Remove another instance of the banner from the user guide
[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.
18 <indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
19 <indexterm><primary>Foreign Function
20 Interface</primary><secondary>GHCi support</secondary></indexterm>.
21 GHCi also includes an interactive debugger (see <xref linkend="ghci-debugger"/>).</para>
22
23 <sect1 id="ghci-introduction">
24 <title>Introduction to GHCi</title>
25
26 <para>Let's start with an example GHCi session. You can fire up
27 GHCi with the command <literal>ghci</literal>:</para>
28
29 <screen>
30 $ ghci
31 GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
32 Loading package base ... linking ... done.
33 Prelude>
34 </screen>
35
36 <para>There may be a short pause while GHCi loads the prelude and
37 standard libraries, after which the prompt is shown. As the banner
38 says, you can type <literal>:?</literal> to see the list of commands
39 available, and a half line description of each of them.</para>
40
41 <para>We'll explain most of these commands as we go along. For
42 Hugs users: many things work the same as in Hugs, so you should be
43 able to get going straight away.</para>
44
45 <para>Haskell expressions can be typed at the prompt:</para>
46 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
47 </indexterm>
48
49 <screen>
50 Prelude> 1+2
51 3
52 Prelude> let x = 42 in x / 9
53 4.666666666666667
54 Prelude>
55 </screen>
56
57 <para>GHCi interprets the whole line as an expression to evaluate.
58 The expression may not span several lines - as soon as you press
59 enter, GHCi will attempt to evaluate it.</para>
60 </sect1>
61
62 <sect1 id="loading-source-files">
63 <title>Loading source files</title>
64
65 <para>Suppose we have the following Haskell source code, which we
66 place in a file <filename>Main.hs</filename>:</para>
67
68 <programlisting>
69 main = print (fac 20)
70
71 fac 0 = 1
72 fac n = n * fac (n-1)
73 </programlisting>
74
75 <para>You can save <filename>Main.hs</filename> anywhere you like,
76 but if you save it somewhere other than the current
77 directory<footnote><para>If you started up GHCi from the command
78 line then GHCi's current directory is the same as the current
79 directory of the shell from which it was started. If you started
80 GHCi from the &ldquo;Start&rdquo; menu in Windows, then the
81 current directory is probably something like
82 <filename>C:\Documents and Settings\<replaceable>user
83 name</replaceable></filename>.</para> </footnote> then we will
84 need to change to the right directory in GHCi:</para>
85
86 <screen>
87 Prelude> :cd <replaceable>dir</replaceable>
88 </screen>
89
90 <para>where <replaceable>dir</replaceable> is the directory (or
91 folder) in which you saved <filename>Main.hs</filename>.</para>
92
93 <para>To load a Haskell source file into GHCi, use the
94 <literal>:load</literal> command:</para>
95 <indexterm><primary><literal>:load</literal></primary></indexterm>
96
97 <screen>
98 Prelude> :load Main
99 Compiling Main ( Main.hs, interpreted )
100 Ok, modules loaded: Main.
101 *Main>
102 </screen>
103
104 <para>GHCi has loaded the <literal>Main</literal> module, and the
105 prompt has changed to &ldquo;<literal>*Main></literal>&rdquo; to
106 indicate that the current context for expressions typed at the
107 prompt is the <literal>Main</literal> module we just loaded (we'll
108 explain what the <literal>*</literal> means later in <xref
109 linkend="ghci-scope"/>). So we can now type expressions involving
110 the functions from <filename>Main.hs</filename>:</para>
111
112 <screen>
113 *Main> fac 17
114 355687428096000
115 </screen>
116
117 <para>Loading a multi-module program is just as straightforward;
118 just give the name of the &ldquo;topmost&rdquo; module to the
119 <literal>:load</literal> command (hint: <literal>:load</literal>
120 can be abbreviated to <literal>:l</literal>). The topmost module
121 will normally be <literal>Main</literal>, but it doesn't have to
122 be. GHCi will discover which modules are required, directly or
123 indirectly, by the topmost module, and load them all in dependency
124 order.</para>
125
126 <sect2 id="ghci-modules-filenames">
127 <title>Modules vs. filenames</title>
128 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
129 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
130
131 <para>Question: How does GHC find the filename which contains
132 module <replaceable>M</replaceable>? Answer: it looks for the
133 file <literal><replaceable>M</replaceable>.hs</literal>, or
134 <literal><replaceable>M</replaceable>.lhs</literal>. This means
135 that for most modules, the module name must match the filename.
136 If it doesn't, GHCi won't be able to find it.</para>
137
138 <para>There is one exception to this general rule: when you load
139 a program with <literal>:load</literal>, or specify it when you
140 invoke <literal>ghci</literal>, you can give a filename rather
141 than a module name. This filename is loaded if it exists, and
142 it may contain any module you like. This is particularly
143 convenient if you have several <literal>Main</literal> modules
144 in the same directory and you can't call them all
145 <filename>Main.hs</filename>.</para>
146
147 <para>The search path for finding source files is specified with
148 the <option>-i</option> option on the GHCi command line, like
149 so:</para>
150 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
151
152 <para>or it can be set using the <literal>:set</literal> command
153 from within GHCi (see <xref
154 linkend="ghci-cmd-line-options"/>)<footnote><para>Note that in
155 GHCi, and <option>&ndash;&ndash;make</option> mode, the <option>-i</option>
156 option is used to specify the search path for
157 <emphasis>source</emphasis> files, whereas in standard
158 batch-compilation mode the <option>-i</option> option is used to
159 specify the search path for interface files, see <xref
160 linkend="search-path"/>.</para> </footnote></para>
161
162 <para>One consequence of the way that GHCi follows dependencies
163 to find modules to load is that every module must have a source
164 file. The only exception to the rule is modules that come from
165 a package, including the <literal>Prelude</literal> and standard
166 libraries such as <literal>IO</literal> and
167 <literal>Complex</literal>. If you attempt to load a module for
168 which GHCi can't find a source file, even if there are object
169 and interface files for the module, you'll get an error
170 message.</para>
171 </sect2>
172
173 <sect2>
174 <title>Making changes and recompilation</title>
175 <indexterm><primary><literal>:reload</literal></primary></indexterm>
176
177 <para>If you make some changes to the source code and want GHCi
178 to recompile the program, give the <literal>:reload</literal>
179 command. The program will be recompiled as necessary, with GHCi
180 doing its best to avoid actually recompiling modules if their
181 external dependencies haven't changed. This is the same
182 mechanism we use to avoid re-compiling modules in the batch
183 compilation setting (see <xref linkend="recomp"/>).</para>
184 </sect2>
185 </sect1>
186
187 <sect1 id="ghci-compiled">
188 <title>Loading compiled code</title>
189 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
190
191 <para>When you load a Haskell source module into GHCi, it is
192 normally converted to byte-code and run using the interpreter.
193 However, interpreted code can also run alongside compiled code in
194 GHCi; indeed, normally when GHCi starts, it loads up a compiled
195 copy of the <literal>base</literal> package, which contains the
196 <literal>Prelude</literal>.</para>
197
198 <para>Why should we want to run compiled code? Well, compiled
199 code is roughly 10x faster than interpreted code, but takes about
200 2x longer to produce (perhaps longer if optimisation is on). So
201 it pays to compile the parts of a program that aren't changing
202 very often, and use the interpreter for the code being actively
203 developed.</para>
204
205 <para>When loading up source files with <literal>:load</literal>,
206 GHCi looks for any corresponding compiled object files, and will
207 use one in preference to interpreting the source if possible. For
208 example, suppose we have a 4-module program consisting of modules
209 A, B, C, and D. Modules B and C both import D only,
210 and A imports both B &amp; C:</para>
211 <screen>
212 A
213 / \
214 B C
215 \ /
216 D
217 </screen>
218 <para>We can compile D, then load the whole program, like this:</para>
219 <screen>
220 Prelude> :! ghc -c D.hs
221 Prelude> :load A
222 Compiling B ( B.hs, interpreted )
223 Compiling C ( C.hs, interpreted )
224 Compiling A ( A.hs, interpreted )
225 Ok, modules loaded: A, B, C, D.
226 *Main>
227 </screen>
228
229 <para>In the messages from the compiler, we see that there is no line
230 for <literal>D</literal>. This is because
231 it isn't necessary to compile <literal>D</literal>,
232 because the source and everything it depends on
233 is unchanged since the last compilation.</para>
234
235 <para>At any time you can use the command
236 <literal>:show modules</literal>
237 to get a list of the modules currently loaded
238 into GHCi:</para>
239
240 <screen>
241 *Main> :show modules
242 D ( D.hs, D.o )
243 C ( C.hs, interpreted )
244 B ( B.hs, interpreted )
245 A ( A.hs, interpreted )
246 *Main></screen>
247
248 <para>If we now modify the source of D (or pretend to: using the Unix
249 command <literal>touch</literal> on the source file is handy for
250 this), the compiler will no longer be able to use the object file,
251 because it might be out of date:</para>
252
253 <screen>
254 *Main> :! touch D.hs
255 *Main> :reload
256 Compiling D ( D.hs, interpreted )
257 Ok, modules loaded: A, B, C, D.
258 *Main>
259 </screen>
260
261 <para>Note that module D was compiled, but in this instance
262 because its source hadn't really changed, its interface remained
263 the same, and the recompilation checker determined that A, B and C
264 didn't need to be recompiled.</para>
265
266 <para>So let's try compiling one of the other modules:</para>
267
268 <screen>
269 *Main> :! ghc -c C.hs
270 *Main> :load A
271 Compiling D ( D.hs, interpreted )
272 Compiling B ( B.hs, interpreted )
273 Compiling C ( C.hs, interpreted )
274 Compiling A ( A.hs, interpreted )
275 Ok, modules loaded: A, B, C, D.
276 </screen>
277
278 <para>We didn't get the compiled version of C! What happened?
279 Well, in GHCi a compiled module may only depend on other compiled
280 modules, and in this case C depends on D, which doesn't have an
281 object file, so GHCi also rejected C's object file. Ok, so let's
282 also compile D:</para>
283
284 <screen>
285 *Main> :! ghc -c D.hs
286 *Main> :reload
287 Ok, modules loaded: A, B, C, D.
288 </screen>
289
290 <para>Nothing happened! Here's another lesson: newly compiled
291 modules aren't picked up by <literal>:reload</literal>, only
292 <literal>:load</literal>:</para>
293
294 <screen>
295 *Main> :load A
296 Compiling B ( B.hs, interpreted )
297 Compiling A ( A.hs, interpreted )
298 Ok, modules loaded: A, B, C, D.
299 </screen>
300
301 <para>HINT: since GHCi will only use a compiled object file if it
302 can be sure that the compiled version is up-to-date, a good technique
303 when working on a large program is to occasionally run
304 <literal>ghc &ndash;&ndash;make</literal> to compile the whole project (say
305 before you go for lunch :-), then continue working in the
306 interpreter. As you modify code, the changed modules will be
307 interpreted, but the rest of the project will remain
308 compiled.</para>
309
310 </sect1>
311
312 <sect1 id="interactive-evaluation">
313 <title>Interactive evaluation at the prompt</title>
314
315 <para>When you type an expression at the prompt, GHCi immediately
316 evaluates and prints the result:
317 <screen>
318 Prelude> reverse "hello"
319 "olleh"
320 Prelude> 5+5
321 10
322 </screen>
323 </para>
324
325 <sect2><title>I/O actions at the prompt</title>
326
327 <para>GHCi does more than simple expression evaluation at the prompt.
328 If you type something of type <literal>IO a</literal> for some
329 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
330 as an IO-computation.
