1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <sect1 id="separate-compilation">
3 <title>Filenames and separate compilation</title>
5 <indexterm><primary>separate compilation</primary></indexterm>
6 <indexterm><primary>recompilation checker</primary></indexterm>
7 <indexterm><primary>make and recompilation</primary></indexterm>
9 <para>This section describes what files GHC expects to find, what
10 files it creates, where these files are stored, and what options
11 affect this behaviour.</para>
13 <para>Note that this section is written with
14 <firstterm>hierarchical modules</firstterm> in mind (see <xref
15 linkend="hierarchical-modules"/>); hierarchical modules are an
16 extension to Haskell 98 which extends the lexical syntax of
17 module names to include a dot ‘.’. Non-hierarchical
18 modules are thus a special case in which none of the module names
21 <para>Pathname conventions vary from system to system. In
22 particular, the directory separator is
23 ‘<literal>/</literal>’ on Unix systems and
24 ‘<literal>\</literal>’ on Windows systems. In the
25 sections that follow, we shall consistently use
26 ‘<literal>/</literal>’ as the directory separator;
27 substitute this for the appropriate character for your
30 <sect2 id="source-files">
31 <title>Haskell source files</title>
33 <para>Each Haskell source module should be placed in a file on
36 <para>The file should usually be named after the module name, by
37 replacing dots in the module name by directory separators. For
38 example, on a Unix system, the module <literal>A.B.C</literal>
39 should be placed in the file <literal>A/B/C.hs</literal>,
40 relative to some base directory. GHC's behaviour if this rule
41 is not followed is fully defined by the following section (<xref
42 linkend="output-files"/>).</para>
45 <sect2 id="output-files">
46 <title>Output files</title>
48 <indexterm><primary>interface files</primary></indexterm>
49 <indexterm><primary><literal>.hi</literal> files</primary></indexterm>
50 <indexterm><primary>object files</primary></indexterm>
51 <indexterm><primary><literal>.o</literal> files</primary></indexterm>
53 <para>When asked to compile a source file, GHC normally
54 generates two files: an <firstterm>object file</firstterm>, and
55 an <firstterm>interface file</firstterm>. </para>
57 <para>The object file, which normally ends in a
58 <literal>.o</literal> suffix (or <literal>.obj</literal> if
59 you're on Windows), contains the compiled code for the module.</para>
61 <para>The interface file,
62 which normally ends in a <literal>.hi</literal> suffix, contains
63 the information that GHC needs in order to compile further
64 modules that depend on this module. It contains things like the
65 types of exported functions, definitions of data types, and so
66 on. It is stored in a binary format, so don't try to read one;
67 use the <option>--show-iface</option> option instead (see <xref
68 linkend="hi-options"/>).</para>
70 <para>You should think of the object file and the interface file as a
71 pair, since the interface file is in a sense a compiler-readable
72 description of the contents of the object file. If the
73 interface file and object file get out of sync for any reason,
74 then the compiler may end up making assumptions about the object
75 file that aren't true; trouble will almost certainly follow.
76 For this reason, we recommend keeping object files and interface
77 files in the same place (GHC does this by default, but it is
78 possible to override the defaults as we'll explain
81 <para>Every module has a <emphasis>module name</emphasis>
82 defined in its source code (<literal>module A.B.C where
83 ...</literal>).</para>
85 <para>The name of the object file generated by GHC is derived
86 according to the following rules, where
87 <replaceable>osuf</replaceable> is the object-file suffix (this
88 can be changed with the <option>-osuf</option> option).</para>
92 <para>If there is no <option>-odir</option> option (the
93 default), then the object filename is derived from the
94 source filename (ignoring the module name) by replacing the
95 suffix with <replaceable>osuf</replaceable>.</para>
99 <option>-odir</option> <replaceable>dir</replaceable>
100 has been specified, then the object filename is
101 <replaceable>dir</replaceable>/<replaceable>mod</replaceable>.<replaceable>osuf</replaceable>,
102 where <replaceable>mod</replaceable> is the module name with
103 dots replaced by slashes.</para>
107 <para>The name of the interface file is derived using the same
108 rules, except that the suffix is
109 <replaceable>hisuf</replaceable> (<literal>.hi</literal> by
110 default) instead of <replaceable>osuf</replaceable>, and the
111 relevant options are <option>-hidir</option> and
112 <option>-hisuf</option> instead of <option>-odir</option> and
113 <option>-osuf</option> respectively.</para>
115 <para>For example, if GHC compiles the module
116 <literal>A.B.C</literal> in the file
117 <filename>src/A/B/C.hs</filename>, with no
118 <literal>-odir</literal> or <literal>-hidir</literal> flags, the
119 interface file will be put in <literal>src/A/B/C.hi</literal>
120 and the object file in <literal>src/A/B/C.o</literal>.</para>
122 <para>For any module that is imported, GHC requires that the
123 name of the module in the import statement exactly matches the
124 name of the module in the interface file (or source file) found
125 using the strategy specified in <xref linkend="search-path"/>.
