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…
362 FURTHER EXOTICA: If you are doing a normal <Filename>.hs</Filename>-to-<Filename>.o</Filename> compilation
363 but would like to hang onto the intermediate <Filename>.hc</Filename> C file, just
364 throw in a <Option>-keep-hc-file-too</Option> option<IndexTerm><Primary>-keep-hc-file-too option</Primary></IndexTerm>.
365 If you would like to look at the assembler output, toss in a
366 <Option>-keep-s-file-too</Option>,<IndexTerm><Primary>-keep-s-file-too option</Primary></IndexTerm> too.
369 <Sect2 id="saving-ghc-stderr">
370 <Title>Saving GHC's standard error output
374 <IndexTerm><Primary>standard error, saving</Primary></IndexTerm>
378 Sometimes, you may cause GHC to be rather chatty on standard error;
379 with <Option>-v</Option>, for example. You can instruct GHC to <Emphasis>append</Emphasis> this
380 output to a particular log file with a <Option>-odump <blah></Option><IndexTerm><Primary>-odump
381 <blah> option</Primary></IndexTerm> option.
386 <Sect2 id="temp-files">
387 <Title>Redirecting temporary files
391 <IndexTerm><Primary>temporary files, redirecting</Primary></IndexTerm>
395 If you have trouble because of running out of space in <Filename>/tmp</Filename> (or
396 wherever your installation thinks temporary files should go), you may
397 use the <Option>-tmpdir <dir></Option><IndexTerm><Primary>-tmpdir <dir> option</Primary></IndexTerm> option
398 to specify an alternate directory. For example, <Option>-tmpdir .</Option> says to
399 put temporary files in the current working directory.
403 Alternatively, use your <Constant>TMPDIR</Constant> environment variable.<IndexTerm><Primary>TMPDIR
404 environment variable</Primary></IndexTerm> Set it to the name of the directory where
405 temporary files should be put. GCC and other programs will honour the
406 <Constant>TMPDIR</Constant> variable as well.
410 Even better idea: Set the <Constant>TMPDIR</Constant> variable when building GHC, and
411 never worry about <Constant>TMPDIR</Constant> again. (see the build documentation).
418 <Sect1 id="options-sanity">
419 <Title>Warnings and sanity-checking
423 <IndexTerm><Primary>sanity-checking options</Primary></IndexTerm>
424 <IndexTerm><Primary>warnings</Primary></IndexTerm>
425 GHC has a number of options that select which types of non-fatal error
426 messages, otherwise known as warnings, can be generated during
427 compilation. By default, you get a standard set of warnings which are
428 generally likely to indicate bugs in your program. These are:
429 <Option>-fwarn-overlpapping-patterns</Option>, <Option>-fwarn-duplicate-exports</Option>, and
430 <Option>-fwarn-missing-methods</Option>. The following flags are simple ways to
431 select standard “packages” of warnings:
438 <Term><Option>-Wnot</Option>:</Term>
441 <IndexTerm><Primary>-Wnot option</Primary></IndexTerm>
442 Turns off all warnings, including the standard ones.
448 <Term><Option>-w</Option>:</Term>
451 <IndexTerm><Primary>-w option</Primary></IndexTerm>
452 Synonym for <Option>-Wnot</Option>.
458 <Term><Option>-W</Option>:</Term>
461 <IndexTerm><Primary>-W option</Primary></IndexTerm>
462 Provides the standard warnings plus <Option>-fwarn-incomplete-patterns</Option>,
463 <Option>-fwarn-unused-imports</Option> and <Option>-fwarn-unused-binds</Option>.
469 <Term><Option>-Wall</Option>:</Term>
472 <IndexTerm><Primary>-Wall option</Primary></IndexTerm>
473 Turns on all warning options.
482 The full set of warning options is described below. To turn off any
483 warning, simply give the corresponding <Option>-fno-warn-...</Option> option on
491 <Term><Option>-fwarn-name-shadowing</Option>:</Term>
494 <IndexTerm><Primary>-fwarn-name-shadowing option</Primary></IndexTerm>
495 <IndexTerm><Primary>shadowing, warning</Primary></IndexTerm>This option causes a warning to be emitted whenever an inner-scope
496 value has the same name as an outer-scope value, i.e. the inner value
497 shadows the outer one. This can catch typographical errors that turn
498 into hard-to-find bugs, e.g., in the inadvertent cyclic definition
499 <Literal>let x = ... x ... in</Literal>.
503 Consequently, this option does <Emphasis>not</Emphasis> allow cyclic recursive
510 <Term><Option>-fwarn-overlapping-patterns</Option>:</Term>
513 <IndexTerm><Primary>-fwarn-overlapping-patterns option</Primary></IndexTerm>
514 <IndexTerm><Primary>overlapping patterns, warning</Primary></IndexTerm>
515 <IndexTerm><Primary>patterns, overlapping</Primary></IndexTerm>
516 By default, the compiler will warn you if a set of patterns are
522 f :: String -> Int
530 where the last pattern match in <Function>f</Function> won't ever be reached, as the
531 second pattern overlaps it. More often than not, redundant patterns
532 is a programmer mistake/error, so this option is enabled by default.
538 <Term><Option>-fwarn-incomplete-patterns</Option>:</Term>
541 <IndexTerm><Primary>-fwarn-incomplete-patterns option</Primary></IndexTerm>
542 <IndexTerm><Primary>incomplete patterns, warning</Primary></IndexTerm>
543 <IndexTerm><Primary>patterns, incomplete</Primary></IndexTerm>
544 Similarly for incomplete patterns, the function <Function>g</Function> below will fail
545 when applied to non-empty lists, so the compiler will emit a warning
546 about this when <Option>-fwarn-incomplete-patterns</Option> is enabled.
556 This option isn't enabled be default because it can be a bit noisy,
557 and it doesn't always indicate a bug in the program. However, it's
558 generally considered good practice to cover all the cases in your
565 <Term><Option>-fwarn-missing-methods</Option>:</Term>
568 <IndexTerm><Primary>-fwarn-missing-methods option</Primary></IndexTerm>
569 <IndexTerm><Primary>missing methods, warning</Primary></IndexTerm>
570 <IndexTerm><Primary>methods, missing</Primary></IndexTerm>
571 This option is on by default, and warns you whenever an instance
572 declaration is missing one or more methods, and the corresponding
573 class declaration has no default declaration for them.
579 <Term><Option>-fwarn-missing-fields</Option>:</Term>
582 <IndexTerm><Primary>-fwarn-missing-fields option</Primary></IndexTerm>
583 <IndexTerm><Primary>missing fields, warning</Primary></IndexTerm>
584 <IndexTerm><Primary>fields, missing</Primary></IndexTerm>
585 This option is on by default, and warns you whenever the construction
586 of a labelled field constructor isn't complete, missing initializers
587 for one or more fields. While not an error (the missing fields are
588 initialised with bottoms), it is often an indication of a programmer
595 <Term><Option>-fwarn-unused-imports</Option>:</Term>
598 <IndexTerm><Primary>-fwarn-unused-imports option</Primary></IndexTerm>
599 <IndexTerm><Primary>unused imports, warning</Primary></IndexTerm>
600 <IndexTerm><Primary>imports, unused</Primary></IndexTerm>
601 Report any objects that are explicitly imported but never used.
607 <Term><Option>-fwarn-unused-binds</Option>:</Term>
610 <IndexTerm><Primary>-fwarn-unused-binds option</Primary></IndexTerm>
611 <IndexTerm><Primary>unused binds, warning</Primary></IndexTerm>
612 <IndexTerm><Primary>binds, unused</Primary></IndexTerm>
613 Report any function definitions (and local bindings) which are unused.
614 For top-level functions, the warning is only given if the binding is
620 <Term><Option>-fwarn-unused-matches</Option>:</Term>
623 <IndexTerm><Primary>-fwarn-unused-matches option</Primary></IndexTerm>
624 <IndexTerm><Primary>unused matches, warning</Primary></IndexTerm>
625 <IndexTerm><Primary>matches, unused</Primary></IndexTerm>
626 Report all unused variables which arise from pattern matches,
627 including patterns consisting of a single variable. For instance <Literal>f x
628 y = []</Literal> would report <VarName>x</VarName> and <VarName>y</VarName> as unused. To eliminate the warning,
629 all unused variables can be replaced with wildcards.
