Add -fpedantic-bottoms, and document it
[ghc.git] / docs / users_guide / using.xml
1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="using-ghc">
3 <title>Using GHC</title>
4
5 <indexterm><primary>GHC, using</primary></indexterm>
6 <indexterm><primary>using GHC</primary></indexterm>
7
8 <sect1>
9 <title>Getting started: compiling programs</title>
10
11 <para>
12 In this chapter you'll find a complete reference to the GHC
13 command-line syntax, including all 400+ flags. It's a large and
14 complex system, and there are lots of details, so it can be
15 quite hard to figure out how to get started. With that in mind,
16 this introductory section provides a quick introduction to the
17 basic usage of GHC for compiling a Haskell program, before the
18 following sections dive into the full syntax.
19 </para>
20
21 <para>
22 Let's create a Hello World program, and compile and run it.
23 First, create a file <filename>hello.hs</filename> containing
24 the Haskell code:
25 </para>
26
27 <programlisting>
28 main = putStrLn "Hello, World!"
29 </programlisting>
30
31 <para>To compile the program, use GHC like this:</para>
32
33 <screen>
34 $ ghc hello.hs</screen>
35
36 <para>(where <literal>$</literal> represents the prompt: don't
37 type it). GHC will compile the source
38 file <filename>hello.hs</filename>, producing
39 an <firstterm>object
40 file</firstterm> <filename>hello.o</filename> and
41 an <firstterm>interface
42 file</firstterm> <filename>hello.hi</filename>, and then it
43 will link the object file to the libraries that come with GHC
44 to produce an executable called <filename>hello</filename> on
45 Unix/Linux/Mac, or <filename>hello.exe</filename> on
46 Windows.</para>
47
48 <para>
49 By default GHC will be very quiet about what it is doing, only
50 printing error messages. If you want to see in more detail
51 what's going on behind the scenes, add <option>-v</option> to
52 the command line.
53 </para>
54
55 <para>
56 Then we can run the program like this:
57 </para>
58
59 <screen>
60 $ ./hello
61 Hello World!</screen>
62
63 <para>
64 If your program contains multiple modules, then you only need to
65 tell GHC the name of the source file containing
66 the <filename>Main</filename> module, and GHC will examine
67 the <literal>import</literal> declarations to find the other
68 modules that make up the program and find their source files.
69 This means that, with the exception of
70 the <literal>Main</literal> module, every source file should be
71 named after the module name that it contains (with dots replaced
72 by directory separators). For example, the
73 module <literal>Data.Person</literal> would be in the
74 file <filename>Data/Person.hs</filename> on Unix/Linux/Mac,
75 or <filename>Data\Person.hs</filename> on Windows.
76 </para>
77 </sect1>
78
79 <sect1>
80 <title>Options overview</title>
81
82 <para>GHC's behaviour is controlled by
83 <firstterm>options</firstterm>, which for historical reasons are
84 also sometimes referred to as command-line flags or arguments.
85 Options can be specified in three ways:</para>
86
87 <sect2>
88 <title>Command-line arguments</title>
89
90 <indexterm><primary>structure, command-line</primary></indexterm>
91 <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
92 <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
93
94 <para>An invocation of GHC takes the following form:</para>
95
96 <screen>
97 ghc [argument...]
98 </screen>
99
100 <para>Command-line arguments are either options or file names.</para>
101
102 <para>Command-line options begin with <literal>-</literal>.
103 They may <emphasis>not</emphasis> be grouped:
104 <option>-vO</option> is different from <option>-v -O</option>.
105 Options need not precede filenames: e.g., <literal>ghc *.o -o
106 foo</literal>. All options are processed and then applied to
107 all files; you cannot, for example, invoke <literal>ghc -c -O1
108 Foo.hs -O2 Bar.hs</literal> to apply different optimisation
109 levels to the files <filename>Foo.hs</filename> and
110 <filename>Bar.hs</filename>.</para>
111 </sect2>
112
113 <sect2 id="source-file-options">
114 <title>Command line options in source files</title>
115
116 <indexterm><primary>source-file options</primary></indexterm>
117
118 <para>Sometimes it is useful to make the connection between a
119 source file and the command-line options it requires quite
120 tight. For instance, if a Haskell source file deliberately
121 uses name shadowing, it should be compiled with the
122 <option>-fno-warn-name-shadowing</option> option. Rather than maintaining
123 the list of per-file options in a <filename>Makefile</filename>,
124 it is possible to do this directly in the source file using the
125 <literal>OPTIONS_GHC</literal> pragma <indexterm><primary>OPTIONS_GHC
126 pragma</primary></indexterm>:</para>
127
128 <programlisting>
129 {-# OPTIONS_GHC -fno-warn-name-shadowing #-}
130 module X where
131 ...
132 </programlisting>
133
134 <para><literal>OPTIONS_GHC</literal> is a <emphasis>file-header pragma</emphasis>
135 (see <xref linkend="pragmas"/>).</para>
136
137 <para>Only <emphasis>dynamic</emphasis> flags can be used in an <literal>OPTIONS_GHC</literal> pragma
138 (see <xref linkend="static-dynamic-flags"/>).</para>
139
140 <para>Note that your command shell does not
141 get to the source file options, they are just included literally
142 in the array of command-line arguments the compiler
143 maintains internally, so you'll be desperately disappointed if
144 you try to glob etc. inside <literal>OPTIONS_GHC</literal>.</para>
145
146 <para>NOTE: the contents of OPTIONS_GHC are appended to the
147 command-line options, so options given in the source file
148 override those given on the command-line.</para>
149
150 <para>It is not recommended to move all the contents of your
151 Makefiles into your source files, but in some circumstances, the
152 <literal>OPTIONS_GHC</literal> pragma is the Right Thing. (If you
153 use <option>-keep-hc-file</option> and have OPTION flags in
154 your module, the OPTIONS_GHC will get put into the generated .hc
155 file).</para>
156 </sect2>
157
158 <sect2>
159 <title>Setting options in GHCi</title>
160
161 <para>Options may also be modified from within GHCi, using the
162 <literal>:set</literal> command. See <xref linkend="ghci-set"/>
163 for more details.</para>
164 </sect2>
165 </sect1>
166
167 <sect1 id="static-dynamic-flags">
168 <title>Static, Dynamic, and Mode options</title>
169 <indexterm><primary>static</primary><secondary>options</secondary>
170 </indexterm>
171 <indexterm><primary>dynamic</primary><secondary>options</secondary>
172 </indexterm>
173 <indexterm><primary>mode</primary><secondary>options</secondary>
174 </indexterm>
175
176 <para>Each of GHC's command line options is classified as
177 <firstterm>static</firstterm>, <firstterm>dynamic</firstterm> or
178 <firstterm>mode</firstterm>:</para>
179
180 <variablelist>
181 <varlistentry>
182 <term>Mode flags</term>
183 <listitem>
184 <para>For example, <option>&ndash;&ndash;make</option> or <option>-E</option>.
185 There may only be a single mode flag on the command line. The
186 available modes are listed in <xref linkend="modes"/>.</para>
187 </listitem>
188 </varlistentry>
189 <varlistentry>
190 <term>Dynamic Flags</term>
191 <listitem>
192 <para>Most non-mode flags fall into this category. A dynamic flag
193 may be used on the command line, in a
194 <literal>OPTIONS_GHC</literal> pragma in a source file, or set
195 using <literal>:set</literal> in GHCi.</para>
196 </listitem>
197 </varlistentry>
198 <varlistentry>
199 <term>Static Flags</term>
200 <listitem>
201 <para>A few flags are "static", which means they can only be used on
202 the command-line, and remain in force over the entire GHC/GHCi
203 run.</para>
204 </listitem>
205 </varlistentry>
206 </variablelist>
207
208 <para>The flag reference tables (<xref
209 linkend="flag-reference"/>) lists the status of each flag.</para>
210
211 <para>There are a few flags that are static except that they can
212 also be used with GHCi's <literal>:set</literal> command; these
213 are listed as &ldquo;static/<literal>:set</literal>&rdquo; in the
214 table.</para>
215 </sect1>
216
217 <sect1 id="file-suffixes">
218 <title>Meaningful file suffixes</title>
219
220 <indexterm><primary>suffixes, file</primary></indexterm>
221 <indexterm><primary>file suffixes for GHC</primary></indexterm>
222
223 <para>File names with &ldquo;meaningful&rdquo; suffixes (e.g.,
224 <filename>.lhs</filename> or <filename>.o</filename>) cause the
225 &ldquo;right thing&rdquo; to happen to those files.</para>
226
227 <variablelist>
228
229 <varlistentry>
230 <term><filename>.hs</filename></term>
231 <listitem>
232 <para>A Haskell module.</para>
233 </listitem>
234 </varlistentry>
235
236 <varlistentry>
237 <term>
238 <filename>.lhs</filename>
239 <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
240 </term>
241 <listitem>
242 <para>A &ldquo;literate Haskell&rdquo; module.</para>
243 </listitem>
244 </varlistentry>
245
246 <varlistentry>
247 <term><filename>.hi</filename></term>
248 <listitem>
249 <para>A Haskell interface file, probably
250 compiler-generated.</para>
251 </listitem>
252 </varlistentry>
253
254 <varlistentry>
255 <term><filename>.hc</filename></term>
256 <listitem>
257 <para>Intermediate C file produced by the Haskell
258 compiler.</para>
259 </listitem>
260 </varlistentry>
261
262 <varlistentry>
263 <term><filename>.c</filename></term>
264 <listitem>
265 <para>A C&nbsp;file not produced by the Haskell
266 compiler.</para>
267 </listitem>
268 </varlistentry>
269
270 <varlistentry>
271 <term><filename>.ll</filename></term>
272 <listitem>
273 <para>An llvm-intermediate-language source file, usually
274 produced by the compiler.</para>
275 </listitem>
276 </varlistentry>
277
278 <varlistentry>
279 <term><filename>.bc</filename></term>
280 <listitem>
281 <para>An llvm-intermediate-language bitcode file, usually
282 produced by the compiler.</para>
283 </listitem>
284 </varlistentry>
285
286 <varlistentry>
287 <term><filename>.s</filename></term>
288 <listitem>
289 <para>An assembly-language source file, usually produced by
290 the compiler.</para>
291 </listitem>
292 </varlistentry>
293
294 <varlistentry>
295 <term><filename>.o</filename></term>
296 <listitem>
297 <para>An object file, produced by an assembler.</para>
298 </listitem>
299 </varlistentry>
300 </variablelist>
301
302 <para>Files with other suffixes (or without suffixes) are passed
303 straight to the linker.</para>
304
305 </sect1>
306
307 <sect1 id="modes">
308 <title>Modes of operation</title>
309
310 <para>
311 GHC's behaviour is firstly controlled by a mode flag. Only one
312 of these flags may be given, but it does not necessarily need to
313 be the first option on the command-line.
