FIX #1765, #1766
[ghc.git] / docs / users_guide / ghci.xml
1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="ghci">
3 <title>Using GHCi</title>
4 <indexterm><primary>GHCi</primary></indexterm>
5 <indexterm><primary>interpreter</primary><see>GHCi</see></indexterm>
6 <indexterm><primary>interactive</primary><see>GHCi</see></indexterm>
7
8 <para>GHCi<footnote>
9 <para>The &lsquo;i&rsquo; stands for &ldquo;Interactive&rdquo;</para>
10 </footnote>
11 is GHC's interactive environment, in which Haskell expressions can
12 be interactively evaluated and programs can be interpreted. If
13 you're familiar with <ulink url="http://www.haskell.org/hugs/">Hugs</ulink><indexterm><primary>Hugs</primary>
14 </indexterm>, then you'll be right at home with GHCi. However, GHCi
15 also has support for interactively loading compiled code, as well as
16 supporting all<footnote><para>except <literal>foreign export</literal>, at the moment</para>
17 </footnote> the language extensions that GHC provides.
18 <indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
19 <indexterm><primary>Foreign Function
20 Interface</primary><secondary>GHCi support</secondary></indexterm>.
21 GHCi also includes an interactive debugger (see <xref linkend="ghci-debugger"/>).</para>
22
23 <sect1 id="ghci-introduction">
24 <title>Introduction to GHCi</title>
25
26 <para>Let's start with an example GHCi session. You can fire up
27 GHCi with the command <literal>ghci</literal>:</para>
28
29 <screen>
30 $ ghci
31 GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
32 Loading package base ... linking ... done.
33 Prelude>
34 </screen>
35
36 <para>There may be a short pause while GHCi loads the prelude and
37 standard libraries, after which the prompt is shown. As the banner
38 says, you can type <literal>:?</literal> to see the list of commands
39 available, and a half line description of each of them.</para>
40
41 <para>We'll explain most of these commands as we go along. For
42 Hugs users: many things work the same as in Hugs, so you should be
43 able to get going straight away.</para>
44
45 <para>Haskell expressions can be typed at the prompt:</para>
46 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
47 </indexterm>
48
49 <screen>
50 Prelude> 1+2
51 3
52 Prelude> let x = 42 in x / 9
53 4.666666666666667
54 Prelude>
55 </screen>
56
57 <para>GHCi interprets the whole line as an expression to evaluate.
58 The expression may not span several lines - as soon as you press
59 enter, GHCi will attempt to evaluate it.</para>
60 </sect1>
61
62 <sect1 id="loading-source-files">
63 <title>Loading source files</title>
64
65 <para>Suppose we have the following Haskell source code, which we
66 place in a file <filename>Main.hs</filename>:</para>
67
68 <programlisting>
69 main = print (fac 20)
70
71 fac 0 = 1
72 fac n = n * fac (n-1)
73 </programlisting>
74
75 <para>You can save <filename>Main.hs</filename> anywhere you like,
76 but if you save it somewhere other than the current
77 directory<footnote><para>If you started up GHCi from the command
78 line then GHCi's current directory is the same as the current
79 directory of the shell from which it was started. If you started
80 GHCi from the &ldquo;Start&rdquo; menu in Windows, then the
81 current directory is probably something like
82 <filename>C:\Documents and Settings\<replaceable>user
83 name</replaceable></filename>.</para> </footnote> then we will
84 need to change to the right directory in GHCi:</para>
85
86 <screen>
87 Prelude> :cd <replaceable>dir</replaceable>
88 </screen>
89
90 <para>where <replaceable>dir</replaceable> is the directory (or
91 folder) in which you saved <filename>Main.hs</filename>.</para>
92
93 <para>To load a Haskell source file into GHCi, use the
94 <literal>:load</literal> command:</para>
95 <indexterm><primary><literal>:load</literal></primary></indexterm>
96
97 <screen>
98 Prelude> :load Main
99 Compiling Main ( Main.hs, interpreted )
100 Ok, modules loaded: Main.
101 *Main>
102 </screen>
103
104 <para>GHCi has loaded the <literal>Main</literal> module, and the
105 prompt has changed to &ldquo;<literal>*Main></literal>&rdquo; to
106 indicate that the current context for expressions typed at the
107 prompt is the <literal>Main</literal> module we just loaded (we'll
108 explain what the <literal>*</literal> means later in <xref
109 linkend="ghci-scope"/>). So we can now type expressions involving
110 the functions from <filename>Main.hs</filename>:</para>
111
112 <screen>
113 *Main> fac 17
114 355687428096000
115 </screen>
116
117 <para>Loading a multi-module program is just as straightforward;
118 just give the name of the &ldquo;topmost&rdquo; module to the
119 <literal>:load</literal> command (hint: <literal>:load</literal>
120 can be abbreviated to <literal>:l</literal>). The topmost module
121 will normally be <literal>Main</literal>, but it doesn't have to
122 be. GHCi will discover which modules are required, directly or
123 indirectly, by the topmost module, and load them all in dependency
124 order.</para>
125
126 <sect2 id="ghci-modules-filenames">
127 <title>Modules vs. filenames</title>
128 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
129 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
130
131 <para>Question: How does GHC find the filename which contains
132 module <replaceable>M</replaceable>? Answer: it looks for the
133 file <literal><replaceable>M</replaceable>.hs</literal>, or
134 <literal><replaceable>M</replaceable>.lhs</literal>. This means
135 that for most modules, the module name must match the filename.
136 If it doesn't, GHCi won't be able to find it.</para>
137
138 <para>There is one exception to this general rule: when you load
139 a program with <literal>:load</literal>, or specify it when you
140 invoke <literal>ghci</literal>, you can give a filename rather
141 than a module name. This filename is loaded if it exists, and
142 it may contain any module you like. This is particularly
143 convenient if you have several <literal>Main</literal> modules
144 in the same directory and you can't call them all
145 <filename>Main.hs</filename>.</para>
146
147 <para>The search path for finding source files is specified with
148 the <option>-i</option> option on the GHCi command line, like
149 so:</para>
150 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
151
152 <para>or it can be set using the <literal>:set</literal> command
153 from within GHCi (see <xref
154 linkend="ghci-cmd-line-options"/>)<footnote><para>Note that in
155 GHCi, and <option>&ndash;&ndash;make</option> mode, the <option>-i</option>
156 option is used to specify the search path for
157 <emphasis>source</emphasis> files, whereas in standard
158 batch-compilation mode the <option>-i</option> option is used to
159 specify the search path for interface files, see <xref
160 linkend="search-path"/>.</para> </footnote></para>
161
162 <para>One consequence of the way that GHCi follows dependencies
163 to find modules to load is that every module must have a source
164 file. The only exception to the rule is modules that come from
165 a package, including the <literal>Prelude</literal> and standard
166 libraries such as <literal>IO</literal> and
167 <literal>Complex</literal>. If you attempt to load a module for
168 which GHCi can't find a source file, even if there are object
169 and interface files for the module, you'll get an error
170 message.</para>
171 </sect2>
172
173 <sect2>
174 <title>Making changes and recompilation</title>
175 <indexterm><primary><literal>:reload</literal></primary></indexterm>
176
177 <para>If you make some changes to the source code and want GHCi
178 to recompile the program, give the <literal>:reload</literal>
179 command. The program will be recompiled as necessary, with GHCi
180 doing its best to avoid actually recompiling modules if their
181 external dependencies haven't changed. This is the same
182 mechanism we use to avoid re-compiling modules in the batch
183 compilation setting (see <xref linkend="recomp"/>).</para>
184 </sect2>
185 </sect1>
186
187 <sect1 id="ghci-compiled">
188 <title>Loading compiled code</title>
189 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
190
191 <para>When you load a Haskell source module into GHCi, it is
192 normally converted to byte-code and run using the interpreter.
193 However, interpreted code can also run alongside compiled code in
194 GHCi; indeed, normally when GHCi starts, it loads up a compiled
195 copy of the <literal>base</literal> package, which contains the
196 <literal>Prelude</literal>.</para>
197
198 <para>Why should we want to run compiled code? Well, compiled
199 code is roughly 10x faster than interpreted code, but takes about
200 2x longer to produce (perhaps longer if optimisation is on). So
201 it pays to compile the parts of a program that aren't changing
202 very often, and use the interpreter for the code being actively
203 developed.</para>
204
205 <para>When loading up source files with <literal>:load</literal>,
206 GHCi looks for any corresponding compiled object files, and will
207 use one in preference to interpreting the source if possible. For
208 example, suppose we have a 4-module program consisting of modules
209 A, B, C, and D. Modules B and C both import D only,
210 and A imports both B &amp; C:</para>
211 <screen>
212 A
213 / \
214 B C
215 \ /
216 D
217 </screen>
218 <para>We can compile D, then load the whole program, like this:</para>
219 <screen>
220 Prelude> :! ghc -c D.hs
221 Prelude> :load A
222 Compiling B ( B.hs, interpreted )
223 Compiling C ( C.hs, interpreted )
224 Compiling A ( A.hs, interpreted )
225 Ok, modules loaded: A, B, C, D.
226 *Main>
227 </screen>
228
229 <para>In the messages from the compiler, we see that there is no line
230 for <literal>D</literal>. This is because
231 it isn't necessary to compile <literal>D</literal>,
232 because the source and everything it depends on
233 is unchanged since the last compilation.</para>
234
235 <para>At any time you can use the command
236 <literal>:show modules</literal>
237 to get a list of the modules currently loaded
238 into GHCi:</para>
239
240 <screen>
241 *Main> :show modules
242 D ( D.hs, D.o )
243 C ( C.hs, interpreted )
244 B ( B.hs, interpreted )
245 A ( A.hs, interpreted )
246 *Main></screen>
247
248 <para>If we now modify the source of D (or pretend to: using the Unix
249 command <literal>touch</literal> on the source file is handy for
250 this), the compiler will no longer be able to use the object file,
251 because it might be out of date:</para>
252
253 <screen>
254 *Main> :! touch D.hs
255 *Main> :reload
256 Compiling D ( D.hs, interpreted )
257 Ok, modules loaded: A, B, C, D.
258 *Main>
259 </screen>
260
261 <para>Note that module D was compiled, but in this instance
262 because its source hadn't really changed, its interface remained
263 the same, and the recompilation checker determined that A, B and C
264 didn't need to be recompiled.</para>
265
266 <para>So let's try compiling one of the other modules:</para>
267
268 <screen>
269 *Main> :! ghc -c C.hs
270 *Main> :load A
271 Compiling D ( D.hs, interpreted )
272 Compiling B ( B.hs, interpreted )
273 Compiling C ( C.hs, interpreted )
274 Compiling A ( A.hs, interpreted )
275 Ok, modules loaded: A, B, C, D.
276 </screen>
277
278 <para>We didn't get the compiled version of C! What happened?
