1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="using-ghc">
3 <title>Using GHC</title>
5 <indexterm><primary>GHC, using</primary></indexterm>
6 <indexterm><primary>using GHC</primary></indexterm>
9 <title>Options overview</title>
11 <para>GHC's behaviour is controlled by
12 <firstterm>options</firstterm>, which for historical reasons are
13 also sometimes referred to as command-line flags or arguments.
14 Options can be specified in three ways:</para>
17 <title>command-line arguments</title>
19 <indexterm><primary>structure, command-line</primary></indexterm>
20 <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
21 <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
23 <para>An invocation of GHC takes the following form:</para>
29 <para>command-line arguments are either options or file names.</para>
31 <para>command-line options begin with <literal>-</literal>.
32 They may <emphasis>not</emphasis> be grouped:
33 <option>-vO</option> is different from <option>-v -O</option>.
34 Options need not precede filenames: e.g., <literal>ghc *.o -o
35 foo</literal>. All options are processed and then applied to
36 all files; you cannot, for example, invoke <literal>ghc -c -O1
37 Foo.hs -O2 Bar.hs</literal> to apply different optimisation
38 levels to the files <filename>Foo.hs</filename> and
39 <filename>Bar.hs</filename>.</para>
42 <sect2 id="source-file-options">
43 <title>command line options in source files</title>
45 <indexterm><primary>source-file options</primary></indexterm>
47 <para>Sometimes it is useful to make the connection between a
48 source file and the command-line options it requires quite
49 tight. For instance, if a Haskell source file uses GHC
50 extensions, it will always need to be compiled with the
51 <option>-fglasgow-exts</option> option. Rather than maintaining
52 the list of per-file options in a <filename>Makefile</filename>,
53 it is possible to do this directly in the source file using the
54 <literal>OPTIONS_GHC</literal> pragma <indexterm><primary>OPTIONS_GHC
55 pragma</primary></indexterm>:</para>
58 {-# OPTIONS_GHC -fglasgow-exts #-}
63 <para><literal>OPTIONS_GHC</literal> pragmas are only looked for at
64 the top of your source files, upto the first
65 (non-literate,non-empty) line not containing
66 <literal>OPTIONS_GHC</literal>. Multiple <literal>OPTIONS_GHC</literal>
67 pragmas are recognised. Do not put comments before, or on the same line
68 as, the <literal>OPTIONS_GHC</literal> pragma.</para>
70 <para>Note that your command shell does not
71 get to the source file options, they are just included literally
72 in the array of command-line arguments the compiler
73 maintains internally, so you'll be desperately disappointed if
74 you try to glob etc. inside <literal>OPTIONS_GHC</literal>.</para>
76 <para>NOTE: the contents of OPTIONS_GHC are prepended to the
77 command-line options, so you <emphasis>do</emphasis> have the
78 ability to override OPTIONS_GHC settings via the command
81 <para>It is not recommended to move all the contents of your
82 Makefiles into your source files, but in some circumstances, the
83 <literal>OPTIONS_GHC</literal> pragma is the Right Thing. (If you
84 use <option>-keep-hc-file-too</option> and have OPTION flags in
85 your module, the OPTIONS_GHC will get put into the generated .hc
90 <title>Setting options in GHCi</title>
92 <para>Options may also be modified from within GHCi, using the
93 <literal>:set</literal> command. See <xref linkend="ghci-set"/>
94 for more details.</para>
98 <sect1 id="static-dynamic-flags">
99 <title>Static, Dynamic, and Mode options</title>
100 <indexterm><primary>static</primary><secondary>options</secondary>
102 <indexterm><primary>dynamic</primary><secondary>options</secondary>
104 <indexterm><primary>mode</primary><secondary>options</secondary>
107 <para>Each of GHC's command line options is classified as either
108 <firstterm>static</firstterm> or <firstterm>dynamic</firstterm> or
109 <firstterm>mode</firstterm>:</para>
113 <term>Mode flags</term>
115 <para>For example, <option>--make</option> or <option>-E</option>.
116 There may be only a single mode flag on the command line. The
117 available modes are listed in <xref linkend="modes"/>.</para>
121 <term>Dynamic Flags</term>
123 <para>Most non-mode flags fall into this category. A dynamic flag
124 may be used on the command line, in a
125 <literal>GHC_OPTIONS</literal> pragma in a source file, or set
126 using <literal>:set</literal> in GHCi.</para>
130 <term>Static Flags</term>
132 <para>A few flags are "static", which means they can only be used on
133 the command-line, and remain in force over the entire GHC/GHCi
139 <para>The flag reference tables (<xref
140 linkend="flag-reference"/>) lists the status of each flag.</para>
143 <sect1 id="file-suffixes">
144 <title>Meaningful file suffixes</title>
146 <indexterm><primary>suffixes, file</primary></indexterm>
147 <indexterm><primary>file suffixes for GHC</primary></indexterm>
149 <para>File names with “meaningful” suffixes (e.g.,
150 <filename>.lhs</filename> or <filename>.o</filename>) cause the
151 “right thing” to happen to those files.</para>
156 <term><filename>.hs</filename></term>
158 <para>A Haskell module.</para>
164 <filename>.lhs</filename>
165 <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
168 <para>A “literate Haskell” module.</para>
173 <term><filename>.hi</filename></term>
175 <para>A Haskell interface file, probably
176 compiler-generated.</para>
181 <term><filename>.hc</filename></term>
183 <para>Intermediate C file produced by the Haskell
189 <term><filename>.c</filename></term>
191 <para>A C file not produced by the Haskell
197 <term><filename>.s</filename></term>
199 <para>An assembly-language source file, usually produced by
205 <term><filename>.o</filename></term>
207 <para>An object file, produced by an assembler.</para>
212 <para>Files with other suffixes (or without suffixes) are passed
213 straight to the linker.</para>
218 <title>Modes of operation</title>
220 <para>GHC's behaviour is firstly controlled by a mode flag. Only
221 one of these flags may be given, but it does not necessarily need
222 to be the first option on the command-line. The available modes
228 <cmdsynopsis><command>ghc --interactive</command>
230 <indexterm><primary>interactive mode</primary></indexterm>
231 <indexterm><primary>ghci</primary></indexterm>
234 <para>Interactive mode, which is also available as
235 <command>ghci</command>. Interactive mode is described in
236 more detail in <xref linkend="ghci"/>.</para>
242 <cmdsynopsis><command>ghc --make</command>
244 <indexterm><primary>make mode</primary></indexterm>
245 <indexterm><primary><option>––make</option></primary></indexterm>
248 <para>In this mode, GHC will build a multi-module Haskell
249 program automatically, figuring out dependencies for itself.
