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 example). The
839 right interface files should then be available. <XRef LinkEnd="ghc-prelude"> lists the
840 libraries available by this mechanism.
846 <Term><Option>-I<dir></Option></Term>
849 <IndexTerm><Primary>-I<dir> option</Primary></IndexTerm>
850 Once a Haskell module has been compiled to C (<Filename>.hc</Filename> file), you may
851 wish to specify where GHC tells the C compiler to look for <Filename>.h</Filename> files.
852 (Or, if you are using the <Option>-cpp</Option> option<IndexTerm><Primary>-cpp option</Primary></IndexTerm>, where
853 it tells the C pre-processor to look…) For this purpose, use a <Option>-I</Option>
854 option in the usual C-ish way.
864 <Sect2 id="hi-options">
865 <Title>Other options related to interface files
869 <IndexTerm><Primary>interface files, options</Primary></IndexTerm>
870 The interface output may be directed to another file
871 <Filename>bar2/Wurble.iface</Filename> with the option <Option>-ohi bar2/Wurble.iface</Option><IndexTerm><Primary>-ohi
872 <file> option</Primary></IndexTerm> (not recommended).
876 To avoid generating an interface file at all, use a <Option>-nohi</Option>
877 option.<IndexTerm><Primary>-nohi option</Primary></IndexTerm>
881 The compiler does not overwrite an existing <Filename>.hi</Filename> interface file if
882 the new one is byte-for-byte the same as the old one; this is friendly
883 to <Command>make</Command>. When an interface does change, it is often enlightening to
884 be informed. The <Option>-hi-diffs</Option><IndexTerm><Primary>-hi-diffs option</Primary></IndexTerm> option will
885 make GHC run <Command>diff</Command> on the old and new <Filename>.hi</Filename> files. You can also
886 record the difference in the interface file itself, the
887 <Option>-keep-hi-diffs</Option><IndexTerm><Primary>-keep-hi-diffs</Primary></IndexTerm> option takes care of that.
891 The <Filename>.hi</Filename> files from GHC contain “usage” information which changes
892 often and uninterestingly. If you really want to see these changes
893 reported, you need to use the
894 <Option>-hi-diffs-with-usages</Option><IndexTerm><Primary>-hi-diffs-with-usages option</Primary></IndexTerm>
899 Interface files are normally jammed full of compiler-produced
900 <Emphasis>pragmas</Emphasis>, which record arities, strictness info, etc. If you
901 think these pragmas are messing you up (or you are doing some kind of
902 weird experiment), you can tell GHC to ignore them with the
903 <Option>-fignore-interface-pragmas</Option><IndexTerm><Primary>-fignore-interface-pragmas
904 option</Primary></IndexTerm> option.
908 When compiling without optimisations on, the compiler is extra-careful
909 about not slurping in data constructors and instance declarations that
910 it will not need. If you believe it is getting it wrong and not
911 importing stuff which you think it should, this optimisation can be
912 turned off with <Option>-fno-prune-tydecls</Option> and <Option>-fno-prune-instdecls</Option>.
913 <IndexTerm><Primary>-fno-prune-tydecls option</Primary></IndexTerm><IndexTerm><Primary>-fno-prune-instdecls
914 option</Primary></IndexTerm>
918 See also <XRef LinkEnd="options-linker">, which describes how the linker finds standard
925 <Title>The recompilation checker
928 <IndexTerm><Primary>recompilation checker</Primary></IndexTerm>
933 <Term><Option>-recomp</Option></Term>
934 <IndexTerm><Primary><option>-recomp</option> option</Primary></IndexTerm>
937 Turn on recompilation checking. This will stop compilation early,
938 leaving an existing <filename>.o</filename> file in place, if it can
939 be determined that the module does not need to be recompiled.
947 In the olden days, GHC compared the newly-generated
948 <Filename>.hi</Filename> file with the previous version; if they were
949 identical, it left the old one alone and didn't change its
950 modification date. In consequence, importers of a module with an
951 unchanged output <Filename>.hi</Filename> file were not recompiled.
955 This doesn't work any more. In our earlier example, module
956 <Literal>C</Literal> does not import module <Literal>A</Literal>
957 directly, yet changes to <Filename>A.hi</Filename> should force a
958 recompilation of <Literal>C</Literal>. And some changes to
959 <Literal>A</Literal> (changing the definition of a function that
960 appears in an inlining of a function exported by <Literal>B</Literal>,
961 say) may conceivably not change <Filename>B.hi</Filename> one jot. So
966 GHC keeps a version number on each interface file, and on each type
967 signature within the interface file. It also keeps in every interface
968 file a list of the version numbers of everything it used when it last
969 compiled the file. If the source file's modification date is earlier
970 than the <Filename>.o</Filename> file's date (i.e. the source hasn't
971 changed since the file was last compiled), and the
972 <option>-recomp</option> is given on the command line, GHC will be
973 clever. It compares the version numbers on the things it needs this
974 time with the version numbers on the things it needed last time
975 (gleaned from the interface file of the module being compiled); if
976 they are all the same it stops compiling rather early in the process
977 saying “Compilation IS NOT required”. What a beautiful
982 GHC <Emphasis>only</Emphasis> keeps detailed dependency information
983 for “user” modules, not for “library” modules.
984 It distinguishes the two by a hack: a module whose
985 <Filename>.hi</Filename> file has an absolute path name is considered
986 a library module, while a relative path name indicates a user module.
987 So if you have a multi-directory application, use
988 <Emphasis>relative</Emphasis> path names in your <Option>-i</Option>
989 path, to force GHC to record detailed dependency information. Use
990 absolute path names only for directories containing slowly-changing
995 A path is considered “absolute” if it starts with
996 “<Filename>/</Filename>”, or
997 “<Filename>A:/</Filename>”, or
998 “<Filename>A:\</Filename>” (or
999 “<Filename>B:/</Filename>”,
1000 “<Filename>B:\</Filename>” etc).
1004 Patrick Sansom had a workshop paper about how all this is done (though
1005 the details have changed quite a bit). <ULink URL="mailto:sansom@dcs.gla.ac.uk">Ask him</ULink> if you want a copy.
