the
1078     <option>-dcore-lint</option>
1079     option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1080     is a good choice.  It turns on heavyweight intra-pass
1081     sanity-checking within GHC.  (It checks GHC's sanity, not
1082     yours.)</para>
1083
1084   </sect1>
1085
1086   &separate;
1087   &packages;
1088
1089   <sect1 id="options-optimise">
1090     <title>Optimisation (code improvement)</title>
1091
1092     <indexterm><primary>optimisation</primary></indexterm>
1093     <indexterm><primary>improvement, code</primary></indexterm>
1094
1095     <para>The <option>-O*</option> options specify convenient
1096     &ldquo;packages&rdquo; of optimisation flags; the
1097     <option>-f*</option> options described later on specify
1098     <emphasis>individual</emphasis> optimisations to be turned on/off;
1099     the <option>-m*</option> options specify
1100     <emphasis>machine-specific</emphasis> optimisations to be turned
1101     on/off.</para>
1102
1103     <sect2 id="optimise-pkgs">
1104       <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1105
1106       <para>There are <emphasis>many</emphasis> options that affect
1107       the quality of code produced by GHC.  Most people only have a
1108       general goal, something like &ldquo;Compile quickly&rdquo; or
1109       &ldquo;Make my program run like greased lightning.&rdquo; The
1110       following &ldquo;packages&rdquo; of optimisations (or lack
1111       thereof) should suffice.</para>
1112
1113       <para>Once you choose a <option>-O*</option>
1114       &ldquo;package,&rdquo; stick with it&mdash;don't chop and
1115       change.  Modules' interfaces <emphasis>will</emphasis> change
1116       with a shift to a new <option>-O*</option> option, and you may
1117       have to recompile a large chunk of all importing modules before
1118       your program can again be run safely (see <XRef
1119       LinkEnd="recomp">).</para>
1120
1121       <variablelist>
1122
1123         <varlistentry>
1124           <term>No <option>-O*</option>-type option specified:</term>
1125           <indexterm><primary>-O* not specified</primary></indexterm>
1126           <listitem>
1127             <para>This is taken to mean: &ldquo;Please compile
1128             quickly; I'm not over-bothered about compiled-code
1129             quality.&rdquo; So, for example: <command>ghc -c
1130             Foo.hs</command></para>
1131           </listitem>
1132         </varlistentry>
1133
1134         <varlistentry>
1135           <term><option>-O0</option>:</term>
1136           <indexterm><primary><option>-O0</option></primary></indexterm>
1137           <listitem>
1138             <para>Means &ldquo;turn off all optimisation&rdquo;,
1139             reverting to the same settings as if no
1140             <option>-O</option> options had been specified.  Saying
1141             <option>-O0</option> can be useful if
1142             eg. <command>make</command> has inserted a
1143             <option>-O</option> on the command line already.</para>
1144           </listitem>
1145         </varlistentry>
1146
1147         <varlistentry>
1148           <term><option>-O</option> or <option>-O1</option>:</term>
1149           <indexterm><primary>-O option</primary></indexterm>
1150           <indexterm><primary>-O1 option</primary></indexterm>
1151           <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1152           <listitem>
1153             <para>Means: &ldquo;Generate good-quality code without
1154             taking too long about it.&rdquo; Thus, for example:
1155             <command>ghc -c -O Main.lhs</command></para>
1156           </listitem>
1157         </varlistentry>
1158
1159         <varlistentry>
1160           <term><option>-O2</option>:</term>
1161           <indexterm><primary>-O2 option</primary></indexterm>
1162           <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1163           <listitem>
1164             <para>Means: &ldquo;Apply every non-dangerous
1165             optimisation, even if it means significantly longer
1166             compile times.&rdquo;</para>
1167
1168             <para>The avoided &ldquo;dangerous&rdquo; optimisations
1169             are those that can make runtime or space
1170             <emphasis>worse</emphasis> if you're unlucky.  They are
1171             normally turned on or off individually.</para>
1172
1173             <para>At the moment, <option>-O2</option> is
