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>
261 <option>-odir</option> <replaceable>dir</replaceable>
262 <indexterm><primary><option>-odir</option></primary></indexterm>
265 <para>Redirects object files to directory
266 <replaceable>dir</replaceable>. For example:</para>
269 $ ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
272 <para>The object files, <filename>Foo.o</filename>,
273 <filename>Bar.o</filename>, and
274 <filename>Bumble.o</filename> would be put into a
275 subdirectory named after the architecture of the executing
276 machine (<filename>x86</filename>,
277 <filename>mips</filename>, etc).</para>
279 <para>Note that the <option>-odir</option> option does
280 <emphasis>not</emphasis> affect where the interface files
281 are put; use the <option>-hidir</option> option for that.
282 In the above example, they would still be put in
283 <filename>parse/Foo.hi</filename>,
284 <filename>parse/Bar.hi</filename>, and
285 <filename>gurgle/Bumble.hi</filename>.</para>
291 <option>-ohi</option> <replaceable>file</replaceable>
292 <indexterm><primary><option>-ohi</option></primary></indexterm>
295 <para>The interface output may be directed to another file
296 <filename>bar2/Wurble.iface</filename> with the option
297 <option>-ohi bar2/Wurble.iface</option> (not
300 <para>WARNING: if you redirect the interface file
301 somewhere that GHC can't find it, then the recompilation
302 checker may get confused (at the least, you won't get any
303 recompilation avoidance). We recommend using a
304 combination of <option>-hidir</option> and
305 <option>-hisuf</option> options instead, if
308 <para>To avoid generating an interface at all, you could
309 use this option to redirect the interface into the bit
310 bucket: <literal>-ohi /dev/null</literal>, for
317 <option>-hidir</option> <replaceable>dir</replaceable>
318 <indexterm><primary><option>-hidir</option></primary></indexterm>
321 <para>Redirects all generated interface files into
322 <replaceable>dir</replaceable>, instead of the
329 <option>-osuf</option> <replaceable>suffix</replaceable>
330 <indexterm><primary><option>-osuf</option></primary></indexterm>
333 <option>-hisuf</option> <replaceable>suffix</replaceable>
334 <indexterm><primary><option>-hisuf</option></primary></indexterm>
337 <option>-hcsuf</option> <replaceable>suffix</replaceable>
338 <indexterm><primary><option>-hcsuf</option></primary></indexterm>
341 <para>The <option>-osuf</option>
342 <replaceable>suffix</replaceable> will change the
343 <literal>.o</literal> file suffix for object files to
344 whatever you specify. We use this when compiling
345 libraries, so that objects for the profiling versions of
346 the libraries don't clobber the normal ones.</para>
348 <para>Similarly, the <option>-hisuf</option>
349 <replaceable>suffix</replaceable> will change the
350 <literal>.hi</literal> file suffix for non-system
351 interface files (see <xref linkend="hi-options"/>).</para>
353 <para>Finally, the option <option>-hcsuf</option>
354 <replaceable>suffix</replaceable> will change the
355 <literal>.hc</literal> file suffix for compiler-generated
356 intermediate C files.</para>
358 <para>The <option>-hisuf</option>/<option>-osuf</option>
359 game is particularly useful if you want to compile a
360 program both with and without profiling, in the same
361 directory. You can say:
364 to get the ordinary version, and
366 ghc ... -osuf prof.o -hisuf prof.hi -prof -auto-all</screen>
367 to get the profiled version.</para>
373 <sect2 id="keeping-intermediates">
374 <title>Keeping Intermediate Files</title>
375 <indexterm><primary>intermediate files, saving</primary>
377 <indexterm><primary><literal>.hc</literal> files, saving</primary>
379 <indexterm><primary><literal>.s</literal> files, saving</primary>
382 <para>The following options are useful for keeping certain
383 intermediate files around, when normally GHC would throw these
384 away after compilation:</para>
389 <option>-keep-hc-files</option>
390 <indexterm><primary><option>-keep-hc-files</option></primary></indexterm>
393 <para>Keep intermediate <literal>.hc</literal> files when
394 doing <literal>.hs</literal>-to-<literal>.o</literal>
395 compilations via C (NOTE: <literal>.hc</literal> files
396 aren't generated when using the native code generator, you
397 may need to use <option>-fvia-C</option> to force them
398 to be produced).</para>
404 <option>-keep-s-files</option>
405 <indexterm><primary><option>-keep-s-files</option></primary></indexterm>
408 <para>Keep intermediate <literal>.s</literal> files.</para>
414 <option>-keep-raw-s-files</option>
415 <indexterm><primary><option>-keep-raw-s-files</option></primary></indexterm>
418 <para>Keep intermediate <literal>.raw-s</literal> files.
