Fix DocBook formatting
[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 6.12.1: http://www.haskell.org/ghc/ :? for help
32 Loading package ghc-prim ... linking ... done.
33 Loading package integer-gmp ... linking ... done.
34 Loading package base ... linking ... done.
35 Loading package ffi-1.0 ... linking ... done.
36 Prelude>
37 </screen>
38
39 <para>There may be a short pause while GHCi loads the prelude and
40 standard libraries, after which the prompt is shown. As the banner
41 says, you can type <literal>:?</literal> to see the list of commands
42 available, and a half line description of each of them.</para>
43
44 <para>We'll explain most of these commands as we go along. For
45 Hugs users: many things work the same as in Hugs, so you should be
46 able to get going straight away.</para>
47
48 <para>Haskell expressions can be typed at the prompt:</para>
49 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
50 </indexterm>
51
52 <screen>
53 Prelude> 1+2
54 3
55 Prelude> let x = 42 in x / 9
56 4.666666666666667
57 Prelude>
58 </screen>
59
60 <para>GHCi interprets the whole line as an expression to evaluate.
61 The expression may not span several lines - as soon as you press enter,
62 GHCi will attempt to evaluate it.</para>
63
64 <para>GHCi also has a multiline mode,
65 <indexterm><primary><literal>:set +m</literal></primary></indexterm>,
66 which is terminated by an empty line:</para>
67
68 <screen>
69 Prelude> :set +m
70 Prelude> let x = 42 in x / 9
71 Prelude|
72 4.666666666666667
73 Prelude>
74 </screen>
75
76 <para>In Haskell, a <literal>let</literal> expression is followed
77 by <literal>in</literal>. However, in GHCi, since the expression
78 can also be interpreted in the <literal>IO</literal> monad,
79 a <literal>let</literal> binding with no accompanying
80 <literal>in</literal> statement can be signalled by an empty line,
81 as in the above example.</para>
82
83 <para>Multiline mode is useful when entering monadic
84 <literal>do</literal> statements:</para>
85
86 <screen>
87 Control.Monad.State> flip evalStateT 0 $ do
88 Control.Monad.State| i &lt;- get
89 Control.Monad.State| lift $ do
90 Control.Monad.State| putStrLn "Hello World!"
91 Control.Monad.State| print i
92 Control.Monad.State|
93 "Hello World!"
94 0
95 Control.Monad.State>
96 </screen>
97
98 <para>During a multiline interaction, the user can interrupt and
99 return to the top-level prompt.</para>
100
101 <screen>
102 Prelude> do
103 Prelude| putStrLn "Hello, World!"
104 Prelude| ^C
105 Prelude>
106 </screen>
107 </sect1>
108
109 <sect1 id="loading-source-files">
110 <title>Loading source files</title>
111
112 <para>Suppose we have the following Haskell source code, which we
113 place in a file <filename>Main.hs</filename>:</para>
114
115 <programlisting>
116 main = print (fac 20)
117
118 fac 0 = 1
119 fac n = n * fac (n-1)
120 </programlisting>
121
122 <para>You can save <filename>Main.hs</filename> anywhere you like,
123 but if you save it somewhere other than the current
124 directory<footnote><para>If you started up GHCi from the command
125 line then GHCi's current directory is the same as the current
126 directory of the shell from which it was started. If you started
127 GHCi from the &ldquo;Start&rdquo; menu in Windows, then the
128 current directory is probably something like
129 <filename>C:\Documents and Settings\<replaceable>user
130 name</replaceable></filename>.</para> </footnote> then we will
131 need to change to the right directory in GHCi:</para>
132
133 <screen>
134 Prelude> :cd <replaceable>dir</replaceable>
135 </screen>
136
137 <para>where <replaceable>dir</replaceable> is the directory (or
138 folder) in which you saved <filename>Main.hs</filename>.</para>
139
140 <para>To load a Haskell source file into GHCi, use the
141 <literal>:load</literal> command:</para>
142 <indexterm><primary><literal>:load</literal></primary></indexterm>
143
144 <screen>
145 Prelude> :load Main
146 Compiling Main ( Main.hs, interpreted )
147 Ok, modules loaded: Main.
148 *Main>
149 </screen>
150
151 <para>GHCi has loaded the <literal>Main</literal> module, and the
152 prompt has changed to &ldquo;<literal>*Main></literal>&rdquo; to
153 indicate that the current context for expressions typed at the
154 prompt is the <literal>Main</literal> module we just loaded (we'll
155 explain what the <literal>*</literal> means later in <xref
156 linkend="ghci-scope"/>). So we can now type expressions involving
157 the functions from <filename>Main.hs</filename>:</para>
158
159 <screen>
160 *Main> fac 17
161 355687428096000
162 </screen>
163
164 <para>Loading a multi-module program is just as straightforward;
165 just give the name of the &ldquo;topmost&rdquo; module to the
166 <literal>:load</literal> command (hint: <literal>:load</literal>
167 can be abbreviated to <literal>:l</literal>). The topmost module
168 will normally be <literal>Main</literal>, but it doesn't have to
169 be. GHCi will discover which modules are required, directly or
170 indirectly, by the topmost module, and load them all in dependency
171 order.</para>
172
173 <sect2 id="ghci-modules-filenames">
174 <title>Modules vs. filenames</title>
175 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
176 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
177
178 <para>Question: How does GHC find the filename which contains
179 module <replaceable>M</replaceable>? Answer: it looks for the
180 file <literal><replaceable>M</replaceable>.hs</literal>, or
181 <literal><replaceable>M</replaceable>.lhs</literal>. This means
182 that for most modules, the module name must match the filename.
183 If it doesn't, GHCi won't be able to find it.</para>
184
185 <para>There is one exception to this general rule: when you load
186 a program with <literal>:load</literal>, or specify it when you
187 invoke <literal>ghci</literal>, you can give a filename rather
188 than a module name. This filename is loaded if it exists, and
189 it may contain any module you like. This is particularly
190 convenient if you have several <literal>Main</literal> modules
191 in the same directory and you can't call them all
192 <filename>Main.hs</filename>.</para>
193
194 <para>The search path for finding source files is specified with
195 the <option>-i</option> option on the GHCi command line, like
196 so:</para>
197 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
198
199 <para>or it can be set using the <literal>:set</literal> command
200 from within GHCi (see <xref
201 linkend="ghci-cmd-line-options"/>)<footnote><para>Note that in
202 GHCi, and <option>&ndash;&ndash;make</option> mode, the <option>-i</option>
203 option is used to specify the search path for
204 <emphasis>source</emphasis> files, whereas in standard
205 batch-compilation mode the <option>-i</option> option is used to
206 specify the search path for interface files, see <xref
207 linkend="search-path"/>.</para> </footnote></para>
208
209 <para>One consequence of the way that GHCi follows dependencies
210 to find modules to load is that every module must have a source
211 file. The only exception to the rule is modules that come from
212 a package, including the <literal>Prelude</literal> and standard
213 libraries such as <literal>IO</literal> and
214 <literal>Complex</literal>. If you attempt to load a module for
215 which GHCi can't find a source file, even if there are object
216 and interface files for the module, you'll get an error
217 message.</para>
218 </sect2>
219
220 <sect2>
221 <title>Making changes and recompilation</title>
222 <indexterm><primary><literal>:reload</literal></primary></indexterm>
223
224 <para>If you make some changes to the source code and want GHCi
225 to recompile the program, give the <literal>:reload</literal>
226 command. The program will be recompiled as necessary, with GHCi
227 doing its best to avoid actually recompiling modules if their
228 external dependencies haven't changed. This is the same
229 mechanism we use to avoid re-compiling modules in the batch
230 compilation setting (see <xref linkend="recomp"/>).</para>
231 </sect2>
232 </sect1>
233
234 <sect1 id="ghci-compiled">
235 <title>Loading compiled code</title>
236 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
237
238 <para>When you load a Haskell source module into GHCi, it is
239 normally converted to byte-code and run using the interpreter.
240 However, interpreted code can also run alongside compiled code in
241 GHCi; indeed, normally when GHCi starts, it loads up a compiled
242 copy of the <literal>base</literal> package, which contains the
243 <literal>Prelude</literal>.</para>
244
245 <para>Why should we want to run compiled code? Well, compiled
246 code is roughly 10x faster than interpreted code, but takes about
247 2x longer to produce (perhaps longer if optimisation is on). So
248 it pays to compile the parts of a program that aren't changing
249 very often, and use the interpreter for the code being actively
250 developed.</para>
251
252 <para>When loading up source modules with <literal>:load</literal>,
253 GHCi normally looks for any corresponding compiled object files,
254 and will use one in preference to interpreting the source if
255 possible. For example, suppose we have a 4-module program
256 consisting of modules A, B, C, and D. Modules B and C both import
257 D only, and A imports both B &amp; C:</para>
258 <screen>
259 A
260 / \
261 B C
262 \ /
263 D
264 </screen>
265 <para>We can compile D, then load the whole program, like this:</para>
266 <screen>
267 Prelude> :! ghc -c D.hs
268 Prelude> :load A
269 Compiling B ( B.hs, interpreted )
270 Compiling C ( C.hs, interpreted )
271 Compiling A ( A.hs, interpreted )
272 Ok, modules loaded: A, B, C, D.
273 *Main>
274 </screen>
275
276 <para>In the messages from the compiler, we see that there is no line
277 for <literal>D</literal>. This is because
278 it isn't necessary to compile <literal>D</literal>,
279 because the source and everything it depends on
280 is unchanged since the last compilation.</para>
281
282 <para>At any time you can use the command
283 <literal>:show modules</literal>
284 to get a list of the modules currently loaded
285 into GHCi:</para>
286
287 <screen>
288 *Main> :show modules
289 D ( D.hs, D.o )
290 C ( C.hs, interpreted )
291 B ( B.hs, interpreted )
292 A ( A.hs, interpreted )
293 *Main></screen>
294
295 <para>If we now modify the source of D (or pretend to: using the Unix
296 command <literal>touch</literal> on the source file is handy for
297 this), the compiler will no longer be able to use the object file,
298 because it might be out of date:</para>
299
300 <screen>
301 *Main> :! touch D.hs
302 *Main> :reload
303 Compiling D ( D.hs, interpreted )
304 Ok, modules loaded: A, B, C, D.
305 *Main>
306 </screen>
307
308 <para>Note that module D was compiled, but in this instance
309 because its source hadn't really changed, its interface remained
310 the same, and the recompilation checker determined that A, B and C
311 didn't need to be recompiled.</para>
312
313 <para>So let's try compiling one of the other modules:</para>
314
315 <screen>
316 *Main> :! ghc -c C.hs
317 *Main> :load A
318 Compiling D ( D.hs, interpreted )
319 Compiling B ( B.hs, interpreted )
320 Compiling C ( C.hs, interpreted )
321 Compiling A ( A.hs, interpreted )
322 Ok, modules loaded: A, B, C, D.
323 </screen>
324
325 <para>We didn't get the compiled version of C! What happened?
326 Well, in GHCi a compiled module may only depend on other compiled
327 modules, and in this case C depends on D, which doesn't have an
328 object file, so GHCi also rejected C's object file. Ok, so let's
329 also compile D:</para>
330
331 <screen>
332 *Main> :! ghc -c D.hs
333 *Main> :reload
334 Ok, modules loaded: A, B, C, D.
335 </screen>
336
337 <para>Nothing happened! Here's another lesson: newly compiled
338 modules aren't picked up by <literal>:reload</literal>, only
339 <literal>:load</literal>:</para>
340
341 <screen>
342 *Main> :load A
343 Compiling B ( B.hs, interpreted )
344 Compiling A ( A.hs, interpreted )
345 Ok, modules loaded: A, B, C, D.
346 </screen>
347
348 <para>The automatic loading of object files can sometimes lead to
349 confusion, because non-exported top-level definitions of a module
350 are only available for use in expressions at the prompt when the
351 module is interpreted (see <xref linkend="ghci-scope" />). For
352 this reason, you might sometimes want to force GHCi to load a
353 module using the interpreter. This can be done by prefixing
354 a <literal>*</literal> to the module name or filename when
355 using <literal>:load</literal>, for example</para>
356
357 <screen>
358 Prelude> :load *A
359 Compiling A ( A.hs, interpreted )
360 *A>
361 </screen>
362
363 <para>When the <literal>*</literal> is used, GHCi ignores any
364 pre-compiled object code and interprets the module. If you have
365 already loaded a number of modules as object code and decide that
366 you wanted to interpret one of them, instead of re-loading the whole
367 set you can use <literal>:add *M</literal> to specify that you want
368 <literal>M</literal> to be interpreted (note that this might cause
369 other modules to be interpreted too, because compiled modules cannot
370 depend on interpreted ones).</para>
371
372 <para>To always compile everything to object code and never use the
373 interpreter, use the <literal>-fobject-code</literal> option (see
374 <xref linkend="ghci-obj" />).</para>
375
376 <para>HINT: since GHCi will only use a compiled object file if it
377 can be sure that the compiled version is up-to-date, a good technique
378 when working on a large program is to occasionally run
379 <literal>ghc &ndash;&ndash;make</literal> to compile the whole project (say
380 before you go for lunch :-), then continue working in the
381 interpreter. As you modify code, the changed modules will be
382 interpreted, but the rest of the project will remain
383 compiled.</para>
384 </sect1>
385
386 <sect1 id="interactive-evaluation">
387 <title>Interactive evaluation at the prompt</title>
388
389 <para>When you type an expression at the prompt, GHCi immediately
390 evaluates and prints the result:
391 <screen>
392 Prelude> reverse "hello"
393 "olleh"
394 Prelude> 5+5
395 10
396 </screen>
397 </para>
398
399 <sect2><title>I/O actions at the prompt</title>
400
401 <para>GHCi does more than simple expression evaluation at the prompt.
402 If you type something of type <literal>IO a</literal> for some
403 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
404 as an IO-computation.