1174             <emphasis>unlikely</emphasis> to produce better code than
1175             <option>-O</option>.</para>
1176           </listitem>
1177         </varlistentry>
1178
1179         <varlistentry>
1180           <term><option>-O2-for-C</option>:</term>
1181           <indexterm><primary>-O2-for-C option</primary></indexterm>
1182           <indexterm><primary>gcc, invoking with -O2</primary></indexterm>
1183           <listitem>
1184             <para>Says to run GCC with <option>-O2</option>, which may
1185             be worth a few percent in execution speed.  Don't forget
1186             <option>-fvia-C</option>, lest you use the native-code
1187             generator and bypass GCC altogether!</para>
1188           </listitem>
1189         </varlistentry>
1190
1191         <varlistentry>
1192           <term><option>-Ofile &lt;file&gt;</option>:</term>
1193           <indexterm><primary>-Ofile &lt;file&gt; option</primary></indexterm>
1194           <indexterm><primary>optimising, customised</primary></indexterm>
1195           <listitem>
1196             <para>(NOTE: not supported yet in GHC 5.x.  Please ask if
1197             you're interested in this.)</para>
1198
1199             <para>For those who need <emphasis>absolute</emphasis>
1200             control over <emphasis>exactly</emphasis> what options are
1201             used (e.g., compiler writers, sometimes :-), a list of
1202             options can be put in a file and then slurped in with
1203             <option>-Ofile</option>.</para>
1204
1205             <para>In that file, comments are of the
1206             <literal>&num;</literal>-to-end-of-line variety; blank
1207             lines and most whitespace is ignored.</para>
1208
1209             <para>Please ask if you are baffled and would like an
1210             example of <option>-Ofile</option>!</para>
1211           </listitem>
1212         </varlistentry>
1213       </variablelist>
1214
1215       <para>We don't use a <option>-O*</option> flag for day-to-day
1216       work.  We use <option>-O</option> to get respectable speed;
1217       e.g., when we want to measure something.  When we want to go for
1218       broke, we tend to use <option>-O -fvia-C -O2-for-C</option> (and
1219       we go for lots of coffee breaks).</para>
1220
1221       <para>The easiest way to see what <option>-O</option> (etc.)
1222       &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1223       then stand back in amazement.</para>
1224     </sect2>
1225
1226     <sect2 id="options-f">
1227       <title><option>-f*</option>: platform-independent flags</title>
1228
1229       <indexterm><primary>-f* options (GHC)</primary></indexterm>
1230       <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1231
1232       <para>These flags turn on and off individual optimisations.
1233       They are normally set via the <option>-O</option> options
1234       described above, and as such, you shouldn't need to set any of
1235       them explicitly (indeed, doing so could lead to unexpected
1236       results).  However, there are one or two that may be of
1237       interest:</para>
1238
1239       <variablelist>
1240         <varlistentry>
1241           <term><option>-fexcess-precision</option>:</term>
1242           <listitem>
1243             <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1244             <para>When this option is given, intermediate floating
1245             point values can have a <emphasis>greater</emphasis>
1246             precision/range than the final type.  Generally this is a
1247             good thing, but some programs may rely on the exact
1248             precision/range of
1249             <literal>Float</literal>/<literal>Double</literal> values
1250             and should not use this option for their compilation.</para>
1251           </listitem>
1252         </varlistentry>
1253
1254         <varlistentry>
1255           <term><option>-fignore-asserts</option>:</term>
1256           <listitem>
1257             <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1258             <para>Causes GHC to ignore uses of the function
1259             <literal>Exception.assert</literal> in source code (in
1260             other words, rewriting <literal>Exception.assert p
1261             e</literal> to <literal>e</literal> (see <xref
1262             linkend="sec-assertions">).  This flag is turned on by
1263             <option>-O</option>.
