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>--make</option></primary>
  53         </indexterm>
  54         <listitem>
  55           <para>This is the traditional batch-compiler mode, in which
  56           GHC can compile source files one at a time, or link objects
  57           together into an executable.</para>
  58         </listitem>
  59       </varlistentry>
  60     </variablelist>
  61
  62   <sect1>
  63     <title>Options overview</title>
  64
  65     <para>GHC's behaviour is controlled by
  66     <firstterm>options</firstterm>, which for historical reasons are
  67     also sometimes referred to as command-line flags or arguments.
  68     Options can be specified in three ways:</para>
  69
  70     <sect2>
  71       <title>Command-line arguments</title>
  72
  73       <indexterm><primary>structure, command-line</primary></indexterm>
  74       <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
  75       <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
  76
  77       <para>An invocation of GHC takes the following form:</para>
  78
  79 <Screen>
  80 ghc [argument...]
  81 </Screen>
  82
  83       <para>Command-line arguments are either options or file names.</para>
  84
  85       <para>Command-line options begin with <literal>-</literal>.
  86       They may <emphasis>not</emphasis> be grouped:
  87       <option>-vO</option> is different from <option>-v -O</option>.
  88       Options need not precede filenames: e.g., <literal>ghc *.o -o
  89       foo</literal>.  All options are processed and then applied to
  90       all files; you cannot, for example, invoke <literal>ghc -c -O1
  91       Foo.hs -O2 Bar.hs</literal> to apply different optimisation
  92       levels to the files <filename>Foo.hs</filename> and
  93       <filename>Bar.hs</filename>.</para>
  94     </sect2>
  95
  96     <Sect2 id="source-file-options">
  97       <title>Command line options in source files</title>
  98
  99       <indexterm><primary>source-file options</primary></indexterm>
 100
 101       <para>Sometimes it is useful to make the connection between a
 102       source file and the command-line options it requires quite
 103       tight. For instance, if a Haskell source file uses GHC
 104       extensions, it will always need to be compiled with the
 105       <option>-fglasgow-exts</option> option.  Rather than maintaining
 106       the list of per-file options in a <filename>Makefile</filename>,
 107       it is possible to do this directly in the source file using the
 108       <literal>OPTIONS</literal> pragma <indexterm><primary>OPTIONS
 109       pragma</primary></indexterm>:</para>
 110
 111 <ProgramListing>
 112 {-# OPTIONS -fglasgow-exts #-}
 113 module X where
 114 ...
 115 </ProgramListing>
 116
 117       <para><literal>OPTIONS</literal> pragmas are only looked for at
 118       the top of your source files, upto the first
 119       (non-literate,non-empty) line not containing
 120       <literal>OPTIONS</literal>. Multiple <literal>OPTIONS</literal>
 121       pragmas are recognised. Note that your command shell does not
 122       get to the source file options, they are just included literally
 123       in the array of command-line arguments the compiler driver
 124       maintains internally, so you'll be desperately disappointed if
 125       you try to glob etc. inside <literal>OPTIONS</literal>.</para>
 126
 127       <para>NOTE: the contents of OPTIONS are prepended to the
 128       command-line options, so you <emphasis>do</emphasis> have the
 129       ability to override OPTIONS settings via the command
 130       line.</para>
 131
 132       <para>It is not recommended to move all the contents of your
 133       Makefiles into your source files, but in some circumstances, the
 134       <literal>OPTIONS</literal> pragma is the Right Thing. (If you
 135       use <option>-keep-hc-file-too</option> and have OPTION flags in
 136       your module, the OPTIONS will get put into the generated .hc
 137       file).</para>
 138     </sect2>
 139
 140     <sect2>
 141       <title>Setting options in GHCi</title>
 142
 143       <para>Options may also be modified from within GHCi, using the
 144       <literal>:set</literal> command.  See <xref linkend="ghci-set">
 145       for more details.</para>
 146     </sect2>
 147   </sect1>
 148
 149   <sect1 id="static-dynamic-flags">
 150     <title>Static vs. Dynamic options</title>
 151     <indexterm><primary>static</primary><secondary>options</secondary>
 152     </indexterm>
 153     <indexterm><primary>dynamic</primary><secondary>options</secondary>
 154     </indexterm>
 155
 156     <para>Each of GHC's command line options is classified as either
 157     <firstterm>static</firstterm> or <firstterm>dynamic</firstterm>.
 158     A static flag may only be specified on the command line, whereas a
 159     dynamic flag may also be given in an <literal>OPTIONS</literal>
 160     pragma in a source file or set from the GHCi command-line with
 161     <literal>:set</literal>.</para>
 162
 163     <para>As a rule of thumb, all the language options are dynamic, as
 164     are the warning options and the debugging options.  The rest are
 165     static, with the notable exceptions of <option>-v</option>,
 166     <option>-cpp</option>, <option>-fasm</option>,
 167     <option>-fvia-C</option>, and <option>-#include</option>.
 168
 169     The flag reference tables (<xref linkend="flag-reference">) lists
 170     the status of each flag.</para>
 171   </sect1>
 172
 173   <sect1 id="file-suffixes">
 174     <title>Meaningful file suffixes</title>
 175
 176     <indexterm><primary>suffixes, file</primary></indexterm>
 177     <indexterm><primary>file suffixes for GHC</primary></indexterm>
 178
 179     <para>File names with &ldquo;meaningful&rdquo; suffixes (e.g.,
 180     <filename>.lhs</filename> or <filename>.o</filename>) cause the
 181     &ldquo;right thing&rdquo; to happen to those files.</para>
 182
 183     <variablelist>
 184
 185       <varlistentry>
 186         <term><filename>.lhs</filename></term>
 187         <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
 188         <listitem>
 189           <para>A &ldquo;literate Haskell&rdquo; module.</para>
 190         </listitem>
 191       </varlistentry>
 192
 193       <varlistentry>
 194         <term><filename>.hs</filename></term>
 195         <listitem>
 196           <para>A not-so-literate Haskell module.</para>
 197         </listitem>
 198       </varlistentry>
 199
 200       <varlistentry>
 201         <term><filename>.hi</filename></term>
 202         <listitem>
 203           <para>A Haskell interface file, probably
 204           compiler-generated.</para>
 205         </listitem>
 206       </varlistentry>
 207
 208       <varlistentry>
 209         <term><filename>.hc</filename></term>
 210         <listitem>
 211           <para>Intermediate C file produced by the Haskell
 212           compiler.</para>
 213         </listitem>
 214       </varlistentry>
 215
 216       <varlistentry>
 217         <term><filename>.c</filename></term>
 218         <listitem>
 219           <para>A C&nbsp;file not produced by the Haskell
 220           compiler.</para>
 221         </listitem>
 222       </varlistentry>
 223
 224       <varlistentry>
 225         <term><filename>.s</filename></term>
 226         <listitem>
 227           <para>An assembly-language source file, usually produced by
 228           the compiler.</para>
 229         </listitem>
 230       </varlistentry>
 231
 232       <varlistentry>
 233         <term><filename>.o</filename></term>
 234         <listitem>
 235           <para>An object file, produced by an assembler.</para>
 236         </listitem>
 237       </varlistentry>
 238     </variablelist>
 239
 240     <para>Files with other suffixes (or without suffixes) are passed
 241     straight to the linker.</para>
 242
 243   </sect1>
 244
 245   <sect1 id="options-help">
 246     <title>Help and verbosity options</title>
 247
 248     <IndexTerm><Primary>help options</Primary></IndexTerm>
 249     <IndexTerm><Primary>verbosity options</Primary></IndexTerm>
 250
 251     <variablelist>
 252       <varlistentry>
 253         <term><literal>-help</literal></term>
 254         <term><literal>-?</literal></term>
 255         <indexterm><primary><literal>-?</literal></primary></indexterm>
 256         <indexterm><primary><literal>-help</literal></primary></indexterm>
 257         <listitem>
 258           <para>Cause GHC to spew a long usage message to standard
