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>
142 <para>There are a few flags that are static except that they can
143 also be used with GHCi's <literal>:set</literal> command; these
144 are listed as “static/<literal>:set</literal>” in the
148 <sect1 id="file-suffixes">
149 <title>Meaningful file suffixes</title>
151 <indexterm><primary>suffixes, file</primary></indexterm>
152 <indexterm><primary>file suffixes for GHC</primary></indexterm>
154 <para>File names with “meaningful” suffixes (e.g.,
155 <filename>.lhs</filename> or <filename>.o</filename>) cause the
156 “right thing” to happen to those files.</para>
161 <term><filename>.hs</filename></term>
163 <para>A Haskell module.</para>
169 <filename>.lhs</filename>
170 <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
173 <para>A “literate Haskell” module.</para>
178 <term><filename>.hi</filename></term>
180 <para>A Haskell interface file, probably
181 compiler-generated.</para>
186 <term><filename>.hc</filename></term>
188 <para>Intermediate C file produced by the Haskell
194 <term><filename>.c</filename></term>
196 <para>A C file not produced by the Haskell
202 <term><filename>.s</filename></term>
204 <para>An assembly-language source file, usually produced by
210 <term><filename>.o</filename></term>
212 <para>An object file, produced by an assembler.</para>
217 <para>Files with other suffixes (or without suffixes) are passed
218 straight to the linker.</para>
223 <title>Modes of operation</title>
225 <para>GHC's behaviour is firstly controlled by a mode flag. Only
226 one of these flags may be given, but it does not necessarily need
227 to be the first option on the command-line. The available modes
233 <cmdsynopsis><command>ghc --interactive</command>
235 <indexterm><primary>interactive mode</primary></indexterm>
236 <indexterm><primary>ghci</primary></indexterm>
239 <para>Interactive mode, which is also available as
240 <command>ghci</command>. Interactive mode is described in
241 more detail in <xref linkend="ghci"/>.</para>
247 <cmdsynopsis><command>ghc --make</command>
249 <indexterm><primary>make mode</primary></indexterm>
250 <indexterm><primary><option>––make</option></primary></indexterm>
253 <para>In this mode, GHC will build a multi-module Haskell
254 program automatically, figuring out dependencies for itself.
255 If you have a straightforward Haskell program, this is
256 likely to be much easier, and faster, than using
257 <command>make</command>. Make mode is described in <xref
258 linkend="make-mode"/>.</para>
264 <cmdsynopsis><command>ghc -e</command>
265 <arg choice='plain'><replaceable>expr</replaceable></arg>
267 <indexterm><primary>eval mode</primary></indexterm>
270 <para>Expression-evaluation mode. This is very similar to
271 interactive mode, except that there is a single expression
272 to evaluate (<replaceable>expr</replaceable>) which is given
273 on the command line. See <xref linkend="eval-mode"/> for
281 <command>ghc -E</command>
282 <command>ghc -c</command>
283 <command>ghc -S</command>
284 <command>ghc -c</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 -M</command>
307 <indexterm><primary>dependency-generation mode</primary></indexterm>
310 <para>Dependency-generation mode. In this mode, GHC can be
311 used to generate dependency information suitable for use in
312 a <literal>Makefile</literal>. See <xref
313 linkend="sec-makefile-dependencies"/>.</para>
320 <command>ghc --mk-dll</command>
322 <indexterm><primary>DLL-creation mode</primary></indexterm>
325 <para>DLL-creation mode (Windows only). See <xref
326 linkend="win32-dlls-create"/>.</para>
333 <command>ghc --help</command> <command>ghc -?</command>
335 <indexterm><primary><option>––help</option></primary></indexterm>
338 <para>Cause GHC to spew a long usage message to standard
339 output and then exit.</para>
346 <command>ghc --version</command> <command>ghc -V</command>
347 <indexterm><primary><option>-V</option></primary></indexterm>
348 <indexterm><primary><option>––version</option></primary></indexterm>
352 <para>Print a one-line string including GHC's version number.</para>
359 <command>ghc --numeric-version</command>
360 <indexterm><primary><option>––numeric-version</option></primary></indexterm>
364 <para>Print GHC's numeric version number only.</para>
371 <command>ghc --print-libdir</command>
372 <indexterm><primary><option>––print-libdir</option></primary></indexterm>
376 <para>Print the path to GHC's library directory. This is
377 the top of the directory tree containing GHC's libraries,
378 interfaces, and include files (usually something like
379 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
381 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary>
382 </indexterm>in the package configuration file (see <xref
383 linkend="packages"/>).</para>
389 <sect2 id="make-mode">
390 <title>Using <command>ghc</command> <option>––make</option></title>
391 <indexterm><primary><option>––make</option></primary></indexterm>
392 <indexterm><primary>separate compilation</primary></indexterm>
394 <para>When given the <option>––make</option> option,
395 GHC will build a multi-module Haskell program by following
396 dependencies from a single root module (usually
397 <literal>Main</literal>). For example, if your
398 <literal>Main</literal> module is in a file called
399 <filename>Main.hs</filename>, you could compile and link the
400 program like this:</para>
403 ghc ––make Main.hs
406 <para>The command line may contain any number of source file
407 names or module names; GHC will figure out all the modules in
408 the program by following the imports from these initial modules.
