Merge branch 'master' of darcs.haskell.org:/home/darcs/ghc
[ghc.git] / docs / users_guide / using.xml
1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="using-ghc">
3 <title>Using GHC</title>
4
5 <indexterm><primary>GHC, using</primary></indexterm>
6 <indexterm><primary>using GHC</primary></indexterm>
7
8 <sect1>
9 <title>Getting started: compiling programs</title>
10
11 <para>
12 In this chapter you'll find a complete reference to the GHC
13 command-line syntax, including all 400+ flags. It's a large and
14 complex system, and there are lots of details, so it can be
15 quite hard to figure out how to get started. With that in mind,
16 this introductory section provides a quick introduction to the
17 basic usage of GHC for compiling a Haskell program, before the
18 following sections dive into the full syntax.
19 </para>
20
21 <para>
22 Let's create a Hello World program, and compile and run it.
23 First, create a file <filename>hello.hs</filename> containing
24 the Haskell code:
25 </para>
26
27 <programlisting>
28 main = putStrLn "Hello, World!"
29 </programlisting>
30
31 <para>To compile the program, use GHC like this:</para>
32
33 <screen>
34 $ ghc hello.hs
35 </screen>
36
37 <para>(where <literal>$</literal> represents the prompt: don't
38 type it). GHC will compile the source
39 file <filename>hello.hs</filename>, producing
40 an <firstterm>object
41 file</firstterm> <filename>hello.o</filename> and
42 an <firstterm>interface
43 file</firstterm> <filename>hello.hi</filename>, and then it
44 will link the object file to the libraries that come with GHC
45 to produce an executable called <filename>hello</filename> on
46 Unix/Linux/Mac, or <filename>hello.exe</filename> on
47 Windows.</para>
48
49 <para>
50 By default GHC will be very quiet about what it is doing, only
51 printing error messages. If you want to see in more detail
52 what's going on behind the scenes, add <option>-v</option> to
53 the command line.
54 </para>
55
56 <para>
57 Then we can run the program like this:
58 </para>
59
60 <screen>
61 $ ./hello
62 Hello World!
63 </screen>
64
65 <para>
66 If your program contains multiple modules, then you only need to
67 tell GHC the name of the source file containing
68 the <filename>Main</filename> module, and GHC will examine
69 the <literal>import</literal> declarations to find the other
70 modules that make up the program and find their source files.
71 This means that, with the exception of
72 the <literal>Main</literal> module, every source file should be
73 named after the module name that it contains (with dots replaced
74 by directory separators). For example, the
75 module <literal>Data.Person</literal> would be in the
76 file <filename>Data/Person.hs</filename> on Unix/Linux/Mac,
77 or <filename>Data\Person.hs</filename> on Windows.
78 </para>
79 </sect1>
80
81 <sect1>
82 <title>Options overview</title>
83
84 <para>GHC's behaviour is controlled by
85 <firstterm>options</firstterm>, which for historical reasons are
86 also sometimes referred to as command-line flags or arguments.
87 Options can be specified in three ways:</para>
88
89 <sect2>
90 <title>Command-line arguments</title>
91
92 <indexterm><primary>structure, command-line</primary></indexterm>
93 <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
94 <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
95
96 <para>An invocation of GHC takes the following form:</para>
97
98 <screen>
99 ghc [argument...]
100 </screen>
101
102 <para>Command-line arguments are either options or file names.</para>
103
104 <para>Command-line options begin with <literal>-</literal>.
105 They may <emphasis>not</emphasis> be grouped:
106 <option>-vO</option> is different from <option>-v -O</option>.
107 Options need not precede filenames: e.g., <literal>ghc *.o -o
108 foo</literal>. All options are processed and then applied to
109 all files; you cannot, for example, invoke <literal>ghc -c -O1
110 Foo.hs -O2 Bar.hs</literal> to apply different optimisation
111 levels to the files <filename>Foo.hs</filename> and
112 <filename>Bar.hs</filename>.</para>
113 </sect2>
114
115 <sect2 id="source-file-options">
116 <title>Command line options in source files</title>
117
118 <indexterm><primary>source-file options</primary></indexterm>
119
120 <para>Sometimes it is useful to make the connection between a
121 source file and the command-line options it requires quite
122 tight. For instance, if a Haskell source file deliberately
123 uses name shadowing, it should be compiled with the
124 <option>-fno-warn-name-shadowing</option> option. Rather than maintaining
125 the list of per-file options in a <filename>Makefile</filename>,
126 it is possible to do this directly in the source file using the
127 <literal>OPTIONS_GHC</literal> pragma <indexterm><primary>OPTIONS_GHC
128 pragma</primary></indexterm>:</para>
129
130 <programlisting>
131 {-# OPTIONS_GHC -fno-warn-name-shadowing #-}
132 module X where
133 ...
134 </programlisting>
135
136 <para><literal>OPTIONS_GHC</literal> is a <emphasis>file-header pragma</emphasis>
137 (see <xref linkend="pragmas"/>).</para>
138
139 <para>Only <emphasis>dynamic</emphasis> flags can be used in an <literal>OPTIONS_GHC</literal> pragma
140 (see <xref linkend="static-dynamic-flags"/>).</para>
141
142 <para>Note that your command shell does not
143 get to the source file options, they are just included literally
144 in the array of command-line arguments the compiler
145 maintains internally, so you'll be desperately disappointed if
146 you try to glob etc. inside <literal>OPTIONS_GHC</literal>.</para>
147
148 <para>NOTE: the contents of OPTIONS_GHC are appended to the
149 command-line options, so options given in the source file
150 override those given on the command-line.</para>
151
152 <para>It is not recommended to move all the contents of your
153 Makefiles into your source files, but in some circumstances, the
154 <literal>OPTIONS_GHC</literal> pragma is the Right Thing. (If you
155 use <option>-keep-hc-file</option> and have OPTION flags in
156 your module, the OPTIONS_GHC will get put into the generated .hc
157 file).</para>
158 </sect2>
159
160 <sect2>
161 <title>Setting options in GHCi</title>
162
163 <para>Options may also be modified from within GHCi, using the
164 <literal>:set</literal> command. See <xref linkend="ghci-set"/>
165 for more details.</para>
166 </sect2>
167 </sect1>
168
169 <sect1 id="static-dynamic-flags">
170 <title>Static, Dynamic, and Mode options</title>
171 <indexterm><primary>static</primary><secondary>options</secondary>
172 </indexterm>
173 <indexterm><primary>dynamic</primary><secondary>options</secondary>
174 </indexterm>
175 <indexterm><primary>mode</primary><secondary>options</secondary>
176 </indexterm>
177
178 <para>Each of GHC's command line options is classified as
179 <firstterm>static</firstterm>, <firstterm>dynamic</firstterm> or
180 <firstterm>mode</firstterm>:</para>
181
182 <variablelist>
183 <varlistentry>
184 <term>Mode flags</term>
185 <listitem>
186 <para>For example, <option>&ndash;&ndash;make</option> or <option>-E</option>.
187 There may only be a single mode flag on the command line. The
188 available modes are listed in <xref linkend="modes"/>.</para>
189 </listitem>
190 </varlistentry>
191 <varlistentry>
192 <term>Dynamic Flags</term>
193 <listitem>
194 <para>Most non-mode flags fall into this category. A dynamic flag
195 may be used on the command line, in a
196 <literal>OPTIONS_GHC</literal> pragma in a source file, or set
197 using <literal>:set</literal> in GHCi.</para>
198 </listitem>
199 </varlistentry>
200 <varlistentry>
201 <term>Static Flags</term>
202 <listitem>
203 <para>A few flags are "static", which means they can only be used on
204 the command-line, and remain in force over the entire GHC/GHCi
205 run.</para>
206 </listitem>
207 </varlistentry>
208 </variablelist>
209
210 <para>The flag reference tables (<xref
211 linkend="flag-reference"/>) lists the status of each flag.</para>
212
213 <para>There are a few flags that are static except that they can
214 also be used with GHCi's <literal>:set</literal> command; these
215 are listed as &ldquo;static/<literal>:set</literal>&rdquo; in the
216 table.</para>
217 </sect1>
218
219 <sect1 id="file-suffixes">
220 <title>Meaningful file suffixes</title>
221
222 <indexterm><primary>suffixes, file</primary></indexterm>
223 <indexterm><primary>file suffixes for GHC</primary></indexterm>
224
225 <para>File names with &ldquo;meaningful&rdquo; suffixes (e.g.,
226 <filename>.lhs</filename> or <filename>.o</filename>) cause the
227 &ldquo;right thing&rdquo; to happen to those files.</para>
228
229 <variablelist>
230
231 <varlistentry>
232 <term><filename>.hs</filename></term>
233 <listitem>
234 <para>A Haskell module.</para>
235 </listitem>
236 </varlistentry>
237
238 <varlistentry>
239 <term>
240 <filename>.lhs</filename>
241 <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
242 </term>
243 <listitem>
244 <para>A &ldquo;literate Haskell&rdquo; module.</para>
245 </listitem>
246 </varlistentry>
247
248 <varlistentry>
249 <term><filename>.hi</filename></term>
250 <listitem>
251 <para>A Haskell interface file, probably
252 compiler-generated.</para>
253 </listitem>
254 </varlistentry>
255
256 <varlistentry>
257 <term><filename>.hc</filename></term>
258 <listitem>
259 <para>Intermediate C file produced by the Haskell
260 compiler.</para>
261 </listitem>
262 </varlistentry>
263
264 <varlistentry>
265 <term><filename>.c</filename></term>
266 <listitem>
267 <para>A C&nbsp;file not produced by the Haskell
268 compiler.</para>
269 </listitem>
270 </varlistentry>
271
272 <varlistentry>
273 <term><filename>.ll</filename></term>
274 <listitem>
275 <para>An llvm-intermediate-language source file, usually
276 produced by the compiler.</para>
277 </listitem>
278 </varlistentry>
279
280 <varlistentry>
281 <term><filename>.bc</filename></term>
282 <listitem>
283 <para>An llvm-intermediate-language bitcode file, usually
284 produced by the compiler.</para>
285 </listitem>
286 </varlistentry>
287
288 <varlistentry>
289 <term><filename>.s</filename></term>
290 <listitem>
291 <para>An assembly-language source file, usually produced by
292 the compiler.</para>
293 </listitem>
294 </varlistentry>
295
296 <varlistentry>
297 <term><filename>.o</filename></term>
298 <listitem>
299 <para>An object file, produced by an assembler.</para>
300 </listitem>
301 </varlistentry>
302 </variablelist>
303
304 <para>Files with other suffixes (or without suffixes) are passed
305 straight to the linker.</para>
306
307 </sect1>
308
309 <sect1 id="modes">
310 <title>Modes of operation</title>
311
312 <para>
313 GHC's behaviour is firstly controlled by a mode flag. Only one
314 of these flags may be given, but it does not necessarily need to
315 be the first option on the command-line.
316 </para>
317
318 <para>
319 If no mode flag is present, then GHC will enter make mode
320 (<xref linkend="make-mode" />) if there are any Haskell source
321 files given on the command line, or else it will link the
322 objects named on the command line to produce an executable.
323 </para>
324
325 <para>The available mode flags are:</para>
326
327 <variablelist>
328 <varlistentry>
329 <term>
330 <cmdsynopsis><command>ghc --interactive</command>
331 </cmdsynopsis>
332 <indexterm><primary>interactive mode</primary></indexterm>
333 <indexterm><primary>ghci</primary></indexterm>
334 </term>
335 <listitem>
336 <para>Interactive mode, which is also available as
337 <command>ghci</command>. Interactive mode is described in
338 more detail in <xref linkend="ghci"/>.</para>
339 </listitem>
340 </varlistentry>
341
342 <varlistentry>
343 <term>
344 <cmdsynopsis><command>ghc &ndash;&ndash;make</command>
345 </cmdsynopsis>
346 <indexterm><primary>make mode</primary></indexterm>
347 <indexterm><primary><option>&ndash;&ndash;make</option></primary></indexterm>
348 </term>
349 <listitem>
350 <para>In this mode, GHC will build a multi-module Haskell
351 program automatically, figuring out dependencies for itself.
