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>.s</filename></term>
273 <para>An assembly-language source file, usually produced by
279 <term><filename>.o</filename></term>
281 <para>An object file, produced by an assembler.</para>
286 <para>Files with other suffixes (or without suffixes) are passed
287 straight to the linker.</para>
292 <title>Modes of operation</title>
295 GHC's behaviour is firstly controlled by a mode flag. Only one
296 of these flags may be given, but it does not necessarily need to
297 be the first option on the command-line.
301 If no mode flag is present, then GHC will enter make mode
302 (<xref linkend="make-mode" />) if there are any Haskell source
303 files given on the command line, or else it will link the
304 objects named on the command line to produce an executable.
307 <para>The available mode flags are:</para>
312 <cmdsynopsis><command>ghc --interactive</command>
314 <indexterm><primary>interactive mode</primary></indexterm>
315 <indexterm><primary>ghci</primary></indexterm>
318 <para>Interactive mode, which is also available as
319 <command>ghci</command>. Interactive mode is described in
320 more detail in <xref linkend="ghci"/>.</para>
326 <cmdsynopsis><command>ghc ––make</command>
328 <indexterm><primary>make mode</primary></indexterm>
329 <indexterm><primary><option>––make</option></primary></indexterm>
332 <para>In this mode, GHC will build a multi-module Haskell
333 program automatically, figuring out dependencies for itself.
334 If you have a straightforward Haskell program, this is
335 likely to be much easier, and faster, than using
336 <command>make</command>. Make mode is described in <xref
337 linkend="make-mode"/>.</para>
340 This mode is the default if there are any Haskell
341 source files mentioned on the command line, and in this case
342 the <option>––make</option> option can be omitted.
349 <cmdsynopsis><command>ghc -e</command>
350 <arg choice='plain'><replaceable>expr</replaceable></arg>
352 <indexterm><primary>eval mode</primary></indexterm>
355 <para>Expression-evaluation mode. This is very similar to
356 interactive mode, except that there is a single expression
357 to evaluate (<replaceable>expr</replaceable>) which is given
358 on the command line. See <xref linkend="eval-mode"/> for
366 <command>ghc -E</command>
367 <command>ghc -c</command>
368 <command>ghc -S</command>
369 <command>ghc -c</command>
371 <indexterm><primary><option>-E</option></primary></indexterm>
372 <indexterm><primary><option>-C</option></primary></indexterm>
373 <indexterm><primary><option>-S</option></primary></indexterm>
374 <indexterm><primary><option>-c</option></primary></indexterm>
377 <para>This is the traditional batch-compiler mode, in which
378 GHC can compile source files one at a time, or link objects
379 together into an executable. This mode also applies if
380 there is no other mode flag specified on the command line,
381 in which case it means that the specified files should be
382 compiled and then linked to form a program. See <xref
383 linkend="options-order"/>.</para>
390 <command>ghc -M</command>
392 <indexterm><primary>dependency-generation mode</primary></indexterm>
395 <para>Dependency-generation mode. In this mode, GHC can be
396 used to generate dependency information suitable for use in
397 a <literal>Makefile</literal>. See <xref
398 linkend="makefile-dependencies"/>.</para>
405 <command>ghc --mk-dll</command>
407 <indexterm><primary>DLL-creation mode</primary></indexterm>
410 <para>DLL-creation mode (Windows only). See <xref
411 linkend="win32-dlls-create"/>.</para>
418 <command>ghc --help</command> <command>ghc -?</command>
420 <indexterm><primary><option>––help</option></primary></indexterm>
423 <para>Cause GHC to spew a long usage message to standard
424 output and then exit.</para>
431 <command>ghc --show-iface <replaceable>file</replaceable></command>
433 <indexterm><primary><option>––--show-iface</option></primary></indexterm>
436 <para>Read the interface in
437 <replaceable>file</replaceable> and dump it as text to
438 <literal>stdout</literal>. For example <literal>ghc --show-iface M.hi</literal>.</para>
445 <command>ghc --supported-extensions</command>
446 <command>ghc --supported-languages</command>
448 <indexterm><primary><option>––supported-extensions</option></primary><primary><option>––supported-languages</option></primary></indexterm>
451 <para>Print the supported language extensions.</para>
458 <command>ghc --info</command>
460 <indexterm><primary><option>––info</option></primary></indexterm>
463 <para>Print information about the compiler.</para>
470 <command>ghc --version</command>
471 <command>ghc -V</command>
473 <indexterm><primary><option>-V</option></primary></indexterm>
474 <indexterm><primary><option>––version</option></primary></indexterm>
477 <para>Print a one-line string including GHC's version number.</para>
484 <command>ghc --numeric-version</command>
486 <indexterm><primary><option>––numeric-version</option></primary></indexterm>
489 <para>Print GHC's numeric version number only.</para>
496 <command>ghc --print-libdir</command>
498 <indexterm><primary><option>––print-libdir</option></primary></indexterm>
501 <para>Print the path to GHC's library directory. This is
502 the top of the directory tree containing GHC's libraries,
503 interfaces, and include files (usually something like
504 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
506 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
507 in the package configuration file
508 (see <xref linkend="packages"/>).</para>
514 <sect2 id="make-mode">
515 <title>Using <command>ghc</command> <option>––make</option></title>
516 <indexterm><primary><option>––make</option></primary></indexterm>
517 <indexterm><primary>separate compilation</primary></indexterm>
519 <para>In this mode, GHC will build a multi-module Haskell program by following
520 dependencies from one or more root modules (usually just
521 <literal>Main</literal>). For example, if your
522 <literal>Main</literal> module is in a file called
523 <filename>Main.hs</filename>, you could compile and link the
524 program like this:</para>
527 ghc ––make Main.hs
531 In fact, GHC enters make mode automatically if there are any
532 Haskell source files on the command line and no other mode is
533 specified, so in this case we could just type
540 <para>Any number of source file names or module names may be
541 specified; GHC will figure out all the modules in the program by
542 following the imports from these initial modules. It will then
543 attempt to compile each module which is out of date, and
544 finally, if there is a <literal>Main</literal> module, the
545 program will also be linked into an executable.</para>
547 <para>The main advantages to using <literal>ghc
548 ––make</literal> over traditional
549 <literal>Makefile</literal>s are:</para>
553 <para>GHC doesn't have to be restarted for each compilation,
554 which means it can cache information between compilations.