331 <screen>
332 Prelude> "hello"
333 "hello"
334 Prelude> putStrLn "hello"
335 hello
336 </screen>
337 Furthermore, GHCi will print the result of the I/O action if (and only
338 if):
339 <itemizedlist>
340 <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
341 <listitem><para>The result type is not
342 <literal>()</literal>.</para></listitem>
343 </itemizedlist>
344 For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
345 <screen>
346 Prelude> putStrLn "hello"
347 hello
348 Prelude> do { putStrLn "hello"; return "yes" }
349 hello
350 "yes"
351 </screen>
352 </para></sect2>
353
354 <sect2 id="ghci-stmts">
355 <title>Using <literal>do-</literal>notation at the prompt</title>
356 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
357 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
358
359 <para>GHCi actually accepts <firstterm>statements</firstterm>
360 rather than just expressions at the prompt. This means you can
361 bind values and functions to names, and use them in future
362 expressions or statements.</para>
363
364 <para>The syntax of a statement accepted at the GHCi prompt is
365 exactly the same as the syntax of a statement in a Haskell
366 <literal>do</literal> expression. However, there's no monad
367 overloading here: statements typed at the prompt must be in the
368 <literal>IO</literal> monad.
369 <screen>
370 Prelude> x &lt;- return 42
371 42
372 Prelude> print x
373 42
374 Prelude>
375 </screen>
376 The statement <literal>x &lt;- return 42</literal> means
377 &ldquo;execute <literal>return 42</literal> in the
378 <literal>IO</literal> monad, and bind the result to
379 <literal>x</literal>&rdquo;. We can then use
380 <literal>x</literal> in future statements, for example to print
381 it as we did above.</para>
382
383 <para>GHCi will print the result of a statement if and only if:
384 <itemizedlist>
385 <listitem>
386 <para>The statement is not a binding, or it is a monadic binding
387 (<literal>p &lt;- e</literal>) that binds exactly one
388 variable.</para>
389 </listitem>
390 <listitem>
391 <para>The variable's type is not polymorphic, is not
392 <literal>()</literal>, and is an instance of
393 <literal>Show</literal></para>
394 </listitem>
395 </itemizedlist>
396 The automatic printing of binding results can be supressed with
397 <option>:set -fno-print-bind-result</option> (this does not
398 supress printing the result of non-binding statements).
399 <indexterm><primary><option>-fno-print-bind-result</option></primary></indexterm><indexterm><primary><option>-fprint-bind-result</option></primary></indexterm>.
400 You might want to do this to prevent the result of binding
401 statements from being fully evaluated by the act of printing
402 them, for example.</para>
403
404 <para>Of course, you can also bind normal non-IO expressions
405 using the <literal>let</literal>-statement:</para>
406 <screen>
407 Prelude> let x = 42
408 Prelude> x
409 42
410 Prelude>
411 </screen>
412 <para>Another important difference between the two types of binding
413 is that the monadic bind (<literal>p &lt;- e</literal>) is
414 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
415 whereas with the <literal>let</literal> form, the expression
416 isn't evaluated immediately:</para>
417 <screen>
418 Prelude> let x = error "help!"
419 Prelude> print x
420 *** Exception: help!
421 Prelude>
422 </screen>
423
424 <para>Note that <literal>let</literal> bindings do not automatically
425 print the value bound, unlike monadic bindings.</para>
426
427 <para>Any exceptions raised during the evaluation or execution
428 of the statement are caught and printed by the GHCi command line
429 interface (for more information on exceptions, see the module
430 <literal>Control.Exception</literal> in the libraries
431 documentation).</para>
432
433 <para>Every new binding shadows any existing bindings of the
434 same name, including entities that are in scope in the current
435 module context.</para>
436
437 <para>WARNING: temporary bindings introduced at the prompt only
438 last until the next <literal>:load</literal> or
439 <literal>:reload</literal> command, at which time they will be
440 simply lost. However, they do survive a change of context with
441 <literal>:module</literal>: the temporary bindings just move to
442 the new location.</para>
443
444 <para>HINT: To get a list of the bindings currently in scope, use the
445 <literal>:show bindings</literal> command:</para>
446
447 <screen>
448 Prelude> :show bindings
449 x :: Int
450 Prelude></screen>
451
452 <para>HINT: if you turn on the <literal>+t</literal> option,
453 GHCi will show the type of each variable bound by a statement.
454 For example:</para>
455 <indexterm><primary><literal>+t</literal></primary></indexterm>
456 <screen>
457 Prelude> :set +t
458 Prelude> let (x:xs) = [1..]
459 x :: Integer
460 xs :: [Integer]
461 </screen>
462
463 </sect2>
464
465 <sect2 id="ghci-scope">
466 <title>What's really in scope at the prompt?</title>
467
468 <para>When you type an expression at the prompt, what
469 identifiers and types are in scope? GHCi provides a flexible
470 way to control exactly how the context for an expression is
471 constructed. Let's start with the simple cases; when you start
472 GHCi the prompt looks like this:</para>
473
474 <screen>Prelude></screen>
475
476 <para>Which indicates that everything from the module
477 <literal>Prelude</literal> is currently in scope. If we now
478 load a file into GHCi, the prompt will change:</para>
479
480 <screen>
481 Prelude> :load Main.hs
482 Compiling Main ( Main.hs, interpreted )
483 *Main>
484 </screen>
485
486 <para>The new prompt is <literal>*Main</literal>, which
487 indicates that we are typing expressions in the context of the
488 top-level of the <literal>Main</literal> module. Everything
489 that is in scope at the top-level in the module
490 <literal>Main</literal> we just loaded is also in scope at the
491 prompt (probably including <literal>Prelude</literal>, as long
492 as <literal>Main</literal> doesn't explicitly hide it).</para>
493
494 <para>The syntax
495 <literal>*<replaceable>module</replaceable></literal> indicates
496 that it is the full top-level scope of
497 <replaceable>module</replaceable> that is contributing to the
498 scope for expressions typed at the prompt. Without the
499 <literal>*</literal>, just the exports of the module are
500 visible.</para>
501
502 <para>We're not limited to a single module: GHCi can combine
503 scopes from multiple modules, in any mixture of
504 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
505 combines the scopes from all of these modules to form the scope
506 that is in effect at the prompt. For technical reasons, GHCi
507 can only support the <literal>*</literal>-form for modules which
508 are interpreted, so compiled modules and package modules can
509 only contribute their exports to the current scope.</para>
510
511 <para>The scope is manipulated using the
512 <literal>:module</literal> command. For example, if the current
513 scope is <literal>Prelude</literal>, then we can bring into
514 scope the exports from the module <literal>IO</literal> like
515 so:</para>
516
517 <screen>
518 Prelude> :module +IO
519 Prelude IO> hPutStrLn stdout "hello\n"
520 hello
521 Prelude IO>
522 </screen>
523
524 <para>(Note: you can use <literal>import M</literal> as an
525 alternative to <literal>:module +M</literal>, and
526 <literal>:module</literal> can also be shortened to
527 <literal>:m</literal>). The full syntax of the
528 <literal>:module</literal> command is:</para>
529
530 <screen>
531 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
532 </screen>
533
534 <para>Using the <literal>+</literal> form of the
535 <literal>module</literal> commands adds modules to the current
536 scope, and <literal>-</literal> removes them. Without either
537 <literal>+</literal> or <literal>-</literal>, the current scope
538 is replaced by the set of modules specified. Note that if you
539 use this form and leave out <literal>Prelude</literal>, GHCi
540 will assume that you really wanted the
541 <literal>Prelude</literal> and add it in for you (if you don't
542 want the <literal>Prelude</literal>, then ask to remove it with
543 <literal>:m -Prelude</literal>).</para>
544
545 <para>The scope is automatically set after a
546 <literal>:load</literal> command, to the most recently loaded
547 "target" module, in a <literal>*</literal>-form if possible.
548 For example, if you say <literal>:load foo.hs bar.hs</literal>
549 and <filename>bar.hs</filename> contains module
550 <literal>Bar</literal>, then the scope will be set to
551 <literal>*Bar</literal> if <literal>Bar</literal> is
552 interpreted, or if <literal>Bar</literal> is compiled it will be
553 set to <literal>Prelude Bar</literal> (GHCi automatically adds
554 <literal>Prelude</literal> if it isn't present and there aren't
555 any <literal>*</literal>-form modules).</para>
556
557 <para>With multiple modules in scope, especially multiple
558 <literal>*</literal>-form modules, it is likely that name
559 clashes will occur. Haskell specifies that name clashes are
560 only reported when an ambiguous identifier is used, and GHCi
561 behaves in the same way for expressions typed at the
562 prompt.</para>
563
564 <para>
565 Hint: GHCi will tab-complete names that are in scope; for
566 example, if you run GHCi and type <literal>J&lt;tab&gt;</literal>
567 then GHCi will expand it to &ldquo;<literal>Just </literal>&rdquo;.
568 </para>
569
570 <sect3>
571 <title>Qualified names</title>
572
573 <para>To make life slightly easier, the GHCi prompt also
574 behaves as if there is an implicit <literal>import
575 qualified</literal> declaration for every module in every
576 package, and every module currently loaded into GHCi.</para>
577 </sect3>
578
579 <sect3>
580 <title>The <literal>:main</literal> command</title>
581
582 <para>
583 When a program is compiled and executed, it can use the
584 <literal>getArgs</literal> function to access the
585 command-line arguments.
586 However, we cannot simply pass the arguments to the
587 <literal>main</literal> function while we are testing in ghci,
588 as the <literal>main</literal> function doesn't take its
589 directly.
590 </para>
591
592 <para>
593 Instead, we can use the <literal>:main</literal> command.
594 This runs whatever <literal>main</literal> is in scope, with
595 any arguments being treated the same as command-line arguments,
596 e.g.:
597 </para>
598
599 <screen>
600 Prelude> let main = System.Environment.getArgs >>= print
601 Prelude> :main foo bar
602 ["foo","bar"]
603 </screen>
604
605 </sect3>
606 </sect2>
607
608
609 <sect2>
610 <title>The <literal>it</literal> variable</title>
611 <indexterm><primary><literal>it</literal></primary>
612 </indexterm>
613
614 <para>Whenever an expression (or a non-binding statement, to be
615 precise) is typed at the prompt, GHCi implicitly binds its value
616 to the variable <literal>it</literal>. For example:</para>
617 <screen>
618 Prelude> 1+2
619 3
620 Prelude> it * 2
621 6
622 </screen>
623 <para>What actually happens is that GHCi typechecks the
624 expression, and if it doesn't have an <literal>IO</literal> type,
625 then it transforms it as follows: an expression
626 <replaceable>e</replaceable> turns into
627 <screen>
628 let it = <replaceable>e</replaceable>;
629 print it
630 </screen>
631 which is then run as an IO-action.</para>
632
633 <para>Hence, the original expression must have a type which is an
634 instance of the <literal>Show</literal> class, or GHCi will
635 complain:</para>
636
637 <screen>
638 Prelude&gt; id
639
640 &lt;interactive&gt;:1:0:
641 No instance for (Show (a -&gt; a))
642 arising from use of `print' at &lt;interactive&gt;:1:0-1
643 Possible fix: add an instance declaration for (Show (a -> a))
644 In the expression: print it
645 In a 'do' expression: print it
646 </screen>
647
648 <para>The error message contains some clues as to the
649 transformation happening internally.</para>
650
651 <para>If the expression was instead of type <literal>IO a</literal> for
652 some <literal>a</literal>, then <literal>it</literal> will be
653 bound to the result of the <literal>IO</literal> computation,
654 which is of type <literal>a</literal>. eg.:</para>
655 <screen>
656 Prelude> Time.getClockTime
657 Wed Mar 14 12:23:13 GMT 2001
658 Prelude> print it
659 Wed Mar 14 12:23:13 GMT 2001
660 </screen>
661
662 <para>The corresponding translation for an IO-typed
663 <replaceable>e</replaceable> is
664 <screen>
665 it &lt;- <replaceable>e</replaceable>
666 </screen>
667 </para>
668
669 <para>Note that <literal>it</literal> is shadowed by the new
670 value each time you evaluate a new expression, and the old value
671 of <literal>it</literal> is lost.</para>
672
673 </sect2>
674
675 <sect2 id="extended-default-rules">
676 <title>Type defaulting in GHCi</title>
677 <indexterm><primary>Type default</primary></indexterm>
678 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
679 <para>
680 Consider this GHCi session:
681 <programlisting>
682 ghci> reverse []
683 </programlisting>
684 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
685 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
686 on the type <literal>a</literal>. For example:
687 <programlisting>
688 ghci> (reverse []) :: String
689 ""
690 ghci> (reverse []) :: [Int]
691 []
692 </programlisting>
693 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
694 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
695 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
696 a)</literal> for each type variable <literal>a</literal>, and defaults the
697 type variable if
698 <orderedlist>
699 <listitem>
700 <para>
701 The type variable <literal>a</literal> appears in no
702 other constraints
703 </para>
704 </listitem>
705 <listitem>
706 <para>
707 All the classes <literal>Ci</literal> are standard.
708 </para>
709 </listitem>
710 <listitem>
711 <para>
712 At least one of the classes <literal>Ci</literal> is
713 numeric.
