1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="using-ghc">
3 <title>Using GHC</title>
5 <indexterm><primary>GHC, using</primary></indexterm>
6 <indexterm><primary>using GHC</primary></indexterm>
9 <title>Options overview</title>
11 <para>GHC's behaviour is controlled by
12 <firstterm>options</firstterm>, which for historical reasons are
13 also sometimes referred to as command-line flags or arguments.
14 Options can be specified in three ways:</para>
17 <title>Command-line arguments</title>
19 <indexterm><primary>structure, command-line</primary></indexterm>
20 <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
21 <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
23 <para>An invocation of GHC takes the following form:</para>
29 <para>Command-line arguments are either options or file names.</para>
31 <para>Command-line options begin with <literal>-</literal>.
32 They may <emphasis>not</emphasis> be grouped:
33 <option>-vO</option> is different from <option>-v -O</option>.
34 Options need not precede filenames: e.g., <literal>ghc *.o -o
35 foo</literal>. All options are processed and then applied to
36 all files; you cannot, for example, invoke <literal>ghc -c -O1
37 Foo.hs -O2 Bar.hs</literal> to apply different optimisation
38 levels to the files <filename>Foo.hs</filename> and
39 <filename>Bar.hs</filename>.</para>
42 <sect2 id="source-file-options">
43 <title>Command line options in source files</title>
45 <indexterm><primary>source-file options</primary></indexterm>
47 <para>Sometimes it is useful to make the connection between a
48 source file and the command-line options it requires quite
49 tight. For instance, if a Haskell source file uses GHC
50 extensions, it will always need to be compiled with the
51 <option>-fglasgow-exts</option> option. Rather than maintaining
52 the list of per-file options in a <filename>Makefile</filename>,
53 it is possible to do this directly in the source file using the
54 <literal>OPTIONS_GHC</literal> pragma <indexterm><primary>OPTIONS_GHC
55 pragma</primary></indexterm>:</para>
58 {-# OPTIONS_GHC -fglasgow-exts #-}
63 <para><literal>OPTIONS_GHC</literal> pragmas are only looked for at
64 the top of your source files, upto the first
65 (non-literate,non-empty) line not containing
66 <literal>OPTIONS_GHC</literal>. Multiple <literal>OPTIONS_GHC</literal>
67 pragmas are recognised. Do not put comments before, or on the same line
68 as, the <literal>OPTIONS_GHC</literal> pragma.</para>
70 <para>Note that your command shell does not
71 get to the source file options, they are just included literally
72 in the array of command-line arguments the compiler
73 maintains internally, so you'll be desperately disappointed if
74 you try to glob etc. inside <literal>OPTIONS_GHC</literal>.</para>
76 <para>NOTE: the contents of OPTIONS_GHC are appended to the
77 command-line options, so options given in the source file
78 override those given on the command-line.</para>
80 <para>It is not recommended to move all the contents of your
81 Makefiles into your source files, but in some circumstances, the
82 <literal>OPTIONS_GHC</literal> pragma is the Right Thing. (If you
83 use <option>-keep-hc-file</option> and have OPTION flags in
84 your module, the OPTIONS_GHC will get put into the generated .hc
89 <title>Setting options in GHCi</title>
91 <para>Options may also be modified from within GHCi, using the
92 <literal>:set</literal> command. See <xref linkend="ghci-set"/>
93 for more details.</para>
97 <sect1 id="static-dynamic-flags">
98 <title>Static, Dynamic, and Mode options</title>
99 <indexterm><primary>static</primary><secondary>options</secondary>
101 <indexterm><primary>dynamic</primary><secondary>options</secondary>
103 <indexterm><primary>mode</primary><secondary>options</secondary>
106 <para>Each of GHC's command line options is classified as
107 <firstterm>static</firstterm>, <firstterm>dynamic</firstterm> or
108 <firstterm>mode</firstterm>:</para>
112 <term>Mode flags</term>
114 <para>For example, <option>--make</option> or <option>-E</option>.
115 There may only be a single mode flag on the command line. The
116 available modes are listed in <xref linkend="modes"/>.</para>
120 <term>Dynamic Flags</term>
122 <para>Most non-mode flags fall into this category. A dynamic flag
123 may be used on the command line, in a
124 <literal>GHC_OPTIONS</literal> pragma in a source file, or set
125 using <literal>:set</literal> in GHCi.</para>
129 <term>Static Flags</term>
131 <para>A few flags are "static", which means they can only be used on
132 the command-line, and remain in force over the entire GHC/GHCi
138 <para>The flag reference tables (<xref
139 linkend="flag-reference"/>) lists the status of each flag.</para>
141 <para>There are a few flags that are static except that they can
142 also be used with GHCi's <literal>:set</literal> command; these
143 are listed as “static/<literal>:set</literal>” in the
147 <sect1 id="file-suffixes">
148 <title>Meaningful file suffixes</title>
150 <indexterm><primary>suffixes, file</primary></indexterm>
151 <indexterm><primary>file suffixes for GHC</primary></indexterm>
153 <para>File names with “meaningful” suffixes (e.g.,
154 <filename>.lhs</filename> or <filename>.o</filename>) cause the
155 “right thing” to happen to those files.</para>
160 <term><filename>.hs</filename></term>
162 <para>A Haskell module.</para>
168 <filename>.lhs</filename>
169 <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
172 <para>A “literate Haskell” module.</para>
177 <term><filename>.hi</filename></term>
179 <para>A Haskell interface file, probably
180 compiler-generated.</para>
185 <term><filename>.hc</filename></term>
187 <para>Intermediate C file produced by the Haskell
193 <term><filename>.c</filename></term>
195 <para>A C file not produced by the Haskell
201 <term><filename>.s</filename></term>
203 <para>An assembly-language source file, usually produced by
209 <term><filename>.o</filename></term>
211 <para>An object file, produced by an assembler.</para>
216 <para>Files with other suffixes (or without suffixes) are passed
217 straight to the linker.</para>
222 <title>Modes of operation</title>
224 <para>GHC's behaviour is firstly controlled by a mode flag. Only
225 one of these flags may be given, but it does not necessarily need
226 to be the first option on the command-line. The available modes
232 <cmdsynopsis><command>ghc --interactive</command>
234 <indexterm><primary>interactive mode</primary></indexterm>
235 <indexterm><primary>ghci</primary></indexterm>
238 <para>Interactive mode, which is also available as
239 <command>ghci</command>. Interactive mode is described in
240 more detail in <xref linkend="ghci"/>.</para>
246 <cmdsynopsis><command>ghc --make</command>
248 <indexterm><primary>make mode</primary></indexterm>
249 <indexterm><primary><option>––make</option></primary></indexterm>
252 <para>In this mode, GHC will build a multi-module Haskell
253 program automatically, figuring out dependencies for itself.