 259           output and then exit.</para>
 260         </listitem>
 261       </varlistentry>
 262
 263       <varlistentry>
 264         <term><literal>-v</literal></term>
 265         <indexterm><primary><literal>-v</literal></primary></indexterm>
 266         <listitem>
 267           <para>The <option>-v</option> option makes GHC
 268           <emphasis>verbose</emphasis>: it reports its version number
 269           and shows (on stderr) exactly how it invokes each phase of
 270           the compilation system.  Moreover, it passes the
 271           <option>-v</option> flag to most phases; each reports its
 272           version number (and possibly some other information).</para>
 273
 274           <para>Please, oh please, use the <option>-v</option> option
 275           when reporting bugs!  Knowing that you ran the right bits in
 276           the right order is always the first thing we want to
 277           verify.</para>
 278         </listitem>
 279       </varlistentry>
 280
 281       <varlistentry>
 282         <term><literal>-v</literal><replaceable>n</replaceable></term>
 283         <indexterm><primary><option>-v</option></primary></indexterm>
 284         <listitem>
 285           <para>To provide more control over the compiler's verbosity,
 286           the <option>-v</option> flag takes an optional numeric
 287           argument.  Specifying <option>-v</option> on its own is
 288           equivalent to <option>-v3</option>, and the other levels
 289           have the following meanings:</para>
 290
 291           <variablelist>
 292             <varlistentry>
 293               <term><literal>-v0</literal></term>
 294               <listitem>
 295                 <para>Disable all non-essential messages (this is the
 296                 default).</para>
 297               </listitem>
 298             </varlistentry>
 299
 300             <varlistentry>
 301               <term><literal>-v1</literal></term>
 302               <listitem>
 303                 <para>Minimal verbosity: print one line per
 304                 compilation (this is the default when
 305                 <option>--make</option> or
 306                 <option>--interactive</option> is on).</para>
 307               </listitem>
 308             </varlistentry>
 309
 310             <varlistentry>
 311               <term><literal>-v2</literal></term>
 312               <listitem>
 313                 <para>Print the name of each compilation phase as it
 314                 is executed. (equivalent to
 315                 <option>-dshow-passes</option>).</para>
 316               </listitem>
 317             </varlistentry>
 318
 319             <varlistentry>
 320               <term><literal>-v3</literal></term>
 321               <listitem>
 322                 <para>The same as <option>-v2</option>, except that in
 323                 addition the full command line (if appropriate) for
 324                 each compilation phase is also printed.</para>
 325               </listitem>
 326             </varlistentry>
 327
 328             <varlistentry>
 329               <term><literal>-v4</literal></term>
 330               <listitem>
 331                 <para>The same as <option>-v3</option> except that the
 332                 intermediate program representation after each
 333                 compilation phase is also printed (excluding
 334                 preprocessed and C/assembly files).</para>
 335               </listitem>
 336             </varlistentry>
 337           </variablelist>
 338         </listitem>
 339       </varlistentry>
 340
 341       <varlistentry>
 342         <term><literal>--version</literal></term>
 343         <indexterm><primary><literal>--version</literal></primary></indexterm>
 344         <listitem>
 345           <para>Print a one-line string including GHC's version number.</para>
 346         </listitem>
 347       </varlistentry>
 348
 349       <varlistentry>
 350         <term><literal>--numeric-version</literal></term>
 351         <indexterm><primary><literal>--numeric-version</literal></primary></indexterm>
 352         <listitem>
 353           <para>Print GHC's numeric version number only.</para>
 354         </listitem>
 355       </varlistentry>
 356     </variablelist>
 357   </sect1>
 358
 359   <sect1 id="make-mode">
 360     <title>Using <command>ghc</command> <option>--make</option></title>
 361
 362     <indexterm><primary><option>--make</option></primary>
 363     </indexterm>
 364     <indexterm><primary>separate compilation</primary>
 365     </indexterm>
 366
 367     <para>When given the <option>--make</option> option, GHC will
 368     build a multi-module Haskell program by following dependencies
 369     from a single root module (usually <literal>Main</literal>).  For
 370     example, if your <literal>Main</literal> module is in a file
 371     called <filename>Main.hs</filename>, you could compile and link
 372     the program like this:</para>
 373
 374 <screen>
 375 ghc --make Main.hs
 376 </screen>
 377
 378     <para>The command line must contain one source file or module
 379     name; GHC will figure out all the modules in the program by
 380     following the imports from this initial module.  It will then
 381     attempt to compile each module which is out of date, and finally
 382     if the top module is <literal>Main</literal>, the program
 383     will also be linked into an executable.</para>
 384
 385     <para>The main advantages to using <literal>ghc --make</literal>
 386     over traditional <literal>Makefile</literal>s are:</para>
 387
 388     <itemizedlist>
 389       <listitem>
 390         <para>GHC doesn't have to be restarted for each compilation,
 391         which means it can cache information between compilations.
 392         Compiling a muli-module program with <literal>ghc
 393         --make</literal> can be up to twice as fast as running
 394         <literal>ghc</literal> individually on each source
 395         file.</para>
 396       </listitem>
 397       <listitem>
 398         <para>You don't have to write a
 399         <literal>Makefile</literal>.</para>
 400       </listitem>
 401       <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary>
 402       </indexterm>
 403       <listitem>
 404         <para>GHC re-calculates the dependencies each time it is
 405         invoked, so the dependencies never get out of sync with the
 406         source.</para>
 407       </listitem>
 408     </itemizedlist>
 409
 410     <para>Any of the command-line options described in the rest of
 411     this chapter can be used with <option>--make</option>, but note
 412     that any options you give on the command line will apply to all
 413     the source files compiled, so if you want any options to apply to
 414     a single source file only, you'll need to use an
 415     <literal>OPTIONS</literal> pragma (see <xref
 416     linkend="source-file-options">).</para>
 417
 418     <para>If the program needs to be linked with additional objects
 419     (say, some auxilliary C code), these can be specified on the
 420     command line as usual.</para>
 421   </sect1>
 422
 423   <Sect1 id="options-order">
 424     <title>GHC without <option>--make</option></title>
 425
 426     <para>Without <option>--make</option>, GHC will compile one or
 427     more source files given on the command line.</para>
 428
 429     <para>The first phase to run is determined by each input-file
 430     suffix, and the last phase is determined by a flag.  If no
 431     relevant flag is present, then go all the way through linking.
 432     This table summarises:</para>
 433
 434     <informaltable>
 435       <tgroup cols="4">
 436         <colspec align="left">
 437         <colspec align="left">
 438         <colspec align="left">
 439         <colspec align="left">
 440
 441         <thead>
 442           <row>
 443             <entry>Phase of the compilation system</entry>
 444             <entry>Suffix saying &ldquo;start here&rdquo;</entry>
 445             <entry>Flag saying &ldquo;stop after&rdquo;</entry>
 446             <entry>(suffix of) output file</entry>
 447           </row>
 448         </thead>
 449         <tbody>
 450           <row>
 451             <entry>literate pre-processor</entry>
 452             <entry><literal>.lhs</literal></entry>
 453             <entry>-</entry>
 454             <entry><literal>.hs</literal></entry>
 455           </row>
 456
 457           <row>
 458             <entry>C pre-processor (opt.)
