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>-v</option></primary></indexterm>
599 <para>The <option>-v</option> option makes GHC
600 <emphasis>verbose</emphasis>: it reports its version number
601 and shows (on stderr) exactly how it invokes each phase of
602 the compilation system. Moreover, it passes the
603 <option>-v</option> flag to most phases; each reports its
604 version number (and possibly some other information).</para>
606 <para>Please, oh please, use the <option>-v</option> option
607 when reporting bugs! Knowing that you ran the right bits in
608 the right order is always the first thing we want to
615 <option>-v</option><replaceable>n</replaceable>
616 <indexterm><primary><option>-v</option></primary></indexterm>
619 <para>To provide more control over the compiler's verbosity,
620 the <option>-v</option> flag takes an optional numeric
621 argument. Specifying <option>-v</option> on its own is
622 equivalent to <option>-v3</option>, and the other levels
623 have the following meanings:</para>
627 <term><option>-v0</option></term>
629 <para>Disable all non-essential messages (this is the
635 <term><option>-v1</option></term>
637 <para>Minimal verbosity: print one line per
638 compilation (this is the default when
639 <option>––make</option> or
640 <option>––interactive</option> is on).</para>
645 <term><option>-v2</option></term>
647 <para>Print the name of each compilation phase as it
648 is executed. (equivalent to
649 <option>-dshow-passes</option>).</para>
654 <term><option>-v3</option></term>
656 <para>The same as <option>-v2</option>, except that in
657 addition the full command line (if appropriate) for
658 each compilation phase is also printed.</para>
663 <term><option>-v4</option></term>
665 <para>The same as <option>-v3</option> except that the
666 intermediate program representation after each
667 compilation phase is also printed (excluding
668 preprocessed and C/assembly files).</para>
678 <indexterm><primary><option>-V</option></primary></indexterm>
681 <option>––version</option>
682 <indexterm><primary><option>––version</option></primary></indexterm>
685 <para>Print a one-line string including GHC's version number.</para>
691 <option>––numeric-version</option>
692 <indexterm><primary><option>––numeric-version</option></primary></indexterm>
695 <para>Print GHC's numeric version number only.</para>
701 <option>––print-libdir</option>
702 <indexterm><primary><option>––print-libdir</option></primary></indexterm>
705 <para>Print the path to GHC's library directory. This is
706 the top of the directory tree containing GHC's libraries,
707 interfaces, and include files (usually something like
708 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
710 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary>
711 </indexterm>in the package configuration file (see <xref
712 linkend="packages"/>).</para>
717 <term><option>-ferror-spans</option>
718 <indexterm><primary><option>-ferror-spans</option></primary>
722 <para>Causes GHC to emit the full source span of the
723 syntactic entity relating to an error message. Normally, GHC
724 emits the source location of the start of the syntactic
727 <para>For example:</para>
729 <screen>test.hs:3:6: parse error on input `where'</screen>
731 <para>becomes:</para>
733 <screen>test296.hs:3:6-10: parse error on input `where'</screen>
735 <para>And multi-line spans are possible too:</para>
737 <screen>test.hs:(5,4)-(6,7):
738 Conflicting definitions for `a'
739 Bound at: test.hs:5:4
741 In the binding group for: a, b, a</screen>
743 <para>Note that line numbers start counting at one, but
744 column numbers start at zero. This choice was made to
745 follow existing convention (i.e. this is how Emacs does
751 <term><option>-Rghc-timing</option>
752 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
755 <para>Prints a one-line summary of timing statistics for the
756 GHC run. This option is equivalent to
757 <literal>+RTS -tstderr</literal>, see <xref
758 linkend="rts-options-gc" />.
