GHCi debugger: Added a -fprint-evld-with-show flag
[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> :set -fprint-evld-with-show
933 [qsort.hs:2:15-46] *Main> :print left
934 left = (_t1::[a])
935 </screen>
936
937 <para>This isn't particularly enlightening. What happened is that
938 <literal>left</literal> is bound to an unevaluated computation (a
939 suspension, or <firstterm>thunk</firstterm>), and
940 <literal>:print</literal> does not force any evaluation. The idea is
941 that <literal>:print</literal> can be used to inspect values at a
942 breakpoint without any unfortunate side effects. It won't force any
943 evaluation, which could cause the program to give a different answer
944 than it would normally, and hence it won't cause any exceptions to be
945 raised, infinite loops, or further breakpoints to be triggered (see
946 <xref linkend="nested-breakpoints" />).
947 Rather than forcing thunks, <literal>:print</literal>
948 binds each thunk to a fresh variable beginning with an
949 underscore, in this case
950 <literal>_t1</literal>.</para>
951
952 <para>The flag <literal>-fprint-evld-with-show</literal> instructs
953 <literal>:print</literal> to reuse
954 available <literal>Show</literal> instances when possible. This happens
955 only when the contents of the variable being inspected
956 are completely evaluated.</para>
957
958
959 <para>If we aren't concerned about preserving the evaluatedness of a
960 variable, we can use <literal>:force</literal> instead of
961 <literal>:print</literal>. The <literal>:force</literal> command
962 behaves exactly like <literal>:print</literal>, except that it forces
963 the evaluation of any thunks it encounters:</para>
964
965 <screen>
966 [qsort.hs:2:15-46] *Main> :force left
967 left = [4,0,3,1]
968 </screen>
969
970 <para>Now, since <literal>:force</literal> has inspected the runtime
971 value of <literal>left</literal>, it has reconstructed its type. We
972 can see the results of this type reconstruction:</para>
973
974 <screen>
975 [qsort.hs:2:15-46] *Main> :show bindings
976 _result :: [Integer]
977 a :: Integer
978 left :: [Integer]
979 right :: [Integer]
980 _t1 :: [Integer]
981 </screen>
982
983 <para>Not only do we now know the type of <literal>left</literal>, but
984 all the other partial types have also been resolved. So we can ask
985 for the value of <literal>a</literal>, for example:</para>
986
987 <screen>
988 [qsort.hs:2:15-46] *Main> a
989 8
990 </screen>
991
992 <para>You might find it useful to use Haskell's
993 <literal>seq</literal> function to evaluate individual thunks rather
994 than evaluating the whole expression with <literal>:force</literal>.
995 For example:</para>
996
997 <screen>
998 [qsort.hs:2:15-46] *Main> :print right
999 right = (_t1::[Integer])
1000 [qsort.hs:2:15-46] *Main> seq _t1 ()
1001 ()
1002 [qsort.hs:2:15-46] *Main> :print right
1003 right = 23 : (_t2::[Integer])
1004 </screen>
1005
1006 <para>We evaluated only the <literal>_t1</literal> thunk, revealing the
1007 head of the list, and the tail is another thunk now bound to
1008 <literal>_t2</literal>. The <literal>seq</literal> function is a
1009 little inconvenient to use here, so you might want to use
1010 <literal>:def</literal> to make a nicer interface (left as an exercise
1011 for the reader!).</para>
1012
1013 <para>Finally, we can continue the current execution:</para>
1014
1015 <screen>
1016 [qsort.hs:2:15-46] *Main> :continue
1017 Stopped at qsort.hs:2:15-46
1018 _result :: [a]
1019 a :: a
1020 left :: [a]
1021 right :: [a]
1022 [qsort.hs:2:15-46] *Main>
1023 </screen>
1024
1025 <para>The execution continued at the point it previously stopped, and has
1026 now stopped at the breakpoint for a second time.</para>
1027
1028
1029 <sect3 id="setting-breakpoints">
1030 <title>Setting breakpoints</title>
1031
1032 <para>Breakpoints can be set in various ways. Perhaps the easiest way to
1033 set a breakpoint is to name a top-level function:</para>
1034
1035 <screen>
1036 :break <replaceable>identifier</replaceable>
1037 </screen>
1038
1039 <para>Where <replaceable>identifier</replaceable> names any top-level
1040 function in an interpreted module currently loaded into GHCi (qualified
1041 names may be used). The breakpoint will be set on the body of the
1042 function, when it is fully applied but before any pattern matching has
1043 taken place.</para>
1044
1045 <para>Breakpoints can also be set by line (and optionally column)
1046 number:</para>
1047
1048 <screen>
1049 :break <replaceable>line</replaceable>
1050 :break <replaceable>line</replaceable> <replaceable>column</replaceable>
1051 :break <replaceable>module</replaceable> <replaceable>line</replaceable>
1052 :break <replaceable>module</replaceable> <replaceable>line</replaceable> <replaceable>column</replaceable>
1053 </screen>
1054
1055 <para>When a breakpoint is set on a particular line, GHCi sets the
1056 breakpoint on the
1057 leftmost subexpression that begins and ends on that line. If two
1058 complete subexpressions start at the same
1059 column, the longest one is picked. If there is no complete
1060 subexpression on the line, then the leftmost expression starting on
1061 the line is picked, and failing that the rightmost expression that
1062 partially or completely covers the line.</para>
1063
1064 <para>When a breakpoint is set on a particular line and column, GHCi
1065 picks the smallest subexpression that encloses that location on which
1066 to set the breakpoint. Note: GHC considers the TAB character to have a
1067 width of 1, wherever it occurs; in other words it counts
1068 characters, rather than columns. This matches what some editors do,
1069 and doesn't match others. The best advice is to avoid tab
1070 characters in your source code altogether (see
1071 <option>-fwarn-tabs</option> in <xref linkend="options-sanity"
1072 />).</para>
1073
1074 <para>If the module is omitted, then the most recently-loaded module is
1075 used.</para>
1076
1077 <para>Not all subexpressions are potential breakpoint locations. Single
1078 variables are typically not considered to be breakpoint locations
1079 (unless the variable is the right-hand-side of a function definition,
1080 lambda, or case alternative). The rule of thumb is that all redexes
1081 are breakpoint locations, together with the bodies of functions,
1082 lambdas, case alternatives and binding statements. There is normally
1083 no breakpoint on a let expression, but there will always be a
1084 breakpoint on its body, because we are usually interested in inspecting
1085 the values of the variables bound by the let.</para>
1086
1087 </sect3>
1088 <sect3>
1089 <title>Listing and deleting breakpoints</title>
1090
1091 <para>The list of breakpoints currently enabled can be displayed using
1092 <literal>:show&nbsp;breaks</literal>:</para>
1093 <screen>
1094 *Main> :show breaks
1095 [0] Main qsort.hs:1:11-12
1096 [1] Main qsort.hs:2:15-46
1097 </screen>
1098
1099 <para>To delete a breakpoint, use the <literal>:delete</literal>
1100 command with the number given in the output from <literal>:show&nbsp;breaks</literal>:</para>
1101
1102 <screen>
1103 *Main> :delete 0
1104 *Main> :show breaks
1105 [1] Main qsort.hs:2:15-46
1106 </screen>
1107
1108 <para>To delete all breakpoints at once, use <literal>:delete *</literal>.</para>
1109
1110 </sect3>
1111 </sect2>
1112
1113 <sect2 id="single-stepping">
1114 <title>Single-stepping</title>
1115
1116 <para>Single-stepping is a great way to visualise the execution of your
1117 program, and it is also a useful tool for identifying the source of a
1118 bug. GHCi offers two variants of stepping. Use
1119 <literal>:step</literal> to enable all the
1120 breakpoints in the program, and execute until the next breakpoint is
1121 reached. Use <literal>:steplocal</literal> to limit the set
1122 of enabled breakpoints to those in the current top level function.
1123 Similarly, use <literal>:stepmodule</literal> to single step only on
1124 breakpoints contained in the current module.