254 If you have a straightforward Haskell program, this is
255 likely to be much easier, and faster, than using
256 <command>make</command>. Make mode is described in <xref
257 linkend="make-mode"/>.</para>
263 <cmdsynopsis><command>ghc -e</command>
264 <arg choice='plain'><replaceable>expr</replaceable></arg>
266 <indexterm><primary>eval mode</primary></indexterm>
269 <para>Expression-evaluation mode. This is very similar to
270 interactive mode, except that there is a single expression
271 to evaluate (<replaceable>expr</replaceable>) which is given
272 on the command line. See <xref linkend="eval-mode"/> for
280 <command>ghc -E</command>
281 <command>ghc -c</command>
282 <command>ghc -S</command>
283 <command>ghc -c</command>
285 <indexterm><primary><option>-E</option></primary></indexterm>
286 <indexterm><primary><option>-C</option></primary></indexterm>
287 <indexterm><primary><option>-S</option></primary></indexterm>
288 <indexterm><primary><option>-c</option></primary></indexterm>
291 <para>This is the traditional batch-compiler mode, in which
292 GHC can compile source files one at a time, or link objects
293 together into an executable. This mode also applies if
294 there is no other mode flag specified on the command line,
295 in which case it means that the specified files should be
296 compiled and then linked to form a program. See <xref
297 linkend="options-order"/>.</para>
304 <command>ghc -M</command>
306 <indexterm><primary>dependency-generation mode</primary></indexterm>
309 <para>Dependency-generation mode. In this mode, GHC can be
310 used to generate dependency information suitable for use in
311 a <literal>Makefile</literal>. See <xref
312 linkend="makefile-dependencies"/>.</para>
319 <command>ghc --mk-dll</command>
321 <indexterm><primary>DLL-creation mode</primary></indexterm>
324 <para>DLL-creation mode (Windows only). See <xref
325 linkend="win32-dlls-create"/>.</para>
332 <command>ghc --help</command> <command>ghc -?</command>
334 <indexterm><primary><option>––help</option></primary></indexterm>
337 <para>Cause GHC to spew a long usage message to standard
338 output and then exit.</para>
345 <command>ghc --show-iface <replaceable>file</replaceable></command>
347 <indexterm><primary><option>––--show-iface</option></primary></indexterm>
350 <para>Read the interface in
351 <replaceable>file</replaceable> and dump it as text to
352 <literal>stdout</literal>. For example <literal>ghc --show-iface M.hi</literal>.</para>
359 <command>ghc --supported-languages</command>
361 <indexterm><primary><option>––supported-languages</option></primary></indexterm>
364 <para>Print the supported language extensions.</para>
371 <command>ghc --info</command>
373 <indexterm><primary><option>––info</option></primary></indexterm>
376 <para>Print information about the compiler.</para>
383 <command>ghc --version</command>
384 <command>ghc -V</command>
386 <indexterm><primary><option>-V</option></primary></indexterm>
387 <indexterm><primary><option>––version</option></primary></indexterm>
390 <para>Print a one-line string including GHC's version number.</para>
397 <command>ghc --numeric-version</command>
399 <indexterm><primary><option>––numeric-version</option></primary></indexterm>
402 <para>Print GHC's numeric version number only.</para>
409 <command>ghc --print-libdir</command>
411 <indexterm><primary><option>––print-libdir</option></primary></indexterm>
414 <para>Print the path to GHC's library directory. This is
415 the top of the directory tree containing GHC's libraries,
416 interfaces, and include files (usually something like
417 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
419 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
420 in the package configuration file
421 (see <xref linkend="packages"/>).</para>
427 <sect2 id="make-mode">
428 <title>Using <command>ghc</command> <option>––make</option></title>
429 <indexterm><primary><option>––make</option></primary></indexterm>
430 <indexterm><primary>separate compilation</primary></indexterm>
432 <para>When given the <option>––make</option> option,
433 GHC will build a multi-module Haskell program by following
434 dependencies from one or more root modules (usually just
435 <literal>Main</literal>). For example, if your
436 <literal>Main</literal> module is in a file called
437 <filename>Main.hs</filename>, you could compile and link the
438 program like this:</para>
441 ghc ––make Main.hs
444 <para>The command line may contain any number of source file
445 names or module names; GHC will figure out all the modules in
446 the program by following the imports from these initial modules.
447 It will then attempt to compile each module which is out of
448 date, and finally, if there is a <literal>Main</literal> module,
449 the program will also be linked into an executable.</para>
451 <para>The main advantages to using <literal>ghc
452 ––make</literal> over traditional
453 <literal>Makefile</literal>s are:</para>
457 <para>GHC doesn't have to be restarted for each compilation,
458 which means it can cache information between compilations.
459 Compiling a multi-module program with <literal>ghc
460 ––make</literal> can be up to twice as fast as
461 running <literal>ghc</literal> individually on each source
465 <para>You don't have to write a <literal>Makefile</literal>.</para>
466 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
469 <para>GHC re-calculates the dependencies each time it is
470 invoked, so the dependencies never get out of sync with the
475 <para>Any of the command-line options described in the rest of
476 this chapter can be used with
477 <option>––make</option>, but note that any options
478 you give on the command line will apply to all the source files
479 compiled, so if you want any options to apply to a single source
480 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
481 pragma (see <xref linkend="source-file-options"/>).</para>
483 <para>If the program needs to be linked with additional objects
484 (say, some auxiliary C code), then the object files can be
485 given on the command line and GHC will include them when linking
486 the executable.</para>
488 <para>Note that GHC can only follow dependencies if it has the
489 source file available, so if your program includes a module for
490 which there is no source file, even if you have an object and an
491 interface file for the module, then GHC will complain. The
492 exception to this rule is for package modules, which may or may
493 not have source files.</para>
495 <para>The source files for the program don't all need to be in
496 the same directory; the <option>-i</option> option can be used
497 to add directories to the search path (see <xref
498 linkend="search-path"/>).</para>
501 <sect2 id="eval-mode">
502 <title>Expression evaluation mode</title>
504 <para>This mode is very similar to interactive mode, except that
505 there is a single expression to evaluate which is specified on
506 the command line as an argument to the <option>-e</option>
510 ghc -e <replaceable>expr</replaceable>
513 <para>Haskell source files may be named on the command line, and
514 they will be loaded exactly as in interactive mode. The
515 expression is evaluated in the context of the loaded
518 <para>For example, to load and run a Haskell program containing
519 a module <literal>Main</literal>, we might say</para>
522 ghc -e Main.main Main.hs
525 <para>or we can just use this mode to evaluate expressions in
526 the context of the <literal>Prelude</literal>:</para>
529 $ ghc -e "interact (unlines.map reverse.lines)"
535 <sect2 id="options-order">
536 <title>Batch compiler mode</title>
538 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
539 given on the command line.</para>
541 <para>The first phase to run is determined by each input-file
542 suffix, and the last phase is determined by a flag. If no
543 relevant flag is present, then go all the way through to linking.