314 </para>
315
316 <para>
317 If no mode flag is present, then GHC will enter make mode
318 (<xref linkend="make-mode" />) if there are any Haskell source
319 files given on the command line, or else it will link the
320 objects named on the command line to produce an executable.
321 </para>
322
323 <para>The available mode flags are:</para>
324
325 <variablelist>
326 <varlistentry>
327 <term>
328 <cmdsynopsis><command>ghc --interactive</command>
329 </cmdsynopsis>
330 <indexterm><primary>interactive mode</primary></indexterm>
331 <indexterm><primary>ghci</primary></indexterm>
332 </term>
333 <listitem>
334 <para>Interactive mode, which is also available as
335 <command>ghci</command>. Interactive mode is described in
336 more detail in <xref linkend="ghci"/>.</para>
337 </listitem>
338 </varlistentry>
339
340 <varlistentry>
341 <term>
342 <cmdsynopsis><command>ghc &ndash;&ndash;make</command>
343 </cmdsynopsis>
344 <indexterm><primary>make mode</primary></indexterm>
345 <indexterm><primary><option>&ndash;&ndash;make</option></primary></indexterm>
346 </term>
347 <listitem>
348 <para>In this mode, GHC will build a multi-module Haskell
349 program automatically, figuring out dependencies for itself.
350 If you have a straightforward Haskell program, this is
351 likely to be much easier, and faster, than using
352 <command>make</command>. Make mode is described in <xref
353 linkend="make-mode"/>.</para>
354
355 <para>
356 This mode is the default if there are any Haskell
357 source files mentioned on the command line, and in this case
358 the <option>&ndash;&ndash;make</option> option can be omitted.
359 </para>
360 </listitem>
361 </varlistentry>
362
363 <varlistentry>
364 <term>
365 <cmdsynopsis><command>ghc -e</command>
366 <arg choice='plain'><replaceable>expr</replaceable></arg>
367 </cmdsynopsis>
368 <indexterm><primary>eval mode</primary></indexterm>
369 </term>
370 <listitem>
371 <para>Expression-evaluation mode. This is very similar to
372 interactive mode, except that there is a single expression
373 to evaluate (<replaceable>expr</replaceable>) which is given
374 on the command line. See <xref linkend="eval-mode"/> for
375 more details.</para>
376 </listitem>
377 </varlistentry>
378
379 <varlistentry>
380 <term>
381 <cmdsynopsis>
382 <command>ghc -E</command>
383 <command>ghc -c</command>
384 <command>ghc -S</command>
385 <command>ghc -c</command>
386 </cmdsynopsis>
387 <indexterm><primary><option>-E</option></primary></indexterm>
388 <indexterm><primary><option>-C</option></primary></indexterm>
389 <indexterm><primary><option>-S</option></primary></indexterm>
390 <indexterm><primary><option>-c</option></primary></indexterm>
391 </term>
392 <listitem>
393 <para>This is the traditional batch-compiler mode, in which
394 GHC can compile source files one at a time, or link objects
395 together into an executable. This mode also applies if
396 there is no other mode flag specified on the command line,
397 in which case it means that the specified files should be
398 compiled and then linked to form a program. See <xref
399 linkend="options-order"/>.</para>
400 </listitem>
401 </varlistentry>
402
403 <varlistentry>
404 <term>
405 <cmdsynopsis>
406 <command>ghc -M</command>
407 </cmdsynopsis>
408 <indexterm><primary>dependency-generation mode</primary></indexterm>
409 </term>
410 <listitem>
411 <para>Dependency-generation mode. In this mode, GHC can be
412 used to generate dependency information suitable for use in
413 a <literal>Makefile</literal>. See <xref
414 linkend="makefile-dependencies"/>.</para>
415 </listitem>
416 </varlistentry>
417
418 <varlistentry>
419 <term>
420 <cmdsynopsis>
421 <command>ghc --mk-dll</command>
422 </cmdsynopsis>
423 <indexterm><primary>DLL-creation mode</primary></indexterm>
424 </term>
425 <listitem>
426 <para>DLL-creation mode (Windows only). See <xref
427 linkend="win32-dlls-create"/>.</para>
428 </listitem>
429 </varlistentry>
430
431 <varlistentry>
432 <term>
433 <cmdsynopsis>
434 <command>ghc --help</command> <command>ghc -?</command>
435 </cmdsynopsis>
436 <indexterm><primary><option>&ndash;&ndash;help</option></primary></indexterm>
437 </term>
438 <listitem>
439 <para>Cause GHC to spew a long usage message to standard
440 output and then exit.</para>
441 </listitem>
442 </varlistentry>
443
444 <varlistentry>
445 <term>
446 <cmdsynopsis>
447 <command>ghc --show-iface <replaceable>file</replaceable></command>
448 </cmdsynopsis>
449 <indexterm><primary><option>&ndash;&ndash;--show-iface</option></primary></indexterm>
450 </term>
451 <listitem>
452 <para>Read the interface in
453 <replaceable>file</replaceable> and dump it as text to
454 <literal>stdout</literal>. For example <literal>ghc --show-iface M.hi</literal>.</para>
455 </listitem>
456 </varlistentry>
457
458 <varlistentry>
459 <term>
460 <cmdsynopsis>
461 <command>ghc --supported-extensions</command>
462 <command>ghc --supported-languages</command>
463 </cmdsynopsis>
464 <indexterm><primary><option>&ndash;&ndash;supported-extensions</option></primary><primary><option>&ndash;&ndash;supported-languages</option></primary></indexterm>
465 </term>
466 <listitem>
467 <para>Print the supported language extensions.</para>
468 </listitem>
469 </varlistentry>
470
471 <varlistentry>
472 <term>
473 <cmdsynopsis>
474 <command>ghc --info</command>
475 </cmdsynopsis>
476 <indexterm><primary><option>&ndash;&ndash;info</option></primary></indexterm>
477 </term>
478 <listitem>
479 <para>Print information about the compiler.</para>
480 </listitem>
481 </varlistentry>
482
483 <varlistentry>
484 <term>
485 <cmdsynopsis>
486 <command>ghc --version</command>
487 <command>ghc -V</command>
488 </cmdsynopsis>
489 <indexterm><primary><option>-V</option></primary></indexterm>
490 <indexterm><primary><option>&ndash;&ndash;version</option></primary></indexterm>
491 </term>
492 <listitem>
493 <para>Print a one-line string including GHC's version number.</para>
494 </listitem>
495 </varlistentry>
496
497 <varlistentry>
498 <term>
499 <cmdsynopsis>
500 <command>ghc --numeric-version</command>
501 </cmdsynopsis>
502 <indexterm><primary><option>&ndash;&ndash;numeric-version</option></primary></indexterm>
503 </term>
504 <listitem>
505 <para>Print GHC's numeric version number only.</para>
506 </listitem>
507 </varlistentry>
508
509 <varlistentry>
510 <term>
511 <cmdsynopsis>
512 <command>ghc --print-libdir</command>
513 </cmdsynopsis>
514 <indexterm><primary><option>&ndash;&ndash;print-libdir</option></primary></indexterm>
515 </term>
516 <listitem>
517 <para>Print the path to GHC's library directory. This is
518 the top of the directory tree containing GHC's libraries,
519 interfaces, and include files (usually something like
520 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
521 is the value of
522 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
523 in the package configuration file
524 (see <xref linkend="packages"/>).</para>
525 </listitem>
526 </varlistentry>
527
528 </variablelist>
529
530 <sect2 id="make-mode">
531 <title>Using <command>ghc</command> <option>&ndash;&ndash;make</option></title>
532 <indexterm><primary><option>&ndash;&ndash;make</option></primary></indexterm>
533 <indexterm><primary>separate compilation</primary></indexterm>
534
535 <para>In this mode, GHC will build a multi-module Haskell program by following
536 dependencies from one or more root modules (usually just
537 <literal>Main</literal>). For example, if your
538 <literal>Main</literal> module is in a file called
539 <filename>Main.hs</filename>, you could compile and link the
540 program like this:</para>
541
542 <screen>
543 ghc &ndash;&ndash;make Main.hs
544 </screen>
545
546 <para>
547 In fact, GHC enters make mode automatically if there are any
548 Haskell source files on the command line and no other mode is
549 specified, so in this case we could just type
550 </para>
551
552 <screen>
553 ghc Main.hs
554 </screen>
555
556 <para>Any number of source file names or module names may be
557 specified; GHC will figure out all the modules in the program by
558 following the imports from these initial modules. It will then
559 attempt to compile each module which is out of date, and
560 finally, if there is a <literal>Main</literal> module, the
561 program will also be linked into an executable.</para>
562
563 <para>The main advantages to using <literal>ghc
564 &ndash;&ndash;make</literal> over traditional
565 <literal>Makefile</literal>s are:</para>
566
567 <itemizedlist>
568 <listitem>
569 <para>GHC doesn't have to be restarted for each compilation,
570 which means it can cache information between compilations.
