1 <Chapter id="using-GHC">
6 <IndexTerm><Primary>GHC, using</Primary></IndexTerm>
7 <IndexTerm><Primary>using GHC</Primary></IndexTerm>
8 GHC is a command-line compiler: in order to compile a Haskell program,
9 GHC must be invoked on the source file(s) by typing a command to the
10 shell. The steps involved in compiling a program can be automated
11 using the <Command>make</Command> tool (this is especially useful if the program
12 consists of multiple source files which depend on each other). This
13 section describes how to use GHC from the command-line.
16 <Sect1 id="command-line-structure">
17 <Title>Overall command-line structure
21 <IndexTerm><Primary>structure, command-line</Primary></IndexTerm>
22 <IndexTerm><Primary>command-line structure</Primary></IndexTerm>
26 An invocation of GHC takes the following form:
38 Command-line arguments are either options or file names.
42 Command-line options begin with <Literal>-</Literal>. They may <Emphasis>not</Emphasis> be
43 grouped: <Option>-vO</Option> is different from <Option>-v -O</Option>. Options need not
44 precede filenames: e.g., <Command>ghc *.o -o foo</Command>. All options are
45 processed and then applied to all files; you cannot, for example, invoke
46 <Command>ghc -c -O1 Foo.hs -O2 Bar.hs</Command> to apply different optimisation
47 levels to the files <Filename>Foo.hs</Filename> and <Filename>Bar.hs</Filename>. For conflicting
48 options, e.g., <Option>-c -S</Option>, we reserve the right to do anything we
49 want. (Usually, the last one applies.)
54 <Sect1 id="file-suffixes">
55 <Title>Meaningful file suffixes
59 <IndexTerm><Primary>suffixes, file</Primary></IndexTerm>
60 <IndexTerm><Primary>file suffixes for GHC</Primary></IndexTerm>
64 File names with “meaningful” suffixes (e.g., <Filename>.lhs</Filename> or <Filename>.o</Filename>)
65 cause the “right thing” to happen to those files.
72 <Term><Filename>.lhs</Filename>:</Term>
75 <IndexTerm><Primary>lhs suffix</Primary></IndexTerm>
76 A “literate Haskell” module.
81 <Term><Filename>.hs</Filename>:</Term>
84 A not-so-literate Haskell module.
89 <Term><Filename>.hi</Filename>:</Term>
92 A Haskell interface file, probably compiler-generated.
97 <Term><Filename>.hc</Filename>:</Term>
100 Intermediate C file produced by the Haskell compiler.
105 <Term><Filename>.c</Filename>:</Term>
108 A C file not produced by the Haskell compiler.
113 <Term><Filename>.s</Filename>:</Term>
116 An assembly-language source file, usually
117 produced by the compiler.
122 <Term><Filename>.o</Filename>:</Term>
125 An object file, produced by an assembler.
133 Files with other suffixes (or without suffixes) are passed straight
139 <Sect1 id="options-help">
140 <Title>Help and verbosity options
144 <IndexTerm><Primary>help options (GHC)</Primary></IndexTerm>
145 <IndexTerm><Primary>verbose option (GHC)</Primary></IndexTerm>
149 A good option to start with is the <Option>-help</Option> (or <Option>-?</Option>) option.
150 <IndexTerm><Primary>-help option</Primary></IndexTerm>
151 <IndexTerm><Primary>-? option</Primary></IndexTerm>
152 GHC spews a long message to standard output and then exits.
156 The <Option>-v</Option><IndexTerm><Primary>-v option</Primary></IndexTerm> option makes GHC <Emphasis>verbose</Emphasis>: it
157 reports its version number and shows (on stderr) exactly how it invokes each
158 phase of the compilation system. Moreover, it passes
159 the <Option>-v</Option> flag to most phases; each reports
160 its version number (and possibly some other information).
164 Please, oh please, use the <Option>-v</Option> option when reporting bugs!
165 Knowing that you ran the right bits in the right order is always the
166 first thing we want to verify.
170 If you're just interested in the compiler version number, the
171 <Option>--version</Option><IndexTerm><Primary>--version option</Primary></IndexTerm> option prints out a
172 one-line string containing the requested info.
177 <Sect1 id="options-order">
178 <Title>Running the right phases in the right order
182 <IndexTerm><Primary>order of passes in GHC</Primary></IndexTerm>
183 <IndexTerm><Primary>pass ordering in GHC</Primary></IndexTerm>
184 The basic task of the <Command>ghc</Command> driver is to run each input file
185 through the right phases (compiling, linking, etc.).
189 The first phase to run is determined by the input-file suffix, and the
190 last phase is determined by a flag. If no relevant flag is present,
191 then go all the way through linking. This table summarises:
197 <ColSpec Align="Left">
198 <ColSpec Align="Left">
199 <ColSpec Align="Left">
200 <ColSpec Align="Left">
204 <Entry>Phase of the compilation system</Entry>
205 <Entry>Suffix saying “start here”</Entry>
206 <Entry>Flag saying “stop after”</Entry>
207 <Entry>(suffix of) output file</Entry>
212 literate pre-processor </Entry>
213 <Entry> .lhs </Entry>
219 C pre-processor (opt.) </Entry>
226 Haskell compiler </Entry>
228 <Entry> -C, -S </Entry>
229 <Entry> .hc, .s </Entry>
233 C compiler (opt.) </Entry>
234 <Entry> .hc or .c </Entry>
248 <Entry> other </Entry>
250 <Entry> a.out </Entry>
256 <IndexTerm><Primary>-C option</Primary></IndexTerm>
257 <IndexTerm><Primary>-S option</Primary></IndexTerm>
258 <IndexTerm><Primary>-c option</Primary></IndexTerm>
262 Thus, a common invocation would be: <Command>ghc -c Foo.hs</Command>
266 Note: What the Haskell compiler proper produces depends on whether a
267 native-code generator is used (producing assembly language) or not
272 The option <Option>-cpp</Option><IndexTerm><Primary>-cpp option</Primary></IndexTerm> must be given for the C
273 pre-processor phase to be run, that is, the pre-processor will be run
274 over your Haskell source file before continuing.
278 The option <Option>-E</Option><IndexTerm><Primary>-E option</Primary></IndexTerm> runs just the pre-processing
279 passes of the compiler, outputting the result on stdout before
280 stopping. If used in conjunction with -cpp, the output is the
281 code blocks of the original (literal) source after having put it
282 through the grinder that is the C pre-processor. Sans <Option>-cpp</Option>, the
283 output is the de-litted version of the original source.
287 The option <Option>-optcpp-E</Option><IndexTerm><Primary>-optcpp-E option</Primary></IndexTerm> runs just the
288 pre-processing stage of the C-compiling phase, sending the result to
289 stdout. (For debugging or obfuscation contests, usually.)
294 <Sect1 id="options-output">
295 <Title>Re-directing the compilation output(s)
299 <IndexTerm><Primary>output-directing options</Primary></IndexTerm>
303 GHC's compiled output normally goes into a <Filename>.hc</Filename>, <Filename>.o</Filename>, etc., file,
304 depending on the last-run compilation phase. The option <Option>-o
305 foo</Option><IndexTerm><Primary>-o option</Primary></IndexTerm> re-directs the output of that last-run
306 phase to file <Filename>foo</Filename>.
310 Note: this “feature” can be counterintuitive:
311 <Command>ghc -C -o foo.o foo.hs</Command> will put the intermediate C code in the
312 file <Filename>foo.o</Filename>, name notwithstanding!
316 EXOTICA: But the <Option>-o</Option> option isn't of much use if you have
317 <Emphasis>several</Emphasis> input files… Non-interface output files are
318 normally put in the same directory as their corresponding input file
319 came from. You may specify that they be put in another directory
320 using the <Option>-odir <dir></Option><IndexTerm><Primary>-odir <dir> option</Primary></IndexTerm> (the
321 “Oh, dear” option). For example:
327 % ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
333 The output files, <Filename>Foo.o</Filename>, <Filename>Bar.o</Filename>, and <Filename>Bumble.o</Filename> would be
334 put into a subdirectory named after the architecture of the executing
335 machine (<Filename>sun4</Filename>, <Filename>mips</Filename>, etc). The directory must already
336 exist; it won't be created.
340 Note that the <Option>-odir</Option> option does <Emphasis>not</Emphasis> affect where the
341 interface files are put. In the above example, they would still be
342 put in <Filename>parse/Foo.hi</Filename>, <Filename>parse/Bar.hi</Filename>, and <Filename>gurgle/Bumble.hi</Filename>.
346 MORE EXOTICA: The <Option>-osuf <suffix></Option><IndexTerm><Primary>-osuf <suffix>
347 option</Primary></IndexTerm> will change the <Filename>.o</Filename> file suffix for object files to
348 whatever you specify. (We use this in compiling the prelude.).
349 Similarly, the <Option>-hisuf <suffix></Option><IndexTerm><Primary>-hisuf <suffix>
350 option</Primary></IndexTerm> will change the <Filename>.hi</Filename> file suffix for non-system
351 interface files (see <XRef LinkEnd="hi-options">).
355 The <Option>-hisuf</Option>/<Option>-osuf</Option> game is useful if you want to compile a program
356 with both GHC and HBC (say) in the same directory. Let HBC use the
357 standard <Filename>.hi</Filename>/<Filename>.o</Filename> suffixes; add <Option>-hisuf g_hi -osuf g_o</Option> to your
358 <Command>make</Command> rule for GHC compiling…
361 <sect2 id="keeping-intermediates">
362 <title>Keeping Intermediate Files</title>
363 <indexterm><primary>intermediate files, saving</primary>
365 <indexterm><primary><literal>.hc</literal> files, saving</primary>
367 <indexterm><primary><literal>.s</literal> files, saving</primary>
370 <para>The following options are useful for keeping certain
371 intermediate files around, when normally GHC would throw these
372 away after compilation:</para>
376 <term><literal>-keep-hc-files</literal></term>
378 <primary><literal>-keep-hc-files</literal></primary>
381 <para>Keep intermediate <literal>.hc</literal> files when
382 doing <literal>.hs</literal>-to-<literal>.o</literal>
383 compilations via C (NOTE: <literal>.hc</literal> files
384 aren't generated when using the native code generator, you
385 may need to use <literal>-fvia-C</literal> to force them
386 to be produced).</para>
391 <term><literal>-keep-s-files</literal></term>
393 <primary><literal>-keep-s-files</literal></primary>
396 <para>Keep intermediate <literal>.s</literal> files.</para>
401 <term><literal>-keep-raw-s-files</literal></term>
403 <primary><literal>-keep-raw-s-files</literal></primary>
406 <para>Keep intermediate <literal>.raw-s</literal> files.
