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.
893 The idea is that you can then import a Prelude of your own. (But don't call it <Literal>Prelude</Literal>;
894 the Haskell module namespace is flat, and you must not conflict with any Prelude module.)
897 Even though you have not imported the Prelude, all the built-in syntax still refers to
898 the built-in Haskell Prelude types and values, as specified by the Haskell Report.
899 For example, the type <Literal>[Int]</Literal>
900 still means <Literal>Prelude.[] Int</Literal>; tuples continue to refer to the standard Prelude
901 tuples; the translation for list comprehensions continues to use <Literal>Prelude.map</Literal> etc.
903 <Para> With one group of exceptions! You may want to define your own numeric class hierarchy.
904 It completely defeats that purpose if the literal "1" means "<Literal>Prelude.fromInteger 1</Literal>",
905 which is what the Haskell Report specifies. So the <Option>-fno-implicit-prelude</Option> flag causes
906 the following pieces of built-in syntax to refer to whatever is in scope, not the Prelude versions:
910 Integer and fractional literals mean "<Literal>fromInteger 1</Literal>" and "<Literal>fromRational 3.2</Literal>",
911 not the Prelude-qualified versions; both in expressions and in patterns.
916 Negation (e.g. "<Literal>- (f x)</Literal>") means "<Literal>negate (f x)</Literal>" (not <Literal>Prelude.negate</Literal>).
921 In an n+k pattern, the standard Prelude <Literal>Ord</Literal> class is used for comparison, but the
922 necessary subtraction uses
923 whatever "<Literal>(-)</Literal>" is in scope (not "<Literal>Prelude.(-)</Literal>").
932 <Term><Option>-I<dir></Option></Term>
935 <IndexTerm><Primary>-I<dir> option</Primary></IndexTerm>
936 Once a Haskell module has been compiled to C (<Filename>.hc</Filename> file), you may
937 wish to specify where GHC tells the C compiler to look for <Filename>.h</Filename> files.
938 (Or, if you are using the <Option>-cpp</Option> option<IndexTerm><Primary>-cpp option</Primary></IndexTerm>, where
939 it tells the C pre-processor to look…) For this purpose, use a <Option>-I</Option>
940 option in the usual C-ish way.
950 <Sect2 id="hi-options">
951 <Title>Other options related to interface files
955 <IndexTerm><Primary>interface files, options</Primary></IndexTerm>
956 The interface output may be directed to another file
957 <Filename>bar2/Wurble.iface</Filename> with the option <Option>-ohi bar2/Wurble.iface</Option><IndexTerm><Primary>-ohi
958 <file> option</Primary></IndexTerm> (not recommended).
962 To avoid generating an interface file at all, use a <Option>-nohi</Option>
963 option.<IndexTerm><Primary>-nohi option</Primary></IndexTerm>
967 The compiler does not overwrite an existing <Filename>.hi</Filename> interface file if
968 the new one is byte-for-byte the same as the old one; this is friendly
969 to <Command>make</Command>. When an interface does change, it is often enlightening to
970 be informed. The <Option>-hi-diffs</Option><IndexTerm><Primary>-hi-diffs option</Primary></IndexTerm> option will
971 make GHC run <Command>diff</Command> on the old and new <Filename>.hi</Filename> files. You can also
972 record the difference in the interface file itself, the
973 <Option>-keep-hi-diffs</Option><IndexTerm><Primary>-keep-hi-diffs</Primary></IndexTerm> option takes care of that.
977 The <Filename>.hi</Filename> files from GHC contain “usage” information which changes
978 often and uninterestingly. If you really want to see these changes
979 reported, you need to use the
980 <Option>-hi-diffs-with-usages</Option><IndexTerm><Primary>-hi-diffs-with-usages option</Primary></IndexTerm>
985 Interface files are normally jammed full of compiler-produced
986 <Emphasis>pragmas</Emphasis>, which record arities, strictness info, etc. If you
987 think these pragmas are messing you up (or you are doing some kind of
988 weird experiment), you can tell GHC to ignore them with the
989 <Option>-fignore-interface-pragmas</Option><IndexTerm><Primary>-fignore-interface-pragmas
990 option</Primary></IndexTerm> option.
994 When compiling without optimisations on, the compiler is extra-careful
995 about not slurping in data constructors and instance declarations that
996 it will not need. If you believe it is getting it wrong and not
997 importing stuff which you think it should, this optimisation can be
998 turned off with <Option>-fno-prune-tydecls</Option> and <Option>-fno-prune-instdecls</Option>.
999 <IndexTerm><Primary>-fno-prune-tydecls option</Primary></IndexTerm><IndexTerm><Primary>-fno-prune-instdecls
1000 option</Primary></IndexTerm>
1004 See also <XRef LinkEnd="options-linker">, which describes how the linker finds standard
1011 <Title>The recompilation checker
1014 <IndexTerm><Primary>recompilation checker</Primary></IndexTerm>
1019 <Term><Option>-recomp</Option></Term>
1020 <IndexTerm><Primary><option>-recomp</option> option</Primary></IndexTerm>
1023 (On by default) Turn on recompilation checking. This will stop
1024 compilation early, leaving an existing <filename>.o</filename> file in
1025 place, if it can be determined that the module does not need to be
1031 <Term><Option>-no-recomp</Option></Term>
1032 <IndexTerm><Primary><option>-recomp</option> option</Primary></IndexTerm>
1035 Turn off recompilation checking.
1043 In the olden days, GHC compared the newly-generated
1044 <Filename>.hi</Filename> file with the previous version; if they were
1045 identical, it left the old one alone and didn't change its
1046 modification date. In consequence, importers of a module with an
1047 unchanged output <Filename>.hi</Filename> file were not recompiled.
1051 This doesn't work any more. In our earlier example, module
1052 <Literal>C</Literal> does not import module <Literal>A</Literal>
1053 directly, yet changes to <Filename>A.hi</Filename> should force a
1054 recompilation of <Literal>C</Literal>. And some changes to
1055 <Literal>A</Literal> (changing the definition of a function that
1056 appears in an inlining of a function exported by <Literal>B</Literal>,
1057 say) may conceivably not change <Filename>B.hi</Filename> one jot. So
1062 GHC keeps a version number on each interface file, and on each type
1063 signature within the interface file. It also keeps in every interface
1064 file a list of the version numbers of everything it used when it last
1065 compiled the file. If the source file's modification date is earlier
1066 than the <Filename>.o</Filename> file's date (i.e. the source hasn't
1067 changed since the file was last compiled), and the
1068 <option>-recomp</option> is given on the command line, GHC will be
1069 clever. It compares the version numbers on the things it needs this
1070 time with the version numbers on the things it needed last time
1071 (gleaned from the interface file of the module being compiled); if
1072 they are all the same it stops compiling rather early in the process
1073 saying “Compilation IS NOT required”. What a beautiful
1078 Patrick Sansom had a workshop paper about how all this is done (though
1079 the details have changed quite a bit). <ULink URL="mailto:sansom@dcs.gla.ac.uk">Ask him</ULink> if you want a copy.
1085 <Sect2 id="using-make">
1086 <Title>Using <Command>make</Command>
1090 <IndexTerm><Primary><literal>make</literal></Primary></IndexTerm>
1094 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.
1102 HC_OPTS = -cpp $(EXTRA_HC_OPTS)
1104 SRCS = Main.lhs Foo.lhs Bar.lhs
1105 OBJS = Main.o Foo.o Bar.o
1107 .SUFFIXES : .o .hs .hi .lhs .hc .s
1111 $(HC) -o $@ $(HC_OPTS) $(OBJS)
1113 # Standard suffix rules
1118 $(HC) -c $< $(HC_OPTS)
1121 $(HC) -c $< $(HC_OPTS)
1123 # Inter-module dependencies
1124 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
1125 Main.o Main.hc Main.s : Foo.hi Baz.hi # Main imports Foo and Baz
1131 (Sophisticated <Command>make</Command> variants may achieve some of the above more
1132 elegantly. Notably, <Command>gmake</Command>'s pattern rules let you write the more
1140 $(HC) -c $< $(HC_OPTS)
1146 What we've shown should work with any <Command>make</Command>.)
1150 Note the cheesy <Literal>.o.hi</Literal> rule: It records the dependency of the
1151 interface (<Filename>.hi</Filename>) file on the source. The rule says a <Filename>.hi</Filename> file can
1152 be made from a <Filename>.o</Filename> file by doing…nothing. Which is true.
