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1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="using-ghc">
3 <title>Using GHC</title>
4
5 <indexterm><primary>GHC, using</primary></indexterm>
6 <indexterm><primary>using GHC</primary></indexterm>
7
8 <sect1>
9 <title>Getting started: compiling programs</title>
10
11 <para>
12 In this chapter you'll find a complete reference to the GHC
13 command-line syntax, including all 400+ flags. It's a large and
14 complex system, and there are lots of details, so it can be
15 quite hard to figure out how to get started. With that in mind,
16 this introductory section provides a quick introduction to the
17 basic usage of GHC for compiling a Haskell program, before the
18 following sections dive into the full syntax.
19 </para>
20
21 <para>
22 Let's create a Hello World program, and compile and run it.
23 First, create a file <filename>hello.hs</filename> containing
24 the Haskell code:
25 </para>
26
27 <programlisting>
28 main = putStrLn "Hello, World!"
29 </programlisting>
30
31 <para>To compile the program, use GHC like this:</para>
32
33 <screen>
34 $ ghc hello.hs
35 </screen>
36
37 <para>(where <literal>$</literal> represents the prompt: don't
38 type it). GHC will compile the source
39 file <filename>hello.hs</filename>, producing
40 an <firstterm>object
41 file</firstterm> <filename>hello.o</filename> and
42 an <firstterm>interface
43 file</firstterm> <filename>hello.hi</filename>, and then it
44 will link the object file to the libraries that come with GHC
45 to produce an executable called <filename>hello</filename> on
46 Unix/Linux/Mac, or <filename>hello.exe</filename> on
47 Windows.</para>
48
49 <para>
50 By default GHC will be very quiet about what it is doing, only
51 printing error messages. If you want to see in more detail
52 what's going on behind the scenes, add <option>-v</option> to
53 the command line.
54 </para>
55
56 <para>
57 Then we can run the program like this:
58 </para>
59
60 <screen>
61 $ ./hello
62 Hello World!
63 </screen>
64
65 <para>
66 If your program contains multiple modules, then you only need to
67 tell GHC the name of the source file containing
68 the <filename>Main</filename> module, and GHC will examine
69 the <literal>import</literal> declarations to find the other
70 modules that make up the program and find their source files.
71 This means that, with the exception of
72 the <literal>Main</literal> module, every source file should be
73 named after the module name that it contains (with dots replaced
74 by directory separators). For example, the
75 module <literal>Data.Person</literal> would be in the
76 file <filename>Data/Person.hs</filename> on Unix/Linux/Mac,
77 or <filename>Data\Person.hs</filename> on Windows.
78 </para>
79 </sect1>
80
81 <sect1>
82 <title>Options overview</title>
83
84 <para>GHC's behaviour is controlled by
85 <firstterm>options</firstterm>, which for historical reasons are
86 also sometimes referred to as command-line flags or arguments.
87 Options can be specified in three ways:</para>
88
89 <sect2>
90 <title>Command-line arguments</title>
91
92 <indexterm><primary>structure, command-line</primary></indexterm>
93 <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
94 <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
95
96 <para>An invocation of GHC takes the following form:</para>
97
98 <screen>
99 ghc [argument...]
100 </screen>
101
102 <para>Command-line arguments are either options or file names.</para>
103
104 <para>Command-line options begin with <literal>-</literal>.
105 They may <emphasis>not</emphasis> be grouped:
106 <option>-vO</option> is different from <option>-v -O</option>.
107 Options need not precede filenames: e.g., <literal>ghc *.o -o
108 foo</literal>. All options are processed and then applied to
109 all files; you cannot, for example, invoke <literal>ghc -c -O1
110 Foo.hs -O2 Bar.hs</literal> to apply different optimisation
111 levels to the files <filename>Foo.hs</filename> and
112 <filename>Bar.hs</filename>.</para>
113 </sect2>
114
115 <sect2 id="source-file-options">
116 <title>Command line options in source files</title>
117
118 <indexterm><primary>source-file options</primary></indexterm>
119
120 <para>Sometimes it is useful to make the connection between a
121 source file and the command-line options it requires quite
122 tight. For instance, if a Haskell source file deliberately
123 uses name shadowing, it should be compiled with the
124 <option>-fno-warn-name-shadowing</option> option. Rather than maintaining
125 the list of per-file options in a <filename>Makefile</filename>,
126 it is possible to do this directly in the source file using the
127 <literal>OPTIONS_GHC</literal> pragma <indexterm><primary>OPTIONS_GHC
128 pragma</primary></indexterm>:</para>
129
130 <programlisting>
131 {-# OPTIONS_GHC -fno-warn-name-shadowing #-}
132 module X where
133 ...
134 </programlisting>
135
136 <para><literal>OPTIONS_GHC</literal> is a <emphasis>file-header pragma</emphasis>
137 (see <xref linkend="pragmas"/>).</para>
138
139 <para>Only <emphasis>dynamic</emphasis> flags can be used in an <literal>OPTIONS_GHC</literal> pragma
140 (see <xref linkend="static-dynamic-flags"/>).</para>
141
142 <para>Note that your command shell does not
143 get to the source file options, they are just included literally
144 in the array of command-line arguments the compiler
145 maintains internally, so you'll be desperately disappointed if
146 you try to glob etc. inside <literal>OPTIONS_GHC</literal>.</para>
147
148 <para>NOTE: the contents of OPTIONS_GHC are appended to the
149 command-line options, so options given in the source file
150 override those given on the command-line.</para>
151
152 <para>It is not recommended to move all the contents of your
153 Makefiles into your source files, but in some circumstances, the
154 <literal>OPTIONS_GHC</literal> pragma is the Right Thing. (If you
155 use <option>-keep-hc-file</option> and have OPTION flags in
156 your module, the OPTIONS_GHC will get put into the generated .hc
157 file).</para>
158 </sect2>
159
160 <sect2>
161 <title>Setting options in GHCi</title>
162
163 <para>Options may also be modified from within GHCi, using the
164 <literal>:set</literal> command. See <xref linkend="ghci-set"/>
165 for more details.</para>
166 </sect2>
167 </sect1>
168
169 <sect1 id="static-dynamic-flags">
170 <title>Static, Dynamic, and Mode options</title>
171 <indexterm><primary>static</primary><secondary>options</secondary>
172 </indexterm>
173 <indexterm><primary>dynamic</primary><secondary>options</secondary>
174 </indexterm>
175 <indexterm><primary>mode</primary><secondary>options</secondary>
176 </indexterm>
177
178 <para>Each of GHC's command line options is classified as
179 <firstterm>static</firstterm>, <firstterm>dynamic</firstterm> or
180 <firstterm>mode</firstterm>:</para>
181
182 <variablelist>
183 <varlistentry>
184 <term>Mode flags</term>
185 <listitem>
186 <para>For example, <option>&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>, and
958 <option>-fwarn-dodgy-foreign-imports</option>. The following
959 flags are
960 simple ways to select standard &ldquo;packages&rdquo; of warnings:
961 </para>
962
963 <variablelist>
964
965 <varlistentry>
966 <term><option>-W</option>:</term>
967 <listitem>
968 <indexterm><primary>-W option</primary></indexterm>
969 <para>Provides the standard warnings plus
970 <option>-fwarn-incomplete-patterns</option>,
971 <option>-fwarn-dodgy-exports</option>,
972 <option>-fwarn-dodgy-imports</option>,
973 <option>-fwarn-unused-matches</option>,
974 <option>-fwarn-unused-imports</option>, and
975 <option>-fwarn-unused-binds</option>.</para>
976 </listitem>
977 </varlistentry>
978
979 <varlistentry>
980 <term><option>-Wall</option>:</term>
981 <listitem>
982 <indexterm><primary><option>-Wall</option></primary></indexterm>
983 <para>Turns on all warning options that indicate potentially
984 suspicious code. The warnings that are
985 <emphasis>not</emphasis> enabled by <option>-Wall</option>
986 are
987 <option>-fwarn-tabs</option>,
988 <option>-fwarn-incomplete-uni-patterns</option>,
989 <option>-fwarn-incomplete-record-updates</option>,
990 <option>-fwarn-monomorphism-restriction</option>,
991 <option>-fwarn-auto-orphans</option>,
992 <option>-fwarn-implicit-prelude</option>,
993 <option>-fwarn-missing-local-sigs</option>,
994 <option>-fwarn-missing-import-lists</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-dodgy-foreign-imports</option>:</term>
1108 <listitem>
1109 <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
1110 </indexterm>
1111 <para>Causes a warning to be emitted for foreign imports of
1112 the following form:</para>
1113
1114 <programlisting>
1115 foreign import "f" f :: FunPtr t
1116 </programlisting>
1117
1118 <para>on the grounds that it probably should be</para>
1119
1120 <programlisting>
1121 foreign import "&amp;f" f :: FunPtr t
1122 </programlisting>
1123
1124 <para>The first form declares that `f` is a (pure) C
1125 function that takes no arguments and returns a pointer to a
1126 C function with type `t`, whereas the second form declares
1127 that `f` itself is a C function with type `t`. The first
1128 declaration is usually a mistake, and one that is hard to
1129 debug because it results in a crash, hence this
1130 warning.</para>
1131 </listitem>
1132 </varlistentry>
1133
1134 <varlistentry>
1135 <term><option>-fwarn-dodgy-exports</option>:</term>
1136 <listitem>
1137 <indexterm><primary><option>-fwarn-dodgy-exports</option></primary>
1138 </indexterm>
1139 <para>Causes a warning to be emitted when a datatype
1140 <literal>T</literal> is exported
1141 with all constructors, i.e. <literal>T(..)</literal>, but is it
1142 just a type synonym.</para>
1143 <para>Also causes a warning to be emitted when a module is
1144 re-exported, but that module exports nothing.</para>
1145 </listitem>
1146 </varlistentry>
1147
1148 <varlistentry>
1149 <term><option>-fwarn-dodgy-imports</option>:</term>
1150 <listitem>
1151 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
1152 </indexterm>
1153 <para>Causes a warning to be emitted when a datatype
1154 <literal>T</literal> is imported
1155 with all constructors, i.e. <literal>T(..)</literal>, but has been
1156 exported abstractly, i.e. <literal>T</literal>.</para>
1157 </listitem>
1158 </varlistentry>
1159
1160 <varlistentry>
1161 <term><option>-fwarn-lazy-unlifted-bindings</option>:</term>
1162 <listitem>
1163 <indexterm><primary><option>-fwarn-lazy-unlifted-bindings</option></primary>
1164 </indexterm>
1165 <para>Causes a warning to be emitted when an unlifted type
1166 is bound in a way that looks lazy, e.g.