555 Compiling a multi-module program with <literal>ghc
556 ––make</literal> can be up to twice as fast as
557 running <literal>ghc</literal> individually on each source
561 <para>You don't have to write a <literal>Makefile</literal>.</para>
562 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
565 <para>GHC re-calculates the dependencies each time it is
566 invoked, so the dependencies never get out of sync with the
571 <para>Any of the command-line options described in the rest of
572 this chapter can be used with
573 <option>––make</option>, but note that any options
574 you give on the command line will apply to all the source files
575 compiled, so if you want any options to apply to a single source
576 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
577 pragma (see <xref linkend="source-file-options"/>).</para>
579 <para>If the program needs to be linked with additional objects
580 (say, some auxiliary C code), then the object files can be
581 given on the command line and GHC will include them when linking
582 the executable.</para>
584 <para>Note that GHC can only follow dependencies if it has the
585 source file available, so if your program includes a module for
586 which there is no source file, even if you have an object and an
587 interface file for the module, then GHC will complain. The
588 exception to this rule is for package modules, which may or may
589 not have source files.</para>
591 <para>The source files for the program don't all need to be in
592 the same directory; the <option>-i</option> option can be used
593 to add directories to the search path (see <xref
594 linkend="search-path"/>).</para>
597 <sect2 id="eval-mode">
598 <title>Expression evaluation mode</title>
600 <para>This mode is very similar to interactive mode, except that
601 there is a single expression to evaluate which is specified on
602 the command line as an argument to the <option>-e</option>
606 ghc -e <replaceable>expr</replaceable>
609 <para>Haskell source files may be named on the command line, and
610 they will be loaded exactly as in interactive mode. The
611 expression is evaluated in the context of the loaded
614 <para>For example, to load and run a Haskell program containing
615 a module <literal>Main</literal>, we might say</para>
618 ghc -e Main.main Main.hs
621 <para>or we can just use this mode to evaluate expressions in
622 the context of the <literal>Prelude</literal>:</para>
625 $ ghc -e "interact (unlines.map reverse.lines)"
631 <sect2 id="options-order">
632 <title>Batch compiler mode</title>
634 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
635 given on the command line.</para>
637 <para>The first phase to run is determined by each input-file
638 suffix, and the last phase is determined by a flag. If no
639 relevant flag is present, then go all the way through to linking.
640 This table summarises:</para>
644 <colspec align="left"/>
645 <colspec align="left"/>
646 <colspec align="left"/>
647 <colspec align="left"/>
651 <entry>Phase of the compilation system</entry>
652 <entry>Suffix saying “start here”</entry>
653 <entry>Flag saying “stop after”</entry>
654 <entry>(suffix of) output file</entry>
659 <entry>literate pre-processor</entry>
660 <entry><literal>.lhs</literal></entry>
662 <entry><literal>.hs</literal></entry>
666 <entry>C pre-processor (opt.) </entry>
667 <entry><literal>.hs</literal> (with
668 <option>-cpp</option>)</entry>
669 <entry><option>-E</option></entry>
670 <entry><literal>.hspp</literal></entry>
674 <entry>Haskell compiler</entry>
675 <entry><literal>.hs</literal></entry>
676 <entry><option>-C</option>, <option>-S</option></entry>
677 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
681 <entry>C compiler (opt.)</entry>
682 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
683 <entry><option>-S</option></entry>
684 <entry><literal>.s</literal></entry>
688 <entry>assembler</entry>
689 <entry><literal>.s</literal></entry>
690 <entry><option>-c</option></entry>
691 <entry><literal>.o</literal></entry>
695 <entry>linker</entry>
696 <entry><replaceable>other</replaceable></entry>
698 <entry><filename>a.out</filename></entry>
704 <indexterm><primary><option>-C</option></primary></indexterm>
705 <indexterm><primary><option>-E</option></primary></indexterm>
706 <indexterm><primary><option>-S</option></primary></indexterm>
707 <indexterm><primary><option>-c</option></primary></indexterm>
709 <para>Thus, a common invocation would be: </para>
712 ghc -c Foo.hs</screen>
714 <para>to compile the Haskell source file
715 <filename>Foo.hs</filename> to an object file
716 <filename>Foo.o</filename>.</para>
718 <para>Note: What the Haskell compiler proper produces depends on
719 whether a native-code generator<indexterm><primary>native-code
720 generator</primary></indexterm> is used (producing assembly
721 language) or not (producing C). See <xref
722 linkend="options-codegen"/> for more details.</para>
724 <para>Note: C pre-processing is optional, the
725 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
726 flag turns it on. See <xref linkend="c-pre-processor"/> for more
729 <para>Note: The option <option>-E</option><indexterm><primary>-E
730 option</primary></indexterm> runs just the pre-processing passes
731 of the compiler, dumping the result in a file.</para>
733 <sect3 id="overriding-suffixes">
734 <title>Overriding the default behaviour for a file</title>
736 <para>As described above, the way in which a file is processed by GHC
737 depends on its suffix. This behaviour can be overridden using the
738 <option>-x</option> option:</para>
742 <term><option>-x</option> <replaceable>suffix</replaceable>
743 <indexterm><primary><option>-x</option></primary>
746 <para>Causes all files following this option on the command
747 line to be processed as if they had the suffix
748 <replaceable>suffix</replaceable>. For example, to compile a
749 Haskell module in the file <literal>M.my-hs</literal>,
750 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
759 <sect1 id="options-help">
760 <title>Help and verbosity options</title>
762 <indexterm><primary>help options</primary></indexterm>
763 <indexterm><primary>verbosity options</primary></indexterm>
765 <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
766 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
771 <indexterm><primary><option>-n</option></primary></indexterm>
774 <para>Does a dry-run, i.e. GHC goes through all the motions
775 of compiling as normal, but does not actually run any
776 external commands.</para>
783 <indexterm><primary><option>-v</option></primary></indexterm>
786 <para>The <option>-v</option> option makes GHC
787 <emphasis>verbose</emphasis>: it reports its version number
788 and shows (on stderr) exactly how it invokes each phase of
789 the compilation system. Moreover, it passes the
790 <option>-v</option> flag to most phases; each reports its
791 version number (and possibly some other information).</para>
793 <para>Please, oh please, use the <option>-v</option> option
794 when reporting bugs! Knowing that you ran the right bits in
795 the right order is always the first thing we want to
802 <option>-v</option><replaceable>n</replaceable>
803 <indexterm><primary><option>-v</option></primary></indexterm>
806 <para>To provide more control over the compiler's verbosity,
807 the <option>-v</option> flag takes an optional numeric
808 argument. Specifying <option>-v</option> on its own is
809 equivalent to <option>-v3</option>, and the other levels
810 have the following meanings:</para>
814 <term><option>-v0</option></term>
816 <para>Disable all non-essential messages (this is the
822 <term><option>-v1</option></term>
824 <para>Minimal verbosity: print one line per
825 compilation (this is the default when
826 <option>––make</option> or
827 <option>––interactive</option> is on).</para>
832 <term><option>-v2</option></term>
834 <para>Print the name of each compilation phase as it
835 is executed. (equivalent to
836 <option>-dshow-passes</option>).</para>
841 <term><option>-v3</option></term>
843 <para>The same as <option>-v2</option>, except that in
844 addition the full command line (if appropriate) for
845 each compilation phase is also printed.</para>
850 <term><option>-v4</option></term>
852 <para>The same as <option>-v3</option> except that the
853 intermediate program representation after each
854 compilation phase is also printed (excluding
855 preprocessed and C/assembly files).</para>
863 <term><option>-ferror-spans</option>
864 <indexterm><primary><option>-ferror-spans</option></primary>
868 <para>Causes GHC to emit the full source span of the
869 syntactic entity relating to an error message. Normally, GHC
870 emits the source location of the start of the syntactic
873 <para>For example:</para>
875 <screen>test.hs:3:6: parse error on input `where'</screen>
877 <para>becomes:</para>
879 <screen>test296.hs:3:6-10: parse error on input `where'</screen>
881 <para>And multi-line spans are possible too:</para>
883 <screen>test.hs:(5,4)-(6,7):
884 Conflicting definitions for `a'
885 Bound at: test.hs:5:4
887 In the binding group for: a, b, a</screen>
889 <para>Note that line numbers start counting at one, but
890 column numbers start at zero. This choice was made to
891 follow existing convention (i.e. this is how Emacs does
897 <term><option>-H</option><replaceable>size</replaceable>
898 <indexterm><primary><option>-H</option></primary></indexterm>
901 <para>Set the minimum size of the heap to
902 <replaceable>size</replaceable>.
