[project @ 2003-01-08 10:10:19 by simonmar]
[ghc-hetmet.git] / 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>&ndash;&ndash;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>&ndash;&ndash;make</arg>
28         </cmdsynopsis></term>
29       <indexterm><primary>make mode</primary>
30       </indexterm>
31       <indexterm><primary><option>&ndash;&ndash;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 likely
37         to be much easier, and faster, than using
38         <command>make</command>.</para>
39       </listitem>
40     </varlistentry>
41
42     <varlistentry>
43       <term><cmdsynopsis>
44           <command>ghc</command>
45           <group>
46             <arg>-E</arg>
47             <arg>-C</arg>
48             <arg>-S</arg>
49             <arg>-c</arg>
50           </group>
51         </cmdsynopsis></term>
52       <indexterm><primary><option>-E</option></primary></indexterm>
53       <indexterm><primary><option>-C</option></primary></indexterm>
54       <indexterm><primary><option>-S</option></primary></indexterm>
55       <indexterm><primary><option>-c</option></primary></indexterm>
56       <listitem>
57         <para>This is the traditional batch-compiler mode, in which
58           GHC can compile source files one at a time, or link objects
59           together into an executable.</para>
60       </listitem>
61     </varlistentry>
62   </variablelist>
63   
64   <sect1>
65     <title>Options overview</title>
66     
67     <para>GHC's behaviour is controlled by
68     <firstterm>options</firstterm>, which for historical reasons are
69     also sometimes referred to as command-line flags or arguments.
70     Options can be specified in three ways:</para>
71
72     <sect2>
73       <title>Command-line arguments</title>
74       
75       <indexterm><primary>structure, command-line</primary></indexterm>
76       <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
77       <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
78       
79       <para>An invocation of GHC takes the following form:</para>
80
81 <Screen>
82 ghc [argument...]
83 </Screen>
84
85       <para>Command-line arguments are either options or file names.</para>
86
87       <para>Command-line options begin with <literal>-</literal>.
88       They may <emphasis>not</emphasis> be grouped:
89       <option>-vO</option> is different from <option>-v -O</option>.
90       Options need not precede filenames: e.g., <literal>ghc *.o -o
91       foo</literal>.  All options are processed and then applied to
92       all files; you cannot, for example, invoke <literal>ghc -c -O1
93       Foo.hs -O2 Bar.hs</literal> to apply different optimisation
94       levels to the files <filename>Foo.hs</filename> and
95       <filename>Bar.hs</filename>.</para>
96     </sect2>
97
98     <Sect2 id="source-file-options">
99       <title>Command line options in source files</title>
100     
101       <indexterm><primary>source-file options</primary></indexterm>
102
103       <para>Sometimes it is useful to make the connection between a
104       source file and the command-line options it requires quite
105       tight. For instance, if a Haskell source file uses GHC
106       extensions, it will always need to be compiled with the
107       <option>-fglasgow-exts</option> option.  Rather than maintaining
108       the list of per-file options in a <filename>Makefile</filename>,
109       it is possible to do this directly in the source file using the
110       <literal>OPTIONS</literal> pragma <indexterm><primary>OPTIONS
111       pragma</primary></indexterm>:</para>
112
113 <ProgramListing>
114 {-# OPTIONS -fglasgow-exts #-}
115 module X where
116 ...
117 </ProgramListing>
118       
119       <para><literal>OPTIONS</literal> pragmas are only looked for at
120       the top of your source files, upto the first
121       (non-literate,non-empty) line not containing
122       <literal>OPTIONS</literal>. Multiple <literal>OPTIONS</literal>
123       pragmas are recognised. Note that your command shell does not
124       get to the source file options, they are just included literally
125       in the array of command-line arguments the compiler driver
126       maintains internally, so you'll be desperately disappointed if
127       you try to glob etc. inside <literal>OPTIONS</literal>.</para>
128
129       <para>NOTE: the contents of OPTIONS are prepended to the
130       command-line options, so you <emphasis>do</emphasis> have the
131       ability to override OPTIONS settings via the command
132       line.</para>
133
134       <para>It is not recommended to move all the contents of your
135       Makefiles into your source files, but in some circumstances, the
136       <literal>OPTIONS</literal> pragma is the Right Thing. (If you
137       use <option>-keep-hc-file-too</option> and have OPTION flags in
138       your module, the OPTIONS will get put into the generated .hc
139       file).</para>
140     </sect2>
141
142     <sect2>
143       <title>Setting options in GHCi</title>
144
145       <para>Options may also be modified from within GHCi, using the
146       <literal>:set</literal> command.  See <xref linkend="ghci-set">
147       for more details.</para>
148     </sect2>
149   </sect1>
150     
151   <sect1 id="static-dynamic-flags">
152     <title>Static vs. Dynamic options</title>
153     <indexterm><primary>static</primary><secondary>options</secondary>
154     </indexterm>
155     <indexterm><primary>dynamic</primary><secondary>options</secondary>
156     </indexterm>
157
158     <para>Each of GHC's command line options is classified as either
159     <firstterm>static</firstterm> or <firstterm>dynamic</firstterm>.
160     A static flag may only be specified on the command line, whereas a
161     dynamic flag may also be given in an <literal>OPTIONS</literal>
162     pragma in a source file or set from the GHCi command-line with
163     <literal>:set</literal>.</para>
164
165     <para>As a rule of thumb, all the language options are dynamic, as
166     are the warning options and the debugging options.  The rest are
167     static, with the notable exceptions of <option>-v</option>,
168     <option>-cpp</option>, <option>-fasm</option>,
169     <option>-fvia-C</option>, and <option>-#include</option>.
170
171     The flag reference tables (<xref linkend="flag-reference">) lists
172     the status of each flag.</para>
173   </sect1>
174
175   <sect1 id="file-suffixes">
176     <title>Meaningful file suffixes</title>
177
178     <indexterm><primary>suffixes, file</primary></indexterm>
179     <indexterm><primary>file suffixes for GHC</primary></indexterm>
180
181     <para>File names with &ldquo;meaningful&rdquo; suffixes (e.g.,
182     <filename>.lhs</filename> or <filename>.o</filename>) cause the
183     &ldquo;right thing&rdquo; to happen to those files.</para>
184
185     <variablelist>
186
187       <varlistentry>
188         <term><filename>.lhs</filename></term>
189         <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
190         <listitem>
191           <para>A &ldquo;literate Haskell&rdquo; module.</para>
192         </listitem>
193       </varlistentry>
194
195       <varlistentry>
196         <term><filename>.hs</filename></term>
197         <listitem>
198           <para>A not-so-literate Haskell module.</para>
199         </listitem>
200       </varlistentry>
201
202       <varlistentry>
203         <term><filename>.hi</filename></term>
204         <listitem>
205           <para>A Haskell interface file, probably
206           compiler-generated.</para>
207         </listitem>
208       </varlistentry>
209
210       <varlistentry>
211         <term><filename>.hc</filename></term>
212         <listitem>
213           <para>Intermediate C file produced by the Haskell
214           compiler.</para>
215         </listitem>
216       </varlistentry>
217
218       <varlistentry>
219         <term><filename>.c</filename></term>
220         <listitem>
221           <para>A C&nbsp;file not produced by the Haskell
222           compiler.</para>
223         </listitem>
224       </varlistentry>
225       
226       <varlistentry>
227         <term><filename>.s</filename></term>
228         <listitem>
229           <para>An assembly-language source file, usually produced by
230           the compiler.</para>
231         </listitem>
232       </varlistentry>
233
234       <varlistentry>
235         <term><filename>.o</filename></term>
236         <listitem>
237           <para>An object file, produced by an assembler.</para>
238         </listitem>
239       </varlistentry>
240     </variablelist>
241
242     <para>Files with other suffixes (or without suffixes) are passed
243     straight to the linker.</para>
244
245   </sect1>
246
247   <sect1 id="options-help">
248     <title>Help and verbosity options</title>
249
250     <IndexTerm><Primary>help options</Primary></IndexTerm>
251     <IndexTerm><Primary>verbosity options</Primary></IndexTerm>
252
253     <variablelist>
254       <varlistentry>
255         <term><option>&ndash;&ndash;help</option></term>
256         <term><option>-?</option></term>
257         <indexterm><primary><option>-?</option></primary></indexterm>
258         <indexterm><primary><option>&ndash;&ndash;help</option></primary></indexterm>
259         <listitem>
260           <para>Cause GHC to spew a long usage message to standard
261           output and then exit.</para>
262         </listitem>
263       </varlistentry>
264
265       <varlistentry>
266         <term><option>-v</option></term>
267         <indexterm><primary><option>-v</option></primary></indexterm>
268         <listitem>
269           <para>The <option>-v</option> option makes GHC
