(short) Documentation for :stepover in the Users Guide
[ghc.git] / docs / users_guide / ghci.xml
1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="ghci">
3 <title>Using GHCi</title>
4 <indexterm><primary>GHCi</primary></indexterm>
5 <indexterm><primary>interpreter</primary><see>GHCi</see></indexterm>
6 <indexterm><primary>interactive</primary><see>GHCi</see></indexterm>
7
8 <para>GHCi<footnote>
9 <para>The &lsquo;i&rsquo; stands for &ldquo;Interactive&rdquo;</para>
10 </footnote>
11 is GHC's interactive environment, in which Haskell expressions can
12 be interactively evaluated and programs can be interpreted. If
13 you're familiar with <ulink url="http://www.haskell.org/hugs/">Hugs</ulink><indexterm><primary>Hugs</primary>
14 </indexterm>, then you'll be right at home with GHCi. However, GHCi
15 also has support for interactively loading compiled code, as well as
16 supporting all<footnote><para>except <literal>foreign export</literal>, at the moment</para>
17 </footnote> the language extensions that GHC provides.
18 <indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
19 <indexterm><primary>Foreign Function
20 Interface</primary><secondary>GHCi support</secondary></indexterm>.
21 GHCi also includes an interactive debugger (see <xref linkend="ghci-debugger"/>).</para>
22
23 <sect1 id="ghci-introduction">
24 <title>Introduction to GHCi</title>
25
26 <para>Let's start with an example GHCi session. You can fire up
27 GHCi with the command <literal>ghci</literal>:</para>
28
29 <screen>
30 $ ghci
31 GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
32 Loading package base ... linking ... done.
33 Prelude>
34 </screen>
35
36 <para>There may be a short pause while GHCi loads the prelude and
37 standard libraries, after which the prompt is shown. As the banner
38 says, you can type <literal>:?</literal> to see the list of commands
39 available, and a half line description of each of them.</para>
40
41 <para>We'll explain most of these commands as we go along. For
42 Hugs users: many things work the same as in Hugs, so you should be
43 able to get going straight away.</para>
44
45 <para>Haskell expressions can be typed at the prompt:</para>
46 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
47 </indexterm>
48
49 <screen>
50 Prelude> 1+2
51 3
52 Prelude> let x = 42 in x / 9
53 4.666666666666667
54 Prelude>
55 </screen>
56
57 <para>GHCi interprets the whole line as an expression to evaluate.
58 The expression may not span several lines - as soon as you press
59 enter, GHCi will attempt to evaluate it.</para>
60 </sect1>
61
62 <sect1 id="loading-source-files">
63 <title>Loading source files</title>
64
65 <para>Suppose we have the following Haskell source code, which we
66 place in a file <filename>Main.hs</filename>:</para>
67
68 <programlisting>
69 main = print (fac 20)
70
71 fac 0 = 1
72 fac n = n * fac (n-1)
73 </programlisting>
74
75 <para>You can save <filename>Main.hs</filename> anywhere you like,
76 but if you save it somewhere other than the current
77 directory<footnote><para>If you started up GHCi from the command
78 line then GHCi's current directory is the same as the current
79 directory of the shell from which it was started. If you started
80 GHCi from the &ldquo;Start&rdquo; menu in Windows, then the
81 current directory is probably something like
82 <filename>C:\Documents and Settings\<replaceable>user
83 name</replaceable></filename>.</para> </footnote> then we will
84 need to change to the right directory in GHCi:</para>
85
86 <screen>
87 Prelude> :cd <replaceable>dir</replaceable>
88 </screen>
89
90 <para>where <replaceable>dir</replaceable> is the directory (or
91 folder) in which you saved <filename>Main.hs</filename>.</para>
92
93 <para>To load a Haskell source file into GHCi, use the
94 <literal>:load</literal> command:</para>
95 <indexterm><primary><literal>:load</literal></primary></indexterm>
96
97 <screen>
98 Prelude> :load Main
99 Compiling Main ( Main.hs, interpreted )
100 Ok, modules loaded: Main.
101 *Main>
102 </screen>
103
104 <para>GHCi has loaded the <literal>Main</literal> module, and the
105 prompt has changed to &ldquo;<literal>*Main></literal>&rdquo; to
106 indicate that the current context for expressions typed at the
107 prompt is the <literal>Main</literal> module we just loaded (we'll
108 explain what the <literal>*</literal> means later in <xref
109 linkend="ghci-scope"/>). So we can now type expressions involving
110 the functions from <filename>Main.hs</filename>:</para>
111
112 <screen>
113 *Main> fac 17
114 355687428096000
115 </screen>
116
117 <para>Loading a multi-module program is just as straightforward;
118 just give the name of the &ldquo;topmost&rdquo; module to the
119 <literal>:load</literal> command (hint: <literal>:load</literal>
120 can be abbreviated to <literal>:l</literal>). The topmost module
121 will normally be <literal>Main</literal>, but it doesn't have to
122 be. GHCi will discover which modules are required, directly or
123 indirectly, by the topmost module, and load them all in dependency
124 order.</para>
125
126 <sect2 id="ghci-modules-filenames">
127 <title>Modules vs. filenames</title>
128 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
129 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
130
131 <para>Question: How does GHC find the filename which contains
132 module <replaceable>M</replaceable>? Answer: it looks for the
133 file <literal><replaceable>M</replaceable>.hs</literal>, or
134 <literal><replaceable>M</replaceable>.lhs</literal>. This means
135 that for most modules, the module name must match the filename.
136 If it doesn't, GHCi won't be able to find it.</para>
137
138 <para>There is one exception to this general rule: when you load
139 a program with <literal>:load</literal>, or specify it when you
140 invoke <literal>ghci</literal>, you can give a filename rather
141 than a module name. This filename is loaded if it exists, and
142 it may contain any module you like. This is particularly
143 convenient if you have several <literal>Main</literal> modules
144 in the same directory and you can't call them all
145 <filename>Main.hs</filename>.</para>
146
147 <para>The search path for finding source files is specified with
148 the <option>-i</option> option on the GHCi command line, like
149 so:</para>
150 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
151
152 <para>or it can be set using the <literal>:set</literal> command
153 from within GHCi (see <xref
154 linkend="ghci-cmd-line-options"/>)<footnote><para>Note that in
155 GHCi, and <option>&ndash;&ndash;make</option> mode, the <option>-i</option>
156 option is used to specify the search path for
157 <emphasis>source</emphasis> files, whereas in standard
158 batch-compilation mode the <option>-i</option> option is used to
159 specify the search path for interface files, see <xref
160 linkend="search-path"/>.</para> </footnote></para>
161
162 <para>One consequence of the way that GHCi follows dependencies
163 to find modules to load is that every module must have a source
164 file. The only exception to the rule is modules that come from
165 a package, including the <literal>Prelude</literal> and standard
166 libraries such as <literal>IO</literal> and
167 <literal>Complex</literal>. If you attempt to load a module for
168 which GHCi can't find a source file, even if there are object
169 and interface files for the module, you'll get an error
170 message.</para>
171 </sect2>
172
173 <sect2>
174 <title>Making changes and recompilation</title>
175 <indexterm><primary><literal>:reload</literal></primary></indexterm>
176
177 <para>If you make some changes to the source code and want GHCi
178 to recompile the program, give the <literal>:reload</literal>
179 command. The program will be recompiled as necessary, with GHCi
180 doing its best to avoid actually recompiling modules if their
181 external dependencies haven't changed. This is the same
182 mechanism we use to avoid re-compiling modules in the batch
183 compilation setting (see <xref linkend="recomp"/>).</para>
184 </sect2>
185 </sect1>
186
187 <sect1 id="ghci-compiled">
188 <title>Loading compiled code</title>
189 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
190
191 <para>When you load a Haskell source module into GHCi, it is
192 normally converted to byte-code and run using the interpreter.
193 However, interpreted code can also run alongside compiled code in
194 GHCi; indeed, normally when GHCi starts, it loads up a compiled
195 copy of the <literal>base</literal> package, which contains the
196 <literal>Prelude</literal>.</para>
197
198 <para>Why should we want to run compiled code? Well, compiled
199 code is roughly 10x faster than interpreted code, but takes about
200 2x longer to produce (perhaps longer if optimisation is on). So
201 it pays to compile the parts of a program that aren't changing
202 very often, and use the interpreter for the code being actively
203 developed.</para>
204
205 <para>When loading up source files with <literal>:load</literal>,
206 GHCi looks for any corresponding compiled object files, and will
207 use one in preference to interpreting the source if possible. For
208 example, suppose we have a 4-module program consisting of modules
209 A, B, C, and D. Modules B and C both import D only,
210 and A imports both B &amp; C:</para>
211 <screen>
212 A
213 / \
214 B C
215 \ /
216 D
217 </screen>
218 <para>We can compile D, then load the whole program, like this:</para>
219 <screen>
220 Prelude> :! ghc -c D.hs
221 Prelude> :load A
222 Compiling B ( B.hs, interpreted )
223 Compiling C ( C.hs, interpreted )
224 Compiling A ( A.hs, interpreted )
225 Ok, modules loaded: A, B, C, D.
226 *Main>
227 </screen>
228
229 <para>In the messages from the compiler, we see that there is no line
230 for <literal>D</literal>. This is because
231 it isn't necessary to compile <literal>D</literal>,
232 because the source and everything it depends on
233 is unchanged since the last compilation.</para>
234
235 <para>At any time you can use the command
236 <literal>:show modules</literal>
237 to get a list of the modules currently loaded
238 into GHCi:</para>
239
240 <screen>
241 *Main> :show modules
242 D ( D.hs, D.o )
243 C ( C.hs, interpreted )
244 B ( B.hs, interpreted )
245 A ( A.hs, interpreted )
246 *Main></screen>
247
248 <para>If we now modify the source of D (or pretend to: using the Unix
249 command <literal>touch</literal> on the source file is handy for
250 this), the compiler will no longer be able to use the object file,
251 because it might be out of date:</para>
252
253 <screen>
254 *Main> :! touch D.hs
255 *Main> :reload
256 Compiling D ( D.hs, interpreted )
257 Ok, modules loaded: A, B, C, D.
258 *Main>
259 </screen>
260
261 <para>Note that module D was compiled, but in this instance
262 because its source hadn't really changed, its interface remained
263 the same, and the recompilation checker determined that A, B and C
264 didn't need to be recompiled.</para>
265
266 <para>So let's try compiling one of the other modules:</para>
267
268 <screen>
269 *Main> :! ghc -c C.hs
270 *Main> :load A
271 Compiling D ( D.hs, interpreted )
272 Compiling B ( B.hs, interpreted )
273 Compiling C ( C.hs, interpreted )
274 Compiling A ( A.hs, interpreted )
275 Ok, modules loaded: A, B, C, D.
276 </screen>
277
278 <para>We didn't get the compiled version of C! What happened?
279 Well, in GHCi a compiled module may only depend on other compiled
280 modules, and in this case C depends on D, which doesn't have an
281 object file, so GHCi also rejected C's object file. Ok, so let's
282 also compile D:</para>
283
284 <screen>
285 *Main> :! ghc -c D.hs
286 *Main> :reload
287 Ok, modules loaded: A, B, C, D.
288 </screen>
289
290 <para>Nothing happened! Here's another lesson: newly compiled
291 modules aren't picked up by <literal>:reload</literal>, only
292 <literal>:load</literal>:</para>
293
294 <screen>
295 *Main> :load A
296 Compiling B ( B.hs, interpreted )
297 Compiling A ( A.hs, interpreted )
298 Ok, modules loaded: A, B, C, D.
299 </screen>
300
301 <para>HINT: since GHCi will only use a compiled object file if it
302 can be sure that the compiled version is up-to-date, a good technique
303 when working on a large program is to occasionally run
304 <literal>ghc &ndash;&ndash;make</literal> to compile the whole project (say
305 before you go for lunch :-), then continue working in the
306 interpreter. As you modify code, the changed modules will be
307 interpreted, but the rest of the project will remain
308 compiled.</para>
309
310 </sect1>
311
312 <sect1 id="interactive-evaluation">
313 <title>Interactive evaluation at the prompt</title>
314
315 <para>When you type an expression at the prompt, GHCi immediately
316 evaluates and prints the result:
317 <screen>
318 Prelude> reverse "hello"
319 "olleh"
320 Prelude> 5+5
321 10
322 </screen>
323 </para>
324
325 <sect2><title>I/O actions at the prompt</title>
326
327 <para>GHCi does more than simple expression evaluation at the prompt.
