65ca0acdf2d253d9bd6b3cbd328121a4d5ab575d
[ghc.git] / docs / users_guide / using.xml
1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="using-ghc">
3 <title>Using GHC</title>
4
5 <indexterm><primary>GHC, using</primary></indexterm>
6 <indexterm><primary>using GHC</primary></indexterm>
7
8 <sect1>
9 <title>Options overview</title>
10
11 <para>GHC's behaviour is controlled by
12 <firstterm>options</firstterm>, which for historical reasons are
13 also sometimes referred to as command-line flags or arguments.
14 Options can be specified in three ways:</para>
15
16 <sect2>
17 <title>Command-line arguments</title>
18
19 <indexterm><primary>structure, command-line</primary></indexterm>
20 <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
21 <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
22
23 <para>An invocation of GHC takes the following form:</para>
24
25 <screen>
26 ghc [argument...]
27 </screen>
28
29 <para>Command-line arguments are either options or file names.</para>
30
31 <para>Command-line options begin with <literal>-</literal>.
32 They may <emphasis>not</emphasis> be grouped:
33 <option>-vO</option> is different from <option>-v -O</option>.
34 Options need not precede filenames: e.g., <literal>ghc *.o -o
35 foo</literal>. All options are processed and then applied to
36 all files; you cannot, for example, invoke <literal>ghc -c -O1
37 Foo.hs -O2 Bar.hs</literal> to apply different optimisation
38 levels to the files <filename>Foo.hs</filename> and
39 <filename>Bar.hs</filename>.</para>
40 </sect2>
41
42 <sect2 id="source-file-options">
43 <title>Command line options in source files</title>
44
45 <indexterm><primary>source-file options</primary></indexterm>
46
47 <para>Sometimes it is useful to make the connection between a
48 source file and the command-line options it requires quite
49 tight. For instance, if a Haskell source file deliberately
50 uses name shadowing, it should be compiled with the
51 <option>-fno-warn-name-shadowing</option> option. Rather than maintaining
52 the list of per-file options in a <filename>Makefile</filename>,
53 it is possible to do this directly in the source file using the
54 <literal>OPTIONS_GHC</literal> pragma <indexterm><primary>OPTIONS_GHC
55 pragma</primary></indexterm>:</para>
56
57 <programlisting>
58 {-# OPTIONS_GHC -fno-warn-name-shadowing #-}
59 module X where
60 ...
61 </programlisting>
62
63 <para><literal>OPTIONS_GHC</literal> is a <emphasis>file-header pragma</emphasis>
64 (see <xref linkend="pragmas"/>).</para>
65
66 <para>Only <emphasis>dynamic</emphasis> flags can be used in an <literal>OPTIONS_GHC</literal> pragma
67 (see <xref linkend="static-dynamic-flags"/>).</para>
68
69 <para>Note that your command shell does not
70 get to the source file options, they are just included literally
71 in the array of command-line arguments the compiler
72 maintains internally, so you'll be desperately disappointed if
73 you try to glob etc. inside <literal>OPTIONS_GHC</literal>.</para>
74
75 <para>NOTE: the contents of OPTIONS_GHC are appended to the
76 command-line options, so options given in the source file
77 override those given on the command-line.</para>
78
79 <para>It is not recommended to move all the contents of your
80 Makefiles into your source files, but in some circumstances, the
81 <literal>OPTIONS_GHC</literal> pragma is the Right Thing. (If you
82 use <option>-keep-hc-file</option> and have OPTION flags in
83 your module, the OPTIONS_GHC will get put into the generated .hc
84 file).</para>
85 </sect2>
86
87 <sect2>
88 <title>Setting options in GHCi</title>
89
90 <para>Options may also be modified from within GHCi, using the
91 <literal>:set</literal> command. See <xref linkend="ghci-set"/>
92 for more details.</para>
93 </sect2>
94 </sect1>
95
96 <sect1 id="static-dynamic-flags">
97 <title>Static, Dynamic, and Mode options</title>
98 <indexterm><primary>static</primary><secondary>options</secondary>
99 </indexterm>
100 <indexterm><primary>dynamic</primary><secondary>options</secondary>
101 </indexterm>
102 <indexterm><primary>mode</primary><secondary>options</secondary>
103 </indexterm>
104
105 <para>Each of GHC's command line options is classified as
106 <firstterm>static</firstterm>, <firstterm>dynamic</firstterm> or
107 <firstterm>mode</firstterm>:</para>
108
109 <variablelist>
110 <varlistentry>
111 <term>Mode flags</term>
112 <listitem>
113 <para>For example, <option>--make</option> or <option>-E</option>.
114 There may only be a single mode flag on the command line. The
115 available modes are listed in <xref linkend="modes"/>.</para>
116 </listitem>
117 </varlistentry>
118 <varlistentry>
119 <term>Dynamic Flags</term>
120 <listitem>
121 <para>Most non-mode flags fall into this category. A dynamic flag
122 may be used on the command line, in a
123 <literal>OPTIONS_GHC</literal> pragma in a source file, or set
124 using <literal>:set</literal> in GHCi.</para>
125 </listitem>
126 </varlistentry>
127 <varlistentry>
128 <term>Static Flags</term>
129 <listitem>
130 <para>A few flags are "static", which means they can only be used on
131 the command-line, and remain in force over the entire GHC/GHCi
132 run.</para>
133 </listitem>
134 </varlistentry>
135 </variablelist>
136
137 <para>The flag reference tables (<xref
138 linkend="flag-reference"/>) lists the status of each flag.</para>
139
140 <para>There are a few flags that are static except that they can
141 also be used with GHCi's <literal>:set</literal> command; these
142 are listed as &ldquo;static/<literal>:set</literal>&rdquo; in the
143 table.</para>
144 </sect1>
145
146 <sect1 id="file-suffixes">
147 <title>Meaningful file suffixes</title>
148
149 <indexterm><primary>suffixes, file</primary></indexterm>
150 <indexterm><primary>file suffixes for GHC</primary></indexterm>
151
152 <para>File names with &ldquo;meaningful&rdquo; suffixes (e.g.,
153 <filename>.lhs</filename> or <filename>.o</filename>) cause the
154 &ldquo;right thing&rdquo; to happen to those files.</para>
155
156 <variablelist>
157
158 <varlistentry>
159 <term><filename>.hs</filename></term>
160 <listitem>
161 <para>A Haskell module.</para>
162 </listitem>
163 </varlistentry>
164
165 <varlistentry>
166 <term>
167 <filename>.lhs</filename>
168 <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
169 </term>
170 <listitem>
171 <para>A &ldquo;literate Haskell&rdquo; module.</para>
172 </listitem>
173 </varlistentry>
174
175 <varlistentry>
176 <term><filename>.hi</filename></term>
177 <listitem>
178 <para>A Haskell interface file, probably
179 compiler-generated.</para>
180 </listitem>
181 </varlistentry>
182
183 <varlistentry>
184 <term><filename>.hc</filename></term>
185 <listitem>
186 <para>Intermediate C file produced by the Haskell
187 compiler.</para>
188 </listitem>
189 </varlistentry>
190
191 <varlistentry>
192 <term><filename>.c</filename></term>
193 <listitem>
194 <para>A C&nbsp;file not produced by the Haskell
195 compiler.</para>
196 </listitem>
197 </varlistentry>
198
199 <varlistentry>
200 <term><filename>.s</filename></term>
201 <listitem>
202 <para>An assembly-language source file, usually produced by
203 the compiler.</para>
204 </listitem>
205 </varlistentry>
206
207 <varlistentry>
208 <term><filename>.o</filename></term>
209 <listitem>
210 <para>An object file, produced by an assembler.</para>
211 </listitem>
212 </varlistentry>
213 </variablelist>
214
215 <para>Files with other suffixes (or without suffixes) are passed
216 straight to the linker.</para>
217
218 </sect1>
219
220 <sect1 id="modes">
221 <title>Modes of operation</title>
222
223 <para>GHC's behaviour is firstly controlled by a mode flag. Only
224 one of these flags may be given, but it does not necessarily need
225 to be the first option on the command-line. The available modes
226 are:</para>
227
228 <variablelist>
229 <varlistentry>
230 <term>
231 <cmdsynopsis><command>ghc --interactive</command>
232 </cmdsynopsis>
233 <indexterm><primary>interactive mode</primary></indexterm>
234 <indexterm><primary>ghci</primary></indexterm>
235 </term>
236 <listitem>
237 <para>Interactive mode, which is also available as
238 <command>ghci</command>. Interactive mode is described in
239 more detail in <xref linkend="ghci"/>.</para>
240 </listitem>
241 </varlistentry>
242
243 <varlistentry>
244 <term>
245 <cmdsynopsis><command>ghc --make</command>
246 </cmdsynopsis>
247 <indexterm><primary>make mode</primary></indexterm>
248 <indexterm><primary><option>&ndash;&ndash;make</option></primary></indexterm>
249 </term>
250 <listitem>
251 <para>In this mode, GHC will build a multi-module Haskell
252 program automatically, figuring out dependencies for itself.
