1 <sect1 id="separate-compilation">
2 <title>Filenames and separate compilation</title>
4 <indexterm><primary>separate compilation</primary></indexterm>
5 <indexterm><primary>recompilation checker</primary></indexterm>
6 <indexterm><primary>make and recompilation</primary></indexterm>
8 <para>This section describes what files GHC expects to find, what
9 files it creates, where these files are stored, and what options
10 affect this behaviour.</para>
12 <para>Note that this section is written with
13 <firstterm>hierarchical modules</firstterm> in mind (see <xref
14 linkend="hierarchical-modules">); hierarchical modules are an
15 extension to Haskell 98 which extends the lexical syntax of
16 module names to include a dot ‘.’. Non-hierarchical
17 modules are thus a special case in which none of the module names
20 <para>Pathname conventions vary from system to system. In
21 particular, the directory separator is
22 ‘<literal>/</literal>’ on Unix systems and
23 ‘<literal>\</literal>’ on Windows systems. In the
24 sections that follow, we shall consistently use
25 ‘<literal>/</literal>’ as the directory separator;
26 substitute this for the appropriate character for your
29 <sect2 id="source-files">
30 <title>Haskell source files</title>
32 <para>Each Haskell source module should be placed in a file on
35 <para>The file should usually be named after the module name, by
36 replacing dots in the module name by directory separators. For
37 example, on a Unix system, the module <literal>A.B.C</literal>
38 should be placed in the file <literal>A/B/C.hs</literal>,
39 relative to some base directory. GHC's behaviour if this rule
40 is not followed is fully defined by the following section (<xref
41 linkend="output-files">).</para>
44 <sect2 id="output-files">
45 <title>Output files</title>
47 <indexterm><primary>interface files</primary></indexterm>
48 <indexterm><primary><literal>.hi</literal> files</primary></indexterm>
49 <indexterm><primary>object files</primary></indexterm>
50 <indexterm><primary><literal>.o</literal> files</primary></indexterm>
52 <para>When asked to compile a source file, GHC normally
53 generates two files: an <firstterm>object file</firstterm>, and
54 an <firstterm>interface file</firstterm>. </para>
56 <para>The object file, which normally ends in a
57 <literal>.o</literal> suffix (or <literal>.obj</literal> if
58 you're on Windows), contains the compiled code for the module.</para>
60 <para>The interface file,
61 which normally ends in a <literal>.hi</literal> suffix, contains
62 the information that GHC needs in order to compile further
63 modules that depend on this module. It contains things like the
64 types of exported functions, definitions of data types, and so
65 on. It is stored in a binary format, so don't try to read one;
66 use the <option>--show-iface</option> option instead (see <xref
67 linkend="hi-options">).</para>
69 <para>You should think of the object file and the interface file as a
70 pair, since the interface file is in a sense a compiler-readable
71 description of the contents of the object file. If the
72 interface file and object file get out of sync for any reason,
73 then the compiler may end up making assumptions about the object
74 file that aren't true; trouble will almost certainly follow.
75 For this reason, we recommend keeping object files and interface
76 files in the same place (GHC does this by default, but it is
77 possible to override the defaults as we'll explain
80 <para>Every module has a <emphasis>module name</emphasis>
81 defined in its source code (<literal>module A.B.C where
82 ...</literal>). Unless overridden with the
83 <literal>-o</literal> and <literal>-ohi</literal> flags
84 respectively, GHC always puts the object file for module
85 <literal>A.B.C</literal> in
86 <replaceable>odir</replaceable><literal>/A/B/C.</literal><replaceable>osuf</replaceable>,
87 and the interface file in the file
88 <replaceable>hidir</replaceable><literal>/A/B/C.</literal><replaceable>hisuf</replaceable>,
