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 and 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 `arch`
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>-osuf</option> <replaceable>suffix</replaceable>
380 <indexterm><primary><option>-osuf</option></primary></indexterm>
383 <option>-hisuf</option> <replaceable>suffix</replaceable>
384 <indexterm><primary><option>-hisuf</option></primary></indexterm>
387 <option>-hcsuf</option> <replaceable>suffix</replaceable>
388 <indexterm><primary><option>-hcsuf</option></primary></indexterm>
391 <para>The <option>-osuf</option>
392 <replaceable>suffix</replaceable> will change the
393 <literal>.o</literal> file suffix for object files to
394 whatever you specify. We use this when compiling
395 libraries, so that objects for the profiling versions of
396 the libraries don't clobber the normal ones.</para>
398 <para>Similarly, the <option>-hisuf</option>
399 <replaceable>suffix</replaceable> will change the
400 <literal>.hi</literal> file suffix for non-system
401 interface files (see <xref linkend="hi-options"/>).</para>
403 <para>Finally, the option <option>-hcsuf</option>
404 <replaceable>suffix</replaceable> will change the
405 <literal>.hc</literal> file suffix for compiler-generated
406 intermediate C files.</para>
408 <para>The <option>-hisuf</option>/<option>-osuf</option>
409 game is particularly useful if you want to compile a
410 program both with and without profiling, in the same
411 directory. You can say:
414 to get the ordinary version, and
416 ghc ... -osuf prof.o -hisuf prof.hi -prof -auto-all</screen>
417 to get the profiled version.</para>
423 <sect2 id="keeping-intermediates">
424 <title>Keeping Intermediate Files</title>
425 <indexterm><primary>intermediate files, saving</primary>
427 <indexterm><primary><literal>.hc</literal> files, saving</primary>
429 <indexterm><primary><literal>.s</literal> files, saving</primary>
432 <para>The following options are useful for keeping certain
433 intermediate files around, when normally GHC would throw these
434 away after compilation:</para>
439 <option>-keep-hc-files</option>
440 <indexterm><primary><option>-keep-hc-files</option></primary></indexterm>
443 <para>Keep intermediate <literal>.hc</literal> files when
444 doing <literal>.hs</literal>-to-<literal>.o</literal>
445 compilations via C (NOTE: <literal>.hc</literal> files
446 aren't generated when using the native code generator, you
447 may need to use <option>-fvia-C</option> to force them
448 to be produced).</para>
454 <option>-keep-s-files</option>
455 <indexterm><primary><option>-keep-s-files</option></primary></indexterm>
458 <para>Keep intermediate <literal>.s</literal> files.</para>
464 <option>-keep-raw-s-files</option>
465 <indexterm><primary><option>-keep-raw-s-files</option></primary></indexterm>
468 <para>Keep intermediate <literal>.raw-s</literal> files.
469 These are the direct output from the C compiler, before
470 GHC does “assembly mangling” to produce the
471 <literal>.s</literal> file. Again, these are not produced
472 when using the native code generator.</para>
478 <option>-keep-tmp-files</option>
479 <indexterm><primary><option>-keep-tmp-files</option></primary></indexterm>
480 <indexterm><primary>temporary files</primary><secondary>keeping</secondary></indexterm>
483 <para>Instructs the GHC driver not to delete any of its
484 temporary files, which it normally keeps in
485 <literal>/tmp</literal> (or possibly elsewhere; see <xref
486 linkend="temp-files"/>). Running GHC with
487 <option>-v</option> will show you what temporary files
488 were generated along the way.</para>
494 <sect2 id="temp-files">
495 <title>Redirecting temporary files</title>
498 <primary>temporary files</primary>
499 <secondary>redirecting</secondary>
505 <option>-tmpdir</option>
506 <indexterm><primary><option>-tmpdir</option></primary></indexterm>
509 <para>If you have trouble because of running out of space
510 in <filename>/tmp</filename> (or wherever your
511 installation thinks temporary files should go), you may
512 use the <option>-tmpdir
513 <dir></option><indexterm><primary>-tmpdir
514 <dir> option</primary></indexterm> option to specify
515 an alternate directory. For example, <option>-tmpdir
516 .</option> says to put temporary files in the current
517 working directory.</para>
519 <para>Alternatively, use your <constant>TMPDIR</constant>
520 environment variable.<indexterm><primary>TMPDIR
521 environment variable</primary></indexterm> Set it to the
522 name of the directory where temporary files should be put.