250 If you have a straightforward Haskell program, this is
251 likely to be much easier, and faster, than using
252 <command>make</command>. Make mode is described in <xref
253 linkend="make-mode"/>.</para>
259 <cmdsynopsis><command>ghc -e</command>
260 <arg choice='plain'><replaceable>expr</replaceable></arg>
262 <indexterm><primary>eval mode</primary></indexterm>
265 <para>Expression-evaluation mode. This is very similar to
266 interactive mode, except that there is a single expression
267 to evaluate (<replaceable>expr</replaceable>) which is given
268 on the command line. See <xref linkend="eval-mode"/> for
276 <command>ghc -E</command>
277 <command>ghc -c</command>
278 <command>ghc -S</command>
279 <command>ghc -c</command>
281 <indexterm><primary><option>-E</option></primary></indexterm>
282 <indexterm><primary><option>-C</option></primary></indexterm>
283 <indexterm><primary><option>-S</option></primary></indexterm>
284 <indexterm><primary><option>-c</option></primary></indexterm>
287 <para>This is the traditional batch-compiler mode, in which
288 GHC can compile source files one at a time, or link objects
289 together into an executable. This mode also applies if
290 there is no other mode flag specified on the command line,
291 in which case it means that the specified files should be
292 compiled and then linked to form a program. See <xref
293 linkend="options-order"/>.</para>
300 <command>ghc -M</command>
302 <indexterm><primary>dependency-generation mode</primary></indexterm>
305 <para>Dependency-generation mode. In this mode, GHC can be
306 used to generate dependency information suitable for use in
307 a <literal>Makefile</literal>. See <xref
308 linkend="sec-makefile-dependencies"/>.</para>
315 <command>ghc --mk-dll</command>
317 <indexterm><primary>DLL-creation mode</primary></indexterm>
320 <para>DLL-creation mode (Windows only). See <xref
321 linkend="win32-dlls-create"/>.</para>
328 <command>ghc --help</command> <command>ghc -?</command>
330 <indexterm><primary><option>––help</option></primary></indexterm>
333 <para>Cause GHC to spew a long usage message to standard
334 output and then exit.</para>
341 <command>ghc --version</command> <command>ghc -V</command>
342 <indexterm><primary><option>-V</option></primary></indexterm>
343 <indexterm><primary><option>––version</option></primary></indexterm>
347 <para>Print a one-line string including GHC's version number.</para>
354 <command>ghc --numeric-version</command>
355 <indexterm><primary><option>––numeric-version</option></primary></indexterm>
359 <para>Print GHC's numeric version number only.</para>
366 <command>ghc --print-libdir</command>
367 <indexterm><primary><option>––print-libdir</option></primary></indexterm>
371 <para>Print the path to GHC's library directory. This is
372 the top of the directory tree containing GHC's libraries,
373 interfaces, and include files (usually something like
374 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
376 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary>
377 </indexterm>in the package configuration file (see <xref
378 linkend="packages"/>).</para>
384 <sect2 id="make-mode">
385 <title>Using <command>ghc</command> <option>––make</option></title>
386 <indexterm><primary><option>––make</option></primary></indexterm>
387 <indexterm><primary>separate compilation</primary></indexterm>
389 <para>When given the <option>––make</option> option,
390 GHC will build a multi-module Haskell program by following
391 dependencies from a single root module (usually
392 <literal>Main</literal>). For example, if your
393 <literal>Main</literal> module is in a file called
394 <filename>Main.hs</filename>, you could compile and link the
395 program like this:</para>
398 ghc ––make Main.hs
401 <para>The command line may contain any number of source file
402 names or module names; GHC will figure out all the modules in
403 the program by following the imports from these initial modules.
404 It will then attempt to compile each module which is out of
405 date, and finally if there is a <literal>Main</literal> module,
406 the program will also be linked into an executable.</para>
408 <para>The main advantages to using <literal>ghc
409 ––make</literal> over traditional
410 <literal>Makefile</literal>s are:</para>
414 <para>GHC doesn't have to be restarted for each compilation,
415 which means it can cache information between compilations.