352 If you have a straightforward Haskell program, this is
353 likely to be much easier, and faster, than using
354 <command>make</command>. Make mode is described in <xref
355 linkend="make-mode"/>.</para>
356
357 <para>
358 This mode is the default if there are any Haskell
359 source files mentioned on the command line, and in this case
360 the <option>&ndash;&ndash;make</option> option can be omitted.
361 </para>
362 </listitem>
363 </varlistentry>
364
365 <varlistentry>
366 <term>
367 <cmdsynopsis><command>ghc -e</command>
368 <arg choice='plain'><replaceable>expr</replaceable></arg>
369 </cmdsynopsis>
370 <indexterm><primary>eval mode</primary></indexterm>
371 </term>
372 <listitem>
373 <para>Expression-evaluation mode. This is very similar to
374 interactive mode, except that there is a single expression
375 to evaluate (<replaceable>expr</replaceable>) which is given
376 on the command line. See <xref linkend="eval-mode"/> for
377 more details.</para>
378 </listitem>
379 </varlistentry>
380
381 <varlistentry>
382 <term>
383 <cmdsynopsis>
384 <command>ghc -E</command>
385 <command>ghc -c</command>
386 <command>ghc -S</command>
387 <command>ghc -c</command>
388 </cmdsynopsis>
389 <indexterm><primary><option>-E</option></primary></indexterm>
390 <indexterm><primary><option>-C</option></primary></indexterm>
391 <indexterm><primary><option>-S</option></primary></indexterm>
392 <indexterm><primary><option>-c</option></primary></indexterm>
393 </term>
394 <listitem>
395 <para>This is the traditional batch-compiler mode, in which
396 GHC can compile source files one at a time, or link objects
397 together into an executable. This mode also applies if
398 there is no other mode flag specified on the command line,
399 in which case it means that the specified files should be
400 compiled and then linked to form a program. See <xref
401 linkend="options-order"/>.</para>
402 </listitem>
403 </varlistentry>
404
405 <varlistentry>
406 <term>
407 <cmdsynopsis>
408 <command>ghc -M</command>
409 </cmdsynopsis>
410 <indexterm><primary>dependency-generation mode</primary></indexterm>
411 </term>
412 <listitem>
413 <para>Dependency-generation mode. In this mode, GHC can be
414 used to generate dependency information suitable for use in
415 a <literal>Makefile</literal>. See <xref
416 linkend="makefile-dependencies"/>.</para>
417 </listitem>
418 </varlistentry>
419
420 <varlistentry>
421 <term>
422 <cmdsynopsis>
423 <command>ghc --mk-dll</command>
424 </cmdsynopsis>
425 <indexterm><primary>DLL-creation mode</primary></indexterm>
426 </term>
427 <listitem>
428 <para>DLL-creation mode (Windows only). See <xref
429 linkend="win32-dlls-create"/>.</para>
430 </listitem>
431 </varlistentry>
432
433 <varlistentry>
434 <term>
435 <cmdsynopsis>
436 <command>ghc --help</command> <command>ghc -?</command>
437 </cmdsynopsis>
438 <indexterm><primary><option>&ndash;&ndash;help</option></primary></indexterm>
439 </term>
440 <listitem>
441 <para>Cause GHC to spew a long usage message to standard
442 output and then exit.</para>
443 </listitem>
444 </varlistentry>
445
446 <varlistentry>
447 <term>
448 <cmdsynopsis>
449 <command>ghc --show-iface <replaceable>file</replaceable></command>
450 </cmdsynopsis>
451 <indexterm><primary><option>&ndash;&ndash;--show-iface</option></primary></indexterm>
452 </term>
453 <listitem>
454 <para>Read the interface in
455 <replaceable>file</replaceable> and dump it as text to
456 <literal>stdout</literal>. For example <literal>ghc --show-iface M.hi</literal>.</para>
457 </listitem>
458 </varlistentry>
459
460 <varlistentry>
461 <term>
462 <cmdsynopsis>
463 <command>ghc --supported-extensions</command>
464 <command>ghc --supported-languages</command>
465 </cmdsynopsis>
466 <indexterm><primary><option>&ndash;&ndash;supported-extensions</option></primary><primary><option>&ndash;&ndash;supported-languages</option></primary></indexterm>
467 </term>
468 <listitem>
469 <para>Print the supported language extensions.</para>
470 </listitem>
471 </varlistentry>
472
473 <varlistentry>
474 <term>
475 <cmdsynopsis>
476 <command>ghc --info</command>
477 </cmdsynopsis>
478 <indexterm><primary><option>&ndash;&ndash;info</option></primary></indexterm>
479 </term>
480 <listitem>
481 <para>Print information about the compiler.</para>
482 </listitem>
483 </varlistentry>
484
485 <varlistentry>
486 <term>
487 <cmdsynopsis>
488 <command>ghc --version</command>
489 <command>ghc -V</command>
490 </cmdsynopsis>
491 <indexterm><primary><option>-V</option></primary></indexterm>
492 <indexterm><primary><option>&ndash;&ndash;version</option></primary></indexterm>
493 </term>
494 <listitem>
495 <para>Print a one-line string including GHC's version number.</para>
496 </listitem>
497 </varlistentry>
498
499 <varlistentry>
500 <term>
501 <cmdsynopsis>
502 <command>ghc --numeric-version</command>
503 </cmdsynopsis>
504 <indexterm><primary><option>&ndash;&ndash;numeric-version</option></primary></indexterm>
505 </term>
506 <listitem>
507 <para>Print GHC's numeric version number only.</para>
508 </listitem>
509 </varlistentry>
510
511 <varlistentry>
512 <term>
513 <cmdsynopsis>
514 <command>ghc --print-libdir</command>
515 </cmdsynopsis>
516 <indexterm><primary><option>&ndash;&ndash;print-libdir</option></primary></indexterm>
517 </term>
518 <listitem>
519 <para>Print the path to GHC's library directory. This is
520 the top of the directory tree containing GHC's libraries,
521 interfaces, and include files (usually something like
522 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
523 is the value of
524 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
525 in the package configuration file
526 (see <xref linkend="packages"/>).</para>
527 </listitem>
528 </varlistentry>
529
530 </variablelist>
531
532 <sect2 id="make-mode">
533 <title>Using <command>ghc</command> <option>&ndash;&ndash;make</option></title>
534 <indexterm><primary><option>&ndash;&ndash;make</option></primary></indexterm>
535 <indexterm><primary>separate compilation</primary></indexterm>
536
537 <para>In this mode, GHC will build a multi-module Haskell program by following
538 dependencies from one or more root modules (usually just
539 <literal>Main</literal>). For example, if your
540 <literal>Main</literal> module is in a file called
541 <filename>Main.hs</filename>, you could compile and link the
542 program like this:</para>
543
544 <screen>
545 ghc &ndash;&ndash;make Main.hs
546 </screen>
547
548 <para>
549 In fact, GHC enters make mode automatically if there are any
550 Haskell source files on the command line and no other mode is
551 specified, so in this case we could just type
552 </para>
553
554 <screen>
555 ghc Main.hs
556 </screen>
557
558 <para>Any number of source file names or module names may be
559 specified; GHC will figure out all the modules in the program by
560 following the imports from these initial modules. It will then
561 attempt to compile each module which is out of date, and
562 finally, if there is a <literal>Main</literal> module, the
563 program will also be linked into an executable.</para>
564
565 <para>The main advantages to using <literal>ghc
566 &ndash;&ndash;make</literal> over traditional
567 <literal>Makefile</literal>s are:</para>
568
569 <itemizedlist>
570 <listitem>
571 <para>GHC doesn't have to be restarted for each compilation,
572 which means it can cache information between compilations.
573 Compiling a multi-module program with <literal>ghc
574 &ndash;&ndash;make</literal> can be up to twice as fast as
575 running <literal>ghc</literal> individually on each source
576 file.</para>
577 </listitem>
578 <listitem>
579 <para>You don't have to write a <literal>Makefile</literal>.</para>
580 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
581 </listitem>
582 <listitem>
583 <para>GHC re-calculates the dependencies each time it is
584 invoked, so the dependencies never get out of sync with the
585 source.</para>
586 </listitem>
587 </itemizedlist>
588
589 <para>Any of the command-line options described in the rest of
590 this chapter can be used with
591 <option>&ndash;&ndash;make</option>, but note that any options
592 you give on the command line will apply to all the source files
593 compiled, so if you want any options to apply to a single source
594 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
595 pragma (see <xref linkend="source-file-options"/>).</para>
596
597 <para>If the program needs to be linked with additional objects
598 (say, some auxiliary C code), then the object files can be
599 given on the command line and GHC will include them when linking
600 the executable.</para>
601
602 <para>Note that GHC can only follow dependencies if it has the
603 source file available, so if your program includes a module for
604 which there is no source file, even if you have an object and an
605 interface file for the module, then GHC will complain. The
606 exception to this rule is for package modules, which may or may
607 not have source files.</para>
608
609 <para>The source files for the program don't all need to be in
610 the same directory; the <option>-i</option> option can be used
611 to add directories to the search path (see <xref
612 linkend="search-path"/>).</para>
613 </sect2>
614
615 <sect2 id="eval-mode">
616 <title>Expression evaluation mode</title>
617
618 <para>This mode is very similar to interactive mode, except that
619 there is a single expression to evaluate which is specified on
620 the command line as an argument to the <option>-e</option>
621 option:</para>
622
623 <screen>
624 ghc -e <replaceable>expr</replaceable>
625 </screen>
626
627 <para>Haskell source files may be named on the command line, and
628 they will be loaded exactly as in interactive mode. The
629 expression is evaluated in the context of the loaded
630 modules.</para>
631
632 <para>For example, to load and run a Haskell program containing
633 a module <literal>Main</literal>, we might say</para>
634
635 <screen>
636 ghc -e Main.main Main.hs
637 </screen>
638
639 <para>or we can just use this mode to evaluate expressions in
640 the context of the <literal>Prelude</literal>:</para>
641
642 <screen>
643 $ ghc -e "interact (unlines.map reverse.lines)"
644 hello
645 olleh
646 </screen>
647 </sect2>
648
649 <sect2 id="options-order">
650 <title>Batch compiler mode</title>
651
652 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
653 given on the command line.</para>
654
655 <para>The first phase to run is determined by each input-file
656 suffix, and the last phase is determined by a flag. If no
657 relevant flag is present, then go all the way through to linking.