126 This means that for most modules, the source file name should
127 match the module name.</para>
129 <para>However, note that it is reasonable to have a module
130 <literal>Main</literal> in a file named
131 <filename>foo.hs</filename>, but this only works because GHC
132 never needs to search for the interface for module
133 <literal>Main</literal> (because it is never imported). It is
134 therefore possible to have several <literal>Main</literal>
135 modules in separate source files in the same directory, and GHC
136 will not get confused.</para>
138 <para>In batch compilation mode, the name of the object file can
139 also be overridden using the <option>-o</option> option, and the
140 name of the interface file can be specified directly using the
141 <option>-ohi</option> option.</para>
144 <sect2 id="search-path">
145 <title>The search path</title>
147 <indexterm><primary>search path</primary>
149 <indexterm><primary>interface files, finding them</primary></indexterm>
150 <indexterm><primary>finding interface files</primary></indexterm>
152 <para>In your program, you import a module
153 <literal>Foo</literal> by saying <literal>import Foo</literal>.
154 In <option>--make</option> mode or GHCi, GHC will look for a
155 source file for <literal>Foo</literal> and arrange to compile it
156 first. Without <option>--make</option>, GHC will look for the
157 interface file for <literal>Foo</literal>, which should have
158 been created by an earlier compilation of
159 <literal>Foo</literal>. GHC uses the same strategy in each of
160 these cases for finding the appropriate file.</para>
162 <para>This strategy is as follows: GHC keeps a list of
163 directories called the <firstterm>search path</firstterm>. For
164 each of these directories, it tries appending
165 <replaceable>basename</replaceable><literal>.</literal><replaceable>extension</replaceable>
166 to the directory, and checks whether the file exists. The value
167 of <replaceable>basename</replaceable> is the module name with
168 dots replaced by the directory separator ('/' or '\', depending
169 on the system), and <replaceable>extension</replaceable> is a
170 source extension (<literal>hs</literal>, <literal>lhs</literal>)
171 if we are in <option>--make</option> mode and GHCi, or
172 <replaceable>hisuf</replaceable> otherwise.</para>
174 <para>For example, suppose the search path contains directories
175 <literal>d1</literal>, <literal>d2</literal>, and
176 <literal>d3</literal>, and we are in <literal>--make</literal>
177 mode looking for the source file for a module
178 <literal>A.B.C</literal>. GHC will look in
179 <literal>d1/A/B/C.hs</literal>, <literal>d1/A/B/C.lhs</literal>,
180 <literal>d2/A/B/C.hs</literal>, and so on.</para>
182 <para>The search path by default contains a single directory:
183 <quote>.</quote> (i.e. the current directory). The following
184 options can be used to add to or change the contents of the
189 <term><option>-i<replaceable>dirs</replaceable></option></term>
191 <para><indexterm><primary><option>-i<replaceable>dirs</replaceable></option>
192 </primary></indexterm>This flag appends a colon-separated
193 list of <filename>dirs</filename> to the search path.</para>
198 <term><option>-i</option></term>
200 <para>resets the search path back to nothing.</para>
205 <para>This isn't the whole story: GHC also looks for modules in
206 pre-compiled libraries, known as packages. See the section on
207 packages (<xref linkend="packages"/>), for details.</para>
210 <sect2 id="options-output">
211 <title>Redirecting the compilation output(s)</title>
213 <indexterm><primary>output-directing options</primary></indexterm>
214 <indexterm><primary>redirecting compilation output</primary></indexterm>
219 <option>-o</option> <replaceable>file</replaceable>
220 <indexterm><primary><option>-o</option></primary></indexterm>
223 <para>GHC's compiled output normally goes into a
224 <filename>.hc</filename>, <filename>.o</filename>, etc.,
225 file, depending on the last-run compilation phase. The
226 option <option>-o <replaceable>file</replaceable></option>
227 re-directs the output of that last-run phase to
228 <replaceable>file</replaceable>.</para>
230 <para>Note: this “feature” can be
231 counterintuitive: <command>ghc -C -o foo.o
232 foo.hs</command> will put the intermediate C code in the
233 file <filename>foo.o</filename>, name
234 notwithstanding!</para>
236 <para>This option is most often used when creating an
237 executable file, to set the filename of the executable.
239 <screen> ghc -o prog --make Main</screen>
241 will compile the program starting with module
242 <literal>Main</literal> and put the executable in the
243 file <literal>prog</literal>.</para>
245 <para>Note: on Windows, if the result is an executable
246 file, the extension "<filename>.exe</filename>" is added
247 if the specified filename does not already have an
252 will compile and link the module
253 <filename>Main.hs</filename>, and put the resulting
254 executable in <filename>foo.exe</filename> (not
255 <filename>foo</filename>).</para>
257 <para>If you use <command>ghc --make</command> and you don't
258 use the <option>-o</option>, the name GHC will choose
259 for the executable will be based on the name of the file
260 containing the module <literal>Main</literal>.
261 Note that with GHC the <literal>Main</literal> module doesn't
262 have to be put in file <filename>Main.hs</filename>.
