Rationalise GhcMode, HscTarget and GhcLink
[ghc.git] / docs / users_guide / ghci.xml
1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="ghci">
3 <title>Using GHCi</title>
4 <indexterm><primary>GHCi</primary></indexterm>
5 <indexterm><primary>interpreter</primary><see>GHCi</see></indexterm>
6 <indexterm><primary>interactive</primary><see>GHCi</see></indexterm>
7
8 <para>GHCi<footnote>
9 <para>The &lsquo;i&rsquo; stands for &ldquo;Interactive&rdquo;</para>
10 </footnote>
11 is GHC's interactive environment, in which Haskell expressions can
12 be interactively evaluated and programs can be interpreted. If
13 you're familiar with <ulink url="http://www.haskell.org/hugs/">Hugs</ulink><indexterm><primary>Hugs</primary>
14 </indexterm>, then you'll be right at home with GHCi. However, GHCi
15 also has support for interactively loading compiled code, as well as
16 supporting all<footnote><para>except <literal>foreign export</literal>, at the moment</para>
17 </footnote> the language extensions that GHC provides.</para>
18 <indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
19 <indexterm><primary>Foreign Function Interface</primary><secondary>GHCi support</secondary></indexterm>
20
21 <sect1 id="ghci-introduction">
22 <title>Introduction to GHCi</title>
23
24 <para>Let's start with an example GHCi session. You can fire up
25 GHCi with the command <literal>ghci</literal>:</para>
26
27 <screen>
28 $ ghci
29 ___ ___ _
30 / _ \ /\ /\/ __(_)
31 / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98.
32 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
33 \____/\/ /_/\____/|_| Type :? for help.
34
35 Loading package base ... linking ... done.
36 Prelude>
37 </screen>
38
39 <para>There may be a short pause while GHCi loads the prelude and
40 standard libraries, after which the prompt is shown. If we follow
41 the instructions and type <literal>:?</literal> for help, we
42 get:</para>
43
44 <screen>
45 Commands available from the prompt:
46
47 &lt;stmt&gt; evaluate/run &lt;stmt&gt;
48 :add &lt;filename&gt; ... add module(s) to the current target set
49 :browse [*]&lt;module&gt; display the names defined by &lt;module&gt;
50 :cd &lt;dir&gt; change directory to &lt;dir&gt;
51 :def &lt;cmd&gt; &lt;expr&gt; define a command :&lt;cmd&gt;
52 :edit &lt;file&gt; edit file
53 :edit edit last module
54 :help, :? display this list of commands
55 :info [&lt;name&gt; ...] display information about the given names
56 :load &lt;filename&gt; ... load module(s) and their dependents
57 :module [+/-] [*]&lt;mod&gt; ... set the context for expression evaluation
58 :main [&lt;arguments&gt; ...] run the main function with the given arguments
59 :reload reload the current module set
60
61 :set &lt;option&gt; ... set options
62 :set args &lt;arg&gt; ... set the arguments returned by System.getArgs
63 :set prog &lt;progname&gt; set the value returned by System.getProgName
64 :set prompt &lt;prompt&gt; set the prompt used in GHCi
65 :set editor &lt;cmd&gt; set the command used for :edit
66
67 :show modules show the currently loaded modules
68 :show bindings show the current bindings made at the prompt
69
70 :ctags [&lt;file&gt;] create tags file for Vi (default: "tags")
71 :etags [&lt;file&gt;] create tags file for Emacs (default: "TAGS")
72 :type &lt;expr&gt; show the type of &lt;expr&gt;
73 :kind &lt;type&gt; show the kind of &lt;type&gt;
74 :undef &lt;cmd&gt; undefine user-defined command :&lt;cmd&gt;
75 :unset &lt;option&gt; ... unset options
76 :quit exit GHCi
77 :!&lt;command&gt; run the shell command &lt;command&gt;
78
79 Options for ':set' and ':unset':
80
81 +r revert top-level expressions after each evaluation
82 +s print timing/memory stats after each evaluation
83 +t print type after evaluation
84 -&lt;flags&gt; most GHC command line flags can also be set here
85 (eg. -v2, -fglasgow-exts, etc.)
86 </screen>
87
88 <para>We'll explain most of these commands as we go along. For
89 Hugs users: many things work the same as in Hugs, so you should be
90 able to get going straight away.</para>
91
92 <para>Haskell expressions can be typed at the prompt:</para>
93 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
94 </indexterm>
95
96 <screen>
97 Prelude> 1+2
98 3
99 Prelude> let x = 42 in x / 9
100 4.666666666666667
101 Prelude>
102 </screen>
103
104 <para>GHCi interprets the whole line as an expression to evaluate.
105 The expression may not span several lines - as soon as you press
106 enter, GHCi will attempt to evaluate it.</para>
107 </sect1>
108
109 <sect1 id="loading-source-files">
110 <title>Loading source files</title>
111
112 <para>Suppose we have the following Haskell source code, which we
113 place in a file <filename>Main.hs</filename>:</para>
114
115 <programlisting>
116 main = print (fac 20)
117
118 fac 0 = 1
119 fac n = n * fac (n-1)
120 </programlisting>
121
122 <para>You can save <filename>Main.hs</filename> anywhere you like,
123 but if you save it somewhere other than the current
124 directory<footnote><para>If you started up GHCi from the command
125 line then GHCi's current directory is the same as the current
126 directory of the shell from which it was started. If you started
127 GHCi from the &ldquo;Start&rdquo; menu in Windows, then the
128 current directory is probably something like
129 <filename>C:\Documents and Settings\<replaceable>user
130 name</replaceable></filename>.</para> </footnote> then we will
131 need to change to the right directory in GHCi:</para>
132
133 <screen>
134 Prelude> :cd <replaceable>dir</replaceable>
135 </screen>
136
137 <para>where <replaceable>dir</replaceable> is the directory (or
138 folder) in which you saved <filename>Main.hs</filename>.</para>
139
140 <para>To load a Haskell source file into GHCi, use the
141 <literal>:load</literal> command:</para>
142 <indexterm><primary><literal>:load</literal></primary></indexterm>
143
144 <screen>
145 Prelude> :load Main
146 Compiling Main ( Main.hs, interpreted )
147 Ok, modules loaded: Main.
148 *Main>
149 </screen>
150
151 <para>GHCi has loaded the <literal>Main</literal> module, and the
152 prompt has changed to &ldquo;<literal>*Main></literal>&rdquo; to
153 indicate that the current context for expressions typed at the
154 prompt is the <literal>Main</literal> module we just loaded (we'll
155 explain what the <literal>*</literal> means later in <xref
156 linkend="ghci-scope"/>). So we can now type expressions involving
157 the functions from <filename>Main.hs</filename>:</para>
158
159 <screen>
160 *Main> fac 17
161 355687428096000
162 </screen>
163
164 <para>Loading a multi-module program is just as straightforward;
165 just give the name of the &ldquo;topmost&rdquo; module to the
166 <literal>:load</literal> command (hint: <literal>:load</literal>
167 can be abbreviated to <literal>:l</literal>). The topmost module
168 will normally be <literal>Main</literal>, but it doesn't have to
169 be. GHCi will discover which modules are required, directly or
170 indirectly, by the topmost module, and load them all in dependency
171 order.</para>
172
173 <sect2 id="ghci-modules-filenames">
174 <title>Modules vs. filenames</title>
175 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
176 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
177
178 <para>Question: How does GHC find the filename which contains
179 module <replaceable>M</replaceable>? Answer: it looks for the
180 file <literal><replaceable>M</replaceable>.hs</literal>, or
181 <literal><replaceable>M</replaceable>.lhs</literal>. This means
182 that for most modules, the module name must match the filename.
183 If it doesn't, GHCi won't be able to find it.</para>
184
185 <para>There is one exception to this general rule: when you load
186 a program with <literal>:load</literal>, or specify it when you
187 invoke <literal>ghci</literal>, you can give a filename rather
188 than a module name. This filename is loaded if it exists, and
189 it may contain any module you like. This is particularly
190 convenient if you have several <literal>Main</literal> modules
191 in the same directory and you can't call them all
192 <filename>Main.hs</filename>.</para>
193
194 <para>The search path for finding source files is specified with
195 the <option>-i</option> option on the GHCi command line, like
196 so:</para>
197 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
198
199 <para>or it can be set using the <literal>:set</literal> command
200 from within GHCi (see <xref
201 linkend="ghci-cmd-line-options"/>)<footnote><para>Note that in
202 GHCi, and <option>&ndash;&ndash;make</option> mode, the <option>-i</option>
203 option is used to specify the search path for
204 <emphasis>source</emphasis> files, whereas in standard
205 batch-compilation mode the <option>-i</option> option is used to
206 specify the search path for interface files, see <xref
207 linkend="search-path"/>.</para> </footnote></para>
208
209 <para>One consequence of the way that GHCi follows dependencies
210 to find modules to load is that every module must have a source
211 file. The only exception to the rule is modules that come from
212 a package, including the <literal>Prelude</literal> and standard
213 libraries such as <literal>IO</literal> and
214 <literal>Complex</literal>. If you attempt to load a module for
215 which GHCi can't find a source file, even if there are object
216 and interface files for the module, you'll get an error
217 message.</para>
218 </sect2>
219
220 <sect2>
221 <title>Making changes and recompilation</title>
222 <indexterm><primary><literal>:reload</literal></primary></indexterm>
223
224 <para>If you make some changes to the source code and want GHCi
225 to recompile the program, give the <literal>:reload</literal>
226 command. The program will be recompiled as necessary, with GHCi
227 doing its best to avoid actually recompiling modules if their
228 external dependencies haven't changed. This is the same
229 mechanism we use to avoid re-compiling modules in the batch
230 compilation setting (see <xref linkend="recomp"/>).</para>
231 </sect2>
232 </sect1>
233
234 <sect1 id="ghci-compiled">
235 <title>Loading compiled code</title>
236 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
237
238 <para>When you load a Haskell source module into GHCi, it is
239 normally converted to byte-code and run using the interpreter.
