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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>--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 --make</command>
345 </cmdsynopsis>
346 <indexterm><primary>make mode</primary></indexterm>
347 <indexterm><primary><option>--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>--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>--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>--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>--supported-extensions</option></primary><primary><option>--supported-languages</option></primary></indexterm>
467 </term>
468 <listitem>
469 <para>Print the supported language extensions.</para>
470 </listitem>
471 </varlistentry>
472
473 <varlistentry>
474 <term>
475 <cmdsynopsis>
476 <command>ghc --show-options</command>
477 </cmdsynopsis>
478 <indexterm><primary><option>--show-options</option></primary></indexterm>
479 </term>
480 <listitem>
481 <para>Print the supported command line options. This flag can be used for autocompletion in a shell.</para>
482 </listitem>
483 </varlistentry>
484
485 <varlistentry>
486 <term>
487 <cmdsynopsis>
488 <command>ghc --info</command>
489 </cmdsynopsis>
490 <indexterm><primary><option>--info</option></primary></indexterm>
491 </term>
492 <listitem>
493 <para>Print information about the compiler.</para>
494 </listitem>
495 </varlistentry>
496
497 <varlistentry>
498 <term>
499 <cmdsynopsis>
500 <command>ghc --version</command>
501 <command>ghc -V</command>
502 </cmdsynopsis>
503 <indexterm><primary><option>-V</option></primary></indexterm>
504 <indexterm><primary><option>--version</option></primary></indexterm>
505 </term>
506 <listitem>
507 <para>Print a one-line string including GHC's version number.</para>
508 </listitem>
509 </varlistentry>
510
511 <varlistentry>
512 <term>
513 <cmdsynopsis>
514 <command>ghc --numeric-version</command>
515 </cmdsynopsis>
516 <indexterm><primary><option>--numeric-version</option></primary></indexterm>
517 </term>
518 <listitem>
519 <para>Print GHC's numeric version number only.</para>
520 </listitem>
521 </varlistentry>
522
523 <varlistentry>
524 <term>
525 <cmdsynopsis>
526 <command>ghc --print-libdir</command>
527 </cmdsynopsis>
528 <indexterm><primary><option>--print-libdir</option></primary></indexterm>
529 </term>
530 <listitem>
531 <para>Print the path to GHC's library directory. This is
532 the top of the directory tree containing GHC's libraries,
533 interfaces, and include files (usually something like
534 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
535 is the value of
536 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
537 in the package configuration file
538 (see <xref linkend="packages"/>).</para>
539 </listitem>
540 </varlistentry>
541
542 </variablelist>
543
544 <sect2 id="make-mode">
545 <title>Using <command>ghc</command> <option>--make</option></title>
546 <indexterm><primary><option>--make</option></primary></indexterm>
547 <indexterm><primary>separate compilation</primary></indexterm>
548
549 <para>In this mode, GHC will build a multi-module Haskell program by following
550 dependencies from one or more root modules (usually just
551 <literal>Main</literal>). For example, if your
552 <literal>Main</literal> module is in a file called
553 <filename>Main.hs</filename>, you could compile and link the
554 program like this:</para>
555
556 <screen>
557 ghc --make Main.hs
558 </screen>
559
560 <para>
561 In fact, GHC enters make mode automatically if there are any
562 Haskell source files on the command line and no other mode is
563 specified, so in this case we could just type
564 </para>
565
566 <screen>
567 ghc Main.hs
568 </screen>
569
570 <para>Any number of source file names or module names may be
571 specified; GHC will figure out all the modules in the program by
572 following the imports from these initial modules. It will then
573 attempt to compile each module which is out of date, and
574 finally, if there is a <literal>Main</literal> module, the
575 program will also be linked into an executable.</para>
576
577 <para>The main advantages to using <literal>ghc
578 --make</literal> over traditional
579 <literal>Makefile</literal>s are:</para>
580
581 <itemizedlist>
582 <listitem>
583 <para>GHC doesn't have to be restarted for each compilation,
584 which means it can cache information between compilations.
585 Compiling a multi-module program with <literal>ghc
586 --make</literal> can be up to twice as fast as
587 running <literal>ghc</literal> individually on each source
588 file.</para>
589 </listitem>
590 <listitem>
591 <para>You don't have to write a <literal>Makefile</literal>.</para>
592 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
593 </listitem>
594 <listitem>
595 <para>GHC re-calculates the dependencies each time it is
596 invoked, so the dependencies never get out of sync with the
597 source.</para>
598 </listitem>
599 <listitem>
600 <para>Using the <literal>-j</literal> flag, you can compile
601 modules in parallel. Specify <literal>-jN</literal> to
602 compile <replaceable>N</replaceable> jobs in parallel.</para>
603 </listitem>
604 </itemizedlist>
605
606 <para>Any of the command-line options described in the rest of
607 this chapter can be used with
608 <option>--make</option>, but note that any options
609 you give on the command line will apply to all the source files
610 compiled, so if you want any options to apply to a single source
611 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
612 pragma (see <xref linkend="source-file-options"/>).</para>
613
614 <para>If the program needs to be linked with additional objects
615 (say, some auxiliary C code), then the object files can be
616 given on the command line and GHC will include them when linking
617 the executable.</para>
618
619 <para>Note that GHC can only follow dependencies if it has the
620 source file available, so if your program includes a module for
621 which there is no source file, even if you have an object and an
622 interface file for the module, then GHC will complain. The
623 exception to this rule is for package modules, which may or may
624 not have source files.</para>
625
626 <para>The source files for the program don't all need to be in
627 the same directory; the <option>-i</option> option can be used
628 to add directories to the search path (see <xref
629 linkend="search-path"/>).</para>
630 </sect2>
631
632 <sect2 id="eval-mode">
633 <title>Expression evaluation mode</title>
634
635 <para>This mode is very similar to interactive mode, except that
636 there is a single expression to evaluate which is specified on
637 the command line as an argument to the <option>-e</option>
638 option:</para>
639
640 <screen>
641 ghc -e <replaceable>expr</replaceable>
642 </screen>
643
644 <para>Haskell source files may be named on the command line, and
645 they will be loaded exactly as in interactive mode. The
646 expression is evaluated in the context of the loaded
647 modules.</para>
648
649 <para>For example, to load and run a Haskell program containing
650 a module <literal>Main</literal>, we might say</para>
651
652 <screen>
653 ghc -e Main.main Main.hs
654 </screen>
655
656 <para>or we can just use this mode to evaluate expressions in
657 the context of the <literal>Prelude</literal>:</para>
658
659 <screen>
660 $ ghc -e "interact (unlines.map reverse.lines)"
661 hello
662 olleh
663 </screen>
664 </sect2>
665
666 <sect2 id="options-order">
667 <title>Batch compiler mode</title>
668
669 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
670 given on the command line.</para>
671
672 <para>The first phase to run is determined by each input-file
673 suffix, and the last phase is determined by a flag. If no
674 relevant flag is present, then go all the way through to linking.
675 This table summarises:</para>
676
677 <informaltable>
678 <tgroup cols="4">
679 <colspec align="left"/>
680 <colspec align="left"/>
681 <colspec align="left"/>
682 <colspec align="left"/>
683
684 <thead>
685 <row>
686 <entry>Phase of the compilation system</entry>
687 <entry>Suffix saying &ldquo;start here&rdquo;</entry>
688 <entry>Flag saying &ldquo;stop after&rdquo;</entry>
689 <entry>(suffix of) output file</entry>
690 </row>
691 </thead>
692 <tbody>
693 <row>
694 <entry>literate pre-processor</entry>
695 <entry><literal>.lhs</literal></entry>
696 <entry>-</entry>
697 <entry><literal>.hs</literal></entry>
698 </row>
699
700 <row>
701 <entry>C pre-processor (opt.) </entry>
702 <entry><literal>.hs</literal> (with
703 <option>-cpp</option>)</entry>
704 <entry><option>-E</option></entry>
705 <entry><literal>.hspp</literal></entry>
706 </row>
707
708 <row>
709 <entry>Haskell compiler</entry>
710 <entry><literal>.hs</literal></entry>
711 <entry><option>-C</option>, <option>-S</option></entry>
712 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
713 </row>
714
715 <row>
716 <entry>C compiler (opt.)</entry>
717 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
718 <entry><option>-S</option></entry>
719 <entry><literal>.s</literal></entry>
720 </row>
721
722 <row>
723 <entry>assembler</entry>
724 <entry><literal>.s</literal></entry>
725 <entry><option>-c</option></entry>
726 <entry><literal>.o</literal></entry>
727 </row>
728
729 <row>
730 <entry>linker</entry>
731 <entry><replaceable>other</replaceable></entry>
732 <entry>-</entry>
733 <entry><filename>a.out</filename></entry>
734 </row>
735 </tbody>
736 </tgroup>
737 </informaltable>
738
739 <indexterm><primary><option>-C</option></primary></indexterm>
740 <indexterm><primary><option>-E</option></primary></indexterm>
741 <indexterm><primary><option>-S</option></primary></indexterm>
742 <indexterm><primary><option>-c</option></primary></indexterm>
743
744 <para>Thus, a common invocation would be: </para>
745
746 <screen>
747 ghc -c Foo.hs
748 </screen>
749
750 <para>to compile the Haskell source file
751 <filename>Foo.hs</filename> to an object file
752 <filename>Foo.o</filename>.</para>
753
754 <para>Note: What the Haskell compiler proper produces depends on what
755 backend code generator is used. See <xref linkend="code-generators"/>
756 for more details.</para>
757
758 <para>Note: C pre-processing is optional, the
759 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