271 will produce <filename>Prog</filename> (or
272 <filename>Prog.exe</filename> if you are on Windows).</para>
278 <option>-odir</option> <replaceable>dir</replaceable>
279 <indexterm><primary><option>-odir</option></primary></indexterm>
282 <para>Redirects object files to directory
283 <replaceable>dir</replaceable>. For example:</para>
286 $ ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
289 <para>The object files, <filename>Foo.o</filename>,
290 <filename>Bar.o</filename>, and
291 <filename>Bumble.o</filename> would be put into a
292 subdirectory named after the architecture of the executing
293 machine (<filename>x86</filename>,
294 <filename>mips</filename>, etc).</para>
296 <para>Note that the <option>-odir</option> option does
297 <emphasis>not</emphasis> affect where the interface files
298 are put; use the <option>-hidir</option> option for that.
299 In the above example, they would still be put in
300 <filename>parse/Foo.hi</filename>,
301 <filename>parse/Bar.hi</filename>, and
302 <filename>gurgle/Bumble.hi</filename>.</para>
308 <option>-ohi</option> <replaceable>file</replaceable>
309 <indexterm><primary><option>-ohi</option></primary></indexterm>
312 <para>The interface output may be directed to another file
313 <filename>bar2/Wurble.iface</filename> with the option
314 <option>-ohi bar2/Wurble.iface</option> (not
317 <para>WARNING: if you redirect the interface file
318 somewhere that GHC can't find it, then the recompilation
319 checker may get confused (at the least, you won't get any
320 recompilation avoidance). We recommend using a
321 combination of <option>-hidir</option> and
322 <option>-hisuf</option> options instead, if
325 <para>To avoid generating an interface at all, you could
326 use this option to redirect the interface into the bit
327 bucket: <literal>-ohi /dev/null</literal>, for
334 <option>-hidir</option> <replaceable>dir</replaceable>
335 <indexterm><primary><option>-hidir</option></primary></indexterm>
338 <para>Redirects all generated interface files into
339 <replaceable>dir</replaceable>, instead of the
346 <option>-stubdir</option> <replaceable>dir</replaceable>
347 <indexterm><primary><option>-stubdir</option></primary></indexterm>
350 <para>Redirects all generated FFI stub files into
351 <replaceable>dir</replaceable>. Stub files are generated when the
352 Haskell source contains a <literal>foreign export</literal> or
353 <literal>foreign import "&wrapper"</literal> declaration (see <xref
354 linkend="foreign-export-ghc" />). The <option>-stubdir</option>
355 option behaves in exactly the same way as <option>-odir</option>
356 and <option>-hidir</option> with respect to hierarchical
363 <option>-osuf</option> <replaceable>suffix</replaceable>
364 <indexterm><primary><option>-osuf</option></primary></indexterm>
367 <option>-hisuf</option> <replaceable>suffix</replaceable>
368 <indexterm><primary><option>-hisuf</option></primary></indexterm>
371 <option>-hcsuf</option> <replaceable>suffix</replaceable>
372 <indexterm><primary><option>-hcsuf</option></primary></indexterm>
375 <para>The <option>-osuf</option>
376 <replaceable>suffix</replaceable> will change the
377 <literal>.o</literal> file suffix for object files to
378 whatever you specify. We use this when compiling
379 libraries, so that objects for the profiling versions of
380 the libraries don't clobber the normal ones.</para>
382 <para>Similarly, the <option>-hisuf</option>
383 <replaceable>suffix</replaceable> will change the
384 <literal>.hi</literal> file suffix for non-system
385 interface files (see <xref linkend="hi-options"/>).</para>
387 <para>Finally, the option <option>-hcsuf</option>
388 <replaceable>suffix</replaceable> will change the
389 <literal>.hc</literal> file suffix for compiler-generated
390 intermediate C files.</para>
392 <para>The <option>-hisuf</option>/<option>-osuf</option>
393 game is particularly useful if you want to compile a
394 program both with and without profiling, in the same
395 directory. You can say:
398 to get the ordinary version, and
400 ghc ... -osuf prof.o -hisuf prof.hi -prof -auto-all</screen>
401 to get the profiled version.</para>
407 <sect2 id="keeping-intermediates">
408 <title>Keeping Intermediate Files</title>
409 <indexterm><primary>intermediate files, saving</primary>
411 <indexterm><primary><literal>.hc</literal> files, saving</primary>
413 <indexterm><primary><literal>.s</literal> files, saving</primary>
416 <para>The following options are useful for keeping certain
417 intermediate files around, when normally GHC would throw these
418 away after compilation:</para>
423 <option>-keep-hc-files</option>
424 <indexterm><primary><option>-keep-hc-files</option></primary></indexterm>
427 <para>Keep intermediate <literal>.hc</literal> files when
428 doing <literal>.hs</literal>-to-<literal>.o</literal>
429 compilations via C (NOTE: <literal>.hc</literal> files
430 aren't generated when using the native code generator, you
431 may need to use <option>-fvia-C</option> to force them
432 to be produced).</para>
438 <option>-keep-s-files</option>
439 <indexterm><primary><option>-keep-s-files</option></primary></indexterm>
442 <para>Keep intermediate <literal>.s</literal> files.</para>
448 <option>-keep-raw-s-files</option>
449 <indexterm><primary><option>-keep-raw-s-files</option></primary></indexterm>
452 <para>Keep intermediate <literal>.raw-s</literal> files.