279 Well, in GHCi a compiled module may only depend on other compiled
280 modules, and in this case C depends on D, which doesn't have an
281 object file, so GHCi also rejected C's object file. Ok, so let's
282 also compile D:</para>
283
284 <screen>
285 *Main> :! ghc -c D.hs
286 *Main> :reload
287 Ok, modules loaded: A, B, C, D.
288 </screen>
289
290 <para>Nothing happened! Here's another lesson: newly compiled
291 modules aren't picked up by <literal>:reload</literal>, only
292 <literal>:load</literal>:</para>
293
294 <screen>
295 *Main> :load A
296 Compiling B ( B.hs, interpreted )
297 Compiling A ( A.hs, interpreted )
298 Ok, modules loaded: A, B, C, D.
299 </screen>
300
301 <para>HINT: since GHCi will only use a compiled object file if it
302 can be sure that the compiled version is up-to-date, a good technique
303 when working on a large program is to occasionally run
304 <literal>ghc &ndash;&ndash;make</literal> to compile the whole project (say
305 before you go for lunch :-), then continue working in the
306 interpreter. As you modify code, the changed modules will be
307 interpreted, but the rest of the project will remain
308 compiled.</para>
309
310 </sect1>
311
312 <sect1 id="interactive-evaluation">
313 <title>Interactive evaluation at the prompt</title>
314
315 <para>When you type an expression at the prompt, GHCi immediately
316 evaluates and prints the result:
317 <screen>
318 Prelude> reverse "hello"
319 "olleh"
320 Prelude> 5+5
321 10
322 </screen>
323 </para>
324
325 <sect2><title>I/O actions at the prompt</title>
326
327 <para>GHCi does more than simple expression evaluation at the prompt.
328 If you type something of type <literal>IO a</literal> for some
329 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
330 as an IO-computation.
331 <screen>
332 Prelude> "hello"
333 "hello"
334 Prelude> putStrLn "hello"
335 hello
336 </screen>
337 Furthermore, GHCi will print the result of the I/O action if (and only
338 if):
339 <itemizedlist>
340 <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
341 <listitem><para>The result type is not
342 <literal>()</literal>.</para></listitem>
343 </itemizedlist>
344 For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
345 <screen>
346 Prelude> putStrLn "hello"
347 hello
348 Prelude> do { putStrLn "hello"; return "yes" }
349 hello
350 "yes"
351 </screen>
352 </para></sect2>
353
354 <sect2 id="ghci-stmts">
355 <title>Using <literal>do-</literal>notation at the prompt</title>
356 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
357 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
358
359 <para>GHCi actually accepts <firstterm>statements</firstterm>
360 rather than just expressions at the prompt. This means you can
361 bind values and functions to names, and use them in future
362 expressions or statements.</para>
363
364 <para>The syntax of a statement accepted at the GHCi prompt is
365 exactly the same as the syntax of a statement in a Haskell
366 <literal>do</literal> expression. However, there's no monad
367 overloading here: statements typed at the prompt must be in the
368 <literal>IO</literal> monad.
369 <screen>
370 Prelude> x &lt;- return 42
371 42
372 Prelude> print x
373 42
374 Prelude>
375 </screen>
376 The statement <literal>x &lt;- return 42</literal> means
377 &ldquo;execute <literal>return 42</literal> in the
378 <literal>IO</literal> monad, and bind the result to
379 <literal>x</literal>&rdquo;. We can then use
380 <literal>x</literal> in future statements, for example to print
381 it as we did above.</para>
382
383 <para>GHCi will print the result of a statement if and only if:
384 <itemizedlist>
385 <listitem>
386 <para>The statement is not a binding, or it is a monadic binding
387 (<literal>p &lt;- e</literal>) that binds exactly one
388 variable.</para>
389 </listitem>
390 <listitem>
391 <para>The variable's type is not polymorphic, is not
392 <literal>()</literal>, and is an instance of
393 <literal>Show</literal></para>
394 </listitem>
395 </itemizedlist>
396 The automatic printing of binding results can be suppressed with
397 <option>:set -fno-print-bind-result</option> (this does not
398 suppress printing the result of non-binding statements).
399 <indexterm><primary><option>-fno-print-bind-result</option></primary></indexterm><indexterm><primary><option>-fprint-bind-result</option></primary></indexterm>.
400 You might want to do this to prevent the result of binding
401 statements from being fully evaluated by the act of printing
402 them, for example.</para>
403
404 <para>Of course, you can also bind normal non-IO expressions
405 using the <literal>let</literal>-statement:</para>
406 <screen>
407 Prelude> let x = 42
408 Prelude> x
409 42
410 Prelude>
411 </screen>
412 <para>Another important difference between the two types of binding
413 is that the monadic bind (<literal>p &lt;- e</literal>) is
414 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
415 whereas with the <literal>let</literal> form, the expression
416 isn't evaluated immediately:</para>
417 <screen>
418 Prelude> let x = error "help!"
419 Prelude> print x
420 *** Exception: help!
421 Prelude>
422 </screen>
423
424 <para>Note that <literal>let</literal> bindings do not automatically
425 print the value bound, unlike monadic bindings.</para>
426
427 <para>Hint: you can also use <literal>let</literal>-statements
428 to define functions at the prompt:</para>
429 <screen>
430 Prelude> let add a b = a + b
431 Prelude> add 1 2
432 3
433 Prelude>
434 </screen>
435 <para>However, this quickly gets tedious when defining functions
436 with multiple clauses, or groups of mutually recursive functions,
437 because the complete definition has to be given on a single line,
438 using explicit braces and semicolons instead of layout:</para>
439 <screen>
440 Prelude> let { f op n [] = n ; f op n (h:t) = h `op` f op n t }
441 Prelude> f (+) 0 [1..3]
442 6
443 Prelude>
444 </screen>
445 <para>To alleviate this issue, GHCi commands can be split over
446 multiple lines, by wrapping them in <literal>:{</literal> and
447 <literal>:}</literal> (each on a single line of its own):</para>
448 <screen>
449 Prelude> :{
450 Prelude| let { g op n [] = n
451 Prelude| ; g op n (h:t) = h `op` g op n t
452 Prelude| }
453 Prelude| :}
454 Prelude> g (*) 1 [1..3]
455 6
456 </screen>
457 <para>Such multiline commands can be used with any GHCi command,
458 and the lines between <literal>:{</literal> and
459 <literal>:}</literal> are simply merged into a single line for
460 interpretation. That implies that each such group must form a single
461 valid command when merged, and that no layout rule is used.