658 This table summarises:</para>
659
660 <informaltable>
661 <tgroup cols="4">
662 <colspec align="left"/>
663 <colspec align="left"/>
664 <colspec align="left"/>
665 <colspec align="left"/>
666
667 <thead>
668 <row>
669 <entry>Phase of the compilation system</entry>
670 <entry>Suffix saying &ldquo;start here&rdquo;</entry>
671 <entry>Flag saying &ldquo;stop after&rdquo;</entry>
672 <entry>(suffix of) output file</entry>
673 </row>
674 </thead>
675 <tbody>
676 <row>
677 <entry>literate pre-processor</entry>
678 <entry><literal>.lhs</literal></entry>
679 <entry>-</entry>
680 <entry><literal>.hs</literal></entry>
681 </row>
682
683 <row>
684 <entry>C pre-processor (opt.) </entry>
685 <entry><literal>.hs</literal> (with
686 <option>-cpp</option>)</entry>
687 <entry><option>-E</option></entry>
688 <entry><literal>.hspp</literal></entry>
689 </row>
690
691 <row>
692 <entry>Haskell compiler</entry>
693 <entry><literal>.hs</literal></entry>
694 <entry><option>-C</option>, <option>-S</option></entry>
695 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
696 </row>
697
698 <row>
699 <entry>C compiler (opt.)</entry>
700 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
701 <entry><option>-S</option></entry>
702 <entry><literal>.s</literal></entry>
703 </row>
704
705 <row>
706 <entry>assembler</entry>
707 <entry><literal>.s</literal></entry>
708 <entry><option>-c</option></entry>
709 <entry><literal>.o</literal></entry>
710 </row>
711
712 <row>
713 <entry>linker</entry>
714 <entry><replaceable>other</replaceable></entry>
715 <entry>-</entry>
716 <entry><filename>a.out</filename></entry>
717 </row>
718 </tbody>
719 </tgroup>
720 </informaltable>
721
722 <indexterm><primary><option>-C</option></primary></indexterm>
723 <indexterm><primary><option>-E</option></primary></indexterm>
724 <indexterm><primary><option>-S</option></primary></indexterm>
725 <indexterm><primary><option>-c</option></primary></indexterm>
726
727 <para>Thus, a common invocation would be: </para>
728
729 <screen>
730 ghc -c Foo.hs
731 </screen>
732
733 <para>to compile the Haskell source file
734 <filename>Foo.hs</filename> to an object file
735 <filename>Foo.o</filename>.</para>
736
737 <para>Note: What the Haskell compiler proper produces depends on what
738 backend code generator is used. See <xref linkend="code-generators"/>
739 for more details.</para>
740
741 <para>Note: C pre-processing is optional, the
742 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
743 flag turns it on. See <xref linkend="c-pre-processor"/> for more
744 details.</para>
745
746 <para>Note: The option <option>-E</option><indexterm><primary>-E
747 option</primary></indexterm> runs just the pre-processing passes
748 of the compiler, dumping the result in a file.</para>
749
750 <sect3 id="overriding-suffixes">
751 <title>Overriding the default behaviour for a file</title>
752
753 <para>As described above, the way in which a file is processed by GHC
754 depends on its suffix. This behaviour can be overridden using the
755 <option>-x</option> option:</para>
756
757 <variablelist>
758 <varlistentry>
759 <term><option>-x</option> <replaceable>suffix</replaceable>
760 <indexterm><primary><option>-x</option></primary>
761 </indexterm></term>
762 <listitem>
763 <para>Causes all files following this option on the command
764 line to be processed as if they had the suffix
765 <replaceable>suffix</replaceable>. For example, to compile a
766 Haskell module in the file <literal>M.my-hs</literal>,
767 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
768 </listitem>
769 </varlistentry>
770 </variablelist>
771 </sect3>
772
773 </sect2>
774 </sect1>
775
776 <sect1 id="options-help">
777 <title>Help and verbosity options</title>
778
779 <indexterm><primary>help options</primary></indexterm>
780 <indexterm><primary>verbosity options</primary></indexterm>
781
782 <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
783 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
784 <variablelist>
785 <varlistentry>
786 <term>
787 <option>-v</option>
788 <indexterm><primary><option>-v</option></primary></indexterm>
789 </term>
790 <listitem>
791 <para>The <option>-v</option> option makes GHC
792 <emphasis>verbose</emphasis>: it reports its version number
793 and shows (on stderr) exactly how it invokes each phase of
794 the compilation system. Moreover, it passes the
795 <option>-v</option> flag to most phases; each reports its
796 version number (and possibly some other information).</para>
797
798 <para>Please, oh please, use the <option>-v</option> option
799 when reporting bugs! Knowing that you ran the right bits in
800 the right order is always the first thing we want to
801 verify.</para>
802 </listitem>
803 </varlistentry>
804
805 <varlistentry>
806 <term>
807 <option>-v</option><replaceable>n</replaceable>
808 <indexterm><primary><option>-v</option></primary></indexterm>
809 </term>
810 <listitem>
811 <para>To provide more control over the compiler's verbosity,
812 the <option>-v</option> flag takes an optional numeric
813 argument. Specifying <option>-v</option> on its own is
814 equivalent to <option>-v3</option>, and the other levels
815 have the following meanings:</para>
816
817 <variablelist>
818 <varlistentry>
819 <term><option>-v0</option></term>
820 <listitem>
821 <para>Disable all non-essential messages (this is the
822 default).</para>
823 </listitem>
824 </varlistentry>
825
826 <varlistentry>
827 <term><option>-v1</option></term>
828 <listitem>
829 <para>Minimal verbosity: print one line per
830 compilation (this is the default when
831 <option>&ndash;&ndash;make</option> or
832 <option>&ndash;&ndash;interactive</option> is on).</para>
833 </listitem>
834 </varlistentry>
835
836 <varlistentry>
837 <term><option>-v2</option></term>
838 <listitem>
839 <para>Print the name of each compilation phase as it
840 is executed. (equivalent to
841 <option>-dshow-passes</option>).</para>
842 </listitem>
843 </varlistentry>
844
845 <varlistentry>
846 <term><option>-v3</option></term>
847 <listitem>
848 <para>The same as <option>-v2</option>, except that in
849 addition the full command line (if appropriate) for
850 each compilation phase is also printed.</para>
851 </listitem>
852 </varlistentry>
853
854 <varlistentry>
855 <term><option>-v4</option></term>
856 <listitem>
857 <para>The same as <option>-v3</option> except that the
858 intermediate program representation after each
859 compilation phase is also printed (excluding
860 preprocessed and C/assembly files).</para>
861 </listitem>
862 </varlistentry>
863 </variablelist>
864 </listitem>
865 </varlistentry>
866
867 <varlistentry>
868 <term><option>-ferror-spans</option>
869 <indexterm><primary><option>-ferror-spans</option></primary>
870 </indexterm>
871 </term>
872 <listitem>
873 <para>Causes GHC to emit the full source span of the
874 syntactic entity relating to an error message. Normally, GHC
875 emits the source location of the start of the syntactic
876 entity only.</para>
877
878 <para>For example:</para>
879
880 <screen>
881 test.hs:3:6: parse error on input `where'
882 </screen>
883
884 <para>becomes:</para>
885
886 <screen>
887 test296.hs:3:6-10: parse error on input `where'
888 </screen>
889
890 <para>And multi-line spans are possible too:</para>
891
892 <screen>
893 test.hs:(5,4)-(6,7):
894 Conflicting definitions for `a'
895 Bound at: test.hs:5:4
896 test.hs:6:7
897 In the binding group for: a, b, a
898 </screen>
899
900 <para>Note that line numbers start counting at one, but
901 column numbers start at zero. This choice was made to
902 follow existing convention (i.e. this is how Emacs does
903 it).</para>
904 </listitem>
905 </varlistentry>
906
907 <varlistentry>
908 <term><option>-H</option><replaceable>size</replaceable>
909 <indexterm><primary><option>-H</option></primary></indexterm>
910 </term>
911 <listitem>
912 <para>Set the minimum size of the heap to
913 <replaceable>size</replaceable>.
914 This option is equivalent to
915 <literal>+RTS&nbsp;-H<replaceable>size</replaceable></literal>,
916 see <xref linkend="rts-options-gc" />.
917 </para>
918 </listitem>
919 </varlistentry>
920
921 <varlistentry>
922 <term><option>-Rghc-timing</option>
923 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
924 </term>
925 <listitem>
926 <para>Prints a one-line summary of timing statistics for the
927 GHC run. This option is equivalent to
928 <literal>+RTS&nbsp;-tstderr</literal>, see <xref
929 linkend="rts-options-gc" />.
930 </para>
931 </listitem>
932 </varlistentry>
933 </variablelist>
934 </sect1>
935
936 &separate;
937
938 <sect1 id="options-sanity">
939 <title>Warnings and sanity-checking</title>
940
941 <indexterm><primary>sanity-checking options</primary></indexterm>
942 <indexterm><primary>warnings</primary></indexterm>
943
944
945 <para>GHC has a number of options that select which types of
946 non-fatal error messages, otherwise known as warnings, can be
947 generated during compilation. By default, you get a standard set
948 of warnings which are generally likely to indicate bugs in your
949 program. These are:
950 <option>-fwarn-overlapping-patterns</option>,
951 <option>-fwarn-warnings-deprecations</option>,
952 <option>-fwarn-deprecated-flags</option>,
953 <option>-fwarn-duplicate-exports</option>,
954 <option>-fwarn-missing-fields</option>,
955 <option>-fwarn-missing-methods</option>,
956 <option>-fwarn-lazy-unlifted-bindings</option>,
957 <option>-fwarn-wrong-do-bind</option>,
958 <option>-fwarn-unsupported-calling-conventions</option>, and
959 <option>-fwarn-dodgy-foreign-imports</option>. The following
960 flags are
961 simple ways to select standard &ldquo;packages&rdquo; of warnings:
962 </para>
963
964 <variablelist>
965
966 <varlistentry>
967 <term><option>-W</option>:</term>
968 <listitem>
969 <indexterm><primary>-W option</primary></indexterm>
970 <para>Provides the standard warnings plus
971 <option>-fwarn-incomplete-patterns</option>,
972 <option>-fwarn-dodgy-exports</option>,
973 <option>-fwarn-dodgy-imports</option>,
974 <option>-fwarn-unused-matches</option>,
975 <option>-fwarn-unused-imports</option>, and
976 <option>-fwarn-unused-binds</option>.</para>
977 </listitem>
978 </varlistentry>
979
980 <varlistentry>
981 <term><option>-Wall</option>:</term>
982 <listitem>
983 <indexterm><primary><option>-Wall</option></primary></indexterm>
984 <para>Turns on all warning options that indicate potentially
985 suspicious code. The warnings that are
986 <emphasis>not</emphasis> enabled by <option>-Wall</option>
987 are
988 <option>-fwarn-tabs</option>,
989 <option>-fwarn-incomplete-uni-patterns</option>,
990 <option>-fwarn-incomplete-record-updates</option>,
991 <option>-fwarn-monomorphism-restriction</option>,
992 <option>-fwarn-auto-orphans</option>,
993 <option>-fwarn-implicit-prelude</option>,
994 <option>-fwarn-missing-local-sigs</option>,
995 <option>-fwarn-missing-import-lists</option>.</para>
996 </listitem>
997 </varlistentry>
998
999 <varlistentry>
1000 <term><option>-w</option>:</term>
1001 <listitem>
1002 <indexterm><primary><option>-w</option></primary></indexterm>
1003 <para>Turns off all warnings, including the standard ones and
1004 those that <literal>-Wall</literal> doesn't enable.</para>
1005 </listitem>
1006 </varlistentry>
1007
1008 <varlistentry>
1009 <term><option>-Werror</option>:</term>
1010 <listitem>
1011 <indexterm><primary><option>-Werror</option></primary></indexterm>
1012 <para>Makes any warning into a fatal error. Useful so that you don't
1013 miss warnings when doing batch compilation. </para>
1014 </listitem>
1015 </varlistentry>
1016
1017 <varlistentry>
1018 <term><option>-Wwarn</option>:</term>
1019 <listitem>
1020 <indexterm><primary><option>-Wwarn</option></primary></indexterm>
1021 <para>Warnings are treated only as warnings, not as errors. This is
1022 the default, but can be useful to negate a
1023 <option>-Werror</option> flag.</para>
1024 </listitem>
1025 </varlistentry>
1026
1027 </variablelist>
1028
1029 <para>The full set of warning options is described below. To turn
1030 off any warning, simply give the corresponding
1031 <option>-fno-warn-...</option> option on the command line.</para>
1032
1033 <variablelist>
1034
1035 <varlistentry>
1036 <term><option>-fdefer-type-errors</option>:</term>
1037 <listitem>
1038 <indexterm><primary><option>-fdefer-type-errors</option></primary>
1039 </indexterm>
1040 <indexterm><primary>warnings</primary></indexterm>
1041 <para>Defer as many type errors as possible until runtime.