544 This table summarises:</para>
548 <colspec align="left"/>
549 <colspec align="left"/>
550 <colspec align="left"/>
551 <colspec align="left"/>
555 <entry>Phase of the compilation system</entry>
556 <entry>Suffix saying “start here”</entry>
557 <entry>Flag saying “stop after”</entry>
558 <entry>(suffix of) output file</entry>
563 <entry>literate pre-processor</entry>
564 <entry><literal>.lhs</literal></entry>
566 <entry><literal>.hs</literal></entry>
570 <entry>C pre-processor (opt.) </entry>
571 <entry><literal>.hs</literal> (with
572 <option>-cpp</option>)</entry>
573 <entry><option>-E</option></entry>
574 <entry><literal>.hspp</literal></entry>
578 <entry>Haskell compiler</entry>
579 <entry><literal>.hs</literal></entry>
580 <entry><option>-C</option>, <option>-S</option></entry>
581 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
585 <entry>C compiler (opt.)</entry>
586 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
587 <entry><option>-S</option></entry>
588 <entry><literal>.s</literal></entry>
592 <entry>assembler</entry>
593 <entry><literal>.s</literal></entry>
594 <entry><option>-c</option></entry>
595 <entry><literal>.o</literal></entry>
599 <entry>linker</entry>
600 <entry><replaceable>other</replaceable></entry>
602 <entry><filename>a.out</filename></entry>
608 <indexterm><primary><option>-C</option></primary></indexterm>
609 <indexterm><primary><option>-E</option></primary></indexterm>
610 <indexterm><primary><option>-S</option></primary></indexterm>
611 <indexterm><primary><option>-c</option></primary></indexterm>
613 <para>Thus, a common invocation would be: </para>
616 ghc -c Foo.hs</screen>
618 <para>to compile the Haskell source file
619 <filename>Foo.hs</filename> to an object file
620 <filename>Foo.o</filename>.</para>
622 <para>Note: What the Haskell compiler proper produces depends on
623 whether a native-code generator<indexterm><primary>native-code
624 generator</primary></indexterm> is used (producing assembly
625 language) or not (producing C). See <xref
626 linkend="options-codegen"/> for more details.</para>
628 <para>Note: C pre-processing is optional, the
629 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
630 flag turns it on. See <xref linkend="c-pre-processor"/> for more
633 <para>Note: The option <option>-E</option><indexterm><primary>-E
634 option</primary></indexterm> runs just the pre-processing passes
635 of the compiler, dumping the result in a file.</para>
637 <sect3 id="overriding-suffixes">
638 <title>Overriding the default behaviour for a file</title>
640 <para>As described above, the way in which a file is processed by GHC
641 depends on its suffix. This behaviour can be overridden using the
642 <option>-x</option> option:</para>
646 <term><option>-x</option> <replaceable>suffix</replaceable>
647 <indexterm><primary><option>-x</option></primary>
650 <para>Causes all files following this option on the command
651 line to be processed as if they had the suffix
652 <replaceable>suffix</replaceable>. For example, to compile a
653 Haskell module in the file <literal>M.my-hs</literal>,
654 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
663 <sect1 id="options-help">
664 <title>Help and verbosity options</title>
666 <indexterm><primary>help options</primary></indexterm>
667 <indexterm><primary>verbosity options</primary></indexterm>
669 <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
670 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
675 <indexterm><primary><option>-n</option></primary></indexterm>
678 <para>Does a dry-run, i.e. GHC goes through all the motions
679 of compiling as normal, but does not actually run any
680 external commands.</para>
687 <indexterm><primary><option>-v</option></primary></indexterm>
690 <para>The <option>-v</option> option makes GHC
691 <emphasis>verbose</emphasis>: it reports its version number
692 and shows (on stderr) exactly how it invokes each phase of
693 the compilation system. Moreover, it passes the
694 <option>-v</option> flag to most phases; each reports its
695 version number (and possibly some other information).</para>
697 <para>Please, oh please, use the <option>-v</option> option
698 when reporting bugs! Knowing that you ran the right bits in
699 the right order is always the first thing we want to
706 <option>-v</option><replaceable>n</replaceable>
707 <indexterm><primary><option>-v</option></primary></indexterm>
710 <para>To provide more control over the compiler's verbosity,
711 the <option>-v</option> flag takes an optional numeric
712 argument. Specifying <option>-v</option> on its own is
713 equivalent to <option>-v3</option>, and the other levels
714 have the following meanings:</para>
718 <term><option>-v0</option></term>
720 <para>Disable all non-essential messages (this is the
726 <term><option>-v1</option></term>
728 <para>Minimal verbosity: print one line per
729 compilation (this is the default when
730 <option>––make</option> or
731 <option>––interactive</option> is on).</para>
736 <term><option>-v2</option></term>
738 <para>Print the name of each compilation phase as it
739 is executed. (equivalent to
740 <option>-dshow-passes</option>).</para>
745 <term><option>-v3</option></term>
747 <para>The same as <option>-v2</option>, except that in
748 addition the full command line (if appropriate) for
749 each compilation phase is also printed.</para>
754 <term><option>-v4</option></term>
756 <para>The same as <option>-v3</option> except that the
757 intermediate program representation after each
758 compilation phase is also printed (excluding
759 preprocessed and C/assembly files).</para>
767 <term><option>-ferror-spans</option>
768 <indexterm><primary><option>-ferror-spans</option></primary>
772 <para>Causes GHC to emit the full source span of the
773 syntactic entity relating to an error message. Normally, GHC
774 emits the source location of the start of the syntactic
777 <para>For example:</para>
779 <screen>test.hs:3:6: parse error on input `where'</screen>
781 <para>becomes:</para>
783 <screen>test296.hs:3:6-10: parse error on input `where'</screen>
785 <para>And multi-line spans are possible too:</para>
787 <screen>test.hs:(5,4)-(6,7):
788 Conflicting definitions for `a'
789 Bound at: test.hs:5:4
791 In the binding group for: a, b, a</screen>
793 <para>Note that line numbers start counting at one, but
794 column numbers start at zero. This choice was made to
795 follow existing convention (i.e. this is how Emacs does
801 <term><option>-H</option><replaceable>size</replaceable>
802 <indexterm><primary><option>-H</option></primary></indexterm>
805 <para>Set the minimum size of the heap to
806 <replaceable>size</replaceable>.