89 where <replaceable>hidir</replaceable>,
90 <replaceable>hisuf</replaceable>,
91 <replaceable>odir</replaceable>, and
92 <replaceable>osuf</replaceable>, defined as follows:
96 <term><replaceable>hidir</replaceable></term>
98 <para>is the value of the <option>-hidir</option> option if
99 one was given (see below), or
100 <replaceable>root-path</replaceable> otherwise.</para>
104 <term><replaceable>hisuf</replaceable></term>
106 <para>is the value of the <option>-hisuf</option> option if
107 one was given (see below), or <literal>hi</literal>
113 <term><replaceable>odir</replaceable></term>
115 <para>is the value of the <option>-odir</option> option if
116 one was given (see below), or
117 <replaceable>root-path</replaceable> otherwise.</para>
121 <term><replaceable>osuf</replaceable></term>
123 <para>is the value of the <option>-osuf</option> option if
124 one was given (see below), or <literal>o</literal>
125 otherwise (<literal>obj</literal> on Windows).</para>
130 The <replaceable>root-path</replaceable>, used in the above definitions, is derived from the
131 location of the source file, <replaceable>source-filename</replaceable>, as follows:
137 <para>GHC matches <replaceable>source-filename</replaceable> against the pattern:
139 <screen><replaceable>root-path</replaceable>/<literal>A/B/C.</literal><replaceable>extension</replaceable></screen>
145 <term><replaceable>extension</replaceable></term>
147 <para>is the source file extension (usually
148 <literal>.hs</literal> or <literal>.lhs</literal>).</para>
152 <term><replaceable>root-path</replaceable></term>
154 <para>is what is left after <literal>A/B/C.</literal><replaceable>extension</replaceable>
155 has been stripped off the end of <replaceable>source-file</replaceable>.</para>
166 <para>If <replaceable>source-filename</replaceable> does not match the pattern
167 above (presumably because it doesn't finish with <literal>A/B/C.hs</literal>
168 or <literal>A/B/C.lhs</literal>)
169 then <replaceable>root-path</replaceable> becomes the
170 whole of the directory portion of the filename. </para>
175 For example, if GHC compiles the module
176 <literal>A.B.C</literal> in the file
177 <filename>src/A/B/C.hs</filename>, with no <literal>-odir</literal> or <literal>-hidir</literal> flags,
178 the interface file will be put in <literal>src/A/B/C.hi</literal> and the object file in
179 <literal>src/A/B/C.o</literal> (using Rule 1).
180 If the same module <literal>A.B.C</literal> was in file
181 <filename>src/ABC.hs</filename>,
182 the interface file will still be put in <literal>src/A/B/C.hi</literal> and the object file in
183 <literal>src/A/B/C.o</literal> (using Rule 2).
185 <para>A common use for Rule 2 is to have many modules all called <literal>Main</literal> held in
186 files <literal>Test1.hs</literal> <literal>Test2.hs</literal>, etc. Beware, though: when compiling
187 (say) <literal>Test2.hs</literal>, GHC will consult <literal>Main.hi</literal> for version information
188 from the last recompilation. Currently (a bug, really) GHC is not clever enough to spot that the source file has changed,
189 and so there is a danger that the recompilation checker will declare that no recompilation is needed when in fact it is.
190 Solution: delete the interface file first.
192 <para>Notice that (unless overriden with <option>-o</option> or <option>-ohi</option>) the filenames
193 of the object and interface files are always based on the module name. The reason for this is so that
194 GHC can find the interface file for module <literal>A.B.C</literal> when compiling the declaration
195 "<literal>import A.B.C</literal>".
199 <sect2 id="search-path">
200 <title>The search path</title>
202 <indexterm><primary>search path</primary>
204 <indexterm><primary>interface files, finding them</primary></indexterm>
205 <indexterm><primary>finding interface files</primary></indexterm>
207 <para>In your program, you import a module
208 <literal>Foo</literal> by saying <literal>import Foo</literal>.
