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 deliberately
50 uses name shadowing, it should be compiled with the
51 <option>-fno-warn-name-shadowing</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 -fno-warn-name-shadowing #-}
63 <para><literal>OPTIONS_GHC</literal> is a <emphasis>file-header pragma</emphasis>
64 (see <xref linkend="pragmas"/>).</para>
66 <para>Only <emphasis>dynamic</emphasis> flags can be used in an <literal>OPTIONS_GHC</literal> pragma
67 (see <xref linkend="static-dynamic-flags"/>).</para>
69 <para>Note that your command shell does not
70 get to the source file options, they are just included literally
71 in the array of command-line arguments the compiler
72 maintains internally, so you'll be desperately disappointed if
73 you try to glob etc. inside <literal>OPTIONS_GHC</literal>.</para>
75 <para>NOTE: the contents of OPTIONS_GHC are appended to the
76 command-line options, so options given in the source file
77 override those given on the command-line.</para>
79 <para>It is not recommended to move all the contents of your
80 Makefiles into your source files, but in some circumstances, the
81 <literal>OPTIONS_GHC</literal> pragma is the Right Thing. (If you
82 use <option>-keep-hc-file</option> and have OPTION flags in
83 your module, the OPTIONS_GHC will get put into the generated .hc
88 <title>Setting options in GHCi</title>
90 <para>Options may also be modified from within GHCi, using the
91 <literal>:set</literal> command. See <xref linkend="ghci-set"/>
92 for more details.</para>
96 <sect1 id="static-dynamic-flags">
97 <title>Static, Dynamic, and Mode options</title>
98 <indexterm><primary>static</primary><secondary>options</secondary>
100 <indexterm><primary>dynamic</primary><secondary>options</secondary>
102 <indexterm><primary>mode</primary><secondary>options</secondary>
105 <para>Each of GHC's command line options is classified as
106 <firstterm>static</firstterm>, <firstterm>dynamic</firstterm> or
107 <firstterm>mode</firstterm>:</para>
111 <term>Mode flags</term>
113 <para>For example, <option>--make</option> or <option>-E</option>.
114 There may only be a single mode flag on the command line. The
115 available modes are listed in <xref linkend="modes"/>.</para>
119 <term>Dynamic Flags</term>
121 <para>Most non-mode flags fall into this category. A dynamic flag
122 may be used on the command line, in a
123 <literal>OPTIONS_GHC</literal> pragma in a source file, or set
124 using <literal>:set</literal> in GHCi.</para>
128 <term>Static Flags</term>
130 <para>A few flags are "static", which means they can only be used on
131 the command-line, and remain in force over the entire GHC/GHCi
137 <para>The flag reference tables (<xref
138 linkend="flag-reference"/>) lists the status of each flag.</para>
140 <para>There are a few flags that are static except that they can
141 also be used with GHCi's <literal>:set</literal> command; these
142 are listed as “static/<literal>:set</literal>” in the
146 <sect1 id="file-suffixes">
147 <title>Meaningful file suffixes</title>
149 <indexterm><primary>suffixes, file</primary></indexterm>
150 <indexterm><primary>file suffixes for GHC</primary></indexterm>
152 <para>File names with “meaningful” suffixes (e.g.,
153 <filename>.lhs</filename> or <filename>.o</filename>) cause the
154 “right thing” to happen to those files.</para>
159 <term><filename>.hs</filename></term>
161 <para>A Haskell module.</para>
167 <filename>.lhs</filename>
168 <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
171 <para>A “literate Haskell” module.</para>
176 <term><filename>.hi</filename></term>
178 <para>A Haskell interface file, probably
179 compiler-generated.</para>
184 <term><filename>.hc</filename></term>
186 <para>Intermediate C file produced by the Haskell
192 <term><filename>.c</filename></term>
194 <para>A C file not produced by the Haskell
200 <term><filename>.s</filename></term>
202 <para>An assembly-language source file, usually produced by
208 <term><filename>.o</filename></term>
210 <para>An object file, produced by an assembler.</para>
215 <para>Files with other suffixes (or without suffixes) are passed
216 straight to the linker.</para>
221 <title>Modes of operation</title>
223 <para>GHC's behaviour is firstly controlled by a mode flag. Only
224 one of these flags may be given, but it does not necessarily need
225 to be the first option on the command-line. The available modes
231 <cmdsynopsis><command>ghc --interactive</command>
233 <indexterm><primary>interactive mode</primary></indexterm>
234 <indexterm><primary>ghci</primary></indexterm>
237 <para>Interactive mode, which is also available as
238 <command>ghci</command>. Interactive mode is described in
239 more detail in <xref linkend="ghci"/>.</para>
245 <cmdsynopsis><command>ghc --make</command>
247 <indexterm><primary>make mode</primary></indexterm>
248 <indexterm><primary><option>––make</option></primary></indexterm>
251 <para>In this mode, GHC will build a multi-module Haskell
252 program automatically, figuring out dependencies for itself.