405 <screen>
406 Prelude> "hello"
407 "hello"
408 Prelude> putStrLn "hello"
409 hello
410 </screen>
411 Furthermore, GHCi will print the result of the I/O action if (and only
412 if):
413 <itemizedlist>
414 <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
415 <listitem><para>The result type is not
416 <literal>()</literal>.</para></listitem>
417 </itemizedlist>
418 For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
419 <screen>
420 Prelude> putStrLn "hello"
421 hello
422 Prelude> do { putStrLn "hello"; return "yes" }
423 hello
424 "yes"
425 </screen>
426 </para></sect2>
427
428 <sect2 id="ghci-stmts">
429 <title>Using <literal>do-</literal>notation at the prompt</title>
430 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
431 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
432
433 <para>GHCi actually accepts <firstterm>statements</firstterm>
434 rather than just expressions at the prompt. This means you can
435 bind values and functions to names, and use them in future
436 expressions or statements.</para>
437
438 <para>The syntax of a statement accepted at the GHCi prompt is
439 exactly the same as the syntax of a statement in a Haskell
440 <literal>do</literal> expression. However, there's no monad
441 overloading here: statements typed at the prompt must be in the
442 <literal>IO</literal> monad.
443 <screen>
444 Prelude> x &lt;- return 42
445 Prelude> print x
446 42
447 Prelude>
448 </screen>
449 The statement <literal>x &lt;- return 42</literal> means
450 &ldquo;execute <literal>return 42</literal> in the
451 <literal>IO</literal> monad, and bind the result to
452 <literal>x</literal>&rdquo;. We can then use
453 <literal>x</literal> in future statements, for example to print
454 it as we did above.</para>
455
456 <para>If <option>-fprint-bind-result</option> is set then
457 GHCi will print the result of a statement if and only if:
458 <itemizedlist>
459 <listitem>
460 <para>The statement is not a binding, or it is a monadic binding
461 (<literal>p &lt;- e</literal>) that binds exactly one
462 variable.</para>
463 </listitem>
464 <listitem>
465 <para>The variable's type is not polymorphic, is not
466 <literal>()</literal>, and is an instance of
467 <literal>Show</literal></para>
468 </listitem>
469 </itemizedlist>
470 <indexterm><primary><option>-fprint-bind-result</option></primary></indexterm><indexterm><primary><option>-fno-print-bind-result</option></primary></indexterm>.
471 </para>
472
473 <para>Of course, you can also bind normal non-IO expressions
474 using the <literal>let</literal>-statement:</para>
475 <screen>
476 Prelude> let x = 42
477 Prelude> x
478 42
479 Prelude>
480 </screen>
481 <para>Another important difference between the two types of binding
482 is that the monadic bind (<literal>p &lt;- e</literal>) is
483 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
484 whereas with the <literal>let</literal> form, the expression
485 isn't evaluated immediately:</para>
486 <screen>
487 Prelude> let x = error "help!"
488 Prelude> print x
489 *** Exception: help!
490 Prelude>
491 </screen>
492
493 <para>Note that <literal>let</literal> bindings do not automatically
494 print the value bound, unlike monadic bindings.</para>
495
496 <para>Hint: you can also use <literal>let</literal>-statements
497 to define functions at the prompt:</para>
498 <screen>
499 Prelude> let add a b = a + b
500 Prelude> add 1 2
501 3
502 Prelude>
503 </screen>
504 <para>However, this quickly gets tedious when defining functions
505 with multiple clauses, or groups of mutually recursive functions,
506 because the complete definition has to be given on a single line,
507 using explicit braces and semicolons instead of layout:</para>
508 <screen>
509 Prelude> let { f op n [] = n ; f op n (h:t) = h `op` f op n t }
510 Prelude> f (+) 0 [1..3]
511 6
512 Prelude>
513 </screen>
514 <para>To alleviate this issue, GHCi commands can be split over
515 multiple lines, by wrapping them in <literal>:{</literal> and
516 <literal>:}</literal> (each on a single line of its own):</para>
517 <screen>
518 Prelude> :{
519 Prelude| let { g op n [] = n
520 Prelude| ; g op n (h:t) = h `op` g op n t
521 Prelude| }
522 Prelude| :}
523 Prelude> g (*) 1 [1..3]
524 6
525 </screen>
526 <para>Such multiline commands can be used with any GHCi command,
527 and the lines between <literal>:{</literal> and
528 <literal>:}</literal> are simply merged into a single line for
529 interpretation. That implies that each such group must form a single
530 valid command when merged, and that no layout rule is used.
531 The main purpose of multiline commands is not to replace module
532 loading but to make definitions in .ghci-files (see <xref
533 linkend="ghci-dot-files"/>) more readable and maintainable.</para>
534
535 <para>Any exceptions raised during the evaluation or execution
536 of the statement are caught and printed by the GHCi command line
537 interface (for more information on exceptions, see the module
538 <literal>Control.Exception</literal> in the libraries
539 documentation).</para>
540
541 <para>Every new binding shadows any existing bindings of the
542 same name, including entities that are in scope in the current
543 module context.</para>
544
545 <para>WARNING: temporary bindings introduced at the prompt only
546 last until the next <literal>:load</literal> or
547 <literal>:reload</literal> command, at which time they will be
548 simply lost. However, they do survive a change of context with
549 <literal>:module</literal>: the temporary bindings just move to
550 the new location.</para>
551
552 <para>HINT: To get a list of the bindings currently in scope, use the
553 <literal>:show bindings</literal> command:</para>
554
555 <screen>
556 Prelude> :show bindings
557 x :: Int
558 Prelude></screen>
559
560 <para>HINT: if you turn on the <literal>+t</literal> option,
561 GHCi will show the type of each variable bound by a statement.
562 For example:</para>
563 <indexterm><primary><literal>+t</literal></primary></indexterm>
564 <screen>
565 Prelude> :set +t
566 Prelude> let (x:xs) = [1..]
567 x :: Integer
568 xs :: [Integer]
569 </screen>
570
571 </sect2>
572
573 <sect2 id="ghci-scope">
574 <title>What's really in scope at the prompt?</title>
575
576 <para>When you type an expression at the prompt, what
577 identifiers and types are in scope? GHCi provides a flexible
578 way to control exactly how the context for an expression is
579 constructed. Let's start with the simple cases; when you start
580 GHCi the prompt looks like this:</para>
581
582 <screen>Prelude></screen>
583
584 <para>Which indicates that everything from the module
585 <literal>Prelude</literal> is currently in scope. If we now
586 load a file into GHCi, the prompt will change:</para>
587
588 <screen>
589 Prelude> :load Main.hs
590 Compiling Main ( Main.hs, interpreted )
591 *Main>
592 </screen>
593
594 <para>The new prompt is <literal>*Main</literal>, which
595 indicates that we are typing expressions in the context of the
596 top-level of the <literal>Main</literal> module. Everything
597 that is in scope at the top-level in the module
598 <literal>Main</literal> we just loaded is also in scope at the
599 prompt (probably including <literal>Prelude</literal>, as long
600 as <literal>Main</literal> doesn't explicitly hide it).</para>
601
602 <para>The syntax
603 <literal>*<replaceable>module</replaceable></literal> indicates
604 that it is the full top-level scope of
605 <replaceable>module</replaceable> that is contributing to the
606 scope for expressions typed at the prompt. Without the
607 <literal>*</literal>, just the exports of the module are
608 visible.</para>
609
610 <para>We're not limited to a single module: GHCi can combine
611 scopes from multiple modules, in any mixture of
612 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
613 combines the scopes from all of these modules to form the scope
614 that is in effect at the prompt.</para>
615
616 <para>NOTE: for technical reasons, GHCi can only support the
617 <literal>*</literal>-form for modules that are interpreted.
618 Compiled modules and package modules can only contribute their
619 exports to the current scope. To ensure that GHCi loads the
620 interpreted version of a module, add the <literal>*</literal>
621 when loading the module, e.g. <literal>:load *M</literal>.</para>
622
623 <para>The scope is manipulated using the
624 <literal>:module</literal> command. For example, if the current
625 scope is <literal>Prelude</literal>, then we can bring into
626 scope the exports from the module <literal>IO</literal> like
627 so:</para>
628
629 <screen>
630 Prelude> :module +IO
631 Prelude IO> hPutStrLn stdout "hello\n"
632 hello
633 Prelude IO>
634 </screen>
635
636 <para>(Note: you can use conventional
637 haskell <literal>import</literal> syntax as
638 well, but this does not support
639 <literal>*</literal> forms).
640 <literal>:module</literal> can also be shortened to
641 <literal>:m</literal>. The full syntax of the
642 <literal>:module</literal> command is:</para>
643
644 <screen>
645 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
646 </screen>
647
648 <para>Using the <literal>+</literal> form of the
649 <literal>module</literal> commands adds modules to the current
650 scope, and <literal>-</literal> removes them. Without either
651 <literal>+</literal> or <literal>-</literal>, the current scope
652 is replaced by the set of modules specified. Note that if you
653 use this form and leave out <literal>Prelude</literal>, GHCi
654 will assume that you really wanted the
655 <literal>Prelude</literal> and add it in for you (if you don't
656 want the <literal>Prelude</literal>, then ask to remove it with
657 <literal>:m -Prelude</literal>).</para>
658
659 <para>The scope is automatically set after a
660 <literal>:load</literal> command, to the most recently loaded
661 "target" module, in a <literal>*</literal>-form if possible.
662 For example, if you say <literal>:load foo.hs bar.hs</literal>
663 and <filename>bar.hs</filename> contains module
664 <literal>Bar</literal>, then the scope will be set to
665 <literal>*Bar</literal> if <literal>Bar</literal> is
666 interpreted, or if <literal>Bar</literal> is compiled it will be
667 set to <literal>Prelude Bar</literal> (GHCi automatically adds
668 <literal>Prelude</literal> if it isn't present and there aren't
669 any <literal>*</literal>-form modules).</para>
670
671 <para>With multiple modules in scope, especially multiple
672 <literal>*</literal>-form modules, it is likely that name
673 clashes will occur. Haskell specifies that name clashes are
674 only reported when an ambiguous identifier is used, and GHCi
675 behaves in the same way for expressions typed at the
676 prompt.</para>
677
678 <para>
679 Hint: GHCi will tab-complete names that are in scope; for
680 example, if you run GHCi and type <literal>J&lt;tab&gt;</literal>
681 then GHCi will expand it to &ldquo;<literal>Just </literal>&rdquo;.
682 </para>
683
684 <sect3>
685 <title><literal>:module</literal> and
686 <literal>:load</literal></title>
687
688 <para>It might seem that <literal>:module</literal> and
689 <literal>:load</literal> do similar things: you can use both
690 to bring a module into scope. However, there is a clear
691 difference. GHCi is concerned with two sets of modules:</para>
692
693 <itemizedlist>
694 <listitem>
695 <para>The set of modules that are
696 currently <emphasis>loaded</emphasis>. This set is
697 modified
698 by <literal>:load</literal>, <literal>:add</literal>
699 and <literal>:reload</literal>.
700 </para>
701 </listitem>
702 <listitem>
703 <para>The set of modules that are currently <emphasis>in
704 scope</emphasis> at the prompt. This set is modified
705 by <literal>:module</literal>, and it is also set
706 automatically
707 after <literal>:load</literal>, <literal>:add</literal>,
708 and <literal>:reload</literal>.</para>
709 </listitem>
710 </itemizedlist>
711
712 <para>You cannot add a module to the scope if it is not
713 loaded. This is why trying to
714 use <literal>:module</literal> to load a new module results
715 in the message &ldquo;<literal>module M is not
716 loaded</literal>&rdquo;.</para>
717 </sect3>
718
719 <sect3 id="ghci-import-qualified">
720 <title>Qualified names</title>
721
722 <para>To make life slightly easier, the GHCi prompt also
723 behaves as if there is an implicit <literal>import
724 qualified</literal> declaration for every module in every
725 package, and every module currently loaded into GHCi. This
726 behaviour can be disabled with the flag <option>-fno-implicit-import-qualified</option><indexterm><primary><option>-fno-implicit-import-qualified</option></primary></indexterm>.</para>
727 </sect3>
728
729 <sect3>
730 <title>The <literal>:main</literal> and <literal>:run</literal> commands</title>
731
732 <para>
733 When a program is compiled and executed, it can use the
734 <literal>getArgs</literal> function to access the
735 command-line arguments.
736 However, we cannot simply pass the arguments to the
737 <literal>main</literal> function while we are testing in ghci,
738 as the <literal>main</literal> function doesn't take its
739 directly.