1264             </para>
1265           </listitem>
1266         </varlistentry>
1267
1268         <varlistentry>
1269           <term><option>-fno-strictness</option></term>
1270           <indexterm><primary><option>-fno-strictness</option></primary>
1271           </indexterm>
1272           <listitem>
1273             <para>Turns off the strictness analyser; sometimes it eats
1274             too many cycles.</para>
1275           </listitem>
1276         </varlistentry>
1277
1278         <varlistentry>
1279           <term><option>-fno-cpr-analyse</option></term>
1280           <indexterm><primary><option>-fno-cpr-analyse</option></primary>
1281           </indexterm>
1282           <listitem>
1283             <para>Turns off the CPR (constructed product result)
1284             analysis; it is somewhat experimental.</para>
1285           </listitem>
1286         </varlistentry>
1287
1288         <varlistentry>
1289           <term><option>-funbox-strict-fields</option>:</term>
1290           <listitem>
1291             <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1292             <indexterm><primary>strict constructor fields</primary></indexterm>
1293             <indexterm><primary>constructor fields, strict</primary></indexterm>
1294
1295             <para>This option causes all constructor fields which are
1296             marked strict (i.e. &ldquo;!&rdquo;) to be unboxed or
1297             unpacked if possible.  For example:</para>
1298
1299 <ProgramListing>
1300 data T = T !Float !Float
1301 </ProgramListing>
1302
1303             <para>will create a constructor <literal>T</literal>
1304             containing two unboxed floats if the
1305             <option>-funbox-strict-fields</option> flag is given.
1306             This may not always be an optimisation: if the
1307             <Function>T</Function> constructor is scrutinised and the
1308             floats passed to a non-strict function for example, they
1309             will have to be reboxed (this is done automatically by the
1310             compiler).</para>
1311
1312             <para>This option should only be used in conjunction with
1313             <option>-O</option>, in order to expose unfoldings to the
1314             compiler so the reboxing can be removed as often as
1315             possible.  For example:</para>
1316
1317 <ProgramListing>
1318 f :: T -&#62; Float
1319 f (T f1 f2) = f1 + f2
1320 </ProgramListing>
1321
1322             <para>The compiler will avoid reboxing
1323             <Function>f1</Function> and <Function>f2</Function> by
1324             inlining <Function>+</Function> on floats, but only when
1325             <option>-O</option> is on.</para>
1326
1327             <para>Any single-constructor data is eligible for
1328             unpacking; for example</para>
1329
1330 <ProgramListing>
1331 data T = T !(Int,Int)
1332 </ProgramListing>
1333
1334             <para>will store the two <literal>Int</literal>s directly
1335             in the <Function>T</Function> constructor, by flattening
1336             the pair.  Multi-level unpacking is also supported:</para>
1337
1338 <ProgramListing>
1339 data T = T !S
1340 data S = S !Int !Int
1341 </ProgramListing>
1342
1343             <para>will store two unboxed <literal>Int&num;</literal>s
1344             directly in the <Function>T</Function> constructor.</para>
1345           </listitem>
1346         </varlistentry>
1347
1348         <varlistentry>
1349           <term><option>-funfolding-update-in-place&lt;n&gt;</option></term>
1350           <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1351           <listitem>
1352             <para>Switches on an experimental "optimisation".
1353             Switching it on makes the compiler a little keener to
1354             inline a function that returns a constructor, if the
1355             context is that of a thunk.
