Implememt -fdefer-type-errors (Trac #5624)
[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 what
735 backend code generator is used. See <xref linkend="code-generators"/>
736 for more details.</para>
737
738 <para>Note: C pre-processing is optional, the
739 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
740 flag turns it on. See <xref linkend="c-pre-processor"/> for more
741 details.</para>
742
743 <para>Note: The option <option>-E</option><indexterm><primary>-E
744 option</primary></indexterm> runs just the pre-processing passes
745 of the compiler, dumping the result in a file.</para>
746
747 <sect3 id="overriding-suffixes">
748 <title>Overriding the default behaviour for a file</title>
749
750 <para>As described above, the way in which a file is processed by GHC
751 depends on its suffix. This behaviour can be overridden using the
752 <option>-x</option> option:</para>
753
754 <variablelist>
755 <varlistentry>
756 <term><option>-x</option> <replaceable>suffix</replaceable>
757 <indexterm><primary><option>-x</option></primary>
758 </indexterm></term>
759 <listitem>
760 <para>Causes all files following this option on the command
761 line to be processed as if they had the suffix
762 <replaceable>suffix</replaceable>. For example, to compile a
763 Haskell module in the file <literal>M.my-hs</literal>,
764 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
765 </listitem>
766 </varlistentry>
767 </variablelist>
768 </sect3>
769
770 </sect2>
771 </sect1>
772
773 <sect1 id="options-help">
774 <title>Help and verbosity options</title>
775
776 <indexterm><primary>help options</primary></indexterm>
777 <indexterm><primary>verbosity options</primary></indexterm>
778
779 <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
780 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
781 <variablelist>
782 <varlistentry>
783 <term>
784 <option>-v</option>
785 <indexterm><primary><option>-v</option></primary></indexterm>
786 </term>
787 <listitem>
788 <para>The <option>-v</option> option makes GHC
789 <emphasis>verbose</emphasis>: it reports its version number
790 and shows (on stderr) exactly how it invokes each phase of
791 the compilation system. Moreover, it passes the
792 <option>-v</option> flag to most phases; each reports its
793 version number (and possibly some other information).</para>
794
795 <para>Please, oh please, use the <option>-v</option> option
796 when reporting bugs! Knowing that you ran the right bits in
797 the right order is always the first thing we want to
798 verify.</para>
799 </listitem>
800 </varlistentry>
801
802 <varlistentry>
803 <term>
804 <option>-v</option><replaceable>n</replaceable>
805 <indexterm><primary><option>-v</option></primary></indexterm>
806 </term>
807 <listitem>
808 <para>To provide more control over the compiler's verbosity,
809 the <option>-v</option> flag takes an optional numeric
810 argument. Specifying <option>-v</option> on its own is
811 equivalent to <option>-v3</option>, and the other levels
812 have the following meanings:</para>
813
814 <variablelist>
815 <varlistentry>
816 <term><option>-v0</option></term>
817 <listitem>
818 <para>Disable all non-essential messages (this is the
819 default).</para>
820 </listitem>
821 </varlistentry>
822
823 <varlistentry>
824 <term><option>-v1</option></term>
825 <listitem>
826 <para>Minimal verbosity: print one line per
827 compilation (this is the default when
828 <option>&ndash;&ndash;make</option> or
829 <option>&ndash;&ndash;interactive</option> is on).</para>
830 </listitem>
831 </varlistentry>
832
833 <varlistentry>
834 <term><option>-v2</option></term>
835 <listitem>
836 <para>Print the name of each compilation phase as it
837 is executed. (equivalent to
838 <option>-dshow-passes</option>).</para>
839 </listitem>
840 </varlistentry>
841
842 <varlistentry>
843 <term><option>-v3</option></term>
844 <listitem>
845 <para>The same as <option>-v2</option>, except that in
846 addition the full command line (if appropriate) for
847 each compilation phase is also printed.</para>
848 </listitem>
849 </varlistentry>
850
851 <varlistentry>
852 <term><option>-v4</option></term>
853 <listitem>
854 <para>The same as <option>-v3</option> except that the
855 intermediate program representation after each
856 compilation phase is also printed (excluding
857 preprocessed and C/assembly files).</para>
858 </listitem>
859 </varlistentry>
860 </variablelist>
861 </listitem>
862 </varlistentry>
863
864 <varlistentry>
865 <term><option>-ferror-spans</option>
866 <indexterm><primary><option>-ferror-spans</option></primary>
867 </indexterm>
868 </term>
869 <listitem>
870 <para>Causes GHC to emit the full source span of the
871 syntactic entity relating to an error message. Normally, GHC
872 emits the source location of the start of the syntactic
873 entity only.</para>
874
875 <para>For example:</para>
876
877 <screen>test.hs:3:6: parse error on input `where'</screen>
878
879 <para>becomes:</para>
880
881 <screen>test296.hs:3:6-10: parse error on input `where'</screen>
882
883 <para>And multi-line spans are possible too:</para>
884
885 <screen>test.hs:(5,4)-(6,7):
886 Conflicting definitions for `a'
887 Bound at: test.hs:5:4
888 test.hs:6:7
889 In the binding group for: a, b, a</screen>
890
891 <para>Note that line numbers start counting at one, but
892 column numbers start at zero. This choice was made to
893 follow existing convention (i.e. this is how Emacs does
894 it).</para>
895 </listitem>
896 </varlistentry>
897
898 <varlistentry>
899 <term><option>-H</option><replaceable>size</replaceable>
900 <indexterm><primary><option>-H</option></primary></indexterm>
901 </term>
902 <listitem>
903 <para>Set the minimum size of the heap to
904 <replaceable>size</replaceable>.
905 This option is equivalent to
906 <literal>+RTS&nbsp;-H<replaceable>size</replaceable></literal>,
907 see <xref linkend="rts-options-gc" />.
908 </para>
909 </listitem>
910 </varlistentry>
911
912 <varlistentry>
913 <term><option>-Rghc-timing</option>
914 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
915 </term>
916 <listitem>
917 <para>Prints a one-line summary of timing statistics for the
918 GHC run. This option is equivalent to
919 <literal>+RTS&nbsp;-tstderr</literal>, see <xref
920 linkend="rts-options-gc" />.