760 flag turns it on. See <xref linkend="c-pre-processor"/> for more
761 details.</para>
762
763 <para>Note: The option <option>-E</option><indexterm><primary>-E
764 option</primary></indexterm> runs just the pre-processing passes
765 of the compiler, dumping the result in a file.</para>
766
767 <sect3 id="overriding-suffixes">
768 <title>Overriding the default behaviour for a file</title>
769
770 <para>As described above, the way in which a file is processed by GHC
771 depends on its suffix. This behaviour can be overridden using the
772 <option>-x</option> option:</para>
773
774 <variablelist>
775 <varlistentry>
776 <term><option>-x</option> <replaceable>suffix</replaceable>
777 <indexterm><primary><option>-x</option></primary>
778 </indexterm></term>
779 <listitem>
780 <para>Causes all files following this option on the command
781 line to be processed as if they had the suffix
782 <replaceable>suffix</replaceable>. For example, to compile a
783 Haskell module in the file <literal>M.my-hs</literal>,
784 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
785 </listitem>
786 </varlistentry>
787 </variablelist>
788 </sect3>
789
790 </sect2>
791 </sect1>
792
793 <sect1 id="options-help">
794 <title>Help and verbosity options</title>
795
796 <indexterm><primary>help options</primary></indexterm>
797 <indexterm><primary>verbosity options</primary></indexterm>
798
799 <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
800 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
801 <variablelist>
802 <varlistentry>
803 <term>
804 <option>-v</option>
805 <indexterm><primary><option>-v</option></primary></indexterm>
806 </term>
807 <listitem>
808 <para>The <option>-v</option> option makes GHC
809 <emphasis>verbose</emphasis>: it reports its version number
810 and shows (on stderr) exactly how it invokes each phase of
811 the compilation system. Moreover, it passes the
812 <option>-v</option> flag to most phases; each reports its
813 version number (and possibly some other information).</para>
814
815 <para>Please, oh please, use the <option>-v</option> option
816 when reporting bugs! Knowing that you ran the right bits in
817 the right order is always the first thing we want to
818 verify.</para>
819 </listitem>
820 </varlistentry>
821
822 <varlistentry>
823 <term>
824 <option>-v</option><replaceable>n</replaceable>
825 <indexterm><primary><option>-v</option></primary></indexterm>
826 </term>
827 <listitem>
828 <para>To provide more control over the compiler's verbosity,
829 the <option>-v</option> flag takes an optional numeric
830 argument. Specifying <option>-v</option> on its own is
831 equivalent to <option>-v3</option>, and the other levels
832 have the following meanings:</para>
833
834 <variablelist>
835 <varlistentry>
836 <term><option>-v0</option></term>
837 <listitem>
838 <para>Disable all non-essential messages (this is the
839 default).</para>
840 </listitem>
841 </varlistentry>
842
843 <varlistentry>
844 <term><option>-v1</option></term>
845 <listitem>
846 <para>Minimal verbosity: print one line per
847 compilation (this is the default when
848 <option>--make</option> or
849 <option>--interactive</option> is on).</para>
850 </listitem>
851 </varlistentry>
852
853 <varlistentry>
854 <term><option>-v2</option></term>
855 <listitem>
856 <para>Print the name of each compilation phase as it
857 is executed. (equivalent to
858 <option>-dshow-passes</option>).</para>
859 </listitem>
860 </varlistentry>
861
862 <varlistentry>
863 <term><option>-v3</option></term>
864 <listitem>
865 <para>The same as <option>-v2</option>, except that in
866 addition the full command line (if appropriate) for
867 each compilation phase is also printed.</para>
868 </listitem>
869 </varlistentry>
870
871 <varlistentry>
872 <term><option>-v4</option></term>
873 <listitem>
874 <para>The same as <option>-v3</option> except that the
875 intermediate program representation after each
876 compilation phase is also printed (excluding
877 preprocessed and C/assembly files).</para>
878 </listitem>
879 </varlistentry>
880 </variablelist>
881 </listitem>
882 </varlistentry>
883
884 <varlistentry>
885 <term><option>-ferror-spans</option>
886 <indexterm><primary><option>-ferror-spans</option></primary>
887 </indexterm>
888 </term>
889 <listitem>
890 <para>Causes GHC to emit the full source span of the
891 syntactic entity relating to an error message. Normally, GHC
892 emits the source location of the start of the syntactic
893 entity only.</para>
894
895 <para>For example:</para>
896
897 <screen>
898 test.hs:3:6: parse error on input `where'
899 </screen>
900
901 <para>becomes:</para>
902
903 <screen>
904 test296.hs:3:6-10: parse error on input `where'
905 </screen>
906
907 <para>And multi-line spans are possible too:</para>
908
909 <screen>
910 test.hs:(5,4)-(6,7):
911 Conflicting definitions for `a'
912 Bound at: test.hs:5:4
913 test.hs:6:7
914 In the binding group for: a, b, a
915 </screen>
916
917 <para>Note that line numbers start counting at one, but
918 column numbers start at zero. This choice was made to
919 follow existing convention (i.e. this is how Emacs does
920 it).</para>
921 </listitem>
922 </varlistentry>
923
924 <varlistentry>
925 <term><option>-H</option><replaceable>size</replaceable>
926 <indexterm><primary><option>-H</option></primary></indexterm>
927 </term>
928 <listitem>
929 <para>Set the minimum size of the heap to
930 <replaceable>size</replaceable>.
931 This option is equivalent to
932 <literal>+RTS&nbsp;-H<replaceable>size</replaceable></literal>,
933 see <xref linkend="rts-options-gc" />.
934 </para>
935 </listitem>
936 </varlistentry>
937
938 <varlistentry>
939 <term><option>-Rghc-timing</option>
940 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
941 </term>
942 <listitem>
943 <para>Prints a one-line summary of timing statistics for the
944 GHC run. This option is equivalent to
945 <literal>+RTS&nbsp;-tstderr</literal>, see <xref
946 linkend="rts-options-gc" />.
947 </para>
948 </listitem>
949 </varlistentry>
950 </variablelist>
951 </sect1>
952
953 &separate;
954
955 <sect1 id="options-sanity">
956 <title>Warnings and sanity-checking</title>
957
958 <indexterm><primary>sanity-checking options</primary></indexterm>
959 <indexterm><primary>warnings</primary></indexterm>
960
961
962 <para>GHC has a number of options that select which types of
963 non-fatal error messages, otherwise known as warnings, can be
964 generated during compilation. By default, you get a standard set
965 of warnings which are generally likely to indicate bugs in your
966 program. These are:
967 <option>-fwarn-overlapping-patterns</option>,
968 <option>-fwarn-warnings-deprecations</option>,
969 <option>-fwarn-amp</option>,
970 <option>-fwarn-deprecated-flags</option>,
971 <option>-fwarn-unrecognised-pragmas</option>,
972 <option>-fwarn-pointless-pragmas</option>,
973 <option>-fwarn-duplicate-constraints</option>,
974 <option>-fwarn-duplicate-exports</option>,
975 <option>-fwarn-overflowed-literals</option>,
976 <option>-fwarn-empty-enumerations</option>,
977 <option>-fwarn-missing-fields</option>,
978 <option>-fwarn-missing-methods</option>,
979 <option>-fwarn-wrong-do-bind</option>,
980 <option>-fwarn-unsupported-calling-conventions</option>,
981 <option>-fwarn-dodgy-foreign-imports</option>,
982 <option>-fwarn-inline-rule-shadowing</option>, and
983 <option>-fwarn-unsupported-llvm-version</option>.
984 The following flags are simple ways to select standard
985 &ldquo;packages&rdquo; of warnings:
986 </para>
987
988 <variablelist>
989
990 <varlistentry>
991 <term><option>-W</option>:</term>
992 <listitem>
993 <indexterm><primary>-W option</primary></indexterm>
994 <para>Provides the standard warnings plus
995 <option>-fwarn-incomplete-patterns</option>,
996 <option>-fwarn-dodgy-exports</option>,
997 <option>-fwarn-dodgy-imports</option>,
998 <option>-fwarn-unused-matches</option>,
999 <option>-fwarn-unused-imports</option>, and
1000 <option>-fwarn-unused-binds</option>.</para>
1001 </listitem>
1002 </varlistentry>
1003
1004 <varlistentry>
1005 <term><option>-Wall</option>:</term>
1006 <listitem>
1007 <indexterm><primary><option>-Wall</option></primary></indexterm>
1008 <para>Turns on all warning options that indicate potentially
1009 suspicious code. The warnings that are
1010 <emphasis>not</emphasis> enabled by <option>-Wall</option>
1011 are
1012 <option>-fwarn-tabs</option>,
1013 <option>-fwarn-incomplete-uni-patterns</option>,
1014 <option>-fwarn-incomplete-record-updates</option>,
1015 <option>-fwarn-monomorphism-restriction</option>,
1016 <option>-fwarn-auto-orphans</option>,
1017 <option>-fwarn-implicit-prelude</option>,
1018 <option>-fwarn-missing-local-sigs</option>,
1019 <option>-fwarn-missing-import-lists</option>.</para>
1020 </listitem>
1021 </varlistentry>
1022
1023 <varlistentry>
1024 <term><option>-w</option>:</term>
1025 <listitem>
1026 <indexterm><primary><option>-w</option></primary></indexterm>
1027 <para>Turns off all warnings, including the standard ones and
1028 those that <literal>-Wall</literal> doesn't enable.</para>
1029 </listitem>
1030 </varlistentry>
1031
1032 <varlistentry>
1033 <term><option>-Werror</option>:</term>
1034 <listitem>
1035 <indexterm><primary><option>-Werror</option></primary></indexterm>
1036 <para>Makes any warning into a fatal error. Useful so that you don't
1037 miss warnings when doing batch compilation. </para>
1038 </listitem>
1039 </varlistentry>
1040
1041 <varlistentry>
1042 <term><option>-Wwarn</option>:</term>
1043 <listitem>
1044 <indexterm><primary><option>-Wwarn</option></primary></indexterm>
1045 <para>Warnings are treated only as warnings, not as errors. This is
1046 the default, but can be useful to negate a
1047 <option>-Werror</option> flag.</para>
1048 </listitem>
1049 </varlistentry>
1050
1051 </variablelist>
1052
1053 <para>The full set of warning options is described below. To turn
1054 off any warning, simply give the corresponding
1055 <option>-fno-warn-...</option> option on the command line.</para>
1056
1057 <variablelist>
1058
1059 <varlistentry>
1060 <term><option>-fdefer-type-errors</option>:</term>
1061 <listitem>
1062 <indexterm><primary><option>-fdefer-type-errors</option></primary>
1063 </indexterm>
1064 <indexterm><primary>warnings</primary></indexterm>
1065 <para>Defer as many type errors as possible until runtime.