1042 At compile time you get a warning (instead of an error). At
1043 runtime, if you use a value that depends on a type error, you
1044 get a runtime error; but you can run any type-correct parts of your code
1045 just fine.</para>
1046 </listitem>
1047 </varlistentry>
1048
1049 <varlistentry>
1050 <term><option>-fhelpful-errors</option>:</term>
1051 <listitem>
1052 <indexterm><primary><option>-fhelpful-errors</option></primary>
1053 </indexterm>
1054 <indexterm><primary>warnings</primary></indexterm>
1055 <para>When a name or package is not found in scope, make
1056 suggestions for the name or package you might have meant instead.</para>
1057 <para>This option is on by default.</para>
1058 </listitem>
1059 </varlistentry>
1060
1061 <varlistentry>
1062 <term><option>-fwarn-unrecognised-pragmas</option>:</term>
1063 <listitem>
1064 <indexterm><primary><option>-fwarn-unrecognised-pragmas</option></primary>
1065 </indexterm>
1066 <indexterm><primary>warnings</primary></indexterm>
1067 <indexterm><primary>pragmas</primary></indexterm>
1068 <para>Causes a warning to be emitted when a
1069 pragma that GHC doesn't recognise is used. As well as pragmas
1070 that GHC itself uses, GHC also recognises pragmas known to be used
1071 by other tools, e.g. <literal>OPTIONS_HUGS</literal> and
1072 <literal>DERIVE</literal>.</para>
1073
1074 <para>This option is on by default.</para>
1075 </listitem>
1076 </varlistentry>
1077
1078 <varlistentry>
1079 <term><option>-fwarn-warnings-deprecations</option>:</term>
1080 <listitem>
1081 <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
1082 </indexterm>
1083 <indexterm><primary>warnings</primary></indexterm>
1084 <indexterm><primary>deprecations</primary></indexterm>
1085 <para>Causes a warning to be emitted when a
1086 module, function or type with a WARNING or DEPRECATED pragma
1087 is used. See <xref linkend="warning-deprecated-pragma"/> for more
1088 details on the pragmas.</para>
1089
1090 <para>This option is on by default.</para>
1091 </listitem>
1092 </varlistentry>
1093
1094 <varlistentry>
1095 <term><option>-fwarn-deprecated-flags</option>:</term>
1096 <listitem>
1097 <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
1098 </indexterm>
1099 <indexterm><primary>deprecated-flags</primary></indexterm>
1100 <para>Causes a warning to be emitted when a deprecated
1101 commandline flag is used.</para>
1102
1103 <para>This option is on by default.</para>
1104 </listitem>
1105 </varlistentry>
1106
1107 <varlistentry>
1108 <term><option>-fwarn-unsupported-calling-conventions</option>:</term>
1109 <listitem>
1110 <indexterm><primary><option>-fwarn-unsupported-calling-conventions</option></primary>
1111 </indexterm>
1112 <para>Causes a warning to be emitted for foreign declarations
1113 that use unsupported calling conventions. In particular,
1114 if the <literal>stdcall</literal> calling convention is used
1115 on an architecture other than i386 then it will be treated
1116 as <literal>ccall</literal>.</para>
1117 </listitem>
1118 </varlistentry>
1119
1120 <varlistentry>
1121 <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
1122 <listitem>
1123 <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
1124 </indexterm>
1125 <para>Causes a warning to be emitted for foreign imports of
1126 the following form:</para>
1127
1128 <programlisting>
1129 foreign import "f" f :: FunPtr t
1130 </programlisting>
1131
1132 <para>on the grounds that it probably should be</para>
1133
1134 <programlisting>
1135 foreign import "&amp;f" f :: FunPtr t
1136 </programlisting>
1137
1138 <para>The first form declares that `f` is a (pure) C
1139 function that takes no arguments and returns a pointer to a
1140 C function with type `t`, whereas the second form declares
1141 that `f` itself is a C function with type `t`. The first
1142 declaration is usually a mistake, and one that is hard to
1143 debug because it results in a crash, hence this
1144 warning.</para>
1145 </listitem>
1146 </varlistentry>
1147
1148 <varlistentry>
1149 <term><option>-fwarn-dodgy-exports</option>:</term>
1150 <listitem>
1151 <indexterm><primary><option>-fwarn-dodgy-exports</option></primary>
1152 </indexterm>
1153 <para>Causes a warning to be emitted when a datatype
1154 <literal>T</literal> is exported
1155 with all constructors, i.e. <literal>T(..)</literal>, but is it
1156 just a type synonym.</para>
1157 <para>Also causes a warning to be emitted when a module is
1158 re-exported, but that module exports nothing.</para>
1159 </listitem>
1160 </varlistentry>
1161
1162 <varlistentry>
1163 <term><option>-fwarn-dodgy-imports</option>:</term>
1164 <listitem>
1165 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
1166 </indexterm>
1167 <para>Causes a warning to be emitted when a datatype
1168 <literal>T</literal> is imported
1169 with all constructors, i.e. <literal>T(..)</literal>, but has been
1170 exported abstractly, i.e. <literal>T</literal>.</para>
1171 </listitem>
1172 </varlistentry>
1173
1174 <varlistentry>
1175 <term><option>-fwarn-lazy-unlifted-bindings</option>:</term>
1176 <listitem>
1177 <indexterm><primary><option>-fwarn-lazy-unlifted-bindings</option></primary>
1178 </indexterm>
1179 <para>Causes a warning to be emitted when an unlifted type
1180 is bound in a way that looks lazy, e.g.
1181 <literal>where (I# x) = ...</literal>. Use
1182 <literal>where !(I# x) = ...</literal> instead. This will be an
1183 error, rather than a warning, in GHC 7.2.
1184 </para>
1185 </listitem>
1186 </varlistentry>
1187
1188 <varlistentry>
1189 <term><option>-fwarn-duplicate-exports</option>:</term>
1190 <listitem>
1191 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
1192 <indexterm><primary>duplicate exports, warning</primary></indexterm>
1193 <indexterm><primary>export lists, duplicates</primary></indexterm>
1194
1195 <para>Have the compiler warn about duplicate entries in
1196 export lists. This is useful information if you maintain
1197 large export lists, and want to avoid the continued export
1198 of a definition after you've deleted (one) mention of it in
1199 the export list.</para>
1200
1201 <para>This option is on by default.</para>
1202 </listitem>
1203 </varlistentry>
1204
1205 <varlistentry>
1206 <term><option>-fwarn-hi-shadowing</option>:</term>
1207 <listitem>
1208 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
1209 <indexterm><primary>shadowing</primary>
1210 <secondary>interface files</secondary></indexterm>
1211
1212 <para>Causes the compiler to emit a warning when a module or
1213 interface file in the current directory is shadowing one
1214 with the same module name in a library or other
1215 directory.</para>
1216 </listitem>
1217 </varlistentry>
1218
1219 <varlistentry>
1220 <term><option>-fwarn-identities</option>:</term>
1221 <listitem>
1222 <indexterm><primary><option>-fwarn-identities</option></primary></indexterm>
1223 <para>Causes the compiler to emit a warning when a Prelude numeric
1224 conversion converts a type T to the same type T; such calls
1225 are probably no-ops and can be omitted. The functions checked for
1226 are: <literal>toInteger</literal>,
1227 <literal>toRational</literal>,
1228 <literal>fromIntegral</literal>,
1229 and <literal>realToFrac</literal>.
1230 </para>
1231 </listitem>
1232 </varlistentry>
1233
1234 <varlistentry>
1235 <term><option>-fwarn-implicit-prelude</option>:</term>
1236 <listitem>
1237 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
1238 <indexterm><primary>implicit prelude, warning</primary></indexterm>
1239 <para>Have the compiler warn if the Prelude is implicitly
1240 imported. This happens unless either the Prelude module is
1241 explicitly imported with an <literal>import ... Prelude ...</literal>
1242 line, or this implicit import is disabled (either by
1243 <option>-XNoImplicitPrelude</option> or a
1244 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
1245
1246 <para>Note that no warning is given for syntax that implicitly
1247 refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
1248 would change whether it refers to the Prelude.
1249 For example, no warning is given when
1250 <literal>368</literal> means
1251 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
1252 (where <literal>Prelude</literal> refers to the actual Prelude module,
1253 regardless of the imports of the module being compiled).</para>
1254
1255 <para>This warning is off by default.</para>
1256 </listitem>
1257 </varlistentry>
1258
1259 <varlistentry>
1260 <term><option>-fwarn-incomplete-patterns</option>,
1261 <option>-fwarn-incomplete-uni-patterns</option>:
1262 </term>
1263 <listitem>
1264 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
1265 <indexterm><primary><option>-fwarn-incomplete-uni-patterns</option></primary></indexterm>
1266 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
1267 <indexterm><primary>patterns, incomplete</primary></indexterm>
1268
1269 <para>The option <option>-fwarn-incomplete-patterns</option> warns
1270 about places where
1271 a pattern-match might fail at runtime.
1272 The function
1273 <function>g</function> below will fail when applied to
1274 non-empty lists, so the compiler will emit a warning about
1275 this when <option>-fwarn-incomplete-patterns</option> is
1276 enabled.
1277
1278 <programlisting>
1279 g [] = 2
1280 </programlisting>
1281
1282 This option isn't enabled by default because it can be
1283 a bit noisy, and it doesn't always indicate a bug in the
1284 program. However, it's generally considered good practice
1285 to cover all the cases in your functions, and it is switched
1286 on by <option>-W</option>.</para>
1287
1288 <para>The flag <option>-fwarn-incomplete-uni-patterns</option> is
1289 similar, except that it
1290 applies only to lambda-expressions and pattern bindings, constructs
1291 that only allow a single pattern:
1292
1293 <programlisting>
1294 h = \[] -> 2
1295 Just k = f y
1296 </programlisting>
1297
1298 </para>
1299 </listitem>
1300 </varlistentry>
1301
1302 <varlistentry>
1303 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1304 <listitem>
1305 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1306 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1307 <indexterm><primary>record updates, incomplete</primary></indexterm>
1308
1309 <para>The function
1310 <function>f</function> below will fail when applied to
1311 <literal>Bar</literal>, so the compiler will emit a warning about
1312 this when <option>-fwarn-incomplete-record-updates</option> is
1313 enabled.</para>
1314
1315 <programlisting>
1316 data Foo = Foo { x :: Int }
1317 | Bar
1318
1319 f :: Foo -> Foo
1320 f foo = foo { x = 6 }
1321 </programlisting>
1322
1323 <para>This option isn't enabled by default because it can be
1324 very noisy, and it often doesn't indicate a bug in the
1325 program.</para>
1326 </listitem>
1327 </varlistentry>
1328
1329 <varlistentry>
1330 <term>
1331 <option>-fwarn-missing-fields</option>:
1332 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1333 <indexterm><primary>missing fields, warning</primary></indexterm>
1334 <indexterm><primary>fields, missing</primary></indexterm>
1335 </term>
1336 <listitem>
1337
1338 <para>This option is on by default, and warns you whenever
1339 the construction of a labelled field constructor isn't
1340 complete, missing initializers for one or more fields. While
1341 not an error (the missing fields are initialised with
1342 bottoms), it is often an indication of a programmer error.</para>
1343 </listitem>
1344 </varlistentry>
1345
1346 <varlistentry>
1347 <term>
1348 <option>-fwarn-missing-import-lists</option>:
1349 <indexterm><primary><option>-fwarn-import-lists</option></primary></indexterm>
1350 <indexterm><primary>missing import lists, warning</primary></indexterm>
1351 <indexterm><primary>import lists, missing</primary></indexterm>
1352 </term>
1353 <listitem>
1354
1355 <para>This flag warns if you use an unqualified
1356 <literal>import</literal> declaration
1357 that does not explicitly list the entities brought into scope. For
1358 example
1359 </para>
1360
1361 <programlisting>
1362 module M where
1363 import X( f )
1364 import Y
1365 import qualified Z
1366 p x = f x x
1367 </programlisting>
1368
1369 <para>
1370 The <option>-fwarn-import-lists</option> flag will warn about the import
1371 of <literal>Y</literal> but not <literal>X</literal>
1372 If module <literal>Y</literal> is later changed to export (say) <literal>f</literal>,
1373 then the reference to <literal>f</literal> in <literal>M</literal> will become
1374 ambiguous. No warning is produced for the import of <literal>Z</literal>
1375 because extending <literal>Z</literal>'s exports would be unlikely to produce
1376 ambiguity in <literal>M</literal>.