1167 <literal>where (I# x) = ...</literal>. Use
1168 <literal>where !(I# x) = ...</literal> instead. This will be an
1169 error, rather than a warning, in GHC 7.2.
1170 </para>
1171 </listitem>
1172 </varlistentry>
1173
1174 <varlistentry>
1175 <term><option>-fwarn-duplicate-exports</option>:</term>
1176 <listitem>
1177 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
1178 <indexterm><primary>duplicate exports, warning</primary></indexterm>
1179 <indexterm><primary>export lists, duplicates</primary></indexterm>
1180
1181 <para>Have the compiler warn about duplicate entries in
1182 export lists. This is useful information if you maintain
1183 large export lists, and want to avoid the continued export
1184 of a definition after you've deleted (one) mention of it in
1185 the export list.</para>
1186
1187 <para>This option is on by default.</para>
1188 </listitem>
1189 </varlistentry>
1190
1191 <varlistentry>
1192 <term><option>-fwarn-hi-shadowing</option>:</term>
1193 <listitem>
1194 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
1195 <indexterm><primary>shadowing</primary>
1196 <secondary>interface files</secondary></indexterm>
1197
1198 <para>Causes the compiler to emit a warning when a module or
1199 interface file in the current directory is shadowing one
1200 with the same module name in a library or other
1201 directory.</para>
1202 </listitem>
1203 </varlistentry>
1204
1205 <varlistentry>
1206 <term><option>-fwarn-identities</option>:</term>
1207 <listitem>
1208 <indexterm><primary><option>-fwarn-identities</option></primary></indexterm>
1209 <para>Causes the compiler to emit a warning when a Prelude numeric
1210 conversion converts a type T to the same type T; such calls
1211 are probably no-ops and can be omitted. The functions checked for
1212 are: <literal>toInteger</literal>,
1213 <literal>toRational</literal>,
1214 <literal>fromIntegral</literal>,
1215 and <literal>realToFrac</literal>.
1216 </para>
1217 </listitem>
1218 </varlistentry>
1219
1220 <varlistentry>
1221 <term><option>-fwarn-implicit-prelude</option>:</term>
1222 <listitem>
1223 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
1224 <indexterm><primary>implicit prelude, warning</primary></indexterm>
1225 <para>Have the compiler warn if the Prelude is implicitly
1226 imported. This happens unless either the Prelude module is
1227 explicitly imported with an <literal>import ... Prelude ...</literal>
1228 line, or this implicit import is disabled (either by
1229 <option>-XNoImplicitPrelude</option> or a
1230 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
1231
1232 <para>Note that no warning is given for syntax that implicitly
1233 refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
1234 would change whether it refers to the Prelude.
1235 For example, no warning is given when
1236 <literal>368</literal> means
1237 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
1238 (where <literal>Prelude</literal> refers to the actual Prelude module,
1239 regardless of the imports of the module being compiled).</para>
1240
1241 <para>This warning is off by default.</para>
1242 </listitem>
1243 </varlistentry>
1244
1245 <varlistentry>
1246 <term><option>-fwarn-incomplete-patterns</option>,
1247 <option>-fwarn-incomplete-uni-patterns</option>:
1248 </term>
1249 <listitem>
1250 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
1251 <indexterm><primary><option>-fwarn-incomplete-uni-patterns</option></primary></indexterm>
1252 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
1253 <indexterm><primary>patterns, incomplete</primary></indexterm>
1254
1255 <para>The option <option>-fwarn-incomplete-patterns</option> warns
1256 about places where
1257 a pattern-match might fail at runtime.
1258 The function
1259 <function>g</function> below will fail when applied to
1260 non-empty lists, so the compiler will emit a warning about
1261 this when <option>-fwarn-incomplete-patterns</option> is
1262 enabled.
1263
1264 <programlisting>
1265 g [] = 2
1266 </programlisting>
1267
1268 This option isn't enabled by default because it can be
1269 a bit noisy, and it doesn't always indicate a bug in the
1270 program. However, it's generally considered good practice
1271 to cover all the cases in your functions, and it is switched
1272 on by <option>-W</option>.</para>
1273
1274 <para>The flag <option>-fwarn-incomplete-uni-patterns</option> is
1275 similar, except that it
1276 applies only to lambda-expressions and pattern bindings, constructs
1277 that only allow a single pattern:
1278
1279 <programlisting>
1280 h = \[] -> 2
1281 Just k = f y
1282 </programlisting>
1283
1284 </para>
1285 </listitem>
1286 </varlistentry>
1287
1288 <varlistentry>
1289 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1290 <listitem>
1291 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1292 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1293 <indexterm><primary>record updates, incomplete</primary></indexterm>
1294
1295 <para>The function
1296 <function>f</function> below will fail when applied to
1297 <literal>Bar</literal>, so the compiler will emit a warning about
1298 this when <option>-fwarn-incomplete-record-updates</option> is
1299 enabled.</para>
1300
1301 <programlisting>
1302 data Foo = Foo { x :: Int }
1303 | Bar
1304
1305 f :: Foo -> Foo
1306 f foo = foo { x = 6 }
1307 </programlisting>
1308
1309 <para>This option isn't enabled by default because it can be
1310 very noisy, and it often doesn't indicate a bug in the
1311 program.</para>
1312 </listitem>
1313 </varlistentry>
1314
1315 <varlistentry>
1316 <term>
1317 <option>-fwarn-missing-fields</option>:
1318 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1319 <indexterm><primary>missing fields, warning</primary></indexterm>
1320 <indexterm><primary>fields, missing</primary></indexterm>
1321 </term>
1322 <listitem>
1323
1324 <para>This option is on by default, and warns you whenever
1325 the construction of a labelled field constructor isn't
1326 complete, missing initializers for one or more fields. While
1327 not an error (the missing fields are initialised with
1328 bottoms), it is often an indication of a programmer error.</para>
1329 </listitem>
1330 </varlistentry>
1331
1332 <varlistentry>
1333 <term>
1334 <option>-fwarn-missing-import-lists</option>:
1335 <indexterm><primary><option>-fwarn-import-lists</option></primary></indexterm>
1336 <indexterm><primary>missing import lists, warning</primary></indexterm>
1337 <indexterm><primary>import lists, missing</primary></indexterm>
1338 </term>
1339 <listitem>
1340
1341 <para>This flag warns if you use an unqualified
1342 <literal>import</literal> declaration
1343 that does not explicitly list the entities brought into scope. For
1344 example
1345 </para>
1346
1347 <programlisting>
1348 module M where
1349 import X( f )
1350 import Y
1351 import qualified Z
1352 p x = f x x
1353 </programlisting>
1354
1355 <para>
1356 The <option>-fwarn-import-lists</option> flag will warn about the import
1357 of <literal>Y</literal> but not <literal>X</literal>
1358 If module <literal>Y</literal> is later changed to export (say) <literal>f</literal>,
1359 then the reference to <literal>f</literal> in <literal>M</literal> will become
1360 ambiguous. No warning is produced for the import of <literal>Z</literal>
1361 because extending <literal>Z</literal>'s exports would be unlikely to produce
1362 ambiguity in <literal>M</literal>.
