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>,
 768           <option>-fwarn-misc</option>, and
 769           <option>-fwarn-unused-binds</option>.</para>
 770         </listitem>
 771       </varlistentry>
 772
 773       <varlistentry>
 774         <term><option>-w</option>:</term>
 775         <listitem>
 776           <IndexTerm><Primary><option>-w</option></Primary></IndexTerm>
 777           <para>Turns off all warnings, including the standard ones.</para>
 778         </listitem>
 779       </varlistentry>
 780
 781       <varlistentry>
 782         <term><option>-Wall</option>:</term>
 783         <listitem>
 784           <indexterm><primary><option>-Wall</option></primary></indexterm>
 785           <para>Turns on all warning options.</para>
 786         </listitem>
 787       </varlistentry>
 788
 789     </variablelist>
 790
 791     <para>The full set of warning options is described below.  To turn
 792     off any warning, simply give the corresponding
 793     <option>-fno-warn-...</option> option on the command line.</para>
 794
 795     <variablelist>
 796
 797       <varlistentry>
 798         <term><option>-fwarn-deprecations</option>:</term>
 799         <listitem>
 800           <indexterm><primary><option>-fwarn-deprecations</option></primary>
 801           </indexterm>
 802           <indexterm><primary>deprecations</primary></indexterm>
 803           <para>Causes a warning to be emitted when a deprecated
 804           function or type is used.  Entities can be marked as
 805           deprecated using a pragma, see <xref
 806           linkend="deprecated-pragma">.</para>
 807         </listitem>
 808       </varlistentry>
 809
 810       <varlistentry>
 811         <term><option>-fwarn-duplicate-exports</option>:</term>
 812         <listitem>
 813           <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
 814           <indexterm><primary>duplicate exports, warning</primary></indexterm>
 815           <indexterm><primary>export lists, duplicates</primary></indexterm>
 816
 817           <para>Have the compiler warn about duplicate entries in
 818           export lists. This is useful information if you maintain
 819           large export lists, and want to avoid the continued export
 820           of a definition after you've deleted (one) mention of it in
 821           the export list.</para>
 822
 823           <para>This option is on by default.</para>
 824         </listitem>
 825       </varlistentry>
 826
 827       <varlistentry>
 828         <term><option>-fwarn-hi-shadowing</option>:</term>
 829         <listitem>
 830           <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
 831           <indexterm><primary>shadowing</primary>
 832             <secondary>interface files</secondary></indexterm>
 833
 834           <para>Causes the compiler to emit a warning when a module or
 835           interface file in the current directory is shadowing one
 836           with the same module name in a library or other
 837           directory.</para>
 838         </listitem>
 839       </varlistentry>
 840
 841       <varlistentry>
 842         <term><option>-fwarn-incomplete-patterns</option>:</term>
 843         <listitem>
 844           <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
 845           <indexterm><primary>incomplete patterns, warning</primary></indexterm>
 846           <indexterm><primary>patterns, incomplete</primary></indexterm>
 847
 848           <para>Similarly for incomplete patterns, the function
 849           <function>g</function> below will fail when applied to
 850           non-empty lists, so the compiler will emit a warning about
 851           this when <option>-fwarn-incomplete-patterns</option> is
 852           enabled.</para>
 853
 854 <programlisting>
 855 g [] = 2
 856 </programlisting>
 857
 858           <para>This option isn't enabled be default because it can be
 859           a bit noisy, and it doesn't always indicate a bug in the
 860           program.  However, it's generally considered good practice
 861           to cover all the cases in your functions.</para>
 862         </listitem>
 863       </varlistentry>
 864
 865       <varlistentry>
 866         <term><option>-fwarn-misc</option>:</term>
 867         <indexterm><primary><option>-fwarn-misc</option></primary></indexterm>
 868         <listitem>
 869           <para>Turns on warnings for various harmless but untidy
 870           things.  This currently includes: importing a type with
 871           <literal>(..)</literal> when the export is abstract, and
 872           listing duplicate class assertions in a qualified type.</para>
 873         </listitem>
 874       </varlistentry>
 875
 876       <varlistentry>
 877         <term><option>-fwarn-missing-fields</option>:</term>
 878         <listitem>
 879           <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
 880           <indexterm><primary>missing fields, warning</primary></indexterm>
 881           <indexterm><primary>fields, missing</primary></indexterm>
 882
 883           <para>This option is on by default, and warns you whenever
 884           the construction of a labelled field constructor isn't
 885           complete, missing initializers for one or more fields. While
 886           not an error (the missing fields are initialised with
 887           bottoms), it is often an indication of a programmer error.</para>
 888         </listitem>
 889       </varlistentry>
 890
 891       <varlistentry>
 892         <term><option>-fwarn-missing-methods</option>:</term>
 893         <listitem>
 894           <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
 895           <indexterm><primary>missing methods, warning</primary></indexterm>
 896           <indexterm><primary>methods, missing</primary></indexterm>
 897
 898           <para>This option is on by default, and warns you whenever