1156 Note the inter-module dependencies at the end of the Makefile, which
1163 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
1169 They tell <Command>make</Command> that if any of <Literal>Foo.o</Literal>, <Literal>Foo.hc</Literal> or <Literal>Foo.s</Literal> have an
1170 earlier modification date than <Literal>Baz.hi</Literal>, then the out-of-date file
1171 must be brought up to date. To bring it up to date, <Literal>make</Literal> looks for
1172 a rule to do so; one of the preceding suffix rules does the job
1178 <sect2 id="sec-makefile-dependencies">
1179 <title>Dependency generation</title>
1180 <indexterm><primary>dependencies in Makefiles</primary></indexterm>
1181 <indexterm><primary>Makefile dependencies</primary></indexterm>
1183 <para>Putting inter-dependencies of the form <Literal>Foo.o :
1184 Bar.hi</Literal> into your <Filename>Makefile</Filename> by hand
1185 is rather error-prone. Don't worry, GHC has support for
1186 automatically generating the required dependencies. Add the
1187 following to your <Filename>Makefile</Filename>:</para>
1191 ghc -M $(HC_OPTS) $(SRCS)
1194 <para>Now, before you start compiling, and any time you change
1195 the <Literal>imports</Literal> in your program, do <Command>make
1196 depend</Command> before you do <Command>make
1197 cool_pgm</Command>. <Command>ghc -M</Command> will append
1198 the needed dependencies to your
1199 <Filename>Makefile</Filename>.</Para>
1201 <para>In general, if module <Literal>A</Literal> contains the
1208 then <command>ghc -M</command> will generate a dependency
1215 If module <literal>A</literal> contains the line
1218 import {-# SOURCE #-} B ...blah...
1221 then <command>ghc -M</command> will generate a dependency
1228 (See <xref linkend="hi-files"> for details of interface files.)
1229 If <literal>A</literal> imports multiple modules, then there
1230 will be multiple lines with <filename>A.o</filename> as the
1233 <para>By default, <Command>ghc -M</Command> generates all the
1234 dependencies, and then concatenates them onto the end of
1235 <Filename>makefile</Filename> (or <Filename>Makefile</Filename>
1236 if <Filename>makefile</Filename> doesn't exist) bracketed by the
1237 lines "<Literal># DO NOT DELETE: Beginning of Haskell
1238 dependencies</Literal>" and "<Literal># DO NOT DELETE: End
1239 of Haskell dependencies</Literal>". If these lines already
1240 exist in the <Filename>makefile</Filename>, then the old
1241 dependencies are deleted first.</para>
1243 <para>Internally, GHC uses a script to generate the
1244 dependencies, called <command>mkdependHS</command>. This script
1245 has some options of its own, which you might find useful.
1246 Options can be passed directly to <command>mkdependHS</command>
1247 with GHC's <literal>-optdep</literal> option. For example, to
1248 generate the dependencies into a file called
1249 <literal>.depend</literal> instead of
1250 <literal>Makefile</literal>:</para>
1253 ghc -M -optdep-f optdep.depend ...
1256 <para>The full list of options accepted by
1257 <command>mkdependHS</command> is:</para>
1262 <term><option>-w</option></term>
1264 <para>Turn off warnings about interface file shadowing.</para>
1269 <term><option>-f blah</option></term>
1271 <para>Use <Filename>blah</Filename> as the makefile,
1272 rather than <Filename>makefile</Filename> or
1273 <Filename>Makefile</Filename>. If
1274 <Filename>blah</Filename> doesn't exist,
1275 <Command>mkdependHS</Command> creates it. We often use
1276 <Option>-f .depend</Option> to put the dependencies in
1277 <Filename>.depend</Filename> and then
1278 <Command>include</Command> the file
1279 <Filename>.depend</Filename> into
1280 <Filename>Makefile</Filename>.</para>
1285 <term><option>-o <osuf></option></term>
1287 <para>Use <Filename>.<osuf></Filename> as the
1288 "target file" suffix ( default: <Literal>o</Literal>).
1289 Multiple <Option>-o</Option> flags are permitted (GHC2.05
1290 onwards). Thus "<Option>-o hc -o o</Option>" will
1291 generate dependencies for <Filename>.hc</Filename> and
1292 <Filename>.o</Filename> files.</para>
1297 <term><option>-s <suf></option></term>
1299 <para>Make extra dependencies that declare that files with
1301 <Filename>.<suf>_<osuf></Filename>
1302 depend on interface files with suffix
1303 <Filename>.<suf>_hi</Filename>, or (for
1304 <Literal>{-# SOURCE #-}</Literal>
1305 imports) on <Filename>.hi-boot</Filename>. Multiple
1306 <Option>-s</Option> flags are permitted. For example,
1307 <Option>-o hc -s a -s b</Option> will make dependencies
1308 for <Filename>.hc</Filename> on <Filename>.hi</Filename>,
1309 <Filename>.a_hc</Filename> on
1310 <Filename>.a_hi</Filename>, and
1311 <Filename>.b_hc</Filename> on
1312 <Filename>.b_hi</Filename>. (Useful in conjunction
1313 with NoFib "ways".)</para>
1318 <term><option>--exclude-module=<file></option></term>
1320 <para>Regard <Filename><file></Filename> as
1321 "stable"; i.e., exclude it from having dependencies on
1327 <term><option>-x</option></term>
1329 <para>same as <option>--exclude-module</option></para>
1334 <term><option>--exclude-directory=<dirs></option></term>
1336 <para>Regard the colon-separated list of directories
1337 <Filename><dirs></Filename> as containing stable,
1338 don't generate any dependencies on modules therein.</para>
1343 <term><option>-xdirs</option></term>
1345 <para>same as <Option>--exclude-directory</Option>.</para>
1350 <term><option>--include-module=<file></option></term>
1352 <para>Regard <Filename><file></Filename> as not
1353 "stable"; i.e., generate dependencies on it (if any). This
1354 option is normally used in conjunction with the
1355 <Option>--exclude-directory</Option> option.</para>
1360 <term><option>--include-prelude</option></term>
1362 <para>Regard prelude libraries as unstable, i.e., generate
1363 dependencies on the prelude modules used (including
1364 <Literal>Prelude</Literal>). This option is normally only
1365 used by the various system libraries. If a
1366 <Option>-package</Option> option is used, dependencies will
1367 also be generated on the library's interfaces.</para>
1374 <Sect2 id="mutual-recursion">
1375 <Title>How to compile mutually recursive modules
1379 <IndexTerm><Primary>module system, recursion</Primary></IndexTerm>
1380 <IndexTerm><Primary>recursion, between modules</Primary></IndexTerm>
1384 Currently, the compiler does not have proper support for dealing with
1385 mutually recursive modules:
1395 newtype TA = MkTA Int
1413 When compiling either module A and B, the compiler will try (in vain)
1414 to look for the interface file of the other. So, to get mutually
1415 recursive modules off the ground, you need to hand write an interface
1416 file for A or B, so as to break the loop. These hand-written
1417 interface files are called <Literal>hi-boot</Literal> files, and are placed in a file
1418 called <Filename><module>.hi-boot</Filename>. To import from an <Literal>hi-boot</Literal> file instead
1419 of the standard <Filename>.hi</Filename> file, use the following syntax in the importing module:
1420 <IndexTerm><Primary>hi-boot files</Primary></IndexTerm>
1421 <IndexTerm><Primary>importing, hi-boot files</Primary></IndexTerm>
1427 import {-# SOURCE #-} A
1433 The hand-written interface need only contain the bare minimum of
1434 information needed to get the bootstrapping process started. For
1435 example, it doesn't need to contain declarations for <Emphasis>everything</Emphasis>
1436 that module <Literal>A</Literal> exports, only the things required by the module that
1437 imports <Literal>A</Literal> recursively.