1125 For example:</para>
1126
1127 <screen>
1128 *Main> :step main
1129 Stopped at qsort.hs:5:7-47
1130 _result :: IO ()
1131 </screen>
1132
1133 <para>The command <literal>:step
1134 <replaceable>expr</replaceable></literal> begins the evaluation of
1135 <replaceable>expr</replaceable> in single-stepping mode. If
1136 <replaceable>expr</replaceable> is ommitted, then it single-steps from
1137 the current breakpoint. <literal>:stepover</literal>
1138 works similarly.</para>
1139
1140 <para>The <literal>:list</literal> command is particularly useful when
1141 single-stepping, to see where you currently are:</para>
1142
1143 <screen>
1144 [qsort.hs:5:7-47] *Main> :list
1145 4
1146 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1147 6
1148 [qsort.hs:5:7-47] *Main>
1149 </screen>
1150
1151 <para>In fact, GHCi provides a way to run a command when a breakpoint is
1152 hit, so we can make it automatically do
1153 <literal>:list</literal>:</para>
1154
1155 <screen>
1156 [qsort.hs:5:7-47] *Main> :set stop :list
1157 [qsort.hs:5:7-47] *Main> :step
1158 Stopped at qsort.hs:5:14-46
1159 _result :: [Integer]
1160 4
1161 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1162 6
1163 [qsort.hs:5:14-46] *Main>
1164 </screen>
1165 </sect2>
1166
1167 <sect2 id="nested-breakpoints">
1168 <title>Nested breakpoints</title>
1169 <para>When GHCi is stopped at a breakpoint, and an expression entered at
1170 the prompt triggers a
1171 second breakpoint, the new breakpoint becomes the &ldquo;current&rdquo;
1172 one, and the old one is saved on a stack. An arbitrary number of
1173 breakpoint contexts can be built up in this way. For example:</para>
1174
1175 <screen>
1176 [qsort.hs:2:15-46] *Main> :st qsort [1,3]
1177 Stopped at qsort.hs:(1,0)-(3,55)
1178 _result :: [a]
1179 ... [qsort.hs:(1,0)-(3,55)] *Main>
1180 </screen>
1181
1182 <para>While stopped at the breakpoint on line 2 that we set earlier, we
1183 started a new evaluation with <literal>:step qsort [1,3]</literal>.
1184 This new evaluation stopped after one step (at the definition of
1185 <literal>qsort</literal>). The prompt has changed, now prefixed with
1186 <literal>...</literal>, to indicate that there are saved breakpoints
1187 beyond the current one. To see the stack of contexts, use
1188 <literal>:show context</literal>:</para>
1189
1190 <screen>
1191 ... [qsort.hs:(1,0)-(3,55)] *Main> :show context
1192 --> main
1193 Stopped at qsort.hs:2:15-46
1194 --> qsort [1,3]
1195 Stopped at qsort.hs:(1,0)-(3,55)
1196 ... [qsort.hs:(1,0)-(3,55)] *Main>
1197 </screen>
1198
1199 <para>To abandon the current evaluation, use
1200 <literal>:abandon</literal>:</para>
1201
1202 <screen>
1203 ... [qsort.hs:(1,0)-(3,55)] *Main> :abandon
1204 [qsort.hs:2:15-46] *Main> :abandon
1205 *Main>
1206 </screen>
1207 </sect2>
1208
1209 <sect2 id="ghci-debugger-result">
1210 <title>The <literal>_result</literal> variable</title>
1211 <para>When stopped at a breakpoint or single-step, GHCi binds the
1212 variable <literal>_result</literal> to the value of the currently
1213 active expression. The value of <literal>_result</literal> is
1214 presumably not available yet, because we stopped its evaluation, but it
1215 can be forced: if the type is known and showable, then just entering
1216 <literal>_result</literal> at the prompt will show it. However,
1217 there's one caveat to doing this: evaluating <literal>_result</literal>
1218 will be likely to trigger further breakpoints, starting with the
1219 breakpoint we are currently stopped at (if we stopped at a real
1220 breakpoint, rather than due to <literal>:step</literal>). So it will
1221 probably be necessary to issue a <literal>:continue</literal>
1222 immediately when evaluating <literal>_result</literal>. Alternatively,
1223 you can use <literal>:force</literal> which ignores breakpoints.</para>
1224 </sect2>
1225
1226 <sect2 id="tracing">
1227 <title>Tracing and history</title>
1228
1229 <para>A question that we often want to ask when debugging a program is
1230 &ldquo;how did I get here?&rdquo;. Traditional imperative debuggers
1231 usually provide some kind of stack-tracing feature that lets you see
1232 the stack of active function calls (sometimes called the &ldquo;lexical
1233 call stack&rdquo;), describing a path through the code
1234 to the current location. Unfortunately this is hard to provide in
1235 Haskell, because execution proceeds on a demand-driven basis, rather
1236 than a depth-first basis as in strict languages. The
1237 &ldquo;stack&ldquo; in GHC's execution engine bears little
1238 resemblance to the lexical call stack. Ideally GHCi would maintain a
1239 separate lexical call stack in addition to the dynamic call stack, and
1240 in fact this is exactly
1241 what our profiling system does (<xref linkend="profiling" />), and what
1242 some other Haskell debuggers do. For the time being, however, GHCi
1243 doesn't maintain a lexical call stack (there are some technical
1244 challenges to be overcome). Instead, we provide a way to backtrack from a
1245 breakpoint to previous evaluation steps: essentially this is like
1246 single-stepping backwards, and should in many cases provide enough
1247 information to answer the &ldquo;how did I get here?&rdquo;
1248 question.</para>
1249
1250 <para>To use tracing, evaluate an expression with the
1251 <literal>:trace</literal> command. For example, if we set a breakpoint
1252 on the base case of <literal>qsort</literal>:</para>
1253
1254 <screen>
1255 *Main&gt; :list qsort
1256 1 qsort [] = []
1257 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
1258 3 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
1259 4
1260 *Main&gt; :b 1
1261 Breakpoint 1 activated at qsort.hs:1:11-12
1262 *Main&gt;
1263 </screen>
1264
1265 <para>and then run a small <literal>qsort</literal> with
1266 tracing:</para>
1267
1268 <screen>
1269 *Main> :trace qsort [3,2,1]
1270 Stopped at qsort.hs:1:11-12
1271 _result :: [a]
1272 [qsort.hs:1:11-12] *Main>
1273 </screen>
1274
1275 <para>We can now inspect the history of evaluation steps:</para>
1276
1277 <screen>
1278 [qsort.hs:1:11-12] *Main> :hist
1279 -1 : qsort.hs:3:24-38
1280 -2 : qsort.hs:3:23-55
1281 -3 : qsort.hs:(1,0)-(3,55)
1282 -4 : qsort.hs:2:15-24
1283 -5 : qsort.hs:2:15-46
1284 -6 : qsort.hs:3:24-38
1285 -7 : qsort.hs:3:23-55
1286 -8 : qsort.hs:(1,0)-(3,55)
1287 -9 : qsort.hs:2:15-24
1288 -10 : qsort.hs:2:15-46
1289 -11 : qsort.hs:3:24-38
1290 -12 : qsort.hs:3:23-55
1291 -13 : qsort.hs:(1,0)-(3,55)
1292 -14 : qsort.hs:2:15-24
1293 -15 : qsort.hs:2:15-46
1294 -16 : qsort.hs:(1,0)-(3,55)
1295 &lt;end of history&gt;
1296 </screen>
1297
1298 <para>To examine one of the steps in the history, use
1299 <literal>:back</literal>:</para>
1300
1301 <screen>
1302 [qsort.hs:1:11-12] *Main> :back
1303 Logged breakpoint at qsort.hs:3:24-38
1304 _result :: [a]
1305 as :: [a]
1306 a :: a
1307 [-1: qsort.hs:3:24-38] *Main>
1308 </screen>
1309
1310 <para>Note that the local variables at each step in the history have been
1311 preserved, and can be examined as usual. Also note that the prompt has
1312 changed to indicate that we're currently examining the first step in
1313 the history: <literal>-1</literal>. The command
1314 <literal>:forward</literal> can be used to traverse forward in the
1315 history.</para>
1316
1317 <para>The <literal>:trace</literal> command can be used with or without
1318 an expression. When used without an expression, tracing begins from