921 </para>
922 </listitem>
923 </varlistentry>
924 </variablelist>
925 </sect1>
926
927 &separate;
928
929 <sect1 id="options-sanity">
930 <title>Warnings and sanity-checking</title>
931
932 <indexterm><primary>sanity-checking options</primary></indexterm>
933 <indexterm><primary>warnings</primary></indexterm>
934
935
936 <para>GHC has a number of options that select which types of
937 non-fatal error messages, otherwise known as warnings, can be
938 generated during compilation. By default, you get a standard set
939 of warnings which are generally likely to indicate bugs in your
940 program. These are:
941 <option>-fwarn-overlapping-patterns</option>,
942 <option>-fwarn-warnings-deprecations</option>,
943 <option>-fwarn-deprecated-flags</option>,
944 <option>-fwarn-duplicate-exports</option>,
945 <option>-fwarn-missing-fields</option>,
946 <option>-fwarn-missing-methods</option>,
947 <option>-fwarn-lazy-unlifted-bindings</option>,
948 <option>-fwarn-wrong-do-bind</option>, and
949 <option>-fwarn-dodgy-foreign-imports</option>. The following
950 flags are
951 simple ways to select standard &ldquo;packages&rdquo; of warnings:
952 </para>
953
954 <variablelist>
955
956 <varlistentry>
957 <term><option>-W</option>:</term>
958 <listitem>
959 <indexterm><primary>-W option</primary></indexterm>
960 <para>Provides the standard warnings plus
961 <option>-fwarn-incomplete-patterns</option>,
962 <option>-fwarn-dodgy-exports</option>,
963 <option>-fwarn-dodgy-imports</option>,
964 <option>-fwarn-unused-matches</option>,
965 <option>-fwarn-unused-imports</option>, and
966 <option>-fwarn-unused-binds</option>.</para>
967 </listitem>
968 </varlistentry>
969
970 <varlistentry>
971 <term><option>-Wall</option>:</term>
972 <listitem>
973 <indexterm><primary><option>-Wall</option></primary></indexterm>
974 <para>Turns on all warning options that indicate potentially
975 suspicious code. The warnings that are
976 <emphasis>not</emphasis> enabled by <option>-Wall</option>
977 are
978 <option>-fwarn-tabs</option>,
979 <option>-fwarn-incomplete-uni-patterns</option>,
980 <option>-fwarn-incomplete-record-updates</option>,
981 <option>-fwarn-monomorphism-restriction</option>,
982 <option>-fwarn-unrecognised-pragmas</option>,
983 <option>-fwarn-auto-orphans</option>,
984 <option>-fwarn-implicit-prelude</option>.</para>
985 </listitem>
986 </varlistentry>
987
988 <varlistentry>
989 <term><option>-w</option>:</term>
990 <listitem>
991 <indexterm><primary><option>-w</option></primary></indexterm>
992 <para>Turns off all warnings, including the standard ones and
993 those that <literal>-Wall</literal> doesn't enable.</para>
994 </listitem>
995 </varlistentry>
996
997 <varlistentry>
998 <term><option>-Werror</option>:</term>
999 <listitem>
1000 <indexterm><primary><option>-Werror</option></primary></indexterm>
1001 <para>Makes any warning into a fatal error. Useful so that you don't
1002 miss warnings when doing batch compilation. </para>
1003 </listitem>
1004 </varlistentry>
1005
1006 <varlistentry>
1007 <term><option>-Wwarn</option>:</term>
1008 <listitem>
1009 <indexterm><primary><option>-Wwarn</option></primary></indexterm>
1010 <para>Warnings are treated only as warnings, not as errors. This is
1011 the default, but can be useful to negate a
1012 <option>-Werror</option> flag.</para>
1013 </listitem>
1014 </varlistentry>
1015
1016 </variablelist>
1017
1018 <para>The full set of warning options is described below. To turn
1019 off any warning, simply give the corresponding
1020 <option>-fno-warn-...</option> option on the command line.</para>
1021
1022 <variablelist>
1023
1024 <varlistentry>
1025 <term><option>-fdefer-type-errors</option>:</term>
1026 <listitem>
1027 <indexterm><primary><option>-fdefer-type-errors</option></primary>
1028 </indexterm>
1029 <indexterm><primary>warnings</primary></indexterm>
1030 <para>Defer as many type errors as possible until runtime.
1031 At compile time you get a warning (instead of an error). At
1032 runtime, if you use a value that depends on a type error, you
1033 get a runtime error; but you can run any type-correct parts of your code
1034 just fine.</para>
1035 </listitem>
1036 </varlistentry>
1037
1038 <varlistentry>
1039 <term><option>-fhelpful-errors</option>:</term>
1040 <listitem>
1041 <indexterm><primary><option>-fhelpful-errors</option></primary>
1042 </indexterm>
1043 <indexterm><primary>warnings</primary></indexterm>
1044 <para>When a name or package is not found in scope, make
1045 suggestions for the name or package you might have meant instead.</para>
1046 <para>This option is on by default.</para>
1047 </listitem>
1048 </varlistentry>
1049
1050 <varlistentry>
1051 <term><option>-fwarn-unrecognised-pragmas</option>:</term>
1052 <listitem>
1053 <indexterm><primary><option>-fwarn-unrecognised-pragmas</option></primary>
1054 </indexterm>
1055 <indexterm><primary>warnings</primary></indexterm>
1056 <indexterm><primary>pragmas</primary></indexterm>
1057 <para>Causes a warning to be emitted when a
1058 pragma that GHC doesn't recognise is used. As well as pragmas
1059 that GHC itself uses, GHC also recognises pragmas known to be used
1060 by other tools, e.g. <literal>OPTIONS_HUGS</literal> and
1061 <literal>DERIVE</literal>.</para>
1062
1063 <para>This option is on by default.</para>
1064 </listitem>
1065 </varlistentry>
1066
1067 <varlistentry>
1068 <term><option>-fwarn-warnings-deprecations</option>:</term>
1069 <listitem>
1070 <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
1071 </indexterm>
1072 <indexterm><primary>warnings</primary></indexterm>
1073 <indexterm><primary>deprecations</primary></indexterm>
1074 <para>Causes a warning to be emitted when a
1075 module, function or type with a WARNING or DEPRECATED pragma
1076 is used. See <xref linkend="warning-deprecated-pragma"/> for more
1077 details on the pragmas.</para>
1078
1079 <para>This option is on by default.</para>
1080 </listitem>
1081 </varlistentry>
1082
1083 <varlistentry>
1084 <term><option>-fwarn-deprecated-flags</option>:</term>
1085 <listitem>
1086 <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
1087 </indexterm>
1088 <indexterm><primary>deprecated-flags</primary></indexterm>
1089 <para>Causes a warning to be emitted when a deprecated
1090 commandline flag is used.</para>
1091
1092 <para>This option is on by default.</para>
1093 </listitem>
1094 </varlistentry>
1095
1096 <varlistentry>
1097 <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
1098 <listitem>
1099 <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
1100 </indexterm>
1101 <para>Causes a warning to be emitted for foreign imports of
1102 the following form:</para>
1103 <programlisting>
1104 foreign import "f" f :: FunPtr t
1105 </programlisting>
1106 <para>on the grounds that it probably should be</para>
1107 <programlisting>
1108 foreign import "&amp;f" f :: FunPtr t
1109 </programlisting>
1110 <para>The first form declares that `f` is a (pure) C
1111 function that takes no arguments and returns a pointer to a
1112 C function with type `t`, whereas the second form declares
1113 that `f` itself is a C function with type `t`. The first
1114 declaration is usually a mistake, and one that is hard to
1115 debug because it results in a crash, hence this
1116 warning.</para>
1117 </listitem>
1118 </varlistentry>
1119
1120 <varlistentry>
1121 <term><option>-fwarn-dodgy-exports</option>:</term>
1122 <listitem>
1123 <indexterm><primary><option>-fwarn-dodgy-exports</option></primary>
1124 </indexterm>
1125 <para>Causes a warning to be emitted when a datatype
1126 <literal>T</literal> is exported
1127 with all constructors, i.e. <literal>T(..)</literal>, but is it
1128 just a type synonym.</para>
1129 <para>Also causes a warning to be emitted when a module is
1130 re-exported, but that module exports nothing.</para>
1131 </listitem>
1132 </varlistentry>
1133
1134 <varlistentry>
1135 <term><option>-fwarn-dodgy-imports</option>:</term>
1136 <listitem>
1137 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
1138 </indexterm>
1139 <para>Causes a warning to be emitted when a datatype
1140 <literal>T</literal> is imported
1141 with all constructors, i.e. <literal>T(..)</literal>, but has been
1142 exported abstractly, i.e. <literal>T</literal>.</para>
1143 </listitem>
1144 </varlistentry>
1145
1146 <varlistentry>
1147 <term><option>-fwarn-lazy-unlifted-bindings</option>:</term>
1148 <listitem>
1149 <indexterm><primary><option>-fwarn-lazy-unlifted-bindings</option></primary>
1150 </indexterm>
1151 <para>Causes a warning to be emitted when an unlifted type
1152 is bound in a way that looks lazy, e.g.
