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>Getting started: compiling programs</title>
12 In this chapter you'll find a complete reference to the GHC
13 command-line syntax, including all 400+ flags. It's a large and
14 complex system, and there are lots of details, so it can be
15 quite hard to figure out how to get started. With that in mind,
16 this introductory section provides a quick introduction to the
17 basic usage of GHC for compiling a Haskell program, before the
18 following sections dive into the full syntax.
22 Let's create a Hello World program, and compile and run it.
23 First, create a file <filename>hello.hs</filename> containing
28 main = putStrLn "Hello, World!"
31 <para>To compile the program, use GHC like this:</para>
34 $ ghc hello.hs</screen>
36 <para>(where <literal>$</literal> represents the prompt: don't
37 type it). GHC will compile the source
38 file <filename>hello.hs</filename>, producing
40 file</firstterm> <filename>hello.o</filename> and
41 an <firstterm>interface
42 file</firstterm> <filename>hello.hi</filename>, and then it
43 will link the object file to the libraries that come with GHC
44 to produce an executable called <filename>hello</filename> on
45 Unix/Linux/Mac, or <filename>hello.exe</filename> on
49 By default GHC will be very quiet about what it is doing, only
50 printing error messages. If you want to see in more detail
51 what's going on behind the scenes, add <option>-v</option> to
56 Then we can run the program like this:
64 If your program contains multiple modules, then you only need to
65 tell GHC the name of the source file containing
66 the <filename>Main</filename> module, and GHC will examine
67 the <literal>import</literal> declarations to find the other
68 modules that make up the program and find their source files.
69 This means that, with the exception of
70 the <literal>Main</literal> module, every source file should be
71 named after the module name that it contains (with dots replaced
72 by directory separators). For example, the
73 module <literal>Data.Person</literal> would be in the
74 file <filename>Data/Person.hs</filename> on Unix/Linux/Mac,
75 or <filename>Data\Person.hs</filename> on Windows.
80 <title>Options overview</title>
82 <para>GHC's behaviour is controlled by
83 <firstterm>options</firstterm>, which for historical reasons are
84 also sometimes referred to as command-line flags or arguments.
85 Options can be specified in three ways:</para>
88 <title>Command-line arguments</title>
90 <indexterm><primary>structure, command-line</primary></indexterm>
91 <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
92 <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
94 <para>An invocation of GHC takes the following form:</para>
100 <para>Command-line arguments are either options or file names.</para>
102 <para>Command-line options begin with <literal>-</literal>.
103 They may <emphasis>not</emphasis> be grouped:
104 <option>-vO</option> is different from <option>-v -O</option>.
105 Options need not precede filenames: e.g., <literal>ghc *.o -o
106 foo</literal>. All options are processed and then applied to
107 all files; you cannot, for example, invoke <literal>ghc -c -O1
108 Foo.hs -O2 Bar.hs</literal> to apply different optimisation
109 levels to the files <filename>Foo.hs</filename> and
110 <filename>Bar.hs</filename>.</para>
113 <sect2 id="source-file-options">
114 <title>Command line options in source files</title>
116 <indexterm><primary>source-file options</primary></indexterm>
118 <para>Sometimes it is useful to make the connection between a
119 source file and the command-line options it requires quite
120 tight. For instance, if a Haskell source file deliberately
121 uses name shadowing, it should be compiled with the
122 <option>-fno-warn-name-shadowing</option> option. Rather than maintaining
123 the list of per-file options in a <filename>Makefile</filename>,
124 it is possible to do this directly in the source file using the
125 <literal>OPTIONS_GHC</literal> pragma <indexterm><primary>OPTIONS_GHC
126 pragma</primary></indexterm>:</para>
129 {-# OPTIONS_GHC -fno-warn-name-shadowing #-}
134 <para><literal>OPTIONS_GHC</literal> is a <emphasis>file-header pragma</emphasis>
135 (see <xref linkend="pragmas"/>).</para>
137 <para>Only <emphasis>dynamic</emphasis> flags can be used in an <literal>OPTIONS_GHC</literal> pragma
138 (see <xref linkend="static-dynamic-flags"/>).</para>
140 <para>Note that your command shell does not
141 get to the source file options, they are just included literally
142 in the array of command-line arguments the compiler
143 maintains internally, so you'll be desperately disappointed if
144 you try to glob etc. inside <literal>OPTIONS_GHC</literal>.</para>
146 <para>NOTE: the contents of OPTIONS_GHC are appended to the
147 command-line options, so options given in the source file
148 override those given on the command-line.</para>
150 <para>It is not recommended to move all the contents of your
151 Makefiles into your source files, but in some circumstances, the
152 <literal>OPTIONS_GHC</literal> pragma is the Right Thing. (If you
153 use <option>-keep-hc-file</option> and have OPTION flags in
154 your module, the OPTIONS_GHC will get put into the generated .hc
159 <title>Setting options in GHCi</title>
161 <para>Options may also be modified from within GHCi, using the
162 <literal>:set</literal> command. See <xref linkend="ghci-set"/>
163 for more details.</para>
167 <sect1 id="static-dynamic-flags">
168 <title>Static, Dynamic, and Mode options</title>
169 <indexterm><primary>static</primary><secondary>options</secondary>
171 <indexterm><primary>dynamic</primary><secondary>options</secondary>
173 <indexterm><primary>mode</primary><secondary>options</secondary>
176 <para>Each of GHC's command line options is classified as
177 <firstterm>static</firstterm>, <firstterm>dynamic</firstterm> or
178 <firstterm>mode</firstterm>:</para>
182 <term>Mode flags</term>
184 <para>For example, <option>––make</option> or <option>-E</option>.
185 There may only be a single mode flag on the command line. The
186 available modes are listed in <xref linkend="modes"/>.</para>
190 <term>Dynamic Flags</term>
192 <para>Most non-mode flags fall into this category. A dynamic flag
193 may be used on the command line, in a
194 <literal>OPTIONS_GHC</literal> pragma in a source file, or set
195 using <literal>:set</literal> in GHCi.</para>
199 <term>Static Flags</term>
201 <para>A few flags are "static", which means they can only be used on
202 the command-line, and remain in force over the entire GHC/GHCi
208 <para>The flag reference tables (<xref
209 linkend="flag-reference"/>) lists the status of each flag.</para>
211 <para>There are a few flags that are static except that they can
212 also be used with GHCi's <literal>:set</literal> command; these
213 are listed as “static/<literal>:set</literal>” in the
217 <sect1 id="file-suffixes">
218 <title>Meaningful file suffixes</title>
220 <indexterm><primary>suffixes, file</primary></indexterm>
221 <indexterm><primary>file suffixes for GHC</primary></indexterm>
223 <para>File names with “meaningful” suffixes (e.g.,
224 <filename>.lhs</filename> or <filename>.o</filename>) cause the
225 “right thing” to happen to those files.</para>
230 <term><filename>.hs</filename></term>
232 <para>A Haskell module.</para>
238 <filename>.lhs</filename>
239 <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
242 <para>A “literate Haskell” module.</para>
247 <term><filename>.hi</filename></term>
249 <para>A Haskell interface file, probably
250 compiler-generated.</para>
255 <term><filename>.hc</filename></term>
257 <para>Intermediate C file produced by the Haskell
263 <term><filename>.c</filename></term>
265 <para>A C file not produced by the Haskell
271 <term><filename>.ll</filename></term>
273 <para>An llvm-intermediate-language source file, usually
274 produced by the compiler.</para>
279 <term><filename>.bc</filename></term>
281 <para>An llvm-intermediate-language bitcode file, usually
282 produced by the compiler.</para>
287 <term><filename>.s</filename></term>
289 <para>An assembly-language source file, usually produced by
295 <term><filename>.o</filename></term>
297 <para>An object file, produced by an assembler.</para>
302 <para>Files with other suffixes (or without suffixes) are passed
303 straight to the linker.</para>
308 <title>Modes of operation</title>
311 GHC's behaviour is firstly controlled by a mode flag. Only one
312 of these flags may be given, but it does not necessarily need to
313 be the first option on the command-line.