807 This option is equivalent to
808 <literal>+RTS -H<replaceable>size</replaceable></literal>,
809 see <xref linkend="rts-options-gc" />.
815 <term><option>-Rghc-timing</option>
816 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
819 <para>Prints a one-line summary of timing statistics for the
820 GHC run. This option is equivalent to
821 <literal>+RTS -tstderr</literal>, see <xref
822 linkend="rts-options-gc" />.
831 <sect1 id="options-sanity">
832 <title>Warnings and sanity-checking</title>
834 <indexterm><primary>sanity-checking options</primary></indexterm>
835 <indexterm><primary>warnings</primary></indexterm>
838 <para>GHC has a number of options that select which types of
839 non-fatal error messages, otherwise known as warnings, can be
840 generated during compilation. By default, you get a standard set
841 of warnings which are generally likely to indicate bugs in your
843 <option>-fwarn-overlapping-patterns</option>,
844 <option>-fwarn-deprecations</option>,
845 <option>-fwarn-deprecated-flags</option>,
846 <option>-fwarn-duplicate-exports</option>,
847 <option>-fwarn-missing-fields</option>,
848 <option>-fwarn-missing-methods</option>, and
849 <option>-fwarn-dodgy-foreign-imports</option>. The following
851 simple ways to select standard “packages” of warnings:
857 <term><option>-W</option>:</term>
859 <indexterm><primary>-W option</primary></indexterm>
860 <para>Provides the standard warnings plus
861 <option>-fwarn-incomplete-patterns</option>,
862 <option>-fwarn-dodgy-imports</option>,
863 <option>-fwarn-unused-matches</option>,
864 <option>-fwarn-unused-imports</option>, and
865 <option>-fwarn-unused-binds</option>.</para>
870 <term><option>-Wall</option>:</term>
872 <indexterm><primary><option>-Wall</option></primary></indexterm>
873 <para>Turns on all warning options that indicate potentially
874 suspicious code. The warnings that are
875 <emphasis>not</emphasis> enabled by <option>-Wall</option>
877 <option>-fwarn-simple-patterns</option>,
878 <option>-fwarn-tabs</option>,
879 <option>-fwarn-incomplete-record-updates</option>,
880 <option>-fwarn-monomorphism-restriction</option>, and
881 <option>-fwarn-implicit-prelude</option>.</para>
886 <term><option>-w</option>:</term>
888 <indexterm><primary><option>-w</option></primary></indexterm>
889 <para>Turns off all warnings, including the standard ones and
890 those that <literal>-Wall</literal> doesn't enable.</para>
895 <term><option>-Werror</option>:</term>
897 <indexterm><primary><option>-Werror</option></primary></indexterm>
898 <para>Makes any warning into a fatal error. Useful so that you don't
899 miss warnings when doing batch compilation. </para>
904 <term><option>-Wwarn</option>:</term>
906 <indexterm><primary><option>-Wwarn</option></primary></indexterm>
907 <para>Warnings are treated only as warnings, not as errors. This is
908 the default, but can be useful to negate a
909 <option>-Werror</option> flag.</para>
915 <para>The full set of warning options is described below. To turn
916 off any warning, simply give the corresponding
917 <option>-fno-warn-...</option> option on the command line.</para>
922 <term><option>-fwarn-deprecations</option>:</term>
924 <indexterm><primary><option>-fwarn-deprecations</option></primary>
926 <indexterm><primary>deprecations</primary></indexterm>
927 <para>Causes a warning to be emitted when a deprecated
928 function or type is used. Entities can be marked as
929 deprecated using a pragma, see <xref
930 linkend="deprecated-pragma"/>.</para>
932 <para>This option is on by default.</para>
937 <term><option>-fwarn-deprecated-flags</option>:</term>
939 <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
941 <indexterm><primary>deprecated-flags</primary></indexterm>
942 <para>Causes a warning to be emitted when a deprecated
943 commandline flag is used.</para>
945 <para>This option is on by default.</para>
950 <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
952 <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
954 <para>Causes a warning to be emitted for foreign imports of
955 the following form:</para>
957 foreign import "f" f :: FunPtr t
959 <para>on the grounds that it probably should be</para>
961 foreign import "&f" f :: FunPtr t
963 <para>The first form declares that `f` is a (pure) C
964 function that takes no arguments and returns a pointer to a
965 C function with type `t`, whereas the second form declares
966 that `f` itself is a C function with type `t`. The first
967 declaration is usually a mistake, and one that is hard to
968 debug because it results in a crash, hence this
974 <term><option>-fwarn-dodgy-imports</option>:</term>
976 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
978 <para>Causes a warning to be emitted when a a datatype
979 <literal>T</literal> is imported
980 with all constructors, i.e. <literal>T(..)</literal>, but has been
981 exported abstractly, i.e. <literal>T</literal>.</para>
986 <term><option>-fwarn-duplicate-exports</option>:</term>
988 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
989 <indexterm><primary>duplicate exports, warning</primary></indexterm>
990 <indexterm><primary>export lists, duplicates</primary></indexterm>
992 <para>Have the compiler warn about duplicate entries in
993 export lists. This is useful information if you maintain
994 large export lists, and want to avoid the continued export
995 of a definition after you've deleted (one) mention of it in
996 the export list.</para>
998 <para>This option is on by default.</para>
1003 <term><option>-fwarn-hi-shadowing</option>:</term>
1005 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
1006 <indexterm><primary>shadowing</primary>
1007 <secondary>interface files</secondary></indexterm>
1009 <para>Causes the compiler to emit a warning when a module or
1010 interface file in the current directory is shadowing one
1011 with the same module name in a library or other
1017 <term><option>-fwarn-implicit-prelude</option>:</term>
1019 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
1020 <indexterm><primary>implicit prelude, warning</primary></indexterm>
1021 <para>Have the compiler warn if the Prelude is implicitly
1022 imported. This happens unless either the Prelude module is
1023 explicitly imported with an <literal>import ... Prelude ...</literal>
1024 line, or this implicit import is disabled (either by
1025 <option>-XNoImplicitPrelude</option> or a
1026 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
1028 <para>Note that no warning is given for syntax that implicitly
1029 refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
1030 would change whether it refers to the Prelude.