571 Compiling a multi-module program with <literal>ghc
572 &ndash;&ndash;make</literal> can be up to twice as fast as
573 running <literal>ghc</literal> individually on each source
574 file.</para>
575 </listitem>
576 <listitem>
577 <para>You don't have to write a <literal>Makefile</literal>.</para>
578 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
579 </listitem>
580 <listitem>
581 <para>GHC re-calculates the dependencies each time it is
582 invoked, so the dependencies never get out of sync with the
583 source.</para>
584 </listitem>
585 </itemizedlist>
586
587 <para>Any of the command-line options described in the rest of
588 this chapter can be used with
589 <option>&ndash;&ndash;make</option>, but note that any options
590 you give on the command line will apply to all the source files
591 compiled, so if you want any options to apply to a single source
592 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
593 pragma (see <xref linkend="source-file-options"/>).</para>
594
595 <para>If the program needs to be linked with additional objects
596 (say, some auxiliary C code), then the object files can be
597 given on the command line and GHC will include them when linking
598 the executable.</para>
599
600 <para>Note that GHC can only follow dependencies if it has the
601 source file available, so if your program includes a module for
602 which there is no source file, even if you have an object and an
603 interface file for the module, then GHC will complain. The
604 exception to this rule is for package modules, which may or may
605 not have source files.</para>
606
607 <para>The source files for the program don't all need to be in
608 the same directory; the <option>-i</option> option can be used
609 to add directories to the search path (see <xref
610 linkend="search-path"/>).</para>
611 </sect2>
612
613 <sect2 id="eval-mode">
614 <title>Expression evaluation mode</title>
615
616 <para>This mode is very similar to interactive mode, except that
617 there is a single expression to evaluate which is specified on
618 the command line as an argument to the <option>-e</option>
619 option:</para>
620
621 <screen>
622 ghc -e <replaceable>expr</replaceable>
623 </screen>
624
625 <para>Haskell source files may be named on the command line, and
626 they will be loaded exactly as in interactive mode. The
627 expression is evaluated in the context of the loaded
628 modules.</para>
629
630 <para>For example, to load and run a Haskell program containing
631 a module <literal>Main</literal>, we might say</para>
632
633 <screen>
634 ghc -e Main.main Main.hs
635 </screen>
636
637 <para>or we can just use this mode to evaluate expressions in
638 the context of the <literal>Prelude</literal>:</para>
639
640 <screen>
641 $ ghc -e "interact (unlines.map reverse.lines)"
642 hello
643 olleh
644 </screen>
645 </sect2>
646
647 <sect2 id="options-order">
648 <title>Batch compiler mode</title>
649
650 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
651 given on the command line.</para>
652
653 <para>The first phase to run is determined by each input-file
654 suffix, and the last phase is determined by a flag. If no
655 relevant flag is present, then go all the way through to linking.
656 This table summarises:</para>
657
658 <informaltable>
659 <tgroup cols="4">
660 <colspec align="left"/>
661 <colspec align="left"/>
662 <colspec align="left"/>
663 <colspec align="left"/>
664
665 <thead>
666 <row>
667 <entry>Phase of the compilation system</entry>
668 <entry>Suffix saying &ldquo;start here&rdquo;</entry>
669 <entry>Flag saying &ldquo;stop after&rdquo;</entry>
670 <entry>(suffix of) output file</entry>
671 </row>
672 </thead>
673 <tbody>
674 <row>
675 <entry>literate pre-processor</entry>
676 <entry><literal>.lhs</literal></entry>
677 <entry>-</entry>
678 <entry><literal>.hs</literal></entry>
679 </row>
680
681 <row>
682 <entry>C pre-processor (opt.) </entry>
683 <entry><literal>.hs</literal> (with
684 <option>-cpp</option>)</entry>
685 <entry><option>-E</option></entry>
686 <entry><literal>.hspp</literal></entry>
687 </row>
688
689 <row>
690 <entry>Haskell compiler</entry>
691 <entry><literal>.hs</literal></entry>
692 <entry><option>-C</option>, <option>-S</option></entry>
693 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
694 </row>
695
696 <row>
697 <entry>C compiler (opt.)</entry>
698 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
699 <entry><option>-S</option></entry>
700 <entry><literal>.s</literal></entry>
701 </row>
702
703 <row>
704 <entry>assembler</entry>
705 <entry><literal>.s</literal></entry>
706 <entry><option>-c</option></entry>
707 <entry><literal>.o</literal></entry>
708 </row>
709
710 <row>
711 <entry>linker</entry>
712 <entry><replaceable>other</replaceable></entry>
713 <entry>-</entry>
714 <entry><filename>a.out</filename></entry>
715 </row>
716 </tbody>
717 </tgroup>
718 </informaltable>
719
720 <indexterm><primary><option>-C</option></primary></indexterm>
721 <indexterm><primary><option>-E</option></primary></indexterm>
722 <indexterm><primary><option>-S</option></primary></indexterm>
723 <indexterm><primary><option>-c</option></primary></indexterm>
724
725 <para>Thus, a common invocation would be: </para>
726
727 <screen>
728 ghc -c Foo.hs</screen>
729
730 <para>to compile the Haskell source file
731 <filename>Foo.hs</filename> to an object file
732 <filename>Foo.o</filename>.</para>
733
734 <para>Note: What the Haskell compiler proper produces depends on
735 whether a native-code generator<indexterm><primary>native-code
736 generator</primary></indexterm> is used (producing assembly
737 language) or not (producing C). See <xref
738 linkend="options-codegen"/> for more details.</para>
739
740 <para>Note: C pre-processing is optional, the
741 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
742 flag turns it on. See <xref linkend="c-pre-processor"/> for more
743 details.</para>
744
745 <para>Note: The option <option>-E</option><indexterm><primary>-E
746 option</primary></indexterm> runs just the pre-processing passes
747 of the compiler, dumping the result in a file.</para>
748
749 <sect3 id="overriding-suffixes">
750 <title>Overriding the default behaviour for a file</title>
751
752 <para>As described above, the way in which a file is processed by GHC
753 depends on its suffix. This behaviour can be overridden using the
754 <option>-x</option> option:</para>
755
756 <variablelist>
757 <varlistentry>
758 <term><option>-x</option> <replaceable>suffix</replaceable>
759 <indexterm><primary><option>-x</option></primary>
760 </indexterm></term>
761 <listitem>
762 <para>Causes all files following this option on the command
763 line to be processed as if they had the suffix
764 <replaceable>suffix</replaceable>. For example, to compile a
765 Haskell module in the file <literal>M.my-hs</literal>,
766 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
767 </listitem>
768 </varlistentry>
769 </variablelist>
770 </sect3>
771
772 </sect2>
773 </sect1>
774
775 <sect1 id="options-help">
776 <title>Help and verbosity options</title>
777
778 <indexterm><primary>help options</primary></indexterm>
779 <indexterm><primary>verbosity options</primary></indexterm>
780
781 <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
782 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
783 <variablelist>
784 <varlistentry>
785 <term>
786 <option>-v</option>
787 <indexterm><primary><option>-v</option></primary></indexterm>
788 </term>
789 <listitem>
790 <para>The <option>-v</option> option makes GHC
791 <emphasis>verbose</emphasis>: it reports its version number
792 and shows (on stderr) exactly how it invokes each phase of
793 the compilation system. Moreover, it passes the
794 <option>-v</option> flag to most phases; each reports its
795 version number (and possibly some other information).</para>
796
797 <para>Please, oh please, use the <option>-v</option> option
798 when reporting bugs! Knowing that you ran the right bits in
799 the right order is always the first thing we want to
800 verify.</para>
801 </listitem>
802 </varlistentry>
803
804 <varlistentry>
805 <term>
806 <option>-v</option><replaceable>n</replaceable>
807 <indexterm><primary><option>-v</option></primary></indexterm>
808 </term>
809 <listitem>
810 <para>To provide more control over the compiler's verbosity,
811 the <option>-v</option> flag takes an optional numeric
812 argument. Specifying <option>-v</option> on its own is
813 equivalent to <option>-v3</option>, and the other levels
814 have the following meanings:</para>
815
816 <variablelist>
817 <varlistentry>
818 <term><option>-v0</option></term>
819 <listitem>
820 <para>Disable all non-essential messages (this is the
821 default).</para>
822 </listitem>
823 </varlistentry>
824
825 <varlistentry>
826 <term><option>-v1</option></term>
827 <listitem>
828 <para>Minimal verbosity: print one line per
829 compilation (this is the default when
830 <option>&ndash;&ndash;make</option> or
831 <option>&ndash;&ndash;interactive</option> is on).</para>
832 </listitem>
833 </varlistentry>
834
835 <varlistentry>
836 <term><option>-v2</option></term>
837 <listitem>
838 <para>Print the name of each compilation phase as it
839 is executed. (equivalent to
840 <option>-dshow-passes</option>).</para>
841 </listitem>
842 </varlistentry>
843
844 <varlistentry>
845 <term><option>-v3</option></term>
846 <listitem>
847 <para>The same as <option>-v2</option>, except that in
848 addition the full command line (if appropriate) for
849 each compilation phase is also printed.</para>
850 </listitem>
851 </varlistentry>
852
853 <varlistentry>
854 <term><option>-v4</option></term>
855 <listitem>
856 <para>The same as <option>-v3</option> except that the
857 intermediate program representation after each
858 compilation phase is also printed (excluding
859 preprocessed and C/assembly files).</para>
860 </listitem>
861 </varlistentry>
862 </variablelist>
863 </listitem>
864 </varlistentry>
865
866 <varlistentry>
867 <term><option>-ferror-spans</option>
868 <indexterm><primary><option>-ferror-spans</option></primary>
869 </indexterm>
870 </term>
871 <listitem>
872 <para>Causes GHC to emit the full source span of the
873 syntactic entity relating to an error message. Normally, GHC
874 emits the source location of the start of the syntactic
875 entity only.</para>
876
877 <para>For example:</para>
878
879 <screen>test.hs:3:6: parse error on input `where'</screen>
880
881 <para>becomes:</para>
882
883 <screen>test296.hs:3:6-10: parse error on input `where'</screen>
884
885 <para>And multi-line spans are possible too:</para>
886
887 <screen>test.hs:(5,4)-(6,7):
888 Conflicting definitions for `a'
889 Bound at: test.hs:5:4
890 test.hs:6:7
891 In the binding group for: a, b, a</screen>
892
893 <para>Note that line numbers start counting at one, but
894 column numbers start at zero. This choice was made to
895 follow existing convention (i.e. this is how Emacs does
896 it).</para>
897 </listitem>
898 </varlistentry>
899
900 <varlistentry>
901 <term><option>-H</option><replaceable>size</replaceable>
902 <indexterm><primary><option>-H</option></primary></indexterm>
903 </term>
904 <listitem>
905 <para>Set the minimum size of the heap to
906 <replaceable>size</replaceable>.
907 This option is equivalent to
908 <literal>+RTS&nbsp;-H<replaceable>size</replaceable></literal>,
909 see <xref linkend="rts-options-gc" />.
910 </para>
911 </listitem>
912 </varlistentry>
913
914 <varlistentry>
915 <term><option>-Rghc-timing</option>
916 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
917 </term>
918 <listitem>
919 <para>Prints a one-line summary of timing statistics for the
920 GHC run. This option is equivalent to
921 <literal>+RTS&nbsp;-tstderr</literal>, see <xref
922 linkend="rts-options-gc" />.
