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</command>
229 <arg choice='plain'>––interactive</arg>
231 <indexterm><primary>interactive mode</primary></indexterm>
232 <indexterm><primary>ghci</primary></indexterm>
235 <para>Interactive mode, which is also available as
236 <command>ghci</command>. Interactive mode is described in
237 more detail in <xref linkend="ghci"/>.</para>
243 <cmdsynopsis><command>ghc</command>
244 <arg choice='plain'>––make</arg>
246 <indexterm><primary>make mode</primary></indexterm>
247 <indexterm><primary><option>––make</option></primary></indexterm>
250 <para>In this mode, GHC will build a multi-module Haskell
251 program automatically, figuring out dependencies for itself.
252 If you have a straightforward Haskell program, this is
253 likely to be much easier, and faster, than using
254 <command>make</command>. Make mode is described in <xref
255 linkend="make-mode"/>.</para>
261 <cmdsynopsis><command>ghc</command>
262 <arg choice='plain'>–e</arg> <arg choice='plain'><replaceable>expr</replaceable></arg>
264 <indexterm><primary>eval mode</primary></indexterm>
267 <para>Expression-evaluation mode. This is very similar to
268 interactive mode, except that there is a single expression
269 to evaluate (<replaceable>expr</replaceable>) which is given
270 on the command line. See <xref linkend="eval-mode"/> for
278 <command>ghc</command>
286 <indexterm><primary><option>-E</option></primary></indexterm>
287 <indexterm><primary><option>-C</option></primary></indexterm>
288 <indexterm><primary><option>-S</option></primary></indexterm>
289 <indexterm><primary><option>-c</option></primary></indexterm>
292 <para>This is the traditional batch-compiler mode, in which
293 GHC can compile source files one at a time, or link objects
294 together into an executable. This mode also applies if
295 there is no other mode flag specified on the command line,
296 in which case it means that the specified files should be
297 compiled and then linked to form a program. See <xref
298 linkend="options-order"/>.</para>
305 <command>ghc</command>
306 <arg choice='plain'>–M</arg>
308 <indexterm><primary>dependency-generation mode</primary></indexterm>
311 <para>Dependency-generation mode. In this mode, GHC can be
312 used to generate dependency information suitable for use in
313 a <literal>Makefile</literal>. See <xref
314 linkend="sec-makefile-dependencies"/>.</para>
321 <command>ghc</command>
322 <arg choice='plain'>––mk-dll</arg>
324 <indexterm><primary>dependency-generation mode</primary></indexterm>
327 <para>DLL-creation mode (Windows only). See <xref
328 linkend="win32-dlls-create"/>.</para>
333 <sect2 id="make-mode">
334 <title>Using <command>ghc</command> <option>––make</option></title>
335 <indexterm><primary><option>––make</option></primary></indexterm>
336 <indexterm><primary>separate compilation</primary></indexterm>
338 <para>When given the <option>––make</option> option,
339 GHC will build a multi-module Haskell program by following
340 dependencies from a single root module (usually
341 <literal>Main</literal>). For example, if your
342 <literal>Main</literal> module is in a file called
343 <filename>Main.hs</filename>, you could compile and link the
344 program like this:</para>
347 ghc ––make Main.hs
350 <para>The command line may contain any number of source file
351 names or module names; GHC will figure out all the modules in
352 the program by following the imports from these initial modules.
353 It will then attempt to compile each module which is out of
354 date, and finally if there is a <literal>Main</literal> module,
355 the program will also be linked into an executable.</para>
357 <para>The main advantages to using <literal>ghc
358 ––make</literal> over traditional
359 <literal>Makefile</literal>s are:</para>
363 <para>GHC doesn't have to be restarted for each compilation,
364 which means it can cache information between compilations.
365 Compiling a multi-module program with <literal>ghc
366 ––make</literal> can be up to twice as fast as
367 running <literal>ghc</literal> individually on each source
371 <para>You don't have to write a <literal>Makefile</literal>.</para>
372 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
375 <para>GHC re-calculates the dependencies each time it is
376 invoked, so the dependencies never get out of sync with the
381 <para>Any of the command-line options described in the rest of
382 this chapter can be used with
383 <option>––make</option>, but note that any options
384 you give on the command line will apply to all the source files
385 compiled, so if you want any options to apply to a single source
386 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
387 pragma (see <xref linkend="source-file-options"/>).</para>
389 <para>If the program needs to be linked with additional objects
390 (say, some auxiliary C code), then the object files can be
391 given on the command line and GHC will include them when linking
392 the executable.</para>
394 <para>Note that GHC can only follow dependencies if it has the
395 source file available, so if your program includes a module for
396 which there is no source file, even if you have an object and an
397 interface file for the module, then GHC will complain. The
398 exception to this rule is for package modules, which may or may
399 not have source files.</para>
401 <para>The source files for the program don't all need to be in
402 the same directory; the <option>-i</option> option can be used
403 to add directories to the search path (see <xref
404 linkend="search-path"/>).</para>
407 <sect2 id="eval-mode">
408 <title>Expression evaluation mode</title>
410 <para>This mode is very similar to interactive mode, except that
411 there is a single expression to evaluate which is specified on
412 the command line as an argument to the <option>-e</option>
416 ghc -e <replaceable>expr</replaceable>
419 <para>Haskell source files may be named on the command line, and
420 they will be loaded exactly as in interactive mode. The
421 expression is evaluated in the context of the loaded
424 <para>For example, to load and run a Haskell program containing
425 a module <literal>Main</literal>, we might say</para>
428 ghc -e Main.main Main.hs
431 <para>or we can just use this mode to evaluate expressions in
432 the context of the <literal>Prelude</literal>:</para>
435 $ ghc -e "interact (unlines.map reverse.lines)"
441 <sect2 id="options-order">
442 <title>Batch compiler mode</title>
444 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
445 given on the command line.</para>
447 <para>The first phase to run is determined by each input-file
448 suffix, and the last phase is determined by a flag. If no
449 relevant flag is present, then go all the way through linking.