328 If you type something of type <literal>IO a</literal> for some
329 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
330 as an IO-computation.
331 <screen>
332 Prelude> "hello"
333 "hello"
334 Prelude> putStrLn "hello"
335 hello
336 </screen>
337 Furthermore, GHCi will print the result of the I/O action if (and only
338 if):
339 <itemizedlist>
340 <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
341 <listitem><para>The result type is not
342 <literal>()</literal>.</para></listitem>
343 </itemizedlist>
344 For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
345 <screen>
346 Prelude> putStrLn "hello"
347 hello
348 Prelude> do { putStrLn "hello"; return "yes" }
349 hello
350 "yes"
351 </screen>
352 </para></sect2>
353
354 <sect2 id="ghci-stmts">
355 <title>Using <literal>do-</literal>notation at the prompt</title>
356 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
357 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
358
359 <para>GHCi actually accepts <firstterm>statements</firstterm>
360 rather than just expressions at the prompt. This means you can
361 bind values and functions to names, and use them in future
362 expressions or statements.</para>
363
364 <para>The syntax of a statement accepted at the GHCi prompt is
365 exactly the same as the syntax of a statement in a Haskell
366 <literal>do</literal> expression. However, there's no monad
367 overloading here: statements typed at the prompt must be in the
368 <literal>IO</literal> monad.
369 <screen>
370 Prelude> x &lt;- return 42
371 42
372 Prelude> print x
373 42
374 Prelude>
375 </screen>
376 The statement <literal>x &lt;- return 42</literal> means
377 &ldquo;execute <literal>return 42</literal> in the
378 <literal>IO</literal> monad, and bind the result to
379 <literal>x</literal>&rdquo;. We can then use
380 <literal>x</literal> in future statements, for example to print
381 it as we did above.</para>
382
383 <para>GHCi will print the result of a statement if and only if:
384 <itemizedlist>
385 <listitem>
386 <para>The statement is not a binding, or it is a monadic binding
387 (<literal>p &lt;- e</literal>) that binds exactly one
388 variable.</para>
389 </listitem>
390 <listitem>
391 <para>The variable's type is not polymorphic, is not
392 <literal>()</literal>, and is an instance of
393 <literal>Show</literal></para>
394 </listitem>
395 </itemizedlist>
396 The automatic printing of binding results can be supressed with
397 <option>:set -fno-print-bind-result</option> (this does not
398 supress printing the result of non-binding statements).
399 <indexterm><primary><option>-fno-print-bind-result</option></primary></indexterm><indexterm><primary><option>-fprint-bind-result</option></primary></indexterm>.
400 You might want to do this to prevent the result of binding
401 statements from being fully evaluated by the act of printing
402 them, for example.</para>
403
404 <para>Of course, you can also bind normal non-IO expressions
405 using the <literal>let</literal>-statement:</para>
406 <screen>
407 Prelude> let x = 42
408 Prelude> x
409 42
410 Prelude>
411 </screen>
412 <para>Another important difference between the two types of binding
413 is that the monadic bind (<literal>p &lt;- e</literal>) is
414 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
415 whereas with the <literal>let</literal> form, the expression
416 isn't evaluated immediately:</para>
417 <screen>
418 Prelude> let x = error "help!"
419 Prelude> print x
420 *** Exception: help!
421 Prelude>
422 </screen>
423
424 <para>Note that <literal>let</literal> bindings do not automatically
425 print the value bound, unlike monadic bindings.</para>
426
427 <para>Any exceptions raised during the evaluation or execution
428 of the statement are caught and printed by the GHCi command line
429 interface (for more information on exceptions, see the module
430 <literal>Control.Exception</literal> in the libraries
431 documentation).</para>
432
433 <para>Every new binding shadows any existing bindings of the
434 same name, including entities that are in scope in the current
435 module context.</para>
436
437 <para>WARNING: temporary bindings introduced at the prompt only
438 last until the next <literal>:load</literal> or
439 <literal>:reload</literal> command, at which time they will be
440 simply lost. However, they do survive a change of context with
441 <literal>:module</literal>: the temporary bindings just move to
442 the new location.</para>
443
444 <para>HINT: To get a list of the bindings currently in scope, use the
445 <literal>:show bindings</literal> command:</para>
446
447 <screen>
448 Prelude> :show bindings
449 x :: Int
450 Prelude></screen>
451
452 <para>HINT: if you turn on the <literal>+t</literal> option,
453 GHCi will show the type of each variable bound by a statement.
454 For example:</para>
455 <indexterm><primary><literal>+t</literal></primary></indexterm>
456 <screen>
457 Prelude> :set +t
458 Prelude> let (x:xs) = [1..]
459 x :: Integer
460 xs :: [Integer]
461 </screen>
462
463 </sect2>
464
465 <sect2 id="ghci-scope">
466 <title>What's really in scope at the prompt?</title>
467
468 <para>When you type an expression at the prompt, what
469 identifiers and types are in scope? GHCi provides a flexible
470 way to control exactly how the context for an expression is
471 constructed. Let's start with the simple cases; when you start
472 GHCi the prompt looks like this:</para>
473
474 <screen>Prelude></screen>
475
476 <para>Which indicates that everything from the module
477 <literal>Prelude</literal> is currently in scope. If we now
478 load a file into GHCi, the prompt will change:</para>
479
480 <screen>
481 Prelude> :load Main.hs
482 Compiling Main ( Main.hs, interpreted )
483 *Main>
484 </screen>
485
486 <para>The new prompt is <literal>*Main</literal>, which
487 indicates that we are typing expressions in the context of the
488 top-level of the <literal>Main</literal> module. Everything
489 that is in scope at the top-level in the module
490 <literal>Main</literal> we just loaded is also in scope at the
491 prompt (probably including <literal>Prelude</literal>, as long
492 as <literal>Main</literal> doesn't explicitly hide it).</para>
493
494 <para>The syntax
495 <literal>*<replaceable>module</replaceable></literal> indicates
496 that it is the full top-level scope of
497 <replaceable>module</replaceable> that is contributing to the
498 scope for expressions typed at the prompt. Without the
499 <literal>*</literal>, just the exports of the module are
500 visible.</para>
501
502 <para>We're not limited to a single module: GHCi can combine
503 scopes from multiple modules, in any mixture of
504 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
505 combines the scopes from all of these modules to form the scope
506 that is in effect at the prompt. For technical reasons, GHCi
507 can only support the <literal>*</literal>-form for modules which
508 are interpreted, so compiled modules and package modules can
509 only contribute their exports to the current scope.</para>
510
511 <para>The scope is manipulated using the
512 <literal>:module</literal> command. For example, if the current
513 scope is <literal>Prelude</literal>, then we can bring into
514 scope the exports from the module <literal>IO</literal> like
515 so:</para>
516
517 <screen>
518 Prelude> :module +IO
519 Prelude IO> hPutStrLn stdout "hello\n"
520 hello
521 Prelude IO>
522 </screen>
523
524 <para>(Note: you can use <literal>import M</literal> as an
525 alternative to <literal>:module +M</literal>, and
526 <literal>:module</literal> can also be shortened to
527 <literal>:m</literal>). The full syntax of the
528 <literal>:module</literal> command is:</para>
529
530 <screen>
531 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
532 </screen>
533
534 <para>Using the <literal>+</literal> form of the
535 <literal>module</literal> commands adds modules to the current
536 scope, and <literal>-</literal> removes them. Without either
537 <literal>+</literal> or <literal>-</literal>, the current scope
538 is replaced by the set of modules specified. Note that if you
539 use this form and leave out <literal>Prelude</literal>, GHCi
540 will assume that you really wanted the
541 <literal>Prelude</literal> and add it in for you (if you don't
542 want the <literal>Prelude</literal>, then ask to remove it with
543 <literal>:m -Prelude</literal>).</para>
544
545 <para>The scope is automatically set after a
546 <literal>:load</literal> command, to the most recently loaded
547 "target" module, in a <literal>*</literal>-form if possible.
548 For example, if you say <literal>:load foo.hs bar.hs</literal>
549 and <filename>bar.hs</filename> contains module
550 <literal>Bar</literal>, then the scope will be set to
551 <literal>*Bar</literal> if <literal>Bar</literal> is
552 interpreted, or if <literal>Bar</literal> is compiled it will be
553 set to <literal>Prelude Bar</literal> (GHCi automatically adds
554 <literal>Prelude</literal> if it isn't present and there aren't
555 any <literal>*</literal>-form modules).</para>
556
557 <para>With multiple modules in scope, especially multiple
558 <literal>*</literal>-form modules, it is likely that name
559 clashes will occur. Haskell specifies that name clashes are
560 only reported when an ambiguous identifier is used, and GHCi
561 behaves in the same way for expressions typed at the
562 prompt.</para>
563
564 <para>
565 Hint: GHCi will tab-complete names that are in scope; for
566 example, if you run GHCi and type <literal>J&lt;tab&gt;</literal>
567 then GHCi will expand it to &ldquo;<literal>Just </literal>&rdquo;.
568 </para>
569
570 <sect3>
571 <title>Qualified names</title>
572
573 <para>To make life slightly easier, the GHCi prompt also
574 behaves as if there is an implicit <literal>import
575 qualified</literal> declaration for every module in every
576 package, and every module currently loaded into GHCi.</para>
577 </sect3>
578
579 <sect3>
580 <title>The <literal>:main</literal> command</title>
581
582 <para>
583 When a program is compiled and executed, it can use the
584 <literal>getArgs</literal> function to access the
585 command-line arguments.
586 However, we cannot simply pass the arguments to the
587 <literal>main</literal> function while we are testing in ghci,
588 as the <literal>main</literal> function doesn't take its
589 directly.
590 </para>
591
592 <para>
593 Instead, we can use the <literal>:main</literal> command.
594 This runs whatever <literal>main</literal> is in scope, with
595 any arguments being treated the same as command-line arguments,
596 e.g.:
597 </para>
598
599 <screen>
600 Prelude> let main = System.Environment.getArgs >>= print
601 Prelude> :main foo bar
602 ["foo","bar"]
603 </screen>
604
605 </sect3>
606 </sect2>
607
608
609 <sect2>
610 <title>The <literal>it</literal> variable</title>
611 <indexterm><primary><literal>it</literal></primary>
612 </indexterm>
613
614 <para>Whenever an expression (or a non-binding statement, to be
615 precise) is typed at the prompt, GHCi implicitly binds its value
616 to the variable <literal>it</literal>. For example:</para>
617 <screen>
618 Prelude> 1+2
619 3
620 Prelude> it * 2
621 6
622 </screen>
623 <para>What actually happens is that GHCi typechecks the
624 expression, and if it doesn't have an <literal>IO</literal> type,
625 then it transforms it as follows: an expression
626 <replaceable>e</replaceable> turns into
627 <screen>
628 let it = <replaceable>e</replaceable>;
629 print it
630 </screen>
631 which is then run as an IO-action.</para>
632
633 <para>Hence, the original expression must have a type which is an
634 instance of the <literal>Show</literal> class, or GHCi will
635 complain:</para>
636
637 <screen>
638 Prelude&gt; id
639
640 &lt;interactive&gt;:1:0:
641 No instance for (Show (a -&gt; a))
642 arising from use of `print' at &lt;interactive&gt;:1:0-1
643 Possible fix: add an instance declaration for (Show (a -> a))
644 In the expression: print it
645 In a 'do' expression: print it
646 </screen>
647
648 <para>The error message contains some clues as to the
649 transformation happening internally.</para>
650
651 <para>If the expression was instead of type <literal>IO a</literal> for
652 some <literal>a</literal>, then <literal>it</literal> will be
653 bound to the result of the <literal>IO</literal> computation,
654 which is of type <literal>a</literal>. eg.:</para>
655 <screen>
656 Prelude> Time.getClockTime
657 Wed Mar 14 12:23:13 GMT 2001
658 Prelude> print it
659 Wed Mar 14 12:23:13 GMT 2001
660 </screen>
661
662 <para>The corresponding translation for an IO-typed
663 <replaceable>e</replaceable> is
664 <screen>
665 it &lt;- <replaceable>e</replaceable>
666 </screen>
667 </para>
668
669 <para>Note that <literal>it</literal> is shadowed by the new
670 value each time you evaluate a new expression, and the old value
671 of <literal>it</literal> is lost.</para>
672
673 </sect2>
674
675 <sect2 id="extended-default-rules">
676 <title>Type defaulting in GHCi</title>
677 <indexterm><primary>Type default</primary></indexterm>
678 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
679 <para>
680 Consider this GHCi session:
681 <programlisting>
682 ghci> reverse []
683 </programlisting>
684 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
685 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
686 on the type <literal>a</literal>. For example:
687 <programlisting>
688 ghci> (reverse []) :: String
689 ""
690 ghci> (reverse []) :: [Int]
691 []
692 </programlisting>
693 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
694 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
695 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
696 a)</literal> for each type variable <literal>a</literal>, and defaults the
697 type variable if
698 <orderedlist>
699 <listitem>
700 <para>
701 The type variable <literal>a</literal> appears in no
702 other constraints
703 </para>
704 </listitem>
705 <listitem>
706 <para>
707 All the classes <literal>Ci</literal> are standard.
708 </para>
709 </listitem>
710 <listitem>
711 <para>
712 At least one of the classes <literal>Ci</literal> is
713 numeric.