767 <sect1 id="options-sanity">
768 <title>Warnings and sanity-checking</title>
770 <indexterm><primary>sanity-checking options</primary></indexterm>
771 <indexterm><primary>warnings</primary></indexterm>
774 <para>GHC has a number of options that select which types of
775 non-fatal error messages, otherwise known as warnings, can be
776 generated during compilation. By default, you get a standard set
777 of warnings which are generally likely to indicate bugs in your
779 <option>-fwarn-overlapping-patterns</option>,
780 <option>-fwarn-deprecations</option>,
781 <option>-fwarn-duplicate-exports</option>,
782 <option>-fwarn-missing-fields</option>, and
783 <option>-fwarn-missing-methods</option>. The following flags are
784 simple ways to select standard “packages” of warnings:
790 <term><option>-W</option>:</term>
792 <indexterm><primary>-W option</primary></indexterm>
793 <para>Provides the standard warnings plus
794 <option>-fwarn-incomplete-patterns</option>,
795 <option>-fwarn-unused-matches</option>,
796 <option>-fwarn-unused-imports</option>, and
797 <option>-fwarn-unused-binds</option>.</para>
802 <term><option>-w</option>:</term>
804 <indexterm><primary><option>-w</option></primary></indexterm>
805 <para>Turns off all warnings, including the standard ones.</para>
810 <term><option>-Wall</option>:</term>
812 <indexterm><primary><option>-Wall</option></primary></indexterm>
813 <para>Turns on all warning options.</para>
818 <term><option>-Werror</option>:</term>
820 <indexterm><primary><option>-Werror</option></primary></indexterm>
821 <para>Makes any warning into a fatal error. Useful so that you don't
822 miss warnings when doing batch compilation. </para>
828 <para>The full set of warning options is described below. To turn
829 off any warning, simply give the corresponding
830 <option>-fno-warn-...</option> option on the command line.</para>
835 <term><option>-fwarn-deprecations</option>:</term>
837 <indexterm><primary><option>-fwarn-deprecations</option></primary>
839 <indexterm><primary>deprecations</primary></indexterm>
840 <para>Causes a warning to be emitted when a deprecated
841 function or type is used. Entities can be marked as
842 deprecated using a pragma, see <xref
843 linkend="deprecated-pragma"/>.</para>
848 <term><option>-fwarn-duplicate-exports</option>:</term>
850 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
851 <indexterm><primary>duplicate exports, warning</primary></indexterm>
852 <indexterm><primary>export lists, duplicates</primary></indexterm>
854 <para>Have the compiler warn about duplicate entries in
855 export lists. This is useful information if you maintain
856 large export lists, and want to avoid the continued export
857 of a definition after you've deleted (one) mention of it in
858 the export list.</para>
860 <para>This option is on by default.</para>
865 <term><option>-fwarn-hi-shadowing</option>:</term>
867 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
868 <indexterm><primary>shadowing</primary>
869 <secondary>interface files</secondary></indexterm>
871 <para>Causes the compiler to emit a warning when a module or
872 interface file in the current directory is shadowing one
873 with the same module name in a library or other
879 <term><option>-fwarn-incomplete-patterns</option>:</term>
881 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
882 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
883 <indexterm><primary>patterns, incomplete</primary></indexterm>
885 <para>Similarly for incomplete patterns, the function
886 <function>g</function> below will fail when applied to
887 non-empty lists, so the compiler will emit a warning about
888 this when <option>-fwarn-incomplete-patterns</option> is
895 <para>This option isn't enabled be default because it can be
896 a bit noisy, and it doesn't always indicate a bug in the
897 program. However, it's generally considered good practice
898 to cover all the cases in your functions.</para>
903 <term><option>-fwarn-incomplete-record-updates</option>:</term>
905 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
906 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
907 <indexterm><primary>record updates, incomplete</primary></indexterm>
910 <function>f</function> below will fail when applied to
911 <literal>Bar</literal>, so the compiler will emit a warning about
912 this when <option>-fwarn-incomplete-record-updates</option> is
916 data Foo = Foo { x :: Int }
920 f foo = foo { x = 6 }
923 <para>This option isn't enabled be default because it can be
924 very noisy, and it often doesn't indicate a bug in the
931 <option>-fwarn-missing-fields</option>:
932 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
933 <indexterm><primary>missing fields, warning</primary></indexterm>
934 <indexterm><primary>fields, missing</primary></indexterm>
938 <para>This option is on by default, and warns you whenever
939 the construction of a labelled field constructor isn't
940 complete, missing initializers for one or more fields. While
941 not an error (the missing fields are initialised with
942 bottoms), it is often an indication of a programmer error.</para>
947 <term><option>-fwarn-missing-methods</option>:</term>
949 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
950 <indexterm><primary>missing methods, warning</primary></indexterm>
951 <indexterm><primary>methods, missing</primary></indexterm>
953 <para>This option is on by default, and warns you whenever
954 an instance declaration is missing one or more methods, and
955 the corresponding class declaration has no default
956 declaration for them.</para>
957 <para>The warning is suppressed if the method name
958 begins with an underscore. Here's an example where this is useful:
961 _simpleFn :: a -> String
962 complexFn :: a -> a -> String
963 complexFn x y = ... _simpleFn ...