923 </para>
924 </listitem>
925 </varlistentry>
926 </variablelist>
927 </sect1>
928
929 &separate;
930
931 <sect1 id="options-sanity">
932 <title>Warnings and sanity-checking</title>
933
934 <indexterm><primary>sanity-checking options</primary></indexterm>
935 <indexterm><primary>warnings</primary></indexterm>
936
937
938 <para>GHC has a number of options that select which types of
939 non-fatal error messages, otherwise known as warnings, can be
940 generated during compilation. By default, you get a standard set
941 of warnings which are generally likely to indicate bugs in your
942 program. These are:
943 <option>-fwarn-overlapping-patterns</option>,
944 <option>-fwarn-warnings-deprecations</option>,
945 <option>-fwarn-deprecated-flags</option>,
946 <option>-fwarn-duplicate-exports</option>,
947 <option>-fwarn-missing-fields</option>,
948 <option>-fwarn-missing-methods</option>,
949 <option>-fwarn-lazy-unlifted-bindings</option>,
950 <option>-fwarn-wrong-do-bind</option>, and
951 <option>-fwarn-dodgy-foreign-imports</option>. The following
952 flags are
953 simple ways to select standard &ldquo;packages&rdquo; of warnings:
954 </para>
955
956 <variablelist>
957
958 <varlistentry>
959 <term><option>-W</option>:</term>
960 <listitem>
961 <indexterm><primary>-W option</primary></indexterm>
962 <para>Provides the standard warnings plus
963 <option>-fwarn-incomplete-patterns</option>,
964 <option>-fwarn-dodgy-exports</option>,
965 <option>-fwarn-dodgy-imports</option>,
966 <option>-fwarn-unused-matches</option>,
967 <option>-fwarn-unused-imports</option>, and
968 <option>-fwarn-unused-binds</option>.</para>
969 </listitem>
970 </varlistentry>
971
972 <varlistentry>
973 <term><option>-Wall</option>:</term>
974 <listitem>
975 <indexterm><primary><option>-Wall</option></primary></indexterm>
976 <para>Turns on all warning options that indicate potentially
977 suspicious code. The warnings that are
978 <emphasis>not</emphasis> enabled by <option>-Wall</option>
979 are
980 <option>-fwarn-tabs</option>,
981 <option>-fwarn-incomplete-uni-patterns</option>,
982 <option>-fwarn-incomplete-record-updates</option>,
983 <option>-fwarn-monomorphism-restriction</option>,
984 <option>-fwarn-unrecognised-pragmas</option>,
985 <option>-fwarn-auto-orphans</option>,
986 <option>-fwarn-implicit-prelude</option>.</para>
987 </listitem>
988 </varlistentry>
989
990 <varlistentry>
991 <term><option>-w</option>:</term>
992 <listitem>
993 <indexterm><primary><option>-w</option></primary></indexterm>
994 <para>Turns off all warnings, including the standard ones and
995 those that <literal>-Wall</literal> doesn't enable.</para>
996 </listitem>
997 </varlistentry>
998
999 <varlistentry>
1000 <term><option>-Werror</option>:</term>
1001 <listitem>
1002 <indexterm><primary><option>-Werror</option></primary></indexterm>
1003 <para>Makes any warning into a fatal error. Useful so that you don't
1004 miss warnings when doing batch compilation. </para>
1005 </listitem>
1006 </varlistentry>
1007
1008 <varlistentry>
1009 <term><option>-Wwarn</option>:</term>
1010 <listitem>
1011 <indexterm><primary><option>-Wwarn</option></primary></indexterm>
1012 <para>Warnings are treated only as warnings, not as errors. This is
1013 the default, but can be useful to negate a
1014 <option>-Werror</option> flag.</para>
1015 </listitem>
1016 </varlistentry>
1017
1018 </variablelist>
1019
1020 <para>The full set of warning options is described below. To turn
1021 off any warning, simply give the corresponding
1022 <option>-fno-warn-...</option> option on the command line.</para>
1023
1024 <variablelist>
1025
1026 <varlistentry>
1027 <term><option>-fwarn-unrecognised-pragmas</option>:</term>
1028 <listitem>
1029 <indexterm><primary><option>-fwarn-unrecognised-pragmas</option></primary>
1030 </indexterm>
1031 <indexterm><primary>warnings</primary></indexterm>
1032 <indexterm><primary>pragmas</primary></indexterm>
1033 <para>Causes a warning to be emitted when a
1034 pragma that GHC doesn't recognise is used. As well as pragmas
1035 that GHC itself uses, GHC also recognises pragmas known to be used
1036 by other tools, e.g. <literal>OPTIONS_HUGS</literal> and
1037 <literal>DERIVE</literal>.</para>
1038
1039 <para>This option is on by default.</para>
1040 </listitem>
1041 </varlistentry>
1042
1043 <varlistentry>
1044 <term><option>-fwarn-warnings-deprecations</option>:</term>
1045 <listitem>
1046 <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
1047 </indexterm>
1048 <indexterm><primary>warnings</primary></indexterm>
1049 <indexterm><primary>deprecations</primary></indexterm>
1050 <para>Causes a warning to be emitted when a
1051 module, function or type with a WARNING or DEPRECATED pragma
1052 is used. See <xref linkend="warning-deprecated-pragma"/> for more
1053 details on the pragmas.</para>
1054
1055 <para>This option is on by default.</para>
1056 </listitem>
1057 </varlistentry>
1058
1059 <varlistentry>
1060 <term><option>-fwarn-deprecated-flags</option>:</term>
1061 <listitem>
1062 <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
1063 </indexterm>
1064 <indexterm><primary>deprecated-flags</primary></indexterm>
1065 <para>Causes a warning to be emitted when a deprecated
1066 commandline flag is used.</para>
1067
1068 <para>This option is on by default.</para>
1069 </listitem>
1070 </varlistentry>
1071
1072 <varlistentry>
1073 <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
1074 <listitem>
1075 <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
1076 </indexterm>
1077 <para>Causes a warning to be emitted for foreign imports of
1078 the following form:</para>
1079 <programlisting>
1080 foreign import "f" f :: FunPtr t
1081 </programlisting>
1082 <para>on the grounds that it probably should be</para>
1083 <programlisting>
1084 foreign import "&amp;f" f :: FunPtr t
1085 </programlisting>
1086 <para>The first form declares that `f` is a (pure) C
1087 function that takes no arguments and returns a pointer to a
1088 C function with type `t`, whereas the second form declares
1089 that `f` itself is a C function with type `t`. The first
1090 declaration is usually a mistake, and one that is hard to
1091 debug because it results in a crash, hence this
1092 warning.</para>
1093 </listitem>
1094 </varlistentry>
1095
1096 <varlistentry>
1097 <term><option>-fwarn-dodgy-exports</option>:</term>
1098 <listitem>
1099 <indexterm><primary><option>-fwarn-dodgy-exports</option></primary>
1100 </indexterm>
1101 <para>Causes a warning to be emitted when a datatype
1102 <literal>T</literal> is exported
1103 with all constructors, i.e. <literal>T(..)</literal>, but is it
1104 just a type synonym.</para>
1105 <para>Also causes a warning to be emitted when a module is
1106 re-exported, but that module exports nothing.</para>
1107 </listitem>
1108 </varlistentry>
1109
1110 <varlistentry>
1111 <term><option>-fwarn-dodgy-imports</option>:</term>
1112 <listitem>
1113 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
1114 </indexterm>
1115 <para>Causes a warning to be emitted when a datatype
1116 <literal>T</literal> is imported
1117 with all constructors, i.e. <literal>T(..)</literal>, but has been
1118 exported abstractly, i.e. <literal>T</literal>.</para>
1119 </listitem>
1120 </varlistentry>
1121
1122 <varlistentry>
1123 <term><option>-fwarn-lazy-unlifted-bindings</option>:</term>
1124 <listitem>
1125 <indexterm><primary><option>-fwarn-lazy-unlifted-bindings</option></primary>
1126 </indexterm>
1127 <para>Causes a warning to be emitted when an unlifted type
1128 is bound in a way that looks lazy, e.g.
1129 <literal>where (I# x) = ...</literal>. Use
1130 <literal>where !(I# x) = ...</literal> instead. This will be an
1131 error, rather than a warning, in GHC 7.2.
1132 </para>
1133 </listitem>
1134 </varlistentry>
1135
1136 <varlistentry>
1137 <term><option>-fwarn-duplicate-exports</option>:</term>
1138 <listitem>
1139 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
1140 <indexterm><primary>duplicate exports, warning</primary></indexterm>
1141 <indexterm><primary>export lists, duplicates</primary></indexterm>
1142
1143 <para>Have the compiler warn about duplicate entries in
1144 export lists. This is useful information if you maintain
1145 large export lists, and want to avoid the continued export
1146 of a definition after you've deleted (one) mention of it in
1147 the export list.</para>
1148
1149 <para>This option is on by default.</para>
1150 </listitem>
1151 </varlistentry>
1152
1153 <varlistentry>
1154 <term><option>-fwarn-hi-shadowing</option>:</term>
1155 <listitem>
1156 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
1157 <indexterm><primary>shadowing</primary>
1158 <secondary>interface files</secondary></indexterm>
1159
1160 <para>Causes the compiler to emit a warning when a module or
1161 interface file in the current directory is shadowing one
1162 with the same module name in a library or other
1163 directory.</para>
1164 </listitem>
1165 </varlistentry>
1166
1167 <varlistentry>
1168 <term><option>-fwarn-identities</option>:</term>
1169 <listitem>
1170 <indexterm><primary><option>-fwarn-identities</option></primary></indexterm>
1171 <para>Causes the compiler to emit a warning when a Prelude numeric
1172 conversion converts a type T to the same type T; such calls
1173 are probably no-ops and can be omitted. The functions checked for
1174 are: <literal>toInteger</literal>,
1175 <literal>toRational</literal>,
1176 <literal>fromIntegral</literal>,
1177 and <literal>realToFrac</literal>.
1178 </para>
1179 </listitem>
1180 </varlistentry>
1181
1182 <varlistentry>
1183 <term><option>-fwarn-implicit-prelude</option>:</term>
1184 <listitem>
1185 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
1186 <indexterm><primary>implicit prelude, warning</primary></indexterm>
1187 <para>Have the compiler warn if the Prelude is implicitly
1188 imported. This happens unless either the Prelude module is
1189 explicitly imported with an <literal>import ... Prelude ...</literal>
1190 line, or this implicit import is disabled (either by
1191 <option>-XNoImplicitPrelude</option> or a
1192 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
1193
1194 <para>Note that no warning is given for syntax that implicitly
1195 refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
1196 would change whether it refers to the Prelude.