1066 At compile time you get a warning (instead of an error). At
1067 runtime, if you use a value that depends on a type error, you
1068 get a runtime error; but you can run any type-correct parts of your code
1069 just fine. See <xref linkend="defer-type-errors"/></para>
1070 </listitem>
1071 </varlistentry>
1072
1073 <varlistentry>
1074 <term><option>-fhelpful-errors</option>:</term>
1075 <listitem>
1076 <indexterm><primary><option>-fhelpful-errors</option></primary>
1077 </indexterm>
1078 <indexterm><primary>warnings</primary></indexterm>
1079 <para>When a name or package is not found in scope, make
1080 suggestions for the name or package you might have meant instead.</para>
1081 <para>This option is on by default.</para>
1082 </listitem>
1083 </varlistentry>
1084
1085 <varlistentry>
1086 <term><option>-fwarn-unrecognised-pragmas</option>:</term>
1087 <listitem>
1088 <indexterm><primary><option>-fwarn-unrecognised-pragmas</option></primary>
1089 </indexterm>
1090 <indexterm><primary>warnings</primary></indexterm>
1091 <indexterm><primary>pragmas</primary></indexterm>
1092 <para>Causes a warning to be emitted when a
1093 pragma that GHC doesn't recognise is used. As well as pragmas
1094 that GHC itself uses, GHC also recognises pragmas known to be used
1095 by other tools, e.g. <literal>OPTIONS_HUGS</literal> and
1096 <literal>DERIVE</literal>.</para>
1097
1098 <para>This option is on by default.</para>
1099 </listitem>
1100 </varlistentry>
1101
1102 <varlistentry>
1103 <term><option>-fwarn-pointless-pragmas</option>:</term>
1104 <listitem>
1105 <indexterm><primary><option>-fwarn-pointless-pragmas</option></primary>
1106 </indexterm>
1107 <indexterm><primary>warnings</primary></indexterm>
1108 <indexterm><primary>pragmas</primary></indexterm>
1109 <para>Causes a warning to be emitted when GHC detects that a
1110 module contains a pragma that has no effect.</para>
1111
1112 <para>This option is on by default.</para>
1113 </listitem>
1114 </varlistentry>
1115
1116 <varlistentry>
1117 <term><option>-fwarn-warnings-deprecations</option>:</term>
1118 <listitem>
1119 <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
1120 </indexterm>
1121 <indexterm><primary>warnings</primary></indexterm>
1122 <indexterm><primary>deprecations</primary></indexterm>
1123 <para>Causes a warning to be emitted when a
1124 module, function or type with a WARNING or DEPRECATED pragma
1125 is used. See <xref linkend="warning-deprecated-pragma"/> for more
1126 details on the pragmas.</para>
1127
1128 <para>This option is on by default.</para>
1129 </listitem>
1130 </varlistentry>
1131
1132 <varlistentry>
1133 <term><option>-fwarn-amp</option>:</term>
1134 <listitem>
1135 <indexterm><primary><option>-fwarn-amp</option></primary>
1136 </indexterm>
1137 <indexterm><primary>amp</primary></indexterm>
1138 <indexterm><primary>applicative-monad proposal</primary></indexterm>
1139 <para>Causes a warning to be emitted when a definition
1140 is in conflict with the AMP (Applicative-Monad proosal),
1141 namely:
1142 1. Instance of Monad without Applicative;
1143 2. Instance of MonadPlus without Alternative;
1144 3. Custom definitions of join/pure/&lt;*&gt;</para>
1145
1146 <para>This option is on by default.</para>
1147 </listitem>
1148 </varlistentry>
1149
1150 <varlistentry>
1151 <term><option>-fwarn-deprecated-flags</option>:</term>
1152 <listitem>
1153 <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
1154 </indexterm>
1155 <indexterm><primary>deprecated-flags</primary></indexterm>
1156 <para>Causes a warning to be emitted when a deprecated
1157 commandline flag is used.</para>
1158
1159 <para>This option is on by default.</para>
1160 </listitem>
1161 </varlistentry>
1162
1163 <varlistentry>
1164 <term><option>-fwarn-unsupported-calling-conventions</option>:</term>
1165 <listitem>
1166 <indexterm><primary><option>-fwarn-unsupported-calling-conventions</option></primary>
1167 </indexterm>
1168 <para>Causes a warning to be emitted for foreign declarations
1169 that use unsupported calling conventions. In particular,
1170 if the <literal>stdcall</literal> calling convention is used
1171 on an architecture other than i386 then it will be treated
1172 as <literal>ccall</literal>.</para>
1173 </listitem>
1174 </varlistentry>
1175
1176 <varlistentry>
1177 <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
1178 <listitem>
1179 <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
1180 </indexterm>
1181 <para>Causes a warning to be emitted for foreign imports of
1182 the following form:</para>
1183
1184 <programlisting>
1185 foreign import "f" f :: FunPtr t
1186 </programlisting>
1187
1188 <para>on the grounds that it probably should be</para>
1189
1190 <programlisting>
1191 foreign import "&amp;f" f :: FunPtr t
1192 </programlisting>
1193
1194 <para>The first form declares that `f` is a (pure) C
1195 function that takes no arguments and returns a pointer to a
1196 C function with type `t`, whereas the second form declares
1197 that `f` itself is a C function with type `t`. The first
1198 declaration is usually a mistake, and one that is hard to
1199 debug because it results in a crash, hence this
1200 warning.</para>
1201 </listitem>
1202 </varlistentry>
1203
1204 <varlistentry>
1205 <term><option>-fwarn-dodgy-exports</option>:</term>
1206 <listitem>
1207 <indexterm><primary><option>-fwarn-dodgy-exports</option></primary>
1208 </indexterm>
1209 <para>Causes a warning to be emitted when a datatype
1210 <literal>T</literal> is exported
1211 with all constructors, i.e. <literal>T(..)</literal>, but is it
1212 just a type synonym.</para>
1213 <para>Also causes a warning to be emitted when a module is
1214 re-exported, but that module exports nothing.</para>
1215 </listitem>
1216 </varlistentry>
1217
1218 <varlistentry>
1219 <term><option>-fwarn-dodgy-imports</option>:</term>
1220 <listitem>
1221 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
1222 </indexterm>
1223 <para>Causes a warning to be emitted in the following cases:</para>
1224 <itemizedlist>
1225 <listitem>
1226 <para>When a datatype <literal>T</literal> is imported with all
1227 constructors, i.e. <literal>T(..)</literal>, but has been
1228 exported abstractly, i.e. <literal>T</literal>.
1229 </para>
1230 </listitem>
1231 <listitem>
1232 <para>When an <literal>import</literal> statement hides an
1233 entity that is not exported.</para>
1234 </listitem>
1235 </itemizedlist>
1236 </listitem>
1237 </varlistentry>
1238
1239 <varlistentry>
1240 <term><option>-fwarn-overflowed-literals</option>:</term>
1241 <listitem>
1242 <indexterm><primary><option>-fwarn-overflowed-literals</option></primary>
1243 </indexterm>
1244 <para>
1245 Causes a warning to be emitted if a literal will overflow,
1246 e.g. <literal>300 :: Word8</literal>.
1247 </para>
1248 </listitem>
1249 </varlistentry>
1250
1251 <varlistentry>
1252 <term><option>-fwarn-empty-enumerations</option>:</term>
1253 <listitem>
1254 <indexterm><primary><option>-fwarn-empty-enumerations</option></primary>
1255 </indexterm>
1256 <para>
1257 Causes a warning to be emitted if an enumeration is
1258 empty, e.g. <literal>[5 .. 3]</literal>.
1259 </para>
1260 </listitem>
1261 </varlistentry>
1262
1263 <varlistentry>
1264 <term><option>-fwarn-lazy-unlifted-bindings</option>:</term>
1265 <listitem>
1266 <indexterm><primary><option>-fwarn-lazy-unlifted-bindings</option></primary>
1267 </indexterm>
1268 <para>This flag is a no-op, and will be removed in GHC 7.10.</para>
1269 </listitem>
1270 </varlistentry>
1271
1272 <varlistentry>
1273 <term><option>-fwarn-duplicate-constraints</option>:</term>
1274 <listitem>
1275 <indexterm><primary><option>-fwarn-duplicate-constraints</option></primary></indexterm>
1276 <indexterm><primary>duplicate constraints, warning</primary></indexterm>
1277
1278 <para>Have the compiler warn about duplicate constraints in a type signature. For
1279 example
1280 <programlisting>
1281 f :: (Eq a, Show a, Eq a) => a -> a
1282 </programlisting>
1283 The warning will indicate the duplicated <literal>Eq a</literal> constraint.
1284 </para>
1285
1286 <para>This option is on by default.</para>
1287 </listitem>
1288 </varlistentry>
1289
1290 <varlistentry>
1291 <term><option>-fwarn-duplicate-exports</option>:</term>
1292 <listitem>
1293 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
1294 <indexterm><primary>duplicate exports, warning</primary></indexterm>
1295 <indexterm><primary>export lists, duplicates</primary></indexterm>
1296
1297 <para>Have the compiler warn about duplicate entries in
1298 export lists. This is useful information if you maintain
1299 large export lists, and want to avoid the continued export
1300 of a definition after you've deleted (one) mention of it in
1301 the export list.</para>
1302
1303 <para>This option is on by default.</para>
1304 </listitem>
1305 </varlistentry>
1306
1307 <varlistentry>
1308 <term><option>-fwarn-hi-shadowing</option>:</term>
1309 <listitem>
1310 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
1311 <indexterm><primary>shadowing</primary>
1312 <secondary>interface files</secondary></indexterm>
1313
1314 <para>Causes the compiler to emit a warning when a module or
1315 interface file in the current directory is shadowing one
1316 with the same module name in a library or other
1317 directory.</para>
1318 </listitem>
1319 </varlistentry>
1320
1321 <varlistentry>
1322 <term><option>-fwarn-identities</option>:</term>
1323 <listitem>
1324 <indexterm><primary><option>-fwarn-identities</option></primary></indexterm>
1325 <para>Causes the compiler to emit a warning when a Prelude numeric
1326 conversion converts a type T to the same type T; such calls
1327 are probably no-ops and can be omitted. The functions checked for
1328 are: <literal>toInteger</literal>,
1329 <literal>toRational</literal>,
1330 <literal>fromIntegral</literal>,
1331 and <literal>realToFrac</literal>.
1332 </para>
1333 </listitem>
1334 </varlistentry>
1335
1336 <varlistentry>
1337 <term><option>-fwarn-implicit-prelude</option>:</term>
1338 <listitem>
1339 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
1340 <indexterm><primary>implicit prelude, warning</primary></indexterm>
1341 <para>Have the compiler warn if the Prelude is implicitly
1342 imported. This happens unless either the Prelude module is
1343 explicitly imported with an <literal>import ... Prelude ...</literal>
1344 line, or this implicit import is disabled (either by
1345 <option>-XNoImplicitPrelude</option> or a
1346 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
1347
1348 <para>Note that no warning is given for syntax that implicitly
1349 refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
1350 would change whether it refers to the Prelude.
1351 For example, no warning is given when
1352 <literal>368</literal> means
1353 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
1354 (where <literal>Prelude</literal> refers to the actual Prelude module,
1355 regardless of the imports of the module being compiled).</para>
1356
1357 <para>This warning is off by default.</para>
1358 </listitem>
1359 </varlistentry>
1360
1361 <varlistentry>
1362 <term><option>-fwarn-incomplete-patterns</option>,
1363 <option>-fwarn-incomplete-uni-patterns</option>:
1364 </term>
1365 <listitem>
1366 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
1367 <indexterm><primary><option>-fwarn-incomplete-uni-patterns</option></primary></indexterm>
1368 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
1369 <indexterm><primary>patterns, incomplete</primary></indexterm>
1370
1371 <para>The option <option>-fwarn-incomplete-patterns</option> warns
1372 about places where
1373 a pattern-match might fail at runtime.
1374 The function
1375 <function>g</function> below will fail when applied to
1376 non-empty lists, so the compiler will emit a warning about
1377 this when <option>-fwarn-incomplete-patterns</option> is
1378 enabled.