1441 For the example at hand, the boot interface file for A would look like
1448 __interface A 1 404 where
1449 __export A TA{MkTA} ;
1450 1 newtype TA = MkTA PrelBase.Int ;
1456 The syntax is essentially the same as a normal <Filename>.hi</Filename> file
1457 (unfortunately), but you can usually tailor an existing <Filename>.hi</Filename> file to
1458 make a <Filename>.hi-boot</Filename> file.
1462 Notice that we only put the declaration for the newtype <Literal>TA</Literal> in the
1463 <Literal>hi-boot</Literal> file, not the signature for <Function>f</Function>, since <Function>f</Function> isn't used by
1464 <Literal>B</Literal>.
1468 The number “1” after “__interface A” gives the version number of module A;
1469 it is incremented whenever anything in A's interface file changes. The “404” is
1470 the version number of the interface file <Emphasis>syntax</Emphasis>; we change it when
1471 we change the syntax of interface files so that you get a better error message when
1472 you try to read an old-format file with a new-format compiler.
1476 The number “1” at the beginning of a declaration is the <Emphasis>version
1477 number</Emphasis> of that declaration: for the purposes of <Filename>.hi-boot</Filename> files
1478 these can all be set to 1. All names must be fully qualified with the
1479 <Emphasis>original</Emphasis> module that an object comes from: for example, the
1480 reference to <Literal>Int</Literal> in the interface for <Literal>A</Literal> comes from <Literal>PrelBase</Literal>,
1481 which is a module internal to GHC's prelude. It's a pain, but that's
1486 If you want an hi-boot file to export a data type, but you don't want to give its constructors
1487 (because the constructors aren't used by the SOURCE-importing module), you can write simply:
1493 __interface A 1 404 where
1501 (You must write all the type parameters, but leave out the '=' and everything that follows it.)
1505 <Emphasis>Note:</Emphasis> This is all a temporary solution, a version of the
1506 compiler that handles mutually recursive modules properly without the manual
1507 construction of interface files, is (allegedly) in the works.
1514 <sect1 id="packages">
1515 <title>Packages</title>
1516 <indexterm><primary>packages</primary></indexterm>
1518 <para>Packages are collections of libraries, conveniently grouped
1519 together as a single entity. The package system is flexible: a
1520 package may consist of Haskell code, foreign language code (eg. C
1521 libraries), or a mixture of the two. A package is a good way to
1522 group together related Haskell modules, and is essential if you
1523 intend to make the modules into a Windows DLL (see below).</para>
1525 <para>Because packages can contain both Haskell and C libraries, they
1526 are also a good way to provide convenient access to a Haskell
1527 layer over a C library.</para>
1529 <para>GHC comes with several packages (see <xref
1530 linkend="book-hslibs">), and packages can be added/removed from an
1531 existing GHC installation.</para>
1533 <sect2 id="listing-packages">
1534 <title>Listing the available packages</title>
1535 <indexterm><primary>packages</primary>
1536 <secondary>listing</secondary></indexterm>
1538 <para>To see what packages are currently installed, use the
1539 <literal>--list-packages</literal> option:</para>
1540 <indexterm><primary><literal>--list-packages</literal></primary>
1544 $ ghc --list-packages
1545 gmp, rts, std, lang, concurrent, data, net, posix, text, util
1548 <para>Note that your GHC installation might have a slightly
1549 different set of packages installed.</para>
1551 <para>The <literal>gmp</literal> and <literal>rts</literal>
1552 packages are always present, and represent the multi-precision
1553 integer and runtime system libraries respectively. The
1554 <literal>std</literal> package contains the Haskell prelude.
1555 The rest of the packages are optional libraries.</para>
1559 <sect2 id="using-packages">
1560 <title>Using a package</title>
1561 <indexterm><primary>packages</primary>
1562 <secondary>using</secondary></indexterm>
1564 <para>To use a package, add the <literal>-package</literal> flag
1565 to the command line:</para>
1569 <term><option>-package <lib></option></term>
1570 <indexterm><primary>-package <lib> option</primary></indexterm>
1572 <para>This option brings into scope all the modules from
1573 package <literal><lib></literal> (they still have to
1574 be imported in your Haskell source, however). It also
1575 causes the relevant libraries to be linked when linking is
1581 <para>Some packages depend on other packages, for example the
1582 <literal>text</literal> package makes use of some of the modules
1583 in the <literal>lang</literal> package. The package system
1584 takes care of all these dependencies, so that when you say
1585 <literal>-package text</literal> on the command line, you
1586 automatically get <literal>-package lang</literal> too.</para>
1589 <sect2 id="building-packages">
1590 <title>Building a package from Haskell source</title>
1591 <indexterm><primary>packages</primary>
1592 <secondary>building</secondary></indexterm>
1594 <para>It takes some special considerations to build a new
1599 <para>A package may contain several Haskell modules. A
1600 package may span many directories, or many packages may
1601 exist in a single directory. Packages may not be mutually
1606 <para>A package has a name
1607 (e.g. <filename>std</filename>)</para>
1611 <para>The Haskell code in a package may be built into one or
1612 more Unix libraries (e.g. <Filename>libHSfoo.a</Filename>),
1613 or a single DLL on Windows
1614 (e.g. <Filename>HSfoo.dll</Filename>). The restriction to a
1615 single DLL on Windows is that the package system is used to
1616 tell the compiler when it should make an inter-DLL call
1617 rather than an intra-DLL call (inter-DLL calls require an
1618 extra indirection).</para>
1622 <para>GHC does not maintain detailed cross-package
1623 dependency information. It does remember which modules in
1624 other packages the current module depends on, but not which
1625 things within those imported things.</para>
1629 <para>To compile a module which is to be part of a new package,
1630 use the <literal>-package-name</literal> option:</para>
1634 <term><option>-package-name <foo></option></term>
1635 <indexterm><primary><literal>-package-name</literal></primary>
1636 <secondary>option</secondary></indexterm>
1638 <para>This option is added to the command line when
1639 compiling a module that is destined to be part of package
1640 <literal>foo</literal>. If this flag is omitted then the
1641 default package <literal>Main</literal> is assumed.</para>
1646 <para>Failure to use the <literal>-package-name</literal> option
1647 when compiling a package will result in disaster on Windows, but
1648 is relatively harmless on Unix at the moment (it will just cause
1649 a few extra dependencies in some interface files). However,
1650 bear in mind that we might add support for Unix shared libraries
1651 at some point in the future.</para>
1653 <para>It is worth noting that on Windows, because each package
1654 is built as a DLL, and a reference to a DLL costs an extra
1655 indirection, intra-package references are cheaper than
1656 inter-package references. Of course, this applies to the
1657 <Filename>Main</Filename> package as well.</para>
1660 <sect2 id="package-management">
1661 <title>Package management</title>
1662 <indexterm><primary>packages</primary>
1663 <secondary>management</secondary></indexterm>
1665 <para>GHC uses a package configuration file, called
1666 <literal>packages.conf</literal>, which can be found in your GHC
1667 install directory. This file isn't intended to be edited
1668 directly, instead GHC provides options for adding & removing
1673 <term><option>--add-package</option></term>
1674 <indexterm><primary><literal>--add-package</literal></primary>
1675 <secondary>option</secondary></indexterm>
1677 <para>Reads a package specification (see below) on stdin,
1678 and adds it to the database of installed packages. The
1679 package specification must be a package that isn't already
1685 <term><option>--delete-package <foo></option></term>
1686 <indexterm><primary><literal>--delete-package</literal></primary>
1687 <secondary>option</secondary></indexterm>
1689 <para>Removes the specified package from the installed
1690 configuration.</para>
1695 <para>In both cases, the old package configuration file is saved
1696 in <literal>packages.conf.old</literal> in your GHC install
1697 directory, so in an emergency you can always copy this file into
1698 <literal>package.conf</literal> to restore the old
1701 <para>A package specification looks like this:</para>
1706 import_dirs = ["/usr/local/lib/imports/mypkg"],
1707 library_dirs = ["/usr/local/lib"],
1708 hs_libraries = ["HSmypkg" ],
1709 extra_libraries = ["HSmypkg_cbits"],
1711 c_includes = ["HsMyPkg.h"],
1712 package_deps = ["text", "data"],
1713 extra_ghc_opts = [],
1715 extra_ld_opts = ["-lmy_clib"]
1719 <para>Components of a package specification may be specified in
1720 any order, and are:</para>
1724 <term><literal>name</literal></term>
1725 <indexterm><primary><literal>name</literal></primary>
1726 <secondary>package specification</secondary></indexterm>
1728 <para>The package's name, for use with
1729 the <literal>-package</literal> flag and as listed in the
1730 <literal>--list-packages</literal> list.
