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 <indexterm><primary>filenames</primary></indexterm>
35 <para>Each Haskell source module should be placed in a file on
38 <para>Usually, the file should be named after the module name,
39 replacing dots in the module name by directory separators. For
40 example, on a Unix system, the module <literal>A.B.C</literal>
41 should be placed in the file <literal>A/B/C.hs</literal>,
42 relative to some base directory. If the module is not going to
43 be imported by another module (<literal>Main</literal>, for
44 example), then you are free to use any filename for it.</para>
46 <indexterm><primary>unicode</primary></indexterm>
48 <para> GHC assumes that source files are
49 ASCII<indexterm><primary>ASCII</primary></indexterm> or
50 UTF-8<indexterm><primary>UTF-8</primary></indexterm> only, other
51 encodings<indexterm><primary>encoding</primary></indexterm> are
52 not recognised. However, invalid UTF-8 sequences will be
53 ignored in comments, so it is possible to use other encodings
55 Latin-1<indexterm><primary>Latin-1</primary></indexterm>, as
56 long as the non-comment source code is ASCII only.</para>
59 <sect2 id="output-files">
60 <title>Output files</title>
62 <indexterm><primary>interface files</primary></indexterm>
63 <indexterm><primary><literal>.hi</literal> files</primary></indexterm>
64 <indexterm><primary>object files</primary></indexterm>
65 <indexterm><primary><literal>.o</literal> files</primary></indexterm>
67 <para>When asked to compile a source file, GHC normally
68 generates two files: an <firstterm>object file</firstterm>, and
69 an <firstterm>interface file</firstterm>. </para>
71 <para>The object file, which normally ends in a
72 <literal>.o</literal> suffix, contains the compiled code for the
75 <para>The interface file,
76 which normally ends in a <literal>.hi</literal> suffix, contains
77 the information that GHC needs in order to compile further
78 modules that depend on this module. It contains things like the
79 types of exported functions, definitions of data types, and so
80 on. It is stored in a binary format, so don't try to read one;
81 use the <option>--show-iface</option> option instead (see <xref
82 linkend="hi-options"/>).</para>
84 <para>You should think of the object file and the interface file as a
85 pair, since the interface file is in a sense a compiler-readable
86 description of the contents of the object file. If the
87 interface file and object file get out of sync for any reason,
88 then the compiler may end up making assumptions about the object
89 file that aren't true; trouble will almost certainly follow.
90 For this reason, we recommend keeping object files and interface
91 files in the same place (GHC does this by default, but it is
92 possible to override the defaults as we'll explain
95 <para>Every module has a <emphasis>module name</emphasis>
96 defined in its source code (<literal>module A.B.C where
97 ...</literal>).</para>
99 <para>The name of the object file generated by GHC is derived
100 according to the following rules, where
101 <replaceable>osuf</replaceable> is the object-file suffix (this
102 can be changed with the <option>-osuf</option> option).</para>
106 <para>If there is no <option>-odir</option> option (the
107 default), then the object filename is derived from the
108 source filename (ignoring the module name) by replacing the
109 suffix with <replaceable>osuf</replaceable>.</para>
113 <option>-odir</option> <replaceable>dir</replaceable>
114 has been specified, then the object filename is
115 <replaceable>dir</replaceable>/<replaceable>mod</replaceable>.<replaceable>osuf</replaceable>,
116 where <replaceable>mod</replaceable> is the module name with
117 dots replaced by slashes. GHC will silently create the necessary directory
118 structure underneath <replaceable>dir</replaceable>, if it does not
119 already exist.</para>
123 <para>The name of the interface file is derived using the same
124 rules, except that the suffix is
125 <replaceable>hisuf</replaceable> (<literal>.hi</literal> by
126 default) instead of <replaceable>osuf</replaceable>, and the
127 relevant options are <option>-hidir</option> and
128 <option>-hisuf</option> instead of <option>-odir</option> and
129 <option>-osuf</option> respectively.</para>
131 <para>For example, if GHC compiles the module
132 <literal>A.B.C</literal> in the file
133 <filename>src/A/B/C.hs</filename>, with no
134 <literal>-odir</literal> or <literal>-hidir</literal> flags, the
135 interface file will be put in <literal>src/A/B/C.hi</literal>
136 and the object file in <literal>src/A/B/C.o</literal>.</para>
138 <para>For any module that is imported, GHC requires that the
139 name of the module in the import statement exactly matches the
140 name of the module in the interface file (or source file) found
141 using the strategy specified in <xref linkend="search-path"/>.
142 This means that for most modules, the source file name should
143 match the module name.</para>
145 <para>However, note that it is reasonable to have a module
146 <literal>Main</literal> in a file named
147 <filename>foo.hs</filename>, but this only works because GHC
148 never needs to search for the interface for module
149 <literal>Main</literal> (because it is never imported). It is
150 therefore possible to have several <literal>Main</literal>
151 modules in separate source files in the same directory, and GHC
152 will not get confused.</para>
154 <para>In batch compilation mode, the name of the object file can
155 also be overridden using the <option>-o</option> option, and the
156 name of the interface file can be specified directly using the
157 <option>-ohi</option> option.</para>
160 <sect2 id="search-path">
161 <title>The search path</title>
163 <indexterm><primary>search path</primary>
165 <indexterm><primary>interface files, finding them</primary></indexterm>
166 <indexterm><primary>finding interface files</primary></indexterm>
168 <para>In your program, you import a module
169 <literal>Foo</literal> by saying <literal>import Foo</literal>.