240 However, interpreted code can also run alongside compiled code in
241 GHCi; indeed, normally when GHCi starts, it loads up a compiled
242 copy of the <literal>base</literal> package, which contains the
243 <literal>Prelude</literal>.</para>
244
245 <para>Why should we want to run compiled code? Well, compiled
246 code is roughly 10x faster than interpreted code, but takes about
247 2x longer to produce (perhaps longer if optimisation is on). So
248 it pays to compile the parts of a program that aren't changing
249 very often, and use the interpreter for the code being actively
250 developed.</para>
251
252 <para>When loading up source files with <literal>:load</literal>,
253 GHCi looks for any corresponding compiled object files, and will
254 use one in preference to interpreting the source if possible. For
255 example, suppose we have a 4-module program consisting of modules
256 A, B, C, and D. Modules B and C both import D only,
257 and A imports both B &amp; C:</para>
258 <screen>
259 A
260 / \
261 B C
262 \ /
263 D
264 </screen>
265 <para>We can compile D, then load the whole program, like this:</para>
266 <screen>
267 Prelude> :! ghc -c D.hs
268 Prelude> :load A
269 Skipping D ( D.hs, D.o )
270 Compiling C ( C.hs, interpreted )
271 Compiling B ( B.hs, interpreted )
272 Compiling A ( A.hs, interpreted )
273 Ok, modules loaded: A, B, C, D.
274 *Main>
275 </screen>
276
277 <para>In the messages from the compiler, we see that it skipped D,
278 and used the object file <filename>D.o</filename>. The message
279 <literal>Skipping</literal> <replaceable>module</replaceable>
280 indicates that compilation for <replaceable>module</replaceable>
281 isn't necessary, because the source and everything it depends on
282 is unchanged since the last compilation.</para>
283
284 <para>At any time you can use the command
285 <literal>:show modules</literal>
286 to get a list of the modules currently loaded
287 into GHCi:</para>
288
289 <screen>
290 *Main> :show modules
291 D ( D.hs, D.o )
292 C ( C.hs, interpreted )
293 B ( B.hs, interpreted )
294 A ( A.hs, interpreted )
295 *Main></screen>
296
297 <para>If we now modify the source of D (or pretend to: using Unix
298 command <literal>touch</literal> on the source file is handy for
299 this), the compiler will no longer be able to use the object file,
300 because it might be out of date:</para>
301
302 <screen>
303 *Main> :! touch D.hs
304 *Main> :reload
305 Compiling D ( D.hs, interpreted )
306 Skipping C ( C.hs, interpreted )
307 Skipping B ( B.hs, interpreted )
308 Skipping A ( A.hs, interpreted )
309 Ok, modules loaded: A, B, C, D.
310 *Main>
311 </screen>
312
313 <para>Note that module D was compiled, but in this instance
314 because its source hadn't really changed, its interface remained
315 the same, and the recompilation checker determined that A, B and C
316 didn't need to be recompiled.</para>
317
318 <para>So let's try compiling one of the other modules:</para>
319
320 <screen>
321 *Main> :! ghc -c C.hs
322 *Main> :load A
323 Compiling D ( D.hs, interpreted )
324 Compiling C ( C.hs, interpreted )
325 Compiling B ( B.hs, interpreted )
326 Compiling A ( A.hs, interpreted )
327 Ok, modules loaded: A, B, C, D.
328 </screen>
329
330 <para>We didn't get the compiled version of C! What happened?
331 Well, in GHCi a compiled module may only depend on other compiled
332 modules, and in this case C depends on D, which doesn't have an
333 object file, so GHCi also rejected C's object file. Ok, so let's
334 also compile D:</para>
335
336 <screen>
337 *Main> :! ghc -c D.hs
338 *Main> :reload
339 Ok, modules loaded: A, B, C, D.
340 </screen>
341
342 <para>Nothing happened! Here's another lesson: newly compiled
343 modules aren't picked up by <literal>:reload</literal>, only
344 <literal>:load</literal>:</para>
345
346 <screen>
347 *Main> :load A
348 Skipping D ( D.hs, D.o )
349 Skipping C ( C.hs, C.o )
350 Compiling B ( B.hs, interpreted )
351 Compiling A ( A.hs, interpreted )
352 Ok, modules loaded: A, B, C, D.
353 </screen>
354
355 <para>HINT: since GHCi will only use a compiled object file if it
356 can be sure that the compiled version is up-to-date, a good technique
357 when working on a large program is to occasionally run
358 <literal>ghc &ndash;&ndash;make</literal> to compile the whole project (say
359 before you go for lunch :-), then continue working in the
360 interpreter. As you modify code, the new modules will be
361 interpreted, but the rest of the project will remain
362 compiled.</para>
363
364 </sect1>
365
366 <sect1 id="interactive-evaluation">
367 <title>Interactive evaluation at the prompt</title>
368
369 <para>When you type an expression at the prompt, GHCi immediately
370 evaluates and prints the result:
371 <screen>
372 Prelude> reverse "hello"
373 "olleh"
374 Prelude> 5+5
375 10
376 </screen>
377 </para>
378
379 <sect2><title>I/O actions at the prompt</title>
380
381 <para>GHCi does more than simple expression evaluation at the prompt.
382 If you type something of type <literal>IO a</literal> for some
383 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
384 as an IO-computation.
385 <screen>
386 Prelude> "hello"
387 "hello"
388 Prelude> putStrLn "hello"
389 hello
390 </screen>
391 Furthermore, GHCi will print the result of the I/O action if (and only
392 if):
393 <itemizedlist>
394 <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
395 <listitem><para>The result type is not
396 <literal>()</literal>.</para></listitem>
397 </itemizedlist>
398 For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
399 <screen>
400 Prelude> putStrLn "hello"
401 hello
402 Prelude> do { putStrLn "hello"; return "yes" }
403 hello
404 "yes"
405 </screen>
406 </para></sect2>
407
408 <sect2 id="ghci-stmts">
409 <title>Using <literal>do-</literal>notation at the prompt</title>
410 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
411 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
412
413 <para>GHCi actually accepts <firstterm>statements</firstterm>
414 rather than just expressions at the prompt. This means you can
415 bind values and functions to names, and use them in future
416 expressions or statements.</para>
417
418 <para>The syntax of a statement accepted at the GHCi prompt is
419 exactly the same as the syntax of a statement in a Haskell
420 <literal>do</literal> expression. However, there's no monad
421 overloading here: statements typed at the prompt must be in the
422 <literal>IO</literal> monad.
423 <screen>
424 Prelude> x &lt;- return 42
425 42
426 Prelude> print x
427 42
428 Prelude>
429 </screen>
430 The statement <literal>x &lt;- return 42</literal> means
431 &ldquo;execute <literal>return 42</literal> in the
432 <literal>IO</literal> monad, and bind the result to
433 <literal>x</literal>&rdquo;. We can then use
434 <literal>x</literal> in future statements, for example to print
435 it as we did above.</para>
436
437 <para>GHCi will print the result of a statement if and only if:
438 <itemizedlist>
439 <listitem>
440 <para>The statement is not a binding, or it is a monadic binding
441 (<literal>p &lt;- e</literal>) that binds exactly one
442 variable.</para>
443 </listitem>
444 <listitem>
445 <para>The variable's type is not polymorphic, is not
446 <literal>()</literal>, and is an instance of
447 <literal>Show</literal></para>
448 </listitem>
449 </itemizedlist>
450 The automatic printing of binding results can be supressed with
451 <option>:set -fno-print-bind-result</option> (this does not
452 supress printing the result of non-binding statements).
453 <indexterm><primary><option>-fno-print-bind-result</option></primary></indexterm><indexterm><primary><option>-fprint-bind-result</option></primary></indexterm>.
454 You might want to do this to prevent the result of binding
455 statements from being fully evaluated by the act of printing
456 them, for example.</para>
457
458 <para>Of course, you can also bind normal non-IO expressions
459 using the <literal>let</literal>-statement:</para>
460 <screen>
461 Prelude> let x = 42
462 Prelude> x
463 42
464 Prelude>
465 </screen>
466 <para>Another important difference between the two types of binding
467 is that the monadic bind (<literal>p &lt;- e</literal>) is
468 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
469 whereas with the <literal>let</literal> form, the expression
470 isn't evaluated immediately:</para>
471 <screen>
472 Prelude> let x = error "help!"
473 Prelude> print x
474 *** Exception: help!