714 </para>
715 </listitem>
716 </orderedlist>
717 At the GHCi prompt, or with GHC if the
718 <literal>-XExtendedDefaultRules</literal> flag is given,
719 the following additional differences apply:
720 <itemizedlist>
721 <listitem>
722 <para>
723 Rule 2 above is relaxed thus:
724 <emphasis>All</emphasis> of the classes
725 <literal>Ci</literal> are single-parameter type classes.
726 </para>
727 </listitem>
728 <listitem>
729 <para>
730 Rule 3 above is relaxed this:
731 At least one of the classes <literal>Ci</literal> is
732 numeric, <emphasis>or is <literal>Show</literal>,
733 <literal>Eq</literal>, or
734 <literal>Ord</literal></emphasis>.
735 </para>
736 </listitem>
737 <listitem>
738 <para>
739 The unit type <literal>()</literal> is added to the
740 start of the standard list of types which are tried when
741 doing type defaulting.
742 </para>
743 </listitem>
744 </itemizedlist>
745 The last point means that, for example, this program:
746 <programlisting>
747 main :: IO ()
748 main = print def
749
750 instance Num ()
751
752 def :: (Num a, Enum a) => a
753 def = toEnum 0
754 </programlisting>
755 prints <literal>()</literal> rather than <literal>0</literal> as the
756 type is defaulted to <literal>()</literal> rather than
757 <literal>Integer</literal>.
758 </para>
759 <para>
760 The motivation for the change is that it means <literal>IO a</literal>
761 actions default to <literal>IO ()</literal>, which in turn means that
762 ghci won't try to print a result when running them. This is
763 particularly important for <literal>printf</literal>, which has an
764 instance that returns <literal>IO a</literal>.
765 However, it is only able to return
766 <literal>undefined</literal>
767 (the reason for the instance having this type is so that printf
768 doesn't require extensions to the class system), so if the type defaults to
769 <literal>Integer</literal> then ghci gives an error when running a
770 printf.
771 </para>
772 </sect2>
773 </sect1>
774
775 <sect1 id="ghci-debugger">
776 <title>The GHCi Debugger</title>
777 <indexterm><primary>debugger</primary><secondary>in GHCi</secondary>
778 </indexterm>
779
780 <para>GHCi contains a simple imperative-style debugger in which you can
781 stop a running computation in order to examine the values of
782 variables. The debugger is integrated into GHCi, and is turned on by
783 default: no flags are required to enable the debugging facilities. There
784 is one major restriction: breakpoints and single-stepping are only
785 available in <emphasis>interpreted</emphasis> modules; compiled code is
786 invisible to the debugger.</para>
787
788 <para>The debugger provides the following:
789 <itemizedlist>
790 <listitem>
791 <para>The ability to set a <firstterm>breakpoint</firstterm> on a
792 function definition or expression in the program. When the function
793 is called, or the expression evaluated, GHCi suspends
794 execution and returns to the prompt, where you can inspect the
795 values of local variables before continuing with the
796 execution.</para>
797 </listitem>
798 <listitem>
799 <para>Execution can be <firstterm>single-stepped</firstterm>: the
800 evaluator will suspend execution approximately after every
801 reduction, allowing local variables to be inspected. This is
802 equivalent to setting a breakpoint at every point in the
803 program.</para>
804 </listitem>
805 <listitem>
806 <para>Execution can take place in <firstterm>tracing
807 mode</firstterm>, in which the evaluator remembers each
808 evaluation step as it happens, but doesn't suspend execution until
809 an actual breakpoint is reached. When this happens, the history of
810 evaluation steps can be inspected.</para>
811 </listitem>
812 <listitem>
813 <para>Exceptions (e.g. pattern matching failure and
814 <literal>error</literal>) can be treated as breakpoints, to help
815 locate the source of an exception in the program.</para>
816 </listitem>
817 </itemizedlist>
818 </para>
819
820 <para>There is currently no support for obtaining a &ldquo;stack
821 trace&rdquo;, but the tracing and history features provide a useful
822 second-best, which will often be enough to establish the context of an
823 error.</para>
824
825 <sect2 id="breakpoints">
826 <title>Breakpoints and inspecting variables</title>
827
828 <para>Let's use quicksort as a running example. Here's the code:</para>
829
830 <programlisting>
831 qsort [] = []
832 qsort (a:as) = qsort left ++ [a] ++ qsort right
833 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
834
835 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
836 </programlisting>
837
838 <para>First, load the module into GHCi:</para>
839
840 <screen>
841 Prelude> :l qsort.hs
842 [1 of 1] Compiling Main ( qsort.hs, interpreted )
843 Ok, modules loaded: Main.
844 *Main>
845 </screen>
846
847 <para>Now, let's set a breakpoint on the right-hand-side of the second
848 equation of qsort:</para>
849
850 <programlisting>
851 *Main> :break 2
852 Breakpoint 0 activated at qsort.hs:2:15-46
853 *Main>
854 </programlisting>
855
856 <para>The command <literal>:break 2</literal> sets a breakpoint on line
857 2 of the most recently-loaded module, in this case
858 <literal>qsort.hs</literal>. Specifically, it picks the
859 leftmost complete subexpression on that line on which to set the
860 breakpoint, which in this case is the expression
861 <literal>(qsort left ++ [a] ++ qsort right)</literal>.</para>
862
863 <para>Now, we run the program:</para>
864
865 <programlisting>
866 *Main> main
867 Stopped at qsort.hs:2:15-46
868 _result :: [a]
869 a :: a
870 left :: [a]
871 right :: [a]
872 [qsort.hs:2:15-46] *Main>
873 </programlisting>
874
875 <para>Execution has stopped at the breakpoint. The prompt has changed to
876 indicate that we are currently stopped at a breakpoint, and the location:
877 <literal>[qsort.hs:2:15-46]</literal>. To further clarify the
878 location, we can use the <literal>:list</literal> command:</para>
879
880 <programlisting>
881 [qsort.hs:2:15-46] *Main> :list
882 1 qsort [] = []
883 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
884 3 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
885 </programlisting>
886
887 <para>The <literal>:list</literal> command lists the source code around
888 the current breakpoint. If your output device supports it, then GHCi
889 will highlight the active subexpression in bold.</para>
890
891 <para>GHCi has provided bindings for the free variables<footnote><para>We
892 originally provided bindings for all variables in scope, rather
893 than just
894 the free variables of the expression, but found that this affected
895 performance considerably, hence the current restriction to just the
896 free variables.</para>
897 </footnote> of the expression
898 on which the
899 breakpoint was placed (<literal>a</literal>, <literal>left</literal>,
900 <literal>right</literal>), and additionally a binding for the result of
901 the expression (<literal>_result</literal>). These variables are just
902 like other variables that you might define in GHCi; you
903 can use them in expressions that you type at the prompt, you can ask
904 for their types with <literal>:type</literal>, and so on. There is one
905 important difference though: these variables may only have partial
906 types. For example, if we try to display the value of
907 <literal>left</literal>:</para>
908
909 <screen>
910 [qsort.hs:2:15-46] *Main> left
911
912 &lt;interactive&gt;:1:0:
913 Ambiguous type variable `a' in the constraint:
914 `Show a' arising from a use of `print' at &lt;interactive&gt;:1:0-3
915 Cannot resolve unknown runtime types: a
916 Use :print or :force to determine these types
917 </screen>
918
919 <para>This is because <literal>qsort</literal> is a polymorphic function,
920 and because GHCi does not carry type information at runtime, it cannot
921 determine the runtime types of free variables that involve type
922 variables. Hence, when you ask to display <literal>left</literal> at
923 the prompt, GHCi can't figure out which instance of
924 <literal>Show</literal> to use, so it emits the type error above.</para>
925
926 <para>Fortunately, the debugger includes a generic printing command,
927 <literal>:print</literal>, which can inspect the actual runtime value of a
928 variable and attempt to reconstruct its type. If we try it on
929 <literal>left</literal>:</para>
930
931 <screen>
932 [qsort.hs:2:15-46] *Main> :print left
933 left = (_t1::[a])
934 </screen>
935
936 <para>This isn't particularly enlightening. What happened is that
937 <literal>left</literal> is bound to an unevaluated computation (a
938 suspension, or <firstterm>thunk</firstterm>), and
939 <literal>:print</literal> does not force any evaluation. The idea is
940 that <literal>:print</literal> can be used to inspect values at a
941 breakpoint without any unfortunate side effects. It won't force any
942 evaluation, which could cause the program to give a different answer
943 than it would normally, and hence it won't cause any exceptions to be
944 raised, infinite loops, or further breakpoints to be triggered (see
945 <xref linkend="nested-breakpoints" />).
946 Rather than forcing thunks, <literal>:print</literal>
947 binds each thunk to a fresh variable beginning with an
948 underscore, in this case
949 <literal>_t1</literal>.</para>
950
951 <para>If we aren't concerned about preserving the evaluatedness of a
952 variable, we can use <literal>:force</literal> instead of
953 <literal>:print</literal>. The <literal>:force</literal> command
954 behaves exactly like <literal>:print</literal>, except that it forces
955 the evaluation of any thunks it encounters:</para>
956
957 <screen>
958 [qsort.hs:2:15-46] *Main> :force left
959 left = [4,0,3,1]
960 </screen>
961
962 <para>Now, since <literal>:force</literal> has inspected the runtime
963 value of <literal>left</literal>, it has reconstructed its type. We
964 can see the results of this type reconstruction:</para>
965
966 <screen>
967 [qsort.hs:2:15-46] *Main> :show bindings
968 _result :: [Integer]
969 a :: Integer
970 left :: [Integer]
971 right :: [Integer]
972 _t1 :: [Integer]
973 </screen>
974
975 <para>Not only do we now know the type of <literal>left</literal>, but
976 all the other partial types have also been resolved. So we can ask
977 for the value of <literal>a</literal>, for example:</para>
978
979 <screen>
980 [qsort.hs:2:15-46] *Main> a
981 8
982 </screen>
983
984 <para>You might find it useful to use Haskell's
985 <literal>seq</literal> function to evaluate individual thunks rather
986 than evaluating the whole expression with <literal>:force</literal>.
