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
657 <para>To compile these three files, issue the following commands:
659 ghc -c A.hs-boot -- Poduces A.hi-boot, A.o-boot
660 ghc -c B.hs -- Consumes A.hi-boot, produces B.hi, B.o
661 ghc -c A.hs -- Consumes B.hi, produces A.hi, A.o
662 ghc -o foo A.o B.o -- Linking the program
665 <para>There are several points to note here:
668 <para>The file <filename>A.hs-boot</filename> is a programmer-written source file.
669 It must live in the same directory as its parent source file <filename>A.hs</filename>.
670 Currently, if you use a literate source file <filename>A.lhs</filename> you must
671 also use a literate boot file, <filename>A.lhs-boot</filename>; and vice versa.
675 A <filename>hs-boot</filename> file is compiled by GHC, just like a <filename>hs</filename> file:
679 When a hs-boot file <filename>A.hs-boot</filename>
680 is compiled, it is checked for scope and type errors.
681 When its parent module <filename>A.hs</filename> is compiled, the two are compared, and
682 an error is reported if the two are inconsistent.
686 <para> Just as compiling <filename>A.hs</filename> produces an
687 interface file <filename>A.hi</filename>, and an object file
688 <filename>A.o</filename>, so compiling
689 <filename>A.hs-boot</filename> produces an interface file
690 <filename>A.hi-boot</filename>, and an pseudo-object file
691 <filename>A.o-boot</filename>: </para>
695 <para>The pseudo-object file <filename>A.o-boot</filename> is
696 empty (don't link it!), but it is very useful when using a
697 Makefile, to record when the <filename>A.hi-boot</filename> was
698 last brought up to date (see <xref
699 linkend="using-make"/>).</para>
703 <para>The <filename>hi-boot</filename> generated by compiling a
704 <filename>hs-boot</filename> file is in the same
705 machine-generated binary format as any other GHC-generated
706 interface file (e.g. <filename>B.hi</filename>). You can
707 display its contents with <command>ghc
708 --show-iface</command>. If you specify a directory for
709 interface files, the <option>-ohidir</option> flag, then that
710 affects <filename>hi-boot</filename> files
716 <listitem><para> If hs-boot files are considered distinct from their parent source
717 files, and if a <literal>{-# SOURCE #-}</literal> import is considered to refer to the
718 hs-boot file, then the module import graph must have no cycles. The command
719 <command>ghc -M</command> will report an error if a cycle is found.
722 <listitem><para> A module <literal>M</literal> that is
723 <literal>{-# SOURCE #-}</literal>-imported in a program will usually also be
724 ordinarily imported elsewhere. If not, <command>ghc --make</command>
725 automatically adds <literal>M</literal> to the set of moudles it tries to
726 compile and link, to ensure that <literal>M</literal>'s implementation is included in
732 A hs-boot file need only contain the bare
733 minimum of information needed to get the bootstrapping process
734 started. For example, it doesn't need to contain declarations
735 for <emphasis>everything</emphasis> that module
736 <literal>A</literal> exports, only the things required by the
737 module(s) that import <literal>A</literal> recursively.</para>
738 <para>A hs-boot file is written in a subset of Haskell:
740 <listitem><para> The module header (including the export list), and import statements, are exactly as in
741 Haskell, and so are the scoping rules.
742 Hence, to mention a non-Prelude type or class, you must import it.</para></listitem>
744 <listitem><para> There must be no value declarations, but there can be type signatures for
747 double :: Int -> Int
750 <listitem><para> Fixity declarations are exactly as in Haskell.</para></listitem>
751 <listitem><para> Type synonym declarations are exactly as in Haskell.</para></listitem>
752 <listitem><para> A data type declaration can either be given in full, exactly as in Haskell, or it
753 can be given abstractly, by omitting the '=' sign and everything that follows. For example:
757 In a <emphasis>source</emphasis> program
758 this would declare TA to have no constructors (a GHC extension: see <xref linkend="nullary-types"/>),
759 but in an hi-boot file it means "I don't know or care what the constructors are".