462 The main purpose of multiline commands is not to replace module
463 loading but to make definitions in .ghci-files (see <xref
464 linkend="ghci-dot-files"/>) more readable and maintainable.</para>
465
466 <para>Any exceptions raised during the evaluation or execution
467 of the statement are caught and printed by the GHCi command line
468 interface (for more information on exceptions, see the module
469 <literal>Control.Exception</literal> in the libraries
470 documentation).</para>
471
472 <para>Every new binding shadows any existing bindings of the
473 same name, including entities that are in scope in the current
474 module context.</para>
475
476 <para>WARNING: temporary bindings introduced at the prompt only
477 last until the next <literal>:load</literal> or
478 <literal>:reload</literal> command, at which time they will be
479 simply lost. However, they do survive a change of context with
480 <literal>:module</literal>: the temporary bindings just move to
481 the new location.</para>
482
483 <para>HINT: To get a list of the bindings currently in scope, use the
484 <literal>:show bindings</literal> command:</para>
485
486 <screen>
487 Prelude> :show bindings
488 x :: Int
489 Prelude></screen>
490
491 <para>HINT: if you turn on the <literal>+t</literal> option,
492 GHCi will show the type of each variable bound by a statement.
493 For example:</para>
494 <indexterm><primary><literal>+t</literal></primary></indexterm>
495 <screen>
496 Prelude> :set +t
497 Prelude> let (x:xs) = [1..]
498 x :: Integer
499 xs :: [Integer]
500 </screen>
501
502 </sect2>
503
504 <sect2 id="ghci-scope">
505 <title>What's really in scope at the prompt?</title>
506
507 <para>When you type an expression at the prompt, what
508 identifiers and types are in scope? GHCi provides a flexible
509 way to control exactly how the context for an expression is
510 constructed. Let's start with the simple cases; when you start
511 GHCi the prompt looks like this:</para>
512
513 <screen>Prelude></screen>
514
515 <para>Which indicates that everything from the module
516 <literal>Prelude</literal> is currently in scope. If we now
517 load a file into GHCi, the prompt will change:</para>
518
519 <screen>
520 Prelude> :load Main.hs
521 Compiling Main ( Main.hs, interpreted )
522 *Main>
523 </screen>
524
525 <para>The new prompt is <literal>*Main</literal>, which
526 indicates that we are typing expressions in the context of the
527 top-level of the <literal>Main</literal> module. Everything
528 that is in scope at the top-level in the module
529 <literal>Main</literal> we just loaded is also in scope at the
530 prompt (probably including <literal>Prelude</literal>, as long
531 as <literal>Main</literal> doesn't explicitly hide it).</para>
532
533 <para>The syntax
534 <literal>*<replaceable>module</replaceable></literal> indicates
535 that it is the full top-level scope of
536 <replaceable>module</replaceable> that is contributing to the
537 scope for expressions typed at the prompt. Without the
538 <literal>*</literal>, just the exports of the module are
539 visible.</para>
540
541 <para>We're not limited to a single module: GHCi can combine
542 scopes from multiple modules, in any mixture of
543 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
544 combines the scopes from all of these modules to form the scope
545 that is in effect at the prompt. For technical reasons, GHCi
546 can only support the <literal>*</literal>-form for modules which
547 are interpreted, so compiled modules and package modules can
548 only contribute their exports to the current scope.</para>
549
550 <para>The scope is manipulated using the
551 <literal>:module</literal> command. For example, if the current
552 scope is <literal>Prelude</literal>, then we can bring into
553 scope the exports from the module <literal>IO</literal> like
554 so:</para>
555
556 <screen>
557 Prelude> :module +IO
558 Prelude IO> hPutStrLn stdout "hello\n"
559 hello
560 Prelude IO>
561 </screen>
562
563 <para>(Note: you can use <literal>import M</literal> as an
564 alternative to <literal>:module +M</literal>, and
565 <literal>:module</literal> can also be shortened to
566 <literal>:m</literal>). The full syntax of the
567 <literal>:module</literal> command is:</para>
568
569 <screen>
570 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
571 </screen>
572
573 <para>Using the <literal>+</literal> form of the
574 <literal>module</literal> commands adds modules to the current
575 scope, and <literal>-</literal> removes them. Without either
576 <literal>+</literal> or <literal>-</literal>, the current scope
577 is replaced by the set of modules specified. Note that if you
578 use this form and leave out <literal>Prelude</literal>, GHCi
579 will assume that you really wanted the
580 <literal>Prelude</literal> and add it in for you (if you don't
581 want the <literal>Prelude</literal>, then ask to remove it with
582 <literal>:m -Prelude</literal>).</para>
583
584 <para>The scope is automatically set after a
585 <literal>:load</literal> command, to the most recently loaded
586 "target" module, in a <literal>*</literal>-form if possible.
587 For example, if you say <literal>:load foo.hs bar.hs</literal>
588 and <filename>bar.hs</filename> contains module
589 <literal>Bar</literal>, then the scope will be set to
590 <literal>*Bar</literal> if <literal>Bar</literal> is
591 interpreted, or if <literal>Bar</literal> is compiled it will be
592 set to <literal>Prelude Bar</literal> (GHCi automatically adds
593 <literal>Prelude</literal> if it isn't present and there aren't
594 any <literal>*</literal>-form modules).</para>
595
596 <para>With multiple modules in scope, especially multiple
597 <literal>*</literal>-form modules, it is likely that name
598 clashes will occur. Haskell specifies that name clashes are
599 only reported when an ambiguous identifier is used, and GHCi
600 behaves in the same way for expressions typed at the
601 prompt.</para>
602
603 <para>
604 Hint: GHCi will tab-complete names that are in scope; for
605 example, if you run GHCi and type <literal>J&lt;tab&gt;</literal>
606 then GHCi will expand it to &ldquo;<literal>Just </literal>&rdquo;.
607 </para>
608
609 <sect3>
610 <title>Qualified names</title>
611
612 <para>To make life slightly easier, the GHCi prompt also
613 behaves as if there is an implicit <literal>import
614 qualified</literal> declaration for every module in every
615 package, and every module currently loaded into GHCi.</para>
616 </sect3>
617
618 <sect3>
619 <title>The <literal>:main</literal> command</title>
620
621 <para>
622 When a program is compiled and executed, it can use the
623 <literal>getArgs</literal> function to access the
624 command-line arguments.
625 However, we cannot simply pass the arguments to the
626 <literal>main</literal> function while we are testing in ghci,
627 as the <literal>main</literal> function doesn't take its
628 directly.
629 </para>
630
631 <para>
632 Instead, we can use the <literal>:main</literal> command.
633 This runs whatever <literal>main</literal> is in scope, with
634 any arguments being treated the same as command-line arguments,
635 e.g.:
636 </para>
637
638 <screen>
639 Prelude> let main = System.Environment.getArgs >>= print
640 Prelude> :main foo bar
641 ["foo","bar"]
642 </screen>
643
644 </sect3>
645 </sect2>
646
647
648 <sect2>
649 <title>The <literal>it</literal> variable</title>
650 <indexterm><primary><literal>it</literal></primary>
651 </indexterm>
652
653 <para>Whenever an expression (or a non-binding statement, to be
654 precise) is typed at the prompt, GHCi implicitly binds its value
655 to the variable <literal>it</literal>. For example:</para>
656 <screen>
657 Prelude> 1+2
658 3
659 Prelude> it * 2
660 6
661 </screen>
662 <para>What actually happens is that GHCi typechecks the
663 expression, and if it doesn't have an <literal>IO</literal> type,
664 then it transforms it as follows: an expression
665 <replaceable>e</replaceable> turns into
666 <screen>
667 let it = <replaceable>e</replaceable>;
668 print it
669 </screen>
670 which is then run as an IO-action.</para>
671
672 <para>Hence, the original expression must have a type which is an
673 instance of the <literal>Show</literal> class, or GHCi will
674 complain:</para>
675
676 <screen>
677 Prelude&gt; id
678
679 &lt;interactive&gt;:1:0:
680 No instance for (Show (a -&gt; a))
681 arising from use of `print' at &lt;interactive&gt;:1:0-1
682 Possible fix: add an instance declaration for (Show (a -> a))
683 In the expression: print it
684 In a 'do' expression: print it
685 </screen>
686
687 <para>The error message contains some clues as to the
688 transformation happening internally.</para>
689
690 <para>If the expression was instead of type <literal>IO a</literal> for
691 some <literal>a</literal>, then <literal>it</literal> will be
692 bound to the result of the <literal>IO</literal> computation,
693 which is of type <literal>a</literal>. eg.:</para>
694 <screen>
695 Prelude> Time.getClockTime
696 Wed Mar 14 12:23:13 GMT 2001
697 Prelude> print it
698 Wed Mar 14 12:23:13 GMT 2001
699 </screen>
700
701 <para>The corresponding translation for an IO-typed
702 <replaceable>e</replaceable> is
703 <screen>
704 it &lt;- <replaceable>e</replaceable>
705 </screen>
706 </para>
707
708 <para>Note that <literal>it</literal> is shadowed by the new
709 value each time you evaluate a new expression, and the old value
710 of <literal>it</literal> is lost.</para>
711
712 </sect2>
713
714 <sect2 id="extended-default-rules">
715 <title>Type defaulting in GHCi</title>
716 <indexterm><primary>Type default</primary></indexterm>
717 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
718 <para>
719 Consider this GHCi session:
720 <programlisting>
721 ghci> reverse []
722 </programlisting>
723 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
724 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
725 on the type <literal>a</literal>. For example:
726 <programlisting>
727 ghci> (reverse []) :: String
728 ""
729 ghci> (reverse []) :: [Int]
730 []
731 </programlisting>
732 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
733 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
734 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
735 a)</literal> for each type variable <literal>a</literal>, and defaults the
736 type variable if
737 <orderedlist>
738 <listitem>
739 <para>
740 The type variable <literal>a</literal> appears in no
741 other constraints
742 </para>
743 </listitem>
744 <listitem>
745 <para>
746 All the classes <literal>Ci</literal> are standard.
747 </para>
748 </listitem>
749 <listitem>
750 <para>
751 At least one of the classes <literal>Ci</literal> is
752 numeric.
753 </para>
754 </listitem>
755 </orderedlist>
756 At the GHCi prompt, or with GHC if the
757 <literal>-XExtendedDefaultRules</literal> flag is given,
758 the following additional differences apply:
759 <itemizedlist>
760 <listitem>
761 <para>
762 Rule 2 above is relaxed thus:
763 <emphasis>All</emphasis> of the classes
764 <literal>Ci</literal> are single-parameter type classes.
765 </para>
766 </listitem>
767 <listitem>
768 <para>
769 Rule 3 above is relaxed this:
770 At least one of the classes <literal>Ci</literal> is
771 numeric, <emphasis>or is <literal>Show</literal>,
772 <literal>Eq</literal>, or
773 <literal>Ord</literal></emphasis>.
774 </para>
775 </listitem>
776 <listitem>
777 <para>
778 The unit type <literal>()</literal> is added to the
779 start of the standard list of types which are tried when
780 doing type defaulting.
781 </para>
782 </listitem>
783 </itemizedlist>
784 The last point means that, for example, this program:
785 <programlisting>
786 main :: IO ()
787 main = print def
788
789 instance Num ()
790
791 def :: (Num a, Enum a) => a
792 def = toEnum 0
793 </programlisting>
794 prints <literal>()</literal> rather than <literal>0</literal> as the
795 type is defaulted to <literal>()</literal> rather than
796 <literal>Integer</literal>.
797 </para>
798 <para>
799 The motivation for the change is that it means <literal>IO a</literal>
800 actions default to <literal>IO ()</literal>, which in turn means that
801 ghci won't try to print a result when running them. This is
802 particularly important for <literal>printf</literal>, which has an
803 instance that returns <literal>IO a</literal>.