 899           an instance declaration is missing one or more methods, and
 900           the corresponding class declaration has no default
 901           declaration for them.</para>
 902         </listitem>
 903       </varlistentry>
 904
 905       <varlistentry>
 906         <term><option>-fwarn-missing-signatures</option>:</term>
 907         <listitem>
 908           <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
 909           <indexterm><primary>type signatures, missing</primary></indexterm>
 910
 911           <para>If you would like GHC to check that every top-level
 912           function/value has a type signature, use the
 913           <option>-fwarn-missing-signatures</option> option.  This
 914           option is off by default.</para>
 915         </listitem>
 916       </varlistentry>
 917
 918       <varlistentry>
 919         <term><option>-fwarn-name-shadowing</option>:</term>
 920         <listitem>
 921           <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
 922           <indexterm><primary>shadowing, warning</primary></indexterm>
 923
 924           <para>This option causes a warning to be emitted whenever an
 925           inner-scope value has the same name as an outer-scope value,
 926           i.e. the inner value shadows the outer one.  This can catch
 927           typographical errors that turn into hard-to-find bugs, e.g.,
 928           in the inadvertent cyclic definition <literal>let x = ... x
 929           ... in</literal>.</para>
 930
 931           <para>Consequently, this option does
 932           <emphasis>will</emphasis> complain about cyclic recursive
 933           definitions.</para>
 934         </listitem>
 935       </varlistentry>
 936
 937       <varlistentry>
 938         <term><option>-fwarn-overlapping-patterns</option>:</term>
 939         <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
 940         <indexterm><primary>overlapping patterns, warning</primary></indexterm>
 941         <indexterm><primary>patterns, overlapping</primary></indexterm>
 942         <listitem>
 943           <para>By default, the compiler will warn you if a set of
 944           patterns are overlapping, i.e.,</para>
 945
 946 <programlisting>
 947 f :: String -&#62; Int
 948 f []     = 0
 949 f (_:xs) = 1
 950 f "2"    = 2
 951 </programlisting>
 952
 953           <para>where the last pattern match in <Function>f</Function>
 954           won't ever be reached, as the second pattern overlaps
 955           it. More often than not, redundant patterns is a programmer
 956           mistake/error, so this option is enabled by default.</para>
 957         </listitem>
 958       </varlistentry>
 959
 960       <varlistentry>
 961         <term><option>-fwarn-simple-patterns</option>:</term>
 962         <listitem>
 963           <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
 964           </indexterm>
 965           <para>Causes the compiler to warn about lambda-bound
 966           patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
 967           Normally, these aren't treated as incomplete patterns by
 968           <option>-fwarn-incomplete-patterns</option>.</para>
 969         </listitem>
 970       </varlistentry>
 971
 972       <varlistentry>
 973         <term><option>-fwarn-type-defaults</option>:</term>
 974         <listitem>
 975           <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
 976           <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
 977           <para>Have the compiler warn/inform you where in your source
 978           the Haskell defaulting mechanism for numeric types kicks
 979           in. This is useful information when converting code from a
 980           context that assumed one default into one with another,
 981           e.g., the `default default' for Haskell 1.4 caused the
 982           otherwise unconstrained value <Constant>1</Constant> to be
 983           given the type <literal>Int</literal>, whereas Haskell 98
 984           defaults it to <literal>Integer</literal>.  This may lead to
 985           differences in performance and behaviour, hence the
 986           usefulness of being non-silent about this.</para>
 987
 988           <para>This warning is off by default.</para>
 989         </listitem>
 990       </varlistentry>
 991
 992       <varlistentry>
 993         <term><option>-fwarn-unused-binds</option>:</term>
 994         <listitem>
 995           <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
 996           <indexterm><primary>unused binds, warning</primary></indexterm>
 997           <indexterm><primary>binds, unused</primary></indexterm>
 998           <para>Report any function definitions (and local bindings)
 999           which are unused.  For top-level functions, the warning is
1000           only given if the binding is not exported.</para>
1001         </listitem>
1002       </varlistentry>
1003
1004       <varlistentry>
1005         <term><option>-fwarn-unused-imports</option>:</term>
1006         <listitem>
1007           <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1008           <indexterm><primary>unused imports, warning</primary></indexterm>
1009           <indexterm><primary>imports, unused</primary></indexterm>
1010
1011           <para>Report any objects that are explicitly imported but
1012           never used.</para>
1013         </listitem>
1014       </varlistentry>
1015
1016       <varlistentry>
1017         <term><option>-fwarn-unused-matches</option>:</term>
1018         <listitem>
1019           <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1020           <indexterm><primary>unused matches, warning</primary></indexterm>
1021           <indexterm><primary>matches, unused</primary></indexterm>
1022
1023           <para>Report all unused variables which arise from pattern
1024           matches, including patterns consisting of a single variable.