1010 <Sect2 id="using-make">
1011 <Title>Using <Command>make</Command>
1015 <IndexTerm><Primary><literal>make</literal></Primary></IndexTerm>
1019 It is reasonably straightforward to set up a <Filename>Makefile</Filename> to use with
1020 GHC, assuming you name your source files the same as your modules.
1028 HC_OPTS = -cpp $(EXTRA_HC_OPTS)
1030 SRCS = Main.lhs Foo.lhs Bar.lhs
1031 OBJS = Main.o Foo.o Bar.o
1033 .SUFFIXES : .o .hi .lhs .hc .s
1037 $(HC) -o $@ $(HC_OPTS) $(OBJS)
1039 # Standard suffix rules
1044 $(HC) -c $< $(HC_OPTS)
1047 $(HC) -c $< $(HC_OPTS)
1049 # Inter-module dependencies
1050 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
1051 Main.o Main.hc Main.s : Foo.hi Baz.hi # Main imports Foo and Baz
1057 (Sophisticated <Command>make</Command> variants may achieve some of the above more
1058 elegantly. Notably, <Command>gmake</Command>'s pattern rules let you write the more
1066 $(HC) -c $< $(HC_OPTS)
1072 What we've shown should work with any <Command>make</Command>.)
1076 Note the cheesy <Literal>.o.hi</Literal> rule: It records the dependency of the
1077 interface (<Filename>.hi</Filename>) file on the source. The rule says a <Filename>.hi</Filename> file can
1078 be made from a <Filename>.o</Filename> file by doing…nothing. Which is true.
1082 Note the inter-module dependencies at the end of the Makefile, which
1089 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
1095 They tell <Command>make</Command> that if any of <Literal>Foo.o</Literal>, <Literal>Foo.hc</Literal> or <Literal>Foo.s</Literal> have an
1096 earlier modification date than <Literal>Baz.hi</Literal>, then the out-of-date file
1097 must be brought up to date. To bring it up to date, <Literal>make</Literal> looks for
1098 a rule to do so; one of the preceding suffix rules does the job
1103 Putting inter-dependencies of the form <Literal>Foo.o : Bar.hi</Literal> into your
1104 <Filename>Makefile</Filename> by hand is rather error-prone. Don't worry—never fear,
1105 <Command>mkdependHS</Command> is here! (and is distributed as part of GHC) Add the
1106 following to your <Filename>Makefile</Filename>:
1113 mkdependHS -- $(HC_OPTS) -- $(SRCS)
1119 Now, before you start compiling, and any time you change the <Literal>imports</Literal>
1120 in your program, do <Command>make depend</Command> before you do <Command>make cool_pgm</Command>.
1121 <Command>mkdependHS</Command> will append the needed dependencies to your <Filename>Makefile</Filename>.
1122 <Command>mkdependHS</Command> is fully described in <XRef LinkEnd="mkdependHS">.
1126 A few caveats about this simple scheme:
1135 You may need to compile some modules explicitly to create their
1136 interfaces in the first place (e.g., <Command>make Bar.o</Command> to create <Filename>Bar.hi</Filename>).
1143 You may have to type <Command>make</Command> more than once for the dependencies
1144 to have full effect. However, a <Command>make</Command> run that does nothing
1145 <Emphasis>does</Emphasis> mean “everything's up-to-date.”
1152 This scheme will work with mutually-recursive modules but,
1153 again, it may take multiple iterations to “settle.”
1164 <Sect2 id="mutual-recursion">
1165 <Title>How to compile mutually recursive modules
1169 <IndexTerm><Primary>module system, recursion</Primary></IndexTerm>
1170 <IndexTerm><Primary>recursion, between modules</Primary></IndexTerm>
1174 Currently, the compiler does not have proper support for dealing with
1175 mutually recursive modules:
1185 newtype TA = MkTA Int
1203 When compiling either module A and B, the compiler will try (in vain)
1204 to look for the interface file of the other. So, to get mutually
1205 recursive modules off the ground, you need to hand write an interface
1206 file for A or B, so as to break the loop. These hand-written
1207 interface files are called <Literal>hi-boot</Literal> files, and are placed in a file
1208 called <Filename><module>.hi-boot</Filename>. To import from an <Literal>hi-boot</Literal> file instead
1209 of the standard <Filename>.hi</Filename> file, use the following syntax in the importing module:
1210 <IndexTerm><Primary>hi-boot files</Primary></IndexTerm>
1211 <IndexTerm><Primary>importing, hi-boot files</Primary></IndexTerm>
1217 import {-# SOURCE #-} A
1223 The hand-written interface need only contain the bare minimum of
1224 information needed to get the bootstrapping process started. For
1225 example, it doesn't need to contain declarations for <Emphasis>everything</Emphasis>
1226 that module <Literal>A</Literal> exports, only the things required by the module that
1227 imports <Literal>A</Literal> recursively.
1231 For the example at hand, the boot interface file for A would look like
1238 __interface A 1 404 where
1239 __export A TA{MkTA} ;
1240 1 newtype TA = MkTA PrelBase.Int ;
1246 The syntax is essentially the same as a normal <Filename>.hi</Filename> file
1247 (unfortunately), but you can usually tailor an existing <Filename>.hi</Filename> file to
1248 make a <Filename>.hi-boot</Filename> file.
1252 Notice that we only put the declaration for the newtype <Literal>TA</Literal> in the
1253 <Literal>hi-boot</Literal> file, not the signature for <Function>f</Function>, since <Function>f</Function> isn't used by
1254 <Literal>B</Literal>.
1258 The number “1” after “__interface A” gives the version number of module A;
1259 it is incremented whenever anything in A's interface file changes. The “404” is
1260 the version number of the interface file <Emphasis>syntax</Emphasis>; we change it when
1261 we change the syntax of interface files so that you get a better error message when
1262 you try to read an old-format file with a new-format compiler.