523 GCC and other programs will honour the
524 <constant>TMPDIR</constant> variable as well.</para>
526 <para>Even better idea: Set the
527 <constant>DEFAULT_TMPDIR</constant> make variable when
528 building GHC, and never worry about
529 <constant>TMPDIR</constant> again. (see the build
530 documentation).</para>
536 <sect2 id="hi-options">
537 <title>Other options related to interface files</title>
538 <indexterm><primary>interface files, options</primary></indexterm>
543 <option>-ddump-hi</option>
544 <indexterm><primary><option>-ddump-hi</option></primary></indexterm>
547 <para>Dumps the new interface to standard output.</para>
553 <option>-ddump-hi-diffs</option>
554 <indexterm><primary><option>-ddump-hi-diffs</option></primary></indexterm>
557 <para>The compiler does not overwrite an existing
558 <filename>.hi</filename> interface file if the new one is
559 the same as the old one; this is friendly to
560 <command>make</command>. When an interface does change,
561 it is often enlightening to be informed. The
562 <option>-ddump-hi-diffs</option> option will make GHC run
563 <command>diff</command> on the old and new
564 <filename>.hi</filename> files.</para>
570 <option>-ddump-minimal-imports</option>
571 <indexterm><primary><option>-ddump-minimal-imports</option></primary></indexterm>
574 <para>Dump to the file "M.imports" (where M is the module
575 being compiled) a "minimal" set of import declarations.
576 You can safely replace all the import declarations in
577 "M.hs" with those found in "M.imports". Why would you
578 want to do that? Because the "minimal" imports (a) import
579 everything explicitly, by name, and (b) import nothing
580 that is not required. It can be quite painful to maintain
581 this property by hand, so this flag is intended to reduce
588 <option>--show-iface</option> <replaceable>file</replaceable>
589 <indexterm><primary><option>--show-iface</option></primary></indexterm>
592 <para>Where <replaceable>file</replaceable> is the name of
593 an interface file, dumps the contents of that interface in
594 a human-readable (ish) format.</para>
601 <title>The recompilation checker</title>
603 <indexterm><primary>recompilation checker</primary></indexterm>
608 <option>-fforce-recomp</option>
609 <indexterm><primary><option>-fforce-recomp</option></primary></indexterm>
610 <indexterm><primary><option>-fno-force-recomp</option></primary></indexterm>
613 <para>Turn off recompilation checking (which is on by
614 default). Recompilation checking normally stops
615 compilation early, leaving an existing
616 <filename>.o</filename> file in place, if it can be
617 determined that the module does not need to be
623 <para>In the olden days, GHC compared the newly-generated
624 <filename>.hi</filename> file with the previous version; if they
625 were identical, it left the old one alone and didn't change its
626 modification date. In consequence, importers of a module with
627 an unchanged output <filename>.hi</filename> file were not
630 <para>This doesn't work any more. Suppose module
631 <literal>C</literal> imports module <literal>B</literal>, and
632 <literal>B</literal> imports module <literal>A</literal>. So
633 changes to module <literal>A</literal> might require module
634 <literal>C</literal> to be recompiled, and hence when
635 <filename>A.hi</filename> changes we should check whether
636 <literal>C</literal> should be recompiled. However, the
637 dependencies of <literal>C</literal> will only list
638 <literal>B.hi</literal>, not <literal>A.hi</literal>, and some