253 If you have a straightforward Haskell program, this is
254 likely to be much easier, and faster, than using
255 <command>make</command>. Make mode is described in <xref
256 linkend="make-mode"/>.</para>
262 <cmdsynopsis><command>ghc -e</command>
263 <arg choice='plain'><replaceable>expr</replaceable></arg>
265 <indexterm><primary>eval mode</primary></indexterm>
268 <para>Expression-evaluation mode. This is very similar to
269 interactive mode, except that there is a single expression
270 to evaluate (<replaceable>expr</replaceable>) which is given
271 on the command line. See <xref linkend="eval-mode"/> for
279 <command>ghc -E</command>
280 <command>ghc -c</command>
281 <command>ghc -S</command>
282 <command>ghc -c</command>
284 <indexterm><primary><option>-E</option></primary></indexterm>
285 <indexterm><primary><option>-C</option></primary></indexterm>
286 <indexterm><primary><option>-S</option></primary></indexterm>
287 <indexterm><primary><option>-c</option></primary></indexterm>
290 <para>This is the traditional batch-compiler mode, in which
291 GHC can compile source files one at a time, or link objects
292 together into an executable. This mode also applies if
293 there is no other mode flag specified on the command line,
294 in which case it means that the specified files should be
295 compiled and then linked to form a program. See <xref
296 linkend="options-order"/>.</para>
303 <command>ghc -M</command>
305 <indexterm><primary>dependency-generation mode</primary></indexterm>
308 <para>Dependency-generation mode. In this mode, GHC can be
309 used to generate dependency information suitable for use in
310 a <literal>Makefile</literal>. See <xref
311 linkend="makefile-dependencies"/>.</para>
318 <command>ghc --mk-dll</command>
320 <indexterm><primary>DLL-creation mode</primary></indexterm>
323 <para>DLL-creation mode (Windows only). See <xref
324 linkend="win32-dlls-create"/>.</para>
331 <command>ghc --help</command> <command>ghc -?</command>
333 <indexterm><primary><option>––help</option></primary></indexterm>
336 <para>Cause GHC to spew a long usage message to standard
337 output and then exit.</para>
344 <command>ghc --show-iface <replaceable>file</replaceable></command>
346 <indexterm><primary><option>––--show-iface</option></primary></indexterm>
349 <para>Read the interface in
350 <replaceable>file</replaceable> and dump it as text to
351 <literal>stdout</literal>. For example <literal>ghc --show-iface M.hi</literal>.</para>
358 <command>ghc --supported-languages</command>
360 <indexterm><primary><option>––supported-languages</option></primary></indexterm>
363 <para>Print the supported language extensions.</para>
370 <command>ghc --info</command>
372 <indexterm><primary><option>––info</option></primary></indexterm>
375 <para>Print information about the compiler.</para>
382 <command>ghc --version</command>
383 <command>ghc -V</command>
385 <indexterm><primary><option>-V</option></primary></indexterm>
386 <indexterm><primary><option>––version</option></primary></indexterm>
389 <para>Print a one-line string including GHC's version number.</para>
396 <command>ghc --numeric-version</command>
398 <indexterm><primary><option>––numeric-version</option></primary></indexterm>
401 <para>Print GHC's numeric version number only.</para>
408 <command>ghc --print-libdir</command>
410 <indexterm><primary><option>––print-libdir</option></primary></indexterm>
413 <para>Print the path to GHC's library directory. This is
414 the top of the directory tree containing GHC's libraries,
415 interfaces, and include files (usually something like
416 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
418 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
419 in the package configuration file
420 (see <xref linkend="packages"/>).</para>
426 <sect2 id="make-mode">
427 <title>Using <command>ghc</command> <option>––make</option></title>
428 <indexterm><primary><option>––make</option></primary></indexterm>
429 <indexterm><primary>separate compilation</primary></indexterm>
431 <para>When given the <option>––make</option> option,
432 GHC will build a multi-module Haskell program by following
433 dependencies from one or more root modules (usually just
434 <literal>Main</literal>). For example, if your
435 <literal>Main</literal> module is in a file called
436 <filename>Main.hs</filename>, you could compile and link the
437 program like this:</para>
440 ghc ––make Main.hs
443 <para>The command line may contain any number of source file
444 names or module names; GHC will figure out all the modules in
445 the program by following the imports from these initial modules.
446 It will then attempt to compile each module which is out of
447 date, and finally, if there is a <literal>Main</literal> module,
448 the program will also be linked into an executable.</para>
450 <para>The main advantages to using <literal>ghc
451 ––make</literal> over traditional
452 <literal>Makefile</literal>s are:</para>
456 <para>GHC doesn't have to be restarted for each compilation,
457 which means it can cache information between compilations.
458 Compiling a multi-module program with <literal>ghc
459 ––make</literal> can be up to twice as fast as
460 running <literal>ghc</literal> individually on each source
464 <para>You don't have to write a <literal>Makefile</literal>.</para>
465 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
468 <para>GHC re-calculates the dependencies each time it is
469 invoked, so the dependencies never get out of sync with the
474 <para>Any of the command-line options described in the rest of
475 this chapter can be used with
476 <option>––make</option>, but note that any options
477 you give on the command line will apply to all the source files
478 compiled, so if you want any options to apply to a single source
479 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
480 pragma (see <xref linkend="source-file-options"/>).</para>
482 <para>If the program needs to be linked with additional objects
483 (say, some auxiliary C code), then the object files can be
484 given on the command line and GHC will include them when linking
485 the executable.</para>
487 <para>Note that GHC can only follow dependencies if it has the
488 source file available, so if your program includes a module for
489 which there is no source file, even if you have an object and an
490 interface file for the module, then GHC will complain. The
491 exception to this rule is for package modules, which may or may
492 not have source files.</para>
494 <para>The source files for the program don't all need to be in
495 the same directory; the <option>-i</option> option can be used
496 to add directories to the search path (see <xref
497 linkend="search-path"/>).</para>
500 <sect2 id="eval-mode">
501 <title>Expression evaluation mode</title>
503 <para>This mode is very similar to interactive mode, except that
504 there is a single expression to evaluate which is specified on
505 the command line as an argument to the <option>-e</option>
509 ghc -e <replaceable>expr</replaceable>
512 <para>Haskell source files may be named on the command line, and
513 they will be loaded exactly as in interactive mode. The
514 expression is evaluated in the context of the loaded
517 <para>For example, to load and run a Haskell program containing
518 a module <literal>Main</literal>, we might say</para>
521 ghc -e Main.main Main.hs
524 <para>or we can just use this mode to evaluate expressions in
525 the context of the <literal>Prelude</literal>:</para>
528 $ ghc -e "interact (unlines.map reverse.lines)"
534 <sect2 id="options-order">
535 <title>Batch compiler mode</title>
537 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
538 given on the command line.</para>
540 <para>The first phase to run is determined by each input-file
541 suffix, and the last phase is determined by a flag. If no
542 relevant flag is present, then go all the way through to linking.