407 These are the direct output from the C compiler, before
408 GHC does “assembly mangling” to produce the
409 <literal>.s</literal> file. Again, these are not produced
410 when using the native code generator.</para>
415 <term><literal>-keep-tmp-files</literal></term>
417 <primary><literal>-keep-tmp-files</literal></primary>
420 <primary>temporary files</primary>
421 <secondary>keeping</secondary>
424 <para>Instructs the GHC driver not to delete any of its
425 temporary files, which it normally keeps in
426 <literal>/tmp</literal> (or possibly elsewhere; see <xref
427 linkend="temp-files">). Running GHC with
428 <literal>-v</literal> will show you what temporary files
429 were generated along the way.</para>
435 <Sect2 id="saving-ghc-stderr">
436 <Title>Saving GHC's standard error output
440 <IndexTerm><Primary>standard error, saving</Primary></IndexTerm>
444 Sometimes, you may cause GHC to be rather chatty on standard error;
445 with <Option>-v</Option>, for example. You can instruct GHC to <Emphasis>append</Emphasis> this
446 output to a particular log file with a <Option>-odump <blah></Option><IndexTerm><Primary>-odump
447 <blah> option</Primary></IndexTerm> option.
452 <Sect2 id="temp-files">
453 <Title>Redirecting temporary files
457 <primary>temporary files</primary>
458 <secondary>redirecting</secondary>
462 If you have trouble because of running out of space in
463 <Filename>/tmp</Filename> (or wherever your installation thinks
464 temporary files should go), you may use the <Option>-tmpdir
465 <dir></Option><IndexTerm><Primary>-tmpdir <dir>
466 option</Primary></IndexTerm> option to specify an alternate directory.
467 For example, <Option>-tmpdir .</Option> says to put temporary files in
468 the current working directory.
472 Alternatively, use your <Constant>TMPDIR</Constant> environment
473 variable.<IndexTerm><Primary>TMPDIR environment
474 variable</Primary></IndexTerm> Set it to the name of the directory
475 where temporary files should be put. GCC and other programs will
476 honour the <Constant>TMPDIR</Constant> variable as well.
480 Even better idea: Set the <Constant>TMPDIR</Constant> variable when building GHC, and
481 never worry about <Constant>TMPDIR</Constant> again. (see the build documentation).
488 <Sect1 id="options-sanity">
489 <Title>Warnings and sanity-checking
493 <IndexTerm><Primary>sanity-checking options</Primary></IndexTerm>
494 <IndexTerm><Primary>warnings</Primary></IndexTerm>
495 GHC has a number of options that select which types of non-fatal error
496 messages, otherwise known as warnings, can be generated during
497 compilation. By default, you get a standard set of warnings which are
498 generally likely to indicate bugs in your program. These are:
499 <Option>-fwarn-overlpapping-patterns</Option>, <Option>-fwarn-duplicate-exports</Option>, and
500 <Option>-fwarn-missing-methods</Option>. The following flags are simple ways to
501 select standard “packages” of warnings:
508 <Term><Option>-Wnot</Option>:</Term>
511 <IndexTerm><Primary>-Wnot option</Primary></IndexTerm>
512 Turns off all warnings, including the standard ones.
518 <Term><Option>-w</Option>:</Term>
521 <IndexTerm><Primary>-w option</Primary></IndexTerm>
522 Synonym for <Option>-Wnot</Option>.
528 <Term><Option>-W</Option>:</Term>
531 <IndexTerm><Primary>-W option</Primary></IndexTerm>
532 Provides the standard warnings plus <Option>-fwarn-incomplete-patterns</Option>,
533 <Option>-fwarn-unused-imports</Option> and <Option>-fwarn-unused-binds</Option>.
539 <Term><Option>-Wall</Option>:</Term>
542 <IndexTerm><Primary>-Wall option</Primary></IndexTerm>
543 Turns on all warning options.
552 The full set of warning options is described below. To turn off any
553 warning, simply give the corresponding <Option>-fno-warn-...</Option> option on
561 <Term><Option>-fwarn-name-shadowing</Option>:</Term>
564 <IndexTerm><Primary>-fwarn-name-shadowing option</Primary></IndexTerm>
565 <IndexTerm><Primary>shadowing, warning</Primary></IndexTerm>This option causes a warning to be emitted whenever an inner-scope
566 value has the same name as an outer-scope value, i.e. the inner value
567 shadows the outer one. This can catch typographical errors that turn
568 into hard-to-find bugs, e.g., in the inadvertent cyclic definition
569 <Literal>let x = ... x ... in</Literal>.
573 Consequently, this option does <Emphasis>not</Emphasis> allow cyclic recursive
580 <Term><Option>-fwarn-overlapping-patterns</Option>:</Term>
583 <IndexTerm><Primary>-fwarn-overlapping-patterns option</Primary></IndexTerm>
584 <IndexTerm><Primary>overlapping patterns, warning</Primary></IndexTerm>
585 <IndexTerm><Primary>patterns, overlapping</Primary></IndexTerm>
586 By default, the compiler will warn you if a set of patterns are
592 f :: String -> Int
600 where the last pattern match in <Function>f</Function> won't ever be reached, as the
601 second pattern overlaps it. More often than not, redundant patterns
602 is a programmer mistake/error, so this option is enabled by default.
608 <Term><Option>-fwarn-incomplete-patterns</Option>:</Term>
611 <IndexTerm><Primary>-fwarn-incomplete-patterns option</Primary></IndexTerm>
612 <IndexTerm><Primary>incomplete patterns, warning</Primary></IndexTerm>
613 <IndexTerm><Primary>patterns, incomplete</Primary></IndexTerm>
614 Similarly for incomplete patterns, the function <Function>g</Function> below will fail
615 when applied to non-empty lists, so the compiler will emit a warning
616 about this when <Option>-fwarn-incomplete-patterns</Option> is enabled.
626 This option isn't enabled be default because it can be a bit noisy,
627 and it doesn't always indicate a bug in the program. However, it's
628 generally considered good practice to cover all the cases in your
635 <Term><Option>-fwarn-missing-methods</Option>:</Term>
638 <IndexTerm><Primary>-fwarn-missing-methods option</Primary></IndexTerm>
639 <IndexTerm><Primary>missing methods, warning</Primary></IndexTerm>
640 <IndexTerm><Primary>methods, missing</Primary></IndexTerm>
641 This option is on by default, and warns you whenever an instance
642 declaration is missing one or more methods, and the corresponding
643 class declaration has no default declaration for them.
649 <Term><Option>-fwarn-missing-fields</Option>:</Term>
652 <IndexTerm><Primary>-fwarn-missing-fields option</Primary></IndexTerm>
653 <IndexTerm><Primary>missing fields, warning</Primary></IndexTerm>
654 <IndexTerm><Primary>fields, missing</Primary></IndexTerm>
655 This option is on by default, and warns you whenever the construction
656 of a labelled field constructor isn't complete, missing initializers
657 for one or more fields. While not an error (the missing fields are
658 initialised with bottoms), it is often an indication of a programmer
665 <Term><Option>-fwarn-unused-imports</Option>:</Term>
668 <IndexTerm><Primary>-fwarn-unused-imports option</Primary></IndexTerm>
669 <IndexTerm><Primary>unused imports, warning</Primary></IndexTerm>
670 <IndexTerm><Primary>imports, unused</Primary></IndexTerm>
671 Report any objects that are explicitly imported but never used.
677 <Term><Option>-fwarn-unused-binds</Option>:</Term>
680 <IndexTerm><Primary>-fwarn-unused-binds option</Primary></IndexTerm>
681 <IndexTerm><Primary>unused binds, warning</Primary></IndexTerm>
682 <IndexTerm><Primary>binds, unused</Primary></IndexTerm>
683 Report any function definitions (and local bindings) which are unused.
684 For top-level functions, the warning is only given if the binding is
690 <Term><Option>-fwarn-unused-matches</Option>:</Term>
693 <IndexTerm><Primary>-fwarn-unused-matches option</Primary></IndexTerm>
694 <IndexTerm><Primary>unused matches, warning</Primary></IndexTerm>
695 <IndexTerm><Primary>matches, unused</Primary></IndexTerm>
696 Report all unused variables which arise from pattern matches,
697 including patterns consisting of a single variable. For instance <Literal>f x
698 y = []</Literal> would report <VarName>x</VarName> and <VarName>y</VarName> as unused. To eliminate the warning,
699 all unused variables can be replaced with wildcards.
705 <Term><Option>-fwarn-duplicate-exports</Option>:</Term>
708 <IndexTerm><Primary>-fwarn-duplicate-exports option</Primary></IndexTerm>
709 <IndexTerm><Primary>duplicate exports, warning</Primary></IndexTerm>
710 <IndexTerm><Primary>export lists, duplicates</Primary></IndexTerm>
711 Have the compiler warn about duplicate entries in export lists. This
712 is useful information if you maintain large export lists, and want to
713 avoid the continued export of a definition after you've deleted (one)
714 mention of it in the export list.
718 This option is on by default.
724 <Term><Option>-fwarn-type-defaults</Option>:</Term>
727 <IndexTerm><Primary>-fwarn-type-defaults option</Primary></IndexTerm>
728 <IndexTerm><Primary>defaulting mechanism, warning</Primary></IndexTerm>
729 Have the compiler warn/inform you where in your source the Haskell
730 defaulting mechanism for numeric types kicks in. This is useful
731 information when converting code from a context that assumed one
732 default into one with another, e.g., the `default default' for Haskell
733 1.4 caused the otherwise unconstrained value <Constant>1</Constant> to be given
734 the type <Literal>Int</Literal>, whereas Haskell 98 defaults it to
735 <Literal>Integer</Literal>. This may lead to differences in performance and
736 behaviour, hence the usefulness of being non-silent about this.
740 This warning is off by default.
746 <Term><Option>-fwarn-missing-signatures</Option>:</Term>
749 <IndexTerm><Primary>-fwarn-missing-signatures option</Primary></IndexTerm>
750 <IndexTerm><Primary>type signatures, missing</Primary></IndexTerm>
751 If you would like GHC to check that every top-level function/value has
752 a type signature, use the <Option>-fwarn-missing-signatures</Option> option. This
753 option is off by default.
761 If you're feeling really paranoid, the <Option>-dcore-lint</Option>
762 option<IndexTerm><Primary>-dcore-lint option</Primary></IndexTerm> is a good choice. It turns on
763 heavyweight intra-pass sanity-checking within GHC. (It checks GHC's
769 <Sect1 id="separate-compilation">
770 <Title>Separate compilation
774 <IndexTerm><Primary>separate compilation</Primary></IndexTerm>
775 <IndexTerm><Primary>recompilation checker</Primary></IndexTerm>
776 <IndexTerm><Primary>make and recompilation</Primary></IndexTerm>
777 This section describes how GHC supports separate compilation.
780 <Sect2 id="hi-files">
781 <Title>Interface files
785 <IndexTerm><Primary>interface files</Primary></IndexTerm>
786 <IndexTerm><Primary>.hi files</Primary></IndexTerm>
790 When GHC compiles a source file <Filename>F</Filename> which contains a module <Literal>A</Literal>, say,
791 it generates an object <Filename>F.o</Filename>, <Emphasis>and</Emphasis> a companion <Emphasis>interface
792 file</Emphasis> <Filename>A.hi</Filename>. The interface file is not intended for human
793 consumption, as you'll see if you take a look at one. It's merely
794 there to help the compiler compile other modules in the same program.