1266 The number “1” at the beginning of a declaration is the <Emphasis>version
1267 number</Emphasis> of that declaration: for the purposes of <Filename>.hi-boot</Filename> files
1268 these can all be set to 1. All names must be fully qualified with the
1269 <Emphasis>original</Emphasis> module that an object comes from: for example, the
1270 reference to <Literal>Int</Literal> in the interface for <Literal>A</Literal> comes from <Literal>PrelBase</Literal>,
1271 which is a module internal to GHC's prelude. It's a pain, but that's
1276 If you want an hi-boot file to export a data type, but you don't want to give its constructors
1277 (because the constructors aren't used by the SOURCE-importing module), you can write simply:
1283 __interface A 1 404 where
1291 (You must write all the type parameters, but leave out the '=' and everything that follows it.)
1295 <Emphasis>Note:</Emphasis> This is all a temporary solution, a version of the
1296 compiler that handles mutually recursive modules properly without the manual
1297 construction of interface files, is (allegedly) in the works.
1304 <Sect1 id="options-optimise">
1305 <Title>Optimisation (code improvement)
1309 <IndexTerm><Primary>optimisation (GHC)</Primary></IndexTerm>
1310 <IndexTerm><Primary>improvement, code (GHC)</Primary></IndexTerm>
1314 The <Option>-O*</Option> options specify convenient “packages” of optimisation
1315 flags; the <Option>-f*</Option> options described later on specify
1316 <Emphasis>individual</Emphasis> optimisations to be turned on/off; the <Option>-m*</Option>
1317 options specify <Emphasis>machine-specific</Emphasis> optimisations to be turned
1321 <Sect2 id="optimise-pkgs">
1322 <Title><Option>-O*</Option>: convenient “packages” of optimisation flags.
1326 <IndexTerm><Primary>-O options</Primary></IndexTerm>
1330 There are <Emphasis>many</Emphasis> options that affect the quality of code
1331 produced by GHC. Most people only have a general goal, something like
1332 “Compile quickly” or “Make my program run like greased lightning.”
1333 The following “packages” of optimisations (or lack thereof) should
1338 Once you choose a <Option>-O*</Option> “package,” stick with it—don't chop and
1339 change. Modules' interfaces <Emphasis>will</Emphasis> change with a shift to a new
1340 <Option>-O*</Option> option, and you may have to recompile a large chunk of all
1341 importing modules before your program can again be run
1342 safely (see <XRef LinkEnd="recomp">).
1349 <Term>No <Option>-O*</Option>-type option specified:</Term>
1352 <IndexTerm><Primary>-O* not specified</Primary></IndexTerm>
1353 This is taken to mean: “Please compile quickly; I'm not over-bothered
1354 about compiled-code quality.” So, for example: <Command>ghc -c Foo.hs</Command>
1359 <Term><Option>-O</Option> or <Option>-O1</Option>:</Term>
1362 <IndexTerm><Primary>-O option</Primary></IndexTerm>
1363 <IndexTerm><Primary>-O1 option</Primary></IndexTerm>
1364 <IndexTerm><Primary>optimise normally</Primary></IndexTerm>
1365 Means: “Generate good-quality code without taking too long about it.”
1366 Thus, for example: <Command>ghc -c -O Main.lhs</Command>
1371 <Term><Option>-O2</Option>:</Term>
1374 <IndexTerm><Primary>-O2 option</Primary></IndexTerm>
1375 <IndexTerm><Primary>optimise aggressively</Primary></IndexTerm>
1376 Means: “Apply every non-dangerous optimisation, even if it means
1377 significantly longer compile times.”
1381 The avoided “dangerous” optimisations are those that can make
1382 runtime or space <Emphasis>worse</Emphasis> if you're unlucky. They are
1383 normally turned on or off individually.
1387 At the moment, <Option>-O2</Option> is <Emphasis>unlikely</Emphasis> to produce
1388 better code than <Option>-O</Option>.
1393 <Term><Option>-O2-for-C</Option>:</Term>
1396 <IndexTerm><Primary>-O2-for-C option</Primary></IndexTerm>
1397 <IndexTerm><Primary>gcc, invoking with -O2</Primary></IndexTerm>
1401 Says to run GCC with <Option>-O2</Option>, which may be worth a few percent in
1402 execution speed. Don't forget <Option>-fvia-C</Option>, lest you use the native-code
1403 generator and bypass GCC altogether!
1408 <Term><Option>-Onot</Option>:</Term>
1411 <IndexTerm><Primary>-Onot option</Primary></IndexTerm>
1412 <IndexTerm><Primary>optimising, reset</Primary></IndexTerm>
1416 This option will make GHC “forget” any <Option>-O</Option>ish options it has seen so
1417 far. Sometimes useful; for example: <Command>make all EXTRA_HC_OPTS=-Onot</Command>.
1422 <Term><Option>-Ofile <file></Option>:</Term>
1425 <IndexTerm><Primary>-Ofile <file> option</Primary></IndexTerm>
1426 <IndexTerm><Primary>optimising, customised</Primary></IndexTerm>
1430 For those who need <Emphasis>absolute</Emphasis> control over <Emphasis>exactly</Emphasis>
1431 what options are used (e.g., compiler writers, sometimes :-), a list
1432 of options can be put in a file and then slurped in with <Option>-Ofile</Option>.
1436 In that file, comments are of the <Literal>#</Literal>-to-end-of-line variety; blank
1437 lines and most whitespace is ignored.
1441 Please ask if you are baffled and would like an example of <Option>-Ofile</Option>!
1449 At Glasgow, we don't use a <Option>-O*</Option> flag for day-to-day work. We use
1450 <Option>-O</Option> to get respectable speed; e.g., when we want to measure
1451 something. When we want to go for broke, we tend to use <Option>-O -fvia-C
1452 -O2-for-C</Option> (and we go for lots of coffee breaks).
1456 The easiest way to see what <Option>-O</Option> (etc.) “really mean” is to run with
1457 <Option>-v</Option>, then stand back in amazement. Alternatively, just look at the
1458 <Literal>HsC_minus<blah></Literal> lists in the GHC driver script.