253 If you have a straightforward Haskell program, this is
254 likely to be much easier, and faster, than using
255 <command>make</command>. Make mode is described in <xref
256 linkend="make-mode"/>.</para>
257 </listitem>
258 </varlistentry>
259
260 <varlistentry>
261 <term>
262 <cmdsynopsis><command>ghc -e</command>
263 <arg choice='plain'><replaceable>expr</replaceable></arg>
264 </cmdsynopsis>
265 <indexterm><primary>eval mode</primary></indexterm>
266 </term>
267 <listitem>
268 <para>Expression-evaluation mode. This is very similar to
269 interactive mode, except that there is a single expression
270 to evaluate (<replaceable>expr</replaceable>) which is given
271 on the command line. See <xref linkend="eval-mode"/> for
272 more details.</para>
273 </listitem>
274 </varlistentry>
275
276 <varlistentry>
277 <term>
278 <cmdsynopsis>
279 <command>ghc -E</command>
280 <command>ghc -c</command>
281 <command>ghc -S</command>
282 <command>ghc -c</command>
283 </cmdsynopsis>
284 <indexterm><primary><option>-E</option></primary></indexterm>
285 <indexterm><primary><option>-C</option></primary></indexterm>
286 <indexterm><primary><option>-S</option></primary></indexterm>
287 <indexterm><primary><option>-c</option></primary></indexterm>
288 </term>
289 <listitem>
290 <para>This is the traditional batch-compiler mode, in which
291 GHC can compile source files one at a time, or link objects
292 together into an executable. This mode also applies if
293 there is no other mode flag specified on the command line,
294 in which case it means that the specified files should be
295 compiled and then linked to form a program. See <xref
296 linkend="options-order"/>.</para>
297 </listitem>
298 </varlistentry>
299
300 <varlistentry>
301 <term>
302 <cmdsynopsis>
303 <command>ghc -M</command>
304 </cmdsynopsis>
305 <indexterm><primary>dependency-generation mode</primary></indexterm>
306 </term>
307 <listitem>
308 <para>Dependency-generation mode. In this mode, GHC can be
309 used to generate dependency information suitable for use in
310 a <literal>Makefile</literal>. See <xref
311 linkend="makefile-dependencies"/>.</para>
312 </listitem>
313 </varlistentry>
314
315 <varlistentry>
316 <term>
317 <cmdsynopsis>
318 <command>ghc --mk-dll</command>
319 </cmdsynopsis>
320 <indexterm><primary>DLL-creation mode</primary></indexterm>
321 </term>
322 <listitem>
323 <para>DLL-creation mode (Windows only). See <xref
324 linkend="win32-dlls-create"/>.</para>
325 </listitem>
326 </varlistentry>
327
328 <varlistentry>
329 <term>
330 <cmdsynopsis>
331 <command>ghc --help</command> <command>ghc -?</command>
332 </cmdsynopsis>
333 <indexterm><primary><option>&ndash;&ndash;help</option></primary></indexterm>
334 </term>
335 <listitem>
336 <para>Cause GHC to spew a long usage message to standard
337 output and then exit.</para>
338 </listitem>
339 </varlistentry>
340
341 <varlistentry>
342 <term>
343 <cmdsynopsis>
344 <command>ghc --show-iface <replaceable>file</replaceable></command>
345 </cmdsynopsis>
346 <indexterm><primary><option>&ndash;&ndash;--show-iface</option></primary></indexterm>
347 </term>
348 <listitem>
349 <para>Read the interface in
350 <replaceable>file</replaceable> and dump it as text to
351 <literal>stdout</literal>. For example <literal>ghc --show-iface M.hi</literal>.</para>
352 </listitem>
353 </varlistentry>
354
355 <varlistentry>
356 <term>
357 <cmdsynopsis>
358 <command>ghc --supported-languages</command>
359 </cmdsynopsis>
360 <indexterm><primary><option>&ndash;&ndash;supported-languages</option></primary></indexterm>
361 </term>
362 <listitem>
363 <para>Print the supported language extensions.</para>
364 </listitem>
365 </varlistentry>
366
367 <varlistentry>
368 <term>
369 <cmdsynopsis>
370 <command>ghc --info</command>
371 </cmdsynopsis>
372 <indexterm><primary><option>&ndash;&ndash;info</option></primary></indexterm>
373 </term>
374 <listitem>
375 <para>Print information about the compiler.</para>
376 </listitem>
377 </varlistentry>
378
379 <varlistentry>
380 <term>
381 <cmdsynopsis>
382 <command>ghc --version</command>
383 <command>ghc -V</command>
384 </cmdsynopsis>
385 <indexterm><primary><option>-V</option></primary></indexterm>
386 <indexterm><primary><option>&ndash;&ndash;version</option></primary></indexterm>
387 </term>
388 <listitem>
389 <para>Print a one-line string including GHC's version number.</para>
390 </listitem>
391 </varlistentry>
392
393 <varlistentry>
394 <term>
395 <cmdsynopsis>
396 <command>ghc --numeric-version</command>
397 </cmdsynopsis>
398 <indexterm><primary><option>&ndash;&ndash;numeric-version</option></primary></indexterm>
399 </term>
400 <listitem>
401 <para>Print GHC's numeric version number only.</para>
402 </listitem>
403 </varlistentry>
404
405 <varlistentry>
406 <term>
407 <cmdsynopsis>
408 <command>ghc --print-libdir</command>
409 </cmdsynopsis>
410 <indexterm><primary><option>&ndash;&ndash;print-libdir</option></primary></indexterm>
411 </term>
412 <listitem>
413 <para>Print the path to GHC's library directory. This is
414 the top of the directory tree containing GHC's libraries,
415 interfaces, and include files (usually something like
416 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
417 is the value of
418 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
419 in the package configuration file
420 (see <xref linkend="packages"/>).</para>
421 </listitem>
422 </varlistentry>
423
424 </variablelist>
425
426 <sect2 id="make-mode">
427 <title>Using <command>ghc</command> <option>&ndash;&ndash;make</option></title>
428 <indexterm><primary><option>&ndash;&ndash;make</option></primary></indexterm>
429 <indexterm><primary>separate compilation</primary></indexterm>
430
431 <para>When given the <option>&ndash;&ndash;make</option> option,
432 GHC will build a multi-module Haskell program by following
433 dependencies from one or more root modules (usually just
434 <literal>Main</literal>). For example, if your
435 <literal>Main</literal> module is in a file called
436 <filename>Main.hs</filename>, you could compile and link the
437 program like this:</para>
438
439 <screen>
440 ghc &ndash;&ndash;make Main.hs
441 </screen>
442
443 <para>The command line may contain any number of source file
444 names or module names; GHC will figure out all the modules in
445 the program by following the imports from these initial modules.
446 It will then attempt to compile each module which is out of
447 date, and finally, if there is a <literal>Main</literal> module,
448 the program will also be linked into an executable.</para>
449
450 <para>The main advantages to using <literal>ghc
451 &ndash;&ndash;make</literal> over traditional
452 <literal>Makefile</literal>s are:</para>
453
454 <itemizedlist>
455 <listitem>
456 <para>GHC doesn't have to be restarted for each compilation,
457 which means it can cache information between compilations.
458 Compiling a multi-module program with <literal>ghc
459 &ndash;&ndash;make</literal> can be up to twice as fast as
460 running <literal>ghc</literal> individually on each source
461 file.</para>
462 </listitem>
463 <listitem>
464 <para>You don't have to write a <literal>Makefile</literal>.</para>
465 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
466 </listitem>
467 <listitem>
468 <para>GHC re-calculates the dependencies each time it is
469 invoked, so the dependencies never get out of sync with the
470 source.</para>
471 </listitem>
472 </itemizedlist>
473
474 <para>Any of the command-line options described in the rest of
475 this chapter can be used with
476 <option>&ndash;&ndash;make</option>, but note that any options
477 you give on the command line will apply to all the source files
478 compiled, so if you want any options to apply to a single source
479 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
480 pragma (see <xref linkend="source-file-options"/>).</para>
481
482 <para>If the program needs to be linked with additional objects
483 (say, some auxiliary C code), then the object files can be
484 given on the command line and GHC will include them when linking
485 the executable.</para>
486
487 <para>Note that GHC can only follow dependencies if it has the
488 source file available, so if your program includes a module for
489 which there is no source file, even if you have an object and an
490 interface file for the module, then GHC will complain. The
491 exception to this rule is for package modules, which may or may
492 not have source files.</para>
493
494 <para>The source files for the program don't all need to be in
495 the same directory; the <option>-i</option> option can be used
496 to add directories to the search path (see <xref
497 linkend="search-path"/>).</para>
498 </sect2>
499
500 <sect2 id="eval-mode">
501 <title>Expression evaluation mode</title>
502
503 <para>This mode is very similar to interactive mode, except that
504 there is a single expression to evaluate which is specified on
505 the command line as an argument to the <option>-e</option>
506 option:</para>
507
508 <screen>
509 ghc -e <replaceable>expr</replaceable>
510 </screen>
511
512 <para>Haskell source files may be named on the command line, and
513 they will be loaded exactly as in interactive mode. The
514 expression is evaluated in the context of the loaded
515 modules.</para>
516
517 <para>For example, to load and run a Haskell program containing
518 a module <literal>Main</literal>, we might say</para>
519
520 <screen>
521 ghc -e Main.main Main.hs
522 </screen>
523
524 <para>or we can just use this mode to evaluate expressions in
525 the context of the <literal>Prelude</literal>:</para>
526
527 <screen>
528 $ ghc -e "interact (unlines.map reverse.lines)"
529 hello
530 olleh
531 </screen>
532 </sect2>
533
534 <sect2 id="options-order">
535 <title>Batch compiler mode</title>
536
537 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
538 given on the command line.</para>
539
540 <para>The first phase to run is determined by each input-file
541 suffix, and the last phase is determined by a flag. If no
542 relevant flag is present, then go all the way through to linking.