798 NOTE: Having the name of the interface file follow the module name and
799 not the file name, means that working with tools such as <Command>make</Command>
800 become harder. <Command>make</Command> implicitly assumes that any output files
801 produced by processing a translation unit will have file names that
802 can be derived from the file name of the translation unit. For
803 instance, pattern rules becomes unusable. For this reason, we
804 recommend you stick to using the same file name as the module name.
808 The interface file for <Literal>A</Literal> contains information needed by the compiler
809 when it compiles any module <Literal>B</Literal> that imports <Literal>A</Literal>, whether directly or
810 indirectly. When compiling <Literal>B</Literal>, GHC will read <Filename>A.hi</Filename> to find the
811 details that it needs to know about things defined in <Literal>A</Literal>.
815 Furthermore, when compiling module <Literal>C</Literal> which imports <Literal>B</Literal>, GHC may
816 decide that it needs to know something about <Literal>A</Literal>—for example, <Literal>B</Literal>
817 might export a function that involves a type defined in <Literal>A</Literal>. In this
818 case, GHC will go and read <Command>A.hi</Command> even though <Literal>C</Literal> does not explicitly
819 import <Literal>A</Literal> at all.
823 The interface file may contain all sorts of things that aren't
824 explicitly exported from <Literal>A</Literal> by the programmer. For example, even
825 though a data type is exported abstractly, <Filename>A.hi</Filename> will contain the
826 full data type definition. For small function definitions, <Filename>A.hi</Filename>
827 will contain the complete definition of the function. For bigger
828 functions, <Filename>A.hi</Filename> will contain strictness information about the
829 function. And so on. GHC puts much more information into <Filename>.hi</Filename> files
830 when optimisation is turned on with the <Option>-O</Option> flag. Without <Option>-O</Option> it
831 puts in just the minimum; with <Option>-O</Option> it lobs in a whole pile of stuff.
832 <IndexTerm><Primary>optimsation, effect on .hi files</Primary></IndexTerm>
836 <Filename>A.hi</Filename> should really be thought of as a compiler-readable version of
837 <Filename>A.o</Filename>. If you use a <Filename>.hi</Filename> file that wasn't generated by the same
838 compilation run that generates the <Filename>.o</Filename> file the compiler may assume
839 all sorts of incorrect things about <Literal>A</Literal>, resulting in core dumps and
840 other unpleasant happenings.
845 <Sect2 id="options-finding-imports">
846 <Title>Finding interface files
850 <IndexTerm><Primary>interface files, finding them</Primary></IndexTerm>
851 <IndexTerm><Primary>finding interface files</Primary></IndexTerm>
855 In your program, you import a module <Literal>Foo</Literal> by saying
856 <Literal>import Foo</Literal>. GHC goes looking for an interface file, <Filename>Foo.hi</Filename>.
857 It has a builtin list of directories (notably including <Filename>.</Filename>) where
865 <Term><Option>-i<dirs></Option></Term>
868 <IndexTerm><Primary>-i<dirs> option</Primary></IndexTerm>This flag
869 prepends a colon-separated list of <Filename>dirs</Filename> to the “import
870 directories” list.
871 See also <XRef LinkEnd="recomp"> for the significance of using
872 relative and absolute pathnames in the <Option>-i</Option> list.
878 <Term><Option>-i</Option></Term>
881 resets the “import directories” list back to nothing.
887 <Term><Option>-fno-implicit-prelude</Option></Term>
890 <IndexTerm><Primary>-fno-implicit-prelude option</Primary></IndexTerm>
891 GHC normally imports <Filename>Prelude.hi</Filename> files for you. If you'd rather it
892 didn't, then give it a <Option>-fno-implicit-prelude</Option> option. You are
893 unlikely to get very far without a Prelude, but, hey, it's a free
900 <Term><Option>-I<dir></Option></Term>
903 <IndexTerm><Primary>-I<dir> option</Primary></IndexTerm>
904 Once a Haskell module has been compiled to C (<Filename>.hc</Filename> file), you may
905 wish to specify where GHC tells the C compiler to look for <Filename>.h</Filename> files.
906 (Or, if you are using the <Option>-cpp</Option> option<IndexTerm><Primary>-cpp option</Primary></IndexTerm>, where
907 it tells the C pre-processor to look…) For this purpose, use a <Option>-I</Option>
908 option in the usual C-ish way.
918 <Sect2 id="hi-options">
919 <Title>Other options related to interface files
923 <IndexTerm><Primary>interface files, options</Primary></IndexTerm>
924 The interface output may be directed to another file
925 <Filename>bar2/Wurble.iface</Filename> with the option <Option>-ohi bar2/Wurble.iface</Option><IndexTerm><Primary>-ohi
926 <file> option</Primary></IndexTerm> (not recommended).
930 To avoid generating an interface file at all, use a <Option>-nohi</Option>
931 option.<IndexTerm><Primary>-nohi option</Primary></IndexTerm>
935 The compiler does not overwrite an existing <Filename>.hi</Filename> interface file if
936 the new one is byte-for-byte the same as the old one; this is friendly
937 to <Command>make</Command>. When an interface does change, it is often enlightening to
938 be informed. The <Option>-hi-diffs</Option><IndexTerm><Primary>-hi-diffs option</Primary></IndexTerm> option will
939 make GHC run <Command>diff</Command> on the old and new <Filename>.hi</Filename> files. You can also
940 record the difference in the interface file itself, the
941 <Option>-keep-hi-diffs</Option><IndexTerm><Primary>-keep-hi-diffs</Primary></IndexTerm> option takes care of that.
945 The <Filename>.hi</Filename> files from GHC contain “usage” information which changes
946 often and uninterestingly. If you really want to see these changes
947 reported, you need to use the
948 <Option>-hi-diffs-with-usages</Option><IndexTerm><Primary>-hi-diffs-with-usages option</Primary></IndexTerm>
953 Interface files are normally jammed full of compiler-produced
954 <Emphasis>pragmas</Emphasis>, which record arities, strictness info, etc. If you
955 think these pragmas are messing you up (or you are doing some kind of
956 weird experiment), you can tell GHC to ignore them with the
957 <Option>-fignore-interface-pragmas</Option><IndexTerm><Primary>-fignore-interface-pragmas
958 option</Primary></IndexTerm> option.
962 When compiling without optimisations on, the compiler is extra-careful
963 about not slurping in data constructors and instance declarations that
964 it will not need. If you believe it is getting it wrong and not
965 importing stuff which you think it should, this optimisation can be
966 turned off with <Option>-fno-prune-tydecls</Option> and <Option>-fno-prune-instdecls</Option>.
967 <IndexTerm><Primary>-fno-prune-tydecls option</Primary></IndexTerm><IndexTerm><Primary>-fno-prune-instdecls
968 option</Primary></IndexTerm>
972 See also <XRef LinkEnd="options-linker">, which describes how the linker finds standard
979 <Title>The recompilation checker
982 <IndexTerm><Primary>recompilation checker</Primary></IndexTerm>
987 <Term><Option>-recomp</Option></Term>
988 <IndexTerm><Primary><option>-recomp</option> option</Primary></IndexTerm>
991 (On by default) Turn on recompilation checking. This will stop
992 compilation early, leaving an existing <filename>.o</filename> file in
993 place, if it can be determined that the module does not need to be
999 <Term><Option>-no-recomp</Option></Term>
1000 <IndexTerm><Primary><option>-recomp</option> option</Primary></IndexTerm>
1003 Turn off recompilation checking.
1011 In the olden days, GHC compared the newly-generated
1012 <Filename>.hi</Filename> file with the previous version; if they were
1013 identical, it left the old one alone and didn't change its
1014 modification date. In consequence, importers of a module with an
1015 unchanged output <Filename>.hi</Filename> file were not recompiled.
1019 This doesn't work any more. In our earlier example, module
1020 <Literal>C</Literal> does not import module <Literal>A</Literal>
1021 directly, yet changes to <Filename>A.hi</Filename> should force a
1022 recompilation of <Literal>C</Literal>. And some changes to
1023 <Literal>A</Literal> (changing the definition of a function that
1024 appears in an inlining of a function exported by <Literal>B</Literal>,
1025 say) may conceivably not change <Filename>B.hi</Filename> one jot. So
1030 GHC keeps a version number on each interface file, and on each type
1031 signature within the interface file. It also keeps in every interface
1032 file a list of the version numbers of everything it used when it last
1033 compiled the file. If the source file's modification date is earlier
1034 than the <Filename>.o</Filename> file's date (i.e. the source hasn't
1035 changed since the file was last compiled), and the
1036 <option>-recomp</option> is given on the command line, GHC will be
1037 clever. It compares the version numbers on the things it needs this
1038 time with the version numbers on the things it needed last time
1039 (gleaned from the interface file of the module being compiled); if
1040 they are all the same it stops compiling rather early in the process
1041 saying “Compilation IS NOT required”. What a beautiful
1046 Patrick Sansom had a workshop paper about how all this is done (though
1047 the details have changed quite a bit). <ULink URL="mailto:sansom@dcs.gla.ac.uk">Ask him</ULink> if you want a copy.
1053 <Sect2 id="using-make">
1054 <Title>Using <Command>make</Command>
1058 <IndexTerm><Primary><literal>make</literal></Primary></IndexTerm>
1062 It is reasonably straightforward to set up a <Filename>Makefile</Filename> to use with GHC, assuming you name your source files the same as your modules.
1070 HC_OPTS = -cpp $(EXTRA_HC_OPTS)
1072 SRCS = Main.lhs Foo.lhs Bar.lhs
1073 OBJS = Main.o Foo.o Bar.o
1075 .SUFFIXES : .o .hs .hi .lhs .hc .s
1079 $(HC) -o $@ $(HC_OPTS) $(OBJS)
1081 # Standard suffix rules
1086 $(HC) -c $< $(HC_OPTS)
1089 $(HC) -c $< $(HC_OPTS)
1091 # Inter-module dependencies
1092 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
1093 Main.o Main.hc Main.s : Foo.hi Baz.hi # Main imports Foo and Baz
1099 (Sophisticated <Command>make</Command> variants may achieve some of the above more
1100 elegantly. Notably, <Command>gmake</Command>'s pattern rules let you write the more
1108 $(HC) -c $< $(HC_OPTS)
1114 What we've shown should work with any <Command>make</Command>.)
1118 Note the cheesy <Literal>.o.hi</Literal> rule: It records the dependency of the
1119 interface (<Filename>.hi</Filename>) file on the source. The rule says a <Filename>.hi</Filename> file can
1120 be made from a <Filename>.o</Filename> file by doing…nothing. Which is true.