419 These are the direct output from the C compiler, before
420 GHC does “assembly mangling” to produce the
421 <literal>.s</literal> file. Again, these are not produced
422 when using the native code generator.</para>
428 <option>-keep-tmp-files</option>
429 <indexterm><primary><option>-keep-tmp-files</option></primary></indexterm>
430 <indexterm><primary>temporary files</primary><secondary>keeping</secondary></indexterm>
433 <para>Instructs the GHC driver not to delete any of its
434 temporary files, which it normally keeps in
435 <literal>/tmp</literal> (or possibly elsewhere; see <xref
436 linkend="temp-files"/>). Running GHC with
437 <option>-v</option> will show you what temporary files
438 were generated along the way.</para>
444 <sect2 id="temp-files">
445 <title>Redirecting temporary files</title>
448 <primary>temporary files</primary>
449 <secondary>redirecting</secondary>
455 <option>-tmpdir</option>
456 <indexterm><primary><option>-tmpdir</option></primary></indexterm>
459 <para>If you have trouble because of running out of space
460 in <filename>/tmp</filename> (or wherever your
461 installation thinks temporary files should go), you may
462 use the <option>-tmpdir
463 <dir></option><indexterm><primary>-tmpdir
464 <dir> option</primary></indexterm> option to specify
465 an alternate directory. For example, <option>-tmpdir
466 .</option> says to put temporary files in the current
467 working directory.</para>
469 <para>Alternatively, use your <constant>TMPDIR</constant>
470 environment variable.<indexterm><primary>TMPDIR
471 environment variable</primary></indexterm> Set it to the
472 name of the directory where temporary files should be put.
473 GCC and other programs will honour the
474 <constant>TMPDIR</constant> variable as well.</para>
476 <para>Even better idea: Set the
477 <constant>DEFAULT_TMPDIR</constant> make variable when
478 building GHC, and never worry about
479 <constant>TMPDIR</constant> again. (see the build
480 documentation).</para>
486 <sect2 id="hi-options">
487 <title>Other options related to interface files</title>
488 <indexterm><primary>interface files, options</primary></indexterm>
493 <option>-ddump-hi</option>
494 <indexterm><primary><option>-ddump-hi</option></primary></indexterm>
497 <para>Dumps the new interface to standard output.</para>
503 <option>-ddump-hi-diffs</option>
504 <indexterm><primary><option>-ddump-hi-diffs</option></primary></indexterm>
507 <para>The compiler does not overwrite an existing
508 <filename>.hi</filename> interface file if the new one is
509 the same as the old one; this is friendly to
510 <command>make</command>. When an interface does change,
511 it is often enlightening to be informed. The
512 <option>-ddump-hi-diffs</option> option will make GHC run
513 <command>diff</command> on the old and new
514 <filename>.hi</filename> files.</para>
520 <option>-ddump-minimal-imports</option>
521 <indexterm><primary><option>-ddump-minimal-imports</option></primary></indexterm>
524 <para>Dump to the file "M.imports" (where M is the module
525 being compiled) a "minimal" set of import declarations.