1153 <literal>where (I# x) = ...</literal>. Use
1154 <literal>where !(I# x) = ...</literal> instead. This will be an
1155 error, rather than a warning, in GHC 7.2.
1156 </para>
1157 </listitem>
1158 </varlistentry>
1159
1160 <varlistentry>
1161 <term><option>-fwarn-duplicate-exports</option>:</term>
1162 <listitem>
1163 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
1164 <indexterm><primary>duplicate exports, warning</primary></indexterm>
1165 <indexterm><primary>export lists, duplicates</primary></indexterm>
1166
1167 <para>Have the compiler warn about duplicate entries in
1168 export lists. This is useful information if you maintain
1169 large export lists, and want to avoid the continued export
1170 of a definition after you've deleted (one) mention of it in
1171 the export list.</para>
1172
1173 <para>This option is on by default.</para>
1174 </listitem>
1175 </varlistentry>
1176
1177 <varlistentry>
1178 <term><option>-fwarn-hi-shadowing</option>:</term>
1179 <listitem>
1180 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
1181 <indexterm><primary>shadowing</primary>
1182 <secondary>interface files</secondary></indexterm>
1183
1184 <para>Causes the compiler to emit a warning when a module or
1185 interface file in the current directory is shadowing one
1186 with the same module name in a library or other
1187 directory.</para>
1188 </listitem>
1189 </varlistentry>
1190
1191 <varlistentry>
1192 <term><option>-fwarn-identities</option>:</term>
1193 <listitem>
1194 <indexterm><primary><option>-fwarn-identities</option></primary></indexterm>
1195 <para>Causes the compiler to emit a warning when a Prelude numeric
1196 conversion converts a type T to the same type T; such calls
1197 are probably no-ops and can be omitted. The functions checked for
1198 are: <literal>toInteger</literal>,
1199 <literal>toRational</literal>,
1200 <literal>fromIntegral</literal>,
1201 and <literal>realToFrac</literal>.
1202 </para>
1203 </listitem>
1204 </varlistentry>
1205
1206 <varlistentry>
1207 <term><option>-fwarn-implicit-prelude</option>:</term>
1208 <listitem>
1209 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
1210 <indexterm><primary>implicit prelude, warning</primary></indexterm>
1211 <para>Have the compiler warn if the Prelude is implicitly
1212 imported. This happens unless either the Prelude module is
1213 explicitly imported with an <literal>import ... Prelude ...</literal>
1214 line, or this implicit import is disabled (either by
1215 <option>-XNoImplicitPrelude</option> or a
1216 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
1217
1218 <para>Note that no warning is given for syntax that implicitly
1219 refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
1220 would change whether it refers to the Prelude.
1221 For example, no warning is given when
1222 <literal>368</literal> means
1223 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
1224 (where <literal>Prelude</literal> refers to the actual Prelude module,
1225 regardless of the imports of the module being compiled).</para>
1226
1227 <para>This warning is off by default.</para>
1228 </listitem>
1229 </varlistentry>
1230
1231 <varlistentry>
1232 <term><option>-fwarn-incomplete-patterns</option>,
1233 <option>-fwarn-incomplete-uni-patterns</option>:
1234 </term>
1235 <listitem>
1236 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
1237 <indexterm><primary><option>-fwarn-incomplete-uni-patterns</option></primary></indexterm>
1238 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
1239 <indexterm><primary>patterns, incomplete</primary></indexterm>
1240
1241 <para>The option <option>-fwarn-incomplete-patterns</option> warns
1242 about places where
1243 a pattern-match might fail at runtime.
1244 The function
1245 <function>g</function> below will fail when applied to
1246 non-empty lists, so the compiler will emit a warning about
1247 this when <option>-fwarn-incomplete-patterns</option> is
1248 enabled.
1249 <programlisting>
1250 g [] = 2
1251 </programlisting>
1252 This option isn't enabled by default because it can be
1253 a bit noisy, and it doesn't always indicate a bug in the
1254 program. However, it's generally considered good practice
1255 to cover all the cases in your functions, and it is switched
1256 on by <option>-W</option>.</para>
1257
1258 <para>The flag <option>-fwarn-incomplete-uni-patterns</option> is
1259 similar, except that it
1260 applies only to lambda-expressions and pattern bindings, constructs
1261 that only allow a single pattern:
1262 <programlisting>
1263 h = \[] -> 2
1264 Just k = f y
1265 </programlisting>
1266 </para>
1267 </listitem>
1268 </varlistentry>
1269
1270 <varlistentry>
1271 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1272 <listitem>
1273 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1274 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1275 <indexterm><primary>record updates, incomplete</primary></indexterm>
1276
1277 <para>The function
1278 <function>f</function> below will fail when applied to
1279 <literal>Bar</literal>, so the compiler will emit a warning about
1280 this when <option>-fwarn-incomplete-record-updates</option> is
1281 enabled.</para>
1282
1283 <programlisting>
1284 data Foo = Foo { x :: Int }
1285 | Bar
1286
1287 f :: Foo -> Foo
1288 f foo = foo { x = 6 }
1289 </programlisting>
1290
1291 <para>This option isn't enabled by default because it can be
1292 very noisy, and it often doesn't indicate a bug in the
1293 program.</para>
1294 </listitem>
1295 </varlistentry>
1296
1297 <varlistentry>
1298 <term>
1299 <option>-fwarn-missing-fields</option>:
1300 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1301 <indexterm><primary>missing fields, warning</primary></indexterm>
1302 <indexterm><primary>fields, missing</primary></indexterm>
1303 </term>
1304 <listitem>
1305
1306 <para>This option is on by default, and warns you whenever
1307 the construction of a labelled field constructor isn't
1308 complete, missing initializers for one or more fields. While
1309 not an error (the missing fields are initialised with
1310 bottoms), it is often an indication of a programmer error.</para>
1311 </listitem>
1312 </varlistentry>
1313
1314 <varlistentry>
1315 <term>
1316 <option>-fwarn-missing-import-lists</option>:
1317 <indexterm><primary><option>-fwarn-import-lists</option></primary></indexterm>
1318 <indexterm><primary>missing import lists, warning</primary></indexterm>
1319 <indexterm><primary>import lists, missing</primary></indexterm>
1320 </term>
1321 <listitem>
1322
1323 <para>This flag warns if you use an unqualified
1324 <literal>import</literal> declaration
1325 that does not explicitly list the entities brought into scope. For
1326 example
1327 </para>
1328 <programlisting>
1329 module M where
1330 import X( f )
1331 import Y
1332 import qualified Z
1333 p x = f x x
1334 </programlisting>
1335 <para>
1336 The <option>-fwarn-import-lists</option> flag will warn about the import
1337 of <literal>Y</literal> but not <literal>X</literal>
1338 If module <literal>Y</literal> is later changed to export (say) <literal>f</literal>,
1339 then the reference to <literal>f</literal> in <literal>M</literal> will become
1340 ambiguous. No warning is produced for the import of <literal>Z</literal>
1341 because extending <literal>Z</literal>'s exports would be unlikely to produce
1342 ambiguity in <literal>M</literal>.