987 For example:</para>
988
989 <screen>
990 [qsort.hs:2:15-46] *Main> :print right
991 right = (_t1::[Integer])
992 [qsort.hs:2:15-46] *Main> seq _t1 ()
993 ()
994 [qsort.hs:2:15-46] *Main> :print right
995 right = 23 : (_t2::[Integer])
996 </screen>
997
998 <para>We evaluated only the <literal>_t1</literal> thunk, revealing the
999 head of the list, and the tail is another thunk now bound to
1000 <literal>_t2</literal>. The <literal>seq</literal> function is a
1001 little inconvenient to use here, so you might want to use
1002 <literal>:def</literal> to make a nicer interface (left as an exercise
1003 for the reader!).</para>
1004
1005 <para>Finally, we can continue the current execution:</para>
1006
1007 <screen>
1008 [qsort.hs:2:15-46] *Main> :continue
1009 Stopped at qsort.hs:2:15-46
1010 _result :: [a]
1011 a :: a
1012 left :: [a]
1013 right :: [a]
1014 [qsort.hs:2:15-46] *Main>
1015 </screen>
1016
1017 <para>The execution continued at the point it previously stopped, and has
1018 now stopped at the breakpoint for a second time.</para>
1019
1020 <sect3 id="setting-breakpoings">
1021 <title>Setting breakpoints</title>
1022
1023 <para>Breakpoints can be set in various ways. Perhaps the easiest way to
1024 set a breakpoint is to name a top-level function:</para>
1025
1026 <screen>
1027 :break <replaceable>identifier</replaceable>
1028 </screen>
1029
1030 <para>Where <replaceable>identifier</replaceable> names any top-level
1031 function in an interpreted module currently loaded into GHCi (qualified
1032 names may be used). The breakpoint will be set on the body of the
1033 function, when it is fully applied but before any pattern matching has
1034 taken place.</para>
1035
1036 <para>Breakpoints can also be set by line (and optionally column)
1037 number:</para>
1038
1039 <screen>
1040 :break <replaceable>line</replaceable>
1041 :break <replaceable>line</replaceable> <replaceable>column</replaceable>
1042 :break <replaceable>module</replaceable> <replaceable>line</replaceable>
1043 :break <replaceable>module</replaceable> <replaceable>line</replaceable> <replaceable>column</replaceable>
1044 </screen>
1045
1046 <para>When a breakpoint is set on a particular line, GHCi sets the
1047 breakpoint on the
1048 leftmost subexpression that begins and ends on that line. If two
1049 complete subexpressions start at the same
1050 column, the longest one is picked. If there is no complete
1051 subexpression on the line, then the leftmost expression starting on
1052 the line is picked, and failing that the rightmost expression that
1053 partially or completely covers the line.</para>
1054
1055 <para>When a breakpoint is set on a particular line and column, GHCi
1056 picks the smallest subexpression that encloses that location on which
1057 to set the breakpoint. Note: GHC considers the TAB character to have a
1058 width of 1, wherever it occurs; in other words it counts
1059 characters, rather than columns. This matches what some editors do,
1060 and doesn't match others. The best advice is to avoid tab
1061 characters in your source code altogether (see
1062 <option>-fwarn-tabs</option> in <xref linkend="options-sanity"
1063 />).</para>
1064
1065 <para>If the module is omitted, then the most recently-loaded module is
1066 used.</para>
1067
1068 <para>Not all subexpressions are potential breakpoint locations. Single
1069 variables are typically not considered to be breakpoint locations
1070 (unless the variable is the right-hand-side of a function definition,
1071 lambda, or case alternative). The rule of thumb is that all redexes
1072 are breakpoint locations, together with the bodies of functions,
1073 lambdas, case alternatives and binding statements. There is normally
1074 no breakpoint on a let expression, but there will always be a
1075 breakpoint on its body, because we are usually interested in inspecting
1076 the values of the variables bound by the let.</para>
1077
1078 </sect3>
1079 <sect3>
1080 <title>Listing and deleting breakpoints</title>
1081
1082 <para>The list of breakpoints currently enabled can be displayed using
1083 <literal>:show&nbsp;breaks</literal>:</para>
1084 <screen>
1085 *Main> :show breaks
1086 [0] Main qsort.hs:1:11-12
1087 [1] Main qsort.hs:2:15-46
1088 </screen>
1089
1090 <para>To delete a breakpoint, use the <literal>:delete</literal>
1091 command with the number given in the output from <literal>:show&nbsp;breaks</literal>:</para>
1092
1093 <screen>
1094 *Main> :delete 0
1095 *Main> :show breaks
1096 [1] Main qsort.hs:2:15-46
1097 </screen>
1098
1099 <para>To delete all breakpoints at once, use <literal>:delete *</literal>.</para>
1100
1101 </sect3>
1102 </sect2>
1103
1104 <sect2 id="single-stepping">
1105 <title>Single-stepping</title>
1106
1107 <para>Single-stepping is a great way to visualise the execution of your
1108 program, and it is also a useful tool for identifying the source of a
1109 bug. The concept is simple: single-stepping enables all the
1110 breakpoints in the program and executes until the next breakpoint is
1111 reached, at which point you can single-step again, or continue
1112 normally. For example:</para>
1113
1114 <screen>
1115 *Main> :step main
1116 Stopped at qsort.hs:5:7-47
1117 _result :: IO ()
1118 </screen>
1119
1120 <para>The command <literal>:step
1121 <replaceable>expr</replaceable></literal> begins the evaluation of
1122 <replaceable>expr</replaceable> in single-stepping mode. If
1123 <replaceable>expr</replaceable> is ommitted, then it single-steps from
1124 the current breakpoint.</para>
1125
1126 <para>The <literal>:list</literal> command is particularly useful when
1127 single-stepping, to see where you currently are:</para>
1128
1129 <screen>
1130 [qsort.hs:5:7-47] *Main> :list
1131 4
1132 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1133 6
1134 [qsort.hs:5:7-47] *Main>
1135 </screen>
1136
1137 <para>In fact, GHCi provides a way to run a command when a breakpoint is
1138 hit, so we can make it automatically do
1139 <literal>:list</literal>:</para>
1140
1141 <screen>
1142 [qsort.hs:5:7-47] *Main> :set stop :list
1143 [qsort.hs:5:7-47] *Main> :step
1144 Stopped at qsort.hs:5:14-46
1145 _result :: [Integer]
1146 4
1147 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1148 6
1149 [qsort.hs:5:14-46] *Main>
1150 </screen>
1151 </sect2>
1152
1153 <sect2 id="nested-breakpoints">
1154 <title>Nested breakpoints</title>
1155 <para>When GHCi is stopped at a breakpoint, and an expression entered at
1156 the prompt triggers a
1157 second breakpoint, the new breakpoint becomes the &ldquo;current&rdquo;
1158 one, and the old one is saved on a stack. An arbitrary number of
1159 breakpoint contexts can be built up in this way. For example:</para>
1160
1161 <screen>
1162 [qsort.hs:2:15-46] *Main> :st qsort [1,3]
1163 Stopped at qsort.hs:(1,0)-(3,55)
1164 _result :: [a]
1165 ... [qsort.hs:(1,0)-(3,55)] *Main>
1166 </screen>
1167
1168 <para>While stopped at the breakpoint on line 2 that we set earlier, we
1169 started a new evaluation with <literal>:step qsort [1,3]</literal>.
1170 This new evaluation stopped after one step (at the definition of
1171 <literal>qsort</literal>). The prompt has changed, now prefixed with
1172 <literal>...</literal>, to indicate that there are saved breakpoints
1173 beyond the current one. To see the stack of contexts, use
1174 <literal>:show context</literal>:</para>
1175
1176 <screen>
1177 ... [qsort.hs:(1,0)-(3,55)] *Main> :show context
1178 --> main
1179 Stopped at qsort.hs:2:15-46
1180 --> qsort [1,3]
1181 Stopped at qsort.hs:(1,0)-(3,55)
1182 ... [qsort.hs:(1,0)-(3,55)] *Main>
1183 </screen>
1184
1185 <para>To abandon the current evaluation, use
1186 <literal>:abandon</literal>:</para>
1187
1188 <screen>
1189 ... [qsort.hs:(1,0)-(3,55)] *Main> :abandon
1190 [qsort.hs:2:15-46] *Main> :abandon
1191 *Main>
1192 </screen>
1193 </sect2>
1194
1195 <sect2 id="ghci-debugger-result">
1196 <title>The <literal>_result</literal> variable</title>
1197 <para>When stopped at a breakpoint or single-step, GHCi binds the
1198 variable <literal>_result</literal> to the value of the currently
1199 active expression. The value of <literal>_result</literal> is
1200 presumably not available yet, because we stopped its evaluation, but it
1201 can be forced: if the type is known and showable, then just entering
1202 <literal>_result</literal> at the prompt will show it. However,
1203 there's one caveat to doing this: evaluating <literal>_result</literal>
1204 will be likely to trigger further breakpoints, starting with the
1205 breakpoint we are currently stopped at (if we stopped at a real
1206 breakpoint, rather than due to <literal>:step</literal>). So it will
1207 probably be necessary to issue a <literal>:continue</literal>
1208 immediately when evaluating <literal>_result</literal>. Alternatively,
1209 you can use <literal>:force</literal> which ignores breakpoints.</para>
1210 </sect2>
1211
1212 <sect2 id="tracing">
1213 <title>Tracing and history</title>
1214
1215 <para>A question that we often want to ask when debugging a program is
1216 &ldquo;how did I get here?&rdquo;. Traditional imperative debuggers
1217 usually provide some kind of stack-tracing feature that lets you see
1218 the stack of active function calls (sometimes called the &ldquo;lexical
1219 call stack&rdquo;), describing a path through the code
1220 to the current location. Unfortunately this is hard to provide in
1221 Haskell, because execution proceeds on a demand-driven basis, rather
1222 than a depth-first basis as in strict languages. The
1223 &ldquo;stack&ldquo; in GHC's execution engine bears little
1224 resemblance to the lexical call stack. Ideally GHCi would maintain a
1225 separate lexical call stack in addition to the dynamic call stack, and
1226 in fact this is exactly
1227 what our profiling system does (<xref linkend="profiling" />), and what
1228 some other Haskell debuggers do. For the time being, however, GHCi
1229 doesn't maintain a lexical call stack (there are some technical
1230 challenges to be overcome). Instead, we provide a way to backtrack from a
1231 breakpoint to previous evaluation steps: essentially this is like
1232 single-stepping backwards, and should in many cases provide enough
1233 information to answer the &ldquo;how did I get here?&rdquo;
1234 question.</para>
1235
1236 <para>To use tracing, evaluate an expression with the
1237 <literal>:trace</literal> command. For example, if we set a breakpoint
1238 on the base case of <literal>qsort</literal>:</para>
1239
1240 <screen>
1241 *Main&gt; :list qsort
1242 1 qsort [] = []
1243 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
1244 3 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
1245 4
1246 *Main&gt; :b 1
1247 Breakpoint 1 activated at qsort.hs:1:11-12
1248 *Main&gt;
1249 </screen>
1250
1251 <para>and then run a small <literal>qsort</literal> with
1252 tracing:</para>
1253
1254 <screen>
1255 *Main> :trace qsort [3,2,1]
1256 Stopped at qsort.hs:1:11-12
1257 _result :: [a]
1258 [qsort.hs:1:11-12] *Main>
1259 </screen>
1260
1261 <para>We can now inspect the history of evaluation steps:</para>
1262
1263 <screen>
1264 [qsort.hs:1:11-12] *Main> :hist
1265 -1 : qsort.hs:3:24-38
1266 -2 : qsort.hs:3:23-55
1267 -3 : qsort.hs:(1,0)-(3,55)
1268 -4 : qsort.hs:2:15-24
1269 -5 : qsort.hs:2:15-46
1270 -6 : qsort.hs:3:24-38
1271 -7 : qsort.hs:3:23-55
1272 -8 : qsort.hs:(1,0)-(3,55)
1273 -9 : qsort.hs:2:15-24
1274 -10 : qsort.hs:2:15-46
1275 -11 : qsort.hs:3:24-38
1276 -12 : qsort.hs:3:23-55
1277 -13 : qsort.hs:(1,0)-(3,55)
1278 -14 : qsort.hs:2:15-24
1279 -15 : qsort.hs:2:15-46
1280 -16 : qsort.hs:(1,0)-(3,55)
1281 &lt;end of history&gt;
1282 </screen>
1283
1284 <para>To examine one of the steps in the history, use
1285 <literal>:back</literal>:</para>
1286
1287 <screen>
1288 [qsort.hs:1:11-12] *Main> :back
1289 Logged breakpoint at qsort.hs:3:24-38
1290 _result :: [a]
1291 as :: [a]
1292 a :: a
1293 [-1: qsort.hs:3:24-38] *Main>
1294 </screen>
1295
1296 <para>Note that the local variables at each step in the history have been
1297 preserved, and can be examined as usual. Also note that the prompt has
1298 changed to indicate that we're currently examining the first step in
1299 the history: <literal>-1</literal>. The command
1300 <literal>:forward</literal> can be used to traverse forward in the
1301 history.</para>
1302
1303 <para>The <literal>:trace</literal> command can be used with or without
1304 an expression. When used without an expression, tracing begins from
1305 the current breakpoint, just like <literal>:step</literal>.</para>
1306
1307 <para>The history is only available when
1308 using <literal>:trace</literal>; the reason for this is we found that
1309 logging each breakpoint in the history cuts performance by a factor of
1310 2 or more. GHCi remembers the last 50 steps in the history (perhaps in
1311 the future we'll make this configurable).</para>
1312 </sect2>
1313
1314 <sect2 id="ghci-debugger-exceptions">
1315 <title>Debugging exceptions</title>
1316 <para>Another common question that comes up when debugging is
1317 &ldquo;where did this exception come from?&rdquo;. Exceptions such as
1318 those raised by <literal>error</literal> or <literal>head []</literal>
1319 have no context information attached to them. Finding which
1320 particular call to <literal>head</literal> in your program resulted in
1321 the error can be a painstaking process, usually involving
1322 <literal>Debug.Trace.trace</literal>, or compiling with
1323 profiling and using <literal>+RTS -xc</literal> (see <xref
1324 linkend="prof-time-options" />).</para>
1325
1326 <para>The GHCi debugger offers a way to hopefully shed some light on
1327 these errors quickly and without modifying or recompiling the source
1328 code. One way would be to set a breakpoint on the location in the
1329 source code that throws the exception, and then use
1330 <literal>:trace</literal> and <literal>:history</literal> to establish
1331 the context. However, <literal>head</literal> is in a library and
1332 we can't set a breakpoint on it directly. For this reason, GHCi
1333 provides the flag <literal>-fbreak-on-exception</literal> which causes
1334 the evaluator to stop when an exception is thrown, just as it does when
1335 a breakpoint is hit. This is only really useful in conjunction with
1336 <literal>:trace</literal>, in order to log the steps leading up to the
1337 exception. For example:</para>
1338
1339 <screen>
1340 *Main> :set -fbreak-on-exception
1341 *Main> :trace qsort ("abc" ++ undefined)
1342 "Stopped at &lt;exception thrown&gt;
1343 _exception :: e
1344 [&lt;exception thrown&gt;] *Main&gt; :hist
1345 -1 : qsort.hs:3:24-38
1346 -2 : qsort.hs:3:23-55
1347 -3 : qsort.hs:(1,0)-(3,55)
1348 -4 : qsort.hs:2:15-24
1349 -5 : qsort.hs:2:15-46
1350 -6 : qsort.hs:(1,0)-(3,55)
1351 &lt;end of history&gt;
1352 [&lt;exception thrown&gt;] *Main&gt; :back
1353 Logged breakpoint at qsort.hs:3:24-38
1354 _result :: [a]
1355 as :: [a]
1356 a :: a
1357 [-1: qsort.hs:3:24-38] *Main&gt; :force as
1358 *** Exception: Prelude.undefined
1359 [-1: qsort.hs:3:24-38] *Main&gt; :print as
1360 as = 'b' : 'c' : (_t1::[Char])
