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-warnings-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-warnings-deprecations</option>:</term>
924 <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
926 <indexterm><primary>warnings</primary></indexterm>
927 <indexterm><primary>deprecations</primary></indexterm>
928 <para>Causes a warning to be emitted when a
929 module, function or type with a WARNING or DEPRECATED pragma
930 is used. See <xref linkend="warning-deprecated-pragma"/> for more
931 details on the pragmas.</para>
933 <para>This option is on by default.</para>
938 <term><option>-fwarn-deprecated-flags</option>:</term>
940 <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
942 <indexterm><primary>deprecated-flags</primary></indexterm>
943 <para>Causes a warning to be emitted when a deprecated
944 commandline flag is used.</para>
946 <para>This option is on by default.</para>
951 <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
953 <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
955 <para>Causes a warning to be emitted for foreign imports of
956 the following form:</para>
958 foreign import "f" f :: FunPtr t
960 <para>on the grounds that it probably should be</para>
962 foreign import "&f" f :: FunPtr t
964 <para>The first form declares that `f` is a (pure) C
965 function that takes no arguments and returns a pointer to a
966 C function with type `t`, whereas the second form declares
967 that `f` itself is a C function with type `t`. The first
968 declaration is usually a mistake, and one that is hard to
969 debug because it results in a crash, hence this
975 <term><option>-fwarn-dodgy-imports</option>:</term>
977 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
979 <para>Causes a warning to be emitted when a a datatype
980 <literal>T</literal> is imported
981 with all constructors, i.e. <literal>T(..)</literal>, but has been
982 exported abstractly, i.e. <literal>T</literal>.</para>
987 <term><option>-fwarn-duplicate-exports</option>:</term>
989 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
990 <indexterm><primary>duplicate exports, warning</primary></indexterm>
991 <indexterm><primary>export lists, duplicates</primary></indexterm>
993 <para>Have the compiler warn about duplicate entries in
994 export lists. This is useful information if you maintain
995 large export lists, and want to avoid the continued export
996 of a definition after you've deleted (one) mention of it in
997 the export list.</para>
999 <para>This option is on by default.</para>
1004 <term><option>-fwarn-hi-shadowing</option>:</term>
1006 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
1007 <indexterm><primary>shadowing</primary>
1008 <secondary>interface files</secondary></indexterm>
1010 <para>Causes the compiler to emit a warning when a module or
1011 interface file in the current directory is shadowing one
1012 with the same module name in a library or other
1018 <term><option>-fwarn-implicit-prelude</option>:</term>
1020 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
1021 <indexterm><primary>implicit prelude, warning</primary></indexterm>
1022 <para>Have the compiler warn if the Prelude is implicitly
1023 imported. This happens unless either the Prelude module is
1024 explicitly imported with an <literal>import ... Prelude ...</literal>
1025 line, or this implicit import is disabled (either by
1026 <option>-XNoImplicitPrelude</option> or a
1027 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
1029 <para>Note that no warning is given for syntax that implicitly
1030 refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
1031 would change whether it refers to the Prelude.