1377 </para>
1378 </listitem>
1379 </varlistentry>
1380
1381 <varlistentry>
1382 <term><option>-fwarn-missing-methods</option>:</term>
1383 <listitem>
1384 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1385 <indexterm><primary>missing methods, warning</primary></indexterm>
1386 <indexterm><primary>methods, missing</primary></indexterm>
1387
1388 <para>This option is on by default, and warns you whenever
1389 an instance declaration is missing one or more methods, and
1390 the corresponding class declaration has no default
1391 declaration for them.</para>
1392 <para>The warning is suppressed if the method name
1393 begins with an underscore. Here's an example where this is useful:
1394 <programlisting>
1395 class C a where
1396 _simpleFn :: a -> String
1397 complexFn :: a -> a -> String
1398 complexFn x y = ... _simpleFn ...
1399 </programlisting>
1400 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1401 never <literal>_simpleFn</literal>; and (b)
1402 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1403 </para>
1404 </listitem>
1405 </varlistentry>
1406
1407 <varlistentry>
1408 <term><option>-fwarn-missing-signatures</option>:</term>
1409 <listitem>
1410 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1411 <indexterm><primary>type signatures, missing</primary></indexterm>
1412
1413 <para>If you would like GHC to check that every top-level
1414 function/value has a type signature, use the
1415 <option>-fwarn-missing-signatures</option> option. As part of
1416 the warning GHC also reports the inferred type. The
1417 option is off by default.</para>
1418 </listitem>
1419 </varlistentry>
1420
1421 <varlistentry>
1422 <term><option>-fwarn-missing-local-sigs</option>:</term>
1423 <listitem>
1424 <indexterm><primary><option>-fwarn-missing-local-sigs</option></primary></indexterm>
1425 <indexterm><primary>type signatures, missing</primary></indexterm>
1426
1427 <para>If you use the
1428 <option>-fwarn-missing-local-sigs</option> flag GHC will warn
1429 you about any polymorphic local bindings. As part of
1430 the warning GHC also reports the inferred type. The
1431 option is off by default.</para>
1432 </listitem>
1433 </varlistentry>
1434
1435 <varlistentry>
1436 <term><option>-fwarn-name-shadowing</option>:</term>
1437 <listitem>
1438 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1439 <indexterm><primary>shadowing, warning</primary></indexterm>
1440
1441 <para>This option causes a warning to be emitted whenever an
1442 inner-scope value has the same name as an outer-scope value,
1443 i.e. the inner value shadows the outer one. This can catch
1444 typographical errors that turn into hard-to-find bugs, e.g.,
1445 in the inadvertent capture of what would be a recursive call in
1446 <literal>f = ... let f = id in ... f ...</literal>.</para>
1447 <para>The warning is suppressed for names beginning with an underscore. For example
1448 <programlisting>
1449 f x = do { _ignore &lt;- this; _ignore &lt;- that; return (the other) }
1450 </programlisting>
1451 </para>
1452 </listitem>
1453 </varlistentry>
1454
1455 <varlistentry>
1456 <term><option>-fwarn-orphans, -fwarn-auto-orphans</option>:</term>
1457 <listitem>
1458 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1459 <indexterm><primary><option>-fwarn-auto-orphans</option></primary></indexterm>
1460 <indexterm><primary>orphan instances, warning</primary></indexterm>
1461 <indexterm><primary>orphan rules, warning</primary></indexterm>
1462
1463 <para>These flags cause a warning to be emitted whenever the
1464 module contains an "orphan" instance declaration or rewrite rule.
1465 An instance declaration is an orphan if it appears in a module in
1466 which neither the class nor the type being instanced are declared
1467 in the same module. A rule is an orphan if it is a rule for a
1468 function declared in another module. A module containing any
1469 orphans is called an orphan module.</para>
1470 <para>The trouble with orphans is that GHC must pro-actively read the interface
1471 files for all orphan modules, just in case their instances or rules
1472 play a role, whether or not the module's interface would otherwise
1473 be of any use. See <xref linkend="orphan-modules"/> for details.
1474 </para>
1475 <para>The flag <option>-fwarn-orphans</option> warns about user-written
1476 orphan rules or instances. The flag <option>-fwarn-auto-orphans</option>
1477 warns about automatically-generated orphan rules, notably as a result of
1478 specialising functions, for type classes (<literal>Specialise</literal>)
1479 or argument values (<literal>SpecConstr</literal>).</para>
1480 </listitem>
1481 </varlistentry>
1482
1483 <varlistentry>
1484 <term>
1485 <option>-fwarn-overlapping-patterns</option>:
1486 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1487 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1488 <indexterm><primary>patterns, overlapping</primary></indexterm>
1489 </term>
1490 <listitem>
1491 <para>By default, the compiler will warn you if a set of
1492 patterns are overlapping, e.g.,</para>
1493
1494 <programlisting>
1495 f :: String -&#62; Int
1496 f [] = 0
1497 f (_:xs) = 1
1498 f "2" = 2
1499 </programlisting>
1500
1501 <para>where the last pattern match in <function>f</function>
1502 won't ever be reached, as the second pattern overlaps
1503 it. More often than not, redundant patterns is a programmer
1504 mistake/error, so this option is enabled by default.</para>
1505 </listitem>
1506 </varlistentry>
1507
1508 <varlistentry>
1509 <term><option>-fwarn-tabs</option>:</term>
1510 <listitem>
1511 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1512 <indexterm><primary>tabs, warning</primary></indexterm>
1513 <para>Have the compiler warn if there are tabs in your source
1514 file.</para>
1515
1516 <para>This warning is off by default.</para>
1517 </listitem>
1518 </varlistentry>
1519
1520 <varlistentry>
1521 <term><option>-fwarn-type-defaults</option>:</term>
1522 <listitem>
1523 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1524 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1525 <para>Have the compiler warn/inform you where in your source
1526 the Haskell defaulting mechanism for numeric types kicks
1527 in. This is useful information when converting code from a
1528 context that assumed one default into one with another,
1529 e.g., the &lsquo;default default&rsquo; for Haskell 1.4 caused the
1530 otherwise unconstrained value <constant>1</constant> to be
1531 given the type <literal>Int</literal>, whereas Haskell 98
1532 and later
1533 defaults it to <literal>Integer</literal>. This may lead to
1534 differences in performance and behaviour, hence the
1535 usefulness of being non-silent about this.</para>
1536
1537 <para>This warning is off by default.</para>
1538 </listitem>
1539 </varlistentry>
1540
1541 <varlistentry>
1542 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1543 <listitem>
1544 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1545 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1546 <para>Have the compiler warn/inform you where in your source
1547 the Haskell Monomorphism Restriction is applied. If applied silently
1548 the MR can give rise to unexpected behaviour, so it can be helpful
1549 to have an explicit warning that it is being applied.</para>
1550
1551 <para>This warning is off by default.</para>
1552 </listitem>
1553 </varlistentry>
1554
1555 <varlistentry>
1556 <term><option>-fwarn-unused-binds</option>:</term>
1557 <listitem>
1558 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1559 <indexterm><primary>unused binds, warning</primary></indexterm>
1560 <indexterm><primary>binds, unused</primary></indexterm>
1561 <para>Report any function definitions (and local bindings)
1562 which are unused. For top-level functions, the warning is
1563 only given if the binding is not exported.</para>
1564 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1565 mentioned in the right hand side of another definition that is used, or (c) the
1566 function it defines begins with an underscore. The last case provides a
1567 way to suppress unused-binding warnings selectively. </para>
1568 <para> Notice that a variable
1569 is reported as unused even if it appears in the right-hand side of another
1570 unused binding. </para>
1571 </listitem>
1572 </varlistentry>
1573
1574 <varlistentry>
1575 <term><option>-fwarn-unused-imports</option>:</term>
1576 <listitem>
1577 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1578 <indexterm><primary>unused imports, warning</primary></indexterm>
1579 <indexterm><primary>imports, unused</primary></indexterm>
1580
1581 <para>Report any modules that are explicitly imported but
1582 never used. However, the form <literal>import M()</literal> is
1583 never reported as an unused import, because it is a useful idiom
1584 for importing instance declarations, which are anonymous in Haskell.</para>
1585 </listitem>
1586 </varlistentry>
1587
1588 <varlistentry>
1589 <term><option>-fwarn-unused-matches</option>:</term>
1590 <listitem>
1591 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1592 <indexterm><primary>unused matches, warning</primary></indexterm>
1593 <indexterm><primary>matches, unused</primary></indexterm>
1594
1595 <para>Report all unused variables which arise from pattern
1596 matches, including patterns consisting of a single variable.
1597 For instance <literal>f x y = []</literal> would report
1598 <varname>x</varname> and <varname>y</varname> as unused. The
1599 warning is suppressed if the variable name begins with an underscore, thus:
1600 <programlisting>
1601 f _x = True
1602 </programlisting>
1603 </para>
1604 </listitem>
1605 </varlistentry>
1606
1607 <varlistentry>
1608 <term><option>-fwarn-unused-do-bind</option>:</term>
1609 <listitem>
1610 <indexterm><primary><option>-fwarn-unused-do-bind</option></primary></indexterm>
1611 <indexterm><primary>unused do binding, warning</primary></indexterm>
1612 <indexterm><primary>do binding, unused</primary></indexterm>
1613
1614 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1615 that appear to silently throw information away.
1616 For instance <literal>do { mapM popInt xs ; return 10 }</literal> would report
1617 the first statement in the <literal>do</literal> block as suspicious,
1618 as it has the type <literal>StackM [Int]</literal> and not <literal>StackM ()</literal>, but that
1619 <literal>[Int]</literal> value is not bound to anything. The warning is suppressed by
1620 explicitly mentioning in the source code that your program is throwing something away:
1621 <programlisting>
1622 do { _ &lt;- mapM popInt xs ; return 10 }
1623 </programlisting>
1624 Of course, in this particular situation you can do even better:
1625 <programlisting>
1626 do { mapM_ popInt xs ; return 10 }
1627 </programlisting>
1628 </para>
1629 </listitem>
1630 </varlistentry>
1631
1632 <varlistentry>
1633 <term><option>-fwarn-wrong-do-bind</option>:</term>
1634 <listitem>
1635 <indexterm><primary><option>-fwarn-wrong-do-bind</option></primary></indexterm>
1636 <indexterm><primary>apparently erroneous do binding, warning</primary></indexterm>
1637 <indexterm><primary>do binding, apparently erroneous</primary></indexterm>
1638
1639 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1640 that appear to lack a binding.
1641 For instance <literal>do { return (popInt 10) ; return 10 }</literal> would report
1642 the first statement in the <literal>do</literal> block as suspicious,
1643 as it has the type <literal>StackM (StackM Int)</literal> (which consists of two nested applications
1644 of the same monad constructor), but which is not then &quot;unpacked&quot; by binding the result.