1319 the current breakpoint, just like <literal>:step</literal>.</para>
1320
1321 <para>The history is only available when
1322 using <literal>:trace</literal>; the reason for this is we found that
1323 logging each breakpoint in the history cuts performance by a factor of
1324 2 or more. GHCi remembers the last 50 steps in the history (perhaps in
1325 the future we'll make this configurable).</para>
1326 </sect2>
1327
1328 <sect2 id="ghci-debugger-exceptions">
1329 <title>Debugging exceptions</title>
1330 <para>Another common question that comes up when debugging is
1331 &ldquo;where did this exception come from?&rdquo;. Exceptions such as
1332 those raised by <literal>error</literal> or <literal>head []</literal>
1333 have no context information attached to them. Finding which
1334 particular call to <literal>head</literal> in your program resulted in
1335 the error can be a painstaking process, usually involving
1336 <literal>Debug.Trace.trace</literal>, or compiling with
1337 profiling and using <literal>+RTS -xc</literal> (see <xref
1338 linkend="prof-time-options" />).</para>
1339
1340 <para>The GHCi debugger offers a way to hopefully shed some light on
1341 these errors quickly and without modifying or recompiling the source
1342 code. One way would be to set a breakpoint on the location in the
1343 source code that throws the exception, and then use
1344 <literal>:trace</literal> and <literal>:history</literal> to establish
1345 the context. However, <literal>head</literal> is in a library and
1346 we can't set a breakpoint on it directly. For this reason, GHCi
1347 provides the flags <literal>-fbreak-on-exception</literal> which causes
1348 the evaluator to stop when an exception is thrown, and <literal>
1349 -fbreak-on-error</literal>, which works similarly but stops only on
1350 uncaught exceptions. When stopping at an exception, GHCi will act
1351 just as it does when a breakpoint is hit, with the deviation that it
1352 will not show you any source code location. Due to this, these
1353 commands are only really useful in conjunction with
1354 <literal>:trace</literal>, in order to log the steps leading up to the
1355 exception. For example:</para>
1356
1357 <screen>
1358 *Main> :set -fbreak-on-exception
1359 *Main> :trace qsort ("abc" ++ undefined)
1360 "Stopped at &lt;exception thrown&gt;
1361 _exception :: e
1362 [&lt;exception thrown&gt;] *Main&gt; :hist
1363 -1 : qsort.hs:3:24-38
1364 -2 : qsort.hs:3:23-55
1365 -3 : qsort.hs:(1,0)-(3,55)
1366 -4 : qsort.hs:2:15-24
1367 -5 : qsort.hs:2:15-46
1368 -6 : qsort.hs:(1,0)-(3,55)
1369 &lt;end of history&gt;
1370 [&lt;exception thrown&gt;] *Main&gt; :back
1371 Logged breakpoint at qsort.hs:3:24-38
1372 _result :: [a]
1373 as :: [a]
1374 a :: a
1375 [-1: qsort.hs:3:24-38] *Main&gt; :force as
1376 *** Exception: Prelude.undefined
1377 [-1: qsort.hs:3:24-38] *Main&gt; :print as
1378 as = 'b' : 'c' : (_t1::[Char])
1379 </screen>
1380
1381 <para>The exception itself is bound to a new variable,
1382 <literal>_exception</literal>.</para>
1383
1384 <para>Breaking on exceptions is particularly useful for finding out what
1385 your program was doing when it was in an infinite loop. Just hit
1386 Control-C, and examine the history to find out what was going
1387 on.</para>
1388 </sect2>
1389
1390 <sect2><title>Example: inspecting functions</title>
1391 <para>
1392 It is possible to use the debugger to examine function values.
1393 When we are at a breakpoint and a function is in scope, the debugger
1394 cannot show
1395 you the source code for it; however, it is possible to get some
1396 information by applying it to some arguments and observing the result.
1397 </para>
1398
1399 <para>
1400 The process is slightly complicated when the binding is polymorphic.
1401 We show the process by means of an example.
1402 To keep things simple, we will use the well known <literal>map</literal> function:
1403 <programlisting>
1404 import Prelude hiding (map)
1405
1406 map :: (a->b) -> a -> b
1407 map f [] = []
1408 map f (x:xs) = f x : map f xs
1409 </programlisting>
1410 </para>
1411
1412 <para>
1413 We set a breakpoint on <literal>map</literal>, and call it.
1414 <screen>
1415 *Main> :break 5
1416 Breakpoint 0 activated at map.hs:5:15-28
1417 *Main> map Just [1..5]
1418 Stopped at map.hs:(4,0)-(5,12)
1419 _result :: [b]
1420 x :: a
1421 f :: a -> b
1422 xs :: [a]
1423 </screen>
1424 GHCi tells us that, among other bindings, <literal>f</literal> is in scope.
1425 However, its type is not fully known yet,
1426 and thus it is not possible to apply it to any
1427 arguments. Nevertheless, observe that the type of its first argument is the
1428 same as the type of <literal>x</literal>, and its result type is shared
1429 with <literal>_result</literal>.
1430 </para>
1431
1432 <para>
1433 As we demonstrated earlier (<xref linkend="breakpoints" />), the
1434 debugger has some intelligence built-in to update the type of
1435 <literal>f</literal> whenever the types of <literal>x</literal> or
1436 <literal>_result</literal> are discovered. So what we do in this
1437 scenario is
1438 force <literal>x</literal> a bit, in order to recover both its type
1439 and the argument part of <literal>f</literal>.
1440 <screen>
1441 *Main> seq x ()
1442 *Main> :print x
1443 x = 1
1444 </screen>
1445 </para>
1446 <para>
1447 We can check now that as expected, the type of <literal>x</literal>
1448 has been reconstructed, and with it the
1449 type of <literal>f</literal> has been too:</para>
1450 <screen>
1451 *Main> :t x
1452 x :: Integer
1453 *Main> :t f
1454 f :: Integer -> b
1455 </screen>
1456 <para>
1457 From here, we can apply f to any argument of type Integer and observe
1458 the results.
1459 <screen><![CDATA[
1460 *Main> let b = f 10
1461 *Main> :t b
1462 b :: b
1463 *Main> b
1464 <interactive>:1:0:
1465 Ambiguous type variable `b' in the constraint:
1466 `Show b' arising from a use of `print' at <interactive>:1:0
1467 *Main> :p b
1468 b = (_t2::a)
1469 *Main> seq b ()
1470 ()
1471 *Main> :t b
1472 b :: a
1473 *Main> :p b
1474 b = Just 10
1475 *Main> :t b
1476 b :: Maybe Integer
1477 *Main> :t f
1478 f :: Integer -> Maybe Integer
1479 *Main> f 20
1480 Just 20
1481 *Main> map f [1..5]
1482 [Just 1, Just 2, Just 3, Just 4, Just 5]
1483 ]]></screen>
1484 In the first application of <literal>f</literal>, we had to do
1485 some more type reconstruction
1486 in order to recover the result type of <literal>f</literal>.
1487 But after that, we are free to use
1488 <literal>f</literal> normally.
1489 </para>
1490 </sect2>
1491
1492 <sect2><title>Limitations</title>
1493 <itemizedlist>
1494 <listitem>
1495 <para>When stopped at a breakpoint, if you try to evaluate a variable
1496 that is already under evaluation, the second evaluation will hang.
1497 The reason is
1498 that GHC knows the variable is under evaluation, so the new
1499 evaluation just waits for the result before continuing, but of
1500 course this isn't going to happen because the first evaluation is
1501 stopped at a breakpoint. Control-C can interrupt the hung
1502 evaluation and return to the prompt.</para>
1503 <para>The most common way this can happen is when you're evaluating a
1504 CAF (e.g. main), stop at a breakpoint, and ask for the value of the
1505 CAF at the prompt again.</para>
1506 </listitem>
1507 <listitem><para>
1508 Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
1509 at the scope of a breakpoint if there is an explicit type signature.
