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 (<xref linkend="options-output">), 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 (<xref linkend="options-output">), 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 (<xref linkend="options-output">), 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 (<xref linkend="options-output">), 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 module <literal>A</literal> might require module
623 <literal>C</literal> to be recompiled, and hence when
624 <filename>A.hi</filename> changes we should check whether
625 <literal>C</literal> should be recompiled. However, the
626 dependencies of <literal>C</literal> will only list
627 <literal>B.hi</literal>, not <literal>A.hi</literal>, and some
628 changes to <literal>A</literal> (changing the definition of a
629 function that appears in an inlining of a function exported by
630 <literal>B</literal>, say) may conceivably not change
631 <filename>B.hi</filename> one jot. So now…</para>
633 <para>GHC keeps a version number on each interface file, and on
634 each type signature within the interface file. It also keeps in
635 every interface file a list of the version numbers of everything
636 it used when it last compiled the file. If the source file's
637 modification date is earlier than the <filename>.o</filename>
638 file's date (i.e. the source hasn't changed since the file was
639 last compiled), and the reompilation checking is on, GHC will be
640 clever. It compares the version numbers on the things it needs
641 this time with the version numbers on the things it needed last
642 time (gleaned from the interface file of the module being
643 compiled); if they are all the same it stops compiling rather
644 early in the process saying “Compilation IS NOT
645 required”. What a beautiful sight!</para>
647 <para>Patrick Sansom had a workshop paper about how all this is
648 done (though the details have changed quite a bit). <ULink
649 URL="mailto:sansom@dcs.gla.ac.uk">Ask him</ULink> if you want a
654 <sect2 id="using-make">
655 <title>Using <command>make</command></title>
657 <indexterm><primary><literal>make</literal></primary></indexterm>
659 <para>It is reasonably straightforward to set up a
660 <filename>Makefile</filename> to use with GHC, assuming you name
661 your source files the same as your modules. Thus:</para>
665 HC_OPTS = -cpp $(EXTRA_HC_OPTS)
667 SRCS = Main.lhs Foo.lhs Bar.lhs
668 OBJS = Main.o Foo.o Bar.o
670 .SUFFIXES : .o .hs .hi .lhs .hc .s
674 $(HC) -o $@ $(HC_OPTS) $(OBJS)
676 # Standard suffix rules
681 $(HC) -c $< $(HC_OPTS)
684 $(HC) -c $< $(HC_OPTS)
686 # Inter-module dependencies
687 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
688 Main.o Main.hc Main.s : Foo.hi Baz.hi # Main imports Foo and Baz
691 <para>(Sophisticated <command>make</command> variants may
692 achieve some of the above more elegantly. Notably,
693 <command>gmake</command>'s pattern rules let you write the more
694 comprehensible:</para>
698 $(HC) -c $< $(HC_OPTS)
701 <para>What we've shown should work with any
702 <command>make</command>.)</para>
704 <para>Note the cheesy <literal>.o.hi</literal> rule: It records
705 the dependency of the interface (<filename>.hi</filename>) file
706 on the source. The rule says a <filename>.hi</filename> file
707 can be made from a <filename>.o</filename> file by
708 doing…nothing. Which is true.</para>
710 <para>Note the inter-module dependencies at the end of the
711 Makefile, which take the form</para>
714 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
717 <para>They tell <command>make</command> that if any of
718 <literal>Foo.o</literal>, <literal>Foo.hc</literal> or
719 <literal>Foo.s</literal> have an earlier modification date than
720 <literal>Baz.hi</literal>, then the out-of-date file must be
721 brought up to date. To bring it up to date,
722 <literal>make</literal> looks for a rule to do so; one of the
723 preceding suffix rules does the job nicely.</para>
725 <sect3 id="sec-makefile-dependencies">
726 <title>Dependency generation</title>
727 <indexterm><primary>dependencies in Makefiles</primary></indexterm>
728 <indexterm><primary>Makefile dependencies</primary></indexterm>
730 <para>Putting inter-dependencies of the form <literal>Foo.o :
731 Bar.hi</literal> into your <filename>Makefile</filename> by
732 hand is rather error-prone. Don't worry, GHC has support for
733 automatically generating the required dependencies. Add the
734 following to your <filename>Makefile</filename>:</para>
738 ghc -M $(HC_OPTS) $(SRCS)
741 <para>Now, before you start compiling, and any time you change
742 the <literal>imports</literal> in your program, do
743 <command>make depend</command> before you do <command>make
744 cool_pgm</command>. <command>ghc -M</command> will
745 append the needed dependencies to your
746 <filename>Makefile</filename>.</para>
748 <para>In general, if module <literal>A</literal> contains the
755 then <command>ghc -M</command> will generate a dependency line
762 If module <literal>A</literal> contains the line
765 import {-# SOURCE #-} B ...blah...