1032 For example, no warning is given when
1033 <literal>368</literal> means
1034 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
1035 (where <literal>Prelude</literal> refers to the actual Prelude module,
1036 regardless of the imports of the module being compiled).</para>
1038 <para>This warning is off by default.</para>
1043 <term><option>-fwarn-incomplete-patterns</option>:</term>
1045 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
1046 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
1047 <indexterm><primary>patterns, incomplete</primary></indexterm>
1049 <para>Similarly for incomplete patterns, the function
1050 <function>g</function> below will fail when applied to
1051 non-empty lists, so the compiler will emit a warning about
1052 this when <option>-fwarn-incomplete-patterns</option> is
1059 <para>This option isn't enabled by default because it can be
1060 a bit noisy, and it doesn't always indicate a bug in the
1061 program. However, it's generally considered good practice
1062 to cover all the cases in your functions.</para>
1067 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1069 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1070 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1071 <indexterm><primary>record updates, incomplete</primary></indexterm>
1074 <function>f</function> below will fail when applied to
1075 <literal>Bar</literal>, so the compiler will emit a warning about
1076 this when <option>-fwarn-incomplete-record-updates</option> is
1080 data Foo = Foo { x :: Int }
1084 f foo = foo { x = 6 }
1087 <para>This option isn't enabled by default because it can be
1088 very noisy, and it often doesn't indicate a bug in the
1095 <option>-fwarn-missing-fields</option>:
1096 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1097 <indexterm><primary>missing fields, warning</primary></indexterm>
1098 <indexterm><primary>fields, missing</primary></indexterm>
1102 <para>This option is on by default, and warns you whenever
1103 the construction of a labelled field constructor isn't
1104 complete, missing initializers for one or more fields. While
1105 not an error (the missing fields are initialised with
1106 bottoms), it is often an indication of a programmer error.</para>
1111 <term><option>-fwarn-missing-methods</option>:</term>
1113 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1114 <indexterm><primary>missing methods, warning</primary></indexterm>
1115 <indexterm><primary>methods, missing</primary></indexterm>
1117 <para>This option is on by default, and warns you whenever
1118 an instance declaration is missing one or more methods, and
1119 the corresponding class declaration has no default
1120 declaration for them.</para>
1121 <para>The warning is suppressed if the method name
1122 begins with an underscore. Here's an example where this is useful:
1125 _simpleFn :: a -> String
1126 complexFn :: a -> a -> String
1127 complexFn x y = ... _simpleFn ...
1129 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1130 never <literal>_simpleFn</literal>; and (b)
1131 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1137 <term><option>-fwarn-missing-signatures</option>:</term>
1139 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1140 <indexterm><primary>type signatures, missing</primary></indexterm>
1142 <para>If you would like GHC to check that every top-level
1143 function/value has a type signature, use the
1144 <option>-fwarn-missing-signatures</option> option. As part of
1145 the warning GHC also reports the inferred type. The
1146 option is off by default.</para>
1151 <term><option>-fwarn-name-shadowing</option>:</term>
1153 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1154 <indexterm><primary>shadowing, warning</primary></indexterm>
1156 <para>This option causes a warning to be emitted whenever an
1157 inner-scope value has the same name as an outer-scope value,
1158 i.e. the inner value shadows the outer one. This can catch
1159 typographical errors that turn into hard-to-find bugs, e.g.,
1160 in the inadvertent capture of what would be a recursive call in
1161 <literal>f = ... let f = id in ... f ...</literal>.</para>
1166 <term><option>-fwarn-orphans</option>:</term>
1168 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1169 <indexterm><primary>orphan instances, warning</primary></indexterm>
1170 <indexterm><primary>orphan rules, warning</primary></indexterm>
1172 <para>This option causes a warning to be emitted whenever the
1173 module contains an "orphan" instance declaration or rewrite rule.
1174 An instance declaration is an orphan if it appears in a module in
1175 which neither the class nor the type being instanced are declared
1176 in the same module. A rule is an orphan if it is a rule for a
1177 function declared in another module. A module containing any
1178 orphans is called an orphan module.</para>
1179 <para>The trouble with orphans is that GHC must pro-actively read the interface
1180 files for all orphan modules, just in case their instances or rules
1181 play a role, whether or not the module's interface would otherwise
1182 be of any use. See <xref linkend="orphan-modules"/> for details.