317 If no mode flag is present, then GHC will enter make mode
318 (<xref linkend="make-mode" />) if there are any Haskell source
319 files given on the command line, or else it will link the
320 objects named on the command line to produce an executable.
323 <para>The available mode flags are:</para>
328 <cmdsynopsis><command>ghc --interactive</command>
330 <indexterm><primary>interactive mode</primary></indexterm>
331 <indexterm><primary>ghci</primary></indexterm>
334 <para>Interactive mode, which is also available as
335 <command>ghci</command>. Interactive mode is described in
336 more detail in <xref linkend="ghci"/>.</para>
342 <cmdsynopsis><command>ghc ––make</command>
344 <indexterm><primary>make mode</primary></indexterm>
345 <indexterm><primary><option>––make</option></primary></indexterm>
348 <para>In this mode, GHC will build a multi-module Haskell
349 program automatically, figuring out dependencies for itself.
350 If you have a straightforward Haskell program, this is
351 likely to be much easier, and faster, than using
352 <command>make</command>. Make mode is described in <xref
353 linkend="make-mode"/>.</para>
356 This mode is the default if there are any Haskell
357 source files mentioned on the command line, and in this case
358 the <option>––make</option> option can be omitted.
365 <cmdsynopsis><command>ghc -e</command>
366 <arg choice='plain'><replaceable>expr</replaceable></arg>
368 <indexterm><primary>eval mode</primary></indexterm>
371 <para>Expression-evaluation mode. This is very similar to
372 interactive mode, except that there is a single expression
373 to evaluate (<replaceable>expr</replaceable>) which is given
374 on the command line. See <xref linkend="eval-mode"/> for
382 <command>ghc -E</command>
383 <command>ghc -c</command>
384 <command>ghc -S</command>
385 <command>ghc -c</command>
387 <indexterm><primary><option>-E</option></primary></indexterm>
388 <indexterm><primary><option>-C</option></primary></indexterm>
389 <indexterm><primary><option>-S</option></primary></indexterm>
390 <indexterm><primary><option>-c</option></primary></indexterm>
393 <para>This is the traditional batch-compiler mode, in which
394 GHC can compile source files one at a time, or link objects
395 together into an executable. This mode also applies if
396 there is no other mode flag specified on the command line,
397 in which case it means that the specified files should be
398 compiled and then linked to form a program. See <xref
399 linkend="options-order"/>.</para>
406 <command>ghc -M</command>
408 <indexterm><primary>dependency-generation mode</primary></indexterm>
411 <para>Dependency-generation mode. In this mode, GHC can be
412 used to generate dependency information suitable for use in
413 a <literal>Makefile</literal>. See <xref
414 linkend="makefile-dependencies"/>.</para>
421 <command>ghc --mk-dll</command>
423 <indexterm><primary>DLL-creation mode</primary></indexterm>
426 <para>DLL-creation mode (Windows only). See <xref
427 linkend="win32-dlls-create"/>.</para>
434 <command>ghc --help</command> <command>ghc -?</command>
436 <indexterm><primary><option>––help</option></primary></indexterm>
439 <para>Cause GHC to spew a long usage message to standard
440 output and then exit.</para>
447 <command>ghc --show-iface <replaceable>file</replaceable></command>
449 <indexterm><primary><option>––--show-iface</option></primary></indexterm>
452 <para>Read the interface in
453 <replaceable>file</replaceable> and dump it as text to
454 <literal>stdout</literal>. For example <literal>ghc --show-iface M.hi</literal>.</para>
461 <command>ghc --supported-extensions</command>
462 <command>ghc --supported-languages</command>
464 <indexterm><primary><option>––supported-extensions</option></primary><primary><option>––supported-languages</option></primary></indexterm>
467 <para>Print the supported language extensions.</para>
474 <command>ghc --info</command>
476 <indexterm><primary><option>––info</option></primary></indexterm>
479 <para>Print information about the compiler.</para>
486 <command>ghc --version</command>
487 <command>ghc -V</command>
489 <indexterm><primary><option>-V</option></primary></indexterm>
490 <indexterm><primary><option>––version</option></primary></indexterm>
493 <para>Print a one-line string including GHC's version number.</para>
500 <command>ghc --numeric-version</command>
502 <indexterm><primary><option>––numeric-version</option></primary></indexterm>
505 <para>Print GHC's numeric version number only.</para>
512 <command>ghc --print-libdir</command>
514 <indexterm><primary><option>––print-libdir</option></primary></indexterm>
517 <para>Print the path to GHC's library directory. This is
518 the top of the directory tree containing GHC's libraries,
519 interfaces, and include files (usually something like
520 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
522 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
523 in the package configuration file
524 (see <xref linkend="packages"/>).</para>
530 <sect2 id="make-mode">
531 <title>Using <command>ghc</command> <option>––make</option></title>
532 <indexterm><primary><option>––make</option></primary></indexterm>
533 <indexterm><primary>separate compilation</primary></indexterm>
535 <para>In this mode, GHC will build a multi-module Haskell program by following
536 dependencies from one or more root modules (usually just
537 <literal>Main</literal>). For example, if your
538 <literal>Main</literal> module is in a file called
539 <filename>Main.hs</filename>, you could compile and link the
540 program like this:</para>
543 ghc ––make Main.hs
547 In fact, GHC enters make mode automatically if there are any
548 Haskell source files on the command line and no other mode is
549 specified, so in this case we could just type
556 <para>Any number of source file names or module names may be
557 specified; GHC will figure out all the modules in the program by
558 following the imports from these initial modules. It will then
559 attempt to compile each module which is out of date, and
560 finally, if there is a <literal>Main</literal> module, the
561 program will also be linked into an executable.</para>
563 <para>The main advantages to using <literal>ghc
564 ––make</literal> over traditional
565 <literal>Makefile</literal>s are:</para>
569 <para>GHC doesn't have to be restarted for each compilation,
570 which means it can cache information between compilations.
571 Compiling a multi-module program with <literal>ghc
572 ––make</literal> can be up to twice as fast as
573 running <literal>ghc</literal> individually on each source
577 <para>You don't have to write a <literal>Makefile</literal>.</para>
578 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
581 <para>GHC re-calculates the dependencies each time it is
582 invoked, so the dependencies never get out of sync with the
587 <para>Any of the command-line options described in the rest of
588 this chapter can be used with
589 <option>––make</option>, but note that any options
590 you give on the command line will apply to all the source files
591 compiled, so if you want any options to apply to a single source
592 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
593 pragma (see <xref linkend="source-file-options"/>).</para>
595 <para>If the program needs to be linked with additional objects
596 (say, some auxiliary C code), then the object files can be
597 given on the command line and GHC will include them when linking
598 the executable.</para>
600 <para>Note that GHC can only follow dependencies if it has the
601 source file available, so if your program includes a module for
602 which there is no source file, even if you have an object and an
603 interface file for the module, then GHC will complain. The
604 exception to this rule is for package modules, which may or may
605 not have source files.</para>
607 <para>The source files for the program don't all need to be in
608 the same directory; the <option>-i</option> option can be used
609 to add directories to the search path (see <xref
610 linkend="search-path"/>).</para>
613 <sect2 id="eval-mode">
614 <title>Expression evaluation mode</title>
616 <para>This mode is very similar to interactive mode, except that
617 there is a single expression to evaluate which is specified on
618 the command line as an argument to the <option>-e</option>
622 ghc -e <replaceable>expr</replaceable>
625 <para>Haskell source files may be named on the command line, and
626 they will be loaded exactly as in interactive mode. The
627 expression is evaluated in the context of the loaded
630 <para>For example, to load and run a Haskell program containing
631 a module <literal>Main</literal>, we might say</para>
634 ghc -e Main.main Main.hs
637 <para>or we can just use this mode to evaluate expressions in
638 the context of the <literal>Prelude</literal>:</para>
641 $ ghc -e "interact (unlines.map reverse.lines)"
647 <sect2 id="options-order">
648 <title>Batch compiler mode</title>
650 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
651 given on the command line.</para>
653 <para>The first phase to run is determined by each input-file
654 suffix, and the last phase is determined by a flag. If no
655 relevant flag is present, then go all the way through to linking.