1124 Note the inter-module dependencies at the end of the Makefile, which
1131 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
1137 They tell <Command>make</Command> that if any of <Literal>Foo.o</Literal>, <Literal>Foo.hc</Literal> or <Literal>Foo.s</Literal> have an
1138 earlier modification date than <Literal>Baz.hi</Literal>, then the out-of-date file
1139 must be brought up to date. To bring it up to date, <Literal>make</Literal> looks for
1140 a rule to do so; one of the preceding suffix rules does the job
1146 <sect2 id="sec-makefile-dependencies">
1147 <title>Dependency generation</title>
1148 <indexterm><primary>dependencies in Makefiles</primary></indexterm>
1149 <indexterm><primary>Makefile dependencies</primary></indexterm>
1151 <para>Putting inter-dependencies of the form <Literal>Foo.o :
1152 Bar.hi</Literal> into your <Filename>Makefile</Filename> by hand
1153 is rather error-prone. Don't worry, GHC has support for
1154 automatically generating the required dependencies. Add the
1155 following to your <Filename>Makefile</Filename>:</para>
1159 ghc -M $(HC_OPTS) $(SRCS)
1162 <para>Now, before you start compiling, and any time you change
1163 the <Literal>imports</Literal> in your program, do <Command>make
1164 depend</Command> before you do <Command>make
1165 cool_pgm</Command>. <Command>ghc -M</Command> will append
1166 the needed dependencies to your
1167 <Filename>Makefile</Filename>.</Para>
1169 <para>In general, if module <Literal>A</Literal> contains the
1176 then <command>ghc -M</command> will generate a dependency
1183 If module <literal>A</literal> contains the line
1186 import {-# SOURCE #-} B ...blah...
1189 then <command>ghc -M</command> will generate a dependency
1196 (See <xref linkend="hi-files"> for details of interface files.)
1197 If <literal>A</literal> imports multiple modules, then there
1198 will be multiple lines with <filename>A.o</filename> as the
1201 <para>By default, <Command>ghc -M</Command> generates all the
1202 dependencies, and then concatenates them onto the end of
1203 <Filename>makefile</Filename> (or <Filename>Makefile</Filename>
1204 if <Filename>makefile</Filename> doesn't exist) bracketed by the
1205 lines "<Literal># DO NOT DELETE: Beginning of Haskell
1206 dependencies</Literal>" and "<Literal># DO NOT DELETE: End
1207 of Haskell dependencies</Literal>". If these lines already
1208 exist in the <Filename>makefile</Filename>, then the old
1209 dependencies are deleted first.</para>
1211 <para>Internally, GHC uses a script to generate the
1212 dependencies, called <command>mkdependHS</command>. This script
1213 has some options of its own, which you might find useful.
1214 Options can be passed directly to <command>mkdependHS</command>
1215 with GHC's <literal>-optdep</literal> option. For example, to
1216 generate the dependencies into a file called
1217 <literal>.depend</literal> instead of
1218 <literal>Makefile</literal>:</para>
1221 ghc -M -optdep-f optdep.depend ...
1224 <para>The full list of options accepted by
1225 <command>mkdependHS</command> is:</para>
1230 <term><option>-w</option></term>
1232 <para>Turn off warnings about interface file shadowing.</para>
1237 <term><option>-f blah</option></term>
1239 <para>Use <Filename>blah</Filename> as the makefile,
1240 rather than <Filename>makefile</Filename> or
1241 <Filename>Makefile</Filename>. If
1242 <Filename>blah</Filename> doesn't exist,
1243 <Command>mkdependHS</Command> creates it. We often use
1244 <Option>-f .depend</Option> to put the dependencies in
1245 <Filename>.depend</Filename> and then
1246 <Command>include</Command> the file
1247 <Filename>.depend</Filename> into
1248 <Filename>Makefile</Filename>.</para>
1253 <term><option>-o <osuf></option></term>
1255 <para>Use <Filename>.<osuf></Filename> as the
1256 "target file" suffix ( default: <Literal>o</Literal>).
1257 Multiple <Option>-o</Option> flags are permitted (GHC2.05
1258 onwards). Thus "<Option>-o hc -o o</Option>" will
1259 generate dependencies for <Filename>.hc</Filename> and
1260 <Filename>.o</Filename> files.</para>
1265 <term><option>-s <suf></option></term>
1267 <para>Make extra dependencies that declare that files with
1269 <Filename>.<suf>_<osuf></Filename>
1270 depend on interface files with suffix
1271 <Filename>.<suf>_hi</Filename>, or (for
1272 <Literal>{-# SOURCE #-}</Literal>
1273 imports) on <Filename>.hi-boot</Filename>. Multiple
1274 <Option>-s</Option> flags are permitted. For example,
1275 <Option>-o hc -s a -s b</Option> will make dependencies
1276 for <Filename>.hc</Filename> on <Filename>.hi</Filename>,
1277 <Filename>.a_hc</Filename> on
1278 <Filename>.a_hi</Filename>, and
1279 <Filename>.b_hc</Filename> on
1280 <Filename>.b_hi</Filename>. (Useful in conjunction
1281 with NoFib "ways".)</para>
1286 <term><option>--exclude-module=<file></option></term>
1288 <para>Regard <Filename><file></Filename> as
1289 "stable"; i.e., exclude it from having dependencies on
1295 <term><option>-x</option></term>
1297 <para>same as <option>--exclude-module</option></para>
1302 <term><option>--exclude-directory=<dirs></option></term>
1304 <para>Regard the colon-separated list of directories
1305 <Filename><dirs></Filename> as containing stable,
1306 don't generate any dependencies on modules therein.</para>
1311 <term><option>-xdirs</option></term>
1313 <para>same as <Option>--exclude-directory</Option>.</para>
1318 <term><option>--include-module=<file></option></term>
1320 <para>Regard <Filename><file></Filename> as not
1321 "stable"; i.e., generate dependencies on it (if any). This
1322 option is normally used in conjunction with the
1323 <Option>--exclude-directory</Option> option.</para>
1328 <term><option>--include-prelude</option></term>
1330 <para>Regard prelude libraries as unstable, i.e., generate
1331 dependencies on the prelude modules used (including
1332 <Literal>Prelude</Literal>). This option is normally only
1333 used by the various system libraries. If a
1334 <Option>-package</Option> option is used, dependencies will
1335 also be generated on the library's interfaces.</para>
1342 <Sect2 id="mutual-recursion">
1343 <Title>How to compile mutually recursive modules
1347 <IndexTerm><Primary>module system, recursion</Primary></IndexTerm>
1348 <IndexTerm><Primary>recursion, between modules</Primary></IndexTerm>
1352 Currently, the compiler does not have proper support for dealing with
1353 mutually recursive modules:
1363 newtype TA = MkTA Int
1381 When compiling either module A and B, the compiler will try (in vain)
1382 to look for the interface file of the other. So, to get mutually
1383 recursive modules off the ground, you need to hand write an interface
1384 file for A or B, so as to break the loop. These hand-written
1385 interface files are called <Literal>hi-boot</Literal> files, and are placed in a file
1386 called <Filename><module>.hi-boot</Filename>. To import from an <Literal>hi-boot</Literal> file instead
1387 of the standard <Filename>.hi</Filename> file, use the following syntax in the importing module:
1388 <IndexTerm><Primary>hi-boot files</Primary></IndexTerm>
1389 <IndexTerm><Primary>importing, hi-boot files</Primary></IndexTerm>
1395 import {-# SOURCE #-} A
1401 The hand-written interface need only contain the bare minimum of
1402 information needed to get the bootstrapping process started. For
1403 example, it doesn't need to contain declarations for <Emphasis>everything</Emphasis>
1404 that module <Literal>A</Literal> exports, only the things required by the module that
1405 imports <Literal>A</Literal> recursively.
1409 For the example at hand, the boot interface file for A would look like
1416 __interface A 1 404 where
1417 __export A TA{MkTA} ;
1418 1 newtype TA = MkTA PrelBase.Int ;
1424 The syntax is essentially the same as a normal <Filename>.hi</Filename> file
1425 (unfortunately), but you can usually tailor an existing <Filename>.hi</Filename> file to
1426 make a <Filename>.hi-boot</Filename> file.
1430 Notice that we only put the declaration for the newtype <Literal>TA</Literal> in the
1431 <Literal>hi-boot</Literal> file, not the signature for <Function>f</Function>, since <Function>f</Function> isn't used by
1432 <Literal>B</Literal>.
1436 The number “1” after “__interface A” gives the version number of module A;
1437 it is incremented whenever anything in A's interface file changes. The “404” is
1438 the version number of the interface file <Emphasis>syntax</Emphasis>; we change it when
1439 we change the syntax of interface files so that you get a better error message when
1440 you try to read an old-format file with a new-format compiler.
1444 The number “1” at the beginning of a declaration is the <Emphasis>version
1445 number</Emphasis> of that declaration: for the purposes of <Filename>.hi-boot</Filename> files
1446 these can all be set to 1. All names must be fully qualified with the
1447 <Emphasis>original</Emphasis> module that an object comes from: for example, the
1448 reference to <Literal>Int</Literal> in the interface for <Literal>A</Literal> comes from <Literal>PrelBase</Literal>,
1449 which is a module internal to GHC's prelude. It's a pain, but that's
1454 If you want an hi-boot file to export a data type, but you don't want to give its constructors
1455 (because the constructors aren't used by the SOURCE-importing module), you can write simply:
1461 __interface A 1 404 where
1469 (You must write all the type parameters, but leave out the '=' and everything that follows it.)
1473 <Emphasis>Note:</Emphasis> This is all a temporary solution, a version of the
1474 compiler that handles mutually recursive modules properly without the manual
1475 construction of interface files, is (allegedly) in the works.
1482 <sect1 id="packages">
1483 <title>Packages</title>
1484 <indexterm><primary>packages</primary></indexterm>
1486 <para>Packages are collections of libraries, conveniently grouped
1487 together as a single entity. The package system is flexible: a
1488 package may consist of Haskell code, foreign language code (eg. C
1489 libraries), or a mixture of the two. A package is a good way to
1490 group together related Haskell modules, and is essential if you
1491 intend to make the modules into a Windows DLL (see below).</para>
1493 <para>Because packages can contain both Haskell and C libraries, they
1494 are also a good way to provide convenient access to a Haskell
1495 layer over a C library.</para>
1497 <para>GHC comes with several packages (see <xref
1498 linkend="book-hslibs">), and packages can be added/removed from an
1499 existing GHC installation.</para>
1501 <sect2 id="listing-packages">
1502 <title>Listing the available packages</title>
1503 <indexterm><primary>packages</primary>
1504 <secondary>listing</secondary></indexterm>
1506 <para>To see what packages are currently installed, use the
1507 <literal>--list-packages</literal> option:</para>
1508 <indexterm><primary><literal>--list-packages</literal></primary>
1512 $ ghc --list-packages
1513 gmp, rts, std, lang, concurrent, data, net, posix, text, util
1516 <para>Note that your GHC installation might have a slightly
1517 different set of packages installed.</para>
1519 <para>The <literal>gmp</literal> and <literal>rts</literal>
1520 packages are always present, and represent the multi-precision
1521 integer and runtime system libraries respectively. The
1522 <literal>std</literal> package contains the Haskell prelude.