1379
1380 <programlisting>
1381 g [] = 2
1382 </programlisting>
1383
1384 This option isn't enabled by default because it can be
1385 a bit noisy, and it doesn't always indicate a bug in the
1386 program. However, it's generally considered good practice
1387 to cover all the cases in your functions, and it is switched
1388 on by <option>-W</option>.</para>
1389
1390 <para>The flag <option>-fwarn-incomplete-uni-patterns</option> is
1391 similar, except that it
1392 applies only to lambda-expressions and pattern bindings, constructs
1393 that only allow a single pattern:
1394
1395 <programlisting>
1396 h = \[] -> 2
1397 Just k = f y
1398 </programlisting>
1399
1400 </para>
1401 </listitem>
1402 </varlistentry>
1403
1404 <varlistentry>
1405 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1406 <listitem>
1407 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1408 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1409 <indexterm><primary>record updates, incomplete</primary></indexterm>
1410
1411 <para>The function
1412 <function>f</function> below will fail when applied to
1413 <literal>Bar</literal>, so the compiler will emit a warning about
1414 this when <option>-fwarn-incomplete-record-updates</option> is
1415 enabled.</para>
1416
1417 <programlisting>
1418 data Foo = Foo { x :: Int }
1419 | Bar
1420
1421 f :: Foo -> Foo
1422 f foo = foo { x = 6 }
1423 </programlisting>
1424
1425 <para>This option isn't enabled by default because it can be
1426 very noisy, and it often doesn't indicate a bug in the
1427 program.</para>
1428 </listitem>
1429 </varlistentry>
1430
1431 <varlistentry>
1432 <term>
1433 <option>-fwarn-missing-fields</option>:
1434 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1435 <indexterm><primary>missing fields, warning</primary></indexterm>
1436 <indexterm><primary>fields, missing</primary></indexterm>
1437 </term>
1438 <listitem>
1439
1440 <para>This option is on by default, and warns you whenever
1441 the construction of a labelled field constructor isn't
1442 complete, missing initializers for one or more fields. While
1443 not an error (the missing fields are initialised with
1444 bottoms), it is often an indication of a programmer error.</para>
1445 </listitem>
1446 </varlistentry>
1447
1448 <varlistentry>
1449 <term>
1450 <option>-fwarn-missing-import-lists</option>:
1451 <indexterm><primary><option>-fwarn-import-lists</option></primary></indexterm>
1452 <indexterm><primary>missing import lists, warning</primary></indexterm>
1453 <indexterm><primary>import lists, missing</primary></indexterm>
1454 </term>
1455 <listitem>
1456
1457 <para>This flag warns if you use an unqualified
1458 <literal>import</literal> declaration
1459 that does not explicitly list the entities brought into scope. For
1460 example
1461 </para>
1462
1463 <programlisting>
1464 module M where
1465 import X( f )
1466 import Y
1467 import qualified Z
1468 p x = f x x
1469 </programlisting>
1470
1471 <para>
1472 The <option>-fwarn-import-lists</option> flag will warn about the import
1473 of <literal>Y</literal> but not <literal>X</literal>
1474 If module <literal>Y</literal> is later changed to export (say) <literal>f</literal>,
1475 then the reference to <literal>f</literal> in <literal>M</literal> will become
1476 ambiguous. No warning is produced for the import of <literal>Z</literal>
1477 because extending <literal>Z</literal>'s exports would be unlikely to produce
1478 ambiguity in <literal>M</literal>.
1479 </para>
1480 </listitem>
1481 </varlistentry>
1482
1483 <varlistentry>
1484 <term><option>-fwarn-missing-methods</option>:</term>
1485 <listitem>
1486 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1487 <indexterm><primary>missing methods, warning</primary></indexterm>
1488 <indexterm><primary>methods, missing</primary></indexterm>
1489
1490 <para>This option is on by default, and warns you whenever
1491 an instance declaration is missing one or more methods, and
1492 the corresponding class declaration has no default
1493 declaration for them.</para>
1494 <para>The warning is suppressed if the method name
1495 begins with an underscore. Here's an example where this is useful:
1496 <programlisting>
1497 class C a where
1498 _simpleFn :: a -> String
1499 complexFn :: a -> a -> String
1500 complexFn x y = ... _simpleFn ...
1501 </programlisting>
1502 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1503 never <literal>_simpleFn</literal>; and (b)
1504 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1505 </para>
1506 <para>The MINIMAL pragma can be used to change which combination of methods will be required for instances of a particular class. See <xref linkend="minimal-pragma"/>.</para>
1507 </listitem>
1508 </varlistentry>
1509
1510 <varlistentry>
1511 <term><option>-fwarn-missing-signatures</option>:</term>
1512 <listitem>
1513 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1514 <indexterm><primary>type signatures, missing</primary></indexterm>
1515
1516 <para>If you would like GHC to check that every top-level
1517 function/value has a type signature, use the
1518 <option>-fwarn-missing-signatures</option> option. As part of
1519 the warning GHC also reports the inferred type. The
1520 option is off by default.</para>
1521 </listitem>
1522 </varlistentry>
1523
1524 <varlistentry>
1525 <term><option>-fwarn-missing-local-sigs</option>:</term>
1526 <listitem>
1527 <indexterm><primary><option>-fwarn-missing-local-sigs</option></primary></indexterm>
1528 <indexterm><primary>type signatures, missing</primary></indexterm>
1529
1530 <para>If you use the
1531 <option>-fwarn-missing-local-sigs</option> flag GHC will warn
1532 you about any polymorphic local bindings. As part of
1533 the warning GHC also reports the inferred type. The
1534 option is off by default.</para>
1535 </listitem>
1536 </varlistentry>
1537
1538 <varlistentry>
1539 <term><option>-fwarn-name-shadowing</option>:</term>
1540 <listitem>
1541 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1542 <indexterm><primary>shadowing, warning</primary></indexterm>
1543
1544 <para>This option causes a warning to be emitted whenever an
1545 inner-scope value has the same name as an outer-scope value,
1546 i.e. the inner value shadows the outer one. This can catch
1547 typographical errors that turn into hard-to-find bugs, e.g.,
1548 in the inadvertent capture of what would be a recursive call in
1549 <literal>f = ... let f = id in ... f ...</literal>.</para>
1550 <para>The warning is suppressed for names beginning with an underscore. For example
1551 <programlisting>
1552 f x = do { _ignore &lt;- this; _ignore &lt;- that; return (the other) }
1553 </programlisting>
1554 </para>
1555 </listitem>
1556 </varlistentry>
1557
1558 <varlistentry>
1559 <term><option>-fwarn-orphans, -fwarn-auto-orphans</option>:</term>
1560 <listitem>
1561 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1562 <indexterm><primary><option>-fwarn-auto-orphans</option></primary></indexterm>
1563 <indexterm><primary>orphan instances, warning</primary></indexterm>
1564 <indexterm><primary>orphan rules, warning</primary></indexterm>
1565
1566 <para>These flags cause a warning to be emitted whenever the
1567 module contains an "orphan" instance declaration or rewrite rule.
1568 An instance declaration is an orphan if it appears in a module in
1569 which neither the class nor the type being instanced are declared
1570 in the same module. A rule is an orphan if it is a rule for a
1571 function declared in another module. A module containing any
1572 orphans is called an orphan module.</para>
1573 <para>The trouble with orphans is that GHC must pro-actively read the interface
1574 files for all orphan modules, just in case their instances or rules
1575 play a role, whether or not the module's interface would otherwise
1576 be of any use. See <xref linkend="orphan-modules"/> for details.
1577 </para>
1578 <para>The flag <option>-fwarn-orphans</option> warns about user-written
1579 orphan rules or instances. The flag <option>-fwarn-auto-orphans</option>
1580 warns about automatically-generated orphan rules, notably as a result of
1581 specialising functions, for type classes (<literal>Specialise</literal>)
1582 or argument values (<literal>SpecConstr</literal>).</para>
1583 </listitem>
1584 </varlistentry>
1585
1586 <varlistentry>
1587 <term>
1588 <option>-fwarn-overlapping-patterns</option>:
1589 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1590 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1591 <indexterm><primary>patterns, overlapping</primary></indexterm>
1592 </term>
1593 <listitem>
1594 <para>By default, the compiler will warn you if a set of
1595 patterns are overlapping, e.g.,</para>
1596
1597 <programlisting>
1598 f :: String -&#62; Int
1599 f [] = 0
1600 f (_:xs) = 1
1601 f "2" = 2
1602 </programlisting>
1603
1604 <para>where the last pattern match in <function>f</function>
1605 won't ever be reached, as the second pattern overlaps
1606 it. More often than not, redundant patterns is a programmer
1607 mistake/error, so this option is enabled by default.</para>
1608 </listitem>
1609 </varlistentry>
1610
1611 <varlistentry>
1612 <term><option>-fwarn-tabs</option>:</term>
1613 <listitem>
1614 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1615 <indexterm><primary>tabs, warning</primary></indexterm>
1616 <para>Have the compiler warn if there are tabs in your source
1617 file.</para>
1618
1619 <para>This warning is off by default.</para>
1620 </listitem>
1621 </varlistentry>
1622
1623 <varlistentry>
1624 <term><option>-fwarn-type-defaults</option>:</term>
1625 <listitem>
1626 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1627 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1628 <para>Have the compiler warn/inform you where in your source
1629 the Haskell defaulting mechanism for numeric types kicks
1630 in. This is useful information when converting code from a
1631 context that assumed one default into one with another,
1632 e.g., the &lsquo;default default&rsquo; for Haskell 1.4 caused the
1633 otherwise unconstrained value <constant>1</constant> to be
1634 given the type <literal>Int</literal>, whereas Haskell 98
1635 and later
1636 defaults it to <literal>Integer</literal>. This may lead to
1637 differences in performance and behaviour, hence the
1638 usefulness of being non-silent about this.</para>
1639
1640 <para>This warning is off by default.</para>
1641 </listitem>
1642 </varlistentry>
1643
1644 <varlistentry>
1645 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1646 <listitem>
1647 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1648 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1649 <para>Have the compiler warn/inform you where in your source
1650 the Haskell Monomorphism Restriction is applied. If applied silently
1651 the MR can give rise to unexpected behaviour, so it can be helpful
1652 to have an explicit warning that it is being applied.</para>
1653
1654 <para>This warning is off by default.</para>
1655 </listitem>
1656 </varlistentry>
1657
1658 <varlistentry>
1659 <term><option>-fwarn-unused-binds</option>:</term>
1660 <listitem>
1661 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1662 <indexterm><primary>unused binds, warning</primary></indexterm>
1663 <indexterm><primary>binds, unused</primary></indexterm>
1664 <para>Report any function definitions (and local bindings)
1665 which are unused. For top-level functions, the warning is
1666 only given if the binding is not exported.</para>
1667 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1668 mentioned in the right hand side of another definition that is used, or (c) the
1669 function it defines begins with an underscore. The last case provides a
1670 way to suppress unused-binding warnings selectively. </para>
1671 <para> Notice that a variable
1672 is reported as unused even if it appears in the right-hand side of another
1673 unused binding. </para>
1674 </listitem>
1675 </varlistentry>
1676
1677 <varlistentry>
1678 <term><option>-fwarn-unused-imports</option>:</term>
1679 <listitem>
1680 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1681 <indexterm><primary>unused imports, warning</primary></indexterm>
1682 <indexterm><primary>imports, unused</primary></indexterm>
1683
1684 <para>Report any modules that are explicitly imported but
1685 never used. However, the form <literal>import M()</literal> is
1686 never reported as an unused import, because it is a useful idiom
1687 for importing instance declarations, which are anonymous in Haskell.</para>
1688 </listitem>
1689 </varlistentry>
1690
1691 <varlistentry>
1692 <term><option>-fwarn-unused-matches</option>:</term>
1693 <listitem>
1694 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1695 <indexterm><primary>unused matches, warning</primary></indexterm>
1696 <indexterm><primary>matches, unused</primary></indexterm>
1697
1698 <para>Report all unused variables which arise from pattern
1699 matches, including patterns consisting of a single variable.
1700 For instance <literal>f x y = []</literal> would report
1701 <varname>x</varname> and <varname>y</varname> as unused. The
1702 warning is suppressed if the variable name begins with an underscore, thus:
1703 <programlisting>
1704 f _x = True
1705 </programlisting>
1706 </para>
1707 </listitem>
1708 </varlistentry>
1709
1710 <varlistentry>
1711 <term><option>-fwarn-unused-do-bind</option>:</term>
1712 <listitem>
1713 <indexterm><primary><option>-fwarn-unused-do-bind</option></primary></indexterm>
1714 <indexterm><primary>unused do binding, warning</primary></indexterm>
1715 <indexterm><primary>do binding, unused</primary></indexterm>
1716
1717 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1718 that appear to silently throw information away.