1189 <option>-fwarn-overlapping-patterns</option>:
1190 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1191 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1192 <indexterm><primary>patterns, overlapping</primary></indexterm>
1195 <para>By default, the compiler will warn you if a set of
1196 patterns are overlapping, e.g.,</para>
1199 f :: String -> Int
1205 <para>where the last pattern match in <function>f</function>
1206 won't ever be reached, as the second pattern overlaps
1207 it. More often than not, redundant patterns is a programmer
1208 mistake/error, so this option is enabled by default.</para>
1213 <term><option>-fwarn-simple-patterns</option>:</term>
1215 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1217 <para>Causes the compiler to warn about lambda-bound
1218 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1219 Normally, these aren't treated as incomplete patterns by
1220 <option>-fwarn-incomplete-patterns</option>.</para>
1221 <para>“Lambda-bound patterns” includes all places where there is a single pattern,
1222 including list comprehensions and do-notation. In these cases, a pattern-match
1223 failure is quite legitimate, and triggers filtering (list comprehensions) or
1224 the monad <literal>fail</literal> operation (monads). For example:
1226 f :: [Maybe a] -> [a]
1227 f xs = [y | Just y <- xs]
1229 Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
1230 these probably-innocent cases, which is why the flag is off by default. </para>
1235 <term><option>-fwarn-tabs</option>:</term>
1237 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1238 <indexterm><primary>tabs, warning</primary></indexterm>
1239 <para>Have the compiler warn if there are tabs in your source
1242 <para>This warning is off by default.</para>
1247 <term><option>-fwarn-type-defaults</option>:</term>
1249 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1250 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1251 <para>Have the compiler warn/inform you where in your source
1252 the Haskell defaulting mechanism for numeric types kicks
1253 in. This is useful information when converting code from a
1254 context that assumed one default into one with another,
1255 e.g., the ‘default default’ for Haskell 1.4 caused the
1256 otherwise unconstrained value <constant>1</constant> to be
1257 given the type <literal>Int</literal>, whereas Haskell 98
1258 defaults it to <literal>Integer</literal>. This may lead to
1259 differences in performance and behaviour, hence the
1260 usefulness of being non-silent about this.</para>
1262 <para>This warning is off by default.</para>
1267 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1269 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1270 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1271 <para>Have the compiler warn/inform you where in your source
1272 the Haskell Monomorphism Restriction is applied. If applied silently
1273 the MR can give rise to unexpected behaviour, so it can be helpful
1274 to have an explicit warning that it is being applied.</para>
1276 <para>This warning is off by default.</para>
1281 <term><option>-fwarn-unused-binds</option>:</term>
1283 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1284 <indexterm><primary>unused binds, warning</primary></indexterm>
1285 <indexterm><primary>binds, unused</primary></indexterm>
1286 <para>Report any function definitions (and local bindings)
1287 which are unused. For top-level functions, the warning is
1288 only given if the binding is not exported.</para>
1289 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1290 mentioned in the right hand side of another definition that is used, or (c) the
1291 function it defines begins with an underscore. The last case provides a
1292 way to suppress unused-binding warnings selectively. </para>
1293 <para> Notice that a variable
1294 is reported as unused even if it appears in the right-hand side of another
1295 unused binding. </para>
1300 <term><option>-fwarn-unused-imports</option>:</term>
1302 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1303 <indexterm><primary>unused imports, warning</primary></indexterm>
1304 <indexterm><primary>imports, unused</primary></indexterm>
1306 <para>Report any modules that are explicitly imported but
1307 never used. However, the form <literal>import M()</literal> is
1308 never reported as an unused import, because it is a useful idiom
1309 for importing instance declarations, which are anonymous in Haskell.</para>
1314 <term><option>-fwarn-unused-matches</option>:</term>
1316 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1317 <indexterm><primary>unused matches, warning</primary></indexterm>
1318 <indexterm><primary>matches, unused</primary></indexterm>
1320 <para>Report all unused variables which arise from pattern
1321 matches, including patterns consisting of a single variable.