714 </para>
715 </listitem>
716 </orderedlist>
717 At the GHCi prompt, or with GHC if the
718 <literal>-XExtendedDefaultRules</literal> flag is given,
719 the following additional differences apply:
720 <itemizedlist>
721 <listitem>
722 <para>
723 Rule 2 above is relaxed thus:
724 <emphasis>All</emphasis> of the classes
725 <literal>Ci</literal> are single-parameter type classes.
726 </para>
727 </listitem>
728 <listitem>
729 <para>
730 Rule 3 above is relaxed this:
731 At least one of the classes <literal>Ci</literal> is
732 numeric, <emphasis>or is <literal>Show</literal>,
733 <literal>Eq</literal>, or
734 <literal>Ord</literal></emphasis>.
735 </para>
736 </listitem>
737 <listitem>
738 <para>
739 The unit type <literal>()</literal> is added to the
740 start of the standard list of types which are tried when
741 doing type defaulting.
742 </para>
743 </listitem>
744 </itemizedlist>
745 The last point means that, for example, this program:
746 <programlisting>
747 main :: IO ()
748 main = print def
749
750 instance Num ()
751
752 def :: (Num a, Enum a) => a
753 def = toEnum 0
754 </programlisting>
755 prints <literal>()</literal> rather than <literal>0</literal> as the
756 type is defaulted to <literal>()</literal> rather than
757 <literal>Integer</literal>.
758 </para>
759 <para>
760 The motivation for the change is that it means <literal>IO a</literal>
761 actions default to <literal>IO ()</literal>, which in turn means that
762 ghci won't try to print a result when running them. This is
763 particularly important for <literal>printf</literal>, which has an
764 instance that returns <literal>IO a</literal>.
765 However, it is only able to return
766 <literal>undefined</literal>
767 (the reason for the instance having this type is so that printf
768 doesn't require extensions to the class system), so if the type defaults to
769 <literal>Integer</literal> then ghci gives an error when running a
770 printf.
771 </para>
772 </sect2>
773 </sect1>
774
775 <sect1 id="ghci-debugger">
776 <title>The GHCi Debugger</title>
777 <indexterm><primary>debugger</primary><secondary>in GHCi</secondary>
778 </indexterm>
779
780 <para>GHCi contains a simple imperative-style debugger in which you can
781 stop a running computation in order to examine the values of
782 variables. The debugger is integrated into GHCi, and is turned on by
783 default: no flags are required to enable the debugging facilities. There
784 is one major restriction: breakpoints and single-stepping are only
785 available in <emphasis>interpreted</emphasis> modules; compiled code is
786 invisible to the debugger.</para>
787
788 <para>The debugger provides the following:
789 <itemizedlist>
790 <listitem>
791 <para>The ability to set a <firstterm>breakpoint</firstterm> on a
792 function definition or expression in the program. When the function
793 is called, or the expression evaluated, GHCi suspends
794 execution and returns to the prompt, where you can inspect the
795 values of local variables before continuing with the
796 execution.</para>
797 </listitem>
798 <listitem>
799 <para>Execution can be <firstterm>single-stepped</firstterm>: the
800 evaluator will suspend execution approximately after every
801 reduction, allowing local variables to be inspected. This is
802 equivalent to setting a breakpoint at every point in the
803 program.</para>
804 </listitem>
805 <listitem>
806 <para>Execution can take place in <firstterm>tracing
807 mode</firstterm>, in which the evaluator remembers each
808 evaluation step as it happens, but doesn't suspend execution until
809 an actual breakpoint is reached. When this happens, the history of
810 evaluation steps can be inspected.</para>
811 </listitem>
812 <listitem>
813 <para>Exceptions (e.g. pattern matching failure and
814 <literal>error</literal>) can be treated as breakpoints, to help
815 locate the source of an exception in the program.</para>
816 </listitem>
817 </itemizedlist>
818 </para>
819
820 <para>There is currently no support for obtaining a &ldquo;stack
821 trace&rdquo;, but the tracing and history features provide a useful
822 second-best, which will often be enough to establish the context of an
823 error.</para>
824
825 <sect2 id="breakpoints">
826 <title>Breakpoints and inspecting variables</title>
827
828 <para>Let's use quicksort as a running example. Here's the code:</para>
829
830 <programlisting>
831 qsort [] = []
832 qsort (a:as) = qsort left ++ [a] ++ qsort right
833 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
834
835 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
836 </programlisting>
837
838 <para>First, load the module into GHCi:</para>
839
840 <screen>
841 Prelude> :l qsort.hs
842 [1 of 1] Compiling Main ( qsort.hs, interpreted )
843 Ok, modules loaded: Main.
844 *Main>
845 </screen>
846
847 <para>Now, let's set a breakpoint on the right-hand-side of the second
848 equation of qsort:</para>
849
850 <programlisting>
851 *Main> :break 2
852 Breakpoint 0 activated at qsort.hs:2:15-46
853 *Main>
854 </programlisting>
855
856 <para>The command <literal>:break 2</literal> sets a breakpoint on line
857 2 of the most recently-loaded module, in this case
858 <literal>qsort.hs</literal>. Specifically, it picks the
859 leftmost complete subexpression on that line on which to set the
860 breakpoint, which in this case is the expression
861 <literal>(qsort left ++ [a] ++ qsort right)</literal>.</para>
862
863 <para>Now, we run the program:</para>
864
865 <programlisting>
866 *Main> main
867 Stopped at qsort.hs:2:15-46
868 _result :: [a]
869 a :: a
870 left :: [a]
871 right :: [a]
872 [qsort.hs:2:15-46] *Main>
873 </programlisting>
874
875 <para>Execution has stopped at the breakpoint. The prompt has changed to
876 indicate that we are currently stopped at a breakpoint, and the location:
877 <literal>[qsort.hs:2:15-46]</literal>. To further clarify the
878 location, we can use the <literal>:list</literal> command:</para>
879
880 <programlisting>
881 [qsort.hs:2:15-46] *Main> :list
882 1 qsort [] = []
883 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
884 3 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
885 </programlisting>
886
887 <para>The <literal>:list</literal> command lists the source code around
888 the current breakpoint. If your output device supports it, then GHCi
889 will highlight the active subexpression in bold.</para>
890
891 <para>GHCi has provided bindings for the free variables<footnote><para>We
892 originally provided bindings for all variables in scope, rather
893 than just
894 the free variables of the expression, but found that this affected
895 performance considerably, hence the current restriction to just the
896 free variables.</para>
897 </footnote> of the expression
898 on which the
899 breakpoint was placed (<literal>a</literal>, <literal>left</literal>,
900 <literal>right</literal>), and additionally a binding for the result of
901 the expression (<literal>_result</literal>). These variables are just
902 like other variables that you might define in GHCi; you
903 can use them in expressions that you type at the prompt, you can ask
904 for their types with <literal>:type</literal>, and so on. There is one
905 important difference though: these variables may only have partial
906 types. For example, if we try to display the value of
907 <literal>left</literal>:</para>
908
909 <screen>
910 [qsort.hs:2:15-46] *Main> left
911
912 &lt;interactive&gt;:1:0:
913 Ambiguous type variable `a' in the constraint:
914 `Show a' arising from a use of `print' at &lt;interactive&gt;:1:0-3
915 Cannot resolve unknown runtime types: a
916 Use :print or :force to determine these types
917 </screen>
918
919 <para>This is because <literal>qsort</literal> is a polymorphic function,
920 and because GHCi does not carry type information at runtime, it cannot
921 determine the runtime types of free variables that involve type
922 variables. Hence, when you ask to display <literal>left</literal> at
923 the prompt, GHCi can't figure out which instance of
924 <literal>Show</literal> to use, so it emits the type error above.</para>
925
926 <para>Fortunately, the debugger includes a generic printing command,
927 <literal>:print</literal>, which can inspect the actual runtime value of a
928 variable and attempt to reconstruct its type. If we try it on
929 <literal>left</literal>:</para>
930
931 <screen>
932 [qsort.hs:2:15-46] *Main> :print left
933 left = (_t1::[a])
934 </screen>
935
936 <para>This isn't particularly enlightening. What happened is that
937 <literal>left</literal> is bound to an unevaluated computation (a
938 suspension, or <firstterm>thunk</firstterm>), and
939 <literal>:print</literal> does not force any evaluation. The idea is
940 that <literal>:print</literal> can be used to inspect values at a
941 breakpoint without any unfortunate side effects. It won't force any
942 evaluation, which could cause the program to give a different answer
943 than it would normally, and hence it won't cause any exceptions to be
944 raised, infinite loops, or further breakpoints to be triggered (see
945 <xref linkend="nested-breakpoints" />).
946 Rather than forcing thunks, <literal>:print</literal>
947 binds each thunk to a fresh variable beginning with an
948 underscore, in this case
949 <literal>_t1</literal>.</para>
950
951 <para>If we aren't concerned about preserving the evaluatedness of a
952 variable, we can use <literal>:force</literal> instead of
953 <literal>:print</literal>. The <literal>:force</literal> command
954 behaves exactly like <literal>:print</literal>, except that it forces
955 the evaluation of any thunks it encounters:</para>
956
957 <screen>
958 [qsort.hs:2:15-46] *Main> :force left
959 left = [4,0,3,1]
960 </screen>
961
962 <para>Now, since <literal>:force</literal> has inspected the runtime
963 value of <literal>left</literal>, it has reconstructed its type. We
964 can see the results of this type reconstruction:</para>
965
966 <screen>
967 [qsort.hs:2:15-46] *Main> :show bindings
968 _result :: [Integer]
969 a :: Integer
970 left :: [Integer]
971 right :: [Integer]
972 _t1 :: [Integer]
973 </screen>
974
975 <para>Not only do we now know the type of <literal>left</literal>, but
976 all the other partial types have also been resolved. So we can ask
977 for the value of <literal>a</literal>, for example:</para>
978
979 <screen>
980 [qsort.hs:2:15-46] *Main> a
981 8
982 </screen>
983
984 <para>You might find it useful to use Haskell's
985 <literal>seq</literal> function to evaluate individual thunks rather
986 than evaluating the whole expression with <literal>:force</literal>.