1736 <term><literal>import_dirs</literal></term>
1737 <indexterm><primary><literal>import_dirs</literal></primary>
1738 <secondary>package specification</secondary></indexterm>
1740 <para>A list of directories containing interface files
1741 (<literal>.hi</literal> files) for this package.</para>
1746 <term><literal>library_dirs</literal></term>
1747 <indexterm><primary><literal>library_dirs</literal></primary>
1748 <secondary>package specification</secondary></indexterm>
1750 <para>A list of directories containing libraries for this
1756 <term><literal>hs_libraries</literal></term>
1757 <indexterm><primary><literal>hs_libraries</literal></primary>
1758 <secondary>package specification</secondary></indexterm>
1760 <para>A list of libraries containing Haskell code for this
1761 package, with the <literal>.a</literal> or
1762 <literal>.dll</literal> suffix omitted. On Unix, the
1763 <literal>lib</literal> prefix is also omitted.</para>
1768 <term><literal>extra_libraries</literal></term>
1769 <indexterm><primary><literal>extra_libraries</literal></primary>
1770 <secondary>package specification</secondary></indexterm>
1772 <para>A list of extra libraries for this package. The
1773 difference between <literal>hs_libraries</literal> and
1774 <literal>extra_libraries</literal> is that
1775 <literal>hs_libraries</literal> normally have several
1776 versions, to support profiling, parallel and other build
1777 options. The various versions are given different
1778 suffixes to distinguish them, for example the profiling
1779 version of the standard prelude library is named
1780 <filename>libHSstd_p.a</filename>, with the
1781 <literal>_p</literal> indicating that this is a profiling
1782 version. The suffix is added automatically by GHC for
1783 <literal>hs_libraries</literal> only, no suffix is added
1785 <literal>extra_libraries</literal>.</para>
1787 <para>Also, <literal>extra_libraries</literal> are placed
1788 on the linker command line before the
1789 <literal>hs_libraries</literal> for the same package. If
1790 your package has dependencies in the other direction, you
1791 might need to make two separate packages.</para>
1796 <term><literal>include_dirs</literal></term>
1797 <indexterm><primary><literal>include_dirs</literal></primary>
1798 <secondary>package specification</secondary></indexterm>
1800 <para>A list of directories containing C includes for this
1801 package (maybe the empty list).</para>
1806 <term><literal>c_includes</literal></term>
1807 <indexterm><primary><literal>c_includes</literal></primary>
1808 <secondary>package specification</secondary></indexterm>
1810 <para>A list of files to include for via-C compilations
1811 using this package. Typically this include file will
1812 contain function prototypes for any C functions used in
1813 the package, in case they end up being called as a result
1814 of Haskell functions from the package being
1820 <term><literal>package_deps</literal></term>
1821 <indexterm><primary><literal>package_deps</literal></primary>
1822 <secondary>package specification</secondary></indexterm>
1824 <para>A list of packages which this package depends
1830 <term><literal>extra_ghc_opts</literal></term>
1831 <indexterm><primary><literal>extra_ghc_opts</literal></primary>
1832 <secondary>package specification</secondary></indexterm>
1834 <para>Extra arguments to be added to the GHC command line
1835 when this package is being used.</para>
1840 <term><literal>extra_cc_opts</literal></term>
1841 <indexterm><primary><literal>extra_cc_opts</literal></primary>
1842 <secondary>package specification</secondary></indexterm>
1844 <para>Extra arguments to be added to the gcc command line
1845 when this package is being used (only for via-C
1846 compilations).</para>
1851 <term><literal>extra_ld_opts</literal></term>
1852 <indexterm><primary><literal>extra_ld_opts</literal></primary>
1853 <secondary>package specification</secondary></indexterm>
1855 <para>Extra arguments to be added to the gcc command line
1856 (for linking) when this package is being used.</para>
1861 <para>For examples of more package specifications, take a look
1862 at the <literal>package.conf</literal> in your GHC
1863 installation.</para>
1868 <Sect1 id="options-optimise">
1869 <Title>Optimisation (code improvement)
1873 <IndexTerm><Primary>optimisation (GHC)</Primary></IndexTerm>
1874 <IndexTerm><Primary>improvement, code (GHC)</Primary></IndexTerm>
1878 The <Option>-O*</Option> options specify convenient “packages” of optimisation
1879 flags; the <Option>-f*</Option> options described later on specify
1880 <Emphasis>individual</Emphasis> optimisations to be turned on/off; the <Option>-m*</Option>
1881 options specify <Emphasis>machine-specific</Emphasis> optimisations to be turned
1885 <Sect2 id="optimise-pkgs">
1886 <Title><Option>-O*</Option>: convenient “packages” of optimisation flags.
1890 <IndexTerm><Primary>-O options</Primary></IndexTerm>
1894 There are <Emphasis>many</Emphasis> options that affect the quality of code
1895 produced by GHC. Most people only have a general goal, something like
1896 “Compile quickly” or “Make my program run like greased lightning.”
1897 The following “packages” of optimisations (or lack thereof) should
1902 Once you choose a <Option>-O*</Option> “package,” stick with it—don't chop and
1903 change. Modules' interfaces <Emphasis>will</Emphasis> change with a shift to a new
1904 <Option>-O*</Option> option, and you may have to recompile a large chunk of all
1905 importing modules before your program can again be run
1906 safely (see <XRef LinkEnd="recomp">).
1913 <Term>No <Option>-O*</Option>-type option specified:</Term>
1914 <IndexTerm><Primary>-O* not specified</Primary></IndexTerm>
1917 This is taken to mean: “Please compile quickly; I'm not over-bothered
1918 about compiled-code quality.” So, for example: <Command>ghc -c Foo.hs</Command>
1923 <Term><Option>-O</Option> or <Option>-O1</Option>:</Term>
1924 <IndexTerm><Primary>-O option</Primary></IndexTerm>
1925 <IndexTerm><Primary>-O1 option</Primary></IndexTerm>
1926 <IndexTerm><Primary>optimise</Primary><secondary>normally</secondary></IndexTerm>
1929 Means: “Generate good-quality code without taking too long about
1930 it.” Thus, for example: <Command>ghc -c -O Main.lhs</Command>
1935 <Term><Option>-O2</Option>:</Term>
1936 <IndexTerm><Primary>-O2 option</Primary></IndexTerm>
1937 <IndexTerm><Primary>optimise</Primary><secondary>aggressively</secondary></IndexTerm>
1940 Means: “Apply every non-dangerous optimisation, even if it means
1941 significantly longer compile times.”
1945 The avoided “dangerous” optimisations are those that can make
1946 runtime or space <Emphasis>worse</Emphasis> if you're unlucky. They are
1947 normally turned on or off individually.
1951 At the moment, <Option>-O2</Option> is <Emphasis>unlikely</Emphasis> to produce
1952 better code than <Option>-O</Option>.
1957 <Term><Option>-O2-for-C</Option>:</Term>
1958 <IndexTerm><Primary>-O2-for-C option</Primary></IndexTerm>
1959 <IndexTerm><Primary>gcc, invoking with -O2</Primary></IndexTerm>
1962 Says to run GCC with <Option>-O2</Option>, which may be worth a few percent in
1963 execution speed. Don't forget <Option>-fvia-C</Option>, lest you use the native-code
1964 generator and bypass GCC altogether!