903 This option is equivalent to
904 <literal>+RTS -H<replaceable>size</replaceable></literal>,
905 see <xref linkend="rts-options-gc" />.
911 <term><option>-Rghc-timing</option>
912 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
915 <para>Prints a one-line summary of timing statistics for the
916 GHC run. This option is equivalent to
917 <literal>+RTS -tstderr</literal>, see <xref
918 linkend="rts-options-gc" />.
927 <sect1 id="options-sanity">
928 <title>Warnings and sanity-checking</title>
930 <indexterm><primary>sanity-checking options</primary></indexterm>
931 <indexterm><primary>warnings</primary></indexterm>
934 <para>GHC has a number of options that select which types of
935 non-fatal error messages, otherwise known as warnings, can be
936 generated during compilation. By default, you get a standard set
937 of warnings which are generally likely to indicate bugs in your
939 <option>-fwarn-overlapping-patterns</option>,
940 <option>-fwarn-warnings-deprecations</option>,
941 <option>-fwarn-deprecated-flags</option>,
942 <option>-fwarn-duplicate-exports</option>,
943 <option>-fwarn-missing-fields</option>,
944 <option>-fwarn-missing-methods</option>,
945 <option>-fwarn-lazy-unlifted-bindings</option>,
946 <option>-fwarn-wrong-do-bind</option>, and
947 <option>-fwarn-dodgy-foreign-imports</option>. The following
949 simple ways to select standard “packages” of warnings:
955 <term><option>-W</option>:</term>
957 <indexterm><primary>-W option</primary></indexterm>
958 <para>Provides the standard warnings plus
959 <option>-fwarn-incomplete-patterns</option>,
960 <option>-fwarn-dodgy-exports</option>,
961 <option>-fwarn-dodgy-imports</option>,
962 <option>-fwarn-unused-matches</option>,
963 <option>-fwarn-unused-imports</option>, and
964 <option>-fwarn-unused-binds</option>.</para>
969 <term><option>-Wall</option>:</term>
971 <indexterm><primary><option>-Wall</option></primary></indexterm>
972 <para>Turns on all warning options that indicate potentially
973 suspicious code. The warnings that are
974 <emphasis>not</emphasis> enabled by <option>-Wall</option>
976 <option>-fwarn-simple-patterns</option>,
977 <option>-fwarn-tabs</option>,
978 <option>-fwarn-incomplete-record-updates</option>,
979 <option>-fwarn-monomorphism-restriction</option>,
980 <option>-fwarn-unused-do-bind</option>, and
981 <option>-fwarn-implicit-prelude</option>.</para>
986 <term><option>-w</option>:</term>
988 <indexterm><primary><option>-w</option></primary></indexterm>
989 <para>Turns off all warnings, including the standard ones and
990 those that <literal>-Wall</literal> doesn't enable.</para>
995 <term><option>-Werror</option>:</term>
997 <indexterm><primary><option>-Werror</option></primary></indexterm>
998 <para>Makes any warning into a fatal error. Useful so that you don't
999 miss warnings when doing batch compilation. </para>
1004 <term><option>-Wwarn</option>:</term>
1006 <indexterm><primary><option>-Wwarn</option></primary></indexterm>
1007 <para>Warnings are treated only as warnings, not as errors. This is
1008 the default, but can be useful to negate a
1009 <option>-Werror</option> flag.</para>
1015 <para>The full set of warning options is described below. To turn
1016 off any warning, simply give the corresponding
1017 <option>-fno-warn-...</option> option on the command line.</para>
1022 <term><option>-fwarn-unrecognised-pragmas</option>:</term>
1024 <indexterm><primary><option>-fwarn-unrecognised-pragmas</option></primary>
1026 <indexterm><primary>warnings</primary></indexterm>
1027 <indexterm><primary>pragmas</primary></indexterm>
1028 <para>Causes a warning to be emitted when a
1029 pragma that GHC doesn't recognise is used. As well as pragmas
1030 that GHC itself uses, GHC also recognises pragmas known to be used
1031 by other tools, e.g. <literal>OPTIONS_HUGS</literal> and
1032 <literal>DERIVE</literal>.</para>
1034 <para>This option is on by default.</para>
1039 <term><option>-fwarn-warnings-deprecations</option>:</term>
1041 <indexterm><primary><option>-fwarn-warnings-deprecations</option></primary>
1043 <indexterm><primary>warnings</primary></indexterm>
1044 <indexterm><primary>deprecations</primary></indexterm>
1045 <para>Causes a warning to be emitted when a
1046 module, function or type with a WARNING or DEPRECATED pragma
1047 is used. See <xref linkend="warning-deprecated-pragma"/> for more
1048 details on the pragmas.</para>
1050 <para>This option is on by default.</para>
1055 <term><option>-fwarn-deprecated-flags</option>:</term>
1057 <indexterm><primary><option>-fwarn-deprecated-flags</option></primary>
1059 <indexterm><primary>deprecated-flags</primary></indexterm>
1060 <para>Causes a warning to be emitted when a deprecated
1061 commandline flag is used.</para>
1063 <para>This option is on by default.</para>
1068 <term><option>-fwarn-dodgy-foreign-imports</option>:</term>
1070 <indexterm><primary><option>-fwarn-dodgy-foreign-imports</option></primary>
1072 <para>Causes a warning to be emitted for foreign imports of
1073 the following form:</para>
1075 foreign import "f" f :: FunPtr t
1077 <para>on the grounds that it probably should be</para>
1079 foreign import "&f" f :: FunPtr t
1081 <para>The first form declares that `f` is a (pure) C
1082 function that takes no arguments and returns a pointer to a
1083 C function with type `t`, whereas the second form declares
1084 that `f` itself is a C function with type `t`. The first
1085 declaration is usually a mistake, and one that is hard to
1086 debug because it results in a crash, hence this
1092 <term><option>-fwarn-dodgy-exports</option>:</term>
1094 <indexterm><primary><option>-fwarn-dodgy-exports</option></primary>
1096 <para>Causes a warning to be emitted when a datatype
1097 <literal>T</literal> is exported
1098 with all constructors, i.e. <literal>T(..)</literal>, but is it
1099 just a type synonym.</para>
1100 <para>Also causes a warning to be emitted when a module is
1101 re-exported, but that module exports nothing.</para>
1106 <term><option>-fwarn-dodgy-imports</option>:</term>
1108 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
1110 <para>Causes a warning to be emitted when a datatype
1111 <literal>T</literal> is imported
1112 with all constructors, i.e. <literal>T(..)</literal>, but has been
1113 exported abstractly, i.e. <literal>T</literal>.</para>
1118 <term><option>-fwarn-lazy-unlifted-bindings</option>:</term>
1120 <indexterm><primary><option>-fwarn-lazy-unlifted-bindings</option></primary>
1122 <para>Causes a warning to be emitted when an unlifted type
1123 is bound in a way that looks lazy, e.g.