409 It will then attempt to compile each module which is out of
410 date, and finally if there is a <literal>Main</literal> module,
411 the program will also be linked into an executable.</para>
413 <para>The main advantages to using <literal>ghc
414 ––make</literal> over traditional
415 <literal>Makefile</literal>s are:</para>
419 <para>GHC doesn't have to be restarted for each compilation,
420 which means it can cache information between compilations.
421 Compiling a multi-module program with <literal>ghc
422 ––make</literal> can be up to twice as fast as
423 running <literal>ghc</literal> individually on each source
427 <para>You don't have to write a <literal>Makefile</literal>.</para>
428 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
431 <para>GHC re-calculates the dependencies each time it is
432 invoked, so the dependencies never get out of sync with the
437 <para>Any of the command-line options described in the rest of
438 this chapter can be used with
439 <option>––make</option>, but note that any options
440 you give on the command line will apply to all the source files
441 compiled, so if you want any options to apply to a single source
442 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
443 pragma (see <xref linkend="source-file-options"/>).</para>
445 <para>If the program needs to be linked with additional objects
446 (say, some auxiliary C code), then the object files can be
447 given on the command line and GHC will include them when linking
448 the executable.</para>
450 <para>Note that GHC can only follow dependencies if it has the
451 source file available, so if your program includes a module for
452 which there is no source file, even if you have an object and an
453 interface file for the module, then GHC will complain. The
454 exception to this rule is for package modules, which may or may
455 not have source files.</para>
457 <para>The source files for the program don't all need to be in
458 the same directory; the <option>-i</option> option can be used
459 to add directories to the search path (see <xref
460 linkend="search-path"/>).</para>
463 <sect2 id="eval-mode">
464 <title>Expression evaluation mode</title>
466 <para>This mode is very similar to interactive mode, except that
467 there is a single expression to evaluate which is specified on
468 the command line as an argument to the <option>-e</option>
472 ghc -e <replaceable>expr</replaceable>
475 <para>Haskell source files may be named on the command line, and
476 they will be loaded exactly as in interactive mode. The
477 expression is evaluated in the context of the loaded
480 <para>For example, to load and run a Haskell program containing
481 a module <literal>Main</literal>, we might say</para>
484 ghc -e Main.main Main.hs
487 <para>or we can just use this mode to evaluate expressions in
488 the context of the <literal>Prelude</literal>:</para>
491 $ ghc -e "interact (unlines.map reverse.lines)"
497 <sect2 id="options-order">
498 <title>Batch compiler mode</title>
500 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
501 given on the command line.</para>
503 <para>The first phase to run is determined by each input-file
504 suffix, and the last phase is determined by a flag. If no
505 relevant flag is present, then go all the way through linking.
506 This table summarises:</para>
510 <colspec align="left"/>
511 <colspec align="left"/>
512 <colspec align="left"/>
513 <colspec align="left"/>
517 <entry>Phase of the compilation system</entry>
518 <entry>Suffix saying “start here”</entry>
519 <entry>Flag saying “stop after”</entry>
520 <entry>(suffix of) output file</entry>
525 <entry>literate pre-processor</entry>
526 <entry><literal>.lhs</literal></entry>
528 <entry><literal>.hs</literal></entry>
532 <entry>C pre-processor (opt.) </entry>
533 <entry><literal>.hs</literal> (with
534 <option>-cpp</option>)</entry>
535 <entry><option>-E</option></entry>
536 <entry><literal>.hspp</literal></entry>
540 <entry>Haskell compiler</entry>
541 <entry><literal>.hs</literal></entry>
542 <entry><option>-C</option>, <option>-S</option></entry>
543 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
547 <entry>C compiler (opt.)</entry>
548 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
549 <entry><option>-S</option></entry>
550 <entry><literal>.s</literal></entry>
554 <entry>assembler</entry>
555 <entry><literal>.s</literal></entry>
556 <entry><option>-c</option></entry>
557 <entry><literal>.o</literal></entry>
561 <entry>linker</entry>
562 <entry><replaceable>other</replaceable></entry>
564 <entry><filename>a.out</filename></entry>
570 <indexterm><primary><option>-C</option></primary></indexterm>
571 <indexterm><primary><option>-E</option></primary></indexterm>
572 <indexterm><primary><option>-S</option></primary></indexterm>
573 <indexterm><primary><option>-c</option></primary></indexterm>
575 <para>Thus, a common invocation would be: </para>
578 ghc -c Foo.hs</screen>
580 <para>to compile the Haskell source file
581 <filename>Foo.hs</filename> to an object file
582 <filename>Foo.o</filename>.</para>
584 <para>Note: What the Haskell compiler proper produces depends on
585 whether a native-code generator<indexterm><primary>native-code
586 generator</primary></indexterm> is used (producing assembly
587 language) or not (producing C). See <xref
588 linkend="options-codegen"/> for more details.</para>
590 <para>Note: C pre-processing is optional, the
591 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
592 flag turns it on. See <xref linkend="c-pre-processor"/> for more
595 <para>Note: The option <option>-E</option><indexterm><primary>-E
596 option</primary></indexterm> runs just the pre-processing passes
597 of the compiler, dumping the result in a file. Note that this
598 differs from the previous behaviour of dumping the file to
599 standard output.</para>
601 <sect3 id="overriding-suffixes">
602 <title>Overriding the default behaviour for a file</title>
604 <para>As described above, the way in which a file is processed by GHC
605 depends on its suffix. This behaviour can be overriden using the
606 <option>-x</option> option:</para>
610 <term><option>-x</option> <replaceable>suffix</replaceable>
611 <indexterm><primary><option>-x</option></primary>
614 <para>Causes all files following this option on the command
615 line to be processed as if they had the suffix
616 <replaceable>suffix</replaceable>. For example, to compile a
617 Haskell module in the file <literal>M.my-hs</literal>,
618 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
627 <sect1 id="options-help">
628 <title>Help and verbosity options</title>
630 <indexterm><primary>help options</primary></indexterm>
631 <indexterm><primary>verbosity options</primary></indexterm>
633 See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
634 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.