635 <Term><Option>-fwarn-duplicate-exports</Option>:</Term>
638 <IndexTerm><Primary>-fwarn-duplicate-exports option</Primary></IndexTerm>
639 <IndexTerm><Primary>duplicate exports, warning</Primary></IndexTerm>
640 <IndexTerm><Primary>export lists, duplicates</Primary></IndexTerm>
641 Have the compiler warn about duplicate entries in export lists. This
642 is useful information if you maintain large export lists, and want to
643 avoid the continued export of a definition after you've deleted (one)
644 mention of it in the export list.
648 This option is on by default.
654 <Term><Option>-fwarn-type-defaults</Option>:</Term>
657 <IndexTerm><Primary>-fwarn-type-defaults option</Primary></IndexTerm>
658 <IndexTerm><Primary>defaulting mechanism, warning</Primary></IndexTerm>
659 Have the compiler warn/inform you where in your source the Haskell
660 defaulting mechanism for numeric types kicks in. This is useful
661 information when converting code from a context that assumed one
662 default into one with another, e.g., the `default default' for Haskell
663 1.4 caused the otherwise unconstrained value <Constant>1</Constant> to be given
664 the type <Literal>Int</Literal>, whereas Haskell 98 defaults it to
665 <Literal>Integer</Literal>. This may lead to differences in performance and
666 behaviour, hence the usefulness of being non-silent about this.
670 This warning is off by default.
676 <Term><Option>-fwarn-missing-signatures</Option>:</Term>
679 <IndexTerm><Primary>-fwarn-missing-signatures option</Primary></IndexTerm>
680 <IndexTerm><Primary>type signatures, missing</Primary></IndexTerm>
681 If you would like GHC to check that every top-level function/value has
682 a type signature, use the <Option>-fwarn-missing-signatures</Option> option. This
683 option is off by default.
691 If you're feeling really paranoid, the <Option>-dcore-lint</Option>
692 option<IndexTerm><Primary>-dcore-lint option</Primary></IndexTerm> is a good choice. It turns on
693 heavyweight intra-pass sanity-checking within GHC. (It checks GHC's
699 <Sect1 id="separate-compilation">
700 <Title>Separate compilation
704 <IndexTerm><Primary>separate compilation</Primary></IndexTerm>
705 <IndexTerm><Primary>recompilation checker</Primary></IndexTerm>
706 <IndexTerm><Primary>make and recompilation</Primary></IndexTerm>
710 This section describes how GHC supports separate compilation.
713 <Sect2 id="hi-files">
714 <Title>Interface files
718 <IndexTerm><Primary>interface files</Primary></IndexTerm>
719 <IndexTerm><Primary>.hi files</Primary></IndexTerm>
723 When GHC compiles a source file <Filename>F</Filename> which contains a module <Literal>A</Literal>, say,
724 it generates an object <Filename>F.o</Filename>, <Emphasis>and</Emphasis> a companion <Emphasis>interface
725 file</Emphasis> <Filename>A.hi</Filename>. The interface file is not intended for human
726 consumption, as you'll see if you take a look at one. It's merely
727 there to help the compiler compile other modules in the same program.
731 NOTE: Having the name of the interface file follow the module name and
732 not the file name, means that working with tools such as <Command>make</Command>
733 become harder. <Command>make</Command> implicitly assumes that any output files
734 produced by processing a translation unit will have file names that
735 can be derived from the file name of the translation unit. For
736 instance, pattern rules becomes unusable. For this reason, we
737 recommend you stick to using the same file name as the module name.
741 The interface file for <Literal>A</Literal> contains information needed by the compiler
742 when it compiles any module <Literal>B</Literal> that imports <Literal>A</Literal>, whether directly or
743 indirectly. When compiling <Literal>B</Literal>, GHC will read <Filename>A.hi</Filename> to find the
744 details that it needs to know about things defined in <Literal>A</Literal>.
748 Furthermore, when compiling module <Literal>C</Literal> which imports <Literal>B</Literal>, GHC may
749 decide that it needs to know something about <Literal>A</Literal>—for example, <Literal>B</Literal>
750 might export a function that involves a type defined in <Literal>A</Literal>. In this
751 case, GHC will go and read <Command>A.hi</Command> even though <Literal>C</Literal> does not explicitly
752 import <Literal>A</Literal> at all.
756 The interface file may contain all sorts of things that aren't
757 explicitly exported from <Literal>A</Literal> by the programmer. For example, even
758 though a data type is exported abstractly, <Filename>A.hi</Filename> will contain the
759 full data type definition. For small function definitions, <Filename>A.hi</Filename>
760 will contain the complete definition of the function. For bigger
761 functions, <Filename>A.hi</Filename> will contain strictness information about the
762 function. And so on. GHC puts much more information into <Filename>.hi</Filename> files
763 when optimisation is turned on with the <Option>-O</Option> flag. Without <Option>-O</Option> it
764 puts in just the minimum; with <Option>-O</Option> it lobs in a whole pile of stuff.
765 <IndexTerm><Primary>optimsation, effect on .hi files</Primary></IndexTerm>
769 <Filename>A.hi</Filename> should really be thought of as a compiler-readable version of
770 <Filename>A.o</Filename>. If you use a <Filename>.hi</Filename> file that wasn't generated by the same
771 compilation run that generates the <Filename>.o</Filename> file the compiler may assume
772 all sorts of incorrect things about <Literal>A</Literal>, resulting in core dumps and
773 other unpleasant happenings.
778 <Sect2 id="options-finding-imports">
779 <Title>Finding interface files
783 <IndexTerm><Primary>interface files, finding them</Primary></IndexTerm>
784 <IndexTerm><Primary>finding interface files</Primary></IndexTerm>
788 In your program, you import a module <Literal>Foo</Literal> by saying
789 <Literal>import Foo</Literal>. GHC goes looking for an interface file, <Filename>Foo.hi</Filename>.
790 It has a builtin list of directories (notably including <Filename>.</Filename>) where
798 <Term><Option>-i<dirs></Option></Term>
801 <IndexTerm><Primary>-i<dirs> option</Primary></IndexTerm>This flag
802 prepends a colon-separated list of <Filename>dirs</Filename> to the “import
803 directories” list.
804 See also <XRef LinkEnd="recomp"> for the significance of using
805 relative and absolute pathnames in the <Option>-i</Option> list.
811 <Term><Option>-i</Option></Term>
814 resets the “import directories” list back to nothing.
820 <Term><Option>-fno-implicit-prelude</Option></Term>
823 <IndexTerm><Primary>-fno-implicit-prelude option</Primary></IndexTerm>
824 GHC normally imports <Filename>Prelude.hi</Filename> files for you. If you'd rather it
825 didn't, then give it a <Option>-fno-implicit-prelude</Option> option. You are
826 unlikely to get very far without a Prelude, but, hey, it's a free
833 <Term><Option>-syslib <lib></Option></Term>
836 <IndexTerm><Primary>-syslib <lib> option</Primary></IndexTerm>
837 If you are using a system-supplied non-Prelude library (e.g., the
838 POSIX library), just use a <Option>-syslib posix</Option> option (for
839 example). The right interface files should then be available. The
840 accompanying HsLibs document lists the libraries available by this
847 <Term><Option>-I<dir></Option></Term>
850 <IndexTerm><Primary>-I<dir> option</Primary></IndexTerm>
851 Once a Haskell module has been compiled to C (<Filename>.hc</Filename> file), you may
852 wish to specify where GHC tells the C compiler to look for <Filename>.h</Filename> files.
853 (Or, if you are using the <Option>-cpp</Option> option<IndexTerm><Primary>-cpp option</Primary></IndexTerm>, where
854 it tells the C pre-processor to look…) For this purpose, use a <Option>-I</Option>
855 option in the usual C-ish way.