170 In <option>--make</option> mode or GHCi, GHC will look for a
171 source file for <literal>Foo</literal> and arrange to compile it
172 first. Without <option>--make</option>, GHC will look for the
173 interface file for <literal>Foo</literal>, which should have
174 been created by an earlier compilation of
175 <literal>Foo</literal>. GHC uses the same strategy in each of
176 these cases for finding the appropriate file.</para>
178 <para>This strategy is as follows: GHC keeps a list of
179 directories called the <firstterm>search path</firstterm>. For
180 each of these directories, it tries appending
181 <replaceable>basename</replaceable><literal>.</literal><replaceable>extension</replaceable>
182 to the directory, and checks whether the file exists. The value
183 of <replaceable>basename</replaceable> is the module name with
184 dots replaced by the directory separator ('/' or '\', depending
185 on the system), and <replaceable>extension</replaceable> is a
186 source extension (<literal>hs</literal>, <literal>lhs</literal>)
187 if we are in <option>--make</option> mode or GHCi, or
188 <replaceable>hisuf</replaceable> otherwise.</para>
190 <para>For example, suppose the search path contains directories
191 <literal>d1</literal>, <literal>d2</literal>, and
192 <literal>d3</literal>, and we are in <literal>--make</literal>
193 mode looking for the source file for a module
194 <literal>A.B.C</literal>. GHC will look in
195 <literal>d1/A/B/C.hs</literal>, <literal>d1/A/B/C.lhs</literal>,
196 <literal>d2/A/B/C.hs</literal>, and so on.</para>
198 <para>The search path by default contains a single directory:
199 <quote>.</quote> (i.e. the current directory). The following
200 options can be used to add to or change the contents of the
205 <term><option>-i<replaceable>dirs</replaceable></option></term>
207 <para><indexterm><primary><option>-i<replaceable>dirs</replaceable></option>
208 </primary></indexterm>This flag appends a colon-separated
209 list of <filename>dirs</filename> to the search path.</para>
214 <term><option>-i</option></term>
216 <para>resets the search path back to nothing.</para>
221 <para>This isn't the whole story: GHC also looks for modules in
222 pre-compiled libraries, known as packages. See the section on
223 packages (<xref linkend="packages"/>) for details.</para>
226 <sect2 id="options-output">
227 <title>Redirecting the compilation output(s)</title>
229 <indexterm><primary>output-directing options</primary></indexterm>
230 <indexterm><primary>redirecting compilation output</primary></indexterm>
235 <option>-o</option> <replaceable>file</replaceable>
236 <indexterm><primary><option>-o</option></primary></indexterm>
239 <para>GHC's compiled output normally goes into a
240 <filename>.hc</filename>, <filename>.o</filename>, etc.,
241 file, depending on the last-run compilation phase. The
242 option <option>-o <replaceable>file</replaceable></option>
243 re-directs the output of that last-run phase to
244 <replaceable>file</replaceable>.</para>
246 <para>Note: this “feature” can be
247 counterintuitive: <command>ghc -C -o foo.o
248 foo.hs</command> will put the intermediate C code in the
249 file <filename>foo.o</filename>, name
250 notwithstanding!</para>
252 <para>This option is most often used when creating an
253 executable file, to set the filename of the executable.
255 <screen> ghc -o prog --make Main</screen>
257 will compile the program starting with module
258 <literal>Main</literal> and put the executable in the
259 file <literal>prog</literal>.</para>
261 <para>Note: on Windows, if the result is an executable
262 file, the extension "<filename>.exe</filename>" is added
263 if the specified filename does not already have an
268 will compile and link the module
269 <filename>Main.hs</filename>, and put the resulting
270 executable in <filename>foo.exe</filename> (not
271 <filename>foo</filename>).</para>
273 <para>If you use <command>ghc --make</command> and you don't
274 use the <option>-o</option>, the name GHC will choose
275 for the executable will be based on the name of the file
276 containing the module <literal>Main</literal>.
277 Note that with GHC the <literal>Main</literal> module doesn't
278 have to be put in file <filename>Main.hs</filename>.
287 will produce <filename>Prog</filename> (or
288 <filename>Prog.exe</filename> if you are on Windows).</para>
294 <option>-odir</option> <replaceable>dir</replaceable>
295 <indexterm><primary><option>-odir</option></primary></indexterm>
298 <para>Redirects object files to directory
299 <replaceable>dir</replaceable>. For example:</para>
302 $ ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `uname -m`
305 <para>The object files, <filename>Foo.o</filename>,
306 <filename>Bar.o</filename>, and
307 <filename>Bumble.o</filename> would be put into a
308 subdirectory named after the architecture of the executing
309 machine (<filename>x86</filename>,
310 <filename>mips</filename>, etc).</para>
312 <para>Note that the <option>-odir</option> option does
313 <emphasis>not</emphasis> affect where the interface files
314 are put; use the <option>-hidir</option> option for that.