1025           For instance <literal>f x y = []</literal> would report
1026           <VarName>x</VarName> and <VarName>y</VarName> as unused.  To
1027           eliminate the warning, all unused variables can be replaced
1028           with wildcards.</para>
1029         </listitem>
1030       </varlistentry>
1031
1032     </VariableList>
1033
1034     <para>If you're feeling really paranoid, the
1035     <option>-dcore-lint</option>
1036     option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1037     is a good choice.  It turns on heavyweight intra-pass
1038     sanity-checking within GHC.  (It checks GHC's sanity, not
1039     yours.)</para>
1040
1041   </sect1>
1042
1043   &separate;
1044   &packages;
1045
1046   <sect1 id="options-optimise">
1047     <title>Optimisation (code improvement)</title>
1048
1049     <indexterm><primary>optimisation</primary></indexterm>
1050     <indexterm><primary>improvement, code</primary></indexterm>
1051
1052     <para>The <option>-O*</option> options specify convenient
1053     &ldquo;packages&rdquo; of optimisation flags; the
1054     <option>-f*</option> options described later on specify
1055     <emphasis>individual</emphasis> optimisations to be turned on/off;
1056     the <option>-m*</option> options specify
1057     <emphasis>machine-specific</emphasis> optimisations to be turned
1058     on/off.</para>
1059
1060     <sect2 id="optimise-pkgs">
1061       <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1062
1063       <para>There are <emphasis>many</emphasis> options that affect
1064       the quality of code produced by GHC.  Most people only have a
1065       general goal, something like &ldquo;Compile quickly&rdquo; or
1066       &ldquo;Make my program run like greased lightning.&rdquo; The
1067       following &ldquo;packages&rdquo; of optimisations (or lack
1068       thereof) should suffice.</para>
1069
1070       <para>Once you choose a <option>-O*</option>
1071       &ldquo;package,&rdquo; stick with it&mdash;don't chop and
1072       change.  Modules' interfaces <emphasis>will</emphasis> change
1073       with a shift to a new <option>-O*</option> option, and you may
1074       have to recompile a large chunk of all importing modules before
1075       your program can again be run safely (see <XRef
1076       LinkEnd="recomp">).</para>
1077
1078       <variablelist>
1079
1080         <varlistentry>
1081           <term>No <option>-O*</option>-type option specified:</term>
1082           <indexterm><primary>-O* not specified</primary></indexterm>
1083           <listitem>
1084             <para>This is taken to mean: &ldquo;Please compile
1085             quickly; I'm not over-bothered about compiled-code
1086             quality.&rdquo; So, for example: <command>ghc -c
1087             Foo.hs</command></para>
1088           </listitem>
1089         </varlistentry>
1090
1091         <varlistentry>
1092           <term><option>-O0</option>:</term>
1093           <indexterm><primary><option>-O0</option></primary></indexterm>
1094           <listitem>
1095             <para>Means &ldquo;turn off all optimisation&rdquo;,
1096             reverting to the same settings as if no
1097             <option>-O</option> options had been specified.  Saying
1098             <option>-O0</option> can be useful if
1099             eg. <command>make</command> has inserted a
1100             <option>-O</option> on the command line already.</para>
1101           </listitem>
1102         </varlistentry>
1103
1104         <varlistentry>
1105           <term><option>-O</option> or <option>-O1</option>:</term>
1106           <indexterm><primary>-O option</primary></indexterm>
1107           <indexterm><primary>-O1 option</primary></indexterm>
1108           <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1109           <listitem>
1110             <para>Means: &ldquo;Generate good-quality code without
1111             taking too long about it.&rdquo; Thus, for example:
1112             <command>ghc -c -O Main.lhs</command></para>
1113           </listitem>
1114         </varlistentry>
1115
1116         <varlistentry>
1117           <term><option>-O2</option>:</term>
1118           <indexterm><primary>-O2 option</primary></indexterm>
1119           <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1120           <listitem>
1121             <para>Means: &ldquo;Apply every non-dangerous
1122             optimisation, even if it means significantly longer
1123             compile times.&rdquo;</para>
1124
1125             <para>The avoided &ldquo;dangerous&rdquo; optimisations
1126             are those that can make runtime or space
1127             <emphasis>worse</emphasis> if you're unlucky.  They are
1128             normally turned on or off individually.</para>
1129
1130             <para>At the moment, <option>-O2</option> is
1131             <emphasis>unlikely</emphasis> to produce better code than
1132             <option>-O</option>.</para>
1133           </listitem>
1134         </varlistentry>
1135
1136         <varlistentry>
1137           <term><option>-O2-for-C</option>:</term>
1138           <indexterm><primary>-O2-for-C option</primary></indexterm>
1139           <indexterm><primary>gcc, invoking with -O2</primary></indexterm>
1140           <listitem>
1141             <para>Says to run GCC with <option>-O2</option>, which may
1142             be worth a few percent in execution speed.  Don't forget
1143             <option>-fvia-C</option>, lest you use the native-code
1144             generator and bypass GCC altogether!</para>
1145           </listitem>
1146         </varlistentry>
1147
1148         <varlistentry>
1149           <term><option>-Ofile &lt;file&gt;</option>:</term>
1150           <indexterm><primary>-Ofile &lt;file&gt; option</primary></indexterm>
1151           <indexterm><primary>optimising, customised</primary></indexterm>
1152           <listitem>
1153             <para>(NOTE: not supported yet in GHC 5.x.  Please ask if
1154             you're interested in this.)</para>
1155
1156             <para>For those who need <emphasis>absolute</emphasis>
1157             control over <emphasis>exactly</emphasis> what options are
1158             used (e.g., compiler writers, sometimes :-), a list of
1159             options can be put in a file and then slurped in with
1160             <option>-Ofile</option>.</para>
1161
1162             <para>In that file, comments are of the
1163             <literal>&num;</literal>-to-end-of-line variety; blank
1164             lines and most whitespace is ignored.</para>
1165
1166             <para>Please ask if you are baffled and would like an
1167             example of <option>-Ofile</option>!</para>
1168           </listitem>
1169         </varlistentry>
1170       </variablelist>
1171
1172       <para>We don't use a <option>-O*</option> flag for day-to-day
1173       work.  We use <option>-O</option> to get respectable speed;
1174       e.g., when we want to measure something.  When we want to go for
1175       broke, we tend to use <option>-O -fvia-C -O2-for-C</option> (and
1176       we go for lots of coffee breaks).</para>
1177
1178       <para>The easiest way to see what <option>-O</option> (etc.)