450 This table summarises:</para>
454 <colspec align="left"/>
455 <colspec align="left"/>
456 <colspec align="left"/>
457 <colspec align="left"/>
461 <entry>Phase of the compilation system</entry>
462 <entry>Suffix saying “start here”</entry>
463 <entry>Flag saying “stop after”</entry>
464 <entry>(suffix of) output file</entry>
469 <entry>literate pre-processor</entry>
470 <entry><literal>.lhs</literal></entry>
472 <entry><literal>.hs</literal></entry>
476 <entry>C pre-processor (opt.) </entry>
477 <entry><literal>.hs</literal> (with
478 <option>-cpp</option>)</entry>
479 <entry><option>-E</option></entry>
480 <entry><literal>.hspp</literal></entry>
484 <entry>Haskell compiler</entry>
485 <entry><literal>.hs</literal></entry>
486 <entry><option>-C</option>, <option>-S</option></entry>
487 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
491 <entry>C compiler (opt.)</entry>
492 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
493 <entry><option>-S</option></entry>
494 <entry><literal>.s</literal></entry>
498 <entry>assembler</entry>
499 <entry><literal>.s</literal></entry>
500 <entry><option>-c</option></entry>
501 <entry><literal>.o</literal></entry>
505 <entry>linker</entry>
506 <entry><replaceable>other</replaceable></entry>
508 <entry><filename>a.out</filename></entry>
514 <indexterm><primary><option>-C</option></primary></indexterm>
515 <indexterm><primary><option>-E</option></primary></indexterm>
516 <indexterm><primary><option>-S</option></primary></indexterm>
517 <indexterm><primary><option>-c</option></primary></indexterm>
519 <para>Thus, a common invocation would be: </para>
522 ghc -c Foo.hs</screen>
524 <para>to compile the Haskell source file
525 <filename>Foo.hs</filename> to an object file
526 <filename>Foo.o</filename>.</para>
528 <para>Note: What the Haskell compiler proper produces depends on
529 whether a native-code generator<indexterm><primary>native-code
530 generator</primary></indexterm> is used (producing assembly
531 language) or not (producing C). See <xref
532 linkend="options-codegen"/> for more details.</para>
534 <para>Note: C pre-processing is optional, the
535 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
536 flag turns it on. See <xref linkend="c-pre-processor"/> for more
539 <para>Note: The option <option>-E</option><indexterm><primary>-E
540 option</primary></indexterm> runs just the pre-processing passes
541 of the compiler, dumping the result in a file. Note that this
542 differs from the previous behaviour of dumping the file to
543 standard output.</para>
545 <sect3 id="overriding-suffixes">
546 <title>Overriding the default behaviour for a file</title>
548 <para>As described above, the way in which a file is processed by GHC
549 depends on its suffix. This behaviour can be overriden using the
550 <option>-x</option> option:</para>
554 <term><option>-x</option> <replaceable>suffix</replaceable>
555 <indexterm><primary><option>-x</option></primary>
558 <para>Causes all files following this option on the command
559 line to be processed as if they had the suffix
560 <replaceable>suffix</replaceable>. For example, to compile a
561 Haskell module in the file <literal>M.my-hs</literal>,
562 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
571 <sect1 id="options-help">
572 <title>Help and verbosity options</title>
574 <indexterm><primary>help options</primary></indexterm>
575 <indexterm><primary>verbosity options</primary></indexterm>
580 <option>––help</option>
581 <indexterm><primary><option>––help</option></primary></indexterm>
585 <indexterm><primary><option>-?</option></primary></indexterm>
588 <para>Cause GHC to spew a long usage message to standard
589 output and then exit.</para>
596 <indexterm><primary><option>-n</option></primary></indexterm>
599 <para>Does a dry-run, i.e. GHC goes through all the motions
600 of compiling as normal, but does not actually run any
601 external commands.</para>
608 <indexterm><primary><option>-v</option></primary></indexterm>
611 <para>The <option>-v</option> option makes GHC
612 <emphasis>verbose</emphasis>: it reports its version number