 459            </entry>
 460             <entry><literal>.hs</literal> (with
 461             <option>-cpp</option>)</entry>
 462             <entry><option>-E</option></entry>
 463             <entry><literal>.hspp</literal></entry>
 464           </row>
 465
 466           <row>
 467             <entry>Haskell compiler</entry>
 468             <entry><literal>.hs</literal></entry>
 469             <entry><option>-C</option>, <option>-S</option></entry>
 470             <entry><literal>.hc</literal>, <literal>.s</literal></entry>
 471           </row>
 472
 473           <row>
 474             <entry>C compiler (opt.)</entry>
 475             <entry><literal>.hc</literal> or <literal>.c</literal></entry>
 476             <entry><option>-S</option></entry>
 477             <entry><literal>.s</literal></entry>
 478           </row>
 479
 480           <row>
 481             <entry>assembler</entry>
 482             <entry><literal>.s</literal></entry>
 483             <entry><option>-c</option></entry>
 484             <entry><literal>.o</literal></entry>
 485           </row>
 486
 487           <row>
 488             <entry>linker</entry>
 489             <entry><replaceable>other</replaceable></entry>
 490             <entry>-</entry>
 491             <entry><filename>a.out</filename></entry>
 492           </row>
 493         </tbody>
 494       </tgroup>
 495     </informaltable>
 496
 497     <indexterm><primary><option>-C</option></primary></indexterm>
 498     <indexterm><primary><option>-E</option></primary></indexterm>
 499     <indexterm><primary><option>-S</option></primary></indexterm>
 500     <indexterm><primary><option>-c</option></primary></indexterm>
 501
 502     <para>Thus, a common invocation would be: <literal>ghc -c
 503     Foo.hs</literal></para>
 504
 505     <para>Note: What the Haskell compiler proper produces depends on
 506     whether a native-code generator<indexterm><primary>native-code
 507     generator</primary></indexterm> is used (producing assembly
 508     language) or not (producing C).  See <xref
 509     linkend="options-codegen"> for more details.</para>
 510
 511     <para>Note: C pre-processing is optional, the
 512     <option>-ccp</option><indexterm><primary><option>-cpp</option></primary>
 513       </indexterm>flag turns it on.  See <xref
 514     linkend="c-pre-processor"> for more details.</para>
 515
 516     <para>Note: The option <option>-E</option><IndexTerm><Primary>-E
 517     option</Primary></IndexTerm> runs just the pre-processing passes
 518     of the compiler, dumping the result in a file.  Note that this
 519     differs from the previous behaviour of dumping the file to
 520     standard output.</para>
 521   </sect1>
 522
 523   <sect1 id="options-output">
 524     <title>Re-directing the compilation output(s)</title>
 525
 526     <indexterm><primary>output-directing options</primary></indexterm>
 527     <indexterm><primary>redirecting compilation output</primary></indexterm>
 528
 529
 530     <variablelist>
 531       <varlistentry>
 532         <term><literal>-o</literal></term>
 533         <indexterm><primary><literal>-o</literal></primary></indexterm>
 534         <listitem>
 535           <para>GHC's compiled output normally goes into a
 536           <filename>.hc</filename>, <filename>.o</filename>, etc.,
 537           file, depending on the last-run compilation phase.  The
 538           option <option>-o foo</option><IndexTerm><Primary>-o
 539           option</Primary></IndexTerm> re-directs the output of that
 540           last-run phase to file <filename>foo</filename>.</para>
 541
 542           <para>Note: this &ldquo;feature&rdquo; can be
 543           counterintuitive: <command>ghc -C -o foo.o foo.hs</command>
 544           will put the intermediate C code in the file
 545           <filename>foo.o</filename>, name notwithstanding!</para>
 546         </listitem>
 547       </varlistentry>
 548
 549       <varlistentry>
 550         <term><literal>-odir</literal></term>
 551         <indexterm><primary><literal>-odir</literal></primary></indexterm>
 552         <listitem>
 553           <para>The <option>-o</option> option isn't of much use if
 554           you have <emphasis>several</emphasis> input files&hellip;
 555           Non-interface output files are normally put in the same
 556           directory as their corresponding input file came from.  You
 557           may specify that they be put in another directory using the
 558           <option>-odir &lt;dir&gt;</option><IndexTerm><Primary>-odir
 559           &lt;dir&gt; option</Primary></IndexTerm> (the &ldquo;Oh,
 560           dear&rdquo; option).  For example:</para>
 561
 562 <Screen>
 563 % ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
 564 </Screen>
 565
 566           <para>The output files, <filename>Foo.o</filename>,
 567           <filename>Bar.o</filename>, and
 568           <filename>Bumble.o</filename> would be put into a
 569           subdirectory named after the architecture of the executing
 570           machine (<filename>sun4</filename>,
 571           <filename>mips</filename>, etc).  The directory must already
 572           exist; it won't be created.</para>
 573
 574           <para>Note that the <option>-odir</option> option does
 575           <emphasis>not</emphasis> affect where the interface files
 576           are put.  In the above example, they would still be put in
 577           <filename>parse/Foo.hi</filename>,
 578           <filename>parse/Bar.hi</filename>, and
 579           <filename>gurgle/Bumble.hi</filename>.</para>
 580         </listitem>
 581       </varlistentry>
 582
 583       <varlistentry>
 584         <term><literal>-ohi</literal></term>
 585         <indexterm><primary><literal>-ohi</literal></primary></indexterm>
 586         <listitem>
 587           <para></para>
 588         </listitem>
 589       </varlistentry>
 590
 591       <varlistentry>
 592         <term><literal>-osuf</literal></term>
 593         <term><literal>-hisuf</literal></term>
 594         <indexterm><primary><literal>-osuf</literal></primary></indexterm>
 595         <indexterm><primary><literal>-hisuf</literal></primary></indexterm>
 596         <listitem>
 597           <para>EXOTICA: The <option>-osuf
 598           &lt;suffix&gt;</option><IndexTerm><Primary>-osuf
 599           &lt;suffix&gt; option</Primary></IndexTerm> will change the
 600           <filename>.o</filename> file suffix for object files to
 601           whatever you specify.  (We use this in compiling the
 602           prelude.).</para>
 603
 604           <para>Similarly, the <option>-hisuf
 605           &lt;suffix&gt;</option><IndexTerm><Primary>-hisuf
 606           &lt;suffix&gt; option</Primary></IndexTerm> will change the
 607           <filename>.hi</filename> file suffix for non-system
 608           interface files (see <XRef LinkEnd="hi-options">).</para>
 609
 610           <para>The <option>-hisuf</option>/<option>-osuf</option>
 611           game is useful if you want to compile a program with both
 612           GHC and HBC (say) in the same directory.  Let HBC use the
 613           standard <filename>.hi</filename>/<filename>.o</filename>
 614           suffixes; add <option>-hisuf g&lowbar;hi -osuf
 615           g&lowbar;o</option> to your <command>make</command> rule for
 616           GHC compiling&hellip;</para>
 617         </listitem>
 618       </varlistentry>
 619     </variablelist>
 620
 621     <sect2 id="keeping-intermediates">
 622       <title>Keeping Intermediate Files</title>
 623       <indexterm><primary>intermediate files, saving</primary>
 624       </indexterm>
 625       <indexterm><primary><literal>.hc</literal> files, saving</primary>
 626       </indexterm>
 627       <indexterm><primary><literal>.s</literal> files, saving</primary>
 628       </indexterm>
 629
 630
 631       <para>The following options are useful for keeping certain
 632       intermediate files around, when normally GHC would throw these
 633       away after compilation:</para>
 634
 635       <variablelist>
 636         <varlistentry>
 637           <term><literal>-keep-hc-files</literal></term>
 638           <indexterm>
 639             <primary><literal>-keep-hc-files</literal></primary>
 640           </indexterm>
 641           <listitem>
 642             <para>Keep intermediate <literal>.hc</literal> files when
 643             doing <literal>.hs</literal>-to-<literal>.o</literal>
 644             compilations via C (NOTE: <literal>.hc</literal> files
 645             aren't generated when using the native code generator, you
 646             may need to use <literal>-fvia-C</literal> to force them
 647             to be produced).</para>
 648           </listitem>
 649         </varlistentry>
 650
 651         <varlistentry>
 652           <term><literal>-keep-s-files</literal></term>
 653           <indexterm>
 654             <primary><literal>-keep-s-files</literal></primary>
 655           </indexterm>
 656           <listitem>
 657             <para>Keep intermediate <literal>.s</literal> files.</para>
 658           </listitem>
 659         </varlistentry>
 660
 661         <varlistentry>
 662           <term><literal>-keep-raw-s-files</literal></term>
 663           <indexterm>
 664             <primary><literal>-keep-raw-s-files</literal></primary>
 665           </indexterm>
 666           <listitem>
 667             <para>Keep intermediate <literal>.raw-s</literal> files.