740 </para>
741
742 <para>
743 Instead, we can use the <literal>:main</literal> command.
744 This runs whatever <literal>main</literal> is in scope, with
745 any arguments being treated the same as command-line arguments,
746 e.g.:
747 </para>
748
749 <screen>
750 Prelude> let main = System.Environment.getArgs >>= print
751 Prelude> :main foo bar
752 ["foo","bar"]
753 </screen>
754
755 <para>
756 We can also quote arguments which contains characters like
757 spaces, and they are treated like Haskell strings, or we can
758 just use Haskell list syntax:
759 </para>
760
761 <screen>
762 Prelude> :main foo "bar baz"
763 ["foo","bar baz"]
764 Prelude> :main ["foo", "bar baz"]
765 ["foo","bar baz"]
766 </screen>
767
768 <para>
769 Finally, other functions can be called, either with the
770 <literal>-main-is</literal> flag or the <literal>:run</literal>
771 command:
772 </para>
773
774 <screen>
775 Prelude> let foo = putStrLn "foo" >> System.Environment.getArgs >>= print
776 Prelude> let bar = putStrLn "bar" >> System.Environment.getArgs >>= print
777 Prelude> :set -main-is foo
778 Prelude> :main foo "bar baz"
779 foo
780 ["foo","bar baz"]
781 Prelude> :run bar ["foo", "bar baz"]
782 bar
783 ["foo","bar baz"]
784 </screen>
785
786 </sect3>
787 </sect2>
788
789
790 <sect2>
791 <title>The <literal>it</literal> variable</title>
792 <indexterm><primary><literal>it</literal></primary>
793 </indexterm>
794
795 <para>Whenever an expression (or a non-binding statement, to be
796 precise) is typed at the prompt, GHCi implicitly binds its value
797 to the variable <literal>it</literal>. For example:</para>
798 <screen>
799 Prelude> 1+2
800 3
801 Prelude> it * 2
802 6
803 </screen>
804 <para>What actually happens is that GHCi typechecks the
805 expression, and if it doesn't have an <literal>IO</literal> type,
806 then it transforms it as follows: an expression
807 <replaceable>e</replaceable> turns into
808 <screen>
809 let it = <replaceable>e</replaceable>;
810 print it
811 </screen>
812 which is then run as an IO-action.</para>
813
814 <para>Hence, the original expression must have a type which is an
815 instance of the <literal>Show</literal> class, or GHCi will
816 complain:</para>
817
818 <screen>
819 Prelude&gt; id
820
821 &lt;interactive&gt;:1:0:
822 No instance for (Show (a -&gt; a))
823 arising from use of `print' at &lt;interactive&gt;:1:0-1
824 Possible fix: add an instance declaration for (Show (a -> a))
825 In the expression: print it
826 In a 'do' expression: print it
827 </screen>
828
829 <para>The error message contains some clues as to the
830 transformation happening internally.</para>
831
832 <para>If the expression was instead of type <literal>IO a</literal> for
833 some <literal>a</literal>, then <literal>it</literal> will be
834 bound to the result of the <literal>IO</literal> computation,
835 which is of type <literal>a</literal>. eg.:</para>
836 <screen>
837 Prelude> Time.getClockTime
838 Wed Mar 14 12:23:13 GMT 2001
839 Prelude> print it
840 Wed Mar 14 12:23:13 GMT 2001
841 </screen>
842
843 <para>The corresponding translation for an IO-typed
844 <replaceable>e</replaceable> is
845 <screen>
846 it &lt;- <replaceable>e</replaceable>
847 </screen>
848 </para>
849
850 <para>Note that <literal>it</literal> is shadowed by the new
851 value each time you evaluate a new expression, and the old value
852 of <literal>it</literal> is lost.</para>
853
854 </sect2>
855
856 <sect2 id="extended-default-rules">
857 <title>Type defaulting in GHCi</title>
858 <indexterm><primary>Type default</primary></indexterm>
859 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
860 <para>
861 Consider this GHCi session:
862 <programlisting>
863 ghci> reverse []
864 </programlisting>
865 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
866 (which is what GHCi computes here) has type <literal>Show a => String</literal> and how that displays depends
867 on the type <literal>a</literal>. For example:
868 <programlisting>
869 ghci> reverse ([] :: String)
870 ""
871 ghci> reverse ([] :: [Int])
872 []
873 </programlisting>
874 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
875 rules (Section 4.3.4 of the Haskell 2010 Report) as follows. The
876 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
877 a)</literal> for each type variable <literal>a</literal>, and defaults the
878 type variable if
879 <orderedlist>
880 <listitem>
881 <para>
882 The type variable <literal>a</literal> appears in no
883 other constraints
884 </para>
885 </listitem>
886 <listitem>
887 <para>
888 All the classes <literal>Ci</literal> are standard.
889 </para>
890 </listitem>
891 <listitem>
892 <para>
893 At least one of the classes <literal>Ci</literal> is
894 numeric.
895 </para>
896 </listitem>
897 </orderedlist>
898 At the GHCi prompt, or with GHC if the
899 <literal>-XExtendedDefaultRules</literal> flag is given,
900 the following additional differences apply:
901 <itemizedlist>
902 <listitem>
903 <para>
904 Rule 2 above is relaxed thus:
905 <emphasis>All</emphasis> of the classes
906 <literal>Ci</literal> are single-parameter type classes.
907 </para>
908 </listitem>
909 <listitem>
910 <para>
911 Rule 3 above is relaxed this:
912 At least one of the classes <literal>Ci</literal> is
913 numeric, <emphasis>or is <literal>Show</literal>,
914 <literal>Eq</literal>, or
915 <literal>Ord</literal></emphasis>.
916 </para>
917 </listitem>
918 <listitem>
919 <para>
920 The unit type <literal>()</literal> is added to the
921 start of the standard list of types which are tried when
922 doing type defaulting.
923 </para>
924 </listitem>
925 </itemizedlist>
926 The last point means that, for example, this program:
927 <programlisting>
928 main :: IO ()
929 main = print def
930
931 instance Num ()
932
933 def :: (Num a, Enum a) => a
934 def = toEnum 0
935 </programlisting>
936 prints <literal>()</literal> rather than <literal>0</literal> as the
937 type is defaulted to <literal>()</literal> rather than
938 <literal>Integer</literal>.
939 </para>
940 <para>
941 The motivation for the change is that it means <literal>IO a</literal>
942 actions default to <literal>IO ()</literal>, which in turn means that
943 ghci won't try to print a result when running them. This is
944 particularly important for <literal>printf</literal>, which has an
945 instance that returns <literal>IO a</literal>.
946 However, it is only able to return
947 <literal>undefined</literal>
948 (the reason for the instance having this type is so that printf
949 doesn't require extensions to the class system), so if the type defaults to
950 <literal>Integer</literal> then ghci gives an error when running a
951 printf.
952 </para>
953 </sect2>
954 </sect1>
955
956 <sect1 id="ghci-debugger">
957 <title>The GHCi Debugger</title>
958 <indexterm><primary>debugger</primary><secondary>in GHCi</secondary>
959 </indexterm>
960
961 <para>GHCi contains a simple imperative-style debugger in which you can
962 stop a running computation in order to examine the values of
963 variables. The debugger is integrated into GHCi, and is turned on by
964 default: no flags are required to enable the debugging
965 facilities. There is one major restriction: breakpoints and
966 single-stepping are only available in interpreted modules;
967 compiled code is invisible to the debugger<footnote><para>Note that packages
968 only contain compiled code, so debugging a package requires
969 finding its source and loading that directly.</para></footnote>.</para>
970
971 <para>The debugger provides the following:
972 <itemizedlist>
973 <listitem>
974 <para>The ability to set a <firstterm>breakpoint</firstterm> on a
975 function definition or expression in the program. When the function
976 is called, or the expression evaluated, GHCi suspends
977 execution and returns to the prompt, where you can inspect the
978 values of local variables before continuing with the
979 execution.</para>
980 </listitem>
981 <listitem>
982 <para>Execution can be <firstterm>single-stepped</firstterm>: the
983 evaluator will suspend execution approximately after every
984 reduction, allowing local variables to be inspected. This is
985 equivalent to setting a breakpoint at every point in the
986 program.</para>
987 </listitem>
988 <listitem>
989 <para>Execution can take place in <firstterm>tracing
990 mode</firstterm>, in which the evaluator remembers each
991 evaluation step as it happens, but doesn't suspend execution until
992 an actual breakpoint is reached. When this happens, the history of
993 evaluation steps can be inspected.</para>
994 </listitem>
995 <listitem>
996 <para>Exceptions (e.g. pattern matching failure and
997 <literal>error</literal>) can be treated as breakpoints, to help
998 locate the source of an exception in the program.</para>
999 </listitem>
1000 </itemizedlist>
1001 </para>
1002
1003 <para>There is currently no support for obtaining a &ldquo;stack
1004 trace&rdquo;, but the tracing and history features provide a
1005 useful second-best, which will often be enough to establish the
1006 context of an error. For instance, it is possible to break
1007 automatically when an exception is thrown, even if it is thrown
1008 from within compiled code (see <xref
1009 linkend="ghci-debugger-exceptions" />).</para>
1010
1011 <sect2 id="breakpoints">
1012 <title>Breakpoints and inspecting variables</title>
1013
1014 <para>Let's use quicksort as a running example. Here's the code:</para>
1015
1016 <programlisting>
1017 qsort [] = []
1018 qsort (a:as) = qsort left ++ [a] ++ qsort right
1019 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
1020
1021 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1022 </programlisting>
1023
1024 <para>First, load the module into GHCi:</para>
1025
1026 <screen>
1027 Prelude> :l qsort.hs
1028 [1 of 1] Compiling Main ( qsort.hs, interpreted )
1029 Ok, modules loaded: Main.
1030 *Main>
1031 </screen>
1032
1033 <para>Now, let's set a breakpoint on the right-hand-side of the second
1034 equation of qsort:</para>
1035
1036 <programlisting>
1037 *Main> :break 2
1038 Breakpoint 0 activated at qsort.hs:2:15-46
1039 *Main>
1040 </programlisting>
1041
1042 <para>The command <literal>:break 2</literal> sets a breakpoint on line
1043 2 of the most recently-loaded module, in this case
1044 <literal>qsort.hs</literal>. Specifically, it picks the
1045 leftmost complete subexpression on that line on which to set the
1046 breakpoint, which in this case is the expression
1047 <literal>(qsort left ++ [a] ++ qsort right)</literal>.</para>
1048
1049 <para>Now, we run the program:</para>
1050
1051 <programlisting>
1052 *Main> main
1053 Stopped at qsort.hs:2:15-46
1054 _result :: [a]
1055 a :: a
1056 left :: [a]
1057 right :: [a]
1058 [qsort.hs:2:15-46] *Main>
1059 </programlisting>
1060
1061 <para>Execution has stopped at the breakpoint. The prompt has changed to
1062 indicate that we are currently stopped at a breakpoint, and the location:
1063 <literal>[qsort.hs:2:15-46]</literal>. To further clarify the
1064 location, we can use the <literal>:list</literal> command:</para>
1065
1066 <programlisting>
1067 [qsort.hs:2:15-46] *Main> :list
1068 1 qsort [] = []
1069 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
1070 3 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
1071 </programlisting>
1072
1073 <para>The <literal>:list</literal> command lists the source code around
1074 the current breakpoint. If your output device supports it, then GHCi
1075 will highlight the active subexpression in bold.</para>
1076
1077 <para>GHCi has provided bindings for the free variables<footnote><para>We
1078 originally provided bindings for all variables in scope, rather
1079 than just
1080 the free variables of the expression, but found that this affected
1081 performance considerably, hence the current restriction to just the
1082 free variables.</para>
1083 </footnote> of the expression
1084 on which the
1085 breakpoint was placed (<literal>a</literal>, <literal>left</literal>,
1086 <literal>right</literal>), and additionally a binding for the result of
1087 the expression (<literal>_result</literal>). These variables are just
1088 like other variables that you might define in GHCi; you
1089 can use them in expressions that you type at the prompt, you can ask
1090 for their types with <literal>:type</literal>, and so on. There is one
1091 important difference though: these variables may only have partial
1092 types. For example, if we try to display the value of
1093 <literal>left</literal>:</para>
1094
1095 <screen>
1096 [qsort.hs:2:15-46] *Main> left
1097
1098 &lt;interactive&gt;:1:0:
1099 Ambiguous type variable `a' in the constraint:
1100 `Show a' arising from a use of `print' at &lt;interactive&gt;:1:0-3
1101 Cannot resolve unknown runtime types: a
1102 Use :print or :force to determine these types
1103 </screen>
1104
1105 <para>This is because <literal>qsort</literal> is a polymorphic function,
1106 and because GHCi does not carry type information at runtime, it cannot
1107 determine the runtime types of free variables that involve type
1108 variables. Hence, when you ask to display <literal>left</literal> at
1109 the prompt, GHCi can't figure out which instance of
1110 <literal>Show</literal> to use, so it emits the type error above.</para>
1111
1112 <para>Fortunately, the debugger includes a generic printing command,
1113 <literal>:print</literal>, which can inspect the actual runtime value of a
1114 variable and attempt to reconstruct its type. If we try it on
1115 <literal>left</literal>:</para>
1116
1117 <screen>
1118 [qsort.hs:2:15-46] *Main> :set -fprint-evld-with-show
1119 [qsort.hs:2:15-46] *Main> :print left
1120 left = (_t1::[a])
1121 </screen>
1122
1123 <para>This isn't particularly enlightening. What happened is that
1124 <literal>left</literal> is bound to an unevaluated computation (a
1125 suspension, or <firstterm>thunk</firstterm>), and
1126 <literal>:print</literal> does not force any evaluation. The idea is
1127 that <literal>:print</literal> can be used to inspect values at a
1128 breakpoint without any unfortunate side effects. It won't force any
1129 evaluation, which could cause the program to give a different answer
1130 than it would normally, and hence it won't cause any exceptions to be
1131 raised, infinite loops, or further breakpoints to be triggered (see
1132 <xref linkend="nested-breakpoints" />).
1133 Rather than forcing thunks, <literal>:print</literal>
1134 binds each thunk to a fresh variable beginning with an
1135 underscore, in this case
1136 <literal>_t1</literal>.</para>
1137
1138 <para>The flag <literal>-fprint-evld-with-show</literal> instructs
1139 <literal>:print</literal> to reuse
1140 available <literal>Show</literal> instances when possible. This happens
1141 only when the contents of the variable being inspected
1142 are completely evaluated.</para>
1143
1144
1145 <para>If we aren't concerned about preserving the evaluatedness of a
1146 variable, we can use <literal>:force</literal> instead of
1147 <literal>:print</literal>. The <literal>:force</literal> command
1148 behaves exactly like <literal>:print</literal>, except that it forces
1149 the evaluation of any thunks it encounters:</para>
1150
1151 <screen>
1152 [qsort.hs:2:15-46] *Main> :force left
1153 left = [4,0,3,1]
1154 </screen>
1155
1156 <para>Now, since <literal>:force</literal> has inspected the runtime
1157 value of <literal>left</literal>, it has reconstructed its type. We
1158 can see the results of this type reconstruction:</para>
1159
1160 <screen>
1161 [qsort.hs:2:15-46] *Main> :show bindings
1162 _result :: [Integer]
1163 a :: Integer
1164 left :: [Integer]
1165 right :: [Integer]
1166 _t1 :: [Integer]
1167 </screen>
1168
1169 <para>Not only do we now know the type of <literal>left</literal>, but
1170 all the other partial types have also been resolved. So we can ask
1171 for the value of <literal>a</literal>, for example:</para>
1172
1173 <screen>
1174 [qsort.hs:2:15-46] *Main> a
1175 8
1176 </screen>
1177
1178 <para>You might find it useful to use Haskell's
1179 <literal>seq</literal> function to evaluate individual thunks rather
1180 than evaluating the whole expression with <literal>:force</literal>.