1361 </screen>
1362
1363 <para>The exception itself is bound to a new variable,
1364 <literal>_exception</literal>.</para>
1365
1366 <para>Breaking on exceptions is particularly useful for finding out what
1367 your program was doing when it was in an infinite loop. Just hit
1368 Control-C, and examine the history to find out what was going
1369 on.</para>
1370 </sect2>
1371
1372 <sect2><title>Example: inspecting functions</title>
1373 <para>
1374 It is possible to use the debugger to examine function values.
1375 When we are at a breakpoint and a function is in scope, the debugger
1376 cannot show
1377 you the source code for it; however, it is possible to get some
1378 information by applying it to some arguments and observing the result.
1379 </para>
1380
1381 <para>
1382 The process is slightly complicated when the binding is polymorphic.
1383 We show the process by means of an example.
1384 To keep things simple, we will use the well known <literal>map</literal> function:
1385 <programlisting>
1386 import Prelude hiding (map)
1387
1388 map :: (a->b) -> a -> b
1389 map f [] = []
1390 map f (x:xs) = f x : map f xs
1391 </programlisting>
1392 </para>
1393
1394 <para>
1395 We set a breakpoint on <literal>map</literal>, and call it.
1396 <screen>
1397 *Main> :break 5
1398 Breakpoint 0 activated at map.hs:5:15-28
1399 *Main> map Just [1..5]
1400 Stopped at map.hs:(4,0)-(5,12)
1401 _result :: [b]
1402 x :: a
1403 f :: a -> b
1404 xs :: [a]
1405 </screen>
1406 GHCi tells us that, among other bindings, <literal>f</literal> is in scope.
1407 However, its type is not fully known yet,
1408 and thus it is not possible to apply it to any
1409 arguments. Nevertheless, observe that the type of its first argument is the
1410 same as the type of <literal>x</literal>, and its result type is shared
1411 with <literal>_result</literal>.
1412 </para>
1413
1414 <para>
1415 As we demonstrated earlier (<xref linkend="breakpoints" />), the
1416 debugger has some intelligence built-in to update the type of
1417 <literal>f</literal> whenever the types of <literal>x</literal> or
1418 <literal>_result</literal> are discovered. So what we do in this
1419 scenario is
1420 force <literal>x</literal> a bit, in order to recover both its type
1421 and the argument part of <literal>f</literal>.
1422 <screen>
1423 *Main> seq x ()
1424 *Main> :print x
1425 x = 1
1426 </screen>
1427 </para>
1428 <para>
1429 We can check now that as expected, the type of <literal>x</literal>
1430 has been reconstructed, and with it the
1431 type of <literal>f</literal> has been too:</para>
1432 <screen>
1433 *Main> :t x
1434 x :: Integer
1435 *Main> :t f
1436 f :: Integer -> b
1437 </screen>
1438 <para>
1439 From here, we can apply f to any argument of type Integer and observe
1440 the results.
1441 <screen><![CDATA[
1442 *Main> let b = f 10
1443 *Main> :t b
1444 b :: b
1445 *Main> b
1446 <interactive>:1:0:
1447 Ambiguous type variable `b' in the constraint:
1448 `Show b' arising from a use of `print' at <interactive>:1:0
1449 *Main> :p b
1450 b = (_t2::a)
1451 *Main> seq b ()
1452 ()
1453 *Main> :t b
1454 b :: a
1455 *Main> :p b
1456 b = Just 10
1457 *Main> :t b
1458 b :: Maybe Integer
1459 *Main> :t f
1460 f :: Integer -> Maybe Integer
1461 *Main> f 20
1462 Just 20
1463 *Main> map f [1..5]
1464 [Just 1, Just 2, Just 3, Just 4, Just 5]
1465 ]]></screen>
1466 In the first application of <literal>f</literal>, we had to do
1467 some more type reconstruction
1468 in order to recover the result type of <literal>f</literal>.
1469 But after that, we are free to use
1470 <literal>f</literal> normally.
1471 </para>
1472 </sect2>
1473
1474 <sect2><title>Limitations</title>
1475 <itemizedlist>
1476 <listitem>
1477 <para>When stopped at a breakpoint, if you try to evaluate a variable
1478 that is already under evaluation, the second evaluation will hang.
1479 The reason is
1480 that GHC knows the variable is under evaluation, so the new
1481 evaluation just waits for the result before continuing, but of
1482 course this isn't going to happen because the first evaluation is
1483 stopped at a breakpoint. Control-C can interrupt the hung
1484 evaluation and return to the prompt.</para>
1485 <para>The most common way this can happen is when you're evaluating a
1486 CAF (e.g. main), stop at a breakpoint, and ask for the value of the
1487 CAF at the prompt again.</para>
1488 </listitem>
1489 <listitem><para>
1490 Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
1491 at the scope of a breakpoint if there is an explicit type signature.
1492 </para>
1493 </listitem>
1494 </itemizedlist>
1495 </sect2>
1496 </sect1>
1497
1498 <sect1 id="ghci-invocation">
1499 <title>Invoking GHCi</title>
1500 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
1501 <indexterm><primary><option>&ndash;&ndash;interactive</option></primary></indexterm>
1502
1503 <para>GHCi is invoked with the command <literal>ghci</literal> or
1504 <literal>ghc &ndash;&ndash;interactive</literal>. One or more modules or
1505 filenames can also be specified on the command line; this
1506 instructs GHCi to load the specified modules or filenames (and all
1507 the modules they depend on), just as if you had said
1508 <literal>:load <replaceable>modules</replaceable></literal> at the
1509 GHCi prompt (see <xref linkend="ghci-commands" />). For example, to
1510 start GHCi and load the program whose topmost module is in the
1511 file <literal>Main.hs</literal>, we could say:</para>
1512
1513 <screen>
1514 $ ghci Main.hs
1515 </screen>
1516
1517 <para>Most of the command-line options accepted by GHC (see <xref
1518 linkend="using-ghc"/>) also make sense in interactive mode. The ones
1519 that don't make sense are mostly obvious.</para>
1520
1521 <sect2>
1522 <title>Packages</title>
1523 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
1524
1525 <para>Most packages (see <xref linkend="using-packages"/>) are
1526 available without needing to specify any extra flags at all:
1527 they will be automatically loaded the first time they are
1528 needed.</para>
1529
1530 <para>For hidden packages, however, you need to request the
1531 package be loaded by using the <literal>-package</literal> flag:</para>
1532
1533 <screen>
1534 $ ghci -package readline
1535 GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
1536 Loading package base ... linking ... done.
1537 Loading package readline-1.0 ... linking ... done.
1538 Prelude>
1539 </screen>
1540
1541 <para>The following command works to load new packages into a
1542 running GHCi:</para>
1543
1544 <screen>
1545 Prelude> :set -package <replaceable>name</replaceable>
1546 </screen>
1547
1548 <para>But note that doing this will cause all currently loaded
1549 modules to be unloaded, and you'll be dumped back into the
1550 <literal>Prelude</literal>.</para>
1551 </sect2>
1552
1553 <sect2>
1554 <title>Extra libraries</title>
1555 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
1556
1557 <para>Extra libraries may be specified on the command line using
1558 the normal <literal>-l<replaceable>lib</replaceable></literal>
1559 option. (The term <emphasis>library</emphasis> here refers to
1560 libraries of foreign object code; for using libraries of Haskell
1561 source code, see <xref linkend="ghci-modules-filenames"/>.) For
1562 example, to load the &ldquo;m&rdquo; library:</para>
1563
1564 <screen>
1565 $ ghci -lm
1566 </screen>
1567
1568 <para>On systems with <literal>.so</literal>-style shared
1569 libraries, the actual library loaded will the
1570 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
1571 searches the following places for libraries, in this order:</para>
1572
1573 <itemizedlist>
1574 <listitem>
1575 <para>Paths specified using the
1576 <literal>-L<replaceable>path</replaceable></literal>
1577 command-line option,</para>
1578 </listitem>
1579 <listitem>
1580 <para>the standard library search path for your system,
1581 which on some systems may be overridden by setting the
1582 <literal>LD_LIBRARY_PATH</literal> environment
1583 variable.</para>
1584 </listitem>
1585 </itemizedlist>
1586
1587 <para>On systems with <literal>.dll</literal>-style shared
1588 libraries, the actual library loaded will be
1589 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
1590 GHCi will signal an error if it can't find the library.</para>
1591
1592 <para>GHCi can also load plain object files
1593 (<literal>.o</literal> or <literal>.obj</literal> depending on
1594 your platform) from the command-line. Just add the name the
1595 object file to the command line.</para>
1596
1597 <para>Ordering of <option>-l</option> options matters: a library
1598 should be mentioned <emphasis>before</emphasis> the libraries it
1599 depends on (see <xref linkend="options-linker"/>).</para>
1600 </sect2>
1601
1602 </sect1>
1603
1604 <sect1 id="ghci-commands">
1605 <title>GHCi commands</title>
1606
1607 <para>GHCi commands all begin with
1608 &lsquo;<literal>:</literal>&rsquo; and consist of a single command
1609 name followed by zero or more parameters. The command name may be
1610 abbreviated, with ambiguities being resolved in favour of the more
1611 commonly used commands.</para>
1612
1613 <variablelist>
1614 <varlistentry>
1615 <term>
1616 <literal>:abandon</literal>
1617 <indexterm><primary><literal>:abandon</literal></primary></indexterm>
1618 </term>
1619 <listitem>
1620 <para>Abandons the current evaluation (only available when stopped at
1621 a breakpoint).</para>
1622 </listitem>
1623 </varlistentry>
1624
1625 <varlistentry>
1626 <term>
1627 <literal>:add</literal> <replaceable>module</replaceable> ...