613 and shows (on stderr) exactly how it invokes each phase of
614 the compilation system. Moreover, it passes the
615 <option>-v</option> flag to most phases; each reports its
616 version number (and possibly some other information).</para>
618 <para>Please, oh please, use the <option>-v</option> option
619 when reporting bugs! Knowing that you ran the right bits in
620 the right order is always the first thing we want to
627 <option>-v</option><replaceable>n</replaceable>
628 <indexterm><primary><option>-v</option></primary></indexterm>
631 <para>To provide more control over the compiler's verbosity,
632 the <option>-v</option> flag takes an optional numeric
633 argument. Specifying <option>-v</option> on its own is
634 equivalent to <option>-v3</option>, and the other levels
635 have the following meanings:</para>
639 <term><option>-v0</option></term>
641 <para>Disable all non-essential messages (this is the
647 <term><option>-v1</option></term>
649 <para>Minimal verbosity: print one line per
650 compilation (this is the default when
651 <option>––make</option> or
652 <option>––interactive</option> is on).</para>
657 <term><option>-v2</option></term>
659 <para>Print the name of each compilation phase as it
660 is executed. (equivalent to
661 <option>-dshow-passes</option>).</para>
666 <term><option>-v3</option></term>
668 <para>The same as <option>-v2</option>, except that in
669 addition the full command line (if appropriate) for
670 each compilation phase is also printed.</para>
675 <term><option>-v4</option></term>
677 <para>The same as <option>-v3</option> except that the
678 intermediate program representation after each
679 compilation phase is also printed (excluding
680 preprocessed and C/assembly files).</para>
690 <indexterm><primary><option>-V</option></primary></indexterm>
693 <option>––version</option>
694 <indexterm><primary><option>––version</option></primary></indexterm>
697 <para>Print a one-line string including GHC's version number.</para>
703 <option>––numeric-version</option>
704 <indexterm><primary><option>––numeric-version</option></primary></indexterm>
707 <para>Print GHC's numeric version number only.</para>
713 <option>––print-libdir</option>
714 <indexterm><primary><option>––print-libdir</option></primary></indexterm>
717 <para>Print the path to GHC's library directory. This is
718 the top of the directory tree containing GHC's libraries,
719 interfaces, and include files (usually something like
720 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
722 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary>
723 </indexterm>in the package configuration file (see <xref
724 linkend="packages"/>).</para>
729 <term><option>-ferror-spans</option>
730 <indexterm><primary><option>-ferror-spans</option></primary>
734 <para>Causes GHC to emit the full source span of the
735 syntactic entity relating to an error message. Normally, GHC
736 emits the source location of the start of the syntactic
739 <para>For example:</para>
741 <screen>test.hs:3:6: parse error on input `where'</screen>
743 <para>becomes:</para>
745 <screen>test296.hs:3:6-10: parse error on input `where'</screen>
747 <para>And multi-line spans are possible too:</para>
749 <screen>test.hs:(5,4)-(6,7):
750 Conflicting definitions for `a'
751 Bound at: test.hs:5:4
753 In the binding group for: a, b, a</screen>
755 <para>Note that line numbers start counting at one, but
756 column numbers start at zero. This choice was made to
757 follow existing convention (i.e. this is how Emacs does
763 <term><option>-H</option><replaceable>size</replaceable>
764 <indexterm><primary><option>-H</option></primary></indexterm>
767 <para>Set the minimum size of the heap to
768 <replaceable>size</replaceable>.
769 This option is equivalent to
770 <literal>+RTS -H<replaceable>size</replaceable></literal>,
771 see <xref linkend="rts-options-gc" />.
777 <term><option>-Rghc-timing</option>
778 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
781 <para>Prints a one-line summary of timing statistics for the
782 GHC run. This option is equivalent to
783 <literal>+RTS -tstderr</literal>, see <xref
784 linkend="rts-options-gc" />.