475 Prelude>
476 </screen>
477
478 <para>Note that <literal>let</literal> bindings do not automatically
479 print the value bound, unlike monadic bindings.</para>
480
481 <para>Any exceptions raised during the evaluation or execution
482 of the statement are caught and printed by the GHCi command line
483 interface (for more information on exceptions, see the module
484 <literal>Control.Exception</literal> in the libraries
485 documentation).</para>
486
487 <para>Every new binding shadows any existing bindings of the
488 same name, including entities that are in scope in the current
489 module context.</para>
490
491 <para>WARNING: temporary bindings introduced at the prompt only
492 last until the next <literal>:load</literal> or
493 <literal>:reload</literal> command, at which time they will be
494 simply lost. However, they do survive a change of context with
495 <literal>:module</literal>: the temporary bindings just move to
496 the new location.</para>
497
498 <para>HINT: To get a list of the bindings currently in scope, use the
499 <literal>:show bindings</literal> command:</para>
500
501 <screen>
502 Prelude> :show bindings
503 x :: Int
504 Prelude></screen>
505
506 <para>HINT: if you turn on the <literal>+t</literal> option,
507 GHCi will show the type of each variable bound by a statement.
508 For example:</para>
509 <indexterm><primary><literal>+t</literal></primary></indexterm>
510 <screen>
511 Prelude> :set +t
512 Prelude> let (x:xs) = [1..]
513 x :: Integer
514 xs :: [Integer]
515 </screen>
516
517 </sect2>
518
519 <sect2 id="ghci-scope">
520 <title>What's really in scope at the prompt?</title>
521
522 <para>When you type an expression at the prompt, what
523 identifiers and types are in scope? GHCi provides a flexible
524 way to control exactly how the context for an expression is
525 constructed. Let's start with the simple cases; when you start
526 GHCi the prompt looks like this:</para>
527
528 <screen>Prelude></screen>
529
530 <para>Which indicates that everything from the module
531 <literal>Prelude</literal> is currently in scope. If we now
532 load a file into GHCi, the prompt will change:</para>
533
534 <screen>
535 Prelude> :load Main.hs
536 Compiling Main ( Main.hs, interpreted )
537 *Main>
538 </screen>
539
540 <para>The new prompt is <literal>*Main</literal>, which
541 indicates that we are typing expressions in the context of the
542 top-level of the <literal>Main</literal> module. Everything
543 that is in scope at the top-level in the module
544 <literal>Main</literal> we just loaded is also in scope at the
545 prompt (probably including <literal>Prelude</literal>, as long
546 as <literal>Main</literal> doesn't explicitly hide it).</para>
547
548 <para>The syntax
549 <literal>*<replaceable>module</replaceable></literal> indicates
550 that it is the full top-level scope of
551 <replaceable>module</replaceable> that is contributing to the
552 scope for expressions typed at the prompt. Without the
553 <literal>*</literal>, just the exports of the module are
554 visible.</para>
555
556 <para>We're not limited to a single module: GHCi can combine
557 scopes from multiple modules, in any mixture of
558 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
559 combines the scopes from all of these modules to form the scope
560 that is in effect at the prompt. For technical reasons, GHCi
561 can only support the <literal>*</literal>-form for modules which
562 are interpreted, so compiled modules and package modules can
563 only contribute their exports to the current scope.</para>
564
565 <para>The scope is manipulated using the
566 <literal>:module</literal> command. For example, if the current
567 scope is <literal>Prelude</literal>, then we can bring into
568 scope the exports from the module <literal>IO</literal> like
569 so:</para>
570
571 <screen>
572 Prelude> :module +IO
573 Prelude IO> hPutStrLn stdout "hello\n"
574 hello
575 Prelude IO>
576 </screen>
577
578 <para>(Note: <literal>:module</literal> can be shortened to
579 <literal>:m</literal>). The full syntax of the
580 <literal>:module</literal> command is:</para>
581
582 <screen>
583 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
584 </screen>
585
586 <para>Using the <literal>+</literal> form of the
587 <literal>module</literal> commands adds modules to the current
588 scope, and <literal>-</literal> removes them. Without either
589 <literal>+</literal> or <literal>-</literal>, the current scope
590 is replaced by the set of modules specified. Note that if you
591 use this form and leave out <literal>Prelude</literal>, GHCi
592 will assume that you really wanted the
593 <literal>Prelude</literal> and add it in for you (if you don't
594 want the <literal>Prelude</literal>, then ask to remove it with
595 <literal>:m -Prelude</literal>).</para>
596
597 <para>The scope is automatically set after a
598 <literal>:load</literal> command, to the most recently loaded
599 "target" module, in a <literal>*</literal>-form if possible.
600 For example, if you say <literal>:load foo.hs bar.hs</literal>
601 and <filename>bar.hs</filename> contains module
602 <literal>Bar</literal>, then the scope will be set to
603 <literal>*Bar</literal> if <literal>Bar</literal> is
604 interpreted, or if <literal>Bar</literal> is compiled it will be
605 set to <literal>Prelude Bar</literal> (GHCi automatically adds
606 <literal>Prelude</literal> if it isn't present and there aren't
607 any <literal>*</literal>-form modules).</para>
608
609 <para>With multiple modules in scope, especially multiple
610 <literal>*</literal>-form modules, it is likely that name
611 clashes will occur. Haskell specifies that name clashes are
612 only reported when an ambiguous identifier is used, and GHCi
613 behaves in the same way for expressions typed at the
614 prompt.</para>
615
616 <para>
617 Hint: GHCi will tab-complete names that are in scope; for
618 example, if you run GHCi and type <literal>J&lt;tab&gt;</literal>
619 then GHCi will expand it to <literal>Just </literal>.
620 </para>
621
622 <sect3>
623 <title>Qualified names</title>
624
625 <para>To make life slightly easier, the GHCi prompt also
626 behaves as if there is an implicit <literal>import
627 qualified</literal> declaration for every module in every
628 package, and every module currently loaded into GHCi.</para>
629 </sect3>
630
631 <sect3>
632 <title>The <literal>:main</literal> command</title>
633
634 <para>
635 When a program is compiled and executed, it can use the
636 <literal>getArgs</literal> function to access the
637 command-line arguments.
638 However, we cannot simply pass the arguments to the
639 <literal>main</literal> function while we are testing in ghci,
640 as the <literal>main</literal> function doesn't take its
641 directly.
642 </para>
643
644 <para>
645 Instead, we can use the <literal>:main</literal> command.
646 This runs whatever <literal>main</literal> is in scope, with
647 any arguments being treated the same as command-line arguments,
648 e.g.:
649 </para>
650
651 <screen>
652 Prelude> let main = System.Environment.getArgs >>= print
653 Prelude> :main foo bar
654 ["foo","bar"]
655 </screen>
656
657 </sect3>
658 </sect2>
659
660
661 <sect2>
662 <title>The <literal>it</literal> variable</title>
663 <indexterm><primary><literal>it</literal></primary>
664 </indexterm>
665
666 <para>Whenever an expression (or a non-binding statement, to be
667 precise) is typed at the prompt, GHCi implicitly binds its value
668 to the variable <literal>it</literal>. For example:</para>
669 <screen>
670 Prelude> 1+2
671 3
672 Prelude> it * 2
673 6
674 </screen>
675 <para>What actually happens is that GHCi typechecks the
676 expression, and if it doesn't have an <literal>IO</literal> type,
677 then it transforms it as follows: an expression
678 <replaceable>e</replaceable> turns into
679 <screen>
680 let it = <replaceable>e</replaceable>;
681 print it
682 </screen>
683 which is then run as an IO-action.</para>
684
685 <para>Hence, the original expression must have a type which is an
686 instance of the <literal>Show</literal> class, or GHCi will
687 complain:</para>
688
689 <screen>
690 Prelude&gt; id
691
692 &lt;interactive&gt;:1:0:
693 No instance for (Show (a -&gt; a))
694 arising from use of `print' at &lt;interactive&gt;:1:0-1
695 Possible fix: add an instance declaration for (Show (a -> a))
696 In the expression: print it
697 In a 'do' expression: print it
698 </screen>
699
700 <para>The error message contains some clues as to the
701 transformation happening internally.</para>
702
703 <para>If the expression was instead of type <literal>IO a</literal> for
704 some <literal>a</literal>, then <literal>it</literal> will be
705 bound to the result of the <literal>IO</literal> computation,
706 which is of type <literal>a</literal>. eg.:</para>
707 <screen>
708 Prelude> Time.getClockTime
709 Wed Mar 14 12:23:13 GMT 2001
710 Prelude> print it
711 Wed Mar 14 12:23:13 GMT 2001
712 </screen>
713
714 <para>The corresponding translation for an IO-typed
715 <replaceable>e</replaceable> is
716 <screen>
717 it &lt;- <replaceable>e</replaceable>
718 </screen>
719 </para>
720
721 <para>Note that <literal>it</literal> is shadowed by the new
722 value each time you evaluate a new expression, and the old value
723 of <literal>it</literal> is lost.</para>
724
725 </sect2>
726
727 <sect2 id="extended-default-rules">
728 <title>Type defaulting in GHCi</title>
729 <indexterm><primary>Type default</primary></indexterm>
730 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
731 <para>
732 Consider this GHCi session:
733 <programlisting>
734 ghci> reverse []
735 </programlisting>
736 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
737 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
738 on the type <literal>a</literal>. For example:
739 <programlisting>
740 ghci> (reverse []) :: String
741 ""
742 ghci> (reverse []) :: [Int]
743 []
744 </programlisting>
745 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
746 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
747 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
748 a)</literal> for each type variable <literal>a</literal>, and defaults the
749 type variable if
750 <orderedlist>
751 <listitem>
752 <para>
753 The type variable <literal>a</literal> appears in no
754 other constraints
755 </para>
756 </listitem>
757 <listitem>
758 <para>
759 All the classes <literal>Ci</literal> are standard.
760 </para>
761 </listitem>
762 <listitem>
763 <para>
764 At least one of the classes <literal>Ci</literal> is
765 numeric.
766 </para>
767 </listitem>
768 </orderedlist>
769 At the GHCi prompt, or with GHC if the
770 <literal>-fextended-default-rules</literal> flag is given,
771 the following additional differences apply:
772 <itemizedlist>
773 <listitem>
774 <para>
775 Rule 2 above is relaxed thus:
776 <emphasis>All</emphasis> of the classes
777 <literal>Ci</literal> are single-parameter type classes.
778 </para>
779 </listitem>
780 <listitem>
781 <para>
782 Rule 3 above is relaxed this:
783 At least one of the classes <literal>Ci</literal> is
784 numeric, <emphasis>or is <literal>Show</literal>,
785 <literal>Eq</literal>, or
786 <literal>Ord</literal></emphasis>.
787 </para>
788 </listitem>
789 <listitem>
790 <para>
791 The unit type <literal>()</literal> is added to the
792 start of the standard list of types which are tried when
793 doing type defaulting.