543 This table summarises:</para>
544
545 <informaltable>
546 <tgroup cols="4">
547 <colspec align="left"/>
548 <colspec align="left"/>
549 <colspec align="left"/>
550 <colspec align="left"/>
551
552 <thead>
553 <row>
554 <entry>Phase of the compilation system</entry>
555 <entry>Suffix saying &ldquo;start here&rdquo;</entry>
556 <entry>Flag saying &ldquo;stop after&rdquo;</entry>
557 <entry>(suffix of) output file</entry>
558 </row>
559 </thead>
560 <tbody>
561 <row>
562 <entry>literate pre-processor</entry>
563 <entry><literal>.lhs</literal></entry>
564 <entry>-</entry>
565 <entry><literal>.hs</literal></entry>
566 </row>
567
568 <row>
569 <entry>C pre-processor (opt.) </entry>
570 <entry><literal>.hs</literal> (with
571 <option>-cpp</option>)</entry>
572 <entry><option>-E</option></entry>
573 <entry><literal>.hspp</literal></entry>
574 </row>
575
576 <row>
577 <entry>Haskell compiler</entry>
578 <entry><literal>.hs</literal></entry>
579 <entry><option>-C</option>, <option>-S</option></entry>
580 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
581 </row>
582
583 <row>
584 <entry>C compiler (opt.)</entry>
585 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
586 <entry><option>-S</option></entry>
587 <entry><literal>.s</literal></entry>
588 </row>
589
590 <row>
591 <entry>assembler</entry>
592 <entry><literal>.s</literal></entry>
593 <entry><option>-c</option></entry>
594 <entry><literal>.o</literal></entry>
595 </row>
596
597 <row>
598 <entry>linker</entry>
599 <entry><replaceable>other</replaceable></entry>
600 <entry>-</entry>
601 <entry><filename>a.out</filename></entry>
602 </row>
603 </tbody>
604 </tgroup>
605 </informaltable>
606
607 <indexterm><primary><option>-C</option></primary></indexterm>
608 <indexterm><primary><option>-E</option></primary></indexterm>
609 <indexterm><primary><option>-S</option></primary></indexterm>
610 <indexterm><primary><option>-c</option></primary></indexterm>
611
612 <para>Thus, a common invocation would be: </para>
613
614 <screen>
615 ghc -c Foo.hs</screen>
616
617 <para>to compile the Haskell source file
618 <filename>Foo.hs</filename> to an object file
619 <filename>Foo.o</filename>.</para>
620
621 <para>Note: What the Haskell compiler proper produces depends on
622 whether a native-code generator<indexterm><primary>native-code
623 generator</primary></indexterm> is used (producing assembly
624 language) or not (producing C). See <xref
625 linkend="options-codegen"/> for more details.</para>
626
627 <para>Note: C pre-processing is optional, the
628 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
629 flag turns it on. See <xref linkend="c-pre-processor"/> for more
630 details.</para>
631
632 <para>Note: The option <option>-E</option><indexterm><primary>-E
633 option</primary></indexterm> runs just the pre-processing passes
634 of the compiler, dumping the result in a file.</para>
635
636 <sect3 id="overriding-suffixes">
637 <title>Overriding the default behaviour for a file</title>
638
639 <para>As described above, the way in which a file is processed by GHC
640 depends on its suffix. This behaviour can be overridden using the
641 <option>-x</option> option:</para>
642
643 <variablelist>
644 <varlistentry>
645 <term><option>-x</option> <replaceable>suffix</replaceable>
646 <indexterm><primary><option>-x</option></primary>
647 </indexterm></term>
648 <listitem>
649 <para>Causes all files following this option on the command
650 line to be processed as if they had the suffix
651 <replaceable>suffix</replaceable>. For example, to compile a
652 Haskell module in the file <literal>M.my-hs</literal>,
653 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
654 </listitem>
655 </varlistentry>
656 </variablelist>
657 </sect3>
658
659 </sect2>
660 </sect1>
661
662 <sect1 id="options-help">
663 <title>Help and verbosity options</title>
664
665 <indexterm><primary>help options</primary></indexterm>
666 <indexterm><primary>verbosity options</primary></indexterm>
667
668 <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
669 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
670 <variablelist>
671 <varlistentry>
672 <term>
673 <option>-n</option>
674 <indexterm><primary><option>-n</option></primary></indexterm>
675 </term>
676 <listitem>
677 <para>Does a dry-run, i.e. GHC goes through all the motions
678 of compiling as normal, but does not actually run any
679 external commands.</para>
680 </listitem>
681 </varlistentry>
682
683 <varlistentry>
684 <term>
685 <option>-v</option>
686 <indexterm><primary><option>-v</option></primary></indexterm>
687 </term>
688 <listitem>
689 <para>The <option>-v</option> option makes GHC
690 <emphasis>verbose</emphasis>: it reports its version number
691 and shows (on stderr) exactly how it invokes each phase of
692 the compilation system. Moreover, it passes the
693 <option>-v</option> flag to most phases; each reports its
694 version number (and possibly some other information).</para>
695
696 <para>Please, oh please, use the <option>-v</option> option
697 when reporting bugs! Knowing that you ran the right bits in
698 the right order is always the first thing we want to
699 verify.</para>
700 </listitem>
701 </varlistentry>
702
703 <varlistentry>
704 <term>
705 <option>-v</option><replaceable>n</replaceable>
706 <indexterm><primary><option>-v</option></primary></indexterm>
707 </term>
708 <listitem>
709 <para>To provide more control over the compiler's verbosity,
710 the <option>-v</option> flag takes an optional numeric
711 argument. Specifying <option>-v</option> on its own is
712 equivalent to <option>-v3</option>, and the other levels
713 have the following meanings:</para>
714
715 <variablelist>
716 <varlistentry>
717 <term><option>-v0</option></term>
718 <listitem>
719 <para>Disable all non-essential messages (this is the
720 default).</para>
721 </listitem>
722 </varlistentry>
723
724 <varlistentry>
725 <term><option>-v1</option></term>
726 <listitem>
727 <para>Minimal verbosity: print one line per
728 compilation (this is the default when
729 <option>&ndash;&ndash;make</option> or
730 <option>&ndash;&ndash;interactive</option> is on).</para>
731 </listitem>
732 </varlistentry>
733
734 <varlistentry>
735 <term><option>-v2</option></term>
736 <listitem>
737 <para>Print the name of each compilation phase as it
738 is executed. (equivalent to
739 <option>-dshow-passes</option>).</para>
740 </listitem>
741 </varlistentry>
742
743 <varlistentry>
744 <term><option>-v3</option></term>
745 <listitem>
746 <para>The same as <option>-v2</option>, except that in
747 addition the full command line (if appropriate) for
748 each compilation phase is also printed.</para>
749 </listitem>
750 </varlistentry>
751
752 <varlistentry>
753 <term><option>-v4</option></term>
754 <listitem>
755 <para>The same as <option>-v3</option> except that the
756 intermediate program representation after each
757 compilation phase is also printed (excluding
758 preprocessed and C/assembly files).</para>
759 </listitem>
760 </varlistentry>
761 </variablelist>
762 </listitem>
763 </varlistentry>
764
765 <varlistentry>
766 <term><option>-ferror-spans</option>
767 <indexterm><primary><option>-ferror-spans</option></primary>
768 </indexterm>
769 </term>
770 <listitem>
771 <para>Causes GHC to emit the full source span of the
772 syntactic entity relating to an error message. Normally, GHC
773 emits the source location of the start of the syntactic
774 entity only.</para>
775
776 <para>For example:</para>
777
778 <screen>test.hs:3:6: parse error on input `where'</screen>
779
780 <para>becomes:</para>
781
782 <screen>test296.hs:3:6-10: parse error on input `where'</screen>
783
784 <para>And multi-line spans are possible too:</para>
785
786 <screen>test.hs:(5,4)-(6,7):
787 Conflicting definitions for `a'
788 Bound at: test.hs:5:4
789 test.hs:6:7
790 In the binding group for: a, b, a</screen>
791
792 <para>Note that line numbers start counting at one, but
793 column numbers start at zero. This choice was made to
794 follow existing convention (i.e. this is how Emacs does
795 it).</para>
796 </listitem>
797 </varlistentry>
798
799 <varlistentry>
800 <term><option>-H</option><replaceable>size</replaceable>
801 <indexterm><primary><option>-H</option></primary></indexterm>
802 </term>
803 <listitem>
804 <para>Set the minimum size of the heap to
805 <replaceable>size</replaceable>.