793 <sect1 id="options-sanity">
794 <title>Warnings and sanity-checking</title>
796 <indexterm><primary>sanity-checking options</primary></indexterm>
797 <indexterm><primary>warnings</primary></indexterm>
800 <para>GHC has a number of options that select which types of
801 non-fatal error messages, otherwise known as warnings, can be
802 generated during compilation. By default, you get a standard set
803 of warnings which are generally likely to indicate bugs in your
805 <option>-fwarn-overlapping-patterns</option>,
806 <option>-fwarn-deprecations</option>,
807 <option>-fwarn-duplicate-exports</option>,
808 <option>-fwarn-missing-fields</option>, and
809 <option>-fwarn-missing-methods</option>. The following flags are
810 simple ways to select standard “packages” of warnings:
816 <term><option>-W</option>:</term>
818 <indexterm><primary>-W option</primary></indexterm>
819 <para>Provides the standard warnings plus
820 <option>-fwarn-incomplete-patterns</option>,
821 <option>-fwarn-unused-matches</option>,
822 <option>-fwarn-unused-imports</option>, and
823 <option>-fwarn-unused-binds</option>.</para>
828 <term><option>-w</option>:</term>
830 <indexterm><primary><option>-w</option></primary></indexterm>
831 <para>Turns off all warnings, including the standard ones.</para>
836 <term><option>-Wall</option>:</term>
838 <indexterm><primary><option>-Wall</option></primary></indexterm>
839 <para>Turns on all warning options.</para>
844 <term><option>-Werror</option>:</term>
846 <indexterm><primary><option>-Werror</option></primary></indexterm>
847 <para>Makes any warning into a fatal error. Useful so that you don't
848 miss warnings when doing batch compilation. </para>
854 <para>The full set of warning options is described below. To turn
855 off any warning, simply give the corresponding
856 <option>-fno-warn-...</option> option on the command line.</para>
861 <term><option>-fwarn-deprecations</option>:</term>
863 <indexterm><primary><option>-fwarn-deprecations</option></primary>
865 <indexterm><primary>deprecations</primary></indexterm>
866 <para>Causes a warning to be emitted when a deprecated
867 function or type is used. Entities can be marked as
868 deprecated using a pragma, see <xref
869 linkend="deprecated-pragma"/>.</para>
874 <term><option>-fwarn-duplicate-exports</option>:</term>
876 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
877 <indexterm><primary>duplicate exports, warning</primary></indexterm>
878 <indexterm><primary>export lists, duplicates</primary></indexterm>
880 <para>Have the compiler warn about duplicate entries in
881 export lists. This is useful information if you maintain
882 large export lists, and want to avoid the continued export
883 of a definition after you've deleted (one) mention of it in
884 the export list.</para>
886 <para>This option is on by default.</para>
891 <term><option>-fwarn-hi-shadowing</option>:</term>
893 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
894 <indexterm><primary>shadowing</primary>
895 <secondary>interface files</secondary></indexterm>
897 <para>Causes the compiler to emit a warning when a module or
898 interface file in the current directory is shadowing one
899 with the same module name in a library or other
905 <term><option>-fwarn-incomplete-patterns</option>:</term>
907 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
908 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
909 <indexterm><primary>patterns, incomplete</primary></indexterm>
911 <para>Similarly for incomplete patterns, the function
912 <function>g</function> below will fail when applied to
913 non-empty lists, so the compiler will emit a warning about
914 this when <option>-fwarn-incomplete-patterns</option> is
921 <para>This option isn't enabled be default because it can be
922 a bit noisy, and it doesn't always indicate a bug in the
923 program. However, it's generally considered good practice
924 to cover all the cases in your functions.</para>
929 <term><option>-fwarn-incomplete-record-updates</option>:</term>
931 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
932 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
933 <indexterm><primary>record updates, incomplete</primary></indexterm>
936 <function>f</function> below will fail when applied to
937 <literal>Bar</literal>, so the compiler will emit a warning about
938 this when <option>-fwarn-incomplete-record-updates</option> is
942 data Foo = Foo { x :: Int }
946 f foo = foo { x = 6 }
949 <para>This option isn't enabled be default because it can be
950 very noisy, and it often doesn't indicate a bug in the
957 <option>-fwarn-missing-fields</option>:
958 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
959 <indexterm><primary>missing fields, warning</primary></indexterm>
960 <indexterm><primary>fields, missing</primary></indexterm>
964 <para>This option is on by default, and warns you whenever
965 the construction of a labelled field constructor isn't
966 complete, missing initializers for one or more fields. While
967 not an error (the missing fields are initialised with
968 bottoms), it is often an indication of a programmer error.</para>
973 <term><option>-fwarn-missing-methods</option>:</term>
975 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
976 <indexterm><primary>missing methods, warning</primary></indexterm>
977 <indexterm><primary>methods, missing</primary></indexterm>
979 <para>This option is on by default, and warns you whenever
980 an instance declaration is missing one or more methods, and
981 the corresponding class declaration has no default
982 declaration for them.</para>
983 <para>The warning is suppressed if the method name
984 begins with an underscore. Here's an example where this is useful:
987 _simpleFn :: a -> String
988 complexFn :: a -> a -> String
989 complexFn x y = ... _simpleFn ...
991 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
992 never <literal>_simpleFn</literal>; and (b)
993 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
999 <term><option>-fwarn-missing-signatures</option>:</term>
1001 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1002 <indexterm><primary>type signatures, missing</primary></indexterm>
1004 <para>If you would like GHC to check that every top-level
1005 function/value has a type signature, use the
1006 <option>-fwarn-missing-signatures</option> option. This
1007 option is off by default.</para>
1012 <term><option>-fwarn-name-shadowing</option>:</term>
1014 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1015 <indexterm><primary>shadowing, warning</primary></indexterm>
1017 <para>This option causes a warning to be emitted whenever an
1018 inner-scope value has the same name as an outer-scope value,
1019 i.e. the inner value shadows the outer one. This can catch
1020 typographical errors that turn into hard-to-find bugs, e.g.,
1021 in the inadvertent cyclic definition <literal>let x = ... x
1022 ... in</literal>.</para>
1024 <para>Consequently, this option does
1025 <emphasis>will</emphasis> complain about cyclic recursive
1031 <term><option>-fwarn-orphans</option>:</term>
1033 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1034 <indexterm><primary>orphan instances, warning</primary></indexterm>
1035 <indexterm><primary>orphan rules, warning</primary></indexterm>
1037 <para>This option causes a warning to be emitted whenever the
1038 module contains an "orphan" instance declaration or rewrite rule.