639 changes to <literal>A</literal> (changing the definition of a
640 function that appears in an inlining of a function exported by
641 <literal>B</literal>, say) may conceivably not change
642 <filename>B.hi</filename> one jot. So now…</para>
644 <para>GHC keeps a version number on each interface file, and on
645 each type signature within the interface file. It also keeps in
646 every interface file a list of the version numbers of everything
647 it used when it last compiled the file. If the source file's
648 modification date is earlier than the <filename>.o</filename>
649 file's date (i.e. the source hasn't changed since the file was
650 last compiled), and the recompilation checking is on, GHC will be
651 clever. It compares the version numbers on the things it needs
652 this time with the version numbers on the things it needed last
653 time (gleaned from the interface file of the module being
654 compiled); if they are all the same it stops compiling rather
655 early in the process saying “Compilation IS NOT
656 required”. What a beautiful sight!</para>
658 <para>Patrick Sansom had a workshop paper about how all this is
659 done (though the details have changed quite a bit). <ulink
660 url="mailto:sansom@dcs.gla.ac.uk">Ask him</ulink> if you want a
665 <sect2 id="mutual-recursion">
666 <title>How to compile mutually recursive modules</title>
668 <indexterm><primary>module system, recursion</primary></indexterm>
669 <indexterm><primary>recursion, between modules</primary></indexterm>
671 <para>GHC supports the compilation of mutually recursive modules.
672 This section explains how.</para>
674 <para>Every cycle in the module import graph must be broken by a <filename>hs-boot</filename> file.
675 Suppose that modules <filename>A.hs</filename> and <filename>B.hs</filename> are Haskell source files,
681 newtype TA = MkTA Int
687 import {-# SOURCE #-} A( TA(..) )
694 <indexterm><primary><literal>hs-boot</literal>
695 files</primary></indexterm> <indexterm><primary>importing,
696 <literal>hi-boot</literal> files</primary></indexterm>
697 Here <filename>A</filename> imports <filename>B</filename>, but <filename>B</filename> imports
698 <filename>A</filename> with a <literal>{-# SOURCE #-}</literal> pragma, which breaks the
699 circular dependency. For every module <filename>A.hs</filename> that is <literal>{-# SOURCE #-}</literal>-imported
700 in this way there must exist a souce file <literal>A.hs-boot</literal>. This file contains an abbreviated
701 version of <filename>A.hs</filename>, thus:
704 newtype TA = MkTA Int
707 <para>To compile these three files, issue the following commands:
709 ghc -c A.hs-boot -- Poduces A.hi-boot, A.o-boot
710 ghc -c B.hs -- Consumes A.hi-boot, produces B.hi, B.o
711 ghc -c A.hs -- Consumes B.hi, produces A.hi, A.o
712 ghc -o foo A.o B.o -- Linking the program
715 <para>There are several points to note here:
718 <para>The file <filename>A.hs-boot</filename> is a programmer-written source file.
719 It must live in the same directory as its parent source file <filename>A.hs</filename>.
720 Currently, if you use a literate source file <filename>A.lhs</filename> you must
721 also use a literate boot file, <filename>A.lhs-boot</filename>; and vice versa.
725 A <filename>hs-boot</filename> file is compiled by GHC, just like a <filename>hs</filename> file:
729 When a hs-boot file <filename>A.hs-boot</filename>
730 is compiled, it is checked for scope and type errors.
731 When its parent module <filename>A.hs</filename> is compiled, the two are compared, and
732 an error is reported if the two are inconsistent.
