Document the -fwarn-unsupported-calling-conventions flag
[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-unrecognised-pragmas</option>,
993 <option>-fwarn-auto-orphans</option>,
994 <option>-fwarn-implicit-prelude</option>.</para>
995 </listitem>
996 </varlistentry>
997
998 <varlistentry>
999 <term><option>-w</option>:</term>
1000 <listitem>
1001 <indexterm><primary><option>-w</option></primary></indexterm>
1002 <para>Turns off all warnings, including the standard ones and
1003 those that <literal>-Wall</literal> doesn't enable.</para>
1004 </listitem>
1005 </varlistentry>
1006
1007 <varlistentry>
1008 <term><option>-Werror</option>:</term>
1009 <listitem>
1010 <indexterm><primary><option>-Werror</option></primary></indexterm>
1011 <para>Makes any warning into a fatal error. Useful so that you don't
1012 miss warnings when doing batch compilation. </para>
1013 </listitem>
1014 </varlistentry>
1015
1016 <varlistentry>
1017 <term><option>-Wwarn</option>:</term>
1018 <listitem>
1019 <indexterm><primary><option>-Wwarn</option></primary></indexterm>
1020 <para>Warnings are treated only as warnings, not as errors. This is
1021 the default, but can be useful to negate a
1022 <option>-Werror</option> flag.</para>
1023 </listitem>
1024 </varlistentry>
1025
1026 </variablelist>
1027
1028 <para>The full set of warning options is described below. To turn
1029 off any warning, simply give the corresponding
1030 <option>-fno-warn-...</option> option on the command line.</para>
1031
1032 <variablelist>
1033
1034 <varlistentry>
1035 <term><option>-fdefer-type-errors</option>:</term>
1036 <listitem>
1037 <indexterm><primary><option>-fdefer-type-errors</option></primary>
1038 </indexterm>
1039 <indexterm><primary>warnings</primary></indexterm>
1040 <para>Defer as many type errors as possible until runtime.
1041 At compile time you get a warning (instead of an error). At
1042 runtime, if you use a value that depends on a type error, you
1043 get a runtime error; but you can run any type-correct parts of your code
1044 just fine.</para>
1045 </listitem>
1046 </varlistentry>
1047
1048 <varlistentry>
1049 <term><option>-fhelpful-errors</option>:</term>
1050 <listitem>
1051 <indexterm><primary><option>-fhelpful-errors</option></primary>
1052 </indexterm>
1053 <indexterm><primary>warnings</primary></indexterm>
1054 <para>When a name or package is not found in scope, make
1055 suggestions for the name or package you might have meant instead.</para>
1056 <para>This option is on by default.</para>
1057 </listitem>
1058 </varlistentry>
1059
1060 <varlistentry>
1061 <term><option>-fwarn-unrecognised-pragmas</option>:</term>
1062 <listitem>
1063 <indexterm><primary><option>-fwarn-unrecognised-pragmas</option></primary>
1064 </indexterm>
1065 <indexterm><primary>warnings</primary></indexterm>
1066 <indexterm><primary>pragmas</primary></indexterm>
1067 <para>Causes a warning to be emitted when a
1068 pragma that GHC doesn't recognise is used. As well as pragmas
1069 that GHC itself uses, GHC also recognises pragmas known to be used
1070 by other tools, e.g. <literal>OPTIONS_HUGS</literal> and
1071 <literal>DERIVE</literal>.</para>
1072
1073 <para>This option is on by default.</para>
1074 </listitem>
1075 </varlistentry>
1076
1077 <varlistentry>
1078 <term><option>-fwarn-warnings-deprecations</option>:</term>
1079 <listitem>
1080 <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
1081 </indexterm>
1082 <indexterm><primary>warnings</primary></indexterm>
1083 <indexterm><primary>deprecations</primary></indexterm>
1084 <para>Causes a warning to be emitted when a
1085 module, function or type with a WARNING or DEPRECATED pragma
1086 is used. See <xref linkend="warning-deprecated-pragma"/> for more
1087 details on the pragmas.</para>
1088
1089 <para>This option is on by default.</para>
1090 </listitem>
1091 </varlistentry>
1092
1093 <varlistentry>
1094 <term><option>-fwarn-deprecated-flags</option>:</term>
1095 <listitem>
1096 <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
1097 </indexterm>
1098 <indexterm><primary>deprecated-flags</primary></indexterm>
1099 <para>Causes a warning to be emitted when a deprecated
1100 commandline flag is used.</para>
1101
1102 <para>This option is on by default.</para>
1103 </listitem>
1104 </varlistentry>
1105
1106 <varlistentry>
1107 <term><option>-fwarn-unsupported-calling-conventions</option>:</term>
1108 <listitem>
1109 <indexterm><primary><option>-fwarn-unsupported-calling-conventions</option></primary>
1110 </indexterm>
1111 <para>Causes a warning to be emitted for foreign declarations
1112 that use unsupported calling conventions. In particular,
1113 if the <literal>stdcall</literal> calling convention is used
1114 on an architecture other than i386 then it will be treated
1115 as <literal>ccall</literal>.</para>
1116 </listitem>
1117 </varlistentry>
1118
1119 <varlistentry>
1120 <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
1121 <listitem>
1122 <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
1123 </indexterm>
1124 <para>Causes a warning to be emitted for foreign imports of
1125 the following form:</para>
1126
1127 <programlisting>
1128 foreign import "f" f :: FunPtr t
1129 </programlisting>
1130
1131 <para>on the grounds that it probably should be</para>
1132
1133 <programlisting>
1134 foreign import "&amp;f" f :: FunPtr t
1135 </programlisting>
1136
1137 <para>The first form declares that `f` is a (pure) C
1138 function that takes no arguments and returns a pointer to a
1139 C function with type `t`, whereas the second form declares
1140 that `f` itself is a C function with type `t`. The first
1141 declaration is usually a mistake, and one that is hard to
1142 debug because it results in a crash, hence this
1143 warning.</para>
1144 </listitem>
1145 </varlistentry>
1146
1147 <varlistentry>
1148 <term><option>-fwarn-dodgy-exports</option>:</term>
1149 <listitem>
1150 <indexterm><primary><option>-fwarn-dodgy-exports</option></primary>
1151 </indexterm>
1152 <para>Causes a warning to be emitted when a datatype
1153 <literal>T</literal> is exported
1154 with all constructors, i.e. <literal>T(..)</literal>, but is it
1155 just a type synonym.</para>
1156 <para>Also causes a warning to be emitted when a module is
1157 re-exported, but that module exports nothing.</para>
1158 </listitem>
1159 </varlistentry>
1160
1161 <varlistentry>
1162 <term><option>-fwarn-dodgy-imports</option>:</term>
1163 <listitem>
1164 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
1165 </indexterm>
1166 <para>Causes a warning to be emitted when a datatype
1167 <literal>T</literal> is imported
1168 with all constructors, i.e. <literal>T(..)</literal>, but has been
1169 exported abstractly, i.e. <literal>T</literal>.</para>
1170 </listitem>
1171 </varlistentry>
1172
1173 <varlistentry>
1174 <term><option>-fwarn-lazy-unlifted-bindings</option>:</term>
1175 <listitem>
1176 <indexterm><primary><option>-fwarn-lazy-unlifted-bindings</option></primary>
1177 </indexterm>
1178 <para>Causes a warning to be emitted when an unlifted type
1179 is bound in a way that looks lazy, e.g.
1180 <literal>where (I# x) = ...</literal>. Use
1181 <literal>where !(I# x) = ...</literal> instead. This will be an
1182 error, rather than a warning, in GHC 7.2.