1179       &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1180       then stand back in amazement.</para>
1181     </sect2>
1182
1183     <sect2 id="options-f">
1184       <title><option>-f*</option>: platform-independent flags</title>
1185
1186       <indexterm><primary>-f* options (GHC)</primary></indexterm>
1187       <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1188
1189       <para>These flags turn on and off individual optimisations.
1190       They are normally set via the <option>-O</option> options
1191       described above, and as such, you shouldn't need to set any of
1192       them explicitly (indeed, doing so could lead to unexpected
1193       results).  However, there are one or two that may be of
1194       interest:</para>
1195
1196       <variablelist>
1197         <varlistentry>
1198           <term><option>-fexcess-precision</option>:</term>
1199           <listitem>
1200             <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1201             <para>When this option is given, intermediate floating
1202             point values can have a <emphasis>greater</emphasis>
1203             precision/range than the final type.  Generally this is a
1204             good thing, but some programs may rely on the exact
1205             precision/range of
1206             <literal>Float</literal>/<literal>Double</literal> values
1207             and should not use this option for their compilation.</para>
1208           </listitem>
1209         </varlistentry>
1210
1211         <varlistentry>
1212           <term><option>-fignore-asserts</option>:</term>
1213           <listitem>
1214             <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1215             <para>Causes GHC to ignore uses of the function
1216             <literal>Exception.assert</literal> in source code (in
1217             other words, rewriting <literal>Exception.assert p
1218             e</literal> to <literal>e</literal> (see <xref
1219             linkend="sec-assertions">).  This flag is turned on by
1220             <option>-O</option>.
1221             </para>
1222           </listitem>
1223         </varlistentry>
1224
1225         <varlistentry>
1226           <term><option>-fno-strictness</option></term>
1227           <indexterm><primary><option>-fno-strictness</option></primary>
1228           </indexterm>
1229           <listitem>
1230             <para>Turns off the strictness analyser; sometimes it eats
1231             too many cycles.</para>
1232           </listitem>
1233         </varlistentry>
1234
1235         <varlistentry>
1236           <term><option>-fno-cpr-analyse</option></term>
1237           <indexterm><primary><option>-fno-cpr-analyse</option></primary>
1238           </indexterm>
1239           <listitem>
1240             <para>Turns off the CPR (constructed product result)
1241             analysis; it is somewhat experimental.</para>
1242           </listitem>
1243         </varlistentry>
1244
1245         <varlistentry>
1246           <term><option>-funbox-strict-fields</option>:</term>
1247           <listitem>
1248             <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1249             <indexterm><primary>strict constructor fields</primary></indexterm>
1250             <indexterm><primary>constructor fields, strict</primary></indexterm>
1251
1252             <para>This option causes all constructor fields which are
1253             marked strict (i.e. &ldquo;!&rdquo;) to be unboxed or
1254             unpacked if possible.  For example:</para>
1255
1256 <ProgramListing>
1257 data T = T !Float !Float
1258 </ProgramListing>
1259
1260             <para>will create a constructor <literal>T</literal>
1261             containing two unboxed floats if the
1262             <option>-funbox-strict-fields</option> flag is given.
1263             This may not always be an optimisation: if the
1264             <Function>T</Function> constructor is scrutinised and the
1265             floats passed to a non-strict function for example, they
1266             will have to be reboxed (this is done automatically by the
1267             compiler).</para>
1268
1269             <para>This option should only be used in conjunction with
1270             <option>-O</option>, in order to expose unfoldings to the
1271             compiler so the reboxing can be removed as often as
1272             possible.  For example:</para>
1273
1274 <ProgramListing>
1275 f :: T -&#62; Float
1276 f (T f1 f2) = f1 + f2
1277 </ProgramListing>
1278
1279             <para>The compiler will avoid reboxing
1280             <Function>f1</Function> and <Function>f2</Function> by
1281             inlining <Function>+</Function> on floats, but only when
1282             <option>-O</option> is on.</para>
1283
1284             <para>Any single-constructor data is eligible for
1285             unpacking; for example</para>
1286
1287 <ProgramListing>
1288 data T = T !(Int,Int)
1289 </ProgramListing>
1290
1291             <para>will store the two <literal>Int</literal>s directly
1292             in the <Function>T</Function> constructor, by flattening
1293             the pair.  Multi-level unpacking is also supported:</para>
1294
1295 <ProgramListing>
1296 data T = T !S
1297 data S = S !Int !Int
1298 </ProgramListing>
1299
1300             <para>will store two unboxed <literal>Int&num;</literal>s
1301             directly in the <Function>T</Function> constructor.</para>
1302           </listitem>
1303         </varlistentry>
1304
1305         <varlistentry>
1306           <term><option>-funfolding-update-in-place&lt;n&gt;</option></term>
1307           <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1308           <listitem>
1309             <para>Switches on an experimental "optimisation".