760 This is the most common form of data type declaration, because it's easy to get right.
761 You <emphasis>can</emphasis> also write out the constructors but, if you do so, you must write
762 it out precisely as in its real definition.</para>
764 If you do not write out the constructors, you may need to give a kind
765 annotation (<xref linkend="sec-kinding"/>), to tell
766 GHC the kind of the type variable, if it is not "*". (In source files, this is worked out
767 from the way the type variable is used in the constructors.) For example:
769 data R (x :: * -> *) y
772 <listitem><para> Class declarations is exactly as in Haskell, except that you may not put
773 default method declarations. You can also omit all the class methods entirely.
775 <listitem><para> Do not include instance declarations. There is a complication to do with
776 how the dictionary functions are named. It may well work, but it's not a well-tested feature.
783 <sect2 id="using-make">
784 <title>Using <command>make</command></title>
786 <indexterm><primary><literal>make</literal></primary></indexterm>
788 <para>It is reasonably straightforward to set up a
789 <filename>Makefile</filename> to use with GHC, assuming you name
790 your source files the same as your modules. Thus:</para>
794 HC_OPTS = -cpp $(EXTRA_HC_OPTS)
796 SRCS = Main.lhs Foo.lhs Bar.lhs
797 OBJS = Main.o Foo.o Bar.o
799 .SUFFIXES : .o .hs .hi .lhs .hc .s
803 $(HC) -o $@ $(HC_OPTS) $(OBJS)
805 # Standard suffix rules
810 $(HC) -c $< $(HC_OPTS)
813 $(HC) -c $< $(HC_OPTS)
819 $(HC) -c $< $(HC_OPTS)
822 $(HC) -c $< $(HC_OPTS)
824 # Inter-module dependencies
825 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
826 Main.o Main.hc Main.s : Foo.hi Baz.hi # Main imports Foo and Baz
829 <para>(Sophisticated <command>make</command> variants may
830 achieve some of the above more elegantly. Notably,
831 <command>gmake</command>'s pattern rules let you write the more
832 comprehensible:</para>
836 $(HC) -c $< $(HC_OPTS)
839 <para>What we've shown should work with any
840 <command>make</command>.)</para>
842 <para>Note the cheesy <literal>.o.hi</literal> rule: It records
843 the dependency of the interface (<filename>.hi</filename>) file
844 on the source. The rule says a <filename>.hi</filename> file
845 can be made from a <filename>.o</filename> file by
846 doing…nothing. Which is true.</para>
847 <para> Note that the suffix rules are all repeated twice, once
848 for normal Haskell source files, and once for <filename>hs-boot</filename>
849 files (see <xref linkend="mutual-recursion"/>).</para>
851 <para>Note also the inter-module dependencies at the end of the
852 Makefile, which take the form
855 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
858 They tell <command>make</command> that if any of
859 <literal>Foo.o</literal>, <literal>Foo.hc</literal> or
860 <literal>Foo.s</literal> have an earlier modification date than
861 <literal>Baz.hi</literal>, then the out-of-date file must be
862 brought up to date. To bring it up to date,
863 <literal>make</literal> looks for a rule to do so; one of the
864 preceding suffix rules does the job nicely. These dependencies
865 can be generated automatically by <command>ghc</command>; see
866 <xref linkend="sec-makefile-dependencies"/></para>
870 <sect2 id="sec-makefile-dependencies">
871 <title>Dependency generation</title>
872 <indexterm><primary>dependencies in Makefiles</primary></indexterm>
873 <indexterm><primary>Makefile dependencies</primary></indexterm>
875 <para>Putting inter-dependencies of the form <literal>Foo.o :
876 Bar.hi</literal> into your <filename>Makefile</filename> by
877 hand is rather error-prone. Don't worry, GHC has support for
878 automatically generating the required dependencies. Add the
879 following to your <filename>Makefile</filename>:</para>
883 ghc -M $(HC_OPTS) $(SRCS)
886 <para>Now, before you start compiling, and any time you change
887 the <literal>imports</literal> in your program, do
888 <command>make depend</command> before you do <command>make
889 cool_pgm</command>. The command <command>ghc -M</command> will
890 append the needed dependencies to your
891 <filename>Makefile</filename>.</para>
893 <para>In general, <command>ghc -M Foo</command> does the following.
894 For each module <literal>M</literal> in the set
895 <literal>Foo</literal> plus all its imports (transitively),
896 it adds to the Makefile:
898 <listitem><para>A line recording the dependence of the object file on the source file.
902 (or <literal>M.lhs</literal> if that is the filename you used).