1039 An instance declartion is an orphan if it appears in a module in
1040 which neither the class nor the type being instanced are declared
1041 in the same module. A rule is an orphan if it is a rule for a
1042 function declared in another module. A module containing any
1043 orphans is called an orphan module.</para>
1044 <para>The trouble with orphans is that GHC must pro-actively read the interface
1045 files for all orphan modules, just in case their instances or rules
1046 play a role, whether or not the module's interface would otherwise
1047 be of any use. Other things being equal, avoid orphan modules.</para>
1053 <option>-fwarn-overlapping-patterns</option>:
1054 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1055 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1056 <indexterm><primary>patterns, overlapping</primary></indexterm>
1059 <para>By default, the compiler will warn you if a set of
1060 patterns are overlapping, i.e.,</para>
1063 f :: String -> Int
1069 <para>where the last pattern match in <function>f</function>
1070 won't ever be reached, as the second pattern overlaps
1071 it. More often than not, redundant patterns is a programmer
1072 mistake/error, so this option is enabled by default.</para>
1077 <term><option>-fwarn-simple-patterns</option>:</term>
1079 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1081 <para>Causes the compiler to warn about lambda-bound
1082 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1083 Normally, these aren't treated as incomplete patterns by
1084 <option>-fwarn-incomplete-patterns</option>.</para>
1085 <para>``Lambda-bound patterns'' includes all places where there is a single pattern,
1086 including list comprehensions and do-notation. In these cases, a pattern-match
1087 failure is quite legitimate, and triggers filtering (list comprehensions) or
1088 the monad <literal>fail</literal> operation (monads). For example:
1090 f :: [Maybe a] -> [a]
1091 f xs = [y | Just y <- xs]
1093 Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
1094 these probably-innocent cases, which is why the flag is off by default. </para>
1095 <para> The <literal>deriving( Read )</literal> mechanism produces monadic code with
1096 pattern matches, so you will also get misleading warnings about the compiler-generated
1097 code. (This is arguably a Bad Thing, but it's awkward to fix.)</para>
1103 <term><option>-fwarn-type-defaults</option>:</term>
1105 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1106 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1107 <para>Have the compiler warn/inform you where in your source
1108 the Haskell defaulting mechanism for numeric types kicks
1109 in. This is useful information when converting code from a
1110 context that assumed one default into one with another,
1111 e.g., the `default default' for Haskell 1.4 caused the
1112 otherwise unconstrained value <constant>1</constant> to be
1113 given the type <literal>Int</literal>, whereas Haskell 98
1114 defaults it to <literal>Integer</literal>. This may lead to
1115 differences in performance and behaviour, hence the
1116 usefulness of being non-silent about this.</para>
1118 <para>This warning is off by default.</para>
1123 <term><option>-fwarn-unused-binds</option>:</term>
1125 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1126 <indexterm><primary>unused binds, warning</primary></indexterm>
1127 <indexterm><primary>binds, unused</primary></indexterm>
1128 <para>Report any function definitions (and local bindings)
1129 which are unused. For top-level functions, the warning is
1130 only given if the binding is not exported.</para>
1131 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1132 mentioned in the right hand side of another definition that is used, or (c) the
1133 function it defines begins with an underscore. The last case provides a
1134 way to suppress unused-binding warnings selectively. </para>
1135 <para> Notice that a variable
1136 is reported as unused even if it appears in the right-hand side of another
1137 unused binding. </para>
1142 <term><option>-fwarn-unused-imports</option>:</term>
1144 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1145 <indexterm><primary>unused imports, warning</primary></indexterm>
1146 <indexterm><primary>imports, unused</primary></indexterm>
1148 <para>Report any modules that are explicitly imported but
1149 never used. However, the form <literal>import M()</literal> is
1150 never reported as an unused import, because it is a useful idiom
1151 for importing instance declarations, which are anonymous in Haskell.</para>
1156 <term><option>-fwarn-unused-matches</option>:</term>
1158 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1159 <indexterm><primary>unused matches, warning</primary></indexterm>
1160 <indexterm><primary>matches, unused</primary></indexterm>
1162 <para>Report all unused variables which arise from pattern
1163 matches, including patterns consisting of a single variable.