1183 </para>
1184 </listitem>
1185 </varlistentry>
1186
1187 <varlistentry>
1188 <term><option>-fwarn-duplicate-exports</option>:</term>
1189 <listitem>
1190 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
1191 <indexterm><primary>duplicate exports, warning</primary></indexterm>
1192 <indexterm><primary>export lists, duplicates</primary></indexterm>
1193
1194 <para>Have the compiler warn about duplicate entries in
1195 export lists. This is useful information if you maintain
1196 large export lists, and want to avoid the continued export
1197 of a definition after you've deleted (one) mention of it in
1198 the export list.</para>
1199
1200 <para>This option is on by default.</para>
1201 </listitem>
1202 </varlistentry>
1203
1204 <varlistentry>
1205 <term><option>-fwarn-hi-shadowing</option>:</term>
1206 <listitem>
1207 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
1208 <indexterm><primary>shadowing</primary>
1209 <secondary>interface files</secondary></indexterm>
1210
1211 <para>Causes the compiler to emit a warning when a module or
1212 interface file in the current directory is shadowing one
1213 with the same module name in a library or other
1214 directory.</para>
1215 </listitem>
1216 </varlistentry>
1217
1218 <varlistentry>
1219 <term><option>-fwarn-identities</option>:</term>
1220 <listitem>
1221 <indexterm><primary><option>-fwarn-identities</option></primary></indexterm>
1222 <para>Causes the compiler to emit a warning when a Prelude numeric
1223 conversion converts a type T to the same type T; such calls
1224 are probably no-ops and can be omitted. The functions checked for
1225 are: <literal>toInteger</literal>,
1226 <literal>toRational</literal>,
1227 <literal>fromIntegral</literal>,
1228 and <literal>realToFrac</literal>.
1229 </para>
1230 </listitem>
1231 </varlistentry>
1232
1233 <varlistentry>
1234 <term><option>-fwarn-implicit-prelude</option>:</term>
1235 <listitem>
1236 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
1237 <indexterm><primary>implicit prelude, warning</primary></indexterm>
1238 <para>Have the compiler warn if the Prelude is implicitly
1239 imported. This happens unless either the Prelude module is
1240 explicitly imported with an <literal>import ... Prelude ...</literal>
1241 line, or this implicit import is disabled (either by
1242 <option>-XNoImplicitPrelude</option> or a
1243 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
1244
1245 <para>Note that no warning is given for syntax that implicitly
1246 refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
1247 would change whether it refers to the Prelude.
1248 For example, no warning is given when
1249 <literal>368</literal> means
1250 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
1251 (where <literal>Prelude</literal> refers to the actual Prelude module,
1252 regardless of the imports of the module being compiled).</para>
1253
1254 <para>This warning is off by default.</para>
1255 </listitem>
1256 </varlistentry>
1257
1258 <varlistentry>
1259 <term><option>-fwarn-incomplete-patterns</option>,
1260 <option>-fwarn-incomplete-uni-patterns</option>:
1261 </term>
1262 <listitem>
1263 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
1264 <indexterm><primary><option>-fwarn-incomplete-uni-patterns</option></primary></indexterm>
1265 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
1266 <indexterm><primary>patterns, incomplete</primary></indexterm>
1267
1268 <para>The option <option>-fwarn-incomplete-patterns</option> warns
1269 about places where
1270 a pattern-match might fail at runtime.
1271 The function
1272 <function>g</function> below will fail when applied to
1273 non-empty lists, so the compiler will emit a warning about
1274 this when <option>-fwarn-incomplete-patterns</option> is
1275 enabled.
1276
1277 <programlisting>
1278 g [] = 2
1279 </programlisting>
1280
1281 This option isn't enabled by default because it can be
1282 a bit noisy, and it doesn't always indicate a bug in the
1283 program. However, it's generally considered good practice
1284 to cover all the cases in your functions, and it is switched
1285 on by <option>-W</option>.</para>
1286
1287 <para>The flag <option>-fwarn-incomplete-uni-patterns</option> is
1288 similar, except that it
1289 applies only to lambda-expressions and pattern bindings, constructs
1290 that only allow a single pattern:
1291
1292 <programlisting>
1293 h = \[] -> 2
1294 Just k = f y
1295 </programlisting>
1296
1297 </para>
1298 </listitem>
1299 </varlistentry>
1300
1301 <varlistentry>
1302 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1303 <listitem>
1304 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1305 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1306 <indexterm><primary>record updates, incomplete</primary></indexterm>
1307
1308 <para>The function
1309 <function>f</function> below will fail when applied to
1310 <literal>Bar</literal>, so the compiler will emit a warning about
1311 this when <option>-fwarn-incomplete-record-updates</option> is
1312 enabled.</para>
1313
1314 <programlisting>
1315 data Foo = Foo { x :: Int }
1316 | Bar
1317
1318 f :: Foo -> Foo
1319 f foo = foo { x = 6 }
1320 </programlisting>
1321
1322 <para>This option isn't enabled by default because it can be
1323 very noisy, and it often doesn't indicate a bug in the
1324 program.</para>
1325 </listitem>
1326 </varlistentry>
1327
1328 <varlistentry>
1329 <term>
1330 <option>-fwarn-missing-fields</option>:
1331 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1332 <indexterm><primary>missing fields, warning</primary></indexterm>
1333 <indexterm><primary>fields, missing</primary></indexterm>
1334 </term>
1335 <listitem>
1336
1337 <para>This option is on by default, and warns you whenever
1338 the construction of a labelled field constructor isn't
1339 complete, missing initializers for one or more fields. While
1340 not an error (the missing fields are initialised with
1341 bottoms), it is often an indication of a programmer error.</para>
1342 </listitem>
1343 </varlistentry>
1344
1345 <varlistentry>
1346 <term>
1347 <option>-fwarn-missing-import-lists</option>:
1348 <indexterm><primary><option>-fwarn-import-lists</option></primary></indexterm>
1349 <indexterm><primary>missing import lists, warning</primary></indexterm>
1350 <indexterm><primary>import lists, missing</primary></indexterm>
1351 </term>
1352 <listitem>
1353
1354 <para>This flag warns if you use an unqualified
1355 <literal>import</literal> declaration
1356 that does not explicitly list the entities brought into scope. For
1357 example
1358 </para>
1359
1360 <programlisting>
1361 module M where
1362 import X( f )
1363 import Y
1364 import qualified Z
1365 p x = f x x
1366 </programlisting>
1367
1368 <para>
1369 The <option>-fwarn-import-lists</option> flag will warn about the import
1370 of <literal>Y</literal> but not <literal>X</literal>
1371 If module <literal>Y</literal> is later changed to export (say) <literal>f</literal>,
1372 then the reference to <literal>f</literal> in <literal>M</literal> will become
1373 ambiguous. No warning is produced for the import of <literal>Z</literal>
1374 because extending <literal>Z</literal>'s exports would be unlikely to produce
1375 ambiguity in <literal>M</literal>.
1376 </para>
1377 </listitem>
1378 </varlistentry>
1379
1380 <varlistentry>
1381 <term><option>-fwarn-missing-methods</option>:</term>
1382 <listitem>
1383 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1384 <indexterm><primary>missing methods, warning</primary></indexterm>
1385 <indexterm><primary>methods, missing</primary></indexterm>
1386
1387 <para>This option is on by default, and warns you whenever
1388 an instance declaration is missing one or more methods, and
1389 the corresponding class declaration has no default
1390 declaration for them.</para>
1391 <para>The warning is suppressed if the method name
1392 begins with an underscore. Here's an example where this is useful:
1393 <programlisting>
1394 class C a where
1395 _simpleFn :: a -> String
1396 complexFn :: a -> a -> String
1397 complexFn x y = ... _simpleFn ...