965 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
966 never <literal>_simpleFn</literal>; and (b)
967 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
973 <term><option>-fwarn-missing-signatures</option>:</term>
975 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
976 <indexterm><primary>type signatures, missing</primary></indexterm>
978 <para>If you would like GHC to check that every top-level
979 function/value has a type signature, use the
980 <option>-fwarn-missing-signatures</option> option. This
981 option is off by default.</para>
986 <term><option>-fwarn-name-shadowing</option>:</term>
988 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
989 <indexterm><primary>shadowing, warning</primary></indexterm>
991 <para>This option causes a warning to be emitted whenever an
992 inner-scope value has the same name as an outer-scope value,
993 i.e. the inner value shadows the outer one. This can catch
994 typographical errors that turn into hard-to-find bugs, e.g.,
995 in the inadvertent cyclic definition <literal>let x = ... x
996 ... in</literal>.</para>
998 <para>Consequently, this option does
999 <emphasis>will</emphasis> complain about cyclic recursive
1005 <term><option>-fwarn-orphans</option>:</term>
1007 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1008 <indexterm><primary>orphan instances, warning</primary></indexterm>
1009 <indexterm><primary>orphan rules, warning</primary></indexterm>
1011 <para>This option causes a warning to be emitted whenever the
1012 module contains an "orphan" instance declaration or rewrite rule.
1013 An instance declartion is an orphan if it appears in a module in
1014 which neither the class nor the type being instanced are declared
1015 in the same module. A rule is an orphan if it is a rule for a
1016 function declared in another module. A module containing any
1017 orphans is called an orphan module.</para>
1018 <para>The trouble with orphans is that GHC must pro-actively read the interface
1019 files for all orphan modules, just in case their instances or rules
1020 play a role, whether or not the module's interface would otherwise
1021 be of any use. Other things being equal, avoid orphan modules.</para>
1027 <option>-fwarn-overlapping-patterns</option>:
1028 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1029 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1030 <indexterm><primary>patterns, overlapping</primary></indexterm>
1033 <para>By default, the compiler will warn you if a set of
1034 patterns are overlapping, i.e.,</para>
1037 f :: String -> Int
1043 <para>where the last pattern match in <function>f</function>
1044 won't ever be reached, as the second pattern overlaps
1045 it. More often than not, redundant patterns is a programmer
1046 mistake/error, so this option is enabled by default.</para>
1051 <term><option>-fwarn-simple-patterns</option>:</term>
1053 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1055 <para>Causes the compiler to warn about lambda-bound
1056 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1057 Normally, these aren't treated as incomplete patterns by
1058 <option>-fwarn-incomplete-patterns</option>.</para>
1059 <para>``Lambda-bound patterns'' includes all places where there is a single pattern,
1060 including list comprehensions and do-notation. In these cases, a pattern-match
1061 failure is quite legitimate, and triggers filtering (list comprehensions) or
1062 the monad <literal>fail</literal> operation (monads). For example:
1064 f :: [Maybe a] -> [a]
1065 f xs = [y | Just y <- xs]
1067 Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
1068 these probably-innocent cases, which is why the flag is off by default. </para>
1069 <para> The <literal>deriving( Read )</literal> mechanism produces monadic code with
1070 pattern matches, so you will also get misleading warnings about the compiler-generated
1071 code. (This is arguably a Bad Thing, but it's awkward to fix.)</para>
1077 <term><option>-fwarn-type-defaults</option>:</term>
1079 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1080 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1081 <para>Have the compiler warn/inform you where in your source
1082 the Haskell defaulting mechanism for numeric types kicks
1083 in. This is useful information when converting code from a
1084 context that assumed one default into one with another,
1085 e.g., the `default default' for Haskell 1.4 caused the
1086 otherwise unconstrained value <constant>1</constant> to be
1087 given the type <literal>Int</literal>, whereas Haskell 98
