Remove extra dashes in a flag name in the user guide
[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>Getting started: compiling programs</title>
10
11 <para>
12 In this chapter you'll find a complete reference to the GHC
13 command-line syntax, including all 400+ flags. It's a large and
14 complex system, and there are lots of details, so it can be
15 quite hard to figure out how to get started. With that in mind,
16 this introductory section provides a quick introduction to the
17 basic usage of GHC for compiling a Haskell program, before the
18 following sections dive into the full syntax.
19 </para>
20
21 <para>
22 Let's create a Hello World program, and compile and run it.
23 First, create a file <filename>hello.hs</filename> containing
24 the Haskell code:
25 </para>
26
27 <programlisting>
28 main = putStrLn "Hello, World!"
29 </programlisting>
30
31 <para>To compile the program, use GHC like this:</para>
32
33 <screen>
34 $ ghc hello.hs
35 </screen>
36
37 <para>(where <literal>$</literal> represents the prompt: don't
38 type it). GHC will compile the source
39 file <filename>hello.hs</filename>, producing
40 an <firstterm>object
41 file</firstterm> <filename>hello.o</filename> and
42 an <firstterm>interface
43 file</firstterm> <filename>hello.hi</filename>, and then it
44 will link the object file to the libraries that come with GHC
45 to produce an executable called <filename>hello</filename> on
46 Unix/Linux/Mac, or <filename>hello.exe</filename> on
47 Windows.</para>
48
49 <para>
50 By default GHC will be very quiet about what it is doing, only
51 printing error messages. If you want to see in more detail
52 what's going on behind the scenes, add <option>-v</option> to
53 the command line.
54 </para>
55
56 <para>
57 Then we can run the program like this:
58 </para>
59
60 <screen>
61 $ ./hello
62 Hello World!
63 </screen>
64
65 <para>
66 If your program contains multiple modules, then you only need to
67 tell GHC the name of the source file containing
68 the <filename>Main</filename> module, and GHC will examine
69 the <literal>import</literal> declarations to find the other
70 modules that make up the program and find their source files.
71 This means that, with the exception of
72 the <literal>Main</literal> module, every source file should be
73 named after the module name that it contains (with dots replaced
74 by directory separators). For example, the
75 module <literal>Data.Person</literal> would be in the
76 file <filename>Data/Person.hs</filename> on Unix/Linux/Mac,
77 or <filename>Data\Person.hs</filename> on Windows.
78 </para>
79 </sect1>
80
81 <sect1>
82 <title>Options overview</title>
83
84 <para>GHC's behaviour is controlled by
85 <firstterm>options</firstterm>, which for historical reasons are
86 also sometimes referred to as command-line flags or arguments.
87 Options can be specified in three ways:</para>
88
89 <sect2>
90 <title>Command-line arguments</title>
91
92 <indexterm><primary>structure, command-line</primary></indexterm>
93 <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
94 <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
95
96 <para>An invocation of GHC takes the following form:</para>
97
98 <screen>
99 ghc [argument...]
100 </screen>
101
102 <para>Command-line arguments are either options or file names.</para>
103
104 <para>Command-line options begin with <literal>-</literal>.
105 They may <emphasis>not</emphasis> be grouped:
106 <option>-vO</option> is different from <option>-v -O</option>.
107 Options need not precede filenames: e.g., <literal>ghc *.o -o
108 foo</literal>. All options are processed and then applied to
109 all files; you cannot, for example, invoke <literal>ghc -c -O1
110 Foo.hs -O2 Bar.hs</literal> to apply different optimisation
111 levels to the files <filename>Foo.hs</filename> and
112 <filename>Bar.hs</filename>.</para>
113 </sect2>
114
115 <sect2 id="source-file-options">
116 <title>Command line options in source files</title>
117
118 <indexterm><primary>source-file options</primary></indexterm>
119
120 <para>Sometimes it is useful to make the connection between a
121 source file and the command-line options it requires quite
122 tight. For instance, if a Haskell source file deliberately
123 uses name shadowing, it should be compiled with the
124 <option>-fno-warn-name-shadowing</option> option. Rather than maintaining
125 the list of per-file options in a <filename>Makefile</filename>,
126 it is possible to do this directly in the source file using the
127 <literal>OPTIONS_GHC</literal> pragma <indexterm><primary>OPTIONS_GHC
128 pragma</primary></indexterm>:</para>
129
130 <programlisting>
131 {-# OPTIONS_GHC -fno-warn-name-shadowing #-}
132 module X where
133 ...
134 </programlisting>
135
136 <para><literal>OPTIONS_GHC</literal> is a <emphasis>file-header pragma</emphasis>
137 (see <xref linkend="pragmas"/>).</para>
138
139 <para>Only <emphasis>dynamic</emphasis> flags can be used in an <literal>OPTIONS_GHC</literal> pragma
140 (see <xref linkend="static-dynamic-flags"/>).</para>
141
142 <para>Note that your command shell does not
143 get to the source file options, they are just included literally
144 in the array of command-line arguments the compiler
145 maintains internally, so you'll be desperately disappointed if
146 you try to glob etc. inside <literal>OPTIONS_GHC</literal>.</para>
147
148 <para>NOTE: the contents of OPTIONS_GHC are appended to the
149 command-line options, so options given in the source file
150 override those given on the command-line.</para>
151
152 <para>It is not recommended to move all the contents of your
153 Makefiles into your source files, but in some circumstances, the
154 <literal>OPTIONS_GHC</literal> pragma is the Right Thing. (If you
155 use <option>-keep-hc-file</option> and have OPTION flags in
156 your module, the OPTIONS_GHC will get put into the generated .hc
157 file).</para>
158 </sect2>
159
160 <sect2>
161 <title>Setting options in GHCi</title>
162
163 <para>Options may also be modified from within GHCi, using the
164 <literal>:set</literal> command. See <xref linkend="ghci-set"/>
165 for more details.</para>
166 </sect2>
167 </sect1>
168
169 <sect1 id="static-dynamic-flags">
170 <title>Static, Dynamic, and Mode options</title>
171 <indexterm><primary>static</primary><secondary>options</secondary>
172 </indexterm>
173 <indexterm><primary>dynamic</primary><secondary>options</secondary>
174 </indexterm>
175 <indexterm><primary>mode</primary><secondary>options</secondary>
176 </indexterm>
177
178 <para>Each of GHC's command line options is classified as
179 <firstterm>static</firstterm>, <firstterm>dynamic</firstterm> or
180 <firstterm>mode</firstterm>:</para>
181
182 <variablelist>
183 <varlistentry>
184 <term>Mode flags</term>
185 <listitem>
186 <para>For example, <option>&ndash;&ndash;make</option> or <option>-E</option>.
187 There may only be a single mode flag on the command line. The
188 available modes are listed in <xref linkend="modes"/>.</para>
189 </listitem>
190 </varlistentry>
191 <varlistentry>
192 <term>Dynamic Flags</term>
193 <listitem>
194 <para>Most non-mode flags fall into this category. A dynamic flag
195 may be used on the command line, in a
196 <literal>OPTIONS_GHC</literal> pragma in a source file, or set
197 using <literal>:set</literal> in GHCi.</para>
198 </listitem>
199 </varlistentry>
200 <varlistentry>
201 <term>Static Flags</term>
202 <listitem>
203 <para>A few flags are "static", which means they can only be used on
204 the command-line, and remain in force over the entire GHC/GHCi
205 run.</para>
206 </listitem>
207 </varlistentry>
208 </variablelist>
209
210 <para>The flag reference tables (<xref
211 linkend="flag-reference"/>) lists the status of each flag.</para>
212
213 <para>There are a few flags that are static except that they can
214 also be used with GHCi's <literal>:set</literal> command; these
215 are listed as &ldquo;static/<literal>:set</literal>&rdquo; in the
216 table.</para>
217 </sect1>
218
219 <sect1 id="file-suffixes">
220 <title>Meaningful file suffixes</title>
221
222 <indexterm><primary>suffixes, file</primary></indexterm>
223 <indexterm><primary>file suffixes for GHC</primary></indexterm>
224
225 <para>File names with &ldquo;meaningful&rdquo; suffixes (e.g.,
226 <filename>.lhs</filename> or <filename>.o</filename>) cause the
227 &ldquo;right thing&rdquo; to happen to those files.</para>
228
229 <variablelist>
230
231 <varlistentry>
232 <term><filename>.hs</filename></term>
233 <listitem>
234 <para>A Haskell module.</para>
235 </listitem>
236 </varlistentry>
237
238 <varlistentry>
239 <term>
240 <filename>.lhs</filename>
241 <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
242 </term>
243 <listitem>
244 <para>A &ldquo;literate Haskell&rdquo; module.</para>
245 </listitem>
246 </varlistentry>
247
248 <varlistentry>
249 <term><filename>.hi</filename></term>
250 <listitem>
251 <para>A Haskell interface file, probably
252 compiler-generated.</para>
253 </listitem>
254 </varlistentry>
255
256 <varlistentry>
257 <term><filename>.hc</filename></term>
258 <listitem>
259 <para>Intermediate C file produced by the Haskell
260 compiler.</para>
261 </listitem>
262 </varlistentry>
263
264 <varlistentry>
265 <term><filename>.c</filename></term>
266 <listitem>
267 <para>A C&nbsp;file not produced by the Haskell
268 compiler.</para>
269 </listitem>
270 </varlistentry>
271
272 <varlistentry>
273 <term><filename>.ll</filename></term>
274 <listitem>
275 <para>An llvm-intermediate-language source file, usually
276 produced by the compiler.</para>
277 </listitem>
278 </varlistentry>
279
280 <varlistentry>
281 <term><filename>.bc</filename></term>
282 <listitem>
283 <para>An llvm-intermediate-language bitcode file, usually
284 produced by the compiler.</para>
285 </listitem>
286 </varlistentry>
287
288 <varlistentry>
289 <term><filename>.s</filename></term>
290 <listitem>
291 <para>An assembly-language source file, usually produced by
292 the compiler.</para>
293 </listitem>
294 </varlistentry>
295
296 <varlistentry>
297 <term><filename>.o</filename></term>
298 <listitem>
299 <para>An object file, produced by an assembler.</para>
300 </listitem>
301 </varlistentry>
302 </variablelist>
303
304 <para>Files with other suffixes (or without suffixes) are passed
305 straight to the linker.</para>
306
307 </sect1>
308
309 <sect1 id="modes">
310 <title>Modes of operation</title>
311
312 <para>
313 GHC's behaviour is firstly controlled by a mode flag. Only one
314 of these flags may be given, but it does not necessarily need to
315 be the first option on the command-line.
316 </para>
317
318 <para>
319 If no mode flag is present, then GHC will enter make mode
320 (<xref linkend="make-mode" />) if there are any Haskell source
321 files given on the command line, or else it will link the
322 objects named on the command line to produce an executable.
323 </para>
324
325 <para>The available mode flags are:</para>
326
327 <variablelist>
328 <varlistentry>
329 <term>
330 <cmdsynopsis><command>ghc --interactive</command>
331 </cmdsynopsis>
332 <indexterm><primary>interactive mode</primary></indexterm>
333 <indexterm><primary>ghci</primary></indexterm>
334 </term>
335 <listitem>
336 <para>Interactive mode, which is also available as
337 <command>ghci</command>. Interactive mode is described in
338 more detail in <xref linkend="ghci"/>.</para>
339 </listitem>
340 </varlistentry>
341
342 <varlistentry>
343 <term>
344 <cmdsynopsis><command>ghc &ndash;&ndash;make</command>
345 </cmdsynopsis>
346 <indexterm><primary>make mode</primary></indexterm>
347 <indexterm><primary><option>&ndash;&ndash;make</option></primary></indexterm>
348 </term>
349 <listitem>
350 <para>In this mode, GHC will build a multi-module Haskell
351 program automatically, figuring out dependencies for itself.
352 If you have a straightforward Haskell program, this is
353 likely to be much easier, and faster, than using
354 <command>make</command>. Make mode is described in <xref
355 linkend="make-mode"/>.</para>
356
357 <para>
358 This mode is the default if there are any Haskell
359 source files mentioned on the command line, and in this case
360 the <option>&ndash;&ndash;make</option> option can be omitted.