1398 </programlisting>
1399 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1400 never <literal>_simpleFn</literal>; and (b)
1401 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1402 </para>
1403 </listitem>
1404 </varlistentry>
1405
1406 <varlistentry>
1407 <term><option>-fwarn-missing-signatures</option>:</term>
1408 <listitem>
1409 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1410 <indexterm><primary>type signatures, missing</primary></indexterm>
1411
1412 <para>If you would like GHC to check that every top-level
1413 function/value has a type signature, use the
1414 <option>-fwarn-missing-signatures</option> option. As part of
1415 the warning GHC also reports the inferred type. The
1416 option is off by default.</para>
1417 </listitem>
1418 </varlistentry>
1419
1420 <varlistentry>
1421 <term><option>-fwarn-missing-local-sigs</option>:</term>
1422 <listitem>
1423 <indexterm><primary><option>-fwarn-missing-local-sigs</option></primary></indexterm>
1424 <indexterm><primary>type signatures, missing</primary></indexterm>
1425
1426 <para>If you use the
1427 <option>-fwarn-missing-local-sigs</option> flag GHC will warn
1428 you about any polymorphic local bindings. As part of
1429 the warning GHC also reports the inferred type. The
1430 option is off by default.</para>
1431 </listitem>
1432 </varlistentry>
1433
1434 <varlistentry>
1435 <term><option>-fwarn-name-shadowing</option>:</term>
1436 <listitem>
1437 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1438 <indexterm><primary>shadowing, warning</primary></indexterm>
1439
1440 <para>This option causes a warning to be emitted whenever an
1441 inner-scope value has the same name as an outer-scope value,
1442 i.e. the inner value shadows the outer one. This can catch
1443 typographical errors that turn into hard-to-find bugs, e.g.,
1444 in the inadvertent capture of what would be a recursive call in
1445 <literal>f = ... let f = id in ... f ...</literal>.</para>
1446 <para>The warning is suppressed for names beginning with an underscore. For example
1447 <programlisting>
1448 f x = do { _ignore &lt;- this; _ignore &lt;- that; return (the other) }
1449 </programlisting>
1450 </para>
1451 </listitem>
1452 </varlistentry>
1453
1454 <varlistentry>
1455 <term><option>-fwarn-orphans, -fwarn-auto-orphans</option>:</term>
1456 <listitem>
1457 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1458 <indexterm><primary><option>-fwarn-auto-orphans</option></primary></indexterm>
1459 <indexterm><primary>orphan instances, warning</primary></indexterm>
1460 <indexterm><primary>orphan rules, warning</primary></indexterm>
1461
1462 <para>These flags cause a warning to be emitted whenever the
1463 module contains an "orphan" instance declaration or rewrite rule.
1464 An instance declaration is an orphan if it appears in a module in
1465 which neither the class nor the type being instanced are declared
1466 in the same module. A rule is an orphan if it is a rule for a
1467 function declared in another module. A module containing any
1468 orphans is called an orphan module.</para>
1469 <para>The trouble with orphans is that GHC must pro-actively read the interface
1470 files for all orphan modules, just in case their instances or rules
1471 play a role, whether or not the module's interface would otherwise
1472 be of any use. See <xref linkend="orphan-modules"/> for details.
1473 </para>
1474 <para>The flag <option>-fwarn-orphans</option> warns about user-written
1475 orphan rules or instances. The flag <option>-fwarn-auto-orphans</option>
1476 warns about automatically-generated orphan rules, notably as a result of
1477 specialising functions, for type classes (<literal>Specialise</literal>)
1478 or argument values (<literal>SpecConstr</literal>).</para>
1479 </listitem>
1480 </varlistentry>
1481
1482 <varlistentry>
1483 <term>
1484 <option>-fwarn-overlapping-patterns</option>:
1485 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1486 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1487 <indexterm><primary>patterns, overlapping</primary></indexterm>
1488 </term>
1489 <listitem>
1490 <para>By default, the compiler will warn you if a set of
1491 patterns are overlapping, e.g.,</para>
1492
1493 <programlisting>
1494 f :: String -&#62; Int
1495 f [] = 0
1496 f (_:xs) = 1
1497 f "2" = 2
1498 </programlisting>
1499
1500 <para>where the last pattern match in <function>f</function>
1501 won't ever be reached, as the second pattern overlaps
1502 it. More often than not, redundant patterns is a programmer
1503 mistake/error, so this option is enabled by default.</para>
1504 </listitem>
1505 </varlistentry>
1506
1507 <varlistentry>
1508 <term><option>-fwarn-tabs</option>:</term>
1509 <listitem>
1510 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1511 <indexterm><primary>tabs, warning</primary></indexterm>
1512 <para>Have the compiler warn if there are tabs in your source
1513 file.</para>
1514
1515 <para>This warning is off by default.</para>
1516 </listitem>
1517 </varlistentry>
1518
1519 <varlistentry>
1520 <term><option>-fwarn-type-defaults</option>:</term>
1521 <listitem>
1522 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1523 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1524 <para>Have the compiler warn/inform you where in your source
1525 the Haskell defaulting mechanism for numeric types kicks
1526 in. This is useful information when converting code from a
1527 context that assumed one default into one with another,
1528 e.g., the &lsquo;default default&rsquo; for Haskell 1.4 caused the
1529 otherwise unconstrained value <constant>1</constant> to be
1530 given the type <literal>Int</literal>, whereas Haskell 98
1531 and later
1532 defaults it to <literal>Integer</literal>. This may lead to
1533 differences in performance and behaviour, hence the
1534 usefulness of being non-silent about this.</para>
1535
1536 <para>This warning is off by default.</para>
1537 </listitem>
1538 </varlistentry>
1539
1540 <varlistentry>
1541 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1542 <listitem>
1543 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1544 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1545 <para>Have the compiler warn/inform you where in your source
1546 the Haskell Monomorphism Restriction is applied. If applied silently
1547 the MR can give rise to unexpected behaviour, so it can be helpful
1548 to have an explicit warning that it is being applied.</para>
1549
1550 <para>This warning is off by default.</para>
1551 </listitem>
1552 </varlistentry>
1553
1554 <varlistentry>
1555 <term><option>-fwarn-unused-binds</option>:</term>
1556 <listitem>
1557 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1558 <indexterm><primary>unused binds, warning</primary></indexterm>
1559 <indexterm><primary>binds, unused</primary></indexterm>
1560 <para>Report any function definitions (and local bindings)
1561 which are unused. For top-level functions, the warning is
1562 only given if the binding is not exported.</para>
1563 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1564 mentioned in the right hand side of another definition that is used, or (c) the
1565 function it defines begins with an underscore. The last case provides a
1566 way to suppress unused-binding warnings selectively. </para>
1567 <para> Notice that a variable
1568 is reported as unused even if it appears in the right-hand side of another
1569 unused binding. </para>
1570 </listitem>
1571 </varlistentry>
1572
1573 <varlistentry>
1574 <term><option>-fwarn-unused-imports</option>:</term>
1575 <listitem>
1576 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1577 <indexterm><primary>unused imports, warning</primary></indexterm>
1578 <indexterm><primary>imports, unused</primary></indexterm>
1579
1580 <para>Report any modules that are explicitly imported but
1581 never used. However, the form <literal>import M()</literal> is
1582 never reported as an unused import, because it is a useful idiom
1583 for importing instance declarations, which are anonymous in Haskell.</para>
1584 </listitem>
1585 </varlistentry>
1586
1587 <varlistentry>
1588 <term><option>-fwarn-unused-matches</option>:</term>
1589 <listitem>
1590 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1591 <indexterm><primary>unused matches, warning</primary></indexterm>
1592 <indexterm><primary>matches, unused</primary></indexterm>
1593
1594 <para>Report all unused variables which arise from pattern
1595 matches, including patterns consisting of a single variable.
1596 For instance <literal>f x y = []</literal> would report
1597 <varname>x</varname> and <varname>y</varname> as unused. The
1598 warning is suppressed if the variable name begins with an underscore, thus:
1599 <programlisting>
1600 f _x = True
1601 </programlisting>
1602 </para>
1603 </listitem>
1604 </varlistentry>
1605
1606 <varlistentry>
1607 <term><option>-fwarn-unused-do-bind</option>:</term>
1608 <listitem>
1609 <indexterm><primary><option>-fwarn-unused-do-bind</option></primary></indexterm>
1610 <indexterm><primary>unused do binding, warning</primary></indexterm>
1611 <indexterm><primary>do binding, unused</primary></indexterm>
1612
1613 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1614 that appear to silently throw information away.