656 This table summarises:</para>
660 <colspec align="left"/>
661 <colspec align="left"/>
662 <colspec align="left"/>
663 <colspec align="left"/>
667 <entry>Phase of the compilation system</entry>
668 <entry>Suffix saying “start here”</entry>
669 <entry>Flag saying “stop after”</entry>
670 <entry>(suffix of) output file</entry>
675 <entry>literate pre-processor</entry>
676 <entry><literal>.lhs</literal></entry>
678 <entry><literal>.hs</literal></entry>
682 <entry>C pre-processor (opt.) </entry>
683 <entry><literal>.hs</literal> (with
684 <option>-cpp</option>)</entry>
685 <entry><option>-E</option></entry>
686 <entry><literal>.hspp</literal></entry>
690 <entry>Haskell compiler</entry>
691 <entry><literal>.hs</literal></entry>
692 <entry><option>-C</option>, <option>-S</option></entry>
693 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
697 <entry>C compiler (opt.)</entry>
698 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
699 <entry><option>-S</option></entry>
700 <entry><literal>.s</literal></entry>
704 <entry>assembler</entry>
705 <entry><literal>.s</literal></entry>
706 <entry><option>-c</option></entry>
707 <entry><literal>.o</literal></entry>
711 <entry>linker</entry>
712 <entry><replaceable>other</replaceable></entry>
714 <entry><filename>a.out</filename></entry>
720 <indexterm><primary><option>-C</option></primary></indexterm>
721 <indexterm><primary><option>-E</option></primary></indexterm>
722 <indexterm><primary><option>-S</option></primary></indexterm>
723 <indexterm><primary><option>-c</option></primary></indexterm>
725 <para>Thus, a common invocation would be: </para>
728 ghc -c Foo.hs</screen>
730 <para>to compile the Haskell source file
731 <filename>Foo.hs</filename> to an object file
732 <filename>Foo.o</filename>.</para>
734 <para>Note: What the Haskell compiler proper produces depends on
735 whether a native-code generator<indexterm><primary>native-code
736 generator</primary></indexterm> is used (producing assembly
737 language) or not (producing C). See <xref
738 linkend="options-codegen"/> for more details.</para>
740 <para>Note: C pre-processing is optional, the
741 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
742 flag turns it on. See <xref linkend="c-pre-processor"/> for more
745 <para>Note: The option <option>-E</option><indexterm><primary>-E
746 option</primary></indexterm> runs just the pre-processing passes
747 of the compiler, dumping the result in a file.</para>
749 <sect3 id="overriding-suffixes">
750 <title>Overriding the default behaviour for a file</title>
752 <para>As described above, the way in which a file is processed by GHC
753 depends on its suffix. This behaviour can be overridden using the
754 <option>-x</option> option:</para>
758 <term><option>-x</option> <replaceable>suffix</replaceable>
759 <indexterm><primary><option>-x</option></primary>
762 <para>Causes all files following this option on the command
763 line to be processed as if they had the suffix
764 <replaceable>suffix</replaceable>. For example, to compile a
765 Haskell module in the file <literal>M.my-hs</literal>,
766 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
775 <sect1 id="options-help">
776 <title>Help and verbosity options</title>
778 <indexterm><primary>help options</primary></indexterm>
779 <indexterm><primary>verbosity options</primary></indexterm>
781 <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
782 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
787 <indexterm><primary><option>-n</option></primary></indexterm>
790 <para>Does a dry-run, i.e. GHC goes through all the motions
791 of compiling as normal, but does not actually run any
792 external commands.</para>
799 <indexterm><primary><option>-v</option></primary></indexterm>
802 <para>The <option>-v</option> option makes GHC
803 <emphasis>verbose</emphasis>: it reports its version number
804 and shows (on stderr) exactly how it invokes each phase of
805 the compilation system. Moreover, it passes the
806 <option>-v</option> flag to most phases; each reports its
807 version number (and possibly some other information).</para>
809 <para>Please, oh please, use the <option>-v</option> option
810 when reporting bugs! Knowing that you ran the right bits in
811 the right order is always the first thing we want to
818 <option>-v</option><replaceable>n</replaceable>
819 <indexterm><primary><option>-v</option></primary></indexterm>
822 <para>To provide more control over the compiler's verbosity,
823 the <option>-v</option> flag takes an optional numeric
824 argument. Specifying <option>-v</option> on its own is
825 equivalent to <option>-v3</option>, and the other levels
826 have the following meanings:</para>
830 <term><option>-v0</option></term>
832 <para>Disable all non-essential messages (this is the
838 <term><option>-v1</option></term>
840 <para>Minimal verbosity: print one line per
841 compilation (this is the default when
842 <option>––make</option> or
843 <option>––interactive</option> is on).</para>
848 <term><option>-v2</option></term>
850 <para>Print the name of each compilation phase as it
851 is executed. (equivalent to
852 <option>-dshow-passes</option>).</para>
857 <term><option>-v3</option></term>
859 <para>The same as <option>-v2</option>, except that in
860 addition the full command line (if appropriate) for
861 each compilation phase is also printed.</para>
866 <term><option>-v4</option></term>
868 <para>The same as <option>-v3</option> except that the
869 intermediate program representation after each
870 compilation phase is also printed (excluding
871 preprocessed and C/assembly files).</para>
879 <term><option>-ferror-spans</option>
880 <indexterm><primary><option>-ferror-spans</option></primary>
884 <para>Causes GHC to emit the full source span of the
885 syntactic entity relating to an error message. Normally, GHC
886 emits the source location of the start of the syntactic
889 <para>For example:</para>
891 <screen>test.hs:3:6: parse error on input `where'</screen>
893 <para>becomes:</para>
895 <screen>test296.hs:3:6-10: parse error on input `where'</screen>
897 <para>And multi-line spans are possible too:</para>
899 <screen>test.hs:(5,4)-(6,7):
900 Conflicting definitions for `a'
901 Bound at: test.hs:5:4
903 In the binding group for: a, b, a</screen>
905 <para>Note that line numbers start counting at one, but
906 column numbers start at zero. This choice was made to
907 follow existing convention (i.e. this is how Emacs does
913 <term><option>-H</option><replaceable>size</replaceable>
914 <indexterm><primary><option>-H</option></primary></indexterm>
917 <para>Set the minimum size of the heap to
918 <replaceable>size</replaceable>.
919 This option is equivalent to
920 <literal>+RTS -H<replaceable>size</replaceable></literal>,
921 see <xref linkend="rts-options-gc" />.
927 <term><option>-Rghc-timing</option>
928 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
931 <para>Prints a one-line summary of timing statistics for the
932 GHC run. This option is equivalent to
933 <literal>+RTS -tstderr</literal>, see <xref
934 linkend="rts-options-gc" />.