526 You can safely replace all the import declarations in
527 "M.hs" with those found in "M.imports". Why would you
528 want to do that? Because the "minimal" imports (a) import
529 everything explicitly, by name, and (b) import nothing
530 that is not required. It can be quite painful to maintain
531 this property by hand, so this flag is intended to reduce
538 <option>--show-iface</option> <replaceable>file</replaceable>
539 <indexterm><primary><option>--show-iface</option></primary></indexterm>
542 <para>Where <replaceable>file</replaceable> is the name of
543 an interface file, dumps the contents of that interface in
544 a human-readable (ish) format.</para>
551 <title>The recompilation checker</title>
553 <indexterm><primary>recompilation checker</primary></indexterm>
558 <option>-no-recomp</option>
559 <indexterm><primary><option>-recomp</option></primary></indexterm>
560 <indexterm><primary><option>-no-recomp</option></primary></indexterm>
563 <para>Turn off recompilation checking (which is on by
564 default). Recompilation checking normally stops
565 compilation early, leaving an existing
566 <filename>.o</filename> file in place, if it can be
567 determined that the module does not need to be
573 <para>In the olden days, GHC compared the newly-generated
574 <filename>.hi</filename> file with the previous version; if they
575 were identical, it left the old one alone and didn't change its
576 modification date. In consequence, importers of a module with
577 an unchanged output <filename>.hi</filename> file were not
580 <para>This doesn't work any more. Suppose module
581 <literal>C</literal> imports module <literal>B</literal>, and
582 <literal>B</literal> imports module <literal>A</literal>. So
583 changes to module <literal>A</literal> might require module
584 <literal>C</literal> to be recompiled, and hence when
585 <filename>A.hi</filename> changes we should check whether
586 <literal>C</literal> should be recompiled. However, the
587 dependencies of <literal>C</literal> will only list
588 <literal>B.hi</literal>, not <literal>A.hi</literal>, and some
589 changes to <literal>A</literal> (changing the definition of a
590 function that appears in an inlining of a function exported by
591 <literal>B</literal>, say) may conceivably not change
592 <filename>B.hi</filename> one jot. So now…</para>
594 <para>GHC keeps a version number on each interface file, and on
595 each type signature within the interface file. It also keeps in
596 every interface file a list of the version numbers of everything
597 it used when it last compiled the file. If the source file's
598 modification date is earlier than the <filename>.o</filename>
599 file's date (i.e. the source hasn't changed since the file was
600 last compiled), and the recompilation checking is on, GHC will be
601 clever. It compares the version numbers on the things it needs
602 this time with the version numbers on the things it needed last
603 time (gleaned from the interface file of the module being
604 compiled); if they are all the same it stops compiling rather
605 early in the process saying “Compilation IS NOT
606 required”. What a beautiful sight!</para>
608 <para>Patrick Sansom had a workshop paper about how all this is
609 done (though the details have changed quite a bit). <ulink
610 url="mailto:sansom@dcs.gla.ac.uk">Ask him</ulink> if you want a
615 <sect2 id="mutual-recursion">
616 <title>How to compile mutually recursive modules</title>
618 <indexterm><primary>module system, recursion</primary></indexterm>
619 <indexterm><primary>recursion, between modules</primary></indexterm>
621 <para>GHC supports the compilation of mutually recursive modules.
622 This section explains how.</para>
624 <para>Every cycle in the module import graph must be broken by a <filename>hs-boot</filename> file.
625 Suppose that modules <filename>A.hs</filename> and <filename>B.hs</filename> are Haskell source files,
631 newtype TA = MkTA Int
637 import {-# SOURCE #-} A( TA(..) )
644 <indexterm><primary><literal>hs-boot</literal>
645 files</primary></indexterm> <indexterm><primary>importing,
646 <literal>hi-boot</literal> files</primary></indexterm>
647 Here <filename>A</filename> imports <filename>B</filename>, but <filename>B</filename> imports
648 <filename>A</filename> with a <literal>{-# SOURCE #-}</literal> pragma, which breaks the
649 circular dependency. For every module <filename>A.hs</filename> that is <literal>{-# SOURCE #-}</literal>-imported
650 in this way there must exist a souce file <literal>A.hs-boot</literal>. This file contains an abbreviated
651 version of <filename>A.hs</filename>, thus:
654 newtype TA = MkTA Int
656 A <filename>hs-boot</filename> file is compiled by GHC, just like a <filename>hs</filename> file:
660 Just as compiling <filename>A.hs</filename> produces an
661 interface file <filename>A.hi</filename>, and an object file
662 <filename>A.o</filename>, so compiling <filename>A.hs-boot</filename>
663 produces an interface file
664 <filename>A.hi-boot</filename>, and an pseudo-object file
665 <filename>A.o-boot</filename>. The interface file
666 <filename>A.hi-boot</filename> has exactly the same format as any
667 other interface file. The pseudo-object file is empty (don't link it!), but it is
668 very useful when using a Makefile, to record when the <filename>A.hi-boot</filename> was
669 last brought up to date.