1343 </para>
1344 </listitem>
1345 </varlistentry>
1346
1347 <varlistentry>
1348 <term><option>-fwarn-missing-methods</option>:</term>
1349 <listitem>
1350 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1351 <indexterm><primary>missing methods, warning</primary></indexterm>
1352 <indexterm><primary>methods, missing</primary></indexterm>
1353
1354 <para>This option is on by default, and warns you whenever
1355 an instance declaration is missing one or more methods, and
1356 the corresponding class declaration has no default
1357 declaration for them.</para>
1358 <para>The warning is suppressed if the method name
1359 begins with an underscore. Here's an example where this is useful:
1360 <programlisting>
1361 class C a where
1362 _simpleFn :: a -> String
1363 complexFn :: a -> a -> String
1364 complexFn x y = ... _simpleFn ...
1365 </programlisting>
1366 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1367 never <literal>_simpleFn</literal>; and (b)
1368 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1369 </para>
1370 </listitem>
1371 </varlistentry>
1372
1373 <varlistentry>
1374 <term><option>-fwarn-missing-signatures</option>:</term>
1375 <listitem>
1376 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1377 <indexterm><primary>type signatures, missing</primary></indexterm>
1378
1379 <para>If you would like GHC to check that every top-level
1380 function/value has a type signature, use the
1381 <option>-fwarn-missing-signatures</option> option. As part of
1382 the warning GHC also reports the inferred type. The
1383 option is off by default.</para>
1384 </listitem>
1385 </varlistentry>
1386
1387 <varlistentry>
1388 <term><option>-fwarn-missing-local-sigs</option>:</term>
1389 <listitem>
1390 <indexterm><primary><option>-fwarn-missing-local-sigs</option></primary></indexterm>
1391 <indexterm><primary>type signatures, missing</primary></indexterm>
1392
1393 <para>If you use the
1394 <option>-fwarn-missing-local-sigs</option> flag GHC will warn
1395 you about any polymorphic local bindings. As part of
1396 the warning GHC also reports the inferred type. The
1397 option is off by default.</para>
1398 </listitem>
1399 </varlistentry>
1400
1401 <varlistentry>
1402 <term><option>-fwarn-name-shadowing</option>:</term>
1403 <listitem>
1404 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1405 <indexterm><primary>shadowing, warning</primary></indexterm>
1406
1407 <para>This option causes a warning to be emitted whenever an
1408 inner-scope value has the same name as an outer-scope value,
1409 i.e. the inner value shadows the outer one. This can catch
1410 typographical errors that turn into hard-to-find bugs, e.g.,
1411 in the inadvertent capture of what would be a recursive call in
1412 <literal>f = ... let f = id in ... f ...</literal>.</para>
1413 <para>The warning is suppressed for names beginning with an underscore. For example
1414 <programlisting>
1415 f x = do { _ignore &lt;- this; _ignore &lt;- that; return (the other) }
1416 </programlisting>
1417 </para>
1418 </listitem>
1419 </varlistentry>
1420
1421 <varlistentry>
1422 <term><option>-fwarn-orphans</option>:</term>
1423 <listitem>
1424 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1425 <indexterm><primary>orphan instances, warning</primary></indexterm>
1426 <indexterm><primary>orphan rules, warning</primary></indexterm>
1427
1428 <para>This option causes a warning to be emitted whenever the
1429 module contains an "orphan" instance declaration or rewrite rule.
1430 An instance declaration is an orphan if it appears in a module in
1431 which neither the class nor the type being instanced are declared
1432 in the same module. A rule is an orphan if it is a rule for a
1433 function declared in another module. A module containing any
1434 orphans is called an orphan module.</para>
1435 <para>The trouble with orphans is that GHC must pro-actively read the interface
1436 files for all orphan modules, just in case their instances or rules
1437 play a role, whether or not the module's interface would otherwise
1438 be of any use. See <xref linkend="orphan-modules"/> for details.
1439 </para>
1440 </listitem>
1441 </varlistentry>
1442
1443 <varlistentry>
1444 <term>
1445 <option>-fwarn-overlapping-patterns</option>:
1446 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1447 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1448 <indexterm><primary>patterns, overlapping</primary></indexterm>
1449 </term>
1450 <listitem>
1451 <para>By default, the compiler will warn you if a set of
1452 patterns are overlapping, e.g.,</para>
1453
1454 <programlisting>
1455 f :: String -&#62; Int
1456 f [] = 0
1457 f (_:xs) = 1
1458 f "2" = 2
1459 </programlisting>
1460
1461 <para>where the last pattern match in <function>f</function>
1462 won't ever be reached, as the second pattern overlaps
1463 it. More often than not, redundant patterns is a programmer
1464 mistake/error, so this option is enabled by default.</para>
1465 </listitem>
1466 </varlistentry>
1467
1468 <varlistentry>
1469 <term><option>-fwarn-tabs</option>:</term>
1470 <listitem>
1471 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1472 <indexterm><primary>tabs, warning</primary></indexterm>
1473 <para>Have the compiler warn if there are tabs in your source
1474 file.</para>
1475
1476 <para>This warning is off by default.</para>
1477 </listitem>
1478 </varlistentry>
1479
1480 <varlistentry>
1481 <term><option>-fwarn-type-defaults</option>:</term>
1482 <listitem>
1483 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1484 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1485 <para>Have the compiler warn/inform you where in your source
1486 the Haskell defaulting mechanism for numeric types kicks
1487 in. This is useful information when converting code from a
1488 context that assumed one default into one with another,
1489 e.g., the &lsquo;default default&rsquo; for Haskell 1.4 caused the
1490 otherwise unconstrained value <constant>1</constant> to be
1491 given the type <literal>Int</literal>, whereas Haskell 98
1492 and later
1493 defaults it to <literal>Integer</literal>. This may lead to
1494 differences in performance and behaviour, hence the
1495 usefulness of being non-silent about this.</para>
1496
1497 <para>This warning is off by default.</para>
1498 </listitem>
1499 </varlistentry>
1500
1501 <varlistentry>
1502 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1503 <listitem>
1504 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1505 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1506 <para>Have the compiler warn/inform you where in your source
1507 the Haskell Monomorphism Restriction is applied. If applied silently
1508 the MR can give rise to unexpected behaviour, so it can be helpful
1509 to have an explicit warning that it is being applied.</para>
1510
1511 <para>This warning is off by default.</para>
1512 </listitem>
1513 </varlistentry>
1514
1515 <varlistentry>
1516 <term><option>-fwarn-unused-binds</option>:</term>
1517 <listitem>
1518 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1519 <indexterm><primary>unused binds, warning</primary></indexterm>
1520 <indexterm><primary>binds, unused</primary></indexterm>
1521 <para>Report any function definitions (and local bindings)
1522 which are unused. For top-level functions, the warning is
1523 only given if the binding is not exported.</para>
1524 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1525 mentioned in the right hand side of another definition that is used, or (c) the
1526 function it defines begins with an underscore. The last case provides a
1527 way to suppress unused-binding warnings selectively. </para>
1528 <para> Notice that a variable
1529 is reported as unused even if it appears in the right-hand side of another
1530 unused binding. </para>
1531 </listitem>
1532 </varlistentry>
1533
1534 <varlistentry>
1535 <term><option>-fwarn-unused-imports</option>:</term>
1536 <listitem>
1537 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1538 <indexterm><primary>unused imports, warning</primary></indexterm>
1539 <indexterm><primary>imports, unused</primary></indexterm>
1540
1541 <para>Report any modules that are explicitly imported but
1542 never used. However, the form <literal>import M()</literal> is
1543 never reported as an unused import, because it is a useful idiom
1544 for importing instance declarations, which are anonymous in Haskell.</para>
1545 </listitem>
1546 </varlistentry>
1547
1548 <varlistentry>
1549 <term><option>-fwarn-unused-matches</option>:</term>
1550 <listitem>
1551 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1552 <indexterm><primary>unused matches, warning</primary></indexterm>
1553 <indexterm><primary>matches, unused</primary></indexterm>
1554
1555 <para>Report all unused variables which arise from pattern
1556 matches, including patterns consisting of a single variable.