1645 The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away:
1646 <programlisting>
1647 do { _ &lt;- return (popInt 10) ; return 10 }
1648 </programlisting>
1649 For almost all sensible programs this will indicate a bug, and you probably intended to write:
1650 <programlisting>
1651 do { popInt 10 ; return 10 }
1652 </programlisting>
1653 </para>
1654 </listitem>
1655 </varlistentry>
1656
1657 </variablelist>
1658
1659 <para>If you're feeling really paranoid, the
1660 <option>-dcore-lint</option>
1661 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1662 is a good choice. It turns on heavyweight intra-pass
1663 sanity-checking within GHC. (It checks GHC's sanity, not
1664 yours.)</para>
1665
1666 </sect1>
1667
1668 &packages;
1669
1670 <sect1 id="options-optimise">
1671 <title>Optimisation (code improvement)</title>
1672
1673 <indexterm><primary>optimisation</primary></indexterm>
1674 <indexterm><primary>improvement, code</primary></indexterm>
1675
1676 <para>The <option>-O*</option> options specify convenient
1677 &ldquo;packages&rdquo; of optimisation flags; the
1678 <option>-f*</option> options described later on specify
1679 <emphasis>individual</emphasis> optimisations to be turned on/off;
1680 the <option>-m*</option> options specify
1681 <emphasis>machine-specific</emphasis> optimisations to be turned
1682 on/off.</para>
1683
1684 <sect2 id="optimise-pkgs">
1685 <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1686
1687 <para>There are <emphasis>many</emphasis> options that affect
1688 the quality of code produced by GHC. Most people only have a
1689 general goal, something like &ldquo;Compile quickly&rdquo; or
1690 &ldquo;Make my program run like greased lightning.&rdquo; The
1691 following &ldquo;packages&rdquo; of optimisations (or lack
1692 thereof) should suffice.</para>
1693
1694 <para>Note that higher optimisation levels cause more
1695 cross-module optimisation to be performed, which can have an
1696 impact on how much of your program needs to be recompiled when
1697 you change something. This is one reason to stick to
1698 no-optimisation when developing code.</para>
1699
1700 <variablelist>
1701
1702 <varlistentry>
1703 <term>
1704 No <option>-O*</option>-type option specified:
1705 <indexterm><primary>-O* not specified</primary></indexterm>
1706 </term>
1707 <listitem>
1708 <para>This is taken to mean: &ldquo;Please compile
1709 quickly; I'm not over-bothered about compiled-code
1710 quality.&rdquo; So, for example: <command>ghc -c
1711 Foo.hs</command></para>
1712 </listitem>
1713 </varlistentry>
1714
1715 <varlistentry>
1716 <term>
1717 <option>-O0</option>:
1718 <indexterm><primary><option>-O0</option></primary></indexterm>
1719 </term>
1720 <listitem>
1721 <para>Means &ldquo;turn off all optimisation&rdquo;,
1722 reverting to the same settings as if no
1723 <option>-O</option> options had been specified. Saying
1724 <option>-O0</option> can be useful if
1725 eg. <command>make</command> has inserted a
1726 <option>-O</option> on the command line already.</para>
1727 </listitem>
1728 </varlistentry>
1729
1730 <varlistentry>
1731 <term>
1732 <option>-O</option> or <option>-O1</option>:
1733 <indexterm><primary>-O option</primary></indexterm>
1734 <indexterm><primary>-O1 option</primary></indexterm>
1735 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1736 </term>
1737 <listitem>
1738 <para>Means: &ldquo;Generate good-quality code without
1739 taking too long about it.&rdquo; Thus, for example:
1740 <command>ghc -c -O Main.lhs</command></para>
1741 </listitem>
1742 </varlistentry>
1743
1744 <varlistentry>
1745 <term>
1746 <option>-O2</option>:
1747 <indexterm><primary>-O2 option</primary></indexterm>
1748 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1749 </term>
1750 <listitem>
1751 <para>Means: &ldquo;Apply every non-dangerous
1752 optimisation, even if it means significantly longer
1753 compile times.&rdquo;</para>
1754
1755 <para>The avoided &ldquo;dangerous&rdquo; optimisations
1756 are those that can make runtime or space
1757 <emphasis>worse</emphasis> if you're unlucky. They are
1758 normally turned on or off individually.</para>
1759
1760 <para>At the moment, <option>-O2</option> is
1761 <emphasis>unlikely</emphasis> to produce better code than
1762 <option>-O</option>.</para>
1763 </listitem>
1764 </varlistentry>
1765 </variablelist>
1766
1767 <para>We don't use a <option>-O*</option> flag for day-to-day
1768 work. We use <option>-O</option> to get respectable speed;
1769 e.g., when we want to measure something. When we want to go for
1770 broke, we tend to use <option>-O2</option> (and we go for
1771 lots of coffee breaks).</para>
1772
1773 <para>The easiest way to see what <option>-O</option> (etc.)
1774 &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1775 then stand back in amazement.</para>
1776 </sect2>
1777
1778 <sect2 id="options-f">
1779 <title><option>-f*</option>: platform-independent flags</title>
1780
1781 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1782 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1783
1784 <para>These flags turn on and off individual optimisations.
1785 They are normally set via the <option>-O</option> options
1786 described above, and as such, you shouldn't need to set any of
1787 them explicitly (indeed, doing so could lead to unexpected
1788 results). A flag <option>-fwombat</option> can be negated by
1789 saying <option>-fno-wombat</option>. The flags below are off
1790 by default, except where noted below.
1791 </para>
1792
1793 <variablelist>
1794 <varlistentry>
1795 <term>
1796 <option>-fcse</option>
1797 <indexterm><primary><option>-fcse</option></primary></indexterm>
1798 </term>
1799 <listitem>
1800 <para><emphasis>On by default.</emphasis>. Enables the common-sub-expression
1801 elimination optimisation.
1802 Switching this off can be useful if you have some <literal>unsafePerformIO</literal>
1803 expressions that you don't want commoned-up.</para>
1804 </listitem>
1805 </varlistentry>
1806
1807 <varlistentry>
1808 <term>
1809 <option>-fstrictness</option>
1810 <indexterm><primary><option></option></primary></indexterm>
1811 </term>
1812 <listitem>
1813 <para> <emphasis>On by default.</emphasis>.
1814 Switch on the strictness analyser. There is a very old paper about GHC's
1815 strictness analyser, <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/simple-strictnes-analyser.ps.gz">
1816 Measuring the effectiveness of a simple strictness analyser</ulink>,
1817 but the current one is quite a bit different.
1818 </para>
1819
1820 <para>The strictness analyser figures out when arguments and
1821 variables in a function can be treated 'strictly' (that is they
1822 are always evaluated in the function at some point). This allow
1823 GHC to apply certain optimisations such as unboxing that
1824 otherwise don't apply as they change the semantics of the program
1825 when applied to lazy arguments.
1826 </para>
1827 </listitem>
1828 </varlistentry>
1829
1830 <varlistentry>
1831 <term>
1832 <option>-funbox-strict-fields</option>:
1833 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1834 <indexterm><primary>strict constructor fields</primary></indexterm>
1835 <indexterm><primary>constructor fields, strict</primary></indexterm>
1836 </term>
1837 <listitem>
1838 <para>This option causes all constructor fields which are marked
1839 strict (i.e. &ldquo;!&rdquo;) to be unpacked if possible. It is
1840 equivalent to adding an <literal>UNPACK</literal> pragma to every
1841 strict constructor field (see <xref linkend="unpack-pragma"/>).
1842 </para>
1843
1844 <para>This option is a bit of a sledgehammer: it might sometimes
1845 make things worse. Selectively unboxing fields by using
1846 <literal>UNPACK</literal> pragmas might be better. An alternative
1847 is to use <option>-funbox-strict-fields</option> to turn on
1848 unboxing by default but disable it for certain constructor
1849 fields using the <literal>NOUNPACK</literal> pragma (see
1850 <xref linkend="nounpack-pragma"/>).</para>
1851 </listitem>
1852 </varlistentry>
1853
1854 <varlistentry>
1855 <term>
1856 <option>-fspec-constr</option>
1857 <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
1858 </term>
1859 <listitem>
1860 <para><emphasis>Off by default, but enabled by -O2.</emphasis>
1861 Turn on call-pattern specialisation; see
1862 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/spec-constr/index.htm">
1863 Call-pattern specialisation for Haskell programs</ulink>.
1864 </para>
1865
1866 <para>This optimisation specializes recursive functions according to
1867 their argument "shapes". This is best explained by example so
1868 consider:
1869 <programlisting>
1870 last :: [a] -> a
1871 last [] = error "last"
1872 last (x : []) = x
1873 last (x : xs) = last xs
1874 </programlisting>
1875 In this code, once we pass the initial check for an empty list we
1876 know that in the recursive case this pattern match is redundant. As
1877 such <option>-fspec-constr</option> will transform the above code
1878 to:
1879 <programlisting>
1880 last :: [a] -> a
1881 last [] = error "last"
1882 last (x : xs) = last' x xs
1883 where
1884 last' x [] = x
1885 last' x (y : ys) = last' y ys
1886 </programlisting>
1887 </para>
1888
1889 <para>As well avoid unnecessary pattern matching it also helps avoid
1890 unnecessary allocation. This applies when a argument is strict in
1891 the recursive call to itself but not on the initial entry. As
1892 strict recursive branch of the function is created similar to the
1893 above example.
1894 </para>
1895 </listitem>
1896 </varlistentry>
1897
1898 <varlistentry>
1899 <term>
1900 <option>-fspecialise</option>
1901 <indexterm><primary><option>-fspecialise</option></primary></indexterm>
1902 </term>
1903 <listitem>
1904 <para><emphasis>On by default.</emphasis>
1905 Specialise each type-class-overloaded function defined in this
1906 module for the types at which it is called in this module. Also
1907 specialise imported functions that have an INLINABLE pragma
1908 (<xref linkend="inlinable-pragma"/>) for the types at which they
1909 are called in this module.
1910 </para>
1911 </listitem>
1912 </varlistentry>
1913
1914 <varlistentry>
1915 <term>
1916 <option>-fstatic-argument-transformation</option>
1917 <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
1918 </term>
1919 <listitem>
1920 <para>Turn on the static argument transformation, which turns a
1921 recursive function into a non-recursive one with a local
1922 recursive loop. See Chapter 7 of
1923 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/santos-thesis.ps.gz">
1924 Andre Santos's PhD thesis</ulink>
1925 </para>
1926 </listitem>
1927 </varlistentry>
1928
1929 <varlistentry>
1930 <term>
1931 <option>-ffloat-in</option>
1932 <indexterm><primary><option></option></primary></indexterm>
1933 </term>
1934 <listitem>
1935 <para><emphasis>On by default.</emphasis>
1936 Float let-bindings inwards, nearer their binding site. See
1937 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">
1938 Let-floating: moving bindings to give faster programs (ICFP'96)</ulink>.
1939 </para>
1940
1941 <para>This optimisation moves let bindings closer to their use
1942 site. The benefit here is that this may avoid unnecessary
1943 allocation if the branch the let is now on is never executed. It
1944 also enables other optimisation passes to work more effectively
1945 as they have more information locally.
1946 </para>
1947
1948 <para>This optimisation isn't always beneficial though (so GHC
1949 applies some heuristics to decide when to apply it). The details
1950 get complicated but a simple example is that it is often beneficial
1951 to move let bindings outwards so that multiple let bindings can be
1952 grouped into a larger single let binding, effectively batching
1953 their allocation and helping the garbage collector and allocator.
1954 </para>
1955 </listitem>
1956 </varlistentry>
1957
1958 <varlistentry>
1959 <term>
1960 <option>-ffull-laziness</option>
1961 <indexterm><primary><option>-ffull-laziness</option></primary></indexterm>
1962 </term>
1963 <listitem>
1964 <para><emphasis>On by default.</emphasis>
1965 Run the full laziness optimisation (also known as let-floating),
1966 which floats let-bindings outside enclosing lambdas, in the hope
1967 they will be thereby be computed less often. See
1968 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">Let-floating:
1969 moving bindings to give faster programs (ICFP'96)</ulink>.
1970 Full laziness increases sharing, which can lead to increased memory
1971 residency.
1972 </para>
1973
1974 <para>NOTE: GHC doesn't implement complete full-laziness.
1975 When optimisation in on, and <option>-fno-full-laziness</option>
1976 is not given, some transformations that increase sharing are
1977 performed, such as extracting repeated computations from a loop.
1978 These are the same transformations that a fully lazy
1979 implementation would do, the difference is that GHC doesn't
1980 consistently apply full-laziness, so don't rely on it.
1981 </para>
1982 </listitem>
1983 </varlistentry>
1984
1985 <varlistentry>
1986 <term>
1987 <option>-fdo-lambda-eta-expansion</option>
1988 <indexterm><primary><option></option></primary></indexterm>
1989 </term>
1990 <listitem>
1991 <para><emphasis>On by default.</emphasis>
1992 Eta-expand let-bindings to increase their arity.
1993 </para>
1994 </listitem>
1995 </varlistentry>
1996
1997 <varlistentry>
1998 <term>
1999 <option>-fdo-eta-reduction</option>
2000 <indexterm><primary><option></option></primary></indexterm>
2001 </term>
2002 <listitem>
2003 <para><emphasis>On by default.</emphasis>
2004 Eta-reduce lambda expressions, if doing so gets rid of a whole
2005 group of lambdas.