639 <indexterm><primary><option>-n</option></primary></indexterm>
642 <para>Does a dry-run, i.e. GHC goes through all the motions
643 of compiling as normal, but does not actually run any
644 external commands.</para>
651 <indexterm><primary><option>-v</option></primary></indexterm>
654 <para>The <option>-v</option> option makes GHC
655 <emphasis>verbose</emphasis>: it reports its version number
656 and shows (on stderr) exactly how it invokes each phase of
657 the compilation system. Moreover, it passes the
658 <option>-v</option> flag to most phases; each reports its
659 version number (and possibly some other information).</para>
661 <para>Please, oh please, use the <option>-v</option> option
662 when reporting bugs! Knowing that you ran the right bits in
663 the right order is always the first thing we want to
670 <option>-v</option><replaceable>n</replaceable>
671 <indexterm><primary><option>-v</option></primary></indexterm>
674 <para>To provide more control over the compiler's verbosity,
675 the <option>-v</option> flag takes an optional numeric
676 argument. Specifying <option>-v</option> on its own is
677 equivalent to <option>-v3</option>, and the other levels
678 have the following meanings:</para>
682 <term><option>-v0</option></term>
684 <para>Disable all non-essential messages (this is the
690 <term><option>-v1</option></term>
692 <para>Minimal verbosity: print one line per
693 compilation (this is the default when
694 <option>––make</option> or
695 <option>––interactive</option> is on).</para>
700 <term><option>-v2</option></term>
702 <para>Print the name of each compilation phase as it
703 is executed. (equivalent to
704 <option>-dshow-passes</option>).</para>
709 <term><option>-v3</option></term>
711 <para>The same as <option>-v2</option>, except that in
712 addition the full command line (if appropriate) for
713 each compilation phase is also printed.</para>
718 <term><option>-v4</option></term>
720 <para>The same as <option>-v3</option> except that the
721 intermediate program representation after each
722 compilation phase is also printed (excluding
723 preprocessed and C/assembly files).</para>
731 <term><option>-ferror-spans</option>
732 <indexterm><primary><option>-ferror-spans</option></primary>
736 <para>Causes GHC to emit the full source span of the
737 syntactic entity relating to an error message. Normally, GHC
738 emits the source location of the start of the syntactic
741 <para>For example:</para>
743 <screen>test.hs:3:6: parse error on input `where'</screen>
745 <para>becomes:</para>
747 <screen>test296.hs:3:6-10: parse error on input `where'</screen>
749 <para>And multi-line spans are possible too:</para>
751 <screen>test.hs:(5,4)-(6,7):
752 Conflicting definitions for `a'
753 Bound at: test.hs:5:4
755 In the binding group for: a, b, a</screen>
757 <para>Note that line numbers start counting at one, but
758 column numbers start at zero. This choice was made to
759 follow existing convention (i.e. this is how Emacs does
765 <term><option>-H</option><replaceable>size</replaceable>
766 <indexterm><primary><option>-H</option></primary></indexterm>
769 <para>Set the minimum size of the heap to
770 <replaceable>size</replaceable>.
771 This option is equivalent to
772 <literal>+RTS -H<replaceable>size</replaceable></literal>,
773 see <xref linkend="rts-options-gc" />.
779 <term><option>-Rghc-timing</option>
780 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
783 <para>Prints a one-line summary of timing statistics for the
784 GHC run. This option is equivalent to
785 <literal>+RTS -tstderr</literal>, see <xref
786 linkend="rts-options-gc" />.