361 </para>
362 </listitem>
363 </varlistentry>
364
365 <varlistentry>
366 <term>
367 <cmdsynopsis><command>ghc -e</command>
368 <arg choice='plain'><replaceable>expr</replaceable></arg>
369 </cmdsynopsis>
370 <indexterm><primary>eval mode</primary></indexterm>
371 </term>
372 <listitem>
373 <para>Expression-evaluation mode. This is very similar to
374 interactive mode, except that there is a single expression
375 to evaluate (<replaceable>expr</replaceable>) which is given
376 on the command line. See <xref linkend="eval-mode"/> for
377 more details.</para>
378 </listitem>
379 </varlistentry>
380
381 <varlistentry>
382 <term>
383 <cmdsynopsis>
384 <command>ghc -E</command>
385 <command>ghc -c</command>
386 <command>ghc -S</command>
387 <command>ghc -c</command>
388 </cmdsynopsis>
389 <indexterm><primary><option>-E</option></primary></indexterm>
390 <indexterm><primary><option>-C</option></primary></indexterm>
391 <indexterm><primary><option>-S</option></primary></indexterm>
392 <indexterm><primary><option>-c</option></primary></indexterm>
393 </term>
394 <listitem>
395 <para>This is the traditional batch-compiler mode, in which
396 GHC can compile source files one at a time, or link objects
397 together into an executable. This mode also applies if
398 there is no other mode flag specified on the command line,
399 in which case it means that the specified files should be
400 compiled and then linked to form a program. See <xref
401 linkend="options-order"/>.</para>
402 </listitem>
403 </varlistentry>
404
405 <varlistentry>
406 <term>
407 <cmdsynopsis>
408 <command>ghc -M</command>
409 </cmdsynopsis>
410 <indexterm><primary>dependency-generation mode</primary></indexterm>
411 </term>
412 <listitem>
413 <para>Dependency-generation mode. In this mode, GHC can be
414 used to generate dependency information suitable for use in
415 a <literal>Makefile</literal>. See <xref
416 linkend="makefile-dependencies"/>.</para>
417 </listitem>
418 </varlistentry>
419
420 <varlistentry>
421 <term>
422 <cmdsynopsis>
423 <command>ghc --mk-dll</command>
424 </cmdsynopsis>
425 <indexterm><primary>DLL-creation mode</primary></indexterm>
426 </term>
427 <listitem>
428 <para>DLL-creation mode (Windows only). See <xref
429 linkend="win32-dlls-create"/>.</para>
430 </listitem>
431 </varlistentry>
432
433 <varlistentry>
434 <term>
435 <cmdsynopsis>
436 <command>ghc --help</command> <command>ghc -?</command>
437 </cmdsynopsis>
438 <indexterm><primary><option>&ndash;&ndash;help</option></primary></indexterm>
439 </term>
440 <listitem>
441 <para>Cause GHC to spew a long usage message to standard
442 output and then exit.</para>
443 </listitem>
444 </varlistentry>
445
446 <varlistentry>
447 <term>
448 <cmdsynopsis>
449 <command>ghc --show-iface <replaceable>file</replaceable></command>
450 </cmdsynopsis>
451 <indexterm><primary><option>&ndash;&ndash;show-iface</option></primary></indexterm>
452 </term>
453 <listitem>
454 <para>Read the interface in
455 <replaceable>file</replaceable> and dump it as text to
456 <literal>stdout</literal>. For example <literal>ghc --show-iface M.hi</literal>.</para>
457 </listitem>
458 </varlistentry>
459
460 <varlistentry>
461 <term>
462 <cmdsynopsis>
463 <command>ghc --supported-extensions</command>
464 <command>ghc --supported-languages</command>
465 </cmdsynopsis>
466 <indexterm><primary><option>&ndash;&ndash;supported-extensions</option></primary><primary><option>&ndash;&ndash;supported-languages</option></primary></indexterm>
467 </term>
468 <listitem>
469 <para>Print the supported language extensions.</para>
470 </listitem>
471 </varlistentry>
472
473 <varlistentry>
474 <term>
475 <cmdsynopsis>
476 <command>ghc --show-options</command>
477 </cmdsynopsis>
478 <indexterm><primary><option>&ndash;&ndash;show-options</option></primary></indexterm>
479 </term>
480 <listitem>
481 <para>Print the supported command line options. This flag can be used for autocompletion in a shell.</para>
482 </listitem>
483 </varlistentry>
484
485 <varlistentry>
486 <term>
487 <cmdsynopsis>
488 <command>ghc --info</command>
489 </cmdsynopsis>
490 <indexterm><primary><option>&ndash;&ndash;info</option></primary></indexterm>
491 </term>
492 <listitem>
493 <para>Print information about the compiler.</para>
494 </listitem>
495 </varlistentry>
496
497 <varlistentry>
498 <term>
499 <cmdsynopsis>
500 <command>ghc --version</command>
501 <command>ghc -V</command>
502 </cmdsynopsis>
503 <indexterm><primary><option>-V</option></primary></indexterm>
504 <indexterm><primary><option>&ndash;&ndash;version</option></primary></indexterm>
505 </term>
506 <listitem>
507 <para>Print a one-line string including GHC's version number.</para>
508 </listitem>
509 </varlistentry>
510
511 <varlistentry>
512 <term>
513 <cmdsynopsis>
514 <command>ghc --numeric-version</command>
515 </cmdsynopsis>
516 <indexterm><primary><option>&ndash;&ndash;numeric-version</option></primary></indexterm>
517 </term>
518 <listitem>
519 <para>Print GHC's numeric version number only.</para>
520 </listitem>
521 </varlistentry>
522
523 <varlistentry>
524 <term>
525 <cmdsynopsis>
526 <command>ghc --print-libdir</command>
527 </cmdsynopsis>
528 <indexterm><primary><option>&ndash;&ndash;print-libdir</option></primary></indexterm>
529 </term>
530 <listitem>
531 <para>Print the path to GHC's library directory. This is
532 the top of the directory tree containing GHC's libraries,
533 interfaces, and include files (usually something like
534 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
535 is the value of
536 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
537 in the package configuration file
538 (see <xref linkend="packages"/>).</para>
539 </listitem>
540 </varlistentry>
541
542 </variablelist>
543
544 <sect2 id="make-mode">
545 <title>Using <command>ghc</command> <option>&ndash;&ndash;make</option></title>
546 <indexterm><primary><option>&ndash;&ndash;make</option></primary></indexterm>
547 <indexterm><primary>separate compilation</primary></indexterm>
548
549 <para>In this mode, GHC will build a multi-module Haskell program by following
550 dependencies from one or more root modules (usually just
551 <literal>Main</literal>). For example, if your
552 <literal>Main</literal> module is in a file called
553 <filename>Main.hs</filename>, you could compile and link the
554 program like this:</para>
555
556 <screen>
557 ghc &ndash;&ndash;make Main.hs
558 </screen>
559
560 <para>
561 In fact, GHC enters make mode automatically if there are any
562 Haskell source files on the command line and no other mode is
563 specified, so in this case we could just type
564 </para>
565
566 <screen>
567 ghc Main.hs
568 </screen>
569
570 <para>Any number of source file names or module names may be
571 specified; GHC will figure out all the modules in the program by
572 following the imports from these initial modules. It will then
573 attempt to compile each module which is out of date, and
574 finally, if there is a <literal>Main</literal> module, the
575 program will also be linked into an executable.</para>
576
577 <para>The main advantages to using <literal>ghc
578 &ndash;&ndash;make</literal> over traditional
579 <literal>Makefile</literal>s are:</para>
580
581 <itemizedlist>
582 <listitem>
583 <para>GHC doesn't have to be restarted for each compilation,
584 which means it can cache information between compilations.
585 Compiling a multi-module program with <literal>ghc
586 &ndash;&ndash;make</literal> can be up to twice as fast as
587 running <literal>ghc</literal> individually on each source
588 file.</para>
589 </listitem>
590 <listitem>
591 <para>You don't have to write a <literal>Makefile</literal>.</para>
592 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
593 </listitem>
594 <listitem>
595 <para>GHC re-calculates the dependencies each time it is
596 invoked, so the dependencies never get out of sync with the
597 source.</para>
598 </listitem>
599 </itemizedlist>
600
601 <para>Any of the command-line options described in the rest of
602 this chapter can be used with
603 <option>&ndash;&ndash;make</option>, but note that any options
604 you give on the command line will apply to all the source files
605 compiled, so if you want any options to apply to a single source
606 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
607 pragma (see <xref linkend="source-file-options"/>).</para>
608
609 <para>If the program needs to be linked with additional objects
610 (say, some auxiliary C code), then the object files can be
611 given on the command line and GHC will include them when linking
612 the executable.</para>
613
614 <para>Note that GHC can only follow dependencies if it has the
615 source file available, so if your program includes a module for
616 which there is no source file, even if you have an object and an
617 interface file for the module, then GHC will complain. The
618 exception to this rule is for package modules, which may or may
619 not have source files.</para>
620
621 <para>The source files for the program don't all need to be in
622 the same directory; the <option>-i</option> option can be used
623 to add directories to the search path (see <xref
624 linkend="search-path"/>).</para>
625 </sect2>
626
627 <sect2 id="eval-mode">
628 <title>Expression evaluation mode</title>
629
630 <para>This mode is very similar to interactive mode, except that
631 there is a single expression to evaluate which is specified on
632 the command line as an argument to the <option>-e</option>
633 option:</para>
634
635 <screen>
636 ghc -e <replaceable>expr</replaceable>
637 </screen>
638
639 <para>Haskell source files may be named on the command line, and
640 they will be loaded exactly as in interactive mode. The
641 expression is evaluated in the context of the loaded
642 modules.</para>
643
644 <para>For example, to load and run a Haskell program containing
645 a module <literal>Main</literal>, we might say</para>
646
647 <screen>
648 ghc -e Main.main Main.hs
649 </screen>
650
651 <para>or we can just use this mode to evaluate expressions in
652 the context of the <literal>Prelude</literal>:</para>
653
654 <screen>
655 $ ghc -e "interact (unlines.map reverse.lines)"
656 hello
657 olleh
658 </screen>
659 </sect2>
660
661 <sect2 id="options-order">
662 <title>Batch compiler mode</title>
663
664 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
665 given on the command line.</para>
666
667 <para>The first phase to run is determined by each input-file
668 suffix, and the last phase is determined by a flag. If no
669 relevant flag is present, then go all the way through to linking.