1969 <Term><Option>-Onot</Option>:</Term>
1970 <IndexTerm><Primary>-Onot option</Primary></IndexTerm>
1971 <IndexTerm><Primary>optimising, reset</Primary></IndexTerm>
1974 This option will make GHC “forget” any
1975 <Option>-O</Option>ish options it has seen so far. Sometimes useful;
1976 for example: <Command>make all
1977 EXTRA_HC_OPTS=-Onot</Command>.
1982 <Term><Option>-Ofile <file></Option>:</Term>
1983 <IndexTerm><Primary>-Ofile <file> option</Primary></IndexTerm>
1984 <IndexTerm><Primary>optimising, customised</Primary></IndexTerm>
1987 For those who need <Emphasis>absolute</Emphasis> control over
1988 <Emphasis>exactly</Emphasis> what options are used (e.g., compiler
1989 writers, sometimes :-), a list of options can be put in a file and
1990 then slurped in with <Option>-Ofile</Option>.
1994 In that file, comments are of the
1995 <Literal>#</Literal>-to-end-of-line variety; blank lines and most
1996 whitespace is ignored.
2000 Please ask if you are baffled and would like an example of <Option>-Ofile</Option>!
2008 At Glasgow, we don't use a <Option>-O*</Option> flag for day-to-day work. We use
2009 <Option>-O</Option> to get respectable speed; e.g., when we want to measure
2010 something. When we want to go for broke, we tend to use <Option>-O -fvia-C
2011 -O2-for-C</Option> (and we go for lots of coffee breaks).
2015 The easiest way to see what <Option>-O</Option> (etc.) “really mean” is to run with
2016 <Option>-v</Option>, then stand back in amazement. Alternatively, just look at the
2017 <Literal>HsC_minus<blah></Literal> lists in the GHC driver script.
2023 <Title><Option>-f*</Option>: platform-independent flags</Title>
2026 <IndexTerm><Primary>-f* options (GHC)</Primary></IndexTerm>
2027 <IndexTerm><Primary>-fno-* options (GHC)</Primary></IndexTerm>
2031 Flags can be turned <Emphasis>off</Emphasis> individually. (NB: I hope you have a
2032 good reason for doing this…) To turn off the <Option>-ffoo</Option> flag, just use
2033 the <Option>-fno-foo</Option> flag.<IndexTerm><Primary>-fno-<opt> anti-option</Primary></IndexTerm> So, for
2034 example, you can say <Option>-O2 -fno-strictness</Option>, which will then drop out
2035 any running of the strictness analyser.
2039 The options you are most likely to want to turn off are:
2045 <Option>-fno-strictness</Option><IndexTerm><Primary>-fno-strictness option</Primary></IndexTerm> (strictness
2046 analyser, because it is sometimes slow),
2052 <Option>-fno-specialise</Option><IndexTerm><Primary>-fno-specialise option</Primary></IndexTerm> (automatic
2053 specialisation of overloaded functions, because it can make your code
2054 bigger) (US spelling also accepted), and
2060 <Option>-fno-cpr-analyse</Option><IndexTerm><Primary>-fno-cpr-analyse option</Primary></IndexTerm> switches off the CPR (constructed product
2070 Should you wish to turn individual flags <Emphasis>on</Emphasis>, you are advised
2071 to use the <Option>-Ofile</Option> option, described above. Because the order in
2072 which optimisation passes are run is sometimes crucial, it's quite
2073 hard to do with command-line options.
2077 Here are some “dangerous” optimisations you <Emphasis>might</Emphasis> want to try:
2081 <Term><Option>-fvia-C</Option>:</Term>
2084 <IndexTerm><Primary>-fvia-C option</Primary></IndexTerm>
2085 <IndexTerm><Primary>native code generator, turning off</Primary></IndexTerm>
2089 Compile via C, and don't use the native-code generator. (There are many
2090 cases when GHC does this on its own.) You might pick up a little bit of
2091 speed by compiling via C (e.g. for floating-point intensive code on Intel).
2092 If you use <Function>_casm_</Function>s (which are utterly
2093 deprecated), you probably <Emphasis>have</Emphasis> to use
2094 <Option>-fvia-C</Option>.
2098 The lower-case incantation, <Option>-fvia-c</Option>, is synonymous.
2102 Compiling via C will probably be slower (in compilation time) than
2103 using GHC's native code generator.
2108 <Term><Option>-funfolding-interface-threshold<n></Option>:</Term>
2111 <IndexTerm><Primary>-funfolding-interface-threshold option</Primary></IndexTerm>
2112 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
2113 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
2114 (Default: 30) By raising or lowering this number, you can raise or
2115 lower the amount of pragmatic junk that gets spewed into interface
2116 files. (An unfolding has a “size” that reflects the cost in terms
2117 of “code bloat” of expanding that unfolding in another module. A
2118 bigger function would be assigned a bigger cost.)
2123 <Term><Option>-funfolding-creation-threshold<n></Option>:</Term>
2126 <IndexTerm><Primary>-funfolding-creation-threshold option</Primary></IndexTerm>
2127 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
2128 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
2129 (Default: 30) This option is similar to
2130 <Option>-funfolding-interface-threshold</Option>, except that it governs unfoldings
2131 within a single module. Increasing this figure is more likely to
2132 result in longer compile times than faster code. The next option is
2138 <Term><Option>-funfolding-use-threshold<n></Option>:</Term>
2141 <IndexTerm><Primary>-funfolding-use-threshold option</Primary></IndexTerm>
2142 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
2143 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
2144 (Default: 8) This is the magic cut-off figure for unfolding: below
2145 this size, a function definition will be unfolded at the call-site,
2146 any bigger and it won't. The size computed for a function depends on
2147 two things: the actual size of the expression minus any discounts that
2148 apply (see <Option>-funfolding-con-discount</Option>).
2154 <Term><Option>-funfolding-update-in-place<n></Option>:</Term>
2157 Switches on an experimental "optimisation". Switching it on makes the compiler
2158 a little keener to inline a function that returns a constructor, if the context is
2163 If we inlined plusInt we might get an opportunity to use update-in-place for
2170 <Term><Option>-funbox-strict-fields</Option>:</Term>
2173 <IndexTerm><Primary>-funbox-strict-fields option</Primary></IndexTerm>
2174 <IndexTerm><Primary>strict constructor fields</Primary></IndexTerm>
2175 <IndexTerm><Primary>constructor fields, strict</Primary></IndexTerm>
2179 This option causes all constructor fields which are marked strict
2180 (i.e. “!”) to be unboxed or unpacked if possible. For example:
2186 data T = T !Float !Float
2192 will create a constructor <Literal>T</Literal> containing two unboxed floats if the
2193 <Option>-funbox-strict-fields</Option> flag is given. This may not always be an
2194 optimisation: if the <Function>T</Function> constructor is scrutinised and the floats
2195 passed to a non-strict function for example, they will have to be
2196 reboxed (this is done automatically by the compiler).
2200 This option should only be used in conjunction with <Option>-O</Option>, in order to
2201 expose unfoldings to the compiler so the reboxing can be removed as
2202 often as possible. For example:
2209 f (T f1 f2) = f1 + f2
2215 The compiler will avoid reboxing <Function>f1</Function> and <Function>f2</Function> by inlining <Function>+</Function> on
2216 floats, but only when <Option>-O</Option> is on.
2220 Any single-constructor data is eligible for unpacking; for example
2226 data T = T !(Int,Int)
2232 will store the two <Literal>Int</Literal>s directly in the <Function>T</Function> constructor, by flattening
2233 the pair. Multi-level unpacking is also supported:
2240 data S = S !Int !Int
2246 will store two unboxed <Literal>Int#</Literal>s directly in the <Function>T</Function> constructor.
2251 <Term><Option>-fsemi-tagging</Option>:</Term>
2254 This option (which <Emphasis>does not work</Emphasis> with the native-code generator)
2255 tells the compiler to add extra code to test for already-evaluated
2256 values. You win if you have lots of such values during a run of your
2257 program, you lose otherwise. (And you pay in extra code space.)