795 <sect1 id="options-sanity">
796 <title>Warnings and sanity-checking</title>
798 <indexterm><primary>sanity-checking options</primary></indexterm>
799 <indexterm><primary>warnings</primary></indexterm>
802 <para>GHC has a number of options that select which types of
803 non-fatal error messages, otherwise known as warnings, can be
804 generated during compilation. By default, you get a standard set
805 of warnings which are generally likely to indicate bugs in your
807 <option>-fwarn-overlapping-patterns</option>,
808 <option>-fwarn-deprecations</option>,
809 <option>-fwarn-duplicate-exports</option>,
810 <option>-fwarn-missing-fields</option>, and
811 <option>-fwarn-missing-methods</option>. The following flags are
812 simple ways to select standard “packages” of warnings:
818 <term><option>-W</option>:</term>
820 <indexterm><primary>-W option</primary></indexterm>
821 <para>Provides the standard warnings plus
822 <option>-fwarn-incomplete-patterns</option>,
823 <option>-fwarn-unused-matches</option>,
824 <option>-fwarn-unused-imports</option>, and
825 <option>-fwarn-unused-binds</option>.</para>
830 <term><option>-w</option>:</term>
832 <indexterm><primary><option>-w</option></primary></indexterm>
833 <para>Turns off all warnings, including the standard ones.</para>
838 <term><option>-Wall</option>:</term>
840 <indexterm><primary><option>-Wall</option></primary></indexterm>
841 <para>Turns on all warning options.</para>
846 <term><option>-Werror</option>:</term>
848 <indexterm><primary><option>-Werror</option></primary></indexterm>
849 <para>Makes any warning into a fatal error. Useful so that you don't
850 miss warnings when doing batch compilation. </para>
856 <para>The full set of warning options is described below. To turn
857 off any warning, simply give the corresponding
858 <option>-fno-warn-...</option> option on the command line.</para>
863 <term><option>-fwarn-deprecations</option>:</term>
865 <indexterm><primary><option>-fwarn-deprecations</option></primary>
867 <indexterm><primary>deprecations</primary></indexterm>
868 <para>Causes a warning to be emitted when a deprecated
869 function or type is used. Entities can be marked as
870 deprecated using a pragma, see <xref
871 linkend="deprecated-pragma"/>.</para>
876 <term><option>-fwarn-duplicate-exports</option>:</term>
878 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
879 <indexterm><primary>duplicate exports, warning</primary></indexterm>
880 <indexterm><primary>export lists, duplicates</primary></indexterm>
882 <para>Have the compiler warn about duplicate entries in
883 export lists. This is useful information if you maintain
884 large export lists, and want to avoid the continued export
885 of a definition after you've deleted (one) mention of it in
886 the export list.</para>
888 <para>This option is on by default.</para>
893 <term><option>-fwarn-hi-shadowing</option>:</term>
895 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
896 <indexterm><primary>shadowing</primary>
897 <secondary>interface files</secondary></indexterm>
899 <para>Causes the compiler to emit a warning when a module or
900 interface file in the current directory is shadowing one
901 with the same module name in a library or other
907 <term><option>-fwarn-incomplete-patterns</option>:</term>
909 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
910 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
911 <indexterm><primary>patterns, incomplete</primary></indexterm>
913 <para>Similarly for incomplete patterns, the function
914 <function>g</function> below will fail when applied to
915 non-empty lists, so the compiler will emit a warning about
916 this when <option>-fwarn-incomplete-patterns</option> is
923 <para>This option isn't enabled be default because it can be
924 a bit noisy, and it doesn't always indicate a bug in the
925 program. However, it's generally considered good practice
926 to cover all the cases in your functions.</para>
931 <term><option>-fwarn-incomplete-record-updates</option>:</term>
933 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
934 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
935 <indexterm><primary>record updates, incomplete</primary></indexterm>
938 <function>f</function> below will fail when applied to
939 <literal>Bar</literal>, so the compiler will emit a warning about
940 this when <option>-fwarn-incomplete-record-updates</option> is
944 data Foo = Foo { x :: Int }
948 f foo = foo { x = 6 }
951 <para>This option isn't enabled be default because it can be
952 very noisy, and it often doesn't indicate a bug in the
959 <option>-fwarn-missing-fields</option>:
960 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
961 <indexterm><primary>missing fields, warning</primary></indexterm>
962 <indexterm><primary>fields, missing</primary></indexterm>
966 <para>This option is on by default, and warns you whenever
967 the construction of a labelled field constructor isn't
968 complete, missing initializers for one or more fields. While
969 not an error (the missing fields are initialised with
970 bottoms), it is often an indication of a programmer error.</para>
975 <term><option>-fwarn-missing-methods</option>:</term>
977 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
978 <indexterm><primary>missing methods, warning</primary></indexterm>
979 <indexterm><primary>methods, missing</primary></indexterm>
981 <para>This option is on by default, and warns you whenever
982 an instance declaration is missing one or more methods, and
983 the corresponding class declaration has no default
984 declaration for them.</para>
985 <para>The warning is suppressed if the method name
986 begins with an underscore. Here's an example where this is useful:
989 _simpleFn :: a -> String
990 complexFn :: a -> a -> String
991 complexFn x y = ... _simpleFn ...
993 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
994 never <literal>_simpleFn</literal>; and (b)
995 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1001 <term><option>-fwarn-missing-signatures</option>:</term>
1003 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1004 <indexterm><primary>type signatures, missing</primary></indexterm>
1006 <para>If you would like GHC to check that every top-level
1007 function/value has a type signature, use the
1008 <option>-fwarn-missing-signatures</option> option. This
1009 option is off by default.</para>
1014 <term><option>-fwarn-name-shadowing</option>:</term>
1016 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1017 <indexterm><primary>shadowing, warning</primary></indexterm>
1019 <para>This option causes a warning to be emitted whenever an
1020 inner-scope value has the same name as an outer-scope value,
1021 i.e. the inner value shadows the outer one. This can catch
1022 typographical errors that turn into hard-to-find bugs, e.g.,
1023 in the inadvertent cyclic definition <literal>let x = ... x
1024 ... in</literal>.</para>
1026 <para>Consequently, this option does
1027 <emphasis>will</emphasis> complain about cyclic recursive
1033 <term><option>-fwarn-orphans</option>:</term>
1035 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1036 <indexterm><primary>orphan instances, warning</primary></indexterm>
1037 <indexterm><primary>orphan rules, warning</primary></indexterm>
1039 <para>This option causes a warning to be emitted whenever the
1040 module contains an "orphan" instance declaration or rewrite rule.