1363 </para>
1364 </listitem>
1365 </varlistentry>
1366
1367 <varlistentry>
1368 <term><option>-fwarn-missing-methods</option>:</term>
1369 <listitem>
1370 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1371 <indexterm><primary>missing methods, warning</primary></indexterm>
1372 <indexterm><primary>methods, missing</primary></indexterm>
1373
1374 <para>This option is on by default, and warns you whenever
1375 an instance declaration is missing one or more methods, and
1376 the corresponding class declaration has no default
1377 declaration for them.</para>
1378 <para>The warning is suppressed if the method name
1379 begins with an underscore. Here's an example where this is useful:
1380 <programlisting>
1381 class C a where
1382 _simpleFn :: a -> String
1383 complexFn :: a -> a -> String
1384 complexFn x y = ... _simpleFn ...
1385 </programlisting>
1386 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1387 never <literal>_simpleFn</literal>; and (b)
1388 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1389 </para>
1390 </listitem>
1391 </varlistentry>
1392
1393 <varlistentry>
1394 <term><option>-fwarn-missing-signatures</option>:</term>
1395 <listitem>
1396 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1397 <indexterm><primary>type signatures, missing</primary></indexterm>
1398
1399 <para>If you would like GHC to check that every top-level
1400 function/value has a type signature, use the
1401 <option>-fwarn-missing-signatures</option> option. As part of
1402 the warning GHC also reports the inferred type. The
1403 option is off by default.</para>
1404 </listitem>
1405 </varlistentry>
1406
1407 <varlistentry>
1408 <term><option>-fwarn-missing-local-sigs</option>:</term>
1409 <listitem>
1410 <indexterm><primary><option>-fwarn-missing-local-sigs</option></primary></indexterm>
1411 <indexterm><primary>type signatures, missing</primary></indexterm>
1412
1413 <para>If you use the
1414 <option>-fwarn-missing-local-sigs</option> flag GHC will warn
1415 you about any polymorphic local bindings. As part of
1416 the warning GHC also reports the inferred type. The
1417 option is off by default.</para>
1418 </listitem>
1419 </varlistentry>
1420
1421 <varlistentry>
1422 <term><option>-fwarn-name-shadowing</option>:</term>
1423 <listitem>
1424 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1425 <indexterm><primary>shadowing, warning</primary></indexterm>
1426
1427 <para>This option causes a warning to be emitted whenever an
1428 inner-scope value has the same name as an outer-scope value,
1429 i.e. the inner value shadows the outer one. This can catch
1430 typographical errors that turn into hard-to-find bugs, e.g.,
1431 in the inadvertent capture of what would be a recursive call in
1432 <literal>f = ... let f = id in ... f ...</literal>.</para>
1433 <para>The warning is suppressed for names beginning with an underscore. For example
1434 <programlisting>
1435 f x = do { _ignore &lt;- this; _ignore &lt;- that; return (the other) }
1436 </programlisting>
1437 </para>
1438 </listitem>
1439 </varlistentry>
1440
1441 <varlistentry>
1442 <term><option>-fwarn-orphans, -fwarn-auto-orphans</option>:</term>
1443 <listitem>
1444 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1445 <indexterm><primary><option>-fwarn-auto-orphans</option></primary></indexterm>
1446 <indexterm><primary>orphan instances, warning</primary></indexterm>
1447 <indexterm><primary>orphan rules, warning</primary></indexterm>
1448
1449 <para>These flags cause a warning to be emitted whenever the
1450 module contains an "orphan" instance declaration or rewrite rule.
1451 An instance declaration is an orphan if it appears in a module in
1452 which neither the class nor the type being instanced are declared
1453 in the same module. A rule is an orphan if it is a rule for a
1454 function declared in another module. A module containing any
1455 orphans is called an orphan module.</para>
1456 <para>The trouble with orphans is that GHC must pro-actively read the interface
1457 files for all orphan modules, just in case their instances or rules
1458 play a role, whether or not the module's interface would otherwise
1459 be of any use. See <xref linkend="orphan-modules"/> for details.
1460 </para>
1461 <para>The flag <option>-fwarn-orphans</option> warns about user-written
1462 orphan rules or instances. The flag <option>-fwarn-auto-orphans</option>
1463 warns about automatically-generated orphan rules, notably as a result of
1464 specialising functions, for type classes (<literal>Specialise</literal>)
1465 or argument values (<literal>SpecConstr</literal>).</para>
1466 </listitem>
1467 </varlistentry>
1468
1469 <varlistentry>
1470 <term>
1471 <option>-fwarn-overlapping-patterns</option>:
1472 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1473 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1474 <indexterm><primary>patterns, overlapping</primary></indexterm>
1475 </term>
1476 <listitem>
1477 <para>By default, the compiler will warn you if a set of
1478 patterns are overlapping, e.g.,</para>
1479
1480 <programlisting>
1481 f :: String -&#62; Int
1482 f [] = 0
1483 f (_:xs) = 1
1484 f "2" = 2
1485 </programlisting>
1486
1487 <para>where the last pattern match in <function>f</function>
1488 won't ever be reached, as the second pattern overlaps
1489 it. More often than not, redundant patterns is a programmer
1490 mistake/error, so this option is enabled by default.</para>
1491 </listitem>
1492 </varlistentry>
1493
1494 <varlistentry>
1495 <term><option>-fwarn-tabs</option>:</term>
1496 <listitem>
1497 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1498 <indexterm><primary>tabs, warning</primary></indexterm>
1499 <para>Have the compiler warn if there are tabs in your source
1500 file.</para>
1501
1502 <para>This warning is off by default.</para>
1503 </listitem>
1504 </varlistentry>
1505
1506 <varlistentry>
1507 <term><option>-fwarn-type-defaults</option>:</term>
1508 <listitem>
1509 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1510 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1511 <para>Have the compiler warn/inform you where in your source
1512 the Haskell defaulting mechanism for numeric types kicks
1513 in. This is useful information when converting code from a
1514 context that assumed one default into one with another,
1515 e.g., the &lsquo;default default&rsquo; for Haskell 1.4 caused the
1516 otherwise unconstrained value <constant>1</constant> to be
1517 given the type <literal>Int</literal>, whereas Haskell 98
1518 and later
1519 defaults it to <literal>Integer</literal>. This may lead to
1520 differences in performance and behaviour, hence the
1521 usefulness of being non-silent about this.</para>
1522
1523 <para>This warning is off by default.</para>
1524 </listitem>
1525 </varlistentry>
1526
1527 <varlistentry>
1528 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1529 <listitem>
1530 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1531 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1532 <para>Have the compiler warn/inform you where in your source
1533 the Haskell Monomorphism Restriction is applied. If applied silently
1534 the MR can give rise to unexpected behaviour, so it can be helpful
1535 to have an explicit warning that it is being applied.</para>
1536
1537 <para>This warning is off by default.</para>
1538 </listitem>
1539 </varlistentry>
1540
1541 <varlistentry>
1542 <term><option>-fwarn-unused-binds</option>:</term>
1543 <listitem>
1544 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1545 <indexterm><primary>unused binds, warning</primary></indexterm>
1546 <indexterm><primary>binds, unused</primary></indexterm>
1547 <para>Report any function definitions (and local bindings)
1548 which are unused. For top-level functions, the warning is
1549 only given if the binding is not exported.</para>
1550 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1551 mentioned in the right hand side of another definition that is used, or (c) the
1552 function it defines begins with an underscore. The last case provides a
1553 way to suppress unused-binding warnings selectively. </para>
1554 <para> Notice that a variable
1555 is reported as unused even if it appears in the right-hand side of another
1556 unused binding. </para>
1557 </listitem>
1558 </varlistentry>
1559
1560 <varlistentry>
1561 <term><option>-fwarn-unused-imports</option>:</term>
1562 <listitem>
1563 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1564 <indexterm><primary>unused imports, warning</primary></indexterm>
1565 <indexterm><primary>imports, unused</primary></indexterm>
1566
1567 <para>Report any modules that are explicitly imported but
1568 never used. However, the form <literal>import M()</literal> is
1569 never reported as an unused import, because it is a useful idiom
1570 for importing instance declarations, which are anonymous in Haskell.</para>
1571 </listitem>
1572 </varlistentry>
1573
1574 <varlistentry>
1575 <term><option>-fwarn-unused-matches</option>:</term>
1576 <listitem>
1577 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1578 <indexterm><primary>unused matches, warning</primary></indexterm>
1579 <indexterm><primary>matches, unused</primary></indexterm>
1580
1581 <para>Report all unused variables which arise from pattern
1582 matches, including patterns consisting of a single variable.