1310             Switching it on makes the compiler a little keener to
1311             inline a function that returns a constructor, if the
1312             context is that of a thunk.
1313 <ProgramListing>
1314    x = plusInt a b
1315 </ProgramListing>
1316             If we inlined plusInt we might get an opportunity to use
1317             update-in-place for the thunk 'x'.</para>
1318           </listitem>
1319         </varlistentry>
1320
1321         <varlistentry>
1322           <term><option>-funfolding-creation-threshold&lt;n&gt;</option>:</term>
1323           <listitem>
1324             <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1325             <indexterm><primary>inlining, controlling</primary></indexterm>
1326             <indexterm><primary>unfolding, controlling</primary></indexterm>
1327
1328             <para>(Default: 45) Governs the maximum size that GHC will
1329             allow a function unfolding to be.   (An unfolding has a
1330             &ldquo;size&rdquo; that reflects the cost in terms of
1331             &ldquo;code bloat&rdquo; of expanding that unfolding at
1332             at a call site. A bigger function would be assigned a
1333             bigger cost.) </para>
1334
1335             <para> Consequences: (a) nothing larger than this will be
1336             inlined (unless it has an INLINE pragma); (b) nothing
1337             larger than this will be spewed into an interface
1338             file. </para>
1339
1340
1341             <para> Increasing this figure is more likely to result in longer
1342             compile times than faster code.  The next option is more
1343             useful:</para>
1344           </listitem>
1345         </varlistentry>
1346
1347         <varlistentry>
1348           <term><option>-funfolding-use-threshold&lt;n&gt;</option>:</term>
1349           <listitem>
1350             <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1351             <indexterm><primary>inlining, controlling</primary></indexterm>
1352             <indexterm><primary>unfolding, controlling</primary></indexterm>
1353
1354             <para>(Default: 8) This is the magic cut-off figure for
1355             unfolding: below this size, a function definition will be
1356             unfolded at the call-site, any bigger and it won't.  The
1357             size computed for a function depends on two things: the
1358             actual size of the expression minus any discounts that
1359             apply (see <option>-funfolding-con-discount</option>).</para>
1360           </listitem>
1361         </varlistentry>
1362       </variablelist>
1363
1364     </sect2>
1365
1366   </sect1>
1367
1368 &phases;
1369
1370 <Sect1 id="sec-using-concurrent">
1371 <title>Using Concurrent Haskell</title>
1372
1373 <para>
1374 <indexterm><primary>Concurrent Haskell&mdash;use</primary></indexterm>
1375 </para>
1376
1377 <para>
1378 GHC (as of version 4.00) supports Concurrent Haskell by default,
1379 without requiring a special option or libraries compiled in a certain
1380 way.  To get access to the support libraries for Concurrent Haskell
1381 (i.e. <literal>Concurrent</literal> and friends), use the
1382 <option>-package concurrent</option> option.
1383 </para>
1384
1385 <para>
1386 Three RTS options are provided for modifying the behaviour of the
1387 threaded runtime system.  See the descriptions of
1388 <option>-C[&lt;us&gt;]</option>, <option>-q</option>, and
1389 <option>-t&lt;num&gt;</option> in <XRef LinkEnd="parallel-rts-opts">.
1390 </para>
1391
1392 <para>
1393 Concurrent Haskell is described in more detail in <XRef
1394 LinkEnd="sec-Concurrent">.
1395 </para>
1396
1397 </Sect1>
1398
1399 <Sect1 id="sec-using-parallel">
1400 <title>Using Parallel Haskell</title>
1401
1402 <para>
1403 <indexterm><primary>Parallel Haskell&mdash;use</primary></indexterm>
1404 </para>
1405
1406 <para>
1407 &lsqb;You won't be able to execute parallel Haskell programs unless PVM3
1408 (Parallel Virtual Machine, version 3) is installed at your site.&rsqb;