804 However, it is only able to return
805 <literal>undefined</literal>
806 (the reason for the instance having this type is so that printf
807 doesn't require extensions to the class system), so if the type defaults to
808 <literal>Integer</literal> then ghci gives an error when running a
809 printf.
810 </para>
811 </sect2>
812 </sect1>
813
814 <sect1 id="ghci-debugger">
815 <title>The GHCi Debugger</title>
816 <indexterm><primary>debugger</primary><secondary>in GHCi</secondary>
817 </indexterm>
818
819 <para>GHCi contains a simple imperative-style debugger in which you can
820 stop a running computation in order to examine the values of
821 variables. The debugger is integrated into GHCi, and is turned on by
822 default: no flags are required to enable the debugging facilities. There
823 is one major restriction: breakpoints and single-stepping are only
824 available in <emphasis>interpreted</emphasis> modules; compiled code is
825 invisible to the debugger.</para>
826
827 <para>The debugger provides the following:
828 <itemizedlist>
829 <listitem>
830 <para>The ability to set a <firstterm>breakpoint</firstterm> on a
831 function definition or expression in the program. When the function
832 is called, or the expression evaluated, GHCi suspends
833 execution and returns to the prompt, where you can inspect the
834 values of local variables before continuing with the
835 execution.</para>
836 </listitem>
837 <listitem>
838 <para>Execution can be <firstterm>single-stepped</firstterm>: the
839 evaluator will suspend execution approximately after every
840 reduction, allowing local variables to be inspected. This is
841 equivalent to setting a breakpoint at every point in the
842 program.</para>
843 </listitem>
844 <listitem>
845 <para>Execution can take place in <firstterm>tracing
846 mode</firstterm>, in which the evaluator remembers each
847 evaluation step as it happens, but doesn't suspend execution until
848 an actual breakpoint is reached. When this happens, the history of
849 evaluation steps can be inspected.</para>
850 </listitem>
851 <listitem>
852 <para>Exceptions (e.g. pattern matching failure and
853 <literal>error</literal>) can be treated as breakpoints, to help
854 locate the source of an exception in the program.</para>
855 </listitem>
856 </itemizedlist>
857 </para>
858
859 <para>There is currently no support for obtaining a &ldquo;stack
860 trace&rdquo;, but the tracing and history features provide a useful
861 second-best, which will often be enough to establish the context of an
862 error.</para>
863
864 <sect2 id="breakpoints">
865 <title>Breakpoints and inspecting variables</title>
866
867 <para>Let's use quicksort as a running example. Here's the code:</para>
868
869 <programlisting>
870 qsort [] = []
871 qsort (a:as) = qsort left ++ [a] ++ qsort right
872 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
873
874 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
875 </programlisting>
876
877 <para>First, load the module into GHCi:</para>
878
879 <screen>
880 Prelude> :l qsort.hs
881 [1 of 1] Compiling Main ( qsort.hs, interpreted )
882 Ok, modules loaded: Main.
883 *Main>
884 </screen>
885
886 <para>Now, let's set a breakpoint on the right-hand-side of the second
887 equation of qsort:</para>
888
889 <programlisting>
890 *Main> :break 2
891 Breakpoint 0 activated at qsort.hs:2:15-46
892 *Main>
893 </programlisting>
894
895 <para>The command <literal>:break 2</literal> sets a breakpoint on line
896 2 of the most recently-loaded module, in this case
897 <literal>qsort.hs</literal>. Specifically, it picks the
898 leftmost complete subexpression on that line on which to set the
899 breakpoint, which in this case is the expression
900 <literal>(qsort left ++ [a] ++ qsort right)</literal>.</para>
901
902 <para>Now, we run the program:</para>
903
904 <programlisting>
905 *Main> main
906 Stopped at qsort.hs:2:15-46
907 _result :: [a]
908 a :: a
909 left :: [a]
910 right :: [a]
911 [qsort.hs:2:15-46] *Main>
912 </programlisting>
913
914 <para>Execution has stopped at the breakpoint. The prompt has changed to
915 indicate that we are currently stopped at a breakpoint, and the location:
916 <literal>[qsort.hs:2:15-46]</literal>. To further clarify the
917 location, we can use the <literal>:list</literal> command:</para>
918
919 <programlisting>
920 [qsort.hs:2:15-46] *Main> :list
921 1 qsort [] = []
922 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
923 3 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
924 </programlisting>
925
926 <para>The <literal>:list</literal> command lists the source code around
927 the current breakpoint. If your output device supports it, then GHCi
928 will highlight the active subexpression in bold.</para>
929
930 <para>GHCi has provided bindings for the free variables<footnote><para>We
931 originally provided bindings for all variables in scope, rather
932 than just
933 the free variables of the expression, but found that this affected
934 performance considerably, hence the current restriction to just the
935 free variables.</para>
936 </footnote> of the expression
937 on which the
938 breakpoint was placed (<literal>a</literal>, <literal>left</literal>,
939 <literal>right</literal>), and additionally a binding for the result of
940 the expression (<literal>_result</literal>). These variables are just
941 like other variables that you might define in GHCi; you
942 can use them in expressions that you type at the prompt, you can ask
943 for their types with <literal>:type</literal>, and so on. There is one
944 important difference though: these variables may only have partial
945 types. For example, if we try to display the value of
946 <literal>left</literal>:</para>
947
948 <screen>
949 [qsort.hs:2:15-46] *Main> left
950
951 &lt;interactive&gt;:1:0:
952 Ambiguous type variable `a' in the constraint:
953 `Show a' arising from a use of `print' at &lt;interactive&gt;:1:0-3
954 Cannot resolve unknown runtime types: a
955 Use :print or :force to determine these types
956 </screen>
957
958 <para>This is because <literal>qsort</literal> is a polymorphic function,
959 and because GHCi does not carry type information at runtime, it cannot
960 determine the runtime types of free variables that involve type
961 variables. Hence, when you ask to display <literal>left</literal> at
962 the prompt, GHCi can't figure out which instance of
963 <literal>Show</literal> to use, so it emits the type error above.</para>
964
965 <para>Fortunately, the debugger includes a generic printing command,
966 <literal>:print</literal>, which can inspect the actual runtime value of a
967 variable and attempt to reconstruct its type. If we try it on
968 <literal>left</literal>:</para>
969
970 <screen>
971 [qsort.hs:2:15-46] *Main> :set -fprint-evld-with-show
972 [qsort.hs:2:15-46] *Main> :print left
973 left = (_t1::[a])
974 </screen>
975
976 <para>This isn't particularly enlightening. What happened is that
977 <literal>left</literal> is bound to an unevaluated computation (a
978 suspension, or <firstterm>thunk</firstterm>), and
979 <literal>:print</literal> does not force any evaluation. The idea is
980 that <literal>:print</literal> can be used to inspect values at a
981 breakpoint without any unfortunate side effects. It won't force any
982 evaluation, which could cause the program to give a different answer
983 than it would normally, and hence it won't cause any exceptions to be
984 raised, infinite loops, or further breakpoints to be triggered (see
985 <xref linkend="nested-breakpoints" />).
986 Rather than forcing thunks, <literal>:print</literal>
987 binds each thunk to a fresh variable beginning with an
988 underscore, in this case
989 <literal>_t1</literal>.</para>
990
991 <para>The flag <literal>-fprint-evld-with-show</literal> instructs
992 <literal>:print</literal> to reuse
993 available <literal>Show</literal> instances when possible. This happens
994 only when the contents of the variable being inspected
995 are completely evaluated.</para>
996
997
998 <para>If we aren't concerned about preserving the evaluatedness of a
999 variable, we can use <literal>:force</literal> instead of
1000 <literal>:print</literal>. The <literal>:force</literal> command
1001 behaves exactly like <literal>:print</literal>, except that it forces
1002 the evaluation of any thunks it encounters:</para>
1003
1004 <screen>
1005 [qsort.hs:2:15-46] *Main> :force left
1006 left = [4,0,3,1]
1007 </screen>
1008
1009 <para>Now, since <literal>:force</literal> has inspected the runtime
1010 value of <literal>left</literal>, it has reconstructed its type. We
1011 can see the results of this type reconstruction:</para>
1012
1013 <screen>
1014 [qsort.hs:2:15-46] *Main> :show bindings
1015 _result :: [Integer]
1016 a :: Integer
1017 left :: [Integer]
1018 right :: [Integer]
1019 _t1 :: [Integer]
1020 </screen>
1021
1022 <para>Not only do we now know the type of <literal>left</literal>, but
1023 all the other partial types have also been resolved. So we can ask
1024 for the value of <literal>a</literal>, for example:</para>
1025
1026 <screen>
1027 [qsort.hs:2:15-46] *Main> a
1028 8
1029 </screen>
1030
1031 <para>You might find it useful to use Haskell's
1032 <literal>seq</literal> function to evaluate individual thunks rather
1033 than evaluating the whole expression with <literal>:force</literal>.