1583 For instance <literal>f x y = []</literal> would report
1584 <varname>x</varname> and <varname>y</varname> as unused. The
1585 warning is suppressed if the variable name begins with an underscore, thus:
1586 <programlisting>
1587 f _x = True
1588 </programlisting>
1589 </para>
1590 </listitem>
1591 </varlistentry>
1592
1593 <varlistentry>
1594 <term><option>-fwarn-unused-do-bind</option>:</term>
1595 <listitem>
1596 <indexterm><primary><option>-fwarn-unused-do-bind</option></primary></indexterm>
1597 <indexterm><primary>unused do binding, warning</primary></indexterm>
1598 <indexterm><primary>do binding, unused</primary></indexterm>
1599
1600 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1601 that appear to silently throw information away.
1602 For instance <literal>do { mapM popInt xs ; return 10 }</literal> would report
1603 the first statement in the <literal>do</literal> block as suspicious,
1604 as it has the type <literal>StackM [Int]</literal> and not <literal>StackM ()</literal>, but that
1605 <literal>[Int]</literal> value is not bound to anything. The warning is suppressed by
1606 explicitly mentioning in the source code that your program is throwing something away:
1607 <programlisting>
1608 do { _ &lt;- mapM popInt xs ; return 10 }
1609 </programlisting>
1610 Of course, in this particular situation you can do even better:
1611 <programlisting>
1612 do { mapM_ popInt xs ; return 10 }
1613 </programlisting>
1614 </para>
1615 </listitem>
1616 </varlistentry>
1617
1618 <varlistentry>
1619 <term><option>-fwarn-wrong-do-bind</option>:</term>
1620 <listitem>
1621 <indexterm><primary><option>-fwarn-wrong-do-bind</option></primary></indexterm>
1622 <indexterm><primary>apparently erroneous do binding, warning</primary></indexterm>
1623 <indexterm><primary>do binding, apparently erroneous</primary></indexterm>
1624
1625 <para>Report expressions occurring in <literal>do</literal> and <literal>mdo</literal> blocks
1626 that appear to lack a binding.
1627 For instance <literal>do { return (popInt 10) ; return 10 }</literal> would report
1628 the first statement in the <literal>do</literal> block as suspicious,
1629 as it has the type <literal>StackM (StackM Int)</literal> (which consists of two nested applications
1630 of the same monad constructor), but which is not then &quot;unpacked&quot; by binding the result.
1631 The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away:
1632 <programlisting>
1633 do { _ &lt;- return (popInt 10) ; return 10 }
1634 </programlisting>
1635 For almost all sensible programs this will indicate a bug, and you probably intended to write:
1636 <programlisting>
1637 do { popInt 10 ; return 10 }
1638 </programlisting>
1639 </para>
1640 </listitem>
1641 </varlistentry>
1642
1643 </variablelist>
1644
1645 <para>If you're feeling really paranoid, the
1646 <option>-dcore-lint</option>
1647 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1648 is a good choice. It turns on heavyweight intra-pass
1649 sanity-checking within GHC. (It checks GHC's sanity, not
1650 yours.)</para>
1651
1652 </sect1>
1653
1654 &packages;
1655
1656 <sect1 id="options-optimise">
1657 <title>Optimisation (code improvement)</title>
1658
1659 <indexterm><primary>optimisation</primary></indexterm>
1660 <indexterm><primary>improvement, code</primary></indexterm>
1661
1662 <para>The <option>-O*</option> options specify convenient
1663 &ldquo;packages&rdquo; of optimisation flags; the
1664 <option>-f*</option> options described later on specify
1665 <emphasis>individual</emphasis> optimisations to be turned on/off;
1666 the <option>-m*</option> options specify
1667 <emphasis>machine-specific</emphasis> optimisations to be turned
1668 on/off.</para>
1669
1670 <sect2 id="optimise-pkgs">
1671 <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1672
1673 <para>There are <emphasis>many</emphasis> options that affect
1674 the quality of code produced by GHC. Most people only have a
1675 general goal, something like &ldquo;Compile quickly&rdquo; or
1676 &ldquo;Make my program run like greased lightning.&rdquo; The
1677 following &ldquo;packages&rdquo; of optimisations (or lack
1678 thereof) should suffice.</para>
1679
1680 <para>Note that higher optimisation levels cause more
1681 cross-module optimisation to be performed, which can have an
1682 impact on how much of your program needs to be recompiled when
1683 you change something. This is one reason to stick to
1684 no-optimisation when developing code.</para>
1685
1686 <variablelist>
1687
1688 <varlistentry>
1689 <term>
1690 No <option>-O*</option>-type option specified:
1691 <indexterm><primary>-O* not specified</primary></indexterm>
1692 </term>
1693 <listitem>
1694 <para>This is taken to mean: &ldquo;Please compile
1695 quickly; I'm not over-bothered about compiled-code
1696 quality.&rdquo; So, for example: <command>ghc -c
1697 Foo.hs</command></para>
1698 </listitem>
1699 </varlistentry>
1700
1701 <varlistentry>
1702 <term>
1703 <option>-O0</option>:
1704 <indexterm><primary><option>-O0</option></primary></indexterm>
1705 </term>
1706 <listitem>
1707 <para>Means &ldquo;turn off all optimisation&rdquo;,
1708 reverting to the same settings as if no
1709 <option>-O</option> options had been specified. Saying
1710 <option>-O0</option> can be useful if
1711 eg. <command>make</command> has inserted a
1712 <option>-O</option> on the command line already.</para>
1713 </listitem>
1714 </varlistentry>
1715
1716 <varlistentry>
1717 <term>
1718 <option>-O</option> or <option>-O1</option>:
1719 <indexterm><primary>-O option</primary></indexterm>
1720 <indexterm><primary>-O1 option</primary></indexterm>
1721 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1722 </term>
1723 <listitem>
1724 <para>Means: &ldquo;Generate good-quality code without
1725 taking too long about it.&rdquo; Thus, for example:
1726 <command>ghc -c -O Main.lhs</command></para>
1727 </listitem>
1728 </varlistentry>
1729
1730 <varlistentry>
1731 <term>
1732 <option>-O2</option>:
1733 <indexterm><primary>-O2 option</primary></indexterm>
1734 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1735 </term>
1736 <listitem>
1737 <para>Means: &ldquo;Apply every non-dangerous
1738 optimisation, even if it means significantly longer
1739 compile times.&rdquo;</para>
1740
1741 <para>The avoided &ldquo;dangerous&rdquo; optimisations
1742 are those that can make runtime or space
1743 <emphasis>worse</emphasis> if you're unlucky. They are
1744 normally turned on or off individually.</para>
1745
1746 <para>At the moment, <option>-O2</option> is
1747 <emphasis>unlikely</emphasis> to produce better code than
1748 <option>-O</option>.</para>
1749 </listitem>
1750 </varlistentry>
1751 </variablelist>
1752
1753 <para>We don't use a <option>-O*</option> flag for day-to-day
1754 work. We use <option>-O</option> to get respectable speed;
1755 e.g., when we want to measure something. When we want to go for
1756 broke, we tend to use <option>-O2</option> (and we go for
1757 lots of coffee breaks).</para>
1758
1759 <para>The easiest way to see what <option>-O</option> (etc.)