1409 </Para>
1410
1411 <para>
1412 To compile a Haskell program for parallel execution under PVM, use the
1413 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
1414 option</Primary></IndexTerm> both when compiling <Emphasis>and
1415 linking</Emphasis>.  You will probably want to <Literal>import
1416 Parallel</Literal> into your Haskell modules.
1417 </Para>
1418
1419 <para>
1420 To run your parallel program, once PVM is going, just invoke it
1421 &ldquo;as normal&rdquo;.  The main extra RTS option is
1422 <Option>-qp&lt;n&gt;</Option>, to say how many PVM
1423 &ldquo;processors&rdquo; your program to run on.  (For more details of
1424 all relevant RTS options, please see <XRef
1425 LinkEnd="parallel-rts-opts">.)
1426 </para>
1427
1428 <para>
1429 In truth, running Parallel Haskell programs and getting information
1430 out of them (e.g., parallelism profiles) is a battle with the vagaries of
1431 PVM, detailed in the following sections.
1432 </para>
1433
1434 <Sect2 id="pvm-dummies">
1435 <Title>Dummy's guide to using PVM</Title>
1436
1437 <para>
1438 <indexterm><primary>PVM, how to use</primary></indexterm>
1439 <indexterm><primary>Parallel Haskell&mdash;PVM use</primary></indexterm>
1440 Before you can run a parallel program under PVM, you must set the
1441 required environment variables (PVM's idea, not ours); something like,
1442 probably in your <filename>.cshrc</filename> or equivalent:
1443
1444 <ProgramListing>
1445 setenv PVM_ROOT /wherever/you/put/it
1446 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
1447 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
1448 </ProgramListing>
1449
1450 </para>
1451
1452 <para>
1453 Creating and/or controlling your &ldquo;parallel machine&rdquo; is a purely-PVM
1454 business; nothing specific to Parallel Haskell. The following paragraphs
1455 describe how to configure your parallel machine interactively.
1456 </Para>
1457
1458 <Para>
1459 If you use parallel Haskell regularly on the same machine configuration it
1460 is a good idea to maintain a file with all machine names and to make the
1461 environment variable PVM_HOST_FILE point to this file. Then you can avoid
1462 the interactive operations described below by just saying
1463 </Para>
1464
1465 <ProgramListing>
1466 pvm $PVM_HOST_FILE
1467 </ProgramListing>
1468
1469 <Para>
1470 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
1471 machine.  You can then do various things to control/monitor your
1472 &ldquo;parallel machine;&rdquo; the most useful being:
1473 </para>
1474
1475 <para>
1476 <InformalTable>
1477 <TGroup Cols=2>
1478 <ColSpec Align="Left">
1479 <TBody>
1480
1481 <row>
1482 <entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></entry>
1483 <entry>exit <command>pvm</command>, leaving it running</entry>
1484 </row>
1485
1486 <row>
1487 <entry><command>halt</command></entry>
1488 <entry>kill off this &ldquo;parallel machine&rdquo; &amp; exit</entry>
1489 </row>
1490
1491 <row>
1492 <entry><command>add &lt;host&gt;</command></entry>
1493 <entry>add <command>&lt;host&gt;</command> as a processor</entry>
1494 </row>
1495
1496 <row>
1497 <entry><command>delete &lt;host&gt;</command></entry>
1498 <entry>delete <command>&lt;host&gt;</command></entry>
1499 </row>
1500
1501 <row>
1502 <entry><command>reset</command></entry>
1503 <entry>kill what's going, but leave PVM up</entry>
1504 </row>
1505
1506 <row>
1507 <entry><command>conf</command></entry>
1508 <entry>list the current configuration</entry>
1509 </row>
1510
1511 <row>
1512 <entry><command>ps</command></entry>
1513 <entry>report processes' status</entry>
1514 </row>
1515
1516 <row>
1517 <entry><command>pstat &lt;pid&gt;</command></entry>
1518 <entry>status of a particular process</entry>
1519 </row>
1520
1521 </TBody>
1522 </TGroup>
1523 </InformalTable>
1524 </para>
1525
1526 <para>
1527 The PVM documentation can tell you much, much more about <command>pvm</command>!
1528 </para>
1529
1530 </sect2>
1531
1532 <Sect2 id="par-profiles">
1533 <Title>Parallelism profiles</Title>
1534
1535 <para>
1536 <indexterm><primary>parallelism profiles</primary></indexterm>
1537 <indexterm><primary>profiles, parallelism</primary></indexterm>
1538 <indexterm><primary>visualisation tools</primary></indexterm>
1539 </para>
1540
1541 <para>
1542 With Parallel Haskell programs, we usually don't care about the
1543 results&mdash;only with &ldquo;how parallel&rdquo; it was!  We want pretty pictures.
1544 </para>
1545
1546 <Para>
1547 Parallelism profiles (&agrave; la <Command>hbcpp</Command>) can be generated with the
1548 <Option>-qP</Option><IndexTerm><Primary>-qP RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option.  The
1549 per-processor profiling info is dumped into files named
1550 <Filename>&lt;full-path&gt;&lt;program&gt;.gr</Filename>.  These are then munged into a PostScript picture,
1551 which you can then display.  For example, to run your program
1552 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
1553 </Para>
1554
1555 <Para>
1556
1557 <Screen>
1558 <prompt>&dollar;</prompt> ./a.out +RTS -qP -qp8
1559 <prompt>&dollar;</prompt> grs2gr *.???.gr &#62; temp.gr # combine the 8 .gr files into one
1560 <prompt>&dollar;</prompt> gr2ps -O temp.gr              # cvt to .ps; output in temp.ps
1561 <prompt>&dollar;</prompt> ghostview -seascape temp.ps   # look at it!