865 <Sect2 id="hi-options">
866 <Title>Other options related to interface files
870 <IndexTerm><Primary>interface files, options</Primary></IndexTerm>
871 The interface output may be directed to another file
872 <Filename>bar2/Wurble.iface</Filename> with the option <Option>-ohi bar2/Wurble.iface</Option><IndexTerm><Primary>-ohi
873 <file> option</Primary></IndexTerm> (not recommended).
877 To avoid generating an interface file at all, use a <Option>-nohi</Option>
878 option.<IndexTerm><Primary>-nohi option</Primary></IndexTerm>
882 The compiler does not overwrite an existing <Filename>.hi</Filename> interface file if
883 the new one is byte-for-byte the same as the old one; this is friendly
884 to <Command>make</Command>. When an interface does change, it is often enlightening to
885 be informed. The <Option>-hi-diffs</Option><IndexTerm><Primary>-hi-diffs option</Primary></IndexTerm> option will
886 make GHC run <Command>diff</Command> on the old and new <Filename>.hi</Filename> files. You can also
887 record the difference in the interface file itself, the
888 <Option>-keep-hi-diffs</Option><IndexTerm><Primary>-keep-hi-diffs</Primary></IndexTerm> option takes care of that.
892 The <Filename>.hi</Filename> files from GHC contain “usage” information which changes
893 often and uninterestingly. If you really want to see these changes
894 reported, you need to use the
895 <Option>-hi-diffs-with-usages</Option><IndexTerm><Primary>-hi-diffs-with-usages option</Primary></IndexTerm>
900 Interface files are normally jammed full of compiler-produced
901 <Emphasis>pragmas</Emphasis>, which record arities, strictness info, etc. If you
902 think these pragmas are messing you up (or you are doing some kind of
903 weird experiment), you can tell GHC to ignore them with the
904 <Option>-fignore-interface-pragmas</Option><IndexTerm><Primary>-fignore-interface-pragmas
905 option</Primary></IndexTerm> option.
909 When compiling without optimisations on, the compiler is extra-careful
910 about not slurping in data constructors and instance declarations that
911 it will not need. If you believe it is getting it wrong and not
912 importing stuff which you think it should, this optimisation can be
913 turned off with <Option>-fno-prune-tydecls</Option> and <Option>-fno-prune-instdecls</Option>.
914 <IndexTerm><Primary>-fno-prune-tydecls option</Primary></IndexTerm><IndexTerm><Primary>-fno-prune-instdecls
915 option</Primary></IndexTerm>
919 See also <XRef LinkEnd="options-linker">, which describes how the linker finds standard
926 <Title>The recompilation checker
929 <IndexTerm><Primary>recompilation checker</Primary></IndexTerm>
934 <Term><Option>-recomp</Option></Term>
935 <IndexTerm><Primary><option>-recomp</option> option</Primary></IndexTerm>
938 (On by default) Turn on recompilation checking. This will stop
939 compilation early, leaving an existing <filename>.o</filename> file in
940 place, if it can be determined that the module does not need to be
946 <Term><Option>-no-recomp</Option></Term>
947 <IndexTerm><Primary><option>-recomp</option> option</Primary></IndexTerm>
950 Turn off recompilation checking.
958 In the olden days, GHC compared the newly-generated
959 <Filename>.hi</Filename> file with the previous version; if they were
960 identical, it left the old one alone and didn't change its
961 modification date. In consequence, importers of a module with an
962 unchanged output <Filename>.hi</Filename> file were not recompiled.
966 This doesn't work any more. In our earlier example, module
967 <Literal>C</Literal> does not import module <Literal>A</Literal>
968 directly, yet changes to <Filename>A.hi</Filename> should force a
969 recompilation of <Literal>C</Literal>. And some changes to
970 <Literal>A</Literal> (changing the definition of a function that
971 appears in an inlining of a function exported by <Literal>B</Literal>,
972 say) may conceivably not change <Filename>B.hi</Filename> one jot. So
977 GHC keeps a version number on each interface file, and on each type
978 signature within the interface file. It also keeps in every interface
979 file a list of the version numbers of everything it used when it last
980 compiled the file. If the source file's modification date is earlier
981 than the <Filename>.o</Filename> file's date (i.e. the source hasn't
982 changed since the file was last compiled), and the
983 <option>-recomp</option> is given on the command line, GHC will be
984 clever. It compares the version numbers on the things it needs this
985 time with the version numbers on the things it needed last time
986 (gleaned from the interface file of the module being compiled); if
987 they are all the same it stops compiling rather early in the process
988 saying “Compilation IS NOT required”. What a beautiful
993 GHC <Emphasis>only</Emphasis> keeps detailed dependency information
994 for “user” modules, not for “Prelude” modules.
995 It distinguishes Prelude modules because their names start with
996 "Prel", so don't start your modules that way!
998 <!-- April 2000: hack disabled. Now we use a different hack!
1000 It distinguishes the two by a hack: a module whose
1001 <Filename>.hi</Filename> file has an absolute path name is considered
1002 a library module, while a relative path name indicates a user module.
1003 So if you have a multi-directory application, use
1004 <Emphasis>relative</Emphasis> path names in your <Option>-i</Option>
1005 path, to force GHC to record detailed dependency information. Use
1006 absolute path names only for directories containing slowly-changing
1011 A path is considered “absolute” if it starts with
1012 “<Filename>/</Filename>”, or
1013 “<Filename>A:/</Filename>”, or
1014 “<Filename>A:\</Filename>” (or
1015 “<Filename>B:/</Filename>”,
1016 “<Filename>B:\</Filename>” etc).
1021 Patrick Sansom had a workshop paper about how all this is done (though
1022 the details have changed quite a bit). <ULink URL="mailto:sansom@dcs.gla.ac.uk">Ask him</ULink> if you want a copy.
1027 <Sect2 id="using-make">
1028 <Title>Using <Command>make</Command>
1032 <IndexTerm><Primary><literal>make</literal></Primary></IndexTerm>
1036 It is reasonably straightforward to set up a <Filename>Makefile</Filename> to use with
1037 GHC, assuming you name your source files the same as your modules.
1045 HC_OPTS = -cpp $(EXTRA_HC_OPTS)
1047 SRCS = Main.lhs Foo.lhs Bar.lhs
1048 OBJS = Main.o Foo.o Bar.o
1050 .SUFFIXES : .o .hi .lhs .hc .s
1054 $(HC) -o $@ $(HC_OPTS) $(OBJS)
1056 # Standard suffix rules
1061 $(HC) -c $< $(HC_OPTS)
1064 $(HC) -c $< $(HC_OPTS)
1066 # Inter-module dependencies
1067 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
1068 Main.o Main.hc Main.s : Foo.hi Baz.hi # Main imports Foo and Baz
1074 (Sophisticated <Command>make</Command> variants may achieve some of the above more
1075 elegantly. Notably, <Command>gmake</Command>'s pattern rules let you write the more
1083 $(HC) -c $< $(HC_OPTS)
1089 What we've shown should work with any <Command>make</Command>.)
1093 Note the cheesy <Literal>.o.hi</Literal> rule: It records the dependency of the
1094 interface (<Filename>.hi</Filename>) file on the source. The rule says a <Filename>.hi</Filename> file can
1095 be made from a <Filename>.o</Filename> file by doing…nothing. Which is true.
1099 Note the inter-module dependencies at the end of the Makefile, which
1106 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
1112 They tell <Command>make</Command> that if any of <Literal>Foo.o</Literal>, <Literal>Foo.hc</Literal> or <Literal>Foo.s</Literal> have an
1113 earlier modification date than <Literal>Baz.hi</Literal>, then the out-of-date file
1114 must be brought up to date. To bring it up to date, <Literal>make</Literal> looks for
1115 a rule to do so; one of the preceding suffix rules does the job
1120 Putting inter-dependencies of the form <Literal>Foo.o : Bar.hi</Literal> into your
1121 <Filename>Makefile</Filename> by hand is rather error-prone. Don't worry—never fear,
1122 <Command>mkdependHS</Command> is here! (and is distributed as part of GHC) Add the
1123 following to your <Filename>Makefile</Filename>:
1130 mkdependHS -- $(HC_OPTS) -- $(SRCS)
1136 Now, before you start compiling, and any time you change the <Literal>imports</Literal>
1137 in your program, do <Command>make depend</Command> before you do <Command>make cool_pgm</Command>.