1615 For instance <literal>do { mapM popInt xs ; return 10 }</literal> would report
1616 the first statement in the <literal>do</literal> block as suspicious,
1617 as it has the type <literal>StackM [Int]</literal> and not <literal>StackM ()</literal>, but that
1618 <literal>[Int]</literal> value is not bound to anything. The warning is suppressed by
1619 explicitly mentioning in the source code that your program is throwing something away:
1620 <programlisting>
1621 do { _ &lt;- mapM popInt xs ; return 10 }
1622 </programlisting>
1623 Of course, in this particular situation you can do even better:
1624 <programlisting>
1625 do { mapM_ popInt xs ; return 10 }
1626 </programlisting>
1627 </para>
1628 </listitem>
1629 </varlistentry>
1630
1631 <varlistentry>
1632 <term><option>-fwarn-wrong-do-bind</option>:</term>
1633 <listitem>
1634 <indexterm><primary><option>-fwarn-wrong-do-bind</option></primary></indexterm>
1635 <indexterm><primary>apparently erroneous do binding, warning</primary></indexterm>
1636 <indexterm><primary>do binding, apparently erroneous</primary></indexterm>
1637
1638 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1639 that appear to lack a binding.
1640 For instance <literal>do { return (popInt 10) ; return 10 }</literal> would report
1641 the first statement in the <literal>do</literal> block as suspicious,
1642 as it has the type <literal>StackM (StackM Int)</literal> (which consists of two nested applications
1643 of the same monad constructor), but which is not then &quot;unpacked&quot; by binding the result.
1644 The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away:
1645 <programlisting>
1646 do { _ &lt;- return (popInt 10) ; return 10 }
1647 </programlisting>
1648 For almost all sensible programs this will indicate a bug, and you probably intended to write:
1649 <programlisting>
1650 do { popInt 10 ; return 10 }
1651 </programlisting>
1652 </para>
1653 </listitem>
1654 </varlistentry>
1655
1656 </variablelist>
1657
1658 <para>If you're feeling really paranoid, the
1659 <option>-dcore-lint</option>
1660 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1661 is a good choice. It turns on heavyweight intra-pass
1662 sanity-checking within GHC. (It checks GHC's sanity, not
1663 yours.)</para>
1664
1665 </sect1>
1666
1667 &packages;
1668
1669 <sect1 id="options-optimise">
1670 <title>Optimisation (code improvement)</title>
1671
1672 <indexterm><primary>optimisation</primary></indexterm>
1673 <indexterm><primary>improvement, code</primary></indexterm>
1674
1675 <para>The <option>-O*</option> options specify convenient
1676 &ldquo;packages&rdquo; of optimisation flags; the
1677 <option>-f*</option> options described later on specify
1678 <emphasis>individual</emphasis> optimisations to be turned on/off;
1679 the <option>-m*</option> options specify
1680 <emphasis>machine-specific</emphasis> optimisations to be turned
1681 on/off.</para>
1682
1683 <sect2 id="optimise-pkgs">
1684 <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1685
1686 <para>There are <emphasis>many</emphasis> options that affect
1687 the quality of code produced by GHC. Most people only have a
1688 general goal, something like &ldquo;Compile quickly&rdquo; or
1689 &ldquo;Make my program run like greased lightning.&rdquo; The
1690 following &ldquo;packages&rdquo; of optimisations (or lack
1691 thereof) should suffice.</para>
1692
1693 <para>Note that higher optimisation levels cause more
1694 cross-module optimisation to be performed, which can have an
1695 impact on how much of your program needs to be recompiled when
1696 you change something. This is one reason to stick to
1697 no-optimisation when developing code.</para>
1698
1699 <variablelist>
1700
1701 <varlistentry>
1702 <term>
1703 No <option>-O*</option>-type option specified:
1704 <indexterm><primary>-O* not specified</primary></indexterm>
1705 </term>
1706 <listitem>
1707 <para>This is taken to mean: &ldquo;Please compile
1708 quickly; I'm not over-bothered about compiled-code
1709 quality.&rdquo; So, for example: <command>ghc -c
1710 Foo.hs</command></para>
1711 </listitem>
1712 </varlistentry>
1713
1714 <varlistentry>
1715 <term>
1716 <option>-O0</option>:
1717 <indexterm><primary><option>-O0</option></primary></indexterm>
1718 </term>
1719 <listitem>
1720 <para>Means &ldquo;turn off all optimisation&rdquo;,
1721 reverting to the same settings as if no
1722 <option>-O</option> options had been specified. Saying
1723 <option>-O0</option> can be useful if
1724 eg. <command>make</command> has inserted a
1725 <option>-O</option> on the command line already.</para>
1726 </listitem>
1727 </varlistentry>
1728
1729 <varlistentry>
1730 <term>
1731 <option>-O</option> or <option>-O1</option>:
1732 <indexterm><primary>-O option</primary></indexterm>
1733 <indexterm><primary>-O1 option</primary></indexterm>
1734 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1735 </term>
1736 <listitem>
1737 <para>Means: &ldquo;Generate good-quality code without
1738 taking too long about it.&rdquo; Thus, for example:
1739 <command>ghc -c -O Main.lhs</command></para>
1740 </listitem>
1741 </varlistentry>
1742
1743 <varlistentry>
1744 <term>
1745 <option>-O2</option>:
1746 <indexterm><primary>-O2 option</primary></indexterm>
1747 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1748 </term>
1749 <listitem>
1750 <para>Means: &ldquo;Apply every non-dangerous
1751 optimisation, even if it means significantly longer
1752 compile times.&rdquo;</para>
1753
1754 <para>The avoided &ldquo;dangerous&rdquo; optimisations
1755 are those that can make runtime or space
1756 <emphasis>worse</emphasis> if you're unlucky. They are
1757 normally turned on or off individually.</para>
1758
1759 <para>At the moment, <option>-O2</option> is
1760 <emphasis>unlikely</emphasis> to produce better code than
1761 <option>-O</option>.</para>
1762 </listitem>
1763 </varlistentry>
1764 </variablelist>
1765
1766 <para>We don't use a <option>-O*</option> flag for day-to-day
1767 work. We use <option>-O</option> to get respectable speed;
1768 e.g., when we want to measure something. When we want to go for
1769 broke, we tend to use <option>-O2</option> (and we go for
1770 lots of coffee breaks).</para>
1771
1772 <para>The easiest way to see what <option>-O</option> (etc.)
1773 &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1774 then stand back in amazement.</para>
1775 </sect2>
1776
1777 <sect2 id="options-f">
1778 <title><option>-f*</option>: platform-independent flags</title>
1779
1780 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1781 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1782
1783 <para>These flags turn on and off individual optimisations.
1784 They are normally set via the <option>-O</option> options
1785 described above, and as such, you shouldn't need to set any of
1786 them explicitly (indeed, doing so could lead to unexpected
1787 results). A flag <option>-fwombat</option> can be negated by
1788 saying <option>-fno-wombat</option>. The flags below are off
1789 by default, except where noted below.
1790 </para>
1791
1792 <variablelist>
1793 <varlistentry>
1794 <term>
1795 <option>-fcse</option>
1796 <indexterm><primary><option>-fcse</option></primary></indexterm>
1797 </term>
1798 <listitem>
1799 <para><emphasis>On by default.</emphasis>. Enables the common-sub-expression
1800 elimination optimisation.
1801 Switching this off can be useful if you have some <literal>unsafePerformIO</literal>
1802 expressions that you don't want commoned-up.</para>
1803 </listitem>
1804 </varlistentry>
1805
1806 <varlistentry>
1807 <term>
1808 <option>-fstrictness</option>
1809 <indexterm><primary><option></option></primary></indexterm>
1810 </term>
1811 <listitem>
1812 <para> <emphasis>On by default.</emphasis>.