1510 </para>
1511 </listitem>
1512 </itemizedlist>
1513 </sect2>
1514 </sect1>
1515
1516 <sect1 id="ghci-invocation">
1517 <title>Invoking GHCi</title>
1518 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
1519 <indexterm><primary><option>&ndash;&ndash;interactive</option></primary></indexterm>
1520
1521 <para>GHCi is invoked with the command <literal>ghci</literal> or
1522 <literal>ghc &ndash;&ndash;interactive</literal>. One or more modules or
1523 filenames can also be specified on the command line; this
1524 instructs GHCi to load the specified modules or filenames (and all
1525 the modules they depend on), just as if you had said
1526 <literal>:load <replaceable>modules</replaceable></literal> at the
1527 GHCi prompt (see <xref linkend="ghci-commands" />). For example, to
1528 start GHCi and load the program whose topmost module is in the
1529 file <literal>Main.hs</literal>, we could say:</para>
1530
1531 <screen>
1532 $ ghci Main.hs
1533 </screen>
1534
1535 <para>Most of the command-line options accepted by GHC (see <xref
1536 linkend="using-ghc"/>) also make sense in interactive mode. The ones
1537 that don't make sense are mostly obvious.</para>
1538
1539 <sect2>
1540 <title>Packages</title>
1541 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
1542
1543 <para>Most packages (see <xref linkend="using-packages"/>) are
1544 available without needing to specify any extra flags at all:
1545 they will be automatically loaded the first time they are
1546 needed.</para>
1547
1548 <para>For hidden packages, however, you need to request the
1549 package be loaded by using the <literal>-package</literal> flag:</para>
1550
1551 <screen>
1552 $ ghci -package readline
1553 GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
1554 Loading package base ... linking ... done.
1555 Loading package readline-1.0 ... linking ... done.
1556 Prelude>
1557 </screen>
1558
1559 <para>The following command works to load new packages into a
1560 running GHCi:</para>
1561
1562 <screen>
1563 Prelude> :set -package <replaceable>name</replaceable>
1564 </screen>
1565
1566 <para>But note that doing this will cause all currently loaded
1567 modules to be unloaded, and you'll be dumped back into the
1568 <literal>Prelude</literal>.</para>
1569 </sect2>
1570
1571 <sect2>
1572 <title>Extra libraries</title>
1573 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
1574
1575 <para>Extra libraries may be specified on the command line using
1576 the normal <literal>-l<replaceable>lib</replaceable></literal>
1577 option. (The term <emphasis>library</emphasis> here refers to
1578 libraries of foreign object code; for using libraries of Haskell
1579 source code, see <xref linkend="ghci-modules-filenames"/>.) For
1580 example, to load the &ldquo;m&rdquo; library:</para>
1581
1582 <screen>
1583 $ ghci -lm
1584 </screen>
1585
1586 <para>On systems with <literal>.so</literal>-style shared
1587 libraries, the actual library loaded will the
1588 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
1589 searches the following places for libraries, in this order:</para>
1590
1591 <itemizedlist>
1592 <listitem>
1593 <para>Paths specified using the
1594 <literal>-L<replaceable>path</replaceable></literal>
1595 command-line option,</para>
1596 </listitem>
1597 <listitem>
1598 <para>the standard library search path for your system,
1599 which on some systems may be overridden by setting the
1600 <literal>LD_LIBRARY_PATH</literal> environment
1601 variable.</para>
1602 </listitem>
1603 </itemizedlist>
1604
1605 <para>On systems with <literal>.dll</literal>-style shared
1606 libraries, the actual library loaded will be
1607 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
1608 GHCi will signal an error if it can't find the library.</para>
1609
1610 <para>GHCi can also load plain object files
1611 (<literal>.o</literal> or <literal>.obj</literal> depending on
1612 your platform) from the command-line. Just add the name the
1613 object file to the command line.</para>
1614
1615 <para>Ordering of <option>-l</option> options matters: a library
1616 should be mentioned <emphasis>before</emphasis> the libraries it
1617 depends on (see <xref linkend="options-linker"/>).</para>
1618 </sect2>
1619
1620 </sect1>
1621
1622 <sect1 id="ghci-commands">
1623 <title>GHCi commands</title>
1624
1625 <para>GHCi commands all begin with
1626 &lsquo;<literal>:</literal>&rsquo; and consist of a single command
1627 name followed by zero or more parameters. The command name may be
1628 abbreviated, with ambiguities being resolved in favour of the more
1629 commonly used commands.</para>
1630
1631 <variablelist>
1632 <varlistentry>
1633 <term>
1634 <literal>:abandon</literal>
1635 <indexterm><primary><literal>:abandon</literal></primary></indexterm>
1636 </term>
1637 <listitem>
1638 <para>Abandons the current evaluation (only available when stopped at
1639 a breakpoint).</para>
1640 </listitem>
1641 </varlistentry>
1642
1643 <varlistentry>
1644 <term>
1645 <literal>:add</literal> <replaceable>module</replaceable> ...
1646 <indexterm><primary><literal>:add</literal></primary></indexterm>
1647 </term>
1648 <listitem>
1649 <para>Add <replaceable>module</replaceable>(s) to the
1650 current <firstterm>target set</firstterm>, and perform a
1651 reload.</para>
1652 </listitem>
1653 </varlistentry>
1654
1655 <varlistentry>
1656 <term>
1657 <literal>:back</literal>
1658 <indexterm><primary><literal>:back</literal></primary></indexterm>
1659 </term>
1660 <listitem>
1661 <para>Travel back one step in the history. See <xref
1662 linkend="tracing" />. See also:
1663 <literal>:trace</literal>, <literal>:history</literal>,
1664 <literal>:forward</literal>.</para>
1665 </listitem>
1666 </varlistentry>
1667
1668 <varlistentry>
1669 <term>
1670 <literal>:break [<replaceable>identifier</replaceable> |
1671 [<replaceable>module</replaceable>] <replaceable>line</replaceable>
1672 [<replaceable>column</replaceable>]]</literal>
1673 </term>
1674 <indexterm><primary><literal>:break</literal></primary></indexterm>
1675 <listitem>
1676 <para>Set a breakpoint on the specified function or line and
1677 column. See <xref linkend="setting-breakpoints" />.</para>
1678 </listitem>
1679 </varlistentry>
1680
1681 <varlistentry>
1682 <term>
1683 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
1684 <indexterm><primary><literal>:browse</literal></primary></indexterm>
1685 </term>
1686 <listitem>
1687 <para>Displays the identifiers defined by the module
1688 <replaceable>module</replaceable>, which must be either
1689 loaded into GHCi or be a member of a package. If the
1690 <literal>*</literal> symbol is placed before the module
1691 name, then <emphasis>all</emphasis> the identifiers defined
1692 in <replaceable>module</replaceable> are shown; otherwise
1693 the list is limited to the exports of
1694 <replaceable>module</replaceable>. The
1695 <literal>*</literal>-form is only available for modules
1696 which are interpreted; for compiled modules (including
1697 modules from packages) only the non-<literal>*</literal>
1698 form of <literal>:browse</literal> is available.</para>
1699 </listitem>
1700 </varlistentry>
1701
1702 <varlistentry>
1703 <term>
1704 <literal>:cd</literal> <replaceable>dir</replaceable>
1705 <indexterm><primary><literal>:cd</literal></primary></indexterm>
1706 </term>
1707 <listitem>
1708 <para>Changes the current working directory to
1709 <replaceable>dir</replaceable>. A
1710 &lsquo;<literal>&tilde;</literal>&rsquo; symbol at the
1711 beginning of <replaceable>dir</replaceable> will be replaced
1712 by the contents of the environment variable
1713 <literal>HOME</literal>.</para>
1714
1715 <para>NOTE: changing directories causes all currently loaded
1716 modules to be unloaded. This is because the search path is
1717 usually expressed using relative directories, and changing
1718 the search path in the middle of a session is not
1719 supported.</para>
1720 </listitem>
1721 </varlistentry>
1722
1723 <varlistentry>
1724 <term>
1725 <literal>:cmd</literal> <replaceable>expr</replaceable>
1726 <indexterm><primary><literal>:cmd</literal></primary></indexterm>
1727 </term>
1728 <listitem>
1729 <para>Executes <replaceable>expr</replaceable> as a computation of
1730 type <literal>IO String</literal>, and then executes the resulting
1731 string as a list of GHCi commands. Multiple commands are separated
1732 by newlines. The <literal>:cmd</literal> command is useful with
1733 <literal>:def</literal> and <literal>:set stop</literal>.</para>
1734 </listitem>
1735 </varlistentry>
1736
1737 <varlistentry>
1738 <term>