 668             These are the direct output from the C compiler, before
 669             GHC does &ldquo;assembly mangling&rdquo; to produce the
 670             <literal>.s</literal> file.  Again, these are not produced
 671             when using the native code generator.</para>
 672           </listitem>
 673         </varlistentry>
 674
 675         <varlistentry>
 676           <term><literal>-keep-tmp-files</literal></term>
 677           <indexterm>
 678             <primary><literal>-keep-tmp-files</literal></primary>
 679           </indexterm>
 680           <indexterm>
 681             <primary>temporary files</primary>
 682             <secondary>keeping</secondary>
 683           </indexterm>
 684           <listitem>
 685             <para>Instructs the GHC driver not to delete any of its
 686             temporary files, which it normally keeps in
 687             <literal>/tmp</literal> (or possibly elsewhere; see <xref
 688             linkend="temp-files">).  Running GHC with
 689             <literal>-v</literal> will show you what temporary files
 690             were generated along the way.</para>
 691           </listitem>
 692         </varlistentry>
 693       </variablelist>
 694     </sect2>
 695
 696     <sect2 id="temp-files">
 697       <title>Redirecting temporary files</title>
 698
 699       <indexterm>
 700         <primary>temporary files</primary>
 701         <secondary>redirecting</secondary>
 702       </indexterm>
 703
 704       <variablelist>
 705         <varlistentry>
 706           <term><literal>-tmpdir</literal></term>
 707           <indexterm><primary><literal>-tmpdir</literal></primary></indexterm>
 708           <listitem>
 709             <para>If you have trouble because of running out of space
 710             in <filename>/tmp</filename> (or wherever your
 711             installation thinks temporary files should go), you may
 712             use the <option>-tmpdir
 713             &lt;dir&gt;</option><IndexTerm><Primary>-tmpdir
 714             &lt;dir&gt; option</Primary></IndexTerm> option to specify
 715             an alternate directory.  For example, <option>-tmpdir
 716             .</option> says to put temporary files in the current
 717             working directory.</para>
 718
 719             <para>Alternatively, use your <Constant>TMPDIR</Constant>
 720             environment variable.<IndexTerm><Primary>TMPDIR
 721             environment variable</Primary></IndexTerm> Set it to the
 722             name of the directory where temporary files should be put.
 723             GCC and other programs will honour the
 724             <Constant>TMPDIR</Constant> variable as well.</para>
 725
 726             <para>Even better idea: Set the
 727             <Constant>DEFAULT_TMPDIR</Constant> make variable when
 728             building GHC, and never worry about
 729             <Constant>TMPDIR</Constant> again. (see the build
 730             documentation).</para>
 731           </listitem>
 732         </varlistentry>
 733       </variablelist>
 734     </sect2>
 735
 736   </sect1>
 737
 738   <sect1 id="options-sanity">
 739     <title>Warnings and sanity-checking</title>
 740
 741     <indexterm><primary>sanity-checking options</primary></indexterm>
 742     <indexterm><primary>warnings</primary></indexterm>
 743
 744
 745     <para>GHC has a number of options that select which types of
 746     non-fatal error messages, otherwise known as warnings, can be
 747     generated during compilation.  By default, you get a standard set
 748     of warnings which are generally likely to indicate bugs in your
 749     program.  These are:
 750     <option>-fwarn-overlpapping-patterns</option>,
 751     <option>-fwarn-deprecations</option>,
 752     <option>-fwarn-duplicate-exports</option>,
 753     <option>-fwarn-missing-fields</option>, and
 754     <option>-fwarn-missing-methods</option>.  The following flags are
 755     simple ways to select standard &ldquo;packages&rdquo; of warnings:
 756     </para>
 757
 758     <VariableList>
 759
 760       <varlistentry>
 761         <term><option>-W</option>:</term>
 762         <listitem>
 763           <IndexTerm><Primary>-W option</Primary></IndexTerm>
 764           <para>Provides the standard warnings plus
 765           <option>-fwarn-incomplete-patterns</option>,
 766           <option>-fwarn-unused-matches</option>,
 767           <option>-fwarn-unused-imports</option> and
 768           <option>-fwarn-unused-binds</option>.</para>
 769         </listitem>
 770       </varlistentry>
 771
 772       <varlistentry>
 773         <term><option>-w</option>:</term>
 774         <listitem>
 775           <IndexTerm><Primary>-w option</Primary></IndexTerm>
 776           <para>Turns off all warnings, including the standard ones.</para>
 777         </listitem>
 778       </varlistentry>
 779
 780       <varlistentry>
 781         <term><option>-Wall</option>:</term>
 782         <listitem>
 783           <indexterm><primary>-Wall option</primary></indexterm>
 784           <para>Turns on all warning options.</para>
 785         </listitem>
 786       </varlistentry>
 787
 788     </variablelist>
 789
 790     <para>The full set of warning options is described below.  To turn
 791     off any warning, simply give the corresponding
 792     <option>-fno-warn-...</option> option on the command line.</para>
 793
 794     <variablelist>
 795
 796       <varlistentry>
 797         <term><option>-fwarn-deprecations</option>:</term>
 798         <listitem>
 799           <indexterm><primary><option>-fwarn-deprecations</option></primary>
 800           </indexterm>
 801           <indexterm><primary>deprecations</primary></indexterm>
 802           <para>Causes a warning to be emitted when a deprecated
 803           function or type is used.  Entities can be marked as
 804           deprecated using a pragma, see <xref
 805           linkend="deprecated-pragma">.</para>
 806         </listitem>
 807       </varlistentry>
 808
 809       <varlistentry>
 810         <term><option>-fwarn-duplicate-exports</option>:</term>
 811         <listitem>
 812           <indexterm><primary>-fwarn-duplicate-exports option</primary></indexterm>
 813           <indexterm><primary>duplicate exports, warning</primary></indexterm>
 814           <indexterm><primary>export lists, duplicates</primary></indexterm>
 815
 816           <para>Have the compiler warn about duplicate entries in
 817           export lists. This is useful information if you maintain
 818           large export lists, and want to avoid the continued export
 819           of a definition after you've deleted (one) mention of it in
 820           the export list.</para>
 821
 822           <para>This option is on by default.</para>
 823         </listitem>
 824       </varlistentry>
 825
 826       <varlistentry>
 827         <term><option>-fwarn-hi-shadowing</option>:</term>
 828         <listitem>
 829           <indexterm><primary>-fwarn-hi-shadowing option</primary></indexterm>
 830           <indexterm><primary>shadowing</primary>
 831             <secondary>interface files</secondary></indexterm>
 832
 833           <para>Causes the compiler to emit a warning when a module or
 834           interface file in the current directory is shadowing one
 835           with the same module name in a library or other
 836           directory.</para>
 837         </listitem>
 838       </varlistentry>
 839
 840       <varlistentry>
 841         <term><option>-fwarn-incomplete-patterns</option>:</term>
 842         <listitem>
 843           <indexterm><primary>-fwarn-incomplete-patterns option</primary></indexterm>
 844           <indexterm><primary>incomplete patterns, warning</primary></indexterm>
 845           <indexterm><primary>patterns, incomplete</primary></indexterm>
 846
 847           <para>Similarly for incomplete patterns, the function
 848           <function>g</function> below will fail when applied to
 849           non-empty lists, so the compiler will emit a warning about
 850           this when <option>-fwarn-incomplete-patterns</option> is
 851           enabled.</para>
 852
 853 <programlisting>
 854 g [] = 2
 855 </programlisting>
 856
 857           <para>This option isn't enabled be default because it can be
 858           a bit noisy, and it doesn't always indicate a bug in the
 859           program.  However, it's generally considered good practice
 860           to cover all the cases in your functions.</para>
 861         </listitem>
 862       </varlistentry>
 863
 864       <varlistentry>
 865         <term><option>-fwarn-missing-fields</option>:</term>
 866         <listitem>
 867           <indexterm><primary>-fwarn-missing-fields option</primary></indexterm>
 868           <indexterm><primary>missing fields, warning</primary></indexterm>
 869           <indexterm><primary>fields, missing</primary></indexterm>
 870
 871           <para>This option is on by default, and warns you whenever
 872           the construction of a labelled field constructor isn't
 873           complete, missing initializers for one or more fields. While
 874           not an error (the missing fields are initialised with
 875           bottoms), it is often an indication of a programmer error.</para>
 876         </listitem>
 877       </varlistentry>
 878
 879       <varlistentry>
 880         <term><option>-fwarn-missing-methods</option>:</term>
 881         <listitem>
 882           <indexterm><primary>-fwarn-missing-methods option</primary></indexterm>
 883           <indexterm><primary>missing methods, warning</primary></indexterm>
 884           <indexterm><primary>methods, missing</primary></indexterm>