1813 Switch on the strictness analyser. There is a very old paper about GHC's
1814 strictness analyser, <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/simple-strictnes-analyser.ps.gz">
1815 Measuring the effectiveness of a simple strictness analyser</ulink>,
1816 but the current one is quite a bit different.
1817 </para>
1818
1819 <para>The strictness analyser figures out when arguments and
1820 variables in a function can be treated 'strictly' (that is they
1821 are always evaluated in the function at some point). This allow
1822 GHC to apply certain optimisations such as unboxing that
1823 otherwise don't apply as they change the semantics of the program
1824 when applied to lazy arguments.
1825 </para>
1826 </listitem>
1827 </varlistentry>
1828
1829 <varlistentry>
1830 <term>
1831 <option>-funbox-strict-fields</option>:
1832 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1833 <indexterm><primary>strict constructor fields</primary></indexterm>
1834 <indexterm><primary>constructor fields, strict</primary></indexterm>
1835 </term>
1836 <listitem>
1837 <para>This option causes all constructor fields which are marked
1838 strict (i.e. &ldquo;!&rdquo;) to be unpacked if possible. It is
1839 equivalent to adding an <literal>UNPACK</literal> pragma to every
1840 strict constructor field (see <xref linkend="unpack-pragma"/>).
1841 </para>
1842
1843 <para>This option is a bit of a sledgehammer: it might sometimes
1844 make things worse. Selectively unboxing fields by using
1845 <literal>UNPACK</literal> pragmas might be better. An alternative
1846 is to use <option>-funbox-strict-fields</option> to turn on
1847 unboxing by default but disable it for certain constructor
1848 fields using the <literal>NOUNPACK</literal> pragma (see
1849 <xref linkend="nounpack-pragma"/>).</para>
1850 </listitem>
1851 </varlistentry>
1852
1853 <varlistentry>
1854 <term>
1855 <option>-fspec-constr</option>
1856 <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
1857 </term>
1858 <listitem>
1859 <para><emphasis>Off by default, but enabled by -O2.</emphasis>
1860 Turn on call-pattern specialisation; see
1861 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/spec-constr/index.htm">
1862 Call-pattern specialisation for Haskell programs</ulink>.
1863 </para>
1864
1865 <para>This optimisation specializes recursive functions according to
1866 their argument "shapes". This is best explained by example so
1867 consider:
1868 <programlisting>
1869 last :: [a] -> a
1870 last [] = error "last"
1871 last (x : []) = x
1872 last (x : xs) = last xs
1873 </programlisting>
1874 In this code, once we pass the initial check for an empty list we
1875 know that in the recursive case this pattern match is redundant. As
1876 such <option>-fspec-constr</option> will transform the above code
1877 to:
1878 <programlisting>
1879 last :: [a] -> a
1880 last [] = error "last"
1881 last (x : xs) = last' x xs
1882 where
1883 last' x [] = x
1884 last' x (y : ys) = last' y ys
1885 </programlisting>
1886 </para>
1887
1888 <para>As well avoid unnecessary pattern matching it also helps avoid
1889 unnecessary allocation. This applies when a argument is strict in
1890 the recursive call to itself but not on the initial entry. As
1891 strict recursive branch of the function is created similar to the
1892 above example.
1893 </para>
1894 </listitem>
1895 </varlistentry>
1896
1897 <varlistentry>
1898 <term>
1899 <option>-fspecialise</option>
1900 <indexterm><primary><option>-fspecialise</option></primary></indexterm>
1901 </term>
1902 <listitem>
1903 <para><emphasis>On by default.</emphasis>
1904 Specialise each type-class-overloaded function defined in this
1905 module for the types at which it is called in this module. Also
1906 specialise imported functions that have an INLINABLE pragma
1907 (<xref linkend="inlinable-pragma"/>) for the types at which they
1908 are called in this module.
1909 </para>
1910 </listitem>
1911 </varlistentry>
1912
1913 <varlistentry>
1914 <term>
1915 <option>-fstatic-argument-transformation</option>
1916 <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
1917 </term>
1918 <listitem>
1919 <para>Turn on the static argument transformation, which turns a
1920 recursive function into a non-recursive one with a local
1921 recursive loop. See Chapter 7 of
1922 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/santos-thesis.ps.gz">
1923 Andre Santos's PhD thesis</ulink>
1924 </para>
1925 </listitem>
1926 </varlistentry>
1927
1928 <varlistentry>
1929 <term>
1930 <option>-ffloat-in</option>
1931 <indexterm><primary><option></option></primary></indexterm>
1932 </term>
1933 <listitem>
1934 <para><emphasis>On by default.</emphasis>
1935 Float let-bindings inwards, nearer their binding site. See
1936 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">
1937 Let-floating: moving bindings to give faster programs (ICFP'96)</ulink>.
1938 </para>
1939
1940 <para>This optimisation moves let bindings closer to their use
1941 site. The benefit here is that this may avoid unnecessary
1942 allocation if the branch the let is now on is never executed. It
1943 also enables other optimisation passes to work more effectively
1944 as they have more information locally.
1945 </para>
1946
1947 <para>This optimisation isn't always beneficial though (so GHC
1948 applies some heuristics to decide when to apply it). The details
1949 get complicated but a simple example is that it is often beneficial
1950 to move let bindings outwards so that multiple let bindings can be
1951 grouped into a larger single let binding, effectively batching
1952 their allocation and helping the garbage collector and allocator.
1953 </para>
1954 </listitem>
1955 </varlistentry>
1956
1957 <varlistentry>
1958 <term>
1959 <option>-ffull-laziness</option>
1960 <indexterm><primary><option>-ffull-laziness</option></primary></indexterm>
1961 </term>
1962 <listitem>
1963 <para><emphasis>On by default.</emphasis>
1964 Run the full laziness optimisation (also known as let-floating),
1965 which floats let-bindings outside enclosing lambdas, in the hope
1966 they will be thereby be computed less often. See
1967 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">Let-floating:
1968 moving bindings to give faster programs (ICFP'96)</ulink>.
1969 Full laziness increases sharing, which can lead to increased memory
1970 residency.
1971 </para>
1972
1973 <para>NOTE: GHC doesn't implement complete full-laziness.
1974 When optimisation in on, and <option>-fno-full-laziness</option>
1975 is not given, some transformations that increase sharing are
1976 performed, such as extracting repeated computations from a loop.
1977 These are the same transformations that a fully lazy
1978 implementation would do, the difference is that GHC doesn't
1979 consistently apply full-laziness, so don't rely on it.
1980 </para>
1981 </listitem>
1982 </varlistentry>
1983
1984 <varlistentry>
1985 <term>
1986 <option>-fdo-lambda-eta-expansion</option>
1987 <indexterm><primary><option></option></primary></indexterm>
1988 </term>
1989 <listitem>
1990 <para><emphasis>On by default.</emphasis>
1991 Eta-expand let-bindings to increase their arity.
1992 </para>
1993 </listitem>
1994 </varlistentry>
1995
1996 <varlistentry>
1997 <term>
1998 <option>-fdo-eta-reduction</option>
1999 <indexterm><primary><option></option></primary></indexterm>
2000 </term>
2001 <listitem>
2002 <para><emphasis>On by default.</emphasis>
2003 Eta-reduce lambda expressions, if doing so gets rid of a whole
2004 group of lambdas.