1628 <indexterm><primary><literal>:add</literal></primary></indexterm>
1629 </term>
1630 <listitem>
1631 <para>Add <replaceable>module</replaceable>(s) to the
1632 current <firstterm>target set</firstterm>, and perform a
1633 reload.</para>
1634 </listitem>
1635 </varlistentry>
1636
1637 <varlistentry>
1638 <term>
1639 <literal>:back</literal>
1640 <indexterm><primary><literal>:back</literal></primary></indexterm>
1641 </term>
1642 <listitem>
1643 <para>Travel back one step in the history. See <xref
1644 linkend="tracing" />. See also:
1645 <literal>:trace</literal>, <literal>:history</literal>,
1646 <literal>:forward</literal>.</para>
1647 </listitem>
1648 </varlistentry>
1649
1650 <varlistentry>
1651 <term>
1652 <literal>:break [<replaceable>identifier</replaceable> |
1653 [<replaceable>module</replaceable>] <replaceable>line</replaceable>
1654 [<replaceable>column</replaceable>]]</literal>
1655 </term>
1656 <indexterm><primary><literal>:break</literal></primary></indexterm>
1657 <listitem>
1658 <para>Set a breakpoint on the specified function or line and
1659 column. See <xref linkend="setting-breakpoints" />.</para>
1660 </listitem>
1661 </varlistentry>
1662
1663 <varlistentry>
1664 <term>
1665 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
1666 <indexterm><primary><literal>:browse</literal></primary></indexterm>
1667 </term>
1668 <listitem>
1669 <para>Displays the identifiers defined by the module
1670 <replaceable>module</replaceable>, which must be either
1671 loaded into GHCi or be a member of a package. If the
1672 <literal>*</literal> symbol is placed before the module
1673 name, then <emphasis>all</emphasis> the identifiers defined
1674 in <replaceable>module</replaceable> are shown; otherwise
1675 the list is limited to the exports of
1676 <replaceable>module</replaceable>. The
1677 <literal>*</literal>-form is only available for modules
1678 which are interpreted; for compiled modules (including
1679 modules from packages) only the non-<literal>*</literal>
1680 form of <literal>:browse</literal> is available.</para>
1681 </listitem>
1682 </varlistentry>
1683
1684 <varlistentry>
1685 <term>
1686 <literal>:cd</literal> <replaceable>dir</replaceable>
1687 <indexterm><primary><literal>:cd</literal></primary></indexterm>
1688 </term>
1689 <listitem>
1690 <para>Changes the current working directory to
1691 <replaceable>dir</replaceable>. A
1692 &lsquo;<literal>&tilde;</literal>&rsquo; symbol at the
1693 beginning of <replaceable>dir</replaceable> will be replaced
1694 by the contents of the environment variable
1695 <literal>HOME</literal>.</para>
1696
1697 <para>NOTE: changing directories causes all currently loaded
1698 modules to be unloaded. This is because the search path is
1699 usually expressed using relative directories, and changing
1700 the search path in the middle of a session is not
1701 supported.</para>
1702 </listitem>
1703 </varlistentry>
1704
1705 <varlistentry>
1706 <term>
1707 <literal>:continue</literal>
1708 <indexterm><primary><literal>:continue</literal></primary></indexterm>
1709 </term>
1710 <listitem><para>Continue the current evaluation, when stopped at a
1711 breakpoint.</para>
1712 </listitem>
1713 </varlistentry>
1714
1715 <varlistentry>
1716 <term>
1717 <literal>:cmd</literal> <replaceable>expr</replaceable>
1718 <indexterm><primary><literal>:cmd</literal></primary></indexterm>
1719 </term>
1720 <listitem>
1721 <para>Executes <replaceable>expr</replaceable> as a computation of
1722 type <literal>IO String</literal>, and then executes the resulting
1723 string as a list of GHCi commands. Multiple commands are separated
1724 by newlines. The <literal>:cmd</literal> command is useful with
1725 <literal>:def</literal> and <literal>:set stop</literal>.</para>
1726 </listitem>
1727 </varlistentry>
1728
1729 <varlistentry>
1730 <term>
1731 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1732 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1733 <indexterm><primary><literal>:etags</literal></primary>
1734 </indexterm>
1735 <indexterm><primary><literal>:etags</literal></primary>
1736 </indexterm>
1737 </term>
1738 <listitem>
1739 <para>Generates a &ldquo;tags&rdquo; file for Vi-style editors
1740 (<literal>:ctags</literal>) or Emacs-style editors (<literal>etags</literal>). If
1741 no filename is specified, the defaulit <filename>tags</filename> or
1742 <filename>TAGS</filename> is
1743 used, respectively. Tags for all the functions, constructors and
1744 types in the currently loaded modules are created. All modules must
1745 be interpreted for these commands to work.</para>
1746 <para>See also <xref linkend="hasktags" />.</para>
1747 </listitem>
1748 </varlistentry>
1749
1750 <varlistentry>
1751 <term>
1752 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
1753 <indexterm><primary><literal>:def</literal></primary></indexterm>
1754 </term>
1755 <listitem>
1756 <para>The command <literal>:def</literal>
1757 <replaceable>name</replaceable>
1758 <replaceable>expr</replaceable> defines a new GHCi command
1759 <literal>:<replaceable>name</replaceable></literal>,
1760 implemented by the Haskell expression
1761 <replaceable>expr</replaceable>, which must have type
1762 <literal>String -> IO String</literal>. When
1763 <literal>:<replaceable>name</replaceable>
1764 <replaceable>args</replaceable></literal> is typed at the
1765 prompt, GHCi will run the expression
1766 <literal>(<replaceable>name</replaceable>
1767 <replaceable>args</replaceable>)</literal>, take the
1768 resulting <literal>String</literal>, and feed it back into
1769 GHCi as a new sequence of commands. Separate commands in
1770 the result must be separated by
1771 &lsquo;<literal>\n</literal>&rsquo;.</para>
1772
1773 <para>That's all a little confusing, so here's a few
1774 examples. To start with, here's a new GHCi command which
1775 doesn't take any arguments or produce any results, it just
1776 outputs the current date &amp; time:</para>
1777
1778 <screen>
1779 Prelude> let date _ = Time.getClockTime >>= print >> return ""
1780 Prelude> :def date date
1781 Prelude> :date
1782 Fri Mar 23 15:16:40 GMT 2001
1783 </screen>
1784
1785 <para>Here's an example of a command that takes an argument.
1786 It's a re-implementation of <literal>:cd</literal>:</para>
1787
1788 <screen>
1789 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
1790 Prelude> :def mycd mycd
1791 Prelude> :mycd ..
1792 </screen>
1793
1794 <para>Or I could define a simple way to invoke
1795 &ldquo;<literal>ghc &ndash;&ndash;make Main</literal>&rdquo; in the
1796 current directory:</para>
1797
1798 <screen>
1799 Prelude> :def make (\_ -> return ":! ghc &ndash;&ndash;make Main")
1800 </screen>
1801
1802 <para>We can define a command that reads GHCi input from a
1803 file. This might be useful for creating a set of bindings
1804 that we want to repeatedly load into the GHCi session:</para>
1805
1806 <screen>
1807 Prelude> :def . readFile
1808 Prelude> :. cmds.ghci
1809 </screen>
1810
1811 <para>Notice that we named the command
1812 <literal>:.</literal>, by analogy with the
1813 &lsquo;<literal>.</literal>&rsquo; Unix shell command that
1814 does the same thing.</para>
1815 </listitem>
1816 </varlistentry>
1817
1818 <varlistentry>
1819 <term>
1820 <literal>:delete * | <replaceable>num</replaceable> ...</literal>
1821 <indexterm><primary><literal>:delete</literal></primary></indexterm>
1822 </term>
1823 <listitem>
1824 <para>Delete one or more breakpoints by number (use <literal>:show
1825 breaks</literal> to see the number of each breakpoint). The
1826 <literal>*</literal> form deletes all the breakpoints.</para>
1827 </listitem>
1828 </varlistentry>
1829
1830 <varlistentry>
1831 <term>
1832 <literal>:edit <optional><replaceable>file</replaceable></optional></literal>
1833 <indexterm><primary><literal>:edit</literal></primary></indexterm>
1834 </term>
1835 <listitem>
1836 <para>Opens an editor to edit the file
1837 <replaceable>file</replaceable>, or the most recently loaded
1838 module if <replaceable>file</replaceable> is omitted. The
1839 editor to invoke is taken from the <literal>EDITOR</literal>
1840 environment variable, or a default editor on your system if
1841 <literal>EDITOR</literal> is not set. You can change the
1842 editor using <literal>:set editor</literal>.</para>
1843 </listitem>
1844 </varlistentry>
1845
1846 <varlistentry>
1847 <term>
1848 <literal>:force <replaceable>identifier</replaceable> ...</literal>
1849 <indexterm><primary><literal>:force</literal></primary></indexterm>
1850 </term>
1851 <listitem>
1852 <para>Prints the value of <replaceable>identifier</replaceable> in
1853 the same way as <literal>:print</literal>. Unlike
1854 <literal>:print</literal>, <literal>:force</literal> evaluates each
1855 thunk that it encounters while traversing the value. This may
1856 cause exceptions or infinite loops, or further breakpoints (which
1857 are ignored, but displayed).</para>
1858 </listitem>
1859 </varlistentry>
1860
1861 <varlistentry>
1862 <term>
1863 <literal>:forward</literal>
1864 <indexterm><primary><literal>:forward</literal></primary></indexterm>
1865 </term>
1866 <listitem>
1867 <para>Move forward in the history. See <xref
1868 linkend="tracing" />. See also:
1869 <literal>:trace</literal>, <literal>:history</literal>,
1870 <literal>:back</literal>.</para>
1871 </listitem>
1872 </varlistentry>
1873
1874 <varlistentry>
1875 <term>
1876 <literal>:help</literal>
1877 <indexterm><primary><literal>:help</literal></primary></indexterm>
1878 </term>
1879 <term>
1880 <literal>:?</literal>
1881 <indexterm><primary><literal>:?</literal></primary></indexterm>
1882 </term>
1883 <listitem>
1884 <para>Displays a list of the available commands.</para>
1885 </listitem>
1886 </varlistentry>
1887
1888 <varlistentry>
1889 <term>
1890 <literal>:history [<replaceable>num</replaceable>]</literal>
1891 <indexterm><primary><literal>:history</literal></primary></indexterm>
1892 </term>
1893 <listitem>
1894 <para>Display the history of evaluation steps. With a number,
1895 displays that many steps (default: 20). For use with
1896 <literal>:trace</literal>; see <xref
1897 linkend="tracing" />.</para>
1898 </listitem>
1899 </varlistentry>
1900
1901 <varlistentry>
1902 <term>
1903 <literal>:info</literal> <replaceable>name</replaceable> ...
1904 <indexterm><primary><literal>:info</literal></primary></indexterm>
1905 </term>
1906 <listitem>
1907 <para>Displays information about the given name(s). For
1908 example, if <replaceable>name</replaceable> is a class, then
1909 the class methods and their types will be printed; if
1910 <replaceable>name</replaceable> is a type constructor, then
1911 its definition will be printed; if
1912 <replaceable>name</replaceable> is a function, then its type
1913 will be printed. If <replaceable>name</replaceable> has
1914 been loaded from a source file, then GHCi will also display
1915 the location of its definition in the source.</para>
1916 </listitem>
1917 </varlistentry>
1918
1919 <varlistentry>
1920 <term>
1921 <literal>:kind</literal> <replaceable>type</replaceable>
1922 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1923 </term>
1924 <listitem>
1925 <para>Infers and prints the kind of
1926 <replaceable>type</replaceable>. The latter can be an arbitrary
1927 type expression, including a partial application of a type constructor,
1928 such as <literal>Either Int</literal>.</para>
1929 </listitem>
1930 </varlistentry>
1931
1932 <varlistentry>
1933 <term>
1934 <literal>:load</literal> <replaceable>module</replaceable> ...