1031 For example, no warning is given when
1032 <literal>368</literal> means
1033 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
1034 (where <literal>Prelude</literal> refers to the actual Prelude module,
1035 regardless of the imports of the module being compiled).</para>
1037 <para>This warning is off by default.</para>
1042 <term><option>-fwarn-incomplete-patterns</option>:</term>
1044 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
1045 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
1046 <indexterm><primary>patterns, incomplete</primary></indexterm>
1048 <para>Similarly for incomplete patterns, the function
1049 <function>g</function> below will fail when applied to
1050 non-empty lists, so the compiler will emit a warning about
1051 this when <option>-fwarn-incomplete-patterns</option> is
1058 <para>This option isn't enabled by default because it can be
1059 a bit noisy, and it doesn't always indicate a bug in the
1060 program. However, it's generally considered good practice
1061 to cover all the cases in your functions.</para>
1066 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1068 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1069 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1070 <indexterm><primary>record updates, incomplete</primary></indexterm>
1073 <function>f</function> below will fail when applied to
1074 <literal>Bar</literal>, so the compiler will emit a warning about
1075 this when <option>-fwarn-incomplete-record-updates</option> is
1079 data Foo = Foo { x :: Int }
1083 f foo = foo { x = 6 }
1086 <para>This option isn't enabled by default because it can be
1087 very noisy, and it often doesn't indicate a bug in the
1094 <option>-fwarn-missing-fields</option>:
1095 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1096 <indexterm><primary>missing fields, warning</primary></indexterm>
1097 <indexterm><primary>fields, missing</primary></indexterm>
1101 <para>This option is on by default, and warns you whenever
1102 the construction of a labelled field constructor isn't
1103 complete, missing initializers for one or more fields. While
1104 not an error (the missing fields are initialised with
1105 bottoms), it is often an indication of a programmer error.</para>
1110 <term><option>-fwarn-missing-methods</option>:</term>
1112 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1113 <indexterm><primary>missing methods, warning</primary></indexterm>
1114 <indexterm><primary>methods, missing</primary></indexterm>
1116 <para>This option is on by default, and warns you whenever
1117 an instance declaration is missing one or more methods, and
1118 the corresponding class declaration has no default
1119 declaration for them.</para>
1120 <para>The warning is suppressed if the method name
1121 begins with an underscore. Here's an example where this is useful:
1124 _simpleFn :: a -> String
1125 complexFn :: a -> a -> String
1126 complexFn x y = ... _simpleFn ...
1128 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1129 never <literal>_simpleFn</literal>; and (b)
1130 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1136 <term><option>-fwarn-missing-signatures</option>:</term>
1138 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1139 <indexterm><primary>type signatures, missing</primary></indexterm>
1141 <para>If you would like GHC to check that every top-level
1142 function/value has a type signature, use the
1143 <option>-fwarn-missing-signatures</option> option. As part of
1144 the warning GHC also reports the inferred type. The
1145 option is off by default.</para>
1150 <term><option>-fwarn-name-shadowing</option>:</term>
1152 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1153 <indexterm><primary>shadowing, warning</primary></indexterm>
1155 <para>This option causes a warning to be emitted whenever an
1156 inner-scope value has the same name as an outer-scope value,
1157 i.e. the inner value shadows the outer one. This can catch
1158 typographical errors that turn into hard-to-find bugs, e.g.,
1159 in the inadvertent capture of what would be a recursive call in
1160 <literal>f = ... let f = id in ... f ...</literal>.</para>
1165 <term><option>-fwarn-orphans</option>:</term>
1167 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1168 <indexterm><primary>orphan instances, warning</primary></indexterm>
1169 <indexterm><primary>orphan rules, warning</primary></indexterm>
1171 <para>This option causes a warning to be emitted whenever the
1172 module contains an "orphan" instance declaration or rewrite rule.
1173 An instance declaration is an orphan if it appears in a module in
1174 which neither the class nor the type being instanced are declared
1175 in the same module. A rule is an orphan if it is a rule for a
1176 function declared in another module. A module containing any
1177 orphans is called an orphan module.</para>
1178 <para>The trouble with orphans is that GHC must pro-actively read the interface
1179 files for all orphan modules, just in case their instances or rules
1180 play a role, whether or not the module's interface would otherwise
1181 be of any use. Other things being equal, avoid orphan modules.</para>
1187 <option>-fwarn-overlapping-patterns</option>:
1188 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1189 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1190 <indexterm><primary>patterns, overlapping</primary></indexterm>
1193 <para>By default, the compiler will warn you if a set of
1194 patterns are overlapping, e.g.,</para>
1197 f :: String -> Int
1203 <para>where the last pattern match in <function>f</function>
1204 won't ever be reached, as the second pattern overlaps
1205 it. More often than not, redundant patterns is a programmer
1206 mistake/error, so this option is enabled by default.</para>
1211 <term><option>-fwarn-simple-patterns</option>:</term>
1213 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1215 <para>Causes the compiler to warn about lambda-bound
1216 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1217 Normally, these aren't treated as incomplete patterns by
1218 <option>-fwarn-incomplete-patterns</option>.</para>
1219 <para>“Lambda-bound patterns” includes all places where there is a single pattern,
1220 including list comprehensions and do-notation. In these cases, a pattern-match
1221 failure is quite legitimate, and triggers filtering (list comprehensions) or
1222 the monad <literal>fail</literal> operation (monads). For example:
1224 f :: [Maybe a] -> [a]
1225 f xs = [y | Just y <- xs]
1227 Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
1228 these probably-innocent cases, which is why the flag is off by default. </para>
1233 <term><option>-fwarn-tabs</option>:</term>
1235 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1236 <indexterm><primary>tabs, warning</primary></indexterm>
1237 <para>Have the compiler warn if there are tabs in your source
1240 <para>This warning is off by default.</para>
1245 <term><option>-fwarn-type-defaults</option>:</term>
1247 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1248 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1249 <para>Have the compiler warn/inform you where in your source
1250 the Haskell defaulting mechanism for numeric types kicks
1251 in. This is useful information when converting code from a