671 <para>To compile these three files, issue the following commands:
673 ghc -c A.hs-boot -- Poduces A.hi-boot, A.o-boot
674 ghc -c B.hs -- Consumes A.hi-boot, produces B.hi, B.o
675 ghc -c A.hs -- Consumes B.hi, produces A.hi, A.o
676 ghc -o foo A.o B.o -- Linking the program
679 <para>There are several points to note here:
682 <para>The file <filename>A.hs-boot</filename> is a programmer-written source file.
683 It must live in the same directory as its parent source file <filename>A.hs</filename>.
684 (Currently, if you use a literate source file <filename>A.lhs</filename> you must
685 also use a literate boot file, <filename>A.lhs-boot</filename>.)
688 <listitem><para> The <filename>hi-boot</filename> generated by compiling a <filename>hs-boot</filename>
689 file is in machine-generated binary format.
690 You can display its contents with <command>ghc --show-iface</commaond>. If you
691 specify a directory for interface files, the <option>-ohidir</option> flag, then that affects
692 <filename>hi-boot</filename> files too.</para></listitem>b
694 <listitem><para> Hs-boot files are written in a subset of Haskell. In particular, the module
695 exports and imports, and the scoping rules are exactly the same as in Haskell. Hence, to
696 mention a non-Prelude type or class, you must import it.</para></listitem>
698 <listitem><para> When a hs-boot file <filename>A.hs-boot</filename>
699 is compiled, it is checked for scope and type errors.
700 When its parent module <filename>A.hs</filename> is compiled, the two are compared, and
701 an error is reported if the two are inconsistent.
704 <listitem><para> If hs-boot files are considered distinct from their parent source
705 files, and if a <literal>{-# SOURCE #-}</literal> import is considered to refer to the
706 hs-boot file, then the module import graph must have no cycles. The <command>ghc -M</command>
707 will report an error if a cycle is found.
712 A hs-boot file need only contain the bare
713 minimum of information needed to get the bootstrapping process
714 started. For example, it doesn't need to contain declarations
715 for <emphasis>everything</emphasis> that module
716 <literal>A</literal> exports, only the things required by the
717 module that imports <literal>A</literal> recursively.</para>
718 <para>A hs-boot file is written in a subset of Haskell:
720 <listitem><para> The module header, and import statements, are exactly as in Haskell.</para></listitem>
721 <listitem><para> There must be no value declarations, but there can be type signatures for
724 double :: Int -> Int
727 <listitem><para> Fixity declarations are exactly as in Haskell.</para></listitem>
728 <listitem><para> Type synonym declarations are exactly as in Haskell.</para></listitem>
729 <listitem><para> A data type declaration can either be given in full, exactly as in Haskell, or it
730 can be given abstractly, by omitting the '=' sign and everything that follows. For example:
734 In a <emphasis>source</emphasis> program
735 this would declare TA to have no constructors (a GHC extension: see <xref linkend="nullary-types"/>),
736 but in an hi-boot file it means "I don't know or care what the constructors are".