1935 <indexterm><primary><literal>:load</literal></primary></indexterm>
1936 </term>
1937 <listitem>
1938 <para>Recursively loads the specified
1939 <replaceable>module</replaceable>s, and all the modules they
1940 depend on. Here, each <replaceable>module</replaceable>
1941 must be a module name or filename, but may not be the name
1942 of a module in a package.</para>
1943
1944 <para>All previously loaded modules, except package modules,
1945 are forgotten. The new set of modules is known as the
1946 <firstterm>target set</firstterm>. Note that
1947 <literal>:load</literal> can be used without any arguments
1948 to unload all the currently loaded modules and
1949 bindings.</para>
1950
1951 <para>After a <literal>:load</literal> command, the current
1952 context is set to:</para>
1953
1954 <itemizedlist>
1955 <listitem>
1956 <para><replaceable>module</replaceable>, if it was loaded
1957 successfully, or</para>
1958 </listitem>
1959 <listitem>
1960 <para>the most recently successfully loaded module, if
1961 any other modules were loaded as a result of the current
1962 <literal>:load</literal>, or</para>
1963 </listitem>
1964 <listitem>
1965 <para><literal>Prelude</literal> otherwise.</para>
1966 </listitem>
1967 </itemizedlist>
1968 </listitem>
1969 </varlistentry>
1970
1971 <varlistentry>
1972 <term>
1973 <literal>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
1974 <indexterm><primary><literal>:main</literal></primary></indexterm>
1975 </term>
1976 <listitem>
1977 <para>
1978 When a program is compiled and executed, it can use the
1979 <literal>getArgs</literal> function to access the
1980 command-line arguments.
1981 However, we cannot simply pass the arguments to the
1982 <literal>main</literal> function while we are testing in ghci,
1983 as the <literal>main</literal> function doesn't take its
1984 directly.
1985 </para>
1986
1987 <para>
1988 Instead, we can use the <literal>:main</literal> command.
1989 This runs whatever <literal>main</literal> is in scope, with
1990 any arguments being treated the same as command-line arguments,
1991 e.g.:
1992 </para>
1993
1994 <screen>
1995 Prelude> let main = System.Environment.getArgs >>= print
1996 Prelude> :main foo bar
1997 ["foo","bar"]
1998 </screen>
1999
2000 </listitem>
2001 </varlistentry>
2002
2003 <varlistentry>
2004 <term>
2005 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
2006 <indexterm><primary><literal>:module</literal></primary></indexterm>
2007 </term>
2008 <term>
2009 <literal>import <replaceable>mod</replaceable></literal>
2010 </term>
2011 <listitem>
2012 <para>Sets or modifies the current context for statements
2013 typed at the prompt. The form <literal>import
2014 <replaceable>mod</replaceable></literal> is equivalent to
2015 <literal>:module +<replaceable>mod</replaceable></literal>.
2016 See <xref linkend="ghci-scope"/> for
2017 more details.</para>
2018 </listitem>
2019 </varlistentry>
2020
2021 <varlistentry>
2022 <term>
2023 <literal>:print </literal> <replaceable>names</replaceable> ...
2024 <indexterm><primary><literal>:print</literal></primary></indexterm>
2025 </term>
2026 <listitem>
2027 <para>Prints a value without forcing its evaluation.
2028 <literal>:print</literal> may be used on values whose types are
2029 unkonwn or partially known, which might be the case for local
2030 variables with polymorphic types at a breakpoint. While inspecting
2031 the runtime value, <literal>:print</literal> attempts to
2032 reconstruct the type of the value, and will elaborate the type in
2033 GHCi's environment if possible. If any unevaluated components
2034 (thunks) are encountered, then <literal>:print</literal> binds
2035 a fresh variable with a name beginning with <literal>_t</literal>
2036 to each thunk. See <xref linkend="breakpoints" /> for more
2037 information. See also the <literal>:sprint</literal> command,
2038 which works like <literal>:print</literal> but does not bind new
2039 variables.</para>
2040 </listitem>
2041 </varlistentry>
2042
2043 <varlistentry>
2044 <term>
2045 <literal>:quit</literal>
2046 <indexterm><primary><literal>:quit</literal></primary></indexterm>
2047 </term>
2048 <listitem>
2049 <para>Quits GHCi. You can also quit by typing a control-D
2050 at the prompt.</para>
2051 </listitem>
2052 </varlistentry>
2053
2054 <varlistentry>
2055 <term>
2056 <literal>:reload</literal>
2057 <indexterm><primary><literal>:reload</literal></primary></indexterm>
2058 </term>
2059 <listitem>
2060 <para>Attempts to reload the current target set (see
2061 <literal>:load</literal>) if any of the modules in the set,
2062 or any dependent module, has changed. Note that this may
2063 entail loading new modules, or dropping modules which are no
2064 longer indirectly required by the target.</para>
2065 </listitem>
2066 </varlistentry>
2067
2068 <varlistentry>
2069 <term>
2070 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
2071 <indexterm><primary><literal>:set</literal></primary></indexterm>
2072 </term>
2073 <listitem>
2074 <para>Sets various options. See <xref linkend="ghci-set"/>
2075 for a list of available options. The
2076 <literal>:set</literal> command by itself shows which
2077 options are currently set.</para>
2078 </listitem>
2079 </varlistentry>
2080
2081 <varlistentry>
2082 <term>
2083 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
2084 <indexterm><primary><literal>:set args</literal></primary></indexterm>
2085 </term>
2086 <listitem>
2087 <para>Sets the list of arguments which are returned when the
2088 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
2089 </indexterm>.</para>
2090 </listitem>
2091 </varlistentry>
2092
2093 <varlistentry>
2094 <term>
2095 <literal>:set</literal> <literal>editor</literal> <replaceable>cmd</replaceable>
2096 </term>
2097 <listitem>
2098 <para>Sets the command used by <literal>:edit</literal> to
2099 <replaceable>cmd</replaceable>.</para>
2100 </listitem>
2101 </varlistentry>
2102
2103 <varlistentry>
2104 <term>
2105 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
2106 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
2107 </term>
2108 <listitem>
2109 <para>Sets the string to be returned when the program calls
2110 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
2111 </indexterm>.</para>
2112 </listitem>
2113 </varlistentry>
2114
2115 <varlistentry>
2116 <term>
2117 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
2118 </term>
2119 <listitem>
2120 <para>Sets the string to be used as the prompt in GHCi.
2121 Inside <replaceable>prompt</replaceable>, the sequence
2122 <literal>%s</literal> is replaced by the names of the
2123 modules currently in scope, and <literal>%%</literal> is
2124 replaced by <literal>%</literal>.</para>
2125 </listitem>
2126 </varlistentry>
2127
2128 <varlistentry>
2129 <term>
2130 <literal>:set</literal> <literal>stop</literal>
2131 [<replaceable>num</replaceable>] <replaceable>cmd</replaceable>
2132 </term>
2133 <listitem>
2134 <para>Set a command to be executed when a breakpoint is hit, or a new
2135 item in the history is selected. The most common use of
2136 <literal>:set stop</literal> is to display the source code at the
2137 current location, e.g. <literal>:set stop :list</literal>.</para>
2138
2139 <para>If a number is given before the command, then the commands are
2140 run when the specified breakpoint (only) is hit. This can be quite
2141 useful: for example, <literal>:set stop 1 :continue</literal>
2142 effectively disables breakpoint 1, by running
2143 <literal>:continue</literal> whenever it is hit (although GHCi will
2144 still emit a message to say the breakpoint was hit). What's more,
2145 with cunning use of <literal>:def</literal> and
2146 <literal>:cmd</literal> you can use <literal>:set stop</literal> to
2147 implement conditional breakpoints:</para>
2148 <screen>
2149 *Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
2150 *Main> :set stop 0 :cond (x &lt; 3)
2151 </screen>
2152 <para>Ignoring breakpoints for a specified number of iterations is
2153 also possible using similar techniques.</para>
2154 </listitem>
2155 </varlistentry>
2156
2157 <varlistentry>
2158 <term>
2159 <literal>:show bindings</literal>
2160 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
2161 </term>
2162 <listitem>
2163 <para>Show the bindings made at the prompt and their
2164 types.</para>
2165 </listitem>
2166 </varlistentry>
2167
2168 <varlistentry>
2169 <term>
2170 <literal>:show breaks</literal>
2171 <indexterm><primary><literal>:show breaks</literal></primary></indexterm>
2172 </term>
2173 <listitem>
2174 <para>List the active breakpoints.</para>
2175 </listitem>
2176 </varlistentry>
2177
2178 <varlistentry>
2179 <term>
2180 <literal>:show context</literal>
2181 <indexterm><primary><literal>:show context</literal></primary></indexterm>
2182 </term>
2183 <listitem>
2184 <para>List the active evaluations that are stopped at breakpoints.</para>
2185 </listitem>
2186 </varlistentry>
2187
2188 <varlistentry>
2189 <term>
2190 <literal>:show modules</literal>
2191 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
2192 </term>
2193 <listitem>
2194 <para>Show the list of modules currently load.</para>
2195 </listitem>
2196 </varlistentry>
2197
2198 <varlistentry>
2199 <term>
2200 <literal>:show [args|prog|prompt|editor|stop]</literal>
2201 <indexterm><primary><literal>:show</literal></primary></indexterm>
2202 </term>
2203 <listitem>
2204 <para>Displays the specified setting (see
2205 <literal>:set</literal>).</para>
2206 </listitem>
2207 </varlistentry>
2208
2209 <varlistentry>
2210 <term>
2211 <literal>:sprint</literal>
2212 <indexterm><primary><literal>:sprint</literal></primary></indexterm>
2213 </term>
2214 <listitem>
2215 <para>Prints a value without forcing its evaluation.
2216 <literal>:sprint</literal> is similar to <literal>:print</literal>,
2217 with the difference that unevaluated subterms are not bound to new
2218 variables, they are simply denoted by &lsquo;_&rsquo;.</para>
2219 </listitem>
2220 </varlistentry>
2221
2222 <varlistentry>
2223 <term>
2224 <literal>:step [<replaceable>expr</replaceable>]</literal>
2225 <indexterm><primary><literal>:step</literal></primary></indexterm>
2226 </term>
2227 <listitem>
2228 <para>Single-step from the last breakpoint. With an expression
2229 argument, begins evaluation of the expression with a
2230 single-step.</para>
2231 </listitem>
2232 </varlistentry>
2233
2234 <varlistentry>
2235 <term>
2236 <literal>:trace [<replaceable>expr</replaceable>]</literal>
2237 <indexterm><primary><literal>:trace</literal></primary></indexterm>
2238 </term>
2239 <listitem>
2240 <para>Evaluates the given expression (or from the last breakpoint if
2241 no expression is given), and additionally logs the evaluation
2242 steps for later inspection using <literal>:history</literal>. See
2243 <xref linkend="tracing" />.</para>
2244 </listitem>
2245 </varlistentry>
2246
2247 <varlistentry>
2248 <term>
2249 <literal>:type</literal> <replaceable>expression</replaceable>
2250 <indexterm><primary><literal>:type</literal></primary></indexterm>
2251 </term>
2252 <listitem>
2253 <para>Infers and prints the type of
2254 <replaceable>expression</replaceable>, including explicit
2255 forall quantifiers for polymorphic types. The monomorphism
2256 restriction is <emphasis>not</emphasis> applied to the
2257 expression during type inference.</para>
2258 </listitem>
2259 </varlistentry>
2260
2261 <varlistentry>
2262 <term>
2263 <literal>:undef</literal> <replaceable>name</replaceable>
2264 <indexterm><primary><literal>:undef</literal></primary></indexterm>
2265 </term>
2266 <listitem>
2267 <para>Undefines the user-defined command
2268 <replaceable>name</replaceable> (see <literal>:def</literal>
2269 above).</para>
2270 </listitem>
2271 </varlistentry>
2272
2273 <varlistentry>
2274 <term>
2275 <literal>:unset</literal> <replaceable>option</replaceable>...