794 </para>
795 </listitem>
796 </itemizedlist>
797 The last point means that, for example, this program:
798 <programlisting>
799 main :: IO ()
800 main = print def
801
802 instance Num ()
803
804 def :: (Num a, Enum a) => a
805 def = toEnum 0
806 </programlisting>
807 prints <literal>()</literal> rather than <literal>0</literal> as the
808 type is defaulted to <literal>()</literal> rather than
809 <literal>Integer</literal>.
810 </para>
811 <para>
812 The motivation for the change is that it means <literal>IO a</literal>
813 actions default to <literal>IO ()</literal>, which in turn means that
814 ghci won't try to print a result when running them. This is
815 particularly important for <literal>printf</literal>, which has an
816 instance that returns <literal>IO a</literal>.
817 However, it is only able to return
818 <literal>undefined</literal>
819 (the reason for the instance having this type is to not require
820 extensions to the class system), so if the type defaults to
821 <literal>Integer</literal> then ghci gives an error when running a
822 printf.
823 </para>
824 </sect2>
825 </sect1>
826
827 <sect1 id="ghci-invocation">
828 <title>Invoking GHCi</title>
829 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
830 <indexterm><primary><option>&ndash;&ndash;interactive</option></primary></indexterm>
831
832 <para>GHCi is invoked with the command <literal>ghci</literal> or
833 <literal>ghc &ndash;&ndash;interactive</literal>. One or more modules or
834 filenames can also be specified on the command line; this
835 instructs GHCi to load the specified modules or filenames (and all
836 the modules they depend on), just as if you had said
837 <literal>:load <replaceable>modules</replaceable></literal> at the
838 GHCi prompt (see <xref linkend="ghci-commands"/>). For example, to
839 start GHCi and load the program whose topmost module is in the
840 file <literal>Main.hs</literal>, we could say:</para>
841
842 <screen>
843 $ ghci Main.hs
844 </screen>
845
846 <para>Most of the command-line options accepted by GHC (see <xref
847 linkend="using-ghc"/>) also make sense in interactive mode. The ones
848 that don't make sense are mostly obvious; for example, GHCi
849 doesn't generate interface files, so options related to interface
850 file generation won't have any effect.</para>
851
852 <sect2>
853 <title>Packages</title>
854 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
855
856 <para>Most packages (see <xref linkend="using-packages"/>) are
857 available without needing to specify any extra flags at all:
858 they will be automatically loaded the first time they are
859 needed.</para>
860
861 <para>For non-auto packages, however, you need to request the
862 package be loaded by using the <literal>-package</literal> flag:</para>
863
864 <screen>
865 $ ghci -package readline
866 ___ ___ _
867 / _ \ /\ /\/ __(_)
868 / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98.
869 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
870 \____/\/ /_/\____/|_| Type :? for help.
871
872 Loading package base ... linking ... done.
873 Loading package readline-1.0 ... linking ... done.
874 Prelude>
875 </screen>
876
877 <para>The following command works to load new packages into a
878 running GHCi:</para>
879
880 <screen>
881 Prelude> :set -package <replaceable>name</replaceable>
882 </screen>
883
884 <para>But note that doing this will cause all currently loaded
885 modules to be unloaded, and you'll be dumped back into the
886 <literal>Prelude</literal>.</para>
887 </sect2>
888
889 <sect2>
890 <title>Extra libraries</title>
891 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
892
893 <para>Extra libraries may be specified on the command line using
894 the normal <literal>-l<replaceable>lib</replaceable></literal>
895 option. (The term <emphasis>library</emphasis> here refers to
896 libraries of foreign object code; for using libraries of Haskell
897 source code, see <xref linkend="ghci-modules-filenames"/>.) For
898 example, to load the &ldquo;m&rdquo; library:</para>
899
900 <screen>
901 $ ghci -lm
902 </screen>
903
904 <para>On systems with <literal>.so</literal>-style shared
905 libraries, the actual library loaded will the
906 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
907 searches the following places for libraries, in this order:</para>
908
909 <itemizedlist>
910 <listitem>
911 <para>Paths specified using the
912 <literal>-L<replaceable>path</replaceable></literal>
913 command-line option,</para>
914 </listitem>
915 <listitem>
916 <para>the standard library search path for your system,
917 which on some systems may be overridden by setting the
918 <literal>LD_LIBRARY_PATH</literal> environment
919 variable.</para>
920 </listitem>
921 </itemizedlist>
922
923 <para>On systems with <literal>.dll</literal>-style shared
924 libraries, the actual library loaded will be
925 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
926 GHCi will signal an error if it can't find the library.</para>
927
928 <para>GHCi can also load plain object files
929 (<literal>.o</literal> or <literal>.obj</literal> depending on
930 your platform) from the command-line. Just add the name the
931 object file to the command line.</para>
932
933 <para>Ordering of <option>-l</option> options matters: a library
934 should be mentioned <emphasis>before</emphasis> the libraries it
935 depends on (see <xref linkend="options-linker"/>).</para>
936 </sect2>
937
938 </sect1>
939
940 <sect1 id="ghci-commands">
941 <title>GHCi commands</title>
942
943 <para>GHCi commands all begin with
944 &lsquo;<literal>:</literal>&rsquo; and consist of a single command
945 name followed by zero or more parameters. The command name may be
946 abbreviated, with ambiguities being resolved in favour of the more
947 commonly used commands.</para>
948
949 <variablelist>
950 <varlistentry>
951 <term>
952 <literal>:add</literal> <replaceable>module</replaceable> ...
953 <indexterm><primary><literal>:add</literal></primary></indexterm>
954 </term>
955 <listitem>
956 <para>Add <replaceable>module</replaceable>(s) to the
957 current <firstterm>target set</firstterm>, and perform a
958 reload.</para>
959 </listitem>
960 </varlistentry>
961
962 <varlistentry>
963 <term>
964 <literal>:breakpoint</literal> <replaceable>list|add|continue|del|stop|step</replaceable> ...
965 <indexterm><primary><literal>:breakpoint</literal></primary></indexterm>
966 </term>
967 <listitem>
968 <para>Permits to add, delete or list the breakpoints in a debugging session.
969 In order to make this command available, the
970 <literal>-fdebugging</literal> flag must be active. The easiest way is to launch
971 GHCi with the <literal>-fdebugging</literal> option. For more
972 details on how the debugger works, see <xref linkend="ghci-debugger"/>.
973 </para>
974 </listitem>
975 </varlistentry>
976
977 <varlistentry>
978 <term>
979 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
980 <indexterm><primary><literal>:browse</literal></primary></indexterm>
981 </term>
982 <listitem>
983 <para>Displays the identifiers defined by the module
984 <replaceable>module</replaceable>, which must be either
985 loaded into GHCi or be a member of a package. If the
986 <literal>*</literal> symbol is placed before the module
987 name, then <emphasis>all</emphasis> the identifiers defined
988 in <replaceable>module</replaceable> are shown; otherwise
989 the list is limited to the exports of
990 <replaceable>module</replaceable>. The
991 <literal>*</literal>-form is only available for modules
992 which are interpreted; for compiled modules (including
993 modules from packages) only the non-<literal>*</literal>
994 form of <literal>:browse</literal> is available.</para>
995 </listitem>
996 </varlistentry>
997
998 <varlistentry>
999 <term>
1000 <literal>:cd</literal> <replaceable>dir</replaceable>
1001 <indexterm><primary><literal>:cd</literal></primary></indexterm>
1002 </term>
1003 <listitem>
1004 <para>Changes the current working directory to
1005 <replaceable>dir</replaceable>. A
1006 &lsquo;<literal>&tilde;</literal>&rsquo; symbol at the
1007 beginning of <replaceable>dir</replaceable> will be replaced
1008 by the contents of the environment variable
1009 <literal>HOME</literal>.</para>
1010
1011 <para>NOTE: changing directories causes all currently loaded
1012 modules to be unloaded. This is because the search path is
1013 usually expressed using relative directories, and changing
1014 the search path in the middle of a session is not
1015 supported.</para>
1016 </listitem>
1017 </varlistentry>
1018
1019 <varlistentry>
1020 <term>
1021 <literal>:continue</literal>
1022 <indexterm><primary><literal>:continue</literal></primary></indexterm>
1023 </term>
1024 <listitem><para>Shortcut to <literal>:breakpoint continue</literal></para>
1025 </listitem>
1026 </varlistentry>
1027
1028 <varlistentry>
1029 <term>
1030 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
1031 <indexterm><primary><literal>:def</literal></primary></indexterm>
1032 </term>
1033 <listitem>
1034 <para>The command <literal>:def</literal>
1035 <replaceable>name</replaceable>
1036 <replaceable>expr</replaceable> defines a new GHCi command
1037 <literal>:<replaceable>name</replaceable></literal>,
1038 implemented by the Haskell expression
1039 <replaceable>expr</replaceable>, which must have type
1040 <literal>String -> IO String</literal>. When
1041 <literal>:<replaceable>name</replaceable>
1042 <replaceable>args</replaceable></literal> is typed at the
1043 prompt, GHCi will run the expression
1044 <literal>(<replaceable>name</replaceable>
1045 <replaceable>args</replaceable>)</literal>, take the
1046 resulting <literal>String</literal>, and feed it back into
1047 GHCi as a new sequence of commands. Separate commands in
1048 the result must be separated by
1049 &lsquo;<literal>\n</literal>&rsquo;.</para>
1050
1051 <para>That's all a little confusing, so here's a few
1052 examples. To start with, here's a new GHCi command which
1053 doesn't take any arguments or produce any results, it just
1054 outputs the current date &amp; time:</para>
1055
1056 <screen>
1057 Prelude> let date _ = Time.getClockTime >>= print >> return ""
1058 Prelude> :def date date
1059 Prelude> :date
1060 Fri Mar 23 15:16:40 GMT 2001
1061 </screen>
1062
1063 <para>Here's an example of a command that takes an argument.