1739 <literal>:continue</literal>
1740 <indexterm><primary><literal>:continue</literal></primary></indexterm>
1741 </term>
1742 <listitem><para>Continue the current evaluation, when stopped at a
1743 breakpoint.</para>
1744 </listitem>
1745 </varlistentry>
1746
1747 <varlistentry>
1748 <term>
1749 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1750 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1751 <indexterm><primary><literal>:etags</literal></primary>
1752 </indexterm>
1753 <indexterm><primary><literal>:etags</literal></primary>
1754 </indexterm>
1755 </term>
1756 <listitem>
1757 <para>Generates a &ldquo;tags&rdquo; file for Vi-style editors
1758 (<literal>:ctags</literal>) or
1759 Emacs-style editors (<literal>:etags</literal>). If
1760 no filename is specified, the defaulit <filename>tags</filename> or
1761 <filename>TAGS</filename> is
1762 used, respectively. Tags for all the functions, constructors and
1763 types in the currently loaded modules are created. All modules must
1764 be interpreted for these commands to work.</para>
1765 <para>See also <xref linkend="hasktags" />.</para>
1766 </listitem>
1767 </varlistentry>
1768
1769 <varlistentry>
1770 <term>
1771 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
1772 <indexterm><primary><literal>:def</literal></primary></indexterm>
1773 </term>
1774 <listitem>
1775 <para>The command <literal>:def</literal>
1776 <replaceable>name</replaceable>
1777 <replaceable>expr</replaceable> defines a new GHCi command
1778 <literal>:<replaceable>name</replaceable></literal>,
1779 implemented by the Haskell expression
1780 <replaceable>expr</replaceable>, which must have type
1781 <literal>String -> IO String</literal>. When
1782 <literal>:<replaceable>name</replaceable>
1783 <replaceable>args</replaceable></literal> is typed at the
1784 prompt, GHCi will run the expression
1785 <literal>(<replaceable>name</replaceable>
1786 <replaceable>args</replaceable>)</literal>, take the
1787 resulting <literal>String</literal>, and feed it back into
1788 GHCi as a new sequence of commands. Separate commands in
1789 the result must be separated by
1790 &lsquo;<literal>\n</literal>&rsquo;.</para>
1791
1792 <para>That's all a little confusing, so here's a few
1793 examples. To start with, here's a new GHCi command which
1794 doesn't take any arguments or produce any results, it just
1795 outputs the current date &amp; time:</para>
1796
1797 <screen>
1798 Prelude> let date _ = Time.getClockTime >>= print >> return ""
1799 Prelude> :def date date
1800 Prelude> :date
1801 Fri Mar 23 15:16:40 GMT 2001
1802 </screen>
1803
1804 <para>Here's an example of a command that takes an argument.
1805 It's a re-implementation of <literal>:cd</literal>:</para>
1806
1807 <screen>
1808 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
1809 Prelude> :def mycd mycd
1810 Prelude> :mycd ..
1811 </screen>
1812
1813 <para>Or I could define a simple way to invoke
1814 &ldquo;<literal>ghc &ndash;&ndash;make Main</literal>&rdquo; in the
1815 current directory:</para>
1816
1817 <screen>
1818 Prelude> :def make (\_ -> return ":! ghc &ndash;&ndash;make Main")
1819 </screen>
1820
1821 <para>We can define a command that reads GHCi input from a
1822 file. This might be useful for creating a set of bindings
1823 that we want to repeatedly load into the GHCi session:</para>
1824
1825 <screen>
1826 Prelude> :def . readFile
1827 Prelude> :. cmds.ghci
1828 </screen>
1829
1830 <para>Notice that we named the command
1831 <literal>:.</literal>, by analogy with the
1832 &lsquo;<literal>.</literal>&rsquo; Unix shell command that
1833 does the same thing.</para>
1834 </listitem>
1835 </varlistentry>
1836
1837 <varlistentry>
1838 <term>
1839 <literal>:delete * | <replaceable>num</replaceable> ...</literal>
1840 <indexterm><primary><literal>:delete</literal></primary></indexterm>
1841 </term>
1842 <listitem>
1843 <para>Delete one or more breakpoints by number (use <literal>:show
1844 breaks</literal> to see the number of each breakpoint). The
1845 <literal>*</literal> form deletes all the breakpoints.</para>
1846 </listitem>
1847 </varlistentry>
1848
1849 <varlistentry>
1850 <term>
1851 <literal>:edit <optional><replaceable>file</replaceable></optional></literal>
1852 <indexterm><primary><literal>:edit</literal></primary></indexterm>
1853 </term>
1854 <listitem>
1855 <para>Opens an editor to edit the file
1856 <replaceable>file</replaceable>, or the most recently loaded
1857 module if <replaceable>file</replaceable> is omitted. The
1858 editor to invoke is taken from the <literal>EDITOR</literal>
1859 environment variable, or a default editor on your system if
1860 <literal>EDITOR</literal> is not set. You can change the
1861 editor using <literal>:set editor</literal>.</para>
1862 </listitem>
1863 </varlistentry>
1864
1865 <varlistentry>
1866 <term>
1867 <literal>:etags</literal>
1868 </term>
1869 <listitem>
1870 <para>See <literal>:ctags</literal>.</para>
1871 </listitem>
1872 </varlistentry>
1873
1874 <varlistentry>
1875 <term>
1876 <literal>:force <replaceable>identifier</replaceable> ...</literal>
1877 <indexterm><primary><literal>:force</literal></primary></indexterm>
1878 </term>
1879 <listitem>
1880 <para>Prints the value of <replaceable>identifier</replaceable> in
1881 the same way as <literal>:print</literal>. Unlike
1882 <literal>:print</literal>, <literal>:force</literal> evaluates each
1883 thunk that it encounters while traversing the value. This may
1884 cause exceptions or infinite loops, or further breakpoints (which
1885 are ignored, but displayed).</para>
1886 </listitem>
1887 </varlistentry>
1888
1889 <varlistentry>
1890 <term>
1891 <literal>:forward</literal>
1892 <indexterm><primary><literal>:forward</literal></primary></indexterm>
1893 </term>
1894 <listitem>
1895 <para>Move forward in the history. See <xref
1896 linkend="tracing" />. See also:
1897 <literal>:trace</literal>, <literal>:history</literal>,
1898 <literal>:back</literal>.</para>
1899 </listitem>
1900 </varlistentry>
1901
1902 <varlistentry>
1903 <term>
1904 <literal>:help</literal>
1905 <indexterm><primary><literal>:help</literal></primary></indexterm>
1906 </term>
1907 <term>
1908 <literal>:?</literal>
1909 <indexterm><primary><literal>:?</literal></primary></indexterm>
1910 </term>
1911 <listitem>
1912 <para>Displays a list of the available commands.</para>
1913 </listitem>
1914 </varlistentry>
1915
1916 <varlistentry>
1917 <term>
1918 <literal>:history [<replaceable>num</replaceable>]</literal>
1919 <indexterm><primary><literal>:history</literal></primary></indexterm>
1920 </term>
1921 <listitem>
1922 <para>Display the history of evaluation steps. With a number,
1923 displays that many steps (default: 20). For use with
1924 <literal>:trace</literal>; see <xref
1925 linkend="tracing" />.</para>
1926 </listitem>
1927 </varlistentry>
1928
1929 <varlistentry>
1930 <term>
1931 <literal>:info</literal> <replaceable>name</replaceable> ...
1932 <indexterm><primary><literal>:info</literal></primary></indexterm>
1933 </term>
1934 <listitem>
1935 <para>Displays information about the given name(s). For
1936 example, if <replaceable>name</replaceable> is a class, then
1937 the class methods and their types will be printed; if
1938 <replaceable>name</replaceable> is a type constructor, then
1939 its definition will be printed; if
1940 <replaceable>name</replaceable> is a function, then its type
1941 will be printed. If <replaceable>name</replaceable> has
1942 been loaded from a source file, then GHCi will also display
1943 the location of its definition in the source.</para>
1944 <para>For types and classes, GHCi also summarises instances that
1945 mention them. To avoid showing irrelevant information, an instance
1946 is shown only if (a) its head mentions <replaceable>name</replaceable>,
1947 and (b) all the other things mentioned in the instance
1948 are in scope (either qualified or otherwise) as a result of
1949 a <literal>:load</literal> or <literal>:module</literal> commands. </para>
1950 </listitem>
1951 </varlistentry>
1952
1953 <varlistentry>
1954 <term>
1955 <literal>:kind</literal> <replaceable>type</replaceable>
1956 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1957 </term>
1958 <listitem>
1959 <para>Infers and prints the kind of
1960 <replaceable>type</replaceable>. The latter can be an arbitrary
1961 type expression, including a partial application of a type constructor,
1962 such as <literal>Either Int</literal>.</para>
1963 </listitem>
1964 </varlistentry>
1965
1966 <varlistentry>
1967 <term>
1968 <literal>:load</literal> <replaceable>module</replaceable> ...