736 <para> Just as compiling <filename>A.hs</filename> produces an
737 interface file <filename>A.hi</filename>, and an object file
738 <filename>A.o</filename>, so compiling
739 <filename>A.hs-boot</filename> produces an interface file
740 <filename>A.hi-boot</filename>, and an pseudo-object file
741 <filename>A.o-boot</filename>: </para>
745 <para>The pseudo-object file <filename>A.o-boot</filename> is
746 empty (don't link it!), but it is very useful when using a
747 Makefile, to record when the <filename>A.hi-boot</filename> was
748 last brought up to date (see <xref
749 linkend="using-make"/>).</para>
753 <para>The <filename>hi-boot</filename> generated by compiling a
754 <filename>hs-boot</filename> file is in the same
755 machine-generated binary format as any other GHC-generated
756 interface file (e.g. <filename>B.hi</filename>). You can
757 display its contents with <command>ghc
758 --show-iface</command>. If you specify a directory for
759 interface files, the <option>-ohidir</option> flag, then that
760 affects <filename>hi-boot</filename> files
766 <listitem><para> If hs-boot files are considered distinct from their parent source
767 files, and if a <literal>{-# SOURCE #-}</literal> import is considered to refer to the
768 hs-boot file, then the module import graph must have no cycles. The command
769 <command>ghc -M</command> will report an error if a cycle is found.
772 <listitem><para> A module <literal>M</literal> that is
773 <literal>{-# SOURCE #-}</literal>-imported in a program will usually also be
774 ordinarily imported elsewhere. If not, <command>ghc --make</command>
775 automatically adds <literal>M</literal> to the set of moudles it tries to
776 compile and link, to ensure that <literal>M</literal>'s implementation is included in
782 A hs-boot file need only contain the bare
783 minimum of information needed to get the bootstrapping process
784 started. For example, it doesn't need to contain declarations
785 for <emphasis>everything</emphasis> that module
786 <literal>A</literal> exports, only the things required by the
787 module(s) that import <literal>A</literal> recursively.</para>
788 <para>A hs-boot file is written in a subset of Haskell:
790 <listitem><para> The module header (including the export list), and import statements, are exactly as in
791 Haskell, and so are the scoping rules.
792 Hence, to mention a non-Prelude type or class, you must import it.</para></listitem>
794 <listitem><para> There must be no value declarations, but there can be type signatures for
797 double :: Int -> Int
800 <listitem><para> Fixity declarations are exactly as in Haskell.</para></listitem>
801 <listitem><para> Type synonym declarations are exactly as in Haskell.</para></listitem>
802 <listitem><para> A data type declaration can either be given in full, exactly as in Haskell, or it
803 can be given abstractly, by omitting the '=' sign and everything that follows. For example:
807 In a <emphasis>source</emphasis> program
808 this would declare TA to have no constructors (a GHC extension: see <xref linkend="nullary-types"/>),
809 but in an hi-boot file it means "I don't know or care what the constructors are".
810 This is the most common form of data type declaration, because it's easy to get right.
811 You <emphasis>can</emphasis> also write out the constructors but, if you do so, you must write
812 it out precisely as in its real definition.</para>
814 If you do not write out the constructors, you may need to give a kind
815 annotation (<xref linkend="kinding"/>), to tell
816 GHC the kind of the type variable, if it is not "*". (In source files, this is worked out
817 from the way the type variable is used in the constructors.) For example:
819 data R (x :: * -> *) y
821 You cannot use <literal>deriving</literal> on a data type declaration; write in
822 <literal>instance</literal> declaration instead.
824 <listitem><para> Class declarations is exactly as in Haskell, except that you may not put
825 default method declarations. You can also omit all the superclasses and class
826 methods entirely; but you must either omit them all or put them all in.
828 <listitem><para> You can include instance declarations just as in Haskell; but omit the "where" part.