270           <emphasis>verbose</emphasis>: it reports its version number
271           and shows (on stderr) exactly how it invokes each phase of
272           the compilation system.  Moreover, it passes the
273           <option>-v</option> flag to most phases; each reports its
274           version number (and possibly some other information).</para>
275
276           <para>Please, oh please, use the <option>-v</option> option
277           when reporting bugs!  Knowing that you ran the right bits in
278           the right order is always the first thing we want to
279           verify.</para>
280         </listitem>
281       </varlistentry>
282         
283       <varlistentry>
284         <term><option>-v</option><replaceable>n</replaceable></term>
285         <indexterm><primary><option>-v</option></primary></indexterm>
286         <listitem>
287           <para>To provide more control over the compiler's verbosity,
288           the <option>-v</option> flag takes an optional numeric
289           argument.  Specifying <option>-v</option> on its own is
290           equivalent to <option>-v3</option>, and the other levels
291           have the following meanings:</para>
292           
293           <variablelist>
294             <varlistentry>
295               <term><option>-v0</option></term>
296               <listitem>
297                 <para>Disable all non-essential messages (this is the
298                 default).</para>
299               </listitem>
300             </varlistentry>
301
302             <varlistentry>
303               <term><option>-v1</option></term>
304               <listitem>
305                 <para>Minimal verbosity: print one line per
306                 compilation (this is the default when
307                 <option>&ndash;&ndash;make</option> or
308                 <option>&ndash;&ndash;interactive</option> is on).</para>
309               </listitem>
310             </varlistentry>
311
312             <varlistentry>
313               <term><option>-v2</option></term>
314               <listitem>
315                 <para>Print the name of each compilation phase as it
316                 is executed. (equivalent to
317                 <option>-dshow-passes</option>).</para>
318               </listitem>
319             </varlistentry>
320
321             <varlistentry>
322               <term><option>-v3</option></term>
323               <listitem>
324                 <para>The same as <option>-v2</option>, except that in
325                 addition the full command line (if appropriate) for
326                 each compilation phase is also printed.</para>
327               </listitem>
328             </varlistentry>
329
330             <varlistentry>
331               <term><option>-v4</option></term>
332               <listitem>
333                 <para>The same as <option>-v3</option> except that the
334                 intermediate program representation after each
335                 compilation phase is also printed (excluding
336                 preprocessed and C/assembly files).</para>
337               </listitem>
338             </varlistentry>
339           </variablelist>
340         </listitem>
341       </varlistentry>
342       
343       <varlistentry>
344         <term><option>&ndash;&ndash;version</option></term>
345         <indexterm><primary><option>&ndash;&ndash;version</option></primary></indexterm>
346         <listitem>
347           <para>Print a one-line string including GHC's version number.</para>
348         </listitem>
349       </varlistentry>
350
351       <varlistentry>
352         <term><option>&ndash;&ndash;numeric-version</option></term>
353         <indexterm><primary><option>&ndash;&ndash;numeric-version</option></primary></indexterm>
354         <listitem>
355           <para>Print GHC's numeric version number only.</para>
356         </listitem>
357       </varlistentry>
358
359       <varlistentry>
360         <term><option>&ndash;&ndash;print-libdir</option></term>
361         <indexterm><primary><option>&ndash;&ndash;print-libdir</option></primary></indexterm>
362         <listitem>
363           <para>Print the path to GHC's library directory.  This is
364           the top of the directory tree containing GHC's libraries,
365           interfaces, and include files (usually something like
366           <literal>/usr/local/lib/ghc-5.04</literal> on Unix).  This
367           is the value of
368           <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary>
369           </indexterm>in the package configuration file (see <xref
370           linkend="packages">).</para>
371         </listitem>
372       </varlistentry>
373
374     </variablelist>
375   </sect1>
376
377   <sect1 id="make-mode">
378     <title>Using <command>ghc</command> <option>&ndash;&ndash;make</option></title>
379
380     <indexterm><primary><option>&ndash;&ndash;make</option></primary>
381     </indexterm>
382     <indexterm><primary>separate compilation</primary>
383     </indexterm>
384     
385     <para>When given the <option>&ndash;&ndash;make</option> option, GHC will
386     build a multi-module Haskell program by following dependencies
387     from a single root module (usually <literal>Main</literal>).  For
388     example, if your <literal>Main</literal> module is in a file
389     called <filename>Main.hs</filename>, you could compile and link
390     the program like this:</para>
391
392 <screen>
393 ghc &ndash;&ndash;make Main.hs
394 </screen>
395
396     <para>The command line may contain any number of source file names
397     or module names; GHC will figure out all the modules in the
398     program by following the imports from these initial modules.  It
399     will then attempt to compile each module which is out of date, and
400     finally if there is a <literal>Main</literal> module, the program
401     will also be linked into an executable.</para>
402
403     <para>The main advantages to using <literal>ghc &ndash;&ndash;make</literal>
404     over traditional <literal>Makefile</literal>s are:</para>
405
406     <itemizedlist>
407       <listitem>
408         <para>GHC doesn't have to be restarted for each compilation,
409         which means it can cache information between compilations.
410         Compiling a muli-module program with <literal>ghc
411         &ndash;&ndash;make</literal> can be up to twice as fast as running
412         <literal>ghc</literal> individually on each source
413         file.</para>
414       </listitem>
415       <listitem>
416         <para>You don't have to write a
417         <literal>Makefile</literal>.</para>
418       </listitem>
419       <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary>
420       </indexterm>
421       <listitem>
422         <para>GHC re-calculates the dependencies each time it is
423         invoked, so the dependencies never get out of sync with the
424         source.</para>
425       </listitem>
426     </itemizedlist>
427
428     <para>Any of the command-line options described in the rest of
429     this chapter can be used with <option>&ndash;&ndash;make</option>, but note
430     that any options you give on the command line will apply to all
431     the source files compiled, so if you want any options to apply to
432     a single source file only, you'll need to use an
433     <literal>OPTIONS</literal> pragma (see <xref
434     linkend="source-file-options">).</para>
435
436     <para>If the program needs to be linked with additional objects
437     (say, some auxilliary C code), then the object files can be
438     given on the command line and GHC will include them when linking
439     the executable.</para>
440
441     <para>Note that GHC can only follow dependencies if it has the
442     source file available, so if your program includes a module for
443     which there is no source file, even if you have an object and an
444     interface file for the module, then GHC will complain.  The
445     exception to this rule is for package modules, which may or may
446     not have source files.</para>
447
448     <para>The source files for the program don't all need to be in the
449     same directory; the <option>-i</option> option can be used to add
450     directories to the search path (see <xref
451     linkend="options-finding-imports">).</para>
452
453   </sect1>
454   
455   <Sect1 id="options-order">
456     <title>GHC without <option>&ndash;&ndash;make</option></title>
457
458     <para>Without <option>&ndash;&ndash;make</option>, GHC will compile one or
459     more source files given on the command line.</para>
460
461     <para>The first phase to run is determined by each input-file
462     suffix, and the last phase is determined by a flag.  If no
463     relevant flag is present, then go all the way through linking.
464     This table summarises:</para>
465
466     <informaltable>
467       <tgroup cols="4">
468         <colspec align="left">
469         <colspec align="left">
470         <colspec align="left">
471         <colspec align="left">
472
473         <thead>
474           <row>
475             <entry>Phase of the compilation system</entry>
476             <entry>Suffix saying &ldquo;start here&rdquo;</entry>
477             <entry>Flag saying &ldquo;stop after&rdquo;</entry>
478             <entry>(suffix of) output file</entry>
479           </row>
480         </thead>
481         <tbody>
482           <row>
483             <entry>literate pre-processor</entry>
484             <entry><literal>.lhs</literal></entry>
485             <entry>-</entry>
486             <entry><literal>.hs</literal></entry>
487           </row>
488
489           <row>
490             <entry>C pre-processor (opt.)