1464 <Title><Option>-f*</Option>: platform-independent flags</Title>
1467 <IndexTerm><Primary>-f* options (GHC)</Primary></IndexTerm>
1468 <IndexTerm><Primary>-fno-* options (GHC)</Primary></IndexTerm>
1472 Flags can be turned <Emphasis>off</Emphasis> individually. (NB: I hope you have a
1473 good reason for doing this…) To turn off the <Option>-ffoo</Option> flag, just use
1474 the <Option>-fno-foo</Option> flag.<IndexTerm><Primary>-fno-<opt> anti-option</Primary></IndexTerm> So, for
1475 example, you can say <Option>-O2 -fno-strictness</Option>, which will then drop out
1476 any running of the strictness analyser.
1480 The options you are most likely to want to turn off are:
1486 <Option>-fno-strictness</Option><IndexTerm><Primary>-fno-strictness option</Primary></IndexTerm> (strictness
1487 analyser, because it is sometimes slow),
1493 <Option>-fno-specialise</Option><IndexTerm><Primary>-fno-specialise option</Primary></IndexTerm> (automatic
1494 specialisation of overloaded functions, because it can make your code
1495 bigger) (US spelling also accepted), and
1501 <Option>-fno-cpr-analyse</Option><IndexTerm><Primary>-fno-cpr-analyse option</Primary></IndexTerm> switches off the CPR (constructed product
1511 Should you wish to turn individual flags <Emphasis>on</Emphasis>, you are advised
1512 to use the <Option>-Ofile</Option> option, described above. Because the order in
1513 which optimisation passes are run is sometimes crucial, it's quite
1514 hard to do with command-line options.
1518 Here are some “dangerous” optimisations you <Emphasis>might</Emphasis> want to try:
1522 <Term><Option>-fvia-C</Option>:</Term>
1525 <IndexTerm><Primary>-fvia-C option</Primary></IndexTerm>
1526 <IndexTerm><Primary>native code generator, turning off</Primary></IndexTerm>
1530 Compile via C, and don't use the native-code generator. (There are
1531 many cases when GHC does this on its own.) You might pick up a little
1532 bit of speed by compiling via C. If you use <Function>_ccall_gc_</Function>s or
1533 <Function>_casm_</Function>s, you probably <Emphasis>have</Emphasis> to use <Option>-fvia-C</Option>.
1537 The lower-case incantation, <Option>-fvia-c</Option>, is synonymous.
1541 Compiling via C will probably be slower (in compilation time) than
1542 using GHC's native code generator.
1547 <Term><Option>-funfolding-interface-threshold<n></Option>:</Term>
1550 <IndexTerm><Primary>-funfolding-interface-threshold option</Primary></IndexTerm>
1551 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
1552 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
1553 (Default: 30) By raising or lowering this number, you can raise or
1554 lower the amount of pragmatic junk that gets spewed into interface
1555 files. (An unfolding has a “size” that reflects the cost in terms
1556 of “code bloat” of expanding that unfolding in another module. A
1557 bigger function would be assigned a bigger cost.)
1562 <Term><Option>-funfolding-creation-threshold<n></Option>:</Term>
1565 <IndexTerm><Primary>-funfolding-creation-threshold option</Primary></IndexTerm>
1566 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
1567 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
1568 (Default: 30) This option is similar to
1569 <Option>-funfolding-interface-threshold</Option>, except that it governs unfoldings
1570 within a single module. Increasing this figure is more likely to
1571 result in longer compile times than faster code. The next option is
1577 <Term><Option>-funfolding-use-threshold<n></Option>:</Term>
1580 <IndexTerm><Primary>-funfolding-use-threshold option</Primary></IndexTerm>
1581 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
1582 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
1583 (Default: 8) This is the magic cut-off figure for unfolding: below
1584 this size, a function definition will be unfolded at the call-site,
1585 any bigger and it won't. The size computed for a function depends on
1586 two things: the actual size of the expression minus any discounts that
1587 apply (see <Option>-funfolding-con-discount</Option>).
1592 <Term><Option>-funfolding-con-discount<n></Option>:</Term>
1595 <IndexTerm><Primary>-funfolding-con-discount option</Primary></IndexTerm>
1596 <IndexTerm><Primary>inlining, controlling</Primary></IndexTerm>
1597 <IndexTerm><Primary>unfolding, controlling</Primary></IndexTerm>
1598 (Default: 2) If the compiler decides that it can eliminate some
1599 computation by performing an unfolding, then this is a discount factor
1600 that it applies to the funciton size before deciding whether to unfold
1605 OK, folks, these magic numbers `30', `8', and '2' are mildly
1606 arbitrary; they are of the “seem to be OK” variety. The `8' is the
1607 more critical one; it's what determines how eager GHC is about
1608 expanding unfoldings.
1613 <Term><Option>-funbox-strict-fields</Option>:</Term>
1616 <IndexTerm><Primary>-funbox-strict-fields option</Primary></IndexTerm>
1617 <IndexTerm><Primary>strict constructor fields</Primary></IndexTerm>
1618 <IndexTerm><Primary>constructor fields, strict</Primary></IndexTerm>
1622 This option causes all constructor fields which are marked strict
1623 (i.e. “!”) to be unboxed or unpacked if possible. For example:
1629 data T = T !Float !Float
1635 will create a constructor <Literal>T</Literal> containing two unboxed floats if the
1636 <Option>-funbox-strict-fields</Option> flag is given. This may not always be an
1637 optimisation: if the <Function>T</Function> constructor is scrutinised and the floats
1638 passed to a non-strict function for example, they will have to be
1639 reboxed (this is done automatically by the compiler).
1643 This option should only be used in conjunction with <Option>-O</Option>, in order to
1644 expose unfoldings to the compiler so the reboxing can be removed as
1645 often as possible. For example:
1652 f (T f1 f2) = f1 + f2
1658 The compiler will avoid reboxing <Function>f1</Function> and <Function>f2</Function> by inlining <Function>+</Function> on
1659 floats, but only when <Option>-O</Option> is on.
1663 Any single-constructor data is eligible for unpacking; for example
1669 data T = T !(Int,Int)
1675 will store the two <Literal>Int</Literal>s directly in the <Function>T</Function> constructor, by flattening
1676 the pair. Multi-level unpacking is also supported:
1683 data S = S !Int !Int
1689 will store two unboxed <Literal>Int#</Literal>s directly in the <Function>T</Function> constructor.