1034 For example:</para>
1035
1036 <screen>
1037 [qsort.hs:2:15-46] *Main> :print right
1038 right = (_t1::[Integer])
1039 [qsort.hs:2:15-46] *Main> seq _t1 ()
1040 ()
1041 [qsort.hs:2:15-46] *Main> :print right
1042 right = 23 : (_t2::[Integer])
1043 </screen>
1044
1045 <para>We evaluated only the <literal>_t1</literal> thunk, revealing the
1046 head of the list, and the tail is another thunk now bound to
1047 <literal>_t2</literal>. The <literal>seq</literal> function is a
1048 little inconvenient to use here, so you might want to use
1049 <literal>:def</literal> to make a nicer interface (left as an exercise
1050 for the reader!).</para>
1051
1052 <para>Finally, we can continue the current execution:</para>
1053
1054 <screen>
1055 [qsort.hs:2:15-46] *Main> :continue
1056 Stopped at qsort.hs:2:15-46
1057 _result :: [a]
1058 a :: a
1059 left :: [a]
1060 right :: [a]
1061 [qsort.hs:2:15-46] *Main>
1062 </screen>
1063
1064 <para>The execution continued at the point it previously stopped, and has
1065 now stopped at the breakpoint for a second time.</para>
1066
1067
1068 <sect3 id="setting-breakpoints">
1069 <title>Setting breakpoints</title>
1070
1071 <para>Breakpoints can be set in various ways. Perhaps the easiest way to
1072 set a breakpoint is to name a top-level function:</para>
1073
1074 <screen>
1075 :break <replaceable>identifier</replaceable>
1076 </screen>
1077
1078 <para>Where <replaceable>identifier</replaceable> names any top-level
1079 function in an interpreted module currently loaded into GHCi (qualified
1080 names may be used). The breakpoint will be set on the body of the
1081 function, when it is fully applied but before any pattern matching has
1082 taken place.</para>
1083
1084 <para>Breakpoints can also be set by line (and optionally column)
1085 number:</para>
1086
1087 <screen>
1088 :break <replaceable>line</replaceable>
1089 :break <replaceable>line</replaceable> <replaceable>column</replaceable>
1090 :break <replaceable>module</replaceable> <replaceable>line</replaceable>
1091 :break <replaceable>module</replaceable> <replaceable>line</replaceable> <replaceable>column</replaceable>
1092 </screen>
1093
1094 <para>When a breakpoint is set on a particular line, GHCi sets the
1095 breakpoint on the
1096 leftmost subexpression that begins and ends on that line. If two
1097 complete subexpressions start at the same
1098 column, the longest one is picked. If there is no complete
1099 subexpression on the line, then the leftmost expression starting on
1100 the line is picked, and failing that the rightmost expression that
1101 partially or completely covers the line.</para>
1102
1103 <para>When a breakpoint is set on a particular line and column, GHCi
1104 picks the smallest subexpression that encloses that location on which
1105 to set the breakpoint. Note: GHC considers the TAB character to have a
1106 width of 1, wherever it occurs; in other words it counts
1107 characters, rather than columns. This matches what some editors do,
1108 and doesn't match others. The best advice is to avoid tab
1109 characters in your source code altogether (see
1110 <option>-fwarn-tabs</option> in <xref linkend="options-sanity"
1111 />).</para>
1112
1113 <para>If the module is omitted, then the most recently-loaded module is
1114 used.</para>
1115
1116 <para>Not all subexpressions are potential breakpoint locations. Single
1117 variables are typically not considered to be breakpoint locations
1118 (unless the variable is the right-hand-side of a function definition,
1119 lambda, or case alternative). The rule of thumb is that all redexes
1120 are breakpoint locations, together with the bodies of functions,
1121 lambdas, case alternatives and binding statements. There is normally
1122 no breakpoint on a let expression, but there will always be a
1123 breakpoint on its body, because we are usually interested in inspecting
1124 the values of the variables bound by the let.</para>
1125
1126 </sect3>
1127 <sect3>
1128 <title>Listing and deleting breakpoints</title>
1129
1130 <para>The list of breakpoints currently enabled can be displayed using
1131 <literal>:show&nbsp;breaks</literal>:</para>
1132 <screen>
1133 *Main> :show breaks
1134 [0] Main qsort.hs:1:11-12
1135 [1] Main qsort.hs:2:15-46
1136 </screen>
1137
1138 <para>To delete a breakpoint, use the <literal>:delete</literal>
1139 command with the number given in the output from <literal>:show&nbsp;breaks</literal>:</para>
1140
1141 <screen>
1142 *Main> :delete 0
1143 *Main> :show breaks
1144 [1] Main qsort.hs:2:15-46
1145 </screen>
1146
1147 <para>To delete all breakpoints at once, use <literal>:delete *</literal>.</para>
1148
1149 </sect3>
1150 </sect2>
1151
1152 <sect2 id="single-stepping">
1153 <title>Single-stepping</title>
1154
1155 <para>Single-stepping is a great way to visualise the execution of your
1156 program, and it is also a useful tool for identifying the source of a
1157 bug. GHCi offers two variants of stepping. Use
1158 <literal>:step</literal> to enable all the
1159 breakpoints in the program, and execute until the next breakpoint is
1160 reached. Use <literal>:steplocal</literal> to limit the set
1161 of enabled breakpoints to those in the current top level function.
1162 Similarly, use <literal>:stepmodule</literal> to single step only on
1163 breakpoints contained in the current module.
1164 For example:</para>
1165
1166 <screen>
1167 *Main> :step main
1168 Stopped at qsort.hs:5:7-47
1169 _result :: IO ()
1170 </screen>
1171
1172 <para>The command <literal>:step
1173 <replaceable>expr</replaceable></literal> begins the evaluation of
1174 <replaceable>expr</replaceable> in single-stepping mode. If
1175 <replaceable>expr</replaceable> is omitted, then it single-steps from
1176 the current breakpoint. <literal>:stepover</literal>
1177 works similarly.</para>
1178
1179 <para>The <literal>:list</literal> command is particularly useful when
1180 single-stepping, to see where you currently are:</para>
1181
1182 <screen>
1183 [qsort.hs:5:7-47] *Main> :list
1184 4
1185 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1186 6
1187 [qsort.hs:5:7-47] *Main>
1188 </screen>
1189
1190 <para>In fact, GHCi provides a way to run a command when a breakpoint is
1191 hit, so we can make it automatically do
1192 <literal>:list</literal>:</para>
1193
1194 <screen>
1195 [qsort.hs:5:7-47] *Main> :set stop :list
1196 [qsort.hs:5:7-47] *Main> :step
1197 Stopped at qsort.hs:5:14-46
1198 _result :: [Integer]
1199 4
1200 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1201 6
1202 [qsort.hs:5:14-46] *Main>
1203 </screen>
1204 </sect2>
1205
1206 <sect2 id="nested-breakpoints">
1207 <title>Nested breakpoints</title>
1208 <para>When GHCi is stopped at a breakpoint, and an expression entered at
1209 the prompt triggers a
1210 second breakpoint, the new breakpoint becomes the &ldquo;current&rdquo;
1211 one, and the old one is saved on a stack. An arbitrary number of
1212 breakpoint contexts can be built up in this way. For example:</para>
1213
1214 <screen>
1215 [qsort.hs:2:15-46] *Main> :st qsort [1,3]
1216 Stopped at qsort.hs:(1,0)-(3,55)
1217 _result :: [a]
1218 ... [qsort.hs:(1,0)-(3,55)] *Main>
1219 </screen>
1220
1221 <para>While stopped at the breakpoint on line 2 that we set earlier, we
1222 started a new evaluation with <literal>:step qsort [1,3]</literal>.