209 In <option>--make</option> mode or GHCi, GHC will look for a
210 source file for <literal>Foo</literal> and arrange to compile it
211 first. Without <option>--make</option>, GHC will look for the
212 interface file for <literal>Foo</literal>, which should have
213 been created by an earlier compilation of
214 <literal>Foo</literal>. GHC uses the same strategy in each of
215 these cases for finding the appropriate file.</para>
217 <para>This strategy is as follows: GHC keeps a list of
218 directories called the <firstterm>search path</firstterm>. For
219 each of these directories, it tries appending
220 <replaceable>basename</replaceable><literal>.</literal><replaceable>extension</replaceable>
221 to the directory, and checks whether the file exists. The value
222 of <replaceable>basename</replaceable> is the module name with
223 dots replaced by the directory separator ('/' or '\', depending
224 on the system), and <replaceable>extension</replaceable> is a
225 source extension (<literal>hs</literal>, <literal>lhs</literal>)
226 if we are in <option>--make</option> mode and GHCi, or
227 <replaceable>hisuf</replaceable> otherwise.</para>
229 <para>For example, suppose the search path contains directories
230 <literal>d1</literal>, <literal>d2</literal>, and
231 <literal>d3</literal>, and we are in <literal>--make</literal>
232 mode looking for the source file for a module
233 <literal>A.B.C</literal>. GHC will look in
234 <literal>d1/A/B/C.hs</literal>, <literal>d1/A/B/C.lhs</literal>,
235 <literal>d2/A/B/C.hs</literal>, and so on.</para>
237 <para>The search path by default contains a single directory:
238 <quote>.</quote> (i.e. the current directory). The following
239 options can be used to add to or change the contents of the
244 <term><option>-i<replaceable>dirs</replaceable></option></term>
246 <para><indexterm><primary><option>-i<replaceable>dirs</replaceable></option>
247 </primary></indexterm>This flag appends a colon-separated
248 list of <filename>dirs</filename> to the search path.</para>
253 <term><option>-i</option></term>
255 <para>resets the search path back to nothing.</para>
260 <para>This isn't the whole story: GHC also looks for modules in
261 pre-compiled libraries, known as packages. See the section on
262 packages (<xref linkend="packages">), for details.</para>
265 <sect2 id="options-output">
266 <title>Redirecting the compilation output(s)</title>
268 <indexterm><primary>output-directing options</primary></indexterm>
269 <indexterm><primary>redirecting compilation output</primary></indexterm>
273 <term><option>-o</option> <replaceable>file</replaceable></term>
274 <indexterm><primary><option>-o</option></primary></indexterm>
276 <para>GHC's compiled output normally goes into a
277 <filename>.hc</filename>, <filename>.o</filename>, etc.,
278 file, depending on the last-run compilation phase. The
279 option <option>-o <replaceable>file</replaceable></option>
280 re-directs the output of that last-run phase to
281 <replaceable>file</replaceable>.</para>
283 <para>Note: this “feature” can be
284 counterintuitive: <command>ghc -C -o foo.o
285 foo.hs</command> will put the intermediate C code in the
286 file <filename>foo.o</filename>, name
287 notwithstanding!</para>
289 <para>This option is most often used when creating an
290 executable file, to set the filename of the executable.
292 <screen> ghc -o prog --make Main</screen>
294 will compile the program starting with module
295 <literal>Main</literal> and put the executable in the
296 file <literal>prog</literal>.</para>
298 <para>Note: on Windows, if the result is an executable
299 file, the extension "<filename>.exe</filename>" is added
300 if the specified filename does not already have an
305 will compile and link the module
306 <filename>Main.hs</filename>, and put the resulting
307 executable in <filename>foo.exe</filename> (not
308 <filename>foo</filename>).</para>
313 <term><option>-odir</option> <replaceable>dir</replaceable></term>
314 <indexterm><primary><option>-odir</option></primary></indexterm>
316 <para>Redirects object files to directory
317 <replaceable>dir</replaceable>. For example:</para>
320 $ ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
323 <para>The object files, <filename>Foo.o</filename>,
324 <filename>Bar.o</filename>, and
325 <filename>Bumble.o</filename> would be put into a
326 subdirectory named after the architecture of the executing
327 machine (<filename>x86</filename>,
328 <filename>mips</filename>, etc).</para>
330 <para>Note that the <option>-odir</option> option does
331 <emphasis>not</emphasis> affect where the interface files
332 are put; use the <option>-hidir</option> option for that.