1181 For example:</para>
1182
1183 <screen>
1184 [qsort.hs:2:15-46] *Main> :print right
1185 right = (_t1::[Integer])
1186 [qsort.hs:2:15-46] *Main> seq _t1 ()
1187 ()
1188 [qsort.hs:2:15-46] *Main> :print right
1189 right = 23 : (_t2::[Integer])
1190 </screen>
1191
1192 <para>We evaluated only the <literal>_t1</literal> thunk, revealing the
1193 head of the list, and the tail is another thunk now bound to
1194 <literal>_t2</literal>. The <literal>seq</literal> function is a
1195 little inconvenient to use here, so you might want to use
1196 <literal>:def</literal> to make a nicer interface (left as an exercise
1197 for the reader!).</para>
1198
1199 <para>Finally, we can continue the current execution:</para>
1200
1201 <screen>
1202 [qsort.hs:2:15-46] *Main> :continue
1203 Stopped at qsort.hs:2:15-46
1204 _result :: [a]
1205 a :: a
1206 left :: [a]
1207 right :: [a]
1208 [qsort.hs:2:15-46] *Main>
1209 </screen>
1210
1211 <para>The execution continued at the point it previously stopped, and has
1212 now stopped at the breakpoint for a second time.</para>
1213
1214
1215 <sect3 id="setting-breakpoints">
1216 <title>Setting breakpoints</title>
1217
1218 <para>Breakpoints can be set in various ways. Perhaps the easiest way to
1219 set a breakpoint is to name a top-level function:</para>
1220
1221 <screen>
1222 :break <replaceable>identifier</replaceable>
1223 </screen>
1224
1225 <para>Where <replaceable>identifier</replaceable> names any top-level
1226 function in an interpreted module currently loaded into GHCi (qualified
1227 names may be used). The breakpoint will be set on the body of the
1228 function, when it is fully applied but before any pattern matching has
1229 taken place.</para>
1230
1231 <para>Breakpoints can also be set by line (and optionally column)
1232 number:</para>
1233
1234 <screen>
1235 :break <replaceable>line</replaceable>
1236 :break <replaceable>line</replaceable> <replaceable>column</replaceable>
1237 :break <replaceable>module</replaceable> <replaceable>line</replaceable>
1238 :break <replaceable>module</replaceable> <replaceable>line</replaceable> <replaceable>column</replaceable>
1239 </screen>
1240
1241 <para>When a breakpoint is set on a particular line, GHCi sets the
1242 breakpoint on the
1243 leftmost subexpression that begins and ends on that line. If two
1244 complete subexpressions start at the same
1245 column, the longest one is picked. If there is no complete
1246 subexpression on the line, then the leftmost expression starting on
1247 the line is picked, and failing that the rightmost expression that
1248 partially or completely covers the line.</para>
1249
1250 <para>When a breakpoint is set on a particular line and column, GHCi
1251 picks the smallest subexpression that encloses that location on which
1252 to set the breakpoint. Note: GHC considers the TAB character to have a
1253 width of 1, wherever it occurs; in other words it counts
1254 characters, rather than columns. This matches what some editors do,
1255 and doesn't match others. The best advice is to avoid tab
1256 characters in your source code altogether (see
1257 <option>-fwarn-tabs</option> in <xref linkend="options-sanity"
1258 />).</para>
1259
1260 <para>If the module is omitted, then the most recently-loaded module is
1261 used.</para>
1262
1263 <para>Not all subexpressions are potential breakpoint locations. Single
1264 variables are typically not considered to be breakpoint locations
1265 (unless the variable is the right-hand-side of a function definition,
1266 lambda, or case alternative). The rule of thumb is that all redexes
1267 are breakpoint locations, together with the bodies of functions,
1268 lambdas, case alternatives and binding statements. There is normally
1269 no breakpoint on a let expression, but there will always be a
1270 breakpoint on its body, because we are usually interested in inspecting
1271 the values of the variables bound by the let.</para>
1272
1273 </sect3>
1274 <sect3>
1275 <title>Listing and deleting breakpoints</title>
1276
1277 <para>The list of breakpoints currently enabled can be displayed using
1278 <literal>:show&nbsp;breaks</literal>:</para>
1279 <screen>
1280 *Main> :show breaks
1281 [0] Main qsort.hs:1:11-12
1282 [1] Main qsort.hs:2:15-46
1283 </screen>
1284
1285 <para>To delete a breakpoint, use the <literal>:delete</literal>
1286 command with the number given in the output from <literal>:show&nbsp;breaks</literal>:</para>
1287
1288 <screen>
1289 *Main> :delete 0
1290 *Main> :show breaks
1291 [1] Main qsort.hs:2:15-46
1292 </screen>
1293
1294 <para>To delete all breakpoints at once, use <literal>:delete *</literal>.</para>
1295
1296 </sect3>
1297 </sect2>
1298
1299 <sect2 id="single-stepping">
1300 <title>Single-stepping</title>
1301
1302 <para>Single-stepping is a great way to visualise the execution of your
1303 program, and it is also a useful tool for identifying the source of a
1304 bug. GHCi offers two variants of stepping. Use
1305 <literal>:step</literal> to enable all the
1306 breakpoints in the program, and execute until the next breakpoint is
1307 reached. Use <literal>:steplocal</literal> to limit the set
1308 of enabled breakpoints to those in the current top level function.
1309 Similarly, use <literal>:stepmodule</literal> to single step only on
1310 breakpoints contained in the current module.
1311 For example:</para>
1312
1313 <screen>
1314 *Main> :step main
1315 Stopped at qsort.hs:5:7-47
1316 _result :: IO ()
1317 </screen>
1318
1319 <para>The command <literal>:step
1320 <replaceable>expr</replaceable></literal> begins the evaluation of
1321 <replaceable>expr</replaceable> in single-stepping mode. If
1322 <replaceable>expr</replaceable> is omitted, then it single-steps from
1323 the current breakpoint. <literal>:stepover</literal>
1324 works similarly.</para>
1325
1326 <para>The <literal>:list</literal> command is particularly useful when
1327 single-stepping, to see where you currently are:</para>
1328
1329 <screen>
1330 [qsort.hs:5:7-47] *Main> :list
1331 4
1332 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1333 6
1334 [qsort.hs:5:7-47] *Main>
1335 </screen>
1336
1337 <para>In fact, GHCi provides a way to run a command when a breakpoint is
1338 hit, so we can make it automatically do
1339 <literal>:list</literal>:</para>
1340
1341 <screen>
1342 [qsort.hs:5:7-47] *Main> :set stop :list
1343 [qsort.hs:5:7-47] *Main> :step
1344 Stopped at qsort.hs:5:14-46
1345 _result :: [Integer]
1346 4
1347 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1348 6
1349 [qsort.hs:5:14-46] *Main>
1350 </screen>
1351 </sect2>
1352
1353 <sect2 id="nested-breakpoints">
1354 <title>Nested breakpoints</title>
1355 <para>When GHCi is stopped at a breakpoint, and an expression entered at
1356 the prompt triggers a
1357 second breakpoint, the new breakpoint becomes the &ldquo;current&rdquo;
1358 one, and the old one is saved on a stack. An arbitrary number of
1359 breakpoint contexts can be built up in this way. For example:</para>
1360
1361 <screen>
1362 [qsort.hs:2:15-46] *Main> :st qsort [1,3]
1363 Stopped at qsort.hs:(1,0)-(3,55)
1364 _result :: [a]
1365 ... [qsort.hs:(1,0)-(3,55)] *Main>
1366 </screen>
1367
1368 <para>While stopped at the breakpoint on line 2 that we set earlier, we
1369 started a new evaluation with <literal>:step qsort [1,3]</literal>.
1370 This new evaluation stopped after one step (at the definition of
1371 <literal>qsort</literal>). The prompt has changed, now prefixed with
1372 <literal>...</literal>, to indicate that there are saved breakpoints
1373 beyond the current one. To see the stack of contexts, use
1374 <literal>:show context</literal>:</para>
1375
1376 <screen>
1377 ... [qsort.hs:(1,0)-(3,55)] *Main> :show context
1378 --> main
1379 Stopped at qsort.hs:2:15-46
1380 --> qsort [1,3]
1381 Stopped at qsort.hs:(1,0)-(3,55)
1382 ... [qsort.hs:(1,0)-(3,55)] *Main>
1383 </screen>
1384
1385 <para>To abandon the current evaluation, use
1386 <literal>:abandon</literal>:</para>
1387
1388 <screen>
1389 ... [qsort.hs:(1,0)-(3,55)] *Main> :abandon
1390 [qsort.hs:2:15-46] *Main> :abandon
1391 *Main>
1392 </screen>
1393 </sect2>
1394
1395 <sect2 id="ghci-debugger-result">
1396 <title>The <literal>_result</literal> variable</title>
1397 <para>When stopped at a breakpoint or single-step, GHCi binds the
1398 variable <literal>_result</literal> to the value of the currently
1399 active expression. The value of <literal>_result</literal> is
1400 presumably not available yet, because we stopped its evaluation, but it
1401 can be forced: if the type is known and showable, then just entering
1402 <literal>_result</literal> at the prompt will show it. However,
1403 there's one caveat to doing this: evaluating <literal>_result</literal>
1404 will be likely to trigger further breakpoints, starting with the
1405 breakpoint we are currently stopped at (if we stopped at a real
1406 breakpoint, rather than due to <literal>:step</literal>). So it will
1407 probably be necessary to issue a <literal>:continue</literal>
1408 immediately when evaluating <literal>_result</literal>. Alternatively,
1409 you can use <literal>:force</literal> which ignores breakpoints.</para>
1410 </sect2>
1411
1412 <sect2 id="tracing">
1413 <title>Tracing and history</title>
1414
1415 <para>A question that we often want to ask when debugging a program is
1416 &ldquo;how did I get here?&rdquo;. Traditional imperative debuggers
1417 usually provide some kind of stack-tracing feature that lets you see
1418 the stack of active function calls (sometimes called the &ldquo;lexical
1419 call stack&rdquo;), describing a path through the code
1420 to the current location. Unfortunately this is hard to provide in
1421 Haskell, because execution proceeds on a demand-driven basis, rather
1422 than a depth-first basis as in strict languages. The
1423 &ldquo;stack&ldquo; in GHC's execution engine bears little
1424 resemblance to the lexical call stack. Ideally GHCi would maintain a
1425 separate lexical call stack in addition to the dynamic call stack, and
1426 in fact this is exactly
1427 what our profiling system does (<xref linkend="profiling" />), and what
1428 some other Haskell debuggers do. For the time being, however, GHCi
1429 doesn't maintain a lexical call stack (there are some technical
1430 challenges to be overcome). Instead, we provide a way to backtrack from a
1431 breakpoint to previous evaluation steps: essentially this is like
1432 single-stepping backwards, and should in many cases provide enough
1433 information to answer the &ldquo;how did I get here?&rdquo;
1434 question.</para>
1435
1436 <para>To use tracing, evaluate an expression with the
1437 <literal>:trace</literal> command. For example, if we set a breakpoint
1438 on the base case of <literal>qsort</literal>:</para>
1439
1440 <screen>
1441 *Main&gt; :list qsort
1442 1 qsort [] = []
1443 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
1444 3 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
1445 4
1446 *Main&gt; :b 1
1447 Breakpoint 1 activated at qsort.hs:1:11-12
1448 *Main&gt;
1449 </screen>
1450
1451 <para>and then run a small <literal>qsort</literal> with
1452 tracing:</para>
1453
1454 <screen>
1455 *Main> :trace qsort [3,2,1]
1456 Stopped at qsort.hs:1:11-12
1457 _result :: [a]
1458 [qsort.hs:1:11-12] *Main>
1459 </screen>
1460
1461 <para>We can now inspect the history of evaluation steps:</para>
1462
1463 <screen>
1464 [qsort.hs:1:11-12] *Main> :hist
1465 -1 : qsort.hs:3:24-38
1466 -2 : qsort.hs:3:23-55
1467 -3 : qsort.hs:(1,0)-(3,55)
1468 -4 : qsort.hs:2:15-24
1469 -5 : qsort.hs:2:15-46
1470 -6 : qsort.hs:3:24-38
1471 -7 : qsort.hs:3:23-55
1472 -8 : qsort.hs:(1,0)-(3,55)
1473 -9 : qsort.hs:2:15-24
1474 -10 : qsort.hs:2:15-46
1475 -11 : qsort.hs:3:24-38
1476 -12 : qsort.hs:3:23-55
1477 -13 : qsort.hs:(1,0)-(3,55)
1478 -14 : qsort.hs:2:15-24
1479 -15 : qsort.hs:2:15-46
1480 -16 : qsort.hs:(1,0)-(3,55)
1481 &lt;end of history&gt;
1482 </screen>
1483
1484 <para>To examine one of the steps in the history, use
1485 <literal>:back</literal>:</para>
1486
1487 <screen>
1488 [qsort.hs:1:11-12] *Main> :back
1489 Logged breakpoint at qsort.hs:3:24-38
1490 _result :: [a]
1491 as :: [a]
1492 a :: a
1493 [-1: qsort.hs:3:24-38] *Main>
1494 </screen>
1495
1496 <para>Note that the local variables at each step in the history have been
1497 preserved, and can be examined as usual. Also note that the prompt has
1498 changed to indicate that we're currently examining the first step in
1499 the history: <literal>-1</literal>. The command
1500 <literal>:forward</literal> can be used to traverse forward in the
1501 history.</para>
1502
1503 <para>The <literal>:trace</literal> command can be used with or without
1504 an expression. When used without an expression, tracing begins from
1505 the current breakpoint, just like <literal>:step</literal>.</para>
1506
1507 <para>The history is only available when
1508 using <literal>:trace</literal>; the reason for this is we found that
1509 logging each breakpoint in the history cuts performance by a factor of
1510 2 or more. GHCi remembers the last 50 steps in the history (perhaps in
1511 the future we'll make this configurable).</para>
1512 </sect2>
1513
1514 <sect2 id="ghci-debugger-exceptions">
1515 <title>Debugging exceptions</title>
1516 <para>Another common question that comes up when debugging is
1517 &ldquo;where did this exception come from?&rdquo;. Exceptions such as
1518 those raised by <literal>error</literal> or <literal>head []</literal>
1519 have no context information attached to them. Finding which
1520 particular call to <literal>head</literal> in your program resulted in
1521 the error can be a painstaking process, usually involving
1522 <literal>Debug.Trace.trace</literal>, or compiling with
1523 profiling and using <literal>+RTS -xc</literal> (see <xref
1524 linkend="prof-time-options" />).</para>
1525
1526 <para>The GHCi debugger offers a way to hopefully shed some light on
1527 these errors quickly and without modifying or recompiling the source
1528 code. One way would be to set a breakpoint on the location in the
1529 source code that throws the exception, and then use
1530 <literal>:trace</literal> and <literal>:history</literal> to establish
1531 the context. However, <literal>head</literal> is in a library and
1532 we can't set a breakpoint on it directly. For this reason, GHCi
1533 provides the flags <literal>-fbreak-on-exception</literal> which causes
1534 the evaluator to stop when an exception is thrown, and <literal>
1535 -fbreak-on-error</literal>, which works similarly but stops only on
1536 uncaught exceptions. When stopping at an exception, GHCi will act
1537 just as it does when a breakpoint is hit, with the deviation that it
1538 will not show you any source code location. Due to this, these
1539 commands are only really useful in conjunction with
1540 <literal>:trace</literal>, in order to log the steps leading up to the
1541 exception. For example:</para>
1542
1543 <screen>
1544 *Main> :set -fbreak-on-exception
1545 *Main> :trace qsort ("abc" ++ undefined)
1546 &ldquo;Stopped at &lt;exception thrown&gt;
1547 _exception :: e
1548 [&lt;exception thrown&gt;] *Main&gt; :hist
1549 -1 : qsort.hs:3:24-38
1550 -2 : qsort.hs:3:23-55
1551 -3 : qsort.hs:(1,0)-(3,55)
1552 -4 : qsort.hs:2:15-24
1553 -5 : qsort.hs:2:15-46
1554 -6 : qsort.hs:(1,0)-(3,55)
1555 &lt;end of history&gt;
1556 [&lt;exception thrown&gt;] *Main&gt; :back
1557 Logged breakpoint at qsort.hs:3:24-38
1558 _result :: [a]
1559 as :: [a]
1560 a :: a
1561 [-1: qsort.hs:3:24-38] *Main&gt; :force as
1562 *** Exception: Prelude.undefined
1563 [-1: qsort.hs:3:24-38] *Main&gt; :print as
1564 as = 'b' : 'c' : (_t1::[Char])