543 This table summarises:</para>
547 <colspec align="left"/>
548 <colspec align="left"/>
549 <colspec align="left"/>
550 <colspec align="left"/>
554 <entry>Phase of the compilation system</entry>
555 <entry>Suffix saying “start here”</entry>
556 <entry>Flag saying “stop after”</entry>
557 <entry>(suffix of) output file</entry>
562 <entry>literate pre-processor</entry>
563 <entry><literal>.lhs</literal></entry>
565 <entry><literal>.hs</literal></entry>
569 <entry>C pre-processor (opt.) </entry>
570 <entry><literal>.hs</literal> (with
571 <option>-cpp</option>)</entry>
572 <entry><option>-E</option></entry>
573 <entry><literal>.hspp</literal></entry>
577 <entry>Haskell compiler</entry>
578 <entry><literal>.hs</literal></entry>
579 <entry><option>-C</option>, <option>-S</option></entry>
580 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
584 <entry>C compiler (opt.)</entry>
585 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
586 <entry><option>-S</option></entry>
587 <entry><literal>.s</literal></entry>
591 <entry>assembler</entry>
592 <entry><literal>.s</literal></entry>
593 <entry><option>-c</option></entry>
594 <entry><literal>.o</literal></entry>
598 <entry>linker</entry>
599 <entry><replaceable>other</replaceable></entry>
601 <entry><filename>a.out</filename></entry>
607 <indexterm><primary><option>-C</option></primary></indexterm>
608 <indexterm><primary><option>-E</option></primary></indexterm>
609 <indexterm><primary><option>-S</option></primary></indexterm>
610 <indexterm><primary><option>-c</option></primary></indexterm>
612 <para>Thus, a common invocation would be: </para>
615 ghc -c Foo.hs</screen>
617 <para>to compile the Haskell source file
618 <filename>Foo.hs</filename> to an object file
619 <filename>Foo.o</filename>.</para>
621 <para>Note: What the Haskell compiler proper produces depends on
622 whether a native-code generator<indexterm><primary>native-code
623 generator</primary></indexterm> is used (producing assembly
624 language) or not (producing C). See <xref
625 linkend="options-codegen"/> for more details.</para>
627 <para>Note: C pre-processing is optional, the
628 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
629 flag turns it on. See <xref linkend="c-pre-processor"/> for more
632 <para>Note: The option <option>-E</option><indexterm><primary>-E
633 option</primary></indexterm> runs just the pre-processing passes
634 of the compiler, dumping the result in a file.</para>
636 <sect3 id="overriding-suffixes">
637 <title>Overriding the default behaviour for a file</title>
639 <para>As described above, the way in which a file is processed by GHC
640 depends on its suffix. This behaviour can be overridden using the
641 <option>-x</option> option:</para>
645 <term><option>-x</option> <replaceable>suffix</replaceable>
646 <indexterm><primary><option>-x</option></primary>
649 <para>Causes all files following this option on the command
650 line to be processed as if they had the suffix
651 <replaceable>suffix</replaceable>. For example, to compile a
652 Haskell module in the file <literal>M.my-hs</literal>,
653 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
662 <sect1 id="options-help">
663 <title>Help and verbosity options</title>
665 <indexterm><primary>help options</primary></indexterm>
666 <indexterm><primary>verbosity options</primary></indexterm>
668 <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
669 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
674 <indexterm><primary><option>-n</option></primary></indexterm>
677 <para>Does a dry-run, i.e. GHC goes through all the motions
678 of compiling as normal, but does not actually run any
679 external commands.</para>
686 <indexterm><primary><option>-v</option></primary></indexterm>
689 <para>The <option>-v</option> option makes GHC
690 <emphasis>verbose</emphasis>: it reports its version number
691 and shows (on stderr) exactly how it invokes each phase of
692 the compilation system. Moreover, it passes the
693 <option>-v</option> flag to most phases; each reports its
694 version number (and possibly some other information).</para>
696 <para>Please, oh please, use the <option>-v</option> option
697 when reporting bugs! Knowing that you ran the right bits in
698 the right order is always the first thing we want to
705 <option>-v</option><replaceable>n</replaceable>
706 <indexterm><primary><option>-v</option></primary></indexterm>
709 <para>To provide more control over the compiler's verbosity,
710 the <option>-v</option> flag takes an optional numeric
711 argument. Specifying <option>-v</option> on its own is
712 equivalent to <option>-v3</option>, and the other levels
713 have the following meanings:</para>
717 <term><option>-v0</option></term>
719 <para>Disable all non-essential messages (this is the
725 <term><option>-v1</option></term>
727 <para>Minimal verbosity: print one line per
728 compilation (this is the default when
729 <option>––make</option> or
730 <option>––interactive</option> is on).</para>
735 <term><option>-v2</option></term>
737 <para>Print the name of each compilation phase as it
738 is executed. (equivalent to
739 <option>-dshow-passes</option>).</para>
744 <term><option>-v3</option></term>
746 <para>The same as <option>-v2</option>, except that in
747 addition the full command line (if appropriate) for
748 each compilation phase is also printed.</para>
753 <term><option>-v4</option></term>
755 <para>The same as <option>-v3</option> except that the
756 intermediate program representation after each
757 compilation phase is also printed (excluding
758 preprocessed and C/assembly files).</para>
766 <term><option>-ferror-spans</option>
767 <indexterm><primary><option>-ferror-spans</option></primary>
771 <para>Causes GHC to emit the full source span of the
772 syntactic entity relating to an error message. Normally, GHC
773 emits the source location of the start of the syntactic
776 <para>For example:</para>
778 <screen>test.hs:3:6: parse error on input `where'</screen>
780 <para>becomes:</para>
782 <screen>test296.hs:3:6-10: parse error on input `where'</screen>
784 <para>And multi-line spans are possible too:</para>
786 <screen>test.hs:(5,4)-(6,7):
787 Conflicting definitions for `a'
788 Bound at: test.hs:5:4
790 In the binding group for: a, b, a</screen>
792 <para>Note that line numbers start counting at one, but
793 column numbers start at zero. This choice was made to
794 follow existing convention (i.e. this is how Emacs does
800 <term><option>-H</option><replaceable>size</replaceable>
801 <indexterm><primary><option>-H</option></primary></indexterm>
804 <para>Set the minimum size of the heap to
805 <replaceable>size</replaceable>.
806 This option is equivalent to
807 <literal>+RTS -H<replaceable>size</replaceable></literal>,
808 see <xref linkend="rts-options-gc" />.
814 <term><option>-Rghc-timing</option>
815 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
818 <para>Prints a one-line summary of timing statistics for the
819 GHC run. This option is equivalent to
820 <literal>+RTS -tstderr</literal>, see <xref
821 linkend="rts-options-gc" />.