987 For example:</para>
988
989 <screen>
990 [qsort.hs:2:15-46] *Main> :print right
991 right = (_t1::[Integer])
992 [qsort.hs:2:15-46] *Main> seq _t1 ()
993 ()
994 [qsort.hs:2:15-46] *Main> :print right
995 right = 23 : (_t2::[Integer])
996 </screen>
997
998 <para>We evaluated only the <literal>_t1</literal> thunk, revealing the
999 head of the list, and the tail is another thunk now bound to
1000 <literal>_t2</literal>. The <literal>seq</literal> function is a
1001 little inconvenient to use here, so you might want to use
1002 <literal>:def</literal> to make a nicer interface (left as an exercise
1003 for the reader!).</para>
1004
1005 <para>Finally, we can continue the current execution:</para>
1006
1007 <screen>
1008 [qsort.hs:2:15-46] *Main> :continue
1009 Stopped at qsort.hs:2:15-46
1010 _result :: [a]
1011 a :: a
1012 left :: [a]
1013 right :: [a]
1014 [qsort.hs:2:15-46] *Main>
1015 </screen>
1016
1017 <para>The execution continued at the point it previously stopped, and has
1018 now stopped at the breakpoint for a second time.</para>
1019
1020 <sect3 id="setting-breakpoints">
1021 <title>Setting breakpoints</title>
1022
1023 <para>Breakpoints can be set in various ways. Perhaps the easiest way to
1024 set a breakpoint is to name a top-level function:</para>
1025
1026 <screen>
1027 :break <replaceable>identifier</replaceable>
1028 </screen>
1029
1030 <para>Where <replaceable>identifier</replaceable> names any top-level
1031 function in an interpreted module currently loaded into GHCi (qualified
1032 names may be used). The breakpoint will be set on the body of the
1033 function, when it is fully applied but before any pattern matching has
1034 taken place.</para>
1035
1036 <para>Breakpoints can also be set by line (and optionally column)
1037 number:</para>
1038
1039 <screen>
1040 :break <replaceable>line</replaceable>
1041 :break <replaceable>line</replaceable> <replaceable>column</replaceable>
1042 :break <replaceable>module</replaceable> <replaceable>line</replaceable>
1043 :break <replaceable>module</replaceable> <replaceable>line</replaceable> <replaceable>column</replaceable>
1044 </screen>
1045
1046 <para>When a breakpoint is set on a particular line, GHCi sets the
1047 breakpoint on the
1048 leftmost subexpression that begins and ends on that line. If two
1049 complete subexpressions start at the same
1050 column, the longest one is picked. If there is no complete
1051 subexpression on the line, then the leftmost expression starting on
1052 the line is picked, and failing that the rightmost expression that
1053 partially or completely covers the line.</para>
1054
1055 <para>When a breakpoint is set on a particular line and column, GHCi
1056 picks the smallest subexpression that encloses that location on which
1057 to set the breakpoint. Note: GHC considers the TAB character to have a
1058 width of 1, wherever it occurs; in other words it counts
1059 characters, rather than columns. This matches what some editors do,
1060 and doesn't match others. The best advice is to avoid tab
1061 characters in your source code altogether (see
1062 <option>-fwarn-tabs</option> in <xref linkend="options-sanity"
1063 />).</para>
1064
1065 <para>If the module is omitted, then the most recently-loaded module is
1066 used.</para>
1067
1068 <para>Not all subexpressions are potential breakpoint locations. Single
1069 variables are typically not considered to be breakpoint locations
1070 (unless the variable is the right-hand-side of a function definition,
1071 lambda, or case alternative). The rule of thumb is that all redexes
1072 are breakpoint locations, together with the bodies of functions,
1073 lambdas, case alternatives and binding statements. There is normally
1074 no breakpoint on a let expression, but there will always be a
1075 breakpoint on its body, because we are usually interested in inspecting
1076 the values of the variables bound by the let.</para>
1077
1078 </sect3>
1079 <sect3>
1080 <title>Listing and deleting breakpoints</title>
1081
1082 <para>The list of breakpoints currently enabled can be displayed using
1083 <literal>:show&nbsp;breaks</literal>:</para>
1084 <screen>
1085 *Main> :show breaks
1086 [0] Main qsort.hs:1:11-12
1087 [1] Main qsort.hs:2:15-46
1088 </screen>
1089
1090 <para>To delete a breakpoint, use the <literal>:delete</literal>
1091 command with the number given in the output from <literal>:show&nbsp;breaks</literal>:</para>
1092
1093 <screen>
1094 *Main> :delete 0
1095 *Main> :show breaks
1096 [1] Main qsort.hs:2:15-46
1097 </screen>
1098
1099 <para>To delete all breakpoints at once, use <literal>:delete *</literal>.</para>
1100
1101 </sect3>
1102 </sect2>
1103
1104 <sect2 id="single-stepping">
1105 <title>Single-stepping</title>
1106
1107 <para>Single-stepping is a great way to visualise the execution of your
1108 program, and it is also a useful tool for identifying the source of a
1109 bug. GHCi offers two variants of stepping. Use
1110 <literal>:step</literal> to enable all the
1111 breakpoints in the program, and execute until the next breakpoint is
1112 reached. Use <literal>:stepover</literal> to step over function
1113 applications, which of course are executed all the same.
1114 For example:</para>
1115
1116 <screen>
1117 *Main> :step main
1118 Stopped at qsort.hs:5:7-47
1119 _result :: IO ()
1120 </screen>
1121
1122 <para>The command <literal>:step
1123 <replaceable>expr</replaceable></literal> begins the evaluation of
1124 <replaceable>expr</replaceable> in single-stepping mode. If
1125 <replaceable>expr</replaceable> is ommitted, then it single-steps from
1126 the current breakpoint. <literal>:stepover</literal>
1127 works similarly.</para>
1128
1129 <para>The <literal>:list</literal> command is particularly useful when
1130 single-stepping, to see where you currently are:</para>
1131
1132 <screen>
1133 [qsort.hs:5:7-47] *Main> :list
1134 4
1135 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1136 6
1137 [qsort.hs:5:7-47] *Main>
1138 </screen>
1139
1140 <para>In fact, GHCi provides a way to run a command when a breakpoint is
1141 hit, so we can make it automatically do
1142 <literal>:list</literal>:</para>
1143
1144 <screen>
1145 [qsort.hs:5:7-47] *Main> :set stop :list
1146 [qsort.hs:5:7-47] *Main> :step
1147 Stopped at qsort.hs:5:14-46
1148 _result :: [Integer]
1149 4
1150 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1151 6
1152 [qsort.hs:5:14-46] *Main>
1153 </screen>
1154 </sect2>
1155
1156 <sect2 id="nested-breakpoints">
1157 <title>Nested breakpoints</title>
1158 <para>When GHCi is stopped at a breakpoint, and an expression entered at
1159 the prompt triggers a
1160 second breakpoint, the new breakpoint becomes the &ldquo;current&rdquo;
1161 one, and the old one is saved on a stack. An arbitrary number of
1162 breakpoint contexts can be built up in this way. For example:</para>
1163
1164 <screen>
1165 [qsort.hs:2:15-46] *Main> :st qsort [1,3]
1166 Stopped at qsort.hs:(1,0)-(3,55)
1167 _result :: [a]
1168 ... [qsort.hs:(1,0)-(3,55)] *Main>
1169 </screen>
1170
1171 <para>While stopped at the breakpoint on line 2 that we set earlier, we
1172 started a new evaluation with <literal>:step qsort [1,3]</literal>.
1173 This new evaluation stopped after one step (at the definition of
1174 <literal>qsort</literal>). The prompt has changed, now prefixed with
1175 <literal>...</literal>, to indicate that there are saved breakpoints
1176 beyond the current one. To see the stack of contexts, use
1177 <literal>:show context</literal>:</para>
1178
1179 <screen>
1180 ... [qsort.hs:(1,0)-(3,55)] *Main> :show context
1181 --> main
1182 Stopped at qsort.hs:2:15-46
1183 --> qsort [1,3]
1184 Stopped at qsort.hs:(1,0)-(3,55)
1185 ... [qsort.hs:(1,0)-(3,55)] *Main>
1186 </screen>
1187
1188 <para>To abandon the current evaluation, use
1189 <literal>:abandon</literal>:</para>
1190
1191 <screen>
1192 ... [qsort.hs:(1,0)-(3,55)] *Main> :abandon
1193 [qsort.hs:2:15-46] *Main> :abandon
1194 *Main>
1195 </screen>
1196 </sect2>
1197
1198 <sect2 id="ghci-debugger-result">
1199 <title>The <literal>_result</literal> variable</title>
1200 <para>When stopped at a breakpoint or single-step, GHCi binds the
1201 variable <literal>_result</literal> to the value of the currently
1202 active expression. The value of <literal>_result</literal> is
1203 presumably not available yet, because we stopped its evaluation, but it
1204 can be forced: if the type is known and showable, then just entering
1205 <literal>_result</literal> at the prompt will show it. However,
1206 there's one caveat to doing this: evaluating <literal>_result</literal>
1207 will be likely to trigger further breakpoints, starting with the
1208 breakpoint we are currently stopped at (if we stopped at a real
1209 breakpoint, rather than due to <literal>:step</literal>). So it will
1210 probably be necessary to issue a <literal>:continue</literal>
1211 immediately when evaluating <literal>_result</literal>. Alternatively,
1212 you can use <literal>:force</literal> which ignores breakpoints.</para>
1213 </sect2>
1214
1215 <sect2 id="tracing">
1216 <title>Tracing and history</title>
1217
1218 <para>A question that we often want to ask when debugging a program is
1219 &ldquo;how did I get here?&rdquo;. Traditional imperative debuggers
1220 usually provide some kind of stack-tracing feature that lets you see
1221 the stack of active function calls (sometimes called the &ldquo;lexical
1222 call stack&rdquo;), describing a path through the code
1223 to the current location. Unfortunately this is hard to provide in
1224 Haskell, because execution proceeds on a demand-driven basis, rather
1225 than a depth-first basis as in strict languages. The
1226 &ldquo;stack&ldquo; in GHC's execution engine bears little
1227 resemblance to the lexical call stack. Ideally GHCi would maintain a
1228 separate lexical call stack in addition to the dynamic call stack, and
1229 in fact this is exactly
1230 what our profiling system does (<xref linkend="profiling" />), and what
1231 some other Haskell debuggers do. For the time being, however, GHCi
1232 doesn't maintain a lexical call stack (there are some technical
1233 challenges to be overcome). Instead, we provide a way to backtrack from a
1234 breakpoint to previous evaluation steps: essentially this is like
1235 single-stepping backwards, and should in many cases provide enough
1236 information to answer the &ldquo;how did I get here?&rdquo;
1237 question.</para>
1238
1239 <para>To use tracing, evaluate an expression with the
1240 <literal>:trace</literal> command. For example, if we set a breakpoint
1241 on the base case of <literal>qsort</literal>:</para>
1242
1243 <screen>
1244 *Main&gt; :list qsort
1245 1 qsort [] = []
1246 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
1247 3 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
1248 4
1249 *Main&gt; :b 1
1250 Breakpoint 1 activated at qsort.hs:1:11-12
1251 *Main&gt;
1252 </screen>
1253
1254 <para>and then run a small <literal>qsort</literal> with
1255 tracing:</para>
1256
1257 <screen>
1258 *Main> :trace qsort [3,2,1]
1259 Stopped at qsort.hs:1:11-12
1260 _result :: [a]
1261 [qsort.hs:1:11-12] *Main>
1262 </screen>
1263
1264 <para>We can now inspect the history of evaluation steps:</para>
1265
1266 <screen>
1267 [qsort.hs:1:11-12] *Main> :hist
1268 -1 : qsort.hs:3:24-38
1269 -2 : qsort.hs:3:23-55
1270 -3 : qsort.hs:(1,0)-(3,55)
1271 -4 : qsort.hs:2:15-24
1272 -5 : qsort.hs:2:15-46
1273 -6 : qsort.hs:3:24-38
1274 -7 : qsort.hs:3:23-55
1275 -8 : qsort.hs:(1,0)-(3,55)
1276 -9 : qsort.hs:2:15-24
1277 -10 : qsort.hs:2:15-46
1278 -11 : qsort.hs:3:24-38
1279 -12 : qsort.hs:3:23-55
1280 -13 : qsort.hs:(1,0)-(3,55)
1281 -14 : qsort.hs:2:15-24
1282 -15 : qsort.hs:2:15-46
1283 -16 : qsort.hs:(1,0)-(3,55)
1284 &lt;end of history&gt;
1285 </screen>
1286
1287 <para>To examine one of the steps in the history, use
1288 <literal>:back</literal>:</para>
1289
1290 <screen>
1291 [qsort.hs:1:11-12] *Main> :back
1292 Logged breakpoint at qsort.hs:3:24-38
1293 _result :: [a]
1294 as :: [a]
1295 a :: a
1296 [-1: qsort.hs:3:24-38] *Main>
1297 </screen>
1298
1299 <para>Note that the local variables at each step in the history have been
1300 preserved, and can be examined as usual. Also note that the prompt has
1301 changed to indicate that we're currently examining the first step in
1302 the history: <literal>-1</literal>. The command
1303 <literal>:forward</literal> can be used to traverse forward in the
1304 history.</para>
1305
1306 <para>The <literal>:trace</literal> command can be used with or without
1307 an expression. When used without an expression, tracing begins from
1308 the current breakpoint, just like <literal>:step</literal>.</para>
1309
1310 <para>The history is only available when
1311 using <literal>:trace</literal>; the reason for this is we found that
1312 logging each breakpoint in the history cuts performance by a factor of
1313 2 or more. GHCi remembers the last 50 steps in the history (perhaps in
1314 the future we'll make this configurable).</para>
1315 </sect2>
1316
1317 <sect2 id="ghci-debugger-exceptions">
1318 <title>Debugging exceptions</title>
1319 <para>Another common question that comes up when debugging is
1320 &ldquo;where did this exception come from?&rdquo;. Exceptions such as
1321 those raised by <literal>error</literal> or <literal>head []</literal>
1322 have no context information attached to them. Finding which
1323 particular call to <literal>head</literal> in your program resulted in
1324 the error can be a painstaking process, usually involving
1325 <literal>Debug.Trace.trace</literal>, or compiling with
1326 profiling and using <literal>+RTS -xc</literal> (see <xref
1327 linkend="prof-time-options" />).</para>
1328
1329 <para>The GHCi debugger offers a way to hopefully shed some light on
1330 these errors quickly and without modifying or recompiling the source
1331 code. One way would be to set a breakpoint on the location in the
1332 source code that throws the exception, and then use
1333 <literal>:trace</literal> and <literal>:history</literal> to establish
1334 the context. However, <literal>head</literal> is in a library and
1335 we can't set a breakpoint on it directly. For this reason, GHCi
1336 provides the flag <literal>-fbreak-on-exception</literal> which causes
1337 the evaluator to stop when an exception is thrown, just as it does when
1338 a breakpoint is hit. This is only really useful in conjunction with
1339 <literal>:trace</literal>, in order to log the steps leading up to the
1340 exception. For example:</para>
1341
1342 <screen>
1343 *Main> :set -fbreak-on-exception
1344 *Main> :trace qsort ("abc" ++ undefined)
1345 "Stopped at &lt;exception thrown&gt;
1346 _exception :: e
1347 [&lt;exception thrown&gt;] *Main&gt; :hist
1348 -1 : qsort.hs:3:24-38
1349 -2 : qsort.hs:3:23-55
1350 -3 : qsort.hs:(1,0)-(3,55)
1351 -4 : qsort.hs:2:15-24
1352 -5 : qsort.hs:2:15-46
1353 -6 : qsort.hs:(1,0)-(3,55)
1354 &lt;end of history&gt;
1355 [&lt;exception thrown&gt;] *Main&gt; :back
1356 Logged breakpoint at qsort.hs:3:24-38
1357 _result :: [a]
1358 as :: [a]
1359 a :: a
1360 [-1: qsort.hs:3:24-38] *Main&gt; :force as
1361 *** Exception: Prelude.undefined
1362 [-1: qsort.hs:3:24-38] *Main&gt; :print as
1363 as = 'b' : 'c' : (_t1::[Char])