904 <listitem><para> For each import declaration <literal>import X</literal> in <literal>M</literal>,
905 a line recording the dependence of <literal>M</literal> on <literal>X</literal>:
908 </programlisting></para></listitem>
909 <listitem><para> For each import declaration <literal>import {-# SOURCE #-} X</literal> in <literal>M</literal>,
910 a line recording the dependence of <literal>M</literal> on <literal>X</literal>:
914 (See <xref linkend="mutual-recursion"/> for details of
915 <literal>hi-boot</literal> style interface files.)
918 If <literal>M</literal> imports multiple modules, then there will
919 be multiple lines with <filename>M.o</filename> as the
921 <para>There is no need to list all of the source files as arguments to the <command>ghc -M</command> command;
922 <command>ghc</command> traces the dependencies, just like <command>ghc --make</command>
923 (a new feature in GHC 6.4).</para>
925 <para>Note that <literal>ghc -M</literal> needs to find a <emphasis>source
926 file</emphasis> for each module in the dependency graph, so that it can
927 parse the import declarations and follow dependencies. Any pre-compiled
928 modules without source files must therefore belong to a
929 package<footnote><para>This is a change in behaviour relative to 6.2 and
933 <para>By default, <command>ghc -M</command> generates all the
934 dependencies, and then concatenates them onto the end of
935 <filename>makefile</filename> (or
936 <filename>Makefile</filename> if <filename>makefile</filename>
937 doesn't exist) bracketed by the lines "<literal># DO NOT
938 DELETE: Beginning of Haskell dependencies</literal>" and
939 "<literal># DO NOT DELETE: End of Haskell
940 dependencies</literal>". If these lines already exist in the
941 <filename>makefile</filename>, then the old dependencies are
942 deleted first.</para>
944 <para>Don't forget to use the same <option>-package</option>
945 options on the <literal>ghc -M</literal> command line as you
946 would when compiling; this enables the dependency generator to
947 locate any imported modules that come from packages. The
948 package modules won't be included in the dependencies
949 generated, though (but see the
950 <option>––include-pkg-deps</option> option below).</para>
952 <para>The dependency generation phase of GHC can take some
953 additional options, which you may find useful. For historical
954 reasons, each option passed to the dependency generator from
955 the GHC command line must be preceded by
956 <literal>-optdep</literal>. For example, to pass <literal>-f
957 .depend</literal> to the dependency generator, you say
960 ghc -M -optdep-f -optdep.depend ...
963 The options which affect dependency generation are:</para>
967 <term><option>-w</option></term>
969 <para>Turn off warnings about interface file shadowing.</para>
974 <term><option>-v2</option></term>
976 <para>Print a full list of the module depenencies to stdout.
977 (This is the standard verbosity flag, so the list will
978 also be displayed with <option>-v3</option> and
979 <option>-v4</option>;
980 <xref linkend ="options-help"/>.)</para>
985 <term><option>-f</option> <replaceable>file</replaceable></term>
987 <para>Use <replaceable>file</replaceable> as the makefile,
988 rather than <filename>makefile</filename> or
989 <filename>Makefile</filename>. If
990 <replaceable>file</replaceable> doesn't exist,
991 <command>mkdependHS</command> creates it. We often use
992 <option>-f .depend</option> to put the dependencies in
993 <filename>.depend</filename> and then
994 <command>include</command> the file
995 <filename>.depend</filename> into
996 <filename>Makefile</filename>.</para>
1000 <!-- Retired with the move away from 'mkdependHS'.
1002 <term><option>-o <osuf></option></term>
1004 <para>Use <filename>.<osuf></filename> as the
1005 "target file" suffix ( default: <literal>o</literal>).