830 <sect1 id="options-sanity">
831 <title>Warnings and sanity-checking</title>
833 <indexterm><primary>sanity-checking options</primary></indexterm>
834 <indexterm><primary>warnings</primary></indexterm>
837 <para>GHC has a number of options that select which types of
838 non-fatal error messages, otherwise known as warnings, can be
839 generated during compilation. By default, you get a standard set
840 of warnings which are generally likely to indicate bugs in your
842 <option>-fwarn-overlapping-patterns</option>,
843 <option>-fwarn-warnings-deprecations</option>,
844 <option>-fwarn-deprecated-flags</option>,
845 <option>-fwarn-duplicate-exports</option>,
846 <option>-fwarn-missing-fields</option>,
847 <option>-fwarn-missing-methods</option>, and
848 <option>-fwarn-dodgy-foreign-imports</option>. The following
850 simple ways to select standard “packages” of warnings:
856 <term><option>-W</option>:</term>
858 <indexterm><primary>-W option</primary></indexterm>
859 <para>Provides the standard warnings plus
860 <option>-fwarn-incomplete-patterns</option>,
861 <option>-fwarn-dodgy-imports</option>,
862 <option>-fwarn-unused-matches</option>,
863 <option>-fwarn-unused-imports</option>, and
864 <option>-fwarn-unused-binds</option>.</para>
869 <term><option>-Wall</option>:</term>
871 <indexterm><primary><option>-Wall</option></primary></indexterm>
872 <para>Turns on all warning options that indicate potentially
873 suspicious code. The warnings that are
874 <emphasis>not</emphasis> enabled by <option>-Wall</option>
876 <option>-fwarn-simple-patterns</option>,
877 <option>-fwarn-tabs</option>,
878 <option>-fwarn-incomplete-record-updates</option>,
879 <option>-fwarn-monomorphism-restriction</option>, and
880 <option>-fwarn-implicit-prelude</option>.</para>
885 <term><option>-w</option>:</term>
887 <indexterm><primary><option>-w</option></primary></indexterm>
888 <para>Turns off all warnings, including the standard ones and
889 those that <literal>-Wall</literal> doesn't enable.</para>
894 <term><option>-Werror</option>:</term>
896 <indexterm><primary><option>-Werror</option></primary></indexterm>
897 <para>Makes any warning into a fatal error. Useful so that you don't
898 miss warnings when doing batch compilation. </para>
903 <term><option>-Wwarn</option>:</term>
905 <indexterm><primary><option>-Wwarn</option></primary></indexterm>
906 <para>Warnings are treated only as warnings, not as errors. This is
907 the default, but can be useful to negate a
908 <option>-Werror</option> flag.</para>
914 <para>The full set of warning options is described below. To turn
915 off any warning, simply give the corresponding
916 <option>-fno-warn-...</option> option on the command line.</para>
921 <term><option>-fwarn-unrecognised-pragmas</option>:</term>
923 <indexterm><primary><option>-fwarn-unrecognised-pragmas</option></primary>
925 <indexterm><primary>warnings</primary></indexterm>
926 <indexterm><primary>pragmas</primary></indexterm>
927 <para>Causes a warning to be emitted when a
928 pragma that GHC doesn't recognise is used. As well as pragmas
929 that GHC itself uses, GHC also recognises pragmas known to be used
930 by other tools, e.g. <literal>OPTIONS_HUGS</literal> and
931 <literal>DERIVE</literal>.</para>
933 <para>This option is on by default.</para>
938 <term><option>-fwarn-warnings-deprecations</option>:</term>
940 <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
942 <indexterm><primary>warnings</primary></indexterm>
943 <indexterm><primary>deprecations</primary></indexterm>
944 <para>Causes a warning to be emitted when a
945 module, function or type with a WARNING or DEPRECATED pragma
946 is used. See <xref linkend="warning-deprecated-pragma"/> for more
947 details on the pragmas.</para>
949 <para>This option is on by default.</para>
954 <term><option>-fwarn-deprecated-flags</option>:</term>
956 <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
958 <indexterm><primary>deprecated-flags</primary></indexterm>
959 <para>Causes a warning to be emitted when a deprecated
960 commandline flag is used.</para>
962 <para>This option is on by default.</para>
967 <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
969 <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
971 <para>Causes a warning to be emitted for foreign imports of
972 the following form:</para>
974 foreign import "f" f :: FunPtr t
976 <para>on the grounds that it probably should be</para>
978 foreign import "&f" f :: FunPtr t
980 <para>The first form declares that `f` is a (pure) C
981 function that takes no arguments and returns a pointer to a
982 C function with type `t`, whereas the second form declares
983 that `f` itself is a C function with type `t`. The first
984 declaration is usually a mistake, and one that is hard to
985 debug because it results in a crash, hence this
991 <term><option>-fwarn-dodgy-imports</option>:</term>
993 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
995 <para>Causes a warning to be emitted when a a datatype
996 <literal>T</literal> is imported
997 with all constructors, i.e. <literal>T(..)</literal>, but has been
998 exported abstractly, i.e. <literal>T</literal>.</para>
1003 <term><option>-fwarn-duplicate-exports</option>:</term>
1005 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
1006 <indexterm><primary>duplicate exports, warning</primary></indexterm>
1007 <indexterm><primary>export lists, duplicates</primary></indexterm>
1009 <para>Have the compiler warn about duplicate entries in
1010 export lists. This is useful information if you maintain
1011 large export lists, and want to avoid the continued export
1012 of a definition after you've deleted (one) mention of it in
1013 the export list.</para>
1015 <para>This option is on by default.</para>
1020 <term><option>-fwarn-hi-shadowing</option>:</term>
1022 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
1023 <indexterm><primary>shadowing</primary>
1024 <secondary>interface files</secondary></indexterm>
1026 <para>Causes the compiler to emit a warning when a module or
1027 interface file in the current directory is shadowing one
1028 with the same module name in a library or other
1034 <term><option>-fwarn-implicit-prelude</option>:</term>
1036 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
1037 <indexterm><primary>implicit prelude, warning</primary></indexterm>
1038 <para>Have the compiler warn if the Prelude is implicitly
1039 imported. This happens unless either the Prelude module is
1040 explicitly imported with an <literal>import ... Prelude ...</literal>
1041 line, or this implicit import is disabled (either by
1042 <option>-XNoImplicitPrelude</option> or a
1043 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
1045 <para>Note that no warning is given for syntax that implicitly
1046 refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
1047 would change whether it refers to the Prelude.