 885
 886           <para>This option is on by default, and warns you whenever
 887           an instance declaration is missing one or more methods, and
 888           the corresponding class declaration has no default
 889           declaration for them.</para>
 890         </listitem>
 891       </varlistentry>
 892
 893       <varlistentry>
 894         <term><option>-fwarn-missing-signatures</option>:</term>
 895         <listitem>
 896           <indexterm><primary>-fwarn-missing-signatures option</primary></indexterm>
 897           <indexterm><primary>type signatures, missing</primary></indexterm>
 898
 899           <para>If you would like GHC to check that every top-level
 900           function/value has a type signature, use the
 901           <option>-fwarn-missing-signatures</option> option.  This
 902           option is off by default.</para>
 903         </listitem>
 904       </varlistentry>
 905
 906       <varlistentry>
 907         <term><option>-fwarn-name-shadowing</option>:</term>
 908         <listitem>
 909           <indexterm><primary>-fwarn-name-shadowing option</primary></indexterm>
 910           <indexterm><primary>shadowing, warning</primary></indexterm>
 911
 912           <para>This option causes a warning to be emitted whenever an
 913           inner-scope value has the same name as an outer-scope value,
 914           i.e. the inner value shadows the outer one.  This can catch
 915           typographical errors that turn into hard-to-find bugs, e.g.,
 916           in the inadvertent cyclic definition <literal>let x = ... x
 917           ... in</literal>.</para>
 918
 919           <para>Consequently, this option does
 920           <emphasis>will</emphasis> complain about cyclic recursive
 921           definitions.</para>
 922         </listitem>
 923       </varlistentry>
 924
 925       <varlistentry>
 926         <term><option>-fwarn-overlapping-patterns</option>:</term>
 927         <indexterm><primary>-fwarn-overlapping-patterns option</primary></indexterm>
 928         <indexterm><primary>overlapping patterns, warning</primary></indexterm>
 929         <indexterm><primary>patterns, overlapping</primary></indexterm>
 930         <listitem>
 931           <para>By default, the compiler will warn you if a set of
 932           patterns are overlapping, i.e.,</para>
 933
 934 <programlisting>
 935 f :: String -&#62; Int
 936 f []     = 0
 937 f (_:xs) = 1
 938 f "2"    = 2
 939 </programlisting>
 940
 941           <para>where the last pattern match in <Function>f</Function>
 942           won't ever be reached, as the second pattern overlaps
 943           it. More often than not, redundant patterns is a programmer
 944           mistake/error, so this option is enabled by default.</para>
 945         </listitem>
 946       </varlistentry>
 947
 948       <varlistentry>
 949         <term><option>-fwarn-simple-patterns</option>:</term>
 950         <listitem>
 951           <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
 952           </indexterm>
 953           <para>Causes the compiler to warn about lambda-bound
 954           patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
 955           Normally, these aren't treated as incomplete patterns by
 956           <option>-fwarn-incomplete-patterns</option>.</para>
 957         </listitem>
 958       </varlistentry>
 959
 960       <varlistentry>
 961         <term><option>-fwarn-type-defaults</option>:</term>
 962         <listitem>
 963           <indexterm><primary>-fwarn-type-defaults option</primary></indexterm>
 964           <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
 965           <para>Have the compiler warn/inform you where in your source
 966           the Haskell defaulting mechanism for numeric types kicks
 967           in. This is useful information when converting code from a
 968           context that assumed one default into one with another,
 969           e.g., the `default default' for Haskell 1.4 caused the
 970           otherwise unconstrained value <Constant>1</Constant> to be
 971           given the type <literal>Int</literal>, whereas Haskell 98
 972           defaults it to <literal>Integer</literal>.  This may lead to
 973           differences in performance and behaviour, hence the
 974           usefulness of being non-silent about this.</para>
 975
 976           <para>This warning is off by default.</para>
 977         </listitem>
 978       </varlistentry>
 979
 980       <varlistentry>
 981         <term><option>-fwarn-unused-binds</option>:</term>
 982         <listitem>
 983           <indexterm><primary>-fwarn-unused-binds option</primary></indexterm>
 984           <indexterm><primary>unused binds, warning</primary></indexterm>
 985           <indexterm><primary>binds, unused</primary></indexterm>
 986           <para>Report any function definitions (and local bindings)
 987           which are unused.  For top-level functions, the warning is
 988           only given if the binding is not exported.</para>
 989         </listitem>
 990       </varlistentry>
 991
 992       <varlistentry>
 993         <term><option>-fwarn-unused-imports</option>:</term>
 994         <listitem>
 995           <indexterm><primary>-fwarn-unused-imports option</primary></indexterm>
 996           <indexterm><primary>unused imports, warning</primary></indexterm>
 997           <indexterm><primary>imports, unused</primary></indexterm>
 998
 999           <para>Report any objects that are explicitly imported but
1000           never used.</para>
1001         </listitem>
1002       </varlistentry>
1003
1004       <varlistentry>
1005         <term><option>-fwarn-unused-matches</option>:</term>
1006         <listitem>
1007           <indexterm><primary>-fwarn-unused-matches option</primary></indexterm>
1008           <indexterm><primary>unused matches, warning</primary></indexterm>
1009           <indexterm><primary>matches, unused</primary></indexterm>
1010
1011           <para>Report all unused variables which arise from pattern
1012           matches, including patterns consisting of a single variable.
1013           For instance <literal>f x y = []</literal> would report
1014           <VarName>x</VarName> and <VarName>y</VarName> as unused.  To
1015           eliminate the warning, all unused variables can be replaced
1016           with wildcards.</para>
1017         </listitem>
1018       </varlistentry>
1019
1020     </VariableList>
1021
1022     <para>If you're feeling really paranoid, the
1023     <option>-dcore-lint</option> option<indexterm><primary>-dcore-lint
1024     option</primary></indexterm> is a good choice.  It turns on
1025     heavyweight intra-pass sanity-checking within GHC.  (It checks
1026     GHC's sanity, not yours.)</para>
1027
1028   </sect1>
1029
1030   &separate;
1031   &packages;
1032
1033   <sect1 id="options-optimise">
1034     <title>Optimisation (code improvement)</title>
1035
1036     <indexterm><primary>optimisation</primary></indexterm>
1037     <indexterm><primary>improvement, code</primary></indexterm>
1038
1039     <para>The <option>-O*</option> options specify convenient
1040     &ldquo;packages&rdquo; of optimisation flags; the
1041     <option>-f*</option> options described later on specify
1042     <emphasis>individual</emphasis> optimisations to be turned on/off;
1043     the <option>-m*</option> options specify
1044     <emphasis>machine-specific</emphasis> optimisations to be turned
1045     on/off.</para>
1046
1047     <sect2 id="optimise-pkgs">
1048       <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1049
1050       <para>There are <emphasis>many</emphasis> options that affect
1051       the quality of code produced by GHC.  Most people only have a
1052       general goal, something like &ldquo;Compile quickly&rdquo; or
1053       &ldquo;Make my program run like greased lightning.&rdquo; The
1054       following &ldquo;packages&rdquo; of optimisations (or lack
1055       thereof) should suffice.</para>
1056
1057       <para>Once you choose a <option>-O*</option>
1058       &ldquo;package,&rdquo; stick with it&mdash;don't chop and
1059       change.  Modules' interfaces <emphasis>will</emphasis> change
1060       with a shift to a new <option>-O*</option> option, and you may
1061       have to recompile a large chunk of all importing modules before
1062       your program can again be run safely (see <XRef
1063       LinkEnd="recomp">).</para>
1064
1065       <variablelist>
1066
1067         <varlistentry>
1068           <term>No <option>-O*</option>-type option specified:</term>
1069           <indexterm><primary>-O* not specified</primary></indexterm>
1070           <listitem>
1071             <para>This is taken to mean: &ldquo;Please compile
1072             quickly; I'm not over-bothered about compiled-code
1073             quality.&rdquo; So, for example: <command>ghc -c
1074             Foo.hs</command></para>
1075           </listitem>
1076         </varlistentry>
1077
1078         <varlistentry>
1079           <term><option>-O0</option>:</term>
1080           <indexterm><primary><option>-O0</option></primary></indexterm>
1081           <listitem>
1082             <para>Means &ldquo;turn off all optimisation&rdquo;,
1083             reverting to the same settings as if no
1084             <option>-O</option> options had been specified.  Saying
1085             <option>-O0</option> can be useful if
1086             eg. <command>make</command> has inserted a
1087             <option>-O</option> on the command line already.</para>
1088           </listitem>
1089         </varlistentry>
1090
1091         <varlistentry>