491            </entry> 
492             <entry><literal>.hs</literal> (with
493             <option>-cpp</option>)</entry>
494             <entry><option>-E</option></entry>
495             <entry><literal>.hspp</literal></entry>
496           </row>
497           
498           <row>
499             <entry>Haskell compiler</entry>
500             <entry><literal>.hs</literal></entry>
501             <entry><option>-C</option>, <option>-S</option></entry>
502             <entry><literal>.hc</literal>, <literal>.s</literal></entry>
503           </row>
504
505           <row>
506             <entry>C compiler (opt.)</entry>
507             <entry><literal>.hc</literal> or <literal>.c</literal></entry>
508             <entry><option>-S</option></entry>
509             <entry><literal>.s</literal></entry>
510           </row>
511
512           <row>
513             <entry>assembler</entry>
514             <entry><literal>.s</literal></entry>
515             <entry><option>-c</option></entry>
516             <entry><literal>.o</literal></entry>
517           </row>
518           
519           <row>
520             <entry>linker</entry>
521             <entry><replaceable>other</replaceable></entry>
522             <entry>-</entry>
523             <entry><filename>a.out</filename></entry>
524           </row>
525         </tbody>
526       </tgroup>
527     </informaltable>
528
529     <indexterm><primary><option>-C</option></primary></indexterm>
530     <indexterm><primary><option>-E</option></primary></indexterm>
531     <indexterm><primary><option>-S</option></primary></indexterm>
532     <indexterm><primary><option>-c</option></primary></indexterm>
533
534     <para>Thus, a common invocation would be: <literal>ghc -c
535     Foo.hs</literal></para>
536
537     <para>Note: What the Haskell compiler proper produces depends on
538     whether a native-code generator<indexterm><primary>native-code
539     generator</primary></indexterm> is used (producing assembly
540     language) or not (producing C).  See <xref
541     linkend="options-codegen"> for more details.</para>
542
543     <para>Note: C pre-processing is optional, the
544     <option>-ccp</option><indexterm><primary><option>-cpp</option></primary>
545       </indexterm>flag turns it on.  See <xref
546     linkend="c-pre-processor"> for more details.</para>
547
548     <para>Note: The option <option>-E</option><IndexTerm><Primary>-E
549     option</Primary></IndexTerm> runs just the pre-processing passes
550     of the compiler, dumping the result in a file.  Note that this
551     differs from the previous behaviour of dumping the file to
552     standard output.</para>
553   </sect1>
554
555   <sect1 id="options-output">
556     <title>Re-directing the compilation output(s)</title>
557
558     <indexterm><primary>output-directing options</primary></indexterm>
559     <indexterm><primary>redirecting compilation output</primary></indexterm>
560
561
562     <variablelist>
563       <varlistentry>
564         <term><option>-o</option></term>
565         <indexterm><primary><option>-o</option></primary></indexterm>
566         <listitem>
567           <para>GHC's compiled output normally goes into a
568           <filename>.hc</filename>, <filename>.o</filename>, etc.,
569           file, depending on the last-run compilation phase.  The
570           option <option>-o foo</option><IndexTerm><Primary>-o
571           option</Primary></IndexTerm> re-directs the output of that
572           last-run phase to file <filename>foo</filename>.</para>
573
574           <para>Note: this &ldquo;feature&rdquo; can be
575           counterintuitive: <command>ghc -C -o foo.o foo.hs</command>
576           will put the intermediate C code in the file
577           <filename>foo.o</filename>, name notwithstanding!</para>
578
579           <para>Note: on Windows, if the result is an executable file, the
580           extension "<filename>.exe</filename>" is added if the specified filename
581             does not already have an extension.  Thus
582            <programlisting>
583                 ghc -o foo Main.hs
584            </programlisting>
585           will compile and link the module <filename>Main.hs</filename>, and put the
586           resulting executable in <filename>foo.exe</filename> (not <filename>foo</filename>).
587           </para>
588         </listitem>
589       </varlistentry>
590
591       <varlistentry>
592         <term><option>-odir</option></term>
593         <indexterm><primary><option>-odir</option></primary></indexterm>
594         <listitem>
595           <para>The <option>-o</option> option isn't of much use if
596           you have <emphasis>several</emphasis> input files&hellip;
597           Non-interface output files are normally put in the same
598           directory as their corresponding input file came from.  You
599           may specify that they be put in another directory using the
600           <option>-odir &lt;dir&gt;</option><IndexTerm><Primary>-odir
601           &lt;dir&gt; option</Primary></IndexTerm> (the &ldquo;Oh,
602           dear&rdquo; option).  For example:</para>
603
604 <Screen>
605 % ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
606 </Screen>
607
608           <para>The output files, <filename>Foo.o</filename>,
609           <filename>Bar.o</filename>, and
610           <filename>Bumble.o</filename> would be put into a
611           subdirectory named after the architecture of the executing
612           machine (<filename>sun4</filename>,
613           <filename>mips</filename>, etc).  The directory must already
614           exist; it won't be created.</para>
615
616           <para>Note that the <option>-odir</option> option does
617           <emphasis>not</emphasis> affect where the interface files
618           are put.  In the above example, they would still be put in
619           <filename>parse/Foo.hi</filename>,
620           <filename>parse/Bar.hi</filename>, and
621           <filename>gurgle/Bumble.hi</filename>.</para>
622         </listitem>
623       </varlistentry>
624
625       <varlistentry>
626         <term><option>-ohi</option>  <replaceable>file</replaceable></term>
627         <indexterm><primary><option>-ohi</option></primary>
628         </indexterm>
629         <listitem>
630           <para>The interface output may be directed to another file
631           <filename>bar2/Wurble.iface</filename> with the option
632           <option>-ohi bar2/Wurble.iface</option> (not
633           recommended).</para>
634
635           <para>WARNING: if you redirect the interface file somewhere
636           that GHC can't find it, then the recompilation checker may
637           get confused (at the least, you won't get any recompilation
638           avoidance).  We recommend using a combination of
639           <option>-hidir</option> and <option>-hisuf</option> options
640           instead, if possible.</para>
641
642           <para>To avoid generating an interface at all, you could use
643           this option to redirect the interface into the bit bucket:
644           <literal>-ohi /dev/null</literal>, for example.</para>
645         </listitem>
646       </varlistentry>
647       
648       <varlistentry>
649         <term><option>-hidir</option>  <replaceable>directory</replaceable></term>
650         <indexterm><primary><option>-hidir</option></primary>
651         </indexterm>
652         <listitem>
653           <para>Redirects all generated interface files into
654           <replaceable>directory</replaceable>, instead of the default
655           which is to place the interface file in the same directory
656           as the source file.</para>
657         </listitem>
658       </varlistentry>
659
660       <varlistentry>
661         <term><option>-osuf</option> <replaceable>suffix</replaceable></term>
662         <term><option>-hisuf</option> <replaceable>suffix</replaceable></term>
663         <term><option>-hcsuf</option> <replaceable>suffix</replaceable></term>
664         <indexterm><primary><option>-osuf</option></primary></indexterm>
665         <indexterm><primary><option>-hisuf</option></primary></indexterm>
666         <indexterm><primary><option>-hcsuf</option></primary></indexterm>
667         <listitem>
668           <para>EXOTICA: The <option>-osuf</option>
669           <replaceable>suffix</replaceable> will change the
670           <literal>.o</literal> file suffix for object files to
671           whatever you specify.  We use this when compiling libraries,
672           so that objects for the profiling versions of the libraries
673           don't clobber the normal ones.</para>
674
675           <para>Similarly, the <option>-hisuf</option>
676           <replaceable>suffix</replaceable> will change the
677           <literal>.hi</literal> file suffix for non-system interface
678           files (see <XRef LinkEnd="hi-options">).</para>
679
680           <para>Finally, the option <option>-hcsuf</option>
681           <replaceable>suffix</replaceable> will change the
682           <literal>.hc</literal> file suffix for compiler-generated
683           intermediate C files.</para>
684
685           <para>The <option>-hisuf</option>/<option>-osuf</option>
686           game is particularly useful if you want to compile a program both with and without
687             profiling, in the same directory.  You can say:
688             <Screen>
689               ghc ... 
690             </Screen>
691             to get the ordinary version, and
692             <Screen>
693               ghc ... -osuf prof.o -hisuf prof.hi -prof -auto-all
694             </Screen>
695             to get the profiled version.</para>
696         </listitem>
697       </varlistentry>
698     </variablelist>
699         
700     <sect2 id="keeping-intermediates">
701       <title>Keeping Intermediate Files</title>
702       <indexterm><primary>intermediate files, saving</primary>
703       </indexterm>
704       <indexterm><primary><literal>.hc</literal> files, saving</primary>
705       </indexterm>
706       <indexterm><primary><literal>.s</literal> files, saving</primary>
707       </indexterm>
708
709
710       <para>The following options are useful for keeping certain
711       intermediate files around, when normally GHC would throw these
712       away after compilation:</para>
713
714       <variablelist>
715         <varlistentry>
716           <term><option>-keep-hc-files</option></term>
717           <indexterm>
718             <primary><option>-keep-hc-files</option></primary>
719           </indexterm>
720           <listitem>
721             <para>Keep intermediate <literal>.hc</literal> files when
722             doing <literal>.hs</literal>-to-<literal>.o</literal>
723             compilations via C (NOTE: <literal>.hc</literal> files
724             aren't generated when using the native code generator, you
725             may need to use <option>-fvia-C</option> to force them
726             to be produced).</para>
727           </listitem>
728         </varlistentry>
729
730         <varlistentry>
731           <term><option>-keep-s-files</option></term>
732           <indexterm>
733             <primary><option>-keep-s-files</option></primary>
734           </indexterm>
735           <listitem>
736             <para>Keep intermediate <literal>.s</literal> files.</para>
737           </listitem>
738         </varlistentry>
739
740         <varlistentry>
741           <term><option>-keep-raw-s-files</option></term>
742           <indexterm>
743             <primary><option>-keep-raw-s-files</option></primary>
744           </indexterm>
745           <listitem>
746             <para>Keep intermediate <literal>.raw-s</literal> files.