1364 </screen>
1365
1366 <para>The exception itself is bound to a new variable,
1367 <literal>_exception</literal>.</para>
1368
1369 <para>Breaking on exceptions is particularly useful for finding out what
1370 your program was doing when it was in an infinite loop. Just hit
1371 Control-C, and examine the history to find out what was going
1372 on.</para>
1373 </sect2>
1374
1375 <sect2><title>Example: inspecting functions</title>
1376 <para>
1377 It is possible to use the debugger to examine function values.
1378 When we are at a breakpoint and a function is in scope, the debugger
1379 cannot show
1380 you the source code for it; however, it is possible to get some
1381 information by applying it to some arguments and observing the result.
1382 </para>
1383
1384 <para>
1385 The process is slightly complicated when the binding is polymorphic.
1386 We show the process by means of an example.
1387 To keep things simple, we will use the well known <literal>map</literal> function:
1388 <programlisting>
1389 import Prelude hiding (map)
1390
1391 map :: (a->b) -> a -> b
1392 map f [] = []
1393 map f (x:xs) = f x : map f xs
1394 </programlisting>
1395 </para>
1396
1397 <para>
1398 We set a breakpoint on <literal>map</literal>, and call it.
1399 <screen>
1400 *Main> :break 5
1401 Breakpoint 0 activated at map.hs:5:15-28
1402 *Main> map Just [1..5]
1403 Stopped at map.hs:(4,0)-(5,12)
1404 _result :: [b]
1405 x :: a
1406 f :: a -> b
1407 xs :: [a]
1408 </screen>
1409 GHCi tells us that, among other bindings, <literal>f</literal> is in scope.
1410 However, its type is not fully known yet,
1411 and thus it is not possible to apply it to any
1412 arguments. Nevertheless, observe that the type of its first argument is the
1413 same as the type of <literal>x</literal>, and its result type is shared
1414 with <literal>_result</literal>.
1415 </para>
1416
1417 <para>
1418 As we demonstrated earlier (<xref linkend="breakpoints" />), the
1419 debugger has some intelligence built-in to update the type of
1420 <literal>f</literal> whenever the types of <literal>x</literal> or
1421 <literal>_result</literal> are discovered. So what we do in this
1422 scenario is
1423 force <literal>x</literal> a bit, in order to recover both its type
1424 and the argument part of <literal>f</literal>.
1425 <screen>
1426 *Main> seq x ()
1427 *Main> :print x
1428 x = 1
1429 </screen>
1430 </para>
1431 <para>
1432 We can check now that as expected, the type of <literal>x</literal>
1433 has been reconstructed, and with it the
1434 type of <literal>f</literal> has been too:</para>
1435 <screen>
1436 *Main> :t x
1437 x :: Integer
1438 *Main> :t f
1439 f :: Integer -> b
1440 </screen>
1441 <para>
1442 From here, we can apply f to any argument of type Integer and observe
1443 the results.
1444 <screen><![CDATA[
1445 *Main> let b = f 10
1446 *Main> :t b
1447 b :: b
1448 *Main> b
1449 <interactive>:1:0:
1450 Ambiguous type variable `b' in the constraint:
1451 `Show b' arising from a use of `print' at <interactive>:1:0
1452 *Main> :p b
1453 b = (_t2::a)
1454 *Main> seq b ()
1455 ()
1456 *Main> :t b
1457 b :: a
1458 *Main> :p b
1459 b = Just 10
1460 *Main> :t b
1461 b :: Maybe Integer
1462 *Main> :t f
1463 f :: Integer -> Maybe Integer
1464 *Main> f 20
1465 Just 20
1466 *Main> map f [1..5]
1467 [Just 1, Just 2, Just 3, Just 4, Just 5]
1468 ]]></screen>
1469 In the first application of <literal>f</literal>, we had to do
1470 some more type reconstruction
1471 in order to recover the result type of <literal>f</literal>.
1472 But after that, we are free to use
1473 <literal>f</literal> normally.
1474 </para>
1475 </sect2>
1476
1477 <sect2><title>Limitations</title>
1478 <itemizedlist>
1479 <listitem>
1480 <para>When stopped at a breakpoint, if you try to evaluate a variable
1481 that is already under evaluation, the second evaluation will hang.
1482 The reason is
1483 that GHC knows the variable is under evaluation, so the new
1484 evaluation just waits for the result before continuing, but of
1485 course this isn't going to happen because the first evaluation is
1486 stopped at a breakpoint. Control-C can interrupt the hung
1487 evaluation and return to the prompt.</para>
1488 <para>The most common way this can happen is when you're evaluating a
1489 CAF (e.g. main), stop at a breakpoint, and ask for the value of the
1490 CAF at the prompt again.</para>
1491 </listitem>
1492 <listitem><para>
1493 Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
1494 at the scope of a breakpoint if there is an explicit type signature.
1495 </para>
1496 </listitem>
1497 </itemizedlist>
1498 </sect2>
1499 </sect1>
1500
1501 <sect1 id="ghci-invocation">
1502 <title>Invoking GHCi</title>
1503 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
1504 <indexterm><primary><option>&ndash;&ndash;interactive</option></primary></indexterm>
1505
1506 <para>GHCi is invoked with the command <literal>ghci</literal> or
1507 <literal>ghc &ndash;&ndash;interactive</literal>. One or more modules or
1508 filenames can also be specified on the command line; this
1509 instructs GHCi to load the specified modules or filenames (and all
1510 the modules they depend on), just as if you had said
1511 <literal>:load <replaceable>modules</replaceable></literal> at the
1512 GHCi prompt (see <xref linkend="ghci-commands" />). For example, to
1513 start GHCi and load the program whose topmost module is in the
1514 file <literal>Main.hs</literal>, we could say:</para>
1515
1516 <screen>
1517 $ ghci Main.hs
1518 </screen>
1519
1520 <para>Most of the command-line options accepted by GHC (see <xref
1521 linkend="using-ghc"/>) also make sense in interactive mode. The ones
1522 that don't make sense are mostly obvious.</para>
1523
1524 <sect2>
1525 <title>Packages</title>
1526 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
1527
1528 <para>Most packages (see <xref linkend="using-packages"/>) are
1529 available without needing to specify any extra flags at all:
1530 they will be automatically loaded the first time they are
1531 needed.</para>
1532
1533 <para>For hidden packages, however, you need to request the
1534 package be loaded by using the <literal>-package</literal> flag:</para>
1535
1536 <screen>
1537 $ ghci -package readline
1538 GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
1539 Loading package base ... linking ... done.
1540 Loading package readline-1.0 ... linking ... done.
1541 Prelude>
1542 </screen>
1543
1544 <para>The following command works to load new packages into a
1545 running GHCi:</para>
1546
1547 <screen>
1548 Prelude> :set -package <replaceable>name</replaceable>
1549 </screen>
1550
1551 <para>But note that doing this will cause all currently loaded
1552 modules to be unloaded, and you'll be dumped back into the
1553 <literal>Prelude</literal>.</para>
1554 </sect2>
1555
1556 <sect2>
1557 <title>Extra libraries</title>
1558 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
1559
1560 <para>Extra libraries may be specified on the command line using
1561 the normal <literal>-l<replaceable>lib</replaceable></literal>
1562 option. (The term <emphasis>library</emphasis> here refers to
1563 libraries of foreign object code; for using libraries of Haskell
1564 source code, see <xref linkend="ghci-modules-filenames"/>.) For
1565 example, to load the &ldquo;m&rdquo; library:</para>
1566
1567 <screen>
1568 $ ghci -lm
1569 </screen>
1570
1571 <para>On systems with <literal>.so</literal>-style shared
1572 libraries, the actual library loaded will the
1573 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
1574 searches the following places for libraries, in this order:</para>
1575
1576 <itemizedlist>
1577 <listitem>
1578 <para>Paths specified using the
1579 <literal>-L<replaceable>path</replaceable></literal>
1580 command-line option,</para>
1581 </listitem>
1582 <listitem>
1583 <para>the standard library search path for your system,
1584 which on some systems may be overridden by setting the
1585 <literal>LD_LIBRARY_PATH</literal> environment
1586 variable.</para>
1587 </listitem>
1588 </itemizedlist>
1589
1590 <para>On systems with <literal>.dll</literal>-style shared
1591 libraries, the actual library loaded will be
1592 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
1593 GHCi will signal an error if it can't find the library.</para>
1594
1595 <para>GHCi can also load plain object files
1596 (<literal>.o</literal> or <literal>.obj</literal> depending on
1597 your platform) from the command-line. Just add the name the
1598 object file to the command line.</para>
1599
1600 <para>Ordering of <option>-l</option> options matters: a library
1601 should be mentioned <emphasis>before</emphasis> the libraries it
1602 depends on (see <xref linkend="options-linker"/>).</para>
1603 </sect2>
1604
1605 </sect1>
1606
1607 <sect1 id="ghci-commands">
1608 <title>GHCi commands</title>
1609
1610 <para>GHCi commands all begin with
1611 &lsquo;<literal>:</literal>&rsquo; and consist of a single command
1612 name followed by zero or more parameters. The command name may be
1613 abbreviated, with ambiguities being resolved in favour of the more
1614 commonly used commands.</para>
1615
1616 <variablelist>
1617 <varlistentry>
1618 <term>
1619 <literal>:abandon</literal>
1620 <indexterm><primary><literal>:abandon</literal></primary></indexterm>
1621 </term>
1622 <listitem>
1623 <para>Abandons the current evaluation (only available when stopped at
1624 a breakpoint).</para>
1625 </listitem>
1626 </varlistentry>
1627
1628 <varlistentry>
1629 <term>
1630 <literal>:add</literal> <replaceable>module</replaceable> ...