1092           <term><option>-O</option> or <option>-O1</option>:</term>
1093           <indexterm><primary>-O option</primary></indexterm>
1094           <indexterm><primary>-O1 option</primary></indexterm>
1095           <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1096           <listitem>
1097             <para>Means: &ldquo;Generate good-quality code without
1098             taking too long about it.&rdquo; Thus, for example:
1099             <command>ghc -c -O Main.lhs</command></para>
1100           </listitem>
1101         </varlistentry>
1102
1103         <varlistentry>
1104           <term><option>-O2</option>:</term>
1105           <indexterm><primary>-O2 option</primary></indexterm>
1106           <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1107           <listitem>
1108             <para>Means: &ldquo;Apply every non-dangerous
1109             optimisation, even if it means significantly longer
1110             compile times.&rdquo;</para>
1111
1112             <para>The avoided &ldquo;dangerous&rdquo; optimisations
1113             are those that can make runtime or space
1114             <emphasis>worse</emphasis> if you're unlucky.  They are
1115             normally turned on or off individually.</para>
1116
1117             <para>At the moment, <option>-O2</option> is
1118             <emphasis>unlikely</emphasis> to produce better code than
1119             <option>-O</option>.</para>
1120           </listitem>
1121         </varlistentry>
1122
1123         <varlistentry>
1124           <term><option>-O2-for-C</option>:</term>
1125           <indexterm><primary>-O2-for-C option</primary></indexterm>
1126           <indexterm><primary>gcc, invoking with -O2</primary></indexterm>
1127           <listitem>
1128             <para>Says to run GCC with <option>-O2</option>, which may
1129             be worth a few percent in execution speed.  Don't forget
1130             <option>-fvia-C</option>, lest you use the native-code
1131             generator and bypass GCC altogether!</para>
1132           </listitem>
1133         </varlistentry>
1134
1135         <varlistentry>
1136           <term><option>-Ofile &lt;file&gt;</option>:</term>
1137           <indexterm><primary>-Ofile &lt;file&gt; option</primary></indexterm>
1138           <indexterm><primary>optimising, customised</primary></indexterm>
1139           <listitem>
1140             <para>(NOTE: not supported yet in GHC 5.x.  Please ask if
1141             you're interested in this.)</para>
1142
1143             <para>For those who need <emphasis>absolute</emphasis>
1144             control over <emphasis>exactly</emphasis> what options are
1145             used (e.g., compiler writers, sometimes :-), a list of
1146             options can be put in a file and then slurped in with
1147             <option>-Ofile</option>.</para>
1148
1149             <para>In that file, comments are of the
1150             <literal>&num;</literal>-to-end-of-line variety; blank
1151             lines and most whitespace is ignored.</para>
1152
1153             <para>Please ask if you are baffled and would like an
1154             example of <option>-Ofile</option>!</para>
1155           </listitem>
1156         </varlistentry>
1157       </variablelist>
1158
1159       <para>We don't use a <option>-O*</option> flag for day-to-day
1160       work.  We use <option>-O</option> to get respectable speed;
1161       e.g., when we want to measure something.  When we want to go for
1162       broke, we tend to use <option>-O -fvia-C -O2-for-C</option> (and
1163       we go for lots of coffee breaks).</para>
1164
1165       <para>The easiest way to see what <option>-O</option> (etc.)
1166       &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1167       then stand back in amazement.</para>
1168     </sect2>
1169
1170     <sect2 id="options-f">
1171       <title><option>-f*</option>: platform-independent flags</title>
1172
1173       <indexterm><primary>-f* options (GHC)</primary></indexterm>
1174       <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1175
1176       <para>These flags turn on and off individual optimisations.
1177       They are normally set via the <option>-O</option> options
1178       described above, and as such, you shouldn't need to set any of
1179       them explicitly (indeed, doing so could lead to unexpected
1180       results).  However, there are one or two that may be of
1181       interest:</para>
1182
1183       <variablelist>
1184         <varlistentry>
1185           <term><option>-fexcess-precision</option>:</term>
1186           <listitem>
1187             <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1188             <para>When this option is given, intermediate floating
1189             point values can have a <emphasis>greater</emphasis>
1190             precision/range than the final type.  Generally this is a
1191             good thing, but some programs may rely on the exact
1192             precision/range of
1193             <literal>Float</literal>/<literal>Double</literal> values
1194             and should not use this option for their compilation.</para>
1195           </listitem>
1196         </varlistentry>
1197
1198         <varlistentry>
1199           <term><option>-fignore-asserts</option>:</term>
1200           <listitem>
1201             <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1202             <para>Causes GHC to ignore uses of the function
1203             <literal>Exception.assert</literal> in source code (in
1204             other words, rewriting <literal>Exception.assert p
1205             e</literal> to <literal>e</literal> (see <xref
1206             linkend="sec-assertions">).  This flag is turned on by
1207             <option>-O</option>.
1208             </para>
1209           </listitem>
1210         </varlistentry>
1211
1212         <varlistentry>
1213           <term><option>-fno-strictness</option></term>
1214           <indexterm><primary><option>-fno-strictness</option></primary>
1215           </indexterm>
1216           <listitem>
1217             <para>Turns off the strictness analyser; sometimes it eats
1218             too many cycles.</para>
1219           </listitem>
1220         </varlistentry>
1221
1222         <varlistentry>
1223           <term><option>-fno-cpr-analyse</option></term>
1224           <indexterm><primary><option>-fno-cpr-analyse</option></primary>
1225           </indexterm>
1226           <listitem>
1227             <para>Turns off the CPR (constructed product result)
1228             analysis; it is somewhat experimental.</para>
1229           </listitem>
1230         </varlistentry>
1231
1232         <varlistentry>
1233           <term><option>-funbox-strict-fields</option>:</term>
1234           <listitem>
1235             <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1236             <indexterm><primary>strict constructor fields</primary></indexterm>
1237             <indexterm><primary>constructor fields, strict</primary></indexterm>
1238
1239             <para>This option causes all constructor fields which are
1240             marked strict (i.e. &ldquo;!&rdquo;) to be unboxed or
1241             unpacked if possible.  For example:</para>
1242
1243 <ProgramListing>
1244 data T = T !Float !Float
1245 </ProgramListing>
1246
1247             <para>will create a constructor <literal>T</literal>
1248             containing two unboxed floats if the
1249             <option>-funbox-strict-fields</option> flag is given.
1250             This may not always be an optimisation: if the
1251             <Function>T</Function> constructor is scrutinised and the
1252             floats passed to a non-strict function for example, they
1253             will have to be reboxed (this is done automatically by the
1254             compiler).</para>
1255
1256             <para>This option should only be used in conjunction with
1257             <option>-O</option>, in order to expose unfoldings to the
1258             compiler so the reboxing can be removed as often as
1259             possible.  For example:</para>
1260
1261 <ProgramListing>
1262 f :: T -&#62; Float
1263 f (T f1 f2) = f1 + f2
1264 </ProgramListing>
1265
1266             <para>The compiler will avoid reboxing
1267             <Function>f1</Function> and <Function>f2</Function> by
1268             inlining <Function>+</Function> on floats, but only when
1269             <option>-O</option> is on.</para>
1270
1271             <para>Any single-constructor data is eligible for
1272             unpacking; for example</para>
1273
1274 <ProgramListing>
1275 data T = T !(Int,Int)
1276 </ProgramListing>
1277
1278             <para>will store the two <literal>Int</literal>s directly
1279             in the <Function>T</Function> constructor, by flattening
1280             the pair.  Multi-level unpacking is also supported:</para>
1281
1282 <ProgramListing>
1283 data T = T !S
1284 data S = S !Int !Int
1285 </ProgramListing>
1286
1287             <para>will store two unboxed <literal>Int&num;</literal>s
1288             directly in the <Function>T</Function> constructor.</para>
1289           </listitem>
1290         </varlistentry>
1291
1292         <varlistentry>
1293           <term><option>-funfolding-update-in-place&lt;n&gt;</option></term>
1294           <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1295           <listitem>
1296             <para>Switches on an experimental "optimisation".
1297             Switching it on makes the compiler a little keener to
1298             inline a function that returns a constructor, if the
1299             context is that of a thunk.