1523 The rest of the packages are optional libraries.</para>
1527 <sect2 id="using-packages">
1528 <title>Using a package</title>
1529 <indexterm><primary>packages</primary>
1530 <secondary>using</secondary></indexterm>
1532 <para>To use a package, add the <literal>-package</literal> flag
1533 to the command line:</para>
1537 <term><option>-package <lib></option></term>
1538 <indexterm><primary>-package <lib> option</primary></indexterm>
1540 <para>This option brings into scope all the modules from
1541 package <literal><lib></literal> (they still have to
1542 be imported in your Haskell source, however). It also
1543 causes the relevant libraries to be linked when linking is
1549 <para>Some packages depend on other packages, for example the
1550 <literal>text</literal> package makes use of some of the modules
1551 in the <literal>lang</literal> package. The package system
1552 takes care of all these dependencies, so that when you say
1553 <literal>-package text</literal> on the command line, you
1554 automatically get <literal>-package lang</literal> too.</para>
1557 <sect2 id="building-packages">
1558 <title>Building a package from Haskell source</title>
1559 <indexterm><primary>packages</primary>
1560 <secondary>building</secondary></indexterm>
1562 <para>It takes some special considerations to build a new
1567 <para>A package may contain several Haskell modules. A
1568 package may span many directories, or many packages may
1569 exist in a single directory. Packages may not be mutually
1574 <para>A package has a name
1575 (e.g. <filename>std</filename>)</para>
1579 <para>The Haskell code in a package may be built into one or
1580 more Unix libraries (e.g. <Filename>libHSfoo.a</Filename>),
1581 or a single DLL on Windows
1582 (e.g. <Filename>HSfoo.dll</Filename>). The restriction to a
1583 single DLL on Windows is that the package system is used to
1584 tell the compiler when it should make an inter-DLL call
1585 rather than an intra-DLL call (inter-DLL calls require an
1586 extra indirection).</para>
1590 <para>GHC does not maintain detailed cross-package
1591 dependency information. It does remember which modules in
1592 other packages the current module depends on, but not which
1593 things within those imported things.</para>
1597 <para>To compile a module which is to be part of a new package,
1598 use the <literal>-package-name</literal> option:</para>
1602 <term><option>-package-name <foo></option></term>
1603 <indexterm><primary><literal>-package-name</literal></primary>
1604 <secondary>option</secondary></indexterm>
1606 <para>This option is added to the command line when
1607 compiling a module that is destined to be part of package
1608 <literal>foo</literal>. If this flag is omitted then the
1609 default package <literal>Main</literal> is assumed.</para>
1614 <para>Failure to use the <literal>-package-name</literal> option
1615 when compiling a package will result in disaster on Windows, but
1616 is relatively harmless on Unix at the moment (it will just cause
1617 a few extra dependencies in some interface files). However,
1618 bear in mind that we might add support for Unix shared libraries
1619 at some point in the future.</para>
1621 <para>It is worth noting that on Windows, because each package
1622 is built as a DLL, and a reference to a DLL costs an extra
1623 indirection, intra-package references are cheaper than
1624 inter-package references. Of course, this applies to the
1625 <Filename>Main</Filename> package as well.</para>
1628 <sect2 id="package-management">
1629 <title>Package management</title>
1630 <indexterm><primary>packages</primary>
1631 <secondary>management</secondary></indexterm>
1633 <para>GHC uses a package configuration file, called
1634 <literal>packages.conf</literal>, which can be found in your GHC
1635 install directory. This file isn't intended to be edited
1636 directly, instead GHC provides options for adding & removing
1641 <term><option>--add-package</option></term>
1642 <indexterm><primary><literal>--add-package</literal></primary>
1643 <secondary>option</secondary></indexterm>
1645 <para>Reads a package specification (see below) on stdin,
1646 and adds it to the database of installed packages. The
1647 package specification must be a package that isn't already
1653 <term><option>--delete-package <foo></option></term>
1654 <indexterm><primary><literal>--delete-package</literal></primary>
1655 <secondary>option</secondary></indexterm>
1657 <para>Removes the specified package from the installed
1658 configuration.</para>
1663 <para>In both cases, the old package configuration file is saved
1664 in <literal>packages.conf.old</literal> in your GHC install
1665 directory, so in an emergency you can always copy this file into
1666 <literal>package.conf</literal> to restore the old
1669 <para>A package specification looks like this:</para>
1676 import_dirs = ["/usr/local/lib/imports/mypkg"],
1677 library_dirs = ["/usr/local/lib"],
1678 hs_libraries = ["HSmypkg" ],
1679 extra_libraries = ["HSmypkg_cbits"],
1681 c_includes = ["HsMyPkg.h"],
1682 package_deps = ["text", "data"],
1683 extra_ghc_opts = [],
1685 extra_ld_opts = ["-lmy_clib"]
1690 <para>The first line is the name of the package, for use with
1691 the <literal>-package</literal> flag and as listed in the
1692 <literal>--list-packages</literal> list. The second line is the
1693 version of GHC that was used to compile any Haskell code in this
1694 package (GHC will refuse to add the package if its version
1695 number differs from this one). The rest of the components of
1696 the package specification may be specified in any order, and
1701 <term><literal>import_dirs</literal></term>
1702 <indexterm><primary><literal>import_dirs</literal></primary>
1703 <secondary>package specification</secondary></indexterm>
1705 <para>A list of directories containing interface files
1706 (<literal>.hi</literal> files) for this package.</para>
1711 <term><literal>library_dirs</literal></term>
1712 <indexterm><primary><literal>library_dirs</literal></primary>
1713 <secondary>package specification</secondary></indexterm>
1715 <para>A list of directories containing libraries for this
1721 <term><literal>hs_libraries</literal></term>
1722 <indexterm><primary><literal>hs_libraries</literal></primary>
1723 <secondary>package specification</secondary></indexterm>
1725 <para>A list of libraries containing Haskell code for this
1726 package, with the <literal>.a</literal> or
1727 <literal>.dll</literal> suffix omitted. On Unix, the
1728 <literal>lib</literal> prefix is also omitted.</para>
1733 <term><literal>extra_libraries</literal></term>
1734 <indexterm><primary><literal>extra_libraries</literal></primary>
1735 <secondary>package specification</secondary></indexterm>
1737 <para>A list of extra libraries for this package. The
1738 difference between <literal>hs_libraries</literal> and
1739 <literal>extra_libraries</literal> is that
1740 <literal>hs_libraries</literal> normally have several
1741 versions, to support profiling, parallel and other build
1742 options. The various versions are given different
1743 suffixes to distinguish them, for example the profiling
1744 version of the standard prelude library is named
1745 <filename>libHSstd_p.a</filename>, with the
1746 <literal>_p</literal> indicating that this is a profiling
1747 version. The suffix is added automatically by GHC for
1748 <literal>hs_libraries</literal> only, no suffix is added
1750 <literal>extra_libraries</literal>.</para>
1752 <para>Also, <literal>extra_libraries</literal> are placed
1753 on the linker command line before the
1754 <literal>hs_libraries</literal> for the same package. If
1755 your package has dependencies in the other direction, you
1756 might need to make two separate packages.</para>
1761 <term><literal>include_dirs</literal></term>
1762 <indexterm><primary><literal>include_dirs</literal></primary>
1763 <secondary>package specification</secondary></indexterm>
1765 <para>A list of directories containing C includes for this
1766 package (maybe the empty list).</para>
1771 <term><literal>c_includes</literal></term>
1772 <indexterm><primary><literal>c_includes</literal></primary>
1773 <secondary>package specification</secondary></indexterm>
1775 <para>A list of files to include for via-C compilations
1776 using this package. Typically this include file will
1777 contain function prototypes for any C functions used in
1778 the package, in case they end up being called as a result
1779 of Haskell functions from the package being
1785 <term><literal>package_deps</literal></term>
1786 <indexterm><primary><literal>package_deps</literal></primary>
1787 <secondary>package specification</secondary></indexterm>
1789 <para>A list of packages which this package depends
1795 <term><literal>extra_ghc_opts</literal></term>
1796 <indexterm><primary><literal>extra_ghc_opts</literal></primary>
1797 <secondary>package specification</secondary></indexterm>
1799 <para>Extra arguments to be added to the GHC command line
1800 when this package is being used.</para>
1805 <term><literal>extra_cc_opts</literal></term>
1806 <indexterm><primary><literal>extra_cc_opts</literal></primary>
1807 <secondary>package specification</secondary></indexterm>
1809 <para>Extra arguments to be added to the gcc command line
1810 when this package is being used (only for via-C
1811 compilations).</para>
1816 <term><literal>extra_ld_opts</literal></term>
1817 <indexterm><primary><literal>extra_ld_opts</literal></primary>
1818 <secondary>package specification</secondary></indexterm>
1820 <para>Extra arguments to be added to the gcc command line
1821 (for linking) when this package is being used.</para>
1826 <para>For examples of more package specifications, take a look
1827 at the <literal>package.conf</literal> in your GHC
1828 installation.</para>
1833 <Sect1 id="options-optimise">
1834 <Title>Optimisation (code improvement)
1838 <IndexTerm><Primary>optimisation (GHC)</Primary></IndexTerm>
1839 <IndexTerm><Primary>improvement, code (GHC)</Primary></IndexTerm>
1843 The <Option>-O*</Option> options specify convenient “packages” of optimisation
1844 flags; the <Option>-f*</Option> options described later on specify
1845 <Emphasis>individual</Emphasis> optimisations to be turned on/off; the <Option>-m*</Option>
1846 options specify <Emphasis>machine-specific</Emphasis> optimisations to be turned
1850 <Sect2 id="optimise-pkgs">
1851 <Title><Option>-O*</Option>: convenient “packages” of optimisation flags.
1855 <IndexTerm><Primary>-O options</Primary></IndexTerm>
1859 There are <Emphasis>many</Emphasis> options that affect the quality of code
1860 produced by GHC. Most people only have a general goal, something like
1861 “Compile quickly” or “Make my program run like greased lightning.”
1862 The following “packages” of optimisations (or lack thereof) should
1867 Once you choose a <Option>-O*</Option> “package,” stick with it—don't chop and
1868 change. Modules' interfaces <Emphasis>will</Emphasis> change with a shift to a new
1869 <Option>-O*</Option> option, and you may have to recompile a large chunk of all
1870 importing modules before your program can again be run
1871 safely (see <XRef LinkEnd="recomp">).
1878 <Term>No <Option>-O*</Option>-type option specified:</Term>
1879 <IndexTerm><Primary>-O* not specified</Primary></IndexTerm>
1882 This is taken to mean: “Please compile quickly; I'm not over-bothered
1883 about compiled-code quality.” So, for example: <Command>ghc -c Foo.hs</Command>
1888 <Term><Option>-O</Option> or <Option>-O1</Option>:</Term>
1889 <IndexTerm><Primary>-O option</Primary></IndexTerm>
1890 <IndexTerm><Primary>-O1 option</Primary></IndexTerm>
1891 <IndexTerm><Primary>optimise</Primary><secondary>normally</secondary></IndexTerm>
1894 Means: “Generate good-quality code without taking too long about
1895 it.” Thus, for example: <Command>ghc -c -O Main.lhs</Command>
1900 <Term><Option>-O2</Option>:</Term>
1901 <IndexTerm><Primary>-O2 option</Primary></IndexTerm>
1902 <IndexTerm><Primary>optimise</Primary><secondary>aggressively</secondary></IndexTerm>
1905 Means: “Apply every non-dangerous optimisation, even if it means
1906 significantly longer compile times.”