1322 For instance <literal>f x y = []</literal> would report
1323 <varname>x</varname> and <varname>y</varname> as unused. The
1324 warning is suppressed if the variable name begins with an underscore, thus:
1334 <para>If you're feeling really paranoid, the
1335 <option>-dcore-lint</option>
1336 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1337 is a good choice. It turns on heavyweight intra-pass
1338 sanity-checking within GHC. (It checks GHC's sanity, not
1345 <sect1 id="options-optimise">
1346 <title>Optimisation (code improvement)</title>
1348 <indexterm><primary>optimisation</primary></indexterm>
1349 <indexterm><primary>improvement, code</primary></indexterm>
1351 <para>The <option>-O*</option> options specify convenient
1352 “packages” of optimisation flags; the
1353 <option>-f*</option> options described later on specify
1354 <emphasis>individual</emphasis> optimisations to be turned on/off;
1355 the <option>-m*</option> options specify
1356 <emphasis>machine-specific</emphasis> optimisations to be turned
1359 <sect2 id="optimise-pkgs">
1360 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1362 <para>There are <emphasis>many</emphasis> options that affect
1363 the quality of code produced by GHC. Most people only have a
1364 general goal, something like “Compile quickly” or
1365 “Make my program run like greased lightning.” The
1366 following “packages” of optimisations (or lack
1367 thereof) should suffice.</para>
1369 <para>Note that higher optimisation levels cause more
1370 cross-module optimisation to be performed, which can have an
1371 impact on how much of your program needs to be recompiled when
1372 you change something. This is one reason to stick to
1373 no-optimisation when developing code.</para>
1379 No <option>-O*</option>-type option specified:
1380 <indexterm><primary>-O* not specified</primary></indexterm>
1383 <para>This is taken to mean: “Please compile
1384 quickly; I'm not over-bothered about compiled-code
1385 quality.” So, for example: <command>ghc -c
1386 Foo.hs</command></para>
1392 <option>-O0</option>:
1393 <indexterm><primary><option>-O0</option></primary></indexterm>
1396 <para>Means “turn off all optimisation”,
1397 reverting to the same settings as if no
1398 <option>-O</option> options had been specified. Saying
1399 <option>-O0</option> can be useful if
1400 eg. <command>make</command> has inserted a
1401 <option>-O</option> on the command line already.</para>
1407 <option>-O</option> or <option>-O1</option>:
1408 <indexterm><primary>-O option</primary></indexterm>
1409 <indexterm><primary>-O1 option</primary></indexterm>
1410 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1413 <para>Means: “Generate good-quality code without
1414 taking too long about it.” Thus, for example:
1415 <command>ghc -c -O Main.lhs</command></para>
1421 <option>-O2</option>:
1422 <indexterm><primary>-O2 option</primary></indexterm>
1423 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1426 <para>Means: “Apply every non-dangerous
1427 optimisation, even if it means significantly longer
1428 compile times.”</para>
1430 <para>The avoided “dangerous” optimisations
1431 are those that can make runtime or space
1432 <emphasis>worse</emphasis> if you're unlucky. They are
1433 normally turned on or off individually.</para>
1435 <para>At the moment, <option>-O2</option> is
1436 <emphasis>unlikely</emphasis> to produce better code than
1437 <option>-O</option>.</para>
1443 <option>-Ofile <file></option>:
1444 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1445 <indexterm><primary>optimising, customised</primary></indexterm>
1448 <para>(NOTE: not supported since GHC 4.x. Please ask if
1449 you're interested in this.)</para>
1451 <para>For those who need <emphasis>absolute</emphasis>
1452 control over <emphasis>exactly</emphasis> what options are
1453 used (e.g., compiler writers, sometimes :-), a list of
1454 options can be put in a file and then slurped in with
1455 <option>-Ofile</option>.</para>
1457 <para>In that file, comments are of the
1458 <literal>#</literal>-to-end-of-line variety; blank
1459 lines and most whitespace is ignored.</para>
1461 <para>Please ask if you are baffled and would like an
1462 example of <option>-Ofile</option>!</para>
1467 <para>We don't use a <option>-O*</option> flag for day-to-day
1468 work. We use <option>-O</option> to get respectable speed;
1469 e.g., when we want to measure something. When we want to go for
1470 broke, we tend to use <option>-O2 -fvia-C</option> (and we go for
1471 lots of coffee breaks).</para>
1473 <para>The easiest way to see what <option>-O</option> (etc.)
1474 “really mean” is to run with <option>-v</option>,
1475 then stand back in amazement.</para>
1478 <sect2 id="options-f">
1479 <title><option>-f*</option>: platform-independent flags</title>
1481 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1482 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1484 <para>These flags turn on and off individual optimisations.