2261 We have not played with <Option>-fsemi-tagging</Option> enough to recommend it.
2262 (For all we know, it doesn't even work anymore… Sigh.)
2267 <Term><Option>-fexcess-precision</Option>:</Term>
2270 When this option is given, intermediate floating point values can have
2271 a <Emphasis>greater</Emphasis> precision/range than the final type.
2272 Generally this is a good thing, but some programs may rely on the
2273 exact precision/range of <Literal>Float</Literal>/<Literal>Double</Literal>
2274 values and should not use this option for their compilation.
2284 <Title><Option>-m*</Option>: platform-specific flags</Title>
2287 <IndexTerm><Primary>-m* options (GHC)</Primary></IndexTerm>
2288 <IndexTerm><Primary>platform-specific options</Primary></IndexTerm>
2289 <IndexTerm><Primary>machine-specific options</Primary></IndexTerm>
2293 Some flags only make sense for particular target platforms.
2300 <Term><Option>-mv8</Option>:</Term>
2303 (SPARC machines)<IndexTerm><Primary>-mv8 option (SPARC only)</Primary></IndexTerm>
2304 Means to pass the like-named option to GCC; it says to use the
2305 Version 8 SPARC instructions, notably integer multiply and divide.
2306 The similiar <Option>-m*</Option> GCC options for SPARC also work, actually.
2311 <Term><Option>-mlong-calls</Option>:</Term>
2314 (HPPA machines)<IndexTerm><Primary>-mlong-calls option (HPPA only)</Primary></IndexTerm>
2315 Means to pass the like-named option to GCC. Required for Very Big
2316 modules, maybe. (Probably means you're in trouble…)
2321 <Term><Option>-monly-[32]-regs</Option>:</Term>
2324 (iX86 machines)<IndexTerm><Primary>-monly-N-regs option (iX86 only)</Primary></IndexTerm>
2325 GHC tries to “steal” four registers from GCC, for performance
2326 reasons; it almost always works. However, when GCC is compiling some
2327 modules with four stolen registers, it will crash, probably saying:
2330 Foo.hc:533: fixed or forbidden register was spilled.
2331 This may be due to a compiler bug or to impossible asm
2332 statements or clauses.
2335 Just give some registers back with <Option>-monly-N-regs</Option>. Try `3' first,
2336 then `2'. If `2' doesn't work, please report the bug to us.
2345 <Sect2 id="optimise-C-compiler">
2346 <Title>Code improvement by the C compiler.
2350 <IndexTerm><Primary>optimisation by GCC</Primary></IndexTerm>
2351 <IndexTerm><Primary>GCC optimisation</Primary></IndexTerm>
2355 The C compiler (GCC) is run with <Option>-O</Option> turned on. (It has
2360 If you want to run GCC with <Option>-O2</Option>—which may be worth a few
2361 percent in execution speed—you can give a
2362 <Option>-O2-for-C</Option><IndexTerm><Primary>-O2-for-C option</Primary></IndexTerm> option.
2369 <Sect1 id="options-phases">
2370 <Title>Options related to a particular phase
2373 <Sect2 id="c-pre-processor">
2374 <Title>The C pre-processor
2378 <IndexTerm><Primary>pre-processing: cpp</Primary></IndexTerm>
2379 <IndexTerm><Primary>C pre-processor options</Primary></IndexTerm>
2380 <IndexTerm><Primary>cpp, pre-processing with</Primary></IndexTerm>
2384 The C pre-processor <Command>cpp</Command> is run over your Haskell code only if the
2385 <Option>-cpp</Option> option <IndexTerm><Primary>-cpp option</Primary></IndexTerm> is given. Unless you are
2386 building a large system with significant doses of conditional
2387 compilation, you really shouldn't need it.
2391 <Term><Option>-D<foo></Option>:</Term>
2394 <IndexTerm><Primary>-D<name> option</Primary></IndexTerm>
2395 Define macro <Constant><foo></Constant> in the usual way. NB: does <Emphasis>not</Emphasis> affect
2396 <Option>-D</Option> macros passed to the C compiler when compiling via C! For those,
2397 use the <Option>-optc-Dfoo</Option> hack… (see <XRef LinkEnd="forcing-options-through">).
2402 <Term><Option>-U<foo></Option>:</Term>
2405 <IndexTerm><Primary>-U<name> option</Primary></IndexTerm>
2406 Undefine macro <Command><foo></Command> in the usual way.
2411 <Term><Option>-I<dir></Option>:</Term>
2414 <IndexTerm><Primary>-I<dir> option</Primary></IndexTerm>
2415 Specify a directory in which to look for <Literal>#include</Literal> files, in
2424 The GHC driver pre-defines several macros when processing Haskell
2425 source code (<Filename>.hs</Filename> or <Filename>.lhs</Filename> files):
2432 <Term><Constant>__HASKELL98__</Constant>:</Term>
2435 <IndexTerm><Primary>__HASKELL98__</Primary></IndexTerm>
2436 If defined, this means that GHC supports the language defined by the
2442 <Term><Constant>__HASKELL__=98</Constant>:</Term>
2445 <IndexTerm><Primary>__HASKELL__</Primary></IndexTerm>
2446 In GHC 4.04 and later, the <Constant>__HASKELL__</Constant> macro is defined as having
2447 the value <Constant>98</Constant>.
2452 <Term><Constant>__HASKELL1__</Constant>:</Term>
2455 <IndexTerm><Primary>__HASKELL1__ macro</Primary></IndexTerm>
2456 If defined to <Emphasis>n</Emphasis>, that means GHC supports the Haskell language
2457 defined in the Haskell report version <Emphasis>1.n</Emphasis>. Currently 5. This
2458 macro is deprecated, and will probably disappear in future versions.
2463 <Term><Constant>__GLASGOW_HASKELL__</Constant>:</Term>
2466 <IndexTerm><Primary>__GLASGOW_HASKELL__ macro</Primary></IndexTerm>
2467 For version <Emphasis>n</Emphasis> of the GHC system, this will be <Literal>#define</Literal>d to
2468 <Emphasis>100n</Emphasis>. So, for version 4.00, it is 400.
2472 With any luck, <Constant>__GLASGOW_HASKELL__</Constant> will be undefined in all other
2473 implementations that support C-style pre-processing.
2477 (For reference: the comparable symbols for other systems are:
2478 <Constant>__HUGS__</Constant> for Hugs and <Constant>__HBC__</Constant> for Chalmers.)
2482 NB. This macro is set when pre-processing both Haskell source and C
2483 source, including the C source generated from a Haskell module
2484 (i.e. <Filename>.hs</Filename>, <Filename>.lhs</Filename>, <Filename>.c</Filename> and <Filename>.hc</Filename> files).
2489 <Term><Constant>__CONCURRENT_HASKELL__</Constant>:</Term>
2492 <IndexTerm><Primary>__CONCURRENT_HASKELL__ macro</Primary></IndexTerm>
2493 This symbol is defined when pre-processing Haskell (input) and
2494 pre-processing C (GHC output). Since GHC from verion 4.00 now
2495 supports concurrent haskell by default, this symbol is always defined.
2500 <Term><Constant>__PARALLEL_HASKELL__</Constant>:</Term>
2503 <IndexTerm><Primary>__PARALLEL_HASKELL__ macro</Primary></IndexTerm>
2504 Only defined when <Option>-parallel</Option> is in use! This symbol is defined when
2505 pre-processing Haskell (input) and pre-processing C (GHC output).
2513 Options other than the above can be forced through to the C
2514 pre-processor with the <Option>-opt</Option> flags (see
2515 <XRef LinkEnd="forcing-options-through">).
2519 A small word of warning: <Option>-cpp</Option> is not friendly to “string
2520 gaps”.<IndexTerm><Primary>-cpp vs string gaps</Primary></IndexTerm><IndexTerm><Primary>string gaps vs
2521 -cpp</Primary></IndexTerm>. In other words, strings such as the following:
2535 don't work with <Option>-cpp</Option>; <Filename>/usr/bin/cpp</Filename> elides the
2536 backslash-newline pairs.