333 In the above example, they would still be put in
334 <filename>parse/Foo.hi</filename>,
335 <filename>parse/Bar.hi</filename>, and
336 <filename>gurgle/Bumble.hi</filename>.</para>
341 <term><option>-ohi</option> <replaceable>file</replaceable></term>
342 <indexterm><primary><option>-ohi</option></primary>
345 <para>The interface output may be directed to another file
346 <filename>bar2/Wurble.iface</filename> with the option
347 <option>-ohi bar2/Wurble.iface</option> (not
350 <para>WARNING: if you redirect the interface file
351 somewhere that GHC can't find it, then the recompilation
352 checker may get confused (at the least, you won't get any
353 recompilation avoidance). We recommend using a
354 combination of <option>-hidir</option> and
355 <option>-hisuf</option> options instead, if
358 <para>To avoid generating an interface at all, you could
359 use this option to redirect the interface into the bit
360 bucket: <literal>-ohi /dev/null</literal>, for
366 <term><option>-hidir</option> <replaceable>dir</replaceable></term>
367 <indexterm><primary><option>-hidir</option></primary>
370 <para>Redirects all generated interface files into
371 <replaceable>dir</replaceable>, instead of the
377 <term><option>-osuf</option> <replaceable>suffix</replaceable></term>
378 <term><option>-hisuf</option> <replaceable>suffix</replaceable></term>
379 <term><option>-hcsuf</option> <replaceable>suffix</replaceable></term>
380 <indexterm><primary><option>-osuf</option></primary></indexterm>
381 <indexterm><primary><option>-hisuf</option></primary></indexterm>
382 <indexterm><primary><option>-hcsuf</option></primary></indexterm>
384 <para>EXOTICA: The <option>-osuf</option>
385 <replaceable>suffix</replaceable> will change the
386 <literal>.o</literal> file suffix for object files to
387 whatever you specify. We use this when compiling
388 libraries, so that objects for the profiling versions of
389 the libraries don't clobber the normal ones.</para>
391 <para>Similarly, the <option>-hisuf</option>
392 <replaceable>suffix</replaceable> will change the
393 <literal>.hi</literal> file suffix for non-system
394 interface files (see <XRef LinkEnd="hi-options">).</para>
396 <para>Finally, the option <option>-hcsuf</option>
397 <replaceable>suffix</replaceable> will change the
398 <literal>.hc</literal> file suffix for compiler-generated
399 intermediate C files.</para>
401 <para>The <option>-hisuf</option>/<option>-osuf</option>
402 game is particularly useful if you want to compile a
403 program both with and without profiling, in the same
404 directory. You can say:
407 to get the ordinary version, and
409 ghc ... -osuf prof.o -hisuf prof.hi -prof -auto-all</Screen>
410 to get the profiled version.</para>
416 <sect2 id="keeping-intermediates">
417 <title>Keeping Intermediate Files</title>
418 <indexterm><primary>intermediate files, saving</primary>
420 <indexterm><primary><literal>.hc</literal> files, saving</primary>
422 <indexterm><primary><literal>.s</literal> files, saving</primary>
425 <para>The following options are useful for keeping certain
426 intermediate files around, when normally GHC would throw these
427 away after compilation:</para>
431 <term><option>-keep-hc-files</option></term>
433 <primary><option>-keep-hc-files</option></primary>
436 <para>Keep intermediate <literal>.hc</literal> files when
437 doing <literal>.hs</literal>-to-<literal>.o</literal>
438 compilations via C (NOTE: <literal>.hc</literal> files
439 aren't generated when using the native code generator, you
440 may need to use <option>-fvia-C</option> to force them
441 to be produced).</para>
446 <term><option>-keep-s-files</option></term>
448 <primary><option>-keep-s-files</option></primary>
451 <para>Keep intermediate <literal>.s</literal> files.</para>
456 <term><option>-keep-raw-s-files</option></term>
458 <primary><option>-keep-raw-s-files</option></primary>
461 <para>Keep intermediate <literal>.raw-s</literal> files.
462 These are the direct output from the C compiler, before
463 GHC does “assembly mangling” to produce the
464 <literal>.s</literal> file. Again, these are not produced
465 when using the native code generator.</para>
470 <term><option>-keep-tmp-files</option></term>
472 <primary><option>-keep-tmp-files</option></primary>
475 <primary>temporary files</primary>
476 <secondary>keeping</secondary>
479 <para>Instructs the GHC driver not to delete any of its
480 temporary files, which it normally keeps in
481 <literal>/tmp</literal> (or possibly elsewhere; see <xref
482 linkend="temp-files">). Running GHC with
483 <option>-v</option> will show you what temporary files
484 were generated along the way.</para>
490 <sect2 id="temp-files">
491 <title>Redirecting temporary files</title>
494 <primary>temporary files</primary>
495 <secondary>redirecting</secondary>
500 <term><option>-tmpdir</option></term>
501 <indexterm><primary><option>-tmpdir</option></primary></indexterm>
503 <para>If you have trouble because of running out of space
504 in <filename>/tmp</filename> (or wherever your
505 installation thinks temporary files should go), you may
506 use the <option>-tmpdir
507 <dir></option><IndexTerm><Primary>-tmpdir
508 <dir> option</Primary></IndexTerm> option to specify
509 an alternate directory. For example, <option>-tmpdir
510 .</option> says to put temporary files in the current
511 working directory.</para>
513 <para>Alternatively, use your <Constant>TMPDIR</Constant>
514 environment variable.<IndexTerm><Primary>TMPDIR
515 environment variable</Primary></IndexTerm> Set it to the
516 name of the directory where temporary files should be put.