1064 It's a re-implementation of <literal>:cd</literal>:</para>
1065
1066 <screen>
1067 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
1068 Prelude> :def mycd mycd
1069 Prelude> :mycd ..
1070 </screen>
1071
1072 <para>Or I could define a simple way to invoke
1073 &ldquo;<literal>ghc &ndash;&ndash;make Main</literal>&rdquo; in the
1074 current directory:</para>
1075
1076 <screen>
1077 Prelude> :def make (\_ -> return ":! ghc &ndash;&ndash;make Main")
1078 </screen>
1079
1080 <para>We can define a command that reads GHCi input from a
1081 file. This might be useful for creating a set of bindings
1082 that we want to repeatedly load into the GHCi session:</para>
1083
1084 <screen>
1085 Prelude> :def . readFile
1086 Prelude> :. cmds.ghci
1087 </screen>
1088
1089 <para>Notice that we named the command
1090 <literal>:.</literal>, by analogy with the
1091 &lsquo;<literal>.</literal>&rsquo; Unix shell command that
1092 does the same thing.</para>
1093 </listitem>
1094 </varlistentry>
1095
1096 <varlistentry>
1097 <term>
1098 <literal>:edit <optional><replaceable>file</replaceable></optional></literal>
1099 <indexterm><primary><literal>:edit</literal></primary></indexterm>
1100 </term>
1101 <listitem>
1102 <para>Opens an editor to edit the file
1103 <replaceable>file</replaceable>, or the most recently loaded
1104 module if <replaceable>file</replaceable> is omitted. The
1105 editor to invoke is taken from the <literal>EDITOR</literal>
1106 environment variable, or a default editor on your system if
1107 <literal>EDITOR</literal> is not set. You can change the
1108 editor using <literal>:set editor</literal>.</para>
1109 </listitem>
1110 </varlistentry>
1111
1112 <varlistentry>
1113 <term>
1114 <literal>:help</literal>
1115 <indexterm><primary><literal>:help</literal></primary></indexterm>
1116 </term>
1117 <term>
1118 <literal>:?</literal>
1119 <indexterm><primary><literal>:?</literal></primary></indexterm>
1120 </term>
1121 <listitem>
1122 <para>Displays a list of the available commands.</para>
1123 </listitem>
1124 </varlistentry>
1125
1126 <varlistentry>
1127 <term>
1128 <literal>:info</literal> <replaceable>name</replaceable> ...
1129 <indexterm><primary><literal>:info</literal></primary></indexterm>
1130 </term>
1131 <listitem>
1132 <para>Displays information about the given name(s). For
1133 example, if <replaceable>name</replaceable> is a class, then
1134 the class methods and their types will be printed; if
1135 <replaceable>name</replaceable> is a type constructor, then
1136 its definition will be printed; if
1137 <replaceable>name</replaceable> is a function, then its type
1138 will be printed. If <replaceable>name</replaceable> has
1139 been loaded from a source file, then GHCi will also display
1140 the location of its definition in the source.</para>
1141 </listitem>
1142 </varlistentry>
1143
1144 <varlistentry>
1145 <term>
1146 <literal>:load</literal> <replaceable>module</replaceable> ...
1147 <indexterm><primary><literal>:load</literal></primary></indexterm>
1148 </term>
1149 <listitem>
1150 <para>Recursively loads the specified
1151 <replaceable>module</replaceable>s, and all the modules they
1152 depend on. Here, each <replaceable>module</replaceable>
1153 must be a module name or filename, but may not be the name
1154 of a module in a package.</para>
1155
1156 <para>All previously loaded modules, except package modules,
1157 are forgotten. The new set of modules is known as the
1158 <firstterm>target set</firstterm>. Note that
1159 <literal>:load</literal> can be used without any arguments
1160 to unload all the currently loaded modules and
1161 bindings.</para>
1162
1163 <para>After a <literal>:load</literal> command, the current
1164 context is set to:</para>
1165
1166 <itemizedlist>
1167 <listitem>
1168 <para><replaceable>module</replaceable>, if it was loaded
1169 successfully, or</para>
1170 </listitem>
1171 <listitem>
1172 <para>the most recently successfully loaded module, if
1173 any other modules were loaded as a result of the current
1174 <literal>:load</literal>, or</para>
1175 </listitem>
1176 <listitem>
1177 <para><literal>Prelude</literal> otherwise.</para>
1178 </listitem>
1179 </itemizedlist>
1180 </listitem>
1181 </varlistentry>
1182
1183 <varlistentry>
1184 <term>
1185 <literal>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
1186 <indexterm><primary><literal>:main</literal></primary></indexterm>
1187 </term>
1188 <listitem>
1189 <para>
1190 When a program is compiled and executed, it can use the
1191 <literal>getArgs</literal> function to access the
1192 command-line arguments.
1193 However, we cannot simply pass the arguments to the
1194 <literal>main</literal> function while we are testing in ghci,
1195 as the <literal>main</literal> function doesn't take its
1196 directly.
1197 </para>
1198
1199 <para>
1200 Instead, we can use the <literal>:main</literal> command.
1201 This runs whatever <literal>main</literal> is in scope, with
1202 any arguments being treated the same as command-line arguments,
1203 e.g.:
1204 </para>
1205
1206 <screen>
1207 Prelude> let main = System.Environment.getArgs >>= print
1208 Prelude> :main foo bar
1209 ["foo","bar"]
1210 </screen>
1211
1212 </listitem>
1213 </varlistentry>
1214
1215 <varlistentry>
1216 <term>
1217 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
1218 <indexterm><primary><literal>:module</literal></primary></indexterm>
1219 </term>
1220 <listitem>
1221 <para>Sets or modifies the current context for statements
1222 typed at the prompt. See <xref linkend="ghci-scope"/> for
1223 more details.</para>
1224 </listitem>
1225 </varlistentry>
1226
1227 <varlistentry>
1228 <term>
1229 <literal>:print </literal> <replaceable>names</replaceable> ...
1230 <indexterm><primary><literal>:print</literal></primary></indexterm>
1231 </term>
1232 <listitem>
1233 <para> Prints a semievaluated value without forcing its evaluation.
1234 <literal>:print </literal> works just like <literal>:sprint</literal> but additionally,
1235 <literal>:print</literal> binds the unevaluated parts -called
1236 <quote>suspensions</quote>-
1237 to names which you can play with. For example:
1238 <screen>
1239 Prelude> let li = map Just [1..5]
1240 Prelude> :sp li
1241 li - _
1242 Prelude> :p li
1243 li - (_t1::[Maybe Integer])
1244 Prelude> head li
1245 Just 1
1246 Prelude> :sp li
1247 li - [Just 1 | _]
1248 Prelude> :p li
1249 li - [Just 1 | (_t2::[Maybe Integer])]
1250 Prelude> last li
1251 Just 5
1252 Prelude> :sp li
1253 li - [Just 1,_,_,_,Just 5]
1254 Prelude> :p li
1255 li - [Just 1,(_t3::Maybe Integer),(_t4::Maybe Integer),(_t5::Maybe Integer),Just 4]
1256 Prelude> _t4
1257 Just 3
1258 Prelude> :p li
1259 li - [Just 1,(_t6::Maybe Integer),Just 3,(_t7::Maybe Integer),Just 4]
1260 </screen>
1261 The example uses <literal>:print</literal> and <literal>:sprint</literal>
1262 to help us observe how the <literal>li</literal> variable is evaluated progressively as we operate
1263 with it. Note for instance how <quote>last</quote> traverses all the elements of
1264 the list to compute its result, but without evaluating the individual elements.</para>
1265 <para>Finally note that the Prolog convention of [head | tail] is used by
1266 <literal>:sprint</literal> to display unevaluated lists.
1267 </para>
1268 </listitem>
1269 </varlistentry>
1270
1271 <varlistentry>
1272 <term>
1273 <literal>:quit</literal>
1274 <indexterm><primary><literal>:quit</literal></primary></indexterm>
1275 </term>
1276 <listitem>
1277 <para>Quits GHCi. You can also quit by typing a control-D
1278 at the prompt.</para>
1279 </listitem>
1280 </varlistentry>
1281
1282 <varlistentry>
1283 <term>
1284 <literal>:reload</literal>
1285 <indexterm><primary><literal>:reload</literal></primary></indexterm>
1286 </term>
1287 <listitem>
1288 <para>Attempts to reload the current target set (see
1289 <literal>:load</literal>) if any of the modules in the set,
1290 or any dependent module, has changed. Note that this may
1291 entail loading new modules, or dropping modules which are no
1292 longer indirectly required by the target.</para>
1293 </listitem>
1294 </varlistentry>
1295
1296 <varlistentry>
1297 <term>
1298 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
1299 <indexterm><primary><literal>:set</literal></primary></indexterm>
1300 </term>
1301 <listitem>
1302 <para>Sets various options. See <xref linkend="ghci-set"/>
1303 for a list of available options. The
1304 <literal>:set</literal> command by itself shows which
1305 options are currently set.</para>
1306 </listitem>
1307 </varlistentry>
1308
1309 <varlistentry>
1310 <term>
1311 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
1312 <indexterm><primary><literal>:set args</literal></primary></indexterm>
1313 </term>
1314 <listitem>
1315 <para>Sets the list of arguments which are returned when the
1316 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
1317 </indexterm>.</para>
1318 </listitem>
1319 </varlistentry>
1320
1321 <varlistentry>
1322 <term>
1323 <literal>:set</literal> <literal>editor</literal> <replaceable>cmd</replaceable>
1324 </term>
1325 <listitem>
1326 <para>Sets the command used by <literal>:edit</literal> to
1327 <replaceable>cmd</replaceable>.</para>
1328 </listitem>
1329 </varlistentry>
1330
1331 <varlistentry>
1332 <term>
1333 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
1334 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
1335 </term>
1336 <listitem>
1337 <para>Sets the string to be returned when the program calls
1338 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
1339 </indexterm>.</para>
1340 </listitem>
1341 </varlistentry>
1342
1343 <varlistentry>
1344 <term>
1345 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
1346 </term>
1347 <listitem>
1348 <para>Sets the string to be used as the prompt in GHCi.