670 This table summarises:</para>
671
672 <informaltable>
673 <tgroup cols="4">
674 <colspec align="left"/>
675 <colspec align="left"/>
676 <colspec align="left"/>
677 <colspec align="left"/>
678
679 <thead>
680 <row>
681 <entry>Phase of the compilation system</entry>
682 <entry>Suffix saying &ldquo;start here&rdquo;</entry>
683 <entry>Flag saying &ldquo;stop after&rdquo;</entry>
684 <entry>(suffix of) output file</entry>
685 </row>
686 </thead>
687 <tbody>
688 <row>
689 <entry>literate pre-processor</entry>
690 <entry><literal>.lhs</literal></entry>
691 <entry>-</entry>
692 <entry><literal>.hs</literal></entry>
693 </row>
694
695 <row>
696 <entry>C pre-processor (opt.) </entry>
697 <entry><literal>.hs</literal> (with
698 <option>-cpp</option>)</entry>
699 <entry><option>-E</option></entry>
700 <entry><literal>.hspp</literal></entry>
701 </row>
702
703 <row>
704 <entry>Haskell compiler</entry>
705 <entry><literal>.hs</literal></entry>
706 <entry><option>-C</option>, <option>-S</option></entry>
707 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
708 </row>
709
710 <row>
711 <entry>C compiler (opt.)</entry>
712 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
713 <entry><option>-S</option></entry>
714 <entry><literal>.s</literal></entry>
715 </row>
716
717 <row>
718 <entry>assembler</entry>
719 <entry><literal>.s</literal></entry>
720 <entry><option>-c</option></entry>
721 <entry><literal>.o</literal></entry>
722 </row>
723
724 <row>
725 <entry>linker</entry>
726 <entry><replaceable>other</replaceable></entry>
727 <entry>-</entry>
728 <entry><filename>a.out</filename></entry>
729 </row>
730 </tbody>
731 </tgroup>
732 </informaltable>
733
734 <indexterm><primary><option>-C</option></primary></indexterm>
735 <indexterm><primary><option>-E</option></primary></indexterm>
736 <indexterm><primary><option>-S</option></primary></indexterm>
737 <indexterm><primary><option>-c</option></primary></indexterm>
738
739 <para>Thus, a common invocation would be: </para>
740
741 <screen>
742 ghc -c Foo.hs
743 </screen>
744
745 <para>to compile the Haskell source file
746 <filename>Foo.hs</filename> to an object file
747 <filename>Foo.o</filename>.</para>
748
749 <para>Note: What the Haskell compiler proper produces depends on what
750 backend code generator is used. See <xref linkend="code-generators"/>
751 for more details.</para>
752
753 <para>Note: C pre-processing is optional, the
754 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
755 flag turns it on. See <xref linkend="c-pre-processor"/> for more
756 details.</para>
757
758 <para>Note: The option <option>-E</option><indexterm><primary>-E
759 option</primary></indexterm> runs just the pre-processing passes
760 of the compiler, dumping the result in a file.</para>
761
762 <sect3 id="overriding-suffixes">
763 <title>Overriding the default behaviour for a file</title>
764
765 <para>As described above, the way in which a file is processed by GHC
766 depends on its suffix. This behaviour can be overridden using the
767 <option>-x</option> option:</para>
768
769 <variablelist>
770 <varlistentry>
771 <term><option>-x</option> <replaceable>suffix</replaceable>
772 <indexterm><primary><option>-x</option></primary>
773 </indexterm></term>
774 <listitem>
775 <para>Causes all files following this option on the command
776 line to be processed as if they had the suffix
777 <replaceable>suffix</replaceable>. For example, to compile a
778 Haskell module in the file <literal>M.my-hs</literal>,
779 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
780 </listitem>
781 </varlistentry>
782 </variablelist>
783 </sect3>
784
785 </sect2>
786 </sect1>
787
788 <sect1 id="options-help">
789 <title>Help and verbosity options</title>
790
791 <indexterm><primary>help options</primary></indexterm>
792 <indexterm><primary>verbosity options</primary></indexterm>
793
794 <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
795 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
796 <variablelist>
797 <varlistentry>
798 <term>
799 <option>-v</option>
800 <indexterm><primary><option>-v</option></primary></indexterm>
801 </term>
802 <listitem>
803 <para>The <option>-v</option> option makes GHC
804 <emphasis>verbose</emphasis>: it reports its version number
805 and shows (on stderr) exactly how it invokes each phase of
806 the compilation system. Moreover, it passes the
807 <option>-v</option> flag to most phases; each reports its
808 version number (and possibly some other information).</para>
809
810 <para>Please, oh please, use the <option>-v</option> option
811 when reporting bugs! Knowing that you ran the right bits in
812 the right order is always the first thing we want to
813 verify.</para>
814 </listitem>
815 </varlistentry>
816
817 <varlistentry>
818 <term>
819 <option>-v</option><replaceable>n</replaceable>
820 <indexterm><primary><option>-v</option></primary></indexterm>
821 </term>
822 <listitem>
823 <para>To provide more control over the compiler's verbosity,
824 the <option>-v</option> flag takes an optional numeric
825 argument. Specifying <option>-v</option> on its own is
826 equivalent to <option>-v3</option>, and the other levels
827 have the following meanings:</para>
828
829 <variablelist>
830 <varlistentry>
831 <term><option>-v0</option></term>
832 <listitem>
833 <para>Disable all non-essential messages (this is the
834 default).</para>
835 </listitem>
836 </varlistentry>
837
838 <varlistentry>
839 <term><option>-v1</option></term>
840 <listitem>
841 <para>Minimal verbosity: print one line per
842 compilation (this is the default when
843 <option>&ndash;&ndash;make</option> or
844 <option>&ndash;&ndash;interactive</option> is on).</para>
845 </listitem>
846 </varlistentry>
847
848 <varlistentry>
849 <term><option>-v2</option></term>
850 <listitem>
851 <para>Print the name of each compilation phase as it
852 is executed. (equivalent to
853 <option>-dshow-passes</option>).</para>
854 </listitem>
855 </varlistentry>
856
857 <varlistentry>
858 <term><option>-v3</option></term>
859 <listitem>
860 <para>The same as <option>-v2</option>, except that in
861 addition the full command line (if appropriate) for
862 each compilation phase is also printed.</para>
863 </listitem>
864 </varlistentry>
865
866 <varlistentry>
867 <term><option>-v4</option></term>
868 <listitem>
869 <para>The same as <option>-v3</option> except that the
870 intermediate program representation after each
871 compilation phase is also printed (excluding
872 preprocessed and C/assembly files).</para>
873 </listitem>
874 </varlistentry>
875 </variablelist>
876 </listitem>
877 </varlistentry>
878
879 <varlistentry>
880 <term><option>-ferror-spans</option>
881 <indexterm><primary><option>-ferror-spans</option></primary>
882 </indexterm>
883 </term>
884 <listitem>
885 <para>Causes GHC to emit the full source span of the
886 syntactic entity relating to an error message. Normally, GHC
887 emits the source location of the start of the syntactic
888 entity only.</para>
889
890 <para>For example:</para>
891
892 <screen>
893 test.hs:3:6: parse error on input `where'
894 </screen>
895
896 <para>becomes:</para>
897
898 <screen>
899 test296.hs:3:6-10: parse error on input `where'
900 </screen>
901
902 <para>And multi-line spans are possible too:</para>
903
904 <screen>
905 test.hs:(5,4)-(6,7):
906 Conflicting definitions for `a'
907 Bound at: test.hs:5:4
908 test.hs:6:7
909 In the binding group for: a, b, a
910 </screen>
911
912 <para>Note that line numbers start counting at one, but
913 column numbers start at zero. This choice was made to
914 follow existing convention (i.e. this is how Emacs does
915 it).</para>
916 </listitem>
917 </varlistentry>
918
919 <varlistentry>
920 <term><option>-H</option><replaceable>size</replaceable>
921 <indexterm><primary><option>-H</option></primary></indexterm>
922 </term>
923 <listitem>
924 <para>Set the minimum size of the heap to
925 <replaceable>size</replaceable>.
926 This option is equivalent to
927 <literal>+RTS&nbsp;-H<replaceable>size</replaceable></literal>,
928 see <xref linkend="rts-options-gc" />.
929 </para>
930 </listitem>
931 </varlistentry>
932
933 <varlistentry>
934 <term><option>-Rghc-timing</option>
935 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
936 </term>
937 <listitem>
938 <para>Prints a one-line summary of timing statistics for the
939 GHC run. This option is equivalent to
940 <literal>+RTS&nbsp;-tstderr</literal>, see <xref
941 linkend="rts-options-gc" />.
942 </para>
943 </listitem>
944 </varlistentry>
945 </variablelist>
946 </sect1>
947
948 &separate;
949
950 <sect1 id="options-sanity">
951 <title>Warnings and sanity-checking</title>
952
953 <indexterm><primary>sanity-checking options</primary></indexterm>
954 <indexterm><primary>warnings</primary></indexterm>
955
956
957 <para>GHC has a number of options that select which types of
958 non-fatal error messages, otherwise known as warnings, can be
959 generated during compilation. By default, you get a standard set
960 of warnings which are generally likely to indicate bugs in your
961 program. These are:
962 <option>-fwarn-overlapping-patterns</option>,
963 <option>-fwarn-warnings-deprecations</option>,
964 <option>-fwarn-deprecated-flags</option>,
965 <option>-fwarn-duplicate-constraints</option>,
966 <option>-fwarn-duplicate-exports</option>,
967 <option>-fwarn-missing-fields</option>,
968 <option>-fwarn-missing-methods</option>,
969 <option>-fwarn-lazy-unlifted-bindings</option>,
970 <option>-fwarn-wrong-do-bind</option>,
971 <option>-fwarn-unsupported-calling-conventions</option>,
972 <option>-fwarn-dodgy-foreign-imports</option>, and
973 <option>-fwarn-typeable-instances</option>. The following
974 flags are
975 simple ways to select standard &ldquo;packages&rdquo; of warnings:
976 </para>
977
978 <variablelist>
979
980 <varlistentry>
981 <term><option>-W</option>:</term>
982 <listitem>
983 <indexterm><primary>-W option</primary></indexterm>
984 <para>Provides the standard warnings plus
985 <option>-fwarn-incomplete-patterns</option>,
986 <option>-fwarn-dodgy-exports</option>,
987 <option>-fwarn-dodgy-imports</option>,
988 <option>-fwarn-unused-matches</option>,
989 <option>-fwarn-unused-imports</option>, and
990 <option>-fwarn-unused-binds</option>.</para>
991 </listitem>
992 </varlistentry>
993
994 <varlistentry>
995 <term><option>-Wall</option>:</term>
996 <listitem>
997 <indexterm><primary><option>-Wall</option></primary></indexterm>
998 <para>Turns on all warning options that indicate potentially
999 suspicious code. The warnings that are
1000 <emphasis>not</emphasis> enabled by <option>-Wall</option>
1001 are
1002 <option>-fwarn-tabs</option>,
1003 <option>-fwarn-incomplete-uni-patterns</option>,
1004 <option>-fwarn-incomplete-record-updates</option>,
1005 <option>-fwarn-monomorphism-restriction</option>,
1006 <option>-fwarn-auto-orphans</option>,
1007 <option>-fwarn-implicit-prelude</option>,
1008 <option>-fwarn-missing-local-sigs</option>,
1009 <option>-fwarn-missing-import-lists</option>.</para>
1010 </listitem>
1011 </varlistentry>
1012
1013 <varlistentry>
1014 <term><option>-w</option>:</term>
1015 <listitem>
1016 <indexterm><primary><option>-w</option></primary></indexterm>
1017 <para>Turns off all warnings, including the standard ones and
1018 those that <literal>-Wall</literal> doesn't enable.</para>
1019 </listitem>
1020 </varlistentry>
1021
1022 <varlistentry>
1023 <term><option>-Werror</option>:</term>
1024 <listitem>
1025 <indexterm><primary><option>-Werror</option></primary></indexterm>
1026 <para>Makes any warning into a fatal error. Useful so that you don't
1027 miss warnings when doing batch compilation. </para>
1028 </listitem>
1029 </varlistentry>
1030
1031 <varlistentry>
1032 <term><option>-Wwarn</option>:</term>
1033 <listitem>
1034 <indexterm><primary><option>-Wwarn</option></primary></indexterm>
1035 <para>Warnings are treated only as warnings, not as errors. This is
1036 the default, but can be useful to negate a
1037 <option>-Werror</option> flag.</para>
1038 </listitem>
1039 </varlistentry>
1040
1041 </variablelist>
1042
1043 <para>The full set of warning options is described below. To turn
1044 off any warning, simply give the corresponding
1045 <option>-fno-warn-...</option> option on the command line.</para>
1046
1047 <variablelist>
1048
1049 <varlistentry>
1050 <term><option>-fdefer-type-errors</option>:</term>
1051 <listitem>
1052 <indexterm><primary><option>-fdefer-type-errors</option></primary>
1053 </indexterm>
1054 <indexterm><primary>warnings</primary></indexterm>
1055 <para>Defer as many type errors as possible until runtime.