1138 <Command>mkdependHS</Command> will append the needed dependencies to your <Filename>Makefile</Filename>.
1139 <Command>mkdependHS</Command> is fully described in <XRef LinkEnd="mkdependHS">.
1143 A few caveats about this simple scheme:
1152 You may need to compile some modules explicitly to create their
1153 interfaces in the first place (e.g., <Command>make Bar.o</Command> to create <Filename>Bar.hi</Filename>).
1160 You may have to type <Command>make</Command> more than once for the dependencies
1161 to have full effect. However, a <Command>make</Command> run that does nothing
1162 <Emphasis>does</Emphasis> mean “everything's up-to-date.”
1169 This scheme will work with mutually-recursive modules but,
1170 again, it may take multiple iterations to “settle.”
1181 <Sect2 id="mutual-recursion">
1182 <Title>How to compile mutually recursive modules
1186 <IndexTerm><Primary>module system, recursion</Primary></IndexTerm>
1187 <IndexTerm><Primary>recursion, between modules</Primary></IndexTerm>
1191 Currently, the compiler does not have proper support for dealing with
1192 mutually recursive modules:
1202 newtype TA = MkTA Int
1220 When compiling either module A and B, the compiler will try (in vain)
1221 to look for the interface file of the other. So, to get mutually
1222 recursive modules off the ground, you need to hand write an interface
1223 file for A or B, so as to break the loop. These hand-written
1224 interface files are called <Literal>hi-boot</Literal> files, and are placed in a file
1225 called <Filename><module>.hi-boot</Filename>. To import from an <Literal>hi-boot</Literal> file instead
1226 of the standard <Filename>.hi</Filename> file, use the following syntax in the importing module:
1227 <IndexTerm><Primary>hi-boot files</Primary></IndexTerm>
1228 <IndexTerm><Primary>importing, hi-boot files</Primary></IndexTerm>
1234 import {-# SOURCE #-} A
1240 The hand-written interface need only contain the bare minimum of
1241 information needed to get the bootstrapping process started. For
1242 example, it doesn't need to contain declarations for <Emphasis>everything</Emphasis>
1243 that module <Literal>A</Literal> exports, only the things required by the module that
1244 imports <Literal>A</Literal> recursively.
1248 For the example at hand, the boot interface file for A would look like
1255 __interface A 1 404 where
1256 __export A TA{MkTA} ;
1257 1 newtype TA = MkTA PrelBase.Int ;
1263 The syntax is essentially the same as a normal <Filename>.hi</Filename> file
1264 (unfortunately), but you can usually tailor an existing <Filename>.hi</Filename> file to
1265 make a <Filename>.hi-boot</Filename> file.
1269 Notice that we only put the declaration for the newtype <Literal>TA</Literal> in the
1270 <Literal>hi-boot</Literal> file, not the signature for <Function>f</Function>, since <Function>f</Function> isn't used by
1271 <Literal>B</Literal>.
1275 The number “1” after “__interface A” gives the version number of module A;
1276 it is incremented whenever anything in A's interface file changes. The “404” is
1277 the version number of the interface file <Emphasis>syntax</Emphasis>; we change it when
1278 we change the syntax of interface files so that you get a better error message when
1279 you try to read an old-format file with a new-format compiler.
1283 The number “1” at the beginning of a declaration is the <Emphasis>version
1284 number</Emphasis> of that declaration: for the purposes of <Filename>.hi-boot</Filename> files
1285 these can all be set to 1. All names must be fully qualified with the
1286 <Emphasis>original</Emphasis> module that an object comes from: for example, the
1287 reference to <Literal>Int</Literal> in the interface for <Literal>A</Literal> comes from <Literal>PrelBase</Literal>,
1288 which is a module internal to GHC's prelude. It's a pain, but that's
1293 If you want an hi-boot file to export a data type, but you don't want to give its constructors
1294 (because the constructors aren't used by the SOURCE-importing module), you can write simply:
1300 __interface A 1 404 where
1308 (You must write all the type parameters, but leave out the '=' and everything that follows it.)
1312 <Emphasis>Note:</Emphasis> This is all a temporary solution, a version of the
1313 compiler that handles mutually recursive modules properly without the manual
1314 construction of interface files, is (allegedly) in the works.
1321 <Sect1 id="options-optimise">
1322 <Title>Optimisation (code improvement)
1326 <IndexTerm><Primary>optimisation (GHC)</Primary></IndexTerm>
1327 <IndexTerm><Primary>improvement, code (GHC)</Primary></IndexTerm>
1331 The <Option>-O*</Option> options specify convenient “packages” of optimisation
1332 flags; the <Option>-f*</Option> options described later on specify
1333 <Emphasis>individual</Emphasis> optimisations to be turned on/off; the <Option>-m*</Option>
1334 options specify <Emphasis>machine-specific</Emphasis> optimisations to be turned
1338 <Sect2 id="optimise-pkgs">
1339 <Title><Option>-O*</Option>: convenient “packages” of optimisation flags.
1343 <IndexTerm><Primary>-O options</Primary></IndexTerm>
1347 There are <Emphasis>many</Emphasis> options that affect the quality of code
1348 produced by GHC. Most people only have a general goal, something like
1349 “Compile quickly” or “Make my program run like greased lightning.”
1350 The following “packages” of optimisations (or lack thereof) should
1355 Once you choose a <Option>-O*</Option> “package,” stick with it—don't chop and
1356 change. Modules' interfaces <Emphasis>will</Emphasis> change with a shift to a new
1357 <Option>-O*</Option> option, and you may have to recompile a large chunk of all
1358 importing modules before your program can again be run
1359 safely (see <XRef LinkEnd="recomp">).
1366 <Term>No <Option>-O*</Option>-type option specified:</Term>
1367 <IndexTerm><Primary>-O* not specified</Primary></IndexTerm>
1370 This is taken to mean: “Please compile quickly; I'm not over-bothered
1371 about compiled-code quality.” So, for example: <Command>ghc -c Foo.hs</Command>
1376 <Term><Option>-O</Option> or <Option>-O1</Option>:</Term>
1377 <IndexTerm><Primary>-O option</Primary></IndexTerm>
1378 <IndexTerm><Primary>-O1 option</Primary></IndexTerm>
1379 <IndexTerm><Primary>optimise</Primary><secondary>normally</secondary></IndexTerm>
1382 Means: “Generate good-quality code without taking too long about
1383 it.” Thus, for example: <Command>ghc -c -O Main.lhs</Command>
1388 <Term><Option>-O2</Option>:</Term>
1389 <IndexTerm><Primary>-O2 option</Primary></IndexTerm>
1390 <IndexTerm><Primary>optimise</Primary><secondary>aggressively</secondary></IndexTerm>
1393 Means: “Apply every non-dangerous optimisation, even if it means
1394 significantly longer compile times.”
1398 The avoided “dangerous” optimisations are those that can make
1399 runtime or space <Emphasis>worse</Emphasis> if you're unlucky. They are
1400 normally turned on or off individually.
1404 At the moment, <Option>-O2</Option> is <Emphasis>unlikely</Emphasis> to produce
1405 better code than <Option>-O</Option>.
1410 <Term><Option>-O2-for-C</Option>:</Term>
1411 <IndexTerm><Primary>-O2-for-C option</Primary></IndexTerm>
1412 <IndexTerm><Primary>gcc, invoking with -O2</Primary></IndexTerm>
1415 Says to run GCC with <Option>-O2</Option>, which may be worth a few percent in
1416 execution speed. Don't forget <Option>-fvia-C</Option>, lest you use the native-code
1417 generator and bypass GCC altogether!