1969 <indexterm><primary><literal>:load</literal></primary></indexterm>
1970 </term>
1971 <listitem>
1972 <para>Recursively loads the specified
1973 <replaceable>module</replaceable>s, and all the modules they
1974 depend on. Here, each <replaceable>module</replaceable>
1975 must be a module name or filename, but may not be the name
1976 of a module in a package.</para>
1977
1978 <para>All previously loaded modules, except package modules,
1979 are forgotten. The new set of modules is known as the
1980 <firstterm>target set</firstterm>. Note that
1981 <literal>:load</literal> can be used without any arguments
1982 to unload all the currently loaded modules and
1983 bindings.</para>
1984
1985 <para>After a <literal>:load</literal> command, the current
1986 context is set to:</para>
1987
1988 <itemizedlist>
1989 <listitem>
1990 <para><replaceable>module</replaceable>, if it was loaded
1991 successfully, or</para>
1992 </listitem>
1993 <listitem>
1994 <para>the most recently successfully loaded module, if
1995 any other modules were loaded as a result of the current
1996 <literal>:load</literal>, or</para>
1997 </listitem>
1998 <listitem>
1999 <para><literal>Prelude</literal> otherwise.</para>
2000 </listitem>
2001 </itemizedlist>
2002 </listitem>
2003 </varlistentry>
2004
2005 <varlistentry>
2006 <term>
2007 <literal>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
2008 <indexterm><primary><literal>:main</literal></primary></indexterm>
2009 </term>
2010 <listitem>
2011 <para>
2012 When a program is compiled and executed, it can use the
2013 <literal>getArgs</literal> function to access the
2014 command-line arguments.
2015 However, we cannot simply pass the arguments to the
2016 <literal>main</literal> function while we are testing in ghci,
2017 as the <literal>main</literal> function doesn't take its
2018 arguments directly.
2019 </para>
2020
2021 <para>
2022 Instead, we can use the <literal>:main</literal> command.
2023 This runs whatever <literal>main</literal> is in scope, with
2024 any arguments being treated the same as command-line arguments,
2025 e.g.:
2026 </para>
2027
2028 <screen>
2029 Prelude> let main = System.Environment.getArgs >>= print
2030 Prelude> :main foo bar
2031 ["foo","bar"]
2032 </screen>
2033
2034 </listitem>
2035 </varlistentry>
2036
2037 <varlistentry>
2038 <term>
2039 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
2040 <indexterm><primary><literal>:module</literal></primary></indexterm>
2041 </term>
2042 <term>
2043 <literal>import <replaceable>mod</replaceable></literal>
2044 </term>
2045 <listitem>
2046 <para>Sets or modifies the current context for statements
2047 typed at the prompt. The form <literal>import
2048 <replaceable>mod</replaceable></literal> is equivalent to
2049 <literal>:module +<replaceable>mod</replaceable></literal>.
2050 See <xref linkend="ghci-scope"/> for
2051 more details.</para>
2052 </listitem>
2053 </varlistentry>
2054
2055 <varlistentry>
2056 <term>
2057 <literal>:print </literal> <replaceable>names</replaceable> ...
2058 <indexterm><primary><literal>:print</literal></primary></indexterm>
2059 </term>
2060 <listitem>
2061 <para>Prints a value without forcing its evaluation.
2062 <literal>:print</literal> may be used on values whose types are
2063 unknown or partially known, which might be the case for local
2064 variables with polymorphic types at a breakpoint. While inspecting
2065 the runtime value, <literal>:print</literal> attempts to
2066 reconstruct the type of the value, and will elaborate the type in
2067 GHCi's environment if possible. If any unevaluated components
2068 (thunks) are encountered, then <literal>:print</literal> binds
2069 a fresh variable with a name beginning with <literal>_t</literal>
2070 to each thunk. See <xref linkend="breakpoints" /> for more
2071 information. See also the <literal>:sprint</literal> command,
2072 which works like <literal>:print</literal> but does not bind new
2073 variables.</para>
2074 </listitem>
2075 </varlistentry>
2076
2077 <varlistentry>
2078 <term>
2079 <literal>:quit</literal>
2080 <indexterm><primary><literal>:quit</literal></primary></indexterm>
2081 </term>
2082 <listitem>
2083 <para>Quits GHCi. You can also quit by typing control-D
2084 at the prompt.</para>
2085 </listitem>
2086 </varlistentry>
2087
2088 <varlistentry>
2089 <term>
2090 <literal>:reload</literal>
2091 <indexterm><primary><literal>:reload</literal></primary></indexterm>
2092 </term>
2093 <listitem>
2094 <para>Attempts to reload the current target set (see
2095 <literal>:load</literal>) if any of the modules in the set,
2096 or any dependent module, has changed. Note that this may
2097 entail loading new modules, or dropping modules which are no
2098 longer indirectly required by the target.</para>
2099 </listitem>
2100 </varlistentry>
2101
2102 <varlistentry>
2103 <term>
2104 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
2105 <indexterm><primary><literal>:set</literal></primary></indexterm>
2106 </term>
2107 <listitem>
2108 <para>Sets various options. See <xref linkend="ghci-set"/>
2109 for a list of available options. The
2110 <literal>:set</literal> command by itself shows which
2111 options are currently set.</para>
2112 </listitem>
2113 </varlistentry>
2114
2115 <varlistentry>
2116 <term>
2117 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
2118 <indexterm><primary><literal>:set args</literal></primary></indexterm>
2119 </term>
2120 <listitem>
2121 <para>Sets the list of arguments which are returned when the
2122 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
2123 </indexterm>.</para>
2124 </listitem>
2125 </varlistentry>
2126
2127 <varlistentry>
2128 <term>
2129 <literal>:set</literal> <literal>editor</literal> <replaceable>cmd</replaceable>
2130 </term>
2131 <listitem>
2132 <para>Sets the command used by <literal>:edit</literal> to
2133 <replaceable>cmd</replaceable>.</para>
2134 </listitem>
2135 </varlistentry>
2136
2137 <varlistentry>
2138 <term>
2139 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
2140 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
2141 </term>
2142 <listitem>
2143 <para>Sets the string to be returned when the program calls
2144 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
2145 </indexterm>.</para>
2146 </listitem>
2147 </varlistentry>
2148
2149 <varlistentry>
2150 <term>
2151 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
2152 </term>
2153 <listitem>
2154 <para>Sets the string to be used as the prompt in GHCi.