806 This option is equivalent to
807 <literal>+RTS&nbsp;-H<replaceable>size</replaceable></literal>,
808 see <xref linkend="rts-options-gc" />.
809 </para>
810 </listitem>
811 </varlistentry>
812
813 <varlistentry>
814 <term><option>-Rghc-timing</option>
815 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
816 </term>
817 <listitem>
818 <para>Prints a one-line summary of timing statistics for the
819 GHC run. This option is equivalent to
820 <literal>+RTS&nbsp;-tstderr</literal>, see <xref
821 linkend="rts-options-gc" />.
822 </para>
823 </listitem>
824 </varlistentry>
825 </variablelist>
826 </sect1>
827
828 &separate;
829
830 <sect1 id="options-sanity">
831 <title>Warnings and sanity-checking</title>
832
833 <indexterm><primary>sanity-checking options</primary></indexterm>
834 <indexterm><primary>warnings</primary></indexterm>
835
836
837 <para>GHC has a number of options that select which types of
838 non-fatal error messages, otherwise known as warnings, can be
839 generated during compilation. By default, you get a standard set
840 of warnings which are generally likely to indicate bugs in your
841 program. These are:
842 <option>-fwarn-overlapping-patterns</option>,
843 <option>-fwarn-warnings-deprecations</option>,
844 <option>-fwarn-deprecated-flags</option>,
845 <option>-fwarn-duplicate-exports</option>,
846 <option>-fwarn-missing-fields</option>,
847 <option>-fwarn-missing-methods</option>, and
848 <option>-fwarn-dodgy-foreign-imports</option>. The following
849 flags are
850 simple ways to select standard &ldquo;packages&rdquo; of warnings:
851 </para>
852
853 <variablelist>
854
855 <varlistentry>
856 <term><option>-W</option>:</term>
857 <listitem>
858 <indexterm><primary>-W option</primary></indexterm>
859 <para>Provides the standard warnings plus
860 <option>-fwarn-incomplete-patterns</option>,
861 <option>-fwarn-dodgy-imports</option>,
862 <option>-fwarn-unused-matches</option>,
863 <option>-fwarn-unused-imports</option>, and
864 <option>-fwarn-unused-binds</option>.</para>
865 </listitem>
866 </varlistentry>
867
868 <varlistentry>
869 <term><option>-Wall</option>:</term>
870 <listitem>
871 <indexterm><primary><option>-Wall</option></primary></indexterm>
872 <para>Turns on all warning options that indicate potentially
873 suspicious code. The warnings that are
874 <emphasis>not</emphasis> enabled by <option>-Wall</option>
875 are
876 <option>-fwarn-simple-patterns</option>,
877 <option>-fwarn-tabs</option>,
878 <option>-fwarn-incomplete-record-updates</option>,
879 <option>-fwarn-monomorphism-restriction</option>, and
880 <option>-fwarn-implicit-prelude</option>.</para>
881 </listitem>
882 </varlistentry>
883
884 <varlistentry>
885 <term><option>-w</option>:</term>
886 <listitem>
887 <indexterm><primary><option>-w</option></primary></indexterm>
888 <para>Turns off all warnings, including the standard ones and
889 those that <literal>-Wall</literal> doesn't enable.</para>
890 </listitem>
891 </varlistentry>
892
893 <varlistentry>
894 <term><option>-Werror</option>:</term>
895 <listitem>
896 <indexterm><primary><option>-Werror</option></primary></indexterm>
897 <para>Makes any warning into a fatal error. Useful so that you don't
898 miss warnings when doing batch compilation. </para>
899 </listitem>
900 </varlistentry>
901
902 <varlistentry>
903 <term><option>-Wwarn</option>:</term>
904 <listitem>
905 <indexterm><primary><option>-Wwarn</option></primary></indexterm>
906 <para>Warnings are treated only as warnings, not as errors. This is
907 the default, but can be useful to negate a
908 <option>-Werror</option> flag.</para>
909 </listitem>
910 </varlistentry>
911
912 </variablelist>
913
914 <para>The full set of warning options is described below. To turn
915 off any warning, simply give the corresponding
916 <option>-fno-warn-...</option> option on the command line.</para>
917
918 <variablelist>
919
920 <varlistentry>
921 <term><option>-fwarn-unrecognised-pragmas</option>:</term>
922 <listitem>
923 <indexterm><primary><option>-fwarn-unrecognised-pragmas</option></primary>
924 </indexterm>
925 <indexterm><primary>warnings</primary></indexterm>
926 <indexterm><primary>pragmas</primary></indexterm>
927 <para>Causes a warning to be emitted when a
928 pragma that GHC doesn't recognise is used. As well as pragmas
929 that GHC itself uses, GHC also recognises pragmas known to be used
930 by other tools, e.g. <literal>OPTIONS_HUGS</literal> and
931 <literal>DERIVE</literal>.</para>
932
933 <para>This option is on by default.</para>
934 </listitem>
935 </varlistentry>
936
937 <varlistentry>
938 <term><option>-fwarn-warnings-deprecations</option>:</term>
939 <listitem>
940 <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
941 </indexterm>
942 <indexterm><primary>warnings</primary></indexterm>
943 <indexterm><primary>deprecations</primary></indexterm>
944 <para>Causes a warning to be emitted when a
945 module, function or type with a WARNING or DEPRECATED pragma
946 is used. See <xref linkend="warning-deprecated-pragma"/> for more
947 details on the pragmas.</para>
948
949 <para>This option is on by default.</para>
950 </listitem>
951 </varlistentry>
952
953 <varlistentry>
954 <term><option>-fwarn-deprecated-flags</option>:</term>
955 <listitem>
956 <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
957 </indexterm>
958 <indexterm><primary>deprecated-flags</primary></indexterm>
959 <para>Causes a warning to be emitted when a deprecated
960 commandline flag is used.</para>
961
962 <para>This option is on by default.</para>
963 </listitem>
964 </varlistentry>
965
966 <varlistentry>
967 <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
968 <listitem>
969 <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
970 </indexterm>
971 <para>Causes a warning to be emitted for foreign imports of
972 the following form:</para>
973 <programlisting>
974 foreign import "f" f :: FunPtr t
975 </programlisting>
976 <para>on the grounds that it probably should be</para>
977 <programlisting>
978 foreign import "&amp;f" f :: FunPtr t
979 </programlisting>
980 <para>The first form declares that `f` is a (pure) C
981 function that takes no arguments and returns a pointer to a
982 C function with type `t`, whereas the second form declares
983 that `f` itself is a C function with type `t`. The first
984 declaration is usually a mistake, and one that is hard to
985 debug because it results in a crash, hence this
986 warning.</para>
987 </listitem>
988 </varlistentry>
989
990 <varlistentry>
991 <term><option>-fwarn-dodgy-imports</option>:</term>
992 <listitem>
993 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
994 </indexterm>
995 <para>Causes a warning to be emitted when a datatype
996 <literal>T</literal> is imported
997 with all constructors, i.e. <literal>T(..)</literal>, but has been
998 exported abstractly, i.e. <literal>T</literal>.</para>
999 </listitem>
1000 </varlistentry>
1001
1002 <varlistentry>
1003 <term><option>-fwarn-duplicate-exports</option>:</term>
1004 <listitem>
1005 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
1006 <indexterm><primary>duplicate exports, warning</primary></indexterm>
1007 <indexterm><primary>export lists, duplicates</primary></indexterm>
1008
1009 <para>Have the compiler warn about duplicate entries in
1010 export lists. This is useful information if you maintain
1011 large export lists, and want to avoid the continued export
1012 of a definition after you've deleted (one) mention of it in
1013 the export list.</para>
1014
1015 <para>This option is on by default.</para>
1016 </listitem>
1017 </varlistentry>
1018
1019 <varlistentry>
1020 <term><option>-fwarn-hi-shadowing</option>:</term>
1021 <listitem>
1022 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
1023 <indexterm><primary>shadowing</primary>
1024 <secondary>interface files</secondary></indexterm>
1025
1026 <para>Causes the compiler to emit a warning when a module or
1027 interface file in the current directory is shadowing one
1028 with the same module name in a library or other
1029 directory.</para>
1030 </listitem>
1031 </varlistentry>
1032
1033 <varlistentry>
1034 <term><option>-fwarn-implicit-prelude</option>:</term>
1035 <listitem>
1036 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
1037 <indexterm><primary>implicit prelude, warning</primary></indexterm>
1038 <para>Have the compiler warn if the Prelude is implicitly
1039 imported. This happens unless either the Prelude module is
1040 explicitly imported with an <literal>import ... Prelude ...</literal>
1041 line, or this implicit import is disabled (either by
1042 <option>-XNoImplicitPrelude</option> or a
1043 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
1044
1045 <para>Note that no warning is given for syntax that implicitly
1046 refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
1047 would change whether it refers to the Prelude.