416 Compiling a multi-module program with <literal>ghc
417 ––make</literal> can be up to twice as fast as
418 running <literal>ghc</literal> individually on each source
422 <para>You don't have to write a <literal>Makefile</literal>.</para>
423 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
426 <para>GHC re-calculates the dependencies each time it is
427 invoked, so the dependencies never get out of sync with the
432 <para>Any of the command-line options described in the rest of
433 this chapter can be used with
434 <option>––make</option>, but note that any options
435 you give on the command line will apply to all the source files
436 compiled, so if you want any options to apply to a single source
437 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
438 pragma (see <xref linkend="source-file-options"/>).</para>
440 <para>If the program needs to be linked with additional objects
441 (say, some auxiliary C code), then the object files can be
442 given on the command line and GHC will include them when linking
443 the executable.</para>
445 <para>Note that GHC can only follow dependencies if it has the
446 source file available, so if your program includes a module for
447 which there is no source file, even if you have an object and an
448 interface file for the module, then GHC will complain. The
449 exception to this rule is for package modules, which may or may
450 not have source files.</para>
452 <para>The source files for the program don't all need to be in
453 the same directory; the <option>-i</option> option can be used
454 to add directories to the search path (see <xref
455 linkend="search-path"/>).</para>
458 <sect2 id="eval-mode">
459 <title>Expression evaluation mode</title>
461 <para>This mode is very similar to interactive mode, except that
462 there is a single expression to evaluate which is specified on
463 the command line as an argument to the <option>-e</option>
467 ghc -e <replaceable>expr</replaceable>
470 <para>Haskell source files may be named on the command line, and
471 they will be loaded exactly as in interactive mode. The
472 expression is evaluated in the context of the loaded
475 <para>For example, to load and run a Haskell program containing
476 a module <literal>Main</literal>, we might say</para>
479 ghc -e Main.main Main.hs
482 <para>or we can just use this mode to evaluate expressions in
483 the context of the <literal>Prelude</literal>:</para>
486 $ ghc -e "interact (unlines.map reverse.lines)"
492 <sect2 id="options-order">
493 <title>Batch compiler mode</title>
495 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
496 given on the command line.</para>
498 <para>The first phase to run is determined by each input-file
499 suffix, and the last phase is determined by a flag. If no
500 relevant flag is present, then go all the way through linking.
501 This table summarises:</para>
505 <colspec align="left"/>
506 <colspec align="left"/>
507 <colspec align="left"/>
508 <colspec align="left"/>
512 <entry>Phase of the compilation system</entry>
513 <entry>Suffix saying “start here”</entry>
514 <entry>Flag saying “stop after”</entry>
515 <entry>(suffix of) output file</entry>
520 <entry>literate pre-processor</entry>
521 <entry><literal>.lhs</literal></entry>
523 <entry><literal>.hs</literal></entry>
527 <entry>C pre-processor (opt.) </entry>
528 <entry><literal>.hs</literal> (with
529 <option>-cpp</option>)</entry>
530 <entry><option>-E</option></entry>
531 <entry><literal>.hspp</literal></entry>
535 <entry>Haskell compiler</entry>
536 <entry><literal>.hs</literal></entry>
537 <entry><option>-C</option>, <option>-S</option></entry>
538 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
542 <entry>C compiler (opt.)</entry>
543 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
544 <entry><option>-S</option></entry>
545 <entry><literal>.s</literal></entry>
549 <entry>assembler</entry>
550 <entry><literal>.s</literal></entry>
551 <entry><option>-c</option></entry>
552 <entry><literal>.o</literal></entry>
556 <entry>linker</entry>
557 <entry><replaceable>other</replaceable></entry>
559 <entry><filename>a.out</filename></entry>
565 <indexterm><primary><option>-C</option></primary></indexterm>
566 <indexterm><primary><option>-E</option></primary></indexterm>
567 <indexterm><primary><option>-S</option></primary></indexterm>
568 <indexterm><primary><option>-c</option></primary></indexterm>
570 <para>Thus, a common invocation would be: </para>
573 ghc -c Foo.hs</screen>
575 <para>to compile the Haskell source file
576 <filename>Foo.hs</filename> to an object file
577 <filename>Foo.o</filename>.</para>
579 <para>Note: What the Haskell compiler proper produces depends on
580 whether a native-code generator<indexterm><primary>native-code
581 generator</primary></indexterm> is used (producing assembly
582 language) or not (producing C). See <xref
583 linkend="options-codegen"/> for more details.</para>
585 <para>Note: C pre-processing is optional, the
586 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
587 flag turns it on. See <xref linkend="c-pre-processor"/> for more
590 <para>Note: The option <option>-E</option><indexterm><primary>-E
591 option</primary></indexterm> runs just the pre-processing passes
592 of the compiler, dumping the result in a file. Note that this
593 differs from the previous behaviour of dumping the file to
594 standard output.</para>
596 <sect3 id="overriding-suffixes">
597 <title>Overriding the default behaviour for a file</title>
599 <para>As described above, the way in which a file is processed by GHC
600 depends on its suffix. This behaviour can be overriden using the
601 <option>-x</option> option:</para>
605 <term><option>-x</option> <replaceable>suffix</replaceable>
606 <indexterm><primary><option>-x</option></primary>
609 <para>Causes all files following this option on the command
610 line to be processed as if they had the suffix
611 <replaceable>suffix</replaceable>. For example, to compile a
612 Haskell module in the file <literal>M.my-hs</literal>,
613 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
622 <sect1 id="options-help">
623 <title>Help and verbosity options</title>
625 <indexterm><primary>help options</primary></indexterm>
626 <indexterm><primary>verbosity options</primary></indexterm>
628 See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
629 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.