315 In the above example, they would still be put in
316 <filename>parse/Foo.hi</filename>,
317 <filename>parse/Bar.hi</filename>, and
318 <filename>gurgle/Bumble.hi</filename>.</para>
324 <option>-ohi</option> <replaceable>file</replaceable>
325 <indexterm><primary><option>-ohi</option></primary></indexterm>
328 <para>The interface output may be directed to another file
329 <filename>bar2/Wurble.iface</filename> with the option
330 <option>-ohi bar2/Wurble.iface</option> (not
333 <para>WARNING: if you redirect the interface file
334 somewhere that GHC can't find it, then the recompilation
335 checker may get confused (at the least, you won't get any
336 recompilation avoidance). We recommend using a
337 combination of <option>-hidir</option> and
338 <option>-hisuf</option> options instead, if
341 <para>To avoid generating an interface at all, you could
342 use this option to redirect the interface into the bit
343 bucket: <literal>-ohi /dev/null</literal>, for
350 <option>-hidir</option> <replaceable>dir</replaceable>
351 <indexterm><primary><option>-hidir</option></primary></indexterm>
354 <para>Redirects all generated interface files into
355 <replaceable>dir</replaceable>, instead of the
362 <option>-stubdir</option> <replaceable>dir</replaceable>
363 <indexterm><primary><option>-stubdir</option></primary></indexterm>
366 <para>Redirects all generated FFI stub files into
367 <replaceable>dir</replaceable>. Stub files are generated when the
368 Haskell source contains a <literal>foreign export</literal> or
369 <literal>foreign import "&wrapper"</literal> declaration (see <xref
370 linkend="foreign-export-ghc" />). The <option>-stubdir</option>
371 option behaves in exactly the same way as <option>-odir</option>
372 and <option>-hidir</option> with respect to hierarchical
379 <option>-outputdir</option> <replaceable>dir</replaceable>
380 <indexterm><primary><option>-outputdir</option></primary></indexterm>
383 <para>The <option>-outputdir</option> option is shorthand for
385 of <option>-odir</option>, <option>-hidir</option>,
386 and <option>-stubdir</option>.
393 <option>-osuf</option> <replaceable>suffix</replaceable>
394 <indexterm><primary><option>-osuf</option></primary></indexterm>
397 <option>-hisuf</option> <replaceable>suffix</replaceable>
398 <indexterm><primary><option>-hisuf</option></primary></indexterm>
401 <option>-hcsuf</option> <replaceable>suffix</replaceable>
402 <indexterm><primary><option>-hcsuf</option></primary></indexterm>
405 <para>The <option>-osuf</option>
406 <replaceable>suffix</replaceable> will change the
407 <literal>.o</literal> file suffix for object files to
408 whatever you specify. We use this when compiling
409 libraries, so that objects for the profiling versions of
410 the libraries don't clobber the normal ones.</para>
412 <para>Similarly, the <option>-hisuf</option>
413 <replaceable>suffix</replaceable> will change the
414 <literal>.hi</literal> file suffix for non-system
415 interface files (see <xref linkend="hi-options"/>).</para>
417 <para>Finally, the option <option>-hcsuf</option>
418 <replaceable>suffix</replaceable> will change the
419 <literal>.hc</literal> file suffix for compiler-generated
420 intermediate C files.</para>
422 <para>The <option>-hisuf</option>/<option>-osuf</option>
423 game is particularly useful if you want to compile a
424 program both with and without profiling, in the same
425 directory. You can say:
428 to get the ordinary version, and
430 ghc ... -osuf prof.o -hisuf prof.hi -prof -auto-all</screen>
431 to get the profiled version.</para>
437 <sect2 id="keeping-intermediates">
438 <title>Keeping Intermediate Files</title>
439 <indexterm><primary>intermediate files, saving</primary>
441 <indexterm><primary><literal>.hc</literal> files, saving</primary>
443 <indexterm><primary><literal>.ll</literal> files, saving</primary>
445 <indexterm><primary><literal>.s</literal> files, saving</primary>
448 <para>The following options are useful for keeping certain
449 intermediate files around, when normally GHC would throw these
450 away after compilation:</para>
455 <option>-keep-hc-file</option>,
456 <option>-keep-hc-files</option>
457 <indexterm><primary><option>-keep-hc-file</option></primary></indexterm>
458 <indexterm><primary><option>-keep-hc-files</option></primary></indexterm>
461 <para>Keep intermediate <literal>.hc</literal> files when
462 doing <literal>.hs</literal>-to-<literal>.o</literal>
463 compilations via C (NOTE: <literal>.hc</literal> files
464 are only generated by unregisterised compilers).</para>
470 <option>-keep-llvm-file</option>,
471 <option>-keep-llvm-files</option>
472 <indexterm><primary><option>-keep-llvm-file</option></primary></indexterm>
473 <indexterm><primary><option>-keep-llvm-files</option></primary></indexterm>
476 <para>Keep intermediate <literal>.ll</literal> files when
477 doing <literal>.hs</literal>-to-<literal>.o</literal>
478 compilations via LLVM (NOTE: <literal>.ll</literal> files
479 aren't generated when using the native code generator, you
480 may need to use <option>-fllvm</option> to force them
481 to be produced).</para>
487 <option>-keep-s-file</option>,
488 <option>-keep-s-files</option>
489 <indexterm><primary><option>-keep-s-file</option></primary></indexterm>
490 <indexterm><primary><option>-keep-s-files</option></primary></indexterm>
493 <para>Keep intermediate <literal>.s</literal> files.</para>
499 <option>-keep-raw-s-file</option>,
500 <option>-keep-raw-s-files</option>
501 <indexterm><primary><option>-keep-raw-s-file</option></primary></indexterm>
502 <indexterm><primary><option>-keep-raw-s-files</option></primary></indexterm>
505 <para>Keep intermediate <literal>.raw-s</literal> files.