1197 For example, no warning is given when
1198 <literal>368</literal> means
1199 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
1200 (where <literal>Prelude</literal> refers to the actual Prelude module,
1201 regardless of the imports of the module being compiled).</para>
1202
1203 <para>This warning is off by default.</para>
1204 </listitem>
1205 </varlistentry>
1206
1207 <varlistentry>
1208 <term><option>-fwarn-incomplete-patterns</option>,
1209 <option>-fwarn-incomplete-uni-patterns</option>:
1210 </term>
1211 <listitem>
1212 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
1213 <indexterm><primary><option>-fwarn-incomplete-uni-patterns</option></primary></indexterm>
1214 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
1215 <indexterm><primary>patterns, incomplete</primary></indexterm>
1216
1217 <para>The option <option>-fwarn-incomplete-patterns</option> warns
1218 about places where
1219 a pattern-match might fail at runtime.
1220 The function
1221 <function>g</function> below will fail when applied to
1222 non-empty lists, so the compiler will emit a warning about
1223 this when <option>-fwarn-incomplete-patterns</option> is
1224 enabled.
1225 <programlisting>
1226 g [] = 2
1227 </programlisting>
1228 This option isn't enabled by default because it can be
1229 a bit noisy, and it doesn't always indicate a bug in the
1230 program. However, it's generally considered good practice
1231 to cover all the cases in your functions, and it is switched
1232 on by <option>-W</option>.</para>
1233
1234 <para>The flag <option>-fwarn-incomplete-uni-patterns</option> is
1235 similar, except that it
1236 applies only to lambda-expressions and pattern bindings, constructs
1237 that only allow a single pattern:
1238 <programlisting>
1239 h = \[] -> 2
1240 Just k = f y
1241 </programlisting>
1242 </para>
1243 </listitem>
1244 </varlistentry>
1245
1246 <varlistentry>
1247 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1248 <listitem>
1249 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1250 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1251 <indexterm><primary>record updates, incomplete</primary></indexterm>
1252
1253 <para>The function
1254 <function>f</function> below will fail when applied to
1255 <literal>Bar</literal>, so the compiler will emit a warning about
1256 this when <option>-fwarn-incomplete-record-updates</option> is
1257 enabled.</para>
1258
1259 <programlisting>
1260 data Foo = Foo { x :: Int }
1261 | Bar
1262
1263 f :: Foo -> Foo
1264 f foo = foo { x = 6 }
1265 </programlisting>
1266
1267 <para>This option isn't enabled by default because it can be
1268 very noisy, and it often doesn't indicate a bug in the
1269 program.</para>
1270 </listitem>
1271 </varlistentry>
1272
1273 <varlistentry>
1274 <term>
1275 <option>-fwarn-missing-fields</option>:
1276 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1277 <indexterm><primary>missing fields, warning</primary></indexterm>
1278 <indexterm><primary>fields, missing</primary></indexterm>
1279 </term>
1280 <listitem>
1281
1282 <para>This option is on by default, and warns you whenever
1283 the construction of a labelled field constructor isn't
1284 complete, missing initializers for one or more fields. While
1285 not an error (the missing fields are initialised with
1286 bottoms), it is often an indication of a programmer error.</para>
1287 </listitem>
1288 </varlistentry>
1289
1290 <varlistentry>
1291 <term>
1292 <option>-fwarn-missing-import-lists</option>:
1293 <indexterm><primary><option>-fwarn-import-lists</option></primary></indexterm>
1294 <indexterm><primary>missing import lists, warning</primary></indexterm>
1295 <indexterm><primary>import lists, missing</primary></indexterm>
1296 </term>
1297 <listitem>
1298
1299 <para>This flag warns if you use an unqualified
1300 <literal>import</literal> declaration
1301 that does not explicitly list the entities brought into scope. For
1302 example
1303 </para>
1304 <programlisting>
1305 module M where
1306 import X( f )
1307 import Y
1308 import qualified Z
1309 p x = f x x
1310 </programlisting>
1311 <para>
1312 The <option>-fwarn-import-lists</option> flag will warn about the import
1313 of <literal>Y</literal> but not <literal>X</literal>
1314 If module <literal>Y</literal> is later changed to export (say) <literal>f</literal>,
1315 then the reference to <literal>f</literal> in <literal>M</literal> will become
1316 ambiguous. No warning is produced for the import of <literal>Z</literal>
1317 because extending <literal>Z</literal>'s exports would be unlikely to produce
1318 ambiguity in <literal>M</literal>.
1319 </para>
1320 </listitem>
1321 </varlistentry>
1322
1323 <varlistentry>
1324 <term><option>-fwarn-missing-methods</option>:</term>
1325 <listitem>
1326 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1327 <indexterm><primary>missing methods, warning</primary></indexterm>
1328 <indexterm><primary>methods, missing</primary></indexterm>
1329
1330 <para>This option is on by default, and warns you whenever
1331 an instance declaration is missing one or more methods, and
1332 the corresponding class declaration has no default
1333 declaration for them.</para>
1334 <para>The warning is suppressed if the method name
1335 begins with an underscore. Here's an example where this is useful:
1336 <programlisting>
1337 class C a where
1338 _simpleFn :: a -> String
1339 complexFn :: a -> a -> String
1340 complexFn x y = ... _simpleFn ...
1341 </programlisting>
1342 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1343 never <literal>_simpleFn</literal>; and (b)
1344 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1345 </para>
1346 </listitem>
1347 </varlistentry>
1348
1349 <varlistentry>
1350 <term><option>-fwarn-missing-signatures</option>:</term>
1351 <listitem>
1352 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1353 <indexterm><primary>type signatures, missing</primary></indexterm>
1354
1355 <para>If you would like GHC to check that every top-level
1356 function/value has a type signature, use the
1357 <option>-fwarn-missing-signatures</option> option. As part of
1358 the warning GHC also reports the inferred type. The
1359 option is off by default.</para>
1360 </listitem>
1361 </varlistentry>
1362
1363 <varlistentry>
1364 <term><option>-fwarn-missing-local-sigs</option>:</term>
1365 <listitem>
1366 <indexterm><primary><option>-fwarn-missing-local-sigs</option></primary></indexterm>
1367 <indexterm><primary>type signatures, missing</primary></indexterm>
1368
1369 <para>If you use the
1370 <option>-fwarn-missing-local-sigs</option> flag GHC will warn
1371 you about any polymorphic local bindings. As part of
1372 the warning GHC also reports the inferred type. The
1373 option is off by default.</para>
1374 </listitem>
1375 </varlistentry>
1376
1377 <varlistentry>
1378 <term><option>-fwarn-name-shadowing</option>:</term>
1379 <listitem>
1380 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1381 <indexterm><primary>shadowing, warning</primary></indexterm>
1382
1383 <para>This option causes a warning to be emitted whenever an
1384 inner-scope value has the same name as an outer-scope value,
1385 i.e. the inner value shadows the outer one. This can catch
1386 typographical errors that turn into hard-to-find bugs, e.g.,
1387 in the inadvertent capture of what would be a recursive call in
1388 <literal>f = ... let f = id in ... f ...</literal>.</para>
1389 <para>The warning is suppressed for names beginning with an underscore. For example
1390 <programlisting>
1391 f x = do { _ignore &lt;- this; _ignore &lt;- that; return (the other) }
1392 </programlisting>
1393 </para>
1394 </listitem>
1395 </varlistentry>
1396
1397 <varlistentry>
1398 <term><option>-fwarn-orphans</option>:</term>
1399 <listitem>
1400 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1401 <indexterm><primary>orphan instances, warning</primary></indexterm>
1402 <indexterm><primary>orphan rules, warning</primary></indexterm>
1403
1404 <para>This option causes a warning to be emitted whenever the
1405 module contains an "orphan" instance declaration or rewrite rule.
1406 An instance declaration is an orphan if it appears in a module in
1407 which neither the class nor the type being instanced are declared
1408 in the same module. A rule is an orphan if it is a rule for a
1409 function declared in another module. A module containing any
1410 orphans is called an orphan module.</para>
1411 <para>The trouble with orphans is that GHC must pro-actively read the interface
1412 files for all orphan modules, just in case their instances or rules
1413 play a role, whether or not the module's interface would otherwise
1414 be of any use. See <xref linkend="orphan-modules"/> for details.