1349 Inside <replaceable>prompt</replaceable>, the sequence
1350 <literal>%s</literal> is replaced by the names of the
1351 modules currently in scope, and <literal>%%</literal> is
1352 replaced by <literal>%</literal>.</para>
1353 </listitem>
1354 </varlistentry>
1355
1356 <varlistentry>
1357 <term>
1358 <literal>:show bindings</literal>
1359 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
1360 </term>
1361 <listitem>
1362 <para>Show the bindings made at the prompt and their
1363 types.</para>
1364 </listitem>
1365 </varlistentry>
1366
1367 <varlistentry>
1368 <term>
1369 <literal>:show modules</literal>
1370 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
1371 </term>
1372 <listitem>
1373 <para>Show the list of modules currently load.</para>
1374 </listitem>
1375 </varlistentry>
1376 <varlistentry>
1377 <term>
1378 <literal>:sprint</literal>
1379 <indexterm><primary><literal>:sprint</literal></primary></indexterm>
1380 </term>
1381 <listitem>
1382 <para>Prints a semievaluated value without forcing its evaluation.
1383 <literal>:sprint</literal> and its sibling <literal>:print</literal>
1384 are very useful to observe how lazy evaluation works in your code. For example:
1385 <screen>
1386 Prelude> let li = map Just [1..5]
1387 Prelude> :sp li
1388 li - _
1389 Prelude> head li
1390 Just 1
1391 Prelude> :sp li
1392 li - [Just 1 | _]
1393 Prelude> last li
1394 Just 5
1395 Prelude> :sp li
1396 li - [Just 1,_,_,_,Just 5]
1397 </screen>
1398 The example uses <literal>:sprint</literal> to help us observe how the <literal>li</literal> variable is evaluated progressively as we operate
1399 with it. Note for instance how <quote>last</quote> traverses all the elements of
1400 the list to compute its result, but without evaluating the individual elements.</para>
1401 <para>Finally note that the Prolog convention of [head | tail] is used by
1402 <literal>:sprint</literal> to display unevaluated lists.
1403 </para>
1404 </listitem>
1405 </varlistentry>
1406 <varlistentry>
1407 <term>
1408 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1409 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1410 <indexterm><primary><literal>:etags</literal></primary>
1411 </indexterm>
1412 <indexterm><primary><literal>:etags</literal></primary>
1413 </indexterm>
1414 </term>
1415 <listitem>
1416 <para>Generates a &ldquo;tags&rdquo; file for Vi-style editors
1417 (<literal>:ctags</literal>) or Emacs-style editors (<literal>etags</literal>). If
1418 no filename is specified, the defaulit <filename>tags</filename> or
1419 <filename>TAGS</filename> is
1420 used, respectively. Tags for all the functions, constructors and
1421 types in the currently loaded modules are created. All modules must
1422 be interpreted for these commands to work.</para>
1423 <para>See also <xref linkend="hasktags" />.</para>
1424 </listitem>
1425 </varlistentry>
1426
1427 <varlistentry>
1428 <term>
1429 <literal>:type</literal> <replaceable>expression</replaceable>
1430 <indexterm><primary><literal>:type</literal></primary></indexterm>
1431 </term>
1432 <listitem>
1433 <para>Infers and prints the type of
1434 <replaceable>expression</replaceable>, including explicit
1435 forall quantifiers for polymorphic types. The monomorphism
1436 restriction is <emphasis>not</emphasis> applied to the
1437 expression during type inference.</para>
1438 </listitem>
1439 </varlistentry>
1440
1441 <varlistentry>
1442 <term>
1443 <literal>:kind</literal> <replaceable>type</replaceable>
1444 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1445 </term>
1446 <listitem>
1447 <para>Infers and prints the kind of
1448 <replaceable>type</replaceable>. The latter can be an arbitrary
1449 type expression, including a partial application of a type constructor,
1450 such as <literal>Either Int</literal>.</para>
1451 </listitem>
1452 </varlistentry>
1453
1454 <varlistentry>
1455 <term>
1456 <literal>:undef</literal> <replaceable>name</replaceable>
1457 <indexterm><primary><literal>:undef</literal></primary></indexterm>
1458 </term>
1459 <listitem>
1460 <para>Undefines the user-defined command
1461 <replaceable>name</replaceable> (see <literal>:def</literal>
1462 above).</para>
1463 </listitem>
1464 </varlistentry>
1465
1466 <varlistentry>
1467 <term>
1468 <literal>:unset</literal> <replaceable>option</replaceable>...
1469 <indexterm><primary><literal>:unset</literal></primary></indexterm>
1470 </term>
1471 <listitem>
1472 <para>Unsets certain options. See <xref linkend="ghci-set"/>
1473 for a list of available options.</para>
1474 </listitem>
1475 </varlistentry>
1476
1477 <varlistentry>
1478 <term>
1479 <literal>:!</literal> <replaceable>command</replaceable>...
1480 <indexterm><primary><literal>:!</literal></primary></indexterm>
1481 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
1482 </term>
1483 <listitem>
1484 <para>Executes the shell command
1485 <replaceable>command</replaceable>.</para>
1486 </listitem>
1487 </varlistentry>
1488
1489 </variablelist>
1490 </sect1>
1491
1492 <sect1 id="ghci-set">
1493 <title>The <literal>:set</literal> command</title>
1494 <indexterm><primary><literal>:set</literal></primary></indexterm>
1495
1496 <para>The <literal>:set</literal> command sets two types of
1497 options: GHCi options, which begin with
1498 &lsquo;<literal>+</literal>&rdquo; and &ldquo;command-line&rdquo;
1499 options, which begin with &lsquo;-&rsquo;. </para>
1500
1501 <para>NOTE: at the moment, the <literal>:set</literal> command
1502 doesn't support any kind of quoting in its arguments: quotes will
1503 not be removed and cannot be used to group words together. For
1504 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
1505 you expect.</para>
1506
1507 <sect2>
1508 <title>GHCi options</title>
1509 <indexterm><primary>options</primary><secondary>GHCi</secondary>
1510 </indexterm>
1511
1512 <para>GHCi options may be set using <literal>:set</literal> and
1513 unset using <literal>:unset</literal>.</para>
1514
1515 <para>The available GHCi options are:</para>
1516
1517 <variablelist>
1518 <varlistentry>
1519 <term>
1520 <literal>+r</literal>
1521 <indexterm><primary><literal>+r</literal></primary></indexterm>
1522 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
1523 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
1524 </term>
1525 <listitem>
1526 <para>Normally, any evaluation of top-level expressions
1527 (otherwise known as CAFs or Constant Applicative Forms) in
1528 loaded modules is retained between evaluations. Turning
1529 on <literal>+r</literal> causes all evaluation of
1530 top-level expressions to be discarded after each
1531 evaluation (they are still retained
1532 <emphasis>during</emphasis> a single evaluation).</para>
1533
1534 <para>This option may help if the evaluated top-level
1535 expressions are consuming large amounts of space, or if
1536 you need repeatable performance measurements.</para>
1537 </listitem>
1538 </varlistentry>
1539
1540 <varlistentry>
1541 <term>
1542 <literal>+s</literal>
1543 <indexterm><primary><literal>+s</literal></primary></indexterm>
1544 </term>
1545 <listitem>
1546 <para>Display some stats after evaluating each expression,
1547 including the elapsed time and number of bytes allocated.
1548 NOTE: the allocation figure is only accurate to the size
1549 of the storage manager's allocation area, because it is
1550 calculated at every GC. Hence, you might see values of
1551 zero if no GC has occurred.</para>
1552 </listitem>
1553 </varlistentry>
1554
1555 <varlistentry>
1556 <term>
1557 <literal>+t</literal>
1558 <indexterm><primary><literal>+t</literal></primary></indexterm>
1559 </term>
1560 <listitem>
1561 <para>Display the type of each variable bound after a
1562 statement is entered at the prompt. If the statement is a
1563 single expression, then the only variable binding will be
1564 for the variable
1565 &lsquo;<literal>it</literal>&rsquo;.</para>
1566 </listitem>
1567 </varlistentry>
1568 </variablelist>
1569 </sect2>
1570
1571 <sect2 id="ghci-cmd-line-options">
1572 <title>Setting GHC command-line options in GHCi</title>
1573
1574 <para>Normal GHC command-line options may also be set using
1575 <literal>:set</literal>. For example, to turn on
1576 <option>-fglasgow-exts</option>, you would say:</para>
1577
1578 <screen>
1579 Prelude> :set -fglasgow-exts
1580 </screen>
1581
1582 <para>Any GHC command-line option that is designated as
1583 <firstterm>dynamic</firstterm> (see the table in <xref
1584 linkend="flag-reference"/>), may be set using
1585 <literal>:set</literal>. To unset an option, you can set the
1586 reverse option:</para>
1587 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
1588
1589 <screen>
1590 Prelude> :set -fno-glasgow-exts
1591 </screen>
1592
1593 <para><xref linkend="flag-reference"/> lists the reverse for each
1594 option where applicable.</para>
1595
1596 <para>Certain static options (<option>-package</option>,
1597 <option>-I</option>, <option>-i</option>, and
1598 <option>-l</option> in particular) will also work, but some may
1599 not take effect until the next reload.</para>
1600 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
1601 </sect2>
1602 </sect1>
1603 <sect1 id="ghci-debugger">
1604 <title>The GHCi debugger</title>
1605 <indexterm><primary>debugger</primary></indexterm>
1606 <para>GHCi embeds an utility debugger with a very basic set of operations. The debugger
1607 is always available in ghci, you do not need to do anything to activate it. </para>
1608 <para>The following conditions must hold before a module can be debugged in GHCi:
1609 <itemizedlist>
1610 <listitem>
1611 <para>The module must have been loaded interpreted, i.e. not loaded from an <filename>.o</filename> file compiled by ghc </para>
1612 </listitem>
1613 <listitem>
1614 <para>The module must have been loaded with the <literal>-fdebugging</literal> flag
1615 </para></listitem>
1616 </itemizedlist></para>
1617 <sect2><title>Using the debugger</title>
1618 <para>The debugger allows the insertion of breakpoints at specific locations in the source code. These locations are governed by event sites, and not by line as in traditional debuggers such as gdb. </para> <para>
1619 Once a breakpointed event is hit, the debugger stops the execution and you can examine the local variables in scope
1620 in the context of the event, as well as evaluate arbitrary Haskell expressions in
1621 a special interactive prompt. </para><para>
1622
1623 When you are done you issue the <literal>:continue</literal>
1624 command to leave the breakpoint and let the execution go on.