506 These are the direct output from the C compiler, before
507 GHC does “assembly mangling” to produce the
508 <literal>.s</literal> file. Again, these are not produced
509 when using the native code generator.</para>
515 <option>-keep-tmp-files</option>
516 <indexterm><primary><option>-keep-tmp-files</option></primary></indexterm>
517 <indexterm><primary>temporary files</primary><secondary>keeping</secondary></indexterm>
520 <para>Instructs the GHC driver not to delete any of its
521 temporary files, which it normally keeps in
522 <literal>/tmp</literal> (or possibly elsewhere; see <xref
523 linkend="temp-files"/>). Running GHC with
524 <option>-v</option> will show you what temporary files
525 were generated along the way.</para>
531 <sect2 id="temp-files">
532 <title>Redirecting temporary files</title>
535 <primary>temporary files</primary>
536 <secondary>redirecting</secondary>
542 <option>-tmpdir</option>
543 <indexterm><primary><option>-tmpdir</option></primary></indexterm>
546 <para>If you have trouble because of running out of space
547 in <filename>/tmp</filename> (or wherever your
548 installation thinks temporary files should go), you may
549 use the <option>-tmpdir
550 <dir></option><indexterm><primary>-tmpdir
551 <dir> option</primary></indexterm> option to specify
552 an alternate directory. For example, <option>-tmpdir
553 .</option> says to put temporary files in the current
554 working directory.</para>
556 <para>Alternatively, use your <constant>TMPDIR</constant>
557 environment variable.<indexterm><primary>TMPDIR
558 environment variable</primary></indexterm> Set it to the
559 name of the directory where temporary files should be put.
560 GCC and other programs will honour the
561 <constant>TMPDIR</constant> variable as well.</para>
563 <para>Even better idea: Set the
564 <constant>DEFAULT_TMPDIR</constant> make variable when
565 building GHC, and never worry about
566 <constant>TMPDIR</constant> again. (see the build
567 documentation).</para>
573 <sect2 id="hi-options">
574 <title>Other options related to interface files</title>
575 <indexterm><primary>interface files, options</primary></indexterm>
580 <option>-ddump-hi</option>
581 <indexterm><primary><option>-ddump-hi</option></primary></indexterm>
584 <para>Dumps the new interface to standard output.</para>
590 <option>-ddump-hi-diffs</option>
591 <indexterm><primary><option>-ddump-hi-diffs</option></primary></indexterm>
594 <para>The compiler does not overwrite an existing
595 <filename>.hi</filename> interface file if the new one is
596 the same as the old one; this is friendly to
597 <command>make</command>. When an interface does change,
598 it is often enlightening to be informed. The
599 <option>-ddump-hi-diffs</option> option will make GHC
600 report the differences between the old and
601 new <filename>.hi</filename> files.</para>
607 <option>-ddump-minimal-imports</option>
608 <indexterm><primary><option>-ddump-minimal-imports</option></primary></indexterm>
611 <para>Dump to the file "M.imports" (where M is the module
612 being compiled) a "minimal" set of import declarations.