1164 For instance <literal>f x y = []</literal> would report
1165 <varname>x</varname> and <varname>y</varname> as unused. The
1166 warning is suppressed if the variable name begins with an underscore, thus:
1176 <para>If you're feeling really paranoid, the
1177 <option>-dcore-lint</option>
1178 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1179 is a good choice. It turns on heavyweight intra-pass
1180 sanity-checking within GHC. (It checks GHC's sanity, not
1187 <sect1 id="options-optimise">
1188 <title>Optimisation (code improvement)</title>
1190 <indexterm><primary>optimisation</primary></indexterm>
1191 <indexterm><primary>improvement, code</primary></indexterm>
1193 <para>The <option>-O*</option> options specify convenient
1194 “packages” of optimisation flags; the
1195 <option>-f*</option> options described later on specify
1196 <emphasis>individual</emphasis> optimisations to be turned on/off;
1197 the <option>-m*</option> options specify
1198 <emphasis>machine-specific</emphasis> optimisations to be turned
1201 <sect2 id="optimise-pkgs">
1202 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1204 <para>There are <emphasis>many</emphasis> options that affect
1205 the quality of code produced by GHC. Most people only have a
1206 general goal, something like “Compile quickly” or
1207 “Make my program run like greased lightning.” The
1208 following “packages” of optimisations (or lack
1209 thereof) should suffice.</para>
1211 <para>Note that higher optimisation levels cause more
1212 cross-module optimisation to be performed, which can have an
1213 impact on how much of your program needs to be recompiled when
1214 you change something. This is one reaosn to stick to
1215 no-optimisation when developing code.</para>
1221 No <option>-O*</option>-type option specified:
1222 <indexterm><primary>-O* not specified</primary></indexterm>
1225 <para>This is taken to mean: “Please compile
1226 quickly; I'm not over-bothered about compiled-code
1227 quality.” So, for example: <command>ghc -c
1228 Foo.hs</command></para>
1234 <option>-O0</option>:
1235 <indexterm><primary><option>-O0</option></primary></indexterm>
1238 <para>Means “turn off all optimisation”,
1239 reverting to the same settings as if no
1240 <option>-O</option> options had been specified. Saying
1241 <option>-O0</option> can be useful if
1242 eg. <command>make</command> has inserted a
1243 <option>-O</option> on the command line already.</para>
1249 <option>-O</option> or <option>-O1</option>:
1250 <indexterm><primary>-O option</primary></indexterm>
1251 <indexterm><primary>-O1 option</primary></indexterm>
1252 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1255 <para>Means: “Generate good-quality code without
1256 taking too long about it.” Thus, for example:
1257 <command>ghc -c -O Main.lhs</command></para>
1259 <para><option>-O</option> currently also implies
1260 <option>-fvia-C</option>. This may change in the
1267 <option>-O2</option>:
1268 <indexterm><primary>-O2 option</primary></indexterm>
1269 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1272 <para>Means: “Apply every non-dangerous
1273 optimisation, even if it means significantly longer
1274 compile times.”</para>
1276 <para>The avoided “dangerous” optimisations
1277 are those that can make runtime or space
1278 <emphasis>worse</emphasis> if you're unlucky. They are
1279 normally turned on or off individually.</para>
1281 <para>At the moment, <option>-O2</option> is
1282 <emphasis>unlikely</emphasis> to produce better code than
1283 <option>-O</option>.</para>
1289 <option>-Ofile <file></option>:
1290 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1291 <indexterm><primary>optimising, customised</primary></indexterm>
1294 <para>(NOTE: not supported since GHC 4.x. Please ask if
1295 you're interested in this.)</para>
1297 <para>For those who need <emphasis>absolute</emphasis>
1298 control over <emphasis>exactly</emphasis> what options are
1299 used (e.g., compiler writers, sometimes :-), a list of
1300 options can be put in a file and then slurped in with
1301 <option>-Ofile</option>.</para>
1303 <para>In that file, comments are of the
1304 <literal>#</literal>-to-end-of-line variety; blank
1305 lines and most whitespace is ignored.</para>
1307 <para>Please ask if you are baffled and would like an
1308 example of <option>-Ofile</option>!</para>
1313 <para>We don't use a <option>-O*</option> flag for day-to-day
1314 work. We use <option>-O</option> to get respectable speed;
1315 e.g., when we want to measure something. When we want to go for
1316 broke, we tend to use <option>-O2 -fvia-C</option> (and we go for
1317 lots of coffee breaks).</para>
1319 <para>The easiest way to see what <option>-O</option> (etc.)
1320 “really mean” is to run with <option>-v</option>,
1321 then stand back in amazement.</para>
1324 <sect2 id="options-f">
1325 <title><option>-f*</option>: platform-independent flags</title>
1327 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1328 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1330 <para>These flags turn on and off individual optimisations.
1331 They are normally set via the <option>-O</option> options
1332 described above, and as such, you shouldn't need to set any of
1333 them explicitly (indeed, doing so could lead to unexpected
1334 results). However, there are one or two that may be of
1339 <term><option>-fexcess-precision</option>:</term>
1341 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1342 <para>When this option is given, intermediate floating
1343 point values can have a <emphasis>greater</emphasis>
1344 precision/range than the final type. Generally this is a
1345 good thing, but some programs may rely on the exact
1347 <literal>Float</literal>/<literal>Double</literal> values
1348 and should not use this option for their compilation.</para>
1353 <term><option>-fignore-asserts</option>:</term>
1355 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1356 <para>Causes GHC to ignore uses of the function
1357 <literal>Exception.assert</literal> in source code (in
1358 other words, rewriting <literal>Exception.assert p
1359 e</literal> to <literal>e</literal> (see <xref
1360 linkend="sec-assertions"/>). This flag is turned on by
1361 <option>-O</option>.