1719 For instance <literal>do { mapM popInt xs ; return 10 }</literal> would report
1720 the first statement in the <literal>do</literal> block as suspicious,
1721 as it has the type <literal>StackM [Int]</literal> and not <literal>StackM ()</literal>, but that
1722 <literal>[Int]</literal> value is not bound to anything. The warning is suppressed by
1723 explicitly mentioning in the source code that your program is throwing something away:
1724 <programlisting>
1725 do { _ &lt;- mapM popInt xs ; return 10 }
1726 </programlisting>
1727 Of course, in this particular situation you can do even better:
1728 <programlisting>
1729 do { mapM_ popInt xs ; return 10 }
1730 </programlisting>
1731 </para>
1732 </listitem>
1733 </varlistentry>
1734
1735 <varlistentry>
1736 <term><option>-fwarn-wrong-do-bind</option>:</term>
1737 <listitem>
1738 <indexterm><primary><option>-fwarn-wrong-do-bind</option></primary></indexterm>
1739 <indexterm><primary>apparently erroneous do binding, warning</primary></indexterm>
1740 <indexterm><primary>do binding, apparently erroneous</primary></indexterm>
1741
1742 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1743 that appear to lack a binding.
1744 For instance <literal>do { return (popInt 10) ; return 10 }</literal> would report
1745 the first statement in the <literal>do</literal> block as suspicious,
1746 as it has the type <literal>StackM (StackM Int)</literal> (which consists of two nested applications
1747 of the same monad constructor), but which is not then &quot;unpacked&quot; by binding the result.
1748 The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away:
1749 <programlisting>
1750 do { _ &lt;- return (popInt 10) ; return 10 }
1751 </programlisting>
1752 For almost all sensible programs this will indicate a bug, and you probably intended to write:
1753 <programlisting>
1754 do { popInt 10 ; return 10 }
1755 </programlisting>
1756 </para>
1757 </listitem>
1758 </varlistentry>
1759
1760 </variablelist>
1761
1762 <para>If you're feeling really paranoid, the
1763 <option>-dcore-lint</option>
1764 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1765 is a good choice. It turns on heavyweight intra-pass
1766 sanity-checking within GHC. (It checks GHC's sanity, not
1767 yours.)</para>
1768
1769 </sect1>
1770
1771 &packages;
1772
1773 <sect1 id="options-optimise">
1774 <title>Optimisation (code improvement)</title>
1775
1776 <indexterm><primary>optimisation</primary></indexterm>
1777 <indexterm><primary>improvement, code</primary></indexterm>
1778
1779 <para>The <option>-O*</option> options specify convenient
1780 &ldquo;packages&rdquo; of optimisation flags; the
1781 <option>-f*</option> options described later on specify
1782 <emphasis>individual</emphasis> optimisations to be turned on/off;
1783 the <option>-m*</option> options specify
1784 <emphasis>machine-specific</emphasis> optimisations to be turned
1785 on/off.</para>
1786
1787 <sect2 id="optimise-pkgs">
1788 <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1789
1790 <para>There are <emphasis>many</emphasis> options that affect
1791 the quality of code produced by GHC. Most people only have a
1792 general goal, something like &ldquo;Compile quickly&rdquo; or
1793 &ldquo;Make my program run like greased lightning.&rdquo; The
1794 following &ldquo;packages&rdquo; of optimisations (or lack
1795 thereof) should suffice.</para>
1796
1797 <para>Note that higher optimisation levels cause more
1798 cross-module optimisation to be performed, which can have an
1799 impact on how much of your program needs to be recompiled when
1800 you change something. This is one reason to stick to
1801 no-optimisation when developing code.</para>
1802
1803 <variablelist>
1804
1805 <varlistentry>
1806 <term>
1807 No <option>-O*</option>-type option specified:
1808 <indexterm><primary>-O* not specified</primary></indexterm>
1809 </term>
1810 <listitem>
1811 <para>This is taken to mean: &ldquo;Please compile
1812 quickly; I'm not over-bothered about compiled-code
1813 quality.&rdquo; So, for example: <command>ghc -c
1814 Foo.hs</command></para>
1815 </listitem>
1816 </varlistentry>
1817
1818 <varlistentry>
1819 <term>
1820 <option>-O0</option>:
1821 <indexterm><primary><option>-O0</option></primary></indexterm>
1822 </term>
1823 <listitem>
1824 <para>Means &ldquo;turn off all optimisation&rdquo;,
1825 reverting to the same settings as if no
1826 <option>-O</option> options had been specified. Saying
1827 <option>-O0</option> can be useful if
1828 eg. <command>make</command> has inserted a
1829 <option>-O</option> on the command line already.</para>
1830 </listitem>
1831 </varlistentry>
1832
1833 <varlistentry>
1834 <term>
1835 <option>-O</option> or <option>-O1</option>:
1836 <indexterm><primary>-O option</primary></indexterm>
1837 <indexterm><primary>-O1 option</primary></indexterm>
1838 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1839 </term>
1840 <listitem>
1841 <para>Means: &ldquo;Generate good-quality code without
1842 taking too long about it.&rdquo; Thus, for example:
1843 <command>ghc -c -O Main.lhs</command></para>
1844 </listitem>
1845 </varlistentry>
1846
1847 <varlistentry>
1848 <term>
1849 <option>-O2</option>:
1850 <indexterm><primary>-O2 option</primary></indexterm>
1851 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1852 </term>
1853 <listitem>
1854 <para>Means: &ldquo;Apply every non-dangerous
1855 optimisation, even if it means significantly longer
1856 compile times.&rdquo;</para>
1857
1858 <para>The avoided &ldquo;dangerous&rdquo; optimisations
1859 are those that can make runtime or space
1860 <emphasis>worse</emphasis> if you're unlucky. They are
1861 normally turned on or off individually.</para>
1862
1863 <para>At the moment, <option>-O2</option> is
1864 <emphasis>unlikely</emphasis> to produce better code than
1865 <option>-O</option>.</para>
1866 </listitem>
1867 </varlistentry>
1868 </variablelist>
1869
1870 <para>We don't use a <option>-O*</option> flag for day-to-day
1871 work. We use <option>-O</option> to get respectable speed;
1872 e.g., when we want to measure something. When we want to go for
1873 broke, we tend to use <option>-O2</option> (and we go for
1874 lots of coffee breaks).</para>
1875
1876 <para>The easiest way to see what <option>-O</option> (etc.)
1877 &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1878 then stand back in amazement.</para>
1879 </sect2>
1880
1881 <sect2 id="options-f">
1882 <title><option>-f*</option>: platform-independent flags</title>
1883
1884 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1885 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1886
1887 <para>These flags turn on and off individual optimisations.
1888 They are normally set via the <option>-O</option> options
1889 described above, and as such, you shouldn't need to set any of
1890 them explicitly (indeed, doing so could lead to unexpected
1891 results). A flag <option>-fwombat</option> can be negated by
1892 saying <option>-fno-wombat</option>. The flags below are off
1893 by default, except where noted below. See <xref linkend="options-f-compact"/>
1894 for a compact list.
1895 </para>
1896
1897 <variablelist>
1898 <varlistentry>
1899 <term>
1900 <option>-favoid-vect</option>
1901 <indexterm><primary><option></option></primary></indexterm>
1902 </term>
1903 <listitem>
1904 <para>Part of <link linkend="dph">Data Parallel Haskell
1905 (DPH)</link>.</para>
1906
1907 <para><emphasis>Off by default.</emphasis> Enable the
1908 <emphasis>vectorisation</emphasis> avoidance optimisation. This
1909 optimisation only works when used in combination with the
1910 <option>-fvectorise</option> transformation.</para>
1911
1912 <para>While vectorisation of code using DPH is often a big win, it
1913 can also produce worse results for some kinds of code. This
1914 optimisation modifies the vectorisation transformation to try to
1915 determine if a function would be better of unvectorised and if
1916 so, do just that.</para>
1917 </listitem>
1918 </varlistentry>
1919
1920 <varlistentry>
1921 <term>
1922 <option>-fcase-merge</option>
1923 <indexterm><primary><option></option></primary></indexterm>
1924 </term>
1925 <listitem>
1926 <para><emphasis>On by default.</emphasis>
1927 Merge immediately-nested case expressions that scrutinse the same variable. Example
1928 <programlisting>
1929 case x of
1930 Red -> e1
1931 _ -> case x of
1932 Blue -> e2
1933 Green -> e3
1934 ==>
1935 case x of
1936 Red -> e1
1937 Blue -> e2
1938 Green -> e2
1939 </programlisting>
1940 </para>
1941 </listitem>
1942 </varlistentry>
1943
1944 <varlistentry>
1945 <term>
1946 <option>-fcse</option>
1947 <indexterm><primary><option>-fcse</option></primary></indexterm>
1948 </term>
1949 <listitem>
1950 <para><emphasis>On by default.</emphasis>. Enables the common-sub-expression
1951 elimination optimisation.
1952 Switching this off can be useful if you have some <literal>unsafePerformIO</literal>
1953 expressions that you don't want commoned-up.</para>
1954 </listitem>
1955 </varlistentry>
1956
1957 <varlistentry>
1958 <term>
1959 <option>-fdicts-cheap</option>
1960 <indexterm><primary><option></option></primary></indexterm>
1961 </term>
1962 <listitem>
1963 <para>A very experimental flag that makes dictionary-valued
1964 expressions seem cheap to the optimiser.
1965 </para>
1966 </listitem>
1967 </varlistentry>
1968
1969 <varlistentry>
1970 <term>
1971 <option>-fdo-lambda-eta-expansion</option>
1972 <indexterm><primary><option></option></primary></indexterm>
1973 </term>
1974 <listitem>
1975 <para><emphasis>On by default.</emphasis>
1976 Eta-expand let-bindings to increase their arity.
1977 </para>
1978 </listitem>
1979 </varlistentry>
1980
1981 <varlistentry>
1982 <term>
1983 <option>-fdo-eta-reduction</option>
1984 <indexterm><primary><option></option></primary></indexterm>
1985 </term>
1986 <listitem>
1987 <para><emphasis>On by default.</emphasis>
1988 Eta-reduce lambda expressions, if doing so gets rid of a whole
1989 group of lambdas.
1990 </para>
1991 </listitem>
1992 </varlistentry>
1993
1994 <varlistentry>
1995 <term>
1996 <option>-feager-blackholing</option>
1997 <indexterm><primary><option></option></primary></indexterm>
1998 </term>
1999 <listitem>
2000 <para>Usually GHC black-holes a thunk only when it switches
2001 threads. This flag makes it do so as soon as the thunk is
2002 entered. See <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/">
2003 Haskell on a shared-memory multiprocessor</ulink>.
2004 </para>
2005 </listitem>
2006 </varlistentry>
2007
2008 <varlistentry>
2009 <term>
2010 <option>-fexcess-precision</option>
2011 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
2012 </term>
2013 <listitem>
2014 <para>When this option is given, intermediate floating
2015 point values can have a <emphasis>greater</emphasis>
2016 precision/range than the final type. Generally this is a
2017 good thing, but some programs may rely on the exact
2018 precision/range of
2019 <literal>Float</literal>/<literal>Double</literal> values
2020 and should not use this option for their compilation.</para>
2021
2022 <para>
2023 Note that the 32-bit x86 native code generator only
2024 supports excess-precision mode, so neither
2025 <option>-fexcess-precision</option> nor
2026 <option>-fno-excess-precision</option> has any effect.