1557 For instance <literal>f x y = []</literal> would report
1558 <varname>x</varname> and <varname>y</varname> as unused. The
1559 warning is suppressed if the variable name begins with an underscore, thus:
1560 <programlisting>
1561 f _x = True
1562 </programlisting>
1563 </para>
1564 </listitem>
1565 </varlistentry>
1566
1567 <varlistentry>
1568 <term><option>-fwarn-unused-do-bind</option>:</term>
1569 <listitem>
1570 <indexterm><primary><option>-fwarn-unused-do-bind</option></primary></indexterm>
1571 <indexterm><primary>unused do binding, warning</primary></indexterm>
1572 <indexterm><primary>do binding, unused</primary></indexterm>
1573
1574 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1575 that appear to silently throw information away.
1576 For instance <literal>do { mapM popInt xs ; return 10 }</literal> would report
1577 the first statement in the <literal>do</literal> block as suspicious,
1578 as it has the type <literal>StackM [Int]</literal> and not <literal>StackM ()</literal>, but that
1579 <literal>[Int]</literal> value is not bound to anything. The warning is suppressed by
1580 explicitly mentioning in the source code that your program is throwing something away:
1581 <programlisting>
1582 do { _ &lt;- mapM popInt xs ; return 10 }
1583 </programlisting>
1584 Of course, in this particular situation you can do even better:
1585 <programlisting>
1586 do { mapM_ popInt xs ; return 10 }
1587 </programlisting>
1588 </para>
1589 </listitem>
1590 </varlistentry>
1591
1592 <varlistentry>
1593 <term><option>-fwarn-wrong-do-bind</option>:</term>
1594 <listitem>
1595 <indexterm><primary><option>-fwarn-wrong-do-bind</option></primary></indexterm>
1596 <indexterm><primary>apparently erroneous do binding, warning</primary></indexterm>
1597 <indexterm><primary>do binding, apparently erroneous</primary></indexterm>
1598
1599 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1600 that appear to lack a binding.
1601 For instance <literal>do { return (popInt 10) ; return 10 }</literal> would report
1602 the first statement in the <literal>do</literal> block as suspicious,
1603 as it has the type <literal>StackM (StackM Int)</literal> (which consists of two nested applications
1604 of the same monad constructor), but which is not then &quot;unpacked&quot; by binding the result.
1605 The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away:
1606 <programlisting>
1607 do { _ &lt;- return (popInt 10) ; return 10 }
1608 </programlisting>
1609 For almost all sensible programs this will indicate a bug, and you probably intended to write:
1610 <programlisting>
1611 do { popInt 10 ; return 10 }
1612 </programlisting>
1613 </para>
1614 </listitem>
1615 </varlistentry>
1616
1617 </variablelist>
1618
1619 <para>If you're feeling really paranoid, the
1620 <option>-dcore-lint</option>
1621 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1622 is a good choice. It turns on heavyweight intra-pass
1623 sanity-checking within GHC. (It checks GHC's sanity, not
1624 yours.)</para>
1625
1626 </sect1>
1627
1628 &packages;
1629
1630 <sect1 id="options-optimise">
1631 <title>Optimisation (code improvement)</title>
1632
1633 <indexterm><primary>optimisation</primary></indexterm>
1634 <indexterm><primary>improvement, code</primary></indexterm>
1635
1636 <para>The <option>-O*</option> options specify convenient
1637 &ldquo;packages&rdquo; of optimisation flags; the
1638 <option>-f*</option> options described later on specify
1639 <emphasis>individual</emphasis> optimisations to be turned on/off;
1640 the <option>-m*</option> options specify
1641 <emphasis>machine-specific</emphasis> optimisations to be turned
1642 on/off.</para>
1643
1644 <sect2 id="optimise-pkgs">
1645 <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1646
1647 <para>There are <emphasis>many</emphasis> options that affect
1648 the quality of code produced by GHC. Most people only have a
1649 general goal, something like &ldquo;Compile quickly&rdquo; or
1650 &ldquo;Make my program run like greased lightning.&rdquo; The
1651 following &ldquo;packages&rdquo; of optimisations (or lack
1652 thereof) should suffice.</para>
1653
1654 <para>Note that higher optimisation levels cause more
1655 cross-module optimisation to be performed, which can have an
1656 impact on how much of your program needs to be recompiled when
1657 you change something. This is one reason to stick to
1658 no-optimisation when developing code.</para>
1659
1660 <variablelist>
1661
1662 <varlistentry>
1663 <term>
1664 No <option>-O*</option>-type option specified:
1665 <indexterm><primary>-O* not specified</primary></indexterm>
1666 </term>
1667 <listitem>
1668 <para>This is taken to mean: &ldquo;Please compile
1669 quickly; I'm not over-bothered about compiled-code
1670 quality.&rdquo; So, for example: <command>ghc -c
1671 Foo.hs</command></para>
1672 </listitem>
1673 </varlistentry>
1674
1675 <varlistentry>
1676 <term>
1677 <option>-O0</option>:
1678 <indexterm><primary><option>-O0</option></primary></indexterm>
1679 </term>
1680 <listitem>
1681 <para>Means &ldquo;turn off all optimisation&rdquo;,
1682 reverting to the same settings as if no
1683 <option>-O</option> options had been specified. Saying
1684 <option>-O0</option> can be useful if
1685 eg. <command>make</command> has inserted a
1686 <option>-O</option> on the command line already.</para>
1687 </listitem>
1688 </varlistentry>
1689
1690 <varlistentry>
1691 <term>
1692 <option>-O</option> or <option>-O1</option>:
1693 <indexterm><primary>-O option</primary></indexterm>
1694 <indexterm><primary>-O1 option</primary></indexterm>
1695 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1696 </term>
1697 <listitem>
1698 <para>Means: &ldquo;Generate good-quality code without
1699 taking too long about it.&rdquo; Thus, for example:
1700 <command>ghc -c -O Main.lhs</command></para>
1701 </listitem>
1702 </varlistentry>
1703
1704 <varlistentry>
1705 <term>
1706 <option>-O2</option>:
1707 <indexterm><primary>-O2 option</primary></indexterm>
1708 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1709 </term>
1710 <listitem>
1711 <para>Means: &ldquo;Apply every non-dangerous
1712 optimisation, even if it means significantly longer
1713 compile times.&rdquo;</para>
1714
1715 <para>The avoided &ldquo;dangerous&rdquo; optimisations
1716 are those that can make runtime or space
1717 <emphasis>worse</emphasis> if you're unlucky. They are
1718 normally turned on or off individually.</para>
1719
1720 <para>At the moment, <option>-O2</option> is
1721 <emphasis>unlikely</emphasis> to produce better code than
1722 <option>-O</option>.</para>
1723 </listitem>
1724 </varlistentry>
1725 </variablelist>
1726
1727 <para>We don't use a <option>-O*</option> flag for day-to-day
1728 work. We use <option>-O</option> to get respectable speed;
1729 e.g., when we want to measure something. When we want to go for
1730 broke, we tend to use <option>-O2</option> (and we go for
1731 lots of coffee breaks).</para>
1732
1733 <para>The easiest way to see what <option>-O</option> (etc.)
1734 &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1735 then stand back in amazement.</para>
1736 </sect2>
1737
1738 <sect2 id="options-f">
1739 <title><option>-f*</option>: platform-independent flags</title>
1740
1741 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1742 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1743
1744 <para>These flags turn on and off individual optimisations.