634 <indexterm><primary><option>-n</option></primary></indexterm>
637 <para>Does a dry-run, i.e. GHC goes through all the motions
638 of compiling as normal, but does not actually run any
639 external commands.</para>
646 <indexterm><primary><option>-v</option></primary></indexterm>
649 <para>The <option>-v</option> option makes GHC
650 <emphasis>verbose</emphasis>: it reports its version number
651 and shows (on stderr) exactly how it invokes each phase of
652 the compilation system. Moreover, it passes the
653 <option>-v</option> flag to most phases; each reports its
654 version number (and possibly some other information).</para>
656 <para>Please, oh please, use the <option>-v</option> option
657 when reporting bugs! Knowing that you ran the right bits in
658 the right order is always the first thing we want to
665 <option>-v</option><replaceable>n</replaceable>
666 <indexterm><primary><option>-v</option></primary></indexterm>
669 <para>To provide more control over the compiler's verbosity,
670 the <option>-v</option> flag takes an optional numeric
671 argument. Specifying <option>-v</option> on its own is
672 equivalent to <option>-v3</option>, and the other levels
673 have the following meanings:</para>
677 <term><option>-v0</option></term>
679 <para>Disable all non-essential messages (this is the
685 <term><option>-v1</option></term>
687 <para>Minimal verbosity: print one line per
688 compilation (this is the default when
689 <option>––make</option> or
690 <option>––interactive</option> is on).</para>
695 <term><option>-v2</option></term>
697 <para>Print the name of each compilation phase as it
698 is executed. (equivalent to
699 <option>-dshow-passes</option>).</para>
704 <term><option>-v3</option></term>
706 <para>The same as <option>-v2</option>, except that in
707 addition the full command line (if appropriate) for
708 each compilation phase is also printed.</para>
713 <term><option>-v4</option></term>
715 <para>The same as <option>-v3</option> except that the
716 intermediate program representation after each
717 compilation phase is also printed (excluding
718 preprocessed and C/assembly files).</para>
726 <term><option>-ferror-spans</option>
727 <indexterm><primary><option>-ferror-spans</option></primary>
731 <para>Causes GHC to emit the full source span of the
732 syntactic entity relating to an error message. Normally, GHC
733 emits the source location of the start of the syntactic
736 <para>For example:</para>
738 <screen>test.hs:3:6: parse error on input `where'</screen>
740 <para>becomes:</para>
742 <screen>test296.hs:3:6-10: parse error on input `where'</screen>
744 <para>And multi-line spans are possible too:</para>
746 <screen>test.hs:(5,4)-(6,7):
747 Conflicting definitions for `a'
748 Bound at: test.hs:5:4
750 In the binding group for: a, b, a</screen>
752 <para>Note that line numbers start counting at one, but
753 column numbers start at zero. This choice was made to
754 follow existing convention (i.e. this is how Emacs does
760 <term><option>-H</option><replaceable>size</replaceable>
761 <indexterm><primary><option>-H</option></primary></indexterm>
764 <para>Set the minimum size of the heap to
765 <replaceable>size</replaceable>.
766 This option is equivalent to
767 <literal>+RTS -H<replaceable>size</replaceable></literal>,
768 see <xref linkend="rts-options-gc" />.
774 <term><option>-Rghc-timing</option>
775 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
778 <para>Prints a one-line summary of timing statistics for the
779 GHC run. This option is equivalent to
780 <literal>+RTS -tstderr</literal>, see <xref
781 linkend="rts-options-gc" />.
790 <sect1 id="options-sanity">
791 <title>Warnings and sanity-checking</title>
793 <indexterm><primary>sanity-checking options</primary></indexterm>
794 <indexterm><primary>warnings</primary></indexterm>
797 <para>GHC has a number of options that select which types of
798 non-fatal error messages, otherwise known as warnings, can be
799 generated during compilation. By default, you get a standard set
800 of warnings which are generally likely to indicate bugs in your
802 <option>-fwarn-overlapping-patterns</option>,
803 <option>-fwarn-deprecations</option>,
804 <option>-fwarn-duplicate-exports</option>,
805 <option>-fwarn-missing-fields</option>, and
806 <option>-fwarn-missing-methods</option>. The following flags are
807 simple ways to select standard “packages” of warnings:
813 <term><option>-W</option>:</term>
815 <indexterm><primary>-W option</primary></indexterm>
816 <para>Provides the standard warnings plus
817 <option>-fwarn-incomplete-patterns</option>,
818 <option>-fwarn-unused-matches</option>,
819 <option>-fwarn-unused-imports</option>, and
820 <option>-fwarn-unused-binds</option>.</para>
825 <term><option>-w</option>:</term>
827 <indexterm><primary><option>-w</option></primary></indexterm>
828 <para>Turns off all warnings, including the standard ones.</para>
833 <term><option>-Wall</option>:</term>
835 <indexterm><primary><option>-Wall</option></primary></indexterm>
836 <para>Turns on all warning options.</para>
841 <term><option>-Werror</option>:</term>
843 <indexterm><primary><option>-Werror</option></primary></indexterm>
844 <para>Makes any warning into a fatal error. Useful so that you don't
845 miss warnings when doing batch compilation. </para>
851 <para>The full set of warning options is described below. To turn
852 off any warning, simply give the corresponding
853 <option>-fno-warn-...</option> option on the command line.</para>
858 <term><option>-fwarn-deprecations</option>:</term>
860 <indexterm><primary><option>-fwarn-deprecations</option></primary>
862 <indexterm><primary>deprecations</primary></indexterm>
863 <para>Causes a warning to be emitted when a deprecated
864 function or type is used. Entities can be marked as
865 deprecated using a pragma, see <xref
866 linkend="deprecated-pragma"/>.</para>
871 <term><option>-fwarn-duplicate-exports</option>:</term>
873 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
874 <indexterm><primary>duplicate exports, warning</primary></indexterm>
875 <indexterm><primary>export lists, duplicates</primary></indexterm>
877 <para>Have the compiler warn about duplicate entries in
878 export lists. This is useful information if you maintain
879 large export lists, and want to avoid the continued export
880 of a definition after you've deleted (one) mention of it in
881 the export list.</para>
883 <para>This option is on by default.</para>
888 <term><option>-fwarn-hi-shadowing</option>:</term>
890 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
891 <indexterm><primary>shadowing</primary>
892 <secondary>interface files</secondary></indexterm>
894 <para>Causes the compiler to emit a warning when a module or
895 interface file in the current directory is shadowing one
896 with the same module name in a library or other
902 <term><option>-fwarn-incomplete-patterns</option>:</term>
904 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
905 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
906 <indexterm><primary>patterns, incomplete</primary></indexterm>
908 <para>Similarly for incomplete patterns, the function
909 <function>g</function> below will fail when applied to
910 non-empty lists, so the compiler will emit a warning about
911 this when <option>-fwarn-incomplete-patterns</option> is
918 <para>This option isn't enabled be default because it can be
919 a bit noisy, and it doesn't always indicate a bug in the
920 program. However, it's generally considered good practice
921 to cover all the cases in your functions.</para>
926 <term><option>-fwarn-incomplete-record-updates</option>:</term>
928 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
929 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
930 <indexterm><primary>record updates, incomplete</primary></indexterm>
933 <function>f</function> below will fail when applied to
934 <literal>Bar</literal>, so the compiler will emit a warning about
935 this when <option>-fwarn-incomplete-record-updates</option> is