1760 &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1761 then stand back in amazement.</para>
1762 </sect2>
1763
1764 <sect2 id="options-f">
1765 <title><option>-f*</option>: platform-independent flags</title>
1766
1767 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1768 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1769
1770 <para>These flags turn on and off individual optimisations.
1771 They are normally set via the <option>-O</option> options
1772 described above, and as such, you shouldn't need to set any of
1773 them explicitly (indeed, doing so could lead to unexpected
1774 results). A flag <option>-fwombat</option> can be negated by
1775 saying <option>-fno-wombat</option>. The flags below are off
1776 by default, except where noted below.
1777 </para>
1778
1779 <variablelist>
1780 <varlistentry>
1781 <term>
1782 <option>-fcse</option>
1783 <indexterm><primary><option>-fcse</option></primary></indexterm>
1784 </term>
1785 <listitem>
1786 <para><emphasis>On by default.</emphasis>. Enables the common-sub-expression
1787 elimination optimisation.
1788 Switching this off can be useful if you have some <literal>unsafePerformIO</literal>
1789 expressions that you don't want commoned-up.</para>
1790 </listitem>
1791 </varlistentry>
1792
1793 <varlistentry>
1794 <term>
1795 <option>-fstrictness</option>
1796 <indexterm><primary><option></option></primary></indexterm>
1797 </term>
1798 <listitem>
1799 <para> <emphasis>On by default.</emphasis>.
1800 Switch on the strictness analyser. There is a very old paper about GHC's
1801 strictness analyser, <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/simple-strictnes-analyser.ps.gz">
1802 Measuring the effectiveness of a simple strictness analyser</ulink>,
1803 but the current one is quite a bit different.
1804 </para>
1805
1806 <para>The strictness analyser figures out when arguments and
1807 variables in a function can be treated 'strictly' (that is they
1808 are always evaluated in the function at some point). This allow
1809 GHC to apply certain optimisations such as unboxing that
1810 otherwise don't apply as they change the semantics of the program
1811 when applied to lazy arguments.
1812 </para>
1813 </listitem>
1814 </varlistentry>
1815
1816 <varlistentry>
1817 <term>
1818 <option>-funbox-strict-fields</option>:
1819 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1820 <indexterm><primary>strict constructor fields</primary></indexterm>
1821 <indexterm><primary>constructor fields, strict</primary></indexterm>
1822 </term>
1823 <listitem>
1824 <para>This option causes all constructor fields which are marked
1825 strict (i.e. &ldquo;!&rdquo;) to be unpacked if possible. It is
1826 equivalent to adding an <literal>UNPACK</literal> pragma to every
1827 strict constructor field (see <xref linkend="unpack-pragma"/>).
1828 </para>
1829
1830 <para>This option is a bit of a sledgehammer: it might sometimes
1831 make things worse. Selectively unboxing fields by using
1832 <literal>UNPACK</literal> pragmas might be better. An alternative
1833 is to use <option>-funbox-strict-fields</option> to turn on
1834 unboxing by default but disable it for certain constructor
1835 fields using the <literal>NOUNPACK</literal> pragma (see
1836 <xref linkend="nounpack-pragma"/>).</para>
1837 </listitem>
1838 </varlistentry>
1839
1840 <varlistentry>
1841 <term>
1842 <option>-fspec-constr</option>
1843 <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
1844 </term>
1845 <listitem>
1846 <para><emphasis>Off by default, but enabled by -O2.</emphasis>
1847 Turn on call-pattern specialisation; see
1848 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/spec-constr/index.htm">
1849 Call-pattern specialisation for Haskell programs</ulink>.
1850 </para>
1851
1852 <para>This optimisation specializes recursive functions according to
1853 their argument "shapes". This is best explained by example so
1854 consider:
1855 <programlisting>
1856 last :: [a] -> a
1857 last [] = error "last"
1858 last (x : []) = x
1859 last (x : xs) = last xs
1860 </programlisting>
1861 In this code, once we pass the initial check for an empty list we
1862 know that in the recursive case this pattern match is redundant. As
1863 such <option>-fspec-constr</option> will transform the above code
1864 to:
1865 <programlisting>
1866 last :: [a] -> a
1867 last [] = error "last"
1868 last (x : xs) = last' x xs
1869 where
1870 last' x [] = x
1871 last' x (y : ys) = last' y ys
1872 </programlisting>
1873 </para>
1874
1875 <para>As well avoid unnecessary pattern matching it also helps avoid
1876 unnecessary allocation. This applies when a argument is strict in
1877 the recursive call to itself but not on the initial entry. As
1878 strict recursive branch of the function is created similar to the
1879 above example.
1880 </para>
1881 </listitem>
1882 </varlistentry>
1883
1884 <varlistentry>
1885 <term>
1886 <option>-fspecialise</option>
1887 <indexterm><primary><option>-fspecialise</option></primary></indexterm>
1888 </term>
1889 <listitem>
1890 <para><emphasis>On by default.</emphasis>
1891 Specialise each type-class-overloaded function defined in this
1892 module for the types at which it is called in this module. Also
1893 specialise imported functions that have an INLINABLE pragma
1894 (<xref linkend="inlinable-pragma"/>) for the types at which they
1895 are called in this module.
1896 </para>
1897 </listitem>
1898 </varlistentry>
1899
1900 <varlistentry>
1901 <term>
1902 <option>-fstatic-argument-transformation</option>
1903 <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
1904 </term>
1905 <listitem>
1906 <para>Turn on the static argument transformation, which turns a
1907 recursive function into a non-recursive one with a local
1908 recursive loop. See Chapter 7 of
1909 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/santos-thesis.ps.gz">
1910 Andre Santos's PhD thesis</ulink>
1911 </para>
1912 </listitem>
1913 </varlistentry>
1914
1915 <varlistentry>
1916 <term>
1917 <option>-ffloat-in</option>
1918 <indexterm><primary><option></option></primary></indexterm>
1919 </term>
1920 <listitem>
1921 <para><emphasis>On by default.</emphasis>
1922 Float let-bindings inwards, nearer their binding site. See
1923 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">
1924 Let-floating: moving bindings to give faster programs (ICFP'96)</ulink>.
1925 </para>
1926
1927 <para>This optimisation moves let bindings closer to their use
1928 site. The benefit here is that this may avoid unnecessary
1929 allocation if the branch the let is now on is never executed. It
1930 also enables other optimisation passes to work more effectively
1931 as they have more information locally.
1932 </para>
1933
1934 <para>This optimisation isn't always beneficial though (so GHC
1935 applies some heuristics to decide when to apply it). The details
1936 get complicated but a simple example is that it is often beneficial
1937 to move let bindings outwards so that multiple let bindings can be
1938 grouped into a larger single let binding, effectively batching
1939 their allocation and helping the garbage collector and allocator.
1940 </para>
1941 </listitem>
1942 </varlistentry>
1943
1944 <varlistentry>
1945 <term>
1946 <option>-ffull-laziness</option>
1947 <indexterm><primary><option>-ffull-laziness</option></primary></indexterm>
1948 </term>
1949 <listitem>
1950 <para><emphasis>On by default.</emphasis>
1951 Run the full laziness optimisation (also known as let-floating),
1952 which floats let-bindings outside enclosing lambdas, in the hope
1953 they will be thereby be computed less often. See
1954 <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">Let-floating:
1955 moving bindings to give faster programs (ICFP'96)</ulink>.
1956 Full laziness increases sharing, which can lead to increased memory
1957 residency.
1958 </para>
1959
1960 <para>NOTE: GHC doesn't implement complete full-laziness.
1961 When optimisation in on, and <option>-fno-full-laziness</option>
1962 is not given, some transformations that increase sharing are
1963 performed, such as extracting repeated computations from a loop.
1964 These are the same transformations that a fully lazy
1965 implementation would do, the difference is that GHC doesn't
1966 consistently apply full-laziness, so don't rely on it.