2540 However, it appears that if you add a space at the end of the line,
2541 then <Command>cpp</Command> (at least GNU <Command>cpp</Command> and possibly other <Command>cpp</Command>s)
2542 leaves the backslash-space pairs alone and the string gap works as
2548 <Sect2 id="options-C-compiler">
2549 <Title>Options affecting the C compiler (if applicable)
2553 <IndexTerm><Primary>include-file options</Primary></IndexTerm>
2554 <IndexTerm><Primary>C compiler options</Primary></IndexTerm>
2555 <IndexTerm><Primary>GCC options</Primary></IndexTerm>
2559 At the moment, quite a few common C-compiler options are passed on
2560 quietly to the C compilation of Haskell-compiler-generated C files.
2561 THIS MAY CHANGE. Meanwhile, options so sent are:
2568 <ColSpec Align="Left" Colsep="0">
2569 <ColSpec Align="Left" Colsep="0">
2572 <Entry><Option>-ansi</Option> </Entry>
2573 <Entry> do ANSI C (not K&R) </Entry>
2577 <Option>-pedantic</Option> </Entry>
2578 <Entry> be so</Entry>
2582 <Option>-dgcc-lint</Option> </Entry>
2583 <Entry> (hack) short for “make GCC very paranoid”</Entry>
2591 <IndexTerm><Primary>-ansi option (for GCC)</Primary></IndexTerm>
2592 <IndexTerm><Primary>-pedantic option (for GCC)</Primary></IndexTerm>
2593 <IndexTerm><Primary>-dgcc-lint option (GCC paranoia)</Primary></IndexTerm>
2597 If you are compiling with lots of foreign calls, you may need to
2598 tell the C compiler about some <Literal>#include</Literal> files. There is no real
2599 pretty way to do this, but you can use this hack from the
2606 % ghc -c '-#include <X/Xlib.h>' Xstuff.lhs
2613 <Sect2 id="options-linker">
2614 <Title>Linking and consistency-checking
2618 <IndexTerm><Primary>linker options</Primary></IndexTerm>
2619 <IndexTerm><Primary>ld options</Primary></IndexTerm>
2623 GHC has to link your code with various libraries, possibly including:
2624 user-supplied, GHC-supplied, and system-supplied (<Option>-lm</Option> math
2625 library, for example).
2632 <Term><Option>-l<FOO></Option>:</Term>
2635 <IndexTerm><Primary>-l<lib> option</Primary></IndexTerm>
2636 Link in a library named <Filename>lib<FOO>.a</Filename> which resides somewhere on the
2637 library directories path.
2641 Because of the sad state of most UNIX linkers, the order of such
2642 options does matter. Thus: <Command>ghc -lbar *.o</Command> is almost certainly
2643 wrong, because it will search <Filename>libbar.a</Filename> <Emphasis>before</Emphasis> it has
2644 collected unresolved symbols from the <Filename>*.o</Filename> files.
2645 <Command>ghc *.o -lbar</Command> is probably better.
2649 The linker will of course be informed about some GHC-supplied
2650 libraries automatically; these are:
2657 <ColSpec Align="Left" Colsep="0">
2658 <ColSpec Align="Left" Colsep="0">
2661 <Entry><Emphasis>-l equivalent</Emphasis> </Entry>
2662 <Entry> <Emphasis>description</Emphasis> </Entry>
2667 <Option>-lHSrts,-lHSclib</Option> </Entry>
2668 <Entry> basic runtime libraries </Entry>
2672 <Option>-lHS</Option> </Entry>
2673 <Entry> standard Prelude library </Entry>
2677 <Option>-lHS_cbits</Option> </Entry>
2678 <Entry> C support code for standard Prelude library </Entry>
2682 <Option>-lgmp</Option> </Entry>
2683 <Entry> GNU multi-precision library (for Integers)</Entry>
2694 <IndexTerm><Primary>-lHS library</Primary></IndexTerm>
2695 <IndexTerm><Primary>-lHS_cbits library</Primary></IndexTerm>
2696 <IndexTerm><Primary>-lHSrts library</Primary></IndexTerm>
2697 <IndexTerm><Primary>-lgmp library</Primary></IndexTerm>
2702 <Term><Option>-package <name></Option>:</Term>
2705 <IndexTerm><Primary>-package <name> option</Primary></IndexTerm>
2709 If you are using a Haskell “package” (e.g., the POSIX
2710 library), just use the <Option>-package posix</Option> option, and the
2711 correct code should be linked in. See <xref linkend="packages"> for
2717 <Term><Option>-L<dir></Option>:</Term>
2720 <IndexTerm><Primary>-L<dir> option</Primary></IndexTerm>
2721 Where to find user-supplied libraries… Prepend the directory
2722 <Filename><dir></Filename> to the library directories path.
2727 <Term><Option>-static</Option>:</Term>
2730 <IndexTerm><Primary>-static option</Primary></IndexTerm>
2731 Tell the linker to avoid shared libraries.
2736 <Term><Option>-no-link-chk</Option> and <Option>-link-chk</Option>:</Term>
2739 <IndexTerm><Primary>-no-link-chk option</Primary></IndexTerm>
2740 <IndexTerm><Primary>-link-chk option</Primary></IndexTerm>
2741 <IndexTerm><Primary>consistency checking of executables</Primary></IndexTerm>
2742 By default, immediately after linking an executable, GHC verifies that
2743 the pieces that went into it were compiled with compatible flags; a
2744 “consistency check”.
2745 (This is to avoid mysterious failures caused by non-meshing of
2746 incompatibly-compiled programs; e.g., if one <Filename>.o</Filename> file was compiled
2747 for a parallel machine and the others weren't.) You may turn off this
2748 check with <Option>-no-link-chk</Option>. You can turn it (back) on with
2749 <Option>-link-chk</Option> (the default).
2754 <Term><Option>-no-hs-main</Option>:</Term>
2757 <IndexTerm><Primary>-no-hs-main option</Primary></IndexTerm>
2758 <IndexTerm><Primary>linking Haskell libraries with foreign code</Primary></IndexTerm>
2762 In the event you want to include ghc-compiled code as part of another
2763 (non-Haskell) program, the RTS will not be supplying its definition of
2764 <Function>main()</Function> at link-time, you will have to. To signal that to the
2765 driver script when linking, use <Option>-no-hs-main</Option>.
2769 Notice that since the command-line passed to the linker is rather
2770 involved, you probably want to use the ghc driver script to do the
2771 final link of your `mixed-language' application. This is not a
2772 requirement though, just try linking once with <Option>-v</Option> on to see what
2773 options the driver passes through to the linker.
2784 <Sect1 id="sec-using-concurrent">
2785 <Title>Using Concurrent Haskell</Title>
2788 <IndexTerm><Primary>Concurrent Haskell—use</Primary></IndexTerm>
2792 GHC (as of version 4.00) supports Concurrent Haskell by default,
2793 without requiring a special option or libraries compiled in a certain
2794 way. To get access to the support libraries for Concurrent Haskell
2795 (i.e. <Literal>Concurrent</Literal> and friends), use the
2796 <Option>-package concurrent</Option> option.
2800 Three RTS options are provided for modifying the behaviour of the
2801 threaded runtime system. See the descriptions of
2802 <Option>-C[<us>]</Option>, <Option>-q</Option>, and
2803 <Option>-t<num></Option> in <XRef LinkEnd="parallel-rts-opts">.
2807 Concurrent Haskell is described in more detail in <XRef
2808 LinkEnd="sec-Concurrent">.
2813 <Sect1 id="sec-using-parallel">
2814 <Title>Using Parallel Haskell</Title>
2817 <IndexTerm><Primary>Parallel Haskell—use</Primary></IndexTerm>
2821 [You won't be able to execute parallel Haskell programs unless PVM3
2822 (Parallel Virtual Machine, version 3) is installed at your site.]