453 These are the direct output from the C compiler, before
454 GHC does “assembly mangling” to produce the
455 <literal>.s</literal> file. Again, these are not produced
456 when using the native code generator.</para>
462 <option>-keep-tmp-files</option>
463 <indexterm><primary><option>-keep-tmp-files</option></primary></indexterm>
464 <indexterm><primary>temporary files</primary><secondary>keeping</secondary></indexterm>
467 <para>Instructs the GHC driver not to delete any of its
468 temporary files, which it normally keeps in
469 <literal>/tmp</literal> (or possibly elsewhere; see <xref
470 linkend="temp-files"/>). Running GHC with
471 <option>-v</option> will show you what temporary files
472 were generated along the way.</para>
478 <sect2 id="temp-files">
479 <title>Redirecting temporary files</title>
482 <primary>temporary files</primary>
483 <secondary>redirecting</secondary>
489 <option>-tmpdir</option>
490 <indexterm><primary><option>-tmpdir</option></primary></indexterm>
493 <para>If you have trouble because of running out of space
494 in <filename>/tmp</filename> (or wherever your
495 installation thinks temporary files should go), you may
496 use the <option>-tmpdir
497 <dir></option><indexterm><primary>-tmpdir
498 <dir> option</primary></indexterm> option to specify
499 an alternate directory. For example, <option>-tmpdir
500 .</option> says to put temporary files in the current
501 working directory.</para>
503 <para>Alternatively, use your <constant>TMPDIR</constant>
504 environment variable.<indexterm><primary>TMPDIR
505 environment variable</primary></indexterm> Set it to the
506 name of the directory where temporary files should be put.
507 GCC and other programs will honour the
508 <constant>TMPDIR</constant> variable as well.</para>
510 <para>Even better idea: Set the
511 <constant>DEFAULT_TMPDIR</constant> make variable when
512 building GHC, and never worry about
513 <constant>TMPDIR</constant> again. (see the build
514 documentation).</para>
520 <sect2 id="hi-options">
521 <title>Other options related to interface files</title>
522 <indexterm><primary>interface files, options</primary></indexterm>
527 <option>-ddump-hi</option>
528 <indexterm><primary><option>-ddump-hi</option></primary></indexterm>
531 <para>Dumps the new interface to standard output.</para>
537 <option>-ddump-hi-diffs</option>
538 <indexterm><primary><option>-ddump-hi-diffs</option></primary></indexterm>
541 <para>The compiler does not overwrite an existing
542 <filename>.hi</filename> interface file if the new one is
543 the same as the old one; this is friendly to
544 <command>make</command>. When an interface does change,
545 it is often enlightening to be informed. The
546 <option>-ddump-hi-diffs</option> option will make GHC run
547 <command>diff</command> on the old and new
548 <filename>.hi</filename> files.</para>
554 <option>-ddump-minimal-imports</option>
555 <indexterm><primary><option>-ddump-minimal-imports</option></primary></indexterm>
558 <para>Dump to the file "M.imports" (where M is the module
559 being compiled) a "minimal" set of import declarations.