1056 At compile time you get a warning (instead of an error). At
1057 runtime, if you use a value that depends on a type error, you
1058 get a runtime error; but you can run any type-correct parts of your code
1059 just fine. See <xref linkend="defer-type-errors"/></para>
1060 </listitem>
1061 </varlistentry>
1062
1063 <varlistentry>
1064 <term><option>-fhelpful-errors</option>:</term>
1065 <listitem>
1066 <indexterm><primary><option>-fhelpful-errors</option></primary>
1067 </indexterm>
1068 <indexterm><primary>warnings</primary></indexterm>
1069 <para>When a name or package is not found in scope, make
1070 suggestions for the name or package you might have meant instead.</para>
1071 <para>This option is on by default.</para>
1072 </listitem>
1073 </varlistentry>
1074
1075 <varlistentry>
1076 <term><option>-fwarn-unrecognised-pragmas</option>:</term>
1077 <listitem>
1078 <indexterm><primary><option>-fwarn-unrecognised-pragmas</option></primary>
1079 </indexterm>
1080 <indexterm><primary>warnings</primary></indexterm>
1081 <indexterm><primary>pragmas</primary></indexterm>
1082 <para>Causes a warning to be emitted when a
1083 pragma that GHC doesn't recognise is used. As well as pragmas
1084 that GHC itself uses, GHC also recognises pragmas known to be used
1085 by other tools, e.g. <literal>OPTIONS_HUGS</literal> and
1086 <literal>DERIVE</literal>.</para>
1087
1088 <para>This option is on by default.</para>
1089 </listitem>
1090 </varlistentry>
1091
1092 <varlistentry>
1093 <term><option>-fwarn-warnings-deprecations</option>:</term>
1094 <listitem>
1095 <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
1096 </indexterm>
1097 <indexterm><primary>warnings</primary></indexterm>
1098 <indexterm><primary>deprecations</primary></indexterm>
1099 <para>Causes a warning to be emitted when a
1100 module, function or type with a WARNING or DEPRECATED pragma
1101 is used. See <xref linkend="warning-deprecated-pragma"/> for more
1102 details on the pragmas.</para>
1103
1104 <para>This option is on by default.</para>
1105 </listitem>
1106 </varlistentry>
1107
1108 <varlistentry>
1109 <term><option>-fwarn-deprecated-flags</option>:</term>
1110 <listitem>
1111 <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
1112 </indexterm>
1113 <indexterm><primary>deprecated-flags</primary></indexterm>
1114 <para>Causes a warning to be emitted when a deprecated
1115 commandline flag is used.</para>
1116
1117 <para>This option is on by default.</para>
1118 </listitem>
1119 </varlistentry>
1120
1121 <varlistentry>
1122 <term><option>-fwarn-unsupported-calling-conventions</option>:</term>
1123 <listitem>
1124 <indexterm><primary><option>-fwarn-unsupported-calling-conventions</option></primary>
1125 </indexterm>
1126 <para>Causes a warning to be emitted for foreign declarations
1127 that use unsupported calling conventions. In particular,
1128 if the <literal>stdcall</literal> calling convention is used
1129 on an architecture other than i386 then it will be treated
1130 as <literal>ccall</literal>.</para>
1131 </listitem>
1132 </varlistentry>
1133
1134 <varlistentry>
1135 <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
1136 <listitem>
1137 <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
1138 </indexterm>
1139 <para>Causes a warning to be emitted for foreign imports of
1140 the following form:</para>
1141
1142 <programlisting>
1143 foreign import "f" f :: FunPtr t
1144 </programlisting>
1145
1146 <para>on the grounds that it probably should be</para>
1147
1148 <programlisting>
1149 foreign import "&amp;f" f :: FunPtr t
1150 </programlisting>
1151
1152 <para>The first form declares that `f` is a (pure) C
1153 function that takes no arguments and returns a pointer to a
1154 C function with type `t`, whereas the second form declares
1155 that `f` itself is a C function with type `t`. The first
1156 declaration is usually a mistake, and one that is hard to
1157 debug because it results in a crash, hence this
1158 warning.</para>
1159 </listitem>
1160 </varlistentry>
1161
1162 <varlistentry>
1163 <term><option>-fwarn-dodgy-exports</option>:</term>
1164 <listitem>
1165 <indexterm><primary><option>-fwarn-dodgy-exports</option></primary>
1166 </indexterm>
1167 <para>Causes a warning to be emitted when a datatype
1168 <literal>T</literal> is exported
1169 with all constructors, i.e. <literal>T(..)</literal>, but is it
1170 just a type synonym.</para>
1171 <para>Also causes a warning to be emitted when a module is
1172 re-exported, but that module exports nothing.</para>
1173 </listitem>
1174 </varlistentry>
1175
1176 <varlistentry>
1177 <term><option>-fwarn-dodgy-imports</option>:</term>
1178 <listitem>
1179 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
1180 </indexterm>
1181 <para>Causes a warning to be emitted in the following cases:</para>
1182 <itemizedlist>
1183 <listitem>
1184 <para>When a datatype <literal>T</literal> is imported with all
1185 constructors, i.e. <literal>T(..)</literal>, but has been
1186 exported abstractly, i.e. <literal>T</literal>.
1187 </para>
1188 </listitem>
1189 <listitem>
1190 <para>When an <literal>import</literal> statement hides an
1191 entity that is not exported.</para>
1192 </listitem>
1193 </itemizedlist>
1194 </listitem>
1195 </varlistentry>
1196
1197 <varlistentry>
1198 <term><option>-fwarn-lazy-unlifted-bindings</option>:</term>
1199 <listitem>
1200 <indexterm><primary><option>-fwarn-lazy-unlifted-bindings</option></primary>
1201 </indexterm>
1202 <para>Causes a warning to be emitted when an unlifted type
1203 is bound in a way that looks lazy, e.g.
1204 <literal>where (I# x) = ...</literal>. Use
1205 <literal>where !(I# x) = ...</literal> instead. This will be an
1206 error, rather than a warning, in GHC 7.2.
1207 </para>
1208 </listitem>
1209 </varlistentry>
1210
1211 <varlistentry>
1212 <term><option>-fwarn-duplicate-constraints</option>:</term>
1213 <listitem>
1214 <indexterm><primary><option>-fwarn-duplicate-constraints</option></primary></indexterm>
1215 <indexterm><primary>duplicate constraints, warning</primary></indexterm>
1216
1217 <para>Have the compiler warn about duplicate constraints in a type signature. For
1218 example
1219 <programlisting>
1220 f :: (Eq a, Show a, Eq a) => a -> a
1221 </programlisting>
1222 The warning will indicate the duplicated <literal>Eq a</literal> constraint.
1223 </para>
1224
1225 <para>This option is on by default.</para>
1226 </listitem>
1227 </varlistentry>
1228
1229 <varlistentry>
1230 <term><option>-fwarn-duplicate-exports</option>:</term>
1231 <listitem>
1232 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
1233 <indexterm><primary>duplicate exports, warning</primary></indexterm>
1234 <indexterm><primary>export lists, duplicates</primary></indexterm>
1235
1236 <para>Have the compiler warn about duplicate entries in
1237 export lists. This is useful information if you maintain
1238 large export lists, and want to avoid the continued export
1239 of a definition after you've deleted (one) mention of it in
1240 the export list.</para>
1241
1242 <para>This option is on by default.</para>
1243 </listitem>
1244 </varlistentry>
1245
1246 <varlistentry>
1247 <term><option>-fwarn-hi-shadowing</option>:</term>
1248 <listitem>
1249 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
1250 <indexterm><primary>shadowing</primary>
1251 <secondary>interface files</secondary></indexterm>
1252
1253 <para>Causes the compiler to emit a warning when a module or
1254 interface file in the current directory is shadowing one
1255 with the same module name in a library or other
1256 directory.</para>
1257 </listitem>
1258 </varlistentry>
1259
1260 <varlistentry>
1261 <term><option>-fwarn-identities</option>:</term>
1262 <listitem>
1263 <indexterm><primary><option>-fwarn-identities</option></primary></indexterm>
1264 <para>Causes the compiler to emit a warning when a Prelude numeric
1265 conversion converts a type T to the same type T; such calls
1266 are probably no-ops and can be omitted. The functions checked for
1267 are: <literal>toInteger</literal>,
1268 <literal>toRational</literal>,
1269 <literal>fromIntegral</literal>,
1270 and <literal>realToFrac</literal>.
1271 </para>
1272 </listitem>
1273 </varlistentry>
1274
1275 <varlistentry>
1276 <term><option>-fwarn-implicit-prelude</option>:</term>
1277 <listitem>
1278 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
1279 <indexterm><primary>implicit prelude, warning</primary></indexterm>
1280 <para>Have the compiler warn if the Prelude is implicitly
1281 imported. This happens unless either the Prelude module is
1282 explicitly imported with an <literal>import ... Prelude ...</literal>
1283 line, or this implicit import is disabled (either by
1284 <option>-XNoImplicitPrelude</option> or a
1285 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
1286
1287 <para>Note that no warning is given for syntax that implicitly
1288 refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
1289 would change whether it refers to the Prelude.
1290 For example, no warning is given when
1291 <literal>368</literal> means
1292 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
1293 (where <literal>Prelude</literal> refers to the actual Prelude module,
1294 regardless of the imports of the module being compiled).</para>
1295
1296 <para>This warning is off by default.</para>
1297 </listitem>
1298 </varlistentry>
1299
1300 <varlistentry>
1301 <term><option>-fwarn-incomplete-patterns</option>,
1302 <option>-fwarn-incomplete-uni-patterns</option>:
1303 </term>
1304 <listitem>
1305 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
1306 <indexterm><primary><option>-fwarn-incomplete-uni-patterns</option></primary></indexterm>
1307 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
1308 <indexterm><primary>patterns, incomplete</primary></indexterm>
1309
1310 <para>The option <option>-fwarn-incomplete-patterns</option> warns
1311 about places where
1312 a pattern-match might fail at runtime.
1313 The function
1314 <function>g</function> below will fail when applied to
1315 non-empty lists, so the compiler will emit a warning about
1316 this when <option>-fwarn-incomplete-patterns</option> is
1317 enabled.
1318
1319 <programlisting>
1320 g [] = 2
1321 </programlisting>
1322
1323 This option isn't enabled by default because it can be
1324 a bit noisy, and it doesn't always indicate a bug in the
1325 program. However, it's generally considered good practice
1326 to cover all the cases in your functions, and it is switched
1327 on by <option>-W</option>.</para>
1328
1329 <para>The flag <option>-fwarn-incomplete-uni-patterns</option> is
1330 similar, except that it
1331 applies only to lambda-expressions and pattern bindings, constructs
1332 that only allow a single pattern:
1333
1334 <programlisting>
1335 h = \[] -> 2
1336 Just k = f y
1337 </programlisting>
1338
1339 </para>
1340 </listitem>
1341 </varlistentry>
1342
1343 <varlistentry>
1344 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1345 <listitem>
1346 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1347 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1348 <indexterm><primary>record updates, incomplete</primary></indexterm>
1349
1350 <para>The function
1351 <function>f</function> below will fail when applied to
1352 <literal>Bar</literal>, so the compiler will emit a warning about
1353 this when <option>-fwarn-incomplete-record-updates</option> is
1354 enabled.</para>
1355
1356 <programlisting>
1357 data Foo = Foo { x :: Int }
1358 | Bar
1359
1360 f :: Foo -> Foo
1361 f foo = foo { x = 6 }
1362 </programlisting>
1363
1364 <para>This option isn't enabled by default because it can be
1365 very noisy, and it often doesn't indicate a bug in the
1366 program.</para>
1367 </listitem>
1368 </varlistentry>
1369
1370 <varlistentry>
1371 <term>
1372 <option>-fwarn-missing-fields</option>:
1373 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1374 <indexterm><primary>missing fields, warning</primary></indexterm>
1375 <indexterm><primary>fields, missing</primary></indexterm>
1376 </term>
1377 <listitem>
1378
1379 <para>This option is on by default, and warns you whenever
1380 the construction of a labelled field constructor isn't
1381 complete, missing initializers for one or more fields. While
1382 not an error (the missing fields are initialised with
1383 bottoms), it is often an indication of a programmer error.</para>
1384 </listitem>
1385 </varlistentry>
1386
1387 <varlistentry>
1388 <term>
1389 <option>-fwarn-missing-import-lists</option>:
1390 <indexterm><primary><option>-fwarn-import-lists</option></primary></indexterm>
1391 <indexterm><primary>missing import lists, warning</primary></indexterm>
1392 <indexterm><primary>import lists, missing</primary></indexterm>
1393 </term>
1394 <listitem>
1395
1396 <para>This flag warns if you use an unqualified
1397 <literal>import</literal> declaration
1398 that does not explicitly list the entities brought into scope. For
1399 example
1400 </para>
1401
1402 <programlisting>
1403 module M where
1404 import X( f )
1405 import Y
1406 import qualified Z
1407 p x = f x x
1408 </programlisting>
1409
1410 <para>
1411 The <option>-fwarn-import-lists</option> flag will warn about the import
1412 of <literal>Y</literal> but not <literal>X</literal>
1413 If module <literal>Y</literal> is later changed to export (say) <literal>f</literal>,
1414 then the reference to <literal>f</literal> in <literal>M</literal> will become
1415 ambiguous. No warning is produced for the import of <literal>Z</literal>
1416 because extending <literal>Z</literal>'s exports would be unlikely to produce
1417 ambiguity in <literal>M</literal>.
1418 </para>
1419 </listitem>
1420 </varlistentry>
1421
1422 <varlistentry>
1423 <term><option>-fwarn-missing-methods</option>:</term>
1424 <listitem>
1425 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1426 <indexterm><primary>missing methods, warning</primary></indexterm>
1427 <indexterm><primary>methods, missing</primary></indexterm>
1428
1429 <para>This option is on by default, and warns you whenever
1430 an instance declaration is missing one or more methods, and
1431 the corresponding class declaration has no default
1432 declaration for them.</para>
1433 <para>The warning is suppressed if the method name
1434 begins with an underscore. Here's an example where this is useful:
1435 <programlisting>
1436 class C a where
1437 _simpleFn :: a -> String
1438 complexFn :: a -> a -> String
1439 complexFn x y = ... _simpleFn ...