1422 <Term><Option>-Onot</Option>:</Term>
1423 <IndexTerm><Primary>-Onot option</Primary></IndexTerm>
1424 <IndexTerm><Primary>optimising, reset</Primary></IndexTerm>
1427 This option will make GHC “forget” any
1428 <Option>-O</Option>ish options it has seen so far. Sometimes useful;
1429 for example: <Command>make all
1430 EXTRA_HC_OPTS=-Onot</Command>.
1435 <Term><Option>-Ofile <file></Option>:</Term>
1436 <IndexTerm><Primary>-Ofile <file> option</Primary></IndexTerm>
1437 <IndexTerm><Primary>optimising, customised</Primary></IndexTerm>
1440 For those who need <Emphasis>absolute</Emphasis> control over
1441 <Emphasis>exactly</Emphasis> what options are used (e.g., compiler
1442 writers, sometimes :-), a list of options can be put in a file and
1443 then slurped in with <Option>-Ofile</Option>.
1447 In that file, comments are of the
1448 <Literal>#</Literal>-to-end-of-line variety; blank lines and most
1449 whitespace is ignored.
1453 Please ask if you are baffled and would like an example of <Option>-Ofile</Option>!
1461 At Glasgow, we don't use a <Option>-O*</Option> flag for day-to-day work. We use
1462 <Option>-O</Option> to get respectable speed; e.g., when we want to measure
1463 something. When we want to go for broke, we tend to use <Option>-O -fvia-C
1464 -O2-for-C</Option> (and we go for lots of coffee breaks).
1468 The easiest way to see what <Option>-O</Option> (etc.) “really mean” is to run with
1469 <Option>-v</Option>, then stand back in amazement. Alternatively, just look at the
1470 <Literal>HsC_minus<blah></Literal> lists in the GHC driver script.
1476 <Title><Option>-f*</Option>: platform-independent flags</Title>
1479 <IndexTerm><Primary>-f* options (GHC)</Primary></IndexTerm>
1480 <IndexTerm><Primary>-fno-* options (GHC)</Primary></IndexTerm>
1484 Flags can be turned <Emphasis>off</Emphasis> individually. (NB: I hope you have a
1485 good reason for doing this…) To turn off the <Option>-ffoo</Option> flag, just use
1486 the <Option>-fno-foo</Option> flag.<IndexTerm><Primary>-fno-<opt> anti-option</Primary></IndexTerm> So, for
1487 example, you can say <Option>-O2 -fno-strictness</Option>, which will then drop out
1488 any running of the strictness analyser.
1492 The options you are most likely to want to turn off are:
1498 <Option>-fno-strictness</Option><IndexTerm><Primary>-fno-strictness option</Primary></IndexTerm> (strictness
1499 analyser, because it is sometimes slow),
1505 <Option>-fno-specialise</Option><IndexTerm><Primary>-fno-specialise option</Primary></IndexTerm> (automatic
1506 specialisation of overloaded functions, because it can make your code
1507 bigger) (US spelling also accepted), and
1513 <Option>-fno-cpr-analyse</Option><IndexTerm><Primary>-fno-cpr-analyse option</Primary></IndexTerm> switches off the CPR (constructed product
1523 Should you wish to turn individual flags <Emphasis>on</Emphasis>, you are advised
1524 to use the <Option>-Ofile</Option> option, described above. Because the order in
1525 which optimisation passes are run is sometimes crucial, it's quite
1526 hard to do with command-line options.
1530 Here are some “dangerous” optimisations you <Emphasis>might</Emphasis> want to try:
1534 <Term><Option>-fvia-C</Option>:</Term>
1537 <IndexTerm><Primary>-fvia-C option</Primary></IndexTerm>
1538 <IndexTerm><Primary>native code generator, turning off</Primary></IndexTerm>
1542 Compile via C, and don't use the native-code generator. (There are
1543 many cases when GHC does this on its own.) You might pick up a little
1544 bit of speed by compiling via C. If you use <Function>_ccall_gc_</Function>s or
1545 <Function>_casm_</Function>s, you probably <Emphasis>have</Emphasis> to use <Option>-fvia-C</Option>.
1549 The lower-case incantation, <Option>-fvia-c</Option>, is synonymous.
1553 Compiling via C will probably be slower (in compilation time) than
1554 using GHC's native code generator.
1559 <Term><Option>-funfolding-interface-threshold<n></Option>:</Term>
1562 <IndexTerm><Primary>-funfolding-interface-threshold option</Primary></IndexTerm>
1563 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
1564 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
1565 (Default: 30) By raising or lowering this number, you can raise or
1566 lower the amount of pragmatic junk that gets spewed into interface
1567 files. (An unfolding has a “size” that reflects the cost in terms
1568 of “code bloat” of expanding that unfolding in another module. A
1569 bigger function would be assigned a bigger cost.)
1574 <Term><Option>-funfolding-creation-threshold<n></Option>:</Term>
1577 <IndexTerm><Primary>-funfolding-creation-threshold option</Primary></IndexTerm>
1578 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
1579 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
1580 (Default: 30) This option is similar to
1581 <Option>-funfolding-interface-threshold</Option>, except that it governs unfoldings
1582 within a single module. Increasing this figure is more likely to
1583 result in longer compile times than faster code. The next option is
1589 <Term><Option>-funfolding-use-threshold<n></Option>:</Term>
1592 <IndexTerm><Primary>-funfolding-use-threshold option</Primary></IndexTerm>
1593 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
1594 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
1595 (Default: 8) This is the magic cut-off figure for unfolding: below
1596 this size, a function definition will be unfolded at the call-site,
1597 any bigger and it won't. The size computed for a function depends on
1598 two things: the actual size of the expression minus any discounts that
1599 apply (see <Option>-funfolding-con-discount</Option>).
1604 <Term><Option>-funfolding-con-discount<n></Option>:</Term>
1607 <IndexTerm><Primary>-funfolding-con-discount option</Primary></IndexTerm>
1608 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
1609 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
1610 (Default: 2) If the compiler decides that it can eliminate some
1611 computation by performing an unfolding, then this is a discount factor
1612 that it applies to the funciton size before deciding whether to unfold
1617 OK, folks, these magic numbers `30', `8', and '2' are mildly
1618 arbitrary; they are of the “seem to be OK” variety. The `8' is the
1619 more critical one; it's what determines how eager GHC is about
1620 expanding unfoldings.
1625 <Term><Option>-funbox-strict-fields</Option>:</Term>
1628 <IndexTerm><Primary>-funbox-strict-fields option</Primary></IndexTerm>
1629 <IndexTerm><Primary>strict constructor fields</Primary></IndexTerm>
1630 <IndexTerm><Primary>constructor fields, strict</Primary></IndexTerm>
1634 This option causes all constructor fields which are marked strict
1635 (i.e. “!”) to be unboxed or unpacked if possible. For example:
1641 data T = T !Float !Float
1647 will create a constructor <Literal>T</Literal> containing two unboxed floats if the
1648 <Option>-funbox-strict-fields</Option> flag is given. This may not always be an
1649 optimisation: if the <Function>T</Function> constructor is scrutinised and the floats
1650 passed to a non-strict function for example, they will have to be
1651 reboxed (this is done automatically by the compiler).
1655 This option should only be used in conjunction with <Option>-O</Option>, in order to
1656 expose unfoldings to the compiler so the reboxing can be removed as
1657 often as possible. For example:
1664 f (T f1 f2) = f1 + f2
1670 The compiler will avoid reboxing <Function>f1</Function> and <Function>f2</Function> by inlining <Function>+</Function> on
1671 floats, but only when <Option>-O</Option> is on.
1675 Any single-constructor data is eligible for unpacking; for example
1681 data T = T !(Int,Int)
1687 will store the two <Literal>Int</Literal>s directly in the <Function>T</Function> constructor, by flattening
1688 the pair. Multi-level unpacking is also supported:
1695 data S = S !Int !Int
1701 will store two unboxed <Literal>Int#</Literal>s directly in the <Function>T</Function> constructor.