939 data Foo = Foo { x :: Int }
943 f foo = foo { x = 6 }
946 <para>This option isn't enabled be default because it can be
947 very noisy, and it often doesn't indicate a bug in the
954 <option>-fwarn-missing-fields</option>:
955 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
956 <indexterm><primary>missing fields, warning</primary></indexterm>
957 <indexterm><primary>fields, missing</primary></indexterm>
961 <para>This option is on by default, and warns you whenever
962 the construction of a labelled field constructor isn't
963 complete, missing initializers for one or more fields. While
964 not an error (the missing fields are initialised with
965 bottoms), it is often an indication of a programmer error.</para>
970 <term><option>-fwarn-missing-methods</option>:</term>
972 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
973 <indexterm><primary>missing methods, warning</primary></indexterm>
974 <indexterm><primary>methods, missing</primary></indexterm>
976 <para>This option is on by default, and warns you whenever
977 an instance declaration is missing one or more methods, and
978 the corresponding class declaration has no default
979 declaration for them.</para>
980 <para>The warning is suppressed if the method name
981 begins with an underscore. Here's an example where this is useful:
984 _simpleFn :: a -> String
985 complexFn :: a -> a -> String
986 complexFn x y = ... _simpleFn ...
988 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
989 never <literal>_simpleFn</literal>; and (b)
990 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
996 <term><option>-fwarn-missing-signatures</option>:</term>
998 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
999 <indexterm><primary>type signatures, missing</primary></indexterm>
1001 <para>If you would like GHC to check that every top-level
1002 function/value has a type signature, use the
1003 <option>-fwarn-missing-signatures</option> option. This
1004 option is off by default.</para>
1009 <term><option>-fwarn-name-shadowing</option>:</term>
1011 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1012 <indexterm><primary>shadowing, warning</primary></indexterm>
1014 <para>This option causes a warning to be emitted whenever an
1015 inner-scope value has the same name as an outer-scope value,
1016 i.e. the inner value shadows the outer one. This can catch
1017 typographical errors that turn into hard-to-find bugs, e.g.,
1018 in the inadvertent cyclic definition <literal>let x = ... x
1019 ... in</literal>.</para>
1021 <para>Consequently, this option does
1022 <emphasis>will</emphasis> complain about cyclic recursive
1028 <term><option>-fwarn-orphans</option>:</term>
1030 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1031 <indexterm><primary>orphan instances, warning</primary></indexterm>
1032 <indexterm><primary>orphan rules, warning</primary></indexterm>
1034 <para>This option causes a warning to be emitted whenever the
1035 module contains an "orphan" instance declaration or rewrite rule.
1036 An instance declartion is an orphan if it appears in a module in
1037 which neither the class nor the type being instanced are declared
1038 in the same module. A rule is an orphan if it is a rule for a
1039 function declared in another module. A module containing any
1040 orphans is called an orphan module.</para>
1041 <para>The trouble with orphans is that GHC must pro-actively read the interface
1042 files for all orphan modules, just in case their instances or rules
1043 play a role, whether or not the module's interface would otherwise
1044 be of any use. Other things being equal, avoid orphan modules.</para>
1050 <option>-fwarn-overlapping-patterns</option>:
1051 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1052 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1053 <indexterm><primary>patterns, overlapping</primary></indexterm>
1056 <para>By default, the compiler will warn you if a set of
1057 patterns are overlapping, i.e.,</para>
1060 f :: String -> Int
1066 <para>where the last pattern match in <function>f</function>
1067 won't ever be reached, as the second pattern overlaps
1068 it. More often than not, redundant patterns is a programmer
1069 mistake/error, so this option is enabled by default.</para>
1074 <term><option>-fwarn-simple-patterns</option>:</term>
1076 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1078 <para>Causes the compiler to warn about lambda-bound
1079 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1080 Normally, these aren't treated as incomplete patterns by
1081 <option>-fwarn-incomplete-patterns</option>.</para>
1082 <para>``Lambda-bound patterns'' includes all places where there is a single pattern,
1083 including list comprehensions and do-notation. In these cases, a pattern-match
1084 failure is quite legitimate, and triggers filtering (list comprehensions) or
1085 the monad <literal>fail</literal> operation (monads). For example:
1087 f :: [Maybe a] -> [a]
1088 f xs = [y | Just y <- xs]
1090 Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
1091 these probably-innocent cases, which is why the flag is off by default. </para>
1092 <para> The <literal>deriving( Read )</literal> mechanism produces monadic code with
1093 pattern matches, so you will also get misleading warnings about the compiler-generated
1094 code. (This is arguably a Bad Thing, but it's awkward to fix.)</para>
1100 <term><option>-fwarn-type-defaults</option>:</term>
1102 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1103 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1104 <para>Have the compiler warn/inform you where in your source
1105 the Haskell defaulting mechanism for numeric types kicks
1106 in. This is useful information when converting code from a
1107 context that assumed one default into one with another,
1108 e.g., the `default default' for Haskell 1.4 caused the
1109 otherwise unconstrained value <constant>1</constant> to be
1110 given the type <literal>Int</literal>, whereas Haskell 98
1111 defaults it to <literal>Integer</literal>. This may lead to
1112 differences in performance and behaviour, hence the
1113 usefulness of being non-silent about this.</para>
1115 <para>This warning is off by default.</para>
1120 <term><option>-fwarn-unused-binds</option>:</term>
1122 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1123 <indexterm><primary>unused binds, warning</primary></indexterm>
1124 <indexterm><primary>binds, unused</primary></indexterm>
1125 <para>Report any function definitions (and local bindings)
1126 which are unused. For top-level functions, the warning is
1127 only given if the binding is not exported.</para>
1128 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1129 mentioned in the right hand side of another definition that is used, or (c) the
1130 function it defines begins with an underscore. The last case provides a
1131 way to suppress unused-binding warnings selectively. </para>
1132 <para> Notice that a variable
1133 is reported as unused even if it appears in the right-hand side of another
1134 unused binding. </para>
1139 <term><option>-fwarn-unused-imports</option>:</term>
1141 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1142 <indexterm><primary>unused imports, warning</primary></indexterm>
1143 <indexterm><primary>imports, unused</primary></indexterm>
1145 <para>Report any modules that are explicitly imported but
1146 never used. However, the form <literal>import M()</literal> is
1147 never reported as an unused import, because it is a useful idiom
1148 for importing instance declarations, which are anonymous in Haskell.</para>
1153 <term><option>-fwarn-unused-matches</option>:</term>
1155 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1156 <indexterm><primary>unused matches, warning</primary></indexterm>
1157 <indexterm><primary>matches, unused</primary></indexterm>
1159 <para>Report all unused variables which arise from pattern
1160 matches, including patterns consisting of a single variable.