1565 </screen>
1566
1567 <para>The exception itself is bound to a new variable,
1568 <literal>_exception</literal>.</para>
1569
1570 <para>Breaking on exceptions is particularly useful for finding out what
1571 your program was doing when it was in an infinite loop. Just hit
1572 Control-C, and examine the history to find out what was going
1573 on.</para>
1574 </sect2>
1575
1576 <sect2><title>Example: inspecting functions</title>
1577 <para>
1578 It is possible to use the debugger to examine function values.
1579 When we are at a breakpoint and a function is in scope, the debugger
1580 cannot show
1581 you the source code for it; however, it is possible to get some
1582 information by applying it to some arguments and observing the result.
1583 </para>
1584
1585 <para>
1586 The process is slightly complicated when the binding is polymorphic.
1587 We show the process by means of an example.
1588 To keep things simple, we will use the well known <literal>map</literal> function:
1589 <programlisting>
1590 import Prelude hiding (map)
1591
1592 map :: (a->b) -> [a] -> [b]
1593 map f [] = []
1594 map f (x:xs) = f x : map f xs
1595 </programlisting>
1596 </para>
1597
1598 <para>
1599 We set a breakpoint on <literal>map</literal>, and call it.
1600 <screen>
1601 *Main> :break 5
1602 Breakpoint 0 activated at map.hs:5:15-28
1603 *Main> map Just [1..5]
1604 Stopped at map.hs:(4,0)-(5,12)
1605 _result :: [b]
1606 x :: a
1607 f :: a -> b
1608 xs :: [a]
1609 </screen>
1610 GHCi tells us that, among other bindings, <literal>f</literal> is in scope.
1611 However, its type is not fully known yet,
1612 and thus it is not possible to apply it to any
1613 arguments. Nevertheless, observe that the type of its first argument is the
1614 same as the type of <literal>x</literal>, and its result type is shared
1615 with <literal>_result</literal>.
1616 </para>
1617
1618 <para>
1619 As we demonstrated earlier (<xref linkend="breakpoints" />), the
1620 debugger has some intelligence built-in to update the type of
1621 <literal>f</literal> whenever the types of <literal>x</literal> or
1622 <literal>_result</literal> are discovered. So what we do in this
1623 scenario is
1624 force <literal>x</literal> a bit, in order to recover both its type
1625 and the argument part of <literal>f</literal>.
1626 <screen>
1627 *Main> seq x ()
1628 *Main> :print x
1629 x = 1
1630 </screen>
1631 </para>
1632 <para>
1633 We can check now that as expected, the type of <literal>x</literal>
1634 has been reconstructed, and with it the
1635 type of <literal>f</literal> has been too:</para>
1636 <screen>
1637 *Main> :t x
1638 x :: Integer
1639 *Main> :t f
1640 f :: Integer -> b
1641 </screen>
1642 <para>
1643 From here, we can apply f to any argument of type Integer and observe
1644 the results.
1645 <screen><![CDATA[
1646 *Main> let b = f 10
1647 *Main> :t b
1648 b :: b
1649 *Main> b
1650 <interactive>:1:0:
1651 Ambiguous type variable `b' in the constraint:
1652 `Show b' arising from a use of `print' at <interactive>:1:0
1653 *Main> :p b
1654 b = (_t2::a)
1655 *Main> seq b ()
1656 ()
1657 *Main> :t b
1658 b :: a
1659 *Main> :p b
1660 b = Just 10
1661 *Main> :t b
1662 b :: Maybe Integer
1663 *Main> :t f
1664 f :: Integer -> Maybe Integer
1665 *Main> f 20
1666 Just 20
1667 *Main> map f [1..5]
1668 [Just 1, Just 2, Just 3, Just 4, Just 5]
1669 ]]></screen>
1670 In the first application of <literal>f</literal>, we had to do
1671 some more type reconstruction
1672 in order to recover the result type of <literal>f</literal>.
1673 But after that, we are free to use
1674 <literal>f</literal> normally.
1675 </para>
1676 </sect2>
1677
1678 <sect2><title>Limitations</title>
1679 <itemizedlist>
1680 <listitem>
1681 <para>When stopped at a breakpoint, if you try to evaluate a variable
1682 that is already under evaluation, the second evaluation will hang.
1683 The reason is
1684 that GHC knows the variable is under evaluation, so the new
1685 evaluation just waits for the result before continuing, but of
1686 course this isn't going to happen because the first evaluation is
1687 stopped at a breakpoint. Control-C can interrupt the hung
1688 evaluation and return to the prompt.</para>
1689 <para>The most common way this can happen is when you're evaluating a
1690 CAF (e.g. main), stop at a breakpoint, and ask for the value of the
1691 CAF at the prompt again.</para>
1692 </listitem>
1693 <listitem><para>
1694 Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
1695 at the scope of a breakpoint if there is an explicit type signature.
1696 </para>
1697 </listitem>
1698 </itemizedlist>
1699 </sect2>
1700 </sect1>
1701
1702 <sect1 id="ghci-invocation">
1703 <title>Invoking GHCi</title>
1704 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
1705 <indexterm><primary><option>&ndash;&ndash;interactive</option></primary></indexterm>
1706
1707 <para>GHCi is invoked with the command <literal>ghci</literal> or
1708 <literal>ghc &ndash;&ndash;interactive</literal>. One or more modules or
1709 filenames can also be specified on the command line; this
1710 instructs GHCi to load the specified modules or filenames (and all
1711 the modules they depend on), just as if you had said
1712 <literal>:load <replaceable>modules</replaceable></literal> at the
1713 GHCi prompt (see <xref linkend="ghci-commands" />). For example, to
1714 start GHCi and load the program whose topmost module is in the
1715 file <literal>Main.hs</literal>, we could say:</para>
1716
1717 <screen>
1718 $ ghci Main.hs
1719 </screen>
1720
1721 <para>Most of the command-line options accepted by GHC (see <xref
1722 linkend="using-ghc"/>) also make sense in interactive mode. The ones
1723 that don't make sense are mostly obvious.</para>
1724
1725 <sect2>
1726 <title>Packages</title>
1727 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
1728
1729 <para>Most packages (see <xref linkend="using-packages"/>) are
1730 available without needing to specify any extra flags at all:
1731 they will be automatically loaded the first time they are
1732 needed.</para>
1733
1734 <para>For hidden packages, however, you need to request the
1735 package be loaded by using the <literal>-package</literal> flag:</para>
1736
1737 <screen>
1738 $ ghci -package readline
1739 GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
1740 Loading package base ... linking ... done.
1741 Loading package readline-1.0 ... linking ... done.
1742 Prelude>
1743 </screen>
1744
1745 <para>The following command works to load new packages into a
1746 running GHCi:</para>
1747
1748 <screen>
1749 Prelude> :set -package <replaceable>name</replaceable>
1750 </screen>
1751
1752 <para>But note that doing this will cause all currently loaded
1753 modules to be unloaded, and you'll be dumped back into the
1754 <literal>Prelude</literal>.</para>
1755 </sect2>
1756
1757 <sect2>
1758 <title>Extra libraries</title>
1759 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
1760
1761 <para>Extra libraries may be specified on the command line using
1762 the normal <literal>-l<replaceable>lib</replaceable></literal>
1763 option. (The term <emphasis>library</emphasis> here refers to
1764 libraries of foreign object code; for using libraries of Haskell
1765 source code, see <xref linkend="ghci-modules-filenames"/>.) For
1766 example, to load the &ldquo;m&rdquo; library:</para>
1767
1768 <screen>
1769 $ ghci -lm
1770 </screen>
1771
1772 <para>On systems with <literal>.so</literal>-style shared
1773 libraries, the actual library loaded will the
1774 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
1775 searches the following places for libraries, in this order:</para>
1776
1777 <itemizedlist>
1778 <listitem>
1779 <para>Paths specified using the
1780 <literal>-L<replaceable>path</replaceable></literal>
1781 command-line option,</para>
1782 </listitem>
1783 <listitem>
1784 <para>the standard library search path for your system,
1785 which on some systems may be overridden by setting the
1786 <literal>LD_LIBRARY_PATH</literal> environment
1787 variable.</para>
1788 </listitem>
1789 </itemizedlist>
1790
1791 <para>On systems with <literal>.dll</literal>-style shared
1792 libraries, the actual library loaded will be
1793 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
1794 GHCi will signal an error if it can't find the library.</para>
1795
1796 <para>GHCi can also load plain object files
1797 (<literal>.o</literal> or <literal>.obj</literal> depending on
1798 your platform) from the command-line. Just add the name the
1799 object file to the command line.</para>
1800
1801 <para>Ordering of <option>-l</option> options matters: a library
1802 should be mentioned <emphasis>before</emphasis> the libraries it
1803 depends on (see <xref linkend="options-linker"/>).</para>
1804 </sect2>
1805
1806 </sect1>
1807
1808 <sect1 id="ghci-commands">
1809 <title>GHCi commands</title>
1810
1811 <para>GHCi commands all begin with
1812 &lsquo;<literal>:</literal>&rsquo; and consist of a single command
1813 name followed by zero or more parameters. The command name may be
1814 abbreviated, with ambiguities being resolved in favour of the more
1815 commonly used commands.</para>
1816
1817 <variablelist>
1818 <varlistentry>
1819 <term>
1820 <literal>:abandon</literal>
1821 <indexterm><primary><literal>:abandon</literal></primary></indexterm>
1822 </term>
1823 <listitem>
1824 <para>Abandons the current evaluation (only available when stopped at
1825 a breakpoint).</para>
1826 </listitem>
1827 </varlistentry>
1828
1829 <varlistentry>
1830 <term>
1831 <literal>:add</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
1832 <indexterm><primary><literal>:add</literal></primary></indexterm>
1833 </term>
1834 <listitem>
1835 <para>Add <replaceable>module</replaceable>(s) to the
1836 current <firstterm>target set</firstterm>, and perform a
1837 reload. Normally pre-compiled code for the module will be
1838 loaded if available, or otherwise the module will be
1839 compiled to byte-code. Using the <literal>*</literal>
1840 prefix forces the module to be loaded as byte-code.</para>
1841 </listitem>
1842 </varlistentry>
1843
1844 <varlistentry>
1845 <term>
1846 <literal>:back</literal>
1847 <indexterm><primary><literal>:back</literal></primary></indexterm>
1848 </term>
1849 <listitem>
1850 <para>Travel back one step in the history. See <xref
1851 linkend="tracing" />. See also:
1852 <literal>:trace</literal>, <literal>:history</literal>,
1853 <literal>:forward</literal>.</para>
1854 </listitem>
1855 </varlistentry>
1856
1857 <varlistentry>
1858 <term>
1859 <literal>:break [<replaceable>identifier</replaceable> |
1860 [<replaceable>module</replaceable>] <replaceable>line</replaceable>
1861 [<replaceable>column</replaceable>]]</literal>
1862 </term>
1863 <indexterm><primary><literal>:break</literal></primary></indexterm>
1864 <listitem>
1865 <para>Set a breakpoint on the specified function or line and
1866 column. See <xref linkend="setting-breakpoints" />.</para>
1867 </listitem>
1868 </varlistentry>
1869
1870 <varlistentry>
1871 <term>
1872 <literal>:browse</literal><optional><literal>!</literal></optional> <optional><optional><literal>*</literal></optional><replaceable>module</replaceable></optional> ...
1873 <indexterm><primary><literal>:browse</literal></primary></indexterm>
1874 </term>
1875 <listitem>
1876 <para>Displays the identifiers exported by the module
1877 <replaceable>module</replaceable>, which must be either
1878 loaded into GHCi or be a member of a package. If
1879 <replaceable>module</replaceable> is omitted, the most
1880 recently-loaded module is used.</para>
1881
1882 <para>Like all other GHCi commands, the output is always
1883 displayed in the current GHCi scope (<xref linkend="ghci-scope"/>).</para>
1884
1885 <para>There are two variants of the browse command:
1886 <itemizedlist>
1887 <listitem>
1888 <para>If the <literal>*</literal> symbol is placed before
1889 the module name, then <emphasis>all</emphasis> the
1890 identifiers in scope in <replaceable>module</replaceable>
1891 (rather that just its exports) are shown. </para>
1892
1893 <para>The <literal>*</literal>-form is only available for modules
1894 which are interpreted; for compiled modules (including
1895 modules from packages) only the non-<literal>*</literal>
1896 form of <literal>:browse</literal> is available.</para>
1897 </listitem>
1898 <listitem>
1899 <para>Data constructors and class methods are usually
1900 displayed in the context of their data type or class declaration.