1910 The avoided “dangerous” optimisations are those that can make
1911 runtime or space <Emphasis>worse</Emphasis> if you're unlucky. They are
1912 normally turned on or off individually.
1916 At the moment, <Option>-O2</Option> is <Emphasis>unlikely</Emphasis> to produce
1917 better code than <Option>-O</Option>.
1922 <Term><Option>-O2-for-C</Option>:</Term>
1923 <IndexTerm><Primary>-O2-for-C option</Primary></IndexTerm>
1924 <IndexTerm><Primary>gcc, invoking with -O2</Primary></IndexTerm>
1927 Says to run GCC with <Option>-O2</Option>, which may be worth a few percent in
1928 execution speed. Don't forget <Option>-fvia-C</Option>, lest you use the native-code
1929 generator and bypass GCC altogether!
1934 <Term><Option>-Onot</Option>:</Term>
1935 <IndexTerm><Primary>-Onot option</Primary></IndexTerm>
1936 <IndexTerm><Primary>optimising, reset</Primary></IndexTerm>
1939 This option will make GHC “forget” any
1940 <Option>-O</Option>ish options it has seen so far. Sometimes useful;
1941 for example: <Command>make all
1942 EXTRA_HC_OPTS=-Onot</Command>.
1947 <Term><Option>-Ofile <file></Option>:</Term>
1948 <IndexTerm><Primary>-Ofile <file> option</Primary></IndexTerm>
1949 <IndexTerm><Primary>optimising, customised</Primary></IndexTerm>
1952 For those who need <Emphasis>absolute</Emphasis> control over
1953 <Emphasis>exactly</Emphasis> what options are used (e.g., compiler
1954 writers, sometimes :-), a list of options can be put in a file and
1955 then slurped in with <Option>-Ofile</Option>.
1959 In that file, comments are of the
1960 <Literal>#</Literal>-to-end-of-line variety; blank lines and most
1961 whitespace is ignored.
1965 Please ask if you are baffled and would like an example of <Option>-Ofile</Option>!
1973 At Glasgow, we don't use a <Option>-O*</Option> flag for day-to-day work. We use
1974 <Option>-O</Option> to get respectable speed; e.g., when we want to measure
1975 something. When we want to go for broke, we tend to use <Option>-O -fvia-C
1976 -O2-for-C</Option> (and we go for lots of coffee breaks).
1980 The easiest way to see what <Option>-O</Option> (etc.) “really mean” is to run with
1981 <Option>-v</Option>, then stand back in amazement. Alternatively, just look at the
1982 <Literal>HsC_minus<blah></Literal> lists in the GHC driver script.
1988 <Title><Option>-f*</Option>: platform-independent flags</Title>
1991 <IndexTerm><Primary>-f* options (GHC)</Primary></IndexTerm>
1992 <IndexTerm><Primary>-fno-* options (GHC)</Primary></IndexTerm>
1996 Flags can be turned <Emphasis>off</Emphasis> individually. (NB: I hope you have a
1997 good reason for doing this…) To turn off the <Option>-ffoo</Option> flag, just use
1998 the <Option>-fno-foo</Option> flag.<IndexTerm><Primary>-fno-<opt> anti-option</Primary></IndexTerm> So, for
1999 example, you can say <Option>-O2 -fno-strictness</Option>, which will then drop out
2000 any running of the strictness analyser.
2004 The options you are most likely to want to turn off are:
2010 <Option>-fno-strictness</Option><IndexTerm><Primary>-fno-strictness option</Primary></IndexTerm> (strictness
2011 analyser, because it is sometimes slow),
2017 <Option>-fno-specialise</Option><IndexTerm><Primary>-fno-specialise option</Primary></IndexTerm> (automatic
2018 specialisation of overloaded functions, because it can make your code
2019 bigger) (US spelling also accepted), and
2025 <Option>-fno-cpr-analyse</Option><IndexTerm><Primary>-fno-cpr-analyse option</Primary></IndexTerm> switches off the CPR (constructed product
2035 Should you wish to turn individual flags <Emphasis>on</Emphasis>, you are advised
2036 to use the <Option>-Ofile</Option> option, described above. Because the order in
2037 which optimisation passes are run is sometimes crucial, it's quite
2038 hard to do with command-line options.
2042 Here are some “dangerous” optimisations you <Emphasis>might</Emphasis> want to try:
2046 <Term><Option>-fvia-C</Option>:</Term>
2049 <IndexTerm><Primary>-fvia-C option</Primary></IndexTerm>
2050 <IndexTerm><Primary>native code generator, turning off</Primary></IndexTerm>
2054 Compile via C, and don't use the native-code generator. (There are many
2055 cases when GHC does this on its own.) You might pick up a little bit of
2056 speed by compiling via C (e.g. for floating-point intensive code on Intel).
2057 If you use <Function>_casm_</Function>s (which are utterly
2058 deprecated), you probably <Emphasis>have</Emphasis> to use
2059 <Option>-fvia-C</Option>.
2063 The lower-case incantation, <Option>-fvia-c</Option>, is synonymous.
2067 Compiling via C will probably be slower (in compilation time) than
2068 using GHC's native code generator.
2073 <Term><Option>-funfolding-interface-threshold<n></Option>:</Term>
2076 <IndexTerm><Primary>-funfolding-interface-threshold option</Primary></IndexTerm>
2077 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
2078 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
2079 (Default: 30) By raising or lowering this number, you can raise or
2080 lower the amount of pragmatic junk that gets spewed into interface
2081 files. (An unfolding has a “size” that reflects the cost in terms
2082 of “code bloat” of expanding that unfolding in another module. A
2083 bigger function would be assigned a bigger cost.)
2088 <Term><Option>-funfolding-creation-threshold<n></Option>:</Term>
2091 <IndexTerm><Primary>-funfolding-creation-threshold option</Primary></IndexTerm>
2092 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
2093 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
2094 (Default: 30) This option is similar to
2095 <Option>-funfolding-interface-threshold</Option>, except that it governs unfoldings
2096 within a single module. Increasing this figure is more likely to
2097 result in longer compile times than faster code. The next option is
2103 <Term><Option>-funfolding-use-threshold<n></Option>:</Term>
2106 <IndexTerm><Primary>-funfolding-use-threshold option</Primary></IndexTerm>
2107 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
2108 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
2109 (Default: 8) This is the magic cut-off figure for unfolding: below
2110 this size, a function definition will be unfolded at the call-site,
2111 any bigger and it won't. The size computed for a function depends on
2112 two things: the actual size of the expression minus any discounts that
2113 apply (see <Option>-funfolding-con-discount</Option>).
2119 <Term><Option>-funfolding-update-in-place<n></Option>:</Term>
2121 Switches on an experimental "optimisation". Switching it on makes the compiler
2122 a little keener to inline a function that returns a constructor, if the context is
2127 If we inlined plusInt we might get an opportunity to use update-in-place for
2134 <Term><Option>-funbox-strict-fields</Option>:</Term>
2137 <IndexTerm><Primary>-funbox-strict-fields option</Primary></IndexTerm>
2138 <IndexTerm><Primary>strict constructor fields</Primary></IndexTerm>
2139 <IndexTerm><Primary>constructor fields, strict</Primary></IndexTerm>
2143 This option causes all constructor fields which are marked strict
2144 (i.e. “!”) to be unboxed or unpacked if possible. For example:
2150 data T = T !Float !Float
2156 will create a constructor <Literal>T</Literal> containing two unboxed floats if the
2157 <Option>-funbox-strict-fields</Option> flag is given. This may not always be an
2158 optimisation: if the <Function>T</Function> constructor is scrutinised and the floats
2159 passed to a non-strict function for example, they will have to be
2160 reboxed (this is done automatically by the compiler).
2164 This option should only be used in conjunction with <Option>-O</Option>, in order to
2165 expose unfoldings to the compiler so the reboxing can be removed as
2166 often as possible. For example:
2173 f (T f1 f2) = f1 + f2
2179 The compiler will avoid reboxing <Function>f1</Function> and <Function>f2</Function> by inlining <Function>+</Function> on
2180 floats, but only when <Option>-O</Option> is on.
2184 Any single-constructor data is eligible for unpacking; for example
2190 data T = T !(Int,Int)
2196 will store the two <Literal>Int</Literal>s directly in the <Function>T</Function> constructor, by flattening
2197 the pair. Multi-level unpacking is also supported:
2204 data S = S !Int !Int
2210 will store two unboxed <Literal>Int#</Literal>s directly in the <Function>T</Function> constructor.
2215 <Term><Option>-fsemi-tagging</Option>:</Term>
2218 This option (which <Emphasis>does not work</Emphasis> with the native-code generator)
2219 tells the compiler to add extra code to test for already-evaluated
2220 values. You win if you have lots of such values during a run of your
2221 program, you lose otherwise. (And you pay in extra code space.)
2225 We have not played with <Option>-fsemi-tagging</Option> enough to recommend it.
2226 (For all we know, it doesn't even work anymore… Sigh.)
2231 <Term><Option>-fexcess-precision</Option>:</Term>
2234 When this option is given, intermediate floating point values can have
2235 a <Emphasis>greater</Emphasis> precision/range than the final type.
2236 Generally this is a good thing, but some programs may rely on the
2237 exact precision/range of <Literal>Float</Literal>/<Literal>Double</Literal>
2238 values and should not use this option for their compilation.
2248 <Title><Option>-m*</Option>: platform-specific flags</Title>
2251 <IndexTerm><Primary>-m* options (GHC)</Primary></IndexTerm>
2252 <IndexTerm><Primary>platform-specific options</Primary></IndexTerm>
2253 <IndexTerm><Primary>machine-specific options</Primary></IndexTerm>
2257 Some flags only make sense for particular target platforms.
2264 <Term><Option>-mv8</Option>:</Term>
2267 (SPARC machines)<IndexTerm><Primary>-mv8 option (SPARC only)</Primary></IndexTerm>
2268 Means to pass the like-named option to GCC; it says to use the
2269 Version 8 SPARC instructions, notably integer multiply and divide.
2270 The similiar <Option>-m*</Option> GCC options for SPARC also work, actually.
2275 <Term><Option>-mlong-calls</Option>:</Term>
2278 (HPPA machines)<IndexTerm><Primary>-mlong-calls option (HPPA only)</Primary></IndexTerm>
2279 Means to pass the like-named option to GCC. Required for Very Big
2280 modules, maybe. (Probably means you're in trouble…)
2285 <Term><Option>-monly-[32]-regs</Option>:</Term>
2288 (iX86 machines)<IndexTerm><Primary>-monly-N-regs option (iX86 only)</Primary></IndexTerm>
2289 GHC tries to “steal” four registers from GCC, for performance
2290 reasons; it almost always works. However, when GCC is compiling some
2291 modules with four stolen registers, it will crash, probably saying:
2294 Foo.hc:533: fixed or forbidden register was spilled.
2295 This may be due to a compiler bug or to impossible asm
2296 statements or clauses.