1356 <ProgramListing>
1357    x = plusInt a b
1358 </ProgramListing>
1359             If we inlined plusInt we might get an opportunity to use
1360             update-in-place for the thunk 'x'.</para>
1361           </listitem>
1362         </varlistentry>
1363
1364         <varlistentry>
1365           <term><option>-funfolding-creation-threshold&lt;n&gt;</option>:</term>
1366           <listitem>
1367             <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1368             <indexterm><primary>inlining, controlling</primary></indexterm>
1369             <indexterm><primary>unfolding, controlling</primary></indexterm>
1370
1371             <para>(Default: 45) Governs the maximum size that GHC will
1372             allow a function unfolding to be.   (An unfolding has a
1373             &ldquo;size&rdquo; that reflects the cost in terms of
1374             &ldquo;code bloat&rdquo; of expanding that unfolding at
1375             at a call site. A bigger function would be assigned a
1376             bigger cost.) </para>
1377
1378             <para> Consequences: (a) nothing larger than this will be
1379             inlined (unless it has an INLINE pragma); (b) nothing
1380             larger than this will be spewed into an interface
1381             file. </para>
1382
1383
1384             <para> Increasing this figure is more likely to result in longer
1385             compile times than faster code.  The next option is more
1386             useful:</para>
1387           </listitem>
1388         </varlistentry>
1389
1390         <varlistentry>
1391           <term><option>-funfolding-use-threshold&lt;n&gt;</option>:</term>
1392           <listitem>
1393             <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1394             <indexterm><primary>inlining, controlling</primary></indexterm>
1395             <indexterm><primary>unfolding, controlling</primary></indexterm>
1396
1397             <para>(Default: 8) This is the magic cut-off figure for
1398             unfolding: below this size, a function definition will be
1399             unfolded at the call-site, any bigger and it won't.  The
1400             size computed for a function depends on two things: the
1401             actual size of the expression minus any discounts that
1402             apply (see <option>-funfolding-con-discount</option>).</para>
1403           </listitem>
1404         </varlistentry>
1405       </variablelist>
1406
1407     </sect2>
1408
1409   </sect1>
1410
1411 &phases;
1412
1413 <Sect1 id="sec-using-concurrent">
1414 <title>Using Concurrent Haskell</title>
1415
1416 <para>
1417 <indexterm><primary>Concurrent Haskell&mdash;use</primary></indexterm>
1418 </para>
1419
1420 <para>
1421 GHC (as of version 4.00) supports Concurrent Haskell by default,
1422 without requiring a special option or libraries compiled in a certain
1423 way.  To get access to the support libraries for Concurrent Haskell
1424 (i.e. <literal>Concurrent</literal> and friends), use the
1425 <option>-package concurrent</option> option.
1426 </para>
1427
1428 <para>
1429 Three RTS options are provided for modifying the behaviour of the
1430 threaded runtime system.  See the descriptions of
1431 <option>-C[&lt;us&gt;]</option>, <option>-q</option>, and
1432 <option>-t&lt;num&gt;</option> in <XRef LinkEnd="parallel-rts-opts">.
1433 </para>
1434
1435 <para>
1436 Concurrent Haskell is described in more detail in <XRef
1437 LinkEnd="sec-Concurrent">.
1438 </para>
1439
1440 </Sect1>
1441
1442 <Sect1 id="sec-using-parallel">
1443 <title>Using Parallel Haskell</title>
1444
1445 <para>
1446 <indexterm><primary>Parallel Haskell&mdash;use</primary></indexterm>
1447 </para>
1448
1449 <para>
1450 &lsqb;You won't be able to execute parallel Haskell programs unless PVM3
1451 (Parallel Virtual Machine, version 3) is installed at your site.&rsqb;