613 You can safely replace all the import declarations in
614 "M.hs" with those found in "M.imports". Why would you
615 want to do that? Because the "minimal" imports (a) import
616 everything explicitly, by name, and (b) import nothing
617 that is not required. It can be quite painful to maintain
618 this property by hand, so this flag is intended to reduce
625 <option>--show-iface</option> <replaceable>file</replaceable>
626 <indexterm><primary><option>--show-iface</option></primary></indexterm>
629 <para>where <replaceable>file</replaceable> is the name of
630 an interface file, dumps the contents of that interface in
631 a human-readable (ish) format. See <xref linkend="modes"/>.</para>
638 <title>The recompilation checker</title>
640 <indexterm><primary>recompilation checker</primary></indexterm>
645 <option>-fforce-recomp</option>
646 <indexterm><primary><option>-fforce-recomp</option></primary></indexterm>
647 <indexterm><primary><option>-fno-force-recomp</option></primary></indexterm>
650 <para>Turn off recompilation checking (which is on by
651 default). Recompilation checking normally stops
652 compilation early, leaving an existing
653 <filename>.o</filename> file in place, if it can be
654 determined that the module does not need to be
660 <para>In the olden days, GHC compared the newly-generated
661 <filename>.hi</filename> file with the previous version; if they
662 were identical, it left the old one alone and didn't change its
663 modification date. In consequence, importers of a module with
664 an unchanged output <filename>.hi</filename> file were not
667 <para>This doesn't work any more. Suppose module
668 <literal>C</literal> imports module <literal>B</literal>, and
669 <literal>B</literal> imports module <literal>A</literal>. So
670 changes to module <literal>A</literal> might require module
671 <literal>C</literal> to be recompiled, and hence when
672 <filename>A.hi</filename> changes we should check whether
673 <literal>C</literal> should be recompiled. However, the
674 dependencies of <literal>C</literal> will only list
675 <literal>B.hi</literal>, not <literal>A.hi</literal>, and some
676 changes to <literal>A</literal> (changing the definition of a
677 function that appears in an inlining of a function exported by
678 <literal>B</literal>, say) may conceivably not change
679 <filename>B.hi</filename> one jot. So now…</para>
681 <para>GHC calculates a fingerprint (in fact an MD5 hash) of each
682 interface file, and of each declaration within the interface
683 file. It also keeps in every interface file a list of the
684 fingerprints of everything it used when it last compiled the
685 file. If the source file's modification date is earlier than
686 the <filename>.o</filename> file's date (i.e. the source hasn't
687 changed since the file was last compiled), and the recompilation
688 checking is on, GHC will be clever. It compares the fingerprints
689 on the things it needs this time with the fingerprints
690 on the things it needed last time (gleaned from the
691 interface file of the module being compiled); if they are all
692 the same it stops compiling early in the process saying
693 “Compilation IS NOT required”. What a beautiful
697 about <ulink url="http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/RecompilationAvoidance">how
698 all this works</ulink> in the GHC commentary.</para>
702 <sect2 id="mutual-recursion">
703 <title>How to compile mutually recursive modules</title>
705 <indexterm><primary>module system, recursion</primary></indexterm>
706 <indexterm><primary>recursion, between modules</primary></indexterm>
708 <para>GHC supports the compilation of mutually recursive modules.
709 This section explains how.</para>
711 <para>Every cycle in the module import graph must be broken by a <filename>hs-boot</filename> file.
712 Suppose that modules <filename>A.hs</filename> and <filename>B.hs</filename> are Haskell source files,
718 newtype TA = MkTA Int
724 import {-# SOURCE #-} A( TA(..) )
731 <indexterm><primary><literal>hs-boot</literal>
732 files</primary></indexterm> <indexterm><primary>importing,
733 <literal>hi-boot</literal> files</primary></indexterm>
734 Here <filename>A</filename> imports <filename>B</filename>, but <filename>B</filename> imports
735 <filename>A</filename> with a <literal>{-# SOURCE #-}</literal> pragma, which breaks the
736 circular dependency. Every loop in the module import graph must be broken by a <literal>{-# SOURCE #-}</literal> import;
737 or, equivalently, the module import graph must be acyclic if <literal>{-# SOURCE #-}</literal> imports are ignored.
739 <para>For every module <filename>A.hs</filename> that is <literal>{-# SOURCE #-}</literal>-imported
740 in this way there must exist a source file <literal>A.hs-boot</literal>. This file contains an abbreviated
741 version of <filename>A.hs</filename>, thus:
744 newtype TA = MkTA Int
747 <para>To compile these three files, issue the following commands:
749 ghc -c A.hs-boot -- Produces A.hi-boot, A.o-boot
750 ghc -c B.hs -- Consumes A.hi-boot, produces B.hi, B.o
751 ghc -c A.hs -- Consumes B.hi, produces A.hi, A.o
752 ghc -o foo A.o B.o -- Linking the program
755 <para>There are several points to note here:
758 <para>The file <filename>A.hs-boot</filename> is a programmer-written source file.
759 It must live in the same directory as its parent source file <filename>A.hs</filename>.
760 Currently, if you use a literate source file <filename>A.lhs</filename> you must
761 also use a literate boot file, <filename>A.lhs-boot</filename>; and vice versa.
765 A <filename>hs-boot</filename> file is compiled by GHC, just like a <filename>hs</filename> file:
769 When a hs-boot file <filename>A.hs-boot</filename>
770 is compiled, it is checked for scope and type errors.
771 When its parent module <filename>A.hs</filename> is compiled, the two are compared, and
772 an error is reported if the two are inconsistent.
776 <para> Just as compiling <filename>A.hs</filename> produces an
777 interface file <filename>A.hi</filename>, and an object file
778 <filename>A.o</filename>, so compiling
779 <filename>A.hs-boot</filename> produces an interface file
780 <filename>A.hi-boot</filename>, and an pseudo-object file
781 <filename>A.o-boot</filename>: </para>
785 <para>The pseudo-object file <filename>A.o-boot</filename> is
786 empty (don't link it!), but it is very useful when using a
787 Makefile, to record when the <filename>A.hi-boot</filename> was
788 last brought up to date (see <xref
789 linkend="using-make"/>).</para>
793 <para>The <filename>hi-boot</filename> generated by compiling a
794 <filename>hs-boot</filename> file is in the same
795 machine-generated binary format as any other GHC-generated
796 interface file (e.g. <filename>B.hi</filename>). You can
797 display its contents with <command>ghc
798 --show-iface</command>. If you specify a directory for
799 interface files, the <option>-ohidir</option> flag, then that
800 affects <filename>hi-boot</filename> files
806 <listitem><para> If hs-boot files are considered distinct from their parent source
807 files, and if a <literal>{-# SOURCE #-}</literal> import is considered to refer to the
808 hs-boot file, then the module import graph must have no cycles. The command
809 <command>ghc -M</command> will report an error if a cycle is found.