1967 </para>
1968 </listitem>
1969 </varlistentry>
1970
1971 <varlistentry>
1972 <term>
1973 <option>-fdo-lambda-eta-expansion</option>
1974 <indexterm><primary><option></option></primary></indexterm>
1975 </term>
1976 <listitem>
1977 <para><emphasis>On by default.</emphasis>
1978 Eta-expand let-bindings to increase their arity.
1979 </para>
1980 </listitem>
1981 </varlistentry>
1982
1983 <varlistentry>
1984 <term>
1985 <option>-fdo-eta-reduction</option>
1986 <indexterm><primary><option></option></primary></indexterm>
1987 </term>
1988 <listitem>
1989 <para><emphasis>On by default.</emphasis>
1990 Eta-reduce lambda expressions, if doing so gets rid of a whole
1991 group of lambdas.
1992 </para>
1993 </listitem>
1994 </varlistentry>
1995
1996 <varlistentry>
1997 <term>
1998 <option>-fcase-merge</option>
1999 <indexterm><primary><option></option></primary></indexterm>
2000 </term>
2001 <listitem>
2002 <para><emphasis>On by default.</emphasis>
2003 Merge immediately-nested case expressions that scrutinse the same variable. Example
2004 <programlisting>
2005 case x of
2006 Red -> e1
2007 _ -> case x of
2008 Blue -> e2
2009 Green -> e3
2010 ==>
2011 case x of
2012 Red -> e1
2013 Blue -> e2
2014 Green -> e2
2015 </programlisting>
2016 </para>
2017 </listitem>
2018 </varlistentry>
2019
2020 <varlistentry>
2021 <term>
2022 <option>-fliberate-case</option>
2023 <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
2024 </term>
2025 <listitem>
2026 <para><emphasis>Off by default, but enabled by -O2.</emphasis>
2027 Turn on the liberate-case transformation. This unrolls recursive
2028 function once in its own RHS, to avoid repeated case analysis of
2029 free variables. It's a bit like the call-pattern specialiser
2030 (<option>-fspec-constr</option>) but for free variables rather than
2031 arguments.
2032 </para>
2033 </listitem>
2034 </varlistentry>
2035
2036 <varlistentry>
2037 <term>
2038 <option>-fdicts-cheap</option>
2039 <indexterm><primary><option></option></primary></indexterm>
2040 </term>
2041 <listitem>
2042 <para>A very experimental flag that makes dictionary-valued
2043 expressions seem cheap to the optimiser.
2044 </para>
2045 </listitem>
2046 </varlistentry>
2047
2048 <varlistentry>
2049 <term>
2050 <option>-feager-blackholing</option>
2051 <indexterm><primary><option></option></primary></indexterm>
2052 </term>
2053 <listitem>
2054 <para>Usually GHC black-holes a thunk only when it switches
2055 threads. This flag makes it do so as soon as the thunk is
2056 entered. See <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/">
2057 Haskell on a shared-memory multiprocessor</ulink>.
2058 </para>
2059 </listitem>
2060 </varlistentry>
2061
2062 <varlistentry>
2063 <term>
2064 <option>-fno-state-hack</option>
2065 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
2066 </term>
2067 <listitem>
2068 <para>Turn off the "state hack" whereby any lambda with a
2069 <literal>State#</literal> token as argument is considered to be
2070 single-entry, hence it is considered OK to inline things inside
2071 it. This can improve performance of IO and ST monad code, but it
2072 runs the risk of reducing sharing.
2073 </para>
2074 </listitem>
2075 </varlistentry>
2076
2077 <varlistentry>
2078 <term>
2079 <option>-fpedantic-bottoms</option>
2080 <indexterm><primary><option>-fpedantic-bottoms</option></primary></indexterm>
2081 </term>
2082 <listitem>
2083 <para>Make GHC be more precise about its treatment of bottom (but see also
2084 <option>-fno-state-hack</option>). In particular, stop GHC
2085 eta-expanding through a case expression, which is good for
2086 performance, but bad if you are using <literal>seq</literal> on
2087 partial applications.
2088 </para>
2089 </listitem>
2090 </varlistentry>
2091
2092 <varlistentry>
2093 <term>
2094 <option>-fsimpl-tick-factor=<replaceable>n</replaceable></option>
2095 <indexterm><primary><option>-fsimpl-tick-factor</option></primary></indexterm>
2096 </term>
2097 <listitem>
2098 <para>GHC's optimiser can diverge if you write rewrite rules (
2099 <xref linkend="rewrite-rules"/>) that don't terminate, or (less
2100 satisfactorily) if you code up recursion through data types
2101 (<xref linkend="bugs-ghc"/>). To avoid making the compiler fall
2102 into an infinite loop, the optimiser carries a "tick count" and
2103 stops inlining and applying rewrite rules when this count is
2104 exceeded. The limit is set as a multiple of the program size, so
2105 bigger programs get more ticks. The
2106 <option>-fsimpl-tick-factor</option> flag lets you change the
2107 multiplier. The default is 100; numbers larger than 100 give more
2108 ticks, and numbers smaller than 100 give fewer.
2109 </para>
2110
2111 <para>If the tick-count expires, GHC summarises what simplifier
2112 steps it has done; you can use
2113 <option>-fddump-simpl-stats</option> to generate a much more
2114 detailed list. Usually that identifies the loop quite
2115 accurately, because some numbers are very large.
2116 </para>
2117 </listitem>
2118 </varlistentry>
2119
2120 <varlistentry>
2121 <term>
2122 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
2123 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
2124 <indexterm><primary>inlining, controlling</primary></indexterm>
2125 <indexterm><primary>unfolding, controlling</primary></indexterm>
2126 </term>
2127 <listitem>
2128 <para>(Default: 45) Governs the maximum size that GHC will allow a
2129 function unfolding to be. (An unfolding has a &ldquo;size&rdquo;
2130 that reflects the cost in terms of &ldquo;code bloat&rdquo; of
2131 expanding (aka inlining) that unfolding at a call site. A bigger
2132 function would be assigned a bigger cost.)
2133 </para>
2134
2135 <para>Consequences: (a) nothing larger than this will be inlined
2136 (unless it has an INLINE pragma); (b) nothing larger than this
2137 will be spewed into an interface file.
2138 </para>
2139
2140 <para>Increasing this figure is more likely to result in longer
2141 compile times than faster code. The
2142 <option>-funfolding-use-threshold</option> is more useful.
2143 </para>
2144 </listitem>
2145 </varlistentry>
2146
2147 <varlistentry>
2148 <term>
2149 <option>-funfolding-use-threshold=<replaceable>n</replaceable></option>
2150 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
2151 <indexterm><primary>inlining, controlling</primary></indexterm>
2152 <indexterm><primary>unfolding, controlling</primary></indexterm>
2153 </term>
2154 <listitem>
2155 <para>(Default: 8) This is the magic cut-off figure for unfolding
2156 (aka inlining): below this size, a function definition will be
2157 unfolded at the call-site, any bigger and it won't. The size
2158 computed for a function depends on two things: the actual size of
2159 the expression minus any discounts that
2160 apply (see <option>-funfolding-con-discount</option>).
2161 </para>
2162
2163 <para>The difference between this and
2164 <option>-funfolding-creation-threshold</option> is that this one
2165 determines if a function definition will be inlined <emphasis>at
2166 a call site</emphasis>. The other option determines if a
2167 function definition will be kept around at all for potential
2168 inlining.
2169 </para>
2170 </listitem>
2171 </varlistentry>
2172
2173 <varlistentry>
2174 <term>
2175 <option>-fexpose-all-unfoldings</option>
2176 <indexterm><primary><option></option></primary></indexterm>
2177 </term>
2178 <listitem>
2179 <para>An experimental flag to expose all unfoldings, even for very
2180 large or recursive functions. This allows for all functions to be
2181 inlined while usually GHC would avoid inlining larger functions.
2182 </para>
2183 </listitem>
2184 </varlistentry>
2185
2186 <varlistentry>
2187 <term>
2188 <option>-fvectorise</option>
2189 <indexterm><primary><option></option></primary></indexterm>
2190 </term>
2191 <listitem>
2192 <para>Data Parallel Haskell.