1745 They are normally set via the <option>-O</option> options
1746 described above, and as such, you shouldn't need to set any of
1747 them explicitly (indeed, doing so could lead to unexpected
1748 results). However, there are one or two that may be of
1749 interest:</para>
1750
1751 <variablelist>
1752 <varlistentry>
1753 <term><option>-fexcess-precision</option>:</term>
1754 <listitem>
1755 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1756 <para>When this option is given, intermediate floating
1757 point values can have a <emphasis>greater</emphasis>
1758 precision/range than the final type. Generally this is a
1759 good thing, but some programs may rely on the exact
1760 precision/range of
1761 <literal>Float</literal>/<literal>Double</literal> values
1762 and should not use this option for their compilation.</para>
1763 </listitem>
1764 </varlistentry>
1765
1766 <varlistentry>
1767 <term><option>-fignore-asserts</option>:</term>
1768 <listitem>
1769 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1770 <para>Causes GHC to ignore uses of the function
1771 <literal>Exception.assert</literal> in source code (in
1772 other words, rewriting <literal>Exception.assert p
1773 e</literal> to <literal>e</literal> (see <xref
1774 linkend="assertions"/>). This flag is turned on by
1775 <option>-O</option>.
1776 </para>
1777 </listitem>
1778 </varlistentry>
1779
1780 <varlistentry>
1781 <term>
1782 <option>-fignore-interface-pragmas</option>
1783 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
1784 </term>
1785 <listitem>
1786 <para>Tells GHC to ignore all inessential information when reading interface files.
1787 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
1788 for a function, GHC will ignore that information.</para>
1789 </listitem>
1790 </varlistentry>
1791
1792 <varlistentry>
1793 <term>
1794 <option>-fliberate-case</option>
1795 <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
1796 </term>
1797 <listitem>
1798 <para>Turn on the liberate-case transformation.</para>
1799 </listitem>
1800 </varlistentry>
1801
1802 <varlistentry>
1803 <term>
1804 <option>-fno-cse</option>
1805 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1806 </term>
1807 <listitem>
1808 <para>Turns off the common-sub-expression elimination optimisation.
1809 Can be useful if you have some <literal>unsafePerformIO</literal>
1810 expressions that you don't want commoned-up.</para>
1811 </listitem>
1812 </varlistentry>
1813
1814 <varlistentry>
1815 <term>
1816 <option>-fno-strictness</option>
1817 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1818 </term>
1819 <listitem>
1820 <para>Turns off the strictness analyser; sometimes it eats
1821 too many cycles.</para>
1822 </listitem>
1823 </varlistentry>
1824
1825 <varlistentry>
1826 <term>
1827 <option>-fno-full-laziness</option>
1828 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1829 </term>
1830 <listitem>
1831 <para>Turns off the full laziness optimisation (also known as
1832 let-floating). Full laziness increases sharing, which can lead
1833 to increased memory residency.</para>
1834
1835 <para>NOTE: GHC doesn't implement complete full-laziness.
1836 When optimisation in on, and
1837 <option>-fno-full-laziness</option> is not given, some
1838 transformations that increase sharing are performed, such
1839 as extracting repeated computations from a loop. These
1840 are the same transformations that a fully lazy
1841 implementation would do, the difference is that GHC
1842 doesn't consistently apply full-laziness, so don't rely on
1843 it.</para>
1844 </listitem>
1845 </varlistentry>
1846
1847 <varlistentry>
1848 <term>
1849 <option>-fno-float-in</option>
1850 <indexterm><primary><option>-fno-float-in</option></primary></indexterm>
1851 </term>
1852 <listitem>
1853 <para>Turns off the float-in transformation.</para>
1854 </listitem>
1855 </varlistentry>
1856
1857 <varlistentry>
1858 <term>
1859 <option>-fno-specialise</option>
1860 <indexterm><primary><option>-fno-specialise</option></primary></indexterm>
1861 </term>
1862 <listitem>
1863 <para>Turns off the automatic specialisation of overloaded functions.</para>
1864 </listitem>
1865 </varlistentry>
1866
1867 <varlistentry>
1868 <term>
1869 <option>-fno-state-hack</option>
1870 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1871 </term>
1872 <listitem>
1873 <para>Turn off the "state hack" whereby any lambda with a
1874 <literal>State#</literal> token as argument is considered to be
1875 single-entry, hence it is considered OK to inline things inside
1876 it. This can improve performance of IO and ST monad code, but it
1877 runs the risk of reducing sharing.</para>
1878 </listitem>
1879 </varlistentry>
1880
1881 <varlistentry>
1882 <term>
1883 <option>-fpedantic-bottoms</option>
1884 <indexterm><primary><option>-fpedantic-bottoms</option></primary></indexterm>
1885 </term>
1886 <listitem>
1887 <para>Make GHC be more precise about its treatment of bottom (but see also
1888 <option>-fno-state-hack</option>). In particular, stop GHC
1889 eta-expanding through a case expression, which is good for
1890 performance, but bad if you are using <literal>seq</literal> on
1891 partial applications.</para>
1892 </listitem>
1893 </varlistentry>
1894
1895 <varlistentry>
1896 <term>
1897 <option>-fomit-interface-pragmas</option>
1898 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
1899 </term>
1900 <listitem>
1901 <para>Tells GHC to omit all inessential information from the interface file
1902 generated for the module being compiled (say M). This means that a module
1903 importing M will see only the <emphasis>types</emphasis> of the functions that M exports, but not
1904 their unfoldings, strictness info, etc. Hence, for example,
1905 no function exported by M will be inlined
1906 into an importing module. The benefit is that modules that import M will
1907 need to be recompiled less often (only when M's exports change their type,
1908 not when they change their implementation).
1909 </para>
1910 </listitem>
1911 </varlistentry>
1912
1913 <varlistentry>
1914 <term>
1915 <option>-fsimpl-tick-factor=<replaceable>n</replaceable></option>
1916 <indexterm><primary><option>-fsimpl-tick-factor</option></primary></indexterm>
1917 </term>
1918 <listitem>
1919 <para>GHC's optimiser can diverge if you write rewrite rules (<xref linkend="rewrite-rules"/>)
1920 that don't terminate, or (less satisfactorily) if you
1921 code up recursion through data types
1922 (<xref linkend="bugs-ghc"/>). To avoid making the compiler fall into an infinite
1923 loop, the optimiser carries a "tick count" and stops inlining and applying rewrite rules
1924 when this count is exceeded. The limit is set as a multiple of the program size, so
1925 bigger programs get more ticks. The <option>-fsimpl-tick-factor</option> flag lets
1926 you change the multiplier. The default is 100; numbers larger than 100 give more ticks,
1927 and numbers smaller than 100 give fewer.</para>
1928 <para>If the tick-count expires, GHC summarises what simplifier steps it has done;
1929 you can use <option>-fddump-simpl-stats</option> to generate a much more detailed list.