1161 For instance <literal>f x y = []</literal> would report
1162 <varname>x</varname> and <varname>y</varname> as unused. The
1163 warning is suppressed if the variable name begins with an underscore, thus:
1173 <para>If you're feeling really paranoid, the
1174 <option>-dcore-lint</option>
1175 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1176 is a good choice. It turns on heavyweight intra-pass
1177 sanity-checking within GHC. (It checks GHC's sanity, not
1184 <sect1 id="options-optimise">
1185 <title>Optimisation (code improvement)</title>
1187 <indexterm><primary>optimisation</primary></indexterm>
1188 <indexterm><primary>improvement, code</primary></indexterm>
1190 <para>The <option>-O*</option> options specify convenient
1191 “packages” of optimisation flags; the
1192 <option>-f*</option> options described later on specify
1193 <emphasis>individual</emphasis> optimisations to be turned on/off;
1194 the <option>-m*</option> options specify
1195 <emphasis>machine-specific</emphasis> optimisations to be turned
1198 <sect2 id="optimise-pkgs">
1199 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1201 <para>There are <emphasis>many</emphasis> options that affect
1202 the quality of code produced by GHC. Most people only have a
1203 general goal, something like “Compile quickly” or
1204 “Make my program run like greased lightning.” The
1205 following “packages” of optimisations (or lack
1206 thereof) should suffice.</para>
1208 <para>Note that higher optimisation levels cause more
1209 cross-module optimisation to be performed, which can have an
1210 impact on how much of your program needs to be recompiled when
1211 you change something. This is one reaosn to stick to
1212 no-optimisation when developing code.</para>
1218 No <option>-O*</option>-type option specified:
1219 <indexterm><primary>-O* not specified</primary></indexterm>
1222 <para>This is taken to mean: “Please compile
1223 quickly; I'm not over-bothered about compiled-code
1224 quality.” So, for example: <command>ghc -c
1225 Foo.hs</command></para>
1231 <option>-O0</option>:
1232 <indexterm><primary><option>-O0</option></primary></indexterm>
1235 <para>Means “turn off all optimisation”,
1236 reverting to the same settings as if no
1237 <option>-O</option> options had been specified. Saying
1238 <option>-O0</option> can be useful if
1239 eg. <command>make</command> has inserted a
1240 <option>-O</option> on the command line already.</para>
1246 <option>-O</option> or <option>-O1</option>:
1247 <indexterm><primary>-O option</primary></indexterm>
1248 <indexterm><primary>-O1 option</primary></indexterm>
1249 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1252 <para>Means: “Generate good-quality code without
1253 taking too long about it.” Thus, for example:
1254 <command>ghc -c -O Main.lhs</command></para>
1256 <para><option>-O</option> currently also implies
1257 <option>-fvia-C</option>. This may change in the
1264 <option>-O2</option>:
1265 <indexterm><primary>-O2 option</primary></indexterm>
1266 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1269 <para>Means: “Apply every non-dangerous
1270 optimisation, even if it means significantly longer
1271 compile times.”</para>
1273 <para>The avoided “dangerous” optimisations
1274 are those that can make runtime or space
1275 <emphasis>worse</emphasis> if you're unlucky. They are
1276 normally turned on or off individually.</para>
1278 <para>At the moment, <option>-O2</option> is
1279 <emphasis>unlikely</emphasis> to produce better code than
1280 <option>-O</option>.</para>
1286 <option>-Ofile <file></option>:
1287 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1288 <indexterm><primary>optimising, customised</primary></indexterm>
1291 <para>(NOTE: not supported since GHC 4.x. Please ask if
1292 you're interested in this.)</para>
1294 <para>For those who need <emphasis>absolute</emphasis>
1295 control over <emphasis>exactly</emphasis> what options are
1296 used (e.g., compiler writers, sometimes :-), a list of
1297 options can be put in a file and then slurped in with
1298 <option>-Ofile</option>.</para>
1300 <para>In that file, comments are of the
1301 <literal>#</literal>-to-end-of-line variety; blank
1302 lines and most whitespace is ignored.</para>
1304 <para>Please ask if you are baffled and would like an
1305 example of <option>-Ofile</option>!</para>
1310 <para>We don't use a <option>-O*</option> flag for day-to-day
1311 work. We use <option>-O</option> to get respectable speed;
1312 e.g., when we want to measure something. When we want to go for
1313 broke, we tend to use <option>-O2 -fvia-C</option> (and we go for
1314 lots of coffee breaks).</para>
1316 <para>The easiest way to see what <option>-O</option> (etc.)