1485 They are normally set via the <option>-O</option> options
1486 described above, and as such, you shouldn't need to set any of
1487 them explicitly (indeed, doing so could lead to unexpected
1488 results). However, there are one or two that may be of
1493 <term><option>-fexcess-precision</option>:</term>
1495 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1496 <para>When this option is given, intermediate floating
1497 point values can have a <emphasis>greater</emphasis>
1498 precision/range than the final type. Generally this is a
1499 good thing, but some programs may rely on the exact
1501 <literal>Float</literal>/<literal>Double</literal> values
1502 and should not use this option for their compilation.</para>
1507 <term><option>-fignore-asserts</option>:</term>
1509 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1510 <para>Causes GHC to ignore uses of the function
1511 <literal>Exception.assert</literal> in source code (in
1512 other words, rewriting <literal>Exception.assert p
1513 e</literal> to <literal>e</literal> (see <xref
1514 linkend="assertions"/>). This flag is turned on by
1515 <option>-O</option>.
1522 <option>-fno-cse</option>
1523 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1526 <para>Turns off the common-sub-expression elimination optimisation.
1527 Can be useful if you have some <literal>unsafePerformIO</literal>
1528 expressions that you don't want commoned-up.</para>
1534 <option>-fno-strictness</option>
1535 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1538 <para>Turns off the strictness analyser; sometimes it eats
1539 too many cycles.</para>
1545 <option>-fno-full-laziness</option>
1546 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1549 <para>Turns off the full laziness optimisation (also known as
1550 let-floating). Full laziness increases sharing, which can lead
1551 to increased memory residency.</para>
1553 <para>NOTE: GHC doesn't implement complete full-laziness.
1554 When optimisation in on, and
1555 <option>-fno-full-laziness</option> is not given, some
1556 transformations that increase sharing are performed, such
1557 as extracting repeated computations from a loop. These
1558 are the same transformations that a fully lazy
1559 implementation would do, the difference is that GHC
1560 doesn't consistently apply full-laziness, so don't rely on
1567 <option>-fspec-constr</option>
1568 <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
1571 <para>Turn on call-pattern specialisation.</para>
1577 <option>-fliberate-case</option>
1578 <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
1581 <para>Turn on the liberate-case transformation.</para>
1587 <option>-fstatic-argument-transformation</option>
1588 <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
1591 <para>Turn on the static argument transformation.</para>
1597 <option>-fno-state-hack</option>
1598 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1601 <para>Turn off the "state hack" whereby any lambda with a
1602 <literal>State#</literal> token as argument is considered to be
1603 single-entry, hence it is considered OK to inline things inside
1604 it. This can improve performance of IO and ST monad code, but it
1605 runs the risk of reducing sharing.</para>
1611 <option>-fomit-interface-pragmas</option>
1612 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
1615 <para>Tells GHC to omit all inessential information from the interface file
1616 generated for the module being compiled (say M). This means that a module
1617 importing M will see only the <emphasis>types</emphasis> of the functions that M exports, but not
1618 their unfoldings, strictness info, etc. Hence, for example,
1619 no function exported by M will be inlined
1620 into an importing module. The benefit is that modules that import M will
1621 need to be recompiled less often (only when M's exports change their type,
1622 not when they change their implementation).
1629 <option>-fignore-interface-pragmas</option>
1630 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
1633 <para>Tells GHC to ignore all inessential information when reading interface files.