943 <sect1 id="options-sanity">
944 <title>Warnings and sanity-checking</title>
946 <indexterm><primary>sanity-checking options</primary></indexterm>
947 <indexterm><primary>warnings</primary></indexterm>
950 <para>GHC has a number of options that select which types of
951 non-fatal error messages, otherwise known as warnings, can be
952 generated during compilation. By default, you get a standard set
953 of warnings which are generally likely to indicate bugs in your
955 <option>-fwarn-overlapping-patterns</option>,
956 <option>-fwarn-warnings-deprecations</option>,
957 <option>-fwarn-deprecated-flags</option>,
958 <option>-fwarn-duplicate-exports</option>,
959 <option>-fwarn-missing-fields</option>,
960 <option>-fwarn-missing-methods</option>,
961 <option>-fwarn-lazy-unlifted-bindings</option>,
962 <option>-fwarn-wrong-do-bind</option>, and
963 <option>-fwarn-dodgy-foreign-imports</option>. The following
965 simple ways to select standard “packages” of warnings:
971 <term><option>-W</option>:</term>
973 <indexterm><primary>-W option</primary></indexterm>
974 <para>Provides the standard warnings plus
975 <option>-fwarn-incomplete-patterns</option>,
976 <option>-fwarn-dodgy-exports</option>,
977 <option>-fwarn-dodgy-imports</option>,
978 <option>-fwarn-unused-matches</option>,
979 <option>-fwarn-unused-imports</option>, and
980 <option>-fwarn-unused-binds</option>.</para>
985 <term><option>-Wall</option>:</term>
987 <indexterm><primary><option>-Wall</option></primary></indexterm>
988 <para>Turns on all warning options that indicate potentially
989 suspicious code. The warnings that are
990 <emphasis>not</emphasis> enabled by <option>-Wall</option>
992 <option>-fwarn-tabs</option>,
993 <option>-fwarn-incomplete-uni-patterns</option>,
994 <option>-fwarn-incomplete-record-updates</option>,
995 <option>-fwarn-monomorphism-restriction</option>,
996 <option>-fwarn-unrecognised-pragmas</option>,
997 <option>-fwarn-auto-orphans</option>,
998 <option>-fwarn-implicit-prelude</option>.</para>
1003 <term><option>-w</option>:</term>
1005 <indexterm><primary><option>-w</option></primary></indexterm>
1006 <para>Turns off all warnings, including the standard ones and
1007 those that <literal>-Wall</literal> doesn't enable.</para>
1012 <term><option>-Werror</option>:</term>
1014 <indexterm><primary><option>-Werror</option></primary></indexterm>
1015 <para>Makes any warning into a fatal error. Useful so that you don't
1016 miss warnings when doing batch compilation. </para>
1021 <term><option>-Wwarn</option>:</term>
1023 <indexterm><primary><option>-Wwarn</option></primary></indexterm>
1024 <para>Warnings are treated only as warnings, not as errors. This is
1025 the default, but can be useful to negate a
1026 <option>-Werror</option> flag.</para>
1032 <para>The full set of warning options is described below. To turn
1033 off any warning, simply give the corresponding
1034 <option>-fno-warn-...</option> option on the command line.</para>
1039 <term><option>-fwarn-unrecognised-pragmas</option>:</term>
1041 <indexterm><primary><option>-fwarn-unrecognised-pragmas</option></primary>
1043 <indexterm><primary>warnings</primary></indexterm>
1044 <indexterm><primary>pragmas</primary></indexterm>
1045 <para>Causes a warning to be emitted when a
1046 pragma that GHC doesn't recognise is used. As well as pragmas
1047 that GHC itself uses, GHC also recognises pragmas known to be used
1048 by other tools, e.g. <literal>OPTIONS_HUGS</literal> and
1049 <literal>DERIVE</literal>.</para>
1051 <para>This option is on by default.</para>
1056 <term><option>-fwarn-warnings-deprecations</option>:</term>
1058 <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
1060 <indexterm><primary>warnings</primary></indexterm>
1061 <indexterm><primary>deprecations</primary></indexterm>
1062 <para>Causes a warning to be emitted when a
1063 module, function or type with a WARNING or DEPRECATED pragma
1064 is used. See <xref linkend="warning-deprecated-pragma"/> for more
1065 details on the pragmas.</para>
1067 <para>This option is on by default.</para>
1072 <term><option>-fwarn-deprecated-flags</option>:</term>
1074 <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
1076 <indexterm><primary>deprecated-flags</primary></indexterm>
1077 <para>Causes a warning to be emitted when a deprecated
1078 commandline flag is used.</para>
1080 <para>This option is on by default.</para>
1085 <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
1087 <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
1089 <para>Causes a warning to be emitted for foreign imports of
1090 the following form:</para>
1092 foreign import "f" f :: FunPtr t
1094 <para>on the grounds that it probably should be</para>
1096 foreign import "&f" f :: FunPtr t
1098 <para>The first form declares that `f` is a (pure) C
1099 function that takes no arguments and returns a pointer to a
1100 C function with type `t`, whereas the second form declares
1101 that `f` itself is a C function with type `t`. The first
1102 declaration is usually a mistake, and one that is hard to
1103 debug because it results in a crash, hence this
1109 <term><option>-fwarn-dodgy-exports</option>:</term>
1111 <indexterm><primary><option>-fwarn-dodgy-exports</option></primary>
1113 <para>Causes a warning to be emitted when a datatype
1114 <literal>T</literal> is exported
1115 with all constructors, i.e. <literal>T(..)</literal>, but is it
1116 just a type synonym.</para>
1117 <para>Also causes a warning to be emitted when a module is
1118 re-exported, but that module exports nothing.</para>
1123 <term><option>-fwarn-dodgy-imports</option>:</term>
1125 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
1127 <para>Causes a warning to be emitted when a datatype
1128 <literal>T</literal> is imported
1129 with all constructors, i.e. <literal>T(..)</literal>, but has been
1130 exported abstractly, i.e. <literal>T</literal>.</para>
1135 <term><option>-fwarn-lazy-unlifted-bindings</option>:</term>
1137 <indexterm><primary><option>-fwarn-lazy-unlifted-bindings</option></primary>
1139 <para>Causes a warning to be emitted when an unlifted type
1140 is bound in a way that looks lazy, e.g.
1141 <literal>where (I# x) = ...</literal>. Use
1142 <literal>where !(I# x) = ...</literal> instead. This will be an
1143 error, rather than a warning, in GHC 7.2.
1149 <term><option>-fwarn-duplicate-exports</option>:</term>
1151 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
1152 <indexterm><primary>duplicate exports, warning</primary></indexterm>
1153 <indexterm><primary>export lists, duplicates</primary></indexterm>
1155 <para>Have the compiler warn about duplicate entries in
1156 export lists. This is useful information if you maintain
1157 large export lists, and want to avoid the continued export
1158 of a definition after you've deleted (one) mention of it in
1159 the export list.</para>
1161 <para>This option is on by default.</para>
1166 <term><option>-fwarn-hi-shadowing</option>:</term>
1168 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
1169 <indexterm><primary>shadowing</primary>
1170 <secondary>interface files</secondary></indexterm>
1172 <para>Causes the compiler to emit a warning when a module or
1173 interface file in the current directory is shadowing one
1174 with the same module name in a library or other
1180 <term><option>-fwarn-identities</option>:</term>
1182 <indexterm><primary><option>-fwarn-identities</option></primary></indexterm>
1183 <para>Causes the compiler to emit a warning when a Prelude numeric
1184 conversion converts a type T to the same type T; such calls
1185 are probably no-ops and can be omitted. The functions checked for
1186 are: <literal>toInteger</literal>,
1187 <literal>toRational</literal>,
1188 <literal>fromIntegral</literal>,
1189 and <literal>realToFrac</literal>.