1562 </Screen>
1563
1564 </Para>
1565
1566 <para>
1567 The scripts for processing the parallelism profiles are distributed
1568 in <filename>ghc/utils/parallel/</filename>.
1569 </para>
1570
1571 </sect2>
1572
1573 <Sect2>
1574 <Title>Other useful info about running parallel programs</Title>
1575
1576 <Para>
1577 The &ldquo;garbage-collection statistics&rdquo; RTS options can be useful for
1578 seeing what parallel programs are doing.  If you do either
1579 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
1580 you'll get mutator, garbage-collection, etc., times on standard
1581 error. The standard error of all PE's other than the `main thread'
1582 appears in <filename>/tmp/pvml.nnn</filename>, courtesy of PVM.
1583 </para>
1584
1585 <para>
1586 Whether doing <option>+RTS -Sstderr</option> or not, a handy way to watch
1587 what's happening overall is: <command>tail -f /tmp/pvml.nnn</command>.
1588 </para>
1589
1590 </sect2>
1591
1592 <Sect2 id="parallel-rts-opts">
1593 <title>RTS options for Concurrent/Parallel Haskell
1594 </title>
1595
1596 <para>
1597 <indexterm><primary>RTS options, concurrent</primary></indexterm>
1598 <indexterm><primary>RTS options, parallel</primary></indexterm>
1599 <indexterm><primary>Concurrent Haskell&mdash;RTS options</primary></indexterm>
1600 <indexterm><primary>Parallel Haskell&mdash;RTS options</primary></indexterm>
1601 </para>
1602
1603 <para>
1604 Besides the usual runtime system (RTS) options
1605 (<XRef LinkEnd="runtime-control">), there are a few options particularly
1606 for concurrent/parallel execution.
1607 </para>
1608
1609 <para>
1610 <VariableList>
1611
1612 <VarListEntry>
1613 <Term><Option>-qp&lt;N&gt;</Option>:</Term>
1614 <ListItem>
1615 <Para>
1616 <IndexTerm><Primary>-qp&lt;N&gt; RTS option</Primary></IndexTerm>
1617 (PARALLEL ONLY) Use <Literal>&lt;N&gt;</Literal> PVM processors to run this program;
1618 the default is 2.
1619 </para>
1620 </listitem>
1621 </varlistentry>
1622 <varlistentry>
1623 <term><option>-C[&lt;us&gt;]</option>:</term>
1624 <listitem>
1625 <para>
1626 <indexterm><primary>-C&lt;us&gt; RTS option</primary></indexterm> Sets
1627 the context switch interval to <literal>&lt;s&gt;</literal> seconds.
1628 A context switch will occur at the next heap block allocation after
1629 the timer expires (a heap block allocation occurs every 4k of
1630 allocation).  With <option>-C0</option> or <option>-C</option>,
1631 context switches will occur as often as possible (at every heap block
1632 allocation).  By default, context switches occur every 20ms
1633 milliseconds.  Note that GHC's internal timer ticks every 20ms, and
1634 the context switch timer is always a multiple of this timer, so 20ms
1635 is the maximum granularity available for timed context switches.
1636 </para>
1637 </listitem>
1638 </varlistentry>
1639 <varlistentry>
1640 <term><option>-q[v]</option>:</term>
1641 <listitem>
1642 <para>
1643 <indexterm><primary>-q RTS option</primary></indexterm>
1644 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
1645 in the file <FIlename>&lt;program&gt;.qp</FIlename>.  In the style of <command>hbcpp</command>, this profile
1646 records the movement of threads between the green (runnable) and red
1647 (blocked) queues.  If you specify the verbose suboption (<option>-qv</option>), the
1648 green queue is split into green (for the currently running thread
1649 only) and amber (for other runnable threads).  We do not recommend
1650 that you use the verbose suboption if you are planning to use the
1651 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
1652 check (with <Option>-C</Option>).
1653 -->
1654 </Para>
1655 </ListItem>
1656 </VarListEntry>
1657 <VarListEntry>
1658 <Term><Option>-qt&lt;num&gt;</Option>:</Term>
1659 <ListItem>
1660 <Para>
1661 <IndexTerm><Primary>-qt&lt;num&gt; RTS option</Primary></IndexTerm>
1662 (PARALLEL ONLY) Limit the thread pool size, i.e. the number of concurrent
1663 threads per processor to <Literal>&lt;num&gt;</Literal>.  The default is
1664 32.  Each thread requires slightly over 1K <Emphasis>words</Emphasis> in
1665 the heap for thread state and stack objects.  (For 32-bit machines, this
1666 translates to 4K bytes, and for 64-bit machines, 8K bytes.)
1667 </Para>
1668 </ListItem>
1669 </VarListEntry>
1670 <!-- no more -HWL
1671 <VarListEntry>
1672 <Term><Option>-d</Option>:</Term>
1673 <ListItem>
1674 <Para>
1675 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
1676 (PARALLEL ONLY) Turn on debugging.  It pops up one xterm (or GDB, or
1677 something&hellip;) per PVM processor.  We use the standard <Command>debugger</Command>
1678 script that comes with PVM3, but we sometimes meddle with the
1679 <Command>debugger2</Command> script.  We include ours in the GHC distribution,
1680 in <Filename>ghc/utils/pvm/</Filename>.