2276 <indexterm><primary><literal>:unset</literal></primary></indexterm>
2277 </term>
2278 <listitem>
2279 <para>Unsets certain options. See <xref linkend="ghci-set"/>
2280 for a list of available options.</para>
2281 </listitem>
2282 </varlistentry>
2283
2284 <varlistentry>
2285 <term>
2286 <literal>:!</literal> <replaceable>command</replaceable>...
2287 <indexterm><primary><literal>:!</literal></primary></indexterm>
2288 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
2289 </term>
2290 <listitem>
2291 <para>Executes the shell command
2292 <replaceable>command</replaceable>.</para>
2293 </listitem>
2294 </varlistentry>
2295
2296 </variablelist>
2297 </sect1>
2298
2299 <sect1 id="ghci-set">
2300 <title>The <literal>:set</literal> command</title>
2301 <indexterm><primary><literal>:set</literal></primary></indexterm>
2302
2303 <para>The <literal>:set</literal> command sets two types of
2304 options: GHCi options, which begin with
2305 &lsquo;<literal>+</literal>&rdquo; and &ldquo;command-line&rdquo;
2306 options, which begin with &lsquo;-&rsquo;. </para>
2307
2308 <para>NOTE: at the moment, the <literal>:set</literal> command
2309 doesn't support any kind of quoting in its arguments: quotes will
2310 not be removed and cannot be used to group words together. For
2311 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
2312 you expect.</para>
2313
2314 <sect2>
2315 <title>GHCi options</title>
2316 <indexterm><primary>options</primary><secondary>GHCi</secondary>
2317 </indexterm>
2318
2319 <para>GHCi options may be set using <literal>:set</literal> and
2320 unset using <literal>:unset</literal>.</para>
2321
2322 <para>The available GHCi options are:</para>
2323
2324 <variablelist>
2325 <varlistentry>
2326 <term>
2327 <literal>+r</literal>
2328 <indexterm><primary><literal>+r</literal></primary></indexterm>
2329 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
2330 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
2331 </term>
2332 <listitem>
2333 <para>Normally, any evaluation of top-level expressions
2334 (otherwise known as CAFs or Constant Applicative Forms) in
2335 loaded modules is retained between evaluations. Turning
2336 on <literal>+r</literal> causes all evaluation of
2337 top-level expressions to be discarded after each
2338 evaluation (they are still retained
2339 <emphasis>during</emphasis> a single evaluation).</para>
2340
2341 <para>This option may help if the evaluated top-level
2342 expressions are consuming large amounts of space, or if
2343 you need repeatable performance measurements.</para>
2344 </listitem>
2345 </varlistentry>
2346
2347 <varlistentry>
2348 <term>
2349 <literal>+s</literal>
2350 <indexterm><primary><literal>+s</literal></primary></indexterm>
2351 </term>
2352 <listitem>
2353 <para>Display some stats after evaluating each expression,
2354 including the elapsed time and number of bytes allocated.
2355 NOTE: the allocation figure is only accurate to the size
2356 of the storage manager's allocation area, because it is
2357 calculated at every GC. Hence, you might see values of
2358 zero if no GC has occurred.</para>
2359 </listitem>
2360 </varlistentry>
2361
2362 <varlistentry>
2363 <term>
2364 <literal>+t</literal>
2365 <indexterm><primary><literal>+t</literal></primary></indexterm>
2366 </term>
2367 <listitem>
2368 <para>Display the type of each variable bound after a
2369 statement is entered at the prompt. If the statement is a
2370 single expression, then the only variable binding will be
2371 for the variable
2372 &lsquo;<literal>it</literal>&rsquo;.</para>
2373 </listitem>
2374 </varlistentry>
2375 </variablelist>
2376 </sect2>
2377
2378 <sect2 id="ghci-cmd-line-options">
2379 <title>Setting GHC command-line options in GHCi</title>
2380
2381 <para>Normal GHC command-line options may also be set using
2382 <literal>:set</literal>. For example, to turn on
2383 <option>-fglasgow-exts</option>, you would say:</para>
2384
2385 <screen>
2386 Prelude> :set -fglasgow-exts
2387 </screen>
2388
2389 <para>Any GHC command-line option that is designated as
2390 <firstterm>dynamic</firstterm> (see the table in <xref
2391 linkend="flag-reference"/>), may be set using
2392 <literal>:set</literal>. To unset an option, you can set the
2393 reverse option:</para>
2394 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
2395
2396 <screen>
2397 Prelude> :set -fno-glasgow-exts
2398 </screen>
2399
2400 <para><xref linkend="flag-reference"/> lists the reverse for each
2401 option where applicable.</para>
2402
2403 <para>Certain static options (<option>-package</option>,
2404 <option>-I</option>, <option>-i</option>, and
2405 <option>-l</option> in particular) will also work, but some may
2406 not take effect until the next reload.</para>
2407 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
2408 </sect2>
2409 </sect1>
2410 <sect1 id="ghci-dot-files">
2411 <title>The <filename>.ghci</filename> file</title>
2412 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
2413 </indexterm>
2414 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
2415 </indexterm>
2416
2417 <para>When it starts, GHCi always reads and executes commands from
2418 <filename>$HOME/.ghci</filename>, followed by
2419 <filename>./.ghci</filename>.</para>
2420
2421 <para>The <filename>.ghci</filename> in your home directory is
2422 most useful for turning on favourite options (eg. <literal>:set
2423 +s</literal>), and defining useful macros. Placing a
2424 <filename>.ghci</filename> file in a directory with a Haskell
2425 project is a useful way to set certain project-wide options so you
2426 don't have to type them everytime you start GHCi: eg. if your
2427 project uses GHC extensions and CPP, and has source files in three
2428 subdirectories A B and C, you might put the following lines in
2429 <filename>.ghci</filename>:</para>
2430
2431 <screen>
2432 :set -fglasgow-exts -cpp
2433 :set -iA:B:C
2434 </screen>
2435
2436 <para>(Note that strictly speaking the <option>-i</option> flag is
2437 a static one, but in fact it works to set it using
2438 <literal>:set</literal> like this. The changes won't take effect
2439 until the next <literal>:load</literal>, though.)</para>
2440
2441 <para>Two command-line options control whether the
2442 <filename>.ghci</filename> files are read:</para>
2443
2444 <variablelist>
2445 <varlistentry>
2446 <term>
2447 <option>-ignore-dot-ghci</option>
2448 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
2449 </term>
2450 <listitem>
2451 <para>Don't read either <filename>./.ghci</filename> or
2452 <filename>$HOME/.ghci</filename> when starting up.</para>
2453 </listitem>
2454 </varlistentry>
2455 <varlistentry>
2456 <term>
2457 <option>-read-dot-ghci</option>
2458 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
2459 </term>
2460 <listitem>
2461 <para>Read <filename>.ghci</filename> and
2462 <filename>$HOME/.ghci</filename>. This is normally the
2463 default, but the <option>-read-dot-ghci</option> option may
2464 be used to override a previous
2465 <option>-ignore-dot-ghci</option> option.</para>
2466 </listitem>
2467 </varlistentry>
2468 </variablelist>
2469
2470 </sect1>
2471
2472 <sect1 id="ghci-obj">
2473 <title>Compiling to object code inside GHCi</title>
2474
2475 <para>By default, GHCi compiles Haskell source code into byte-code
2476 that is interpreted by the runtime system. GHCi can also compile
2477 Haskell code to object code: to turn on this feature, use the
2478 <option>-fobject-code</option> flag either on the command line or
2479 with <literal>:set</literal> (the option
2480 <option>-fbyte-code</option> restores byte-code compilation
2481 again). Compiling to object code takes longer, but typically the
2482 code will execute 10-20 times faster than byte-code.</para>
2483
2484 <para>Compiling to object code inside GHCi is particularly useful
2485 if you are developing a compiled application, because the
2486 <literal>:reload</literal> command typically runs much faster than
2487 restarting GHC with <option>--make</option> from the command-line,
2488 because all the interface files are already cached in
2489 memory.</para>
2490
2491 <para>There are disadvantages to compiling to object-code: you
2492 can't set breakpoints in object-code modules, for example. Only
2493 the exports of an object-code module will be visible in GHCi,
2494 rather than all top-level bindings as in interpreted
2495 modules.</para>
2496 </sect1>
2497
2498 <sect1 id="ghci-faq">
2499 <title>FAQ and Things To Watch Out For</title>
2500
2501 <variablelist>
2502 <varlistentry>
2503 <term>The interpreter can't load modules with foreign export
2504 declarations!</term>
2505 <listitem>
2506 <para>Unfortunately not. We haven't implemented it yet.
2507 Please compile any offending modules by hand before loading
2508 them into GHCi.</para>
2509 </listitem>
2510 </varlistentry>
2511
2512 <varlistentry>
2513 <term>
2514 <literal>-O</literal> doesn't work with GHCi!
2515 <indexterm><primary><option>-O</option></primary></indexterm>
2516 </term>
2517 <listitem>
2518 <para>For technical reasons, the bytecode compiler doesn't
2519 interact well with one of the optimisation passes, so we
2520 have disabled optimisation when using the interpreter. This
2521 isn't a great loss: you'll get a much bigger win by
2522 compiling the bits of your code that need to go fast, rather
2523 than interpreting them with optimisation turned on.</para>
2524 </listitem>
2525 </varlistentry>
2526
2527 <varlistentry>
2528 <term>Unboxed tuples don't work with GHCi</term>
2529 <listitem>
2530 <para>That's right. You can always compile a module that
2531 uses unboxed tuples and load it into GHCi, however.
2532 (Incidentally the previous point, namely that
2533 <literal>-O</literal> is incompatible with GHCi, is because
2534 the bytecode compiler can't deal with unboxed
2535 tuples).</para>
2536 </listitem>
2537 </varlistentry>
2538
2539 <varlistentry>
2540 <term>Concurrent threads don't carry on running when GHCi is
2541 waiting for input.</term>
2542 <listitem>
2543 <para>This should work, as long as your GHCi was built with
2544 the <option>-threaded</option> switch, which is the default.
2545 Consult whoever supplied your GHCi installation.</para>
2546 </listitem>
2547 </varlistentry>
2548
2549 <varlistentry>
2550 <term>After using <literal>getContents</literal>, I can't use
2551 <literal>stdin</literal> again until I do
2552 <literal>:load</literal> or <literal>:reload</literal>.</term>
2553
2554 <listitem>
2555 <para>This is the defined behaviour of
2556 <literal>getContents</literal>: it puts the stdin Handle in
2557 a state known as <firstterm>semi-closed</firstterm>, wherein
2558 any further I/O operations on it are forbidden. Because I/O
2559 state is retained between computations, the semi-closed
2560 state persists until the next <literal>:load</literal> or
2561 <literal>:reload</literal> command.</para>
2562
2563 <para>You can make <literal>stdin</literal> reset itself
2564 after every evaluation by giving GHCi the command
2565 <literal>:set +r</literal>. This works because
2566 <literal>stdin</literal> is just a top-level expression that
2567 can be reverted to its unevaluated state in the same way as
2568 any other top-level expression (CAF).</para>
2569 </listitem>
2570 </varlistentry>
2571
2572 <varlistentry>
2573 <term>I can't use Control-C to interrupt computations in
2574 GHCi on Windows.</term>
2575 <listitem>
2576 <para>See <xref linkend="ghci-windows"/></para>
2577 </listitem>
2578 </varlistentry>
2579 </variablelist>
2580 </sect1>
2581
2582 </chapter>
2583
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