560 You can safely replace all the import declarations in
561 "M.hs" with those found in "M.imports". Why would you
562 want to do that? Because the "minimal" imports (a) import
563 everything explicitly, by name, and (b) import nothing
564 that is not required. It can be quite painful to maintain
565 this property by hand, so this flag is intended to reduce
572 <option>--show-iface</option> <replaceable>file</replaceable>
573 <indexterm><primary><option>--show-iface</option></primary></indexterm>
576 <para>Where <replaceable>file</replaceable> is the name of
577 an interface file, dumps the contents of that interface in
578 a human-readable (ish) format.</para>
585 <title>The recompilation checker</title>
587 <indexterm><primary>recompilation checker</primary></indexterm>
592 <option>-no-recomp</option>
593 <indexterm><primary><option>-recomp</option></primary></indexterm>
594 <indexterm><primary><option>-no-recomp</option></primary></indexterm>
597 <para>Turn off recompilation checking (which is on by
598 default). Recompilation checking normally stops
599 compilation early, leaving an existing
600 <filename>.o</filename> file in place, if it can be
601 determined that the module does not need to be
607 <para>In the olden days, GHC compared the newly-generated
608 <filename>.hi</filename> file with the previous version; if they
609 were identical, it left the old one alone and didn't change its
610 modification date. In consequence, importers of a module with
611 an unchanged output <filename>.hi</filename> file were not
614 <para>This doesn't work any more. Suppose module
615 <literal>C</literal> imports module <literal>B</literal>, and
616 <literal>B</literal> imports module <literal>A</literal>. So
617 changes to module <literal>A</literal> might require module
618 <literal>C</literal> to be recompiled, and hence when
619 <filename>A.hi</filename> changes we should check whether
620 <literal>C</literal> should be recompiled. However, the
621 dependencies of <literal>C</literal> will only list
622 <literal>B.hi</literal>, not <literal>A.hi</literal>, and some
623 changes to <literal>A</literal> (changing the definition of a
624 function that appears in an inlining of a function exported by
625 <literal>B</literal>, say) may conceivably not change
626 <filename>B.hi</filename> one jot. So now…</para>
628 <para>GHC keeps a version number on each interface file, and on
629 each type signature within the interface file. It also keeps in
630 every interface file a list of the version numbers of everything
631 it used when it last compiled the file. If the source file's
632 modification date is earlier than the <filename>.o</filename>
633 file's date (i.e. the source hasn't changed since the file was
634 last compiled), and the recompilation checking is on, GHC will be
635 clever. It compares the version numbers on the things it needs
636 this time with the version numbers on the things it needed last
637 time (gleaned from the interface file of the module being
638 compiled); if they are all the same it stops compiling rather
639 early in the process saying “Compilation IS NOT
640 required”. What a beautiful sight!</para>
642 <para>Patrick Sansom had a workshop paper about how all this is
643 done (though the details have changed quite a bit). <ulink
644 url="mailto:sansom@dcs.gla.ac.uk">Ask him</ulink> if you want a
649 <sect2 id="mutual-recursion">
650 <title>How to compile mutually recursive modules</title>
652 <indexterm><primary>module system, recursion</primary></indexterm>
653 <indexterm><primary>recursion, between modules</primary></indexterm>
655 <para>GHC supports the compilation of mutually recursive modules.
656 This section explains how.</para>
658 <para>Every cycle in the module import graph must be broken by a <filename>hs-boot</filename> file.
659 Suppose that modules <filename>A.hs</filename> and <filename>B.hs</filename> are Haskell source files,
665 newtype TA = MkTA Int
671 import {-# SOURCE #-} A( TA(..) )
678 <indexterm><primary><literal>hs-boot</literal>
679 files</primary></indexterm> <indexterm><primary>importing,
680 <literal>hi-boot</literal> files</primary></indexterm>
681 Here <filename>A</filename> imports <filename>B</filename>, but <filename>B</filename> imports
682 <filename>A</filename> with a <literal>{-# SOURCE #-}</literal> pragma, which breaks the
683 circular dependency. For every module <filename>A.hs</filename> that is <literal>{-# SOURCE #-}</literal>-imported
684 in this way there must exist a souce file <literal>A.hs-boot</literal>. This file contains an abbreviated
685 version of <filename>A.hs</filename>, thus:
688 newtype TA = MkTA Int
691 <para>To compile these three files, issue the following commands:
693 ghc -c A.hs-boot -- Poduces A.hi-boot, A.o-boot
694 ghc -c B.hs -- Consumes A.hi-boot, produces B.hi, B.o
695 ghc -c A.hs -- Consumes B.hi, produces A.hi, A.o
696 ghc -o foo A.o B.o -- Linking the program
699 <para>There are several points to note here:
702 <para>The file <filename>A.hs-boot</filename> is a programmer-written source file.
703 It must live in the same directory as its parent source file <filename>A.hs</filename>.
704 Currently, if you use a literate source file <filename>A.lhs</filename> you must
705 also use a literate boot file, <filename>A.lhs-boot</filename>; and vice versa.
709 A <filename>hs-boot</filename> file is compiled by GHC, just like a <filename>hs</filename> file:
713 When a hs-boot file <filename>A.hs-boot</filename>
714 is compiled, it is checked for scope and type errors.
715 When its parent module <filename>A.hs</filename> is compiled, the two are compared, and
716 an error is reported if the two are inconsistent.
720 <para> Just as compiling <filename>A.hs</filename> produces an
721 interface file <filename>A.hi</filename>, and an object file
722 <filename>A.o</filename>, so compiling
723 <filename>A.hs-boot</filename> produces an interface file
724 <filename>A.hi-boot</filename>, and an pseudo-object file
725 <filename>A.o-boot</filename>: </para>
729 <para>The pseudo-object file <filename>A.o-boot</filename> is
730 empty (don't link it!), but it is very useful when using a
731 Makefile, to record when the <filename>A.hi-boot</filename> was
732 last brought up to date (see <xref
733 linkend="using-make"/>).</para>
737 <para>The <filename>hi-boot</filename> generated by compiling a
738 <filename>hs-boot</filename> file is in the same
739 machine-generated binary format as any other GHC-generated
740 interface file (e.g. <filename>B.hi</filename>). You can
741 display its contents with <command>ghc
742 --show-iface</command>. If you specify a directory for
743 interface files, the <option>-ohidir</option> flag, then that
744 affects <filename>hi-boot</filename> files
750 <listitem><para> If hs-boot files are considered distinct from their parent source
751 files, and if a <literal>{-# SOURCE #-}</literal> import is considered to refer to the
752 hs-boot file, then the module import graph must have no cycles. The command
753 <command>ghc -M</command> will report an error if a cycle is found.