1901 However, if the <literal>!</literal> symbol is appended to the
1902 command, thus <literal>:browse!</literal>,
1903 they are listed individually.
1904 The <literal>!</literal>-form also annotates the listing
1905 with comments giving possible imports for each group of
1906 entries. Here is an example:
1907 <screen>
1908 Prelude> :browse! Data.Maybe
1909 -- not currently imported
1910 Data.Maybe.catMaybes :: [Maybe a] -> [a]
1911 Data.Maybe.fromJust :: Maybe a -> a
1912 Data.Maybe.fromMaybe :: a -> Maybe a -> a
1913 Data.Maybe.isJust :: Maybe a -> Bool
1914 Data.Maybe.isNothing :: Maybe a -> Bool
1915 Data.Maybe.listToMaybe :: [a] -> Maybe a
1916 Data.Maybe.mapMaybe :: (a -> Maybe b) -> [a] -> [b]
1917 Data.Maybe.maybeToList :: Maybe a -> [a]
1918 -- imported via Prelude
1919 Just :: a -> Maybe a
1920 data Maybe a = Nothing | Just a
1921 Nothing :: Maybe a
1922 maybe :: b -> (a -> b) -> Maybe a -> b
1923 </screen>
1924 This output shows that, in the context of the current session (ie in the scope
1925 of <literal>Prelude</literal>), the first group of items from
1926 <literal>Data.Maybe</literal> are not in scope (althought they are available in
1927 fully qualified form in the GHCi session - see <xref
1928 linkend="ghci-scope"/>), whereas the second group of items are in scope
1929 (via <literal>Prelude</literal>) and are therefore available either
1930 unqualified, or with a <literal>Prelude.</literal> qualifier.
1931 </para>
1932 </listitem>
1933 </itemizedlist>
1934 </para>
1935 </listitem>
1936 </varlistentry>
1937
1938 <varlistentry>
1939 <term>
1940 <literal>:cd</literal> <replaceable>dir</replaceable>
1941 <indexterm><primary><literal>:cd</literal></primary></indexterm>
1942 </term>
1943 <listitem>
1944 <para>Changes the current working directory to
1945 <replaceable>dir</replaceable>. A
1946 &lsquo;<literal>&tilde;</literal>&rsquo; symbol at the
1947 beginning of <replaceable>dir</replaceable> will be replaced
1948 by the contents of the environment variable
1949 <literal>HOME</literal>.</para>
1950
1951 <para>NOTE: changing directories causes all currently loaded
1952 modules to be unloaded. This is because the search path is
1953 usually expressed using relative directories, and changing
1954 the search path in the middle of a session is not
1955 supported.</para>
1956 </listitem>
1957 </varlistentry>
1958
1959 <varlistentry>
1960 <term>
1961 <literal>:cmd</literal> <replaceable>expr</replaceable>
1962 <indexterm><primary><literal>:cmd</literal></primary></indexterm>
1963 </term>
1964 <listitem>
1965 <para>Executes <replaceable>expr</replaceable> as a computation of
1966 type <literal>IO String</literal>, and then executes the resulting
1967 string as a list of GHCi commands. Multiple commands are separated
1968 by newlines. The <literal>:cmd</literal> command is useful with
1969 <literal>:def</literal> and <literal>:set stop</literal>.</para>
1970 </listitem>
1971 </varlistentry>
1972
1973 <varlistentry>
1974 <term>
1975 <literal>:continue</literal>
1976 <indexterm><primary><literal>:continue</literal></primary></indexterm>
1977 </term>
1978 <listitem><para>Continue the current evaluation, when stopped at a
1979 breakpoint.</para>
1980 </listitem>
1981 </varlistentry>
1982
1983 <varlistentry>
1984 <term>
1985 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1986 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1987 <indexterm><primary><literal>:etags</literal></primary>
1988 </indexterm>
1989 <indexterm><primary><literal>:etags</literal></primary>
1990 </indexterm>
1991 </term>
1992 <listitem>
1993 <para>Generates a &ldquo;tags&rdquo; file for Vi-style editors
1994 (<literal>:ctags</literal>) or
1995 Emacs-style editors (<literal>:etags</literal>). If
1996 no filename is specified, the default <filename>tags</filename> or
1997 <filename>TAGS</filename> is
1998 used, respectively. Tags for all the functions, constructors and
1999 types in the currently loaded modules are created. All modules must
2000 be interpreted for these commands to work.</para>
2001 </listitem>
2002 </varlistentry>
2003
2004 <varlistentry>
2005 <term>
2006 <literal>:def<optional>!</optional> <optional><replaceable>name</replaceable> <replaceable>expr</replaceable></optional></literal>
2007 <indexterm><primary><literal>:def</literal></primary></indexterm>
2008 </term>
2009 <listitem>
2010 <para><literal>:def</literal> is used to define new
2011 commands, or macros, in GHCi. The command
2012 <literal>:def</literal> <replaceable>name</replaceable>
2013 <replaceable>expr</replaceable> defines a new GHCi command
2014 <literal>:<replaceable>name</replaceable></literal>,
2015 implemented by the Haskell expression
2016 <replaceable>expr</replaceable>, which must have type
2017 <literal>String -> IO String</literal>. When
2018 <literal>:<replaceable>name</replaceable>
2019 <replaceable>args</replaceable></literal> is typed at the
2020 prompt, GHCi will run the expression
2021 <literal>(<replaceable>name</replaceable>
2022 <replaceable>args</replaceable>)</literal>, take the
2023 resulting <literal>String</literal>, and feed it back into
2024 GHCi as a new sequence of commands. Separate commands in
2025 the result must be separated by
2026 &lsquo;<literal>\n</literal>&rsquo;.</para>
2027
2028 <para>That's all a little confusing, so here's a few
2029 examples. To start with, here's a new GHCi command which
2030 doesn't take any arguments or produce any results, it just
2031 outputs the current date &amp; time:</para>
2032
2033 <screen>
2034 Prelude> let date _ = Time.getClockTime >>= print >> return ""
2035 Prelude> :def date date
2036 Prelude> :date
2037 Fri Mar 23 15:16:40 GMT 2001
2038 </screen>
2039
2040 <para>Here's an example of a command that takes an argument.
2041 It's a re-implementation of <literal>:cd</literal>:</para>
2042
2043 <screen>
2044 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
2045 Prelude> :def mycd mycd
2046 Prelude> :mycd ..
2047 </screen>
2048
2049 <para>Or I could define a simple way to invoke
2050 &ldquo;<literal>ghc &ndash;&ndash;make Main</literal>&rdquo; in the
2051 current directory:</para>
2052
2053 <screen>
2054 Prelude> :def make (\_ -> return ":! ghc &ndash;&ndash;make Main")
2055 </screen>
2056
2057 <para>We can define a command that reads GHCi input from a
2058 file. This might be useful for creating a set of bindings
2059 that we want to repeatedly load into the GHCi session:</para>
2060
2061 <screen>
2062 Prelude> :def . readFile
2063 Prelude> :. cmds.ghci
2064 </screen>
2065
2066 <para>Notice that we named the command
2067 <literal>:.</literal>, by analogy with the
2068 &lsquo;<literal>.</literal>&rsquo; Unix shell command that
2069 does the same thing.</para>
2070
2071 <para>Typing <literal>:def</literal> on its own lists the
2072 currently-defined macros. Attempting to redefine an
2073 existing command name results in an error unless the
2074 <literal>:def!</literal> form is used, in which case the old
2075 command with that name is silently overwritten.</para>
2076 </listitem>
2077 </varlistentry>
2078
2079 <varlistentry>
2080 <term>
2081 <literal>:delete * | <replaceable>num</replaceable> ...</literal>
2082 <indexterm><primary><literal>:delete</literal></primary></indexterm>
2083 </term>
2084 <listitem>
2085 <para>Delete one or more breakpoints by number (use <literal>:show
2086 breaks</literal> to see the number of each breakpoint). The
2087 <literal>*</literal> form deletes all the breakpoints.</para>
2088 </listitem>
2089 </varlistentry>
2090
2091 <varlistentry>
2092 <term>
2093 <literal>:edit <optional><replaceable>file</replaceable></optional></literal>
2094 <indexterm><primary><literal>:edit</literal></primary></indexterm>
2095 </term>
2096 <listitem>
2097 <para>Opens an editor to edit the file
2098 <replaceable>file</replaceable>, or the most recently loaded
2099 module if <replaceable>file</replaceable> is omitted. The
2100 editor to invoke is taken from the <literal>EDITOR</literal>
2101 environment variable, or a default editor on your system if
2102 <literal>EDITOR</literal> is not set. You can change the
2103 editor using <literal>:set editor</literal>.</para>
2104 </listitem>
2105 </varlistentry>
2106
2107 <varlistentry>
2108 <term>
2109 <literal>:etags</literal>
2110 </term>
2111 <listitem>
2112 <para>See <literal>:ctags</literal>.</para>
2113 </listitem>
2114 </varlistentry>
2115
2116 <varlistentry>
2117 <term>
2118 <literal>:force <replaceable>identifier</replaceable> ...</literal>
2119 <indexterm><primary><literal>:force</literal></primary></indexterm>
2120 </term>
2121 <listitem>
2122 <para>Prints the value of <replaceable>identifier</replaceable> in
2123 the same way as <literal>:print</literal>. Unlike
2124 <literal>:print</literal>, <literal>:force</literal> evaluates each
2125 thunk that it encounters while traversing the value. This may
2126 cause exceptions or infinite loops, or further breakpoints (which
2127 are ignored, but displayed).</para>
2128 </listitem>
2129 </varlistentry>
2130
2131 <varlistentry>
2132 <term>
2133 <literal>:forward</literal>
2134 <indexterm><primary><literal>:forward</literal></primary></indexterm>
2135 </term>
2136 <listitem>
2137 <para>Move forward in the history. See <xref
2138 linkend="tracing" />. See also:
2139 <literal>:trace</literal>, <literal>:history</literal>,
2140 <literal>:back</literal>.</para>
2141 </listitem>
2142 </varlistentry>
2143
2144 <varlistentry>
2145 <term>
2146 <literal>:help</literal>
2147 <indexterm><primary><literal>:help</literal></primary></indexterm>
2148 </term>
2149 <term>
2150 <literal>:?</literal>
2151 <indexterm><primary><literal>:?</literal></primary></indexterm>
2152 </term>
2153 <listitem>
2154 <para>Displays a list of the available commands.</para>
2155 </listitem>
2156 </varlistentry>
2157
2158 <varlistentry>
2159 <term>
2160 <literal>:</literal>
2161 <indexterm><primary><literal>:</literal></primary></indexterm>
2162 </term>
2163 <listitem>
2164 <para>Repeat the previous command.</para>
2165 </listitem>
2166 </varlistentry>
2167
2168 <varlistentry>
2169
2170 <term>
2171 <literal>:history [<replaceable>num</replaceable>]</literal>
2172 <indexterm><primary><literal>:history</literal></primary></indexterm>
2173 </term>
2174 <listitem>
2175 <para>Display the history of evaluation steps. With a number,
2176 displays that many steps (default: 20). For use with
2177 <literal>:trace</literal>; see <xref
2178 linkend="tracing" />.</para>
2179 </listitem>
2180 </varlistentry>
2181
2182 <varlistentry>
2183 <term>
2184 <literal>:info</literal> <replaceable>name</replaceable> ...
2185 <indexterm><primary><literal>:info</literal></primary></indexterm>
2186 </term>
2187 <listitem>
2188 <para>Displays information about the given name(s). For
2189 example, if <replaceable>name</replaceable> is a class, then
2190 the class methods and their types will be printed; if
2191 <replaceable>name</replaceable> is a type constructor, then
2192 its definition will be printed; if
2193 <replaceable>name</replaceable> is a function, then its type
2194 will be printed. If <replaceable>name</replaceable> has
2195 been loaded from a source file, then GHCi will also display
2196 the location of its definition in the source.</para>
2197 <para>For types and classes, GHCi also summarises instances that
2198 mention them. To avoid showing irrelevant information, an instance
2199 is shown only if (a) its head mentions <replaceable>name</replaceable>,
2200 and (b) all the other things mentioned in the instance
2201 are in scope (either qualified or otherwise) as a result of
2202 a <literal>:load</literal> or <literal>:module</literal> commands. </para>
2203 </listitem>
2204 </varlistentry>
2205
2206 <varlistentry>
2207 <term>
2208 <literal>:kind</literal> <replaceable>type</replaceable>
2209 <indexterm><primary><literal>:kind</literal></primary></indexterm>
2210 </term>
2211 <listitem>
2212 <para>Infers and prints the kind of
2213 <replaceable>type</replaceable>. The latter can be an arbitrary
2214 type expression, including a partial application of a type constructor,
2215 such as <literal>Either Int</literal>.</para>
2216 </listitem>
2217 </varlistentry>
2218
2219 <varlistentry>
2220 <term>
2221 <literal>:load</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
2222 <indexterm><primary><literal>:load</literal></primary></indexterm>
2223 </term>
2224 <listitem>
2225 <para>Recursively loads the specified
2226 <replaceable>module</replaceable>s, and all the modules they
2227 depend on. Here, each <replaceable>module</replaceable>
2228 must be a module name or filename, but may not be the name
2229 of a module in a package.</para>
2230
2231 <para>All previously loaded modules, except package modules,
2232 are forgotten. The new set of modules is known as the
2233 <firstterm>target set</firstterm>. Note that
2234 <literal>:load</literal> can be used without any arguments
2235 to unload all the currently loaded modules and
2236 bindings.</para>
2237
2238 <para>Normally pre-compiled code for a module will be loaded
2239 if available, or otherwise the module will be compiled to
2240 byte-code. Using the <literal>*</literal> prefix forces a
2241 module to be loaded as byte-code.</para>
2242
2243 <para>After a <literal>:load</literal> command, the current
2244 context is set to:</para>
2245
2246 <itemizedlist>
2247 <listitem>
2248 <para><replaceable>module</replaceable>, if it was loaded
2249 successfully, or</para>
2250 </listitem>
2251 <listitem>
2252 <para>the most recently successfully loaded module, if
2253 any other modules were loaded as a result of the current
2254 <literal>:load</literal>, or</para>
2255 </listitem>
2256 <listitem>
2257 <para><literal>Prelude</literal> otherwise.</para>
2258 </listitem>
2259 </itemizedlist>
2260 </listitem>
2261 </varlistentry>
2262
2263 <varlistentry>
2264 <term>
2265 <literal>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
2266 <indexterm><primary><literal>:main</literal></primary></indexterm>
2267 </term>
2268 <listitem>
2269 <para>
2270 When a program is compiled and executed, it can use the
2271 <literal>getArgs</literal> function to access the
2272 command-line arguments.