1300 <ProgramListing>
1301    x = plusInt a b
1302 </ProgramListing>
1303             If we inlined plusInt we might get an opportunity to use
1304             update-in-place for the thunk 'x'.</para>
1305           </listitem>
1306         </varlistentry>
1307
1308         <varlistentry>
1309           <term><option>-funfolding-creation-threshold&lt;n&gt;</option>:</term>
1310           <listitem>
1311             <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1312             <indexterm><primary>inlining, controlling</primary></indexterm>
1313             <indexterm><primary>unfolding, controlling</primary></indexterm>
1314
1315             <para>(Default: 45) Governs the maximum size that GHC will
1316             allow a function unfolding to be.   (An unfolding has a
1317             &ldquo;size&rdquo; that reflects the cost in terms of
1318             &ldquo;code bloat&rdquo; of expanding that unfolding at
1319             at a call site. A bigger function would be assigned a
1320             bigger cost.) </para>
1321
1322             <para> Consequences: (a) nothing larger than this will be
1323             inlined (unless it has an INLINE pragma); (b) nothing
1324             larger than this will be spewed into an interface
1325             file. </para>
1326
1327
1328             <para> Increasing this figure is more likely to result in longer
1329             compile times than faster code.  The next option is more
1330             useful:</para>
1331           </listitem>
1332         </varlistentry>
1333
1334         <varlistentry>
1335           <term><option>-funfolding-use-threshold&lt;n&gt;</option>:</term>
1336           <listitem>
1337             <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1338             <indexterm><primary>inlining, controlling</primary></indexterm>
1339             <indexterm><primary>unfolding, controlling</primary></indexterm>
1340
1341             <para>(Default: 8) This is the magic cut-off figure for
1342             unfolding: below this size, a function definition will be
1343             unfolded at the call-site, any bigger and it won't.  The
1344             size computed for a function depends on two things: the
1345             actual size of the expression minus any discounts that
1346             apply (see <option>-funfolding-con-discount</option>).</para>
1347           </listitem>
1348         </varlistentry>
1349       </variablelist>
1350
1351     </sect2>
1352
1353   </sect1>
1354
1355 &phases;
1356
1357 <Sect1 id="sec-using-concurrent">
1358 <title>Using Concurrent Haskell</title>
1359
1360 <para>
1361 <indexterm><primary>Concurrent Haskell&mdash;use</primary></indexterm>
1362 </para>
1363
1364 <para>
1365 GHC (as of version 4.00) supports Concurrent Haskell by default,
1366 without requiring a special option or libraries compiled in a certain
1367 way.  To get access to the support libraries for Concurrent Haskell
1368 (i.e. <literal>Concurrent</literal> and friends), use the
1369 <option>-package concurrent</option> option.
1370 </para>
1371
1372 <para>
1373 Three RTS options are provided for modifying the behaviour of the
1374 threaded runtime system.  See the descriptions of
1375 <option>-C[&lt;us&gt;]</option>, <option>-q</option>, and
1376 <option>-t&lt;num&gt;</option> in <XRef LinkEnd="parallel-rts-opts">.
1377 </para>
1378
1379 <para>
1380 Concurrent Haskell is described in more detail in <XRef
1381 LinkEnd="sec-Concurrent">.
1382 </para>
1383
1384 </Sect1>
1385
1386 <Sect1 id="sec-using-parallel">
1387 <title>Using Parallel Haskell</title>
1388
1389 <para>
1390 <indexterm><primary>Parallel Haskell&mdash;use</primary></indexterm>
1391 </para>
1392
1393 <para>
1394 &lsqb;You won't be able to execute parallel Haskell programs unless PVM3
1395 (Parallel Virtual Machine, version 3) is installed at your site.]
1396 </para>
1397
1398 <para>
1399 To compile a Haskell program for parallel execution under PVM, use the
1400 <option>-parallel</option> option,<indexterm><primary>-parallel
1401 option</primary></indexterm> both when compiling <emphasis>and
1402 linking</emphasis>.  You will probably want to <literal>import
1403 Parallel</literal> into your Haskell modules.
1404 </para>
1405
1406 <para>
1407 To run your parallel program, once PVM is going, just invoke it
1408 &ldquo;as normal&rdquo;.  The main extra RTS option is
1409 <option>-N&lt;n&gt;</option>, to say how many PVM
1410 &ldquo;processors&rdquo; your program to run on.  (For more details of
1411 all relevant RTS options, please see <XRef
1412 LinkEnd="parallel-rts-opts">.)
1413 </para>
1414
1415 <para>
1416 In truth, running Parallel Haskell programs and getting information
1417 out of them (e.g., parallelism profiles) is a battle with the vagaries of
1418 PVM, detailed in the following sections.
1419 </para>
1420
1421 <sect2>
1422 <title>Dummy's guide to using PVM</title>
1423
1424 <para>
1425 <indexterm><primary>PVM, how to use</primary></indexterm>
1426 <indexterm><primary>Parallel Haskell&mdash;PVM use</primary></indexterm>
1427 Before you can run a parallel program under PVM, you must set the
1428 required environment variables (PVM's idea, not ours); something like,
1429 probably in your <filename>.cshrc</filename> or equivalent:
1430
1431 <ProgramListing>
1432 setenv PVM_ROOT /wherever/you/put/it
1433 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
1434 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
1435 </ProgramListing>
1436
1437 </para>
1438
1439 <para>
1440 Creating and/or controlling your &ldquo;parallel machine&rdquo; is a purely-PVM
1441 business; nothing specific to Parallel Haskell.
1442 </para>
1443
1444 <para>
1445 You use the <command>pvm</command><indexterm><primary>pvm command</primary></indexterm> command to start PVM on your
1446 machine.  You can then do various things to control/monitor your
1447 &ldquo;parallel machine;&rdquo; the most useful being:
1448 </para>
1449
1450 <para>
1451 <InformalTable>
1452 <TGroup Cols=2>
1453 <ColSpec Align="Left">
1454 <TBody>
1455
1456 <row>
1457 <entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></entry>
1458 <entry>exit <command>pvm</command>, leaving it running</entry>
1459 </row>
1460
1461 <row>
1462 <entry><command>halt</command></entry>
1463 <entry>kill off this &ldquo;parallel machine&rdquo; &amp; exit</entry>
1464 </row>
1465
1466 <row>
1467 <entry><command>add &lt;host&gt;</command></entry>
1468 <entry>add <command>&lt;host&gt;</command> as a processor</entry>
1469 </row>
1470
1471 <row>
1472 <entry><command>delete &lt;host&gt;</command></entry>
1473 <entry>delete <command>&lt;host&gt;</command></entry>
1474 </row>
1475
1476 <row>
1477 <entry><command>reset</command></entry>
1478 <entry>kill what's going, but leave PVM up</entry>
1479 </row>
1480
1481 <row>
1482 <entry><command>conf</command></entry>
1483 <entry>list the current configuration</entry>
1484 </row>
1485
1486 <row>
1487 <entry><command>ps</command></entry>
1488 <entry>report processes' status</entry>
1489 </row>
1490
1491 <row>
1492 <entry><command>pstat &lt;pid&gt;</command></entry>
1493 <entry>status of a particular process</entry>
1494 </row>
1495
1496 </TBody>
1497 </TGroup>
1498 </InformalTable>
1499 </para>
1500
1501 <para>
1502 The PVM documentation can tell you much, much more about <command>pvm</command>!
1503 </para>
1504
1505 </sect2>
1506
1507 <sect2>
1508 <title>Parallelism profiles</title>
1509
1510 <para>
1511 <indexterm><primary>parallelism profiles</primary></indexterm>
1512 <indexterm><primary>profiles, parallelism</primary></indexterm>
1513 <indexterm><primary>visualisation tools</primary></indexterm>
1514 </para>
1515
1516 <para>
1517 With Parallel Haskell programs, we usually don't care about the
1518 results&mdash;only with &ldquo;how parallel&rdquo; it was!  We want pretty pictures.
1519 </para>
1520
1521 <para>
1522 Parallelism profiles (&agrave; la <command>hbcpp</command>) can be generated with the
1523 <option>-q</option><indexterm><primary>-q RTS option (concurrent, parallel)</primary></indexterm> RTS option.  The
1524 per-processor profiling info is dumped into files named
1525 <filename>&lt;full-path&gt;&lt;program&gt;.gr</filename>.  These are then munged into a PostScript picture,
1526 which you can then display.  For example, to run your program
1527 <filename>a.out</filename> on 8 processors, then view the parallelism profile, do:
1528 </para>
1529
1530 <para>
1531
1532 <Screen>
1533 % ./a.out +RTS -N8 -q
1534 % grs2gr *.???.gr &#62; temp.gr     # combine the 8 .gr files into one
1535 % gr2ps -O temp.gr              # cvt to .ps; output in temp.ps
1536 % ghostview -seascape temp.ps   # look at it!
1537 </Screen>
1538
1539 </para>
1540
1541 <para>
1542 The scripts for processing the parallelism profiles are distributed
1543 in <filename>ghc/utils/parallel/</filename>.