1452 </Para>
1453
1454 <para>
1455 To compile a Haskell program for parallel execution under PVM, use the
1456 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
1457 option</Primary></IndexTerm> both when compiling <Emphasis>and
1458 linking</Emphasis>.  You will probably want to <Literal>import
1459 Parallel</Literal> into your Haskell modules.
1460 </Para>
1461
1462 <para>
1463 To run your parallel program, once PVM is going, just invoke it
1464 &ldquo;as normal&rdquo;.  The main extra RTS option is
1465 <Option>-qp&lt;n&gt;</Option>, to say how many PVM
1466 &ldquo;processors&rdquo; your program to run on.  (For more details of
1467 all relevant RTS options, please see <XRef
1468 LinkEnd="parallel-rts-opts">.)
1469 </para>
1470
1471 <para>
1472 In truth, running Parallel Haskell programs and getting information
1473 out of them (e.g., parallelism profiles) is a battle with the vagaries of
1474 PVM, detailed in the following sections.
1475 </para>
1476
1477 <Sect2 id="pvm-dummies">
1478 <Title>Dummy's guide to using PVM</Title>
1479
1480 <para>
1481 <indexterm><primary>PVM, how to use</primary></indexterm>
1482 <indexterm><primary>Parallel Haskell&mdash;PVM use</primary></indexterm>
1483 Before you can run a parallel program under PVM, you must set the
1484 required environment variables (PVM's idea, not ours); something like,
1485 probably in your <filename>.cshrc</filename> or equivalent:
1486
1487 <ProgramListing>
1488 setenv PVM_ROOT /wherever/you/put/it
1489 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
1490 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
1491 </ProgramListing>
1492
1493 </para>
1494
1495 <para>
1496 Creating and/or controlling your &ldquo;parallel machine&rdquo; is a purely-PVM
1497 business; nothing specific to Parallel Haskell. The following paragraphs
1498 describe how to configure your parallel machine interactively.
1499 </Para>
1500
1501 <Para>
1502 If you use parallel Haskell regularly on the same machine configuration it
1503 is a good idea to maintain a file with all machine names and to make the
1504 environment variable PVM_HOST_FILE point to this file. Then you can avoid
1505 the interactive operations described below by just saying
1506 </Para>
1507
1508 <ProgramListing>
1509 pvm $PVM_HOST_FILE
1510 </ProgramListing>
1511
1512 <Para>
1513 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
1514 machine.  You can then do various things to control/monitor your
1515 &ldquo;parallel machine;&rdquo; the most useful being:
1516 </para>
1517
1518 <para>
1519 <InformalTable>
1520 <TGroup Cols=2>
1521 <ColSpec Align="Left">
1522 <TBody>
1523
1524 <row>
1525 <entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></entry>
1526 <entry>exit <command>pvm</command>, leaving it running</entry>
1527 </row>
1528
1529 <row>
1530 <entry><command>halt</command></entry>
1531 <entry>kill off this &ldquo;parallel machine&rdquo; &amp; exit</entry>
1532 </row>
1533
1534 <row>
1535 <entry><command>add &lt;host&gt;</command></entry>
1536 <entry>add <command>&lt;host&gt;</command> as a processor</entry>
1537 </row>
1538
1539 <row>
1540 <entry><command>delete &lt;host&gt;</command></entry>
1541 <entry>delete <command>&lt;host&gt;</command></entry>
1542 </row>
1543
1544 <row>
1545 <entry><command>reset</command></entry>
1546 <entry>kill what's going, but leave PVM up</entry>
1547 </row>
1548
1549 <row>
1550 <entry><command>conf</command></entry>
1551 <entry>list the current configuration</entry>
1552 </row>
1553
1554 <row>
1555 <entry><command>ps</command></entry>
1556 <entry>report processes' status</entry>
1557 </row>
1558
1559 <row>
1560 <entry><command>pstat &lt;pid&gt;</command></entry>
1561 <entry>status of a particular process</entry>
1562 </row>
1563
1564 </TBody>
1565 </TGroup>
1566 </InformalTable>
1567 </para>
1568
1569 <para>
1570 The PVM documentation can tell you much, much more about <command>pvm</command>!
1571 </para>
1572
1573 </sect2>
1574
1575 <Sect2 id="par-profiles">
1576 <Title>Parallelism profiles</Title>
1577
1578 <para>
1579 <indexterm><primary>parallelism profiles</primary></indexterm>
1580 <indexterm><primary>profiles, parallelism</primary></indexterm>
1581 <indexterm><primary>visualisation tools</primary></indexterm>
1582 </para>
1583
1584 <para>
1585 With Parallel Haskell programs, we usually don't care about the
1586 results&mdash;only with &ldquo;how parallel&rdquo; it was!  We want pretty pictures.