1440 </programlisting>
1441 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1442 never <literal>_simpleFn</literal>; and (b)
1443 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1444 </para>
1445 </listitem>
1446 </varlistentry>
1447
1448 <varlistentry>
1449 <term><option>-fwarn-missing-signatures</option>:</term>
1450 <listitem>
1451 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1452 <indexterm><primary>type signatures, missing</primary></indexterm>
1453
1454 <para>If you would like GHC to check that every top-level
1455 function/value has a type signature, use the
1456 <option>-fwarn-missing-signatures</option> option. As part of
1457 the warning GHC also reports the inferred type. The
1458 option is off by default.</para>
1459 </listitem>
1460 </varlistentry>
1461
1462 <varlistentry>
1463 <term><option>-fwarn-missing-local-sigs</option>:</term>
1464 <listitem>
1465 <indexterm><primary><option>-fwarn-missing-local-sigs</option></primary></indexterm>
1466 <indexterm><primary>type signatures, missing</primary></indexterm>
1467
1468 <para>If you use the
1469 <option>-fwarn-missing-local-sigs</option> flag GHC will warn
1470 you about any polymorphic local bindings. As part of
1471 the warning GHC also reports the inferred type. The
1472 option is off by default.</para>
1473 </listitem>
1474 </varlistentry>
1475
1476 <varlistentry>
1477 <term><option>-fwarn-name-shadowing</option>:</term>
1478 <listitem>
1479 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1480 <indexterm><primary>shadowing, warning</primary></indexterm>
1481
1482 <para>This option causes a warning to be emitted whenever an
1483 inner-scope value has the same name as an outer-scope value,
1484 i.e. the inner value shadows the outer one. This can catch
1485 typographical errors that turn into hard-to-find bugs, e.g.,
1486 in the inadvertent capture of what would be a recursive call in
1487 <literal>f = ... let f = id in ... f ...</literal>.</para>
1488 <para>The warning is suppressed for names beginning with an underscore. For example
1489 <programlisting>
1490 f x = do { _ignore &lt;- this; _ignore &lt;- that; return (the other) }
1491 </programlisting>
1492 </para>
1493 </listitem>
1494 </varlistentry>
1495
1496 <varlistentry>
1497 <term><option>-fwarn-orphans, -fwarn-auto-orphans</option>:</term>
1498 <listitem>
1499 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1500 <indexterm><primary><option>-fwarn-auto-orphans</option></primary></indexterm>
1501 <indexterm><primary>orphan instances, warning</primary></indexterm>
1502 <indexterm><primary>orphan rules, warning</primary></indexterm>
1503
1504 <para>These flags cause a warning to be emitted whenever the
1505 module contains an "orphan" instance declaration or rewrite rule.
1506 An instance declaration is an orphan if it appears in a module in
1507 which neither the class nor the type being instanced are declared
1508 in the same module. A rule is an orphan if it is a rule for a
1509 function declared in another module. A module containing any
1510 orphans is called an orphan module.</para>
1511 <para>The trouble with orphans is that GHC must pro-actively read the interface
1512 files for all orphan modules, just in case their instances or rules
1513 play a role, whether or not the module's interface would otherwise
1514 be of any use. See <xref linkend="orphan-modules"/> for details.
1515 </para>
1516 <para>The flag <option>-fwarn-orphans</option> warns about user-written
1517 orphan rules or instances. The flag <option>-fwarn-auto-orphans</option>
1518 warns about automatically-generated orphan rules, notably as a result of
1519 specialising functions, for type classes (<literal>Specialise</literal>)
1520 or argument values (<literal>SpecConstr</literal>).</para>
1521 </listitem>
1522 </varlistentry>
1523
1524 <varlistentry>
1525 <term>
1526 <option>-fwarn-overlapping-patterns</option>:
1527 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1528 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1529 <indexterm><primary>patterns, overlapping</primary></indexterm>
1530 </term>
1531 <listitem>
1532 <para>By default, the compiler will warn you if a set of
1533 patterns are overlapping, e.g.,</para>
1534
1535 <programlisting>
1536 f :: String -&#62; Int
1537 f [] = 0
1538 f (_:xs) = 1
1539 f "2" = 2
1540 </programlisting>
1541
1542 <para>where the last pattern match in <function>f</function>
1543 won't ever be reached, as the second pattern overlaps
1544 it. More often than not, redundant patterns is a programmer
1545 mistake/error, so this option is enabled by default.</para>
1546 </listitem>
1547 </varlistentry>
1548
1549 <varlistentry>
1550 <term><option>-fwarn-tabs</option>:</term>
1551 <listitem>
1552 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1553 <indexterm><primary>tabs, warning</primary></indexterm>
1554 <para>Have the compiler warn if there are tabs in your source
1555 file.</para>
1556
1557 <para>This warning is off by default.</para>
1558 </listitem>
1559 </varlistentry>
1560
1561 <varlistentry>
1562 <term><option>-fwarn-type-defaults</option>:</term>
1563 <listitem>
1564 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1565 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1566 <para>Have the compiler warn/inform you where in your source
1567 the Haskell defaulting mechanism for numeric types kicks
1568 in. This is useful information when converting code from a
1569 context that assumed one default into one with another,
1570 e.g., the &lsquo;default default&rsquo; for Haskell 1.4 caused the
1571 otherwise unconstrained value <constant>1</constant> to be
1572 given the type <literal>Int</literal>, whereas Haskell 98
1573 and later
1574 defaults it to <literal>Integer</literal>. This may lead to
1575 differences in performance and behaviour, hence the
1576 usefulness of being non-silent about this.</para>
1577
1578 <para>This warning is off by default.</para>
1579 </listitem>
1580 </varlistentry>
1581
1582 <varlistentry>
1583 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1584 <listitem>
1585 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1586 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1587 <para>Have the compiler warn/inform you where in your source
1588 the Haskell Monomorphism Restriction is applied. If applied silently
1589 the MR can give rise to unexpected behaviour, so it can be helpful
1590 to have an explicit warning that it is being applied.</para>
1591
1592 <para>This warning is off by default.</para>
1593 </listitem>
1594 </varlistentry>
1595
1596 <varlistentry>
1597 <term><option>-fwarn-unused-binds</option>:</term>
1598 <listitem>
1599 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1600 <indexterm><primary>unused binds, warning</primary></indexterm>
1601 <indexterm><primary>binds, unused</primary></indexterm>
1602 <para>Report any function definitions (and local bindings)
1603 which are unused. For top-level functions, the warning is
1604 only given if the binding is not exported.</para>
1605 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1606 mentioned in the right hand side of another definition that is used, or (c) the
1607 function it defines begins with an underscore. The last case provides a
1608 way to suppress unused-binding warnings selectively. </para>
1609 <para> Notice that a variable
1610 is reported as unused even if it appears in the right-hand side of another
1611 unused binding. </para>
1612 </listitem>
1613 </varlistentry>
1614
1615 <varlistentry>
1616 <term><option>-fwarn-unused-imports</option>:</term>
1617 <listitem>
1618 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1619 <indexterm><primary>unused imports, warning</primary></indexterm>
1620 <indexterm><primary>imports, unused</primary></indexterm>
1621
1622 <para>Report any modules that are explicitly imported but
1623 never used. However, the form <literal>import M()</literal> is
1624 never reported as an unused import, because it is a useful idiom
1625 for importing instance declarations, which are anonymous in Haskell.</para>
1626 </listitem>
1627 </varlistentry>
1628
1629 <varlistentry>
1630 <term><option>-fwarn-unused-matches</option>:</term>
1631 <listitem>
1632 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1633 <indexterm><primary>unused matches, warning</primary></indexterm>
1634 <indexterm><primary>matches, unused</primary></indexterm>
1635
1636 <para>Report all unused variables which arise from pattern
1637 matches, including patterns consisting of a single variable.
1638 For instance <literal>f x y = []</literal> would report
1639 <varname>x</varname> and <varname>y</varname> as unused. The
1640 warning is suppressed if the variable name begins with an underscore, thus:
1641 <programlisting>
1642 f _x = True
1643 </programlisting>
1644 </para>
1645 </listitem>
1646 </varlistentry>
1647
1648 <varlistentry>
1649 <term><option>-fwarn-unused-do-bind</option>:</term>
1650 <listitem>
1651 <indexterm><primary><option>-fwarn-unused-do-bind</option></primary></indexterm>
1652 <indexterm><primary>unused do binding, warning</primary></indexterm>
1653 <indexterm><primary>do binding, unused</primary></indexterm>
1654
1655 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1656 that appear to silently throw information away.
1657 For instance <literal>do { mapM popInt xs ; return 10 }</literal> would report
1658 the first statement in the <literal>do</literal> block as suspicious,
1659 as it has the type <literal>StackM [Int]</literal> and not <literal>StackM ()</literal>, but that
1660 <literal>[Int]</literal> value is not bound to anything. The warning is suppressed by
1661 explicitly mentioning in the source code that your program is throwing something away:
1662 <programlisting>
1663 do { _ &lt;- mapM popInt xs ; return 10 }
1664 </programlisting>
1665 Of course, in this particular situation you can do even better:
1666 <programlisting>
1667 do { mapM_ popInt xs ; return 10 }
1668 </programlisting>
1669 </para>
1670 </listitem>
1671 </varlistentry>
1672
1673 <varlistentry>
1674 <term><option>-fwarn-wrong-do-bind</option>:</term>
1675 <listitem>
1676 <indexterm><primary><option>-fwarn-wrong-do-bind</option></primary></indexterm>
1677 <indexterm><primary>apparently erroneous do binding, warning</primary></indexterm>
1678 <indexterm><primary>do binding, apparently erroneous</primary></indexterm>
1679
1680 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1681 that appear to lack a binding.
1682 For instance <literal>do { return (popInt 10) ; return 10 }</literal> would report
1683 the first statement in the <literal>do</literal> block as suspicious,
1684 as it has the type <literal>StackM (StackM Int)</literal> (which consists of two nested applications
1685 of the same monad constructor), but which is not then &quot;unpacked&quot; by binding the result.
1686 The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away:
1687 <programlisting>
1688 do { _ &lt;- return (popInt 10) ; return 10 }
1689 </programlisting>
1690 For almost all sensible programs this will indicate a bug, and you probably intended to write:
1691 <programlisting>
1692 do { popInt 10 ; return 10 }
1693 </programlisting>
1694 </para>
1695 </listitem>
1696 </varlistentry>
1697
1698 <varlistentry>
1699 <term><option>-fwarn-typeable-instances</option>:</term>
1700 <listitem>
1701 <indexterm><primary><option>-fwarn-typeable-instances</option></primary></indexterm>
1702 <indexterm><primary>typeable instances, warning</primary></indexterm>
1703
1704 <para>Report handwritten (ie. not derived) instances of the
1705 <literal>Typeable</literal> class. These are ignore by the compiler;
1706 only derived instances of <literal>Typeable</literal> are allowed.
1707 </para>
1708 </listitem>
1709 </varlistentry>
1710
1711 </variablelist>
1712
1713 <para>If you're feeling really paranoid, the
1714 <option>-dcore-lint</option>
1715 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1716 is a good choice. It turns on heavyweight intra-pass
1717 sanity-checking within GHC. (It checks GHC's sanity, not
1718 yours.)</para>
1719
1720 </sect1>
1721
1722 &packages;
1723
1724 <sect1 id="options-optimise">
1725 <title>Optimisation (code improvement)</title>
1726
1727 <indexterm><primary>optimisation</primary></indexterm>
1728 <indexterm><primary>improvement, code</primary></indexterm>
1729
1730 <para>The <option>-O*</option> options specify convenient
1731 &ldquo;packages&rdquo; of optimisation flags; the
1732 <option>-f*</option> options described later on specify
1733 <emphasis>individual</emphasis> optimisations to be turned on/off;
1734 the <option>-m*</option> options specify
1735 <emphasis>machine-specific</emphasis> optimisations to be turned
1736 on/off.</para>
1737
1738 <sect2 id="optimise-pkgs">
1739 <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1740
1741 <para>There are <emphasis>many</emphasis> options that affect
1742 the quality of code produced by GHC. Most people only have a
1743 general goal, something like &ldquo;Compile quickly&rdquo; or
1744 &ldquo;Make my program run like greased lightning.&rdquo; The
1745 following &ldquo;packages&rdquo; of optimisations (or lack
1746 thereof) should suffice.</para>
1747
1748 <para>Note that higher optimisation levels cause more
1749 cross-module optimisation to be performed, which can have an
1750 impact on how much of your program needs to be recompiled when
1751 you change something. This is one reason to stick to
1752 no-optimisation when developing code.</para>
1753
1754 <variablelist>
1755
1756 <varlistentry>
1757 <term>
1758 No <option>-O*</option>-type option specified:
1759 <indexterm><primary>-O* not specified</primary></indexterm>
1760 </term>
1761 <listitem>
1762 <para>This is taken to mean: &ldquo;Please compile
1763 quickly; I'm not over-bothered about compiled-code
1764 quality.&rdquo; So, for example: <command>ghc -c
1765 Foo.hs</command></para>
1766 </listitem>
1767 </varlistentry>
1768
1769 <varlistentry>
1770 <term>
1771 <option>-O0</option>:
1772 <indexterm><primary><option>-O0</option></primary></indexterm>
1773 </term>
1774 <listitem>
1775 <para>Means &ldquo;turn off all optimisation&rdquo;,
1776 reverting to the same settings as if no
1777 <option>-O</option> options had been specified. Saying
1778 <option>-O0</option> can be useful if
1779 eg. <command>make</command> has inserted a
1780 <option>-O</option> on the command line already.</para>
1781 </listitem>
1782 </varlistentry>
1783
1784 <varlistentry>
1785 <term>
1786 <option>-O</option> or <option>-O1</option>:
1787 <indexterm><primary>-O option</primary></indexterm>
1788 <indexterm><primary>-O1 option</primary></indexterm>
1789 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1790 </term>
1791 <listitem>
1792 <para>Means: &ldquo;Generate good-quality code without
1793 taking too long about it.&rdquo; Thus, for example:
1794 <command>ghc -c -O Main.lhs</command></para>
1795 </listitem>
1796 </varlistentry>
1797
1798 <varlistentry>
1799 <term>
1800 <option>-O2</option>:
1801 <indexterm><primary>-O2 option</primary></indexterm>
1802 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1803 </term>
1804 <listitem>
1805 <para>Means: &ldquo;Apply every non-dangerous
1806 optimisation, even if it means significantly longer
1807 compile times.&rdquo;</para>
1808
1809 <para>The avoided &ldquo;dangerous&rdquo; optimisations
1810 are those that can make runtime or space
1811 <emphasis>worse</emphasis> if you're unlucky. They are
1812 normally turned on or off individually.</para>
1813
1814 <para>At the moment, <option>-O2</option> is
1815 <emphasis>unlikely</emphasis> to produce better code than
1816 <option>-O</option>.</para>
1817 </listitem>
1818 </varlistentry>
1819 </variablelist>
1820
1821 <para>We don't use a <option>-O*</option> flag for day-to-day
1822 work. We use <option>-O</option> to get respectable speed;
1823 e.g., when we want to measure something. When we want to go for
1824 broke, we tend to use <option>-O2</option> (and we go for
1825 lots of coffee breaks).</para>
1826
1827 <para>The easiest way to see what <option>-O</option> (etc.)