1694 <Term><Option>-fsemi-tagging</Option>:</Term>
1697 This option (which <Emphasis>does not work</Emphasis> with the native-code generator)
1698 tells the compiler to add extra code to test for already-evaluated
1699 values. You win if you have lots of such values during a run of your
1700 program, you lose otherwise. (And you pay in extra code space.)
1704 We have not played with <Option>-fsemi-tagging</Option> enough to recommend it.
1705 (For all we know, it doesn't even work anymore… Sigh.)
1715 <Title><Option>-m*</Option>: platform-specific flags</Title>
1718 <IndexTerm><Primary>-m* options (GHC)</Primary></IndexTerm>
1719 <IndexTerm><Primary>platform-specific options</Primary></IndexTerm>
1720 <IndexTerm><Primary>machine-specific options</Primary></IndexTerm>
1724 Some flags only make sense for particular target platforms.
1731 <Term><Option>-mv8</Option>:</Term>
1734 (SPARC machines)<IndexTerm><Primary>-mv8 option (SPARC only)</Primary></IndexTerm>
1735 Means to pass the like-named option to GCC; it says to use the
1736 Version 8 SPARC instructions, notably integer multiply and divide.
1737 The similiar <Option>-m*</Option> GCC options for SPARC also work, actually.
1742 <Term><Option>-mlong-calls</Option>:</Term>
1745 (HPPA machines)<IndexTerm><Primary>-mlong-calls option (HPPA only)</Primary></IndexTerm>
1746 Means to pass the like-named option to GCC. Required for Very Big
1747 modules, maybe. (Probably means you're in trouble…)
1752 <Term><Option>-monly-[32]-regs</Option>:</Term>
1755 (iX86 machines)<IndexTerm><Primary>-monly-N-regs option (iX86 only)</Primary></IndexTerm>
1756 GHC tries to “steal” four registers from GCC, for performance
1757 reasons; it almost always works. However, when GCC is compiling some
1758 modules with four stolen registers, it will crash, probably saying:
1761 Foo.hc:533: fixed or forbidden register was spilled.
1762 This may be due to a compiler bug or to impossible asm
1763 statements or clauses.
1766 Just give some registers back with <Option>-monly-N-regs</Option>. Try `3' first,
1767 then `2'. If `2' doesn't work, please report the bug to us.
1776 <Sect2 id="optimise-C-compiler">
1777 <Title>Code improvement by the C compiler.
1781 <IndexTerm><Primary>optimisation by GCC</Primary></IndexTerm>
1782 <IndexTerm><Primary>GCC optimisation</Primary></IndexTerm>
1786 The C compiler (GCC) is run with <Option>-O</Option> turned on. (It has
1791 If you want to run GCC with <Option>-O2</Option>—which may be worth a few
1792 percent in execution speed—you can give a
1793 <Option>-O2-for-C</Option><IndexTerm><Primary>-O2-for-C option</Primary></IndexTerm> option.
1800 <Sect1 id="options-phases">
1801 <Title>Options related to a particular phase
1804 <Sect2 id="c-pre-processor">
1805 <Title>The C pre-processor
1809 <IndexTerm><Primary>pre-processing: cpp</Primary></IndexTerm>
1810 <IndexTerm><Primary>C pre-processor options</Primary></IndexTerm>
1811 <IndexTerm><Primary>cpp, pre-processing with</Primary></IndexTerm>
1815 The C pre-processor <Command>cpp</Command> is run over your Haskell code only if the
1816 <Option>-cpp</Option> option <IndexTerm><Primary>-cpp option</Primary></IndexTerm> is given. Unless you are
1817 building a large system with significant doses of conditional
1818 compilation, you really shouldn't need it.
1822 <Term><Option>-D<foo></Option>:</Term>
1825 <IndexTerm><Primary>-D<name> option</Primary></IndexTerm>
1826 Define macro <Constant><foo></Constant> in the usual way. NB: does <Emphasis>not</Emphasis> affect
1827 <Option>-D</Option> macros passed to the C compiler when compiling via C! For those,
1828 use the <Option>-optc-Dfoo</Option> hack… (see <XRef LinkEnd="forcing-options-through">).
1833 <Term><Option>-U<foo></Option>:</Term>
1836 <IndexTerm><Primary>-U<name> option</Primary></IndexTerm>
1837 Undefine macro <Command><foo></Command> in the usual way.
1842 <Term><Option>-I<dir></Option>:</Term>
1845 <IndexTerm><Primary>-I<dir> option</Primary></IndexTerm>
1846 Specify a directory in which to look for <Literal>#include</Literal> files, in
1855 The GHC driver pre-defines several macros when processing Haskell
1856 source code (<Filename>.hs</Filename> or <Filename>.lhs</Filename> files):
1863 <Term><Constant>__HASKELL98__</Constant>:</Term>
1866 <IndexTerm><Primary>__HASKELL98__</Primary></IndexTerm>
1867 If defined, this means that GHC supports the language defined by the
1873 <Term><Constant>__HASKELL__=98</Constant>:</Term>
1876 <IndexTerm><Primary>__HASKELL__</Primary></IndexTerm>
1877 In GHC 4.04 and later, the <Constant>__HASKELL__</Constant> macro is defined as having
1878 the value <Constant>98</Constant>.
1883 <Term><Constant>__HASKELL1__</Constant>:</Term>
1886 <IndexTerm><Primary>__HASKELL1__ macro</Primary></IndexTerm>
1887 If defined to <Emphasis>n</Emphasis>, that means GHC supports the Haskell language
1888 defined in the Haskell report version <Emphasis>1.n</Emphasis>. Currently 5. This
1889 macro is deprecated, and will probably disappear in future versions.
1894 <Term><Constant>__GLASGOW_HASKELL__</Constant>:</Term>
1897 <IndexTerm><Primary>__GLASGOW_HASKELL__ macro</Primary></IndexTerm>
1898 For version <Emphasis>n</Emphasis> of the GHC system, this will be <Literal>#define</Literal>d to
1899 <Emphasis>100n</Emphasis>. So, for version 4.00, it is 400.