1587 </para>
1588
1589 <Para>
1590 Parallelism profiles (&agrave; la <Command>hbcpp</Command>) can be generated with the
1591 <Option>-qP</Option><IndexTerm><Primary>-qP RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option.  The
1592 per-processor profiling info is dumped into files named
1593 <Filename>&lt;full-path&gt;&lt;program&gt;.gr</Filename>.  These are then munged into a PostScript picture,
1594 which you can then display.  For example, to run your program
1595 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
1596 </Para>
1597
1598 <Para>
1599
1600 <Screen>
1601 <prompt>&dollar;</prompt> ./a.out +RTS -qP -qp8
1602 <prompt>&dollar;</prompt> grs2gr *.???.gr &#62; temp.gr # combine the 8 .gr files into one
1603 <prompt>&dollar;</prompt> gr2ps -O temp.gr              # cvt to .ps; output in temp.ps
1604 <prompt>&dollar;</prompt> ghostview -seascape temp.ps   # look at it!
1605 </Screen>
1606
1607 </Para>
1608
1609 <para>
1610 The scripts for processing the parallelism profiles are distributed
1611 in <filename>ghc/utils/parallel/</filename>.
1612 </para>
1613
1614 </sect2>
1615
1616 <Sect2>
1617 <Title>Other useful info about running parallel programs</Title>
1618
1619 <Para>
1620 The &ldquo;garbage-collection statistics&rdquo; RTS options can be useful for
1621 seeing what parallel programs are doing.  If you do either
1622 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
1623 you'll get mutator, garbage-collection, etc., times on standard
1624 error. The standard error of all PE's other than the `main thread'
1625 appears in <filename>/tmp/pvml.nnn</filename>, courtesy of PVM.
1626 </para>
1627
1628 <para>
1629 Whether doing <option>+RTS -Sstderr</option> or not, a handy way to watch
1630 what's happening overall is: <command>tail -f /tmp/pvml.nnn</command>.
1631 </para>
1632
1633 </sect2>
1634
1635 <Sect2 id="parallel-rts-opts">
1636 <title>RTS options for Concurrent/Parallel Haskell
1637 </title>
1638
1639 <para>
1640 <indexterm><primary>RTS options, concurrent</primary></indexterm>
1641 <indexterm><primary>RTS options, parallel</primary></indexterm>
1642 <indexterm><primary>Concurrent Haskell&mdash;RTS options</primary></indexterm>
1643 <indexterm><primary>Parallel Haskell&mdash;RTS options</primary></indexterm>
1644 </para>
1645
1646 <para>
1647 Besides the usual runtime system (RTS) options
1648 (<XRef LinkEnd="runtime-control">), there are a few options particularly
1649 for concurrent/parallel execution.
1650 </para>
1651
1652 <para>
1653 <VariableList>
1654
1655 <VarListEntry>
1656 <Term><Option>-qp&lt;N&gt;</Option>:</Term>
1657 <ListItem>
1658 <Para>
1659 <IndexTerm><Primary>-qp&lt;N&gt; RTS option</Primary></IndexTerm>
1660 (PARALLEL ONLY) Use <Literal>&lt;N&gt;</Literal> PVM processors to run this program;
1661 the default is 2.
1662 </para>
1663 </listitem>
1664 </varlistentry>
1665 <varlistentry>
1666 <term><option>-C[&lt;us&gt;]</option>:</term>
1667 <listitem>
1668 <para>
1669 <indexterm><primary>-C&lt;us&gt; RTS option</primary></indexterm> Sets
1670 the context switch interval to <literal>&lt;s&gt;</literal> seconds.