2155 Inside <replaceable>prompt</replaceable>, the sequence
2156 <literal>%s</literal> is replaced by the names of the
2157 modules currently in scope, and <literal>%%</literal> is
2158 replaced by <literal>%</literal>.</para>
2159 </listitem>
2160 </varlistentry>
2161
2162 <varlistentry>
2163 <term>
2164 <literal>:set</literal> <literal>stop</literal>
2165 [<replaceable>num</replaceable>] <replaceable>cmd</replaceable>
2166 </term>
2167 <listitem>
2168 <para>Set a command to be executed when a breakpoint is hit, or a new
2169 item in the history is selected. The most common use of
2170 <literal>:set stop</literal> is to display the source code at the
2171 current location, e.g. <literal>:set stop :list</literal>.</para>
2172
2173 <para>If a number is given before the command, then the commands are
2174 run when the specified breakpoint (only) is hit. This can be quite
2175 useful: for example, <literal>:set stop 1 :continue</literal>
2176 effectively disables breakpoint 1, by running
2177 <literal>:continue</literal> whenever it is hit (although GHCi will
2178 still emit a message to say the breakpoint was hit). What's more,
2179 with cunning use of <literal>:def</literal> and
2180 <literal>:cmd</literal> you can use <literal>:set stop</literal> to
2181 implement conditional breakpoints:</para>
2182 <screen>
2183 *Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
2184 *Main> :set stop 0 :cond (x &lt; 3)
2185 </screen>
2186 <para>Ignoring breakpoints for a specified number of iterations is
2187 also possible using similar techniques.</para>
2188 </listitem>
2189 </varlistentry>
2190
2191 <varlistentry>
2192 <term>
2193 <literal>:show bindings</literal>
2194 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
2195 </term>
2196 <listitem>
2197 <para>Show the bindings made at the prompt and their
2198 types.</para>
2199 </listitem>
2200 </varlistentry>
2201
2202 <varlistentry>
2203 <term>
2204 <literal>:show breaks</literal>
2205 <indexterm><primary><literal>:show breaks</literal></primary></indexterm>
2206 </term>
2207 <listitem>
2208 <para>List the active breakpoints.</para>
2209 </listitem>
2210 </varlistentry>
2211
2212 <varlistentry>
2213 <term>
2214 <literal>:show context</literal>
2215 <indexterm><primary><literal>:show context</literal></primary></indexterm>
2216 </term>
2217 <listitem>
2218 <para>List the active evaluations that are stopped at breakpoints.</para>
2219 </listitem>
2220 </varlistentry>
2221
2222 <varlistentry>
2223 <term>
2224 <literal>:show modules</literal>
2225 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
2226 </term>
2227 <listitem>
2228 <para>Show the list of modules currently loaded.</para>
2229 </listitem>
2230 </varlistentry>
2231
2232 <varlistentry>
2233 <term>
2234 <literal>:show [args|prog|prompt|editor|stop]</literal>
2235 <indexterm><primary><literal>:show</literal></primary></indexterm>
2236 </term>
2237 <listitem>
2238 <para>Displays the specified setting (see
2239 <literal>:set</literal>).</para>
2240 </listitem>
2241 </varlistentry>
2242
2243 <varlistentry>
2244 <term>
2245 <literal>:sprint</literal>
2246 <indexterm><primary><literal>:sprint</literal></primary></indexterm>
2247 </term>
2248 <listitem>
2249 <para>Prints a value without forcing its evaluation.
2250 <literal>:sprint</literal> is similar to <literal>:print</literal>,
2251 with the difference that unevaluated subterms are not bound to new
2252 variables, they are simply denoted by &lsquo;_&rsquo;.</para>
2253 </listitem>
2254 </varlistentry>
2255
2256 <varlistentry>
2257 <term>
2258 <literal>:step [<replaceable>expr</replaceable>]</literal>
2259 <indexterm><primary><literal>:step</literal></primary></indexterm>
2260 </term>
2261 <listitem>
2262 <para>Single-step from the last breakpoint. With an expression
2263 argument, begins evaluation of the expression with a
2264 single-step.</para>
2265 </listitem>
2266 </varlistentry>
2267
2268 <varlistentry>
2269 <term>
2270 <literal>:trace [<replaceable>expr</replaceable>]</literal>
2271 <indexterm><primary><literal>:trace</literal></primary></indexterm>
2272 </term>
2273 <listitem>
2274 <para>Evaluates the given expression (or from the last breakpoint if
2275 no expression is given), and additionally logs the evaluation
2276 steps for later inspection using <literal>:history</literal>. See
2277 <xref linkend="tracing" />.</para>
2278 </listitem>
2279 </varlistentry>
2280
2281 <varlistentry>
2282 <term>
2283 <literal>:type</literal> <replaceable>expression</replaceable>
2284 <indexterm><primary><literal>:type</literal></primary></indexterm>
2285 </term>
2286 <listitem>
2287 <para>Infers and prints the type of
2288 <replaceable>expression</replaceable>, including explicit
2289 forall quantifiers for polymorphic types. The monomorphism
2290 restriction is <emphasis>not</emphasis> applied to the
2291 expression during type inference.</para>
2292 </listitem>
2293 </varlistentry>
2294
2295 <varlistentry>
2296 <term>
2297 <literal>:undef</literal> <replaceable>name</replaceable>
2298 <indexterm><primary><literal>:undef</literal></primary></indexterm>
2299 </term>
2300 <listitem>
2301 <para>Undefines the user-defined command
2302 <replaceable>name</replaceable> (see <literal>:def</literal>
2303 above).</para>
2304 </listitem>
2305 </varlistentry>
2306
2307 <varlistentry>
2308 <term>
2309 <literal>:unset</literal> <replaceable>option</replaceable>...
2310 <indexterm><primary><literal>:unset</literal></primary></indexterm>
2311 </term>
2312 <listitem>
2313 <para>Unsets certain options. See <xref linkend="ghci-set"/>
2314 for a list of available options.</para>
2315 </listitem>
2316 </varlistentry>
2317
2318 <varlistentry>
2319 <term>
2320 <literal>:!</literal> <replaceable>command</replaceable>...
2321 <indexterm><primary><literal>:!</literal></primary></indexterm>
2322 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
2323 </term>
2324 <listitem>
2325 <para>Executes the shell command
2326 <replaceable>command</replaceable>.</para>
2327 </listitem>
2328 </varlistentry>
2329
2330 </variablelist>
2331 </sect1>
2332
2333 <sect1 id="ghci-set">
2334 <title>The <literal>:set</literal> command</title>
2335 <indexterm><primary><literal>:set</literal></primary></indexterm>
2336
2337 <para>The <literal>:set</literal> command sets two types of
2338 options: GHCi options, which begin with
2339 &lsquo;<literal>+</literal>&rsquo;, and &ldquo;command-line&rdquo;
2340 options, which begin with &lsquo;-&rsquo;. </para>
2341
2342 <para>NOTE: at the moment, the <literal>:set</literal> command
2343 doesn't support any kind of quoting in its arguments: quotes will
2344 not be removed and cannot be used to group words together. For
2345 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
2346 you expect.</para>
2347
2348 <sect2>
2349 <title>GHCi options</title>
2350 <indexterm><primary>options</primary><secondary>GHCi</secondary>
2351 </indexterm>
2352
2353 <para>GHCi options may be set using <literal>:set</literal> and
2354 unset using <literal>:unset</literal>.</para>
2355
2356 <para>The available GHCi options are:</para>
2357
2358 <variablelist>
2359 <varlistentry>
2360 <term>
2361 <literal>+r</literal>
2362 <indexterm><primary><literal>+r</literal></primary></indexterm>
2363 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
2364 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
2365 </term>
2366 <listitem>
2367 <para>Normally, any evaluation of top-level expressions
2368 (otherwise known as CAFs or Constant Applicative Forms) in
2369 loaded modules is retained between evaluations. Turning
2370 on <literal>+r</literal> causes all evaluation of
2371 top-level expressions to be discarded after each
2372 evaluation (they are still retained
2373 <emphasis>during</emphasis> a single evaluation).</para>
2374
2375 <para>This option may help if the evaluated top-level
2376 expressions are consuming large amounts of space, or if
2377 you need repeatable performance measurements.</para>
2378 </listitem>
2379 </varlistentry>
2380
2381 <varlistentry>
2382 <term>
2383 <literal>+s</literal>
2384 <indexterm><primary><literal>+s</literal></primary></indexterm>
2385 </term>
2386 <listitem>
2387 <para>Display some stats after evaluating each expression,
2388 including the elapsed time and number of bytes allocated.