1828 &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1829 then stand back in amazement.</para>
1830 </sect2>
1831
1832 <sect2 id="options-f">
1833 <title><option>-f*</option>: platform-independent flags</title>
1834
1835 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1836 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1837
1838 <para>These flags turn on and off individual optimisations.
1839 They are normally set via the <option>-O</option> options
1840 described above, and as such, you shouldn't need to set any of
1841 them explicitly (indeed, doing so could lead to unexpected
1842 results). A flag <option>-fwombat</option> can be negated by
1843 saying <option>-fno-wombat</option>. The flags below are off
1844 by default, except where noted below.
1845 </para>
1846
1847 <variablelist>
1848 <varlistentry>
1849 <term>
1850 <option>-fcse</option>
1851 <indexterm><primary><option>-fcse</option></primary></indexterm>
1852 </term>
1853 <listitem>
1854 <para><emphasis>On by default.</emphasis>. Enables the common-sub-expression
1855 elimination optimisation.
1856 Switching this off can be useful if you have some <literal>unsafePerformIO</literal>
1857 expressions that you don't want commoned-up.</para>
1858 </listitem>
1859 </varlistentry>
1860
1861 <varlistentry>
1862 <term>
1863 <option>-fstrictness</option>
1864 <indexterm><primary><option></option></primary></indexterm>
1865 </term>
1866 <listitem>
1867 <para> <emphasis>On by default.</emphasis>.
1868 Switch on the strictness analyser. There is a very old paper about GHC's
1869 strictness analyser, <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/simple-strictnes-analyser.ps.gz">
1870 Measuring the effectiveness of a simple strictness analyser</ulink>,
1871 but the current one is quite a bit different.
1872 </para>
1873
1874 <para>The strictness analyser figures out when arguments and
1875 variables in a function can be treated 'strictly' (that is they
1876 are always evaluated in the function at some point). This allow
1877 GHC to apply certain optimisations such as unboxing that
1878 otherwise don't apply as they change the semantics of the program
1879 when applied to lazy arguments.
1880 </para>
1881 </listitem>
1882 </varlistentry>
1883
1884 <varlistentry>
1885 <term>
1886 <option>-funbox-strict-fields</option>:
1887 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1888 <indexterm><primary>strict constructor fields</primary></indexterm>
1889 <indexterm><primary>constructor fields, strict</primary></indexterm>
1890 </term>
1891 <listitem>
1892 <para>This option causes all constructor fields which are marked
1893 strict (i.e. &ldquo;!&rdquo;) to be unpacked if possible. It is
1894 equivalent to adding an <literal>UNPACK</literal> pragma to every
1895 strict constructor field (see <xref linkend="unpack-pragma"/>).
1896 </para>
1897
1898 <para>This option is a bit of a sledgehammer: it might sometimes
1899 make things worse. Selectively unboxing fields by using
1900 <literal>UNPACK</literal> pragmas might be better. An alternative
1901 is to use <option>-funbox-strict-fields</option> to turn on
1902 unboxing by default but disable it for certain constructor
1903 fields using the <literal>NOUNPACK</literal> pragma (see
1904 <xref linkend="nounpack-pragma"/>).</para>
1905 </listitem>
1906 </varlistentry>
1907
1908 <varlistentry>
1909 <term>
1910 <option>-funbox-small-strict-fields</option>:
1911 <indexterm><primary><option>-funbox-small-strict-fields</option></primary></indexterm>
1912 <indexterm><primary>strict constructor fields</primary></indexterm>
1913 <indexterm><primary>constructor fields, strict</primary></indexterm>
1914 </term>
1915 <listitem>
1916 <para><emphasis>On by default.</emphasis>. This option
1917 causes all constructor fields which are marked strict
1918 (i.e. &ldquo;!&rdquo;) and which representation is smaller
1919 or equal to the size of a pointer to be unpacked, if
1920 possible. It is equivalent to adding an
1921 <literal>UNPACK</literal> pragma (see <xref
1922 linkend="unpack-pragma"/>) to every strict constructor
1923 field that fulfils the size restriction.
1924 </para>
1925
1926 <para>For example, the constructor fields in the following
1927 data types
1928 <programlisting>
1929 data A = A !Int
1930 data B = B !A
1931 newtype C = C B
1932 data D = D !C
1933 </programlisting>
1934 would all be represented by a single
1935 <literal>Int#</literal> (see <xref linkend="primitives"/>)
1936 value with
1937 <option>-funbox-small-strict-fields</option> enabled.
1938 </para>
1939
1940 <para>This option is less of a sledgehammer than
1941 <option>-funbox-strict-fields</option>: it should rarely make things
1942 worse. If you use <option>-funbox-small-strict-fields</option>
1943 to turn on unboxing by default you can disable it for certain
1944 constructor fields using the <literal>NOUNPACK</literal> pragma (see
1945 <xref linkend="nounpack-pragma"/>).</para>
1946
1947 <para>
1948 Note that for consistency <literal>Double</literal>,
1949 <literal>Word64</literal>, and <literal>Int64</literal> constructor
1950 fields are unpacked on 32-bit platforms, even though they are
1951 technically larger than a pointer on those platforms.
1952 </para>
1953 </listitem>
1954 </varlistentry>
1955
1956 <varlistentry>
1957 <term>
1958 <option>-fspec-constr</option>
1959 <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
1960 </term>
1961 <listitem>
1962 <para><emphasis>Off by default, but enabled by -O2.</emphasis>
1963 Turn on call-pattern specialisation; see
1964 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/spec-constr/index.htm">
1965 Call-pattern specialisation for Haskell programs</ulink>.
1966 </para>
1967
1968 <para>This optimisation specializes recursive functions according to
1969 their argument "shapes". This is best explained by example so
1970 consider:
1971 <programlisting>
1972 last :: [a] -> a
1973 last [] = error "last"
1974 last (x : []) = x
1975 last (x : xs) = last xs
1976 </programlisting>
1977 In this code, once we pass the initial check for an empty list we
1978 know that in the recursive case this pattern match is redundant. As
1979 such <option>-fspec-constr</option> will transform the above code
1980 to:
1981 <programlisting>
1982 last :: [a] -> a
1983 last [] = error "last"
1984 last (x : xs) = last' x xs
1985 where
1986 last' x [] = x
1987 last' x (y : ys) = last' y ys
1988 </programlisting>
1989 </para>
1990
1991 <para>As well avoid unnecessary pattern matching it also helps avoid
1992 unnecessary allocation. This applies when a argument is strict in
1993 the recursive call to itself but not on the initial entry. As
1994 strict recursive branch of the function is created similar to the
1995 above example.
1996 </para>
1997 </listitem>
1998 </varlistentry>
1999
2000 <varlistentry>
2001 <term>
2002 <option>-fspecialise</option>
2003 <indexterm><primary><option>-fspecialise</option></primary></indexterm>
2004 </term>
2005 <listitem>
2006 <para><emphasis>On by default.</emphasis>
2007 Specialise each type-class-overloaded function defined in this
2008 module for the types at which it is called in this module. Also
2009 specialise imported functions that have an INLINABLE pragma
2010 (<xref linkend="inlinable-pragma"/>) for the types at which they
2011 are called in this module.
2012 </para>
2013 </listitem>
2014 </varlistentry>
2015
2016 <varlistentry>
2017 <term>
2018 <option>-fstatic-argument-transformation</option>
2019 <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
2020 </term>
2021 <listitem>
2022 <para>Turn on the static argument transformation, which turns a
2023 recursive function into a non-recursive one with a local
2024 recursive loop. See Chapter 7 of
2025 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/santos-thesis.ps.gz">
2026 Andre Santos's PhD thesis</ulink>
2027 </para>
2028 </listitem>
2029 </varlistentry>
2030
2031 <varlistentry>
2032 <term>
2033 <option>-ffloat-in</option>
2034 <indexterm><primary><option></option></primary></indexterm>
2035 </term>
2036 <listitem>
2037 <para><emphasis>On by default.</emphasis>
2038 Float let-bindings inwards, nearer their binding site. See
2039 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">
2040 Let-floating: moving bindings to give faster programs (ICFP'96)</ulink>.
2041 </para>
2042
2043 <para>This optimisation moves let bindings closer to their use
2044 site. The benefit here is that this may avoid unnecessary
2045 allocation if the branch the let is now on is never executed. It
2046 also enables other optimisation passes to work more effectively
2047 as they have more information locally.
2048 </para>
2049
2050 <para>This optimisation isn't always beneficial though (so GHC
2051 applies some heuristics to decide when to apply it). The details
2052 get complicated but a simple example is that it is often beneficial
2053 to move let bindings outwards so that multiple let bindings can be
2054 grouped into a larger single let binding, effectively batching
2055 their allocation and helping the garbage collector and allocator.
2056 </para>
2057 </listitem>
2058 </varlistentry>
2059
2060 <varlistentry>
2061 <term>
2062 <option>-ffull-laziness</option>
2063 <indexterm><primary><option>-ffull-laziness</option></primary></indexterm>
2064 </term>
2065 <listitem>
2066 <para><emphasis>On by default.</emphasis>
2067 Run the full laziness optimisation (also known as let-floating),
2068 which floats let-bindings outside enclosing lambdas, in the hope
2069 they will be thereby be computed less often. See
2070 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">Let-floating:
2071 moving bindings to give faster programs (ICFP'96)</ulink>.
2072 Full laziness increases sharing, which can lead to increased memory
2073 residency.
2074 </para>
2075
2076 <para>NOTE: GHC doesn't implement complete full-laziness.
2077 When optimisation in on, and <option>-fno-full-laziness</option>
2078 is not given, some transformations that increase sharing are
2079 performed, such as extracting repeated computations from a loop.
2080 These are the same transformations that a fully lazy
2081 implementation would do, the difference is that GHC doesn't
2082 consistently apply full-laziness, so don't rely on it.
2083 </para>
2084 </listitem>
2085 </varlistentry>
2086
2087 <varlistentry>
2088 <term>
2089 <option>-fdo-lambda-eta-expansion</option>
2090 <indexterm><primary><option></option></primary></indexterm>
2091 </term>
2092 <listitem>
2093 <para><emphasis>On by default.</emphasis>
2094 Eta-expand let-bindings to increase their arity.
2095 </para>
2096 </listitem>
2097 </varlistentry>
2098
2099 <varlistentry>
2100 <term>
2101 <option>-fdo-eta-reduction</option>
2102 <indexterm><primary><option></option></primary></indexterm>
2103 </term>
2104 <listitem>
2105 <para><emphasis>On by default.</emphasis>
2106 Eta-reduce lambda expressions, if doing so gets rid of a whole
2107 group of lambdas.