737 This is the most common form of data type declaration, because it's easy to get right.
738 You <emphasis>can</emphasis> also write out the constructors but, if you do so, you must write
739 it out precisely as in its real definition.</para>
741 If you do not write out the constructors, you may need to give a kind
742 annotation (<xref linkend="sec-kinding"/>), to tell
743 GHC the kind of the type variable, if it is not "*". (In source files, this is worked out
744 from the way the type variable is used in the constructors.) For example:
746 data R (x :: * -> *) y
749 <listitem><para> Class declarations is exactly as in Haskell, except that you may not put
750 default method declarations. You can also omit all the class methods entirely.
752 <listitem><para> Do not include instance declarations. There is a complication to do with
753 how the dictionary functions are named. It may well work, but it's not a well-tested feature.
760 <sect2 id="using-make">
761 <title>Using <command>make</command></title>
763 <indexterm><primary><literal>make</literal></primary></indexterm>
765 <para>It is reasonably straightforward to set up a
766 <filename>Makefile</filename> to use with GHC, assuming you name
767 your source files the same as your modules. Thus:</para>
771 HC_OPTS = -cpp $(EXTRA_HC_OPTS)
773 SRCS = Main.lhs Foo.lhs Bar.lhs
774 OBJS = Main.o Foo.o Bar.o
776 .SUFFIXES : .o .hs .hi .lhs .hc .s
780 $(HC) -o $@ $(HC_OPTS) $(OBJS)
782 # Standard suffix rules
787 $(HC) -c $< $(HC_OPTS)
790 $(HC) -c $< $(HC_OPTS)
796 $(HC) -c $< $(HC_OPTS)
799 $(HC) -c $< $(HC_OPTS)
801 # Inter-module dependencies
802 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
803 Main.o Main.hc Main.s : Foo.hi Baz.hi # Main imports Foo and Baz
806 <para>(Sophisticated <command>make</command> variants may
807 achieve some of the above more elegantly. Notably,
808 <command>gmake</command>'s pattern rules let you write the more
809 comprehensible:</para>
813 $(HC) -c $< $(HC_OPTS)
816 <para>What we've shown should work with any
817 <command>make</command>.)</para>
819 <para>Note the cheesy <literal>.o.hi</literal> rule: It records
820 the dependency of the interface (<filename>.hi</filename>) file
821 on the source. The rule says a <filename>.hi</filename> file
822 can be made from a <filename>.o</filename> file by
823 doing…nothing. Which is true.</para>
824 <para> Note that the suffix rules are all repeated twice, once
825 for normal Haskell source files, and once for <filename>hs-boot</filename>
826 files (see <xref linkend="mutual-recursion"/>).
828 <para>Note the inter-module dependencies at the end of the
829 Makefile, which take the form</para>
832 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
835 <para>They tell <command>make</command> that if any of
836 <literal>Foo.o</literal>, <literal>Foo.hc</literal> or
837 <literal>Foo.s</literal> have an earlier modification date than
838 <literal>Baz.hi</literal>, then the out-of-date file must be
839 brought up to date. To bring it up to date,
840 <literal>make</literal> looks for a rule to do so; one of the
841 preceding suffix rules does the job nicely. These dependencies
842 can be generated automatically by <command>ghc</command>; see
843 <xref linkend="sec-makefile-dependencies"/></para>
846 <sect2 id="sec-makefile-dependencies">
847 <title>Dependency generation</title>
848 <indexterm><primary>dependencies in Makefiles</primary></indexterm>
849 <indexterm><primary>Makefile dependencies</primary></indexterm>
851 <para>Putting inter-dependencies of the form <literal>Foo.o :
852 Bar.hi</literal> into your <filename>Makefile</filename> by
853 hand is rather error-prone. Don't worry, GHC has support for
854 automatically generating the required dependencies. Add the
855 following to your <filename>Makefile</filename>:</para>
859 ghc -M $(HC_OPTS) $(SRCS)
862 <para>Now, before you start compiling, and any time you change
863 the <literal>imports</literal> in your program, do
864 <command>make depend</command> before you do <command>make
865 cool_pgm</command>. The command <command>ghc -M</command> will
866 append the needed dependencies to your
867 <filename>Makefile</filename>.</para>
869 <para>In general, <command>ghc -M Foo</command> does the following.