747             These are the direct output from the C compiler, before
748             GHC does &ldquo;assembly mangling&rdquo; to produce the
749             <literal>.s</literal> file.  Again, these are not produced
750             when using the native code generator.</para>
751           </listitem>
752         </varlistentry>
753
754         <varlistentry>
755           <term><option>-keep-tmp-files</option></term>
756           <indexterm>
757             <primary><option>-keep-tmp-files</option></primary>
758           </indexterm>
759           <indexterm>
760             <primary>temporary files</primary>
761             <secondary>keeping</secondary>
762           </indexterm>
763           <listitem>
764             <para>Instructs the GHC driver not to delete any of its
765             temporary files, which it normally keeps in
766             <literal>/tmp</literal> (or possibly elsewhere; see <xref
767             linkend="temp-files">).  Running GHC with
768             <option>-v</option> will show you what temporary files
769             were generated along the way.</para>
770           </listitem>
771         </varlistentry>
772       </variablelist>
773     </sect2>
774
775     <sect2 id="temp-files">
776       <title>Redirecting temporary files</title>
777
778       <indexterm>
779         <primary>temporary files</primary>
780         <secondary>redirecting</secondary>
781       </indexterm>
782
783       <variablelist>
784         <varlistentry>
785           <term><option>-tmpdir</option></term>
786           <indexterm><primary><option>-tmpdir</option></primary></indexterm>
787           <listitem>
788             <para>If you have trouble because of running out of space
789             in <filename>/tmp</filename> (or wherever your
790             installation thinks temporary files should go), you may
791             use the <option>-tmpdir
792             &lt;dir&gt;</option><IndexTerm><Primary>-tmpdir
793             &lt;dir&gt; option</Primary></IndexTerm> option to specify
794             an alternate directory.  For example, <option>-tmpdir
795             .</option> says to put temporary files in the current
796             working directory.</para>
797
798             <para>Alternatively, use your <Constant>TMPDIR</Constant>
799             environment variable.<IndexTerm><Primary>TMPDIR
800             environment variable</Primary></IndexTerm> Set it to the
801             name of the directory where temporary files should be put.
802             GCC and other programs will honour the
803             <Constant>TMPDIR</Constant> variable as well.</para>
804
805             <para>Even better idea: Set the
806             <Constant>DEFAULT_TMPDIR</Constant> make variable when
807             building GHC, and never worry about
808             <Constant>TMPDIR</Constant> again. (see the build
809             documentation).</para>
810           </listitem>
811         </varlistentry>
812       </variablelist>
813     </sect2>
814
815   </sect1>
816
817   <sect1 id="options-sanity">
818     <title>Warnings and sanity-checking</title>
819
820     <indexterm><primary>sanity-checking options</primary></indexterm>
821     <indexterm><primary>warnings</primary></indexterm>
822
823
824     <para>GHC has a number of options that select which types of
825     non-fatal error messages, otherwise known as warnings, can be
826     generated during compilation.  By default, you get a standard set
827     of warnings which are generally likely to indicate bugs in your
828     program.  These are:
829     <option>-fwarn-overlpapping-patterns</option>,
830     <option>-fwarn-deprecations</option>,
831     <option>-fwarn-duplicate-exports</option>,
832     <option>-fwarn-missing-fields</option>, and
833     <option>-fwarn-missing-methods</option>.  The following flags are
834     simple ways to select standard &ldquo;packages&rdquo; of warnings:
835     </para>
836
837     <VariableList>
838
839       <varlistentry>
840         <term><option>-W</option>:</term>
841         <listitem>
842           <IndexTerm><Primary>-W option</Primary></IndexTerm>
843           <para>Provides the standard warnings plus
844           <option>-fwarn-incomplete-patterns</option>,
845           <option>-fwarn-unused-matches</option>,
846           <option>-fwarn-unused-imports</option>,
847           <option>-fwarn-misc</option>, and
848           <option>-fwarn-unused-binds</option>.</para>
849         </listitem>
850       </varlistentry>
851
852       <varlistentry>
853         <term><option>-w</option>:</term>
854         <listitem>
855           <IndexTerm><Primary><option>-w</option></Primary></IndexTerm>
856           <para>Turns off all warnings, including the standard ones.</para>
857         </listitem>
858       </varlistentry>
859
860       <varlistentry>
861         <term><option>-Wall</option>:</term>
862         <listitem>
863           <indexterm><primary><option>-Wall</option></primary></indexterm>
864           <para>Turns on all warning options.</para>
865         </listitem>
866       </varlistentry>
867
868       <varlistentry>
869         <term><option>-Werror</option>:</term>
870         <listitem>
871           <indexterm><primary><option>-Werror</option></primary></indexterm>
872           <para>Makes any warning into a fatal error. Useful so that you don't 
873             miss warnings when doing batch compilation. </para>
874         </listitem>
875       </varlistentry>
876
877     </variablelist>
878
879     <para>The full set of warning options is described below.  To turn
880     off any warning, simply give the corresponding
881     <option>-fno-warn-...</option> option on the command line.</para>
882
883     <variablelist>
884
885       <varlistentry>
886         <term><option>-fwarn-deprecations</option>:</term>
887         <listitem>
888           <indexterm><primary><option>-fwarn-deprecations</option></primary>
889           </indexterm>
890           <indexterm><primary>deprecations</primary></indexterm>
891           <para>Causes a warning to be emitted when a deprecated
892           function or type is used.  Entities can be marked as
893           deprecated using a pragma, see <xref
894           linkend="deprecated-pragma">.</para>
895         </listitem>
896       </varlistentry>
897
898       <varlistentry>
899         <term><option>-fwarn-duplicate-exports</option>:</term>
900         <listitem>
901           <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
902           <indexterm><primary>duplicate exports, warning</primary></indexterm>
903           <indexterm><primary>export lists, duplicates</primary></indexterm>
904
905           <para>Have the compiler warn about duplicate entries in
906           export lists. This is useful information if you maintain
907           large export lists, and want to avoid the continued export
908           of a definition after you've deleted (one) mention of it in
909           the export list.</para>
910
911           <para>This option is on by default.</para>
912         </listitem>
913       </varlistentry>
914
915       <varlistentry>
916         <term><option>-fwarn-hi-shadowing</option>:</term>
917         <listitem>
918           <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
919           <indexterm><primary>shadowing</primary>
920             <secondary>interface files</secondary></indexterm>
921
922           <para>Causes the compiler to emit a warning when a module or
923           interface file in the current directory is shadowing one
924           with the same module name in a library or other
925           directory.</para>
926         </listitem>
927       </varlistentry>
928
929       <varlistentry>
930         <term><option>-fwarn-incomplete-patterns</option>:</term>
931         <listitem>
932           <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
933           <indexterm><primary>incomplete patterns, warning</primary></indexterm>
934           <indexterm><primary>patterns, incomplete</primary></indexterm>
935
936           <para>Similarly for incomplete patterns, the function
937           <function>g</function> below will fail when applied to
938           non-empty lists, so the compiler will emit a warning about
939           this when <option>-fwarn-incomplete-patterns</option> is
940           enabled.</para>
941
942 <programlisting>
943 g [] = 2
944 </programlisting>
945
946           <para>This option isn't enabled be default because it can be
947           a bit noisy, and it doesn't always indicate a bug in the
948           program.  However, it's generally considered good practice
949           to cover all the cases in your functions.</para>
950         </listitem>
951       </varlistentry>
952
953       <varlistentry>
954         <term><option>-fwarn-misc</option>:</term>
955         <indexterm><primary><option>-fwarn-misc</option></primary></indexterm>
956         <listitem>
957           <para>Turns on warnings for various harmless but untidy
958           things.  This currently includes: importing a type with
959           <literal>(..)</literal> when the export is abstract, and
960           listing duplicate class assertions in a qualified type.</para>
961         </listitem>
962       </varlistentry>
963
964       <varlistentry>
965         <term><option>-fwarn-missing-fields</option>:</term>
966         <listitem>
967           <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
968           <indexterm><primary>missing fields, warning</primary></indexterm>
969           <indexterm><primary>fields, missing</primary></indexterm>
970
971           <para>This option is on by default, and warns you whenever
972           the construction of a labelled field constructor isn't
973           complete, missing initializers for one or more fields. While
974           not an error (the missing fields are initialised with
975           bottoms), it is often an indication of a programmer error.</para>
976         </listitem>
977       </varlistentry>
978
979       <varlistentry>
980         <term><option>-fwarn-missing-methods</option>:</term>
981         <listitem>
982           <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
983           <indexterm><primary>missing methods, warning</primary></indexterm>
984           <indexterm><primary>methods, missing</primary></indexterm>
985
986           <para>This option is on by default, and warns you whenever
987           an instance declaration is missing one or more methods, and
988           the corresponding class declaration has no default
989           declaration for them.</para>
990         </listitem>
991       </varlistentry>
992
993       <varlistentry>
994         <term><option>-fwarn-missing-signatures</option>:</term>
995         <listitem>
996           <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
997           <indexterm><primary>type signatures, missing</primary></indexterm>
998
999           <para>If you would like GHC to check that every top-level
1000           function/value has a type signature, use the
1001           <option>-fwarn-missing-signatures</option> option.  This
1002           option is off by default.</para>
1003         </listitem>
1004       </varlistentry>
1005
1006       <varlistentry>
1007         <term><option>-fwarn-name-shadowing</option>:</term>
1008         <listitem>
1009           <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1010           <indexterm><primary>shadowing, warning</primary></indexterm>