1124 <literal>where (I# x) = ...</literal>. Use
1125 <literal>where !(I# x) = ...</literal> instead. This will be an
1126 error, rather than a warning, in GHC 7.2.
1132 <term><option>-fwarn-duplicate-exports</option>:</term>
1134 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
1135 <indexterm><primary>duplicate exports, warning</primary></indexterm>
1136 <indexterm><primary>export lists, duplicates</primary></indexterm>
1138 <para>Have the compiler warn about duplicate entries in
1139 export lists. This is useful information if you maintain
1140 large export lists, and want to avoid the continued export
1141 of a definition after you've deleted (one) mention of it in
1142 the export list.</para>
1144 <para>This option is on by default.</para>
1149 <term><option>-fwarn-hi-shadowing</option>:</term>
1151 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
1152 <indexterm><primary>shadowing</primary>
1153 <secondary>interface files</secondary></indexterm>
1155 <para>Causes the compiler to emit a warning when a module or
1156 interface file in the current directory is shadowing one
1157 with the same module name in a library or other
1163 <term><option>-fwarn-implicit-prelude</option>:</term>
1165 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
1166 <indexterm><primary>implicit prelude, warning</primary></indexterm>
1167 <para>Have the compiler warn if the Prelude is implicitly
1168 imported. This happens unless either the Prelude module is
1169 explicitly imported with an <literal>import ... Prelude ...</literal>
1170 line, or this implicit import is disabled (either by
1171 <option>-XNoImplicitPrelude</option> or a
1172 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
1174 <para>Note that no warning is given for syntax that implicitly
1175 refers to the Prelude, even if <option>-XNoImplicitPrelude</option>
1176 would change whether it refers to the Prelude.
1177 For example, no warning is given when
1178 <literal>368</literal> means
1179 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
1180 (where <literal>Prelude</literal> refers to the actual Prelude module,
1181 regardless of the imports of the module being compiled).</para>
1183 <para>This warning is off by default.</para>
1188 <term><option>-fwarn-incomplete-patterns</option>:</term>
1190 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
1191 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
1192 <indexterm><primary>patterns, incomplete</primary></indexterm>
1194 <para>Similarly for incomplete patterns, the function
1195 <function>g</function> below will fail when applied to
1196 non-empty lists, so the compiler will emit a warning about
1197 this when <option>-fwarn-incomplete-patterns</option> is
1204 <para>This option isn't enabled by default because it can be
1205 a bit noisy, and it doesn't always indicate a bug in the
1206 program. However, it's generally considered good practice
1207 to cover all the cases in your functions.</para>
1212 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1214 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1215 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1216 <indexterm><primary>record updates, incomplete</primary></indexterm>
1219 <function>f</function> below will fail when applied to
1220 <literal>Bar</literal>, so the compiler will emit a warning about
1221 this when <option>-fwarn-incomplete-record-updates</option> is
1225 data Foo = Foo { x :: Int }
1229 f foo = foo { x = 6 }
1232 <para>This option isn't enabled by default because it can be
1233 very noisy, and it often doesn't indicate a bug in the
1240 <option>-fwarn-missing-fields</option>:
1241 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1242 <indexterm><primary>missing fields, warning</primary></indexterm>
1243 <indexterm><primary>fields, missing</primary></indexterm>
1247 <para>This option is on by default, and warns you whenever
1248 the construction of a labelled field constructor isn't
1249 complete, missing initializers for one or more fields. While
1250 not an error (the missing fields are initialised with
1251 bottoms), it is often an indication of a programmer error.</para>
1256 <term><option>-fwarn-missing-methods</option>:</term>
1258 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1259 <indexterm><primary>missing methods, warning</primary></indexterm>
1260 <indexterm><primary>methods, missing</primary></indexterm>
1262 <para>This option is on by default, and warns you whenever
1263 an instance declaration is missing one or more methods, and
1264 the corresponding class declaration has no default
1265 declaration for them.</para>
1266 <para>The warning is suppressed if the method name
1267 begins with an underscore. Here's an example where this is useful:
1270 _simpleFn :: a -> String
1271 complexFn :: a -> a -> String
1272 complexFn x y = ... _simpleFn ...
1274 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1275 never <literal>_simpleFn</literal>; and (b)
1276 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1282 <term><option>-fwarn-missing-signatures</option>:</term>
1284 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1285 <indexterm><primary>type signatures, missing</primary></indexterm>
1287 <para>If you would like GHC to check that every top-level
1288 function/value has a type signature, use the
1289 <option>-fwarn-missing-signatures</option> option. As part of
1290 the warning GHC also reports the inferred type. The
1291 option is off by default.</para>
1296 <term><option>-fwarn-name-shadowing</option>:</term>
1298 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1299 <indexterm><primary>shadowing, warning</primary></indexterm>
1301 <para>This option causes a warning to be emitted whenever an
1302 inner-scope value has the same name as an outer-scope value,
1303 i.e. the inner value shadows the outer one. This can catch
1304 typographical errors that turn into hard-to-find bugs, e.g.,
1305 in the inadvertent capture of what would be a recursive call in
1306 <literal>f = ... let f = id in ... f ...</literal>.</para>
1307 <para>The warning is suppressed for names beginning with an underscore. For example
1309 f x = do { _ignore <- this; _ignore <- that; return (the other) }
1316 <term><option>-fwarn-orphans</option>:</term>
1318 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1319 <indexterm><primary>orphan instances, warning</primary></indexterm>
1320 <indexterm><primary>orphan rules, warning</primary></indexterm>
1322 <para>This option causes a warning to be emitted whenever the
1323 module contains an "orphan" instance declaration or rewrite rule.