1041 An instance declartion is an orphan if it appears in a module in
1042 which neither the class nor the type being instanced are declared
1043 in the same module. A rule is an orphan if it is a rule for a
1044 function declared in another module. A module containing any
1045 orphans is called an orphan module.</para>
1046 <para>The trouble with orphans is that GHC must pro-actively read the interface
1047 files for all orphan modules, just in case their instances or rules
1048 play a role, whether or not the module's interface would otherwise
1049 be of any use. Other things being equal, avoid orphan modules.</para>
1055 <option>-fwarn-overlapping-patterns</option>:
1056 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1057 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1058 <indexterm><primary>patterns, overlapping</primary></indexterm>
1061 <para>By default, the compiler will warn you if a set of
1062 patterns are overlapping, i.e.,</para>
1065 f :: String -> Int
1071 <para>where the last pattern match in <function>f</function>
1072 won't ever be reached, as the second pattern overlaps
1073 it. More often than not, redundant patterns is a programmer
1074 mistake/error, so this option is enabled by default.</para>
1079 <term><option>-fwarn-simple-patterns</option>:</term>
1081 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1083 <para>Causes the compiler to warn about lambda-bound
1084 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1085 Normally, these aren't treated as incomplete patterns by
1086 <option>-fwarn-incomplete-patterns</option>.</para>
1087 <para>``Lambda-bound patterns'' includes all places where there is a single pattern,
1088 including list comprehensions and do-notation. In these cases, a pattern-match
1089 failure is quite legitimate, and triggers filtering (list comprehensions) or
1090 the monad <literal>fail</literal> operation (monads). For example:
1092 f :: [Maybe a] -> [a]
1093 f xs = [y | Just y <- xs]
1095 Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
1096 these probably-innocent cases, which is why the flag is off by default. </para>
1097 <para> The <literal>deriving( Read )</literal> mechanism produces monadic code with
1098 pattern matches, so you will also get misleading warnings about the compiler-generated
1099 code. (This is arguably a Bad Thing, but it's awkward to fix.)</para>
1105 <term><option>-fwarn-tabs</option>:</term>
1107 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1108 <indexterm><primary>tabs, warning</primary></indexterm>
1109 <para>Have the compiler warn if there are tabs in your source
1112 <para>This warning is off by default.</para>
1117 <term><option>-fwarn-type-defaults</option>:</term>
1119 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1120 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1121 <para>Have the compiler warn/inform you where in your source
1122 the Haskell defaulting mechanism for numeric types kicks
1123 in. This is useful information when converting code from a
1124 context that assumed one default into one with another,
1125 e.g., the `default default' for Haskell 1.4 caused the
1126 otherwise unconstrained value <constant>1</constant> to be
1127 given the type <literal>Int</literal>, whereas Haskell 98
1128 defaults it to <literal>Integer</literal>. This may lead to
1129 differences in performance and behaviour, hence the
1130 usefulness of being non-silent about this.</para>
1132 <para>This warning is off by default.</para>
1137 <term><option>-fwarn-unused-binds</option>:</term>
1139 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1140 <indexterm><primary>unused binds, warning</primary></indexterm>
1141 <indexterm><primary>binds, unused</primary></indexterm>
1142 <para>Report any function definitions (and local bindings)
1143 which are unused. For top-level functions, the warning is
1144 only given if the binding is not exported.</para>
1145 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1146 mentioned in the right hand side of another definition that is used, or (c) the
1147 function it defines begins with an underscore. The last case provides a
1148 way to suppress unused-binding warnings selectively. </para>
1149 <para> Notice that a variable
1150 is reported as unused even if it appears in the right-hand side of another
1151 unused binding. </para>
1156 <term><option>-fwarn-unused-imports</option>:</term>
1158 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1159 <indexterm><primary>unused imports, warning</primary></indexterm>
1160 <indexterm><primary>imports, unused</primary></indexterm>
1162 <para>Report any modules that are explicitly imported but
1163 never used. However, the form <literal>import M()</literal> is
1164 never reported as an unused import, because it is a useful idiom
1165 for importing instance declarations, which are anonymous in Haskell.</para>
1170 <term><option>-fwarn-unused-matches</option>:</term>
1172 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1173 <indexterm><primary>unused matches, warning</primary></indexterm>
1174 <indexterm><primary>matches, unused</primary></indexterm>
1176 <para>Report all unused variables which arise from pattern
1177 matches, including patterns consisting of a single variable.