2006 </para>
2007 </listitem>
2008 </varlistentry>
2009
2010 <varlistentry>
2011 <term>
2012 <option>-fcase-merge</option>
2013 <indexterm><primary><option></option></primary></indexterm>
2014 </term>
2015 <listitem>
2016 <para><emphasis>On by default.</emphasis>
2017 Merge immediately-nested case expressions that scrutinse the same variable. Example
2018 <programlisting>
2019 case x of
2020 Red -> e1
2021 _ -> case x of
2022 Blue -> e2
2023 Green -> e3
2024 ==>
2025 case x of
2026 Red -> e1
2027 Blue -> e2
2028 Green -> e2
2029 </programlisting>
2030 </para>
2031 </listitem>
2032 </varlistentry>
2033
2034 <varlistentry>
2035 <term>
2036 <option>-fliberate-case</option>
2037 <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
2038 </term>
2039 <listitem>
2040 <para><emphasis>Off by default, but enabled by -O2.</emphasis>
2041 Turn on the liberate-case transformation. This unrolls recursive
2042 function once in its own RHS, to avoid repeated case analysis of
2043 free variables. It's a bit like the call-pattern specialiser
2044 (<option>-fspec-constr</option>) but for free variables rather than
2045 arguments.
2046 </para>
2047 </listitem>
2048 </varlistentry>
2049
2050 <varlistentry>
2051 <term>
2052 <option>-fdicts-cheap</option>
2053 <indexterm><primary><option></option></primary></indexterm>
2054 </term>
2055 <listitem>
2056 <para>A very experimental flag that makes dictionary-valued
2057 expressions seem cheap to the optimiser.
2058 </para>
2059 </listitem>
2060 </varlistentry>
2061
2062 <varlistentry>
2063 <term>
2064 <option>-feager-blackholing</option>
2065 <indexterm><primary><option></option></primary></indexterm>
2066 </term>
2067 <listitem>
2068 <para>Usually GHC black-holes a thunk only when it switches
2069 threads. This flag makes it do so as soon as the thunk is
2070 entered. See <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/">
2071 Haskell on a shared-memory multiprocessor</ulink>.
2072 </para>
2073 </listitem>
2074 </varlistentry>
2075
2076 <varlistentry>
2077 <term>
2078 <option>-fno-state-hack</option>
2079 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
2080 </term>
2081 <listitem>
2082 <para>Turn off the "state hack" whereby any lambda with a
2083 <literal>State#</literal> token as argument is considered to be
2084 single-entry, hence it is considered OK to inline things inside
2085 it. This can improve performance of IO and ST monad code, but it
2086 runs the risk of reducing sharing.
2087 </para>
2088 </listitem>
2089 </varlistentry>
2090
2091 <varlistentry>
2092 <term>
2093 <option>-fpedantic-bottoms</option>
2094 <indexterm><primary><option>-fpedantic-bottoms</option></primary></indexterm>
2095 </term>
2096 <listitem>
2097 <para>Make GHC be more precise about its treatment of bottom (but see also
2098 <option>-fno-state-hack</option>). In particular, stop GHC
2099 eta-expanding through a case expression, which is good for
2100 performance, but bad if you are using <literal>seq</literal> on
2101 partial applications.
2102 </para>
2103 </listitem>
2104 </varlistentry>
2105
2106 <varlistentry>
2107 <term>
2108 <option>-fsimpl-tick-factor=<replaceable>n</replaceable></option>
2109 <indexterm><primary><option>-fsimpl-tick-factor</option></primary></indexterm>
2110 </term>
2111 <listitem>
2112 <para>GHC's optimiser can diverge if you write rewrite rules (
2113 <xref linkend="rewrite-rules"/>) that don't terminate, or (less
2114 satisfactorily) if you code up recursion through data types
2115 (<xref linkend="bugs-ghc"/>). To avoid making the compiler fall
2116 into an infinite loop, the optimiser carries a "tick count" and
2117 stops inlining and applying rewrite rules when this count is
2118 exceeded. The limit is set as a multiple of the program size, so
2119 bigger programs get more ticks. The
2120 <option>-fsimpl-tick-factor</option> flag lets you change the
2121 multiplier. The default is 100; numbers larger than 100 give more
2122 ticks, and numbers smaller than 100 give fewer.
2123 </para>
2124
2125 <para>If the tick-count expires, GHC summarises what simplifier
2126 steps it has done; you can use
2127 <option>-fddump-simpl-stats</option> to generate a much more
2128 detailed list. Usually that identifies the loop quite
2129 accurately, because some numbers are very large.
2130 </para>
2131 </listitem>
2132 </varlistentry>
2133
2134 <varlistentry>
2135 <term>
2136 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
2137 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
2138 <indexterm><primary>inlining, controlling</primary></indexterm>
2139 <indexterm><primary>unfolding, controlling</primary></indexterm>
2140 </term>
2141 <listitem>
2142 <para>(Default: 45) Governs the maximum size that GHC will allow a
2143 function unfolding to be. (An unfolding has a &ldquo;size&rdquo;
2144 that reflects the cost in terms of &ldquo;code bloat&rdquo; of
2145 expanding (aka inlining) that unfolding at a call site. A bigger
2146 function would be assigned a bigger cost.)
2147 </para>
2148
2149 <para>Consequences: (a) nothing larger than this will be inlined
2150 (unless it has an INLINE pragma); (b) nothing larger than this
2151 will be spewed into an interface file.
2152 </para>
2153
2154 <para>Increasing this figure is more likely to result in longer
2155 compile times than faster code. The
2156 <option>-funfolding-use-threshold</option> is more useful.
2157 </para>
2158 </listitem>
2159 </varlistentry>
2160
2161 <varlistentry>
2162 <term>
2163 <option>-funfolding-use-threshold=<replaceable>n</replaceable></option>
2164 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
2165 <indexterm><primary>inlining, controlling</primary></indexterm>
2166 <indexterm><primary>unfolding, controlling</primary></indexterm>
2167 </term>
2168 <listitem>
2169 <para>(Default: 8) This is the magic cut-off figure for unfolding
2170 (aka inlining): below this size, a function definition will be
2171 unfolded at the call-site, any bigger and it won't. The size
2172 computed for a function depends on two things: the actual size of
2173 the expression minus any discounts that
2174 apply (see <option>-funfolding-con-discount</option>).
2175 </para>
2176
2177 <para>The difference between this and
2178 <option>-funfolding-creation-threshold</option> is that this one
2179 determines if a function definition will be inlined <emphasis>at
2180 a call site</emphasis>. The other option determines if a
2181 function definition will be kept around at all for potential
2182 inlining.
2183 </para>
2184 </listitem>
2185 </varlistentry>
2186
2187 <varlistentry>
2188 <term>
2189 <option>-fexpose-all-unfoldings</option>
2190 <indexterm><primary><option></option></primary></indexterm>
2191 </term>
2192 <listitem>
2193 <para>An experimental flag to expose all unfoldings, even for very
2194 large or recursive functions. This allows for all functions to be
2195 inlined while usually GHC would avoid inlining larger functions.
2196 </para>
2197 </listitem>
2198 </varlistentry>
2199
2200 <varlistentry>
2201 <term>
2202 <option>-fvectorise</option>
2203 <indexterm><primary><option></option></primary></indexterm>
2204 </term>
2205 <listitem>
2206 <para>Data Parallel Haskell.
2207 </para>
2208 TODO: Document optimisation
2209 </listitem>
2210 </varlistentry>
2211
2212 <varlistentry>
2213 <term>
2214 <option>-favoid-vect</option>
2215 <indexterm><primary><option></option></primary></indexterm>
2216 </term>
2217 <listitem>
2218 <para>Data Parallel Haskell.
2219 </para>
2220 TODO: Document optimisation
2221 </listitem>
2222 </varlistentry>
2223
2224 <varlistentry>
2225 <term>
2226 <option>-fregs-graph</option>
2227 <indexterm><primary><option></option></primary></indexterm>
2228 </term>
2229 <listitem>
2230 <para><emphasis>Off by default, but enabled by -O2. Only applies in
2231 combination with the native code generator.</emphasis>
2232 Use the graph colouring register allocator for register allocation
2233 in the native code generator. By default, GHC uses a simpler,
2234 faster linear register allocator. The downside being that the
2235 linear register allocator usually generates worse code.
2236 </para>
2237 </listitem>
2238 </varlistentry>
2239
2240 <varlistentry>
2241 <term>
2242 <option>-fregs-iterative</option>
2243 <indexterm><primary><option></option></primary></indexterm>
2244 </term>
2245 <listitem>
2246 <para><emphasis>Off by default, only applies in combination with
2247 the native code generator.</emphasis>
2248 Use the iterative coalescing graph colouring register allocator for
2249 register allocation in the native code generator. This is the same
2250 register allocator as the <option>-freg-graph</option> one but also
2251 enables iterative coalescing during register allocation.
2252 </para>
2253 </listitem>
2254 </varlistentry>
2255
2256 <varlistentry>
2257 <term>
2258 <option>-fexcess-precision</option>
2259 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
2260 </term>
2261 <listitem>
2262 <para>When this option is given, intermediate floating
2263 point values can have a <emphasis>greater</emphasis>
2264 precision/range than the final type. Generally this is a
2265 good thing, but some programs may rely on the exact
2266 precision/range of
2267 <literal>Float</literal>/<literal>Double</literal> values
2268 and should not use this option for their compilation.</para>
2269
2270 <para>
2271 Note that the 32-bit x86 native code generator only
2272 supports excess-precision mode, so neither
2273 <option>-fexcess-precision</option> nor
2274 <option>-fno-excess-precision</option> has any effect.
2275 This is a known bug, see <xref linkend="bugs-ghc" />.
2276 </para>
2277 </listitem>
2278 </varlistentry>
2279
2280 <varlistentry>
2281 <term>
2282 <option>-fignore-asserts</option>
2283 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
2284 </term>
2285 <listitem>
2286 <para>Causes GHC to ignore uses of the function
2287 <literal>Exception.assert</literal> in source code (in
2288 other words, rewriting <literal>Exception.assert p
2289 e</literal> to <literal>e</literal> (see <xref
2290 linkend="assertions"/>). This flag is turned on by
2291 <option>-O</option>.
2292 </para>
2293 </listitem>
2294 </varlistentry>
2295
2296 <varlistentry>
2297 <term>
2298 <option>-fignore-interface-pragmas</option>
2299 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
2300 </term>
2301 <listitem>
2302 <para>Tells GHC to ignore all inessential information when reading interface files.
2303 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
2304 for a function, GHC will ignore that information.</para>
2305 </listitem>
2306 </varlistentry>
2307
2308 <varlistentry>
2309 <term>
2310 <option>-fomit-interface-pragmas</option>
2311 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
2312 </term>
2313 <listitem>
2314 <para>Tells GHC to omit all inessential information from the
2315 interface file generated for the module being compiled (say M).
2316 This means that a module importing M will see only the
2317 <emphasis>types</emphasis> of the functions that M exports, but
2318 not their unfoldings, strictness info, etc. Hence, for example,
2319 no function exported by M will be inlined into an importing module.
2320 The benefit is that modules that import M will need to be
2321 recompiled less often (only when M's exports change their type, not
2322 when they change their implementation).</para>
2323 </listitem>
2324 </varlistentry>
2325
2326 </variablelist>
2327
2328 </sect2>
2329
2330 </sect1>
2331
2332 &code-gens;
2333
2334 &phases;
2335
2336 &shared_libs;
2337
2338 <sect1 id="using-concurrent">
2339 <title>Using Concurrent Haskell</title>
2340 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
2341
2342 <para>GHC supports Concurrent Haskell by default, without requiring a
2343 special option or libraries compiled in a certain way. To get access to
2344 the support libraries for Concurrent Haskell, just import
2345 <ulink
2346 url="&libraryBaseLocation;/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
2347
2348 <para>
2349 Optionally, the program may be linked with
2350 the <option>-threaded</option> option (see
2351 <xref linkend="options-linker" />. This provides two benefits:
2352
2353 <itemizedlist>
2354 <listitem>
2355 <para>It enables the <option>-N</option><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm> RTS option to be
2356 used, which allows threads to run in
2357 parallel<indexterm><primary>parallelism</primary></indexterm>
2358 on a
2359 multiprocessor<indexterm><primary>multiprocessor</primary></indexterm><indexterm><primary>SMP</primary></indexterm>
2360 or
2361 multicore<indexterm><primary>multicore</primary></indexterm>
2362 machine. See <xref linkend="using-smp" />.</para>
2363 </listitem>
2364 <listitem>
2365 <para>If a thread makes a foreign call (and the call is
2366 not marked <literal>unsafe</literal>), then other
2367 Haskell threads in the program will continue to run
2368 while the foreign call is in progress.