1324 An instance declaration is an orphan if it appears in a module in
1325 which neither the class nor the type being instanced are declared
1326 in the same module. A rule is an orphan if it is a rule for a
1327 function declared in another module. A module containing any
1328 orphans is called an orphan module.</para>
1329 <para>The trouble with orphans is that GHC must pro-actively read the interface
1330 files for all orphan modules, just in case their instances or rules
1331 play a role, whether or not the module's interface would otherwise
1332 be of any use. See <xref linkend="orphan-modules"/> for details.
1339 <option>-fwarn-overlapping-patterns</option>:
1340 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1341 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1342 <indexterm><primary>patterns, overlapping</primary></indexterm>
1345 <para>By default, the compiler will warn you if a set of
1346 patterns are overlapping, e.g.,</para>
1349 f :: String -> Int
1355 <para>where the last pattern match in <function>f</function>
1356 won't ever be reached, as the second pattern overlaps
1357 it. More often than not, redundant patterns is a programmer
1358 mistake/error, so this option is enabled by default.</para>
1363 <term><option>-fwarn-simple-patterns</option>:</term>
1365 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1367 <para>Causes the compiler to warn about lambda-bound
1368 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1369 Normally, these aren't treated as incomplete patterns by
1370 <option>-fwarn-incomplete-patterns</option>.</para>
1371 <para>“Lambda-bound patterns” includes all places where there is a single pattern,
1372 including list comprehensions and do-notation. In these cases, a pattern-match
1373 failure is quite legitimate, and triggers filtering (list comprehensions) or
1374 the monad <literal>fail</literal> operation (monads). For example:
1376 f :: [Maybe a] -> [a]
1377 f xs = [y | Just y <- xs]
1379 Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
1380 these probably-innocent cases, which is why the flag is off by default. </para>
1385 <term><option>-fwarn-tabs</option>:</term>
1387 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1388 <indexterm><primary>tabs, warning</primary></indexterm>
1389 <para>Have the compiler warn if there are tabs in your source
1392 <para>This warning is off by default.</para>
1397 <term><option>-fwarn-type-defaults</option>:</term>
1399 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1400 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1401 <para>Have the compiler warn/inform you where in your source
1402 the Haskell defaulting mechanism for numeric types kicks
1403 in. This is useful information when converting code from a
1404 context that assumed one default into one with another,
1405 e.g., the ‘default default’ for Haskell 1.4 caused the
1406 otherwise unconstrained value <constant>1</constant> to be
1407 given the type <literal>Int</literal>, whereas Haskell 98
1408 defaults it to <literal>Integer</literal>. This may lead to
1409 differences in performance and behaviour, hence the
1410 usefulness of being non-silent about this.</para>
1412 <para>This warning is off by default.</para>
1417 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1419 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1420 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1421 <para>Have the compiler warn/inform you where in your source
1422 the Haskell Monomorphism Restriction is applied. If applied silently
1423 the MR can give rise to unexpected behaviour, so it can be helpful
1424 to have an explicit warning that it is being applied.</para>
1426 <para>This warning is off by default.</para>
1431 <term><option>-fwarn-unused-binds</option>:</term>
1433 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1434 <indexterm><primary>unused binds, warning</primary></indexterm>
1435 <indexterm><primary>binds, unused</primary></indexterm>
1436 <para>Report any function definitions (and local bindings)
1437 which are unused. For top-level functions, the warning is
1438 only given if the binding is not exported.</para>
1439 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1440 mentioned in the right hand side of another definition that is used, or (c) the
1441 function it defines begins with an underscore. The last case provides a
1442 way to suppress unused-binding warnings selectively. </para>
1443 <para> Notice that a variable
1444 is reported as unused even if it appears in the right-hand side of another
1445 unused binding. </para>
1450 <term><option>-fwarn-unused-imports</option>:</term>
1452 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1453 <indexterm><primary>unused imports, warning</primary></indexterm>
1454 <indexterm><primary>imports, unused</primary></indexterm>
1456 <para>Report any modules that are explicitly imported but
1457 never used. However, the form <literal>import M()</literal> is
1458 never reported as an unused import, because it is a useful idiom
1459 for importing instance declarations, which are anonymous in Haskell.</para>
1464 <term><option>-fwarn-unused-matches</option>:</term>
1466 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1467 <indexterm><primary>unused matches, warning</primary></indexterm>
1468 <indexterm><primary>matches, unused</primary></indexterm>
1470 <para>Report all unused variables which arise from pattern
1471 matches, including patterns consisting of a single variable.
1472 For instance <literal>f x y = []</literal> would report
1473 <varname>x</varname> and <varname>y</varname> as unused. The
1474 warning is suppressed if the variable name begins with an underscore, thus:
1483 <term><option>-fwarn-unused-do-bind</option>:</term>
1485 <indexterm><primary><option>-fwarn-unused-do-bind</option></primary></indexterm>
1486 <indexterm><primary>unused do binding, warning</primary></indexterm>
1487 <indexterm><primary>do binding, unused</primary></indexterm>
1489 <para>Report expressions occuring in <literal>do</literal> and <literal>mdo</literal> blocks
1490 that appear to silently throw information away.
1491 For instance <literal>do { mapM popInt xs ; return 10 }</literal> would report
1492 the first statement in the <literal>do</literal> block as suspicious,
1493 as it has the type <literal>StackM [Int]</literal> and not <literal>StackM ()</literal>, but that
1494 <literal>[Int]</literal> value is not bound to anything. The warning is suppressed by
1495 explicitly mentioning in the source code that your program is throwing something away:
1497 do { _ <- mapM popInt xs ; return 10 }
1499 Of course, in this particular situation you can do even better:
1501 do { mapM_ popInt xs ; return 10 }
1508 <term><option>-fwarn-wrong-do-bind</option>:</term>
1510 <indexterm><primary><option>-fwarn-wrong-do-bind</option></primary></indexterm>
1511 <indexterm><primary>apparently erroneous do binding, warning</primary></indexterm>
1512 <indexterm><primary>do binding, apparently erroneous</primary></indexterm>
1514 <para>Report expressions occuring in <literal>do</literal> and <literal>mdo</literal> blocks
1515 that appear to lack a binding.
1516 For instance <literal>do { return (popInt 10) ; return 10 }</literal> would report
1517 the first statement in the <literal>do</literal> block as suspicious,
1518 as it has the type <literal>StackM (StackM Int)</literal> (which consists of two nested applications
1519 of the same monad constructor), but which is not then "unpacked" by binding the result.