756 <listitem><para> A module <literal>M</literal> that is
757 <literal>{-# SOURCE #-}</literal>-imported in a program will usually also be
758 ordinarily imported elsewhere. If not, <command>ghc --make</command>
759 automatically adds <literal>M</literal> to the set of moudles it tries to
760 compile and link, to ensure that <literal>M</literal>'s implementation is included in
766 A hs-boot file need only contain the bare
767 minimum of information needed to get the bootstrapping process
768 started. For example, it doesn't need to contain declarations
769 for <emphasis>everything</emphasis> that module
770 <literal>A</literal> exports, only the things required by the
771 module(s) that import <literal>A</literal> recursively.</para>
772 <para>A hs-boot file is written in a subset of Haskell:
774 <listitem><para> The module header (including the export list), and import statements, are exactly as in
775 Haskell, and so are the scoping rules.
776 Hence, to mention a non-Prelude type or class, you must import it.</para></listitem>
778 <listitem><para> There must be no value declarations, but there can be type signatures for
781 double :: Int -> Int
784 <listitem><para> Fixity declarations are exactly as in Haskell.</para></listitem>
785 <listitem><para> Type synonym declarations are exactly as in Haskell.</para></listitem>
786 <listitem><para> A data type declaration can either be given in full, exactly as in Haskell, or it
787 can be given abstractly, by omitting the '=' sign and everything that follows. For example:
791 In a <emphasis>source</emphasis> program
792 this would declare TA to have no constructors (a GHC extension: see <xref linkend="nullary-types"/>),
793 but in an hi-boot file it means "I don't know or care what the constructors are".
794 This is the most common form of data type declaration, because it's easy to get right.
795 You <emphasis>can</emphasis> also write out the constructors but, if you do so, you must write
796 it out precisely as in its real definition.</para>
798 If you do not write out the constructors, you may need to give a kind
799 annotation (<xref linkend="sec-kinding"/>), to tell
800 GHC the kind of the type variable, if it is not "*". (In source files, this is worked out
801 from the way the type variable is used in the constructors.) For example:
803 data R (x :: * -> *) y
806 <listitem><para> Class declarations is exactly as in Haskell, except that you may not put
807 default method declarations. You can also omit all the class methods entirely.
809 <listitem><para> Do not include instance declarations. There is a complication to do with
810 how the dictionary functions are named. It may well work, but it's not a well-tested feature.
817 <sect2 id="using-make">
818 <title>Using <command>make</command></title>
820 <indexterm><primary><literal>make</literal></primary></indexterm>
822 <para>It is reasonably straightforward to set up a
823 <filename>Makefile</filename> to use with GHC, assuming you name
824 your source files the same as your modules. Thus:</para>
828 HC_OPTS = -cpp $(EXTRA_HC_OPTS)
830 SRCS = Main.lhs Foo.lhs Bar.lhs
831 OBJS = Main.o Foo.o Bar.o
833 .SUFFIXES : .o .hs .hi .lhs .hc .s
837 $(HC) -o $@ $(HC_OPTS) $(OBJS)
839 # Standard suffix rules
844 $(HC) -c $< $(HC_OPTS)
847 $(HC) -c $< $(HC_OPTS)
853 $(HC) -c $< $(HC_OPTS)
856 $(HC) -c $< $(HC_OPTS)
858 # Inter-module dependencies
859 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
860 Main.o Main.hc Main.s : Foo.hi Baz.hi # Main imports Foo and Baz
863 <para>(Sophisticated <command>make</command> variants may
864 achieve some of the above more elegantly. Notably,
865 <command>gmake</command>'s pattern rules let you write the more
866 comprehensible:</para>
870 $(HC) -c $< $(HC_OPTS)
873 <para>What we've shown should work with any
874 <command>make</command>.)</para>
876 <para>Note the cheesy <literal>.o.hi</literal> rule: It records
877 the dependency of the interface (<filename>.hi</filename>) file
878 on the source. The rule says a <filename>.hi</filename> file
879 can be made from a <filename>.o</filename> file by
880 doing…nothing. Which is true.</para>
881 <para> Note that the suffix rules are all repeated twice, once
882 for normal Haskell source files, and once for <filename>hs-boot</filename>
883 files (see <xref linkend="mutual-recursion"/>).</para>
885 <para>Note also the inter-module dependencies at the end of the
886 Makefile, which take the form
889 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
892 They tell <command>make</command> that if any of
893 <literal>Foo.o</literal>, <literal>Foo.hc</literal> or
894 <literal>Foo.s</literal> have an earlier modification date than
895 <literal>Baz.hi</literal>, then the out-of-date file must be
896 brought up to date. To bring it up to date,
897 <literal>make</literal> looks for a rule to do so; one of the
898 preceding suffix rules does the job nicely. These dependencies
899 can be generated automatically by <command>ghc</command>; see
900 <xref linkend="sec-makefile-dependencies"/></para>
904 <sect2 id="sec-makefile-dependencies">
905 <title>Dependency generation</title>
906 <indexterm><primary>dependencies in Makefiles</primary></indexterm>
907 <indexterm><primary>Makefile dependencies</primary></indexterm>
909 <para>Putting inter-dependencies of the form <literal>Foo.o :
910 Bar.hi</literal> into your <filename>Makefile</filename> by
911 hand is rather error-prone. Don't worry, GHC has support for
912 automatically generating the required dependencies. Add the
913 following to your <filename>Makefile</filename>:</para>
917 ghc -M $(HC_OPTS) $(SRCS)
920 <para>Now, before you start compiling, and any time you change
921 the <literal>imports</literal> in your program, do
922 <command>make depend</command> before you do <command>make
923 cool_pgm</command>. The command <command>ghc -M</command> will
924 append the needed dependencies to your
925 <filename>Makefile</filename>.</para>
927 <para>In general, <command>ghc -M Foo</command> does the following.