1011           
1012           <para>This option causes a warning to be emitted whenever an
1013           inner-scope value has the same name as an outer-scope value,
1014           i.e. the inner value shadows the outer one.  This can catch
1015           typographical errors that turn into hard-to-find bugs, e.g.,
1016           in the inadvertent cyclic definition <literal>let x = ... x
1017           ... in</literal>.</para>
1018
1019           <para>Consequently, this option does
1020           <emphasis>will</emphasis> complain about cyclic recursive
1021           definitions.</para>
1022         </listitem>
1023       </varlistentry>
1024
1025       <varlistentry>
1026         <term><option>-fwarn-overlapping-patterns</option>:</term>
1027         <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1028         <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1029         <indexterm><primary>patterns, overlapping</primary></indexterm>
1030         <listitem>
1031           <para>By default, the compiler will warn you if a set of
1032           patterns are overlapping, i.e.,</para>
1033
1034 <programlisting>
1035 f :: String -&#62; Int
1036 f []     = 0
1037 f (_:xs) = 1
1038 f "2"    = 2
1039 </programlisting>
1040
1041           <para>where the last pattern match in <Function>f</Function>
1042           won't ever be reached, as the second pattern overlaps
1043           it. More often than not, redundant patterns is a programmer
1044           mistake/error, so this option is enabled by default.</para>
1045         </listitem>
1046       </varlistentry>
1047
1048       <varlistentry>
1049         <term><option>-fwarn-simple-patterns</option>:</term>
1050         <listitem>
1051           <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1052           </indexterm>
1053           <para>Causes the compiler to warn about lambda-bound
1054           patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1055           Normally, these aren't treated as incomplete patterns by
1056           <option>-fwarn-incomplete-patterns</option>.</para>
1057         </listitem>
1058       </varlistentry>
1059
1060       <varlistentry>
1061         <term><option>-fwarn-type-defaults</option>:</term>
1062         <listitem>
1063           <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1064           <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1065           <para>Have the compiler warn/inform you where in your source
1066           the Haskell defaulting mechanism for numeric types kicks
1067           in. This is useful information when converting code from a
1068           context that assumed one default into one with another,
1069           e.g., the `default default' for Haskell 1.4 caused the
1070           otherwise unconstrained value <Constant>1</Constant> to be
1071           given the type <literal>Int</literal>, whereas Haskell 98
1072           defaults it to <literal>Integer</literal>.  This may lead to
1073           differences in performance and behaviour, hence the
1074           usefulness of being non-silent about this.</para>
1075
1076           <para>This warning is off by default.</para>
1077         </listitem>
1078       </varlistentry>
1079
1080       <varlistentry>
1081         <term><option>-fwarn-unused-binds</option>:</term>
1082         <listitem>
1083           <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1084           <indexterm><primary>unused binds, warning</primary></indexterm>
1085           <indexterm><primary>binds, unused</primary></indexterm>
1086           <para>Report any function definitions (and local bindings)
1087           which are unused.  For top-level functions, the warning is
1088           only given if the binding is not exported.</para>
1089         </listitem>
1090       </varlistentry>
1091
1092       <varlistentry>
1093         <term><option>-fwarn-unused-imports</option>:</term>
1094         <listitem>
1095           <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1096           <indexterm><primary>unused imports, warning</primary></indexterm>
1097           <indexterm><primary>imports, unused</primary></indexterm>
1098
1099           <para>Report any objects that are explicitly imported but
1100           never used.</para>
1101         </listitem>
1102       </varlistentry>
1103
1104       <varlistentry>
1105         <term><option>-fwarn-unused-matches</option>:</term>
1106         <listitem>
1107           <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1108           <indexterm><primary>unused matches, warning</primary></indexterm>
1109           <indexterm><primary>matches, unused</primary></indexterm>
1110
1111           <para>Report all unused variables which arise from pattern
1112           matches, including patterns consisting of a single variable.
1113           For instance <literal>f x y = []</literal> would report
1114           <VarName>x</VarName> and <VarName>y</VarName> as unused.  To
1115           eliminate the warning, all unused variables can be replaced
1116           with wildcards.</para>
1117         </listitem>
1118       </varlistentry>
1119
1120     </VariableList>
1121
1122     <para>If you're feeling really paranoid, the
1123     <option>-dcore-lint</option>
1124     option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1125     is a good choice.  It turns on heavyweight intra-pass
1126     sanity-checking within GHC.  (It checks GHC's sanity, not
1127     yours.)</para>
1128
1129   </sect1>
1130
1131   &separate;
1132   &packages;
1133
1134   <sect1 id="options-optimise">
1135     <title>Optimisation (code improvement)</title>
1136
1137     <indexterm><primary>optimisation</primary></indexterm>
1138     <indexterm><primary>improvement, code</primary></indexterm>
1139
1140     <para>The <option>-O*</option> options specify convenient
1141     &ldquo;packages&rdquo; of optimisation flags; the
1142     <option>-f*</option> options described later on specify
1143     <emphasis>individual</emphasis> optimisations to be turned on/off;
1144     the <option>-m*</option> options specify
1145     <emphasis>machine-specific</emphasis> optimisations to be turned
1146     on/off.</para>
1147
1148     <sect2 id="optimise-pkgs">
1149       <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1150
1151       <para>There are <emphasis>many</emphasis> options that affect
1152       the quality of code produced by GHC.  Most people only have a
1153       general goal, something like &ldquo;Compile quickly&rdquo; or
1154       &ldquo;Make my program run like greased lightning.&rdquo; The
1155       following &ldquo;packages&rdquo; of optimisations (or lack
1156       thereof) should suffice.</para>
1157
1158       <para>Once you choose a <option>-O*</option>
1159       &ldquo;package,&rdquo; stick with it&mdash;don't chop and
1160       change.  Modules' interfaces <emphasis>will</emphasis> change
1161       with a shift to a new <option>-O*</option> option, and you may
1162       have to recompile a large chunk of all importing modules before
1163       your program can again be run safely (see <XRef
1164       LinkEnd="recomp">).</para>
1165
1166       <variablelist>
1167
1168         <varlistentry>
1169           <term>No <option>-O*</option>-type option specified:</term>
1170           <indexterm><primary>-O* not specified</primary></indexterm>
1171           <listitem>
1172             <para>This is taken to mean: &ldquo;Please compile
1173             quickly; I'm not over-bothered about compiled-code
1174             quality.&rdquo; So, for example: <command>ghc -c
1175             Foo.hs</command></para>
1176           </listitem>
1177         </varlistentry>
1178
1179         <varlistentry>
1180           <term><option>-O0</option>:</term>
1181           <indexterm><primary><option>-O0</option></primary></indexterm>
1182           <listitem>
1183             <para>Means &ldquo;turn off all optimisation&rdquo;,
1184             reverting to the same settings as if no
1185             <option>-O</option> options had been specified.  Saying
1186             <option>-O0</option> can be useful if
1187             eg. <command>make</command> has inserted a
1188             <option>-O</option> on the command line already.</para>
1189           </listitem>
1190         </varlistentry>
1191
1192         <varlistentry>
1193           <term><option>-O</option> or <option>-O1</option>:</term>
1194           <indexterm><primary>-O option</primary></indexterm>
1195           <indexterm><primary>-O1 option</primary></indexterm>
1196           <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1197           <listitem>
1198             <para>Means: &ldquo;Generate good-quality code without
1199             taking too long about it.&rdquo; Thus, for example:
1200             <command>ghc -c -O Main.lhs</command></para>
1201           </listitem>
1202         </varlistentry>
1203
1204         <varlistentry>
1205           <term><option>-O2</option>:</term>
1206           <indexterm><primary>-O2 option</primary></indexterm>
1207           <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1208           <listitem>
1209             <para>Means: &ldquo;Apply every non-dangerous
1210             optimisation, even if it means significantly longer
1211             compile times.&rdquo;</para>
1212
1213             <para>The avoided &ldquo;dangerous&rdquo; optimisations
1214             are those that can make runtime or space
1215             <emphasis>worse</emphasis> if you're unlucky.  They are
1216             normally turned on or off individually.</para>
1217
1218             <para>At the moment, <option>-O2</option> is
1219             <emphasis>unlikely</emphasis> to produce better code than
1220             <option>-O</option>.</para>
1221           </listitem>
1222         </varlistentry>
1223
1224         <varlistentry>
1225           <term><option>-Ofile &lt;file&gt;</option>:</term>
1226           <indexterm><primary>-Ofile &lt;file&gt; option</primary></indexterm>
1227           <indexterm><primary>optimising, customised</primary></indexterm>
1228           <listitem>
1229             <para>(NOTE: not supported yet in GHC 5.x.  Please ask if
1230             you're interested in this.)</para>
1231             
1232             <para>For those who need <emphasis>absolute</emphasis>
1233             control over <emphasis>exactly</emphasis> what options are
1234             used (e.g., compiler writers, sometimes :-), a list of
1235             options can be put in a file and then slurped in with
1236             <option>-Ofile</option>.</para>
1237
1238             <para>In that file, comments are of the
1239             <literal>&num;</literal>-to-end-of-line variety; blank
1240             lines and most whitespace is ignored.</para>
1241
1242             <para>Please ask if you are baffled and would like an
1243             example of <option>-Ofile</option>!</para>
1244           </listitem>
1245         </varlistentry>
1246       </variablelist>
1247
1248       <para>We don't use a <option>-O*</option> flag for day-to-day
1249       work.  We use <option>-O</option> to get respectable speed;
1250       e.g., when we want to measure something.  When we want to go for
1251       broke, we tend to use <option>-O -fvia-C</option> (and we go for
1252       lots of coffee breaks).</para>
1253
1254       <para>The easiest way to see what <option>-O</option> (etc.)