1048 For example, no warning is given when
1049 <literal>368</literal> means
1050 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
1051 (where <literal>Prelude</literal> refers to the actual Prelude module,
1052 regardless of the imports of the module being compiled).</para>
1053
1054 <para>This warning is off by default.</para>
1055 </listitem>
1056 </varlistentry>
1057
1058 <varlistentry>
1059 <term><option>-fwarn-incomplete-patterns</option>:</term>
1060 <listitem>
1061 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
1062 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
1063 <indexterm><primary>patterns, incomplete</primary></indexterm>
1064
1065 <para>Similarly for incomplete patterns, the function
1066 <function>g</function> below will fail when applied to
1067 non-empty lists, so the compiler will emit a warning about
1068 this when <option>-fwarn-incomplete-patterns</option> is
1069 enabled.</para>
1070
1071 <programlisting>
1072 g [] = 2
1073 </programlisting>
1074
1075 <para>This option isn't enabled by default because it can be
1076 a bit noisy, and it doesn't always indicate a bug in the
1077 program. However, it's generally considered good practice
1078 to cover all the cases in your functions.</para>
1079 </listitem>
1080 </varlistentry>
1081
1082 <varlistentry>
1083 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1084 <listitem>
1085 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1086 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1087 <indexterm><primary>record updates, incomplete</primary></indexterm>
1088
1089 <para>The function
1090 <function>f</function> below will fail when applied to
1091 <literal>Bar</literal>, so the compiler will emit a warning about
1092 this when <option>-fwarn-incomplete-record-updates</option> is
1093 enabled.</para>
1094
1095 <programlisting>
1096 data Foo = Foo { x :: Int }
1097 | Bar
1098
1099 f :: Foo -> Foo
1100 f foo = foo { x = 6 }
1101 </programlisting>
1102
1103 <para>This option isn't enabled by default because it can be
1104 very noisy, and it often doesn't indicate a bug in the
1105 program.</para>
1106 </listitem>
1107 </varlistentry>
1108
1109 <varlistentry>
1110 <term>
1111 <option>-fwarn-missing-fields</option>:
1112 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1113 <indexterm><primary>missing fields, warning</primary></indexterm>
1114 <indexterm><primary>fields, missing</primary></indexterm>
1115 </term>
1116 <listitem>
1117
1118 <para>This option is on by default, and warns you whenever
1119 the construction of a labelled field constructor isn't
1120 complete, missing initializers for one or more fields. While
1121 not an error (the missing fields are initialised with
1122 bottoms), it is often an indication of a programmer error.</para>
1123 </listitem>
1124 </varlistentry>
1125
1126 <varlistentry>
1127 <term><option>-fwarn-missing-methods</option>:</term>
1128 <listitem>
1129 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1130 <indexterm><primary>missing methods, warning</primary></indexterm>
1131 <indexterm><primary>methods, missing</primary></indexterm>
1132
1133 <para>This option is on by default, and warns you whenever
1134 an instance declaration is missing one or more methods, and
1135 the corresponding class declaration has no default
1136 declaration for them.</para>
1137 <para>The warning is suppressed if the method name
1138 begins with an underscore. Here's an example where this is useful:
1139 <programlisting>
1140 class C a where
1141 _simpleFn :: a -> String
1142 complexFn :: a -> a -> String
1143 complexFn x y = ... _simpleFn ...
1144 </programlisting>
1145 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1146 never <literal>_simpleFn</literal>; and (b)
1147 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1148 </para>
1149 </listitem>
1150 </varlistentry>
1151
1152 <varlistentry>
1153 <term><option>-fwarn-missing-signatures</option>:</term>
1154 <listitem>
1155 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1156 <indexterm><primary>type signatures, missing</primary></indexterm>
1157
1158 <para>If you would like GHC to check that every top-level
1159 function/value has a type signature, use the
1160 <option>-fwarn-missing-signatures</option> option. As part of
1161 the warning GHC also reports the inferred type. The
1162 option is off by default.</para>
1163 </listitem>
1164 </varlistentry>
1165
1166 <varlistentry>
1167 <term><option>-fwarn-name-shadowing</option>:</term>
1168 <listitem>
1169 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1170 <indexterm><primary>shadowing, warning</primary></indexterm>
1171
1172 <para>This option causes a warning to be emitted whenever an
1173 inner-scope value has the same name as an outer-scope value,
1174 i.e. the inner value shadows the outer one. This can catch
1175 typographical errors that turn into hard-to-find bugs, e.g.,
1176 in the inadvertent capture of what would be a recursive call in
1177 <literal>f = ... let f = id in ... f ...</literal>.</para>
1178 <para>The warning is suppressed for names beginning with an underscore. For example
1179 <programlisting>
1180 f x = do { _ignore &lt;- this; _ignore &lt;- that; return (the other) }
1181 </programlisting>
1182 </para>
1183 </listitem>
1184 </varlistentry>
1185
1186 <varlistentry>
1187 <term><option>-fwarn-orphans</option>:</term>
1188 <listitem>
1189 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1190 <indexterm><primary>orphan instances, warning</primary></indexterm>
1191 <indexterm><primary>orphan rules, warning</primary></indexterm>
1192
1193 <para>This option causes a warning to be emitted whenever the
1194 module contains an "orphan" instance declaration or rewrite rule.
1195 An instance declaration is an orphan if it appears in a module in
1196 which neither the class nor the type being instanced are declared
1197 in the same module. A rule is an orphan if it is a rule for a
1198 function declared in another module. A module containing any
1199 orphans is called an orphan module.</para>
1200 <para>The trouble with orphans is that GHC must pro-actively read the interface
1201 files for all orphan modules, just in case their instances or rules
1202 play a role, whether or not the module's interface would otherwise
1203 be of any use. See <xref linkend="orphan-modules"/> for details.
1204 </para>
1205 </listitem>
1206 </varlistentry>
1207
1208 <varlistentry>
1209 <term>
1210 <option>-fwarn-overlapping-patterns</option>:
1211 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1212 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1213 <indexterm><primary>patterns, overlapping</primary></indexterm>
1214 </term>
1215 <listitem>
1216 <para>By default, the compiler will warn you if a set of
1217 patterns are overlapping, e.g.,</para>
1218
1219 <programlisting>
1220 f :: String -&#62; Int
1221 f [] = 0
1222 f (_:xs) = 1
1223 f "2" = 2
1224 </programlisting>
1225
1226 <para>where the last pattern match in <function>f</function>
1227 won't ever be reached, as the second pattern overlaps
1228 it. More often than not, redundant patterns is a programmer
1229 mistake/error, so this option is enabled by default.</para>
1230 </listitem>
1231 </varlistentry>
1232
1233 <varlistentry>
1234 <term><option>-fwarn-simple-patterns</option>:</term>
1235 <listitem>
1236 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1237 </indexterm>
1238 <para>Causes the compiler to warn about lambda-bound
1239 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1240 Normally, these aren't treated as incomplete patterns by
1241 <option>-fwarn-incomplete-patterns</option>.</para>
1242 <para>&ldquo;Lambda-bound patterns&rdquo; includes all places where there is a single pattern,
1243 including list comprehensions and do-notation. In these cases, a pattern-match
1244 failure is quite legitimate, and triggers filtering (list comprehensions) or
1245 the monad <literal>fail</literal> operation (monads). For example:
1246 <programlisting>
1247 f :: [Maybe a] -> [a]
1248 f xs = [y | Just y &lt;- xs]
1249 </programlisting>
1250 Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
1251 these probably-innocent cases, which is why the flag is off by default. </para>
1252 </listitem>
1253 </varlistentry>
1254
1255 <varlistentry>
1256 <term><option>-fwarn-tabs</option>:</term>
1257 <listitem>
1258 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1259 <indexterm><primary>tabs, warning</primary></indexterm>
1260 <para>Have the compiler warn if there are tabs in your source
1261 file.</para>
1262
1263 <para>This warning is off by default.</para>
1264 </listitem>
1265 </varlistentry>
1266
1267 <varlistentry>
1268 <term><option>-fwarn-type-defaults</option>:</term>
1269 <listitem>
1270 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1271 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1272 <para>Have the compiler warn/inform you where in your source
1273 the Haskell defaulting mechanism for numeric types kicks
1274 in. This is useful information when converting code from a
1275 context that assumed one default into one with another,
1276 e.g., the &lsquo;default default&rsquo; for Haskell 1.4 caused the
1277 otherwise unconstrained value <constant>1</constant> to be
1278 given the type <literal>Int</literal>, whereas Haskell 98
1279 defaults it to <literal>Integer</literal>. This may lead to
1280 differences in performance and behaviour, hence the
1281 usefulness of being non-silent about this.</para>
1282
1283 <para>This warning is off by default.</para>
1284 </listitem>
1285 </varlistentry>
1286
1287 <varlistentry>
1288 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1289 <listitem>
1290 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1291 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1292 <para>Have the compiler warn/inform you where in your source
1293 the Haskell Monomorphism Restriction is applied. If applied silently
1294 the MR can give rise to unexpected behaviour, so it can be helpful
1295 to have an explicit warning that it is being applied.</para>
1296
1297 <para>This warning is off by default.</para>
1298 </listitem>
1299 </varlistentry>
1300
1301 <varlistentry>
1302 <term><option>-fwarn-unused-binds</option>:</term>
1303 <listitem>
1304 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1305 <indexterm><primary>unused binds, warning</primary></indexterm>
1306 <indexterm><primary>binds, unused</primary></indexterm>
1307 <para>Report any function definitions (and local bindings)
1308 which are unused. For top-level functions, the warning is
1309 only given if the binding is not exported.</para>
1310 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1311 mentioned in the right hand side of another definition that is used, or (c) the
1312 function it defines begins with an underscore. The last case provides a
1313 way to suppress unused-binding warnings selectively. </para>
1314 <para> Notice that a variable
1315 is reported as unused even if it appears in the right-hand side of another
1316 unused binding. </para>
1317 </listitem>
1318 </varlistentry>
1319
1320 <varlistentry>
1321 <term><option>-fwarn-unused-imports</option>:</term>
1322 <listitem>
1323 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1324 <indexterm><primary>unused imports, warning</primary></indexterm>
1325 <indexterm><primary>imports, unused</primary></indexterm>
1326
1327 <para>Report any modules that are explicitly imported but
1328 never used. However, the form <literal>import M()</literal> is
1329 never reported as an unused import, because it is a useful idiom
1330 for importing instance declarations, which are anonymous in Haskell.</para>
1331 </listitem>
1332 </varlistentry>
1333
1334 <varlistentry>
1335 <term><option>-fwarn-unused-matches</option>:</term>
1336 <listitem>
1337 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1338 <indexterm><primary>unused matches, warning</primary></indexterm>
1339 <indexterm><primary>matches, unused</primary></indexterm>
1340
1341 <para>Report all unused variables which arise from pattern
1342 matches, including patterns consisting of a single variable.