2005 </para>
2006 </listitem>
2007 </varlistentry>
2008
2009 <varlistentry>
2010 <term>
2011 <option>-fcase-merge</option>
2012 <indexterm><primary><option></option></primary></indexterm>
2013 </term>
2014 <listitem>
2015 <para><emphasis>On by default.</emphasis>
2016 Merge immediately-nested case expressions that scrutinse the same variable. Example
2017 <programlisting>
2018 case x of
2019 Red -> e1
2020 _ -> case x of
2021 Blue -> e2
2022 Green -> e3
2023 ==>
2024 case x of
2025 Red -> e1
2026 Blue -> e2
2027 Green -> e2
2028 </programlisting>
2029 </para>
2030 </listitem>
2031 </varlistentry>
2032
2033 <varlistentry>
2034 <term>
2035 <option>-fliberate-case</option>
2036 <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
2037 </term>
2038 <listitem>
2039 <para><emphasis>Off by default, but enabled by -O2.</emphasis>
2040 Turn on the liberate-case transformation. This unrolls recursive
2041 function once in its own RHS, to avoid repeated case analysis of
2042 free variables. It's a bit like the call-pattern specialiser
2043 (<option>-fspec-constr</option>) but for free variables rather than
2044 arguments.
2045 </para>
2046 </listitem>
2047 </varlistentry>
2048
2049 <varlistentry>
2050 <term>
2051 <option>-fdicts-cheap</option>
2052 <indexterm><primary><option></option></primary></indexterm>
2053 </term>
2054 <listitem>
2055 <para>A very experimental flag that makes dictionary-valued
2056 expressions seem cheap to the optimiser.
2057 </para>
2058 </listitem>
2059 </varlistentry>
2060
2061 <varlistentry>
2062 <term>
2063 <option>-feager-blackholing</option>
2064 <indexterm><primary><option></option></primary></indexterm>
2065 </term>
2066 <listitem>
2067 <para>Usually GHC black-holes a thunk only when it switches
2068 threads. This flag makes it do so as soon as the thunk is
2069 entered. See <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/">
2070 Haskell on a shared-memory multiprocessor</ulink>.
2071 </para>
2072 </listitem>
2073 </varlistentry>
2074
2075 <varlistentry>
2076 <term>
2077 <option>-fno-state-hack</option>
2078 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
2079 </term>
2080 <listitem>
2081 <para>Turn off the "state hack" whereby any lambda with a
2082 <literal>State#</literal> token as argument is considered to be
2083 single-entry, hence it is considered OK to inline things inside
2084 it. This can improve performance of IO and ST monad code, but it
2085 runs the risk of reducing sharing.
2086 </para>
2087 </listitem>
2088 </varlistentry>
2089
2090 <varlistentry>
2091 <term>
2092 <option>-fpedantic-bottoms</option>
2093 <indexterm><primary><option>-fpedantic-bottoms</option></primary></indexterm>
2094 </term>
2095 <listitem>
2096 <para>Make GHC be more precise about its treatment of bottom (but see also
2097 <option>-fno-state-hack</option>). In particular, stop GHC
2098 eta-expanding through a case expression, which is good for
2099 performance, but bad if you are using <literal>seq</literal> on
2100 partial applications.
2101 </para>
2102 </listitem>
2103 </varlistentry>
2104
2105 <varlistentry>
2106 <term>
2107 <option>-fsimpl-tick-factor=<replaceable>n</replaceable></option>
2108 <indexterm><primary><option>-fsimpl-tick-factor</option></primary></indexterm>
2109 </term>
2110 <listitem>
2111 <para>GHC's optimiser can diverge if you write rewrite rules (
2112 <xref linkend="rewrite-rules"/>) that don't terminate, or (less
2113 satisfactorily) if you code up recursion through data types
2114 (<xref linkend="bugs-ghc"/>). To avoid making the compiler fall
2115 into an infinite loop, the optimiser carries a "tick count" and
2116 stops inlining and applying rewrite rules when this count is
2117 exceeded. The limit is set as a multiple of the program size, so
2118 bigger programs get more ticks. The
2119 <option>-fsimpl-tick-factor</option> flag lets you change the
2120 multiplier. The default is 100; numbers larger than 100 give more
2121 ticks, and numbers smaller than 100 give fewer.
2122 </para>
2123
2124 <para>If the tick-count expires, GHC summarises what simplifier
2125 steps it has done; you can use
2126 <option>-fddump-simpl-stats</option> to generate a much more
2127 detailed list. Usually that identifies the loop quite
2128 accurately, because some numbers are very large.
2129 </para>
2130 </listitem>
2131 </varlistentry>
2132
2133 <varlistentry>
2134 <term>
2135 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
2136 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
2137 <indexterm><primary>inlining, controlling</primary></indexterm>
2138 <indexterm><primary>unfolding, controlling</primary></indexterm>
2139 </term>
2140 <listitem>
2141 <para>(Default: 45) Governs the maximum size that GHC will allow a
2142 function unfolding to be. (An unfolding has a &ldquo;size&rdquo;
2143 that reflects the cost in terms of &ldquo;code bloat&rdquo; of
2144 expanding (aka inlining) that unfolding at a call site. A bigger
2145 function would be assigned a bigger cost.)
2146 </para>
2147
2148 <para>Consequences: (a) nothing larger than this will be inlined
2149 (unless it has an INLINE pragma); (b) nothing larger than this
2150 will be spewed into an interface file.
2151 </para>
2152
2153 <para>Increasing this figure is more likely to result in longer
2154 compile times than faster code. The
2155 <option>-funfolding-use-threshold</option> is more useful.
2156 </para>
2157 </listitem>
2158 </varlistentry>
2159
2160 <varlistentry>
2161 <term>
2162 <option>-funfolding-use-threshold=<replaceable>n</replaceable></option>
2163 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
2164 <indexterm><primary>inlining, controlling</primary></indexterm>
2165 <indexterm><primary>unfolding, controlling</primary></indexterm>
2166 </term>
2167 <listitem>
2168 <para>(Default: 8) This is the magic cut-off figure for unfolding
2169 (aka inlining): below this size, a function definition will be
2170 unfolded at the call-site, any bigger and it won't. The size
2171 computed for a function depends on two things: the actual size of
2172 the expression minus any discounts that
2173 apply (see <option>-funfolding-con-discount</option>).
2174 </para>
2175
2176 <para>The difference between this and
2177 <option>-funfolding-creation-threshold</option> is that this one
2178 determines if a function definition will be inlined <emphasis>at
2179 a call site</emphasis>. The other option determines if a
2180 function definition will be kept around at all for potential
2181 inlining.
2182 </para>
2183 </listitem>
2184 </varlistentry>
2185
2186 <varlistentry>
2187 <term>
2188 <option>-fexpose-all-unfoldings</option>
2189 <indexterm><primary><option></option></primary></indexterm>
2190 </term>
2191 <listitem>
2192 <para>An experimental flag to expose all unfoldings, even for very
2193 large or recursive functions. This allows for all functions to be
2194 inlined while usually GHC would avoid inlining larger functions.
2195 </para>
2196 </listitem>
2197 </varlistentry>
2198
2199 <varlistentry>
2200 <term>
2201 <option>-fvectorise</option>
2202 <indexterm><primary><option></option></primary></indexterm>
2203 </term>
2204 <listitem>
2205 <para>Data Parallel Haskell.
2206 </para>
2207 TODO: Document optimisation
2208 </listitem>
2209 </varlistentry>
2210
2211 <varlistentry>
2212 <term>
2213 <option>-favoid-vect</option>
2214 <indexterm><primary><option></option></primary></indexterm>
2215 </term>
2216 <listitem>
2217 <para>Data Parallel Haskell.
2218 </para>
2219 TODO: Document optimisation
2220 </listitem>
2221 </varlistentry>
2222
2223 <varlistentry>
2224 <term>
2225 <option>-fregs-graph</option>
2226 <indexterm><primary><option></option></primary></indexterm>
2227 </term>
2228 <listitem>
2229 <para><emphasis>Off by default, but enabled by -O2. Only applies in
2230 combination with the native code generator.</emphasis>
2231 Use the graph colouring register allocator for register allocation
2232 in the native code generator. By default, GHC uses a simpler,
2233 faster linear register allocator. The downside being that the
2234 linear register allocator usually generates worse code.