1706 <Term><Option>-fsemi-tagging</Option>:</Term>
1709 This option (which <Emphasis>does not work</Emphasis> with the native-code generator)
1710 tells the compiler to add extra code to test for already-evaluated
1711 values. You win if you have lots of such values during a run of your
1712 program, you lose otherwise. (And you pay in extra code space.)
1716 We have not played with <Option>-fsemi-tagging</Option> enough to recommend it.
1717 (For all we know, it doesn't even work anymore… Sigh.)
1727 <Title><Option>-m*</Option>: platform-specific flags</Title>
1730 <IndexTerm><Primary>-m* options (GHC)</Primary></IndexTerm>
1731 <IndexTerm><Primary>platform-specific options</Primary></IndexTerm>
1732 <IndexTerm><Primary>machine-specific options</Primary></IndexTerm>
1736 Some flags only make sense for particular target platforms.
1743 <Term><Option>-mv8</Option>:</Term>
1746 (SPARC machines)<IndexTerm><Primary>-mv8 option (SPARC only)</Primary></IndexTerm>
1747 Means to pass the like-named option to GCC; it says to use the
1748 Version 8 SPARC instructions, notably integer multiply and divide.
1749 The similiar <Option>-m*</Option> GCC options for SPARC also work, actually.
1754 <Term><Option>-mlong-calls</Option>:</Term>
1757 (HPPA machines)<IndexTerm><Primary>-mlong-calls option (HPPA only)</Primary></IndexTerm>
1758 Means to pass the like-named option to GCC. Required for Very Big
1759 modules, maybe. (Probably means you're in trouble…)
1764 <Term><Option>-monly-[32]-regs</Option>:</Term>
1767 (iX86 machines)<IndexTerm><Primary>-monly-N-regs option (iX86 only)</Primary></IndexTerm>
1768 GHC tries to “steal” four registers from GCC, for performance
1769 reasons; it almost always works. However, when GCC is compiling some
1770 modules with four stolen registers, it will crash, probably saying:
1773 Foo.hc:533: fixed or forbidden register was spilled.
1774 This may be due to a compiler bug or to impossible asm
1775 statements or clauses.
1778 Just give some registers back with <Option>-monly-N-regs</Option>. Try `3' first,
1779 then `2'. If `2' doesn't work, please report the bug to us.
1788 <Sect2 id="optimise-C-compiler">
1789 <Title>Code improvement by the C compiler.
1793 <IndexTerm><Primary>optimisation by GCC</Primary></IndexTerm>
1794 <IndexTerm><Primary>GCC optimisation</Primary></IndexTerm>
1798 The C compiler (GCC) is run with <Option>-O</Option> turned on. (It has
1803 If you want to run GCC with <Option>-O2</Option>—which may be worth a few
1804 percent in execution speed—you can give a
1805 <Option>-O2-for-C</Option><IndexTerm><Primary>-O2-for-C option</Primary></IndexTerm> option.
1812 <Sect1 id="options-phases">
1813 <Title>Options related to a particular phase
1816 <Sect2 id="c-pre-processor">
1817 <Title>The C pre-processor
1821 <IndexTerm><Primary>pre-processing: cpp</Primary></IndexTerm>
1822 <IndexTerm><Primary>C pre-processor options</Primary></IndexTerm>
1823 <IndexTerm><Primary>cpp, pre-processing with</Primary></IndexTerm>
1827 The C pre-processor <Command>cpp</Command> is run over your Haskell code only if the
1828 <Option>-cpp</Option> option <IndexTerm><Primary>-cpp option</Primary></IndexTerm> is given. Unless you are
1829 building a large system with significant doses of conditional
1830 compilation, you really shouldn't need it.
1834 <Term><Option>-D<foo></Option>:</Term>
1837 <IndexTerm><Primary>-D<name> option</Primary></IndexTerm>
1838 Define macro <Constant><foo></Constant> in the usual way. NB: does <Emphasis>not</Emphasis> affect
1839 <Option>-D</Option> macros passed to the C compiler when compiling via C! For those,
1840 use the <Option>-optc-Dfoo</Option> hack… (see <XRef LinkEnd="forcing-options-through">).
1845 <Term><Option>-U<foo></Option>:</Term>
1848 <IndexTerm><Primary>-U<name> option</Primary></IndexTerm>
1849 Undefine macro <Command><foo></Command> in the usual way.
1854 <Term><Option>-I<dir></Option>:</Term>
1857 <IndexTerm><Primary>-I<dir> option</Primary></IndexTerm>
1858 Specify a directory in which to look for <Literal>#include</Literal> files, in
1867 The GHC driver pre-defines several macros when processing Haskell
1868 source code (<Filename>.hs</Filename> or <Filename>.lhs</Filename> files):
1875 <Term><Constant>__HASKELL98__</Constant>:</Term>
1878 <IndexTerm><Primary>__HASKELL98__</Primary></IndexTerm>
1879 If defined, this means that GHC supports the language defined by the
1885 <Term><Constant>__HASKELL__=98</Constant>:</Term>
1888 <IndexTerm><Primary>__HASKELL__</Primary></IndexTerm>
1889 In GHC 4.04 and later, the <Constant>__HASKELL__</Constant> macro is defined as having
1890 the value <Constant>98</Constant>.
1895 <Term><Constant>__HASKELL1__</Constant>:</Term>
1898 <IndexTerm><Primary>__HASKELL1__ macro</Primary></IndexTerm>
1899 If defined to <Emphasis>n</Emphasis>, that means GHC supports the Haskell language
1900 defined in the Haskell report version <Emphasis>1.n</Emphasis>. Currently 5. This
1901 macro is deprecated, and will probably disappear in future versions.
1906 <Term><Constant>__GLASGOW_HASKELL__</Constant>:</Term>
1909 <IndexTerm><Primary>__GLASGOW_HASKELL__ macro</Primary></IndexTerm>
1910 For version <Emphasis>n</Emphasis> of the GHC system, this will be <Literal>#define</Literal>d to
1911 <Emphasis>100n</Emphasis>. So, for version 4.00, it is 400.
1915 With any luck, <Constant>__GLASGOW_HASKELL__</Constant> will be undefined in all other
1916 implementations that support C-style pre-processing.
1920 (For reference: the comparable symbols for other systems are:
1921 <Constant>__HUGS__</Constant> for Hugs and <Constant>__HBC__</Constant> for Chalmers.)
1925 NB. This macro is set when pre-processing both Haskell source and C
1926 source, including the C source generated from a Haskell module
1927 (i.e. <Filename>.hs</Filename>, <Filename>.lhs</Filename>, <Filename>.c</Filename> and <Filename>.hc</Filename> files).
1932 <Term><Constant>__CONCURRENT_HASKELL__</Constant>:</Term>
1935 <IndexTerm><Primary>__CONCURRENT_HASKELL__ macro</Primary></IndexTerm>
1936 This symbol is defined when pre-processing Haskell (input) and
1937 pre-processing C (GHC output). Since GHC from verion 4.00 now
1938 supports concurrent haskell by default, this symbol is always defined.
1943 <Term><Constant>__PARALLEL_HASKELL__</Constant>:</Term>
1946 <IndexTerm><Primary>__PARALLEL_HASKELL__ macro</Primary></IndexTerm>
1947 Only defined when <Option>-parallel</Option> is in use! This symbol is defined when
1948 pre-processing Haskell (input) and pre-processing C (GHC output).
1956 Options other than the above can be forced through to the C
1957 pre-processor with the <Option>-opt</Option> flags (see
1958 <XRef LinkEnd="forcing-options-through">).
1962 A small word of warning: <Option>-cpp</Option> is not friendly to “string
1963 gaps”.<IndexTerm><Primary>-cpp vs string gaps</Primary></IndexTerm><IndexTerm><Primary>string gaps vs
1964 -cpp</Primary></IndexTerm>. In other words, strings such as the following:
1978 don't work with <Option>-cpp</Option>; <Filename>/usr/bin/cpp</Filename> elides the
1979 backslash-newline pairs.