1343 For instance <literal>f x y = []</literal> would report
1344 <varname>x</varname> and <varname>y</varname> as unused. The
1345 warning is suppressed if the variable name begins with an underscore, thus:
1346 <programlisting>
1347 f _x = True
1348 </programlisting>
1349 </para>
1350 </listitem>
1351 </varlistentry>
1352
1353 </variablelist>
1354
1355 <para>If you're feeling really paranoid, the
1356 <option>-dcore-lint</option>
1357 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1358 is a good choice. It turns on heavyweight intra-pass
1359 sanity-checking within GHC. (It checks GHC's sanity, not
1360 yours.)</para>
1361
1362 </sect1>
1363
1364 &packages;
1365
1366 <sect1 id="options-optimise">
1367 <title>Optimisation (code improvement)</title>
1368
1369 <indexterm><primary>optimisation</primary></indexterm>
1370 <indexterm><primary>improvement, code</primary></indexterm>
1371
1372 <para>The <option>-O*</option> options specify convenient
1373 &ldquo;packages&rdquo; of optimisation flags; the
1374 <option>-f*</option> options described later on specify
1375 <emphasis>individual</emphasis> optimisations to be turned on/off;
1376 the <option>-m*</option> options specify
1377 <emphasis>machine-specific</emphasis> optimisations to be turned
1378 on/off.</para>
1379
1380 <sect2 id="optimise-pkgs">
1381 <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1382
1383 <para>There are <emphasis>many</emphasis> options that affect
1384 the quality of code produced by GHC. Most people only have a
1385 general goal, something like &ldquo;Compile quickly&rdquo; or
1386 &ldquo;Make my program run like greased lightning.&rdquo; The
1387 following &ldquo;packages&rdquo; of optimisations (or lack
1388 thereof) should suffice.</para>
1389
1390 <para>Note that higher optimisation levels cause more
1391 cross-module optimisation to be performed, which can have an
1392 impact on how much of your program needs to be recompiled when
1393 you change something. This is one reason to stick to
1394 no-optimisation when developing code.</para>
1395
1396 <variablelist>
1397
1398 <varlistentry>
1399 <term>
1400 No <option>-O*</option>-type option specified:
1401 <indexterm><primary>-O* not specified</primary></indexterm>
1402 </term>
1403 <listitem>
1404 <para>This is taken to mean: &ldquo;Please compile
1405 quickly; I'm not over-bothered about compiled-code
1406 quality.&rdquo; So, for example: <command>ghc -c
1407 Foo.hs</command></para>
1408 </listitem>
1409 </varlistentry>
1410
1411 <varlistentry>
1412 <term>
1413 <option>-O0</option>:
1414 <indexterm><primary><option>-O0</option></primary></indexterm>
1415 </term>
1416 <listitem>
1417 <para>Means &ldquo;turn off all optimisation&rdquo;,
1418 reverting to the same settings as if no
1419 <option>-O</option> options had been specified. Saying
1420 <option>-O0</option> can be useful if
1421 eg. <command>make</command> has inserted a
1422 <option>-O</option> on the command line already.</para>
1423 </listitem>
1424 </varlistentry>
1425
1426 <varlistentry>
1427 <term>
1428 <option>-O</option> or <option>-O1</option>:
1429 <indexterm><primary>-O option</primary></indexterm>
1430 <indexterm><primary>-O1 option</primary></indexterm>
1431 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1432 </term>
1433 <listitem>
1434 <para>Means: &ldquo;Generate good-quality code without
1435 taking too long about it.&rdquo; Thus, for example:
1436 <command>ghc -c -O Main.lhs</command></para>
1437 </listitem>
1438 </varlistentry>
1439
1440 <varlistentry>
1441 <term>
1442 <option>-O2</option>:
1443 <indexterm><primary>-O2 option</primary></indexterm>
1444 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1445 </term>
1446 <listitem>
1447 <para>Means: &ldquo;Apply every non-dangerous
1448 optimisation, even if it means significantly longer
1449 compile times.&rdquo;</para>
1450
1451 <para>The avoided &ldquo;dangerous&rdquo; optimisations
1452 are those that can make runtime or space
1453 <emphasis>worse</emphasis> if you're unlucky. They are
1454 normally turned on or off individually.</para>
1455
1456 <para>At the moment, <option>-O2</option> is
1457 <emphasis>unlikely</emphasis> to produce better code than
1458 <option>-O</option>.</para>
1459 </listitem>
1460 </varlistentry>
1461
1462 <varlistentry>
1463 <term>
1464 <option>-Ofile &lt;file&gt;</option>:
1465 <indexterm><primary>-Ofile &lt;file&gt; option</primary></indexterm>
1466 <indexterm><primary>optimising, customised</primary></indexterm>
1467 </term>
1468 <listitem>
1469 <para>(NOTE: not supported since GHC 4.x. Please ask if
1470 you're interested in this.)</para>
1471
1472 <para>For those who need <emphasis>absolute</emphasis>
1473 control over <emphasis>exactly</emphasis> what options are
1474 used (e.g., compiler writers, sometimes :-), a list of
1475 options can be put in a file and then slurped in with
1476 <option>-Ofile</option>.</para>
1477
1478 <para>In that file, comments are of the
1479 <literal>&num;</literal>-to-end-of-line variety; blank
1480 lines and most whitespace is ignored.</para>
1481
1482 <para>Please ask if you are baffled and would like an
1483 example of <option>-Ofile</option>!</para>
1484 </listitem>
1485 </varlistentry>
1486 </variablelist>
1487
1488 <para>We don't use a <option>-O*</option> flag for day-to-day
1489 work. We use <option>-O</option> to get respectable speed;
1490 e.g., when we want to measure something. When we want to go for
1491 broke, we tend to use <option>-O2 -fvia-C</option> (and we go for
1492 lots of coffee breaks).</para>
1493
1494 <para>The easiest way to see what <option>-O</option> (etc.)
1495 &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1496 then stand back in amazement.</para>
1497 </sect2>
1498
1499 <sect2 id="options-f">
1500 <title><option>-f*</option>: platform-independent flags</title>
1501
1502 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1503 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1504
1505 <para>These flags turn on and off individual optimisations.
1506 They are normally set via the <option>-O</option> options
1507 described above, and as such, you shouldn't need to set any of
1508 them explicitly (indeed, doing so could lead to unexpected
1509 results). However, there are one or two that may be of
1510 interest:</para>
1511
1512 <variablelist>
1513 <varlistentry>
1514 <term><option>-fexcess-precision</option>:</term>
1515 <listitem>
1516 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1517 <para>When this option is given, intermediate floating
1518 point values can have a <emphasis>greater</emphasis>
1519 precision/range than the final type. Generally this is a
1520 good thing, but some programs may rely on the exact
1521 precision/range of
1522 <literal>Float</literal>/<literal>Double</literal> values
1523 and should not use this option for their compilation.</para>
1524 </listitem>
1525 </varlistentry>
1526
1527 <varlistentry>
1528 <term><option>-fignore-asserts</option>:</term>
1529 <listitem>
1530 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1531 <para>Causes GHC to ignore uses of the function
1532 <literal>Exception.assert</literal> in source code (in
1533 other words, rewriting <literal>Exception.assert p
1534 e</literal> to <literal>e</literal> (see <xref
1535 linkend="assertions"/>). This flag is turned on by
1536 <option>-O</option>.