1625 Note that not all the GHCi commands are supported in a breakpoint.
1626
1627 </para>
1628 <sidebar><title>Events</title><?dbfo float-type="left"?>
1629 <para> Events are the places in source code where you can set a breakpoint.
1630 <programlisting>
1631 qsort [] = <co id="name-binding-co"/> []
1632 qsort (x:xs) =
1633 <coref linkend="name-binding-co"/> let left = <coref linkend="name-binding-co"/> filter (\y -> <co id="lambda-co"/> y &lt; x) xs
1634 right = <coref linkend="name-binding-co"/> case filter (\y -> <coref linkend="lambda-co"/> y &gt; x) xs of
1635 right_val -> <co id="case-co"/> right_val
1636 in <co id="let-co"/> qsort left ++ [x] ++ qsort right
1637 main = <coref linkend="name-binding-co"/> do {
1638 arg &lt;- <coref linkend="name-binding-co"/> getLine ;
1639 let num = <coref linkend="name-binding-co"/> read arg :: [Int] ;
1640 <co id="stmts-co"/> print (qsort num) ;
1641 <coref linkend="stmts-co"/> putStrLn "GoodBye!" }
1642 </programlisting>
1643 The GHCi debugger recognizes the following event types:
1644 <calloutlist>
1645 <callout arearefs="name-binding-co" id="name-binding">
1646 <para>Function definition and local bindings in let/where</para>
1647 </callout>
1648 <callout arearefs="lambda-co" id="lambda">
1649 <para>Lambda expression entry point</para>
1650 </callout>
1651 <callout arearefs="let-co" id="let">
1652 <para>Let expression body</para>
1653 </callout>
1654 <callout arearefs="case-co" id="case">
1655 <para>Case alternative body</para>
1656 </callout>
1657 <callout arearefs="stmts-co" id="stmts">
1658 <para>do notation statements</para>
1659 </callout>
1660 </calloutlist></para>
1661 <para>In reality however, ghci eliminates some redundant event sites.
1662 For instance, sites with two co-located breakpoint events are coalesced into a single one,
1663 and sites with no bindings in scope are assumed to be uninteresting and no breakpoint can be set in them.</para>
1664 </sidebar>
1665
1666 <para>
1667 You don't need to do anything special in order to start the debugging session.
1668 Simply use ghci to evaluate your Haskell expressions and whenever a breakpoint
1669 is hit, the debugger will enter the stage:
1670 <programlisting>
1671 *main:Main> :break add Main 2
1672 Breakpoint set at (2,15)
1673
1674 *main:Main> qsort [10,9..1]
1675 Local bindings in scope:
1676 x :: a, xs :: [a], left :: [a], right :: [a]
1677
1678 qsort2.hs:2:15-46>
1679 </programlisting>
1680 What is happening here is that GHCi has interrupted the evaluation of
1681 <literal>qsort</literal> at the breakpoint set in line 2, as the prompt indicates.
1682 At this point you can freely explore the contents of the bindings in scope,
1683 but with two catches. </para><para>
1684 First, take into account that due to the lazy nature of Haskell, some of
1685 these bindings may be unevaluated, and that exploring their contents may
1686 trigger a computation. </para><para>
1687 Second: look at the types of the things in scope.
1688 GHCi has left its types parameterised by a variable!
1689 Look at the type of <literal>qsort</literal>, which is
1690 polymorphic on the type of its argument. It does not
1691 tell us really what the types of <literal>x</literal> and <literal>xs</literal> can be.
1692 In general, polymorphic programs deal with polymorphic values,
1693 and this means that some of the bindings available in a breakpoint site
1694 will be parametrically typed.
1695 </para><para>
1696 So, what can we do with a value without concrete type? Very few interesting
1697 things. The <literal>:print</literal> command in ghci allows you to
1698 explore its contents and see if it is evaluated or not.
1699 This is useful because you cannot just type <literal>x</literal> in the
1700 prompt and expect GHCi to return you its value. Perhaps you know for
1701 sure that
1702 <literal>x</literal> is of type <literal>Int</literal>, which is an instance of
1703 <literal>Show</literal>, but GHCi does not have this information.
1704 <literal>:print</literal> however is fine, because it does not need to know the
1705 type to do its work. </para>
1706 <para> Let's go on with the debugging session of the <literal>qsort</literal>
1707 example:
1708 <example id="debuggingEx"><title>A short debugging session</title>
1709 <programlisting>
1710 qsort2.hs:2:15-46> x
1711 This is an untyped, unevaluated computation. You can use seq to
1712 force its evaluation and then :print to recover its type <co id="seq1"/>
1713 qsort2.hs:2:15-46> seq x () <co id="seq2"/>
1714 ()
1715 qsort2.hs:2:15-46> x <co id="seq3"/>
1716 This is an untyped, unevaluated computation. You can use seq to
1717 force its evaluation and then :print to recover its type
1718
1719 qsort2.hs:2:15-46> :t x
1720 x :: GHC.Base.Unknown
1721 qsort2.hs:2:15-46> :p x <co id="seq4"/>
1722 x - 10
1723 qsort2.hs:2:15-46> :t x <co id="seq5"/>
1724 x :: Int
1725 </programlisting>
1726 </example>
1727 <calloutlist>
1728 <callout arearefs="seq1">
1729 <para>GHCi reminds us that this value is untyped, and instructs us to force its evaluation </para>
1730 </callout>
1731 <callout arearefs="seq2">
1732 <para>This line forces the evaluation of <literal>x</literal> </para>
1733 </callout>
1734 <callout arearefs="seq3">
1735 <para>Even though x has been evaluated, we cannot simply use its name to see its value!
1736 This is a bit counterintuitive, but currently in GHCi the type of a binding
1737 cannot be a type variable <literal>a</literal>.
1738 Thus, the binding <literal>x</literal> gets assigned the concrete type Unknown.</para>
1739 </callout>
1740 <callout arearefs="seq4">
1741 <para>We can explore <literal>x</literal> using the <literal>:print</literal>
1742 command, which does find out that <literal>x</literal> is of type Int and prints
1743 its value accordingly.</para>
1744 </callout>
1745 <callout arearefs="seq5">
1746 <para><literal>:print</literal> also updates the type of <literal>x</literal> with
1747 the most concrete type information available.</para>
1748 </callout>
1749 </calloutlist>
1750 The example shows the standard way to proceeed with polymorphic values in a breakpoint.
1751 </para>
1752 </sect2>
1753 <sect2><title>Commands</title>
1754 <para>Breakpoints can be set in several ways using the <literal>:breakpoint</literal> command. Note that you can take advantage of the command abbreviation feature of GHCi and use simply <literal>:bre</literal> to save quite a few keystrokes.
1755 <variablelist>
1756 <varlistentry>
1757 <term>
1758 <literal>:breakpoint add <replaceable>module</replaceable> <replaceable>line</replaceable></literal>
1759 </term>
1760 <listitem><para>
1761 Adds a breakpoint at the first event found at line <literal>line</literal> in <literal>module</literal>, if any.
1762 </para></listitem>
1763 </varlistentry>
1764 <varlistentry>
1765 <term>
1766 <literal>:breakpoint add <replaceable>module</replaceable> <replaceable>line</replaceable> <replaceable>column</replaceable></literal>
1767 </term>
1768 <listitem><para>
1769 Adds a breakpoint at the first event found after column <literal>column</literal>
1770 at line <literal>line</literal> in <literal>module</literal>, if any.
1771 </para></listitem>
1772 </varlistentry>
1773
1774 <varlistentry>
1775 <term>
1776 <literal>:breakpoint continue</literal>
1777 </term>
1778 <listitem><para>
1779 When at a breakpoint, continue execution up to the next breakpoint
1780 or end of evaluation.
1781 </para></listitem>
1782 </varlistentry>
1783
1784 <varlistentry>
1785 <term>
1786 <literal>:continue</literal>
1787 </term>
1788 <listitem><para>
1789 Shortcut for <literal>:breakpoint continue</literal>
1790 </para></listitem>
1791 </varlistentry>
1792
1793 <varlistentry>
1794 <term>
1795 <literal>:breakpoint list</literal>
1796 </term>
1797 <listitem><para>
1798 Lists the currently set up breakpoints.
1799 </para></listitem>
1800 </varlistentry>
1801 <varlistentry>
1802 <term>
1803 <literal>:breakpoint del <replaceable>num</replaceable></literal>
1804 </term>
1805 <listitem><para>
1806 Deletes the breakpoint at position <literal>num</literal> in the list of
1807 breakpoints shown by <literal>:breakpoint list</literal>.
1808 </para></listitem>
1809 </varlistentry>
1810 <varlistentry>
1811 <term>
1812 <literal>:breakpoint del <replaceable>module</replaceable> <replaceable>line</replaceable></literal>
1813 </term>
1814 <listitem><para>
1815 Dels the breakpoint at line <literal>line</literal> in <literal>module</literal>, if any.
1816 </para></listitem>
1817 </varlistentry>
1818 <varlistentry>
1819 <term>
1820 <literal>:breakpoint del <replaceable>module</replaceable> <replaceable>line</replaceable><replaceable>col</replaceable> </literal>
1821 </term>
1822 <listitem><para>
1823 Deletes the first breakpoint found after column <literal>column</literal>
1824 at line <literal>line</literal> in <literal>module</literal>, if any.