1415 </para>
1416 </listitem>
1417 </varlistentry>
1418
1419 <varlistentry>
1420 <term>
1421 <option>-fwarn-overlapping-patterns</option>:
1422 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1423 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1424 <indexterm><primary>patterns, overlapping</primary></indexterm>
1425 </term>
1426 <listitem>
1427 <para>By default, the compiler will warn you if a set of
1428 patterns are overlapping, e.g.,</para>
1429
1430 <programlisting>
1431 f :: String -&#62; Int
1432 f [] = 0
1433 f (_:xs) = 1
1434 f "2" = 2
1435 </programlisting>
1436
1437 <para>where the last pattern match in <function>f</function>
1438 won't ever be reached, as the second pattern overlaps
1439 it. More often than not, redundant patterns is a programmer
1440 mistake/error, so this option is enabled by default.</para>
1441 </listitem>
1442 </varlistentry>
1443
1444 <varlistentry>
1445 <term><option>-fwarn-tabs</option>:</term>
1446 <listitem>
1447 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1448 <indexterm><primary>tabs, warning</primary></indexterm>
1449 <para>Have the compiler warn if there are tabs in your source
1450 file.</para>
1451
1452 <para>This warning is off by default.</para>
1453 </listitem>
1454 </varlistentry>
1455
1456 <varlistentry>
1457 <term><option>-fwarn-type-defaults</option>:</term>
1458 <listitem>
1459 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1460 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1461 <para>Have the compiler warn/inform you where in your source
1462 the Haskell defaulting mechanism for numeric types kicks
1463 in. This is useful information when converting code from a
1464 context that assumed one default into one with another,
1465 e.g., the &lsquo;default default&rsquo; for Haskell 1.4 caused the
1466 otherwise unconstrained value <constant>1</constant> to be
1467 given the type <literal>Int</literal>, whereas Haskell 98
1468 and later
1469 defaults it to <literal>Integer</literal>. This may lead to
1470 differences in performance and behaviour, hence the
1471 usefulness of being non-silent about this.</para>
1472
1473 <para>This warning is off by default.</para>
1474 </listitem>
1475 </varlistentry>
1476
1477 <varlistentry>
1478 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1479 <listitem>
1480 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1481 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1482 <para>Have the compiler warn/inform you where in your source
1483 the Haskell Monomorphism Restriction is applied. If applied silently
1484 the MR can give rise to unexpected behaviour, so it can be helpful
1485 to have an explicit warning that it is being applied.</para>
1486
1487 <para>This warning is off by default.</para>
1488 </listitem>
1489 </varlistentry>
1490
1491 <varlistentry>
1492 <term><option>-fwarn-unused-binds</option>:</term>
1493 <listitem>
1494 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1495 <indexterm><primary>unused binds, warning</primary></indexterm>
1496 <indexterm><primary>binds, unused</primary></indexterm>
1497 <para>Report any function definitions (and local bindings)
1498 which are unused. For top-level functions, the warning is
1499 only given if the binding is not exported.</para>
1500 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1501 mentioned in the right hand side of another definition that is used, or (c) the
1502 function it defines begins with an underscore. The last case provides a
1503 way to suppress unused-binding warnings selectively. </para>
1504 <para> Notice that a variable
1505 is reported as unused even if it appears in the right-hand side of another
1506 unused binding. </para>
1507 </listitem>
1508 </varlistentry>
1509
1510 <varlistentry>
1511 <term><option>-fwarn-unused-imports</option>:</term>
1512 <listitem>
1513 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1514 <indexterm><primary>unused imports, warning</primary></indexterm>
1515 <indexterm><primary>imports, unused</primary></indexterm>
1516
1517 <para>Report any modules that are explicitly imported but
1518 never used. However, the form <literal>import M()</literal> is
1519 never reported as an unused import, because it is a useful idiom
1520 for importing instance declarations, which are anonymous in Haskell.</para>
1521 </listitem>
1522 </varlistentry>
1523
1524 <varlistentry>
1525 <term><option>-fwarn-unused-matches</option>:</term>
1526 <listitem>
1527 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1528 <indexterm><primary>unused matches, warning</primary></indexterm>
1529 <indexterm><primary>matches, unused</primary></indexterm>
1530
1531 <para>Report all unused variables which arise from pattern
1532 matches, including patterns consisting of a single variable.
1533 For instance <literal>f x y = []</literal> would report
1534 <varname>x</varname> and <varname>y</varname> as unused. The
1535 warning is suppressed if the variable name begins with an underscore, thus:
1536 <programlisting>
1537 f _x = True
1538 </programlisting>
1539 </para>
1540 </listitem>
1541 </varlistentry>
1542
1543 <varlistentry>
1544 <term><option>-fwarn-unused-do-bind</option>:</term>
1545 <listitem>
1546 <indexterm><primary><option>-fwarn-unused-do-bind</option></primary></indexterm>
1547 <indexterm><primary>unused do binding, warning</primary></indexterm>
1548 <indexterm><primary>do binding, unused</primary></indexterm>
1549
1550 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1551 that appear to silently throw information away.
1552 For instance <literal>do { mapM popInt xs ; return 10 }</literal> would report
1553 the first statement in the <literal>do</literal> block as suspicious,
1554 as it has the type <literal>StackM [Int]</literal> and not <literal>StackM ()</literal>, but that
1555 <literal>[Int]</literal> value is not bound to anything. The warning is suppressed by
1556 explicitly mentioning in the source code that your program is throwing something away:
1557 <programlisting>
1558 do { _ &lt;- mapM popInt xs ; return 10 }
1559 </programlisting>
1560 Of course, in this particular situation you can do even better:
1561 <programlisting>
1562 do { mapM_ popInt xs ; return 10 }
1563 </programlisting>
1564 </para>
1565 </listitem>
1566 </varlistentry>
1567
1568 <varlistentry>
1569 <term><option>-fwarn-wrong-do-bind</option>:</term>
1570 <listitem>
1571 <indexterm><primary><option>-fwarn-wrong-do-bind</option></primary></indexterm>
1572 <indexterm><primary>apparently erroneous do binding, warning</primary></indexterm>
1573 <indexterm><primary>do binding, apparently erroneous</primary></indexterm>
1574
1575 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1576 that appear to lack a binding.
1577 For instance <literal>do { return (popInt 10) ; return 10 }</literal> would report
1578 the first statement in the <literal>do</literal> block as suspicious,
1579 as it has the type <literal>StackM (StackM Int)</literal> (which consists of two nested applications
1580 of the same monad constructor), but which is not then &quot;unpacked&quot; by binding the result.
1581 The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away:
1582 <programlisting>
1583 do { _ &lt;- return (popInt 10) ; return 10 }
1584 </programlisting>
1585 For almost all sensible programs this will indicate a bug, and you probably intended to write:
1586 <programlisting>
1587 do { popInt 10 ; return 10 }
1588 </programlisting>
1589 </para>
1590 </listitem>
1591 </varlistentry>
1592
1593 </variablelist>
1594
1595 <para>If you're feeling really paranoid, the
1596 <option>-dcore-lint</option>
1597 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1598 is a good choice. It turns on heavyweight intra-pass
1599 sanity-checking within GHC. (It checks GHC's sanity, not
1600 yours.)</para>
1601
1602 </sect1>
1603
1604 &packages;
1605
1606 <sect1 id="options-optimise">
1607 <title>Optimisation (code improvement)</title>
1608
1609 <indexterm><primary>optimisation</primary></indexterm>
1610 <indexterm><primary>improvement, code</primary></indexterm>
1611
1612 <para>The <option>-O*</option> options specify convenient
1613 &ldquo;packages&rdquo; of optimisation flags; the
1614 <option>-f*</option> options described later on specify
1615 <emphasis>individual</emphasis> optimisations to be turned on/off;
1616 the <option>-m*</option> options specify
1617 <emphasis>machine-specific</emphasis> optimisations to be turned
1618 on/off.</para>
1619
1620 <sect2 id="optimise-pkgs">
1621 <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1622
1623 <para>There are <emphasis>many</emphasis> options that affect
1624 the quality of code produced by GHC. Most people only have a
1625 general goal, something like &ldquo;Compile quickly&rdquo; or
1626 &ldquo;Make my program run like greased lightning.&rdquo; The
1627 following &ldquo;packages&rdquo; of optimisations (or lack
1628 thereof) should suffice.</para>
1629
1630 <para>Note that higher optimisation levels cause more
1631 cross-module optimisation to be performed, which can have an
1632 impact on how much of your program needs to be recompiled when
1633 you change something. This is one reason to stick to
1634 no-optimisation when developing code.</para>
1635
1636 <variablelist>
1637
1638 <varlistentry>
1639 <term>
1640 No <option>-O*</option>-type option specified:
1641 <indexterm><primary>-O* not specified</primary></indexterm>
1642 </term>
1643 <listitem>
1644 <para>This is taken to mean: &ldquo;Please compile
1645 quickly; I'm not over-bothered about compiled-code
1646 quality.&rdquo; So, for example: <command>ghc -c
1647 Foo.hs</command></para>
1648 </listitem>
1649 </varlistentry>
1650
1651 <varlistentry>
1652 <term>
1653 <option>-O0</option>:
1654 <indexterm><primary><option>-O0</option></primary></indexterm>
1655 </term>
1656 <listitem>
1657 <para>Means &ldquo;turn off all optimisation&rdquo;,
1658 reverting to the same settings as if no
1659 <option>-O</option> options had been specified. Saying
1660 <option>-O0</option> can be useful if
1661 eg. <command>make</command> has inserted a
1662 <option>-O</option> on the command line already.</para>
1663 </listitem>
1664 </varlistentry>
1665
1666 <varlistentry>
1667 <term>
1668 <option>-O</option> or <option>-O1</option>:
1669 <indexterm><primary>-O option</primary></indexterm>
1670 <indexterm><primary>-O1 option</primary></indexterm>
1671 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1672 </term>
1673 <listitem>
1674 <para>Means: &ldquo;Generate good-quality code without
1675 taking too long about it.&rdquo; Thus, for example:
1676 <command>ghc -c -O Main.lhs</command></para>
1677 </listitem>
1678 </varlistentry>
1679
1680 <varlistentry>
1681 <term>
1682 <option>-O2</option>:
1683 <indexterm><primary>-O2 option</primary></indexterm>
1684 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1685 </term>
1686 <listitem>
1687 <para>Means: &ldquo;Apply every non-dangerous
1688 optimisation, even if it means significantly longer
1689 compile times.&rdquo;</para>
1690
1691 <para>The avoided &ldquo;dangerous&rdquo; optimisations
1692 are those that can make runtime or space
1693 <emphasis>worse</emphasis> if you're unlucky. They are
1694 normally turned on or off individually.</para>
1695
1696 <para>At the moment, <option>-O2</option> is
1697 <emphasis>unlikely</emphasis> to produce better code than
1698 <option>-O</option>.</para>
1699 </listitem>
1700 </varlistentry>
1701 </variablelist>
1702
1703 <para>We don't use a <option>-O*</option> flag for day-to-day
1704 work. We use <option>-O</option> to get respectable speed;
1705 e.g., when we want to measure something. When we want to go for
1706 broke, we tend to use <option>-O2</option> (and we go for
1707 lots of coffee breaks).</para>
1708
1709 <para>The easiest way to see what <option>-O</option> (etc.)
1710 &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1711 then stand back in amazement.</para>
1712 </sect2>
1713
1714 <sect2 id="options-f">
1715 <title><option>-f*</option>: platform-independent flags</title>
1716
1717 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1718 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1719
1720 <para>These flags turn on and off individual optimisations.
1721 They are normally set via the <option>-O</option> options
1722 described above, and as such, you shouldn't need to set any of
1723 them explicitly (indeed, doing so could lead to unexpected
1724 results). However, there are one or two that may be of
1725 interest:</para>
1726
1727 <variablelist>
1728 <varlistentry>
1729 <term><option>-fexcess-precision</option>:</term>
1730 <listitem>
1731 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1732 <para>When this option is given, intermediate floating
1733 point values can have a <emphasis>greater</emphasis>
1734 precision/range than the final type. Generally this is a
1735 good thing, but some programs may rely on the exact
1736 precision/range of
1737 <literal>Float</literal>/<literal>Double</literal> values
1738 and should not use this option for their compilation.</para>
1739 </listitem>
1740 </varlistentry>
1741
1742 <varlistentry>
1743 <term><option>-fignore-asserts</option>:</term>
1744 <listitem>
1745 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1746 <para>Causes GHC to ignore uses of the function
1747 <literal>Exception.assert</literal> in source code (in
1748 other words, rewriting <literal>Exception.assert p
1749 e</literal> to <literal>e</literal> (see <xref
1750 linkend="assertions"/>). This flag is turned on by
1751 <option>-O</option>.