928 For each module <literal>M</literal> in the set
929 <literal>Foo</literal> plus all its imports (transitively),
930 it adds to the Makefile:
932 <listitem><para>A line recording the dependence of the object file on the source file.
936 (or <literal>M.lhs</literal> if that is the filename you used).
938 <listitem><para> For each import declaration <literal>import X</literal> in <literal>M</literal>,
939 a line recording the dependence of <literal>M</literal> on <literal>X</literal>:
942 </programlisting></para></listitem>
943 <listitem><para> For each import declaration <literal>import {-# SOURCE #-} X</literal> in <literal>M</literal>,
944 a line recording the dependence of <literal>M</literal> on <literal>X</literal>:
948 (See <xref linkend="mutual-recursion"/> for details of
949 <literal>hi-boot</literal> style interface files.)
952 If <literal>M</literal> imports multiple modules, then there will
953 be multiple lines with <filename>M.o</filename> as the
955 <para>There is no need to list all of the source files as arguments to the <command>ghc -M</command> command;
956 <command>ghc</command> traces the dependencies, just like <command>ghc --make</command>
957 (a new feature in GHC 6.4).</para>
959 <para>Note that <literal>ghc -M</literal> needs to find a <emphasis>source
960 file</emphasis> for each module in the dependency graph, so that it can
961 parse the import declarations and follow dependencies. Any pre-compiled
962 modules without source files must therefore belong to a
963 package<footnote><para>This is a change in behaviour relative to 6.2 and
967 <para>By default, <command>ghc -M</command> generates all the
968 dependencies, and then concatenates them onto the end of
969 <filename>makefile</filename> (or
970 <filename>Makefile</filename> if <filename>makefile</filename>
971 doesn't exist) bracketed by the lines "<literal># DO NOT
972 DELETE: Beginning of Haskell dependencies</literal>" and
973 "<literal># DO NOT DELETE: End of Haskell
974 dependencies</literal>". If these lines already exist in the
975 <filename>makefile</filename>, then the old dependencies are
976 deleted first.</para>
978 <para>Don't forget to use the same <option>-package</option>
979 options on the <literal>ghc -M</literal> command line as you
980 would when compiling; this enables the dependency generator to
981 locate any imported modules that come from packages. The
982 package modules won't be included in the dependencies
983 generated, though (but see the
984 <option>––include-pkg-deps</option> option below).</para>
986 <para>The dependency generation phase of GHC can take some
987 additional options, which you may find useful. For historical
988 reasons, each option passed to the dependency generator from
989 the GHC command line must be preceded by
990 <literal>-optdep</literal>. For example, to pass <literal>-f
991 .depend</literal> to the dependency generator, you say
994 ghc -M -optdep-f -optdep.depend ...
997 The options which affect dependency generation are:</para>
1001 <term><option>-w</option></term>
1003 <para>Turn off warnings about interface file shadowing.</para>
1008 <term><option>-v2</option></term>
1010 <para>Print a full list of the module depenencies to stdout.
1011 (This is the standard verbosity flag, so the list will
1012 also be displayed with <option>-v3</option> and
1013 <option>-v4</option>;
1014 <xref linkend ="options-help"/>.)</para>
1019 <term><option>-f</option> <replaceable>file</replaceable></term>
1021 <para>Use <replaceable>file</replaceable> as the makefile,
1022 rather than <filename>makefile</filename> or
1023 <filename>Makefile</filename>. If
1024 <replaceable>file</replaceable> doesn't exist,
1025 <command>mkdependHS</command> creates it. We often use
1026 <option>-f .depend</option> to put the dependencies in
1027 <filename>.depend</filename> and then
1028 <command>include</command> the file
1029 <filename>.depend</filename> into
1030 <filename>Makefile</filename>.</para>
1034 <!-- Retired with the move away from 'mkdependHS'.