1255       &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1256       then stand back in amazement.</para>
1257     </sect2>
1258
1259     <sect2 id="options-f">
1260       <title><option>-f*</option>: platform-independent flags</title>
1261
1262       <indexterm><primary>-f* options (GHC)</primary></indexterm>
1263       <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1264
1265       <para>These flags turn on and off individual optimisations.
1266       They are normally set via the <option>-O</option> options
1267       described above, and as such, you shouldn't need to set any of
1268       them explicitly (indeed, doing so could lead to unexpected
1269       results).  However, there are one or two that may be of
1270       interest:</para>
1271
1272       <variablelist>
1273         <varlistentry>
1274           <term><option>-fexcess-precision</option>:</term>
1275           <listitem>
1276             <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1277             <para>When this option is given, intermediate floating
1278             point values can have a <emphasis>greater</emphasis>
1279             precision/range than the final type.  Generally this is a
1280             good thing, but some programs may rely on the exact
1281             precision/range of
1282             <literal>Float</literal>/<literal>Double</literal> values
1283             and should not use this option for their compilation.</para>
1284           </listitem>
1285         </varlistentry>
1286
1287         <varlistentry>
1288           <term><option>-fignore-asserts</option>:</term>
1289           <listitem>
1290             <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1291             <para>Causes GHC to ignore uses of the function
1292             <literal>Exception.assert</literal> in source code (in
1293             other words, rewriting <literal>Exception.assert p
1294             e</literal> to <literal>e</literal> (see <xref
1295             linkend="sec-assertions">).  This flag is turned on by
1296             <option>-O</option>.
1297             </para>
1298           </listitem>
1299         </varlistentry>
1300
1301         <varlistentry>
1302           <term><option>-fno-strictness</option></term>
1303           <indexterm><primary><option>-fno-strictness</option></primary>
1304           </indexterm>
1305           <listitem>
1306             <para>Turns off the strictness analyser; sometimes it eats
1307             too many cycles.</para>
1308           </listitem>
1309         </varlistentry>
1310
1311         <varlistentry>
1312           <term><option>-fno-cpr-analyse</option></term>
1313           <indexterm><primary><option>-fno-cpr-analyse</option></primary>
1314           </indexterm>
1315           <listitem>
1316             <para>Turns off the CPR (constructed product result)
1317             analysis; it is somewhat experimental.</para>
1318           </listitem>
1319         </varlistentry>
1320
1321         <varlistentry>
1322           <term><option>-funbox-strict-fields</option>:</term>
1323           <listitem>
1324             <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1325             <indexterm><primary>strict constructor fields</primary></indexterm>
1326             <indexterm><primary>constructor fields, strict</primary></indexterm>
1327
1328             <para>This option causes all constructor fields which are
1329             marked strict (i.e. &ldquo;!&rdquo;) to be unboxed or
1330             unpacked if possible.  For example:</para>
1331
1332 <ProgramListing>
1333 data T = T !Float !Float
1334 </ProgramListing>
1335
1336             <para>will create a constructor <literal>T</literal>
1337             containing two unboxed floats if the
1338             <option>-funbox-strict-fields</option> flag is given.
1339             This may not always be an optimisation: if the
1340             <Function>T</Function> constructor is scrutinised and the
1341             floats passed to a non-strict function for example, they
1342             will have to be reboxed (this is done automatically by the
1343             compiler).</para>
1344
1345             <para>This option should only be used in conjunction with
1346             <option>-O</option>, in order to expose unfoldings to the
1347             compiler so the reboxing can be removed as often as
1348             possible.  For example:</para>
1349
1350 <ProgramListing>
1351 f :: T -&#62; Float
1352 f (T f1 f2) = f1 + f2
1353 </ProgramListing>
1354
1355             <para>The compiler will avoid reboxing
1356             <Function>f1</Function> and <Function>f2</Function> by
1357             inlining <Function>+</Function> on floats, but only when
1358             <option>-O</option> is on.</para>
1359
1360             <para>Any single-constructor data is eligible for
1361             unpacking; for example</para>
1362
1363 <ProgramListing>
1364 data T = T !(Int,Int)
1365 </ProgramListing>
1366
1367             <para>will store the two <literal>Int</literal>s directly
1368             in the <Function>T</Function> constructor, by flattening
1369             the pair.  Multi-level unpacking is also supported:</para>
1370
1371 <ProgramListing>
1372 data T = T !S
1373 data S = S !Int !Int
1374 </ProgramListing>
1375
1376             <para>will store two unboxed <literal>Int&num;</literal>s
1377             directly in the <Function>T</Function> constructor.</para>
1378           </listitem>
1379         </varlistentry>
1380
1381         <varlistentry>
1382           <term><option>-funfolding-update-in-place&lt;n&gt;</option></term>
1383           <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1384           <listitem>
1385             <para>Switches on an experimental "optimisation".
1386             Switching it on makes the compiler a little keener to
1387             inline a function that returns a constructor, if the
1388             context is that of a thunk.
1389 <ProgramListing>
1390    x = plusInt a b
1391 </ProgramListing>
1392             If we inlined plusInt we might get an opportunity to use
1393             update-in-place for the thunk 'x'.</para>
1394           </listitem>
1395         </varlistentry>
1396
1397         <varlistentry>
1398           <term><option>-funfolding-creation-threshold&lt;n&gt;</option>:</term>
1399           <listitem>
1400             <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1401             <indexterm><primary>inlining, controlling</primary></indexterm>
1402             <indexterm><primary>unfolding, controlling</primary></indexterm>
1403             
1404             <para>(Default: 45) Governs the maximum size that GHC will 
1405             allow a function unfolding to be.   (An unfolding has a
1406             &ldquo;size&rdquo; that reflects the cost in terms of
1407             &ldquo;code bloat&rdquo; of expanding that unfolding at
1408             at a call site. A bigger function would be assigned a
1409             bigger cost.) </para>
1410
1411             <para> Consequences: (a) nothing larger than this will be
1412             inlined (unless it has an INLINE pragma); (b) nothing
1413             larger than this will be spewed into an interface
1414             file. </para>
1415
1416
1417             <para> Increasing this figure is more likely to result in longer
1418             compile times than faster code.  The next option is more
1419             useful:</para>
1420           </listitem>
1421         </varlistentry>
1422
1423         <varlistentry>
1424           <term><option>-funfolding-use-threshold&lt;n&gt;</option>:</term>
1425           <listitem>
1426             <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1427             <indexterm><primary>inlining, controlling</primary></indexterm>
1428             <indexterm><primary>unfolding, controlling</primary></indexterm>
1429
1430             <para>(Default: 8) This is the magic cut-off figure for
1431             unfolding: below this size, a function definition will be
1432             unfolded at the call-site, any bigger and it won't.  The
1433             size computed for a function depends on two things: the
1434             actual size of the expression minus any discounts that
1435             apply (see <option>-funfolding-con-discount</option>).</para>
1436           </listitem>
1437         </varlistentry>
1438       </variablelist>
1439
1440     </sect2>
1441     
1442   </sect1>
1443   
1444   &phases;  
1445   
1446   <sect1 id="sec-using-concurrent">
1447 <title>Using Concurrent Haskell</title>
1448
1449              <indexterm><primary>Concurrent Haskell&mdash;use</primary></indexterm>
1450
1451 <para>
1452 GHC supports Concurrent Haskell by default, without requiring a
1453 special option or libraries compiled in a certain way.  To get access
1454 to the support libraries for Concurrent Haskell, just import
1455 <literal>Control.Concurrent</literal> (details are in the accompanying
1456 library documentation).</para>
1457
1458 <para>
1459 RTS options are provided for modifying the behaviour of the threaded
1460 runtime system.  See <XRef LinkEnd="parallel-rts-opts">.