1825 </para></listitem>
1826 </varlistentry>
1827 <varlistentry>
1828 <term>
1829 <literal>:breakpoint stop </literal>
1830 </term>
1831 <listitem><para>
1832 Stop the program being executed. This interrupts a debugging session
1833 and returns to the top level.
1834 </para></listitem>
1835 </varlistentry>
1836 </variablelist></para>
1837 </sect2>
1838 <sect2><title>Limitations</title>
1839 <para>
1840 <itemizedlist>
1841 <listitem><para>
1842 Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
1843 at the scope of a breakpoint if there is a explicit type signature.
1844 </para></listitem>
1845 </itemizedlist>
1846 <itemizedlist>
1847 <listitem><para>
1848 Modules compiled by GHCi under the <literal>-fdebugging
1849 </literal> flag will perform slower: the debugging mode introduces some overhead.
1850 Modules compiled to object code by ghc are not affected.
1851 </para></listitem>
1852 </itemizedlist>
1853 </para>
1854 </sect2>
1855 <sect2><title>Tips</title>
1856 <variablelist>
1857 <varlistentry><term>* Use PRAGMAs to fine tune which modules are loaded under debugging mode</term>
1858 <listitem>
1859 <programlisting>{-# OPTIONS_GHC -fdebugging #-}</programlisting>
1860 </listitem>
1861 </varlistentry>
1862 <varlistentry> <term>* Repeated use of <literal>seq</literal> and
1863 <literal>:print</literal> may be necessary to observe unevaluated
1864 untyped bindings</term>
1865 <listitem><para>see <xref linkend="debuggingEx"/>
1866 </para></listitem>
1867 </varlistentry>
1868 <varlistentry> <term> * <literal>GHC.Exts.unsafeCoerce</literal> can help if you are positive about the type of a binding</term>
1869 <listitem><para><programlisting>
1870 type MyLongType a = [Maybe [Maybe a]]
1871
1872 main:Main> :m +GHC.Exts
1873 main:Main> main
1874 Local bindings in scope:
1875 x :: a
1876 Main.hs:15> let x' = unsafeCoerce x :: MyLongType Bool
1877 Main.hs:15> x'
1878 [Just [Just False, Just True]]
1879 </programlisting>
1880 Note that a wrong coercion will likely result in your debugging session being interrupted by a segmentation fault
1881 </para></listitem>
1882 </varlistentry>
1883 <varlistentry> <term> * The undocumented (and unsupported) &colon;force command </term>
1884 <listitem><para>
1885 equivalent to <literal> :print</literal> with automatic
1886 <literal>seq</literal> forcing,
1887 may prove useful to replace sequences of <literal>seq</literal> and
1888 <literal>&colon;print</literal> in some situations.
1889 </para></listitem>
1890 </varlistentry>
1891 </variablelist>
1892 </sect2></sect1>
1893 <sect1 id="ghci-dot-files">
1894 <title>The <filename>.ghci</filename> file</title>
1895 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
1896 </indexterm>
1897 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
1898 </indexterm>
1899
1900 <para>When it starts, GHCi always reads and executes commands from
1901 <filename>$HOME/.ghci</filename>, followed by
1902 <filename>./.ghci</filename>.</para>
1903
1904 <para>The <filename>.ghci</filename> in your home directory is
1905 most useful for turning on favourite options (eg. <literal>:set
1906 +s</literal>), and defining useful macros. Placing a
1907 <filename>.ghci</filename> file in a directory with a Haskell
1908 project is a useful way to set certain project-wide options so you
1909 don't have to type them everytime you start GHCi: eg. if your
1910 project uses GHC extensions and CPP, and has source files in three
1911 subdirectories A B and C, you might put the following lines in
1912 <filename>.ghci</filename>:</para>
1913
1914 <screen>
1915 :set -fglasgow-exts -cpp
1916 :set -iA:B:C
1917 </screen>
1918
1919 <para>(Note that strictly speaking the <option>-i</option> flag is
1920 a static one, but in fact it works to set it using
1921 <literal>:set</literal> like this. The changes won't take effect
1922 until the next <literal>:load</literal>, though.)</para>
1923
1924 <para>Two command-line options control whether the
1925 <filename>.ghci</filename> files are read:</para>
1926
1927 <variablelist>
1928 <varlistentry>
1929 <term>
1930 <option>-ignore-dot-ghci</option>
1931 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
1932 </term>
1933 <listitem>
1934 <para>Don't read either <filename>./.ghci</filename> or
1935 <filename>$HOME/.ghci</filename> when starting up.</para>
1936 </listitem>
1937 </varlistentry>
1938 <varlistentry>
1939 <term>
1940 <option>-read-dot-ghci</option>
1941 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
1942 </term>
1943 <listitem>
1944 <para>Read <filename>.ghci</filename> and
1945 <filename>$HOME/.ghci</filename>. This is normally the
1946 default, but the <option>-read-dot-ghci</option> option may
1947 be used to override a previous
1948 <option>-ignore-dot-ghci</option> option.</para>
1949 </listitem>
1950 </varlistentry>
1951 </variablelist>
1952
1953 </sect1>
1954
1955 <sect1 id="ghci-obj">
1956 <title>Compiling to object code inside GHCi</title>
1957
1958 <para>By default, GHCi compiles Haskell source code into byte-code
1959 that is interpreted by the runtime system. GHCi can also compile
1960 Haskell code to object code: to turn on this feature, use the
1961 <option>-fobject-code</option> flag either on the command line or
1962 with <literal>:set</literal> (the option
1963 <option>-fbyte-code</option> restores byte-code compilation
1964 again). Compiling to object code takes longer, but typically the
1965 code will execute 10-20 times faster than byte-code.</para>
1966
1967 <para>Compiling to object code inside GHCi is particularly useful
1968 if you are developing a compiled application, because the
1969 <literal>:reload</literal> command typically runs much faster than
1970 restarting GHC with <option>--make</option> from the command-line,
1971 because all the interface files are already cached in
1972 memory.</para>
1973
1974 <para>There are disadvantages to compiling to object-code: you
1975 can't set breakpoints in object-code modules, for example. Only
1976 the exports of an object-code module will be visible in GHCi,
1977 rather than all top-level bindings as in interpreted
1978 modules.</para>
1979 </sect1>
1980
1981 <sect1 id="ghci-faq">
1982 <title>FAQ and Things To Watch Out For</title>
1983
1984 <variablelist>
1985 <varlistentry>
1986 <term>The interpreter can't load modules with foreign export
1987 declarations!</term>
1988 <listitem>
1989 <para>Unfortunately not. We haven't implemented it yet.
1990 Please compile any offending modules by hand before loading
1991 them into GHCi.</para>
1992 </listitem>
1993 </varlistentry>
1994
1995 <varlistentry>
1996 <term>
1997 <literal>-O</literal> doesn't work with GHCi!
1998 <indexterm><primary><option>-O</option></primary></indexterm>
1999 </term>
2000 <listitem>
2001 <para>For technical reasons, the bytecode compiler doesn't
2002 interact well with one of the optimisation passes, so we
2003 have disabled optimisation when using the interpreter. This
2004 isn't a great loss: you'll get a much bigger win by
2005 compiling the bits of your code that need to go fast, rather
2006 than interpreting them with optimisation turned on.</para>
2007 </listitem>
2008 </varlistentry>
2009
2010 <varlistentry>
2011 <term>Unboxed tuples don't work with GHCi</term>
2012 <listitem>
2013 <para>That's right. You can always compile a module that
2014 uses unboxed tuples and load it into GHCi, however.
2015 (Incidentally the previous point, namely that
2016 <literal>-O</literal> is incompatible with GHCi, is because
2017 the bytecode compiler can't deal with unboxed
2018 tuples).</para>
2019 </listitem>
2020 </varlistentry>
2021
2022 <varlistentry>
2023 <term>Concurrent threads don't carry on running when GHCi is
2024 waiting for input.</term>
2025 <listitem>
2026 <para>This should work, as long as your GHCi was built with
2027 the <option>-threaded</option> switch, which is the default.
2028 Consult whoever supplied your GHCi installation.</para>
2029 </listitem>
2030 </varlistentry>
2031
2032 <varlistentry>
2033 <term>After using <literal>getContents</literal>, I can't use
2034 <literal>stdin</literal> again until I do
2035 <literal>:load</literal> or <literal>:reload</literal>.</term>
2036
2037 <listitem>
2038 <para>This is the defined behaviour of
2039 <literal>getContents</literal>: it puts the stdin Handle in
2040 a state known as <firstterm>semi-closed</firstterm>, wherein
2041 any further I/O operations on it are forbidden. Because I/O
2042 state is retained between computations, the semi-closed
2043 state persists until the next <literal>:load</literal> or
2044 <literal>:reload</literal> command.</para>
2045
2046 <para>You can make <literal>stdin</literal> reset itself
2047 after every evaluation by giving GHCi the command
2048 <literal>:set +r</literal>. This works because
2049 <literal>stdin</literal> is just a top-level expression that
2050 can be reverted to its unevaluated state in the same way as
2051 any other top-level expression (CAF).</para>
2052 </listitem>
2053 </varlistentry>
2054
2055 <varlistentry>
2056 <term>I can't use Control-C to interrupt computations in
2057 GHCi on Windows.</term>
2058 <listitem>
2059 <para>See <xref linkend="ghci-windows"/></para>
2060 </listitem>
2061 </varlistentry>
2062 </variablelist>
2063 </sect1>
2064
2065 </chapter>
2066
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