2027 This is a known bug, see <xref linkend="bugs-ghc" />.
2028 </para>
2029 </listitem>
2030 </varlistentry>
2031
2032 <varlistentry>
2033 <term>
2034 <option>-fexpose-all-unfoldings</option>
2035 <indexterm><primary><option></option></primary></indexterm>
2036 </term>
2037 <listitem>
2038 <para>An experimental flag to expose all unfoldings, even for very
2039 large or recursive functions. This allows for all functions to be
2040 inlined while usually GHC would avoid inlining larger functions.
2041 </para>
2042 </listitem>
2043 </varlistentry>
2044
2045 <varlistentry>
2046 <term>
2047 <option>-ffloat-in</option>
2048 <indexterm><primary><option></option></primary></indexterm>
2049 </term>
2050 <listitem>
2051 <para><emphasis>On by default.</emphasis>
2052 Float let-bindings inwards, nearer their binding site. See
2053 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">
2054 Let-floating: moving bindings to give faster programs (ICFP'96)</ulink>.
2055 </para>
2056
2057 <para>This optimisation moves let bindings closer to their use
2058 site. The benefit here is that this may avoid unnecessary
2059 allocation if the branch the let is now on is never executed. It
2060 also enables other optimisation passes to work more effectively
2061 as they have more information locally.
2062 </para>
2063
2064 <para>This optimisation isn't always beneficial though (so GHC
2065 applies some heuristics to decide when to apply it). The details
2066 get complicated but a simple example is that it is often beneficial
2067 to move let bindings outwards so that multiple let bindings can be
2068 grouped into a larger single let binding, effectively batching
2069 their allocation and helping the garbage collector and allocator.
2070 </para>
2071 </listitem>
2072 </varlistentry>
2073
2074 <varlistentry>
2075 <term>
2076 <option>-ffull-laziness</option>
2077 <indexterm><primary><option>-ffull-laziness</option></primary></indexterm>
2078 </term>
2079 <listitem>
2080 <para><emphasis>On by default.</emphasis>
2081 Run the full laziness optimisation (also known as let-floating),
2082 which floats let-bindings outside enclosing lambdas, in the hope
2083 they will be thereby be computed less often. See
2084 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">Let-floating:
2085 moving bindings to give faster programs (ICFP'96)</ulink>.
2086 Full laziness increases sharing, which can lead to increased memory
2087 residency.
2088 </para>
2089
2090 <para>NOTE: GHC doesn't implement complete full-laziness.
2091 When optimisation in on, and <option>-fno-full-laziness</option>
2092 is not given, some transformations that increase sharing are
2093 performed, such as extracting repeated computations from a loop.
2094 These are the same transformations that a fully lazy
2095 implementation would do, the difference is that GHC doesn't
2096 consistently apply full-laziness, so don't rely on it.
2097 </para>
2098 </listitem>
2099 </varlistentry>
2100
2101 <varlistentry>
2102 <term>
2103 <option>-ffun-to-thunk</option>
2104 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
2105 </term>
2106 <listitem>
2107 <para>Worker-wrapper removes unused arguments, but usually we do
2108 not remove them all, lest it turn a function closure into a thunk,
2109 thereby perhaps creating a space leak and/or disrupting inlining.
2110 This flag allows worker/wrapper to remove <emphasis>all</emphasis>
2111 value lambdas. Off by default.
2112 </para>
2113 </listitem>
2114 </varlistentry>
2115
2116 <varlistentry>
2117 <term>
2118 <option>-fignore-asserts</option>
2119 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
2120 </term>
2121 <listitem>
2122 <para>Causes GHC to ignore uses of the function
2123 <literal>Exception.assert</literal> in source code (in
2124 other words, rewriting <literal>Exception.assert p
2125 e</literal> to <literal>e</literal> (see <xref
2126 linkend="assertions"/>). This flag is turned on by
2127 <option>-O</option>.
2128 </para>
2129 </listitem>
2130 </varlistentry>
2131
2132 <varlistentry>
2133 <term>
2134 <option>-fignore-interface-pragmas</option>
2135 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
2136 </term>
2137 <listitem>
2138 <para>Tells GHC to ignore all inessential information when reading interface files.
2139 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
2140 for a function, GHC will ignore that information.</para>
2141 </listitem>
2142 </varlistentry>
2143
2144 <varlistentry>
2145 <term>
2146 <option>-flate-dmd-anal</option>
2147 <indexterm><primary><option>-flate-dmd-anal</option></primary></indexterm>
2148 </term>
2149 <listitem>
2150 <para><emphasis>Off by default.</emphasis>Run demand analysis
2151 again, at the end of the simplification pipeline. We found some opportunities
2152 for discovering strictness that were not visible earlier; and optimisations like
2153 <literal>SpecConstr</literal> can create functions with unused arguments which
2154 are eliminated by late demand analysis. Improvements are modest, but so is the
2155 cost. See notes on the <ulink href="http://ghc.haskell.org/trac/ghc/wiki/LateDmd">Trac wiki page</ulink>.
2156 </para>
2157 </listitem>
2158 </varlistentry>
2159
2160 <varlistentry>
2161 <term>
2162 <option>-fliberate-case</option>
2163 <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
2164 </term>
2165 <listitem>
2166 <para><emphasis>Off by default, but enabled by -O2.</emphasis>
2167 Turn on the liberate-case transformation. This unrolls recursive
2168 function once in its own RHS, to avoid repeated case analysis of
2169 free variables. It's a bit like the call-pattern specialiser
2170 (<option>-fspec-constr</option>) but for free variables rather than
2171 arguments.
2172 </para>
2173 </listitem>
2174 </varlistentry>
2175
2176 <varlistentry>
2177 <term>
2178 <option>-fliberate-case-threshold=N</option>
2179 <indexterm><primary><option>-fliberate-case-threshold</option></primary></indexterm>
2180 </term>
2181 <listitem>
2182 <para>Set the size threshold for the liberate-case transformation.
2183 </para>
2184 </listitem>
2185 </varlistentry>
2186
2187 <varlistentry>
2188 <term>
2189 <option>-fmax-relevant-bindings=N</option>
2190 <indexterm><primary><option>-fmax-relevant-bindings</option></primary></indexterm>
2191 </term>
2192 <listitem>
2193 <para>The type checker sometimes displays a fragment of the type environment
2194 in error messages, but only up to some maximum number, set by this flag.
2195 The default is 6. Turning it off with <option>-fno-max-relevant-bindings</option>
2196 gives an unlimited number. Syntactically top-level bindings are also
2197 usually excluded (since they may be numerous), but
2198 <option>-fno-max-relevant-bindings</option> includes them too.
2199 </para>
2200 </listitem>
2201 </varlistentry>
2202
2203 <varlistentry>
2204 <term>
2205 <option>-fno-state-hack</option>
2206 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
2207 </term>
2208 <listitem>
2209 <para>Turn off the "state hack" whereby any lambda with a
2210 <literal>State#</literal> token as argument is considered to be
2211 single-entry, hence it is considered OK to inline things inside
2212 it. This can improve performance of IO and ST monad code, but it
2213 runs the risk of reducing sharing.
2214 </para>
2215 </listitem>
2216 </varlistentry>
2217
2218 <varlistentry>
2219 <term>
2220 <option>-fomit-interface-pragmas</option>
2221 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
2222 </term>
2223 <listitem>
2224 <para>Tells GHC to omit all inessential information from the
2225 interface file generated for the module being compiled (say M).
2226 This means that a module importing M will see only the
2227 <emphasis>types</emphasis> of the functions that M exports, but
2228 not their unfoldings, strictness info, etc. Hence, for example,
2229 no function exported by M will be inlined into an importing module.
2230 The benefit is that modules that import M will need to be
2231 recompiled less often (only when M's exports change their type, not
2232 when they change their implementation).</para>
2233 </listitem>
2234 </varlistentry>
2235
2236 <varlistentry>
2237 <term>
2238 <option>-fomit-yields</option>
2239 <indexterm><primary><option>-fomit-yields</option></primary></indexterm>
2240 </term>
2241 <listitem>
2242 <para><emphasis>On by default.</emphasis> Tells GHC to omit
2243 heap checks when no allocation is being performed. While this improves
2244 binary sizes by about 5%, it also means that threads run in
2245 tight non-allocating loops will not get preempted in a timely
2246 fashion. If it is important to always be able to interrupt such
2247 threads, you should turn this optimization off. Consider also
2248 recompiling all libraries with this optimization turned off, if you
2249 need to guarantee interruptibility.
2250 </para>
2251 </listitem>
2252 </varlistentry>
2253
2254 <varlistentry>
2255 <term>
2256 <option>-fpedantic-bottoms</option>
2257 <indexterm><primary><option>-fpedantic-bottoms</option></primary></indexterm>
2258 </term>
2259 <listitem>
2260 <para>Make GHC be more precise about its treatment of bottom (but see also
2261 <option>-fno-state-hack</option>). In particular, stop GHC
2262 eta-expanding through a case expression, which is good for
2263 performance, but bad if you are using <literal>seq</literal> on
2264 partial applications.
2265 </para>
2266 </listitem>
2267 </varlistentry>
2268
2269 <varlistentry>
2270 <term>
2271 <option>-fregs-graph</option>
2272 <indexterm><primary><option></option></primary></indexterm>
2273 </term>
2274 <listitem>
2275 <para><emphasis>Off by default, but enabled by -O2. Only applies in
2276 combination with the native code generator.</emphasis>
2277 Use the graph colouring register allocator for register allocation
2278 in the native code generator. By default, GHC uses a simpler,
2279 faster linear register allocator. The downside being that the
2280 linear register allocator usually generates worse code.
2281 </para>
2282 </listitem>
2283 </varlistentry>
2284
2285 <varlistentry>
2286 <term>
2287 <option>-fregs-iterative</option>
2288 <indexterm><primary><option></option></primary></indexterm>
2289 </term>
2290 <listitem>
2291 <para><emphasis>Off by default, only applies in combination with
2292 the native code generator.</emphasis>
2293 Use the iterative coalescing graph colouring register allocator for
2294 register allocation in the native code generator. This is the same
2295 register allocator as the <option>-freg-graph</option> one but also
2296 enables iterative coalescing during register allocation.
2297 </para>
2298 </listitem>
2299 </varlistentry>
2300
2301 <varlistentry>
2302 <term>
2303 <option>-fsimpl-tick-factor=<replaceable>n</replaceable></option>
2304 <indexterm><primary><option>-fsimpl-tick-factor</option></primary></indexterm>
2305 </term>
2306 <listitem>
2307 <para>GHC's optimiser can diverge if you write rewrite rules (
2308 <xref linkend="rewrite-rules"/>) that don't terminate, or (less
2309 satisfactorily) if you code up recursion through data types
2310 (<xref linkend="bugs-ghc"/>). To avoid making the compiler fall
2311 into an infinite loop, the optimiser carries a "tick count" and
2312 stops inlining and applying rewrite rules when this count is
2313 exceeded. The limit is set as a multiple of the program size, so
2314 bigger programs get more ticks. The
2315 <option>-fsimpl-tick-factor</option> flag lets you change the
2316 multiplier. The default is 100; numbers larger than 100 give more
2317 ticks, and numbers smaller than 100 give fewer.