2369 Additionally, <literal>foreign export</literal>ed
2370 Haskell functions may be called from multiple OS
2371 threads simultaneously. See
2372 <xref linkend="ffi-threads" />.</para>
2373 </listitem>
2374 </itemizedlist>
2375 </para>
2376
2377 <para>The following RTS option(s) affect the behaviour of Concurrent
2378 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
2379
2380 <variablelist>
2381 <varlistentry>
2382 <term><option>-C<replaceable>s</replaceable></option></term>
2383 <listitem>
2384 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2385 Sets the context switch interval to <replaceable>s</replaceable>
2386 seconds. A context switch will occur at the next heap block
2387 allocation after the timer expires (a heap block allocation occurs
2388 every 4k of allocation). With <option>-C0</option> or
2389 <option>-C</option>, context switches will occur as often as
2390 possible (at every heap block allocation). By default, context
2391 switches occur every 20ms.</para>
2392 </listitem>
2393 </varlistentry>
2394 </variablelist>
2395 </sect1>
2396
2397 <sect1 id="using-smp">
2398 <title>Using SMP parallelism</title>
2399 <indexterm><primary>parallelism</primary>
2400 </indexterm>
2401 <indexterm><primary>SMP</primary>
2402 </indexterm>
2403
2404 <para>GHC supports running Haskell programs in parallel on an SMP
2405 (symmetric multiprocessor).</para>
2406
2407 <para>There's a fine distinction between
2408 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
2409 parallelism is all about making your program run
2410 <emphasis>faster</emphasis> by making use of multiple processors
2411 simultaneously. Concurrency, on the other hand, is a means of
2412 abstraction: it is a convenient way to structure a program that must
2413 respond to multiple asynchronous events.</para>
2414
2415 <para>However, the two terms are certainly related. By making use of
2416 multiple CPUs it is possible to run concurrent threads in parallel,
2417 and this is exactly what GHC's SMP parallelism support does. But it
2418 is also possible to obtain performance improvements with parallelism
2419 on programs that do not use concurrency. This section describes how to
2420 use GHC to compile and run parallel programs, in <xref
2421 linkend="lang-parallel" /> we describe the language features that affect
2422 parallelism.</para>
2423
2424 <sect2 id="parallel-compile-options">
2425 <title>Compile-time options for SMP parallelism</title>
2426
2427 <para>In order to make use of multiple CPUs, your program must be
2428 linked with the <option>-threaded</option> option (see <xref
2429 linkend="options-linker" />). Additionally, the following
2430 compiler options affect parallelism:</para>
2431
2432 <variablelist>
2433 <varlistentry>
2434 <term><option>-feager-blackholing</option></term>
2435 <indexterm><primary><option>-feager-blackholing</option></primary></indexterm>
2436 <listitem>
2437 <para>
2438 Blackholing is the act of marking a thunk (lazy
2439 computuation) as being under evaluation. It is useful for
2440 three reasons: firstly it lets us detect certain kinds of
2441 infinite loop (the <literal>NonTermination</literal>
2442 exception), secondly it avoids certain kinds of space
2443 leak, and thirdly it avoids repeating a computation in a
2444 parallel program, because we can tell when a computation
2445 is already in progress.</para>
2446
2447 <para>
2448 The option <option>-feager-blackholing</option> causes
2449 each thunk to be blackholed as soon as evaluation begins.
2450 The default is "lazy blackholing", whereby thunks are only
2451 marked as being under evaluation when a thread is paused
2452 for some reason. Lazy blackholing is typically more
2453 efficient (by 1-2&percnt; or so), because most thunks don't
2454 need to be blackholed. However, eager blackholing can
2455 avoid more repeated computation in a parallel program, and
2456 this often turns out to be important for parallelism.
2457 </para>
2458
2459 <para>
2460 We recommend compiling any code that is intended to be run
2461 in parallel with the <option>-feager-blackholing</option>
2462 flag.
2463 </para>
2464 </listitem>
2465 </varlistentry>
2466 </variablelist>
2467 </sect2>
2468
2469 <sect2 id="parallel-options">
2470 <title>RTS options for SMP parallelism</title>
2471
2472 <para>There are two ways to run a program on multiple
2473 processors:
2474 call <literal>Control.Concurrent.setNumCapabilities</literal> from your
2475 program, or use the RTS <option>-N</option> option.</para>
2476
2477 <variablelist>
2478 <varlistentry>
2479 <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
2480 <listitem>
2481 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2482 Use <replaceable>x</replaceable> simultaneous threads when
2483 running the program. Normally <replaceable>x</replaceable>
2484 should be chosen to match the number of CPU cores on the
2485 machine<footnote><para>Whether hyperthreading cores should be counted or not is an
2486 open question; please feel free to experiment and let us know what
2487 results you find.</para></footnote>. For example,
2488 on a dual-core machine we would probably use
2489 <literal>+RTS -N2 -RTS</literal>.</para>
2490
2491 <para>Omitting <replaceable>x</replaceable>,
2492 i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
2493 choose the value of <replaceable>x</replaceable> itself
2494 based on how many processors are in your machine.</para>
2495
2496 <para>Be careful when using all the processors in your
2497 machine: if some of your processors are in use by other
2498 programs, this can actually harm performance rather than
2499 improve it.</para>
2500
2501 <para>Setting <option>-N</option> also has the effect of
2502 enabling the parallel garbage collector (see
2503 <xref linkend="rts-options-gc" />).</para>
2504
2505 <para>The current value of the <option>-N</option> option
2506 is available to the Haskell program
2507 via <literal>Control.Concurrent.getNumCapabilities</literal>, and
2508 it may be changed while the program is running by
2509 calling <literal>Control.Concurrent.setNumCapabilities</literal>.</para>
2510 </listitem>
2511 </varlistentry>
2512 </variablelist>
2513
2514 <para>The following options affect the way the runtime schedules
2515 threads on CPUs:</para>
2516
2517 <variablelist>
2518 <varlistentry>
2519 <term><option>-qa</option></term>
2520 <indexterm><primary><option>-qa</option></primary><secondary>RTS
2521 option</secondary></indexterm>
2522 <listitem>
2523 <para>Use the OS's affinity facilities to try to pin OS
2524 threads to CPU cores. This is an experimental feature,
2525 and may or may not be useful. Please let us know
2526 whether it helps for you!</para>
2527 </listitem>
2528 </varlistentry>
2529 <varlistentry>
2530 <term><option>-qm</option></term>
2531 <indexterm><primary><option>-qm</option></primary><secondary>RTS
2532 option</secondary></indexterm>
2533 <listitem>
2534 <para>Disable automatic migration for load balancing.
2535 Normally the runtime will automatically try to schedule
2536 threads across the available CPUs to make use of idle
2537 CPUs; this option disables that behaviour. Note that
2538 migration only applies to threads; sparks created
2539 by <literal>par</literal> are load-balanced separately
2540 by work-stealing.</para>
2541
2542 <para>
2543 This option is probably only of use for concurrent
2544 programs that explicitly schedule threads onto CPUs
2545 with <literal>Control.Concurrent.forkOn</literal>.
2546 </para>
2547 </listitem>
2548 </varlistentry>
2549 </variablelist>
2550 </sect2>
2551
2552 <sect2>
2553 <title>Hints for using SMP parallelism</title>
2554
2555 <para>Add the <literal>-s</literal> RTS option when
2556 running the program to see timing stats, which will help to tell you
2557 whether your program got faster by using more CPUs or not. If the user
2558 time is greater than
2559 the elapsed time, then the program used more than one CPU. You should
2560 also run the program without <literal>-N</literal> for
2561 comparison.</para>
2562
2563 <para>The output of <literal>+RTS -s</literal> tells you how
2564 many &ldquo;sparks&rdquo; were created and executed during the
2565 run of the program (see <xref linkend="rts-options-gc" />), which
2566 will give you an idea how well your <literal>par</literal>
2567 annotations are working.</para>
2568
2569 <para>GHC's parallelism support has improved in 6.12.1 as a
2570 result of much experimentation and tuning in the runtime
2571 system. We'd still be interested to hear how well it works
2572 for you, and we're also interested in collecting parallel
2573 programs to add to our benchmarking suite.</para>
2574 </sect2>
2575 </sect1>
2576
2577 <sect1 id="options-platform">
2578 <title>Platform-specific Flags</title>
2579
2580 <indexterm><primary>-m* options</primary></indexterm>
2581 <indexterm><primary>platform-specific options</primary></indexterm>
2582 <indexterm><primary>machine-specific options</primary></indexterm>
2583
2584 <para>Some flags only make sense for particular target
2585 platforms.</para>
2586
2587 <variablelist>
2588
2589 <varlistentry>
2590 <term><option>-msse2</option>:</term>
2591 <listitem>
2592 <para>
2593 (x86 only, added in GHC 7.0.1) Use the SSE2 registers and
2594 instruction set to implement floating point operations when using
2595 the <link linkend="native-code-gen">native code generator</link>.
2596 This gives a substantial performance improvement for floating
2597 point, but the resulting compiled code
2598 will only run on processors that support SSE2 (Intel Pentium 4 and
2599 later, or AMD Athlon 64 and later). The
2600 <link linkend="llvm-code-gen">LLVM backend</link> will also use SSE2
2601 if your processor supports it but detects this automatically so no
2602 flag is required.
2603 </para>
2604 <para>
2605 SSE2 is unconditionally used on x86-64 platforms.
2606 </para>
2607 </listitem>
2608 </varlistentry>
2609
2610 <varlistentry>
2611 <term><option>-msse4.2</option>:</term>
2612 <listitem>
2613 <para>
2614 (x86 only, added in GHC 7.4.1) Use the SSE4.2 instruction set to
2615 implement some floating point and bit operations when using the
2616 <link linkend="native-code-gen">native code generator</link>. The
2617 resulting compiled code will only run on processors that
2618 support SSE4.2 (Intel Core i7 and later). The
2619 <link linkend="llvm-code-gen">LLVM backend</link> will also use
2620 SSE4.2 if your processor supports it but detects this automatically
2621 so no flag is required.
2622 </para>
2623 </listitem>
2624 </varlistentry>
2625
2626 </variablelist>
2627
2628 </sect1>
2629
2630 &runtime;
2631
2632 <sect1 id="ext-core">
2633 <title>Generating and compiling External Core Files</title>
2634
2635 <indexterm><primary>intermediate code generation</primary></indexterm>
2636
2637 <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format)
2638 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
2639 <filename>.hcr</filename>. The Core format is described in <ulink url="../../core.pdf">
2640 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
2641 and sample tools
2642 for manipulating Core files (in Haskell) are available in the
2643 <ulink url="http://hackage.haskell.org/package/extcore">extcore package on Hackage</ulink>. Note that the format of <literal>.hcr</literal>
2644 files is <emphasis>different</emphasis> from the Core output format that GHC generates
2645 for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
2646
2647 <para>The Core format natively supports notes which you can add to
2648 your source code using the <literal>CORE</literal> pragma (see <xref
2649 linkend="pragmas"/>).</para>
2650
2651 <variablelist>
2652
2653 <varlistentry>
2654 <term>
2655 <option>-fext-core</option>
2656 <indexterm><primary><option>-fext-core</option></primary></indexterm>
2657 </term>
2658 <listitem>
2659 <para>Generate <literal>.hcr</literal> files.</para>
2660 </listitem>
2661 </varlistentry>
2662
2663 </variablelist>
2664
2665 <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>
2666
2667 </sect1>
2668
2669 &debug;
2670 &flags;
2671
2672 </chapter>
2673
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