1903 With any luck, <Constant>__GLASGOW_HASKELL__</Constant> will be undefined in all other
1904 implementations that support C-style pre-processing.
1908 (For reference: the comparable symbols for other systems are:
1909 <Constant>__HUGS__</Constant> for Hugs and <Constant>__HBC__</Constant> for Chalmers.)
1913 NB. This macro is set when pre-processing both Haskell source and C
1914 source, including the C source generated from a Haskell module
1915 (i.e. <Filename>.hs</Filename>, <Filename>.lhs</Filename>, <Filename>.c</Filename> and <Filename>.hc</Filename> files).
1920 <Term><Constant>__CONCURRENT_HASKELL__</Constant>:</Term>
1923 <IndexTerm><Primary>__CONCURRENT_HASKELL__ macro</Primary></IndexTerm>
1924 This symbol is defined when pre-processing Haskell (input) and
1925 pre-processing C (GHC output). Since GHC from verion 4.00 now
1926 supports concurrent haskell by default, this symbol is always defined.
1931 <Term><Constant>__PARALLEL_HASKELL__</Constant>:</Term>
1934 <IndexTerm><Primary>__PARALLEL_HASKELL__ macro</Primary></IndexTerm>
1935 Only defined when <Option>-parallel</Option> is in use! This symbol is defined when
1936 pre-processing Haskell (input) and pre-processing C (GHC output).
1944 Options other than the above can be forced through to the C
1945 pre-processor with the <Option>-opt</Option> flags (see
1946 <XRef LinkEnd="forcing-options-through">).
1950 A small word of warning: <Option>-cpp</Option> is not friendly to “string
1951 gaps”.<IndexTerm><Primary>-cpp vs string gaps</Primary></IndexTerm><IndexTerm><Primary>string gaps vs
1952 -cpp</Primary></IndexTerm>. In other words, strings such as the following:
1966 don't work with <Option>-cpp</Option>; <Filename>/usr/bin/cpp</Filename> elides the
1967 backslash-newline pairs.
1971 However, it appears that if you add a space at the end of the line,
1972 then <Command>cpp</Command> (at least GNU <Command>cpp</Command> and possibly other <Command>cpp</Command>s)
1973 leaves the backslash-space pairs alone and the string gap works as
1979 <Sect2 id="options-C-compiler">
1980 <Title>Options affecting the C compiler (if applicable)
1984 <IndexTerm><Primary>include-file options</Primary></IndexTerm>
1985 <IndexTerm><Primary>C compiler options</Primary></IndexTerm>
1986 <IndexTerm><Primary>GCC options</Primary></IndexTerm>
1990 At the moment, quite a few common C-compiler options are passed on
1991 quietly to the C compilation of Haskell-compiler-generated C files.
1992 THIS MAY CHANGE. Meanwhile, options so sent are:
1999 <ColSpec Align="Left" Colsep="0">
2000 <ColSpec Align="Left" Colsep="0">
2003 <Entry><Option>-ansi</Option> </Entry>
2004 <Entry> do ANSI C (not K&R) </Entry>
2008 <Option>-pedantic</Option> </Entry>
2009 <Entry> be so</Entry>
2013 <Option>-dgcc-lint</Option> </Entry>
2014 <Entry> (hack) short for “make GCC very paranoid”</Entry>
2022 <IndexTerm><Primary>-ansi option (for GCC)</Primary></IndexTerm>
2023 <IndexTerm><Primary>-pedantic option (for GCC)</Primary></IndexTerm>
2024 <IndexTerm><Primary>-dgcc-lint option (GCC paranoia)</Primary></IndexTerm>
2028 If you are compiling with lots of <Literal>ccalls</Literal>, etc., you may need to
2029 tell the C compiler about some <Literal>#include</Literal> files. There is no real
2030 pretty way to do this, but you can use this hack from the
2037 % ghc -c '-#include <X/Xlib.h>' Xstuff.lhs
2044 <Sect2 id="options-linker">
2045 <Title>Linking and consistency-checking
2049 <IndexTerm><Primary>linker options</Primary></IndexTerm>
2050 <IndexTerm><Primary>ld options</Primary></IndexTerm>
2054 GHC has to link your code with various libraries, possibly including:
2055 user-supplied, GHC-supplied, and system-supplied (<Option>-lm</Option> math
2056 library, for example).
2063 <Term><Option>-l<FOO></Option>:</Term>
2066 <IndexTerm><Primary>-l<lib> option</Primary></IndexTerm>
2067 Link in a library named <Filename>lib<FOO>.a</Filename> which resides somewhere on the
2068 library directories path.
2072 Because of the sad state of most UNIX linkers, the order of such
2073 options does matter. Thus: <Command>ghc -lbar *.o</Command> is almost certainly
2074 wrong, because it will search <Filename>libbar.a</Filename> <Emphasis>before</Emphasis> it has
2075 collected unresolved symbols from the <Filename>*.o</Filename> files.
2076 <Command>ghc *.o -lbar</Command> is probably better.
2080 The linker will of course be informed about some GHC-supplied
2081 libraries automatically; these are:
2088 <ColSpec Align="Left" Colsep="0">
2089 <ColSpec Align="Left" Colsep="0">
2092 <Entry><Emphasis>-l equivalent</Emphasis> </Entry>
2093 <Entry> <Emphasis>description</Emphasis> </Entry>
2098 <Option>-lHSrts,-lHSclib</Option> </Entry>
2099 <Entry> basic runtime libraries </Entry>
2103 <Option>-lHS</Option> </Entry>
2104 <Entry> standard Prelude library </Entry>
2108 <Option>-lHS_cbits</Option> </Entry>
2109 <Entry> C support code for standard Prelude library </Entry>
2113 <Option>-lgmp</Option> </Entry>
2114 <Entry> GNU multi-precision library (for Integers)</Entry>
2125 <IndexTerm><Primary>-lHS library</Primary></IndexTerm>
2126 <IndexTerm><Primary>-lHS_cbits library</Primary></IndexTerm>
2127 <IndexTerm><Primary>-lHSrts library</Primary></IndexTerm>
2128 <IndexTerm><Primary>-lgmp library</Primary></IndexTerm>
2133 <Term><Option>-syslib <name></Option>:</Term>
2136 <IndexTerm><Primary>-syslib <name> option</Primary></IndexTerm>
2140 If you are using a Haskell “system library” (e.g., the POSIX
2141 library), just use the <Option>-syslib posix</Option> option, and the correct code
2142 should be linked in.