1461 </para>
1462
1463 <para>
1464 Concurrent Haskell is described in more detail in the documentation
1465 for the <literal>Control.Concurrent</literal> module.
1466 </para>
1467
1468 </Sect1>
1469
1470 <Sect1 id="sec-using-parallel">
1471 <title>Using Parallel Haskell</title>
1472
1473 <para>
1474 <indexterm><primary>Parallel Haskell&mdash;use</primary></indexterm>
1475 </para>
1476
1477 <para>
1478 &lsqb;You won't be able to execute parallel Haskell programs unless PVM3
1479 (Parallel Virtual Machine, version 3) is installed at your site.&rsqb;
1480 </Para>
1481
1482 <para>
1483 To compile a Haskell program for parallel execution under PVM, use the
1484 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
1485 option</Primary></IndexTerm> both when compiling <Emphasis>and
1486 linking</Emphasis>.  You will probably want to <Literal>import
1487 Parallel</Literal> into your Haskell modules.
1488 </Para>
1489
1490 <para>
1491 To run your parallel program, once PVM is going, just invoke it
1492 &ldquo;as normal&rdquo;.  The main extra RTS option is
1493 <Option>-qp&lt;n&gt;</Option>, to say how many PVM
1494 &ldquo;processors&rdquo; your program to run on.  (For more details of
1495 all relevant RTS options, please see <XRef
1496 LinkEnd="parallel-rts-opts">.)
1497 </para>
1498
1499 <para>
1500 In truth, running Parallel Haskell programs and getting information
1501 out of them (e.g., parallelism profiles) is a battle with the vagaries of
1502 PVM, detailed in the following sections.
1503 </para>
1504
1505 <Sect2 id="pvm-dummies">
1506 <Title>Dummy's guide to using PVM</Title>
1507
1508 <para>
1509 <indexterm><primary>PVM, how to use</primary></indexterm>
1510 <indexterm><primary>Parallel Haskell&mdash;PVM use</primary></indexterm>
1511 Before you can run a parallel program under PVM, you must set the
1512 required environment variables (PVM's idea, not ours); something like,
1513 probably in your <filename>.cshrc</filename> or equivalent:
1514
1515 <ProgramListing>
1516 setenv PVM_ROOT /wherever/you/put/it
1517 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
1518 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
1519 </ProgramListing>
1520
1521 </para>
1522
1523 <para>
1524 Creating and/or controlling your &ldquo;parallel machine&rdquo; is a purely-PVM
1525 business; nothing specific to Parallel Haskell. The following paragraphs
1526 describe how to configure your parallel machine interactively.
1527 </Para>
1528
1529 <Para>
1530 If you use parallel Haskell regularly on the same machine configuration it
1531 is a good idea to maintain a file with all machine names and to make the
1532 environment variable PVM_HOST_FILE point to this file. Then you can avoid
1533 the interactive operations described below by just saying
1534 </Para>
1535
1536 <ProgramListing>
1537 pvm $PVM_HOST_FILE
1538 </ProgramListing>
1539
1540 <Para>
1541 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
1542 machine.  You can then do various things to control/monitor your
1543 &ldquo;parallel machine;&rdquo; the most useful being:
1544 </para>
1545
1546 <para>
1547 <InformalTable>
1548 <TGroup Cols=2>
1549 <ColSpec Align="Left">
1550 <TBody>
1551
1552 <row>
1553 <entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></entry>
1554 <entry>exit <command>pvm</command>, leaving it running</entry>
1555 </row>
1556
1557 <row>
1558 <entry><command>halt</command></entry>
1559 <entry>kill off this &ldquo;parallel machine&rdquo; &amp; exit</entry>
1560 </row>
1561
1562 <row>
1563 <entry><command>add &lt;host&gt;</command></entry>
1564 <entry>add <command>&lt;host&gt;</command> as a processor</entry>
1565 </row>
1566
1567 <row>
1568 <entry><command>delete &lt;host&gt;</command></entry>
1569 <entry>delete <command>&lt;host&gt;</command></entry>
1570 </row>
1571
1572 <row>
1573 <entry><command>reset</command></entry>
1574 <entry>kill what's going, but leave PVM up</entry>
1575 </row>
1576
1577 <row>
1578 <entry><command>conf</command></entry>
1579 <entry>list the current configuration</entry>
1580 </row>
1581
1582 <row>
1583 <entry><command>ps</command></entry>
1584 <entry>report processes' status</entry>
1585 </row>
1586
1587 <row>
1588 <entry><command>pstat &lt;pid&gt;</command></entry>
1589 <entry>status of a particular process</entry>
1590 </row>
1591
1592 </TBody>
1593 </TGroup>
1594 </InformalTable>
1595 </para>
1596
1597 <para>
1598 The PVM documentation can tell you much, much more about <command>pvm</command>!
1599 </para>
1600
1601 </sect2>
1602
1603 <Sect2 id="par-profiles">
1604 <Title>Parallelism profiles</Title>
1605
1606 <para>
1607 <indexterm><primary>parallelism profiles</primary></indexterm>
1608 <indexterm><primary>profiles, parallelism</primary></indexterm>
1609 <indexterm><primary>visualisation tools</primary></indexterm>
1610 </para>
1611
1612 <para>
1613 With Parallel Haskell programs, we usually don't care about the
1614 results&mdash;only with &ldquo;how parallel&rdquo; it was!  We want pretty pictures.
1615 </para>
1616
1617 <Para>
1618 Parallelism profiles (&agrave; la <Command>hbcpp</Command>) can be generated with the
1619 <Option>-qP</Option><IndexTerm><Primary>-qP RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option.  The
1620 per-processor profiling info is dumped into files named
1621 <Filename>&lt;full-path&gt;&lt;program&gt;.gr</Filename>.  These are then munged into a PostScript picture,
1622 which you can then display.  For example, to run your program
1623 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
1624 </Para>
1625
1626 <Para>
1627
1628 <Screen>
1629 <prompt>&dollar;</prompt> ./a.out +RTS -qP -qp8
1630 <prompt>&dollar;</prompt> grs2gr *.???.gr &#62; temp.gr # combine the 8 .gr files into one
1631 <prompt>&dollar;</prompt> gr2ps -O temp.gr              # cvt to .ps; output in temp.ps
1632 <prompt>&dollar;</prompt> ghostview -seascape temp.ps   # look at it!
1633 </Screen>
1634
1635 </Para>
1636
1637 <para>
1638 The scripts for processing the parallelism profiles are distributed
1639 in <filename>ghc/utils/parallel/</filename>.
1640 </para>
1641
1642 </sect2>
1643
1644 <Sect2>
1645 <Title>Other useful info about running parallel programs</Title>
1646
1647 <Para>
1648 The &ldquo;garbage-collection statistics&rdquo; RTS options can be useful for
1649 seeing what parallel programs are doing.  If you do either
1650 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
1651 you'll get mutator, garbage-collection, etc., times on standard
1652 error. The standard error of all PE's other than the `main thread'
1653 appears in <filename>/tmp/pvml.nnn</filename>, courtesy of PVM.
1654 </para>
1655
1656 <para>
1657 Whether doing <option>+RTS -Sstderr</option> or not, a handy way to watch
1658 what's happening overall is: <command>tail -f /tmp/pvml.nnn</command>.
1659 </para>
1660
1661 </sect2>
1662
1663 <Sect2 id="parallel-rts-opts">
1664 <title>RTS options for Concurrent/Parallel Haskell
1665 </title>
1666
1667 <para>
1668 <indexterm><primary>RTS options, concurrent</primary></indexterm>
1669 <indexterm><primary>RTS options, parallel</primary></indexterm>
1670 <indexterm><primary>Concurrent Haskell&mdash;RTS options</primary></indexterm>
1671 <indexterm><primary>Parallel Haskell&mdash;RTS options</primary></indexterm>
1672 </para>
1673
1674 <para>
1675 Besides the usual runtime system (RTS) options
1676 (<XRef LinkEnd="runtime-control">), there are a few options particularly
1677 for concurrent/parallel execution.
1678 </para>
1679
1680 <para>
1681 <VariableList>
1682
1683 <VarListEntry>
1684 <Term><Option>-qp&lt;N&gt;</Option>:</Term>
1685 <ListItem>
1686 <Para>
1687 <IndexTerm><Primary>-qp&lt;N&gt; RTS option</Primary></IndexTerm>
1688 (PARALLEL ONLY) Use <Literal>&lt;N&gt;</Literal> PVM processors to run this program;
1689 the default is 2.