1006 Multiple <option>-o</option> flags are permitted
1007 (GHC2.05 onwards). Thus "<option>-o hc -o o</option>"
1008 will generate dependencies for <filename>.hc</filename>
1009 and <filename>.o</filename> files.</para>
1014 <term><option>-s <suf></option></term>
1016 <para>Make extra dependencies that declare that files
1018 <filename>.<suf>_<osuf></filename>
1019 depend on interface files with suffix
1020 <filename>.<suf>_hi</filename>, or (for
1021 <literal>{-# SOURCE #-}</literal>
1022 imports) on <filename>.hi-boot</filename>. Multiple
1023 <option>-s</option> flags are permitted. For example,
1024 <option>-o hc -s a -s b</option> will make dependencies
1025 for <filename>.hc</filename> on
1026 <filename>.hi</filename>,
1027 <filename>.a_hc</filename> on
1028 <filename>.a_hi</filename>, and
1029 <filename>.b_hc</filename> on
1030 <filename>.b_hi</filename>. (Useful in
1031 conjunction with NoFib "ways".)</para>
1036 <term><option>––exclude-module=<file></option></term>
1038 <para>Regard <filename><file></filename> as
1039 "stable"; i.e., exclude it from having dependencies on
1045 <term><option>-x</option></term>
1047 <para>same as <option>––exclude-module</option></para>
1052 <term><option>––exclude-directory=<dirs></option></term>
1054 <para>Regard the colon-separated list of directories
1055 <filename><dirs></filename> as containing stable,
1056 don't generate any dependencies on modules
1062 <term><option>––include-module=<file></option></term>
1064 <para>Regard <filename><file></filename> as not
1065 "stable"; i.e., generate dependencies on it (if
1066 any). This option is normally used in conjunction with
1067 the <option>––exclude-directory</option> option.</para>
1072 <term><option>––include-pkg-deps</option></term>
1074 <para>Regard modules imported from packages as unstable,
1075 i.e., generate dependencies on any imported package modules
1076 (including <literal>Prelude</literal>, and all other
1077 standard Haskell libraries). Dependencies are not traced
1078 recursively into packages; dependencies are only generated for
1079 home-package modules on external-package modules directly imported
1080 by the home package module.
1081 This option is normally
1082 only used by the various system libraries.</para>
1089 <sect2 id="orphan-modules">
1090 <title>Orphan modules and instance declarations</title>
1092 <para> Haskell specifies that when compiling module M, any instance
1093 declaration in any module "below" M is visible. (Module A is "below"
1094 M if A is imported directly by M, or if A is below a module that M imports directly.)
1095 In principle, GHC must therefore read the interface files of every module below M,
1096 just in case they contain an instance declaration that matters to M. This would
1097 be a disaster in practice, so GHC tries to be clever. </para>
1099 <para>In particular, if an instance declaration is in the same module as the definition
1100 of any type or class mentioned in the head of the instance declaration, then
1101 GHC has to visit that interface file anyway. Example:</para>
1104 instance C a => D (T a) where ...
1107 <para> The instance declaration is only relevant if the type T is in use, and if
1108 so, GHC will have visited A's interface file to find T's definition. </para>
1110 <para> The only problem comes when a module contains an instance declaration
1111 and GHC has no other reason for visiting the module. Example:
1114 instance C a => D (T a) where ...
1117 Here, neither D nor T is declared in module Orphan.
1118 We call such modules ``orphan modules'',
1119 defined thus:</para>
1121 <listitem> <para> An <emphasis>orphan module</emphasis>
1122 <indexterm><primary>orphan module</primary></indexterm>
1123 contains at least one <emphasis>orphan instance</emphasis> or at
1124 least one <emphasis>orphan rule</emphasis>.</para> </listitem>
1126 <listitem><para> An instance declaration in a module M is an <emphasis>orphan instance</emphasis> if
1127 <indexterm><primary>orphan instance</primary></indexterm>
1128 none of the type constructors
1129 or classes mentioned in the instance head (the part after the ``<literal>=></literal>'') are declared
1132 <para> Only the instance head counts. In the example above, it is not good enough for C's declaration
1133 to be in module A; it must be the declaration of D or T.</para>
1136 <listitem><para> A rewrite rule in a module M is an <emphasis>orphan rule</emphasis>
1137 <indexterm><primary>orphan rule</primary></indexterm>
1138 if none of the variables, type constructors,
1139 or classes that are free in the left hand side of the rule are declared in M.
1144 <para> GHC identifies orphan modules, and visits the interface file of
1145 every orphan module below the module being compiled. This is usually
1146 wasted work, but there is no avoiding it. You should therefore do
1147 your best to have as few orphan modules as possible.
1151 <para> You can identify an orphan module by looking in its interface
1152 file, <filename>M.hi</filename>, using the
1153 <option>--show-iface</option>. If there is a ``!'' on the first line,
1154 GHC considers it an orphan module.
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