1368 <option>-fno-cse</option>
1369 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1372 <para>Turns off the common-sub-expression elimination optimisation.
1373 Can be useful if you have some <literal>unsafePerformIO</literal>
1374 expressions that you don't want commoned-up.</para>
1380 <option>-fno-strictness</option>
1381 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1384 <para>Turns off the strictness analyser; sometimes it eats
1385 too many cycles.</para>
1391 <option>-fno-full-laziness</option>
1392 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1395 <para>Turns off the full laziness optimisation (also known as
1396 let-floating). Full laziness increases sharing, which can lead
1397 to increased memory residency.</para>
1399 <para>NOTE: GHC doesn't implement complete full-laziness.
1400 When optimisation in on, and
1401 <option>-fno-full-laziness</option> is not given, some
1402 transformations that increase sharing are performed, such
1403 as extracting repeated computations from a loop. These
1404 are the same transformations that a fully lazy
1405 implementation would do, the difference is that GHC
1406 doesn't consistently apply full-laziness, so don't rely on
1413 <option>-fno-state-hack</option>
1414 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1417 <para>Turn off the "state hack" whereby any lambda with a
1418 <literal>State#</literal> token as argument is considered to be
1419 single-entry, hence it is considered OK to inline things inside
1420 it. This can improve performance of IO and ST monad code, but it
1421 runs the risk of reducing sharing.</para>
1427 <option>-funbox-strict-fields</option>:
1428 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1429 <indexterm><primary>strict constructor fields</primary></indexterm>
1430 <indexterm><primary>constructor fields, strict</primary></indexterm>
1433 <para>This option causes all constructor fields which are
1434 marked strict (i.e. “!”) to be unboxed or
1435 unpacked if possible. It is equivalent to adding an
1436 <literal>UNPACK</literal> pragma to every strict
1437 constructor field (see <xref
1438 linkend="unpack-pragma"/>).</para>
1440 <para>This option is a bit of a sledgehammer: it might
1441 sometimes make things worse. Selectively unboxing fields
1442 by using <literal>UNPACK</literal> pragmas might be
1449 <option>-funfolding-update-in-place<n></option>
1450 <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1453 <para>Switches on an experimental "optimisation".
1454 Switching it on makes the compiler a little keener to
1455 inline a function that returns a constructor, if the
1456 context is that of a thunk.
1460 If we inlined plusInt we might get an opportunity to use
1461 update-in-place for the thunk 'x'.</para>
1467 <option>-funfolding-creation-threshold<n></option>:
1468 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1469 <indexterm><primary>inlining, controlling</primary></indexterm>
1470 <indexterm><primary>unfolding, controlling</primary></indexterm>
1473 <para>(Default: 45) Governs the maximum size that GHC will
1474 allow a function unfolding to be. (An unfolding has a
1475 “size” that reflects the cost in terms of
1476 “code bloat” of expanding that unfolding at
1477 at a call site. A bigger function would be assigned a
1478 bigger cost.) </para>
1480 <para> Consequences: (a) nothing larger than this will be
1481 inlined (unless it has an INLINE pragma); (b) nothing
1482 larger than this will be spewed into an interface
1486 <para> Increasing this figure is more likely to result in longer
1487 compile times than faster code. The next option is more
1493 <term><option>-funfolding-use-threshold<n></option>:</term>
1495 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1496 <indexterm><primary>inlining, controlling</primary></indexterm>
1497 <indexterm><primary>unfolding, controlling</primary></indexterm>
1499 <para>(Default: 8) This is the magic cut-off figure for
1500 unfolding: below this size, a function definition will be
1501 unfolded at the call-site, any bigger and it won't. The
1502 size computed for a function depends on two things: the
1503 actual size of the expression minus any discounts that
1504 apply (see <option>-funfolding-con-discount</option>).</para>
1515 <sect1 id="sec-using-concurrent">
1516 <title>Using Concurrent Haskell</title>
1517 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
1519 <para>GHC supports Concurrent Haskell by default, without requiring a
1520 special option or libraries compiled in a certain way. To get access to
1521 the support libraries for Concurrent Haskell, just import
1523 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>
1525 <para>The following RTS option(s) affect the behaviour of Concurrent
1526 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
1530 <term><option>-C<replaceable>s</replaceable></option></term>
1532 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1533 Sets the context switch interval to <replaceable>s</replaceable>
1534 seconds. A context switch will occur at the next heap block
1535 allocation after the timer expires (a heap block allocation occurs
1536 every 4k of allocation). With <option>-C0</option> or
1537 <option>-C</option>, context switches will occur as often as
1538 possible (at every heap block allocation). By default, context
1539 switches occur every 20ms.</para>
1545 <sect1 id="sec-using-smp">
1546 <title>Using SMP parallelism</title>
1547 <indexterm><primary>parallelism</primary>
1549 <indexterm><primary>SMP</primary>
1552 <para>GHC supports running Haskell programs in parallel on an SMP
1553 (symmetric multiprocessor).</para>
1555 <para>There's a fine distinction between
1556 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
1557 parallelism is all about making your program run
1558 <emphasis>faster</emphasis> by making use of multiple processors
1559 simultaneously. Concurrency, on the other hand, is a means of
1560 abstraction: it is a convenient way to structure a program that must
1561 respond to multiple asynchronous events.</para>
1563 <para>However, the two terms are certainly related. By making use of
1564 multiple CPUs it is possible to run concurrent threads in parallel,
1565 and this is exactly what GHC's SMP parallelism support does. But it
1566 is also possible to obtain performance improvements with parallelism
1567 on programs that do not use concurrency. This section describes how to
1568 use GHC to compile and run parallel programs, in <xref
1569 linkend="lang-parallel" /> we desribe the language features that affect
1572 <sect2 id="parallel-options">
1573 <title>Options to enable SMP parallelism</title>
1575 <para>In order to make use of multiple CPUs, your program must be
1576 linked with the <option>-threaded</option> option (see <xref
1577 linkend="options-linker" />). Then, to run a program on multiple
1578 CPUs, use the RTS <option>-N</option> option:</para>
1582 <term><option>-N<replaceable>x</replaceable></option></term>
1584 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1585 Use <replaceable>x</replaceable> simultaneous threads when
1586 running the program. Normally <replaceable>x</replaceable>
1587 should be chosen to match the number of CPU cores on the machine.