1671 A context switch will occur at the next heap block allocation after
1672 the timer expires (a heap block allocation occurs every 4k of
1673 allocation).  With <option>-C0</option> or <option>-C</option>,
1674 context switches will occur as often as possible (at every heap block
1675 allocation).  By default, context switches occur every 20ms
1676 milliseconds.  Note that GHC's internal timer ticks every 20ms, and
1677 the context switch timer is always a multiple of this timer, so 20ms
1678 is the maximum granularity available for timed context switches.
1679 </para>
1680 </listitem>
1681 </varlistentry>
1682 <varlistentry>
1683 <term><option>-q[v]</option>:</term>
1684 <listitem>
1685 <para>
1686 <indexterm><primary>-q RTS option</primary></indexterm>
1687 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
1688 in the file <FIlename>&lt;program&gt;.qp</FIlename>.  In the style of <command>hbcpp</command>, this profile
1689 records the movement of threads between the green (runnable) and red
1690 (blocked) queues.  If you specify the verbose suboption (<option>-qv</option>), the
1691 green queue is split into green (for the currently running thread
1692 only) and amber (for other runnable threads).  We do not recommend
1693 that you use the verbose suboption if you are planning to use the
1694 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
1695 check (with <Option>-C</Option>).
1696 -->
1697 </Para>
1698 </ListItem>
1699 </VarListEntry>
1700 <VarListEntry>
1701 <Term><Option>-qt&lt;num&gt;</Option>:</Term>
1702 <ListItem>
1703 <Para>
1704 <IndexTerm><Primary>-qt&lt;num&gt; RTS option</Primary></IndexTerm>
1705 (PARALLEL ONLY) Limit the thread pool size, i.e. the number of concurrent
1706 threads per processor to <Literal>&lt;num&gt;</Literal>.  The default is
1707 32.  Each thread requires slightly over 1K <Emphasis>words</Emphasis> in
1708 the heap for thread state and stack objects.  (For 32-bit machines, this
1709 translates to 4K bytes, and for 64-bit machines, 8K bytes.)
1710 </Para>
1711 </ListItem>
1712 </VarListEntry>
1713 <!-- no more -HWL
1714 <VarListEntry>
1715 <Term><Option>-d</Option>:</Term>
1716 <ListItem>
1717 <Para>
1718 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
1719 (PARALLEL ONLY) Turn on debugging.  It pops up one xterm (or GDB, or
1720 something&hellip;) per PVM processor.  We use the standard <Command>debugger</Command>
1721 script that comes with PVM3, but we sometimes meddle with the
1722 <Command>debugger2</Command> script.  We include ours in the GHC distribution,
1723 in <Filename>ghc/utils/pvm/</Filename>.
1724 </Para>
1725 </ListItem>
1726 </VarListEntry>
1727 -->
1728 <VarListEntry>
1729 <Term><Option>-qe&lt;num&gt;</Option>:</Term>
1730 <ListItem>
1731 <Para>
1732 <IndexTerm><Primary>-qe&lt;num&gt; RTS option
1733 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Limit the spark pool size
1734 i.e. the number of pending sparks per processor to
1735 <Literal>&lt;num&gt;</Literal>. The default is 100. A larger number may be
1736 appropriate if your program generates large amounts of parallelism
1737 initially.
1738 </Para>
1739 </ListItem>
1740 </VarListEntry>
1741 <VarListEntry>
1742 <Term><Option>-qQ&lt;num&gt;</Option>:</Term>
1743 <ListItem>
1744 <Para>
1745 <IndexTerm><Primary>-qQ&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1746 (PARALLEL ONLY) Set the size of packets transmitted between processors
1747 to <Literal>&lt;num&gt;</Literal>. The default is 1024 words. A larger number may be
1748 appropriate if your machine has a high communication cost relative to
1749 computation speed.