2147 <Term><Option>-L<dir></Option>:</Term>
2150 <IndexTerm><Primary>-L<dir> option</Primary></IndexTerm>
2151 Where to find user-supplied libraries… Prepend the directory
2152 <Filename><dir></Filename> to the library directories path.
2157 <Term><Option>-static</Option>:</Term>
2160 <IndexTerm><Primary>-static option</Primary></IndexTerm>
2161 Tell the linker to avoid shared libraries.
2166 <Term><Option>-no-link-chk</Option> and <Option>-link-chk</Option>:</Term>
2169 <IndexTerm><Primary>-no-link-chk option</Primary></IndexTerm>
2170 <IndexTerm><Primary>-link-chk option</Primary></IndexTerm>
2171 <IndexTerm><Primary>consistency checking of executables</Primary></IndexTerm>
2172 By default, immediately after linking an executable, GHC verifies that
2173 the pieces that went into it were compiled with compatible flags; a
2174 “consistency check”.
2175 (This is to avoid mysterious failures caused by non-meshing of
2176 incompatibly-compiled programs; e.g., if one <Filename>.o</Filename> file was compiled
2177 for a parallel machine and the others weren't.) You may turn off this
2178 check with <Option>-no-link-chk</Option>. You can turn it (back) on with
2179 <Option>-link-chk</Option> (the default).
2184 <Term><Option>-no-hs-main</Option>:</Term>
2187 <IndexTerm><Primary>-no-hs-main option</Primary></IndexTerm>
2188 <IndexTerm><Primary>linking Haskell libraries with foreign code</Primary></IndexTerm>
2192 In the event you want to include ghc-compiled code as part of another
2193 (non-Haskell) program, the RTS will not be supplying its definition of
2194 <Function>main()</Function> at link-time, you will have to. To signal that to the
2195 driver script when linking, use <Option>-no-hs-main</Option>.
2199 Notice that since the command-line passed to the linker is rather
2200 involved, you probably want to use the ghc driver script to do the
2201 final link of your `mixed-language' application. This is not a
2202 requirement though, just try linking once with <Option>-v</Option> on to see what
2203 options the driver passes through to the linker.
2215 <Title>Using Concurrent Haskell</Title>
2218 <IndexTerm><Primary>Concurrent Haskell—use</Primary></IndexTerm>
2222 GHC (as of version 4.00) supports Concurrent Haskell by default,
2223 without requiring a special option or libraries compiled in a certain
2224 way. To get access to the support libraries for Concurrent Haskell
2225 (i.e. <Literal>Concurrent</Literal> and friends), use the <Option>-syslib concurrent</Option> option.
2229 Three RTS options are provided for modifying the behaviour of the
2230 threaded runtime system. See the descriptions of <Option>-C[<us>]</Option>, <Option>-q</Option>,
2231 and <Option>-t<num></Option> in <XRef LinkEnd="parallel-rts-opts">.
2235 Concurrent Haskell is described in more detail in <XRef LinkEnd="concurrent-and-parallel">.
2241 <Title>Using Parallel Haskell</Title>
2244 <IndexTerm><Primary>Parallel Haskell—use</Primary></IndexTerm>
2248 [You won't be able to execute parallel Haskell programs unless PVM3
2249 (Parallel Virtual Machine, version 3) is installed at your site.]
2253 To compile a Haskell program for parallel execution under PVM, use the
2254 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel option</Primary></IndexTerm> both when compiling
2255 <Emphasis>and linking</Emphasis>. You will probably want to <Literal>import Parallel</Literal>
2256 into your Haskell modules.
2260 To run your parallel program, once PVM is going, just invoke it “as
2261 normal”. The main extra RTS option is <Option>-N<n></Option>, to say how many
2262 PVM “processors” your program to run on. (For more details of
2263 all relevant RTS options, please see <XRef LinkEnd="parallel-rts-opts">.)
2267 In truth, running Parallel Haskell programs and getting information
2268 out of them (e.g., parallelism profiles) is a battle with the vagaries of
2269 PVM, detailed in the following sections.
2273 <Title>Dummy's guide to using PVM</Title>
2276 <IndexTerm><Primary>PVM, how to use</Primary></IndexTerm>
2277 <IndexTerm><Primary>Parallel Haskell—PVM use</Primary></IndexTerm>
2278 Before you can run a parallel program under PVM, you must set the
2279 required environment variables (PVM's idea, not ours); something like,
2280 probably in your <Filename>.cshrc</Filename> or equivalent:
2283 setenv PVM_ROOT /wherever/you/put/it
2284 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
2285 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
2291 Creating and/or controlling your “parallel machine” is a purely-PVM
2292 business; nothing specific to Parallel Haskell.
2296 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
2297 machine. You can then do various things to control/monitor your
2298 “parallel machine;” the most useful being:
2304 <ColSpec Align="Left">
2308 <Entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></Entry>
2309 <Entry>exit <Command>pvm</Command>, leaving it running</Entry>
2313 <Entry><Command>halt</Command></Entry>
2314 <Entry>kill off this “parallel machine” & exit</Entry>
2318 <Entry><Command>add <host></Command></Entry>
2319 <Entry>add <Command><host></Command> as a processor</Entry>
2323 <Entry><Command>delete <host></Command></Entry>
2324 <Entry>delete <Command><host></Command></Entry>
2328 <Entry><Command>reset</Command></Entry>
2329 <Entry>kill what's going, but leave PVM up</Entry>
2333 <Entry><Command>conf</Command></Entry>
2334 <Entry>list the current configuration</Entry>
2338 <Entry><Command>ps</Command></Entry>
2339 <Entry>report processes' status</Entry>
2343 <Entry><Command>pstat <pid></Command></Entry>
2344 <Entry>status of a particular process</Entry>
2353 The PVM documentation can tell you much, much more about <Command>pvm</Command>!
2359 <Title>Parallelism profiles</Title>
2362 <IndexTerm><Primary>parallelism profiles</Primary></IndexTerm>
2363 <IndexTerm><Primary>profiles, parallelism</Primary></IndexTerm>
2364 <IndexTerm><Primary>visualisation tools</Primary></IndexTerm>
2368 With Parallel Haskell programs, we usually don't care about the
2369 results—only with “how parallel” it was! We want pretty pictures.