1681 </Para>
1682 </ListItem>
1683 </VarListEntry>
1684 -->
1685 <VarListEntry>
1686 <Term><Option>-qe&lt;num&gt;</Option>:</Term>
1687 <ListItem>
1688 <Para>
1689 <IndexTerm><Primary>-qe&lt;num&gt; RTS option
1690 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Limit the spark pool size
1691 i.e. the number of pending sparks per processor to
1692 <Literal>&lt;num&gt;</Literal>. The default is 100. A larger number may be
1693 appropriate if your program generates large amounts of parallelism
1694 initially.
1695 </Para>
1696 </ListItem>
1697 </VarListEntry>
1698 <VarListEntry>
1699 <Term><Option>-qQ&lt;num&gt;</Option>:</Term>
1700 <ListItem>
1701 <Para>
1702 <IndexTerm><Primary>-qQ&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1703 (PARALLEL ONLY) Set the size of packets transmitted between processors
1704 to <Literal>&lt;num&gt;</Literal>. The default is 1024 words. A larger number may be
1705 appropriate if your machine has a high communication cost relative to
1706 computation speed.
1707 </Para>
1708 </ListItem>
1709 </VarListEntry>
1710 <VarListEntry>
1711 <Term><Option>-qh&lt;num&gt;</Option>:</Term>
1712 <ListItem>
1713 <Para>
1714 <IndexTerm><Primary>-qh&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1715 (PARALLEL ONLY) Select a packing scheme. Set the number of non-root thunks to pack in one packet to
1716 &lt;num&gt;-1 (0 means infinity). By default GUM uses full-subgraph
1717 packing, i.e. the entire subgraph with the requested closure as root is
1718 transmitted (provided it fits into one packet). Choosing a smaller value
1719 reduces the amount of pre-fetching of work done in GUM. This can be
1720 advantageous for improving data locality but it can also worsen the balance
1721 of the load in the system.
1722 </Para>
1723 </ListItem>
1724 </VarListEntry>
1725 <VarListEntry>
1726 <Term><Option>-qg&lt;num&gt;</Option>:</Term>
1727 <ListItem>
1728 <Para>
1729 <IndexTerm><Primary>-qg&lt;num&gt; RTS option
1730 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Select a globalisation
1731 scheme. This option affects the
1732 generation of global addresses when transferring data. Global addresses are
1733 globally unique identifiers required to maintain sharing in the distributed
1734 graph structure. Currently this is a binary option. With &lt;num&gt;=0 full globalisation is used
1735 (default). This means a global address is generated for every closure that
1736 is transmitted. With &lt;num&gt;=1 a thunk-only globalisation scheme is
1737 used, which generated global address only for thunks. The latter case may
1738 lose sharing of data but has a reduced overhead in packing graph structures
1739 and maintaining internal tables of global addresses.
1740 </Para>
1741 </ListItem>
1742 </VarListEntry>
1743 </VariableList>
1744 </para>
1745
1746 </sect2>
1747
1748 </Sect1>
1749
1750   <sect1 id="options-platform">
1751     <title>Platform-specific Flags</title>
1752
1753     <indexterm><primary>-m* options</primary></indexterm>
1754     <indexterm><primary>platform-specific options</primary></indexterm>
1755     <indexterm><primary>machine-specific options</primary></indexterm>
1756
1757     <para>Some flags only make sense for particular target
1758     platforms.</para>
1759
1760     <variablelist>
1761
1762       <varlistentry>
1763         <term><option>-mv8</option>:</term>
1764         <listitem>
1765           <para>(SPARC machines)<indexterm><primary>-mv8 option (SPARC
1766           only)</primary></indexterm> Means to pass the like-named
1767           option to GCC; it says to use the Version 8 SPARC
1768           instructions, notably integer multiply and divide.  The
1769           similiar <option>-m*</option> GCC options for SPARC also
1770           work, actually.</para>
1771         </listitem>
1772       </varlistentry>
1773
1774       <varlistentry>
1775         <term><option>-monly-[32]-regs</option>:</term>
1776         <listitem>
1777           <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1778           option (iX86 only)</primary></indexterm> GHC tries to
1779           &ldquo;steal&rdquo; four registers from GCC, for performance
1780           reasons; it almost always works.  However, when GCC is
1781           compiling some modules with four stolen registers, it will
1782           crash, probably saying:
1783
1784 <Screen>
1785 Foo.hc:533: fixed or forbidden register was spilled.
1786 This may be due to a compiler bug or to impossible asm
1787 statements or clauses.
1788 </Screen>
1789
1790           Just give some registers back with
1791           <option>-monly-N-regs</option>.  Try `3' first, then `2'.
1792           If `2' doesn't work, please report the bug to us.</para>
1793         </listitem>
1794       </varlistentry>
1795     </variablelist>
1796
1797   </sect1>
1798
1799 &runtime;
1800 &debug;
1801 &flags;
1802
1803 </Chapter>
1804
1805 <!-- Emacs stuff:
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