1048 For example, no warning is given when
1049 <literal>368</literal> means
1050 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
1051 (where <literal>Prelude</literal> refers to the actual Prelude module,
1052 regardless of the imports of the module being compiled).</para>
1054 <para>This warning is off by default.</para>
1059 <term><option>-fwarn-incomplete-patterns</option>:</term>
1061 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
1062 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
1063 <indexterm><primary>patterns, incomplete</primary></indexterm>
1065 <para>Similarly for incomplete patterns, the function
1066 <function>g</function> below will fail when applied to
1067 non-empty lists, so the compiler will emit a warning about
1068 this when <option>-fwarn-incomplete-patterns</option> is
1075 <para>This option isn't enabled by default because it can be
1076 a bit noisy, and it doesn't always indicate a bug in the
1077 program. However, it's generally considered good practice
1078 to cover all the cases in your functions.</para>
1083 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1085 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1086 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1087 <indexterm><primary>record updates, incomplete</primary></indexterm>
1090 <function>f</function> below will fail when applied to
1091 <literal>Bar</literal>, so the compiler will emit a warning about
1092 this when <option>-fwarn-incomplete-record-updates</option> is
1096 data Foo = Foo { x :: Int }
1100 f foo = foo { x = 6 }
1103 <para>This option isn't enabled by default because it can be
1104 very noisy, and it often doesn't indicate a bug in the
1111 <option>-fwarn-missing-fields</option>:
1112 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1113 <indexterm><primary>missing fields, warning</primary></indexterm>
1114 <indexterm><primary>fields, missing</primary></indexterm>
1118 <para>This option is on by default, and warns you whenever
1119 the construction of a labelled field constructor isn't
1120 complete, missing initializers for one or more fields. While
1121 not an error (the missing fields are initialised with
1122 bottoms), it is often an indication of a programmer error.</para>
1127 <term><option>-fwarn-missing-methods</option>:</term>
1129 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1130 <indexterm><primary>missing methods, warning</primary></indexterm>
1131 <indexterm><primary>methods, missing</primary></indexterm>
1133 <para>This option is on by default, and warns you whenever
1134 an instance declaration is missing one or more methods, and
1135 the corresponding class declaration has no default
1136 declaration for them.</para>
1137 <para>The warning is suppressed if the method name
1138 begins with an underscore. Here's an example where this is useful:
1141 _simpleFn :: a -> String
1142 complexFn :: a -> a -> String
1143 complexFn x y = ... _simpleFn ...
1145 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1146 never <literal>_simpleFn</literal>; and (b)
1147 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1153 <term><option>-fwarn-missing-signatures</option>:</term>
1155 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1156 <indexterm><primary>type signatures, missing</primary></indexterm>
1158 <para>If you would like GHC to check that every top-level
1159 function/value has a type signature, use the
1160 <option>-fwarn-missing-signatures</option> option. As part of
1161 the warning GHC also reports the inferred type. The
1162 option is off by default.</para>
1167 <term><option>-fwarn-name-shadowing</option>:</term>
1169 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1170 <indexterm><primary>shadowing, warning</primary></indexterm>
1172 <para>This option causes a warning to be emitted whenever an
1173 inner-scope value has the same name as an outer-scope value,
1174 i.e. the inner value shadows the outer one. This can catch
1175 typographical errors that turn into hard-to-find bugs, e.g.,
1176 in the inadvertent capture of what would be a recursive call in
1177 <literal>f = ... let f = id in ... f ...</literal>.</para>
1182 <term><option>-fwarn-orphans</option>:</term>
1184 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1185 <indexterm><primary>orphan instances, warning</primary></indexterm>
1186 <indexterm><primary>orphan rules, warning</primary></indexterm>
1188 <para>This option causes a warning to be emitted whenever the
1189 module contains an "orphan" instance declaration or rewrite rule.
1190 An instance declaration is an orphan if it appears in a module in
1191 which neither the class nor the type being instanced are declared
1192 in the same module. A rule is an orphan if it is a rule for a
1193 function declared in another module. A module containing any
1194 orphans is called an orphan module.</para>
1195 <para>The trouble with orphans is that GHC must pro-actively read the interface
1196 files for all orphan modules, just in case their instances or rules
1197 play a role, whether or not the module's interface would otherwise
1198 be of any use. See <xref linkend="orphan-modules"/> for details.
1205 <option>-fwarn-overlapping-patterns</option>:
1206 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1207 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1208 <indexterm><primary>patterns, overlapping</primary></indexterm>
1211 <para>By default, the compiler will warn you if a set of
1212 patterns are overlapping, e.g.,</para>
1215 f :: String -> Int
1221 <para>where the last pattern match in <function>f</function>
1222 won't ever be reached, as the second pattern overlaps
1223 it. More often than not, redundant patterns is a programmer
1224 mistake/error, so this option is enabled by default.</para>
1229 <term><option>-fwarn-simple-patterns</option>:</term>
1231 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1233 <para>Causes the compiler to warn about lambda-bound
1234 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1235 Normally, these aren't treated as incomplete patterns by
1236 <option>-fwarn-incomplete-patterns</option>.</para>
1237 <para>“Lambda-bound patterns” includes all places where there is a single pattern,
1238 including list comprehensions and do-notation. In these cases, a pattern-match
1239 failure is quite legitimate, and triggers filtering (list comprehensions) or
1240 the monad <literal>fail</literal> operation (monads). For example:
1242 f :: [Maybe a] -> [a]
1243 f xs = [y | Just y <- xs]
1245 Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
1246 these probably-innocent cases, which is why the flag is off by default. </para>
1251 <term><option>-fwarn-tabs</option>:</term>
1253 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1254 <indexterm><primary>tabs, warning</primary></indexterm>
1255 <para>Have the compiler warn if there are tabs in your source
1258 <para>This warning is off by default.</para>
1263 <term><option>-fwarn-type-defaults</option>:</term>
1265 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1266 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1267 <para>Have the compiler warn/inform you where in your source
1268 the Haskell defaulting mechanism for numeric types kicks
1269 in. This is useful information when converting code from a
1270 context that assumed one default into one with another,
1271 e.g., the ‘default default’ for Haskell 1.4 caused the
1272 otherwise unconstrained value <constant>1</constant> to be
1273 given the type <literal>Int</literal>, whereas Haskell 98
1274 defaults it to <literal>Integer</literal>. This may lead to
1275 differences in performance and behaviour, hence the
1276 usefulness of being non-silent about this.</para>
1278 <para>This warning is off by default.</para>
1283 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1285 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1286 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1287 <para>Have the compiler warn/inform you where in your source
1288 the Haskell Monomorphism Restriction is applied. If applied silently
1289 the MR can give rise to unexpected behaviour, so it can be helpful
1290 to have an explicit warning that it is being applied.</para>
1292 <para>This warning is off by default.</para>
1297 <term><option>-fwarn-unused-binds</option>:</term>
1299 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1300 <indexterm><primary>unused binds, warning</primary></indexterm>
1301 <indexterm><primary>binds, unused</primary></indexterm>
1302 <para>Report any function definitions (and local bindings)
1303 which are unused. For top-level functions, the warning is
1304 only given if the binding is not exported.</para>
1305 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1306 mentioned in the right hand side of another definition that is used, or (c) the
1307 function it defines begins with an underscore. The last case provides a
1308 way to suppress unused-binding warnings selectively. </para>
1309 <para> Notice that a variable
1310 is reported as unused even if it appears in the right-hand side of another
1311 unused binding. </para>
1316 <term><option>-fwarn-unused-imports</option>:</term>
1318 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1319 <indexterm><primary>unused imports, warning</primary></indexterm>
1320 <indexterm><primary>imports, unused</primary></indexterm>
1322 <para>Report any modules that are explicitly imported but
1323 never used. However, the form <literal>import M()</literal> is
1324 never reported as an unused import, because it is a useful idiom
1325 for importing instance declarations, which are anonymous in Haskell.</para>
1330 <term><option>-fwarn-unused-matches</option>:</term>
1332 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1333 <indexterm><primary>unused matches, warning</primary></indexterm>
1334 <indexterm><primary>matches, unused</primary></indexterm>
1336 <para>Report all unused variables which arise from pattern
1337 matches, including patterns consisting of a single variable.