812 <listitem><para> A module <literal>M</literal> that is
813 <literal>{-# SOURCE #-}</literal>-imported in a program will usually also be
814 ordinarily imported elsewhere. If not, <command>ghc --make</command>
815 automatically adds <literal>M</literal> to the set of modules it tries to
816 compile and link, to ensure that <literal>M</literal>'s implementation is included in
822 A hs-boot file need only contain the bare
823 minimum of information needed to get the bootstrapping process
824 started. For example, it doesn't need to contain declarations
825 for <emphasis>everything</emphasis> that module
826 <literal>A</literal> exports, only the things required by the
827 module(s) that import <literal>A</literal> recursively.</para>
828 <para>A hs-boot file is written in a subset of Haskell:
830 <listitem><para> The module header (including the export list), and import statements, are exactly as in
831 Haskell, and so are the scoping rules.
832 Hence, to mention a non-Prelude type or class, you must import it.</para></listitem>
834 <listitem><para> There must be no value declarations, but there can be type signatures for
837 double :: Int -> Int
840 <listitem><para> Fixity declarations are exactly as in Haskell.</para></listitem>
841 <listitem><para> Type synonym declarations are exactly as in Haskell.</para></listitem>
842 <listitem><para> A data type declaration can either be given in full, exactly as in Haskell, or it
843 can be given abstractly, by omitting the '=' sign and everything that follows. For example:
847 In a <emphasis>source</emphasis> program
848 this would declare TA to have no constructors (a GHC extension: see <xref linkend="nullary-types"/>),
849 but in an hi-boot file it means "I don't know or care what the constructors are".
850 This is the most common form of data type declaration, because it's easy to get right.
851 You <emphasis>can</emphasis> also write out the constructors but, if you do so, you must write
852 it out precisely as in its real definition.</para>
854 If you do not write out the constructors, you may need to give a kind
855 annotation (<xref linkend="kinding"/>), to tell
856 GHC the kind of the type variable, if it is not "*". (In source files, this is worked out
857 from the way the type variable is used in the constructors.) For example:
859 data R (x :: * -> *) y
861 You cannot use <literal>deriving</literal> on a data type declaration; write an
862 <literal>instance</literal> declaration instead.
864 <listitem><para> Class declarations is exactly as in Haskell, except that you may not put
865 default method declarations. You can also omit all the superclasses and class
866 methods entirely; but you must either omit them all or put them all in.
868 <listitem><para> You can include instance declarations just as in Haskell; but omit the "where" part.
875 <sect2 id="using-make">
876 <title>Using <command>make</command></title>
878 <indexterm><primary><literal>make</literal></primary></indexterm>
880 <para>It is reasonably straightforward to set up a
881 <filename>Makefile</filename> to use with GHC, assuming you name
882 your source files the same as your modules. Thus:</para>
886 HC_OPTS = -cpp $(EXTRA_HC_OPTS)
888 SRCS = Main.lhs Foo.lhs Bar.lhs
889 OBJS = Main.o Foo.o Bar.o
891 .SUFFIXES : .o .hs .hi .lhs .hc .s
895 $(HC) -o $@ $(HC_OPTS) $(OBJS)
897 # Standard suffix rules
902 $(HC) -c $< $(HC_OPTS)
905 $(HC) -c $< $(HC_OPTS)
911 $(HC) -c $< $(HC_OPTS)
914 $(HC) -c $< $(HC_OPTS)
916 # Inter-module dependencies
917 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
918 Main.o Main.hc Main.s : Foo.hi Baz.hi # Main imports Foo and Baz
921 <para>(Sophisticated <command>make</command> variants may
922 achieve some of the above more elegantly. Notably,
923 <command>gmake</command>'s pattern rules let you write the more
924 comprehensible:</para>
928 $(HC) -c $< $(HC_OPTS)
931 <para>What we've shown should work with any
932 <command>make</command>.)</para>
934 <para>Note the cheesy <literal>.o.hi</literal> rule: It records
935 the dependency of the interface (<filename>.hi</filename>) file
936 on the source. The rule says a <filename>.hi</filename> file
937 can be made from a <filename>.o</filename> file by
938 doing…nothing. Which is true.</para>
939 <para> Note that the suffix rules are all repeated twice, once
940 for normal Haskell source files, and once for <filename>hs-boot</filename>
941 files (see <xref linkend="mutual-recursion"/>).</para>
943 <para>Note also the inter-module dependencies at the end of the
944 Makefile, which take the form
947 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
950 They tell <command>make</command> that if any of
951 <literal>Foo.o</literal>, <literal>Foo.hc</literal> or
952 <literal>Foo.s</literal> have an earlier modification date than
953 <literal>Baz.hi</literal>, then the out-of-date file must be
954 brought up to date. To bring it up to date,
955 <literal>make</literal> looks for a rule to do so; one of the
956 preceding suffix rules does the job nicely. These dependencies
957 can be generated automatically by <command>ghc</command>; see
958 <xref linkend="makefile-dependencies"/></para>
962 <sect2 id="makefile-dependencies">
963 <title>Dependency generation</title>
964 <indexterm><primary>dependencies in Makefiles</primary></indexterm>
965 <indexterm><primary>Makefile dependencies</primary></indexterm>
967 <para>Putting inter-dependencies of the form <literal>Foo.o :
968 Bar.hi</literal> into your <filename>Makefile</filename> by
969 hand is rather error-prone. Don't worry, GHC has support for
970 automatically generating the required dependencies. Add the
971 following to your <filename>Makefile</filename>:</para>
975 ghc -M $(HC_OPTS) $(SRCS)
978 <para>Now, before you start compiling, and any time you change
979 the <literal>imports</literal> in your program, do
980 <command>make depend</command> before you do <command>make
981 cool_pgm</command>. The command <command>ghc -M</command> will
982 append the needed dependencies to your
983 <filename>Makefile</filename>.</para>
985 <para>In general, <command>ghc -M Foo</command> does the following.