517 GCC and other programs will honour the
518 <Constant>TMPDIR</Constant> variable as well.</para>
520 <para>Even better idea: Set the
521 <Constant>DEFAULT_TMPDIR</Constant> make variable when
522 building GHC, and never worry about
523 <Constant>TMPDIR</Constant> again. (see the build
524 documentation).</para>
530 <Sect2 id="hi-options">
531 <title>Other options related to interface files</title>
532 <indexterm><primary>interface files, options</primary></indexterm>
536 <term><option>-ddump-hi</option></term>
537 <indexterm><primary><option>-ddump-hi</option></primary>
540 <para>Dumps the new interface to standard output.</para>
545 <term><option>-ddump-hi-diffs</option></term>
546 <indexterm><primary><option>-ddump-hi-diffs</option></primary>
549 <para>The compiler does not overwrite an existing
550 <filename>.hi</filename> interface file if the new one is
551 the same as the old one; this is friendly to
552 <command>make</command>. When an interface does change,
553 it is often enlightening to be informed. The
554 <option>-ddump-hi-diffs</option> option will make GHC run
555 <command>diff</command> on the old and new
556 <filename>.hi</filename> files.</para>
561 <term><option>-ddump-minimal-imports</option></term>
562 <indexterm><primary><option>-ddump-minimal-imports</option></primary>
565 <para>Dump to the file "M.imports" (where M is the module
566 being compiled) a "minimal" set of import declarations.
567 You can safely replace all the import declarations in
568 "M.hs" with those found in "M.imports". Why would you
569 want to do that? Because the "minimal" imports (a) import
570 everything explicitly, by name, and (b) import nothing
571 that is not required. It can be quite painful to maintain
572 this property by hand, so this flag is intended to reduce
578 <term><option>--show-iface</option>
579 <replaceable>file</replaceable></term>
580 <indexterm><primary><option>--show-iface</option></primary>
583 <para>Where <replaceable>file</replaceable> is the name of
584 an interface file, dumps the contents of that interface in
585 a human-readable (ish) format.</para>
592 <title>The recompilation checker</title>
594 <indexterm><primary>recompilation checker</primary></indexterm>
598 <term><option>-no-recomp</option></term>
599 <indexterm><primary><option>-recomp</option></primary></indexterm>
600 <indexterm><primary><option>-no-recomp</option></primary></indexterm>
602 <para>Turn off recompilation checking (which is on by
603 default). Recompilation checking normally stops
604 compilation early, leaving an existing
605 <filename>.o</filename> file in place, if it can be
606 determined that the module does not need to be
612 <para>In the olden days, GHC compared the newly-generated
613 <filename>.hi</filename> file with the previous version; if they
614 were identical, it left the old one alone and didn't change its
615 modification date. In consequence, importers of a module with
616 an unchanged output <filename>.hi</filename> file were not
619 <para>This doesn't work any more. Suppose module
620 <literal>C</literal> imports module <literal>B</literal>, and
621 <literal>B</literal> imports module <literal>A</literal>. So
622 changes to <filename>A.hi</filename> should force a
623 recompilation of <literal>C</literal>. And some changes to
624 <literal>A</literal> (changing the definition of a function that
625 appears in an inlining of a function exported by
626 <literal>B</literal>, say) may conceivably not change
627 <filename>B.hi</filename> one jot. So now…</para>
629 <para>GHC keeps a version number on each interface file, and on
630 each type signature within the interface file. It also keeps in
631 every interface file a list of the version numbers of everything
632 it used when it last compiled the file. If the source file's
633 modification date is earlier than the <filename>.o</filename>
634 file's date (i.e. the source hasn't changed since the file was
635 last compiled), and the reompilation checking is on, GHC will be
636 clever. It compares the version numbers on the things it needs
637 this time with the version numbers on the things it needed last
638 time (gleaned from the interface file of the module being
639 compiled); if they are all the same it stops compiling rather
640 early in the process saying “Compilation IS NOT
641 required”. What a beautiful sight!</para>
643 <para>Patrick Sansom had a workshop paper about how all this is
644 done (though the details have changed quite a bit). <ULink
645 URL="mailto:sansom@dcs.gla.ac.uk">Ask him</ULink> if you want a
650 <sect2 id="using-make">
651 <title>Using <command>make</command></title>