1537 </para>
1538 </listitem>
1539 </varlistentry>
1540
1541 <varlistentry>
1542 <term>
1543 <option>-fno-cse</option>
1544 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1545 </term>
1546 <listitem>
1547 <para>Turns off the common-sub-expression elimination optimisation.
1548 Can be useful if you have some <literal>unsafePerformIO</literal>
1549 expressions that you don't want commoned-up.</para>
1550 </listitem>
1551 </varlistentry>
1552
1553 <varlistentry>
1554 <term>
1555 <option>-fno-strictness</option>
1556 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1557 </term>
1558 <listitem>
1559 <para>Turns off the strictness analyser; sometimes it eats
1560 too many cycles.</para>
1561 </listitem>
1562 </varlistentry>
1563
1564 <varlistentry>
1565 <term>
1566 <option>-fno-full-laziness</option>
1567 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1568 </term>
1569 <listitem>
1570 <para>Turns off the full laziness optimisation (also known as
1571 let-floating). Full laziness increases sharing, which can lead
1572 to increased memory residency.</para>
1573
1574 <para>NOTE: GHC doesn't implement complete full-laziness.
1575 When optimisation in on, and
1576 <option>-fno-full-laziness</option> is not given, some
1577 transformations that increase sharing are performed, such
1578 as extracting repeated computations from a loop. These
1579 are the same transformations that a fully lazy
1580 implementation would do, the difference is that GHC
1581 doesn't consistently apply full-laziness, so don't rely on
1582 it.</para>
1583 </listitem>
1584 </varlistentry>
1585
1586 <varlistentry>
1587 <term>
1588 <option>-fspec-constr</option>
1589 <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
1590 </term>
1591 <listitem>
1592 <para>Turn on call-pattern specialisation.</para>
1593 </listitem>
1594 </varlistentry>
1595
1596 <varlistentry>
1597 <term>
1598 <option>-fliberate-case</option>
1599 <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
1600 </term>
1601 <listitem>
1602 <para>Turn on the liberate-case transformation.</para>
1603 </listitem>
1604 </varlistentry>
1605
1606 <varlistentry>
1607 <term>
1608 <option>-fstatic-argument-transformation</option>
1609 <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
1610 </term>
1611 <listitem>
1612 <para>Turn on the static argument transformation.</para>
1613 </listitem>
1614 </varlistentry>
1615
1616 <varlistentry>
1617 <term>
1618 <option>-fno-state-hack</option>
1619 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1620 </term>
1621 <listitem>
1622 <para>Turn off the "state hack" whereby any lambda with a
1623 <literal>State#</literal> token as argument is considered to be
1624 single-entry, hence it is considered OK to inline things inside
1625 it. This can improve performance of IO and ST monad code, but it
1626 runs the risk of reducing sharing.</para>
1627 </listitem>
1628 </varlistentry>
1629
1630 <varlistentry>
1631 <term>
1632 <option>-fomit-interface-pragmas</option>
1633 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
1634 </term>
1635 <listitem>
1636 <para>Tells GHC to omit all inessential information from the interface file
1637 generated for the module being compiled (say M). This means that a module
1638 importing M will see only the <emphasis>types</emphasis> of the functions that M exports, but not
1639 their unfoldings, strictness info, etc. Hence, for example,
1640 no function exported by M will be inlined
1641 into an importing module. The benefit is that modules that import M will
1642 need to be recompiled less often (only when M's exports change their type,
1643 not when they change their implementation).
1644 </para>
1645 </listitem>
1646 </varlistentry>
1647
1648 <varlistentry>
1649 <term>
1650 <option>-fignore-interface-pragmas</option>
1651 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
1652 </term>
1653 <listitem>
1654 <para>Tells GHC to ignore all inessential information when reading interface files.
1655 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
1656 for a function, GHC will ignore that information.</para>
1657 </listitem>
1658 </varlistentry>
1659
1660 <varlistentry>
1661 <term>
1662 <option>-funbox-strict-fields</option>:
1663 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1664 <indexterm><primary>strict constructor fields</primary></indexterm>
1665 <indexterm><primary>constructor fields, strict</primary></indexterm>
1666 </term>
1667 <listitem>
1668 <para>This option causes all constructor fields which are
1669 marked strict (i.e. &ldquo;!&rdquo;) to be unboxed or
1670 unpacked if possible. It is equivalent to adding an
1671 <literal>UNPACK</literal> pragma to every strict
1672 constructor field (see <xref
1673 linkend="unpack-pragma"/>).</para>
1674
1675 <para>This option is a bit of a sledgehammer: it might
1676 sometimes make things worse. Selectively unboxing fields
1677 by using <literal>UNPACK</literal> pragmas might be
1678 better.</para>
1679 </listitem>
1680 </varlistentry>
1681
1682 <varlistentry>
1683 <term>
1684 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
1685 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1686 <indexterm><primary>inlining, controlling</primary></indexterm>
1687 <indexterm><primary>unfolding, controlling</primary></indexterm>
1688 </term>
1689 <listitem>
1690 <para>(Default: 45) Governs the maximum size that GHC will
1691 allow a function unfolding to be. (An unfolding has a
1692 &ldquo;size&rdquo; that reflects the cost in terms of
1693 &ldquo;code bloat&rdquo; of expanding that unfolding at
1694 at a call site. A bigger function would be assigned a
1695 bigger cost.) </para>
1696
1697 <para> Consequences: (a) nothing larger than this will be
1698 inlined (unless it has an INLINE pragma); (b) nothing
1699 larger than this will be spewed into an interface
1700 file. </para>
1701
1702
1703 <para> Increasing this figure is more likely to result in longer
1704 compile times than faster code. The next option is more
1705 useful:</para>
1706 </listitem>
1707 </varlistentry>
1708
1709 <varlistentry>
1710 <term><option>-funfolding-use-threshold=<replaceable>n</replaceable></option></term>
1711 <listitem>
1712 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1713 <indexterm><primary>inlining, controlling</primary></indexterm>
1714 <indexterm><primary>unfolding, controlling</primary></indexterm>
1715
1716 <para>(Default: 8) This is the magic cut-off figure for
1717 unfolding: below this size, a function definition will be
1718 unfolded at the call-site, any bigger and it won't. The
1719 size computed for a function depends on two things: the
1720 actual size of the expression minus any discounts that
1721 apply (see <option>-funfolding-con-discount</option>).</para>
1722 </listitem>
1723 </varlistentry>
1724 </variablelist>
1725
1726 </sect2>
1727
1728 </sect1>
1729
1730 &phases;
1731
1732 <sect1 id="using-concurrent">
1733 <title>Using Concurrent Haskell</title>
1734 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
1735
1736 <para>GHC supports Concurrent Haskell by default, without requiring a
1737 special option or libraries compiled in a certain way. To get access to
1738 the support libraries for Concurrent Haskell, just import
1739 <ulink
1740 url="../libraries/base/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
1741
1742 <para>The following RTS option(s) affect the behaviour of Concurrent
1743 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
1744
1745 <variablelist>
1746 <varlistentry>
1747 <term><option>-C<replaceable>s</replaceable></option></term>
1748 <listitem>
1749 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1750 Sets the context switch interval to <replaceable>s</replaceable>
1751 seconds. A context switch will occur at the next heap block
1752 allocation after the timer expires (a heap block allocation occurs
1753 every 4k of allocation). With <option>-C0</option> or
1754 <option>-C</option>, context switches will occur as often as
1755 possible (at every heap block allocation). By default, context
1756 switches occur every 20ms.</para>
1757 </listitem>
1758 </varlistentry>
1759 </variablelist>
1760 </sect1>
1761
1762 <sect1 id="using-smp">
1763 <title>Using SMP parallelism</title>
1764 <indexterm><primary>parallelism</primary>
1765 </indexterm>
1766 <indexterm><primary>SMP</primary>
1767 </indexterm>
1768
1769 <para>GHC supports running Haskell programs in parallel on an SMP
1770 (symmetric multiprocessor).</para>
1771
1772 <para>There's a fine distinction between
1773 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
1774 parallelism is all about making your program run
1775 <emphasis>faster</emphasis> by making use of multiple processors
1776 simultaneously. Concurrency, on the other hand, is a means of
1777 abstraction: it is a convenient way to structure a program that must
1778 respond to multiple asynchronous events.</para>
1779
1780 <para>However, the two terms are certainly related. By making use of
1781 multiple CPUs it is possible to run concurrent threads in parallel,
1782 and this is exactly what GHC's SMP parallelism support does. But it
1783 is also possible to obtain performance improvements with parallelism
1784 on programs that do not use concurrency. This section describes how to
1785 use GHC to compile and run parallel programs, in <xref
1786 linkend="lang-parallel" /> we describe the language features that affect
1787 parallelism.</para>
1788
1789 <sect2 id="parallel-compile-options">
1790 <title>Compile-time options for SMP parallelism</title>
1791
1792 <para>In order to make use of multiple CPUs, your program must be
1793 linked with the <option>-threaded</option> option (see <xref
1794 linkend="options-linker" />). Additionally, the following
1795 compiler options affect parallelism:</para>
1796
1797 <variablelist>
1798 <varlistentry>
1799 <term><option>-feager-blackholing</option></term>
1800 <indexterm><primary><option>-feager-blackholing</option></primary></indexterm>
1801 <listitem>
1802 <para>
1803 Blackholing is the act of marking a thunk (lazy
1804 computuation) as being under evaluation. It is useful for
1805 three reasons: firstly it lets us detect certain kinds of
1806 infinite loop (the <literal>NonTermination</literal>
1807 exception), secondly it avoids certain kinds of space
1808 leak, and thirdly it avoids repeating a computation in a
1809 parallel program, because we can tell when a computation
1810 is already in progress.</para>
1811
1812 <para>
1813 The option <option>-feager-blackholing</option> causes
1814 each thunk to be blackholed as soon as evaluation begins.