835 <sect2 id="using-make">
836 <title>Using <command>make</command></title>
838 <indexterm><primary><literal>make</literal></primary></indexterm>
840 <para>It is reasonably straightforward to set up a
841 <filename>Makefile</filename> to use with GHC, assuming you name
842 your source files the same as your modules. Thus:</para>
846 HC_OPTS = -cpp $(EXTRA_HC_OPTS)
848 SRCS = Main.lhs Foo.lhs Bar.lhs
849 OBJS = Main.o Foo.o Bar.o
851 .SUFFIXES : .o .hs .hi .lhs .hc .s
855 $(HC) -o $@ $(HC_OPTS) $(OBJS)
857 # Standard suffix rules
862 $(HC) -c $< $(HC_OPTS)
865 $(HC) -c $< $(HC_OPTS)
871 $(HC) -c $< $(HC_OPTS)
874 $(HC) -c $< $(HC_OPTS)
876 # Inter-module dependencies
877 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
878 Main.o Main.hc Main.s : Foo.hi Baz.hi # Main imports Foo and Baz
881 <para>(Sophisticated <command>make</command> variants may
882 achieve some of the above more elegantly. Notably,
883 <command>gmake</command>'s pattern rules let you write the more
884 comprehensible:</para>
888 $(HC) -c $< $(HC_OPTS)
891 <para>What we've shown should work with any
892 <command>make</command>.)</para>
894 <para>Note the cheesy <literal>.o.hi</literal> rule: It records
895 the dependency of the interface (<filename>.hi</filename>) file
896 on the source. The rule says a <filename>.hi</filename> file
897 can be made from a <filename>.o</filename> file by
898 doing…nothing. Which is true.</para>
899 <para> Note that the suffix rules are all repeated twice, once
900 for normal Haskell source files, and once for <filename>hs-boot</filename>
901 files (see <xref linkend="mutual-recursion"/>).</para>
903 <para>Note also the inter-module dependencies at the end of the
904 Makefile, which take the form
907 Foo.o Foo.hc Foo.s : Baz.hi # Foo imports Baz
910 They tell <command>make</command> that if any of
911 <literal>Foo.o</literal>, <literal>Foo.hc</literal> or
912 <literal>Foo.s</literal> have an earlier modification date than
913 <literal>Baz.hi</literal>, then the out-of-date file must be
914 brought up to date. To bring it up to date,
915 <literal>make</literal> looks for a rule to do so; one of the
916 preceding suffix rules does the job nicely. These dependencies
917 can be generated automatically by <command>ghc</command>; see
918 <xref linkend="makefile-dependencies"/></para>
922 <sect2 id="makefile-dependencies">
923 <title>Dependency generation</title>
924 <indexterm><primary>dependencies in Makefiles</primary></indexterm>
925 <indexterm><primary>Makefile dependencies</primary></indexterm>
927 <para>Putting inter-dependencies of the form <literal>Foo.o :
928 Bar.hi</literal> into your <filename>Makefile</filename> by
929 hand is rather error-prone. Don't worry, GHC has support for
930 automatically generating the required dependencies. Add the
931 following to your <filename>Makefile</filename>:</para>
935 ghc -M $(HC_OPTS) $(SRCS)
938 <para>Now, before you start compiling, and any time you change
939 the <literal>imports</literal> in your program, do
940 <command>make depend</command> before you do <command>make
941 cool_pgm</command>. The command <command>ghc -M</command> will
942 append the needed dependencies to your
943 <filename>Makefile</filename>.</para>
945 <para>In general, <command>ghc -M Foo</command> does the following.
946 For each module <literal>M</literal> in the set
947 <literal>Foo</literal> plus all its imports (transitively),
948 it adds to the Makefile:
950 <listitem><para>A line recording the dependence of the object file on the source file.
954 (or <literal>M.lhs</literal> if that is the filename you used).
956 <listitem><para> For each import declaration <literal>import X</literal> in <literal>M</literal>,
957 a line recording the dependence of <literal>M</literal> on <literal>X</literal>:
960 </programlisting></para></listitem>
961 <listitem><para> For each import declaration <literal>import {-# SOURCE #-} X</literal> in <literal>M</literal>,
962 a line recording the dependence of <literal>M</literal> on <literal>X</literal>:
966 (See <xref linkend="mutual-recursion"/> for details of
967 <literal>hi-boot</literal> style interface files.)