1223 This new evaluation stopped after one step (at the definition of
1224 <literal>qsort</literal>). The prompt has changed, now prefixed with
1225 <literal>...</literal>, to indicate that there are saved breakpoints
1226 beyond the current one. To see the stack of contexts, use
1227 <literal>:show context</literal>:</para>
1228
1229 <screen>
1230 ... [qsort.hs:(1,0)-(3,55)] *Main> :show context
1231 --> main
1232 Stopped at qsort.hs:2:15-46
1233 --> qsort [1,3]
1234 Stopped at qsort.hs:(1,0)-(3,55)
1235 ... [qsort.hs:(1,0)-(3,55)] *Main>
1236 </screen>
1237
1238 <para>To abandon the current evaluation, use
1239 <literal>:abandon</literal>:</para>
1240
1241 <screen>
1242 ... [qsort.hs:(1,0)-(3,55)] *Main> :abandon
1243 [qsort.hs:2:15-46] *Main> :abandon
1244 *Main>
1245 </screen>
1246 </sect2>
1247
1248 <sect2 id="ghci-debugger-result">
1249 <title>The <literal>_result</literal> variable</title>
1250 <para>When stopped at a breakpoint or single-step, GHCi binds the
1251 variable <literal>_result</literal> to the value of the currently
1252 active expression. The value of <literal>_result</literal> is
1253 presumably not available yet, because we stopped its evaluation, but it
1254 can be forced: if the type is known and showable, then just entering
1255 <literal>_result</literal> at the prompt will show it. However,
1256 there's one caveat to doing this: evaluating <literal>_result</literal>
1257 will be likely to trigger further breakpoints, starting with the
1258 breakpoint we are currently stopped at (if we stopped at a real
1259 breakpoint, rather than due to <literal>:step</literal>). So it will
1260 probably be necessary to issue a <literal>:continue</literal>
1261 immediately when evaluating <literal>_result</literal>. Alternatively,
1262 you can use <literal>:force</literal> which ignores breakpoints.</para>
1263 </sect2>
1264
1265 <sect2 id="tracing">
1266 <title>Tracing and history</title>
1267
1268 <para>A question that we often want to ask when debugging a program is
1269 &ldquo;how did I get here?&rdquo;. Traditional imperative debuggers
1270 usually provide some kind of stack-tracing feature that lets you see
1271 the stack of active function calls (sometimes called the &ldquo;lexical
1272 call stack&rdquo;), describing a path through the code
1273 to the current location. Unfortunately this is hard to provide in
1274 Haskell, because execution proceeds on a demand-driven basis, rather
1275 than a depth-first basis as in strict languages. The
1276 &ldquo;stack&ldquo; in GHC's execution engine bears little
1277 resemblance to the lexical call stack. Ideally GHCi would maintain a
1278 separate lexical call stack in addition to the dynamic call stack, and
1279 in fact this is exactly
1280 what our profiling system does (<xref linkend="profiling" />), and what
1281 some other Haskell debuggers do. For the time being, however, GHCi
1282 doesn't maintain a lexical call stack (there are some technical
1283 challenges to be overcome). Instead, we provide a way to backtrack from a
1284 breakpoint to previous evaluation steps: essentially this is like
1285 single-stepping backwards, and should in many cases provide enough
1286 information to answer the &ldquo;how did I get here?&rdquo;
1287 question.</para>
1288
1289 <para>To use tracing, evaluate an expression with the
1290 <literal>:trace</literal> command. For example, if we set a breakpoint
1291 on the base case of <literal>qsort</literal>:</para>
1292
1293 <screen>
1294 *Main&gt; :list qsort
1295 1 qsort [] = []
1296 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
1297 3 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
1298 4
1299 *Main&gt; :b 1
1300 Breakpoint 1 activated at qsort.hs:1:11-12
1301 *Main&gt;
1302 </screen>
1303
1304 <para>and then run a small <literal>qsort</literal> with
1305 tracing:</para>
1306
1307 <screen>
1308 *Main> :trace qsort [3,2,1]
1309 Stopped at qsort.hs:1:11-12
1310 _result :: [a]
1311 [qsort.hs:1:11-12] *Main>
1312 </screen>
1313
1314 <para>We can now inspect the history of evaluation steps:</para>
1315
1316 <screen>
1317 [qsort.hs:1:11-12] *Main> :hist
1318 -1 : qsort.hs:3:24-38
1319 -2 : qsort.hs:3:23-55
1320 -3 : qsort.hs:(1,0)-(3,55)
1321 -4 : qsort.hs:2:15-24
1322 -5 : qsort.hs:2:15-46
1323 -6 : qsort.hs:3:24-38
1324 -7 : qsort.hs:3:23-55
1325 -8 : qsort.hs:(1,0)-(3,55)
1326 -9 : qsort.hs:2:15-24
1327 -10 : qsort.hs:2:15-46
1328 -11 : qsort.hs:3:24-38
1329 -12 : qsort.hs:3:23-55
1330 -13 : qsort.hs:(1,0)-(3,55)
1331 -14 : qsort.hs:2:15-24
1332 -15 : qsort.hs:2:15-46
1333 -16 : qsort.hs:(1,0)-(3,55)
1334 &lt;end of history&gt;
1335 </screen>
1336
1337 <para>To examine one of the steps in the history, use
1338 <literal>:back</literal>:</para>
1339
1340 <screen>
1341 [qsort.hs:1:11-12] *Main> :back
1342 Logged breakpoint at qsort.hs:3:24-38
1343 _result :: [a]
1344 as :: [a]
1345 a :: a
1346 [-1: qsort.hs:3:24-38] *Main>
1347 </screen>
1348
1349 <para>Note that the local variables at each step in the history have been
1350 preserved, and can be examined as usual. Also note that the prompt has
1351 changed to indicate that we're currently examining the first step in
1352 the history: <literal>-1</literal>. The command
1353 <literal>:forward</literal> can be used to traverse forward in the
1354 history.</para>
1355
1356 <para>The <literal>:trace</literal> command can be used with or without
1357 an expression. When used without an expression, tracing begins from
1358 the current breakpoint, just like <literal>:step</literal>.</para>
1359
1360 <para>The history is only available when
1361 using <literal>:trace</literal>; the reason for this is we found that
1362 logging each breakpoint in the history cuts performance by a factor of
1363 2 or more. GHCi remembers the last 50 steps in the history (perhaps in
1364 the future we'll make this configurable).</para>
1365 </sect2>
1366
1367 <sect2 id="ghci-debugger-exceptions">
1368 <title>Debugging exceptions</title>
1369 <para>Another common question that comes up when debugging is
1370 &ldquo;where did this exception come from?&rdquo;. Exceptions such as
1371 those raised by <literal>error</literal> or <literal>head []</literal>
1372 have no context information attached to them. Finding which
1373 particular call to <literal>head</literal> in your program resulted in
1374 the error can be a painstaking process, usually involving
1375 <literal>Debug.Trace.trace</literal>, or compiling with
1376 profiling and using <literal>+RTS -xc</literal> (see <xref
1377 linkend="prof-time-options" />).</para>
1378
1379 <para>The GHCi debugger offers a way to hopefully shed some light on
1380 these errors quickly and without modifying or recompiling the source
1381 code. One way would be to set a breakpoint on the location in the
1382 source code that throws the exception, and then use
1383 <literal>:trace</literal> and <literal>:history</literal> to establish
1384 the context. However, <literal>head</literal> is in a library and
1385 we can't set a breakpoint on it directly. For this reason, GHCi
1386 provides the flags <literal>-fbreak-on-exception</literal> which causes
1387 the evaluator to stop when an exception is thrown, and <literal>
1388 -fbreak-on-error</literal>, which works similarly but stops only on
1389 uncaught exceptions. When stopping at an exception, GHCi will act
1390 just as it does when a breakpoint is hit, with the deviation that it
1391 will not show you any source code location. Due to this, these
1392 commands are only really useful in conjunction with
1393 <literal>:trace</literal>, in order to log the steps leading up to the
1394 exception. For example:</para>
1395
1396 <screen>
1397 *Main> :set -fbreak-on-exception
1398 *Main> :trace qsort ("abc" ++ undefined)
1399 "Stopped at &lt;exception thrown&gt;
1400 _exception :: e
1401 [&lt;exception thrown&gt;] *Main&gt; :hist
1402 -1 : qsort.hs:3:24-38
1403 -2 : qsort.hs:3:23-55
1404 -3 : qsort.hs:(1,0)-(3,55)
1405 -4 : qsort.hs:2:15-24
1406 -5 : qsort.hs:2:15-46
1407 -6 : qsort.hs:(1,0)-(3,55)
1408 &lt;end of history&gt;
1409 [&lt;exception thrown&gt;] *Main&gt; :back
1410 Logged breakpoint at qsort.hs:3:24-38
1411 _result :: [a]
1412 as :: [a]
1413 a :: a
1414 [-1: qsort.hs:3:24-38] *Main&gt; :force as
1415 *** Exception: Prelude.undefined
1416 [-1: qsort.hs:3:24-38] *Main&gt; :print as
1417 as = 'b' : 'c' : (_t1::[Char])
1418 </screen>
1419
1420 <para>The exception itself is bound to a new variable,
1421 <literal>_exception</literal>.</para>
1422
1423 <para>Breaking on exceptions is particularly useful for finding out what
1424 your program was doing when it was in an infinite loop. Just hit
1425 Control-C, and examine the history to find out what was going
1426 on.</para>
1427 </sect2>
1428
1429 <sect2><title>Example: inspecting functions</title>
1430 <para>
1431 It is possible to use the debugger to examine function values.
1432 When we are at a breakpoint and a function is in scope, the debugger
1433 cannot show
1434 you the source code for it; however, it is possible to get some
1435 information by applying it to some arguments and observing the result.
1436 </para>
1437
1438 <para>
1439 The process is slightly complicated when the binding is polymorphic.
1440 We show the process by means of an example.
1441 To keep things simple, we will use the well known <literal>map</literal> function:
1442 <programlisting>
1443 import Prelude hiding (map)
1444
1445 map :: (a->b) -> a -> b
1446 map f [] = []
1447 map f (x:xs) = f x : map f xs
1448 </programlisting>
1449 </para>
1450
1451 <para>
1452 We set a breakpoint on <literal>map</literal>, and call it.
1453 <screen>
1454 *Main> :break 5
1455 Breakpoint 0 activated at map.hs:5:15-28
1456 *Main> map Just [1..5]
1457 Stopped at map.hs:(4,0)-(5,12)
1458 _result :: [b]
1459 x :: a
1460 f :: a -> b
1461 xs :: [a]
1462 </screen>
1463 GHCi tells us that, among other bindings, <literal>f</literal> is in scope.
1464 However, its type is not fully known yet,
1465 and thus it is not possible to apply it to any
1466 arguments. Nevertheless, observe that the type of its first argument is the
1467 same as the type of <literal>x</literal>, and its result type is shared
1468 with <literal>_result</literal>.
1469 </para>
1470
1471 <para>
1472 As we demonstrated earlier (<xref linkend="breakpoints" />), the
1473 debugger has some intelligence built-in to update the type of
1474 <literal>f</literal> whenever the types of <literal>x</literal> or
1475 <literal>_result</literal> are discovered. So what we do in this
1476 scenario is
1477 force <literal>x</literal> a bit, in order to recover both its type
1478 and the argument part of <literal>f</literal>.
1479 <screen>
1480 *Main> seq x ()
1481 *Main> :print x
1482 x = 1
1483 </screen>
1484 </para>
1485 <para>
1486 We can check now that as expected, the type of <literal>x</literal>
1487 has been reconstructed, and with it the
1488 type of <literal>f</literal> has been too:</para>
1489 <screen>
1490 *Main> :t x
1491 x :: Integer
1492 *Main> :t f
1493 f :: Integer -> b
1494 </screen>
1495 <para>
1496 From here, we can apply f to any argument of type Integer and observe
1497 the results.