1544 </para>
1545
1546 </sect2>
1547
1548 <sect2>
1549 <title>Other useful info about running parallel programs</title>
1550
1551 <para>
1552 The &ldquo;garbage-collection statistics&rdquo; RTS options can be useful for
1553 seeing what parallel programs are doing.  If you do either
1554 <option>+RTS -Sstderr</option><indexterm><primary>-Sstderr RTS option</primary></indexterm> or <option>+RTS -sstderr</option>, then
1555 you'll get mutator, garbage-collection, etc., times on standard
1556 error. The standard error of all PE's other than the `main thread'
1557 appears in <filename>/tmp/pvml.nnn</filename>, courtesy of PVM.
1558 </para>
1559
1560 <para>
1561 Whether doing <option>+RTS -Sstderr</option> or not, a handy way to watch
1562 what's happening overall is: <command>tail -f /tmp/pvml.nnn</command>.
1563 </para>
1564
1565 </sect2>
1566
1567 <Sect2 id="parallel-rts-opts">
1568 <title>RTS options for Concurrent/Parallel Haskell
1569 </title>
1570
1571 <para>
1572 <indexterm><primary>RTS options, concurrent</primary></indexterm>
1573 <indexterm><primary>RTS options, parallel</primary></indexterm>
1574 <indexterm><primary>Concurrent Haskell&mdash;RTS options</primary></indexterm>
1575 <indexterm><primary>Parallel Haskell&mdash;RTS options</primary></indexterm>
1576 </para>
1577
1578 <para>
1579 Besides the usual runtime system (RTS) options
1580 (<XRef LinkEnd="runtime-control">), there are a few options particularly
1581 for concurrent/parallel execution.
1582 </para>
1583
1584 <para>
1585 <VariableList>
1586
1587 <varlistentry>
1588 <term><option>-N&lt;N&gt;</option>:</term>
1589 <listitem>
1590 <para>
1591 <indexterm><primary>-N&lt;N&gt; RTS option (parallel)</primary></indexterm>
1592 (PARALLEL ONLY) Use <literal>&lt;N&gt;</literal> PVM processors to run this program;
1593 the default is 2.
1594 </para>
1595 </listitem>
1596 </varlistentry>
1597 <varlistentry>
1598 <term><option>-C[&lt;us&gt;]</option>:</term>
1599 <listitem>
1600 <para>
1601 <indexterm><primary>-C&lt;us&gt; RTS option</primary></indexterm> Sets
1602 the context switch interval to <literal>&lt;s&gt;</literal> seconds.
1603 A context switch will occur at the next heap block allocation after
1604 the timer expires (a heap block allocation occurs every 4k of
1605 allocation).  With <option>-C0</option> or <option>-C</option>,
1606 context switches will occur as often as possible (at every heap block
1607 allocation).  By default, context switches occur every 20ms
1608 milliseconds.  Note that GHC's internal timer ticks every 20ms, and
1609 the context switch timer is always a multiple of this timer, so 20ms
1610 is the maximum granularity available for timed context switches.
1611 </para>
1612 </listitem>
1613 </varlistentry>
1614 <varlistentry>
1615 <term><option>-q[v]</option>:</term>
1616 <listitem>
1617 <para>
1618 <indexterm><primary>-q RTS option</primary></indexterm>
1619 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
1620 in the file <FIlename>&lt;program&gt;.qp</FIlename>.  In the style of <command>hbcpp</command>, this profile
1621 records the movement of threads between the green (runnable) and red
1622 (blocked) queues.  If you specify the verbose suboption (<option>-qv</option>), the
1623 green queue is split into green (for the currently running thread
1624 only) and amber (for other runnable threads).  We do not recommend
1625 that you use the verbose suboption if you are planning to use the
1626 <command>hbcpp</command> profiling tools or if you are context switching at every heap
1627 check (with <option>-C</option>).
1628 </para>
1629 </listitem>
1630 </varlistentry>
1631 <varlistentry>
1632 <term><option>-t&lt;num&gt;</option>:</term>
1633 <listitem>
1634 <para>
1635 <indexterm><primary>-t&lt;num&gt; RTS option</primary></indexterm>
1636 (PARALLEL ONLY) Limit the number of concurrent threads per processor
1637 to <literal>&lt;num&gt;</literal>.  The default is 32.  Each thread requires slightly over 1K
1638 <emphasis>words</emphasis> in the heap for thread state and stack objects.  (For
1639 32-bit machines, this translates to 4K bytes, and for 64-bit machines,
1640 8K bytes.)
1641 </para>
1642 </listitem>
1643 </varlistentry>
1644 <varlistentry>
1645 <term><option>-d</option>:</term>
1646 <listitem>
1647 <para>
1648 <indexterm><primary>-d RTS option (parallel)</primary></indexterm>
1649 (PARALLEL ONLY) Turn on debugging.  It pops up one xterm (or GDB, or
1650 something&hellip;) per PVM processor.  We use the standard <command>debugger</command>
1651 script that comes with PVM3, but we sometimes meddle with the
1652 <command>debugger2</command> script.  We include ours in the GHC distribution,
1653 in <filename>ghc/utils/pvm/</filename>.
1654 </para>
1655 </listitem>
1656 </varlistentry>
1657 <varlistentry>
1658 <term><option>-e&lt;num&gt;</option>:</term>
1659 <listitem>
1660 <para>
1661 <indexterm><primary>-e&lt;num&gt; RTS option (parallel)</primary></indexterm>
1662 (PARALLEL ONLY) Limit the number of pending sparks per processor to
1663 <literal>&lt;num&gt;</literal>. The default is 100. A larger number may be appropriate if
1664 your program generates large amounts of parallelism initially.
1665 </para>
1666 </listitem>
1667 </varlistentry>
1668 <varlistentry>
1669 <term><option>-Q&lt;num&gt;</option>:</term>
1670 <listitem>
1671 <para>
1672 <indexterm><primary>-Q&lt;num&gt; RTS option (parallel)</primary></indexterm>
1673 (PARALLEL ONLY) Set the size of packets transmitted between processors
1674 to <literal>&lt;num&gt;</literal>. The default is 1024 words. A larger number may be
1675 appropriate if your machine has a high communication cost relative to
1676 computation speed.
1677 </para>
1678 </listitem>
1679 </varlistentry>
1680 </VariableList>
1681 </para>
1682
1683 </sect2>
1684
1685 </Sect1>
1686
1687   <sect1 id="options-platform">
1688     <title>Platform-specific Flags</title>
1689
1690     <indexterm><primary>-m* options</primary></indexterm>
1691     <indexterm><primary>platform-specific options</primary></indexterm>
1692     <indexterm><primary>machine-specific options</primary></indexterm>
1693
1694     <para>Some flags only make sense for particular target
1695     platforms.</para>
1696
1697     <variablelist>
1698
1699       <varlistentry>
1700         <term><option>-mv8</option>:</term>
1701         <listitem>
1702           <para>(SPARC machines)<indexterm><primary>-mv8 option (SPARC
1703           only)</primary></indexterm> Means to pass the like-named
1704           option to GCC; it says to use the Version 8 SPARC
1705           instructions, notably integer multiply and divide.  The
1706           similiar <option>-m*</option> GCC options for SPARC also
1707           work, actually.</para>
1708         </listitem>
1709       </varlistentry>
1710
1711       <varlistentry>
1712         <term><option>-monly-[32]-regs</option>:</term>
1713         <listitem>
1714           <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1715           option (iX86 only)</primary></indexterm> GHC tries to
1716           &ldquo;steal&rdquo; four registers from GCC, for performance
1717           reasons; it almost always works.  However, when GCC is
1718           compiling some modules with four stolen registers, it will
1719           crash, probably saying:
1720
1721 <Screen>
1722 Foo.hc:533: fixed or forbidden register was spilled.
1723 This may be due to a compiler bug or to impossible asm
1724 statements or clauses.
1725 </Screen>
1726
1727           Just give some registers back with
1728           <option>-monly-N-regs</option>.  Try `3' first, then `2'.
1729           If `2' doesn't work, please report the bug to us.</para>
1730         </listitem>
1731       </varlistentry>
1732     </variablelist>
1733
1734   </sect1>
1735
1736 &runtime;
1737 &debug;
1738 &flags;
1739
1740 </Chapter>
1741
1742 <!-- Emacs stuff:
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