1036 <term><option>-o <osuf></option></term>
1038 <para>Use <filename>.<osuf></filename> as the
1039 "target file" suffix ( default: <literal>o</literal>).
1040 Multiple <option>-o</option> flags are permitted
1041 (GHC2.05 onwards). Thus "<option>-o hc -o o</option>"
1042 will generate dependencies for <filename>.hc</filename>
1043 and <filename>.o</filename> files.</para>
1048 <term><option>-s <suf></option></term>
1050 <para>Make extra dependencies that declare that files
1052 <filename>.<suf>_<osuf></filename>
1053 depend on interface files with suffix
1054 <filename>.<suf>_hi</filename>, or (for
1055 <literal>{-# SOURCE #-}</literal>
1056 imports) on <filename>.hi-boot</filename>. Multiple
1057 <option>-s</option> flags are permitted. For example,
1058 <option>-o hc -s a -s b</option> will make dependencies
1059 for <filename>.hc</filename> on
1060 <filename>.hi</filename>,
1061 <filename>.a_hc</filename> on
1062 <filename>.a_hi</filename>, and
1063 <filename>.b_hc</filename> on
1064 <filename>.b_hi</filename>. (Useful in
1065 conjunction with NoFib "ways".)</para>
1070 <term><option>––exclude-module=<file></option></term>
1072 <para>Regard <filename><file></filename> as
1073 "stable"; i.e., exclude it from having dependencies on
1079 <term><option>-x</option></term>
1081 <para>same as <option>––exclude-module</option></para>
1086 <term><option>––exclude-directory=<dirs></option></term>
1088 <para>Regard the colon-separated list of directories
1089 <filename><dirs></filename> as containing stable,
1090 don't generate any dependencies on modules
1096 <term><option>––include-module=<file></option></term>
1098 <para>Regard <filename><file></filename> as not
1099 "stable"; i.e., generate dependencies on it (if
1100 any). This option is normally used in conjunction with
1101 the <option>––exclude-directory</option> option.</para>
1106 <term><option>––include-pkg-deps</option></term>
1108 <para>Regard modules imported from packages as unstable,
1109 i.e., generate dependencies on any imported package modules
1110 (including <literal>Prelude</literal>, and all other
1111 standard Haskell libraries). Dependencies are not traced
1112 recursively into packages; dependencies are only generated for
1113 home-package modules on external-package modules directly imported
1114 by the home package module.
1115 This option is normally
1116 only used by the various system libraries.</para>
1123 <sect2 id="orphan-modules">
1124 <title>Orphan modules and instance declarations</title>
1126 <para> Haskell specifies that when compiling module M, any instance
1127 declaration in any module "below" M is visible. (Module A is "below"
1128 M if A is imported directly by M, or if A is below a module that M imports directly.)
1129 In principle, GHC must therefore read the interface files of every module below M,
1130 just in case they contain an instance declaration that matters to M. This would
1131 be a disaster in practice, so GHC tries to be clever. </para>
1133 <para>In particular, if an instance declaration is in the same module as the definition
1134 of any type or class mentioned in the head of the instance declaration, then
1135 GHC has to visit that interface file anyway. Example:</para>
1138 instance C a => D (T a) where ...
1141 <para> The instance declaration is only relevant if the type T is in use, and if
1142 so, GHC will have visited A's interface file to find T's definition. </para>
1144 <para> The only problem comes when a module contains an instance declaration
1145 and GHC has no other reason for visiting the module. Example:
1148 instance C a => D (T a) where ...
1151 Here, neither D nor T is declared in module Orphan.
1152 We call such modules ``orphan modules'',
1153 defined thus:</para>
1155 <listitem> <para> An <emphasis>orphan module</emphasis>
1156 <indexterm><primary>orphan module</primary></indexterm>
1157 contains at least one <emphasis>orphan instance</emphasis> or at
1158 least one <emphasis>orphan rule</emphasis>.</para> </listitem>
1160 <listitem><para> An instance declaration in a module M is an <emphasis>orphan instance</emphasis> if
1161 <indexterm><primary>orphan instance</primary></indexterm>
1162 none of the type constructors
1163 or classes mentioned in the instance head (the part after the ``<literal>=></literal>'') are declared
1166 <para> Only the instance head counts. In the example above, it is not good enough for C's declaration
1167 to be in module A; it must be the declaration of D or T.</para>
1170 <listitem><para> A rewrite rule in a module M is an <emphasis>orphan rule</emphasis>
1171 <indexterm><primary>orphan rule</primary></indexterm>
1172 if none of the variables, type constructors,
1173 or classes that are free in the left hand side of the rule are declared in M.
1178 <para> GHC identifies orphan modules, and visits the interface file of
1179 every orphan module below the module being compiled. This is usually
1180 wasted work, but there is no avoiding it. You should therefore do
1181 your best to have as few orphan modules as possible.
1185 <para> You can identify an orphan module by looking in its interface
1186 file, <filename>M.hi</filename>, using the
1187 <option>--show-iface</option>. If there is a ``!'' on the first line,
1188 GHC considers it an orphan module.
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