1195 <term><option>-fwarn-implicit-prelude</option>:</term>
1197 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
1198 <indexterm><primary>implicit prelude, warning</primary></indexterm>
1199 <para>Have the compiler warn if the Prelude is implicitly
1200 imported. This happens unless either the Prelude module is
1201 explicitly imported with an <literal>import ... Prelude ...</literal>
1202 line, or this implicit import is disabled (either by
1203 <option>-XNoImplicitPrelude</option> or a
1204 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
1206 <para>Note that no warning is given for syntax that implicitly
1207 refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
1208 would change whether it refers to the Prelude.
1209 For example, no warning is given when
1210 <literal>368</literal> means
1211 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
1212 (where <literal>Prelude</literal> refers to the actual Prelude module,
1213 regardless of the imports of the module being compiled).</para>
1215 <para>This warning is off by default.</para>
1220 <term><option>-fwarn-incomplete-patterns</option>,
1221 <option>-fwarn-incomplete-uni-patterns</option>:
1224 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
1225 <indexterm><primary><option>-fwarn-incomplete-uni-patterns</option></primary></indexterm>
1226 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
1227 <indexterm><primary>patterns, incomplete</primary></indexterm>
1229 <para>The option <option>-fwarn-incomplete-patterns</option> warns
1231 a pattern-match might fail at runtime.
1233 <function>g</function> below will fail when applied to
1234 non-empty lists, so the compiler will emit a warning about
1235 this when <option>-fwarn-incomplete-patterns</option> is
1240 This option isn't enabled by default because it can be
1241 a bit noisy, and it doesn't always indicate a bug in the
1242 program. However, it's generally considered good practice
1243 to cover all the cases in your functions, and it is switched
1244 on by <option>-W</option>.</para>
1246 <para>The flag <option>-fwarn-incomplete-uni-patterns</option> is
1247 similar, except that it
1248 applies only to lambda-expressions and pattern bindings, constructs
1249 that only allow a single pattern:
1259 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1261 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1262 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1263 <indexterm><primary>record updates, incomplete</primary></indexterm>
1266 <function>f</function> below will fail when applied to
1267 <literal>Bar</literal>, so the compiler will emit a warning about
1268 this when <option>-fwarn-incomplete-record-updates</option> is
1272 data Foo = Foo { x :: Int }
1276 f foo = foo { x = 6 }
1279 <para>This option isn't enabled by default because it can be
1280 very noisy, and it often doesn't indicate a bug in the
1287 <option>-fwarn-missing-fields</option>:
1288 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1289 <indexterm><primary>missing fields, warning</primary></indexterm>
1290 <indexterm><primary>fields, missing</primary></indexterm>
1294 <para>This option is on by default, and warns you whenever
1295 the construction of a labelled field constructor isn't
1296 complete, missing initializers for one or more fields. While
1297 not an error (the missing fields are initialised with
1298 bottoms), it is often an indication of a programmer error.</para>
1304 <option>-fwarn-missing-import-lists</option>:
1305 <indexterm><primary><option>-fwarn-import-lists</option></primary></indexterm>
1306 <indexterm><primary>missing import lists, warning</primary></indexterm>
1307 <indexterm><primary>import lists, missing</primary></indexterm>
1311 <para>This flag warns if you use an unqualified
1312 <literal>import</literal> declaration
1313 that does not explicitly list the entities brought into scope. For
1324 The <option>-fwarn-import-lists</option> flag will warn about the import
1325 of <literal>Y</literal> but not <literal>X</literal>
1326 If module <literal>Y</literal> is later changed to export (say) <literal>f</literal>,
1327 then the reference to <literal>f</literal> in <literal>M</literal> will become
1328 ambiguous. No warning is produced for the import of <literal>Z</literal>
1329 because extending <literal>Z</literal>'s exports would be unlikely to produce
1330 ambiguity in <literal>M</literal>.
1336 <term><option>-fwarn-missing-methods</option>:</term>
1338 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1339 <indexterm><primary>missing methods, warning</primary></indexterm>
1340 <indexterm><primary>methods, missing</primary></indexterm>
1342 <para>This option is on by default, and warns you whenever
1343 an instance declaration is missing one or more methods, and
1344 the corresponding class declaration has no default
1345 declaration for them.</para>
1346 <para>The warning is suppressed if the method name
1347 begins with an underscore. Here's an example where this is useful:
1350 _simpleFn :: a -> String
1351 complexFn :: a -> a -> String
1352 complexFn x y = ... _simpleFn ...
1354 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1355 never <literal>_simpleFn</literal>; and (b)
1356 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1362 <term><option>-fwarn-missing-signatures</option>:</term>
1364 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1365 <indexterm><primary>type signatures, missing</primary></indexterm>
1367 <para>If you would like GHC to check that every top-level
1368 function/value has a type signature, use the
1369 <option>-fwarn-missing-signatures</option> option. As part of
1370 the warning GHC also reports the inferred type. The
1371 option is off by default.</para>
1376 <term><option>-fwarn-missing-local-sigs</option>:</term>
1378 <indexterm><primary><option>-fwarn-missing-local-sigs</option></primary></indexterm>
1379 <indexterm><primary>type signatures, missing</primary></indexterm>
1381 <para>If you use the
1382 <option>-fwarn-missing-local-sigs</option> flag GHC will warn
1383 you about any polymorphic local bindings. As part of
1384 the warning GHC also reports the inferred type. The
1385 option is off by default.</para>
1390 <term><option>-fwarn-name-shadowing</option>:</term>
1392 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1393 <indexterm><primary>shadowing, warning</primary></indexterm>
1395 <para>This option causes a warning to be emitted whenever an
1396 inner-scope value has the same name as an outer-scope value,
1397 i.e. the inner value shadows the outer one. This can catch
1398 typographical errors that turn into hard-to-find bugs, e.g.,
1399 in the inadvertent capture of what would be a recursive call in
1400 <literal>f = ... let f = id in ... f ...</literal>.</para>
1401 <para>The warning is suppressed for names beginning with an underscore. For example
1403 f x = do { _ignore <- this; _ignore <- that; return (the other) }
1410 <term><option>-fwarn-orphans</option>:</term>
1412 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1413 <indexterm><primary>orphan instances, warning</primary></indexterm>
1414 <indexterm><primary>orphan rules, warning</primary></indexterm>
1416 <para>This option causes a warning to be emitted whenever the
1417 module contains an "orphan" instance declaration or rewrite rule.
1418 An instance declaration is an orphan if it appears in a module in
1419 which neither the class nor the type being instanced are declared
1420 in the same module. A rule is an orphan if it is a rule for a
1421 function declared in another module. A module containing any
1422 orphans is called an orphan module.</para>
1423 <para>The trouble with orphans is that GHC must pro-actively read the interface
1424 files for all orphan modules, just in case their instances or rules
1425 play a role, whether or not the module's interface would otherwise
1426 be of any use. See <xref linkend="orphan-modules"/> for details.