1498 <screen><![CDATA[
1499 *Main> let b = f 10
1500 *Main> :t b
1501 b :: b
1502 *Main> b
1503 <interactive>:1:0:
1504 Ambiguous type variable `b' in the constraint:
1505 `Show b' arising from a use of `print' at <interactive>:1:0
1506 *Main> :p b
1507 b = (_t2::a)
1508 *Main> seq b ()
1509 ()
1510 *Main> :t b
1511 b :: a
1512 *Main> :p b
1513 b = Just 10
1514 *Main> :t b
1515 b :: Maybe Integer
1516 *Main> :t f
1517 f :: Integer -> Maybe Integer
1518 *Main> f 20
1519 Just 20
1520 *Main> map f [1..5]
1521 [Just 1, Just 2, Just 3, Just 4, Just 5]
1522 ]]></screen>
1523 In the first application of <literal>f</literal>, we had to do
1524 some more type reconstruction
1525 in order to recover the result type of <literal>f</literal>.
1526 But after that, we are free to use
1527 <literal>f</literal> normally.
1528 </para>
1529 </sect2>
1530
1531 <sect2><title>Limitations</title>
1532 <itemizedlist>
1533 <listitem>
1534 <para>When stopped at a breakpoint, if you try to evaluate a variable
1535 that is already under evaluation, the second evaluation will hang.
1536 The reason is
1537 that GHC knows the variable is under evaluation, so the new
1538 evaluation just waits for the result before continuing, but of
1539 course this isn't going to happen because the first evaluation is
1540 stopped at a breakpoint. Control-C can interrupt the hung
1541 evaluation and return to the prompt.</para>
1542 <para>The most common way this can happen is when you're evaluating a
1543 CAF (e.g. main), stop at a breakpoint, and ask for the value of the
1544 CAF at the prompt again.</para>
1545 </listitem>
1546 <listitem><para>
1547 Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
1548 at the scope of a breakpoint if there is an explicit type signature.
1549 </para>
1550 </listitem>
1551 </itemizedlist>
1552 </sect2>
1553 </sect1>
1554
1555 <sect1 id="ghci-invocation">
1556 <title>Invoking GHCi</title>
1557 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
1558 <indexterm><primary><option>&ndash;&ndash;interactive</option></primary></indexterm>
1559
1560 <para>GHCi is invoked with the command <literal>ghci</literal> or
1561 <literal>ghc &ndash;&ndash;interactive</literal>. One or more modules or
1562 filenames can also be specified on the command line; this
1563 instructs GHCi to load the specified modules or filenames (and all
1564 the modules they depend on), just as if you had said
1565 <literal>:load <replaceable>modules</replaceable></literal> at the
1566 GHCi prompt (see <xref linkend="ghci-commands" />). For example, to
1567 start GHCi and load the program whose topmost module is in the
1568 file <literal>Main.hs</literal>, we could say:</para>
1569
1570 <screen>
1571 $ ghci Main.hs
1572 </screen>
1573
1574 <para>Most of the command-line options accepted by GHC (see <xref
1575 linkend="using-ghc"/>) also make sense in interactive mode. The ones
1576 that don't make sense are mostly obvious.</para>
1577
1578 <sect2>
1579 <title>Packages</title>
1580 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
1581
1582 <para>Most packages (see <xref linkend="using-packages"/>) are
1583 available without needing to specify any extra flags at all:
1584 they will be automatically loaded the first time they are
1585 needed.</para>
1586
1587 <para>For hidden packages, however, you need to request the
1588 package be loaded by using the <literal>-package</literal> flag:</para>
1589
1590 <screen>
1591 $ ghci -package readline
1592 GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
1593 Loading package base ... linking ... done.
1594 Loading package readline-1.0 ... linking ... done.
1595 Prelude>
1596 </screen>
1597
1598 <para>The following command works to load new packages into a
1599 running GHCi:</para>
1600
1601 <screen>
1602 Prelude> :set -package <replaceable>name</replaceable>
1603 </screen>
1604
1605 <para>But note that doing this will cause all currently loaded
1606 modules to be unloaded, and you'll be dumped back into the
1607 <literal>Prelude</literal>.</para>
1608 </sect2>
1609
1610 <sect2>
1611 <title>Extra libraries</title>
1612 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
1613
1614 <para>Extra libraries may be specified on the command line using
1615 the normal <literal>-l<replaceable>lib</replaceable></literal>
1616 option. (The term <emphasis>library</emphasis> here refers to
1617 libraries of foreign object code; for using libraries of Haskell
1618 source code, see <xref linkend="ghci-modules-filenames"/>.) For
1619 example, to load the &ldquo;m&rdquo; library:</para>
1620
1621 <screen>
1622 $ ghci -lm
1623 </screen>
1624
1625 <para>On systems with <literal>.so</literal>-style shared
1626 libraries, the actual library loaded will the
1627 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
1628 searches the following places for libraries, in this order:</para>
1629
1630 <itemizedlist>
1631 <listitem>
1632 <para>Paths specified using the
1633 <literal>-L<replaceable>path</replaceable></literal>
1634 command-line option,</para>
1635 </listitem>
1636 <listitem>
1637 <para>the standard library search path for your system,
1638 which on some systems may be overridden by setting the
1639 <literal>LD_LIBRARY_PATH</literal> environment
1640 variable.</para>
1641 </listitem>
1642 </itemizedlist>
1643
1644 <para>On systems with <literal>.dll</literal>-style shared
1645 libraries, the actual library loaded will be
1646 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
1647 GHCi will signal an error if it can't find the library.</para>
1648
1649 <para>GHCi can also load plain object files
1650 (<literal>.o</literal> or <literal>.obj</literal> depending on
1651 your platform) from the command-line. Just add the name the
1652 object file to the command line.</para>
1653
1654 <para>Ordering of <option>-l</option> options matters: a library
1655 should be mentioned <emphasis>before</emphasis> the libraries it
1656 depends on (see <xref linkend="options-linker"/>).</para>
1657 </sect2>
1658
1659 </sect1>
1660
1661 <sect1 id="ghci-commands">
1662 <title>GHCi commands</title>
1663
1664 <para>GHCi commands all begin with
1665 &lsquo;<literal>:</literal>&rsquo; and consist of a single command
1666 name followed by zero or more parameters. The command name may be
1667 abbreviated, with ambiguities being resolved in favour of the more
1668 commonly used commands.</para>
1669
1670 <variablelist>
1671 <varlistentry>
1672 <term>
1673 <literal>:abandon</literal>
1674 <indexterm><primary><literal>:abandon</literal></primary></indexterm>
1675 </term>
1676 <listitem>
1677 <para>Abandons the current evaluation (only available when stopped at
1678 a breakpoint).</para>
1679 </listitem>
1680 </varlistentry>
1681
1682 <varlistentry>
1683 <term>
1684 <literal>:add</literal> <replaceable>module</replaceable> ...
1685 <indexterm><primary><literal>:add</literal></primary></indexterm>
1686 </term>
1687 <listitem>
1688 <para>Add <replaceable>module</replaceable>(s) to the
1689 current <firstterm>target set</firstterm>, and perform a
1690 reload.</para>
1691 </listitem>
1692 </varlistentry>
1693
1694 <varlistentry>
1695 <term>
1696 <literal>:back</literal>
1697 <indexterm><primary><literal>:back</literal></primary></indexterm>
1698 </term>
1699 <listitem>
1700 <para>Travel back one step in the history. See <xref
1701 linkend="tracing" />. See also:
1702 <literal>:trace</literal>, <literal>:history</literal>,
1703 <literal>:forward</literal>.</para>
1704 </listitem>
1705 </varlistentry>
1706
1707 <varlistentry>
1708 <term>
1709 <literal>:break [<replaceable>identifier</replaceable> |
1710 [<replaceable>module</replaceable>] <replaceable>line</replaceable>
1711 [<replaceable>column</replaceable>]]</literal>
1712 </term>
1713 <indexterm><primary><literal>:break</literal></primary></indexterm>
1714 <listitem>
1715 <para>Set a breakpoint on the specified function or line and
1716 column. See <xref linkend="setting-breakpoints" />.</para>
1717 </listitem>
1718 </varlistentry>
1719
1720 <varlistentry>
1721 <term>
1722 <literal>:browse</literal><optional><literal>!</literal></optional> <optional><optional><literal>*</literal></optional><replaceable>module</replaceable></optional> ...
1723 <indexterm><primary><literal>:browse</literal></primary></indexterm>
1724 </term>
1725 <listitem>
1726 <para>Displays the identifiers defined by the module
1727 <replaceable>module</replaceable>, which must be either
1728 loaded into GHCi or be a member of a package. If
1729 <replaceable>module</replaceable> is omitted, the most
1730 recently-loaded module is used.</para>
1731
1732 <para>If the <literal>*</literal> symbol is placed before
1733 the module name, then <emphasis>all</emphasis> the
1734 identifiers in scope in <replaceable>module</replaceable> are
1735 shown; otherwise the list is limited to the exports of
1736 <replaceable>module</replaceable>. The
1737 <literal>*</literal>-form is only available for modules
1738 which are interpreted; for compiled modules (including
1739 modules from packages) only the non-<literal>*</literal>
1740 form of <literal>:browse</literal> is available.
1741 If the <literal>!</literal> symbol is appended to the
1742 command, data constructors and class methods will be
1743 listed individually, otherwise, they will only be listed
1744 in the context of their data type or class declaration.