1930 Usually that identifies the loop quite accurately, because some numbers are very large.
1931 </para>
1932 </listitem>
1933 </varlistentry>
1934
1935 <varlistentry>
1936 <term>
1937 <option>-fstatic-argument-transformation</option>
1938 <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
1939 </term>
1940 <listitem>
1941 <para>Turn on the static argument transformation.</para>
1942 </listitem>
1943 </varlistentry>
1944
1945 <varlistentry>
1946 <term>
1947 <option>-fspec-constr</option>
1948 <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
1949 </term>
1950 <listitem>
1951 <para>Turn on call-pattern specialisation.</para>
1952 </listitem>
1953 </varlistentry>
1954
1955 <varlistentry>
1956 <term>
1957 <option>-funbox-strict-fields</option>:
1958 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1959 <indexterm><primary>strict constructor fields</primary></indexterm>
1960 <indexterm><primary>constructor fields, strict</primary></indexterm>
1961 </term>
1962 <listitem>
1963 <para>This option causes all constructor fields which are
1964 marked strict (i.e. &ldquo;!&rdquo;) to be unboxed or
1965 unpacked if possible. It is equivalent to adding an
1966 <literal>UNPACK</literal> pragma to every strict
1967 constructor field (see <xref
1968 linkend="unpack-pragma"/>).</para>
1969
1970 <para>This option is a bit of a sledgehammer: it might
1971 sometimes make things worse. Selectively unboxing fields
1972 by using <literal>UNPACK</literal> pragmas might be
1973 better. An alternative is to use
1974 <option>-funbox-strict-fields</option> to turn on
1975 unboxing by default but disable it for certain constructor
1976 fields using the <literal>NOUNPACK</literal> pragma
1977 (see <xref linkend="nounpack-pragma"/>).
1978 </para>
1979 </listitem>
1980 </varlistentry>
1981
1982 <varlistentry>
1983 <term>
1984 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
1985 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1986 <indexterm><primary>inlining, controlling</primary></indexterm>
1987 <indexterm><primary>unfolding, controlling</primary></indexterm>
1988 </term>
1989 <listitem>
1990 <para>(Default: 45) Governs the maximum size that GHC will
1991 allow a function unfolding to be. (An unfolding has a
1992 &ldquo;size&rdquo; that reflects the cost in terms of
1993 &ldquo;code bloat&rdquo; of expanding that unfolding
1994 at a call site. A bigger function would be assigned a
1995 bigger cost.) </para>
1996
1997 <para> Consequences: (a) nothing larger than this will be
1998 inlined (unless it has an INLINE pragma); (b) nothing
1999 larger than this will be spewed into an interface
2000 file. </para>
2001
2002
2003 <para> Increasing this figure is more likely to result in longer
2004 compile times than faster code. The next option is more
2005 useful:</para>
2006 </listitem>
2007 </varlistentry>
2008
2009 <varlistentry>
2010 <term><option>-funfolding-use-threshold=<replaceable>n</replaceable></option></term>
2011 <listitem>
2012 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
2013 <indexterm><primary>inlining, controlling</primary></indexterm>
2014 <indexterm><primary>unfolding, controlling</primary></indexterm>
2015
2016 <para>(Default: 8) This is the magic cut-off figure for
2017 unfolding: below this size, a function definition will be
2018 unfolded at the call-site, any bigger and it won't. The
2019 size computed for a function depends on two things: the
2020 actual size of the expression minus any discounts that
2021 apply (see <option>-funfolding-con-discount</option>).</para>
2022 </listitem>
2023 </varlistentry>
2024 </variablelist>
2025
2026 </sect2>
2027
2028 </sect1>
2029
2030 &code-gens;
2031
2032 &phases;
2033
2034 &shared_libs;
2035
2036 <sect1 id="using-concurrent">
2037 <title>Using Concurrent Haskell</title>
2038 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
2039
2040 <para>GHC supports Concurrent Haskell by default, without requiring a
2041 special option or libraries compiled in a certain way. To get access to
2042 the support libraries for Concurrent Haskell, just import
2043 <ulink
2044 url="&libraryBaseLocation;/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
2045
2046 <para>The following RTS option(s) affect the behaviour of Concurrent
2047 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
2048
2049 <variablelist>
2050 <varlistentry>
2051 <term><option>-C<replaceable>s</replaceable></option></term>
2052 <listitem>
2053 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2054 Sets the context switch interval to <replaceable>s</replaceable>
2055 seconds. A context switch will occur at the next heap block
2056 allocation after the timer expires (a heap block allocation occurs
2057 every 4k of allocation). With <option>-C0</option> or
2058 <option>-C</option>, context switches will occur as often as
2059 possible (at every heap block allocation). By default, context
2060 switches occur every 20ms.</para>
2061 </listitem>
2062 </varlistentry>
2063 </variablelist>
2064 </sect1>
2065
2066 <sect1 id="using-smp">
2067 <title>Using SMP parallelism</title>
2068 <indexterm><primary>parallelism</primary>
2069 </indexterm>
2070 <indexterm><primary>SMP</primary>
2071 </indexterm>
2072
2073 <para>GHC supports running Haskell programs in parallel on an SMP
2074 (symmetric multiprocessor).</para>
2075
2076 <para>There's a fine distinction between
2077 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
2078 parallelism is all about making your program run
2079 <emphasis>faster</emphasis> by making use of multiple processors
2080 simultaneously. Concurrency, on the other hand, is a means of
2081 abstraction: it is a convenient way to structure a program that must
2082 respond to multiple asynchronous events.</para>
2083
2084 <para>However, the two terms are certainly related. By making use of
2085 multiple CPUs it is possible to run concurrent threads in parallel,
2086 and this is exactly what GHC's SMP parallelism support does. But it
2087 is also possible to obtain performance improvements with parallelism
2088 on programs that do not use concurrency. This section describes how to
2089 use GHC to compile and run parallel programs, in <xref
2090 linkend="lang-parallel" /> we describe the language features that affect
2091 parallelism.</para>
2092
2093 <sect2 id="parallel-compile-options">
2094 <title>Compile-time options for SMP parallelism</title>
2095
2096 <para>In order to make use of multiple CPUs, your program must be
2097 linked with the <option>-threaded</option> option (see <xref
2098 linkend="options-linker" />). Additionally, the following
2099 compiler options affect parallelism:</para>
2100
2101 <variablelist>
2102 <varlistentry>
2103 <term><option>-feager-blackholing</option></term>
2104 <indexterm><primary><option>-feager-blackholing</option></primary></indexterm>
2105 <listitem>
2106 <para>
2107 Blackholing is the act of marking a thunk (lazy
2108 computuation) as being under evaluation. It is useful for
2109 three reasons: firstly it lets us detect certain kinds of
2110 infinite loop (the <literal>NonTermination</literal>
2111 exception), secondly it avoids certain kinds of space
2112 leak, and thirdly it avoids repeating a computation in a
2113 parallel program, because we can tell when a computation
2114 is already in progress.</para>
2115
2116 <para>
2117 The option <option>-feager-blackholing</option> causes
2118 each thunk to be blackholed as soon as evaluation begins.
2119 The default is "lazy blackholing", whereby thunks are only
2120 marked as being under evaluation when a thread is paused
2121 for some reason. Lazy blackholing is typically more
2122 efficient (by 1-2&percnt; or so), because most thunks don't
2123 need to be blackholed. However, eager blackholing can
2124 avoid more repeated computation in a parallel program, and
2125 this often turns out to be important for parallelism.
2126 </para>
2127
2128 <para>
2129 We recommend compiling any code that is intended to be run
2130 in parallel with the <option>-feager-blackholing</option>
2131 flag.