1433 <option>-fwarn-overlapping-patterns</option>:
1434 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1435 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1436 <indexterm><primary>patterns, overlapping</primary></indexterm>
1439 <para>By default, the compiler will warn you if a set of
1440 patterns are overlapping, e.g.,</para>
1443 f :: String -> Int
1449 <para>where the last pattern match in <function>f</function>
1450 won't ever be reached, as the second pattern overlaps
1451 it. More often than not, redundant patterns is a programmer
1452 mistake/error, so this option is enabled by default.</para>
1457 <term><option>-fwarn-tabs</option>:</term>
1459 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1460 <indexterm><primary>tabs, warning</primary></indexterm>
1461 <para>Have the compiler warn if there are tabs in your source
1464 <para>This warning is off by default.</para>
1469 <term><option>-fwarn-type-defaults</option>:</term>
1471 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1472 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1473 <para>Have the compiler warn/inform you where in your source
1474 the Haskell defaulting mechanism for numeric types kicks
1475 in. This is useful information when converting code from a
1476 context that assumed one default into one with another,
1477 e.g., the ‘default default’ for Haskell 1.4 caused the
1478 otherwise unconstrained value <constant>1</constant> to be
1479 given the type <literal>Int</literal>, whereas Haskell 98
1481 defaults it to <literal>Integer</literal>. This may lead to
1482 differences in performance and behaviour, hence the
1483 usefulness of being non-silent about this.</para>
1485 <para>This warning is off by default.</para>
1490 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1492 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1493 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1494 <para>Have the compiler warn/inform you where in your source
1495 the Haskell Monomorphism Restriction is applied. If applied silently
1496 the MR can give rise to unexpected behaviour, so it can be helpful
1497 to have an explicit warning that it is being applied.</para>
1499 <para>This warning is off by default.</para>
1504 <term><option>-fwarn-unused-binds</option>:</term>
1506 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1507 <indexterm><primary>unused binds, warning</primary></indexterm>
1508 <indexterm><primary>binds, unused</primary></indexterm>
1509 <para>Report any function definitions (and local bindings)
1510 which are unused. For top-level functions, the warning is
1511 only given if the binding is not exported.</para>
1512 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1513 mentioned in the right hand side of another definition that is used, or (c) the
1514 function it defines begins with an underscore. The last case provides a
1515 way to suppress unused-binding warnings selectively. </para>
1516 <para> Notice that a variable
1517 is reported as unused even if it appears in the right-hand side of another
1518 unused binding. </para>
1523 <term><option>-fwarn-unused-imports</option>:</term>
1525 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1526 <indexterm><primary>unused imports, warning</primary></indexterm>
1527 <indexterm><primary>imports, unused</primary></indexterm>
1529 <para>Report any modules that are explicitly imported but
1530 never used. However, the form <literal>import M()</literal> is
1531 never reported as an unused import, because it is a useful idiom
1532 for importing instance declarations, which are anonymous in Haskell.</para>
1537 <term><option>-fwarn-unused-matches</option>:</term>
1539 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1540 <indexterm><primary>unused matches, warning</primary></indexterm>
1541 <indexterm><primary>matches, unused</primary></indexterm>
1543 <para>Report all unused variables which arise from pattern
1544 matches, including patterns consisting of a single variable.
1545 For instance <literal>f x y = []</literal> would report
1546 <varname>x</varname> and <varname>y</varname> as unused. The
1547 warning is suppressed if the variable name begins with an underscore, thus:
1556 <term><option>-fwarn-unused-do-bind</option>:</term>
1558 <indexterm><primary><option>-fwarn-unused-do-bind</option></primary></indexterm>
1559 <indexterm><primary>unused do binding, warning</primary></indexterm>
1560 <indexterm><primary>do binding, unused</primary></indexterm>
1562 <para>Report expressions occuring in <literal>do</literal> and <literal>mdo</literal> blocks
1563 that appear to silently throw information away.
1564 For instance <literal>do { mapM popInt xs ; return 10 }</literal> would report
1565 the first statement in the <literal>do</literal> block as suspicious,
1566 as it has the type <literal>StackM [Int]</literal> and not <literal>StackM ()</literal>, but that
1567 <literal>[Int]</literal> value is not bound to anything. The warning is suppressed by
1568 explicitly mentioning in the source code that your program is throwing something away:
1570 do { _ <- mapM popInt xs ; return 10 }
1572 Of course, in this particular situation you can do even better:
1574 do { mapM_ popInt xs ; return 10 }
1581 <term><option>-fwarn-wrong-do-bind</option>:</term>
1583 <indexterm><primary><option>-fwarn-wrong-do-bind</option></primary></indexterm>
1584 <indexterm><primary>apparently erroneous do binding, warning</primary></indexterm>
1585 <indexterm><primary>do binding, apparently erroneous</primary></indexterm>
1587 <para>Report expressions occuring in <literal>do</literal> and <literal>mdo</literal> blocks
1588 that appear to lack a binding.
1589 For instance <literal>do { return (popInt 10) ; return 10 }</literal> would report
1590 the first statement in the <literal>do</literal> block as suspicious,
1591 as it has the type <literal>StackM (StackM Int)</literal> (which consists of two nested applications
1592 of the same monad constructor), but which is not then "unpacked" by binding the result.
1593 The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away:
1595 do { _ <- return (popInt 10) ; return 10 }
1597 For almost all sensible programs this will indicate a bug, and you probably intended to write:
1599 do { popInt 10 ; return 10 }
1607 <para>If you're feeling really paranoid, the
1608 <option>-dcore-lint</option>
1609 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1610 is a good choice. It turns on heavyweight intra-pass
1611 sanity-checking within GHC. (It checks GHC's sanity, not
1618 <sect1 id="options-optimise">
1619 <title>Optimisation (code improvement)</title>
1621 <indexterm><primary>optimisation</primary></indexterm>
1622 <indexterm><primary>improvement, code</primary></indexterm>
1624 <para>The <option>-O*</option> options specify convenient
1625 “packages” of optimisation flags; the
1626 <option>-f*</option> options described later on specify
1627 <emphasis>individual</emphasis> optimisations to be turned on/off;
1628 the <option>-m*</option> options specify
1629 <emphasis>machine-specific</emphasis> optimisations to be turned
1632 <sect2 id="optimise-pkgs">
1633 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1635 <para>There are <emphasis>many</emphasis> options that affect
1636 the quality of code produced by GHC. Most people only have a
1637 general goal, something like “Compile quickly” or
1638 “Make my program run like greased lightning.” The
1639 following “packages” of optimisations (or lack
1640 thereof) should suffice.</para>
1642 <para>Note that higher optimisation levels cause more
1643 cross-module optimisation to be performed, which can have an
1644 impact on how much of your program needs to be recompiled when
1645 you change something. This is one reason to stick to
1646 no-optimisation when developing code.</para>
1652 No <option>-O*</option>-type option specified:
1653 <indexterm><primary>-O* not specified</primary></indexterm>
1656 <para>This is taken to mean: “Please compile
1657 quickly; I'm not over-bothered about compiled-code
1658 quality.” So, for example: <command>ghc -c
1659 Foo.hs</command></para>
1665 <option>-O0</option>:
1666 <indexterm><primary><option>-O0</option></primary></indexterm>
1669 <para>Means “turn off all optimisation”,
1670 reverting to the same settings as if no
1671 <option>-O</option> options had been specified. Saying
1672 <option>-O0</option> can be useful if
1673 eg. <command>make</command> has inserted a
1674 <option>-O</option> on the command line already.</para>
1680 <option>-O</option> or <option>-O1</option>:
1681 <indexterm><primary>-O option</primary></indexterm>
1682 <indexterm><primary>-O1 option</primary></indexterm>
1683 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1686 <para>Means: “Generate good-quality code without
1687 taking too long about it.” Thus, for example:
1688 <command>ghc -c -O Main.lhs</command></para>
1694 <option>-O2</option>:
1695 <indexterm><primary>-O2 option</primary></indexterm>
1696 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1699 <para>Means: “Apply every non-dangerous
1700 optimisation, even if it means significantly longer
1701 compile times.”</para>
1703 <para>The avoided “dangerous” optimisations
1704 are those that can make runtime or space
1705 <emphasis>worse</emphasis> if you're unlucky. They are
1706 normally turned on or off individually.</para>
1708 <para>At the moment, <option>-O2</option> is
1709 <emphasis>unlikely</emphasis> to produce better code than
1710 <option>-O</option>.</para>
1715 <para>We don't use a <option>-O*</option> flag for day-to-day
1716 work. We use <option>-O</option> to get respectable speed;
1717 e.g., when we want to measure something. When we want to go for
1718 broke, we tend to use <option>-O2</option> (and we go for
1719 lots of coffee breaks).</para>
1721 <para>The easiest way to see what <option>-O</option> (etc.)