1088 defaults it to <literal>Integer</literal>. This may lead to
1089 differences in performance and behaviour, hence the
1090 usefulness of being non-silent about this.</para>
1092 <para>This warning is off by default.</para>
1097 <term><option>-fwarn-unused-binds</option>:</term>
1099 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1100 <indexterm><primary>unused binds, warning</primary></indexterm>
1101 <indexterm><primary>binds, unused</primary></indexterm>
1102 <para>Report any function definitions (and local bindings)
1103 which are unused. For top-level functions, the warning is
1104 only given if the binding is not exported.</para>
1105 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1106 mentioned in the right hand side of another definition that is used, or (c) the
1107 function it defines begins with an underscore. The last case provides a
1108 way to suppress unused-binding warnings selectively. </para>
1109 <para> Notice that a variable
1110 is reported as unused even if it appears in the right-hand side of another
1111 unused binding. </para>
1116 <term><option>-fwarn-unused-imports</option>:</term>
1118 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1119 <indexterm><primary>unused imports, warning</primary></indexterm>
1120 <indexterm><primary>imports, unused</primary></indexterm>
1122 <para>Report any modules that are explicitly imported but
1123 never used. However, the form <literal>import M()</literal> is
1124 never reported as an unused import, because it is a useful idiom
1125 for importing instance declarations, which are anonymous in Haskell.</para>
1130 <term><option>-fwarn-unused-matches</option>:</term>
1132 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1133 <indexterm><primary>unused matches, warning</primary></indexterm>
1134 <indexterm><primary>matches, unused</primary></indexterm>
1136 <para>Report all unused variables which arise from pattern
1137 matches, including patterns consisting of a single variable.
1138 For instance <literal>f x y = []</literal> would report
1139 <varname>x</varname> and <varname>y</varname> as unused. The
1140 warning is suppressed if the variable name begins with an underscore, thus:
1150 <para>If you're feeling really paranoid, the
1151 <option>-dcore-lint</option>
1152 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1153 is a good choice. It turns on heavyweight intra-pass
1154 sanity-checking within GHC. (It checks GHC's sanity, not
1161 <sect1 id="options-optimise">
1162 <title>Optimisation (code improvement)</title>
1164 <indexterm><primary>optimisation</primary></indexterm>
1165 <indexterm><primary>improvement, code</primary></indexterm>
1167 <para>The <option>-O*</option> options specify convenient
1168 “packages” of optimisation flags; the
1169 <option>-f*</option> options described later on specify
1170 <emphasis>individual</emphasis> optimisations to be turned on/off;
1171 the <option>-m*</option> options specify
1172 <emphasis>machine-specific</emphasis> optimisations to be turned
1175 <sect2 id="optimise-pkgs">
1176 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1178 <para>There are <emphasis>many</emphasis> options that affect
1179 the quality of code produced by GHC. Most people only have a
1180 general goal, something like “Compile quickly” or
1181 “Make my program run like greased lightning.” The
1182 following “packages” of optimisations (or lack
1183 thereof) should suffice.</para>
1185 <para>Note that higher optimisation levels cause more
1186 cross-module optimisation to be performed, which can have an
1187 impact on how much of your program needs to be recompiled when
1188 you change something. This is one reaosn to stick to
1189 no-optimisation when developing code.</para>
1195 No <option>-O*</option>-type option specified:
1196 <indexterm><primary>-O* not specified</primary></indexterm>
1199 <para>This is taken to mean: “Please compile
1200 quickly; I'm not over-bothered about compiled-code
1201 quality.” So, for example: <command>ghc -c
1202 Foo.hs</command></para>
1208 <option>-O0</option>:
1209 <indexterm><primary><option>-O0</option></primary></indexterm>
1212 <para>Means “turn off all optimisation”,
1213 reverting to the same settings as if no
1214 <option>-O</option> options had been specified. Saying
1215 <option>-O0</option> can be useful if
1216 eg. <command>make</command> has inserted a
1217 <option>-O</option> on the command line already.</para>
1223 <option>-O</option> or <option>-O1</option>:
1224 <indexterm><primary>-O option</primary></indexterm>
1225 <indexterm><primary>-O1 option</primary></indexterm>
1226 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1229 <para>Means: “Generate good-quality code without
1230 taking too long about it.” Thus, for example:
1231 <command>ghc -c -O Main.lhs</command></para>
1233 <para><option>-O</option> currently also implies
1234 <option>-fvia-C</option>. This may change in the
1241 <option>-O2</option>:
1242 <indexterm><primary>-O2 option</primary></indexterm>
1243 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1246 <para>Means: “Apply every non-dangerous
1247 optimisation, even if it means significantly longer
1248 compile times.”</para>
1250 <para>The avoided “dangerous” optimisations
1251 are those that can make runtime or space
1252 <emphasis>worse</emphasis> if you're unlucky. They are
1253 normally turned on or off individually.</para>
1255 <para>At the moment, <option>-O2</option> is
1256 <emphasis>unlikely</emphasis> to produce better code than
1257 <option>-O</option>.</para>
1263 <option>-Ofile <file></option>:
1264 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1265 <indexterm><primary>optimising, customised</primary></indexterm>
1268 <para>(NOTE: not supported since GHC 4.x. Please ask if
1269 you're interested in this.)</para>
1271 <para>For those who need <emphasis>absolute</emphasis>
1272 control over <emphasis>exactly</emphasis> what options are
1273 used (e.g., compiler writers, sometimes :-), a list of
1274 options can be put in a file and then slurped in with
1275 <option>-Ofile</option>.</para>
1277 <para>In that file, comments are of the
1278 <literal>#</literal>-to-end-of-line variety; blank
1279 lines and most whitespace is ignored.</para>
1281 <para>Please ask if you are baffled and would like an
1282 example of <option>-Ofile</option>!</para>
1287 <para>We don't use a <option>-O*</option> flag for day-to-day
1288 work. We use <option>-O</option> to get respectable speed;
1289 e.g., when we want to measure something. When we want to go for
1290 broke, we tend to use <option>-O2 -fvia-C</option> (and we go for
1291 lots of coffee breaks).</para>
1293 <para>The easiest way to see what <option>-O</option> (etc.)
1294 “really mean” is to run with <option>-v</option>,
1295 then stand back in amazement.</para>
1298 <sect2 id="options-f">
1299 <title><option>-f*</option>: platform-independent flags</title>
1301 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1302 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1304 <para>These flags turn on and off individual optimisations.
1305 They are normally set via the <option>-O</option> options
1306 described above, and as such, you shouldn't need to set any of
1307 them explicitly (indeed, doing so could lead to unexpected
1308 results). However, there are one or two that may be of
1313 <term><option>-fexcess-precision</option>:</term>
1315 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1316 <para>When this option is given, intermediate floating
1317 point values can have a <emphasis>greater</emphasis>
1318 precision/range than the final type. Generally this is a
1319 good thing, but some programs may rely on the exact
1321 <literal>Float</literal>/<literal>Double</literal> values
1322 and should not use this option for their compilation.</para>
1327 <term><option>-fignore-asserts</option>:</term>
1329 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1330 <para>Causes GHC to ignore uses of the function
1331 <literal>Exception.assert</literal> in source code (in
1332 other words, rewriting <literal>Exception.assert p
1333 e</literal> to <literal>e</literal> (see <xref
1334 linkend="sec-assertions"/>). This flag is turned on by
1335 <option>-O</option>.
1342 <option>-fno-cse</option>
1343 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1346 <para>Turns off the common-sub-expression elimination optimisation.
1347 Can be useful if you have some <literal>unsafePerformIO</literal>
1348 expressions that you don't want commoned-up.</para>
1354 <option>-fno-strictness</option>
1355 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1358 <para>Turns off the strictness analyser; sometimes it eats
1359 too many cycles.</para>
1365 <option>-fno-full-laziness</option>
1366 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1369 <para>Turns off the full laziness optimisation (also known as
1370 let-floating). Full laziness increases sharing, which can lead
1371 to increased memory residency.</para>
1373 <para>NOTE: GHC doesn't implement complete full-laziness.