2193 </para>
2194 TODO: Document optimisation
2195 </listitem>
2196 </varlistentry>
2197
2198 <varlistentry>
2199 <term>
2200 <option>-favoid-vect</option>
2201 <indexterm><primary><option></option></primary></indexterm>
2202 </term>
2203 <listitem>
2204 <para>Data Parallel Haskell.
2205 </para>
2206 TODO: Document optimisation
2207 </listitem>
2208 </varlistentry>
2209
2210 <varlistentry>
2211 <term>
2212 <option>-fregs-graph</option>
2213 <indexterm><primary><option></option></primary></indexterm>
2214 </term>
2215 <listitem>
2216 <para><emphasis>Off by default, but enabled by -O2. Only applies in
2217 combination with the native code generator.</emphasis>
2218 Use the graph colouring register allocator for register allocation
2219 in the native code generator. By default, GHC uses a simpler,
2220 faster linear register allocator. The downside being that the
2221 linear register allocator usually generates worse code.
2222 </para>
2223 </listitem>
2224 </varlistentry>
2225
2226 <varlistentry>
2227 <term>
2228 <option>-fregs-iterative</option>
2229 <indexterm><primary><option></option></primary></indexterm>
2230 </term>
2231 <listitem>
2232 <para><emphasis>Off by default, only applies in combination with
2233 the native code generator.</emphasis>
2234 Use the iterative coalescing graph colouring register allocator for
2235 register allocation in the native code generator. This is the same
2236 register allocator as the <option>-freg-graph</option> one but also
2237 enables iterative coalescing during register allocation.
2238 </para>
2239 </listitem>
2240 </varlistentry>
2241
2242 <varlistentry>
2243 <term>
2244 <option>-fexcess-precision</option>
2245 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
2246 </term>
2247 <listitem>
2248 <para>When this option is given, intermediate floating
2249 point values can have a <emphasis>greater</emphasis>
2250 precision/range than the final type. Generally this is a
2251 good thing, but some programs may rely on the exact
2252 precision/range of
2253 <literal>Float</literal>/<literal>Double</literal> values
2254 and should not use this option for their compilation.</para>
2255 </listitem>
2256 </varlistentry>
2257
2258 <varlistentry>
2259 <term>
2260 <option>-fignore-asserts</option>
2261 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
2262 </term>
2263 <listitem>
2264 <para>Causes GHC to ignore uses of the function
2265 <literal>Exception.assert</literal> in source code (in
2266 other words, rewriting <literal>Exception.assert p
2267 e</literal> to <literal>e</literal> (see <xref
2268 linkend="assertions"/>). This flag is turned on by
2269 <option>-O</option>.
2270 </para>
2271 </listitem>
2272 </varlistentry>
2273
2274 <varlistentry>
2275 <term>
2276 <option>-fignore-interface-pragmas</option>
2277 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
2278 </term>
2279 <listitem>
2280 <para>Tells GHC to ignore all inessential information when reading interface files.
2281 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
2282 for a function, GHC will ignore that information.</para>
2283 </listitem>
2284 </varlistentry>
2285
2286 <varlistentry>
2287 <term>
2288 <option>-fomit-interface-pragmas</option>
2289 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
2290 </term>
2291 <listitem>
2292 <para>Tells GHC to omit all inessential information from the
2293 interface file generated for the module being compiled (say M).
2294 This means that a module importing M will see only the
2295 <emphasis>types</emphasis> of the functions that M exports, but
2296 not their unfoldings, strictness info, etc. Hence, for example,
2297 no function exported by M will be inlined into an importing module.
2298 The benefit is that modules that import M will need to be
2299 recompiled less often (only when M's exports change their type, not
2300 when they change their implementation).</para>
2301 </listitem>
2302 </varlistentry>
2303
2304 </variablelist>
2305
2306 </sect2>
2307
2308 </sect1>
2309
2310 &code-gens;
2311
2312 &phases;
2313
2314 &shared_libs;
2315
2316 <sect1 id="using-concurrent">
2317 <title>Using Concurrent Haskell</title>
2318 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
2319
2320 <para>GHC supports Concurrent Haskell by default, without requiring a
2321 special option or libraries compiled in a certain way. To get access to
2322 the support libraries for Concurrent Haskell, just import
2323 <ulink
2324 url="&libraryBaseLocation;/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
2325
2326 <para>
2327 Optionally, the program may be linked with
2328 the <option>-threaded</option> option (see
2329 <xref linkend="options-linker" />. This provides two benefits:
2330
2331 <itemizedlist>
2332 <listitem>
2333 <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
2334 used, which allows threads to run in
2335 parallel<indexterm><primary>parallelism</primary></indexterm>
2336 on a
2337 multiprocessor<indexterm><primary>multiprocessor</primary></indexterm><indexterm><primary>SMP</primary></indexterm>
2338 or
2339 multicore<indexterm><primary>multicore</primary></indexterm>
2340 machine. See <xref linkend="using-smp" />.</para>
2341 </listitem>
2342 <listitem>
2343 <para>If a thread makes a foreign call (and the call is
2344 not marked <literal>unsafe</literal>), then other
2345 Haskell threads in the program will continue to run
2346 while the foreign call is in progress.
2347 Additionally, <literal>foreign export</literal>ed
2348 Haskell functions may be called from multiple OS
2349 threads simultaneously. See
2350 <xref linkend="ffi-threads" />.</para>
2351 </listitem>
2352 </itemizedlist>
2353 </para>
2354
2355 <para>The following RTS option(s) affect the behaviour of Concurrent
2356 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
2357
2358 <variablelist>
2359 <varlistentry>
2360 <term><option>-C<replaceable>s</replaceable></option></term>
2361 <listitem>
2362 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2363 Sets the context switch interval to <replaceable>s</replaceable>
2364 seconds. A context switch will occur at the next heap block
2365 allocation after the timer expires (a heap block allocation occurs
2366 every 4k of allocation). With <option>-C0</option> or
2367 <option>-C</option>, context switches will occur as often as
2368 possible (at every heap block allocation). By default, context
2369 switches occur every 20ms.</para>
2370 </listitem>
2371 </varlistentry>
2372 </variablelist>
2373 </sect1>
2374
2375 <sect1 id="using-smp">
2376 <title>Using SMP parallelism</title>
2377 <indexterm><primary>parallelism</primary>
2378 </indexterm>
2379 <indexterm><primary>SMP</primary>
2380 </indexterm>
2381
2382 <para>GHC supports running Haskell programs in parallel on an SMP
2383 (symmetric multiprocessor).</para>
2384
2385 <para>There's a fine distinction between
2386 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
2387 parallelism is all about making your program run
2388 <emphasis>faster</emphasis> by making use of multiple processors
2389 simultaneously. Concurrency, on the other hand, is a means of
2390 abstraction: it is a convenient way to structure a program that must
2391 respond to multiple asynchronous events.</para>
2392
2393 <para>However, the two terms are certainly related. By making use of
2394 multiple CPUs it is possible to run concurrent threads in parallel,
2395 and this is exactly what GHC's SMP parallelism support does. But it
2396 is also possible to obtain performance improvements with parallelism
2397 on programs that do not use concurrency. This section describes how to
2398 use GHC to compile and run parallel programs, in <xref
2399 linkend="lang-parallel" /> we describe the language features that affect
2400 parallelism.</para>
2401
2402 <sect2 id="parallel-compile-options">
2403 <title>Compile-time options for SMP parallelism</title>
2404
2405 <para>In order to make use of multiple CPUs, your program must be
2406 linked with the <option>-threaded</option> option (see <xref
2407 linkend="options-linker" />). Additionally, the following
2408 compiler options affect parallelism:</para>
2409
2410 <variablelist>
2411 <varlistentry>
2412 <term><option>-feager-blackholing</option></term>
2413 <indexterm><primary><option>-feager-blackholing</option></primary></indexterm>
2414 <listitem>
2415 <para>
2416 Blackholing is the act of marking a thunk (lazy
2417 computuation) as being under evaluation. It is useful for
2418 three reasons: firstly it lets us detect certain kinds of
2419 infinite loop (the <literal>NonTermination</literal>
2420 exception), secondly it avoids certain kinds of space
2421 leak, and thirdly it avoids repeating a computation in a
2422 parallel program, because we can tell when a computation
2423 is already in progress.</para>
2424
2425 <para>
2426 The option <option>-feager-blackholing</option> causes
2427 each thunk to be blackholed as soon as evaluation begins.