1983 However, it appears that if you add a space at the end of the line,
1984 then <Command>cpp</Command> (at least GNU <Command>cpp</Command> and possibly other <Command>cpp</Command>s)
1985 leaves the backslash-space pairs alone and the string gap works as
1991 <Sect2 id="options-C-compiler">
1992 <Title>Options affecting the C compiler (if applicable)
1996 <IndexTerm><Primary>include-file options</Primary></IndexTerm>
1997 <IndexTerm><Primary>C compiler options</Primary></IndexTerm>
1998 <IndexTerm><Primary>GCC options</Primary></IndexTerm>
2002 At the moment, quite a few common C-compiler options are passed on
2003 quietly to the C compilation of Haskell-compiler-generated C files.
2004 THIS MAY CHANGE. Meanwhile, options so sent are:
2011 <ColSpec Align="Left" Colsep="0">
2012 <ColSpec Align="Left" Colsep="0">
2015 <Entry><Option>-ansi</Option> </Entry>
2016 <Entry> do ANSI C (not K&R) </Entry>
2020 <Option>-pedantic</Option> </Entry>
2021 <Entry> be so</Entry>
2025 <Option>-dgcc-lint</Option> </Entry>
2026 <Entry> (hack) short for “make GCC very paranoid”</Entry>
2034 <IndexTerm><Primary>-ansi option (for GCC)</Primary></IndexTerm>
2035 <IndexTerm><Primary>-pedantic option (for GCC)</Primary></IndexTerm>
2036 <IndexTerm><Primary>-dgcc-lint option (GCC paranoia)</Primary></IndexTerm>
2040 If you are compiling with lots of <Literal>ccalls</Literal>, etc., you may need to
2041 tell the C compiler about some <Literal>#include</Literal> files. There is no real
2042 pretty way to do this, but you can use this hack from the
2049 % ghc -c '-#include <X/Xlib.h>' Xstuff.lhs
2056 <Sect2 id="options-linker">
2057 <Title>Linking and consistency-checking
2061 <IndexTerm><Primary>linker options</Primary></IndexTerm>
2062 <IndexTerm><Primary>ld options</Primary></IndexTerm>
2066 GHC has to link your code with various libraries, possibly including:
2067 user-supplied, GHC-supplied, and system-supplied (<Option>-lm</Option> math
2068 library, for example).
2075 <Term><Option>-l<FOO></Option>:</Term>
2078 <IndexTerm><Primary>-l<lib> option</Primary></IndexTerm>
2079 Link in a library named <Filename>lib<FOO>.a</Filename> which resides somewhere on the
2080 library directories path.
2084 Because of the sad state of most UNIX linkers, the order of such
2085 options does matter. Thus: <Command>ghc -lbar *.o</Command> is almost certainly
2086 wrong, because it will search <Filename>libbar.a</Filename> <Emphasis>before</Emphasis> it has
2087 collected unresolved symbols from the <Filename>*.o</Filename> files.
2088 <Command>ghc *.o -lbar</Command> is probably better.
2092 The linker will of course be informed about some GHC-supplied
2093 libraries automatically; these are:
2100 <ColSpec Align="Left" Colsep="0">
2101 <ColSpec Align="Left" Colsep="0">
2104 <Entry><Emphasis>-l equivalent</Emphasis> </Entry>
2105 <Entry> <Emphasis>description</Emphasis> </Entry>
2110 <Option>-lHSrts,-lHSclib</Option> </Entry>
2111 <Entry> basic runtime libraries </Entry>
2115 <Option>-lHS</Option> </Entry>
2116 <Entry> standard Prelude library </Entry>
2120 <Option>-lHS_cbits</Option> </Entry>
2121 <Entry> C support code for standard Prelude library </Entry>
2125 <Option>-lgmp</Option> </Entry>
2126 <Entry> GNU multi-precision library (for Integers)</Entry>
2137 <IndexTerm><Primary>-lHS library</Primary></IndexTerm>
2138 <IndexTerm><Primary>-lHS_cbits library</Primary></IndexTerm>
2139 <IndexTerm><Primary>-lHSrts library</Primary></IndexTerm>
2140 <IndexTerm><Primary>-lgmp library</Primary></IndexTerm>
2145 <Term><Option>-syslib <name></Option>:</Term>
2148 <IndexTerm><Primary>-syslib <name> option</Primary></IndexTerm>
2152 If you are using a Haskell “system library” (e.g., the POSIX
2153 library), just use the <Option>-syslib posix</Option> option, and the correct code
2154 should be linked in.
2159 <Term><Option>-L<dir></Option>:</Term>
2162 <IndexTerm><Primary>-L<dir> option</Primary></IndexTerm>
2163 Where to find user-supplied libraries… Prepend the directory
2164 <Filename><dir></Filename> to the library directories path.
2169 <Term><Option>-static</Option>:</Term>
2172 <IndexTerm><Primary>-static option</Primary></IndexTerm>
2173 Tell the linker to avoid shared libraries.
2178 <Term><Option>-no-link-chk</Option> and <Option>-link-chk</Option>:</Term>
2181 <IndexTerm><Primary>-no-link-chk option</Primary></IndexTerm>
2182 <IndexTerm><Primary>-link-chk option</Primary></IndexTerm>
2183 <IndexTerm><Primary>consistency checking of executables</Primary></IndexTerm>
2184 By default, immediately after linking an executable, GHC verifies that
2185 the pieces that went into it were compiled with compatible flags; a
2186 “consistency check”.
2187 (This is to avoid mysterious failures caused by non-meshing of
2188 incompatibly-compiled programs; e.g., if one <Filename>.o</Filename> file was compiled
2189 for a parallel machine and the others weren't.) You may turn off this
2190 check with <Option>-no-link-chk</Option>. You can turn it (back) on with
2191 <Option>-link-chk</Option> (the default).
2196 <Term><Option>-no-hs-main</Option>:</Term>
2199 <IndexTerm><Primary>-no-hs-main option</Primary></IndexTerm>
2200 <IndexTerm><Primary>linking Haskell libraries with foreign code</Primary></IndexTerm>
2204 In the event you want to include ghc-compiled code as part of another
2205 (non-Haskell) program, the RTS will not be supplying its definition of
2206 <Function>main()</Function> at link-time, you will have to. To signal that to the
2207 driver script when linking, use <Option>-no-hs-main</Option>.
2211 Notice that since the command-line passed to the linker is rather
2212 involved, you probably want to use the ghc driver script to do the
2213 final link of your `mixed-language' application. This is not a
2214 requirement though, just try linking once with <Option>-v</Option> on to see what
2215 options the driver passes through to the linker.
2227 <Title>Using Concurrent Haskell</Title>
2230 <IndexTerm><Primary>Concurrent Haskell—use</Primary></IndexTerm>
2234 GHC (as of version 4.00) supports Concurrent Haskell by default,
2235 without requiring a special option or libraries compiled in a certain
2236 way. To get access to the support libraries for Concurrent Haskell
2237 (i.e. <Literal>Concurrent</Literal> and friends), use the
2238 <Option>-syslib concurrent</Option> option.
2242 Three RTS options are provided for modifying the behaviour of the
2243 threaded runtime system. See the descriptions of
2244 <Option>-C[<us>]</Option>, <Option>-q</Option>, and
2245 <Option>-t<num></Option> in <XRef LinkEnd="parallel-rts-opts">.
2249 Concurrent Haskell is described in more detail in <XRef
2250 LinkEnd="sec-Concurrent">.
2256 <Title>Using Parallel Haskell</Title>
2259 <IndexTerm><Primary>Parallel Haskell—use</Primary></IndexTerm>
2263 [You won't be able to execute parallel Haskell programs unless PVM3
2264 (Parallel Virtual Machine, version 3) is installed at your site.]