1520 The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away:
1522 do { _ <- return (popInt 10) ; return 10 }
1524 For almost all sensible programs this will indicate a bug, and you probably intended to write:
1526 do { popInt 10 ; return 10 }
1534 <para>If you're feeling really paranoid, the
1535 <option>-dcore-lint</option>
1536 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1537 is a good choice. It turns on heavyweight intra-pass
1538 sanity-checking within GHC. (It checks GHC's sanity, not
1545 <sect1 id="options-optimise">
1546 <title>Optimisation (code improvement)</title>
1548 <indexterm><primary>optimisation</primary></indexterm>
1549 <indexterm><primary>improvement, code</primary></indexterm>
1551 <para>The <option>-O*</option> options specify convenient
1552 “packages” of optimisation flags; the
1553 <option>-f*</option> options described later on specify
1554 <emphasis>individual</emphasis> optimisations to be turned on/off;
1555 the <option>-m*</option> options specify
1556 <emphasis>machine-specific</emphasis> optimisations to be turned
1559 <sect2 id="optimise-pkgs">
1560 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1562 <para>There are <emphasis>many</emphasis> options that affect
1563 the quality of code produced by GHC. Most people only have a
1564 general goal, something like “Compile quickly” or
1565 “Make my program run like greased lightning.” The
1566 following “packages” of optimisations (or lack
1567 thereof) should suffice.</para>
1569 <para>Note that higher optimisation levels cause more
1570 cross-module optimisation to be performed, which can have an
1571 impact on how much of your program needs to be recompiled when
1572 you change something. This is one reason to stick to
1573 no-optimisation when developing code.</para>
1579 No <option>-O*</option>-type option specified:
1580 <indexterm><primary>-O* not specified</primary></indexterm>
1583 <para>This is taken to mean: “Please compile
1584 quickly; I'm not over-bothered about compiled-code
1585 quality.” So, for example: <command>ghc -c
1586 Foo.hs</command></para>
1592 <option>-O0</option>:
1593 <indexterm><primary><option>-O0</option></primary></indexterm>
1596 <para>Means “turn off all optimisation”,
1597 reverting to the same settings as if no
1598 <option>-O</option> options had been specified. Saying
1599 <option>-O0</option> can be useful if
1600 eg. <command>make</command> has inserted a
1601 <option>-O</option> on the command line already.</para>
1607 <option>-O</option> or <option>-O1</option>:
1608 <indexterm><primary>-O option</primary></indexterm>
1609 <indexterm><primary>-O1 option</primary></indexterm>
1610 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1613 <para>Means: “Generate good-quality code without
1614 taking too long about it.” Thus, for example:
1615 <command>ghc -c -O Main.lhs</command></para>
1621 <option>-O2</option>:
1622 <indexterm><primary>-O2 option</primary></indexterm>
1623 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1626 <para>Means: “Apply every non-dangerous
1627 optimisation, even if it means significantly longer
1628 compile times.”</para>
1630 <para>The avoided “dangerous” optimisations
1631 are those that can make runtime or space
1632 <emphasis>worse</emphasis> if you're unlucky. They are
1633 normally turned on or off individually.</para>
1635 <para>At the moment, <option>-O2</option> is
1636 <emphasis>unlikely</emphasis> to produce better code than
1637 <option>-O</option>.</para>
1643 <option>-Ofile <file></option>:
1644 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1645 <indexterm><primary>optimising, customised</primary></indexterm>
1648 <para>(NOTE: not supported since GHC 4.x. Please ask if
1649 you're interested in this.)</para>
1651 <para>For those who need <emphasis>absolute</emphasis>
1652 control over <emphasis>exactly</emphasis> what options are
1653 used (e.g., compiler writers, sometimes :-), a list of
1654 options can be put in a file and then slurped in with
1655 <option>-Ofile</option>.</para>
1657 <para>In that file, comments are of the
1658 <literal>#</literal>-to-end-of-line variety; blank
1659 lines and most whitespace is ignored.</para>
1661 <para>Please ask if you are baffled and would like an
1662 example of <option>-Ofile</option>!</para>
1667 <para>We don't use a <option>-O*</option> flag for day-to-day
1668 work. We use <option>-O</option> to get respectable speed;
1669 e.g., when we want to measure something. When we want to go for
1670 broke, we tend to use <option>-O2 -fvia-C</option> (and we go for
1671 lots of coffee breaks).</para>
1673 <para>The easiest way to see what <option>-O</option> (etc.)
1674 “really mean” is to run with <option>-v</option>,
1675 then stand back in amazement.</para>
1678 <sect2 id="options-f">
1679 <title><option>-f*</option>: platform-independent flags</title>
1681 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1682 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1684 <para>These flags turn on and off individual optimisations.
1685 They are normally set via the <option>-O</option> options
1686 described above, and as such, you shouldn't need to set any of
1687 them explicitly (indeed, doing so could lead to unexpected
1688 results). However, there are one or two that may be of
1693 <term><option>-fexcess-precision</option>:</term>
1695 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1696 <para>When this option is given, intermediate floating
1697 point values can have a <emphasis>greater</emphasis>
1698 precision/range than the final type. Generally this is a
1699 good thing, but some programs may rely on the exact
1701 <literal>Float</literal>/<literal>Double</literal> values
1702 and should not use this option for their compilation.</para>
1707 <term><option>-fignore-asserts</option>:</term>
1709 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1710 <para>Causes GHC to ignore uses of the function
1711 <literal>Exception.assert</literal> in source code (in
1712 other words, rewriting <literal>Exception.assert p
1713 e</literal> to <literal>e</literal> (see <xref
1714 linkend="assertions"/>). This flag is turned on by
1715 <option>-O</option>.
1722 <option>-fno-cse</option>
1723 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1726 <para>Turns off the common-sub-expression elimination optimisation.
1727 Can be useful if you have some <literal>unsafePerformIO</literal>
1728 expressions that you don't want commoned-up.</para>
1734 <option>-fno-strictness</option>
1735 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1738 <para>Turns off the strictness analyser; sometimes it eats
1739 too many cycles.</para>
1745 <option>-fno-full-laziness</option>
1746 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1749 <para>Turns off the full laziness optimisation (also known as
1750 let-floating). Full laziness increases sharing, which can lead
1751 to increased memory residency.</para>
1753 <para>NOTE: GHC doesn't implement complete full-laziness.