2299 Just give some registers back with <Option>-monly-N-regs</Option>. Try `3' first,
2300 then `2'. If `2' doesn't work, please report the bug to us.
2309 <Sect2 id="optimise-C-compiler">
2310 <Title>Code improvement by the C compiler.
2314 <IndexTerm><Primary>optimisation by GCC</Primary></IndexTerm>
2315 <IndexTerm><Primary>GCC optimisation</Primary></IndexTerm>
2319 The C compiler (GCC) is run with <Option>-O</Option> turned on. (It has
2324 If you want to run GCC with <Option>-O2</Option>—which may be worth a few
2325 percent in execution speed—you can give a
2326 <Option>-O2-for-C</Option><IndexTerm><Primary>-O2-for-C option</Primary></IndexTerm> option.
2333 <Sect1 id="options-phases">
2334 <Title>Options related to a particular phase
2337 <Sect2 id="c-pre-processor">
2338 <Title>The C pre-processor
2342 <IndexTerm><Primary>pre-processing: cpp</Primary></IndexTerm>
2343 <IndexTerm><Primary>C pre-processor options</Primary></IndexTerm>
2344 <IndexTerm><Primary>cpp, pre-processing with</Primary></IndexTerm>
2348 The C pre-processor <Command>cpp</Command> is run over your Haskell code only if the
2349 <Option>-cpp</Option> option <IndexTerm><Primary>-cpp option</Primary></IndexTerm> is given. Unless you are
2350 building a large system with significant doses of conditional
2351 compilation, you really shouldn't need it.
2355 <Term><Option>-D<foo></Option>:</Term>
2358 <IndexTerm><Primary>-D<name> option</Primary></IndexTerm>
2359 Define macro <Constant><foo></Constant> in the usual way. NB: does <Emphasis>not</Emphasis> affect
2360 <Option>-D</Option> macros passed to the C compiler when compiling via C! For those,
2361 use the <Option>-optc-Dfoo</Option> hack… (see <XRef LinkEnd="forcing-options-through">).
2366 <Term><Option>-U<foo></Option>:</Term>
2369 <IndexTerm><Primary>-U<name> option</Primary></IndexTerm>
2370 Undefine macro <Command><foo></Command> in the usual way.
2375 <Term><Option>-I<dir></Option>:</Term>
2378 <IndexTerm><Primary>-I<dir> option</Primary></IndexTerm>
2379 Specify a directory in which to look for <Literal>#include</Literal> files, in
2388 The GHC driver pre-defines several macros when processing Haskell
2389 source code (<Filename>.hs</Filename> or <Filename>.lhs</Filename> files):
2396 <Term><Constant>__HASKELL98__</Constant>:</Term>
2399 <IndexTerm><Primary>__HASKELL98__</Primary></IndexTerm>
2400 If defined, this means that GHC supports the language defined by the
2406 <Term><Constant>__HASKELL__=98</Constant>:</Term>
2409 <IndexTerm><Primary>__HASKELL__</Primary></IndexTerm>
2410 In GHC 4.04 and later, the <Constant>__HASKELL__</Constant> macro is defined as having
2411 the value <Constant>98</Constant>.
2416 <Term><Constant>__HASKELL1__</Constant>:</Term>
2419 <IndexTerm><Primary>__HASKELL1__ macro</Primary></IndexTerm>
2420 If defined to <Emphasis>n</Emphasis>, that means GHC supports the Haskell language
2421 defined in the Haskell report version <Emphasis>1.n</Emphasis>. Currently 5. This
2422 macro is deprecated, and will probably disappear in future versions.
2427 <Term><Constant>__GLASGOW_HASKELL__</Constant>:</Term>
2430 <IndexTerm><Primary>__GLASGOW_HASKELL__ macro</Primary></IndexTerm>
2431 For version <Emphasis>n</Emphasis> of the GHC system, this will be <Literal>#define</Literal>d to
2432 <Emphasis>100n</Emphasis>. So, for version 4.00, it is 400.
2436 With any luck, <Constant>__GLASGOW_HASKELL__</Constant> will be undefined in all other
2437 implementations that support C-style pre-processing.
2441 (For reference: the comparable symbols for other systems are:
2442 <Constant>__HUGS__</Constant> for Hugs and <Constant>__HBC__</Constant> for Chalmers.)
2446 NB. This macro is set when pre-processing both Haskell source and C
2447 source, including the C source generated from a Haskell module
2448 (i.e. <Filename>.hs</Filename>, <Filename>.lhs</Filename>, <Filename>.c</Filename> and <Filename>.hc</Filename> files).
2453 <Term><Constant>__CONCURRENT_HASKELL__</Constant>:</Term>
2456 <IndexTerm><Primary>__CONCURRENT_HASKELL__ macro</Primary></IndexTerm>
2457 This symbol is defined when pre-processing Haskell (input) and
2458 pre-processing C (GHC output). Since GHC from verion 4.00 now
2459 supports concurrent haskell by default, this symbol is always defined.
2464 <Term><Constant>__PARALLEL_HASKELL__</Constant>:</Term>
2467 <IndexTerm><Primary>__PARALLEL_HASKELL__ macro</Primary></IndexTerm>
2468 Only defined when <Option>-parallel</Option> is in use! This symbol is defined when
2469 pre-processing Haskell (input) and pre-processing C (GHC output).
2477 Options other than the above can be forced through to the C
2478 pre-processor with the <Option>-opt</Option> flags (see
2479 <XRef LinkEnd="forcing-options-through">).
2483 A small word of warning: <Option>-cpp</Option> is not friendly to “string
2484 gaps”.<IndexTerm><Primary>-cpp vs string gaps</Primary></IndexTerm><IndexTerm><Primary>string gaps vs
2485 -cpp</Primary></IndexTerm>. In other words, strings such as the following:
2499 don't work with <Option>-cpp</Option>; <Filename>/usr/bin/cpp</Filename> elides the
2500 backslash-newline pairs.
2504 However, it appears that if you add a space at the end of the line,
2505 then <Command>cpp</Command> (at least GNU <Command>cpp</Command> and possibly other <Command>cpp</Command>s)
2506 leaves the backslash-space pairs alone and the string gap works as
2512 <Sect2 id="options-C-compiler">
2513 <Title>Options affecting the C compiler (if applicable)
2517 <IndexTerm><Primary>include-file options</Primary></IndexTerm>
2518 <IndexTerm><Primary>C compiler options</Primary></IndexTerm>
2519 <IndexTerm><Primary>GCC options</Primary></IndexTerm>
2523 At the moment, quite a few common C-compiler options are passed on
2524 quietly to the C compilation of Haskell-compiler-generated C files.
2525 THIS MAY CHANGE. Meanwhile, options so sent are:
2532 <ColSpec Align="Left" Colsep="0">
2533 <ColSpec Align="Left" Colsep="0">
2536 <Entry><Option>-ansi</Option> </Entry>
2537 <Entry> do ANSI C (not K&R) </Entry>
2541 <Option>-pedantic</Option> </Entry>
2542 <Entry> be so</Entry>
2546 <Option>-dgcc-lint</Option> </Entry>
2547 <Entry> (hack) short for “make GCC very paranoid”</Entry>
2555 <IndexTerm><Primary>-ansi option (for GCC)</Primary></IndexTerm>
2556 <IndexTerm><Primary>-pedantic option (for GCC)</Primary></IndexTerm>
2557 <IndexTerm><Primary>-dgcc-lint option (GCC paranoia)</Primary></IndexTerm>
2561 If you are compiling with lots of foreign calls, you may need to
2562 tell the C compiler about some <Literal>#include</Literal> files. There is no real
2563 pretty way to do this, but you can use this hack from the
2570 % ghc -c '-#include <X/Xlib.h>' Xstuff.lhs
2577 <Sect2 id="options-linker">
2578 <Title>Linking and consistency-checking
2582 <IndexTerm><Primary>linker options</Primary></IndexTerm>
2583 <IndexTerm><Primary>ld options</Primary></IndexTerm>
2587 GHC has to link your code with various libraries, possibly including:
2588 user-supplied, GHC-supplied, and system-supplied (<Option>-lm</Option> math
2589 library, for example).
2596 <Term><Option>-l<FOO></Option>:</Term>
2599 <IndexTerm><Primary>-l<lib> option</Primary></IndexTerm>
2600 Link in a library named <Filename>lib<FOO>.a</Filename> which resides somewhere on the
2601 library directories path.
2605 Because of the sad state of most UNIX linkers, the order of such
2606 options does matter. Thus: <Command>ghc -lbar *.o</Command> is almost certainly
2607 wrong, because it will search <Filename>libbar.a</Filename> <Emphasis>before</Emphasis> it has
2608 collected unresolved symbols from the <Filename>*.o</Filename> files.
2609 <Command>ghc *.o -lbar</Command> is probably better.
2613 The linker will of course be informed about some GHC-supplied
2614 libraries automatically; these are:
2621 <ColSpec Align="Left" Colsep="0">
2622 <ColSpec Align="Left" Colsep="0">
2625 <Entry><Emphasis>-l equivalent</Emphasis> </Entry>
2626 <Entry> <Emphasis>description</Emphasis> </Entry>
2631 <Option>-lHSrts,-lHSclib</Option> </Entry>
2632 <Entry> basic runtime libraries </Entry>
2636 <Option>-lHS</Option> </Entry>
2637 <Entry> standard Prelude library </Entry>
2641 <Option>-lHS_cbits</Option> </Entry>
2642 <Entry> C support code for standard Prelude library </Entry>
2646 <Option>-lgmp</Option> </Entry>
2647 <Entry> GNU multi-precision library (for Integers)</Entry>
2658 <IndexTerm><Primary>-lHS library</Primary></IndexTerm>
2659 <IndexTerm><Primary>-lHS_cbits library</Primary></IndexTerm>
2660 <IndexTerm><Primary>-lHSrts library</Primary></IndexTerm>
2661 <IndexTerm><Primary>-lgmp library</Primary></IndexTerm>
2666 <Term><Option>-package <name></Option>:</Term>
2669 <IndexTerm><Primary>-package <name> option</Primary></IndexTerm>
2673 If you are using a Haskell “package” (e.g., the POSIX
2674 library), just use the <Option>-package posix</Option> option, and the
2675 correct code should be linked in. See <xref linkend="packages"> for
2681 <Term><Option>-L<dir></Option>:</Term>
2684 <IndexTerm><Primary>-L<dir> option</Primary></IndexTerm>
2685 Where to find user-supplied libraries… Prepend the directory
2686 <Filename><dir></Filename> to the library directories path.
2691 <Term><Option>-static</Option>:</Term>
2694 <IndexTerm><Primary>-static option</Primary></IndexTerm>
2695 Tell the linker to avoid shared libraries.
2700 <Term><Option>-no-link-chk</Option> and <Option>-link-chk</Option>:</Term>
2703 <IndexTerm><Primary>-no-link-chk option</Primary></IndexTerm>
2704 <IndexTerm><Primary>-link-chk option</Primary></IndexTerm>
2705 <IndexTerm><Primary>consistency checking of executables</Primary></IndexTerm>
2706 By default, immediately after linking an executable, GHC verifies that
2707 the pieces that went into it were compiled with compatible flags; a
2708 “consistency check”.