2235 </para>
2236 </listitem>
2237 </varlistentry>
2238
2239 <varlistentry>
2240 <term>
2241 <option>-fregs-iterative</option>
2242 <indexterm><primary><option></option></primary></indexterm>
2243 </term>
2244 <listitem>
2245 <para><emphasis>Off by default, only applies in combination with
2246 the native code generator.</emphasis>
2247 Use the iterative coalescing graph colouring register allocator for
2248 register allocation in the native code generator. This is the same
2249 register allocator as the <option>-freg-graph</option> one but also
2250 enables iterative coalescing during register allocation.
2251 </para>
2252 </listitem>
2253 </varlistentry>
2254
2255 <varlistentry>
2256 <term>
2257 <option>-fexcess-precision</option>
2258 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
2259 </term>
2260 <listitem>
2261 <para>When this option is given, intermediate floating
2262 point values can have a <emphasis>greater</emphasis>
2263 precision/range than the final type. Generally this is a
2264 good thing, but some programs may rely on the exact
2265 precision/range of
2266 <literal>Float</literal>/<literal>Double</literal> values
2267 and should not use this option for their compilation.</para>
2268 </listitem>
2269 </varlistentry>
2270
2271 <varlistentry>
2272 <term>
2273 <option>-fignore-asserts</option>
2274 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
2275 </term>
2276 <listitem>
2277 <para>Causes GHC to ignore uses of the function
2278 <literal>Exception.assert</literal> in source code (in
2279 other words, rewriting <literal>Exception.assert p
2280 e</literal> to <literal>e</literal> (see <xref
2281 linkend="assertions"/>). This flag is turned on by
2282 <option>-O</option>.
2283 </para>
2284 </listitem>
2285 </varlistentry>
2286
2287 <varlistentry>
2288 <term>
2289 <option>-fignore-interface-pragmas</option>
2290 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
2291 </term>
2292 <listitem>
2293 <para>Tells GHC to ignore all inessential information when reading interface files.
2294 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
2295 for a function, GHC will ignore that information.</para>
2296 </listitem>
2297 </varlistentry>
2298
2299 <varlistentry>
2300 <term>
2301 <option>-fomit-interface-pragmas</option>
2302 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
2303 </term>
2304 <listitem>
2305 <para>Tells GHC to omit all inessential information from the
2306 interface file generated for the module being compiled (say M).
2307 This means that a module importing M will see only the
2308 <emphasis>types</emphasis> of the functions that M exports, but
2309 not their unfoldings, strictness info, etc. Hence, for example,
2310 no function exported by M will be inlined into an importing module.
2311 The benefit is that modules that import M will need to be
2312 recompiled less often (only when M's exports change their type, not
2313 when they change their implementation).</para>
2314 </listitem>
2315 </varlistentry>
2316
2317 </variablelist>
2318
2319 </sect2>
2320
2321 </sect1>
2322
2323 &code-gens;
2324
2325 &phases;
2326
2327 &shared_libs;
2328
2329 <sect1 id="using-concurrent">
2330 <title>Using Concurrent Haskell</title>
2331 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
2332
2333 <para>GHC supports Concurrent Haskell by default, without requiring a
2334 special option or libraries compiled in a certain way. To get access to
2335 the support libraries for Concurrent Haskell, just import
2336 <ulink
2337 url="&libraryBaseLocation;/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
2338
2339 <para>
2340 Optionally, the program may be linked with
2341 the <option>-threaded</option> option (see
2342 <xref linkend="options-linker" />. This provides two benefits:
2343
2344 <itemizedlist>
2345 <listitem>
2346 <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
2347 used, which allows threads to run in
2348 parallel<indexterm><primary>parallelism</primary></indexterm>
2349 on a
2350 multiprocessor<indexterm><primary>multiprocessor</primary></indexterm><indexterm><primary>SMP</primary></indexterm>
2351 or
2352 multicore<indexterm><primary>multicore</primary></indexterm>
2353 machine. See <xref linkend="using-smp" />.</para>
2354 </listitem>
2355 <listitem>
2356 <para>If a thread makes a foreign call (and the call is
2357 not marked <literal>unsafe</literal>), then other
2358 Haskell threads in the program will continue to run
2359 while the foreign call is in progress.
2360 Additionally, <literal>foreign export</literal>ed
2361 Haskell functions may be called from multiple OS
2362 threads simultaneously. See
2363 <xref linkend="ffi-threads" />.</para>
2364 </listitem>
2365 </itemizedlist>
2366 </para>
2367
2368 <para>The following RTS option(s) affect the behaviour of Concurrent
2369 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
2370
2371 <variablelist>
2372 <varlistentry>
2373 <term><option>-C<replaceable>s</replaceable></option></term>
2374 <listitem>
2375 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2376 Sets the context switch interval to <replaceable>s</replaceable>
2377 seconds. A context switch will occur at the next heap block
2378 allocation after the timer expires (a heap block allocation occurs
2379 every 4k of allocation). With <option>-C0</option> or
2380 <option>-C</option>, context switches will occur as often as
2381 possible (at every heap block allocation). By default, context
2382 switches occur every 20ms.</para>
2383 </listitem>
2384 </varlistentry>
2385 </variablelist>
2386 </sect1>
2387
2388 <sect1 id="using-smp">
2389 <title>Using SMP parallelism</title>
2390 <indexterm><primary>parallelism</primary>
2391 </indexterm>
2392 <indexterm><primary>SMP</primary>
2393 </indexterm>
2394
2395 <para>GHC supports running Haskell programs in parallel on an SMP
2396 (symmetric multiprocessor).</para>
2397
2398 <para>There's a fine distinction between
2399 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
2400 parallelism is all about making your program run
2401 <emphasis>faster</emphasis> by making use of multiple processors
2402 simultaneously. Concurrency, on the other hand, is a means of
2403 abstraction: it is a convenient way to structure a program that must
2404 respond to multiple asynchronous events.</para>
2405
2406 <para>However, the two terms are certainly related. By making use of
2407 multiple CPUs it is possible to run concurrent threads in parallel,
2408 and this is exactly what GHC's SMP parallelism support does. But it
2409 is also possible to obtain performance improvements with parallelism
2410 on programs that do not use concurrency. This section describes how to
2411 use GHC to compile and run parallel programs, in <xref
2412 linkend="lang-parallel" /> we describe the language features that affect
2413 parallelism.</para>
2414
2415 <sect2 id="parallel-compile-options">
2416 <title>Compile-time options for SMP parallelism</title>
2417
2418 <para>In order to make use of multiple CPUs, your program must be
2419 linked with the <option>-threaded</option> option (see <xref
2420 linkend="options-linker" />). Additionally, the following
2421 compiler options affect parallelism:</para>
2422
2423 <variablelist>
2424 <varlistentry>
2425 <term><option>-feager-blackholing</option></term>
2426 <indexterm><primary><option>-feager-blackholing</option></primary></indexterm>
2427 <listitem>
2428 <para>
2429 Blackholing is the act of marking a thunk (lazy
2430 computuation) as being under evaluation. It is useful for
2431 three reasons: firstly it lets us detect certain kinds of
2432 infinite loop (the <literal>NonTermination</literal>
2433 exception), secondly it avoids certain kinds of space
2434 leak, and thirdly it avoids repeating a computation in a
2435 parallel program, because we can tell when a computation
2436 is already in progress.</para>
2437
2438 <para>
2439 The option <option>-feager-blackholing</option> causes
2440 each thunk to be blackholed as soon as evaluation begins.
2441 The default is "lazy blackholing", whereby thunks are only
2442 marked as being under evaluation when a thread is paused
2443 for some reason. Lazy blackholing is typically more
2444 efficient (by 1-2&percnt; or so), because most thunks don't
2445 need to be blackholed. However, eager blackholing can
2446 avoid more repeated computation in a parallel program, and
2447 this often turns out to be important for parallelism.
2448 </para>
2449
2450 <para>
2451 We recommend compiling any code that is intended to be run
2452 in parallel with the <option>-feager-blackholing</option>
2453 flag.