2108 </para>
2109 </listitem>
2110 </varlistentry>
2111
2112 <varlistentry>
2113 <term>
2114 <option>-fcase-merge</option>
2115 <indexterm><primary><option></option></primary></indexterm>
2116 </term>
2117 <listitem>
2118 <para><emphasis>On by default.</emphasis>
2119 Merge immediately-nested case expressions that scrutinse the same variable. Example
2120 <programlisting>
2121 case x of
2122 Red -> e1
2123 _ -> case x of
2124 Blue -> e2
2125 Green -> e3
2126 ==>
2127 case x of
2128 Red -> e1
2129 Blue -> e2
2130 Green -> e2
2131 </programlisting>
2132 </para>
2133 </listitem>
2134 </varlistentry>
2135
2136 <varlistentry>
2137 <term>
2138 <option>-fliberate-case</option>
2139 <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
2140 </term>
2141 <listitem>
2142 <para><emphasis>Off by default, but enabled by -O2.</emphasis>
2143 Turn on the liberate-case transformation. This unrolls recursive
2144 function once in its own RHS, to avoid repeated case analysis of
2145 free variables. It's a bit like the call-pattern specialiser
2146 (<option>-fspec-constr</option>) but for free variables rather than
2147 arguments.
2148 </para>
2149 </listitem>
2150 </varlistentry>
2151
2152 <varlistentry>
2153 <term>
2154 <option>-fdicts-cheap</option>
2155 <indexterm><primary><option></option></primary></indexterm>
2156 </term>
2157 <listitem>
2158 <para>A very experimental flag that makes dictionary-valued
2159 expressions seem cheap to the optimiser.
2160 </para>
2161 </listitem>
2162 </varlistentry>
2163
2164 <varlistentry>
2165 <term>
2166 <option>-feager-blackholing</option>
2167 <indexterm><primary><option></option></primary></indexterm>
2168 </term>
2169 <listitem>
2170 <para>Usually GHC black-holes a thunk only when it switches
2171 threads. This flag makes it do so as soon as the thunk is
2172 entered. See <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/">
2173 Haskell on a shared-memory multiprocessor</ulink>.
2174 </para>
2175 </listitem>
2176 </varlistentry>
2177
2178 <varlistentry>
2179 <term>
2180 <option>-fno-state-hack</option>
2181 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
2182 </term>
2183 <listitem>
2184 <para>Turn off the "state hack" whereby any lambda with a
2185 <literal>State#</literal> token as argument is considered to be
2186 single-entry, hence it is considered OK to inline things inside
2187 it. This can improve performance of IO and ST monad code, but it
2188 runs the risk of reducing sharing.
2189 </para>
2190 </listitem>
2191 </varlistentry>
2192
2193 <varlistentry>
2194 <term>
2195 <option>-fpedantic-bottoms</option>
2196 <indexterm><primary><option>-fpedantic-bottoms</option></primary></indexterm>
2197 </term>
2198 <listitem>
2199 <para>Make GHC be more precise about its treatment of bottom (but see also
2200 <option>-fno-state-hack</option>). In particular, stop GHC
2201 eta-expanding through a case expression, which is good for
2202 performance, but bad if you are using <literal>seq</literal> on
2203 partial applications.
2204 </para>
2205 </listitem>
2206 </varlistentry>
2207
2208 <varlistentry>
2209 <term>
2210 <option>-fsimpl-tick-factor=<replaceable>n</replaceable></option>
2211 <indexterm><primary><option>-fsimpl-tick-factor</option></primary></indexterm>
2212 </term>
2213 <listitem>
2214 <para>GHC's optimiser can diverge if you write rewrite rules (
2215 <xref linkend="rewrite-rules"/>) that don't terminate, or (less
2216 satisfactorily) if you code up recursion through data types
2217 (<xref linkend="bugs-ghc"/>). To avoid making the compiler fall
2218 into an infinite loop, the optimiser carries a "tick count" and
2219 stops inlining and applying rewrite rules when this count is
2220 exceeded. The limit is set as a multiple of the program size, so
2221 bigger programs get more ticks. The
2222 <option>-fsimpl-tick-factor</option> flag lets you change the
2223 multiplier. The default is 100; numbers larger than 100 give more
2224 ticks, and numbers smaller than 100 give fewer.
2225 </para>
2226
2227 <para>If the tick-count expires, GHC summarises what simplifier
2228 steps it has done; you can use
2229 <option>-fddump-simpl-stats</option> to generate a much more
2230 detailed list. Usually that identifies the loop quite
2231 accurately, because some numbers are very large.
2232 </para>
2233 </listitem>
2234 </varlistentry>
2235
2236 <varlistentry>
2237 <term>
2238 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
2239 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
2240 <indexterm><primary>inlining, controlling</primary></indexterm>
2241 <indexterm><primary>unfolding, controlling</primary></indexterm>
2242 </term>
2243 <listitem>
2244 <para>(Default: 45) Governs the maximum size that GHC will allow a
2245 function unfolding to be. (An unfolding has a &ldquo;size&rdquo;
2246 that reflects the cost in terms of &ldquo;code bloat&rdquo; of
2247 expanding (aka inlining) that unfolding at a call site. A bigger
2248 function would be assigned a bigger cost.)
2249 </para>
2250
2251 <para>Consequences: (a) nothing larger than this will be inlined
2252 (unless it has an INLINE pragma); (b) nothing larger than this
2253 will be spewed into an interface file.
2254 </para>
2255
2256 <para>Increasing this figure is more likely to result in longer
2257 compile times than faster code. The
2258 <option>-funfolding-use-threshold</option> is more useful.
2259 </para>
2260 </listitem>
2261 </varlistentry>
2262
2263 <varlistentry>
2264 <term>
2265 <option>-funfolding-use-threshold=<replaceable>n</replaceable></option>
2266 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
2267 <indexterm><primary>inlining, controlling</primary></indexterm>
2268 <indexterm><primary>unfolding, controlling</primary></indexterm>
2269 </term>
2270 <listitem>
2271 <para>(Default: 8) This is the magic cut-off figure for unfolding
2272 (aka inlining): below this size, a function definition will be
2273 unfolded at the call-site, any bigger and it won't. The size
2274 computed for a function depends on two things: the actual size of
2275 the expression minus any discounts that
2276 apply (see <option>-funfolding-con-discount</option>).
2277 </para>
2278
2279 <para>The difference between this and
2280 <option>-funfolding-creation-threshold</option> is that this one
2281 determines if a function definition will be inlined <emphasis>at
2282 a call site</emphasis>. The other option determines if a
2283 function definition will be kept around at all for potential
2284 inlining.
2285 </para>
2286 </listitem>
2287 </varlistentry>
2288
2289 <varlistentry>
2290 <term>
2291 <option>-fexpose-all-unfoldings</option>
2292 <indexterm><primary><option></option></primary></indexterm>
2293 </term>
2294 <listitem>
2295 <para>An experimental flag to expose all unfoldings, even for very
2296 large or recursive functions. This allows for all functions to be
2297 inlined while usually GHC would avoid inlining larger functions.
2298 </para>
2299 </listitem>
2300 </varlistentry>
2301
2302 <varlistentry>
2303 <term>
2304 <option>-fvectorise</option>
2305 <indexterm><primary><option></option></primary></indexterm>
2306 </term>
2307 <listitem>
2308 <para>Part of <link linkend="dph">Data Parallel Haskell
2309 (DPH)</link>.</para>
2310
2311 <para><emphasis>Off by default.</emphasis> Enable the
2312 <emphasis>vectorisation</emphasis> optimisation transformation. This
2313 optimisation transforms the nested data parallelism code of programs
2314 using DPH into flat data parallelism. Flat data parallel programs
2315 should have better load balancing, enable SIMD parallelism and
2316 friendlier cache behaviour.</para>
2317 </listitem>
2318 </varlistentry>
2319
2320 <varlistentry>
2321 <term>
2322 <option>-favoid-vect</option>
2323 <indexterm><primary><option></option></primary></indexterm>
2324 </term>
2325 <listitem>
2326 <para>Part of <link linkend="dph">Data Parallel Haskell
2327 (DPH)</link>.</para>
2328
2329 <para><emphasis>Off by default.</emphasis> Enable the
2330 <emphasis>vectorisation</emphasis> avoidance optimisation. This
2331 optimisation only works when used in combination with the
2332 <option>-fvectorise</option> transformation.</para>
2333
2334 <para>While vectorisation of code using DPH is often a big win, it
2335 can also produce worse results for some kinds of code. This
2336 optimisation modifies the vectorisation transformation to try to
2337 determine if a function would be better of unvectorised and if
2338 so, do just that.</para>
2339 </listitem>
2340 </varlistentry>
2341
2342 <varlistentry>
2343 <term>
2344 <option>-fregs-graph</option>
2345 <indexterm><primary><option></option></primary></indexterm>
2346 </term>
2347 <listitem>
2348 <para><emphasis>Off by default, but enabled by -O2. Only applies in
2349 combination with the native code generator.</emphasis>
2350 Use the graph colouring register allocator for register allocation
2351 in the native code generator. By default, GHC uses a simpler,
2352 faster linear register allocator. The downside being that the
2353 linear register allocator usually generates worse code.
2354 </para>
2355 </listitem>
2356 </varlistentry>
2357
2358 <varlistentry>
2359 <term>
2360 <option>-fregs-iterative</option>
2361 <indexterm><primary><option></option></primary></indexterm>
2362 </term>
2363 <listitem>
2364 <para><emphasis>Off by default, only applies in combination with
2365 the native code generator.</emphasis>
2366 Use the iterative coalescing graph colouring register allocator for
2367 register allocation in the native code generator. This is the same
2368 register allocator as the <option>-freg-graph</option> one but also
2369 enables iterative coalescing during register allocation.
2370 </para>
2371 </listitem>
2372 </varlistentry>
2373
2374 <varlistentry>
2375 <term>
2376 <option>-fexcess-precision</option>
2377 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
2378 </term>
2379 <listitem>
2380 <para>When this option is given, intermediate floating
2381 point values can have a <emphasis>greater</emphasis>
2382 precision/range than the final type. Generally this is a
2383 good thing, but some programs may rely on the exact
2384 precision/range of
2385 <literal>Float</literal>/<literal>Double</literal> values
2386 and should not use this option for their compilation.</para>
2387
2388 <para>
2389 Note that the 32-bit x86 native code generator only
2390 supports excess-precision mode, so neither
2391 <option>-fexcess-precision</option> nor
2392 <option>-fno-excess-precision</option> has any effect.
2393 This is a known bug, see <xref linkend="bugs-ghc" />.
2394 </para>
2395 </listitem>
2396 </varlistentry>
2397
2398 <varlistentry>
2399 <term>
2400 <option>-fignore-asserts</option>
2401 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
2402 </term>
2403 <listitem>
2404 <para>Causes GHC to ignore uses of the function
2405 <literal>Exception.assert</literal> in source code (in
2406 other words, rewriting <literal>Exception.assert p
2407 e</literal> to <literal>e</literal> (see <xref
2408 linkend="assertions"/>). This flag is turned on by
2409 <option>-O</option>.
2410 </para>
2411 </listitem>
2412 </varlistentry>
2413
2414 <varlistentry>
2415 <term>
2416 <option>-fignore-interface-pragmas</option>
2417 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
2418 </term>
2419 <listitem>
2420 <para>Tells GHC to ignore all inessential information when reading interface files.
2421 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
2422 for a function, GHC will ignore that information.</para>
2423 </listitem>
2424 </varlistentry>
2425
2426 <varlistentry>
2427 <term>
2428 <option>-fomit-interface-pragmas</option>
2429 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
2430 </term>
2431 <listitem>
2432 <para>Tells GHC to omit all inessential information from the
2433 interface file generated for the module being compiled (say M).
2434 This means that a module importing M will see only the
2435 <emphasis>types</emphasis> of the functions that M exports, but
2436 not their unfoldings, strictness info, etc. Hence, for example,
2437 no function exported by M will be inlined into an importing module.
2438 The benefit is that modules that import M will need to be
2439 recompiled less often (only when M's exports change their type, not
2440 when they change their implementation).</para>
2441 </listitem>
2442 </varlistentry>
2443
2444 <varlistentry>
2445 <term>
2446 <option>-fomit-yields</option>
2447 <indexterm><primary><option>-fomit-yields</option></primary></indexterm>
2448 </term>
2449 <listitem>
2450 <para><emphasis>On by default.</emphasis> Tells GHC to omit
2451 heap checks when no allocation is being performed. While this improves
2452 binary sizes by about 5%, it also means that threads run in
2453 tight non-allocating loops will not get preempted in a timely
2454 fashion. If it is important to always be able to interrupt such
2455 threads, you should turn this optimization off. Consider also
2456 recompiling all libraries with this optimization turned off, if you
2457 need to guarantee interruptibility.