1588 There is no means (currently) by which this value may vary after
1589 the program has started.</para>
1591 <para>For example, on a dual-core machine we would probably use
1592 <literal>+RTS -N2 -RTS</literal>.</para>
1594 <para>Whether hyperthreading cores should be counted or not is an
1595 open question; please feel free to experiment and let us know what
1596 results you find.</para>
1603 <title>Hints for using SMP parallelism</title>
1605 <para>Add the <literal>-sstderr</literal> RTS option when
1606 running the program to see timing stats, which will help to tell you
1607 whether your program got faster by using more CPUs or not. If the user
1608 time is greater than
1609 the elapsed time, then the program used more than one CPU. You should
1610 also run the program without <literal>-N</literal> for comparison.</para>
1612 <para>GHC's parallelism support is new and experimental. It may make your
1613 program go faster, or it might slow it down - either way, we'd be
1614 interested to hear from you.</para>
1616 <para>One significant limitation with the current implementation is that
1617 the garbage collector is still single-threaded, and all execution must
1618 stop when GC takes place. This can be a significant bottleneck in a
1619 parallel program, especially if your program does a lot of GC. If this
1620 happens to you, then try reducing the cost of GC by tweaking the GC
1621 settings (<xref linkend="rts-options-gc" />): enlarging the heap or the
1622 allocation area size is a good start.</para>
1626 <sect1 id="options-platform">
1627 <title>Platform-specific Flags</title>
1629 <indexterm><primary>-m* options</primary></indexterm>
1630 <indexterm><primary>platform-specific options</primary></indexterm>
1631 <indexterm><primary>machine-specific options</primary></indexterm>
1633 <para>Some flags only make sense for particular target
1639 <term><option>-monly-[32]-regs</option>:</term>
1641 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1642 option (iX86 only)</primary></indexterm> GHC tries to
1643 “steal” four registers from GCC, for performance
1644 reasons; it almost always works. However, when GCC is
1645 compiling some modules with four stolen registers, it will
1646 crash, probably saying:
1649 Foo.hc:533: fixed or forbidden register was spilled.
1650 This may be due to a compiler bug or to impossible asm
1651 statements or clauses.
1654 Just give some registers back with
1655 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1656 If `2' doesn't work, please report the bug to us.</para>
1665 <sect1 id="ext-core">
1666 <title>Generating and compiling External Core Files</title>
1668 <indexterm><primary>intermediate code generation</primary></indexterm>
1670 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
1671 to a file as a side-effect of compilation. Core files, which are given the suffix
1672 <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1673 tools. The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz">
1674 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1675 and sample tools (in Haskell)
1676 for manipulating Core files are available in the GHC source distribution
1677 directory <literal>/fptools/ghc/utils/ext-core</literal>.
1678 Note that the format of <literal>.hcr</literal>
1679 files is <emphasis>different</emphasis> (though similar) to the Core output format generated
1680 for debugging purposes (<xref linkend="options-debugging"/>).</para>
1682 <para>The Core format natively supports notes which you can add to
1683 your source code using the <literal>CORE</literal> pragma (see <xref
1684 linkend="pragmas"/>).</para>
1690 <option>-fext-core</option>
1691 <indexterm><primary><option>-fext-core</option></primary></indexterm>
1694 <para>Generate <literal>.hcr</literal> files.</para>
1700 <para>GHC can also read in External Core files as source; just give the <literal>.hcr</literal> file on
1701 the command line, instead of the <literal>.hs</literal> or <literal>.lhs</literal> Haskell source.
1702 A current infelicity is that you need to give the <literal>-fglasgow-exts</literal> flag too, because
1703 ordinary Haskell 98, when translated to External Core, uses things like rank-2 types.</para>
1712 ;;; Local Variables: ***
1714 ;;; sgml-parent-document: ("users_guide.xml" "book" "chapter") ***