2268 To compile a Haskell program for parallel execution under PVM, use the
2269 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
2270 option</Primary></IndexTerm> both when compiling <Emphasis>and
2271 linking</Emphasis>. You will probably want to <Literal>import
2272 Parallel</Literal> into your Haskell modules.
2276 To run your parallel program, once PVM is going, just invoke it
2277 “as normal”. The main extra RTS option is
2278 <Option>-N<n></Option>, to say how many PVM
2279 “processors” your program to run on. (For more details of
2280 all relevant RTS options, please see <XRef
2281 LinkEnd="parallel-rts-opts">.)
2285 In truth, running Parallel Haskell programs and getting information
2286 out of them (e.g., parallelism profiles) is a battle with the vagaries of
2287 PVM, detailed in the following sections.
2291 <Title>Dummy's guide to using PVM</Title>
2294 <IndexTerm><Primary>PVM, how to use</Primary></IndexTerm>
2295 <IndexTerm><Primary>Parallel Haskell—PVM use</Primary></IndexTerm>
2296 Before you can run a parallel program under PVM, you must set the
2297 required environment variables (PVM's idea, not ours); something like,
2298 probably in your <Filename>.cshrc</Filename> or equivalent:
2301 setenv PVM_ROOT /wherever/you/put/it
2302 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
2303 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
2309 Creating and/or controlling your “parallel machine” is a purely-PVM
2310 business; nothing specific to Parallel Haskell.
2314 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
2315 machine. You can then do various things to control/monitor your
2316 “parallel machine;” the most useful being:
2322 <ColSpec Align="Left">
2326 <Entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></Entry>
2327 <Entry>exit <Command>pvm</Command>, leaving it running</Entry>
2331 <Entry><Command>halt</Command></Entry>
2332 <Entry>kill off this “parallel machine” & exit</Entry>
2336 <Entry><Command>add <host></Command></Entry>
2337 <Entry>add <Command><host></Command> as a processor</Entry>
2341 <Entry><Command>delete <host></Command></Entry>
2342 <Entry>delete <Command><host></Command></Entry>
2346 <Entry><Command>reset</Command></Entry>
2347 <Entry>kill what's going, but leave PVM up</Entry>
2351 <Entry><Command>conf</Command></Entry>
2352 <Entry>list the current configuration</Entry>
2356 <Entry><Command>ps</Command></Entry>
2357 <Entry>report processes' status</Entry>
2361 <Entry><Command>pstat <pid></Command></Entry>
2362 <Entry>status of a particular process</Entry>
2371 The PVM documentation can tell you much, much more about <Command>pvm</Command>!
2377 <Title>Parallelism profiles</Title>
2380 <IndexTerm><Primary>parallelism profiles</Primary></IndexTerm>
2381 <IndexTerm><Primary>profiles, parallelism</Primary></IndexTerm>
2382 <IndexTerm><Primary>visualisation tools</Primary></IndexTerm>
2386 With Parallel Haskell programs, we usually don't care about the
2387 results—only with “how parallel” it was! We want pretty pictures.
2391 Parallelism profiles (à la <Command>hbcpp</Command>) can be generated with the
2392 <Option>-q</Option><IndexTerm><Primary>-q RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option. The
2393 per-processor profiling info is dumped into files named
2394 <Filename><full-path><program>.gr</Filename>. These are then munged into a PostScript picture,
2395 which you can then display. For example, to run your program
2396 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
2402 % ./a.out +RTS -N8 -q
2403 % grs2gr *.???.gr > temp.gr # combine the 8 .gr files into one
2404 % gr2ps -O temp.gr # cvt to .ps; output in temp.ps
2405 % ghostview -seascape temp.ps # look at it!
2411 The scripts for processing the parallelism profiles are distributed
2412 in <Filename>ghc/utils/parallel/</Filename>.
2418 <Title>Other useful info about running parallel programs</Title>
2421 The “garbage-collection statistics” RTS options can be useful for
2422 seeing what parallel programs are doing. If you do either
2423 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
2424 you'll get mutator, garbage-collection, etc., times on standard
2425 error. The standard error of all PE's other than the `main thread'
2426 appears in <Filename>/tmp/pvml.nnn</Filename>, courtesy of PVM.
2430 Whether doing <Option>+RTS -Sstderr</Option> or not, a handy way to watch
2431 what's happening overall is: <Command>tail -f /tmp/pvml.nnn</Command>.
2436 <Sect2 id="parallel-rts-opts">
2437 <Title>RTS options for Concurrent/Parallel Haskell
2441 <IndexTerm><Primary>RTS options, concurrent</Primary></IndexTerm>
2442 <IndexTerm><Primary>RTS options, parallel</Primary></IndexTerm>
2443 <IndexTerm><Primary>Concurrent Haskell—RTS options</Primary></IndexTerm>
2444 <IndexTerm><Primary>Parallel Haskell—RTS options</Primary></IndexTerm>
2448 Besides the usual runtime system (RTS) options
2449 (<XRef LinkEnd="runtime-control">), there are a few options particularly
2450 for concurrent/parallel execution.
2457 <Term><Option>-N<N></Option>:</Term>
2460 <IndexTerm><Primary>-N<N> RTS option (parallel)</Primary></IndexTerm>
2461 (PARALLEL ONLY) Use <Literal><N></Literal> PVM processors to run this program;
2467 <Term><Option>-C[<us>]</Option>:</Term>
2470 <IndexTerm><Primary>-C<us> RTS option</Primary></IndexTerm>
2471 Sets the context switch interval to <Literal><us></Literal> microseconds. A context
2472 switch will occur at the next heap allocation after the timer expires.
2473 With <Option>-C0</Option> or <Option>-C</Option>, context switches will occur as often as
2474 possible (at every heap allocation). By default, context switches
2475 occur every 10 milliseconds. Note that many interval timers are only
2476 capable of 10 millisecond granularity, so the default setting may be
2477 the finest granularity possible, short of a context switch at every
2482 [NOTE: this option currently has no effect (version 4.00). Context
2483 switches happen when the current heap block is full, i.e. every 4k of
2489 <Term><Option>-q[v]</Option>:</Term>
2492 <IndexTerm><Primary>-q RTS option</Primary></IndexTerm>
2493 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
2494 in the file <FIlename><program>.qp</FIlename>. In the style of <Command>hbcpp</Command>, this profile
2495 records the movement of threads between the green (runnable) and red
2496 (blocked) queues. If you specify the verbose suboption (<Option>-qv</Option>), the
2497 green queue is split into green (for the currently running thread
2498 only) and amber (for other runnable threads). We do not recommend
2499 that you use the verbose suboption if you are planning to use the
2500 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
2501 check (with <Option>-C</Option>).
2506 <Term><Option>-t<num></Option>:</Term>
2509 <IndexTerm><Primary>-t<num> RTS option</Primary></IndexTerm>
2510 (PARALLEL ONLY) Limit the number of concurrent threads per processor
2511 to <Literal><num></Literal>. The default is 32. Each thread requires slightly over 1K
2512 <Emphasis>words</Emphasis> in the heap for thread state and stack objects. (For
2513 32-bit machines, this translates to 4K bytes, and for 64-bit machines,
2519 <Term><Option>-d</Option>:</Term>
2522 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
2523 (PARALLEL ONLY) Turn on debugging. It pops up one xterm (or GDB, or
2524 something…) per PVM processor. We use the standard <Command>debugger</Command>
2525 script that comes with PVM3, but we sometimes meddle with the
2526 <Command>debugger2</Command> script. We include ours in the GHC distribution,
2527 in <Filename>ghc/utils/pvm/</Filename>.
2532 <Term><Option>-e<num></Option>:</Term>
2535 <IndexTerm><Primary>-e<num> RTS option (parallel)</Primary></IndexTerm>
2536 (PARALLEL ONLY) Limit the number of pending sparks per processor to
2537 <Literal><num></Literal>. The default is 100. A larger number may be appropriate if
2538 your program generates large amounts of parallelism initially.
2543 <Term><Option>-Q<num></Option>:</Term>
2546 <IndexTerm><Primary>-Q<num> RTS option (parallel)</Primary></IndexTerm>
2547 (PARALLEL ONLY) Set the size of packets transmitted between processors
2548 to <Literal><num></Literal>. The default is 1024 words. A larger number may be
2549 appropriate if your machine has a high communication cost relative to