2373 Parallelism profiles (à la <Command>hbcpp</Command>) can be generated with the
2374 <Option>-q</Option><IndexTerm><Primary>-q RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option. The
2375 per-processor profiling info is dumped into files named
2376 <Filename><full-path><program>.gr</Filename>. These are then munged into a PostScript picture,
2377 which you can then display. For example, to run your program
2378 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
2384 % ./a.out +RTS -N8 -q
2385 % grs2gr *.???.gr > temp.gr # combine the 8 .gr files into one
2386 % gr2ps -O temp.gr # cvt to .ps; output in temp.ps
2387 % ghostview -seascape temp.ps # look at it!
2393 The scripts for processing the parallelism profiles are distributed
2394 in <Filename>ghc/utils/parallel/</Filename>.
2400 <Title>Other useful info about running parallel programs</Title>
2403 The “garbage-collection statistics” RTS options can be useful for
2404 seeing what parallel programs are doing. If you do either
2405 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
2406 you'll get mutator, garbage-collection, etc., times on standard
2407 error. The standard error of all PE's other than the `main thread'
2408 appears in <Filename>/tmp/pvml.nnn</Filename>, courtesy of PVM.
2412 Whether doing <Option>+RTS -Sstderr</Option> or not, a handy way to watch
2413 what's happening overall is: <Command>tail -f /tmp/pvml.nnn</Command>.
2418 <Sect2 id="parallel-rts-opts">
2419 <Title>RTS options for Concurrent/Parallel Haskell
2423 <IndexTerm><Primary>RTS options, concurrent</Primary></IndexTerm>
2424 <IndexTerm><Primary>RTS options, parallel</Primary></IndexTerm>
2425 <IndexTerm><Primary>Concurrent Haskell—RTS options</Primary></IndexTerm>
2426 <IndexTerm><Primary>Parallel Haskell—RTS options</Primary></IndexTerm>
2430 Besides the usual runtime system (RTS) options
2431 (<XRef LinkEnd="runtime-control">), there are a few options particularly
2432 for concurrent/parallel execution.
2439 <Term><Option>-N<N></Option>:</Term>
2442 <IndexTerm><Primary>-N<N> RTS option (parallel)</Primary></IndexTerm>
2443 (PARALLEL ONLY) Use <Literal><N></Literal> PVM processors to run this program;
2449 <Term><Option>-C[<us>]</Option>:</Term>
2452 <IndexTerm><Primary>-C<us> RTS option</Primary></IndexTerm>
2453 Sets the context switch interval to <Literal><us></Literal> microseconds. A context
2454 switch will occur at the next heap allocation after the timer expires.
2455 With <Option>-C0</Option> or <Option>-C</Option>, context switches will occur as often as
2456 possible (at every heap allocation). By default, context switches
2457 occur every 10 milliseconds. Note that many interval timers are only
2458 capable of 10 millisecond granularity, so the default setting may be
2459 the finest granularity possible, short of a context switch at every
2464 [NOTE: this option currently has no effect (version 4.00). Context
2465 switches happen when the current heap block is full, i.e. every 4k of
2471 <Term><Option>-q[v]</Option>:</Term>
2474 <IndexTerm><Primary>-q RTS option</Primary></IndexTerm>
2475 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
2476 in the file <FIlename><program>.qp</FIlename>. In the style of <Command>hbcpp</Command>, this profile
2477 records the movement of threads between the green (runnable) and red
2478 (blocked) queues. If you specify the verbose suboption (<Option>-qv</Option>), the
2479 green queue is split into green (for the currently running thread
2480 only) and amber (for other runnable threads). We do not recommend
2481 that you use the verbose suboption if you are planning to use the
2482 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
2483 check (with <Option>-C</Option>).
2488 <Term><Option>-t<num></Option>:</Term>
2491 <IndexTerm><Primary>-t<num> RTS option</Primary></IndexTerm>
2492 (PARALLEL ONLY) Limit the number of concurrent threads per processor
2493 to <Literal><num></Literal>. The default is 32. Each thread requires slightly over 1K
2494 <Emphasis>words</Emphasis> in the heap for thread state and stack objects. (For
2495 32-bit machines, this translates to 4K bytes, and for 64-bit machines,
2501 <Term><Option>-d</Option>:</Term>
2504 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
2505 (PARALLEL ONLY) Turn on debugging. It pops up one xterm (or GDB, or
2506 something…) per PVM processor. We use the standard <Command>debugger</Command>
2507 script that comes with PVM3, but we sometimes meddle with the
2508 <Command>debugger2</Command> script. We include ours in the GHC distribution,
2509 in <Filename>ghc/utils/pvm/</Filename>.
2514 <Term><Option>-e<num></Option>:</Term>
2517 <IndexTerm><Primary>-e<num> RTS option (parallel)</Primary></IndexTerm>
2518 (PARALLEL ONLY) Limit the number of pending sparks per processor to
2519 <Literal><num></Literal>. The default is 100. A larger number may be appropriate if
2520 your program generates large amounts of parallelism initially.
2525 <Term><Option>-Q<num></Option>:</Term>
2528 <IndexTerm><Primary>-Q<num> RTS option (parallel)</Primary></IndexTerm>
2529 (PARALLEL ONLY) Set the size of packets transmitted between processors
2530 to <Literal><num></Literal>. The default is 1024 words. A larger number may be
2531 appropriate if your machine has a high communication cost relative to