1752 </para>
1753 </listitem>
1754 </varlistentry>
1755
1756 <varlistentry>
1757 <term>
1758 <option>-fignore-interface-pragmas</option>
1759 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
1760 </term>
1761 <listitem>
1762 <para>Tells GHC to ignore all inessential information when reading interface files.
1763 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
1764 for a function, GHC will ignore that information.</para>
1765 </listitem>
1766 </varlistentry>
1767
1768 <varlistentry>
1769 <term>
1770 <option>-fliberate-case</option>
1771 <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
1772 </term>
1773 <listitem>
1774 <para>Turn on the liberate-case transformation.</para>
1775 </listitem>
1776 </varlistentry>
1777
1778 <varlistentry>
1779 <term>
1780 <option>-fno-cse</option>
1781 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1782 </term>
1783 <listitem>
1784 <para>Turns off the common-sub-expression elimination optimisation.
1785 Can be useful if you have some <literal>unsafePerformIO</literal>
1786 expressions that you don't want commoned-up.</para>
1787 </listitem>
1788 </varlistentry>
1789
1790 <varlistentry>
1791 <term>
1792 <option>-fno-strictness</option>
1793 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1794 </term>
1795 <listitem>
1796 <para>Turns off the strictness analyser; sometimes it eats
1797 too many cycles.</para>
1798 </listitem>
1799 </varlistentry>
1800
1801 <varlistentry>
1802 <term>
1803 <option>-fno-full-laziness</option>
1804 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1805 </term>
1806 <listitem>
1807 <para>Turns off the full laziness optimisation (also known as
1808 let-floating). Full laziness increases sharing, which can lead
1809 to increased memory residency.</para>
1810
1811 <para>NOTE: GHC doesn't implement complete full-laziness.
1812 When optimisation in on, and
1813 <option>-fno-full-laziness</option> is not given, some
1814 transformations that increase sharing are performed, such
1815 as extracting repeated computations from a loop. These
1816 are the same transformations that a fully lazy
1817 implementation would do, the difference is that GHC
1818 doesn't consistently apply full-laziness, so don't rely on
1819 it.</para>
1820 </listitem>
1821 </varlistentry>
1822
1823 <varlistentry>
1824 <term>
1825 <option>-fno-float-in</option>
1826 <indexterm><primary><option>-fno-float-in</option></primary></indexterm>
1827 </term>
1828 <listitem>
1829 <para>Turns off the float-in transformation.</para>
1830 </listitem>
1831 </varlistentry>
1832
1833 <varlistentry>
1834 <term>
1835 <option>-fno-specialise</option>
1836 <indexterm><primary><option>-fno-specialise</option></primary></indexterm>
1837 </term>
1838 <listitem>
1839 <para>Turns off the automatic specialisation of overloaded functions.</para>
1840 </listitem>
1841 </varlistentry>
1842
1843 <varlistentry>
1844 <term>
1845 <option>-fno-state-hack</option>
1846 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1847 </term>
1848 <listitem>
1849 <para>Turn off the "state hack" whereby any lambda with a
1850 <literal>State#</literal> token as argument is considered to be
1851 single-entry, hence it is considered OK to inline things inside
1852 it. This can improve performance of IO and ST monad code, but it
1853 runs the risk of reducing sharing.</para>
1854 </listitem>
1855 </varlistentry>
1856
1857 <varlistentry>
1858 <term>
1859 <option>-fpedantic-bottoms</option>
1860 <indexterm><primary><option>-fpedantic-bottoms</option></primary></indexterm>
1861 </term>
1862 <listitem>
1863 <para>Make GHC be more precise about its treatment of bottom (but see also
1864 <option>-fno-state-hack</option>). In particular, stop GHC
1865 eta-expanding through a case expression, which is good for
1866 performance, but bad if you are using <literal>seq</literal> on
1867 partial applications.</para>
1868 </listitem>
1869 </varlistentry>
1870
1871 <varlistentry>
1872 <term>
1873 <option>-fomit-interface-pragmas</option>
1874 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
1875 </term>
1876 <listitem>
1877 <para>Tells GHC to omit all inessential information from the interface file
1878 generated for the module being compiled (say M). This means that a module
1879 importing M will see only the <emphasis>types</emphasis> of the functions that M exports, but not
1880 their unfoldings, strictness info, etc. Hence, for example,
1881 no function exported by M will be inlined
1882 into an importing module. The benefit is that modules that import M will
1883 need to be recompiled less often (only when M's exports change their type,
1884 not when they change their implementation).
1885 </para>
1886 </listitem>
1887 </varlistentry>
1888
1889 <varlistentry>
1890 <term>
1891 <option>-fsimpl-tick-factor=<replaceable>n</replaceable></option>
1892 <indexterm><primary><option>-fsimpl-tick-factor</option></primary></indexterm>
1893 </term>
1894 <listitem>
1895 <para>GHC's optimiser can diverge if you write rewrite rules (<xref linkend="rewrite-rules"/>)
1896 that don't terminate, or (less satisfactorily) if you
1897 code up recursion through data types
1898 (<xref linkend="bugs-ghc"/>). To avoid making the compiler fall into an infinite
1899 loop, the optimiser carries a "tick count" and stops inlining and applying rewrite rules
1900 when this count is exceeded. The limit is set as a multiple of the program size, so
1901 bigger programs get more ticks. The <option>-fsimpl-tick-factor</option> flag lets
1902 you change the multiplier. The default is 100; numbers larger than 100 give more ticks,
1903 and numbers smaller than 100 give fewer.</para>
1904 <para>If the tick-count expires, GHC summarises what simplifier steps it has done;
1905 you can use <option>-fddump-simpl-stats</option> to generate a much more detailed list.
1906 Usually that identifies the loop quite accurately, because some numbers are very large.
1907 </para>
1908 </listitem>
1909 </varlistentry>
1910
1911 <varlistentry>
1912 <term>
1913 <option>-fstatic-argument-transformation</option>
1914 <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
1915 </term>
1916 <listitem>
1917 <para>Turn on the static argument transformation.</para>
1918 </listitem>
1919 </varlistentry>
1920
1921 <varlistentry>
1922 <term>
1923 <option>-fspec-constr</option>
1924 <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
1925 </term>
1926 <listitem>
1927 <para>Turn on call-pattern specialisation.</para>
1928 </listitem>
1929 </varlistentry>
1930
1931 <varlistentry>
1932 <term>
1933 <option>-funbox-strict-fields</option>:
1934 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1935 <indexterm><primary>strict constructor fields</primary></indexterm>
1936 <indexterm><primary>constructor fields, strict</primary></indexterm>
1937 </term>
1938 <listitem>
1939 <para>This option causes all constructor fields which are
1940 marked strict (i.e. &ldquo;!&rdquo;) to be unboxed or
1941 unpacked if possible. It is equivalent to adding an
1942 <literal>UNPACK</literal> pragma to every strict
1943 constructor field (see <xref
1944 linkend="unpack-pragma"/>).</para>
1945
1946 <para>This option is a bit of a sledgehammer: it might
1947 sometimes make things worse. Selectively unboxing fields
1948 by using <literal>UNPACK</literal> pragmas might be
1949 better. An alternative is to use
1950 <option>-funbox-strict-fields</option> to turn on
1951 unboxing by default but disable it for certain constructor
1952 fields using the <literal>NOUNPACK</literal> pragma
1953 (see <xref linkend="nounpack-pragma"/>).