1754 When optimisation in on, and
1755 <option>-fno-full-laziness</option> is not given, some
1756 transformations that increase sharing are performed, such
1757 as extracting repeated computations from a loop. These
1758 are the same transformations that a fully lazy
1759 implementation would do, the difference is that GHC
1760 doesn't consistently apply full-laziness, so don't rely on
1767 <option>-fno-float-in</option>
1768 <indexterm><primary><option>-fno-float-in</option></primary></indexterm>
1771 <para>Turns off the float-in transformation.</para>
1777 <option>-fno-specialise</option>
1778 <indexterm><primary><option>-fno-specialise</option></primary></indexterm>
1781 <para>Turns off the automatic specialisation of overloaded functions.</para>
1787 <option>-fspec-constr</option>
1788 <indexterm><primary><option>-fspec-constr</option></primary></indexterm>
1791 <para>Turn on call-pattern specialisation.</para>
1797 <option>-fliberate-case</option>
1798 <indexterm><primary><option>-fliberate-case</option></primary></indexterm>
1801 <para>Turn on the liberate-case transformation.</para>
1807 <option>-fstatic-argument-transformation</option>
1808 <indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
1811 <para>Turn on the static argument transformation.</para>
1817 <option>-fno-state-hack</option>
1818 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1821 <para>Turn off the "state hack" whereby any lambda with a
1822 <literal>State#</literal> token as argument is considered to be
1823 single-entry, hence it is considered OK to inline things inside
1824 it. This can improve performance of IO and ST monad code, but it
1825 runs the risk of reducing sharing.</para>
1831 <option>-fomit-interface-pragmas</option>
1832 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
1835 <para>Tells GHC to omit all inessential information from the interface file
1836 generated for the module being compiled (say M). This means that a module
1837 importing M will see only the <emphasis>types</emphasis> of the functions that M exports, but not
1838 their unfoldings, strictness info, etc. Hence, for example,
1839 no function exported by M will be inlined
1840 into an importing module. The benefit is that modules that import M will
1841 need to be recompiled less often (only when M's exports change their type,
1842 not when they change their implementation).
1849 <option>-fignore-interface-pragmas</option>
1850 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
1853 <para>Tells GHC to ignore all inessential information when reading interface files.
1854 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
1855 for a function, GHC will ignore that information.</para>
1861 <option>-funbox-strict-fields</option>:
1862 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1863 <indexterm><primary>strict constructor fields</primary></indexterm>
1864 <indexterm><primary>constructor fields, strict</primary></indexterm>
1867 <para>This option causes all constructor fields which are
1868 marked strict (i.e. “!”) to be unboxed or
1869 unpacked if possible. It is equivalent to adding an
1870 <literal>UNPACK</literal> pragma to every strict
1871 constructor field (see <xref
1872 linkend="unpack-pragma"/>).</para>
1874 <para>This option is a bit of a sledgehammer: it might
1875 sometimes make things worse. Selectively unboxing fields
1876 by using <literal>UNPACK</literal> pragmas might be
1883 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
1884 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1885 <indexterm><primary>inlining, controlling</primary></indexterm>
1886 <indexterm><primary>unfolding, controlling</primary></indexterm>
1889 <para>(Default: 45) Governs the maximum size that GHC will
1890 allow a function unfolding to be. (An unfolding has a
1891 “size” that reflects the cost in terms of
1892 “code bloat” of expanding that unfolding at
1893 at a call site. A bigger function would be assigned a
1894 bigger cost.) </para>
1896 <para> Consequences: (a) nothing larger than this will be
1897 inlined (unless it has an INLINE pragma); (b) nothing
1898 larger than this will be spewed into an interface
1902 <para> Increasing this figure is more likely to result in longer
1903 compile times than faster code. The next option is more
1909 <term><option>-funfolding-use-threshold=<replaceable>n</replaceable></option></term>
1911 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1912 <indexterm><primary>inlining, controlling</primary></indexterm>
1913 <indexterm><primary>unfolding, controlling</primary></indexterm>
1915 <para>(Default: 8) This is the magic cut-off figure for
1916 unfolding: below this size, a function definition will be
1917 unfolded at the call-site, any bigger and it won't. The
1918 size computed for a function depends on two things: the
1919 actual size of the expression minus any discounts that
1920 apply (see <option>-funfolding-con-discount</option>).</para>
1933 <sect1 id="using-concurrent">
1934 <title>Using Concurrent Haskell</title>
1935 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
1937 <para>GHC supports Concurrent Haskell by default, without requiring a
1938 special option or libraries compiled in a certain way. To get access to
1939 the support libraries for Concurrent Haskell, just import
1941 url="&libraryBaseLocation;/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
1943 <para>The following RTS option(s) affect the behaviour of Concurrent
1944 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
1948 <term><option>-C<replaceable>s</replaceable></option></term>
1950 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1951 Sets the context switch interval to <replaceable>s</replaceable>
1952 seconds. A context switch will occur at the next heap block
1953 allocation after the timer expires (a heap block allocation occurs
1954 every 4k of allocation). With <option>-C0</option> or
1955 <option>-C</option>, context switches will occur as often as
1956 possible (at every heap block allocation). By default, context
1957 switches occur every 20ms.</para>
1963 <sect1 id="using-smp">
1964 <title>Using SMP parallelism</title>
1965 <indexterm><primary>parallelism</primary>
1967 <indexterm><primary>SMP</primary>
1970 <para>GHC supports running Haskell programs in parallel on an SMP
1971 (symmetric multiprocessor).</para>
1973 <para>There's a fine distinction between
1974 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
1975 parallelism is all about making your program run
1976 <emphasis>faster</emphasis> by making use of multiple processors
1977 simultaneously. Concurrency, on the other hand, is a means of
1978 abstraction: it is a convenient way to structure a program that must
1979 respond to multiple asynchronous events.</para>
1981 <para>However, the two terms are certainly related. By making use of
1982 multiple CPUs it is possible to run concurrent threads in parallel,
1983 and this is exactly what GHC's SMP parallelism support does. But it
1984 is also possible to obtain performance improvements with parallelism
1985 on programs that do not use concurrency. This section describes how to
1986 use GHC to compile and run parallel programs, in <xref
1987 linkend="lang-parallel" /> we describe the language features that affect
1990 <sect2 id="parallel-compile-options">
1991 <title>Compile-time options for SMP parallelism</title>
1993 <para>In order to make use of multiple CPUs, your program must be
1994 linked with the <option>-threaded</option> option (see <xref
1995 linkend="options-linker" />). Additionally, the following
1996 compiler options affect parallelism:</para>
2000 <term><option>-feager-blackholing</option></term>
2001 <indexterm><primary><option>-feager-blackholing</option></primary></indexterm>
2004 Blackholing is the act of marking a thunk (lazy
2005 computuation) as being under evaluation. It is useful for
2006 three reasons: firstly it lets us detect certain kinds of
2007 infinite loop (the <literal>NonTermination</literal>
2008 exception), secondly it avoids certain kinds of space
2009 leak, and thirdly it avoids repeating a computation in a
2010 parallel program, because we can tell when a computation
2011 is already in progress.</para>
2014 The option <option>-feager-blackholing</option> causes
2015 each thunk to be blackholed as soon as evaluation begins.
2016 The default is "lazy blackholing", whereby thunks are only
2017 marked as being under evaluation when a thread is paused
2018 for some reason. Lazy blackholing is typically more
2019 efficient (by 1-2% or so), because most thunks don't
2020 need to be blackholed. However, eager blackholing can
2021 avoid more repeated computation in a parallel program, and
2022 this often turns out to be important for parallelism.