1690 </para>
1691 </listitem>
1692 </varlistentry>
1693 <varlistentry>
1694 <term><option>-C[&lt;us&gt;]</option>:</term>
1695 <listitem>
1696 <para>
1697 <indexterm><primary>-C&lt;us&gt; RTS option</primary></indexterm> Sets
1698 the context switch interval to <literal>&lt;s&gt;</literal> seconds.
1699 A context switch will occur at the next heap block allocation after
1700 the timer expires (a heap block allocation occurs every 4k of
1701 allocation).  With <option>-C0</option> or <option>-C</option>,
1702 context switches will occur as often as possible (at every heap block
1703 allocation).  By default, context switches occur every 20ms
1704 milliseconds.  Note that GHC's internal timer ticks every 20ms, and
1705 the context switch timer is always a multiple of this timer, so 20ms
1706 is the maximum granularity available for timed context switches.
1707 </para>
1708 </listitem>
1709 </varlistentry>
1710 <varlistentry>
1711 <term><option>-q[v]</option>:</term>
1712 <listitem>
1713 <para>
1714 <indexterm><primary>-q RTS option</primary></indexterm>
1715 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
1716 in the file <FIlename>&lt;program&gt;.qp</FIlename>.  In the style of <command>hbcpp</command>, this profile
1717 records the movement of threads between the green (runnable) and red
1718 (blocked) queues.  If you specify the verbose suboption (<option>-qv</option>), the
1719 green queue is split into green (for the currently running thread
1720 only) and amber (for other runnable threads).  We do not recommend
1721 that you use the verbose suboption if you are planning to use the
1722 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
1723 check (with <Option>-C</Option>).
1724 -->
1725 </Para>
1726 </ListItem>
1727 </VarListEntry>
1728 <VarListEntry>
1729 <Term><Option>-qt&lt;num&gt;</Option>:</Term>
1730 <ListItem>
1731 <Para>
1732 <IndexTerm><Primary>-qt&lt;num&gt; RTS option</Primary></IndexTerm>
1733 (PARALLEL ONLY) Limit the thread pool size, i.e. the number of concurrent
1734 threads per processor to <Literal>&lt;num&gt;</Literal>.  The default is
1735 32.  Each thread requires slightly over 1K <Emphasis>words</Emphasis> in
1736 the heap for thread state and stack objects.  (For 32-bit machines, this
1737 translates to 4K bytes, and for 64-bit machines, 8K bytes.)
1738 </Para>
1739 </ListItem>
1740 </VarListEntry>
1741 <!-- no more -HWL
1742 <VarListEntry>
1743 <Term><Option>-d</Option>:</Term>
1744 <ListItem>
1745 <Para>
1746 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
1747 (PARALLEL ONLY) Turn on debugging.  It pops up one xterm (or GDB, or
1748 something&hellip;) per PVM processor.  We use the standard <Command>debugger</Command>
1749 script that comes with PVM3, but we sometimes meddle with the
1750 <Command>debugger2</Command> script.  We include ours in the GHC distribution,
1751 in <Filename>ghc/utils/pvm/</Filename>.
1752 </Para>
1753 </ListItem>
1754 </VarListEntry>
1755 -->
1756 <VarListEntry>
1757 <Term><Option>-qe&lt;num&gt;</Option>:</Term>
1758 <ListItem>
1759 <Para>
1760 <IndexTerm><Primary>-qe&lt;num&gt; RTS option
1761 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Limit the spark pool size
1762 i.e. the number of pending sparks per processor to
1763 <Literal>&lt;num&gt;</Literal>. The default is 100. A larger number may be
1764 appropriate if your program generates large amounts of parallelism
1765 initially.
1766 </Para>
1767 </ListItem>
1768 </VarListEntry>
1769 <VarListEntry>
1770 <Term><Option>-qQ&lt;num&gt;</Option>:</Term>
1771 <ListItem>
1772 <Para>
1773 <IndexTerm><Primary>-qQ&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1774 (PARALLEL ONLY) Set the size of packets transmitted between processors
1775 to <Literal>&lt;num&gt;</Literal>. The default is 1024 words. A larger number may be
1776 appropriate if your machine has a high communication cost relative to
1777 computation speed.
1778 </Para>
1779 </ListItem>
1780 </VarListEntry>
1781 <VarListEntry>
1782 <Term><Option>-qh&lt;num&gt;</Option>:</Term>
1783 <ListItem>
1784 <Para>
1785 <IndexTerm><Primary>-qh&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1786 (PARALLEL ONLY) Select a packing scheme. Set the number of non-root thunks to pack in one packet to
1787 &lt;num&gt;-1 (0 means infinity). By default GUM uses full-subgraph
1788 packing, i.e. the entire subgraph with the requested closure as root is
1789 transmitted (provided it fits into one packet). Choosing a smaller value
1790 reduces the amount of pre-fetching of work done in GUM. This can be
1791 advantageous for improving data locality but it can also worsen the balance
1792 of the load in the system. 
1793 </Para>
1794 </ListItem>
1795 </VarListEntry>
1796 <VarListEntry>
1797 <Term><Option>-qg&lt;num&gt;</Option>:</Term>
1798 <ListItem>
1799 <Para>
1800 <IndexTerm><Primary>-qg&lt;num&gt; RTS option
1801 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Select a globalisation
1802 scheme. This option affects the
1803 generation of global addresses when transferring data. Global addresses are
1804 globally unique identifiers required to maintain sharing in the distributed
1805 graph structure. Currently this is a binary option. With &lt;num&gt;=0 full globalisation is used
1806 (default). This means a global address is generated for every closure that
1807 is transmitted. With &lt;num&gt;=1 a thunk-only globalisation scheme is
1808 used, which generated global address only for thunks. The latter case may
1809 lose sharing of data but has a reduced overhead in packing graph structures
1810 and maintaining internal tables of global addresses.
1811 </Para>
1812 </ListItem>
1813 </VarListEntry>
1814 </VariableList>
1815 </para>
1816
1817 </sect2>
1818
1819 </Sect1>
1820
1821   <sect1 id="options-platform">
1822     <title>Platform-specific Flags</title>
1823
1824     <indexterm><primary>-m* options</primary></indexterm>
1825     <indexterm><primary>platform-specific options</primary></indexterm>
1826     <indexterm><primary>machine-specific options</primary></indexterm>
1827
1828     <para>Some flags only make sense for particular target
1829     platforms.</para>
1830
1831     <variablelist>
1832
1833       <varlistentry>
1834         <term><option>-mv8</option>:</term>
1835         <listitem>
1836           <para>(SPARC machines)<indexterm><primary>-mv8 option (SPARC
1837           only)</primary></indexterm> Means to pass the like-named
1838           option to GCC; it says to use the Version 8 SPARC
1839           instructions, notably integer multiply and divide.  The
1840           similiar <option>-m*</option> GCC options for SPARC also
1841           work, actually.</para>
1842         </listitem>
1843       </varlistentry>
1844
1845       <varlistentry>
1846         <term><option>-monly-[32]-regs</option>:</term>
1847         <listitem>
1848           <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1849           option (iX86 only)</primary></indexterm> GHC tries to
1850           &ldquo;steal&rdquo; four registers from GCC, for performance
1851           reasons; it almost always works.  However, when GCC is
1852           compiling some modules with four stolen registers, it will
1853           crash, probably saying:
1854
1855 <Screen>
1856 Foo.hc:533: fixed or forbidden register was spilled.
1857 This may be due to a compiler bug or to impossible asm
1858 statements or clauses.
1859 </Screen>
1860
1861           Just give some registers back with
1862           <option>-monly-N-regs</option>.  Try `3' first, then `2'.
1863           If `2' doesn't work, please report the bug to us.</para>
1864         </listitem>
1865       </varlistentry>
1866     </variablelist>
1867
1868   </sect1>
1869
1870 &runtime;
1871
1872 <sect1 id="ext-core">
1873   <title>Generating External Core Files</title>
1874
1875   <indexterm><primary>intermediate code generation</primary></indexterm>
1876
1877   <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format) 
1878   to a file as a side-effect of compilation. Core files, which are given the suffix
1879   <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1880   tools.  The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz"
1881   <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>, 
1882   and sample tools (in Haskell)
1883   for manipulating Core files are available in the GHC source distribution 
1884   directory <literal>/fptools/ghc/utils/ext-core</literal>.  
1885   Note that the format of <literal>.hcr</literal> 
1886   files is <emphasis>different</emphasis> (though similar) to the Core output format generated 
1887   for debugging purposes (<xref linkend="options-debugging">).</para>
1888
1889     <variablelist>
1890
1891         <varlistentry>
1892           <term><option>-fext-core</option></term>
1893           <indexterm>
1894             <primary><option>-fext-core</option></primary>
1895           </indexterm>
1896           <listitem>
1897             <para>Generate <literal>.hcr</literal> files.</para>
1898           </listitem>
1899         </varlistentry>
1900
1901     </variablelist>
1902
1903 </sect1>
1904
1905 &debug;
1906 &flags;
1907
1908 </Chapter>
1909
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