2458 </para>
2459 </listitem>
2460 </varlistentry>
2461
2462 </variablelist>
2463
2464 </sect2>
2465
2466 </sect1>
2467
2468 &code-gens;
2469
2470 &phases;
2471
2472 &shared_libs;
2473
2474 <sect1 id="using-concurrent">
2475 <title>Using Concurrent Haskell</title>
2476 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
2477
2478 <para>GHC supports Concurrent Haskell by default, without requiring a
2479 special option or libraries compiled in a certain way. To get access to
2480 the support libraries for Concurrent Haskell, just import
2481 <ulink
2482 url="&libraryBaseLocation;/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
2483
2484 <para>
2485 Optionally, the program may be linked with
2486 the <option>-threaded</option> option (see
2487 <xref linkend="options-linker" />. This provides two benefits:
2488
2489 <itemizedlist>
2490 <listitem>
2491 <para>It enables the <option>-N</option><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm> RTS option to be
2492 used, which allows threads to run in
2493 parallel<indexterm><primary>parallelism</primary></indexterm>
2494 on a
2495 multiprocessor<indexterm><primary>multiprocessor</primary></indexterm><indexterm><primary>SMP</primary></indexterm>
2496 or
2497 multicore<indexterm><primary>multicore</primary></indexterm>
2498 machine. See <xref linkend="using-smp" />.</para>
2499 </listitem>
2500 <listitem>
2501 <para>If a thread makes a foreign call (and the call is
2502 not marked <literal>unsafe</literal>), then other
2503 Haskell threads in the program will continue to run
2504 while the foreign call is in progress.
2505 Additionally, <literal>foreign export</literal>ed
2506 Haskell functions may be called from multiple OS
2507 threads simultaneously. See
2508 <xref linkend="ffi-threads" />.</para>
2509 </listitem>
2510 </itemizedlist>
2511 </para>
2512
2513 <para>The following RTS option(s) affect the behaviour of Concurrent
2514 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
2515
2516 <variablelist>
2517 <varlistentry>
2518 <term><option>-C<replaceable>s</replaceable></option></term>
2519 <listitem>
2520 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2521 Sets the context switch interval to <replaceable>s</replaceable>
2522 seconds. A context switch will occur at the next heap block
2523 allocation after the timer expires (a heap block allocation occurs
2524 every 4k of allocation). With <option>-C0</option> or
2525 <option>-C</option>, context switches will occur as often as
2526 possible (at every heap block allocation). By default, context
2527 switches occur every 20ms.</para>
2528 </listitem>
2529 </varlistentry>
2530 </variablelist>
2531 </sect1>
2532
2533 <sect1 id="using-smp">
2534 <title>Using SMP parallelism</title>
2535 <indexterm><primary>parallelism</primary>
2536 </indexterm>
2537 <indexterm><primary>SMP</primary>
2538 </indexterm>
2539
2540 <para>GHC supports running Haskell programs in parallel on an SMP
2541 (symmetric multiprocessor).</para>
2542
2543 <para>There's a fine distinction between
2544 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
2545 parallelism is all about making your program run
2546 <emphasis>faster</emphasis> by making use of multiple processors
2547 simultaneously. Concurrency, on the other hand, is a means of
2548 abstraction: it is a convenient way to structure a program that must
2549 respond to multiple asynchronous events.</para>
2550
2551 <para>However, the two terms are certainly related. By making use of
2552 multiple CPUs it is possible to run concurrent threads in parallel,
2553 and this is exactly what GHC's SMP parallelism support does. But it
2554 is also possible to obtain performance improvements with parallelism
2555 on programs that do not use concurrency. This section describes how to
2556 use GHC to compile and run parallel programs, in <xref
2557 linkend="lang-parallel" /> we describe the language features that affect
2558 parallelism.</para>
2559
2560 <sect2 id="parallel-compile-options">
2561 <title>Compile-time options for SMP parallelism</title>
2562
2563 <para>In order to make use of multiple CPUs, your program must be
2564 linked with the <option>-threaded</option> option (see <xref
2565 linkend="options-linker" />). Additionally, the following
2566 compiler options affect parallelism:</para>
2567
2568 <variablelist>
2569 <varlistentry>
2570 <term><option>-feager-blackholing</option></term>
2571 <indexterm><primary><option>-feager-blackholing</option></primary></indexterm>
2572 <listitem>
2573 <para>
2574 Blackholing is the act of marking a thunk (lazy
2575 computuation) as being under evaluation. It is useful for
2576 three reasons: firstly it lets us detect certain kinds of
2577 infinite loop (the <literal>NonTermination</literal>
2578 exception), secondly it avoids certain kinds of space
2579 leak, and thirdly it avoids repeating a computation in a
2580 parallel program, because we can tell when a computation
2581 is already in progress.</para>
2582
2583 <para>
2584 The option <option>-feager-blackholing</option> causes
2585 each thunk to be blackholed as soon as evaluation begins.
2586 The default is "lazy blackholing", whereby thunks are only
2587 marked as being under evaluation when a thread is paused
2588 for some reason. Lazy blackholing is typically more
2589 efficient (by 1-2&percnt; or so), because most thunks don't
2590 need to be blackholed. However, eager blackholing can
2591 avoid more repeated computation in a parallel program, and
2592 this often turns out to be important for parallelism.
2593 </para>
2594
2595 <para>
2596 We recommend compiling any code that is intended to be run
2597 in parallel with the <option>-feager-blackholing</option>
2598 flag.
2599 </para>
2600 </listitem>
2601 </varlistentry>
2602 </variablelist>
2603 </sect2>
2604
2605 <sect2 id="parallel-options">
2606 <title>RTS options for SMP parallelism</title>
2607
2608 <para>There are two ways to run a program on multiple
2609 processors:
2610 call <literal>Control.Concurrent.setNumCapabilities</literal> from your
2611 program, or use the RTS <option>-N</option> option.</para>
2612
2613 <variablelist>
2614 <varlistentry>
2615 <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
2616 <listitem>
2617 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2618 Use <replaceable>x</replaceable> simultaneous threads when
2619 running the program. Normally <replaceable>x</replaceable>
2620 should be chosen to match the number of CPU cores on the
2621 machine<footnote><para>Whether hyperthreading cores should be counted or not is an
2622 open question; please feel free to experiment and let us know what
2623 results you find.</para></footnote>. For example,
2624 on a dual-core machine we would probably use
2625 <literal>+RTS -N2 -RTS</literal>.</para>
2626
2627 <para>Omitting <replaceable>x</replaceable>,
2628 i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
2629 choose the value of <replaceable>x</replaceable> itself
2630 based on how many processors are in your machine.</para>
2631
2632 <para>Be careful when using all the processors in your
2633 machine: if some of your processors are in use by other
2634 programs, this can actually harm performance rather than
2635 improve it.</para>
2636
2637 <para>Setting <option>-N</option> also has the effect of
2638 enabling the parallel garbage collector (see
2639 <xref linkend="rts-options-gc" />).</para>
2640
2641 <para>The current value of the <option>-N</option> option
2642 is available to the Haskell program
2643 via <literal>Control.Concurrent.getNumCapabilities</literal>, and
2644 it may be changed while the program is running by
2645 calling <literal>Control.Concurrent.setNumCapabilities</literal>.</para>
2646 </listitem>
2647 </varlistentry>
2648 </variablelist>
2649
2650 <para>The following options affect the way the runtime schedules
2651 threads on CPUs:</para>
2652
2653 <variablelist>
2654 <varlistentry>
2655 <term><option>-qa</option></term>
2656 <indexterm><primary><option>-qa</option></primary><secondary>RTS
2657 option</secondary></indexterm>
2658 <listitem>
2659 <para>Use the OS's affinity facilities to try to pin OS
2660 threads to CPU cores. This is an experimental feature,
2661 and may or may not be useful. Please let us know
2662 whether it helps for you!</para>
2663 </listitem>
2664 </varlistentry>
2665 <varlistentry>
2666 <term><option>-qm</option></term>
2667 <indexterm><primary><option>-qm</option></primary><secondary>RTS
2668 option</secondary></indexterm>
2669 <listitem>
2670 <para>Disable automatic migration for load balancing.
2671 Normally the runtime will automatically try to schedule
2672 threads across the available CPUs to make use of idle
2673 CPUs; this option disables that behaviour. Note that
2674 migration only applies to threads; sparks created
2675 by <literal>par</literal> are load-balanced separately
2676 by work-stealing.</para>
2677
2678 <para>
2679 This option is probably only of use for concurrent
2680 programs that explicitly schedule threads onto CPUs
2681 with <literal>Control.Concurrent.forkOn</literal>.
2682 </para>
2683 </listitem>
2684 </varlistentry>
2685 </variablelist>
2686 </sect2>
2687
2688 <sect2>
2689 <title>Hints for using SMP parallelism</title>
2690
2691 <para>Add the <literal>-s</literal> RTS option when
2692 running the program to see timing stats, which will help to tell you
2693 whether your program got faster by using more CPUs or not. If the user
2694 time is greater than
2695 the elapsed time, then the program used more than one CPU. You should
2696 also run the program without <literal>-N</literal> for
2697 comparison.</para>
2698
2699 <para>The output of <literal>+RTS -s</literal> tells you how
2700 many &ldquo;sparks&rdquo; were created and executed during the
2701 run of the program (see <xref linkend="rts-options-gc" />), which
2702 will give you an idea how well your <literal>par</literal>
2703 annotations are working.</para>
2704
2705 <para>GHC's parallelism support has improved in 6.12.1 as a
2706 result of much experimentation and tuning in the runtime
2707 system. We'd still be interested to hear how well it works
2708 for you, and we're also interested in collecting parallel
2709 programs to add to our benchmarking suite.</para>
2710 </sect2>
2711 </sect1>
2712
2713 <sect1 id="options-platform">
2714 <title>Platform-specific Flags</title>
2715
2716 <indexterm><primary>-m* options</primary></indexterm>
2717 <indexterm><primary>platform-specific options</primary></indexterm>
2718 <indexterm><primary>machine-specific options</primary></indexterm>
2719
2720 <para>Some flags only make sense for particular target
2721 platforms.</para>
2722
2723 <variablelist>
2724
2725 <varlistentry>
2726 <term><option>-msse2</option>:</term>
2727 <listitem>
2728 <para>
2729 (x86 only, added in GHC 7.0.1) Use the SSE2 registers and
2730 instruction set to implement floating point operations when using
2731 the <link linkend="native-code-gen">native code generator</link>.
2732 This gives a substantial performance improvement for floating
2733 point, but the resulting compiled code
2734 will only run on processors that support SSE2 (Intel Pentium 4 and
2735 later, or AMD Athlon 64 and later). The
2736 <link linkend="llvm-code-gen">LLVM backend</link> will also use SSE2
2737 if your processor supports it but detects this automatically so no
2738 flag is required.
2739 </para>
2740 <para>
2741 SSE2 is unconditionally used on x86-64 platforms.
2742 </para>
2743 </listitem>
2744 </varlistentry>
2745
2746 <varlistentry>
2747 <term><option>-msse4.2</option>:</term>
2748 <listitem>
2749 <para>
2750 (x86 only, added in GHC 7.4.1) Use the SSE4.2 instruction set to
2751 implement some floating point and bit operations when using the
2752 <link linkend="native-code-gen">native code generator</link>. The
2753 resulting compiled code will only run on processors that
2754 support SSE4.2 (Intel Core i7 and later). The
2755 <link linkend="llvm-code-gen">LLVM backend</link> will also use
2756 SSE4.2 if your processor supports it but detects this automatically
2757 so no flag is required.
2758 </para>
2759 </listitem>
2760 </varlistentry>
2761
2762 </variablelist>
2763
2764 </sect1>
2765
2766 &runtime;
2767
2768 <sect1 id="ext-core">
2769 <title>Generating and compiling External Core Files</title>
2770
2771 <indexterm><primary>intermediate code generation</primary></indexterm>
2772
2773 <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format)
2774 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
2775 <filename>.hcr</filename>. The Core format is described in <ulink url="../../core.pdf">
2776 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
2777 and sample tools
2778 for manipulating Core files (in Haskell) are available in the
2779 <ulink url="http://hackage.haskell.org/package/extcore">extcore package on Hackage</ulink>. Note that the format of <literal>.hcr</literal>
2780 files is <emphasis>different</emphasis> from the Core output format that GHC generates
2781 for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
2782
2783 <para>The Core format natively supports notes which you can add to
2784 your source code using the <literal>CORE</literal> pragma (see <xref
2785 linkend="pragmas"/>).</para>
2786
2787 <variablelist>
2788
2789 <varlistentry>
2790 <term>
2791 <option>-fext-core</option>
2792 <indexterm><primary><option>-fext-core</option></primary></indexterm>
2793 </term>
2794 <listitem>
2795 <para>Generate <literal>.hcr</literal> files.</para>
2796 </listitem>
2797 </varlistentry>
2798
2799 </variablelist>
2800
2801 <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>
2802
2803 </sect1>
2804
2805 &debug;
2806 &flags;
2807
2808 </chapter>
2809
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