2709 (This is to avoid mysterious failures caused by non-meshing of
2710 incompatibly-compiled programs; e.g., if one <Filename>.o</Filename> file was compiled
2711 for a parallel machine and the others weren't.) You may turn off this
2712 check with <Option>-no-link-chk</Option>. You can turn it (back) on with
2713 <Option>-link-chk</Option> (the default).
2718 <Term><Option>-no-hs-main</Option>:</Term>
2721 <IndexTerm><Primary>-no-hs-main option</Primary></IndexTerm>
2722 <IndexTerm><Primary>linking Haskell libraries with foreign code</Primary></IndexTerm>
2726 In the event you want to include ghc-compiled code as part of another
2727 (non-Haskell) program, the RTS will not be supplying its definition of
2728 <Function>main()</Function> at link-time, you will have to. To signal that to the
2729 driver script when linking, use <Option>-no-hs-main</Option>.
2733 Notice that since the command-line passed to the linker is rather
2734 involved, you probably want to use the ghc driver script to do the
2735 final link of your `mixed-language' application. This is not a
2736 requirement though, just try linking once with <Option>-v</Option> on to see what
2737 options the driver passes through to the linker.
2748 <Sect1 id="sec-using-concurrent">
2749 <Title>Using Concurrent Haskell</Title>
2752 <IndexTerm><Primary>Concurrent Haskell—use</Primary></IndexTerm>
2756 GHC (as of version 4.00) supports Concurrent Haskell by default,
2757 without requiring a special option or libraries compiled in a certain
2758 way. To get access to the support libraries for Concurrent Haskell
2759 (i.e. <Literal>Concurrent</Literal> and friends), use the
2760 <Option>-package concurrent</Option> option.
2764 Three RTS options are provided for modifying the behaviour of the
2765 threaded runtime system. See the descriptions of
2766 <Option>-C[<us>]</Option>, <Option>-q</Option>, and
2767 <Option>-t<num></Option> in <XRef LinkEnd="parallel-rts-opts">.
2771 Concurrent Haskell is described in more detail in <XRef
2772 LinkEnd="sec-Concurrent">.
2777 <Sect1 id="sec-using-parallel">
2778 <Title>Using Parallel Haskell</Title>
2781 <IndexTerm><Primary>Parallel Haskell—use</Primary></IndexTerm>
2785 [You won't be able to execute parallel Haskell programs unless PVM3
2786 (Parallel Virtual Machine, version 3) is installed at your site.]
2790 To compile a Haskell program for parallel execution under PVM, use the
2791 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
2792 option</Primary></IndexTerm> both when compiling <Emphasis>and
2793 linking</Emphasis>. You will probably want to <Literal>import
2794 Parallel</Literal> into your Haskell modules.
2798 To run your parallel program, once PVM is going, just invoke it
2799 “as normal”. The main extra RTS option is
2800 <Option>-N<n></Option>, to say how many PVM
2801 “processors” your program to run on. (For more details of
2802 all relevant RTS options, please see <XRef
2803 LinkEnd="parallel-rts-opts">.)
2807 In truth, running Parallel Haskell programs and getting information
2808 out of them (e.g., parallelism profiles) is a battle with the vagaries of
2809 PVM, detailed in the following sections.
2813 <Title>Dummy's guide to using PVM</Title>
2816 <IndexTerm><Primary>PVM, how to use</Primary></IndexTerm>
2817 <IndexTerm><Primary>Parallel Haskell—PVM use</Primary></IndexTerm>
2818 Before you can run a parallel program under PVM, you must set the
2819 required environment variables (PVM's idea, not ours); something like,
2820 probably in your <Filename>.cshrc</Filename> or equivalent:
2823 setenv PVM_ROOT /wherever/you/put/it
2824 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
2825 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
2831 Creating and/or controlling your “parallel machine” is a purely-PVM
2832 business; nothing specific to Parallel Haskell.
2836 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
2837 machine. You can then do various things to control/monitor your
2838 “parallel machine;” the most useful being:
2844 <ColSpec Align="Left">
2848 <Entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></Entry>
2849 <Entry>exit <Command>pvm</Command>, leaving it running</Entry>
2853 <Entry><Command>halt</Command></Entry>
2854 <Entry>kill off this “parallel machine” & exit</Entry>
2858 <Entry><Command>add <host></Command></Entry>
2859 <Entry>add <Command><host></Command> as a processor</Entry>
2863 <Entry><Command>delete <host></Command></Entry>
2864 <Entry>delete <Command><host></Command></Entry>
2868 <Entry><Command>reset</Command></Entry>
2869 <Entry>kill what's going, but leave PVM up</Entry>
2873 <Entry><Command>conf</Command></Entry>
2874 <Entry>list the current configuration</Entry>
2878 <Entry><Command>ps</Command></Entry>
2879 <Entry>report processes' status</Entry>
2883 <Entry><Command>pstat <pid></Command></Entry>
2884 <Entry>status of a particular process</Entry>
2893 The PVM documentation can tell you much, much more about <Command>pvm</Command>!
2899 <Title>Parallelism profiles</Title>
2902 <IndexTerm><Primary>parallelism profiles</Primary></IndexTerm>
2903 <IndexTerm><Primary>profiles, parallelism</Primary></IndexTerm>
2904 <IndexTerm><Primary>visualisation tools</Primary></IndexTerm>
2908 With Parallel Haskell programs, we usually don't care about the
2909 results—only with “how parallel” it was! We want pretty pictures.
2913 Parallelism profiles (à la <Command>hbcpp</Command>) can be generated with the
2914 <Option>-q</Option><IndexTerm><Primary>-q RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option. The
2915 per-processor profiling info is dumped into files named
2916 <Filename><full-path><program>.gr</Filename>. These are then munged into a PostScript picture,
2917 which you can then display. For example, to run your program
2918 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
2924 % ./a.out +RTS -N8 -q
2925 % grs2gr *.???.gr > temp.gr # combine the 8 .gr files into one
2926 % gr2ps -O temp.gr # cvt to .ps; output in temp.ps
2927 % ghostview -seascape temp.ps # look at it!
2933 The scripts for processing the parallelism profiles are distributed
2934 in <Filename>ghc/utils/parallel/</Filename>.
2940 <Title>Other useful info about running parallel programs</Title>
2943 The “garbage-collection statistics” RTS options can be useful for
2944 seeing what parallel programs are doing. If you do either
2945 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
2946 you'll get mutator, garbage-collection, etc., times on standard
2947 error. The standard error of all PE's other than the `main thread'
2948 appears in <Filename>/tmp/pvml.nnn</Filename>, courtesy of PVM.
2952 Whether doing <Option>+RTS -Sstderr</Option> or not, a handy way to watch
2953 what's happening overall is: <Command>tail -f /tmp/pvml.nnn</Command>.
2958 <Sect2 id="parallel-rts-opts">
2959 <Title>RTS options for Concurrent/Parallel Haskell
2963 <IndexTerm><Primary>RTS options, concurrent</Primary></IndexTerm>
2964 <IndexTerm><Primary>RTS options, parallel</Primary></IndexTerm>
2965 <IndexTerm><Primary>Concurrent Haskell—RTS options</Primary></IndexTerm>
2966 <IndexTerm><Primary>Parallel Haskell—RTS options</Primary></IndexTerm>
2970 Besides the usual runtime system (RTS) options
2971 (<XRef LinkEnd="runtime-control">), there are a few options particularly
2972 for concurrent/parallel execution.
2979 <Term><Option>-N<N></Option>:</Term>
2982 <IndexTerm><Primary>-N<N> RTS option (parallel)</Primary></IndexTerm>
2983 (PARALLEL ONLY) Use <Literal><N></Literal> PVM processors to run this program;
2989 <Term><Option>-C[<us>]</Option>:</Term>
2992 <IndexTerm><Primary>-C<us> RTS option</Primary></IndexTerm> Sets
2993 the context switch interval to <Literal><s></Literal> seconds.
2994 A context switch will occur at the next heap block allocation after
2995 the timer expires (a heap block allocation occurs every 4k of
2996 allocation). With <Option>-C0</Option> or <Option>-C</Option>,
2997 context switches will occur as often as possible (at every heap block
2998 allocation). By default, context switches occur every 20ms
2999 milliseconds. Note that GHC's internal timer ticks every 20ms, and
3000 the context switch timer is always a multiple of this timer, so 20ms
3001 is the maximum granularity available for timed context switches.
3006 <Term><Option>-q[v]</Option>:</Term>
3009 <IndexTerm><Primary>-q RTS option</Primary></IndexTerm>
3010 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
3011 in the file <FIlename><program>.qp</FIlename>. In the style of <Command>hbcpp</Command>, this profile
3012 records the movement of threads between the green (runnable) and red
3013 (blocked) queues. If you specify the verbose suboption (<Option>-qv</Option>), the
3014 green queue is split into green (for the currently running thread
3015 only) and amber (for other runnable threads). We do not recommend
3016 that you use the verbose suboption if you are planning to use the
3017 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
3018 check (with <Option>-C</Option>).
3023 <Term><Option>-t<num></Option>:</Term>
3026 <IndexTerm><Primary>-t<num> RTS option</Primary></IndexTerm>
3027 (PARALLEL ONLY) Limit the number of concurrent threads per processor
3028 to <Literal><num></Literal>. The default is 32. Each thread requires slightly over 1K
3029 <Emphasis>words</Emphasis> in the heap for thread state and stack objects. (For
3030 32-bit machines, this translates to 4K bytes, and for 64-bit machines,
3036 <Term><Option>-d</Option>:</Term>
3039 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
3040 (PARALLEL ONLY) Turn on debugging. It pops up one xterm (or GDB, or
3041 something…) per PVM processor. We use the standard <Command>debugger</Command>
3042 script that comes with PVM3, but we sometimes meddle with the
3043 <Command>debugger2</Command> script. We include ours in the GHC distribution,
3044 in <Filename>ghc/utils/pvm/</Filename>.
3049 <Term><Option>-e<num></Option>:</Term>
3052 <IndexTerm><Primary>-e<num> RTS option (parallel)</Primary></IndexTerm>
3053 (PARALLEL ONLY) Limit the number of pending sparks per processor to
3054 <Literal><num></Literal>. The default is 100. A larger number may be appropriate if
3055 your program generates large amounts of parallelism initially.
3060 <Term><Option>-Q<num></Option>:</Term>
3063 <IndexTerm><Primary>-Q<num> RTS option (parallel)</Primary></IndexTerm>
3064 (PARALLEL ONLY) Set the size of packets transmitted between processors
3065 to <Literal><num></Literal>. The default is 1024 words. A larger number may be
3066 appropriate if your machine has a high communication cost relative to
3084 ;;; Local Variables: ***
3086 ;;; sgml-parent-document: ("users_guide.sgml" "book" "chapter") ***
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