1178 For instance <literal>f x y = []</literal> would report
1179 <varname>x</varname> and <varname>y</varname> as unused. The
1180 warning is suppressed if the variable name begins with an underscore, thus:
1190 <para>If you're feeling really paranoid, the
1191 <option>-dcore-lint</option>
1192 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1193 is a good choice. It turns on heavyweight intra-pass
1194 sanity-checking within GHC. (It checks GHC's sanity, not
1201 <sect1 id="options-optimise">
1202 <title>Optimisation (code improvement)</title>
1204 <indexterm><primary>optimisation</primary></indexterm>
1205 <indexterm><primary>improvement, code</primary></indexterm>
1207 <para>The <option>-O*</option> options specify convenient
1208 “packages” of optimisation flags; the
1209 <option>-f*</option> options described later on specify
1210 <emphasis>individual</emphasis> optimisations to be turned on/off;
1211 the <option>-m*</option> options specify
1212 <emphasis>machine-specific</emphasis> optimisations to be turned
1215 <sect2 id="optimise-pkgs">
1216 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1218 <para>There are <emphasis>many</emphasis> options that affect
1219 the quality of code produced by GHC. Most people only have a
1220 general goal, something like “Compile quickly” or
1221 “Make my program run like greased lightning.” The
1222 following “packages” of optimisations (or lack
1223 thereof) should suffice.</para>
1225 <para>Note that higher optimisation levels cause more
1226 cross-module optimisation to be performed, which can have an
1227 impact on how much of your program needs to be recompiled when
1228 you change something. This is one reaosn to stick to
1229 no-optimisation when developing code.</para>
1235 No <option>-O*</option>-type option specified:
1236 <indexterm><primary>-O* not specified</primary></indexterm>
1239 <para>This is taken to mean: “Please compile
1240 quickly; I'm not over-bothered about compiled-code
1241 quality.” So, for example: <command>ghc -c
1242 Foo.hs</command></para>
1248 <option>-O0</option>:
1249 <indexterm><primary><option>-O0</option></primary></indexterm>
1252 <para>Means “turn off all optimisation”,
1253 reverting to the same settings as if no
1254 <option>-O</option> options had been specified. Saying
1255 <option>-O0</option> can be useful if
1256 eg. <command>make</command> has inserted a
1257 <option>-O</option> on the command line already.</para>
1263 <option>-O</option> or <option>-O1</option>:
1264 <indexterm><primary>-O option</primary></indexterm>
1265 <indexterm><primary>-O1 option</primary></indexterm>
1266 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1269 <para>Means: “Generate good-quality code without
1270 taking too long about it.” Thus, for example:
1271 <command>ghc -c -O Main.lhs</command></para>
1277 <option>-O2</option>:
1278 <indexterm><primary>-O2 option</primary></indexterm>
1279 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1282 <para>Means: “Apply every non-dangerous
1283 optimisation, even if it means significantly longer
1284 compile times.”</para>
1286 <para>The avoided “dangerous” optimisations
1287 are those that can make runtime or space
1288 <emphasis>worse</emphasis> if you're unlucky. They are
1289 normally turned on or off individually.</para>
1291 <para>At the moment, <option>-O2</option> is
1292 <emphasis>unlikely</emphasis> to produce better code than
1293 <option>-O</option>.</para>
1299 <option>-Ofile <file></option>:
1300 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1301 <indexterm><primary>optimising, customised</primary></indexterm>
1304 <para>(NOTE: not supported since GHC 4.x. Please ask if
1305 you're interested in this.)</para>
1307 <para>For those who need <emphasis>absolute</emphasis>
1308 control over <emphasis>exactly</emphasis> what options are
1309 used (e.g., compiler writers, sometimes :-), a list of
1310 options can be put in a file and then slurped in with
1311 <option>-Ofile</option>.</para>
1313 <para>In that file, comments are of the
1314 <literal>#</literal>-to-end-of-line variety; blank
1315 lines and most whitespace is ignored.</para>
1317 <para>Please ask if you are baffled and would like an
1318 example of <option>-Ofile</option>!</para>
1323 <para>We don't use a <option>-O*</option> flag for day-to-day
1324 work. We use <option>-O</option> to get respectable speed;
1325 e.g., when we want to measure something. When we want to go for
1326 broke, we tend to use <option>-O2 -fvia-C</option> (and we go for
1327 lots of coffee breaks).</para>
1329 <para>The easiest way to see what <option>-O</option> (etc.)
1330 “really mean” is to run with <option>-v</option>,
1331 then stand back in amazement.</para>
1334 <sect2 id="options-f">
1335 <title><option>-f*</option>: platform-independent flags</title>
1337 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1338 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1340 <para>These flags turn on and off individual optimisations.
1341 They are normally set via the <option>-O</option> options
1342 described above, and as such, you shouldn't need to set any of
1343 them explicitly (indeed, doing so could lead to unexpected
1344 results). However, there are one or two that may be of
1349 <term><option>-fexcess-precision</option>:</term>
1351 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1352 <para>When this option is given, intermediate floating
1353 point values can have a <emphasis>greater</emphasis>
1354 precision/range than the final type. Generally this is a
1355 good thing, but some programs may rely on the exact
1357 <literal>Float</literal>/<literal>Double</literal> values
1358 and should not use this option for their compilation.</para>
1363 <term><option>-fignore-asserts</option>:</term>
1365 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1366 <para>Causes GHC to ignore uses of the function
1367 <literal>Exception.assert</literal> in source code (in
1368 other words, rewriting <literal>Exception.assert p
1369 e</literal> to <literal>e</literal> (see <xref
1370 linkend="sec-assertions"/>). This flag is turned on by
1371 <option>-O</option>.