2826 To compile a Haskell program for parallel execution under PVM, use the
2827 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
2828 option</Primary></IndexTerm> both when compiling <Emphasis>and
2829 linking</Emphasis>. You will probably want to <Literal>import
2830 Parallel</Literal> into your Haskell modules.
2834 To run your parallel program, once PVM is going, just invoke it
2835 “as normal”. The main extra RTS option is
2836 <Option>-N<n></Option>, to say how many PVM
2837 “processors” your program to run on. (For more details of
2838 all relevant RTS options, please see <XRef
2839 LinkEnd="parallel-rts-opts">.)
2843 In truth, running Parallel Haskell programs and getting information
2844 out of them (e.g., parallelism profiles) is a battle with the vagaries of
2845 PVM, detailed in the following sections.
2849 <Title>Dummy's guide to using PVM</Title>
2852 <IndexTerm><Primary>PVM, how to use</Primary></IndexTerm>
2853 <IndexTerm><Primary>Parallel Haskell—PVM use</Primary></IndexTerm>
2854 Before you can run a parallel program under PVM, you must set the
2855 required environment variables (PVM's idea, not ours); something like,
2856 probably in your <Filename>.cshrc</Filename> or equivalent:
2859 setenv PVM_ROOT /wherever/you/put/it
2860 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
2861 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
2867 Creating and/or controlling your “parallel machine” is a purely-PVM
2868 business; nothing specific to Parallel Haskell.
2872 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
2873 machine. You can then do various things to control/monitor your
2874 “parallel machine;” the most useful being:
2880 <ColSpec Align="Left">
2884 <Entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></Entry>
2885 <Entry>exit <Command>pvm</Command>, leaving it running</Entry>
2889 <Entry><Command>halt</Command></Entry>
2890 <Entry>kill off this “parallel machine” & exit</Entry>
2894 <Entry><Command>add <host></Command></Entry>
2895 <Entry>add <Command><host></Command> as a processor</Entry>
2899 <Entry><Command>delete <host></Command></Entry>
2900 <Entry>delete <Command><host></Command></Entry>
2904 <Entry><Command>reset</Command></Entry>
2905 <Entry>kill what's going, but leave PVM up</Entry>
2909 <Entry><Command>conf</Command></Entry>
2910 <Entry>list the current configuration</Entry>
2914 <Entry><Command>ps</Command></Entry>
2915 <Entry>report processes' status</Entry>
2919 <Entry><Command>pstat <pid></Command></Entry>
2920 <Entry>status of a particular process</Entry>
2929 The PVM documentation can tell you much, much more about <Command>pvm</Command>!
2935 <Title>Parallelism profiles</Title>
2938 <IndexTerm><Primary>parallelism profiles</Primary></IndexTerm>
2939 <IndexTerm><Primary>profiles, parallelism</Primary></IndexTerm>
2940 <IndexTerm><Primary>visualisation tools</Primary></IndexTerm>
2944 With Parallel Haskell programs, we usually don't care about the
2945 results—only with “how parallel” it was! We want pretty pictures.
2949 Parallelism profiles (à la <Command>hbcpp</Command>) can be generated with the
2950 <Option>-q</Option><IndexTerm><Primary>-q RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option. The
2951 per-processor profiling info is dumped into files named
2952 <Filename><full-path><program>.gr</Filename>. These are then munged into a PostScript picture,
2953 which you can then display. For example, to run your program
2954 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
2960 % ./a.out +RTS -N8 -q
2961 % grs2gr *.???.gr > temp.gr # combine the 8 .gr files into one
2962 % gr2ps -O temp.gr # cvt to .ps; output in temp.ps
2963 % ghostview -seascape temp.ps # look at it!
2969 The scripts for processing the parallelism profiles are distributed
2970 in <Filename>ghc/utils/parallel/</Filename>.
2976 <Title>Other useful info about running parallel programs</Title>
2979 The “garbage-collection statistics” RTS options can be useful for
2980 seeing what parallel programs are doing. If you do either
2981 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
2982 you'll get mutator, garbage-collection, etc., times on standard
2983 error. The standard error of all PE's other than the `main thread'
2984 appears in <Filename>/tmp/pvml.nnn</Filename>, courtesy of PVM.
2988 Whether doing <Option>+RTS -Sstderr</Option> or not, a handy way to watch
2989 what's happening overall is: <Command>tail -f /tmp/pvml.nnn</Command>.
2994 <Sect2 id="parallel-rts-opts">
2995 <Title>RTS options for Concurrent/Parallel Haskell
2999 <IndexTerm><Primary>RTS options, concurrent</Primary></IndexTerm>
3000 <IndexTerm><Primary>RTS options, parallel</Primary></IndexTerm>
3001 <IndexTerm><Primary>Concurrent Haskell—RTS options</Primary></IndexTerm>
3002 <IndexTerm><Primary>Parallel Haskell—RTS options</Primary></IndexTerm>
3006 Besides the usual runtime system (RTS) options
3007 (<XRef LinkEnd="runtime-control">), there are a few options particularly
3008 for concurrent/parallel execution.
3015 <Term><Option>-N<N></Option>:</Term>
3018 <IndexTerm><Primary>-N<N> RTS option (parallel)</Primary></IndexTerm>
3019 (PARALLEL ONLY) Use <Literal><N></Literal> PVM processors to run this program;
3025 <Term><Option>-C[<us>]</Option>:</Term>
3028 <IndexTerm><Primary>-C<us> RTS option</Primary></IndexTerm> Sets
3029 the context switch interval to <Literal><s></Literal> seconds.
3030 A context switch will occur at the next heap block allocation after
3031 the timer expires (a heap block allocation occurs every 4k of
3032 allocation). With <Option>-C0</Option> or <Option>-C</Option>,
3033 context switches will occur as often as possible (at every heap block
3034 allocation). By default, context switches occur every 20ms
3035 milliseconds. Note that GHC's internal timer ticks every 20ms, and
3036 the context switch timer is always a multiple of this timer, so 20ms
3037 is the maximum granularity available for timed context switches.
3042 <Term><Option>-q[v]</Option>:</Term>
3045 <IndexTerm><Primary>-q RTS option</Primary></IndexTerm>
3046 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
3047 in the file <FIlename><program>.qp</FIlename>. In the style of <Command>hbcpp</Command>, this profile
3048 records the movement of threads between the green (runnable) and red
3049 (blocked) queues. If you specify the verbose suboption (<Option>-qv</Option>), the
3050 green queue is split into green (for the currently running thread
3051 only) and amber (for other runnable threads). We do not recommend
3052 that you use the verbose suboption if you are planning to use the
3053 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
3054 check (with <Option>-C</Option>).
3059 <Term><Option>-t<num></Option>:</Term>
3062 <IndexTerm><Primary>-t<num> RTS option</Primary></IndexTerm>
3063 (PARALLEL ONLY) Limit the number of concurrent threads per processor
3064 to <Literal><num></Literal>. The default is 32. Each thread requires slightly over 1K
3065 <Emphasis>words</Emphasis> in the heap for thread state and stack objects. (For
3066 32-bit machines, this translates to 4K bytes, and for 64-bit machines,
3072 <Term><Option>-d</Option>:</Term>
3075 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
3076 (PARALLEL ONLY) Turn on debugging. It pops up one xterm (or GDB, or
3077 something…) per PVM processor. We use the standard <Command>debugger</Command>
3078 script that comes with PVM3, but we sometimes meddle with the
3079 <Command>debugger2</Command> script. We include ours in the GHC distribution,
3080 in <Filename>ghc/utils/pvm/</Filename>.
3085 <Term><Option>-e<num></Option>:</Term>
3088 <IndexTerm><Primary>-e<num> RTS option (parallel)</Primary></IndexTerm>
3089 (PARALLEL ONLY) Limit the number of pending sparks per processor to
3090 <Literal><num></Literal>. The default is 100. A larger number may be appropriate if
3091 your program generates large amounts of parallelism initially.
3096 <Term><Option>-Q<num></Option>:</Term>
3099 <IndexTerm><Primary>-Q<num> RTS option (parallel)</Primary></IndexTerm>
3100 (PARALLEL ONLY) Set the size of packets transmitted between processors
3101 to <Literal><num></Literal>. The default is 1024 words. A larger number may be
3102 appropriate if your machine has a high communication cost relative to
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