2318 </para>
2319
2320 <para>If the tick-count expires, GHC summarises what simplifier
2321 steps it has done; you can use
2322 <option>-fddump-simpl-stats</option> to generate a much more
2323 detailed list. Usually that identifies the loop quite
2324 accurately, because some numbers are very large.
2325 </para>
2326 </listitem>
2327 </varlistentry>
2328
2329 <varlistentry>
2330 <term>
2331 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
2332 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
2333 <indexterm><primary>inlining, controlling</primary></indexterm>
2334 <indexterm><primary>unfolding, controlling</primary></indexterm>
2335 </term>
2336 <listitem>
2337 <para>(Default: 45) Governs the maximum size that GHC will allow a
2338 function unfolding to be. (An unfolding has a &ldquo;size&rdquo;
2339 that reflects the cost in terms of &ldquo;code bloat&rdquo; of
2340 expanding (aka inlining) that unfolding at a call site. A bigger
2341 function would be assigned a bigger cost.)
2342 </para>
2343
2344 <para>Consequences: (a) nothing larger than this will be inlined
2345 (unless it has an INLINE pragma); (b) nothing larger than this
2346 will be spewed into an interface file.
2347 </para>
2348
2349 <para>Increasing this figure is more likely to result in longer
2350 compile times than faster code. The
2351 <option>-funfolding-use-threshold</option> is more useful.
2352 </para>
2353 </listitem>
2354 </varlistentry>
2355
2356 <varlistentry>
2357 <term>
2358 <option>-funfolding-use-threshold=<replaceable>n</replaceable></option>
2359 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
2360 <indexterm><primary>inlining, controlling</primary></indexterm>
2361 <indexterm><primary>unfolding, controlling</primary></indexterm>
2362 </term>
2363 <listitem>
2364 <para>(Default: 8) This is the magic cut-off figure for unfolding
2365 (aka inlining): below this size, a function definition will be
2366 unfolded at the call-site, any bigger and it won't. The size
2367 computed for a function depends on two things: the actual size of
2368 the expression minus any discounts that
2369 apply (see <option>-funfolding-con-discount</option>).
2370 </para>
2371
2372 <para>The difference between this and
2373 <option>-funfolding-creation-threshold</option> is that this one
2374 determines if a function definition will be inlined <emphasis>at
2375 a call site</emphasis>. The other option determines if a
2376 function definition will be kept around at all for potential
2377 inlining.
2378 </para>
2379 </listitem>
2380 </varlistentry>
2381
2382 <varlistentry>
2383 <term>
2384 <option>-fvectorise</option>
2385 <indexterm><primary><option></option></primary></indexterm>
2386 </term>
2387 <listitem>
2388 <para>Part of <link linkend="dph">Data Parallel Haskell
2389 (DPH)</link>.</para>
2390
2391 <para><emphasis>Off by default.</emphasis> Enable the
2392 <emphasis>vectorisation</emphasis> optimisation transformation. This
2393 optimisation transforms the nested data parallelism code of programs
2394 using DPH into flat data parallelism. Flat data parallel programs
2395 should have better load balancing, enable SIMD parallelism and
2396 friendlier cache behaviour.</para>
2397 </listitem>
2398 </varlistentry>
2399
2400 <varlistentry>
2401 <term>
2402 <option>-fspec-constr</option>
2403 <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
2404 </term>
2405 <listitem>
2406 <para><emphasis>Off by default, but enabled by -O2.</emphasis>
2407 Turn on call-pattern specialisation; see
2408 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/spec-constr/index.htm">
2409 Call-pattern specialisation for Haskell programs</ulink>.
2410 </para>
2411
2412 <para>This optimisation specializes recursive functions according to
2413 their argument "shapes". This is best explained by example so
2414 consider:
2415 <programlisting>
2416 last :: [a] -> a
2417 last [] = error "last"
2418 last (x : []) = x
2419 last (x : xs) = last xs
2420 </programlisting>
2421 In this code, once we pass the initial check for an empty list we
2422 know that in the recursive case this pattern match is redundant. As
2423 such <option>-fspec-constr</option> will transform the above code
2424 to:
2425 <programlisting>
2426 last :: [a] -> a
2427 last [] = error "last"
2428 last (x : xs) = last' x xs
2429 where
2430 last' x [] = x
2431 last' x (y : ys) = last' y ys
2432 </programlisting>
2433 </para>
2434
2435 <para>As well avoid unnecessary pattern matching it also helps avoid
2436 unnecessary allocation. This applies when a argument is strict in
2437 the recursive call to itself but not on the initial entry. As
2438 strict recursive branch of the function is created similar to the
2439 above example.
2440 </para>
2441 </listitem>
2442 </varlistentry>
2443
2444 <varlistentry>
2445 <term>
2446 <option>-fspecialise</option>
2447 <indexterm><primary><option>-fspecialise</option></primary></indexterm>
2448 </term>
2449 <listitem>
2450 <para><emphasis>On by default.</emphasis>
2451 Specialise each type-class-overloaded function defined in this
2452 module for the types at which it is called in this module. Also
2453 specialise imported functions that have an INLINABLE pragma
2454 (<xref linkend="inlinable-pragma"/>) for the types at which they
2455 are called in this module.
2456 </para>
2457 </listitem>
2458 </varlistentry>
2459
2460 <varlistentry>
2461 <term>
2462 <option>-fstatic-argument-transformation</option>
2463 <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
2464 </term>
2465 <listitem>
2466 <para>Turn on the static argument transformation, which turns a
2467 recursive function into a non-recursive one with a local
2468 recursive loop. See Chapter 7 of
2469 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/santos-thesis.ps.gz">
2470 Andre Santos's PhD thesis</ulink>
2471 </para>
2472 </listitem>
2473 </varlistentry>
2474
2475 <varlistentry>
2476 <term>
2477 <option>-fstrictness</option>
2478 <indexterm><primary><option></option></primary></indexterm>
2479 </term>
2480 <listitem>
2481 <para> <emphasis>On by default.</emphasis>.
2482 Switch on the strictness analyser. There is a very old paper about GHC's
2483 strictness analyser, <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/simple-strictnes-analyser.ps.gz">
2484 Measuring the effectiveness of a simple strictness analyser</ulink>,
2485 but the current one is quite a bit different.
2486 </para>
2487
2488 <para>The strictness analyser figures out when arguments and
2489 variables in a function can be treated 'strictly' (that is they
2490 are always evaluated in the function at some point). This allow
2491 GHC to apply certain optimisations such as unboxing that
2492 otherwise don't apply as they change the semantics of the program
2493 when applied to lazy arguments.
2494 </para>
2495 </listitem>
2496 </varlistentry>
2497
2498 <varlistentry>
2499 <term>
2500 <option>-funbox-strict-fields</option>:
2501 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
2502 <indexterm><primary>strict constructor fields</primary></indexterm>
2503 <indexterm><primary>constructor fields, strict</primary></indexterm>
2504 </term>
2505 <listitem>
2506 <para>This option causes all constructor fields which are marked
2507 strict (i.e. &ldquo;!&rdquo;) to be unpacked if possible. It is
2508 equivalent to adding an <literal>UNPACK</literal> pragma to every
2509 strict constructor field (see <xref linkend="unpack-pragma"/>).
2510 </para>
2511
2512 <para>This option is a bit of a sledgehammer: it might sometimes
2513 make things worse. Selectively unboxing fields by using
2514 <literal>UNPACK</literal> pragmas might be better. An alternative
2515 is to use <option>-funbox-strict-fields</option> to turn on
2516 unboxing by default but disable it for certain constructor
2517 fields using the <literal>NOUNPACK</literal> pragma (see
2518 <xref linkend="nounpack-pragma"/>).</para>
2519 </listitem>
2520 </varlistentry>
2521
2522 <varlistentry>
2523 <term>
2524 <option>-funbox-small-strict-fields</option>:
2525 <indexterm><primary><option>-funbox-small-strict-fields</option></primary></indexterm>
2526 <indexterm><primary>strict constructor fields</primary></indexterm>
2527 <indexterm><primary>constructor fields, strict</primary></indexterm>
2528 </term>
2529 <listitem>
2530 <para><emphasis>On by default.</emphasis>. This option
2531 causes all constructor fields which are marked strict
2532 (i.e. &ldquo;!&rdquo;) and which representation is smaller
2533 or equal to the size of a pointer to be unpacked, if
2534 possible. It is equivalent to adding an
2535 <literal>UNPACK</literal> pragma (see <xref
2536 linkend="unpack-pragma"/>) to every strict constructor
2537 field that fulfils the size restriction.
2538 </para>
2539
2540 <para>For example, the constructor fields in the following
2541 data types
2542 <programlisting>
2543 data A = A !Int
2544 data B = B !A
2545 newtype C = C B
2546 data D = D !C
2547 </programlisting>
2548 would all be represented by a single
2549 <literal>Int#</literal> (see <xref linkend="primitives"/>)
2550 value with
2551 <option>-funbox-small-strict-fields</option> enabled.
2552 </para>
2553
2554 <para>This option is less of a sledgehammer than
2555 <option>-funbox-strict-fields</option>: it should rarely make things
2556 worse. If you use <option>-funbox-small-strict-fields</option>
2557 to turn on unboxing by default you can disable it for certain
2558 constructor fields using the <literal>NOUNPACK</literal> pragma (see
2559 <xref linkend="nounpack-pragma"/>).</para>
2560
2561 <para>
2562 Note that for consistency <literal>Double</literal>,
2563 <literal>Word64</literal>, and <literal>Int64</literal> constructor
2564 fields are unpacked on 32-bit platforms, even though they are
2565 technically larger than a pointer on those platforms.
2566 </para>
2567 </listitem>
2568 </varlistentry>
2569
2570 </variablelist>
2571
2572 </sect2>
2573
2574 </sect1>
2575
2576 &code-gens;
2577
2578 &phases;
2579
2580 &shared_libs;
2581
2582 <sect1 id="using-concurrent">
2583 <title>Using Concurrent Haskell</title>
2584 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
2585
2586 <para>GHC supports Concurrent Haskell by default, without requiring a
2587 special option or libraries compiled in a certain way. To get access to
2588 the support libraries for Concurrent Haskell, just import
2589 <ulink
2590 url="&libraryBaseLocation;/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
2591
2592 <para>
2593 Optionally, the program may be linked with
2594 the <option>-threaded</option> option (see
2595 <xref linkend="options-linker" />. This provides two benefits:
2596
2597 <itemizedlist>
2598 <listitem>
2599 <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
2600 used, which allows threads to run in
2601 parallel<indexterm><primary>parallelism</primary></indexterm>
2602 on a
2603 multiprocessor<indexterm><primary>multiprocessor</primary></indexterm><indexterm><primary>SMP</primary></indexterm>
2604 or
2605 multicore<indexterm><primary>multicore</primary></indexterm>
2606 machine. See <xref linkend="using-smp" />.</para>
2607 </listitem>
2608 <listitem>
2609 <para>If a thread makes a foreign call (and the call is
2610 not marked <literal>unsafe</literal>), then other
2611 Haskell threads in the program will continue to run
2612 while the foreign call is in progress.