1317 “really mean” is to run with <option>-v</option>,
1318 then stand back in amazement.</para>
1321 <sect2 id="options-f">
1322 <title><option>-f*</option>: platform-independent flags</title>
1324 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1325 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1327 <para>These flags turn on and off individual optimisations.
1328 They are normally set via the <option>-O</option> options
1329 described above, and as such, you shouldn't need to set any of
1330 them explicitly (indeed, doing so could lead to unexpected
1331 results). However, there are one or two that may be of
1336 <term><option>-fexcess-precision</option>:</term>
1338 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1339 <para>When this option is given, intermediate floating
1340 point values can have a <emphasis>greater</emphasis>
1341 precision/range than the final type. Generally this is a
1342 good thing, but some programs may rely on the exact
1344 <literal>Float</literal>/<literal>Double</literal> values
1345 and should not use this option for their compilation.</para>
1350 <term><option>-fignore-asserts</option>:</term>
1352 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1353 <para>Causes GHC to ignore uses of the function
1354 <literal>Exception.assert</literal> in source code (in
1355 other words, rewriting <literal>Exception.assert p
1356 e</literal> to <literal>e</literal> (see <xref
1357 linkend="sec-assertions"/>). This flag is turned on by
1358 <option>-O</option>.
1365 <option>-fno-cse</option>
1366 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1369 <para>Turns off the common-sub-expression elimination optimisation.
1370 Can be useful if you have some <literal>unsafePerformIO</literal>
1371 expressions that you don't want commoned-up.</para>
1377 <option>-fno-strictness</option>
1378 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1381 <para>Turns off the strictness analyser; sometimes it eats
1382 too many cycles.</para>
1388 <option>-fno-full-laziness</option>
1389 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1392 <para>Turns off the full laziness optimisation (also known as
1393 let-floating). Full laziness increases sharing, which can lead
1394 to increased memory residency.</para>
1396 <para>NOTE: GHC doesn't implement complete full-laziness.
1397 When optimisation in on, and
1398 <option>-fno-full-laziness</option> is not given, some
1399 transformations that increase sharing are performed, such
1400 as extracting repeated computations from a loop. These
1401 are the same transformations that a fully lazy
1402 implementation would do, the difference is that GHC
1403 doesn't consistently apply full-laziness, so don't rely on
1410 <option>-fno-state-hack</option>
1411 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1414 <para>Turn off the "state hack" whereby any lambda with a
1415 <literal>State#</literal> token as argument is considered to be
1416 single-entry, hence it is considered OK to inline things inside
1417 it. This can improve performance of IO and ST monad code, but it
1418 runs the risk of reducing sharing.</para>
1424 <option>-funbox-strict-fields</option>:
1425 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1426 <indexterm><primary>strict constructor fields</primary></indexterm>
1427 <indexterm><primary>constructor fields, strict</primary></indexterm>
1430 <para>This option causes all constructor fields which are
1431 marked strict (i.e. “!”) to be unboxed or
1432 unpacked if possible. It is equivalent to adding an
1433 <literal>UNPACK</literal> pragma to every strict
1434 constructor field (see <xref
1435 linkend="unpack-pragma"/>).</para>
1437 <para>This option is a bit of a sledgehammer: it might
1438 sometimes make things worse. Selectively unboxing fields
1439 by using <literal>UNPACK</literal> pragmas might be
1446 <option>-funfolding-update-in-place<n></option>
1447 <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1450 <para>Switches on an experimental "optimisation".
1451 Switching it on makes the compiler a little keener to
1452 inline a function that returns a constructor, if the
1453 context is that of a thunk.
1457 If we inlined plusInt we might get an opportunity to use
1458 update-in-place for the thunk 'x'.</para>
1464 <option>-funfolding-creation-threshold<n></option>:
1465 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1466 <indexterm><primary>inlining, controlling</primary></indexterm>
1467 <indexterm><primary>unfolding, controlling</primary></indexterm>
1470 <para>(Default: 45) Governs the maximum size that GHC will
1471 allow a function unfolding to be. (An unfolding has a
1472 “size” that reflects the cost in terms of
1473 “code bloat” of expanding that unfolding at
1474 at a call site. A bigger function would be assigned a
1475 bigger cost.) </para>
1477 <para> Consequences: (a) nothing larger than this will be
1478 inlined (unless it has an INLINE pragma); (b) nothing
1479 larger than this will be spewed into an interface
1483 <para> Increasing this figure is more likely to result in longer
1484 compile times than faster code. The next option is more
1490 <term><option>-funfolding-use-threshold<n></option>:</term>
1492 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1493 <indexterm><primary>inlining, controlling</primary></indexterm>
1494 <indexterm><primary>unfolding, controlling</primary></indexterm>
1496 <para>(Default: 8) This is the magic cut-off figure for
1497 unfolding: below this size, a function definition will be
1498 unfolded at the call-site, any bigger and it won't. The
1499 size computed for a function depends on two things: the
1500 actual size of the expression minus any discounts that
1501 apply (see <option>-funfolding-con-discount</option>).</para>
1512 <sect1 id="sec-using-concurrent">
1513 <title>Using Concurrent Haskell</title>
1514 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
1516 <para>GHC supports Concurrent Haskell by default, without requiring a
1517 special option or libraries compiled in a certain way. To get access to
1518 the support libraries for Concurrent Haskell, just import
1520 url="../libraries/base/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
1522 <para>The following RTS option(s) affect the behaviour of Concurrent
1523 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
1527 <term><option>-C<replaceable>s</replaceable></option></term>
1529 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1530 Sets the context switch interval to <replaceable>s</replaceable>