768 then <command>ghc -M</command> will generate a dependency
775 (See <xref linkend="mutual-recursion"> for details of
776 <literal>hi-boot</literal> style interface files.) If
777 <literal>A</literal> imports multiple modules, then there will
778 be multiple lines with <filename>A.o</filename> as the
781 <para>By default, <command>ghc -M</command> generates all the
782 dependencies, and then concatenates them onto the end of
783 <filename>makefile</filename> (or
784 <filename>Makefile</filename> if <filename>makefile</filename>
785 doesn't exist) bracketed by the lines "<literal># DO NOT
786 DELETE: Beginning of Haskell dependencies</literal>" and
787 "<literal># DO NOT DELETE: End of Haskell
788 dependencies</literal>". If these lines already exist in the
789 <filename>makefile</filename>, then the old dependencies are
790 deleted first.</para>
792 <para>Don't forget to use the same <option>-package</option>
793 options on the <literal>ghc -M</literal> command line as you
794 would when compiling; this enables the dependency generator to
795 locate any imported modules that come from packages. The
796 package modules won't be included in the dependencies
797 generated, though (but see the
798 <option>––include-prelude</option> option below).</para>
800 <para>The dependency generation phase of GHC can take some
801 additional options, which you may find useful. For historical
802 reasons, each option passed to the dependency generator from
803 the GHC command line must be preceded by
804 <literal>-optdep</literal>. For example, to pass <literal>-f
805 .depend</literal> to the dependency generator, you say
808 ghc -M -optdep-f -optdep.depend ...
811 The options which affect dependency generation are:</para>
815 <term><option>-w</option></term>
817 <para>Turn off warnings about interface file shadowing.</para>
822 <term><option>-f</option> <replaceable>file</replaceable></term>
824 <para>Use <replaceable>file</replaceable> as the makefile,
825 rather than <filename>makefile</filename> or
826 <filename>Makefile</filename>. If
827 <replaceable>file</replaceable> doesn't exist,
828 <command>mkdependHS</command> creates it. We often use
829 <option>-f .depend</option> to put the dependencies in
830 <filename>.depend</filename> and then
831 <command>include</command> the file
832 <filename>.depend</filename> into
833 <filename>Makefile</filename>.</para>
838 <term><option>-o <osuf></option></term>
840 <para>Use <filename>.<osuf></filename> as the
841 "target file" suffix ( default: <literal>o</literal>).
842 Multiple <option>-o</option> flags are permitted
843 (GHC2.05 onwards). Thus "<option>-o hc -o o</option>"
844 will generate dependencies for <filename>.hc</filename>
845 and <filename>.o</filename> files.</para>
850 <term><option>-s <suf></option></term>
852 <para>Make extra dependencies that declare that files
854 <filename>.<suf>_<osuf></filename>
855 depend on interface files with suffix
856 <filename>.<suf>_hi</filename>, or (for
857 <literal>{-# SOURCE #-}</literal>
858 imports) on <filename>.hi-boot</filename>. Multiple
859 <option>-s</option> flags are permitted. For example,
860 <option>-o hc -s a -s b</option> will make dependencies
861 for <filename>.hc</filename> on
862 <filename>.hi</filename>,
863 <filename>.a_hc</filename> on
864 <filename>.a_hi</filename>, and
865 <filename>.b_hc</filename> on
866 <filename>.b_hi</filename>. (Useful in
867 conjunction with NoFib "ways".)</para>
872 <term><option>––exclude-module=<file></option></term>
874 <para>Regard <filename><file></filename> as
875 "stable"; i.e., exclude it from having dependencies on
881 <term><option>-x</option></term>
883 <para>same as <option>––exclude-module</option></para>
888 <term><option>––exclude-directory=<dirs></option></term>
890 <para>Regard the colon-separated list of directories
891 <filename><dirs></filename> as containing stable,
892 don't generate any dependencies on modules
898 <term><option>––include-module=<file></option></term>
900 <para>Regard <filename><file></filename> as not
901 "stable"; i.e., generate dependencies on it (if
902 any). This option is normally used in conjunction with
903 the <option>––exclude-directory</option> option.</para>
908 <term><option>––include-prelude</option></term>
910 <para>Regard modules imported from packages as unstable,
911 i.e., generate dependencies on the package modules used
912 (including <literal>Prelude</literal>, and all other
913 standard Haskell libraries). This option is normally
914 only used by the various system libraries.</para>
922 <sect2 id="mutual-recursion">
923 <title>How to compile mutually recursive modules</title>
925 <indexterm><primary>module system, recursion</primary></indexterm>
926 <indexterm><primary>recursion, between modules</primary></indexterm>
928 <para>Currently, the compiler does not have proper support for
929 dealing with mutually recursive modules:</para>
936 newtype TA = MkTA Int
951 <para>When compiling either module A and B, the compiler will
952 try (in vain) to look for the interface file of the other. So,
953 to get mutually recursive modules off the ground, you need to
954 hand write an interface file for A or B, so as to break the
955 loop. These hand-written interface files are called
956 <literal>hi-boot</literal> files, and are placed in a file
957 called <filename><module>.hi-boot</filename>. To import
958 from an <literal>hi-boot</literal> file instead of the standard