653 <indexterm><primary><literal>make</literal></primary></indexterm>
655 <para>It is reasonably straightforward to set up a
656 <filename>Makefile</filename> to use with GHC, assuming you name
657 your source files the same as your modules. Thus:</para>
661 HC_OPTS = -cpp $(EXTRA_HC_OPTS)
663 SRCS = Main.lhs Foo.lhs Bar.lhs
664 OBJS = Main.o Foo.o Bar.o
666 .SUFFIXES : .o .hs .hi .lhs .hc .s
670 $(HC) -o $@ $(HC_OPTS) $(OBJS)
672 # Standard suffix rules
677 $(HC) -c $< $(HC_OPTS)
680 $(HC) -c $< $(HC_OPTS)
682 # Inter-module dependencies
683 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
684 Main.o Main.hc Main.s : Foo.hi Baz.hi # Main imports Foo and Baz
687 <para>(Sophisticated <command>make</command> variants may
688 achieve some of the above more elegantly. Notably,
689 <command>gmake</command>'s pattern rules let you write the more
690 comprehensible:</para>
694 $(HC) -c $< $(HC_OPTS)
697 <para>What we've shown should work with any
698 <command>make</command>.)</para>
700 <para>Note the cheesy <literal>.o.hi</literal> rule: It records
701 the dependency of the interface (<filename>.hi</filename>) file
702 on the source. The rule says a <filename>.hi</filename> file
703 can be made from a <filename>.o</filename> file by
704 doing…nothing. Which is true.</para>
706 <para>Note the inter-module dependencies at the end of the
707 Makefile, which take the form</para>
710 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
713 <para>They tell <command>make</command> that if any of
714 <literal>Foo.o</literal>, <literal>Foo.hc</literal> or
715 <literal>Foo.s</literal> have an earlier modification date than
716 <literal>Baz.hi</literal>, then the out-of-date file must be
717 brought up to date. To bring it up to date,
718 <literal>make</literal> looks for a rule to do so; one of the
719 preceding suffix rules does the job nicely.</para>
721 <sect3 id="sec-makefile-dependencies">
722 <title>Dependency generation</title>
723 <indexterm><primary>dependencies in Makefiles</primary></indexterm>
724 <indexterm><primary>Makefile dependencies</primary></indexterm>
726 <para>Putting inter-dependencies of the form <literal>Foo.o :
727 Bar.hi</literal> into your <filename>Makefile</filename> by
728 hand is rather error-prone. Don't worry, GHC has support for
729 automatically generating the required dependencies. Add the
730 following to your <filename>Makefile</filename>:</para>
734 ghc -M $(HC_OPTS) $(SRCS)
737 <para>Now, before you start compiling, and any time you change
738 the <literal>imports</literal> in your program, do
739 <command>make depend</command> before you do <command>make
740 cool_pgm</command>. <command>ghc -M</command> will
741 append the needed dependencies to your
742 <filename>Makefile</filename>.</para>
744 <para>In general, if module <literal>A</literal> contains the
751 then <command>ghc -M</command> will generate a dependency line
758 If module <literal>A</literal> contains the line
761 import {-# SOURCE #-} B ...blah...
764 then <command>ghc -M</command> will generate a dependency
771 (See <xref linkend="mutual-recursion"> for details of
772 <literal>hi-boot</literal> style interface files.) If
773 <literal>A</literal> imports multiple modules, then there will
774 be multiple lines with <filename>A.o</filename> as the
777 <para>By default, <command>ghc -M</command> generates all the
778 dependencies, and then concatenates them onto the end of
779 <filename>makefile</filename> (or
780 <filename>Makefile</filename> if <filename>makefile</filename>
781 doesn't exist) bracketed by the lines "<literal># DO NOT
782 DELETE: Beginning of Haskell dependencies</literal>" and
783 "<literal># DO NOT DELETE: End of Haskell
784 dependencies</literal>". If these lines already exist in the
785 <filename>makefile</filename>, then the old dependencies are
786 deleted first.</para>
788 <para>Don't forget to use the same <option>-package</option>
789 options on the <literal>ghc -M</literal> command line as you
790 would when compiling; this enables the dependency generator to
791 locate any imported modules that come from packages. The
792 package modules won't be included in the dependencies
793 generated, though (but see the
794 <option>––include-prelude</option> option below).</para>
796 <para>The dependency generation phase of GHC can take some
797 additional options, which you may find useful. For historical
798 reasons, each option passed to the dependency generator from
799 the GHC command line must be preceded by
800 <literal>-optdep</literal>. For example, to pass <literal>-f
801 .depend</literal> to the dependency generator, you say
804 ghc -M -optdep-f -optdep.depend ...