1252 context that assumed one default into one with another,
1253 e.g., the ‘default default’ for Haskell 1.4 caused the
1254 otherwise unconstrained value <constant>1</constant> to be
1255 given the type <literal>Int</literal>, whereas Haskell 98
1256 defaults it to <literal>Integer</literal>. This may lead to
1257 differences in performance and behaviour, hence the
1258 usefulness of being non-silent about this.</para>
1260 <para>This warning is off by default.</para>
1265 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1267 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1268 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1269 <para>Have the compiler warn/inform you where in your source
1270 the Haskell Monomorphism Restriction is applied. If applied silently
1271 the MR can give rise to unexpected behaviour, so it can be helpful
1272 to have an explicit warning that it is being applied.</para>
1274 <para>This warning is off by default.</para>
1279 <term><option>-fwarn-unused-binds</option>:</term>
1281 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1282 <indexterm><primary>unused binds, warning</primary></indexterm>
1283 <indexterm><primary>binds, unused</primary></indexterm>
1284 <para>Report any function definitions (and local bindings)
1285 which are unused. For top-level functions, the warning is
1286 only given if the binding is not exported.</para>
1287 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1288 mentioned in the right hand side of another definition that is used, or (c) the
1289 function it defines begins with an underscore. The last case provides a
1290 way to suppress unused-binding warnings selectively. </para>
1291 <para> Notice that a variable
1292 is reported as unused even if it appears in the right-hand side of another
1293 unused binding. </para>
1298 <term><option>-fwarn-unused-imports</option>:</term>
1300 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1301 <indexterm><primary>unused imports, warning</primary></indexterm>
1302 <indexterm><primary>imports, unused</primary></indexterm>
1304 <para>Report any modules that are explicitly imported but
1305 never used. However, the form <literal>import M()</literal> is
1306 never reported as an unused import, because it is a useful idiom
1307 for importing instance declarations, which are anonymous in Haskell.</para>
1312 <term><option>-fwarn-unused-matches</option>:</term>
1314 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1315 <indexterm><primary>unused matches, warning</primary></indexterm>
1316 <indexterm><primary>matches, unused</primary></indexterm>
1318 <para>Report all unused variables which arise from pattern
1319 matches, including patterns consisting of a single variable.
1320 For instance <literal>f x y = []</literal> would report
1321 <varname>x</varname> and <varname>y</varname> as unused. The
1322 warning is suppressed if the variable name begins with an underscore, thus:
1332 <para>If you're feeling really paranoid, the
1333 <option>-dcore-lint</option>
1334 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1335 is a good choice. It turns on heavyweight intra-pass
1336 sanity-checking within GHC. (It checks GHC's sanity, not
1343 <sect1 id="options-optimise">
1344 <title>Optimisation (code improvement)</title>
1346 <indexterm><primary>optimisation</primary></indexterm>
1347 <indexterm><primary>improvement, code</primary></indexterm>
1349 <para>The <option>-O*</option> options specify convenient
1350 “packages” of optimisation flags; the
1351 <option>-f*</option> options described later on specify
1352 <emphasis>individual</emphasis> optimisations to be turned on/off;
1353 the <option>-m*</option> options specify
1354 <emphasis>machine-specific</emphasis> optimisations to be turned
1357 <sect2 id="optimise-pkgs">
1358 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1360 <para>There are <emphasis>many</emphasis> options that affect
1361 the quality of code produced by GHC. Most people only have a
1362 general goal, something like “Compile quickly” or
1363 “Make my program run like greased lightning.” The
1364 following “packages” of optimisations (or lack
1365 thereof) should suffice.</para>
1367 <para>Note that higher optimisation levels cause more
1368 cross-module optimisation to be performed, which can have an
1369 impact on how much of your program needs to be recompiled when
1370 you change something. This is one reason to stick to
1371 no-optimisation when developing code.</para>
1377 No <option>-O*</option>-type option specified:
1378 <indexterm><primary>-O* not specified</primary></indexterm>
1381 <para>This is taken to mean: “Please compile
1382 quickly; I'm not over-bothered about compiled-code
1383 quality.” So, for example: <command>ghc -c
1384 Foo.hs</command></para>
1390 <option>-O0</option>:
1391 <indexterm><primary><option>-O0</option></primary></indexterm>
1394 <para>Means “turn off all optimisation”,
1395 reverting to the same settings as if no
1396 <option>-O</option> options had been specified. Saying
1397 <option>-O0</option> can be useful if
1398 eg. <command>make</command> has inserted a
1399 <option>-O</option> on the command line already.</para>
1405 <option>-O</option> or <option>-O1</option>:
1406 <indexterm><primary>-O option</primary></indexterm>
1407 <indexterm><primary>-O1 option</primary></indexterm>
1408 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1411 <para>Means: “Generate good-quality code without
1412 taking too long about it.” Thus, for example:
1413 <command>ghc -c -O Main.lhs</command></para>
1419 <option>-O2</option>:
1420 <indexterm><primary>-O2 option</primary></indexterm>
1421 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1424 <para>Means: “Apply every non-dangerous
1425 optimisation, even if it means significantly longer
1426 compile times.”</para>
1428 <para>The avoided “dangerous” optimisations
1429 are those that can make runtime or space
1430 <emphasis>worse</emphasis> if you're unlucky. They are
1431 normally turned on or off individually.</para>
1433 <para>At the moment, <option>-O2</option> is
1434 <emphasis>unlikely</emphasis> to produce better code than
1435 <option>-O</option>.</para>
1441 <option>-Ofile <file></option>:
1442 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1443 <indexterm><primary>optimising, customised</primary></indexterm>
1446 <para>(NOTE: not supported since GHC 4.x. Please ask if
1447 you're interested in this.)</para>
1449 <para>For those who need <emphasis>absolute</emphasis>
1450 control over <emphasis>exactly</emphasis> what options are
1451 used (e.g., compiler writers, sometimes :-), a list of
1452 options can be put in a file and then slurped in with
1453 <option>-Ofile</option>.</para>
1455 <para>In that file, comments are of the
1456 <literal>#</literal>-to-end-of-line variety; blank
1457 lines and most whitespace is ignored.</para>
1459 <para>Please ask if you are baffled and would like an
1460 example of <option>-Ofile</option>!</para>
1465 <para>We don't use a <option>-O*</option> flag for day-to-day
1466 work. We use <option>-O</option> to get respectable speed;
1467 e.g., when we want to measure something. When we want to go for
1468 broke, we tend to use <option>-O2 -fvia-C</option> (and we go for
1469 lots of coffee breaks).</para>
1471 <para>The easiest way to see what <option>-O</option> (etc.)