2613 Additionally, <literal>foreign export</literal>ed
2614 Haskell functions may be called from multiple OS
2615 threads simultaneously. See
2616 <xref linkend="ffi-threads" />.</para>
2617 </listitem>
2618 </itemizedlist>
2619 </para>
2620
2621 <para>The following RTS option(s) affect the behaviour of Concurrent
2622 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
2623
2624 <variablelist>
2625 <varlistentry>
2626 <term><option>-C<replaceable>s</replaceable></option></term>
2627 <listitem>
2628 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2629 Sets the context switch interval to <replaceable>s</replaceable>
2630 seconds. A context switch will occur at the next heap block
2631 allocation after the timer expires (a heap block allocation occurs
2632 every 4k of allocation). With <option>-C0</option> or
2633 <option>-C</option>, context switches will occur as often as
2634 possible (at every heap block allocation). By default, context
2635 switches occur every 20ms.</para>
2636 </listitem>
2637 </varlistentry>
2638 </variablelist>
2639 </sect1>
2640
2641 <sect1 id="using-smp">
2642 <title>Using SMP parallelism</title>
2643 <indexterm><primary>parallelism</primary>
2644 </indexterm>
2645 <indexterm><primary>SMP</primary>
2646 </indexterm>
2647
2648 <para>GHC supports running Haskell programs in parallel on an SMP
2649 (symmetric multiprocessor).</para>
2650
2651 <para>There's a fine distinction between
2652 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
2653 parallelism is all about making your program run
2654 <emphasis>faster</emphasis> by making use of multiple processors
2655 simultaneously. Concurrency, on the other hand, is a means of
2656 abstraction: it is a convenient way to structure a program that must
2657 respond to multiple asynchronous events.</para>
2658
2659 <para>However, the two terms are certainly related. By making use of
2660 multiple CPUs it is possible to run concurrent threads in parallel,
2661 and this is exactly what GHC's SMP parallelism support does. But it
2662 is also possible to obtain performance improvements with parallelism
2663 on programs that do not use concurrency. This section describes how to
2664 use GHC to compile and run parallel programs, in <xref
2665 linkend="lang-parallel" /> we describe the language features that affect
2666 parallelism.</para>
2667
2668 <sect2 id="parallel-compile-options">
2669 <title>Compile-time options for SMP parallelism</title>
2670
2671 <para>In order to make use of multiple CPUs, your program must be
2672 linked with the <option>-threaded</option> option (see <xref
2673 linkend="options-linker" />). Additionally, the following
2674 compiler options affect parallelism:</para>
2675
2676 <variablelist>
2677 <varlistentry>
2678 <term><option>-feager-blackholing</option></term>
2679 <indexterm><primary><option>-feager-blackholing</option></primary></indexterm>
2680 <listitem>
2681 <para>
2682 Blackholing is the act of marking a thunk (lazy
2683 computuation) as being under evaluation. It is useful for
2684 three reasons: firstly it lets us detect certain kinds of
2685 infinite loop (the <literal>NonTermination</literal>
2686 exception), secondly it avoids certain kinds of space
2687 leak, and thirdly it avoids repeating a computation in a
2688 parallel program, because we can tell when a computation
2689 is already in progress.</para>
2690
2691 <para>
2692 The option <option>-feager-blackholing</option> causes
2693 each thunk to be blackholed as soon as evaluation begins.
2694 The default is "lazy blackholing", whereby thunks are only
2695 marked as being under evaluation when a thread is paused
2696 for some reason. Lazy blackholing is typically more
2697 efficient (by 1-2&percnt; or so), because most thunks don't
2698 need to be blackholed. However, eager blackholing can
2699 avoid more repeated computation in a parallel program, and
2700 this often turns out to be important for parallelism.
2701 </para>
2702
2703 <para>
2704 We recommend compiling any code that is intended to be run
2705 in parallel with the <option>-feager-blackholing</option>
2706 flag.
2707 </para>
2708 </listitem>
2709 </varlistentry>
2710 </variablelist>
2711 </sect2>
2712
2713 <sect2 id="parallel-options">
2714 <title>RTS options for SMP parallelism</title>
2715
2716 <para>There are two ways to run a program on multiple
2717 processors:
2718 call <literal>Control.Concurrent.setNumCapabilities</literal> from your
2719 program, or use the RTS <option>-N</option> option.</para>
2720
2721 <variablelist>
2722 <varlistentry>
2723 <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
2724 <listitem>
2725 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2726 Use <replaceable>x</replaceable> simultaneous threads when
2727 running the program. Normally <replaceable>x</replaceable>
2728 should be chosen to match the number of CPU cores on the
2729 machine<footnote><para>Whether hyperthreading cores should be counted or not is an
2730 open question; please feel free to experiment and let us know what
2731 results you find.</para></footnote>. For example,
2732 on a dual-core machine we would probably use
2733 <literal>+RTS -N2 -RTS</literal>.</para>
2734
2735 <para>Omitting <replaceable>x</replaceable>,
2736 i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
2737 choose the value of <replaceable>x</replaceable> itself
2738 based on how many processors are in your machine.</para>
2739
2740 <para>Be careful when using all the processors in your
2741 machine: if some of your processors are in use by other
2742 programs, this can actually harm performance rather than
2743 improve it.</para>
2744
2745 <para>Setting <option>-N</option> also has the effect of
2746 enabling the parallel garbage collector (see
2747 <xref linkend="rts-options-gc" />).</para>
2748
2749 <para>The current value of the <option>-N</option> option
2750 is available to the Haskell program
2751 via <literal>Control.Concurrent.getNumCapabilities</literal>, and
2752 it may be changed while the program is running by
2753 calling <literal>Control.Concurrent.setNumCapabilities</literal>.</para>
2754 </listitem>
2755 </varlistentry>
2756 </variablelist>
2757
2758 <para>The following options affect the way the runtime schedules
2759 threads on CPUs:</para>
2760
2761 <variablelist>
2762 <varlistentry>
2763 <term><option>-qa</option></term>
2764 <indexterm><primary><option>-qa</option></primary><secondary>RTS
2765 option</secondary></indexterm>
2766 <listitem>
2767 <para>Use the OS's affinity facilities to try to pin OS
2768 threads to CPU cores. This is an experimental feature,
2769 and may or may not be useful. Please let us know
2770 whether it helps for you!</para>
2771 </listitem>
2772 </varlistentry>
2773 <varlistentry>
2774 <term><option>-qm</option></term>
2775 <indexterm><primary><option>-qm</option></primary><secondary>RTS
2776 option</secondary></indexterm>
2777 <listitem>
2778 <para>Disable automatic migration for load balancing.
2779 Normally the runtime will automatically try to schedule
2780 threads across the available CPUs to make use of idle
2781 CPUs; this option disables that behaviour. Note that
2782 migration only applies to threads; sparks created
2783 by <literal>par</literal> are load-balanced separately
2784 by work-stealing.</para>
2785
2786 <para>
2787 This option is probably only of use for concurrent
2788 programs that explicitly schedule threads onto CPUs
2789 with <literal>Control.Concurrent.forkOn</literal>.
2790 </para>
2791 </listitem>
2792 </varlistentry>
2793 </variablelist>
2794 </sect2>
2795
2796 <sect2>
2797 <title>Hints for using SMP parallelism</title>
2798
2799 <para>Add the <literal>-s</literal> RTS option when
2800 running the program to see timing stats, which will help to tell you
2801 whether your program got faster by using more CPUs or not. If the user
2802 time is greater than
2803 the elapsed time, then the program used more than one CPU. You should
2804 also run the program without <literal>-N</literal> for
2805 comparison.</para>
2806
2807 <para>The output of <literal>+RTS -s</literal> tells you how
2808 many &ldquo;sparks&rdquo; were created and executed during the
2809 run of the program (see <xref linkend="rts-options-gc" />), which
2810 will give you an idea how well your <literal>par</literal>
2811 annotations are working.</para>
2812
2813 <para>GHC's parallelism support has improved in 6.12.1 as a
2814 result of much experimentation and tuning in the runtime
2815 system. We'd still be interested to hear how well it works
2816 for you, and we're also interested in collecting parallel
2817 programs to add to our benchmarking suite.</para>
2818 </sect2>
2819 </sect1>
2820
2821 <sect1 id="options-platform">
2822 <title>Platform-specific Flags</title>
2823
2824 <indexterm><primary>-m* options</primary></indexterm>
2825 <indexterm><primary>platform-specific options</primary></indexterm>
2826 <indexterm><primary>machine-specific options</primary></indexterm>
2827
2828 <para>Some flags only make sense for particular target
2829 platforms.</para>
2830
2831 <variablelist>
2832
2833 <varlistentry>
2834 <term><option>-msse2</option>:</term>
2835 <listitem>
2836 <para>
2837 (x86 only, added in GHC 7.0.1) Use the SSE2 registers and
2838 instruction set to implement floating point operations when using
2839 the <link linkend="native-code-gen">native code generator</link>.
2840 This gives a substantial performance improvement for floating
2841 point, but the resulting compiled code
2842 will only run on processors that support SSE2 (Intel Pentium 4 and
2843 later, or AMD Athlon 64 and later). The
2844 <link linkend="llvm-code-gen">LLVM backend</link> will also use SSE2
2845 if your processor supports it but detects this automatically so no
2846 flag is required.
2847 </para>
2848 <para>
2849 SSE2 is unconditionally used on x86-64 platforms.
2850 </para>
2851 </listitem>
2852 </varlistentry>
2853
2854 <varlistentry>
2855 <term><option>-msse4.2</option>:</term>
2856 <listitem>
2857 <para>
2858 (x86 only, added in GHC 7.4.1) Use the SSE4.2 instruction set to
2859 implement some floating point and bit operations when using the
2860 <link linkend="native-code-gen">native code generator</link>. The
2861 resulting compiled code will only run on processors that
2862 support SSE4.2 (Intel Core i7 and later). The
2863 <link linkend="llvm-code-gen">LLVM backend</link> will also use
2864 SSE4.2 if your processor supports it but detects this automatically
2865 so no flag is required.
2866 </para>
2867 </listitem>
2868 </varlistentry>
2869
2870 </variablelist>
2871
2872 </sect1>
2873
2874 &runtime;
2875
2876 <sect1 id="ext-core">
2877 <title>Generating and compiling External Core Files</title>
2878
2879 <indexterm><primary>intermediate code generation</primary></indexterm>
2880
2881 <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format)
2882 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
2883 <filename>.hcr</filename>. The Core format is described in <ulink url="../../core.pdf">
2884 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
2885 and sample tools
2886 for manipulating Core files (in Haskell) are available in the
2887 <ulink url="http://hackage.haskell.org/package/extcore">extcore package on Hackage</ulink>. Note that the format of <literal>.hcr</literal>
2888 files is <emphasis>different</emphasis> from the Core output format that GHC generates
2889 for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
2890
2891 <para>The Core format natively supports notes which you can add to
2892 your source code using the <literal>CORE</literal> pragma (see <xref
2893 linkend="pragmas"/>).</para>
2894
2895 <variablelist>
2896
2897 <varlistentry>
2898 <term>
2899 <option>-fext-core</option>
2900 <indexterm><primary><option>-fext-core</option></primary></indexterm>
2901 </term>
2902 <listitem>
2903 <para>Generate <literal>.hcr</literal> files.</para>
2904 </listitem>
2905 </varlistentry>
2906
2907 </variablelist>
2908
2909 <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://ghc.haskell.org/trac/ghc/wiki/MailingListsAndIRC">make your wishes known to the GHC Team</ulink>.</para>
2910
2911 </sect1>
2912
2913 &debug;
2914 &flags;
2915
2916 </chapter>
2917
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