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