870 For each module <literal>M</literal> in the set
871 <literal>Foo</literal> plus all its imports (transitively),
872 it adds to the Makefile:
874 <listitem><para>A line recording the dependence of the object file on the source file.
878 (or <literal>M.lhs</literal> if that is the filename you used).
880 <listitem><para> For each import declaration <literal>import X</literal> in <literal>M</literal>,
881 a line recording the dependence of <literal>M</literal> on <literal>X</literal>:
884 </programlisting></para></listitem>
885 <listitem><para> For each import declaration <literal>import {-# SOURCE #-} X</literal> in <literal>M</literal>,
886 a line recording the dependence of <literal>M</literal> on <literal>X</literal>:
890 (See <xref linkend="mutual-recursion"/> for details of
891 <literal>hi-boot</literal> style interface files.)
894 If <literal>M</literal> imports multiple modules, then there will
895 be multiple lines with <filename>M.o</filename> as the
897 <para>There is no need to list all of the source files as arguments to the <command>ghc -M</command> command;
898 <command>ghc</command> traces the dependencies, just like <command>ghc --make</command>
899 (a new feature in GHC 6.4).</para>
901 <para>By default, <command>ghc -M</command> generates all the
902 dependencies, and then concatenates them onto the end of
903 <filename>makefile</filename> (or
904 <filename>Makefile</filename> if <filename>makefile</filename>
905 doesn't exist) bracketed by the lines "<literal># DO NOT
906 DELETE: Beginning of Haskell dependencies</literal>" and
907 "<literal># DO NOT DELETE: End of Haskell
908 dependencies</literal>". If these lines already exist in the
909 <filename>makefile</filename>, then the old dependencies are
910 deleted first.</para>
912 <para>Don't forget to use the same <option>-package</option>
913 options on the <literal>ghc -M</literal> command line as you
914 would when compiling; this enables the dependency generator to
915 locate any imported modules that come from packages. The
916 package modules won't be included in the dependencies
917 generated, though (but see the
918 <option>––include-prelude</option> option below).</para>
920 <para>The dependency generation phase of GHC can take some
921 additional options, which you may find useful. For historical
922 reasons, each option passed to the dependency generator from
923 the GHC command line must be preceded by
924 <literal>-optdep</literal>. For example, to pass <literal>-f
925 .depend</literal> to the dependency generator, you say
928 ghc -M -optdep-f -optdep.depend ...
931 The options which affect dependency generation are:</para>
935 <term><option>-w</option></term>
937 <para>Turn off warnings about interface file shadowing.</para>
942 <term><option>-f</option> <replaceable>file</replaceable></term>
944 <para>Use <replaceable>file</replaceable> as the makefile,
945 rather than <filename>makefile</filename> or
946 <filename>Makefile</filename>. If
947 <replaceable>file</replaceable> doesn't exist,
948 <command>mkdependHS</command> creates it. We often use
949 <option>-f .depend</option> to put the dependencies in
950 <filename>.depend</filename> and then
951 <command>include</command> the file
952 <filename>.depend</filename> into
953 <filename>Makefile</filename>.</para>
957 <!-- Retired with the move away from 'mkdependHS'.
959 <term><option>-o <osuf></option></term>
961 <para>Use <filename>.<osuf></filename> as the
962 "target file" suffix ( default: <literal>o</literal>).