1631 <indexterm><primary><literal>:add</literal></primary></indexterm>
1632 </term>
1633 <listitem>
1634 <para>Add <replaceable>module</replaceable>(s) to the
1635 current <firstterm>target set</firstterm>, and perform a
1636 reload.</para>
1637 </listitem>
1638 </varlistentry>
1639
1640 <varlistentry>
1641 <term>
1642 <literal>:back</literal>
1643 <indexterm><primary><literal>:back</literal></primary></indexterm>
1644 </term>
1645 <listitem>
1646 <para>Travel back one step in the history. See <xref
1647 linkend="tracing" />. See also:
1648 <literal>:trace</literal>, <literal>:history</literal>,
1649 <literal>:forward</literal>.</para>
1650 </listitem>
1651 </varlistentry>
1652
1653 <varlistentry>
1654 <term>
1655 <literal>:break [<replaceable>identifier</replaceable> |
1656 [<replaceable>module</replaceable>] <replaceable>line</replaceable>
1657 [<replaceable>column</replaceable>]]</literal>
1658 </term>
1659 <indexterm><primary><literal>:break</literal></primary></indexterm>
1660 <listitem>
1661 <para>Set a breakpoint on the specified function or line and
1662 column. See <xref linkend="setting-breakpoints" />.</para>
1663 </listitem>
1664 </varlistentry>
1665
1666 <varlistentry>
1667 <term>
1668 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
1669 <indexterm><primary><literal>:browse</literal></primary></indexterm>
1670 </term>
1671 <listitem>
1672 <para>Displays the identifiers defined by the module
1673 <replaceable>module</replaceable>, which must be either
1674 loaded into GHCi or be a member of a package. If the
1675 <literal>*</literal> symbol is placed before the module
1676 name, then <emphasis>all</emphasis> the identifiers defined
1677 in <replaceable>module</replaceable> are shown; otherwise
1678 the list is limited to the exports of
1679 <replaceable>module</replaceable>. The
1680 <literal>*</literal>-form is only available for modules
1681 which are interpreted; for compiled modules (including
1682 modules from packages) only the non-<literal>*</literal>
1683 form of <literal>:browse</literal> is available.</para>
1684 </listitem>
1685 </varlistentry>
1686
1687 <varlistentry>
1688 <term>
1689 <literal>:cd</literal> <replaceable>dir</replaceable>
1690 <indexterm><primary><literal>:cd</literal></primary></indexterm>
1691 </term>
1692 <listitem>
1693 <para>Changes the current working directory to
1694 <replaceable>dir</replaceable>. A
1695 &lsquo;<literal>&tilde;</literal>&rsquo; symbol at the
1696 beginning of <replaceable>dir</replaceable> will be replaced
1697 by the contents of the environment variable
1698 <literal>HOME</literal>.</para>
1699
1700 <para>NOTE: changing directories causes all currently loaded
1701 modules to be unloaded. This is because the search path is
1702 usually expressed using relative directories, and changing
1703 the search path in the middle of a session is not
1704 supported.</para>
1705 </listitem>
1706 </varlistentry>
1707
1708 <varlistentry>
1709 <term>
1710 <literal>:cmd</literal> <replaceable>expr</replaceable>
1711 <indexterm><primary><literal>:cmd</literal></primary></indexterm>
1712 </term>
1713 <listitem>
1714 <para>Executes <replaceable>expr</replaceable> as a computation of
1715 type <literal>IO String</literal>, and then executes the resulting
1716 string as a list of GHCi commands. Multiple commands are separated
1717 by newlines. The <literal>:cmd</literal> command is useful with
1718 <literal>:def</literal> and <literal>:set stop</literal>.</para>
1719 </listitem>
1720 </varlistentry>
1721
1722 <varlistentry>
1723 <term>
1724 <literal>:continue</literal>
1725 <indexterm><primary><literal>:continue</literal></primary></indexterm>
1726 </term>
1727 <listitem><para>Continue the current evaluation, when stopped at a
1728 breakpoint.</para>
1729 </listitem>
1730 </varlistentry>
1731
1732 <varlistentry>
1733 <term>
1734 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1735 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1736 <indexterm><primary><literal>:etags</literal></primary>
1737 </indexterm>
1738 <indexterm><primary><literal>:etags</literal></primary>
1739 </indexterm>
1740 </term>
1741 <listitem>
1742 <para>Generates a &ldquo;tags&rdquo; file for Vi-style editors
1743 (<literal>:ctags</literal>) or
1744 Emacs-style editors (<literal>:etags</literal>). If
1745 no filename is specified, the defaulit <filename>tags</filename> or
1746 <filename>TAGS</filename> is
1747 used, respectively. Tags for all the functions, constructors and
1748 types in the currently loaded modules are created. All modules must
1749 be interpreted for these commands to work.</para>
1750 <para>See also <xref linkend="hasktags" />.</para>
1751 </listitem>
1752 </varlistentry>
1753
1754 <varlistentry>
1755 <term>
1756 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
1757 <indexterm><primary><literal>:def</literal></primary></indexterm>
1758 </term>
1759 <listitem>
1760 <para>The command <literal>:def</literal>
1761 <replaceable>name</replaceable>
1762 <replaceable>expr</replaceable> defines a new GHCi command
1763 <literal>:<replaceable>name</replaceable></literal>,
1764 implemented by the Haskell expression
1765 <replaceable>expr</replaceable>, which must have type
1766 <literal>String -> IO String</literal>. When
1767 <literal>:<replaceable>name</replaceable>
1768 <replaceable>args</replaceable></literal> is typed at the
1769 prompt, GHCi will run the expression
1770 <literal>(<replaceable>name</replaceable>
1771 <replaceable>args</replaceable>)</literal>, take the
1772 resulting <literal>String</literal>, and feed it back into
1773 GHCi as a new sequence of commands. Separate commands in
1774 the result must be separated by
1775 &lsquo;<literal>\n</literal>&rsquo;.</para>
1776
1777 <para>That's all a little confusing, so here's a few
1778 examples. To start with, here's a new GHCi command which
1779 doesn't take any arguments or produce any results, it just
1780 outputs the current date &amp; time:</para>
1781
1782 <screen>
1783 Prelude> let date _ = Time.getClockTime >>= print >> return ""
1784 Prelude> :def date date
1785 Prelude> :date
1786 Fri Mar 23 15:16:40 GMT 2001
1787 </screen>
1788
1789 <para>Here's an example of a command that takes an argument.
1790 It's a re-implementation of <literal>:cd</literal>:</para>
1791
1792 <screen>
1793 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
1794 Prelude> :def mycd mycd
1795 Prelude> :mycd ..
1796 </screen>
1797
1798 <para>Or I could define a simple way to invoke
1799 &ldquo;<literal>ghc &ndash;&ndash;make Main</literal>&rdquo; in the
1800 current directory:</para>
1801
1802 <screen>
1803 Prelude> :def make (\_ -> return ":! ghc &ndash;&ndash;make Main")
1804 </screen>
1805
1806 <para>We can define a command that reads GHCi input from a
1807 file. This might be useful for creating a set of bindings
1808 that we want to repeatedly load into the GHCi session:</para>
1809
1810 <screen>
1811 Prelude> :def . readFile
1812 Prelude> :. cmds.ghci
1813 </screen>
1814
1815 <para>Notice that we named the command
1816 <literal>:.</literal>, by analogy with the
1817 &lsquo;<literal>.</literal>&rsquo; Unix shell command that
1818 does the same thing.</para>
1819 </listitem>
1820 </varlistentry>
1821
1822 <varlistentry>
1823 <term>
1824 <literal>:delete * | <replaceable>num</replaceable> ...</literal>
1825 <indexterm><primary><literal>:delete</literal></primary></indexterm>
1826 </term>
1827 <listitem>
1828 <para>Delete one or more breakpoints by number (use <literal>:show
1829 breaks</literal> to see the number of each breakpoint). The
1830 <literal>*</literal> form deletes all the breakpoints.</para>
1831 </listitem>
1832 </varlistentry>
1833
1834 <varlistentry>
1835 <term>
1836 <literal>:edit <optional><replaceable>file</replaceable></optional></literal>
1837 <indexterm><primary><literal>:edit</literal></primary></indexterm>
1838 </term>
1839 <listitem>
1840 <para>Opens an editor to edit the file
1841 <replaceable>file</replaceable>, or the most recently loaded
1842 module if <replaceable>file</replaceable> is omitted. The
1843 editor to invoke is taken from the <literal>EDITOR</literal>
1844 environment variable, or a default editor on your system if
1845 <literal>EDITOR</literal> is not set. You can change the
1846 editor using <literal>:set editor</literal>.</para>
1847 </listitem>
1848 </varlistentry>
1849
1850 <varlistentry>
1851 <term>
1852 <literal>:etags</literal>
1853 </term>
1854 <listitem>
1855 <para>See <literal>:ctags</literal>.</para>
1856 </listitem>
1857 </varlistentry>
1858
1859 <varlistentry>
1860 <term>
1861 <literal>:force <replaceable>identifier</replaceable> ...</literal>
1862 <indexterm><primary><literal>:force</literal></primary></indexterm>
1863 </term>
1864 <listitem>
1865 <para>Prints the value of <replaceable>identifier</replaceable> in
1866 the same way as <literal>:print</literal>. Unlike
1867 <literal>:print</literal>, <literal>:force</literal> evaluates each
1868 thunk that it encounters while traversing the value. This may
1869 cause exceptions or infinite loops, or further breakpoints (which
1870 are ignored, but displayed).</para>
1871 </listitem>
1872 </varlistentry>
1873
1874 <varlistentry>
1875 <term>
1876 <literal>:forward</literal>
1877 <indexterm><primary><literal>:forward</literal></primary></indexterm>
1878 </term>
1879 <listitem>
1880 <para>Move forward in the history. See <xref
1881 linkend="tracing" />. See also:
1882 <literal>:trace</literal>, <literal>:history</literal>,
1883 <literal>:back</literal>.</para>
1884 </listitem>
1885 </varlistentry>
1886
1887 <varlistentry>
1888 <term>
1889 <literal>:help</literal>
1890 <indexterm><primary><literal>:help</literal></primary></indexterm>
1891 </term>
1892 <term>
1893 <literal>:?</literal>
1894 <indexterm><primary><literal>:?</literal></primary></indexterm>
1895 </term>
1896 <listitem>
1897 <para>Displays a list of the available commands.</para>
1898 </listitem>
1899 </varlistentry>
1900
1901 <varlistentry>
1902 <term>
1903 <literal>:history [<replaceable>num</replaceable>]</literal>
1904 <indexterm><primary><literal>:history</literal></primary></indexterm>
1905 </term>
1906 <listitem>
1907 <para>Display the history of evaluation steps. With a number,
1908 displays that many steps (default: 20). For use with
1909 <literal>:trace</literal>; see <xref
1910 linkend="tracing" />.</para>
1911 </listitem>
1912 </varlistentry>
1913
1914 <varlistentry>
1915 <term>
1916 <literal>:info</literal> <replaceable>name</replaceable> ...
1917 <indexterm><primary><literal>:info</literal></primary></indexterm>
1918 </term>
1919 <listitem>
1920 <para>Displays information about the given name(s). For
1921 example, if <replaceable>name</replaceable> is a class, then
1922 the class methods and their types will be printed; if
1923 <replaceable>name</replaceable> is a type constructor, then
1924 its definition will be printed; if
1925 <replaceable>name</replaceable> is a function, then its type
1926 will be printed. If <replaceable>name</replaceable> has
1927 been loaded from a source file, then GHCi will also display
1928 the location of its definition in the source.</para>
1929 <para>For types and classes, GHCi also summarises instances that
1930 mention them. To avoid showing irrelevant information, an instance
1931 is shown only if (a) its head mentions <replaceable>name</replaceable>,
1932 and (b) all the other things mentioned in the instance
1933 are in scope (either qualified or otherwise) as a result of
1934 a <literal>:load</literal> or <literal>:module</literal> commands. </para>
1935 </listitem>
1936 </varlistentry>
1937
1938 <varlistentry>
1939 <term>
1940 <literal>:kind</literal> <replaceable>type</replaceable>
1941 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1942 </term>
1943 <listitem>
1944 <para>Infers and prints the kind of
1945 <replaceable>type</replaceable>. The latter can be an arbitrary
1946 type expression, including a partial application of a type constructor,
1947 such as <literal>Either Int</literal>.</para>
1948 </listitem>
1949 </varlistentry>
1950
1951 <varlistentry>
1952 <term>
1953 <literal>:load</literal> <replaceable>module</replaceable> ...