1531 seconds. A context switch will occur at the next heap block
1532 allocation after the timer expires (a heap block allocation occurs
1533 every 4k of allocation). With <option>-C0</option> or
1534 <option>-C</option>, context switches will occur as often as
1535 possible (at every heap block allocation). By default, context
1536 switches occur every 20ms.</para>
1542 <sect1 id="sec-using-smp">
1543 <title>Using SMP parallelism</title>
1544 <indexterm><primary>parallelism</primary>
1546 <indexterm><primary>SMP</primary>
1549 <para>GHC supports running Haskell programs in parallel on an SMP
1550 (symmetric multiprocessor).</para>
1552 <para>There's a fine distinction between
1553 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
1554 parallelism is all about making your program run
1555 <emphasis>faster</emphasis> by making use of multiple processors
1556 simultaneously. Concurrency, on the other hand, is a means of
1557 abstraction: it is a convenient way to structure a program that must
1558 respond to multiple asynchronous events.</para>
1560 <para>However, the two terms are certainly related. By making use of
1561 multiple CPUs it is possible to run concurrent threads in parallel,
1562 and this is exactly what GHC's SMP parallelism support does. But it
1563 is also possible to obtain performance improvements with parallelism
1564 on programs that do not use concurrency. This section describes how to
1565 use GHC to compile and run parallel programs, in <xref
1566 linkend="lang-parallel" /> we desribe the language features that affect
1569 <sect2 id="parallel-options">
1570 <title>Options to enable SMP parallelism</title>
1572 <para>In order to make use of multiple CPUs, your program must be
1573 linked with the <option>-threaded</option> option (see <xref
1574 linkend="options-linker" />). Then, to run a program on multiple
1575 CPUs, use the RTS <option>-N</option> option:</para>
1579 <term><option>-N<replaceable>x</replaceable></option></term>
1581 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1582 Use <replaceable>x</replaceable> simultaneous threads when
1583 running the program. Normally <replaceable>x</replaceable>
1584 should be chosen to match the number of CPU cores on the machine.
1585 There is no means (currently) by which this value may vary after
1586 the program has started.</para>
1588 <para>For example, on a dual-core machine we would probably use
1589 <literal>+RTS -N2 -RTS</literal>.</para>
1591 <para>Whether hyperthreading cores should be counted or not is an
1592 open question; please feel free to experiment and let us know what
1593 results you find.</para>
1600 <title>Hints for using SMP parallelism</title>
1602 <para>Add the <literal>-sstderr</literal> RTS option when
1603 running the program to see timing stats, which will help to tell you
1604 whether your program got faster by using more CPUs or not. If the user
1605 time is greater than
1606 the elapsed time, then the program used more than one CPU. You should
1607 also run the program without <literal>-N</literal> for comparison.</para>
1609 <para>GHC's parallelism support is new and experimental. It may make your
1610 program go faster, or it might slow it down - either way, we'd be
1611 interested to hear from you.</para>
1613 <para>One significant limitation with the current implementation is that
1614 the garbage collector is still single-threaded, and all execution must
1615 stop when GC takes place. This can be a significant bottleneck in a
1616 parallel program, especially if your program does a lot of GC. If this
1617 happens to you, then try reducing the cost of GC by tweaking the GC
1618 settings (<xref linkend="rts-options-gc" />): enlarging the heap or the
1619 allocation area size is a good start.</para>
1623 <sect1 id="options-platform">
1624 <title>Platform-specific Flags</title>
1626 <indexterm><primary>-m* options</primary></indexterm>
1627 <indexterm><primary>platform-specific options</primary></indexterm>
1628 <indexterm><primary>machine-specific options</primary></indexterm>
1630 <para>Some flags only make sense for particular target
1636 <term><option>-monly-[32]-regs</option>:</term>
1638 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1639 option (iX86 only)</primary></indexterm> GHC tries to
1640 “steal” four registers from GCC, for performance
1641 reasons; it almost always works. However, when GCC is
1642 compiling some modules with four stolen registers, it will
1643 crash, probably saying:
1646 Foo.hc:533: fixed or forbidden register was spilled.
1647 This may be due to a compiler bug or to impossible asm
1648 statements or clauses.
1651 Just give some registers back with
1652 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1653 If `2' doesn't work, please report the bug to us.</para>
1662 <sect1 id="ext-core">
1663 <title>Generating and compiling External Core Files</title>
1665 <indexterm><primary>intermediate code generation</primary></indexterm>
1667 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
1668 to a file as a side-effect of compilation. Core files, which are given the suffix
1669 <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1670 tools. The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz">
1671 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1672 and sample tools (in Haskell)
1673 for manipulating Core files are available in the GHC source distribution
1674 directory <literal>/fptools/ghc/utils/ext-core</literal>.
1675 Note that the format of <literal>.hcr</literal>
1676 files is <emphasis>different</emphasis> (though similar) to the Core output format generated
1677 for debugging purposes (<xref linkend="options-debugging"/>).</para>
1679 <para>The Core format natively supports notes which you can add to
1680 your source code using the <literal>CORE</literal> pragma (see <xref
1681 linkend="pragmas"/>).</para>
1687 <option>-fext-core</option>
1688 <indexterm><primary><option>-fext-core</option></primary></indexterm>
1691 <para>Generate <literal>.hcr</literal> files.</para>
1697 <para>GHC can also read in External Core files as source; just give the <literal>.hcr</literal> file on
1698 the command line, instead of the <literal>.hs</literal> or <literal>.lhs</literal> Haskell source.
1699 A current infelicity is that you need to give the <literal>-fglasgow-exts</literal> flag too, because
1700 ordinary Haskell 98, when translated to External Core, uses things like rank-2 types.</para>
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