1338 For instance <literal>f x y = []</literal> would report
1339 <varname>x</varname> and <varname>y</varname> as unused. The
1340 warning is suppressed if the variable name begins with an underscore, thus:
1350 <para>If you're feeling really paranoid, the
1351 <option>-dcore-lint</option>
1352 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1353 is a good choice. It turns on heavyweight intra-pass
1354 sanity-checking within GHC. (It checks GHC's sanity, not
1361 <sect1 id="options-optimise">
1362 <title>Optimisation (code improvement)</title>
1364 <indexterm><primary>optimisation</primary></indexterm>
1365 <indexterm><primary>improvement, code</primary></indexterm>
1367 <para>The <option>-O*</option> options specify convenient
1368 “packages” of optimisation flags; the
1369 <option>-f*</option> options described later on specify
1370 <emphasis>individual</emphasis> optimisations to be turned on/off;
1371 the <option>-m*</option> options specify
1372 <emphasis>machine-specific</emphasis> optimisations to be turned
1375 <sect2 id="optimise-pkgs">
1376 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1378 <para>There are <emphasis>many</emphasis> options that affect
1379 the quality of code produced by GHC. Most people only have a
1380 general goal, something like “Compile quickly” or
1381 “Make my program run like greased lightning.” The
1382 following “packages” of optimisations (or lack
1383 thereof) should suffice.</para>
1385 <para>Note that higher optimisation levels cause more
1386 cross-module optimisation to be performed, which can have an
1387 impact on how much of your program needs to be recompiled when
1388 you change something. This is one reason to stick to
1389 no-optimisation when developing code.</para>
1395 No <option>-O*</option>-type option specified:
1396 <indexterm><primary>-O* not specified</primary></indexterm>
1399 <para>This is taken to mean: “Please compile
1400 quickly; I'm not over-bothered about compiled-code
1401 quality.” So, for example: <command>ghc -c
1402 Foo.hs</command></para>
1408 <option>-O0</option>:
1409 <indexterm><primary><option>-O0</option></primary></indexterm>
1412 <para>Means “turn off all optimisation”,
1413 reverting to the same settings as if no
1414 <option>-O</option> options had been specified. Saying
1415 <option>-O0</option> can be useful if
1416 eg. <command>make</command> has inserted a
1417 <option>-O</option> on the command line already.</para>
1423 <option>-O</option> or <option>-O1</option>:
1424 <indexterm><primary>-O option</primary></indexterm>
1425 <indexterm><primary>-O1 option</primary></indexterm>
1426 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1429 <para>Means: “Generate good-quality code without
1430 taking too long about it.” Thus, for example:
1431 <command>ghc -c -O Main.lhs</command></para>
1437 <option>-O2</option>:
1438 <indexterm><primary>-O2 option</primary></indexterm>
1439 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1442 <para>Means: “Apply every non-dangerous
1443 optimisation, even if it means significantly longer
1444 compile times.”</para>
1446 <para>The avoided “dangerous” optimisations
1447 are those that can make runtime or space
1448 <emphasis>worse</emphasis> if you're unlucky. They are
1449 normally turned on or off individually.</para>
1451 <para>At the moment, <option>-O2</option> is
1452 <emphasis>unlikely</emphasis> to produce better code than
1453 <option>-O</option>.</para>
1459 <option>-Ofile <file></option>:
1460 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1461 <indexterm><primary>optimising, customised</primary></indexterm>
1464 <para>(NOTE: not supported since GHC 4.x. Please ask if
1465 you're interested in this.)</para>
1467 <para>For those who need <emphasis>absolute</emphasis>
1468 control over <emphasis>exactly</emphasis> what options are
1469 used (e.g., compiler writers, sometimes :-), a list of
1470 options can be put in a file and then slurped in with
1471 <option>-Ofile</option>.</para>
1473 <para>In that file, comments are of the
1474 <literal>#</literal>-to-end-of-line variety; blank
1475 lines and most whitespace is ignored.</para>
1477 <para>Please ask if you are baffled and would like an
1478 example of <option>-Ofile</option>!</para>
1483 <para>We don't use a <option>-O*</option> flag for day-to-day
1484 work. We use <option>-O</option> to get respectable speed;
1485 e.g., when we want to measure something. When we want to go for
1486 broke, we tend to use <option>-O2 -fvia-C</option> (and we go for
1487 lots of coffee breaks).</para>
1489 <para>The easiest way to see what <option>-O</option> (etc.)
1490 “really mean” is to run with <option>-v</option>,
1491 then stand back in amazement.</para>
1494 <sect2 id="options-f">
1495 <title><option>-f*</option>: platform-independent flags</title>
1497 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1498 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1500 <para>These flags turn on and off individual optimisations.
1501 They are normally set via the <option>-O</option> options
1502 described above, and as such, you shouldn't need to set any of
1503 them explicitly (indeed, doing so could lead to unexpected
1504 results). However, there are one or two that may be of
1509 <term><option>-fexcess-precision</option>:</term>
1511 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1512 <para>When this option is given, intermediate floating
1513 point values can have a <emphasis>greater</emphasis>
1514 precision/range than the final type. Generally this is a
1515 good thing, but some programs may rely on the exact
1517 <literal>Float</literal>/<literal>Double</literal> values
1518 and should not use this option for their compilation.</para>
1523 <term><option>-fignore-asserts</option>:</term>
1525 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1526 <para>Causes GHC to ignore uses of the function
1527 <literal>Exception.assert</literal> in source code (in
1528 other words, rewriting <literal>Exception.assert p
1529 e</literal> to <literal>e</literal> (see <xref
1530 linkend="assertions"/>). This flag is turned on by
1531 <option>-O</option>.
1538 <option>-fno-cse</option>
1539 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1542 <para>Turns off the common-sub-expression elimination optimisation.