1634 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
1635 for a function, GHC will ignore that information.</para>
1641 <option>-funbox-strict-fields</option>:
1642 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1643 <indexterm><primary>strict constructor fields</primary></indexterm>
1644 <indexterm><primary>constructor fields, strict</primary></indexterm>
1647 <para>This option causes all constructor fields which are
1648 marked strict (i.e. “!”) to be unboxed or
1649 unpacked if possible. It is equivalent to adding an
1650 <literal>UNPACK</literal> pragma to every strict
1651 constructor field (see <xref
1652 linkend="unpack-pragma"/>).</para>
1654 <para>This option is a bit of a sledgehammer: it might
1655 sometimes make things worse. Selectively unboxing fields
1656 by using <literal>UNPACK</literal> pragmas might be
1663 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
1664 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1665 <indexterm><primary>inlining, controlling</primary></indexterm>
1666 <indexterm><primary>unfolding, controlling</primary></indexterm>
1669 <para>(Default: 45) Governs the maximum size that GHC will
1670 allow a function unfolding to be. (An unfolding has a
1671 “size” that reflects the cost in terms of
1672 “code bloat” of expanding that unfolding at
1673 at a call site. A bigger function would be assigned a
1674 bigger cost.) </para>
1676 <para> Consequences: (a) nothing larger than this will be
1677 inlined (unless it has an INLINE pragma); (b) nothing
1678 larger than this will be spewed into an interface
1682 <para> Increasing this figure is more likely to result in longer
1683 compile times than faster code. The next option is more
1689 <term><option>-funfolding-use-threshold=<replaceable>n</replaceable></option></term>
1691 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1692 <indexterm><primary>inlining, controlling</primary></indexterm>
1693 <indexterm><primary>unfolding, controlling</primary></indexterm>
1695 <para>(Default: 8) This is the magic cut-off figure for
1696 unfolding: below this size, a function definition will be
1697 unfolded at the call-site, any bigger and it won't. The
1698 size computed for a function depends on two things: the
1699 actual size of the expression minus any discounts that
1700 apply (see <option>-funfolding-con-discount</option>).</para>
1711 <sect1 id="using-concurrent">
1712 <title>Using Concurrent Haskell</title>
1713 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
1715 <para>GHC supports Concurrent Haskell by default, without requiring a
1716 special option or libraries compiled in a certain way. To get access to
1717 the support libraries for Concurrent Haskell, just import
1719 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>
1721 <para>The following RTS option(s) affect the behaviour of Concurrent
1722 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
1726 <term><option>-C<replaceable>s</replaceable></option></term>
1728 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1729 Sets the context switch interval to <replaceable>s</replaceable>
1730 seconds. A context switch will occur at the next heap block
1731 allocation after the timer expires (a heap block allocation occurs
1732 every 4k of allocation). With <option>-C0</option> or
1733 <option>-C</option>, context switches will occur as often as
1734 possible (at every heap block allocation). By default, context
1735 switches occur every 20ms.</para>
1741 <sect1 id="using-smp">
1742 <title>Using SMP parallelism</title>
1743 <indexterm><primary>parallelism</primary>
1745 <indexterm><primary>SMP</primary>
1748 <para>GHC supports running Haskell programs in parallel on an SMP
1749 (symmetric multiprocessor).</para>
1751 <para>There's a fine distinction between
1752 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
1753 parallelism is all about making your program run
1754 <emphasis>faster</emphasis> by making use of multiple processors
1755 simultaneously. Concurrency, on the other hand, is a means of
1756 abstraction: it is a convenient way to structure a program that must
1757 respond to multiple asynchronous events.</para>
1759 <para>However, the two terms are certainly related. By making use of
1760 multiple CPUs it is possible to run concurrent threads in parallel,
1761 and this is exactly what GHC's SMP parallelism support does. But it
1762 is also possible to obtain performance improvements with parallelism
1763 on programs that do not use concurrency. This section describes how to
1764 use GHC to compile and run parallel programs, in <xref
1765 linkend="lang-parallel" /> we describe the language features that affect
1768 <sect2 id="parallel-options">
1769 <title>Options for SMP parallelism</title>
1771 <para>In order to make use of multiple CPUs, your program must be
1772 linked with the <option>-threaded</option> option (see <xref
1773 linkend="options-linker" />). Then, to run a program on multiple
1774 CPUs, use the RTS <option>-N</option> option:</para>
1778 <term><option>-N<replaceable>x</replaceable></option></term>
1780 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1781 Use <replaceable>x</replaceable> simultaneous threads when
1782 running the program. Normally <replaceable>x</replaceable>
1783 should be chosen to match the number of CPU cores on the
1784 machine<footnote><para>Whether hyperthreading cores should be counted or not is an
1785 open question; please feel free to experiment and let us know what
1786 results you find.</para></footnote>. For example,
1787 on a dual-core machine we would probably use
1788 <literal>+RTS -N2 -RTS</literal>.</para>
1790 <para>Setting <option>-N</option> also has the effect of
1791 setting <option>-g</option> (the number of OS threads to
1792 use for garbage collection) to the same value.</para>
1794 <para>There is no means (currently) by which this value
1795 may vary after the program has started.</para>
1800 <para>The following options affect the way the runtime schedules
1801 threads on CPUs:</para>
1805 <term><option>-qm</option></term>
1806 <indexterm><primary><option>-qm</option></primary><secondary>RTS
1807 option</secondary></indexterm>
1809 <para>Disable automatic migration for load balancing.