1374 When optimisation in on, and
1375 <option>-fno-full-laziness</option> is not given, some
1376 transformations that increase sharing are performed, such
1377 as extracting repeated computations from a loop. These
1378 are the same transformations that a fully lazy
1379 implementation would do, the difference is that GHC
1380 doesn't consistently apply full-laziness, so don't rely on
1387 <option>-fno-state-hack</option>
1388 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1391 <para>Turn off the "state hack" whereby any lambda with a
1392 <literal>State#</literal> token as argument is considered to be
1393 single-entry, hence it is considered OK to inline things inside
1394 it. This can improve performance of IO and ST monad code, but it
1395 runs the risk of reducing sharing.</para>
1401 <option>-funbox-strict-fields</option>:
1402 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1403 <indexterm><primary>strict constructor fields</primary></indexterm>
1404 <indexterm><primary>constructor fields, strict</primary></indexterm>
1407 <para>This option causes all constructor fields which are
1408 marked strict (i.e. “!”) to be unboxed or
1409 unpacked if possible. It is equivalent to adding an
1410 <literal>UNPACK</literal> pragma to every strict
1411 constructor field (see <xref
1412 linkend="unpack-pragma"/>).</para>
1414 <para>This option is a bit of a sledgehammer: it might
1415 sometimes make things worse. Selectively unboxing fields
1416 by using <literal>UNPACK</literal> pragmas might be
1423 <option>-funfolding-update-in-place<n></option>
1424 <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1427 <para>Switches on an experimental "optimisation".
1428 Switching it on makes the compiler a little keener to
1429 inline a function that returns a constructor, if the
1430 context is that of a thunk.
1434 If we inlined plusInt we might get an opportunity to use
1435 update-in-place for the thunk 'x'.</para>
1441 <option>-funfolding-creation-threshold<n></option>:
1442 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1443 <indexterm><primary>inlining, controlling</primary></indexterm>
1444 <indexterm><primary>unfolding, controlling</primary></indexterm>
1447 <para>(Default: 45) Governs the maximum size that GHC will
1448 allow a function unfolding to be. (An unfolding has a
1449 “size” that reflects the cost in terms of
1450 “code bloat” of expanding that unfolding at
1451 at a call site. A bigger function would be assigned a
1452 bigger cost.) </para>
1454 <para> Consequences: (a) nothing larger than this will be
1455 inlined (unless it has an INLINE pragma); (b) nothing
1456 larger than this will be spewed into an interface
1460 <para> Increasing this figure is more likely to result in longer
1461 compile times than faster code. The next option is more
1467 <term><option>-funfolding-use-threshold<n></option>:</term>
1469 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1470 <indexterm><primary>inlining, controlling</primary></indexterm>
1471 <indexterm><primary>unfolding, controlling</primary></indexterm>
1473 <para>(Default: 8) This is the magic cut-off figure for
1474 unfolding: below this size, a function definition will be
1475 unfolded at the call-site, any bigger and it won't. The
1476 size computed for a function depends on two things: the
1477 actual size of the expression minus any discounts that
1478 apply (see <option>-funfolding-con-discount</option>).</para>
1489 <sect1 id="sec-using-concurrent">
1490 <title>Using Concurrent Haskell</title>
1491 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
1493 <para>GHC supports Concurrent Haskell by default, without requiring a
1494 special option or libraries compiled in a certain way. To get access to
1495 the support libraries for Concurrent Haskell, just import
1497 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>
1499 <para>The following RTS option(s) affect the behaviour of Concurrent
1500 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
1504 <term><option>-C<replaceable>s</replaceable></option></term>
1506 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1507 Sets the context switch interval to <replaceable>s</replaceable>
1508 seconds. A context switch will occur at the next heap block
1509 allocation after the timer expires (a heap block allocation occurs
1510 every 4k of allocation). With <option>-C0</option> or
1511 <option>-C</option>, context switches will occur as often as
1512 possible (at every heap block allocation). By default, context
1513 switches occur every 20ms.</para>
1519 <sect1 id="sec-using-smp">
1520 <title>Using SMP parallelism</title>
1521 <indexterm><primary>parallelism</primary>
1523 <indexterm><primary>SMP</primary>
1526 <para>GHC supports running Haskell programs in parallel on an SMP
1527 (symmetric multiprocessor).</para>
1529 <para>There's a fine distinction between
1530 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
1531 parallelism is all about making your program run
1532 <emphasis>faster</emphasis> by making use of multiple processors
1533 simultaneously. Concurrency, on the other hand, is a means of
1534 abstraction: it is a convenient way to structure a program that must
1535 respond to multiple asynchronous events.</para>
1537 <para>However, the two terms are certainly related. By making use of
1538 multiple CPUs it is possible to run concurrent threads in parallel,
1539 and this is exactly what GHC's SMP parallelism support does. But it
1540 is also possible to obtain performance improvements with parallelism
1541 on programs that do not use concurrency. This section describes how to
1542 use GHC to compile and run parallel programs, in <xref
1543 linkend="lang-parallel" /> we desribe the language features that affect
1546 <sect2 id="parallel-options">
1547 <title>Options to enable SMP parallelism</title>
1549 <para>In order to make use of multiple CPUs, your program must be
1550 linked with the <option>-threaded</option> option (see <xref
1551 linkend="options-linker" />). Then, to run a program on multiple
1552 CPUs, use the RTS <option>-N</option> option:</para>
1556 <term><option>-N<replaceable>x</replaceable></option></term>
1558 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1559 Use <replaceable>x</replaceable> simultaneous threads when
1560 running the program. Normally <replaceable>x</replaceable>
1561 should be chosen to match the number of CPU cores on the machine.