2132 </para>
2133 </listitem>
2134 </varlistentry>
2135 </variablelist>
2136 </sect2>
2137
2138 <sect2 id="parallel-options">
2139 <title>RTS options for SMP parallelism</title>
2140
2141 <para>There are two ways to run a program on multiple
2142 processors:
2143 call <literal>Control.Concurrent.setNumCapabilities</literal> from your
2144 program, or use the RTS <option>-N</option> option.</para>
2145
2146 <variablelist>
2147 <varlistentry>
2148 <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
2149 <listitem>
2150 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2151 Use <replaceable>x</replaceable> simultaneous threads when
2152 running the program. Normally <replaceable>x</replaceable>
2153 should be chosen to match the number of CPU cores on the
2154 machine<footnote><para>Whether hyperthreading cores should be counted or not is an
2155 open question; please feel free to experiment and let us know what
2156 results you find.</para></footnote>. For example,
2157 on a dual-core machine we would probably use
2158 <literal>+RTS -N2 -RTS</literal>.</para>
2159
2160 <para>Omitting <replaceable>x</replaceable>,
2161 i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
2162 choose the value of <replaceable>x</replaceable> itself
2163 based on how many processors are in your machine.</para>
2164
2165 <para>Be careful when using all the processors in your
2166 machine: if some of your processors are in use by other
2167 programs, this can actually harm performance rather than
2168 improve it.</para>
2169
2170 <para>Setting <option>-N</option> also has the effect of
2171 enabling the parallel garbage collector (see
2172 <xref linkend="rts-options-gc" />).</para>
2173
2174 <para>There is no means (currently) by which this value
2175 may vary after the program has started.</para>
2176
2177 <para>The current value of the <option>-N</option> option
2178 is available to the Haskell program
2179 via <literal>Control.Concurrent.getNumCapabilities</literal>, and
2180 it may be changed while the program is running by
2181 calling <literal>Control.Concurrent.setNumCapabilities</literal>.
2182 Note: in the current implementation,
2183 the <option>-N</option> value may only
2184 be <emphasis>increased</emphasis>, not decreased, by
2185 calling <literal>Control.Concurrent.setNumCapabilities</literal>.</para>
2186 </listitem>
2187 </varlistentry>
2188 </variablelist>
2189
2190 <para>The following options affect the way the runtime schedules
2191 threads on CPUs:</para>
2192
2193 <variablelist>
2194 <varlistentry>
2195 <term><option>-qa</option></term>
2196 <indexterm><primary><option>-qa</option></primary><secondary>RTS
2197 option</secondary></indexterm>
2198 <listitem>
2199 <para>Use the OS's affinity facilities to try to pin OS
2200 threads to CPU cores. This is an experimental feature,
2201 and may or may not be useful. Please let us know
2202 whether it helps for you!</para>
2203 </listitem>
2204 </varlistentry>
2205 <varlistentry>
2206 <term><option>-qm</option></term>
2207 <indexterm><primary><option>-qm</option></primary><secondary>RTS
2208 option</secondary></indexterm>
2209 <listitem>
2210 <para>Disable automatic migration for load balancing.
2211 Normally the runtime will automatically try to schedule
2212 threads across the available CPUs to make use of idle
2213 CPUs; this option disables that behaviour. Note that
2214 migration only applies to threads; sparks created
2215 by <literal>par</literal> are load-balanced separately
2216 by work-stealing.</para>
2217
2218 <para>
2219 This option is probably only of use for concurrent
2220 programs that explicitly schedule threads onto CPUs
2221 with <literal>Control.Concurrent.forkOn</literal>.
2222 </para>
2223 </listitem>
2224 </varlistentry>
2225 </variablelist>
2226 </sect2>
2227
2228 <sect2>
2229 <title>Hints for using SMP parallelism</title>
2230
2231 <para>Add the <literal>-s</literal> RTS option when
2232 running the program to see timing stats, which will help to tell you
2233 whether your program got faster by using more CPUs or not. If the user
2234 time is greater than
2235 the elapsed time, then the program used more than one CPU. You should
2236 also run the program without <literal>-N</literal> for
2237 comparison.</para>
2238
2239 <para>The output of <literal>+RTS -s</literal> tells you how
2240 many &ldquo;sparks&rdquo; were created and executed during the
2241 run of the program (see <xref linkend="rts-options-gc" />), which
2242 will give you an idea how well your <literal>par</literal>
2243 annotations are working.</para>
2244
2245 <para>GHC's parallelism support has improved in 6.12.1 as a
2246 result of much experimentation and tuning in the runtime
2247 system. We'd still be interested to hear how well it works
2248 for you, and we're also interested in collecting parallel
2249 programs to add to our benchmarking suite.</para>
2250 </sect2>
2251 </sect1>
2252
2253 <sect1 id="options-platform">
2254 <title>Platform-specific Flags</title>
2255
2256 <indexterm><primary>-m* options</primary></indexterm>
2257 <indexterm><primary>platform-specific options</primary></indexterm>
2258 <indexterm><primary>machine-specific options</primary></indexterm>
2259
2260 <para>Some flags only make sense for particular target
2261 platforms.</para>
2262
2263 <variablelist>
2264
2265 <varlistentry>
2266 <term><option>-msse2</option>:</term>
2267 <listitem>
2268 <para>
2269 (x86 only, added in GHC 7.0.1) Use the SSE2 registers and
2270 instruction set to implement floating point operations when using
2271 the <link linkend="native-code-gen">native code generator</link>.
2272 This gives a substantial performance improvement for floating
2273 point, but the resulting compiled code
2274 will only run on processors that support SSE2 (Intel Pentium 4 and
2275 later, or AMD Athlon 64 and later). The
2276 <link linkend="llvm-code-gen">LLVM backend</link> will also use SSE2
2277 if your processor supports it but detects this automatically so no
2278 flag is required.
2279 </para>
2280 <para>
2281 SSE2 is unconditionally used on x86-64 platforms.
2282 </para>
2283 </listitem>
2284 </varlistentry>
2285
2286 <varlistentry>
2287 <term><option>-msse4.2</option>:</term>
2288 <listitem>
2289 <para>
2290 (x86 only, added in GHC 7.4.1) Use the SSE4.2 instruction set to
2291 implement some floating point and bit operations when using the
2292 <link linkend="native-code-gen">native code generator</link>. The
2293 resulting compiled code will only run on processors that
2294 support SSE4.2 (Intel Core i7 and later). The
2295 <link linkend="llvm-code-gen">LLVM backend</link> will also use
2296 SSE4.2 if your processor supports it but detects this automatically
2297 so no flag is required.
2298 </para>
2299 </listitem>
2300 </varlistentry>
2301
2302 </variablelist>
2303
2304 </sect1>
2305
2306 &runtime;
2307
2308 <sect1 id="ext-core">
2309 <title>Generating and compiling External Core Files</title>
2310
2311 <indexterm><primary>intermediate code generation</primary></indexterm>
2312
2313 <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format)
2314 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
2315 <filename>.hcr</filename>. The Core format is described in <ulink url="../../core.pdf">
2316 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
2317 and sample tools
2318 for manipulating Core files (in Haskell) are available in the
2319 <ulink url="http://hackage.haskell.org/package/extcore">extcore package on Hackage</ulink>. Note that the format of <literal>.hcr</literal>
2320 files is <emphasis>different</emphasis> from the Core output format that GHC generates
2321 for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
2322
2323 <para>The Core format natively supports notes which you can add to
2324 your source code using the <literal>CORE</literal> pragma (see <xref
2325 linkend="pragmas"/>).</para>
2326
2327 <variablelist>
2328
2329 <varlistentry>
2330 <term>
2331 <option>-fext-core</option>
2332 <indexterm><primary><option>-fext-core</option></primary></indexterm>
2333 </term>
2334 <listitem>
2335 <para>Generate <literal>.hcr</literal> files.</para>
2336 </listitem>
2337 </varlistentry>
2338
2339 </variablelist>
2340
2341 <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>
2342
2343 </sect1>
2344
2345 &debug;
2346 &flags;
2347
2348 </chapter>
2349
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