1722 “really mean” is to run with <option>-v</option>,
1723 then stand back in amazement.</para>
1726 <sect2 id="options-f">
1727 <title><option>-f*</option>: platform-independent flags</title>
1729 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1730 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1732 <para>These flags turn on and off individual optimisations.
1733 They are normally set via the <option>-O</option> options
1734 described above, and as such, you shouldn't need to set any of
1735 them explicitly (indeed, doing so could lead to unexpected
1736 results). However, there are one or two that may be of
1741 <term><option>-fexcess-precision</option>:</term>
1743 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1744 <para>When this option is given, intermediate floating
1745 point values can have a <emphasis>greater</emphasis>
1746 precision/range than the final type. Generally this is a
1747 good thing, but some programs may rely on the exact
1749 <literal>Float</literal>/<literal>Double</literal> values
1750 and should not use this option for their compilation.</para>
1755 <term><option>-fignore-asserts</option>:</term>
1757 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1758 <para>Causes GHC to ignore uses of the function
1759 <literal>Exception.assert</literal> in source code (in
1760 other words, rewriting <literal>Exception.assert p
1761 e</literal> to <literal>e</literal> (see <xref
1762 linkend="assertions"/>). This flag is turned on by
1763 <option>-O</option>.
1770 <option>-fno-cse</option>
1771 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1774 <para>Turns off the common-sub-expression elimination optimisation.
1775 Can be useful if you have some <literal>unsafePerformIO</literal>
1776 expressions that you don't want commoned-up.</para>
1782 <option>-fno-strictness</option>
1783 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1786 <para>Turns off the strictness analyser; sometimes it eats
1787 too many cycles.</para>
1793 <option>-fno-full-laziness</option>
1794 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1797 <para>Turns off the full laziness optimisation (also known as
1798 let-floating). Full laziness increases sharing, which can lead
1799 to increased memory residency.</para>
1801 <para>NOTE: GHC doesn't implement complete full-laziness.
1802 When optimisation in on, and
1803 <option>-fno-full-laziness</option> is not given, some
1804 transformations that increase sharing are performed, such
1805 as extracting repeated computations from a loop. These
1806 are the same transformations that a fully lazy
1807 implementation would do, the difference is that GHC
1808 doesn't consistently apply full-laziness, so don't rely on
1815 <option>-fno-float-in</option>
1816 <indexterm><primary><option>-fno-float-in</option></primary></indexterm>
1819 <para>Turns off the float-in transformation.</para>
1825 <option>-fno-specialise</option>
1826 <indexterm><primary><option>-fno-specialise</option></primary></indexterm>
1829 <para>Turns off the automatic specialisation of overloaded functions.</para>
1835 <option>-fspec-constr</option>
1836 <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
1839 <para>Turn on call-pattern specialisation.</para>
1845 <option>-fliberate-case</option>
1846 <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
1849 <para>Turn on the liberate-case transformation.</para>
1855 <option>-fstatic-argument-transformation</option>
1856 <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
1859 <para>Turn on the static argument transformation.</para>
1865 <option>-fno-state-hack</option>
1866 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1869 <para>Turn off the "state hack" whereby any lambda with a
1870 <literal>State#</literal> token as argument is considered to be
1871 single-entry, hence it is considered OK to inline things inside
1872 it. This can improve performance of IO and ST monad code, but it
1873 runs the risk of reducing sharing.</para>
1879 <option>-fomit-interface-pragmas</option>
1880 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
1883 <para>Tells GHC to omit all inessential information from the interface file
1884 generated for the module being compiled (say M). This means that a module
1885 importing M will see only the <emphasis>types</emphasis> of the functions that M exports, but not
1886 their unfoldings, strictness info, etc. Hence, for example,
1887 no function exported by M will be inlined
1888 into an importing module. The benefit is that modules that import M will
1889 need to be recompiled less often (only when M's exports change their type,
1890 not when they change their implementation).
1897 <option>-fignore-interface-pragmas</option>
1898 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
1901 <para>Tells GHC to ignore all inessential information when reading interface files.
1902 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
1903 for a function, GHC will ignore that information.</para>
1909 <option>-funbox-strict-fields</option>:
1910 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1911 <indexterm><primary>strict constructor fields</primary></indexterm>
1912 <indexterm><primary>constructor fields, strict</primary></indexterm>
1915 <para>This option causes all constructor fields which are
1916 marked strict (i.e. “!”) to be unboxed or
1917 unpacked if possible. It is equivalent to adding an
1918 <literal>UNPACK</literal> pragma to every strict
1919 constructor field (see <xref
1920 linkend="unpack-pragma"/>).</para>
1922 <para>This option is a bit of a sledgehammer: it might
1923 sometimes make things worse. Selectively unboxing fields
1924 by using <literal>UNPACK</literal> pragmas might be
1931 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
1932 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1933 <indexterm><primary>inlining, controlling</primary></indexterm>
1934 <indexterm><primary>unfolding, controlling</primary></indexterm>
1937 <para>(Default: 45) Governs the maximum size that GHC will
1938 allow a function unfolding to be. (An unfolding has a
1939 “size” that reflects the cost in terms of
1940 “code bloat” of expanding that unfolding at
1941 at a call site. A bigger function would be assigned a
1942 bigger cost.) </para>
1944 <para> Consequences: (a) nothing larger than this will be
1945 inlined (unless it has an INLINE pragma); (b) nothing
1946 larger than this will be spewed into an interface
1950 <para> Increasing this figure is more likely to result in longer
1951 compile times than faster code. The next option is more
1957 <term><option>-funfolding-use-threshold=<replaceable>n</replaceable></option></term>
1959 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1960 <indexterm><primary>inlining, controlling</primary></indexterm>
1961 <indexterm><primary>unfolding, controlling</primary></indexterm>
1963 <para>(Default: 8) This is the magic cut-off figure for
1964 unfolding: below this size, a function definition will be
1965 unfolded at the call-site, any bigger and it won't. The
1966 size computed for a function depends on two things: the
1967 actual size of the expression minus any discounts that
1968 apply (see <option>-funfolding-con-discount</option>).</para>
1981 <sect1 id="using-concurrent">
1982 <title>Using Concurrent Haskell</title>
1983 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
1985 <para>GHC supports Concurrent Haskell by default, without requiring a
1986 special option or libraries compiled in a certain way. To get access to
1987 the support libraries for Concurrent Haskell, just import
1989 url="&libraryBaseLocation;/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
1991 <para>The following RTS option(s) affect the behaviour of Concurrent
1992 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
1996 <term><option>-C<replaceable>s</replaceable></option></term>
1998 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1999 Sets the context switch interval to <replaceable>s</replaceable>
2000 seconds. A context switch will occur at the next heap block
2001 allocation after the timer expires (a heap block allocation occurs
2002 every 4k of allocation). With <option>-C0</option> or
2003 <option>-C</option>, context switches will occur as often as
2004 possible (at every heap block allocation). By default, context
2005 switches occur every 20ms.</para>
2011 <sect1 id="using-smp">
2012 <title>Using SMP parallelism</title>
2013 <indexterm><primary>parallelism</primary>
2015 <indexterm><primary>SMP</primary>
2018 <para>GHC supports running Haskell programs in parallel on an SMP
2019 (symmetric multiprocessor).</para>
2021 <para>There's a fine distinction between
2022 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
2023 parallelism is all about making your program run
2024 <emphasis>faster</emphasis> by making use of multiple processors
2025 simultaneously. Concurrency, on the other hand, is a means of
2026 abstraction: it is a convenient way to structure a program that must
2027 respond to multiple asynchronous events.</para>
2029 <para>However, the two terms are certainly related. By making use of
2030 multiple CPUs it is possible to run concurrent threads in parallel,
2031 and this is exactly what GHC's SMP parallelism support does. But it
2032 is also possible to obtain performance improvements with parallelism
2033 on programs that do not use concurrency. This section describes how to
2034 use GHC to compile and run parallel programs, in <xref
2035 linkend="lang-parallel" /> we describe the language features that affect
2038 <sect2 id="parallel-compile-options">
2039 <title>Compile-time options for SMP parallelism</title>
2041 <para>In order to make use of multiple CPUs, your program must be
2042 linked with the <option>-threaded</option> option (see <xref
2043 linkend="options-linker" />). Additionally, the following
2044 compiler options affect parallelism:</para>
2048 <term><option>-feager-blackholing</option></term>
2049 <indexterm><primary><option>-feager-blackholing</option></primary></indexterm>
2052 Blackholing is the act of marking a thunk (lazy
2053 computuation) as being under evaluation. It is useful for
2054 three reasons: firstly it lets us detect certain kinds of
2055 infinite loop (the <literal>NonTermination</literal>
2056 exception), secondly it avoids certain kinds of space
2057 leak, and thirdly it avoids repeating a computation in a
2058 parallel program, because we can tell when a computation
2059 is already in progress.</para>
2062 The option <option>-feager-blackholing</option> causes
2063 each thunk to be blackholed as soon as evaluation begins.