959 <filename>.hi</filename> file, use the following syntax in the
960 importing module: <indexterm><primary><literal>hi-boot</literal>
961 files</primary></indexterm> <indexterm><primary>importing,
962 <literal>hi-boot</literal> files</primary></indexterm></para>
965 import {-# SOURCE #-} A
968 <para>The hand-written interface need only contain the bare
969 minimum of information needed to get the bootstrapping process
970 started. For example, it doesn't need to contain declarations
971 for <emphasis>everything</emphasis> that module
972 <literal>A</literal> exports, only the things required by the
973 module that imports <literal>A</literal> recursively.</para>
975 <para>For the example at hand, the boot interface file for A
976 would look like the following:</para>
980 newtype TA = MkTA GHC.Base.Int
983 <para>The syntax is similar to a normal Haskell source file, but
984 with some important differences:</para>
988 <para>Non-local entities must be qualified with their
989 <emphasis>original</emphasis> defining module. Qualifying
990 by a module which just re-exports the entity won't do. In
991 particular, most <literal>Prelude</literal> entities aren't
992 actually defined in the <literal>Prelude</literal> (see for
993 example <literal>GHC.Base.Int</literal> in the above
994 example). HINT: to find out the fully-qualified name for
995 entities in the <literal>Prelude</literal> (or anywhere for
996 that matter), try using GHCi's
997 <literal>:info</literal> command, eg.</para>
998 <programlisting>Prelude> :m -Prelude
1000 -- GHC.IOBase.IO is a type constructor
1001 newtype GHC.IOBase.IO a
1002 ...</programlisting>
1005 <para>Only <literal>data</literal>, <literal>type</literal>,
1006 <literal>newtype</literal>, <literal>class</literal>, and
1007 type signature declarations may be included. You cannot declare
1008 <literal>instances</literal> or derive them automatically.
1013 <para>Notice that we only put the declaration for the newtype
1014 <literal>TA</literal> in the <literal>hi-boot</literal> file,
1015 not the signature for <Function>f</Function>, since
1016 <Function>f</Function> isn't used by <literal>B</literal>.</para>
1018 <para>If you want an <literal>hi-boot</literal> file to export a
1019 data type, but you don't want to give its constructors (because
1020 the constructors aren't used by the SOURCE-importing module),
1021 you can write simply:</para>
1028 <para>(You must write all the type parameters, but leave out the
1029 '=' and everything that follows it.)</para>
1033 <sect2 id="orphan-modules">
1034 <title>Orphan modules and instance declarations</title>
1036 <para> Haskell specifies that when compiling module M, any instance
1037 declaration in any module "below" M is visible. (Module A is "below"
1038 M if A is imported directly by M, or if A is below a module that M imports directly.)
1039 In principle, GHC must therefore read the interface files of every module below M,
1040 just in case they contain an instance declaration that matters to M. This would
1041 be a disaster in practice, so GHC tries to be clever. </para>
1043 <para>In particular, if an instance declaration is in the same module as the definition
1044 of any type or class mentioned in the head of the instance declaration, then
1045 GHC has to visit that interface file anyway. Example:</para>
1048 instance C a => D (T a) where ...
1051 <para> The instance declaration is only relevant if the type T is in use, and if
1052 so, GHC will have visited A's interface file to find T's definition. </para>
1054 <para> The only problem comes when a module contains an instance declaration
1055 and GHC has no other reason for visiting the module. Example:
1058 instance C a => D (T a) where ...
1061 Here, neither D nor T is declared in module Orphan.
1062 We call such modules ``orphan modules'',
1063 defined thus:</para>
1065 <listitem> <para> An <emphasis>orphan module</emphasis>
1066 <indexterm><primary>orphan module</primary></indexterm>
1067 contains at least one <emphasis>orphan instance</emphasis> or at
1068 least one <emphasis>orphan rule</emphasis>.</para> </listitem>
1070 <listitem><para> An instance declaration in a module M is an <emphasis>orphan instance</emphasis> if
1071 <indexterm><primary>orphan instance</primary></indexterm>
1072 none of the type constructors
1073 or classes mentioned in the instance head (the part after the ``<literal>=></literal>'') are declared
1076 <para> Only the instance head counts. In the example above, it is not good enough for C's declaration
1077 to be in module A; it must be the declaration of D or T.</para>
1080 <listitem><para> A rewrite rule in a module M is an <emphasis>orphan rule</emphasis>
1081 <indexterm><primary>orphan rule</primary></indexterm>
1082 if none of the variables, type constructors,
1083 or classes that are free in the left hand side of the rule are declared in M.
1088 <para> GHC identifies orphan modules, and visits the interface file of
1089 every orphan module below the module being compiled. This is usually
1090 wasted work, but there is no avoiding it. You should therefore do
1091 your best to have as few orphan modules as possible.
1095 <para> You can identify an orphan module by looking in its interface
1096 file, <filename>M.hi</filename>, using the
1097 <option>--show-iface</option>. If there is a ``!'' on the first line,
1098 GHC considers it an orphan module.
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