2454 </para>
2455 </listitem>
2456 </varlistentry>
2457 </variablelist>
2458 </sect2>
2459
2460 <sect2 id="parallel-options">
2461 <title>RTS options for SMP parallelism</title>
2462
2463 <para>There are two ways to run a program on multiple
2464 processors:
2465 call <literal>Control.Concurrent.setNumCapabilities</literal> from your
2466 program, or use the RTS <option>-N</option> option.</para>
2467
2468 <variablelist>
2469 <varlistentry>
2470 <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
2471 <listitem>
2472 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2473 Use <replaceable>x</replaceable> simultaneous threads when
2474 running the program. Normally <replaceable>x</replaceable>
2475 should be chosen to match the number of CPU cores on the
2476 machine<footnote><para>Whether hyperthreading cores should be counted or not is an
2477 open question; please feel free to experiment and let us know what
2478 results you find.</para></footnote>. For example,
2479 on a dual-core machine we would probably use
2480 <literal>+RTS -N2 -RTS</literal>.</para>
2481
2482 <para>Omitting <replaceable>x</replaceable>,
2483 i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
2484 choose the value of <replaceable>x</replaceable> itself
2485 based on how many processors are in your machine.</para>
2486
2487 <para>Be careful when using all the processors in your
2488 machine: if some of your processors are in use by other
2489 programs, this can actually harm performance rather than
2490 improve it.</para>
2491
2492 <para>Setting <option>-N</option> also has the effect of
2493 enabling the parallel garbage collector (see
2494 <xref linkend="rts-options-gc" />).</para>
2495
2496 <para>The current value of the <option>-N</option> option
2497 is available to the Haskell program
2498 via <literal>Control.Concurrent.getNumCapabilities</literal>, and
2499 it may be changed while the program is running by
2500 calling <literal>Control.Concurrent.setNumCapabilities</literal>.</para>
2501 </listitem>
2502 </varlistentry>
2503 </variablelist>
2504
2505 <para>The following options affect the way the runtime schedules
2506 threads on CPUs:</para>
2507
2508 <variablelist>
2509 <varlistentry>
2510 <term><option>-qa</option></term>
2511 <indexterm><primary><option>-qa</option></primary><secondary>RTS
2512 option</secondary></indexterm>
2513 <listitem>
2514 <para>Use the OS's affinity facilities to try to pin OS
2515 threads to CPU cores. This is an experimental feature,
2516 and may or may not be useful. Please let us know
2517 whether it helps for you!</para>
2518 </listitem>
2519 </varlistentry>
2520 <varlistentry>
2521 <term><option>-qm</option></term>
2522 <indexterm><primary><option>-qm</option></primary><secondary>RTS
2523 option</secondary></indexterm>
2524 <listitem>
2525 <para>Disable automatic migration for load balancing.
2526 Normally the runtime will automatically try to schedule
2527 threads across the available CPUs to make use of idle
2528 CPUs; this option disables that behaviour. Note that
2529 migration only applies to threads; sparks created
2530 by <literal>par</literal> are load-balanced separately
2531 by work-stealing.</para>
2532
2533 <para>
2534 This option is probably only of use for concurrent
2535 programs that explicitly schedule threads onto CPUs
2536 with <literal>Control.Concurrent.forkOn</literal>.
2537 </para>
2538 </listitem>
2539 </varlistentry>
2540 </variablelist>
2541 </sect2>
2542
2543 <sect2>
2544 <title>Hints for using SMP parallelism</title>
2545
2546 <para>Add the <literal>-s</literal> RTS option when
2547 running the program to see timing stats, which will help to tell you
2548 whether your program got faster by using more CPUs or not. If the user
2549 time is greater than
2550 the elapsed time, then the program used more than one CPU. You should
2551 also run the program without <literal>-N</literal> for
2552 comparison.</para>
2553
2554 <para>The output of <literal>+RTS -s</literal> tells you how
2555 many &ldquo;sparks&rdquo; were created and executed during the
2556 run of the program (see <xref linkend="rts-options-gc" />), which
2557 will give you an idea how well your <literal>par</literal>
2558 annotations are working.</para>
2559
2560 <para>GHC's parallelism support has improved in 6.12.1 as a
2561 result of much experimentation and tuning in the runtime
2562 system. We'd still be interested to hear how well it works
2563 for you, and we're also interested in collecting parallel
2564 programs to add to our benchmarking suite.</para>
2565 </sect2>
2566 </sect1>
2567
2568 <sect1 id="options-platform">
2569 <title>Platform-specific Flags</title>
2570
2571 <indexterm><primary>-m* options</primary></indexterm>
2572 <indexterm><primary>platform-specific options</primary></indexterm>
2573 <indexterm><primary>machine-specific options</primary></indexterm>
2574
2575 <para>Some flags only make sense for particular target
2576 platforms.</para>
2577
2578 <variablelist>
2579
2580 <varlistentry>
2581 <term><option>-msse2</option>:</term>
2582 <listitem>
2583 <para>
2584 (x86 only, added in GHC 7.0.1) Use the SSE2 registers and
2585 instruction set to implement floating point operations when using
2586 the <link linkend="native-code-gen">native code generator</link>.
2587 This gives a substantial performance improvement for floating
2588 point, but the resulting compiled code
2589 will only run on processors that support SSE2 (Intel Pentium 4 and
2590 later, or AMD Athlon 64 and later). The
2591 <link linkend="llvm-code-gen">LLVM backend</link> will also use SSE2
2592 if your processor supports it but detects this automatically so no
2593 flag is required.
2594 </para>
2595 <para>
2596 SSE2 is unconditionally used on x86-64 platforms.
2597 </para>
2598 </listitem>
2599 </varlistentry>
2600
2601 <varlistentry>
2602 <term><option>-msse4.2</option>:</term>
2603 <listitem>
2604 <para>
2605 (x86 only, added in GHC 7.4.1) Use the SSE4.2 instruction set to
2606 implement some floating point and bit operations when using the
2607 <link linkend="native-code-gen">native code generator</link>. The
2608 resulting compiled code will only run on processors that
2609 support SSE4.2 (Intel Core i7 and later). The
2610 <link linkend="llvm-code-gen">LLVM backend</link> will also use
2611 SSE4.2 if your processor supports it but detects this automatically
2612 so no flag is required.
2613 </para>
2614 </listitem>
2615 </varlistentry>
2616
2617 </variablelist>
2618
2619 </sect1>
2620
2621 &runtime;
2622
2623 <sect1 id="ext-core">
2624 <title>Generating and compiling External Core Files</title>
2625
2626 <indexterm><primary>intermediate code generation</primary></indexterm>
2627
2628 <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format)
2629 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
2630 <filename>.hcr</filename>. The Core format is described in <ulink url="../../core.pdf">
2631 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
2632 and sample tools
2633 for manipulating Core files (in Haskell) are available in the
2634 <ulink url="http://hackage.haskell.org/package/extcore">extcore package on Hackage</ulink>. Note that the format of <literal>.hcr</literal>
2635 files is <emphasis>different</emphasis> from the Core output format that GHC generates
2636 for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
2637
2638 <para>The Core format natively supports notes which you can add to
2639 your source code using the <literal>CORE</literal> pragma (see <xref
2640 linkend="pragmas"/>).</para>
2641
2642 <variablelist>
2643
2644 <varlistentry>
2645 <term>
2646 <option>-fext-core</option>
2647 <indexterm><primary><option>-fext-core</option></primary></indexterm>
2648 </term>
2649 <listitem>
2650 <para>Generate <literal>.hcr</literal> files.</para>
2651 </listitem>
2652 </varlistentry>
2653
2654 </variablelist>
2655
2656 <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>
2657
2658 </sect1>
2659
2660 &debug;
2661 &flags;
2662
2663 </chapter>
2664
2665 <!-- Emacs stuff:
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