1472 “really mean” is to run with <option>-v</option>,
1473 then stand back in amazement.</para>
1476 <sect2 id="options-f">
1477 <title><option>-f*</option>: platform-independent flags</title>
1479 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1480 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1482 <para>These flags turn on and off individual optimisations.
1483 They are normally set via the <option>-O</option> options
1484 described above, and as such, you shouldn't need to set any of
1485 them explicitly (indeed, doing so could lead to unexpected
1486 results). However, there are one or two that may be of
1491 <term><option>-fexcess-precision</option>:</term>
1493 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1494 <para>When this option is given, intermediate floating
1495 point values can have a <emphasis>greater</emphasis>
1496 precision/range than the final type. Generally this is a
1497 good thing, but some programs may rely on the exact
1499 <literal>Float</literal>/<literal>Double</literal> values
1500 and should not use this option for their compilation.</para>
1505 <term><option>-fignore-asserts</option>:</term>
1507 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1508 <para>Causes GHC to ignore uses of the function
1509 <literal>Exception.assert</literal> in source code (in
1510 other words, rewriting <literal>Exception.assert p
1511 e</literal> to <literal>e</literal> (see <xref
1512 linkend="assertions"/>). This flag is turned on by
1513 <option>-O</option>.
1520 <option>-fno-cse</option>
1521 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1524 <para>Turns off the common-sub-expression elimination optimisation.
1525 Can be useful if you have some <literal>unsafePerformIO</literal>
1526 expressions that you don't want commoned-up.</para>
1532 <option>-fno-strictness</option>
1533 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1536 <para>Turns off the strictness analyser; sometimes it eats
1537 too many cycles.</para>
1543 <option>-fno-full-laziness</option>
1544 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1547 <para>Turns off the full laziness optimisation (also known as
1548 let-floating). Full laziness increases sharing, which can lead
1549 to increased memory residency.</para>
1551 <para>NOTE: GHC doesn't implement complete full-laziness.
1552 When optimisation in on, and
1553 <option>-fno-full-laziness</option> is not given, some
1554 transformations that increase sharing are performed, such
1555 as extracting repeated computations from a loop. These
1556 are the same transformations that a fully lazy
1557 implementation would do, the difference is that GHC
1558 doesn't consistently apply full-laziness, so don't rely on
1565 <option>-fspec-constr</option>
1566 <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
1569 <para>Turn on call-pattern specialisation.</para>
1575 <option>-fliberate-case</option>
1576 <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
1579 <para>Turn on the liberate-case transformation.</para>
1585 <option>-fstatic-argument-transformation</option>
1586 <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
1589 <para>Turn on the static argument transformation.</para>
1595 <option>-fno-state-hack</option>
1596 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1599 <para>Turn off the "state hack" whereby any lambda with a
1600 <literal>State#</literal> token as argument is considered to be
1601 single-entry, hence it is considered OK to inline things inside
1602 it. This can improve performance of IO and ST monad code, but it
1603 runs the risk of reducing sharing.</para>
1609 <option>-fomit-interface-pragmas</option>
1610 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
1613 <para>Tells GHC to omit all inessential information from the interface file
1614 generated for the module being compiled (say M). This means that a module
1615 importing M will see only the <emphasis>types</emphasis> of the functions that M exports, but not
1616 their unfoldings, strictness info, etc. Hence, for example,
1617 no function exported by M will be inlined
1618 into an importing module. The benefit is that modules that import M will
1619 need to be recompiled less often (only when M's exports change their type,
1620 not when they change their implementation).
1627 <option>-fignore-interface-pragmas</option>
1628 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
1631 <para>Tells GHC to ignore all inessential information when reading interface files.
1632 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
1633 for a function, GHC will ignore that information.</para>
1639 <option>-funbox-strict-fields</option>:
1640 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1641 <indexterm><primary>strict constructor fields</primary></indexterm>
1642 <indexterm><primary>constructor fields, strict</primary></indexterm>
1645 <para>This option causes all constructor fields which are
1646 marked strict (i.e. “!”) to be unboxed or
1647 unpacked if possible. It is equivalent to adding an
1648 <literal>UNPACK</literal> pragma to every strict
1649 constructor field (see <xref
1650 linkend="unpack-pragma"/>).</para>
1652 <para>This option is a bit of a sledgehammer: it might
1653 sometimes make things worse. Selectively unboxing fields
1654 by using <literal>UNPACK</literal> pragmas might be
1661 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
1662 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1663 <indexterm><primary>inlining, controlling</primary></indexterm>
1664 <indexterm><primary>unfolding, controlling</primary></indexterm>
1667 <para>(Default: 45) Governs the maximum size that GHC will
1668 allow a function unfolding to be. (An unfolding has a
1669 “size” that reflects the cost in terms of
1670 “code bloat” of expanding that unfolding at
1671 at a call site. A bigger function would be assigned a
1672 bigger cost.) </para>
1674 <para> Consequences: (a) nothing larger than this will be
1675 inlined (unless it has an INLINE pragma); (b) nothing
1676 larger than this will be spewed into an interface
1680 <para> Increasing this figure is more likely to result in longer
1681 compile times than faster code. The next option is more
1687 <term><option>-funfolding-use-threshold=<replaceable>n</replaceable></option></term>
1689 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1690 <indexterm><primary>inlining, controlling</primary></indexterm>
1691 <indexterm><primary>unfolding, controlling</primary></indexterm>
1693 <para>(Default: 8) This is the magic cut-off figure for
1694 unfolding: below this size, a function definition will be
1695 unfolded at the call-site, any bigger and it won't. The
1696 size computed for a function depends on two things: the
1697 actual size of the expression minus any discounts that
1698 apply (see <option>-funfolding-con-discount</option>).</para>
1709 <sect1 id="using-concurrent">
1710 <title>Using Concurrent Haskell</title>
1711 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
1713 <para>GHC supports Concurrent Haskell by default, without requiring a
1714 special option or libraries compiled in a certain way. To get access to
1715 the support libraries for Concurrent Haskell, just import
1717 url="../libraries/base/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
1719 <para>The following RTS option(s) affect the behaviour of Concurrent
1720 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
1724 <term><option>-C<replaceable>s</replaceable></option></term>
1726 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1727 Sets the context switch interval to <replaceable>s</replaceable>