1750 </Para>
1751 </ListItem>
1752 </VarListEntry>
1753 <VarListEntry>
1754 <Term><Option>-qh&lt;num&gt;</Option>:</Term>
1755 <ListItem>
1756 <Para>
1757 <IndexTerm><Primary>-qh&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1758 (PARALLEL ONLY) Select a packing scheme. Set the number of non-root thunks to pack in one packet to
1759 &lt;num&gt;-1 (0 means infinity). By default GUM uses full-subgraph
1760 packing, i.e. the entire subgraph with the requested closure as root is
1761 transmitted (provided it fits into one packet). Choosing a smaller value
1762 reduces the amount of pre-fetching of work done in GUM. This can be
1763 advantageous for improving data locality but it can also worsen the balance
1764 of the load in the system.
1765 </Para>
1766 </ListItem>
1767 </VarListEntry>
1768 <VarListEntry>
1769 <Term><Option>-qg&lt;num&gt;</Option>:</Term>
1770 <ListItem>
1771 <Para>
1772 <IndexTerm><Primary>-qg&lt;num&gt; RTS option
1773 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Select a globalisation
1774 scheme. This option affects the
1775 generation of global addresses when transferring data. Global addresses are
1776 globally unique identifiers required to maintain sharing in the distributed
1777 graph structure. Currently this is a binary option. With &lt;num&gt;=0 full globalisation is used
1778 (default). This means a global address is generated for every closure that
1779 is transmitted. With &lt;num&gt;=1 a thunk-only globalisation scheme is
1780 used, which generated global address only for thunks. The latter case may
1781 lose sharing of data but has a reduced overhead in packing graph structures
1782 and maintaining internal tables of global addresses.
1783 </Para>
1784 </ListItem>
1785 </VarListEntry>
1786 </VariableList>
1787 </para>
1788
1789 </sect2>
1790
1791 </Sect1>
1792
1793   <sect1 id="options-platform">
1794     <title>Platform-specific Flags</title>
1795
1796     <indexterm><primary>-m* options</primary></indexterm>
1797     <indexterm><primary>platform-specific options</primary></indexterm>
1798     <indexterm><primary>machine-specific options</primary></indexterm>
1799
1800     <para>Some flags only make sense for particular target
1801     platforms.</para>
1802
1803     <variablelist>
1804
1805       <varlistentry>
1806         <term><option>-mv8</option>:</term>
1807         <listitem>
1808           <para>(SPARC machines)<indexterm><primary>-mv8 option (SPARC
1809           only)</primary></indexterm> Means to pass the like-named
1810           option to GCC; it says to use the Version 8 SPARC
1811           instructions, notably integer multiply and divide.  The
1812           similiar <option>-m*</option> GCC options for SPARC also
1813           work, actually.</para>
1814         </listitem>
1815       </varlistentry>
1816
1817       <varlistentry>
1818         <term><option>-monly-[32]-regs</option>:</term>
1819         <listitem>
1820           <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1821           option (iX86 only)</primary></indexterm> GHC tries to
1822           &ldquo;steal&rdquo; four registers from GCC, for performance
1823           reasons; it almost always works.  However, when GCC is
1824           compiling some modules with four stolen registers, it will
1825           crash, probably saying:
1826
1827 <Screen>
1828 Foo.hc:533: fixed or forbidden register was spilled.
1829 This may be due to a compiler bug or to impossible asm
1830 statements or clauses.
1831 </Screen>
1832
1833           Just give some registers back with
1834           <option>-monly-N-regs</option>.  Try `3' first, then `2'.
1835           If `2' doesn't work, please report the bug to us.</para>
1836         </listitem>
1837       </varlistentry>
1838     </variablelist>
1839
1840   </sect1>
1841
1842 &runtime;
1843 &debug;
1844 &flags;
1845
1846 </Chapter>
1847
1848 <!-- Emacs stuff:
1849      ;;; Local Variables: ***
1850      ;;; mode: sgml ***
1851      ;;; sgml-parent-document: ("users_guide.sgml" "book" "chapter") ***
1852      ;;; End: ***
1853  -->