2064 The default is "lazy blackholing", whereby thunks are only
2065 marked as being under evaluation when a thread is paused
2066 for some reason. Lazy blackholing is typically more
2067 efficient (by 1-2% or so), because most thunks don't
2068 need to be blackholed. However, eager blackholing can
2069 avoid more repeated computation in a parallel program, and
2070 this often turns out to be important for parallelism.
2074 We recommend compiling any code that is intended to be run
2075 in parallel with the <option>-feager-blackholing</option>
2083 <sect2 id="parallel-options">
2084 <title>RTS options for SMP parallelism</title>
2086 <para>To run a program on multiple CPUs, use the
2087 RTS <option>-N</option> option:</para>
2091 <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
2093 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2094 Use <replaceable>x</replaceable> simultaneous threads when
2095 running the program. Normally <replaceable>x</replaceable>
2096 should be chosen to match the number of CPU cores on the
2097 machine<footnote><para>Whether hyperthreading cores should be counted or not is an
2098 open question; please feel free to experiment and let us know what
2099 results you find.</para></footnote>. For example,
2100 on a dual-core machine we would probably use
2101 <literal>+RTS -N2 -RTS</literal>.</para>
2103 <para>Omitting <replaceable>x</replaceable>,
2104 i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
2105 choose the value of <replaceable>x</replaceable> itself
2106 based on how many processors are in your machine.</para>
2108 <para>Be careful when using all the processors in your
2109 machine: if some of your processors are in use by other
2110 programs, this can actually harm performance rather than
2113 <para>Setting <option>-N</option> also has the effect of
2114 enabling the parallel garbage collector (see
2115 <xref linkend="rts-options-gc" />).</para>
2117 <para>There is no means (currently) by which this value
2118 may vary after the program has started.</para>
2120 <para>The current value of the <option>-N</option> option
2121 is available to the Haskell program
2122 via <literal>GHC.Conc.numCapabilities</literal>.</para>
2127 <para>The following options affect the way the runtime schedules
2128 threads on CPUs:</para>
2132 <term><option>-qa</option></term>
2133 <indexterm><primary><option>-qa</option></primary><secondary>RTS
2134 option</secondary></indexterm>
2136 <para>Use the OS's affinity facilities to try to pin OS
2137 threads to CPU cores. This is an experimental feature,
2138 and may or may not be useful. Please let us know
2139 whether it helps for you!</para>
2143 <term><option>-qm</option></term>
2144 <indexterm><primary><option>-qm</option></primary><secondary>RTS
2145 option</secondary></indexterm>
2147 <para>Disable automatic migration for load balancing.
2148 Normally the runtime will automatically try to schedule
2149 threads across the available CPUs to make use of idle
2150 CPUs; this option disables that behaviour. Note that
2151 migration only applies to threads; sparks created
2152 by <literal>par</literal> are load-balanced separately
2153 by work-stealing.</para>
2156 This option is probably only of use for concurrent
2157 programs that explicitly schedule threads onto CPUs
2158 with <literal>GHC.Conc.forkOnIO</literal>.
2166 <title>Hints for using SMP parallelism</title>
2168 <para>Add the <literal>-s</literal> RTS option when
2169 running the program to see timing stats, which will help to tell you
2170 whether your program got faster by using more CPUs or not. If the user
2171 time is greater than
2172 the elapsed time, then the program used more than one CPU. You should
2173 also run the program without <literal>-N</literal> for
2176 <para>The output of <literal>+RTS -s</literal> tells you how
2177 many “sparks” were created and executed during the
2178 run of the program (see <xref linkend="rts-options-gc" />), which
2179 will give you an idea how well your <literal>par</literal>
2180 annotations are working.</para>
2182 <para>GHC's parallelism support has improved in 6.12.1 as a
2183 result of much experimentation and tuning in the runtime
2184 system. We'd still be interested to hear how well it works
2185 for you, and we're also interested in collecting parallel
2186 programs to add to our benchmarking suite.</para>
2190 <sect1 id="options-platform">
2191 <title>Platform-specific Flags</title>
2193 <indexterm><primary>-m* options</primary></indexterm>
2194 <indexterm><primary>platform-specific options</primary></indexterm>
2195 <indexterm><primary>machine-specific options</primary></indexterm>
2197 <para>Some flags only make sense for particular target
2203 <term><option>-msse2</option>:</term>
2206 (x86 only, added in GHC 7.0.1) Use the SSE2 registers and
2207 instruction set to implement floating point operations
2208 when using the native code generator. This gives a
2209 substantial performance improvement for floating point,
2210 but the resulting compiled code will only run on
2211 processors that support SSE2 (Intel Pentium 4 and later,
2212 or AMD Athlon 64 and later).
2215 SSE2 is unconditionally used on x86-64 platforms.
2226 <sect1 id="ext-core">
2227 <title>Generating and compiling External Core Files</title>
2229 <indexterm><primary>intermediate code generation</primary></indexterm>
2231 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
2232 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
2233 <filename>.hcr</filename>. The Core format is described in <ulink url="../../core.pdf">
2234 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
2236 for manipulating Core files (in Haskell) are available in the
2237 <ulink url="http://hackage.haskell.org/package/extcore">extcore package on Hackage</ulink>. Note that the format of <literal>.hcr</literal>
2238 files is <emphasis>different</emphasis> from the Core output format that GHC generates
2239 for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
2241 <para>The Core format natively supports notes which you can add to
2242 your source code using the <literal>CORE</literal> pragma (see <xref
2243 linkend="pragmas"/>).</para>
2249 <option>-fext-core</option>
2250 <indexterm><primary><option>-fext-core</option></primary></indexterm>
2253 <para>Generate <literal>.hcr</literal> files.</para>
2259 <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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