963 Multiple <option>-o</option> flags are permitted
964 (GHC2.05 onwards). Thus "<option>-o hc -o o</option>"
965 will generate dependencies for <filename>.hc</filename>
966 and <filename>.o</filename> files.</para>
971 <term><option>-s <suf></option></term>
973 <para>Make extra dependencies that declare that files
975 <filename>.<suf>_<osuf></filename>
976 depend on interface files with suffix
977 <filename>.<suf>_hi</filename>, or (for
978 <literal>{-# SOURCE #-}</literal>
979 imports) on <filename>.hi-boot</filename>. Multiple
980 <option>-s</option> flags are permitted. For example,
981 <option>-o hc -s a -s b</option> will make dependencies
982 for <filename>.hc</filename> on
983 <filename>.hi</filename>,
984 <filename>.a_hc</filename> on
985 <filename>.a_hi</filename>, and
986 <filename>.b_hc</filename> on
987 <filename>.b_hi</filename>. (Useful in
988 conjunction with NoFib "ways".)</para>
993 <term><option>––exclude-module=<file></option></term>
995 <para>Regard <filename><file></filename> as
996 "stable"; i.e., exclude it from having dependencies on
1002 <term><option>-x</option></term>
1004 <para>same as <option>––exclude-module</option></para>
1009 <term><option>––exclude-directory=<dirs></option></term>
1011 <para>Regard the colon-separated list of directories
1012 <filename><dirs></filename> as containing stable,
1013 don't generate any dependencies on modules
1019 <term><option>––include-module=<file></option></term>
1021 <para>Regard <filename><file></filename> as not
1022 "stable"; i.e., generate dependencies on it (if
1023 any). This option is normally used in conjunction with
1024 the <option>––exclude-directory</option> option.</para>
1029 <term><option>––include-prelude</option></term>
1031 <para>Regard modules imported from packages as unstable,
1032 i.e., generate dependencies on the package modules used
1033 (including <literal>Prelude</literal>, and all other
1034 standard Haskell libraries). This option is normally
1035 only used by the various system libraries.</para>
1042 <sect2 id="orphan-modules">
1043 <title>Orphan modules and instance declarations</title>
1045 <para> Haskell specifies that when compiling module M, any instance
1046 declaration in any module "below" M is visible. (Module A is "below"
1047 M if A is imported directly by M, or if A is below a module that M imports directly.)
1048 In principle, GHC must therefore read the interface files of every module below M,
1049 just in case they contain an instance declaration that matters to M. This would
1050 be a disaster in practice, so GHC tries to be clever. </para>
1052 <para>In particular, if an instance declaration is in the same module as the definition
1053 of any type or class mentioned in the head of the instance declaration, then
1054 GHC has to visit that interface file anyway. Example:</para>
1057 instance C a => D (T a) where ...
1060 <para> The instance declaration is only relevant if the type T is in use, and if
1061 so, GHC will have visited A's interface file to find T's definition. </para>
1063 <para> The only problem comes when a module contains an instance declaration
1064 and GHC has no other reason for visiting the module. Example:
1067 instance C a => D (T a) where ...
1070 Here, neither D nor T is declared in module Orphan.
1071 We call such modules ``orphan modules'',
1072 defined thus:</para>
1074 <listitem> <para> An <emphasis>orphan module</emphasis>
1075 <indexterm><primary>orphan module</primary></indexterm>
1076 contains at least one <emphasis>orphan instance</emphasis> or at
1077 least one <emphasis>orphan rule</emphasis>.</para> </listitem>
1079 <listitem><para> An instance declaration in a module M is an <emphasis>orphan instance</emphasis> if
1080 <indexterm><primary>orphan instance</primary></indexterm>
1081 none of the type constructors
1082 or classes mentioned in the instance head (the part after the ``<literal>=></literal>'') are declared
1085 <para> Only the instance head counts. In the example above, it is not good enough for C's declaration
1086 to be in module A; it must be the declaration of D or T.</para>
1089 <listitem><para> A rewrite rule in a module M is an <emphasis>orphan rule</emphasis>
1090 <indexterm><primary>orphan rule</primary></indexterm>
1091 if none of the variables, type constructors,
1092 or classes that are free in the left hand side of the rule are declared in M.
1097 <para> GHC identifies orphan modules, and visits the interface file of
1098 every orphan module below the module being compiled. This is usually
1099 wasted work, but there is no avoiding it. You should therefore do
1100 your best to have as few orphan modules as possible.
1104 <para> You can identify an orphan module by looking in its interface
1105 file, <filename>M.hi</filename>, using the
1106 <option>--show-iface</option>. If there is a ``!'' on the first line,
1107 GHC considers it an orphan module.
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