970 If <literal>M</literal> imports multiple modules, then there will
971 be multiple lines with <filename>M.o</filename> as the
973 <para>There is no need to list all of the source files as arguments to the <command>ghc -M</command> command;
974 <command>ghc</command> traces the dependencies, just like <command>ghc --make</command>
975 (a new feature in GHC 6.4).</para>
977 <para>Note that <literal>ghc -M</literal> needs to find a <emphasis>source
978 file</emphasis> for each module in the dependency graph, so that it can
979 parse the import declarations and follow dependencies. Any pre-compiled
980 modules without source files must therefore belong to a
981 package<footnote><para>This is a change in behaviour relative to 6.2 and
985 <para>By default, <command>ghc -M</command> generates all the
986 dependencies, and then concatenates them onto the end of
987 <filename>makefile</filename> (or
988 <filename>Makefile</filename> if <filename>makefile</filename>
989 doesn't exist) bracketed by the lines "<literal># DO NOT
990 DELETE: Beginning of Haskell dependencies</literal>" and
991 "<literal># DO NOT DELETE: End of Haskell
992 dependencies</literal>". If these lines already exist in the
993 <filename>makefile</filename>, then the old dependencies are
994 deleted first.</para>
996 <para>Don't forget to use the same <option>-package</option>
997 options on the <literal>ghc -M</literal> command line as you
998 would when compiling; this enables the dependency generator to
999 locate any imported modules that come from packages. The
1000 package modules won't be included in the dependencies
1001 generated, though (but see the
1002 <option>––include-pkg-deps</option> option below).</para>
1004 <para>The dependency generation phase of GHC can take some
1005 additional options, which you may find useful. For historical
1006 reasons, each option passed to the dependency generator from
1007 the GHC command line must be preceded by
1008 <literal>-optdep</literal>. For example, to pass <literal>-f
1009 .depend</literal> to the dependency generator, you say
1012 ghc -M -optdep-f -optdep.depend ...
1015 The options which affect dependency generation are:</para>
1019 <term><option>-ddump-mod-cycles</option></term>
1021 <para>Display a list of the cycles in the module graph. This is
1022 useful when trying to eliminate such cycles. You do not need the <literal>-optdep</literal> prefix
1023 for this flag.</para>
1028 <term><option>-w</option></term>
1030 <para>Turn off warnings about interface file shadowing.</para>
1035 <term><option>-v2</option></term>
1037 <para>Print a full list of the module depenencies to stdout.
1038 (This is the standard verbosity flag, so the list will
1039 also be displayed with <option>-v3</option> and
1040 <option>-v4</option>;
1041 <xref linkend ="options-help"/>.)</para>
1046 <term><option>-f</option> <replaceable>file</replaceable></term>
1048 <para>Use <replaceable>file</replaceable> as the makefile,
1049 rather than <filename>makefile</filename> or
1050 <filename>Makefile</filename>. If
1051 <replaceable>file</replaceable> doesn't exist,
1052 <command>mkdependHS</command> creates it. We often use
1053 <option>-f .depend</option> to put the dependencies in
1054 <filename>.depend</filename> and then
1055 <command>include</command> the file
1056 <filename>.depend</filename> into
1057 <filename>Makefile</filename>.</para>
1061 <!-- Retired with the move away from 'mkdependHS'.
1063 <term><option>-o <osuf></option></term>
1065 <para>Use <filename>.<osuf></filename> as the
1066 "target file" suffix ( default: <literal>o</literal>).