1954 <indexterm><primary><literal>:load</literal></primary></indexterm>
1955 </term>
1956 <listitem>
1957 <para>Recursively loads the specified
1958 <replaceable>module</replaceable>s, and all the modules they
1959 depend on. Here, each <replaceable>module</replaceable>
1960 must be a module name or filename, but may not be the name
1961 of a module in a package.</para>
1962
1963 <para>All previously loaded modules, except package modules,
1964 are forgotten. The new set of modules is known as the
1965 <firstterm>target set</firstterm>. Note that
1966 <literal>:load</literal> can be used without any arguments
1967 to unload all the currently loaded modules and
1968 bindings.</para>
1969
1970 <para>After a <literal>:load</literal> command, the current
1971 context is set to:</para>
1972
1973 <itemizedlist>
1974 <listitem>
1975 <para><replaceable>module</replaceable>, if it was loaded
1976 successfully, or</para>
1977 </listitem>
1978 <listitem>
1979 <para>the most recently successfully loaded module, if
1980 any other modules were loaded as a result of the current
1981 <literal>:load</literal>, or</para>
1982 </listitem>
1983 <listitem>
1984 <para><literal>Prelude</literal> otherwise.</para>
1985 </listitem>
1986 </itemizedlist>
1987 </listitem>
1988 </varlistentry>
1989
1990 <varlistentry>
1991 <term>
1992 <literal>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
1993 <indexterm><primary><literal>:main</literal></primary></indexterm>
1994 </term>
1995 <listitem>
1996 <para>
1997 When a program is compiled and executed, it can use the
1998 <literal>getArgs</literal> function to access the
1999 command-line arguments.
2000 However, we cannot simply pass the arguments to the
2001 <literal>main</literal> function while we are testing in ghci,
2002 as the <literal>main</literal> function doesn't take its
2003 arguments directly.
2004 </para>
2005
2006 <para>
2007 Instead, we can use the <literal>:main</literal> command.
2008 This runs whatever <literal>main</literal> is in scope, with
2009 any arguments being treated the same as command-line arguments,
2010 e.g.:
2011 </para>
2012
2013 <screen>
2014 Prelude> let main = System.Environment.getArgs >>= print
2015 Prelude> :main foo bar
2016 ["foo","bar"]
2017 </screen>
2018
2019 </listitem>
2020 </varlistentry>
2021
2022 <varlistentry>
2023 <term>
2024 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
2025 <indexterm><primary><literal>:module</literal></primary></indexterm>
2026 </term>
2027 <term>
2028 <literal>import <replaceable>mod</replaceable></literal>
2029 </term>
2030 <listitem>
2031 <para>Sets or modifies the current context for statements
2032 typed at the prompt. The form <literal>import
2033 <replaceable>mod</replaceable></literal> is equivalent to
2034 <literal>:module +<replaceable>mod</replaceable></literal>.
2035 See <xref linkend="ghci-scope"/> for
2036 more details.</para>
2037 </listitem>
2038 </varlistentry>
2039
2040 <varlistentry>
2041 <term>
2042 <literal>:print </literal> <replaceable>names</replaceable> ...
2043 <indexterm><primary><literal>:print</literal></primary></indexterm>
2044 </term>
2045 <listitem>
2046 <para>Prints a value without forcing its evaluation.
2047 <literal>:print</literal> may be used on values whose types are
2048 unknown or partially known, which might be the case for local
2049 variables with polymorphic types at a breakpoint. While inspecting
2050 the runtime value, <literal>:print</literal> attempts to
2051 reconstruct the type of the value, and will elaborate the type in
2052 GHCi's environment if possible. If any unevaluated components
2053 (thunks) are encountered, then <literal>:print</literal> binds
2054 a fresh variable with a name beginning with <literal>_t</literal>
2055 to each thunk. See <xref linkend="breakpoints" /> for more
2056 information. See also the <literal>:sprint</literal> command,
2057 which works like <literal>:print</literal> but does not bind new
2058 variables.</para>
2059 </listitem>
2060 </varlistentry>
2061
2062 <varlistentry>
2063 <term>
2064 <literal>:quit</literal>
2065 <indexterm><primary><literal>:quit</literal></primary></indexterm>
2066 </term>
2067 <listitem>
2068 <para>Quits GHCi. You can also quit by typing control-D
2069 at the prompt.</para>
2070 </listitem>
2071 </varlistentry>
2072
2073 <varlistentry>
2074 <term>
2075 <literal>:reload</literal>
2076 <indexterm><primary><literal>:reload</literal></primary></indexterm>
2077 </term>
2078 <listitem>
2079 <para>Attempts to reload the current target set (see
2080 <literal>:load</literal>) if any of the modules in the set,
2081 or any dependent module, has changed. Note that this may
2082 entail loading new modules, or dropping modules which are no
2083 longer indirectly required by the target.</para>
2084 </listitem>
2085 </varlistentry>
2086
2087 <varlistentry>
2088 <term>
2089 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
2090 <indexterm><primary><literal>:set</literal></primary></indexterm>
2091 </term>
2092 <listitem>
2093 <para>Sets various options. See <xref linkend="ghci-set"/>
2094 for a list of available options. The
2095 <literal>:set</literal> command by itself shows which
2096 options are currently set.</para>
2097 </listitem>
2098 </varlistentry>
2099
2100 <varlistentry>
2101 <term>
2102 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
2103 <indexterm><primary><literal>:set args</literal></primary></indexterm>
2104 </term>
2105 <listitem>
2106 <para>Sets the list of arguments which are returned when the
2107 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
2108 </indexterm>.</para>
2109 </listitem>
2110 </varlistentry>
2111
2112 <varlistentry>
2113 <term>
2114 <literal>:set</literal> <literal>editor</literal> <replaceable>cmd</replaceable>
2115 </term>
2116 <listitem>
2117 <para>Sets the command used by <literal>:edit</literal> to
2118 <replaceable>cmd</replaceable>.</para>
2119 </listitem>
2120 </varlistentry>
2121
2122 <varlistentry>
2123 <term>
2124 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
2125 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
2126 </term>
2127 <listitem>
2128 <para>Sets the string to be returned when the program calls
2129 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
2130 </indexterm>.</para>
2131 </listitem>
2132 </varlistentry>
2133
2134 <varlistentry>
2135 <term>
2136 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
2137 </term>
2138 <listitem>
2139 <para>Sets the string to be used as the prompt in GHCi.
2140 Inside <replaceable>prompt</replaceable>, the sequence
2141 <literal>%s</literal> is replaced by the names of the
2142 modules currently in scope, and <literal>%%</literal> is
2143 replaced by <literal>%</literal>.</para>
2144 </listitem>
2145 </varlistentry>
2146
2147 <varlistentry>
2148 <term>
2149 <literal>:set</literal> <literal>stop</literal>
2150 [<replaceable>num</replaceable>] <replaceable>cmd</replaceable>
2151 </term>
2152 <listitem>
2153 <para>Set a command to be executed when a breakpoint is hit, or a new
2154 item in the history is selected. The most common use of
2155 <literal>:set stop</literal> is to display the source code at the
2156 current location, e.g. <literal>:set stop :list</literal>.</para>
2157
2158 <para>If a number is given before the command, then the commands are
2159 run when the specified breakpoint (only) is hit. This can be quite
2160 useful: for example, <literal>:set stop 1 :continue</literal>
2161 effectively disables breakpoint 1, by running
2162 <literal>:continue</literal> whenever it is hit (although GHCi will
2163 still emit a message to say the breakpoint was hit). What's more,
2164 with cunning use of <literal>:def</literal> and
2165 <literal>:cmd</literal> you can use <literal>:set stop</literal> to
2166 implement conditional breakpoints:</para>
2167 <screen>
2168 *Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
2169 *Main> :set stop 0 :cond (x &lt; 3)
2170 </screen>
2171 <para>Ignoring breakpoints for a specified number of iterations is
2172 also possible using similar techniques.</para>
2173 </listitem>
2174 </varlistentry>
2175
2176 <varlistentry>
2177 <term>
2178 <literal>:show bindings</literal>
2179 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
2180 </term>
2181 <listitem>
2182 <para>Show the bindings made at the prompt and their
2183 types.</para>
2184 </listitem>
2185 </varlistentry>
2186
2187 <varlistentry>
2188 <term>
2189 <literal>:show breaks</literal>
2190 <indexterm><primary><literal>:show breaks</literal></primary></indexterm>
2191 </term>
2192 <listitem>
2193 <para>List the active breakpoints.</para>
2194 </listitem>
2195 </varlistentry>
2196
2197 <varlistentry>
2198 <term>
2199 <literal>:show context</literal>
2200 <indexterm><primary><literal>:show context</literal></primary></indexterm>
2201 </term>
2202 <listitem>
2203 <para>List the active evaluations that are stopped at breakpoints.</para>
2204 </listitem>
2205 </varlistentry>
2206
2207 <varlistentry>
2208 <term>
2209 <literal>:show modules</literal>
2210 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
2211 </term>
2212 <listitem>
2213 <para>Show the list of modules currently loaded.</para>
2214 </listitem>
2215 </varlistentry>
2216
2217 <varlistentry>
2218 <term>
2219 <literal>:show [args|prog|prompt|editor|stop]</literal>
2220 <indexterm><primary><literal>:show</literal></primary></indexterm>
2221 </term>
2222 <listitem>
2223 <para>Displays the specified setting (see
2224 <literal>:set</literal>).</para>
2225 </listitem>
2226 </varlistentry>
2227
2228 <varlistentry>
2229 <term>
2230 <literal>:sprint</literal>
2231 <indexterm><primary><literal>:sprint</literal></primary></indexterm>
2232 </term>
2233 <listitem>
2234 <para>Prints a value without forcing its evaluation.
2235 <literal>:sprint</literal> is similar to <literal>:print</literal>,
2236 with the difference that unevaluated subterms are not bound to new
2237 variables, they are simply denoted by &lsquo;_&rsquo;.</para>
2238 </listitem>
2239 </varlistentry>
2240
2241 <varlistentry>
2242 <term>
2243 <literal>:step [<replaceable>expr</replaceable>]</literal>
2244 <indexterm><primary><literal>:step</literal></primary></indexterm>
2245 </term>
2246 <listitem>
2247 <para>Single-step from the last breakpoint. With an expression
2248 argument, begins evaluation of the expression with a
2249 single-step.</para>
2250 </listitem>
2251 </varlistentry>
2252
2253 <varlistentry>
2254 <term>
2255 <literal>:trace [<replaceable>expr</replaceable>]</literal>
2256 <indexterm><primary><literal>:trace</literal></primary></indexterm>
2257 </term>
2258 <listitem>
2259 <para>Evaluates the given expression (or from the last breakpoint if
2260 no expression is given), and additionally logs the evaluation
2261 steps for later inspection using <literal>:history</literal>. See
2262 <xref linkend="tracing" />.</para>
2263 </listitem>
2264 </varlistentry>
2265
2266 <varlistentry>
2267 <term>
2268 <literal>:type</literal> <replaceable>expression</replaceable>
2269 <indexterm><primary><literal>:type</literal></primary></indexterm>
2270 </term>
2271 <listitem>
2272 <para>Infers and prints the type of
2273 <replaceable>expression</replaceable>, including explicit
2274 forall quantifiers for polymorphic types. The monomorphism
2275 restriction is <emphasis>not</emphasis> applied to the
2276 expression during type inference.</para>
2277 </listitem>
2278 </varlistentry>
2279
2280 <varlistentry>
2281 <term>
2282 <literal>:undef</literal> <replaceable>name</replaceable>
2283 <indexterm><primary><literal>:undef</literal></primary></indexterm>
2284 </term>
2285 <listitem>
2286 <para>Undefines the user-defined command
2287 <replaceable>name</replaceable> (see <literal>:def</literal>
2288 above).</para>
2289 </listitem>
2290 </varlistentry>
2291
2292 <varlistentry>
2293 <term>
2294 <literal>:unset</literal> <replaceable>option</replaceable>...