1378 <option>-fno-cse</option>
1379 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1382 <para>Turns off the common-sub-expression elimination optimisation.
1383 Can be useful if you have some <literal>unsafePerformIO</literal>
1384 expressions that you don't want commoned-up.</para>
1390 <option>-fno-strictness</option>
1391 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1394 <para>Turns off the strictness analyser; sometimes it eats
1395 too many cycles.</para>
1401 <option>-fno-full-laziness</option>
1402 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1405 <para>Turns off the full laziness optimisation (also known as
1406 let-floating). Full laziness increases sharing, which can lead
1407 to increased memory residency.</para>
1409 <para>NOTE: GHC doesn't implement complete full-laziness.
1410 When optimisation in on, and
1411 <option>-fno-full-laziness</option> is not given, some
1412 transformations that increase sharing are performed, such
1413 as extracting repeated computations from a loop. These
1414 are the same transformations that a fully lazy
1415 implementation would do, the difference is that GHC
1416 doesn't consistently apply full-laziness, so don't rely on
1423 <option>-fno-state-hack</option>
1424 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1427 <para>Turn off the "state hack" whereby any lambda with a
1428 <literal>State#</literal> token as argument is considered to be
1429 single-entry, hence it is considered OK to inline things inside
1430 it. This can improve performance of IO and ST monad code, but it
1431 runs the risk of reducing sharing.</para>
1437 <option>-funbox-strict-fields</option>:
1438 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1439 <indexterm><primary>strict constructor fields</primary></indexterm>
1440 <indexterm><primary>constructor fields, strict</primary></indexterm>
1443 <para>This option causes all constructor fields which are
1444 marked strict (i.e. “!”) to be unboxed or
1445 unpacked if possible. It is equivalent to adding an
1446 <literal>UNPACK</literal> pragma to every strict
1447 constructor field (see <xref
1448 linkend="unpack-pragma"/>).</para>
1450 <para>This option is a bit of a sledgehammer: it might
1451 sometimes make things worse. Selectively unboxing fields
1452 by using <literal>UNPACK</literal> pragmas might be
1459 <option>-funfolding-update-in-place<n></option>
1460 <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1463 <para>Switches on an experimental "optimisation".
1464 Switching it on makes the compiler a little keener to
1465 inline a function that returns a constructor, if the
1466 context is that of a thunk.
1470 If we inlined plusInt we might get an opportunity to use
1471 update-in-place for the thunk 'x'.</para>
1477 <option>-funfolding-creation-threshold<n></option>:
1478 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1479 <indexterm><primary>inlining, controlling</primary></indexterm>
1480 <indexterm><primary>unfolding, controlling</primary></indexterm>
1483 <para>(Default: 45) Governs the maximum size that GHC will
1484 allow a function unfolding to be. (An unfolding has a
1485 “size” that reflects the cost in terms of
1486 “code bloat” of expanding that unfolding at
1487 at a call site. A bigger function would be assigned a
1488 bigger cost.) </para>
1490 <para> Consequences: (a) nothing larger than this will be
1491 inlined (unless it has an INLINE pragma); (b) nothing
1492 larger than this will be spewed into an interface
1496 <para> Increasing this figure is more likely to result in longer
1497 compile times than faster code. The next option is more
1503 <term><option>-funfolding-use-threshold<n></option>:</term>
1505 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1506 <indexterm><primary>inlining, controlling</primary></indexterm>
1507 <indexterm><primary>unfolding, controlling</primary></indexterm>
1509 <para>(Default: 8) This is the magic cut-off figure for
1510 unfolding: below this size, a function definition will be
1511 unfolded at the call-site, any bigger and it won't. The
1512 size computed for a function depends on two things: the
1513 actual size of the expression minus any discounts that
1514 apply (see <option>-funfolding-con-discount</option>).</para>
1525 <sect1 id="sec-using-concurrent">
1526 <title>Using Concurrent Haskell</title>
1527 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
1529 <para>GHC supports Concurrent Haskell by default, without requiring a
1530 special option or libraries compiled in a certain way. To get access to
1531 the support libraries for Concurrent Haskell, just import
1533 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>
1535 <para>The following RTS option(s) affect the behaviour of Concurrent
1536 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
1540 <term><option>-C<replaceable>s</replaceable></option></term>
1542 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1543 Sets the context switch interval to <replaceable>s</replaceable>
1544 seconds. A context switch will occur at the next heap block
1545 allocation after the timer expires (a heap block allocation occurs
1546 every 4k of allocation). With <option>-C0</option> or
1547 <option>-C</option>, context switches will occur as often as
1548 possible (at every heap block allocation). By default, context
1549 switches occur every 20ms.</para>
1555 <sect1 id="sec-using-smp">
1556 <title>Using SMP parallelism</title>
1557 <indexterm><primary>parallelism</primary>
1559 <indexterm><primary>SMP</primary>
1562 <para>GHC supports running Haskell programs in parallel on an SMP
1563 (symmetric multiprocessor).</para>
1565 <para>There's a fine distinction between
1566 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
1567 parallelism is all about making your program run
1568 <emphasis>faster</emphasis> by making use of multiple processors
1569 simultaneously. Concurrency, on the other hand, is a means of
1570 abstraction: it is a convenient way to structure a program that must
1571 respond to multiple asynchronous events.</para>
1573 <para>However, the two terms are certainly related. By making use of
1574 multiple CPUs it is possible to run concurrent threads in parallel,
1575 and this is exactly what GHC's SMP parallelism support does. But it
1576 is also possible to obtain performance improvements with parallelism
1577 on programs that do not use concurrency. This section describes how to
1578 use GHC to compile and run parallel programs, in <xref
1579 linkend="lang-parallel" /> we desribe the language features that affect
1582 <sect2 id="parallel-options">
1583 <title>Options to enable SMP parallelism</title>
1585 <para>In order to make use of multiple CPUs, your program must be
1586 linked with the <option>-threaded</option> option (see <xref
1587 linkend="options-linker" />). Then, to run a program on multiple
1588 CPUs, use the RTS <option>-N</option> option:</para>
1592 <term><option>-N<replaceable>x</replaceable></option></term>
1594 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1595 Use <replaceable>x</replaceable> simultaneous threads when
1596 running the program. Normally <replaceable>x</replaceable>
1597 should be chosen to match the number of CPU cores on the machine.