807 The options which affect dependency generation are:</para>
811 <term><option>-w</option></term>
813 <para>Turn off warnings about interface file shadowing.</para>
818 <term><option>-f</option> <replaceable>file</replaceable></term>
820 <para>Use <replaceable>file</replaceable> as the makefile,
821 rather than <filename>makefile</filename> or
822 <filename>Makefile</filename>. If
823 <replaceable>file</replaceable> doesn't exist,
824 <command>mkdependHS</command> creates it. We often use
825 <option>-f .depend</option> to put the dependencies in
826 <filename>.depend</filename> and then
827 <command>include</command> the file
828 <filename>.depend</filename> into
829 <filename>Makefile</filename>.</para>
834 <term><option>-o <osuf></option></term>
836 <para>Use <filename>.<osuf></filename> as the
837 "target file" suffix ( default: <literal>o</literal>).
838 Multiple <option>-o</option> flags are permitted
839 (GHC2.05 onwards). Thus "<option>-o hc -o o</option>"
840 will generate dependencies for <filename>.hc</filename>
841 and <filename>.o</filename> files.</para>
846 <term><option>-s <suf></option></term>
848 <para>Make extra dependencies that declare that files
850 <filename>.<suf>_<osuf></filename>
851 depend on interface files with suffix
852 <filename>.<suf>_hi</filename>, or (for
853 <literal>{-# SOURCE #-}</literal>
854 imports) on <filename>.hi-boot</filename>. Multiple
855 <option>-s</option> flags are permitted. For example,
856 <option>-o hc -s a -s b</option> will make dependencies
857 for <filename>.hc</filename> on
858 <filename>.hi</filename>,
859 <filename>.a_hc</filename> on
860 <filename>.a_hi</filename>, and
861 <filename>.b_hc</filename> on
862 <filename>.b_hi</filename>. (Useful in
863 conjunction with NoFib "ways".)</para>
868 <term><option>––exclude-module=<file></option></term>
870 <para>Regard <filename><file></filename> as
871 "stable"; i.e., exclude it from having dependencies on
877 <term><option>-x</option></term>
879 <para>same as <option>––exclude-module</option></para>
884 <term><option>––exclude-directory=<dirs></option></term>
886 <para>Regard the colon-separated list of directories
887 <filename><dirs></filename> as containing stable,
888 don't generate any dependencies on modules
894 <term><option>––include-module=<file></option></term>
896 <para>Regard <filename><file></filename> as not
897 "stable"; i.e., generate dependencies on it (if
898 any). This option is normally used in conjunction with
899 the <option>––exclude-directory</option> option.</para>
904 <term><option>––include-prelude</option></term>
906 <para>Regard modules imported from packages as unstable,
907 i.e., generate dependencies on the package modules used
908 (including <literal>Prelude</literal>, and all other
909 standard Haskell libraries). This option is normally
910 only used by the various system libraries.</para>
918 <sect2 id="mutual-recursion">
919 <title>How to compile mutually recursive modules</title>
921 <indexterm><primary>module system, recursion</primary></indexterm>
922 <indexterm><primary>recursion, between modules</primary></indexterm>
924 <para>Currently, the compiler does not have proper support for
925 dealing with mutually recursive modules:</para>
932 newtype TA = MkTA Int
947 <para>When compiling either module A and B, the compiler will
948 try (in vain) to look for the interface file of the other. So,
949 to get mutually recursive modules off the ground, you need to
950 hand write an interface file for A or B, so as to break the
951 loop. These hand-written interface files are called
952 <literal>hi-boot</literal> files, and are placed in a file
953 called <filename><module>.hi-boot</filename>. To import
954 from an <literal>hi-boot</literal> file instead of the standard
955 <filename>.hi</filename> file, use the following syntax in the
956 importing module: <indexterm><primary><literal>hi-boot</literal>
957 files</primary></indexterm> <indexterm><primary>importing,
958 <literal>hi-boot</literal> files</primary></indexterm></para>
961 import {-# SOURCE #-} A
964 <para>The hand-written interface need only contain the bare
965 minimum of information needed to get the bootstrapping process
966 started. For example, it doesn't need to contain declarations
967 for <emphasis>everything</emphasis> that module
968 <literal>A</literal> exports, only the things required by the
969 module that imports <literal>A</literal> recursively.</para>
971 <para>For the example at hand, the boot interface file for A
972 would look like the following:</para>
976 newtype TA = MkTA GHC.Base.Int
979 <para>The syntax is similar to a normal Haskell source file, but
980 with some important differences:</para>
984 <para>Non-local entities must be qualified with their
985 <emphasis>original</emphasis> defining module. Qualifying
986 by a module which just re-exports the entity won't do. In
987 particular, most <literal>Prelude</literal> entities aren't
988 actually defined in the <literal>Prelude</literal> (see for
989 example <literal>GHC.Base.Int</literal> in the above
990 example). HINT: to find out the fully-qualified name for
991 entities in the <literal>Prelude</literal> (or anywhere for
992 that matter), try using GHCi's
993 <literal>:info</literal> command, eg.</para>
994 <programlisting>Prelude> :m -Prelude
996 -- GHC.IOBase.IO is a type constructor
997 newtype GHC.IOBase.IO a
1001 <para>Only <literal>data</literal>, <literal>type</literal>,
1002 <literal>newtype</literal>, <literal>class</literal>, and
1003 type signature declarations may be included. You cannot declare
1004 <literal>instances</literal> or derive them automatically.