2389 NOTE: the allocation figure is only accurate to the size
2390 of the storage manager's allocation area, because it is
2391 calculated at every GC. Hence, you might see values of
2392 zero if no GC has occurred.</para>
2393 </listitem>
2394 </varlistentry>
2395
2396 <varlistentry>
2397 <term>
2398 <literal>+t</literal>
2399 <indexterm><primary><literal>+t</literal></primary></indexterm>
2400 </term>
2401 <listitem>
2402 <para>Display the type of each variable bound after a
2403 statement is entered at the prompt. If the statement is a
2404 single expression, then the only variable binding will be
2405 for the variable
2406 &lsquo;<literal>it</literal>&rsquo;.</para>
2407 </listitem>
2408 </varlistentry>
2409 </variablelist>
2410 </sect2>
2411
2412 <sect2 id="ghci-cmd-line-options">
2413 <title>Setting GHC command-line options in GHCi</title>
2414
2415 <para>Normal GHC command-line options may also be set using
2416 <literal>:set</literal>. For example, to turn on
2417 <option>-fglasgow-exts</option>, you would say:</para>
2418
2419 <screen>
2420 Prelude> :set -fglasgow-exts
2421 </screen>
2422
2423 <para>Any GHC command-line option that is designated as
2424 <firstterm>dynamic</firstterm> (see the table in <xref
2425 linkend="flag-reference"/>), may be set using
2426 <literal>:set</literal>. To unset an option, you can set the
2427 reverse option:</para>
2428 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
2429
2430 <screen>
2431 Prelude> :set -fno-glasgow-exts
2432 </screen>
2433
2434 <para><xref linkend="flag-reference"/> lists the reverse for each
2435 option where applicable.</para>
2436
2437 <para>Certain static options (<option>-package</option>,
2438 <option>-I</option>, <option>-i</option>, and
2439 <option>-l</option> in particular) will also work, but some may
2440 not take effect until the next reload.</para>
2441 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
2442 </sect2>
2443 </sect1>
2444 <sect1 id="ghci-dot-files">
2445 <title>The <filename>.ghci</filename> file</title>
2446 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
2447 </indexterm>
2448 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
2449 </indexterm>
2450
2451 <para>When it starts, unless the <literal>-ignore-dot-ghci</literal>
2452 flag is given, GHCi reads and executes commands from
2453 <filename>./.ghci</filename>, followed by
2454 <filename>$HOME/.ghci</filename>.</para>
2455
2456 <para>The <filename>.ghci</filename> in your home directory is
2457 most useful for turning on favourite options (eg. <literal>:set
2458 +s</literal>), and defining useful macros. Placing a
2459 <filename>.ghci</filename> file in a directory with a Haskell
2460 project is a useful way to set certain project-wide options so you
2461 don't have to type them everytime you start GHCi: eg. if your
2462 project uses GHC extensions and CPP, and has source files in three
2463 subdirectories A, B and C, you might put the following lines in
2464 <filename>.ghci</filename>:</para>
2465
2466 <screen>
2467 :set -fglasgow-exts -cpp
2468 :set -iA:B:C
2469 </screen>
2470
2471 <para>(Note that strictly speaking the <option>-i</option> flag is
2472 a static one, but in fact it works to set it using
2473 <literal>:set</literal> like this. The changes won't take effect
2474 until the next <literal>:load</literal>, though.)</para>
2475
2476 <para>Two command-line options control whether the
2477 <filename>.ghci</filename> files are read:</para>
2478
2479 <variablelist>
2480 <varlistentry>
2481 <term>
2482 <option>-ignore-dot-ghci</option>
2483 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
2484 </term>
2485 <listitem>
2486 <para>Don't read either <filename>./.ghci</filename> or
2487 <filename>$HOME/.ghci</filename> when starting up.</para>
2488 </listitem>
2489 </varlistentry>
2490 <varlistentry>
2491 <term>
2492 <option>-read-dot-ghci</option>
2493 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
2494 </term>
2495 <listitem>
2496 <para>Read <filename>.ghci</filename> and
2497 <filename>$HOME/.ghci</filename>. This is normally the
2498 default, but the <option>-read-dot-ghci</option> option may
2499 be used to override a previous
2500 <option>-ignore-dot-ghci</option> option.</para>
2501 </listitem>
2502 </varlistentry>
2503 </variablelist>
2504
2505 </sect1>
2506
2507 <sect1 id="ghci-obj">
2508 <title>Compiling to object code inside GHCi</title>
2509
2510 <para>By default, GHCi compiles Haskell source code into byte-code
2511 that is interpreted by the runtime system. GHCi can also compile
2512 Haskell code to object code: to turn on this feature, use the
2513 <option>-fobject-code</option> flag either on the command line or
2514 with <literal>:set</literal> (the option
2515 <option>-fbyte-code</option> restores byte-code compilation
2516 again). Compiling to object code takes longer, but typically the
2517 code will execute 10-20 times faster than byte-code.</para>
2518
2519 <para>Compiling to object code inside GHCi is particularly useful
2520 if you are developing a compiled application, because the
2521 <literal>:reload</literal> command typically runs much faster than
2522 restarting GHC with <option>--make</option> from the command-line,
2523 because all the interface files are already cached in
2524 memory.</para>
2525
2526 <para>There are disadvantages to compiling to object-code: you
2527 can't set breakpoints in object-code modules, for example. Only
2528 the exports of an object-code module will be visible in GHCi,
2529 rather than all top-level bindings as in interpreted
2530 modules.</para>
2531 </sect1>
2532
2533 <sect1 id="ghci-faq">
2534 <title>FAQ and Things To Watch Out For</title>
2535
2536 <variablelist>
2537 <varlistentry>
2538 <term>The interpreter can't load modules with foreign export
2539 declarations!</term>
2540 <listitem>
2541 <para>Unfortunately not. We haven't implemented it yet.
2542 Please compile any offending modules by hand before loading
2543 them into GHCi.</para>
2544 </listitem>
2545 </varlistentry>
2546
2547 <varlistentry>
2548 <term>
2549 <literal>-O</literal> doesn't work with GHCi!
2550 <indexterm><primary><option>-O</option></primary></indexterm>
2551 </term>
2552 <listitem>
2553 <para>For technical reasons, the bytecode compiler doesn't
2554 interact well with one of the optimisation passes, so we
2555 have disabled optimisation when using the interpreter. This
2556 isn't a great loss: you'll get a much bigger win by
2557 compiling the bits of your code that need to go fast, rather
2558 than interpreting them with optimisation turned on.</para>
2559 </listitem>
2560 </varlistentry>
2561
2562 <varlistentry>
2563 <term>Unboxed tuples don't work with GHCi</term>
2564 <listitem>
2565 <para>That's right. You can always compile a module that
2566 uses unboxed tuples and load it into GHCi, however.
2567 (Incidentally the previous point, namely that
2568 <literal>-O</literal> is incompatible with GHCi, is because
2569 the bytecode compiler can't deal with unboxed
2570 tuples).</para>
2571 </listitem>
2572 </varlistentry>
2573
2574 <varlistentry>
2575 <term>Concurrent threads don't carry on running when GHCi is
2576 waiting for input.</term>
2577 <listitem>
2578 <para>This should work, as long as your GHCi was built with
2579 the <option>-threaded</option> switch, which is the default.
2580 Consult whoever supplied your GHCi installation.</para>
2581 </listitem>
2582 </varlistentry>
2583
2584 <varlistentry>
2585 <term>After using <literal>getContents</literal>, I can't use
2586 <literal>stdin</literal> again until I do
2587 <literal>:load</literal> or <literal>:reload</literal>.</term>
2588
2589 <listitem>
2590 <para>This is the defined behaviour of
2591 <literal>getContents</literal>: it puts the stdin Handle in
2592 a state known as <firstterm>semi-closed</firstterm>, wherein
2593 any further I/O operations on it are forbidden. Because I/O
2594 state is retained between computations, the semi-closed
2595 state persists until the next <literal>:load</literal> or
2596 <literal>:reload</literal> command.</para>
2597
2598 <para>You can make <literal>stdin</literal> reset itself
2599 after every evaluation by giving GHCi the command
2600 <literal>:set +r</literal>. This works because
2601 <literal>stdin</literal> is just a top-level expression that
2602 can be reverted to its unevaluated state in the same way as
2603 any other top-level expression (CAF).</para>
2604 </listitem>
2605 </varlistentry>
2606
2607 <varlistentry>
2608 <term>I can't use Control-C to interrupt computations in
2609 GHCi on Windows.</term>
2610 <listitem>
2611 <para>See <xref linkend="ghci-windows"/>.</para>
2612 </listitem>
2613 </varlistentry>
2614
2615 <varlistentry>
2616 <term>The default buffering mode is different in GHCi to GHC.</term>
2617 <listitem>
2618 <para>
2619 In GHC, the stdout handle is line-buffered by default.
2620 However, in GHCi we turn off the buffering on stdout,
2621 because this is normally what you want in an interpreter:
2622 output appears as it is generated.
2623 </para>
2624 </listitem>
2625 </varlistentry>
2626 </variablelist>
2627 </sect1>
2628
2629 </chapter>
2630
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