1954 </para>
1955 </listitem>
1956 </varlistentry>
1957
1958 <varlistentry>
1959 <term>
1960 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
1961 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1962 <indexterm><primary>inlining, controlling</primary></indexterm>
1963 <indexterm><primary>unfolding, controlling</primary></indexterm>
1964 </term>
1965 <listitem>
1966 <para>(Default: 45) Governs the maximum size that GHC will
1967 allow a function unfolding to be. (An unfolding has a
1968 &ldquo;size&rdquo; that reflects the cost in terms of
1969 &ldquo;code bloat&rdquo; of expanding that unfolding
1970 at a call site. A bigger function would be assigned a
1971 bigger cost.) </para>
1972
1973 <para> Consequences: (a) nothing larger than this will be
1974 inlined (unless it has an INLINE pragma); (b) nothing
1975 larger than this will be spewed into an interface
1976 file. </para>
1977
1978
1979 <para> Increasing this figure is more likely to result in longer
1980 compile times than faster code. The next option is more
1981 useful:</para>
1982 </listitem>
1983 </varlistentry>
1984
1985 <varlistentry>
1986 <term><option>-funfolding-use-threshold=<replaceable>n</replaceable></option></term>
1987 <listitem>
1988 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1989 <indexterm><primary>inlining, controlling</primary></indexterm>
1990 <indexterm><primary>unfolding, controlling</primary></indexterm>
1991
1992 <para>(Default: 8) This is the magic cut-off figure for
1993 unfolding: below this size, a function definition will be
1994 unfolded at the call-site, any bigger and it won't. The
1995 size computed for a function depends on two things: the
1996 actual size of the expression minus any discounts that
1997 apply (see <option>-funfolding-con-discount</option>).</para>
1998 </listitem>
1999 </varlistentry>
2000 </variablelist>
2001
2002 </sect2>
2003
2004 </sect1>
2005
2006 &phases;
2007
2008 &shared_libs;
2009
2010 <sect1 id="using-concurrent">
2011 <title>Using Concurrent Haskell</title>
2012 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
2013
2014 <para>GHC supports Concurrent Haskell by default, without requiring a
2015 special option or libraries compiled in a certain way. To get access to
2016 the support libraries for Concurrent Haskell, just import
2017 <ulink
2018 url="&libraryBaseLocation;/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
2019
2020 <para>The following RTS option(s) affect the behaviour of Concurrent
2021 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
2022
2023 <variablelist>
2024 <varlistentry>
2025 <term><option>-C<replaceable>s</replaceable></option></term>
2026 <listitem>
2027 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2028 Sets the context switch interval to <replaceable>s</replaceable>
2029 seconds. A context switch will occur at the next heap block
2030 allocation after the timer expires (a heap block allocation occurs
2031 every 4k of allocation). With <option>-C0</option> or
2032 <option>-C</option>, context switches will occur as often as
2033 possible (at every heap block allocation). By default, context
2034 switches occur every 20ms.</para>
2035 </listitem>
2036 </varlistentry>
2037 </variablelist>
2038 </sect1>
2039
2040 <sect1 id="using-smp">
2041 <title>Using SMP parallelism</title>
2042 <indexterm><primary>parallelism</primary>
2043 </indexterm>
2044 <indexterm><primary>SMP</primary>
2045 </indexterm>
2046
2047 <para>GHC supports running Haskell programs in parallel on an SMP
2048 (symmetric multiprocessor).</para>
2049
2050 <para>There's a fine distinction between
2051 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
2052 parallelism is all about making your program run
2053 <emphasis>faster</emphasis> by making use of multiple processors
2054 simultaneously. Concurrency, on the other hand, is a means of
2055 abstraction: it is a convenient way to structure a program that must
2056 respond to multiple asynchronous events.</para>
2057
2058 <para>However, the two terms are certainly related. By making use of
2059 multiple CPUs it is possible to run concurrent threads in parallel,
2060 and this is exactly what GHC's SMP parallelism support does. But it
2061 is also possible to obtain performance improvements with parallelism
2062 on programs that do not use concurrency. This section describes how to
2063 use GHC to compile and run parallel programs, in <xref
2064 linkend="lang-parallel" /> we describe the language features that affect
2065 parallelism.</para>
2066
2067 <sect2 id="parallel-compile-options">
2068 <title>Compile-time options for SMP parallelism</title>
2069
2070 <para>In order to make use of multiple CPUs, your program must be
2071 linked with the <option>-threaded</option> option (see <xref
2072 linkend="options-linker" />). Additionally, the following
2073 compiler options affect parallelism:</para>
2074
2075 <variablelist>
2076 <varlistentry>
2077 <term><option>-feager-blackholing</option></term>
2078 <indexterm><primary><option>-feager-blackholing</option></primary></indexterm>
2079 <listitem>
2080 <para>
2081 Blackholing is the act of marking a thunk (lazy
2082 computuation) as being under evaluation. It is useful for
2083 three reasons: firstly it lets us detect certain kinds of
2084 infinite loop (the <literal>NonTermination</literal>
2085 exception), secondly it avoids certain kinds of space
2086 leak, and thirdly it avoids repeating a computation in a
2087 parallel program, because we can tell when a computation
2088 is already in progress.</para>
2089
2090 <para>
2091 The option <option>-feager-blackholing</option> causes
2092 each thunk to be blackholed as soon as evaluation begins.
2093 The default is "lazy blackholing", whereby thunks are only
2094 marked as being under evaluation when a thread is paused
2095 for some reason. Lazy blackholing is typically more
2096 efficient (by 1-2&percnt; or so), because most thunks don't
2097 need to be blackholed. However, eager blackholing can
2098 avoid more repeated computation in a parallel program, and
2099 this often turns out to be important for parallelism.
2100 </para>
2101
2102 <para>
2103 We recommend compiling any code that is intended to be run
2104 in parallel with the <option>-feager-blackholing</option>
2105 flag.
2106 </para>
2107 </listitem>
2108 </varlistentry>
2109 </variablelist>
2110 </sect2>
2111
2112 <sect2 id="parallel-options">
2113 <title>RTS options for SMP parallelism</title>
2114
2115 <para>To run a program on multiple CPUs, use the
2116 RTS <option>-N</option> option:</para>
2117
2118 <variablelist>
2119 <varlistentry>
2120 <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
2121 <listitem>
2122 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2123 Use <replaceable>x</replaceable> simultaneous threads when
2124 running the program. Normally <replaceable>x</replaceable>
2125 should be chosen to match the number of CPU cores on the
2126 machine<footnote><para>Whether hyperthreading cores should be counted or not is an
2127 open question; please feel free to experiment and let us know what
2128 results you find.</para></footnote>. For example,
2129 on a dual-core machine we would probably use
2130 <literal>+RTS -N2 -RTS</literal>.</para>
2131
2132 <para>Omitting <replaceable>x</replaceable>,
2133 i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
2134 choose the value of <replaceable>x</replaceable> itself
2135 based on how many processors are in your machine.</para>
2136
2137 <para>Be careful when using all the processors in your
2138 machine: if some of your processors are in use by other
2139 programs, this can actually harm performance rather than
2140 improve it.</para>
2141
2142 <para>Setting <option>-N</option> also has the effect of
2143 enabling the parallel garbage collector (see
2144 <xref linkend="rts-options-gc" />).</para>
2145
2146 <para>There is no means (currently) by which this value
2147 may vary after the program has started.</para>
2148
2149 <para>The current value of the <option>-N</option> option
2150 is available to the Haskell program
2151 via <literal>GHC.Conc.numCapabilities</literal>.</para>
2152 </listitem>
2153 </varlistentry>
2154 </variablelist>
2155
2156 <para>The following options affect the way the runtime schedules
2157 threads on CPUs:</para>
2158
2159 <variablelist>
2160 <varlistentry>
2161 <term><option>-qa</option></term>
2162 <indexterm><primary><option>-qa</option></primary><secondary>RTS
2163 option</secondary></indexterm>
2164 <listitem>
2165 <para>Use the OS's affinity facilities to try to pin OS
2166 threads to CPU cores. This is an experimental feature,
2167 and may or may not be useful. Please let us know
2168 whether it helps for you!</para>
2169 </listitem>
2170 </varlistentry>
2171 <varlistentry>
2172 <term><option>-qm</option></term>
2173 <indexterm><primary><option>-qm</option></primary><secondary>RTS
2174 option</secondary></indexterm>
2175 <listitem>
2176 <para>Disable automatic migration for load balancing.
2177 Normally the runtime will automatically try to schedule
2178 threads across the available CPUs to make use of idle
2179 CPUs; this option disables that behaviour. Note that
2180 migration only applies to threads; sparks created
2181 by <literal>par</literal> are load-balanced separately
2182 by work-stealing.</para>
2183
2184 <para>
2185 This option is probably only of use for concurrent
2186 programs that explicitly schedule threads onto CPUs
2187 with <literal>GHC.Conc.forkOnIO</literal>.
2188 </para>
2189 </listitem>
2190 </varlistentry>
2191 </variablelist>
2192 </sect2>
2193
2194 <sect2>
2195 <title>Hints for using SMP parallelism</title>
2196
2197 <para>Add the <literal>-s</literal> RTS option when
2198 running the program to see timing stats, which will help to tell you
2199 whether your program got faster by using more CPUs or not. If the user
2200 time is greater than
2201 the elapsed time, then the program used more than one CPU. You should
2202 also run the program without <literal>-N</literal> for
2203 comparison.</para>
2204
2205 <para>The output of <literal>+RTS -s</literal> tells you how
2206 many &ldquo;sparks&rdquo; were created and executed during the
2207 run of the program (see <xref linkend="rts-options-gc" />), which
2208 will give you an idea how well your <literal>par</literal>
2209 annotations are working.</para>
2210
2211 <para>GHC's parallelism support has improved in 6.12.1 as a
2212 result of much experimentation and tuning in the runtime
2213 system. We'd still be interested to hear how well it works
2214 for you, and we're also interested in collecting parallel
2215 programs to add to our benchmarking suite.</para>
2216 </sect2>
2217 </sect1>
2218
2219 <sect1 id="options-platform">
2220 <title>Platform-specific Flags</title>
2221
2222 <indexterm><primary>-m* options</primary></indexterm>
2223 <indexterm><primary>platform-specific options</primary></indexterm>
2224 <indexterm><primary>machine-specific options</primary></indexterm>
2225
2226 <para>Some flags only make sense for particular target
2227 platforms.</para>
2228
2229 <variablelist>
2230
2231 <varlistentry>
2232 <term><option>-msse2</option>:</term>
2233 <listitem>
2234 <para>
2235 (x86 only, added in GHC 7.0.1) Use the SSE2 registers and
2236 instruction set to implement floating point operations
2237 when using the native code generator. This gives a
2238 substantial performance improvement for floating point,
2239 but the resulting compiled code will only run on
2240 processors that support SSE2 (Intel Pentium 4 and later,
2241 or AMD Athlon 64 and later).
2242 </para>
2243 <para>
2244 SSE2 is unconditionally used on x86-64 platforms.
2245 </para>
2246 </listitem>
2247 </varlistentry>
2248
2249 </variablelist>
2250
2251 </sect1>
2252
2253 &runtime;
2254
2255 <sect1 id="ext-core">
2256 <title>Generating and compiling External Core Files</title>
2257
2258 <indexterm><primary>intermediate code generation</primary></indexterm>
2259
2260 <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format)
2261 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
2262 <filename>.hcr</filename>. The Core format is described in <ulink url="../../core.pdf">
2263 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
2264 and sample tools
2265 for manipulating Core files (in Haskell) are available in the
2266 <ulink url="http://hackage.haskell.org/package/extcore">extcore package on Hackage</ulink>. Note that the format of <literal>.hcr</literal>
2267 files is <emphasis>different</emphasis> from the Core output format that GHC generates
2268 for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
2269
2270 <para>The Core format natively supports notes which you can add to
2271 your source code using the <literal>CORE</literal> pragma (see <xref
2272 linkend="pragmas"/>).</para>
2273
2274 <variablelist>
2275
2276 <varlistentry>
2277 <term>
2278 <option>-fext-core</option>
2279 <indexterm><primary><option>-fext-core</option></primary></indexterm>
2280 </term>
2281 <listitem>
2282 <para>Generate <literal>.hcr</literal> files.</para>
2283 </listitem>
2284 </varlistentry>
2285
2286 </variablelist>
2287
2288 <para>Currently (as of version 6.8.2), GHC does not have the ability to read in External Core files as source. If you would like GHC to have this ability, please <ulink url="http://hackage.haskell.org/trac/ghc/wiki/MailingListsAndIRC">make your wishes known to the GHC Team</ulink>.</para>
2289
2290 </sect1>
2291
2292 &debug;
2293 &flags;
2294
2295 </chapter>
2296
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