1728 seconds. A context switch will occur at the next heap block
1729 allocation after the timer expires (a heap block allocation occurs
1730 every 4k of allocation). With <option>-C0</option> or
1731 <option>-C</option>, context switches will occur as often as
1732 possible (at every heap block allocation). By default, context
1733 switches occur every 20ms.</para>
1739 <sect1 id="using-smp">
1740 <title>Using SMP parallelism</title>
1741 <indexterm><primary>parallelism</primary>
1743 <indexterm><primary>SMP</primary>
1746 <para>GHC supports running Haskell programs in parallel on an SMP
1747 (symmetric multiprocessor).</para>
1749 <para>There's a fine distinction between
1750 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
1751 parallelism is all about making your program run
1752 <emphasis>faster</emphasis> by making use of multiple processors
1753 simultaneously. Concurrency, on the other hand, is a means of
1754 abstraction: it is a convenient way to structure a program that must
1755 respond to multiple asynchronous events.</para>
1757 <para>However, the two terms are certainly related. By making use of
1758 multiple CPUs it is possible to run concurrent threads in parallel,
1759 and this is exactly what GHC's SMP parallelism support does. But it
1760 is also possible to obtain performance improvements with parallelism
1761 on programs that do not use concurrency. This section describes how to
1762 use GHC to compile and run parallel programs, in <xref
1763 linkend="lang-parallel" /> we describe the language features that affect
1766 <sect2 id="parallel-options">
1767 <title>Options for SMP parallelism</title>
1769 <para>In order to make use of multiple CPUs, your program must be
1770 linked with the <option>-threaded</option> option (see <xref
1771 linkend="options-linker" />). Then, to run a program on multiple
1772 CPUs, use the RTS <option>-N</option> option:</para>
1776 <term><option>-N<replaceable>x</replaceable></option></term>
1778 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1779 Use <replaceable>x</replaceable> simultaneous threads when
1780 running the program. Normally <replaceable>x</replaceable>
1781 should be chosen to match the number of CPU cores on the
1782 machine<footnote><para>Whether hyperthreading cores should be counted or not is an
1783 open question; please feel free to experiment and let us know what
1784 results you find.</para></footnote>. For example,
1785 on a dual-core machine we would probably use
1786 <literal>+RTS -N2 -RTS</literal>.</para>
1788 <para>Setting <option>-N</option> also has the effect of
1789 setting <option>-g</option> (the number of OS threads to
1790 use for garbage collection) to the same value.</para>
1792 <para>There is no means (currently) by which this value
1793 may vary after the program has started.</para>
1798 <para>The following options affect the way the runtime schedules
1799 threads on CPUs:</para>
1803 <term><option>-qm</option></term>
1804 <indexterm><primary><option>-qm</option></primary><secondary>RTS
1805 option</secondary></indexterm>
1807 <para>Disable automatic migration for load balancing.
1808 Normally the runtime will automatically try to schedule
1809 threads across the available CPUs to make use of idle
1810 CPUs; this option disables that behaviour. It is probably
1811 only of use if you are explicitly scheduling threads onto
1812 CPUs with <literal>GHC.Conc.forkOnIO</literal>.</para>
1816 <term><option>-qw</option></term>
1817 <indexterm><primary><option>-qw</option></primary><secondary>RTS
1818 option</secondary></indexterm>
1820 <para>Migrate a thread to the current CPU when it is woken
1821 up. Normally when a thread is woken up after being
1822 blocked it will be scheduled on the CPU it was running on
1823 last; this option allows the thread to immediately migrate
1824 to the CPU that unblocked it.</para>
1826 <para>The rationale for allowing this eager migration is
1827 that it tends to move threads that are communicating with
1828 each other onto the same CPU; however there are
1829 pathalogical situations where it turns out to be a poor
1830 strategy. Depending on the communication pattern in your
1831 program, it may or may not be a good idea.</para>
1838 <title>Hints for using SMP parallelism</title>
1840 <para>Add the <literal>-s</literal> RTS option when
1841 running the program to see timing stats, which will help to tell you
1842 whether your program got faster by using more CPUs or not. If the user
1843 time is greater than
1844 the elapsed time, then the program used more than one CPU. You should
1845 also run the program without <literal>-N</literal> for comparison.</para>
1847 <para>GHC's parallelism support is new and experimental. It may make your
1848 program go faster, or it might slow it down - either way, we'd be
1849 interested to hear from you.</para>
1851 <para>One significant limitation with the current implementation is that
1852 the garbage collector is still single-threaded, and all execution must
1853 stop when GC takes place. This can be a significant bottleneck in a
1854 parallel program, especially if your program does a lot of GC. If this
1855 happens to you, then try reducing the cost of GC by tweaking the GC
1856 settings (<xref linkend="rts-options-gc" />): enlarging the heap or the
1857 allocation area size is a good start.</para>
1861 <sect1 id="options-platform">
1862 <title>Platform-specific Flags</title>
1864 <indexterm><primary>-m* options</primary></indexterm>
1865 <indexterm><primary>platform-specific options</primary></indexterm>
1866 <indexterm><primary>machine-specific options</primary></indexterm>
1868 <para>Some flags only make sense for particular target
1874 <term><option>-monly-[32]-regs</option>:</term>
1876 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1877 option (iX86 only)</primary></indexterm> GHC tries to
1878 “steal” four registers from GCC, for performance
1879 reasons; it almost always works. However, when GCC is
1880 compiling some modules with four stolen registers, it will
1881 crash, probably saying:
1884 Foo.hc:533: fixed or forbidden register was spilled.
1885 This may be due to a compiler bug or to impossible asm
1886 statements or clauses.
1889 Just give some registers back with
1890 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1891 If `2' doesn't work, please report the bug to us.</para>
1900 <sect1 id="ext-core">
1901 <title>Generating and compiling External Core Files</title>
1903 <indexterm><primary>intermediate code generation</primary></indexterm>
1905 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
1906 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
1907 <filename>.hcr</filename>. The Core format is described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz">
1908 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1910 for manipulating Core files (in Haskell) are in the GHC source distribution
1911 directory under <literal>utils/ext-core</literal>.
1912 Note that the format of <literal>.hcr</literal>
1913 files is <emphasis>different</emphasis> from the Core output format that GHC generates
1914 for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
1916 <para>The Core format natively supports notes which you can add to
1917 your source code using the <literal>CORE</literal> pragma (see <xref
1918 linkend="pragmas"/>).</para>
1924 <option>-fext-core</option>
1925 <indexterm><primary><option>-fext-core</option></primary></indexterm>
1928 <para>Generate <literal>.hcr</literal> files.</para>
1934 <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>
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