1810 Normally the runtime will automatically try to schedule
1811 threads across the available CPUs to make use of idle
1812 CPUs; this option disables that behaviour. It is probably
1813 only of use if you are explicitly scheduling threads onto
1814 CPUs with <literal>GHC.Conc.forkOnIO</literal>.</para>
1818 <term><option>-qw</option></term>
1819 <indexterm><primary><option>-qw</option></primary><secondary>RTS
1820 option</secondary></indexterm>
1822 <para>Migrate a thread to the current CPU when it is woken
1823 up. Normally when a thread is woken up after being
1824 blocked it will be scheduled on the CPU it was running on
1825 last; this option allows the thread to immediately migrate
1826 to the CPU that unblocked it.</para>
1828 <para>The rationale for allowing this eager migration is
1829 that it tends to move threads that are communicating with
1830 each other onto the same CPU; however there are
1831 pathalogical situations where it turns out to be a poor
1832 strategy. Depending on the communication pattern in your
1833 program, it may or may not be a good idea.</para>
1840 <title>Hints for using SMP parallelism</title>
1842 <para>Add the <literal>-s</literal> RTS option when
1843 running the program to see timing stats, which will help to tell you
1844 whether your program got faster by using more CPUs or not. If the user
1845 time is greater than
1846 the elapsed time, then the program used more than one CPU. You should
1847 also run the program without <literal>-N</literal> for comparison.</para>
1849 <para>GHC's parallelism support is new and experimental. It may make your
1850 program go faster, or it might slow it down - either way, we'd be
1851 interested to hear from you.</para>
1853 <para>One significant limitation with the current implementation is that
1854 the garbage collector is still single-threaded, and all execution must
1855 stop when GC takes place. This can be a significant bottleneck in a
1856 parallel program, especially if your program does a lot of GC. If this
1857 happens to you, then try reducing the cost of GC by tweaking the GC
1858 settings (<xref linkend="rts-options-gc" />): enlarging the heap or the
1859 allocation area size is a good start.</para>
1863 <sect1 id="options-platform">
1864 <title>Platform-specific Flags</title>
1866 <indexterm><primary>-m* options</primary></indexterm>
1867 <indexterm><primary>platform-specific options</primary></indexterm>
1868 <indexterm><primary>machine-specific options</primary></indexterm>
1870 <para>Some flags only make sense for particular target
1876 <term><option>-monly-[32]-regs</option>:</term>
1878 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1879 option (iX86 only)</primary></indexterm> GHC tries to
1880 “steal” four registers from GCC, for performance
1881 reasons; it almost always works. However, when GCC is
1882 compiling some modules with four stolen registers, it will
1883 crash, probably saying:
1886 Foo.hc:533: fixed or forbidden register was spilled.
1887 This may be due to a compiler bug or to impossible asm
1888 statements or clauses.
1891 Just give some registers back with
1892 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1893 If `2' doesn't work, please report the bug to us.</para>
1902 <sect1 id="ext-core">
1903 <title>Generating and compiling External Core Files</title>
1905 <indexterm><primary>intermediate code generation</primary></indexterm>
1907 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
1908 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
1909 <filename>.hcr</filename>. The Core format is described in <ulink url="../ext-core/core.ps.gz">
1910 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1912 for manipulating Core files (in Haskell) are in the GHC source distribution
1913 directory under <literal>utils/ext-core</literal>.
1914 Note that the format of <literal>.hcr</literal>
1915 files is <emphasis>different</emphasis> from the Core output format that GHC generates
1916 for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
1918 <para>The Core format natively supports notes which you can add to
1919 your source code using the <literal>CORE</literal> pragma (see <xref
1920 linkend="pragmas"/>).</para>
1926 <option>-fext-core</option>
1927 <indexterm><primary><option>-fext-core</option></primary></indexterm>
1930 <para>Generate <literal>.hcr</literal> files.</para>
1936 <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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