1543 Can be useful if you have some <literal>unsafePerformIO</literal>
1544 expressions that you don't want commoned-up.</para>
1550 <option>-fno-strictness</option>
1551 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1554 <para>Turns off the strictness analyser; sometimes it eats
1555 too many cycles.</para>
1561 <option>-fno-full-laziness</option>
1562 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1565 <para>Turns off the full laziness optimisation (also known as
1566 let-floating). Full laziness increases sharing, which can lead
1567 to increased memory residency.</para>
1569 <para>NOTE: GHC doesn't implement complete full-laziness.
1570 When optimisation in on, and
1571 <option>-fno-full-laziness</option> is not given, some
1572 transformations that increase sharing are performed, such
1573 as extracting repeated computations from a loop. These
1574 are the same transformations that a fully lazy
1575 implementation would do, the difference is that GHC
1576 doesn't consistently apply full-laziness, so don't rely on
1583 <option>-fspec-constr</option>
1584 <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
1587 <para>Turn on call-pattern specialisation.</para>
1593 <option>-fliberate-case</option>
1594 <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
1597 <para>Turn on the liberate-case transformation.</para>
1603 <option>-fstatic-argument-transformation</option>
1604 <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
1607 <para>Turn on the static argument transformation.</para>
1613 <option>-fno-state-hack</option>
1614 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1617 <para>Turn off the "state hack" whereby any lambda with a
1618 <literal>State#</literal> token as argument is considered to be
1619 single-entry, hence it is considered OK to inline things inside
1620 it. This can improve performance of IO and ST monad code, but it
1621 runs the risk of reducing sharing.</para>
1627 <option>-fomit-interface-pragmas</option>
1628 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
1631 <para>Tells GHC to omit all inessential information from the interface file
1632 generated for the module being compiled (say M). This means that a module
1633 importing M will see only the <emphasis>types</emphasis> of the functions that M exports, but not
1634 their unfoldings, strictness info, etc. Hence, for example,
1635 no function exported by M will be inlined
1636 into an importing module. The benefit is that modules that import M will
1637 need to be recompiled less often (only when M's exports change their type,
1638 not when they change their implementation).
1645 <option>-fignore-interface-pragmas</option>
1646 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
1649 <para>Tells GHC to ignore all inessential information when reading interface files.
1650 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
1651 for a function, GHC will ignore that information.</para>
1657 <option>-funbox-strict-fields</option>:
1658 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1659 <indexterm><primary>strict constructor fields</primary></indexterm>
1660 <indexterm><primary>constructor fields, strict</primary></indexterm>
1663 <para>This option causes all constructor fields which are
1664 marked strict (i.e. “!”) to be unboxed or
1665 unpacked if possible. It is equivalent to adding an
1666 <literal>UNPACK</literal> pragma to every strict
1667 constructor field (see <xref
1668 linkend="unpack-pragma"/>).</para>
1670 <para>This option is a bit of a sledgehammer: it might
1671 sometimes make things worse. Selectively unboxing fields
1672 by using <literal>UNPACK</literal> pragmas might be
1679 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
1680 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1681 <indexterm><primary>inlining, controlling</primary></indexterm>
1682 <indexterm><primary>unfolding, controlling</primary></indexterm>
1685 <para>(Default: 45) Governs the maximum size that GHC will
1686 allow a function unfolding to be. (An unfolding has a
1687 “size” that reflects the cost in terms of
1688 “code bloat” of expanding that unfolding at
1689 at a call site. A bigger function would be assigned a
1690 bigger cost.) </para>
1692 <para> Consequences: (a) nothing larger than this will be
1693 inlined (unless it has an INLINE pragma); (b) nothing
1694 larger than this will be spewed into an interface
1698 <para> Increasing this figure is more likely to result in longer
1699 compile times than faster code. The next option is more
1705 <term><option>-funfolding-use-threshold=<replaceable>n</replaceable></option></term>
1707 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1708 <indexterm><primary>inlining, controlling</primary></indexterm>
1709 <indexterm><primary>unfolding, controlling</primary></indexterm>
1711 <para>(Default: 8) This is the magic cut-off figure for
1712 unfolding: below this size, a function definition will be
1713 unfolded at the call-site, any bigger and it won't. The
1714 size computed for a function depends on two things: the
1715 actual size of the expression minus any discounts that
1716 apply (see <option>-funfolding-con-discount</option>).</para>
1727 <sect1 id="using-concurrent">
1728 <title>Using Concurrent Haskell</title>
1729 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
1731 <para>GHC supports Concurrent Haskell by default, without requiring a
1732 special option or libraries compiled in a certain way. To get access to
1733 the support libraries for Concurrent Haskell, just import
1735 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>
1737 <para>The following RTS option(s) affect the behaviour of Concurrent
1738 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
1742 <term><option>-C<replaceable>s</replaceable></option></term>
1744 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1745 Sets the context switch interval to <replaceable>s</replaceable>
1746 seconds. A context switch will occur at the next heap block
1747 allocation after the timer expires (a heap block allocation occurs
1748 every 4k of allocation). With <option>-C0</option> or
1749 <option>-C</option>, context switches will occur as often as
1750 possible (at every heap block allocation). By default, context
1751 switches occur every 20ms.</para>
1757 <sect1 id="using-smp">
1758 <title>Using SMP parallelism</title>
1759 <indexterm><primary>parallelism</primary>
1761 <indexterm><primary>SMP</primary>
1764 <para>GHC supports running Haskell programs in parallel on an SMP
1765 (symmetric multiprocessor).</para>
1767 <para>There's a fine distinction between
1768 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
1769 parallelism is all about making your program run
1770 <emphasis>faster</emphasis> by making use of multiple processors
1771 simultaneously. Concurrency, on the other hand, is a means of
1772 abstraction: it is a convenient way to structure a program that must
1773 respond to multiple asynchronous events.</para>
1775 <para>However, the two terms are certainly related. By making use of
1776 multiple CPUs it is possible to run concurrent threads in parallel,
1777 and this is exactly what GHC's SMP parallelism support does. But it
1778 is also possible to obtain performance improvements with parallelism
1779 on programs that do not use concurrency. This section describes how to
1780 use GHC to compile and run parallel programs, in <xref
1781 linkend="lang-parallel" /> we describe the language features that affect
1784 <sect2 id="parallel-compile-options">
1785 <title>Compile-time options for SMP parallelism</title>
1787 <para>In order to make use of multiple CPUs, your program must be
1788 linked with the <option>-threaded</option> option (see <xref
1789 linkend="options-linker" />). Additionally, the following
1790 compiler options affect parallelism:</para>
1794 <term><option>-feager-blackholing</option></term>
1795 <indexterm><primary><option>-feager-blackholing</option></primary></indexterm>
1798 Blackholing is the act of marking a thunk (lazy
1799 computuation) as being under evaluation. It is useful for
1800 three reasons: firstly it lets us detect certain kinds of
1801 infinite loop (the <literal>NonTermination</literal>
1802 exception), secondly it avoids certain kinds of space
1803 leak, and thirdly it avoids repeating a computation in a
1804 parallel program, because we can tell when a computation
1805 is already in progress.</para>
1808 The option <option>-feager-blackholing</option> causes
1809 each thunk to be blackholed as soon as evaluation begins.