2273 However, we cannot simply pass the arguments to the
2274 <literal>main</literal> function while we are testing in ghci,
2275 as the <literal>main</literal> function doesn't take its
2276 arguments directly.
2277 </para>
2278
2279 <para>
2280 Instead, we can use the <literal>:main</literal> command.
2281 This runs whatever <literal>main</literal> is in scope, with
2282 any arguments being treated the same as command-line arguments,
2283 e.g.:
2284 </para>
2285
2286 <screen>
2287 Prelude> let main = System.Environment.getArgs >>= print
2288 Prelude> :main foo bar
2289 ["foo","bar"]
2290 </screen>
2291
2292 <para>
2293 We can also quote arguments which contains characters like
2294 spaces, and they are treated like Haskell strings, or we can
2295 just use Haskell list syntax:
2296 </para>
2297
2298 <screen>
2299 Prelude> :main foo "bar baz"
2300 ["foo","bar baz"]
2301 Prelude> :main ["foo", "bar baz"]
2302 ["foo","bar baz"]
2303 </screen>
2304
2305 <para>
2306 Finally, other functions can be called, either with the
2307 <literal>-main-is</literal> flag or the <literal>:run</literal>
2308 command:
2309 </para>
2310
2311 <screen>
2312 Prelude> let foo = putStrLn "foo" >> System.Environment.getArgs >>= print
2313 Prelude> let bar = putStrLn "bar" >> System.Environment.getArgs >>= print
2314 Prelude> :set -main-is foo
2315 Prelude> :main foo "bar baz"
2316 foo
2317 ["foo","bar baz"]
2318 Prelude> :run bar ["foo", "bar baz"]
2319 bar
2320 ["foo","bar baz"]
2321 </screen>
2322
2323 </listitem>
2324 </varlistentry>
2325
2326 <varlistentry>
2327 <term>
2328 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
2329 <indexterm><primary><literal>:module</literal></primary></indexterm>
2330 </term>
2331 <term>
2332 <literal>import <replaceable>mod</replaceable></literal>
2333 </term>
2334 <listitem>
2335 <para>Sets or modifies the current context for statements
2336 typed at the prompt. The form <literal>import
2337 <replaceable>mod</replaceable></literal> is equivalent to
2338 <literal>:module +<replaceable>mod</replaceable></literal>.
2339 See <xref linkend="ghci-scope"/> for
2340 more details.</para>
2341 </listitem>
2342 </varlistentry>
2343
2344 <varlistentry>
2345 <term>
2346 <literal>:print </literal> <replaceable>names</replaceable> ...
2347 <indexterm><primary><literal>:print</literal></primary></indexterm>
2348 </term>
2349 <listitem>
2350 <para>Prints a value without forcing its evaluation.
2351 <literal>:print</literal> may be used on values whose types are
2352 unknown or partially known, which might be the case for local
2353 variables with polymorphic types at a breakpoint. While inspecting
2354 the runtime value, <literal>:print</literal> attempts to
2355 reconstruct the type of the value, and will elaborate the type in
2356 GHCi's environment if possible. If any unevaluated components
2357 (thunks) are encountered, then <literal>:print</literal> binds
2358 a fresh variable with a name beginning with <literal>_t</literal>
2359 to each thunk. See <xref linkend="breakpoints" /> for more
2360 information. See also the <literal>:sprint</literal> command,
2361 which works like <literal>:print</literal> but does not bind new
2362 variables.</para>
2363 </listitem>
2364 </varlistentry>
2365
2366 <varlistentry>
2367 <term>
2368 <literal>:quit</literal>
2369 <indexterm><primary><literal>:quit</literal></primary></indexterm>
2370 </term>
2371 <listitem>
2372 <para>Quits GHCi. You can also quit by typing control-D
2373 at the prompt.</para>
2374 </listitem>
2375 </varlistentry>
2376
2377 <varlistentry>
2378 <term>
2379 <literal>:reload</literal>
2380 <indexterm><primary><literal>:reload</literal></primary></indexterm>
2381 </term>
2382 <listitem>
2383 <para>Attempts to reload the current target set (see
2384 <literal>:load</literal>) if any of the modules in the set,
2385 or any dependent module, has changed. Note that this may
2386 entail loading new modules, or dropping modules which are no
2387 longer indirectly required by the target.</para>
2388 </listitem>
2389 </varlistentry>
2390
2391 <varlistentry>
2392 <term>
2393 <literal>:run</literal>
2394 <indexterm><primary><literal>:run</literal></primary></indexterm>
2395 </term>
2396 <listitem>
2397 <para>See <literal>:main</literal>.</para>
2398 </listitem>
2399 </varlistentry>
2400
2401 <varlistentry>
2402 <term>
2403 <literal>:script</literal> <optional><replaceable>n</replaceable></optional>
2404 <literal>filename</literal>
2405 <indexterm><primary><literal>:script</literal></primary></indexterm>
2406 </term>
2407 <listitem>
2408 <para>Executes the lines of a file as a series of GHCi commands. This command
2409 is compatible with multiline statements as set by <literal>:set +m</literal>
2410 </para>
2411 </listitem>
2412 </varlistentry>
2413
2414 <varlistentry>
2415 <term>
2416 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
2417 <indexterm><primary><literal>:set</literal></primary></indexterm>
2418 </term>
2419 <listitem>
2420 <para>Sets various options. See <xref linkend="ghci-set"/> for a list of
2421 available options and <xref linkend="interactive-mode-options"/> for a
2422 list of GHCi-specific flags. The <literal>:set</literal> command by
2423 itself shows which options are currently set. It also lists the current
2424 dynamic flag settings, with GHCi-specific flags listed separately.</para>
2425 </listitem>
2426 </varlistentry>
2427
2428 <varlistentry>
2429 <term>
2430 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
2431 <indexterm><primary><literal>:set args</literal></primary></indexterm>
2432 </term>
2433 <listitem>
2434 <para>Sets the list of arguments which are returned when the
2435 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
2436 </indexterm>.</para>
2437 </listitem>
2438 </varlistentry>
2439
2440 <varlistentry>
2441 <term>
2442 <literal>:set</literal> <literal>editor</literal> <replaceable>cmd</replaceable>
2443 </term>
2444 <listitem>
2445 <para>Sets the command used by <literal>:edit</literal> to
2446 <replaceable>cmd</replaceable>.</para>
2447 </listitem>
2448 </varlistentry>
2449
2450 <varlistentry>
2451 <term>
2452 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
2453 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
2454 </term>
2455 <listitem>
2456 <para>Sets the string to be returned when the program calls
2457 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
2458 </indexterm>.</para>
2459 </listitem>
2460 </varlistentry>
2461
2462 <varlistentry>
2463 <term>
2464 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
2465 </term>
2466 <listitem>
2467 <para>Sets the string to be used as the prompt in GHCi.
2468 Inside <replaceable>prompt</replaceable>, the sequence
2469 <literal>%s</literal> is replaced by the names of the
2470 modules currently in scope, and <literal>%%</literal> is
2471 replaced by <literal>%</literal>. If <replaceable>prompt</replaceable>
2472 starts with &quot; then it is parsed as a Haskell String;
2473 otherwise it is treated as a literal string.</para>
2474 </listitem>
2475 </varlistentry>
2476
2477 <varlistentry>
2478 <term>
2479 <literal>:set</literal> <literal>stop</literal>
2480 [<replaceable>num</replaceable>] <replaceable>cmd</replaceable>
2481 </term>
2482 <listitem>
2483 <para>Set a command to be executed when a breakpoint is hit, or a new
2484 item in the history is selected. The most common use of
2485 <literal>:set stop</literal> is to display the source code at the
2486 current location, e.g. <literal>:set stop :list</literal>.</para>
2487
2488 <para>If a number is given before the command, then the commands are
2489 run when the specified breakpoint (only) is hit. This can be quite
2490 useful: for example, <literal>:set stop 1 :continue</literal>
2491 effectively disables breakpoint 1, by running
2492 <literal>:continue</literal> whenever it is hit (although GHCi will
2493 still emit a message to say the breakpoint was hit). What's more,
2494 with cunning use of <literal>:def</literal> and
2495 <literal>:cmd</literal> you can use <literal>:set stop</literal> to
2496 implement conditional breakpoints:</para>
2497 <screen>
2498 *Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
2499 *Main> :set stop 0 :cond (x &lt; 3)
2500 </screen>
2501 <para>Ignoring breakpoints for a specified number of iterations is
2502 also possible using similar techniques.</para>
2503 </listitem>
2504 </varlistentry>
2505
2506 <varlistentry>
2507 <term>
2508 <literal>:show bindings</literal>
2509 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
2510 </term>
2511 <listitem>
2512 <para>Show the bindings made at the prompt and their
2513 types.</para>
2514 </listitem>
2515 </varlistentry>
2516
2517 <varlistentry>
2518 <term>
2519 <literal>:show breaks</literal>
2520 <indexterm><primary><literal>:show breaks</literal></primary></indexterm>
2521 </term>
2522 <listitem>
2523 <para>List the active breakpoints.</para>
2524 </listitem>
2525 </varlistentry>
2526
2527 <varlistentry>
2528 <term>
2529 <literal>:show context</literal>
2530 <indexterm><primary><literal>:show context</literal></primary></indexterm>
2531 </term>
2532 <listitem>
2533 <para>List the active evaluations that are stopped at breakpoints.</para>
2534 </listitem>
2535 </varlistentry>
2536
2537 <varlistentry>
2538 <term>
2539 <literal>:show modules</literal>
2540 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
2541 </term>
2542 <listitem>
2543 <para>Show the list of modules currently loaded.</para>
2544 </listitem>
2545 </varlistentry>
2546
2547 <varlistentry>
2548 <term>
2549 <literal>:show packages</literal>
2550 <indexterm><primary><literal>:show packages</literal></primary></indexterm>
2551 </term>
2552 <listitem>
2553 <para>Show the currently active package flags, as well as the list of
2554 packages currently loaded.</para>
2555 </listitem>
2556 </varlistentry>
2557
2558 <varlistentry>
2559 <term>
2560 <literal>:show languages</literal>
2561 <indexterm><primary><literal>:show languages</literal></primary></indexterm>
2562 </term>
2563 <listitem>
2564 <para>Show the currently active language flags.</para>
2565 </listitem>
2566 </varlistentry>
2567
2568
2569 <varlistentry>
2570 <term>
2571 <literal>:show [args|prog|prompt|editor|stop]</literal>
2572 <indexterm><primary><literal>:show</literal></primary></indexterm>
2573 </term>
2574 <listitem>
2575 <para>Displays the specified setting (see
2576 <literal>:set</literal>).</para>
2577 </listitem>
2578 </varlistentry>
2579
2580 <varlistentry>
2581 <term>
2582 <literal>:sprint</literal>
2583 <indexterm><primary><literal>:sprint</literal></primary></indexterm>
2584 </term>
2585 <listitem>
2586 <para>Prints a value without forcing its evaluation.
2587 <literal>:sprint</literal> is similar to <literal>:print</literal>,
2588 with the difference that unevaluated subterms are not bound to new
2589 variables, they are simply denoted by &lsquo;_&rsquo;.</para>
2590 </listitem>
2591 </varlistentry>
2592
2593 <varlistentry>
2594 <term>
2595 <literal>:step [<replaceable>expr</replaceable>]</literal>
2596 <indexterm><primary><literal>:step</literal></primary></indexterm>
2597 </term>
2598 <listitem>
2599 <para>Single-step from the last breakpoint. With an expression
2600 argument, begins evaluation of the expression with a
2601 single-step.</para>
2602 </listitem>
2603 </varlistentry>
2604
2605 <varlistentry>
2606 <term>
2607 <literal>:trace [<replaceable>expr</replaceable>]</literal>
2608 <indexterm><primary><literal>:trace</literal></primary></indexterm>
2609 </term>
2610 <listitem>
2611 <para>Evaluates the given expression (or from the last breakpoint if
2612 no expression is given), and additionally logs the evaluation
2613 steps for later inspection using <literal>:history</literal>. See
2614 <xref linkend="tracing" />.</para>
2615 </listitem>
2616 </varlistentry>
2617
2618 <varlistentry>
2619 <term>
2620 <literal>:type</literal> <replaceable>expression</replaceable>
2621 <indexterm><primary><literal>:type</literal></primary></indexterm>
2622 </term>
2623 <listitem>
2624 <para>Infers and prints the type of
2625 <replaceable>expression</replaceable>, including explicit
2626 forall quantifiers for polymorphic types. The monomorphism
2627 restriction is <emphasis>not</emphasis> applied to the
2628 expression during type inference.</para>
2629 </listitem>
2630 </varlistentry>
2631
2632 <varlistentry>
2633 <term>
2634 <literal>:undef</literal> <replaceable>name</replaceable>
2635 <indexterm><primary><literal>:undef</literal></primary></indexterm>
2636 </term>
2637 <listitem>
2638 <para>Undefines the user-defined command
2639 <replaceable>name</replaceable> (see <literal>:def</literal>
2640 above).</para>
2641 </listitem>
2642 </varlistentry>
2643
2644 <varlistentry>
2645 <term>
2646 <literal>:unset</literal> <replaceable>option</replaceable>...