2026 We recommend compiling any code that is intended to be run
2027 in parallel with the <option>-feager-blackholing</option>
2035 <sect2 id="parallel-options">
2036 <title>RTS options for SMP parallelism</title>
2038 <para>To run a program on multiple CPUs, use the
2039 RTS <option>-N</option> option:</para>
2043 <term><option>-N<optional><replaceable>x</replaceable></optional></option></term>
2045 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
2046 Use <replaceable>x</replaceable> simultaneous threads when
2047 running the program. Normally <replaceable>x</replaceable>
2048 should be chosen to match the number of CPU cores on the
2049 machine<footnote><para>Whether hyperthreading cores should be counted or not is an
2050 open question; please feel free to experiment and let us know what
2051 results you find.</para></footnote>. For example,
2052 on a dual-core machine we would probably use
2053 <literal>+RTS -N2 -RTS</literal>.</para>
2055 <para>Omitting <replaceable>x</replaceable>,
2056 i.e. <literal>+RTS -N -RTS</literal>, lets the runtime
2057 choose the value of <replaceable>x</replaceable> itself
2058 based on how many processors are in your machine.</para>
2060 <para>Be careful when using all the processors in your
2061 machine: if some of your processors are in use by other
2062 programs, this can actually harm performance rather than
2065 <para>Setting <option>-N</option> also has the effect of
2066 enabling the parallel garbage collector (see
2067 <xref linkend="rts-options-gc" />).</para>
2069 <para>There is no means (currently) by which this value
2070 may vary after the program has started.</para>
2072 <para>The current value of the <option>-N</option> option
2073 is available to the Haskell program
2074 via <literal>GHC.Conc.numCapabilities</literal>.</para>
2079 <para>The following options affect the way the runtime schedules
2080 threads on CPUs:</para>
2084 <term><option>-qa</option></term>
2085 <indexterm><primary><option>-qa</option></primary><secondary>RTS
2086 option</secondary></indexterm>
2088 <para>Use the OS's affinity facilities to try to pin OS
2089 threads to CPU cores. This is an experimental feature,
2090 and may or may not be useful. Please let us know
2091 whether it helps for you!</para>
2095 <term><option>-qm</option></term>
2096 <indexterm><primary><option>-qm</option></primary><secondary>RTS
2097 option</secondary></indexterm>
2099 <para>Disable automatic migration for load balancing.
2100 Normally the runtime will automatically try to schedule
2101 threads across the available CPUs to make use of idle
2102 CPUs; this option disables that behaviour. Note that
2103 migration only applies to threads; sparks created
2104 by <literal>par</literal> are load-balanced separately
2105 by work-stealing.</para>
2108 This option is probably only of use for concurrent
2109 programs that explicitly schedule threads onto CPUs
2110 with <literal>GHC.Conc.forkOnIO</literal>.
2115 <term><option>-qw</option></term>
2116 <indexterm><primary><option>-qw</option></primary><secondary>RTS
2117 option</secondary></indexterm>
2119 <para>Migrate a thread to the current CPU when it is woken
2120 up. Normally when a thread is woken up after being
2121 blocked it will be scheduled on the CPU it was running on
2122 last; this option allows the thread to immediately migrate
2123 to the CPU that unblocked it.</para>
2125 <para>The rationale for allowing this eager migration is
2126 that it tends to move threads that are communicating with
2127 each other onto the same CPU; however there are
2128 pathalogical situations where it turns out to be a poor
2129 strategy. Depending on the communication pattern in your
2130 program, it may or may not be a good idea.</para>
2137 <title>Hints for using SMP parallelism</title>
2139 <para>Add the <literal>-s</literal> RTS option when
2140 running the program to see timing stats, which will help to tell you
2141 whether your program got faster by using more CPUs or not. If the user
2142 time is greater than
2143 the elapsed time, then the program used more than one CPU. You should
2144 also run the program without <literal>-N</literal> for
2147 <para>The output of <literal>+RTS -s</literal> tells you how
2148 many “sparks” were created and executed during the
2149 run of the program (see <xref linkend="rts-options-gc" />), which
2150 will give you an idea how well your <literal>par</literal>
2151 annotations are working.</para>
2153 <para>GHC's parallelism support has improved in 6.12.1 as a
2154 result of much experimentation and tuning in the runtime
2155 system. We'd still be interested to hear how well it works
2156 for you, and we're also interested in collecting parallel
2157 programs to add to our benchmarking suite.</para>
2161 <sect1 id="options-platform">
2162 <title>Platform-specific Flags</title>
2164 <indexterm><primary>-m* options</primary></indexterm>
2165 <indexterm><primary>platform-specific options</primary></indexterm>
2166 <indexterm><primary>machine-specific options</primary></indexterm>
2168 <para>Some flags only make sense for particular target
2174 <term><option>-msse2</option>:</term>
2177 (x86 only, added in GHC 7.0.1) Use the SSE2 registers and
2178 instruction set to implement floating point operations
2179 when using the native code generator. This gives a
2180 substantial performance improvement for floating point,
2181 but the resulting compiled code will only run on
2182 processors that support SSE2 (Intel Pentium 4 and later,
2183 or AMD Athlon 64 and later).
2186 SSE2 is unconditionally used on x86-64 platforms.
2192 <term><option>-monly-[32]-regs</option>:</term>
2194 <para>(x86 only)<indexterm><primary>-monly-N-regs
2195 option (iX86 only)</primary></indexterm> GHC tries to
2196 “steal” four registers from GCC, for performance
2197 reasons; it almost always works. However, when GCC is
2198 compiling some modules with four stolen registers, it will
2199 crash, probably saying:
2202 Foo.hc:533: fixed or forbidden register was spilled.
2203 This may be due to a compiler bug or to impossible asm
2204 statements or clauses.
2207 Just give some registers back with
2208 <option>-monly-N-regs</option>. Try `3' first, then `2'.
2209 If `2' doesn't work, please report the bug to us.</para>
2218 <sect1 id="ext-core">
2219 <title>Generating and compiling External Core Files</title>
2221 <indexterm><primary>intermediate code generation</primary></indexterm>
2223 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
2224 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
2225 <filename>.hcr</filename>. The Core format is described in <ulink url="../../core.pdf">
2226 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
2228 for manipulating Core files (in Haskell) are in the GHC source distribution
2229 directory under <literal>utils/ext-core</literal>.
2230 Note that the format of <literal>.hcr</literal>
2231 files is <emphasis>different</emphasis> from the Core output format that GHC generates
2232 for debugging purposes (<xref linkend="options-debugging"/>), though the two formats appear somewhat similar.</para>
2234 <para>The Core format natively supports notes which you can add to
2235 your source code using the <literal>CORE</literal> pragma (see <xref
2236 linkend="pragmas"/>).</para>
2242 <option>-fext-core</option>
2243 <indexterm><primary><option>-fext-core</option></primary></indexterm>
2246 <para>Generate <literal>.hcr</literal> files.</para>
2252 <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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