1815 The default is "lazy blackholing", whereby thunks are only
1816 marked as being under evaluation when a thread is paused
1817 for some reason. Lazy blackholing is typically more
1818 efficient (by 1-2&percnt; or so), because most thunks don't
1819 need to be blackholed. However, eager blackholing can
1820 avoid more repeated computation in a parallel program, and
1821 this often turns out to be important for parallelism.
1822 </para>
1823
1824 <para>
1825 We recommend compiling any code that is intended to be run
1826 in parallel with the <option>-feager-blackholing</option>
1827 flag.
1828 </para>
1829 </listitem>
1830 </varlistentry>
1831 </variablelist>
1832 </sect2>
1833
1834 <sect2 id="parallel-options">
1835 <title>RTS options for SMP parallelism</title>
1836
1837 <para>To run a program on multiple CPUs, use the
1838 RTS <option>-N</option> option:</para>
1839
1840 <variablelist>
1841 <varlistentry>
1842 <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
1843 <listitem>
1844 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1845 Use <replaceable>x</replaceable> simultaneous threads when
1846 running the program. Normally <replaceable>x</replaceable>
1847 should be chosen to match the number of CPU cores on the
1848 machine<footnote><para>Whether hyperthreading cores should be counted or not is an
1849 open question; please feel free to experiment and let us know what
1850 results you find.</para></footnote>. For example,
1851 on a dual-core machine we would probably use
1852 <literal>+RTS -N2 -RTS</literal>.</para>
1853
1854 <para>Omitting <replaceable>x</replaceable>,
1855 i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
1856 choose the value of <replaceable>x</replaceable> itself
1857 based on how many processors are in your machine.</para>
1858
1859 <para>Be careful when using all the processors in your
1860 machine: if some of your processors are in use by other
1861 programs, this can actually harm performance rather than
1862 improve it.</para>
1863
1864 <para>Setting <option>-N</option> also has the effect of
1865 enabling the parallel garbage collector (see
1866 <xref linkend="rts-options-gc" />).</para>
1867
1868 <para>There is no means (currently) by which this value
1869 may vary after the program has started.</para>
1870 </listitem>
1871 </varlistentry>
1872 </variablelist>
1873
1874 <para>The following options affect the way the runtime schedules
1875 threads on CPUs:</para>
1876
1877 <variablelist>
1878 <varlistentry>
1879 <term><option>-qm</option></term>
1880 <indexterm><primary><option>-qm</option></primary><secondary>RTS
1881 option</secondary></indexterm>
1882 <listitem>
1883 <para>Disable automatic migration for load balancing.
1884 Normally the runtime will automatically try to schedule
1885 threads across the available CPUs to make use of idle
1886 CPUs; this option disables that behaviour. It is probably
1887 only of use if you are explicitly scheduling threads onto
1888 CPUs with <literal>GHC.Conc.forkOnIO</literal>.</para>
1889 </listitem>
1890 </varlistentry>
1891 <varlistentry>
1892 <term><option>-qw</option></term>
1893 <indexterm><primary><option>-qw</option></primary><secondary>RTS
1894 option</secondary></indexterm>
1895 <listitem>
1896 <para>Migrate a thread to the current CPU when it is woken
1897 up. Normally when a thread is woken up after being
1898 blocked it will be scheduled on the CPU it was running on
1899 last; this option allows the thread to immediately migrate
1900 to the CPU that unblocked it.</para>
1901
1902 <para>The rationale for allowing this eager migration is
1903 that it tends to move threads that are communicating with
1904 each other onto the same CPU; however there are
1905 pathalogical situations where it turns out to be a poor
1906 strategy. Depending on the communication pattern in your
1907 program, it may or may not be a good idea.</para>
1908 </listitem>
1909 </varlistentry>
1910 </variablelist>
1911 </sect2>
1912
1913 <sect2>
1914 <title>Hints for using SMP parallelism</title>
1915
1916 <para>Add the <literal>-s</literal> RTS option when
1917 running the program to see timing stats, which will help to tell you
1918 whether your program got faster by using more CPUs or not. If the user
1919 time is greater than
1920 the elapsed time, then the program used more than one CPU. You should
1921 also run the program without <literal>-N</literal> for comparison.</para>
1922
1923 <para>GHC's parallelism support is new and experimental. It may make your
1924 program go faster, or it might slow it down - either way, we'd be
1925 interested to hear from you.</para>
1926
1927 <para>One significant limitation with the current implementation is that
1928 the garbage collector is still single-threaded, and all execution must
1929 stop when GC takes place. This can be a significant bottleneck in a
1930 parallel program, especially if your program does a lot of GC. If this
1931 happens to you, then try reducing the cost of GC by tweaking the GC
1932 settings (<xref linkend="rts-options-gc" />): enlarging the heap or the
1933 allocation area size is a good start.</para>
1934 </sect2>
1935 </sect1>
1936
1937 <sect1 id="options-platform">
1938 <title>Platform-specific Flags</title>
1939
1940 <indexterm><primary>-m* options</primary></indexterm>
1941 <indexterm><primary>platform-specific options</primary></indexterm>
1942 <indexterm><primary>machine-specific options</primary></indexterm>
1943
1944 <para>Some flags only make sense for particular target
1945 platforms.</para>
1946
1947 <variablelist>
1948
1949 <varlistentry>
1950 <term><option>-monly-[32]-regs</option>:</term>
1951 <listitem>
1952 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1953 option (iX86 only)</primary></indexterm> GHC tries to
1954 &ldquo;steal&rdquo; four registers from GCC, for performance
1955 reasons; it almost always works. However, when GCC is
1956 compiling some modules with four stolen registers, it will
1957 crash, probably saying:
1958
1959 <screen>
1960 Foo.hc:533: fixed or forbidden register was spilled.
1961 This may be due to a compiler bug or to impossible asm
1962 statements or clauses.
1963 </screen>
1964
1965 Just give some registers back with
1966 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1967 If `2' doesn't work, please report the bug to us.</para>
1968 </listitem>
1969 </varlistentry>
1970 </variablelist>
1971
1972 </sect1>
1973
1974 &runtime;
1975
1976 <sect1 id="ext-core">
1977 <title>Generating and compiling External Core Files</title>
1978
1979 <indexterm><primary>intermediate code generation</primary></indexterm>
1980
1981 <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format)
1982 to a file as a side-effect of compilation. Non-GHC back-end tools can read and process Core files; these files have the suffix
1983 <filename>.hcr</filename>. The Core format is described in <ulink url="../ext-core/core.pdf">
1984 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1985 and sample tools
1986 for manipulating Core files (in Haskell) are in the GHC source distribution
1987 directory under <literal>utils/ext-core</literal>.
1988 Note that the format of <literal>.hcr</literal>
1989 files is <emphasis>different</emphasis> from the Core output format that GHC generates
1990 for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
1991
1992 <para>The Core format natively supports notes which you can add to
1993 your source code using the <literal>CORE</literal> pragma (see <xref
1994 linkend="pragmas"/>).</para>
1995
1996 <variablelist>
1997
1998 <varlistentry>
1999 <term>
2000 <option>-fext-core</option>
2001 <indexterm><primary><option>-fext-core</option></primary></indexterm>
2002 </term>
2003 <listitem>
2004 <para>Generate <literal>.hcr</literal> files.</para>
2005 </listitem>
2006 </varlistentry>
2007
2008 </variablelist>
2009
2010 <para>Currently (as of version 6.8.2), GHC does not have the ability to read in External Core files as source. If you would like GHC to have this ability, please <ulink url="http://hackage.haskell.org/trac/ghc/wiki/MailingListsAndIRC">make your wishes known to the GHC Team</ulink>.</para>
2011
2012 </sect1>
2013
2014 &debug;
2015 &flags;
2016
2017 </chapter>
2018
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