2428 The default is "lazy blackholing", whereby thunks are only
2429 marked as being under evaluation when a thread is paused
2430 for some reason. Lazy blackholing is typically more
2431 efficient (by 1-2&percnt; or so), because most thunks don't
2432 need to be blackholed. However, eager blackholing can
2433 avoid more repeated computation in a parallel program, and
2434 this often turns out to be important for parallelism.
2435 </para>
2436
2437 <para>
2438 We recommend compiling any code that is intended to be run
2439 in parallel with the <option>-feager-blackholing</option>
2440 flag.
2441 </para>
2442 </listitem>
2443 </varlistentry>
2444 </variablelist>
2445 </sect2>
2446
2447 <sect2 id="parallel-options">
2448 <title>RTS options for SMP parallelism</title>
2449
2450 <para>There are two ways to run a program on multiple
2451 processors:
2452 call <literal>Control.Concurrent.setNumCapabilities</literal> from your
2453 program, or use the RTS <option>-N</option> option.</para>
2454
2455 <variablelist>
2456 <varlistentry>
2457 <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
2458 <listitem>
2459 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2460 Use <replaceable>x</replaceable> simultaneous threads when
2461 running the program. Normally <replaceable>x</replaceable>
2462 should be chosen to match the number of CPU cores on the
2463 machine<footnote><para>Whether hyperthreading cores should be counted or not is an
2464 open question; please feel free to experiment and let us know what
2465 results you find.</para></footnote>. For example,
2466 on a dual-core machine we would probably use
2467 <literal>+RTS -N2 -RTS</literal>.</para>
2468
2469 <para>Omitting <replaceable>x</replaceable>,
2470 i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
2471 choose the value of <replaceable>x</replaceable> itself
2472 based on how many processors are in your machine.</para>
2473
2474 <para>Be careful when using all the processors in your
2475 machine: if some of your processors are in use by other
2476 programs, this can actually harm performance rather than
2477 improve it.</para>
2478
2479 <para>Setting <option>-N</option> also has the effect of
2480 enabling the parallel garbage collector (see
2481 <xref linkend="rts-options-gc" />).</para>
2482
2483 <para>The current value of the <option>-N</option> option
2484 is available to the Haskell program
2485 via <literal>Control.Concurrent.getNumCapabilities</literal>, and
2486 it may be changed while the program is running by
2487 calling <literal>Control.Concurrent.setNumCapabilities</literal>.</para>
2488 </listitem>
2489 </varlistentry>
2490 </variablelist>
2491
2492 <para>The following options affect the way the runtime schedules
2493 threads on CPUs:</para>
2494
2495 <variablelist>
2496 <varlistentry>
2497 <term><option>-qa</option></term>
2498 <indexterm><primary><option>-qa</option></primary><secondary>RTS
2499 option</secondary></indexterm>
2500 <listitem>
2501 <para>Use the OS's affinity facilities to try to pin OS
2502 threads to CPU cores. This is an experimental feature,
2503 and may or may not be useful. Please let us know
2504 whether it helps for you!</para>
2505 </listitem>
2506 </varlistentry>
2507 <varlistentry>
2508 <term><option>-qm</option></term>
2509 <indexterm><primary><option>-qm</option></primary><secondary>RTS
2510 option</secondary></indexterm>
2511 <listitem>
2512 <para>Disable automatic migration for load balancing.
2513 Normally the runtime will automatically try to schedule
2514 threads across the available CPUs to make use of idle
2515 CPUs; this option disables that behaviour. Note that
2516 migration only applies to threads; sparks created
2517 by <literal>par</literal> are load-balanced separately
2518 by work-stealing.</para>
2519
2520 <para>
2521 This option is probably only of use for concurrent
2522 programs that explicitly schedule threads onto CPUs
2523 with <literal>Control.Concurrent.forkOn</literal>.
2524 </para>
2525 </listitem>
2526 </varlistentry>
2527 </variablelist>
2528 </sect2>
2529
2530 <sect2>
2531 <title>Hints for using SMP parallelism</title>
2532
2533 <para>Add the <literal>-s</literal> RTS option when
2534 running the program to see timing stats, which will help to tell you
2535 whether your program got faster by using more CPUs or not. If the user
2536 time is greater than
2537 the elapsed time, then the program used more than one CPU. You should
2538 also run the program without <literal>-N</literal> for
2539 comparison.</para>
2540
2541 <para>The output of <literal>+RTS -s</literal> tells you how
2542 many &ldquo;sparks&rdquo; were created and executed during the
2543 run of the program (see <xref linkend="rts-options-gc" />), which
2544 will give you an idea how well your <literal>par</literal>
2545 annotations are working.</para>
2546
2547 <para>GHC's parallelism support has improved in 6.12.1 as a
2548 result of much experimentation and tuning in the runtime
2549 system. We'd still be interested to hear how well it works
2550 for you, and we're also interested in collecting parallel
2551 programs to add to our benchmarking suite.</para>
2552 </sect2>
2553 </sect1>
2554
2555 <sect1 id="options-platform">
2556 <title>Platform-specific Flags</title>
2557
2558 <indexterm><primary>-m* options</primary></indexterm>
2559 <indexterm><primary>platform-specific options</primary></indexterm>
2560 <indexterm><primary>machine-specific options</primary></indexterm>
2561
2562 <para>Some flags only make sense for particular target
2563 platforms.</para>
2564
2565 <variablelist>
2566
2567 <varlistentry>
2568 <term><option>-msse2</option>:</term>
2569 <listitem>
2570 <para>
2571 (x86 only, added in GHC 7.0.1) Use the SSE2 registers and
2572 instruction set to implement floating point operations when using
2573 the <link linkend="native-code-gen">native code generator</link>.
2574 This gives a substantial performance improvement for floating
2575 point, but the resulting compiled code
2576 will only run on processors that support SSE2 (Intel Pentium 4 and
2577 later, or AMD Athlon 64 and later). The
2578 <link linkend="llvm-code-gen">LLVM backend</link> will also use SSE2
2579 if your processor supports it but detects this automatically so no
2580 flag is required.
2581 </para>
2582 <para>
2583 SSE2 is unconditionally used on x86-64 platforms.
2584 </para>
2585 </listitem>
2586 </varlistentry>
2587
2588 <varlistentry>
2589 <term><option>-msse4.2</option>:</term>
2590 <listitem>
2591 <para>
2592 (x86 only, added in GHC 7.4.1) Use the SSE4.2 instruction set to
2593 implement some floating point and bit operations when using the
2594 <link linkend="native-code-gen">native code generator</link>. The
2595 resulting compiled code will only run on processors that
2596 support SSE4.2 (Intel Core i7 and later). The
2597 <link linkend="llvm-code-gen">LLVM backend</link> will also use
2598 SSE4.2 if your processor supports it but detects this automatically
2599 so no flag is required.
2600 </para>
2601 </listitem>
2602 </varlistentry>
2603
2604 </variablelist>
2605
2606 </sect1>
2607
2608 &runtime;
2609
2610 <sect1 id="ext-core">
2611 <title>Generating and compiling External Core Files</title>
2612
2613 <indexterm><primary>intermediate code generation</primary></indexterm>
2614
2615 <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format)
2616 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
2617 <filename>.hcr</filename>. The Core format is described in <ulink url="../../core.pdf">
2618 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
2619 and sample tools
2620 for manipulating Core files (in Haskell) are available in the
2621 <ulink url="http://hackage.haskell.org/package/extcore">extcore package on Hackage</ulink>. Note that the format of <literal>.hcr</literal>
2622 files is <emphasis>different</emphasis> from the Core output format that GHC generates
2623 for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
2624
2625 <para>The Core format natively supports notes which you can add to
2626 your source code using the <literal>CORE</literal> pragma (see <xref
2627 linkend="pragmas"/>).</para>
2628
2629 <variablelist>
2630
2631 <varlistentry>
2632 <term>
2633 <option>-fext-core</option>
2634 <indexterm><primary><option>-fext-core</option></primary></indexterm>
2635 </term>
2636 <listitem>
2637 <para>Generate <literal>.hcr</literal> files.</para>
2638 </listitem>
2639 </varlistentry>
2640
2641 </variablelist>
2642
2643 <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>
2644
2645 </sect1>
2646
2647 &debug;
2648 &flags;
2649
2650 </chapter>
2651
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