2647 <indexterm><primary><literal>:unset</literal></primary></indexterm>
2648 </term>
2649 <listitem>
2650 <para>Unsets certain options. See <xref linkend="ghci-set"/>
2651 for a list of available options.</para>
2652 </listitem>
2653 </varlistentry>
2654
2655 <varlistentry>
2656 <term>
2657 <literal>:!</literal> <replaceable>command</replaceable>...
2658 <indexterm><primary><literal>:!</literal></primary></indexterm>
2659 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
2660 </term>
2661 <listitem>
2662 <para>Executes the shell command
2663 <replaceable>command</replaceable>.</para>
2664 </listitem>
2665 </varlistentry>
2666
2667 </variablelist>
2668 </sect1>
2669
2670 <sect1 id="ghci-set">
2671 <title>The <literal>:set</literal> command</title>
2672 <indexterm><primary><literal>:set</literal></primary></indexterm>
2673
2674 <para>The <literal>:set</literal> command sets two types of
2675 options: GHCi options, which begin with
2676 &lsquo;<literal>+</literal>&rsquo;, and &ldquo;command-line&rdquo;
2677 options, which begin with &lsquo;-&rsquo;. </para>
2678
2679 <para>NOTE: at the moment, the <literal>:set</literal> command
2680 doesn't support any kind of quoting in its arguments: quotes will
2681 not be removed and cannot be used to group words together. For
2682 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
2683 you expect.</para>
2684
2685 <sect2>
2686 <title>GHCi options</title>
2687 <indexterm><primary>options</primary><secondary>GHCi</secondary>
2688 </indexterm>
2689
2690 <para>GHCi options may be set using <literal>:set</literal> and
2691 unset using <literal>:unset</literal>.</para>
2692
2693 <para>The available GHCi options are:</para>
2694
2695 <variablelist>
2696 <varlistentry>
2697 <term>
2698 <literal>+m</literal>
2699 <indexterm><primary><literal>+m</literal></primary></indexterm>
2700 </term>
2701 <listitem>
2702 <para>Enable parsing of multiline commands. A multiline command
2703 is prompted for when the current input line contains open layout
2704 contexts.</para>
2705 </listitem>
2706 </varlistentry>
2707
2708 <varlistentry>
2709 <term>
2710 <literal>+r</literal>
2711 <indexterm><primary><literal>+r</literal></primary></indexterm>
2712 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
2713 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
2714 </term>
2715 <listitem>
2716 <para>Normally, any evaluation of top-level expressions
2717 (otherwise known as CAFs or Constant Applicative Forms) in
2718 loaded modules is retained between evaluations. Turning
2719 on <literal>+r</literal> causes all evaluation of
2720 top-level expressions to be discarded after each
2721 evaluation (they are still retained
2722 <emphasis>during</emphasis> a single evaluation).</para>
2723
2724 <para>This option may help if the evaluated top-level
2725 expressions are consuming large amounts of space, or if
2726 you need repeatable performance measurements.</para>
2727 </listitem>
2728 </varlistentry>
2729
2730 <varlistentry>
2731 <term>
2732 <literal>+s</literal>
2733 <indexterm><primary><literal>+s</literal></primary></indexterm>
2734 </term>
2735 <listitem>
2736 <para>Display some stats after evaluating each expression,
2737 including the elapsed time and number of bytes allocated.
2738 NOTE: the allocation figure is only accurate to the size
2739 of the storage manager's allocation area, because it is
2740 calculated at every GC. Hence, you might see values of
2741 zero if no GC has occurred.</para>
2742 </listitem>
2743 </varlistentry>
2744
2745 <varlistentry>
2746 <term>
2747 <literal>+t</literal>
2748 <indexterm><primary><literal>+t</literal></primary></indexterm>
2749 </term>
2750 <listitem>
2751 <para>Display the type of each variable bound after a
2752 statement is entered at the prompt. If the statement is a
2753 single expression, then the only variable binding will be
2754 for the variable
2755 &lsquo;<literal>it</literal>&rsquo;.</para>
2756 </listitem>
2757 </varlistentry>
2758 </variablelist>
2759 </sect2>
2760
2761 <sect2 id="ghci-cmd-line-options">
2762 <title>Setting GHC command-line options in GHCi</title>
2763
2764 <para>Normal GHC command-line options may also be set using
2765 <literal>:set</literal>. For example, to turn on
2766 <option>-fglasgow-exts</option>, you would say:</para>
2767
2768 <screen>
2769 Prelude> :set -fglasgow-exts
2770 </screen>
2771
2772 <para>Any GHC command-line option that is designated as
2773 <firstterm>dynamic</firstterm> (see the table in <xref
2774 linkend="flag-reference"/>), may be set using
2775 <literal>:set</literal>. To unset an option, you can set the
2776 reverse option:</para>
2777 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
2778
2779 <screen>
2780 Prelude> :set -fno-glasgow-exts
2781 </screen>
2782
2783 <para><xref linkend="flag-reference"/> lists the reverse for each
2784 option where applicable.</para>
2785
2786 <para>Certain static options (<option>-package</option>,
2787 <option>-I</option>, <option>-i</option>, and
2788 <option>-l</option> in particular) will also work, but some may
2789 not take effect until the next reload.</para>
2790 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
2791 </sect2>
2792 </sect1>
2793 <sect1 id="ghci-dot-files">
2794 <title>The <filename>.ghci</filename> file</title>
2795 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
2796 </indexterm>
2797 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
2798 </indexterm>
2799
2800 <para>When it starts, unless the <literal>-ignore-dot-ghci</literal>
2801 flag is given, GHCi reads and executes commands from the following
2802 files, in this order, if they exist:</para>
2803
2804 <orderedlist>
2805 <listitem>
2806 <para><filename>./.ghci</filename></para>
2807 </listitem>
2808 <listitem>
2809 <para><literal><replaceable>appdata</replaceable>/ghc/ghci.conf</literal>,
2810 where <replaceable>appdata</replaceable> depends on your system,
2811 but is usually something like <literal>C:/Documents and Settings/<replaceable>user</replaceable>/Application Data</literal></para>
2812 </listitem>
2813 <listitem>
2814 <para>On Unix: <literal>$HOME/.ghc/ghci.conf</literal></para>
2815 </listitem>
2816 <listitem>
2817 <para><literal>$HOME/.ghci</literal></para>
2818 </listitem>
2819 </orderedlist>
2820
2821 <para>The <filename>ghci.conf</filename> file is most useful for
2822 turning on favourite options (eg. <literal>:set +s</literal>), and
2823 defining useful macros. Placing a <filename>.ghci</filename> file
2824 in a directory with a Haskell project is a useful way to set
2825 certain project-wide options so you don't have to type them
2826 every time you start GHCi: eg. if your project uses GHC extensions
2827 and CPP, and has source files in three subdirectories A, B and C,
2828 you might put the following lines in
2829 <filename>.ghci</filename>:</para>
2830
2831 <screen>
2832 :set -fglasgow-exts -cpp
2833 :set -iA:B:C
2834 </screen>
2835
2836 <para>(Note that strictly speaking the <option>-i</option> flag is
2837 a static one, but in fact it works to set it using
2838 <literal>:set</literal> like this. The changes won't take effect
2839 until the next <literal>:load</literal>, though.)</para>
2840
2841 <para>Once you have a library of GHCi macros, you may want
2842 to source them from separate files, or you may want to source
2843 your <filename>.ghci</filename> file into your running GHCi
2844 session while debugging it</para>
2845
2846 <screen>
2847 :def source readFile
2848 </screen>
2849
2850 <para>With this macro defined in your <filename>.ghci</filename>
2851 file, you can use <literal>:source file</literal> to read GHCi
2852 commands from <literal>file</literal>. You can find (and contribute!-)
2853 other suggestions for <filename>.ghci</filename> files on this Haskell
2854 wiki page: <ulink
2855 url="http://haskell.org/haskellwiki/GHC/GHCi">GHC/GHCi</ulink></para>
2856
2857 <para>Two command-line options control whether the
2858 startup files files are read:</para>
2859
2860 <variablelist>
2861 <varlistentry>
2862 <term>
2863 <option>-ignore-dot-ghci</option>
2864 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
2865 </term>
2866 <listitem>
2867 <para>Don't read either <filename>./.ghci</filename> or the
2868 other startup files when starting up.</para>
2869 </listitem>
2870 </varlistentry>
2871 <varlistentry>
2872 <term>
2873 <option>-read-dot-ghci</option>
2874 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
2875 </term>
2876 <listitem>
2877 <para>Read <filename>./.ghci</filename> and the other
2878 startup files (see above). This is normally the
2879 default, but the <option>-read-dot-ghci</option> option may
2880 be used to override a previous
2881 <option>-ignore-dot-ghci</option> option.</para>
2882 </listitem>
2883 </varlistentry>
2884 </variablelist>
2885
2886 <para>Additional <filename>.ghci</filename> files can be added
2887 through the <option>-ghci-script</option> option. These are
2888 loaded after the normal <filename>.ghci</filename> files.</para>
2889
2890 </sect1>
2891
2892 <sect1 id="ghci-obj">
2893 <title>Compiling to object code inside GHCi</title>
2894
2895 <para>By default, GHCi compiles Haskell source code into byte-code
2896 that is interpreted by the runtime system. GHCi can also compile
2897 Haskell code to object code: to turn on this feature, use the
2898 <option>-fobject-code</option> flag either on the command line or
2899 with <literal>:set</literal> (the option
2900 <option>-fbyte-code</option> restores byte-code compilation
2901 again). Compiling to object code takes longer, but typically the
2902 code will execute 10-20 times faster than byte-code.</para>
2903
2904 <para>Compiling to object code inside GHCi is particularly useful
2905 if you are developing a compiled application, because the
2906 <literal>:reload</literal> command typically runs much faster than
2907 restarting GHC with <option>--make</option> from the command-line,
2908 because all the interface files are already cached in
2909 memory.</para>
2910
2911 <para>There are disadvantages to compiling to object-code: you
2912 can't set breakpoints in object-code modules, for example. Only
2913 the exports of an object-code module will be visible in GHCi,
2914 rather than all top-level bindings as in interpreted
2915 modules.</para>
2916 </sect1>
2917
2918 <sect1 id="ghci-faq">
2919 <title>FAQ and Things To Watch Out For</title>
2920
2921 <variablelist>
2922 <varlistentry>
2923 <term>The interpreter can't load modules with foreign export
2924 declarations!</term>
2925 <listitem>
2926 <para>Unfortunately not. We haven't implemented it yet.
2927 Please compile any offending modules by hand before loading
2928 them into GHCi.</para>
2929 </listitem>
2930 </varlistentry>
2931
2932 <varlistentry>
2933 <term>
2934 <literal>-O</literal> doesn't work with GHCi!
2935 <indexterm><primary><option>-O</option></primary></indexterm>
2936 </term>
2937 <listitem>
2938 <para>For technical reasons, the bytecode compiler doesn't
2939 interact well with one of the optimisation passes, so we
2940 have disabled optimisation when using the interpreter. This
2941 isn't a great loss: you'll get a much bigger win by
2942 compiling the bits of your code that need to go fast, rather
2943 than interpreting them with optimisation turned on.</para>
2944 </listitem>
2945 </varlistentry>
2946
2947 <varlistentry>
2948 <term>Unboxed tuples don't work with GHCi</term>
2949 <listitem>
2950 <para>That's right. You can always compile a module that
2951 uses unboxed tuples and load it into GHCi, however.
2952 (Incidentally the previous point, namely that
2953 <literal>-O</literal> is incompatible with GHCi, is because
2954 the bytecode compiler can't deal with unboxed
2955 tuples).</para>
2956 </listitem>
2957 </varlistentry>
2958
2959 <varlistentry>
2960 <term>Concurrent threads don't carry on running when GHCi is
2961 waiting for input.</term>
2962 <listitem>
2963 <para>This should work, as long as your GHCi was built with
2964 the <option>-threaded</option> switch, which is the default.
2965 Consult whoever supplied your GHCi installation.</para>
2966 </listitem>
2967 </varlistentry>
2968
2969 <varlistentry>
2970 <term>After using <literal>getContents</literal>, I can't use
2971 <literal>stdin</literal> again until I do
2972 <literal>:load</literal> or <literal>:reload</literal>.</term>
2973
2974 <listitem>
2975 <para>This is the defined behaviour of
2976 <literal>getContents</literal>: it puts the stdin Handle in
2977 a state known as <firstterm>semi-closed</firstterm>, wherein
2978 any further I/O operations on it are forbidden. Because I/O
2979 state is retained between computations, the semi-closed
2980 state persists until the next <literal>:load</literal> or
2981 <literal>:reload</literal> command.</para>
2982
2983 <para>You can make <literal>stdin</literal> reset itself
2984 after every evaluation by giving GHCi the command
2985 <literal>:set +r</literal>. This works because
2986 <literal>stdin</literal> is just a top-level expression that
2987 can be reverted to its unevaluated state in the same way as
2988 any other top-level expression (CAF).</para>
2989 </listitem>
2990 </varlistentry>
2991
2992 <varlistentry>
2993 <term>I can't use Control-C to interrupt computations in
2994 GHCi on Windows.</term>
2995 <listitem>
2996 <para>See <xref linkend="ghci-windows"/>.</para>
2997 </listitem>
2998 </varlistentry>
2999
3000 <varlistentry>
3001 <term>The default buffering mode is different in GHCi to GHC.</term>
3002 <listitem>
3003 <para>
3004 In GHC, the stdout handle is line-buffered by default.
3005 However, in GHCi we turn off the buffering on stdout,
3006 because this is normally what you want in an interpreter:
3007 output appears as it is generated.
3008 </para>
3009 <para>
3010 If you want line-buffered behaviour, as in GHC, you can
3011 start your program thus:
3012 <programlisting>
3013 main = do { hSetBuffering stdout LineBuffering; ... }
3014 </programlisting>
3015 </para>
3016 </listitem>
3017 </varlistentry>
3018 </variablelist>
3019 </sect1>
3020
3021 </chapter>
3022
3023 <!-- Emacs stuff:
3024 ;;; Local Variables: ***
3025 ;;; sgml-parent-document: ("users_guide.xml" "book" "chapter") ***
3026 ;;; End: ***
3027 -->