986 For each module <literal>M</literal> in the set
987 <literal>Foo</literal> plus all its imports (transitively),
988 it adds to the Makefile:
990 <listitem><para>A line recording the dependence of the object file on the source file.
994 (or <literal>M.lhs</literal> if that is the filename you used).
996 <listitem><para> For each import declaration <literal>import X</literal> in <literal>M</literal>,
997 a line recording the dependence of <literal>M</literal> on <literal>X</literal>:
1000 </programlisting></para></listitem>
1001 <listitem><para> For each import declaration <literal>import {-# SOURCE #-} X</literal> in <literal>M</literal>,
1002 a line recording the dependence of <literal>M</literal> on <literal>X</literal>:
1006 (See <xref linkend="mutual-recursion"/> for details of
1007 <literal>hi-boot</literal> style interface files.)
1010 If <literal>M</literal> imports multiple modules, then there will
1011 be multiple lines with <filename>M.o</filename> as the
1013 <para>There is no need to list all of the source files as arguments to the <command>ghc -M</command> command;
1014 <command>ghc</command> traces the dependencies, just like <command>ghc --make</command>
1015 (a new feature in GHC 6.4).</para>
1017 <para>Note that <literal>ghc -M</literal> needs to find a <emphasis>source
1018 file</emphasis> for each module in the dependency graph, so that it can
1019 parse the import declarations and follow dependencies. Any pre-compiled
1020 modules without source files must therefore belong to a
1021 package<footnote><para>This is a change in behaviour relative to 6.2 and
1025 <para>By default, <command>ghc -M</command> generates all the
1026 dependencies, and then concatenates them onto the end of
1027 <filename>makefile</filename> (or
1028 <filename>Makefile</filename> if <filename>makefile</filename>
1029 doesn't exist) bracketed by the lines "<literal># DO NOT
1030 DELETE: Beginning of Haskell dependencies</literal>" and
1031 "<literal># DO NOT DELETE: End of Haskell
1032 dependencies</literal>". If these lines already exist in the
1033 <filename>makefile</filename>, then the old dependencies are
1034 deleted first.</para>
1036 <para>Don't forget to use the same <option>-package</option>
1037 options on the <literal>ghc -M</literal> command line as you
1038 would when compiling; this enables the dependency generator to
1039 locate any imported modules that come from packages. The
1040 package modules won't be included in the dependencies
1041 generated, though (but see the
1042 <option>––include-pkg-deps</option> option below).</para>
1044 <para>The dependency generation phase of GHC can take some
1045 additional options, which you may find useful.
1047 The options which affect dependency generation are:</para>
1051 <term><option>-ddump-mod-cycles</option></term>
1053 <para>Display a list of the cycles in the module graph. This is
1054 useful when trying to eliminate such cycles.</para>
1059 <term><option>-v2</option></term>
1061 <para>Print a full list of the module dependencies to stdout.