2295 <indexterm><primary><literal>:unset</literal></primary></indexterm>
2296 </term>
2297 <listitem>
2298 <para>Unsets certain options. See <xref linkend="ghci-set"/>
2299 for a list of available options.</para>
2300 </listitem>
2301 </varlistentry>
2302
2303 <varlistentry>
2304 <term>
2305 <literal>:!</literal> <replaceable>command</replaceable>...
2306 <indexterm><primary><literal>:!</literal></primary></indexterm>
2307 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
2308 </term>
2309 <listitem>
2310 <para>Executes the shell command
2311 <replaceable>command</replaceable>.</para>
2312 </listitem>
2313 </varlistentry>
2314
2315 </variablelist>
2316 </sect1>
2317
2318 <sect1 id="ghci-set">
2319 <title>The <literal>:set</literal> command</title>
2320 <indexterm><primary><literal>:set</literal></primary></indexterm>
2321
2322 <para>The <literal>:set</literal> command sets two types of
2323 options: GHCi options, which begin with
2324 &lsquo;<literal>+</literal>&rsquo;, and &ldquo;command-line&rdquo;
2325 options, which begin with &lsquo;-&rsquo;. </para>
2326
2327 <para>NOTE: at the moment, the <literal>:set</literal> command
2328 doesn't support any kind of quoting in its arguments: quotes will
2329 not be removed and cannot be used to group words together. For
2330 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
2331 you expect.</para>
2332
2333 <sect2>
2334 <title>GHCi options</title>
2335 <indexterm><primary>options</primary><secondary>GHCi</secondary>
2336 </indexterm>
2337
2338 <para>GHCi options may be set using <literal>:set</literal> and
2339 unset using <literal>:unset</literal>.</para>
2340
2341 <para>The available GHCi options are:</para>
2342
2343 <variablelist>
2344 <varlistentry>
2345 <term>
2346 <literal>+r</literal>
2347 <indexterm><primary><literal>+r</literal></primary></indexterm>
2348 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
2349 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
2350 </term>
2351 <listitem>
2352 <para>Normally, any evaluation of top-level expressions
2353 (otherwise known as CAFs or Constant Applicative Forms) in
2354 loaded modules is retained between evaluations. Turning
2355 on <literal>+r</literal> causes all evaluation of
2356 top-level expressions to be discarded after each
2357 evaluation (they are still retained
2358 <emphasis>during</emphasis> a single evaluation).</para>
2359
2360 <para>This option may help if the evaluated top-level
2361 expressions are consuming large amounts of space, or if
2362 you need repeatable performance measurements.</para>
2363 </listitem>
2364 </varlistentry>
2365
2366 <varlistentry>
2367 <term>
2368 <literal>+s</literal>
2369 <indexterm><primary><literal>+s</literal></primary></indexterm>
2370 </term>
2371 <listitem>
2372 <para>Display some stats after evaluating each expression,
2373 including the elapsed time and number of bytes allocated.
2374 NOTE: the allocation figure is only accurate to the size
2375 of the storage manager's allocation area, because it is
2376 calculated at every GC. Hence, you might see values of
2377 zero if no GC has occurred.</para>
2378 </listitem>
2379 </varlistentry>
2380
2381 <varlistentry>
2382 <term>
2383 <literal>+t</literal>
2384 <indexterm><primary><literal>+t</literal></primary></indexterm>
2385 </term>
2386 <listitem>
2387 <para>Display the type of each variable bound after a
2388 statement is entered at the prompt. If the statement is a
2389 single expression, then the only variable binding will be
2390 for the variable
2391 &lsquo;<literal>it</literal>&rsquo;.</para>
2392 </listitem>
2393 </varlistentry>
2394 </variablelist>
2395 </sect2>
2396
2397 <sect2 id="ghci-cmd-line-options">
2398 <title>Setting GHC command-line options in GHCi</title>
2399
2400 <para>Normal GHC command-line options may also be set using
2401 <literal>:set</literal>. For example, to turn on
2402 <option>-fglasgow-exts</option>, you would say:</para>
2403
2404 <screen>
2405 Prelude> :set -fglasgow-exts
2406 </screen>
2407
2408 <para>Any GHC command-line option that is designated as
2409 <firstterm>dynamic</firstterm> (see the table in <xref
2410 linkend="flag-reference"/>), may be set using
2411 <literal>:set</literal>. To unset an option, you can set the
2412 reverse option:</para>
2413 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
2414
2415 <screen>
2416 Prelude> :set -fno-glasgow-exts
2417 </screen>
2418
2419 <para><xref linkend="flag-reference"/> lists the reverse for each
2420 option where applicable.</para>
2421
2422 <para>Certain static options (<option>-package</option>,
2423 <option>-I</option>, <option>-i</option>, and
2424 <option>-l</option> in particular) will also work, but some may
2425 not take effect until the next reload.</para>
2426 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
2427 </sect2>
2428 </sect1>
2429 <sect1 id="ghci-dot-files">
2430 <title>The <filename>.ghci</filename> file</title>
2431 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
2432 </indexterm>
2433 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
2434 </indexterm>
2435
2436 <para>When it starts, unless the <literal>-ignore-dot-ghci</literal>
2437 flag is given, GHCi reads and executes commands from
2438 <filename>./.ghci</filename>, followed by
2439 <filename>$HOME/.ghci</filename>.</para>
2440
2441 <para>The <filename>.ghci</filename> in your home directory is
2442 most useful for turning on favourite options (eg. <literal>:set
2443 +s</literal>), and defining useful macros. Placing a
2444 <filename>.ghci</filename> file in a directory with a Haskell
2445 project is a useful way to set certain project-wide options so you
2446 don't have to type them everytime you start GHCi: eg. if your
2447 project uses GHC extensions and CPP, and has source files in three
2448 subdirectories A, B and C, you might put the following lines in
2449 <filename>.ghci</filename>:</para>
2450
2451 <screen>
2452 :set -fglasgow-exts -cpp
2453 :set -iA:B:C
2454 </screen>
2455
2456 <para>(Note that strictly speaking the <option>-i</option> flag is
2457 a static one, but in fact it works to set it using
2458 <literal>:set</literal> like this. The changes won't take effect
2459 until the next <literal>:load</literal>, though.)</para>
2460
2461 <para>Two command-line options control whether the
2462 <filename>.ghci</filename> files are read:</para>
2463
2464 <variablelist>
2465 <varlistentry>
2466 <term>
2467 <option>-ignore-dot-ghci</option>
2468 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
2469 </term>
2470 <listitem>
2471 <para>Don't read either <filename>./.ghci</filename> or
2472 <filename>$HOME/.ghci</filename> when starting up.</para>
2473 </listitem>
2474 </varlistentry>
2475 <varlistentry>
2476 <term>
2477 <option>-read-dot-ghci</option>
2478 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
2479 </term>
2480 <listitem>
2481 <para>Read <filename>.ghci</filename> and
2482 <filename>$HOME/.ghci</filename>. This is normally the
2483 default, but the <option>-read-dot-ghci</option> option may
2484 be used to override a previous
2485 <option>-ignore-dot-ghci</option> option.</para>
2486 </listitem>
2487 </varlistentry>
2488 </variablelist>
2489
2490 </sect1>
2491
2492 <sect1 id="ghci-obj">
2493 <title>Compiling to object code inside GHCi</title>
2494
2495 <para>By default, GHCi compiles Haskell source code into byte-code
2496 that is interpreted by the runtime system. GHCi can also compile
2497 Haskell code to object code: to turn on this feature, use the
2498 <option>-fobject-code</option> flag either on the command line or
2499 with <literal>:set</literal> (the option
2500 <option>-fbyte-code</option> restores byte-code compilation
2501 again). Compiling to object code takes longer, but typically the
2502 code will execute 10-20 times faster than byte-code.</para>
2503
2504 <para>Compiling to object code inside GHCi is particularly useful
2505 if you are developing a compiled application, because the
2506 <literal>:reload</literal> command typically runs much faster than
2507 restarting GHC with <option>--make</option> from the command-line,
2508 because all the interface files are already cached in
2509 memory.</para>
2510
2511 <para>There are disadvantages to compiling to object-code: you
2512 can't set breakpoints in object-code modules, for example. Only
2513 the exports of an object-code module will be visible in GHCi,
2514 rather than all top-level bindings as in interpreted
2515 modules.</para>
2516 </sect1>
2517
2518 <sect1 id="ghci-faq">
2519 <title>FAQ and Things To Watch Out For</title>
2520
2521 <variablelist>
2522 <varlistentry>
2523 <term>The interpreter can't load modules with foreign export
2524 declarations!</term>
2525 <listitem>
2526 <para>Unfortunately not. We haven't implemented it yet.
2527 Please compile any offending modules by hand before loading
2528 them into GHCi.</para>
2529 </listitem>
2530 </varlistentry>
2531
2532 <varlistentry>
2533 <term>
2534 <literal>-O</literal> doesn't work with GHCi!
2535 <indexterm><primary><option>-O</option></primary></indexterm>
2536 </term>
2537 <listitem>
2538 <para>For technical reasons, the bytecode compiler doesn't
2539 interact well with one of the optimisation passes, so we
2540 have disabled optimisation when using the interpreter. This
2541 isn't a great loss: you'll get a much bigger win by
2542 compiling the bits of your code that need to go fast, rather
2543 than interpreting them with optimisation turned on.</para>
2544 </listitem>
2545 </varlistentry>
2546
2547 <varlistentry>
2548 <term>Unboxed tuples don't work with GHCi</term>
2549 <listitem>
2550 <para>That's right. You can always compile a module that
2551 uses unboxed tuples and load it into GHCi, however.
2552 (Incidentally the previous point, namely that
2553 <literal>-O</literal> is incompatible with GHCi, is because
2554 the bytecode compiler can't deal with unboxed
2555 tuples).</para>
2556 </listitem>
2557 </varlistentry>
2558
2559 <varlistentry>
2560 <term>Concurrent threads don't carry on running when GHCi is
2561 waiting for input.</term>
2562 <listitem>
2563 <para>This should work, as long as your GHCi was built with
2564 the <option>-threaded</option> switch, which is the default.
2565 Consult whoever supplied your GHCi installation.</para>
2566 </listitem>
2567 </varlistentry>
2568
2569 <varlistentry>
2570 <term>After using <literal>getContents</literal>, I can't use
2571 <literal>stdin</literal> again until I do
2572 <literal>:load</literal> or <literal>:reload</literal>.</term>
2573
2574 <listitem>
2575 <para>This is the defined behaviour of
2576 <literal>getContents</literal>: it puts the stdin Handle in
2577 a state known as <firstterm>semi-closed</firstterm>, wherein
2578 any further I/O operations on it are forbidden. Because I/O
2579 state is retained between computations, the semi-closed
2580 state persists until the next <literal>:load</literal> or
2581 <literal>:reload</literal> command.</para>
2582
2583 <para>You can make <literal>stdin</literal> reset itself
2584 after every evaluation by giving GHCi the command
2585 <literal>:set +r</literal>. This works because
2586 <literal>stdin</literal> is just a top-level expression that
2587 can be reverted to its unevaluated state in the same way as
2588 any other top-level expression (CAF).</para>
2589 </listitem>
2590 </varlistentry>
2591
2592 <varlistentry>
2593 <term>I can't use Control-C to interrupt computations in
2594 GHCi on Windows.</term>
2595 <listitem>
2596 <para>See <xref linkend="ghci-windows"/>.</para>
2597 </listitem>
2598 </varlistentry>
2599
2600 <varlistentry>
2601 <term>The default buffering mode is different in GHCi to GHC.</term>
2602 <listitem>
2603 <para>
2604 In GHC, the stdout handle is line-buffered by default.
2605 However, in GHCi we turn off the buffering on stdout,
2606 because this is normally what you want in an interpreter:
2607 output appears as it is generated.
2608 </para>
2609 </listitem>
2610 </varlistentry>
2611 </variablelist>
2612 </sect1>
2613
2614 </chapter>
2615
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