1810 The default is "lazy blackholing", whereby thunks are only
1811 marked as being under evaluation when a thread is paused
1812 for some reason. Lazy blackholing is typically more
1813 efficient (by 1-2% or so), because most thunks don't
1814 need to be blackholed. However, eager blackholing can
1815 avoid more repeated computation in a parallel program, and
1816 this often turns out to be important for parallelism.
1820 We recommend compiling any code that is intended to be run
1821 in parallel with the <option>-feager-blackholing</option>
1829 <sect2 id="parallel-options">
1830 <title>RTS options for SMP parallelism</title>
1832 <para>To run a program on multiple CPUs, use the
1833 RTS <option>-N</option> option:</para>
1837 <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
1839 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1840 Use <replaceable>x</replaceable> simultaneous threads when
1841 running the program. Normally <replaceable>x</replaceable>
1842 should be chosen to match the number of CPU cores on the
1843 machine<footnote><para>Whether hyperthreading cores should be counted or not is an
1844 open question; please feel free to experiment and let us know what
1845 results you find.</para></footnote>. For example,
1846 on a dual-core machine we would probably use
1847 <literal>+RTS -N2 -RTS</literal>.</para>
1849 <para>Omitting <replaceable>x</replaceable>,
1850 i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
1851 choose the value of <replaceable>x</replaceable> itself
1852 based on how many processors are in your machine.</para>
1854 <para>Be careful when using all the processors in your
1855 machine: if some of your processors are in use by other
1856 programs, this can actually harm performance rather than
1859 <para>Setting <option>-N</option> also has the effect of
1860 enabling the parallel garbage collector (see
1861 <xref linkend="rts-options-gc" />).</para>
1863 <para>There is no means (currently) by which this value
1864 may vary after the program has started.</para>
1869 <para>The following options affect the way the runtime schedules
1870 threads on CPUs:</para>
1874 <term><option>-qm</option></term>
1875 <indexterm><primary><option>-qm</option></primary><secondary>RTS
1876 option</secondary></indexterm>
1878 <para>Disable automatic migration for load balancing.
1879 Normally the runtime will automatically try to schedule
1880 threads across the available CPUs to make use of idle
1881 CPUs; this option disables that behaviour. It is probably
1882 only of use if you are explicitly scheduling threads onto
1883 CPUs with <literal>GHC.Conc.forkOnIO</literal>.</para>
1887 <term><option>-qw</option></term>
1888 <indexterm><primary><option>-qw</option></primary><secondary>RTS
1889 option</secondary></indexterm>
1891 <para>Migrate a thread to the current CPU when it is woken
1892 up. Normally when a thread is woken up after being
1893 blocked it will be scheduled on the CPU it was running on
1894 last; this option allows the thread to immediately migrate
1895 to the CPU that unblocked it.</para>
1897 <para>The rationale for allowing this eager migration is
1898 that it tends to move threads that are communicating with
1899 each other onto the same CPU; however there are
1900 pathalogical situations where it turns out to be a poor
1901 strategy. Depending on the communication pattern in your
1902 program, it may or may not be a good idea.</para>
1909 <title>Hints for using SMP parallelism</title>
1911 <para>Add the <literal>-s</literal> RTS option when
1912 running the program to see timing stats, which will help to tell you
1913 whether your program got faster by using more CPUs or not. If the user
1914 time is greater than
1915 the elapsed time, then the program used more than one CPU. You should
1916 also run the program without <literal>-N</literal> for comparison.</para>
1918 <para>GHC's parallelism support is new and experimental. It may make your
1919 program go faster, or it might slow it down - either way, we'd be
1920 interested to hear from you.</para>
1922 <para>One significant limitation with the current implementation is that
1923 the garbage collector is still single-threaded, and all execution must
1924 stop when GC takes place. This can be a significant bottleneck in a
1925 parallel program, especially if your program does a lot of GC. If this
1926 happens to you, then try reducing the cost of GC by tweaking the GC
1927 settings (<xref linkend="rts-options-gc" />): enlarging the heap or the
1928 allocation area size is a good start.</para>
1932 <sect1 id="options-platform">
1933 <title>Platform-specific Flags</title>
1935 <indexterm><primary>-m* options</primary></indexterm>
1936 <indexterm><primary>platform-specific options</primary></indexterm>
1937 <indexterm><primary>machine-specific options</primary></indexterm>
1939 <para>Some flags only make sense for particular target
1945 <term><option>-monly-[32]-regs</option>:</term>
1947 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1948 option (iX86 only)</primary></indexterm> GHC tries to
1949 “steal” four registers from GCC, for performance
1950 reasons; it almost always works. However, when GCC is
1951 compiling some modules with four stolen registers, it will
1952 crash, probably saying:
1955 Foo.hc:533: fixed or forbidden register was spilled.
1956 This may be due to a compiler bug or to impossible asm
1957 statements or clauses.
1960 Just give some registers back with
1961 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1962 If `2' doesn't work, please report the bug to us.</para>
1971 <sect1 id="ext-core">
1972 <title>Generating and compiling External Core Files</title>
1974 <indexterm><primary>intermediate code generation</primary></indexterm>
1976 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
1977 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
1978 <filename>.hcr</filename>. The Core format is described in <ulink url="../ext-core/core.pdf">
1979 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1981 for manipulating Core files (in Haskell) are in the GHC source distribution
1982 directory under <literal>utils/ext-core</literal>.
1983 Note that the format of <literal>.hcr</literal>
1984 files is <emphasis>different</emphasis> from the Core output format that GHC generates
1985 for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
1987 <para>The Core format natively supports notes which you can add to
1988 your source code using the <literal>CORE</literal> pragma (see <xref
1989 linkend="pragmas"/>).</para>
1995 <option>-fext-core</option>
1996 <indexterm><primary><option>-fext-core</option></primary></indexterm>
1999 <para>Generate <literal>.hcr</literal> files.</para>
2005 <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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