1062 (This is the standard verbosity flag, so the list will
1063 also be displayed with <option>-v3</option> and
1064 <option>-v4</option>;
1065 <xref linkend ="options-help"/>.)</para>
1070 <term><option>-dep-makefile</option> <replaceable>file</replaceable></term>
1072 <para>Use <replaceable>file</replaceable> as the makefile,
1073 rather than <filename>makefile</filename> or
1074 <filename>Makefile</filename>. If
1075 <replaceable>file</replaceable> doesn't exist,
1076 <command>mkdependHS</command> creates it. We often use
1077 <option>-dep-makefile .depend</option> to put the dependencies in
1078 <filename>.depend</filename> and then
1079 <command>include</command> the file
1080 <filename>.depend</filename> into
1081 <filename>Makefile</filename>.</para>
1086 <term><option>-dep-suffix <suf></option></term>
1088 <para>Make extra dependencies that declare that files
1090 <filename>.<suf>_<osuf></filename>
1091 depend on interface files with suffix
1092 <filename>.<suf>_hi</filename>, or (for
1093 <literal>{-# SOURCE #-}</literal>
1094 imports) on <filename>.hi-boot</filename>. Multiple
1095 <option>-dep-suffix</option> flags are permitted. For example,
1096 <option>-dep-suffix a -dep-suffix b</option>
1097 will make dependencies
1098 for <filename>.hs</filename> on
1099 <filename>.hi</filename>,
1100 <filename>.a_hs</filename> on
1101 <filename>.a_hi</filename>, and
1102 <filename>.b_hs</filename> on
1103 <filename>.b_hi</filename>. (Useful in
1104 conjunction with NoFib "ways".)</para>
1109 <term><option>––exclude-module=<file></option></term>
1111 <para>Regard <filename><file></filename> as
1112 "stable"; i.e., exclude it from having dependencies on
1118 <term><option>––include-pkg-deps</option></term>
1120 <para>Regard modules imported from packages as unstable,
1121 i.e., generate dependencies on any imported package modules
1122 (including <literal>Prelude</literal>, and all other
1123 standard Haskell libraries). Dependencies are not traced
1124 recursively into packages; dependencies are only generated for
1125 home-package modules on external-package modules directly imported
1126 by the home package module.
1127 This option is normally
1128 only used by the various system libraries.</para>
1135 <sect2 id="orphan-modules">
1136 <title>Orphan modules and instance declarations</title>
1138 <para> Haskell specifies that when compiling module M, any instance
1139 declaration in any module "below" M is visible. (Module A is "below"
1140 M if A is imported directly by M, or if A is below a module that M imports directly.)
1141 In principle, GHC must therefore read the interface files of every module below M,
1142 just in case they contain an instance declaration that matters to M. This would
1143 be a disaster in practice, so GHC tries to be clever. </para>
1145 <para>In particular, if an instance declaration is in the same module as the definition
1146 of any type or class mentioned in the <emphasis>head</emphasis> of the instance declaration
1147 (the part after the “<literal>=></literal>”; see <xref linkend="instance-rules"/>), then
1148 GHC has to visit that interface file anyway. Example:</para>
1151 instance C a => D (T a) where ...
1154 <para> The instance declaration is only relevant if the type T is in use, and if
1155 so, GHC will have visited A's interface file to find T's definition. </para>
1157 <para> The only problem comes when a module contains an instance declaration
1158 and GHC has no other reason for visiting the module. Example:
1161 instance C a => D (T a) where ...
1164 Here, neither D nor T is declared in module Orphan.
1165 We call such modules “orphan modules”.
1166 GHC identifies orphan modules, and visits the interface file of
1167 every orphan module below the module being compiled. This is usually
1168 wasted work, but there is no avoiding it. You should therefore do
1169 your best to have as few orphan modules as possible.
1172 Functional dependencies complicate matters. Suppose we have:
1175 instance E T Int where ...
1178 Is this an orphan module? Apparently not, because <literal>T</literal>
1179 is declared in the same module. But suppose class <literal>E</literal> had a
1180 functional dependency:
1183 class E x y | y -> x where ...
1185 Then in some importing module M, the constraint <literal>(E a Int)</literal> should be "improved" by setting
1186 <literal>a = T</literal>, <emphasis>even though there is no explicit mention
1187 of <literal>T</literal> in M</emphasis>.</para>
1189 These considerations lead to the following definition of an orphan module:
1191 <listitem> <para> An <emphasis>orphan module</emphasis>
1192 <indexterm><primary>orphan module</primary></indexterm>
1193 contains at least one <emphasis>orphan instance</emphasis> or at
1194 least one <emphasis>orphan rule</emphasis>.</para> </listitem>
1196 <listitem><para> An instance declaration in a module M is an <emphasis>orphan instance</emphasis> if
1197 <indexterm><primary>orphan instance</primary></indexterm>
1200 The class of the instance declaration is not declared in M, and
1203 <para> <emphasis>Either</emphasis> the class has no functional dependencies, and none of the type constructors
1204 in the instance head is declared in M; <emphasis>or</emphasis> there
1205 is a functional dependency for which none of the type constructors mentioned
1206 in the <emphasis>non-determined</emphasis> part of the instance head is defined in M.
1210 <para> Only the instance head
1211 counts. In the example above, it is not good enough for C's declaration
1212 to be in module A; it must be the declaration of D or T.</para>
1215 <listitem><para> A rewrite rule in a module M is an <emphasis>orphan rule</emphasis>
1216 <indexterm><primary>orphan rule</primary></indexterm>
1217 if none of the variables, type constructors,
1218 or classes that are free in the left hand side of the rule are declared in M.
1224 <para>If you use the flag <option>-fwarn-orphans</option>, GHC will warn you
1225 if you are creating an orphan module.
1226 Like any warning, you can switch the warning off with <option>-fno-warn-orphans</option>,
1227 and <option>-Werror</option>
1228 will make the compilation fail if the warning is issued.
1231 You can identify an orphan module by looking in its interface
1232 file, <filename>M.hi</filename>, using the
1233 <link linkend="modes"><option>--show-iface</option> mode</link>. If there is a <literal>[orphan module]</literal> on the
1234 first line, GHC considers it an orphan module.
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