1562 There is no means (currently) by which this value may vary after
1563 the program has started.</para>
1565 <para>For example, on a dual-core machine we would probably use
1566 <literal>+RTS -N2 -RTS</literal>.</para>
1568 <para>Whether hyperthreading cores should be counted or not is an
1569 open question; please feel free to experiment and let us know what
1570 results you find.</para>
1577 <title>Hints for using SMP parallelism</title>
1579 <para>Add the <literal>-sstderr</literal> RTS option when
1580 running the program to see timing stats, which will help to tell you
1581 whether your program got faster by using more CPUs or not. If the user
1582 time is greater than
1583 the elapsed time, then the program used more than one CPU. You should
1584 also run the program without <literal>-N</literal> for comparison.</para>
1586 <para>GHC's parallelism support is new and experimental. It may make your
1587 program go faster, or it might slow it down - either way, we'd be
1588 interested to hear from you.</para>
1590 <para>One significant limitation with the current implementation is that
1591 the garbage collector is still single-threaded, and all execution must
1592 stop when GC takes place. This can be a significant bottleneck in a
1593 parallel program, especially if your program does a lot of GC. If this
1594 happens to you, then try reducing the cost of GC by tweaking the GC
1595 settings (<xref linkend="rts-options-gc" />): enlarging the heap or the
1596 allocation area size is a good start.</para>
1600 <sect1 id="options-platform">
1601 <title>Platform-specific Flags</title>
1603 <indexterm><primary>-m* options</primary></indexterm>
1604 <indexterm><primary>platform-specific options</primary></indexterm>
1605 <indexterm><primary>machine-specific options</primary></indexterm>
1607 <para>Some flags only make sense for particular target
1613 <term><option>-monly-[32]-regs</option>:</term>
1615 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1616 option (iX86 only)</primary></indexterm> GHC tries to
1617 “steal” four registers from GCC, for performance
1618 reasons; it almost always works. However, when GCC is
1619 compiling some modules with four stolen registers, it will
1620 crash, probably saying:
1623 Foo.hc:533: fixed or forbidden register was spilled.
1624 This may be due to a compiler bug or to impossible asm
1625 statements or clauses.
1628 Just give some registers back with
1629 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1630 If `2' doesn't work, please report the bug to us.</para>
1639 <sect1 id="ext-core">
1640 <title>Generating and compiling External Core Files</title>
1642 <indexterm><primary>intermediate code generation</primary></indexterm>
1644 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
1645 to a file as a side-effect of compilation. Core files, which are given the suffix
1646 <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1647 tools. The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz">
1648 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1649 and sample tools (in Haskell)
1650 for manipulating Core files are available in the GHC source distribution
1651 directory <literal>/fptools/ghc/utils/ext-core</literal>.
1652 Note that the format of <literal>.hcr</literal>
1653 files is <emphasis>different</emphasis> (though similar) to the Core output format generated
1654 for debugging purposes (<xref linkend="options-debugging"/>).</para>
1656 <para>The Core format natively supports notes which you can add to
1657 your source code using the <literal>CORE</literal> pragma (see <xref
1658 linkend="pragmas"/>).</para>
1664 <option>-fext-core</option>
1665 <indexterm><primary><option>-fext-core</option></primary></indexterm>
1668 <para>Generate <literal>.hcr</literal> files.</para>
1674 <para>GHC can also read in External Core files as source; just give the <literal>.hcr</literal> file on
1675 the command line, instead of the <literal>.hs</literal> or <literal>.lhs</literal> Haskell source.
1676 A current infelicity is that you need to give the <literal>-fglasgow-exts</literal> flag too, because
1677 ordinary Haskell 98, when translated to External Core, uses things like rank-2 types.</para>
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