1067 Multiple <option>-o</option> flags are permitted
1068 (GHC2.05 onwards). Thus "<option>-o hc -o o</option>"
1069 will generate dependencies for <filename>.hc</filename>
1070 and <filename>.o</filename> files.</para>
1075 <term><option>-s <suf></option></term>
1077 <para>Make extra dependencies that declare that files
1079 <filename>.<suf>_<osuf></filename>
1080 depend on interface files with suffix
1081 <filename>.<suf>_hi</filename>, or (for
1082 <literal>{-# SOURCE #-}</literal>
1083 imports) on <filename>.hi-boot</filename>. Multiple
1084 <option>-s</option> flags are permitted. For example,
1085 <option>-o hc -s a -s b</option> will make dependencies
1086 for <filename>.hc</filename> on
1087 <filename>.hi</filename>,
1088 <filename>.a_hc</filename> on
1089 <filename>.a_hi</filename>, and
1090 <filename>.b_hc</filename> on
1091 <filename>.b_hi</filename>. (Useful in
1092 conjunction with NoFib "ways".)</para>
1097 <term><option>––exclude-module=<file></option></term>
1099 <para>Regard <filename><file></filename> as
1100 "stable"; i.e., exclude it from having dependencies on
1106 <term><option>-x</option></term>
1108 <para>same as <option>––exclude-module</option></para>
1112 <!-- Not currently implemented:
1114 <term><option>––exclude-directory=<dirs></option></term>
1116 <para>Regard the colon-separated list of directories
1117 <filename><dirs></filename> as containing stable,
1118 don't generate any dependencies on modules
1124 <term><option>––include-module=<file></option></term>
1126 <para>Regard <filename><file></filename> as not
1127 "stable"; i.e., generate dependencies on it (if
1128 any). This option is normally used in conjunction with
1129 the <option>––exclude-directory</option> option.</para>
1135 <term><option>––include-pkg-deps</option></term>
1137 <para>Regard modules imported from packages as unstable,
1138 i.e., generate dependencies on any imported package modules
1139 (including <literal>Prelude</literal>, and all other
1140 standard Haskell libraries). Dependencies are not traced
1141 recursively into packages; dependencies are only generated for
1142 home-package modules on external-package modules directly imported
1143 by the home package module.
1144 This option is normally
1145 only used by the various system libraries.</para>
1152 <sect2 id="orphan-modules">
1153 <title>Orphan modules and instance declarations</title>
1155 <para> Haskell specifies that when compiling module M, any instance
1156 declaration in any module "below" M is visible. (Module A is "below"
1157 M if A is imported directly by M, or if A is below a module that M imports directly.)
1158 In principle, GHC must therefore read the interface files of every module below M,
1159 just in case they contain an instance declaration that matters to M. This would
1160 be a disaster in practice, so GHC tries to be clever. </para>
1162 <para>In particular, if an instance declaration is in the same module as the definition
1163 of any type or class mentioned in the head of the instance declaration, then
1164 GHC has to visit that interface file anyway. Example:</para>
1167 instance C a => D (T a) where ...
1170 <para> The instance declaration is only relevant if the type T is in use, and if
1171 so, GHC will have visited A's interface file to find T's definition. </para>
1173 <para> The only problem comes when a module contains an instance declaration
1174 and GHC has no other reason for visiting the module. Example:
1177 instance C a => D (T a) where ...
1180 Here, neither D nor T is declared in module Orphan.
1181 We call such modules ``orphan modules'',
1182 defined thus:</para>
1184 <listitem> <para> An <emphasis>orphan module</emphasis>
1185 <indexterm><primary>orphan module</primary></indexterm>
1186 contains at least one <emphasis>orphan instance</emphasis> or at
1187 least one <emphasis>orphan rule</emphasis>.</para> </listitem>
1189 <listitem><para> An instance declaration in a module M is an <emphasis>orphan instance</emphasis> if
1190 <indexterm><primary>orphan instance</primary></indexterm>
1191 none of the type constructors
1192 or classes mentioned in the instance head (the part after the ``<literal>=></literal>'') are declared
1195 <para> Only the instance head counts. In the example above, it is not good enough for C's declaration
1196 to be in module A; it must be the declaration of D or T.</para>
1199 <listitem><para> A rewrite rule in a module M is an <emphasis>orphan rule</emphasis>
1200 <indexterm><primary>orphan rule</primary></indexterm>
1201 if none of the variables, type constructors,
1202 or classes that are free in the left hand side of the rule are declared in M.
1207 <para> GHC identifies orphan modules, and visits the interface file of
1208 every orphan module below the module being compiled. This is usually
1209 wasted work, but there is no avoiding it. You should therefore do
1210 your best to have as few orphan modules as possible.
1214 <para> You can identify an orphan module by looking in its interface
1215 file, <filename>M.hi</filename>, using the
1216 <option>--show-iface</option>. If there is a ``!'' on the first line,
1217 GHC considers it an orphan module.
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