1009 <para>Notice that we only put the declaration for the newtype
1010 <literal>TA</literal> in the <literal>hi-boot</literal> file,
1011 not the signature for <Function>f</Function>, since
1012 <Function>f</Function> isn't used by <literal>B</literal>.</para>
1014 <para>If you want an <literal>hi-boot</literal> file to export a
1015 data type, but you don't want to give its constructors (because
1016 the constructors aren't used by the SOURCE-importing module),
1017 you can write simply:</para>
1024 <para>(You must write all the type parameters, but leave out the
1025 '=' and everything that follows it.)</para>
1029 <sect2 id="orphan-modules">
1030 <title>Orphan modules and instance declarations</title>
1032 <para> Haskell specifies that when compiling module M, any instance
1033 declaration in any module "below" M is visible. (Module A is "below"
1034 M if A is imported directly by M, or if A is below a module that M imports directly.)
1035 In principle, GHC must therefore read the interface files of every module below M,
1036 just in case they contain an instance declaration that matters to M. This would
1037 be a disaster in practice, so GHC tries to be clever. </para>
1039 <para>In particular, if an instance declaration is in the same module as the definition
1040 of any type or class mentioned in the head of the instance declaration, then
1041 GHC has to visit that interface file anyway. Example:</para>
1044 instance C a => D (T a) where ...
1047 <para> The instance declaration is only relevant if the type T is in use, and if
1048 so, GHC will have visited A's interface file to find T's definition. </para>
1050 <para> The only problem comes when a module contains an instance declaration
1051 and GHC has no other reason for visiting the module. Example:
1054 instance C a => D (T a) where ...
1057 Here, neither D nor T is declared in module Orphan.
1058 We call such modules ``orphan modules'',
1059 defined thus:</para>
1061 <listitem> <para> An <emphasis>orphan module</emphasis>
1062 <indexterm><primary>orphan module</primary></indexterm>
1063 contains at least one <emphasis>orphan instance</emphasis> or at
1064 least one <emphasis>orphan rule</emphasis>.</para> </listitem>
1066 <listitem><para> An instance declaration in a module M is an <emphasis>orphan instance</emphasis> if
1067 <indexterm><primary>orphan instance</primary></indexterm>
1068 none of the type constructors
1069 or classes mentioned in the instance head (the part after the ``<literal>=></literal>'') are declared
1072 <para> Only the instance head counts. In the example above, it is not good enough for C's declaration
1073 to be in module A; it must be the declaration of D or T.</para>
1076 <listitem><para> A rewrite rule in a module M is an <emphasis>orphan rule</emphasis>
1077 <indexterm><primary>orphan rule</primary></indexterm>
1078 if none of the variables, type constructors,
1079 or classes that are free in the left hand side of the rule are declared in M.
1084 <para> GHC identifies orphan modules, and visits the interface file of
1085 every orphan module below the module being compiled. This is usually
1086 wasted work, but there is no avoiding it. You should therefore do
1087 your best to have as few orphan modules as possible.
1091 <para> You can identify an orphan module by looking in its interface
1092 file, <filename>M.hi</filename>, using the
1093 <option>--show-iface</option>. If there is a ``!'' on the first line,
1094 GHC considers it an orphan module.
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