1598 There is no means (currently) by which this value may vary after
1599 the program has started.</para>
1601 <para>For example, on a dual-core machine we would probably use
1602 <literal>+RTS -N2 -RTS</literal>.</para>
1604 <para>Whether hyperthreading cores should be counted or not is an
1605 open question; please feel free to experiment and let us know what
1606 results you find.</para>
1613 <title>Hints for using SMP parallelism</title>
1615 <para>Add the <literal>-sstderr</literal> RTS option when
1616 running the program to see timing stats, which will help to tell you
1617 whether your program got faster by using more CPUs or not. If the user
1618 time is greater than
1619 the elapsed time, then the program used more than one CPU. You should
1620 also run the program without <literal>-N</literal> for comparison.</para>
1622 <para>GHC's parallelism support is new and experimental. It may make your
1623 program go faster, or it might slow it down - either way, we'd be
1624 interested to hear from you.</para>
1626 <para>One significant limitation with the current implementation is that
1627 the garbage collector is still single-threaded, and all execution must
1628 stop when GC takes place. This can be a significant bottleneck in a
1629 parallel program, especially if your program does a lot of GC. If this
1630 happens to you, then try reducing the cost of GC by tweaking the GC
1631 settings (<xref linkend="rts-options-gc" />): enlarging the heap or the
1632 allocation area size is a good start.</para>
1636 <sect1 id="options-platform">
1637 <title>Platform-specific Flags</title>
1639 <indexterm><primary>-m* options</primary></indexterm>
1640 <indexterm><primary>platform-specific options</primary></indexterm>
1641 <indexterm><primary>machine-specific options</primary></indexterm>
1643 <para>Some flags only make sense for particular target
1649 <term><option>-monly-[32]-regs</option>:</term>
1651 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1652 option (iX86 only)</primary></indexterm> GHC tries to
1653 “steal” four registers from GCC, for performance
1654 reasons; it almost always works. However, when GCC is
1655 compiling some modules with four stolen registers, it will
1656 crash, probably saying:
1659 Foo.hc:533: fixed or forbidden register was spilled.
1660 This may be due to a compiler bug or to impossible asm
1661 statements or clauses.
1664 Just give some registers back with
1665 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1666 If `2' doesn't work, please report the bug to us.</para>
1675 <sect1 id="ext-core">
1676 <title>Generating and compiling External Core Files</title>
1678 <indexterm><primary>intermediate code generation</primary></indexterm>
1680 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
1681 to a file as a side-effect of compilation. Core files, which are given the suffix
1682 <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1683 tools. The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz">
1684 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1685 and sample tools (in Haskell)
1686 for manipulating Core files are available in the GHC source distribution
1687 directory <literal>/fptools/ghc/utils/ext-core</literal>.
1688 Note that the format of <literal>.hcr</literal>
1689 files is <emphasis>different</emphasis> (though similar) to the Core output format generated
1690 for debugging purposes (<xref linkend="options-debugging"/>).</para>
1692 <para>The Core format natively supports notes which you can add to
1693 your source code using the <literal>CORE</literal> pragma (see <xref
1694 linkend="pragmas"/>).</para>
1700 <option>-fext-core</option>
1701 <indexterm><primary><option>-fext-core</option></primary></indexterm>
1704 <para>Generate <literal>.hcr</literal> files.</para>
1710 <para>GHC can also read in External Core files as source; just give the <literal>.hcr</literal> file on
1711 the command line, instead of the <literal>.hs</literal> or <literal>.lhs</literal> Haskell source.
1712 A current infelicity is that you need to give the <literal>-fglasgow-exts</literal> flag too, because
1713 ordinary Haskell 98, when translated to External Core, uses things like rank-2 types.</para>
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