1745 The <literal>!</literal>-form also annotates the listing
1746 with comments giving possible imports for each group of
1747 entries.</para>
1748 </listitem>
1749 </varlistentry>
1750
1751 <varlistentry>
1752 <term>
1753 <literal>:cd</literal> <replaceable>dir</replaceable>
1754 <indexterm><primary><literal>:cd</literal></primary></indexterm>
1755 </term>
1756 <listitem>
1757 <para>Changes the current working directory to
1758 <replaceable>dir</replaceable>. A
1759 &lsquo;<literal>&tilde;</literal>&rsquo; symbol at the
1760 beginning of <replaceable>dir</replaceable> will be replaced
1761 by the contents of the environment variable
1762 <literal>HOME</literal>.</para>
1763
1764 <para>NOTE: changing directories causes all currently loaded
1765 modules to be unloaded. This is because the search path is
1766 usually expressed using relative directories, and changing
1767 the search path in the middle of a session is not
1768 supported.</para>
1769 </listitem>
1770 </varlistentry>
1771
1772 <varlistentry>
1773 <term>
1774 <literal>:cmd</literal> <replaceable>expr</replaceable>
1775 <indexterm><primary><literal>:cmd</literal></primary></indexterm>
1776 </term>
1777 <listitem>
1778 <para>Executes <replaceable>expr</replaceable> as a computation of
1779 type <literal>IO String</literal>, and then executes the resulting
1780 string as a list of GHCi commands. Multiple commands are separated
1781 by newlines. The <literal>:cmd</literal> command is useful with
1782 <literal>:def</literal> and <literal>:set stop</literal>.</para>
1783 </listitem>
1784 </varlistentry>
1785
1786 <varlistentry>
1787 <term>
1788 <literal>:continue</literal>
1789 <indexterm><primary><literal>:continue</literal></primary></indexterm>
1790 </term>
1791 <listitem><para>Continue the current evaluation, when stopped at a
1792 breakpoint.</para>
1793 </listitem>
1794 </varlistentry>
1795
1796 <varlistentry>
1797 <term>
1798 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1799 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1800 <indexterm><primary><literal>:etags</literal></primary>
1801 </indexterm>
1802 <indexterm><primary><literal>:etags</literal></primary>
1803 </indexterm>
1804 </term>
1805 <listitem>
1806 <para>Generates a &ldquo;tags&rdquo; file for Vi-style editors
1807 (<literal>:ctags</literal>) or
1808 Emacs-style editors (<literal>:etags</literal>). If
1809 no filename is specified, the default <filename>tags</filename> or
1810 <filename>TAGS</filename> is
1811 used, respectively. Tags for all the functions, constructors and
1812 types in the currently loaded modules are created. All modules must
1813 be interpreted for these commands to work.</para>
1814 <para>See also <xref linkend="hasktags" />.</para>
1815 </listitem>
1816 </varlistentry>
1817
1818 <varlistentry>
1819 <term>
1820 <literal>:def<optional>!</optional> <optional><replaceable>name</replaceable> <replaceable>expr</replaceable></optional></literal>
1821 <indexterm><primary><literal>:def</literal></primary></indexterm>
1822 </term>
1823 <listitem>
1824 <para><literal>:def</literal> is used to define new
1825 commands, or macros, in GHCi. The command
1826 <literal>:def</literal> <replaceable>name</replaceable>
1827 <replaceable>expr</replaceable> defines a new GHCi command
1828 <literal>:<replaceable>name</replaceable></literal>,
1829 implemented by the Haskell expression
1830 <replaceable>expr</replaceable>, which must have type
1831 <literal>String -> IO String</literal>. When
1832 <literal>:<replaceable>name</replaceable>
1833 <replaceable>args</replaceable></literal> is typed at the
1834 prompt, GHCi will run the expression
1835 <literal>(<replaceable>name</replaceable>
1836 <replaceable>args</replaceable>)</literal>, take the
1837 resulting <literal>String</literal>, and feed it back into
1838 GHCi as a new sequence of commands. Separate commands in
1839 the result must be separated by
1840 &lsquo;<literal>\n</literal>&rsquo;.</para>
1841
1842 <para>That's all a little confusing, so here's a few
1843 examples. To start with, here's a new GHCi command which
1844 doesn't take any arguments or produce any results, it just
1845 outputs the current date &amp; time:</para>
1846
1847 <screen>
1848 Prelude> let date _ = Time.getClockTime >>= print >> return ""
1849 Prelude> :def date date
1850 Prelude> :date
1851 Fri Mar 23 15:16:40 GMT 2001
1852 </screen>
1853
1854 <para>Here's an example of a command that takes an argument.
1855 It's a re-implementation of <literal>:cd</literal>:</para>
1856
1857 <screen>
1858 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
1859 Prelude> :def mycd mycd
1860 Prelude> :mycd ..
1861 </screen>
1862
1863 <para>Or I could define a simple way to invoke
1864 &ldquo;<literal>ghc &ndash;&ndash;make Main</literal>&rdquo; in the
1865 current directory:</para>
1866
1867 <screen>
1868 Prelude> :def make (\_ -> return ":! ghc &ndash;&ndash;make Main")
1869 </screen>
1870
1871 <para>We can define a command that reads GHCi input from a
1872 file. This might be useful for creating a set of bindings
1873 that we want to repeatedly load into the GHCi session:</para>
1874
1875 <screen>
1876 Prelude> :def . readFile
1877 Prelude> :. cmds.ghci
1878 </screen>
1879
1880 <para>Notice that we named the command
1881 <literal>:.</literal>, by analogy with the
1882 &lsquo;<literal>.</literal>&rsquo; Unix shell command that
1883 does the same thing.</para>
1884
1885 <para>Typing <literal>:def</literal> on its own lists the
1886 currently-defined macros. Attempting to redefine an
1887 existing command name results in an error unless the
1888 <literal>:def!</literal> form is used, in which case the old
1889 command with that name is silently overwritten.</para>
1890 </listitem>
1891 </varlistentry>
1892
1893 <varlistentry>
1894 <term>
1895 <literal>:delete * | <replaceable>num</replaceable> ...</literal>
1896 <indexterm><primary><literal>:delete</literal></primary></indexterm>
1897 </term>
1898 <listitem>
1899 <para>Delete one or more breakpoints by number (use <literal>:show
1900 breaks</literal> to see the number of each breakpoint). The
1901 <literal>*</literal> form deletes all the breakpoints.</para>
1902 </listitem>
1903 </varlistentry>
1904
1905 <varlistentry>
1906 <term>
1907 <literal>:edit <optional><replaceable>file</replaceable></optional></literal>
1908 <indexterm><primary><literal>:edit</literal></primary></indexterm>
1909 </term>
1910 <listitem>
1911 <para>Opens an editor to edit the file
1912 <replaceable>file</replaceable>, or the most recently loaded
1913 module if <replaceable>file</replaceable> is omitted. The
1914 editor to invoke is taken from the <literal>EDITOR</literal>
1915 environment variable, or a default editor on your system if
1916 <literal>EDITOR</literal> is not set. You can change the
1917 editor using <literal>:set editor</literal>.</para>
1918 </listitem>
1919 </varlistentry>
1920
1921 <varlistentry>
1922 <term>
1923 <literal>:etags</literal>
1924 </term>
1925 <listitem>
1926 <para>See <literal>:ctags</literal>.</para>
1927 </listitem>
1928 </varlistentry>
1929
1930 <varlistentry>
1931 <term>
1932 <literal>:force <replaceable>identifier</replaceable> ...</literal>
1933 <indexterm><primary><literal>:force</literal></primary></indexterm>
1934 </term>
1935 <listitem>
1936 <para>Prints the value of <replaceable>identifier</replaceable> in
1937 the same way as <literal>:print</literal>. Unlike
1938 <literal>:print</literal>, <literal>:force</literal> evaluates each
1939 thunk that it encounters while traversing the value. This may
1940 cause exceptions or infinite loops, or further breakpoints (which
1941 are ignored, but displayed).</para>
1942 </listitem>
1943 </varlistentry>
1944
1945 <varlistentry>
1946 <term>
1947 <literal>:forward</literal>
1948 <indexterm><primary><literal>:forward</literal></primary></indexterm>
1949 </term>
1950 <listitem>
1951 <para>Move forward in the history. See <xref
1952 linkend="tracing" />. See also:
1953 <literal>:trace</literal>, <literal>:history</literal>,
1954 <literal>:back</literal>.</para>
1955 </listitem>
1956 </varlistentry>
1957
1958 <varlistentry>
1959 <term>
1960 <literal>:help</literal>
1961 <indexterm><primary><literal>:help</literal></primary></indexterm>
1962 </term>
1963 <term>
1964 <literal>:?</literal>
1965 <indexterm><primary><literal>:?</literal></primary></indexterm>
1966 </term>
1967 <listitem>
1968 <para>Displays a list of the available commands.</para>
1969 </listitem>
1970 </varlistentry>
1971
1972 <varlistentry>
1973 <term>
1974 <literal>:history [<replaceable>num</replaceable>]</literal>
1975 <indexterm><primary><literal>:history</literal></primary></indexterm>
1976 </term>
1977 <listitem>
1978 <para>Display the history of evaluation steps. With a number,
1979 displays that many steps (default: 20). For use with
1980 <literal>:trace</literal>; see <xref
1981 linkend="tracing" />.</para>
1982 </listitem>
1983 </varlistentry>
1984
1985 <varlistentry>
1986 <term>
1987 <literal>:info</literal> <replaceable>name</replaceable> ...
1988 <indexterm><primary><literal>:info</literal></primary></indexterm>
1989 </term>
1990 <listitem>
1991 <para>Displays information about the given name(s). For
1992 example, if <replaceable>name</replaceable> is a class, then
1993 the class methods and their types will be printed; if
1994 <replaceable>name</replaceable> is a type constructor, then
1995 its definition will be printed; if
1996 <replaceable>name</replaceable> is a function, then its type
1997 will be printed. If <replaceable>name</replaceable> has
1998 been loaded from a source file, then GHCi will also display
1999 the location of its definition in the source.</para>
2000 <para>For types and classes, GHCi also summarises instances that
2001 mention them. To avoid showing irrelevant information, an instance
2002 is shown only if (a) its head mentions <replaceable>name</replaceable>,
2003 and (b) all the other things mentioned in the instance
2004 are in scope (either qualified or otherwise) as a result of
2005 a <literal>:load</literal> or <literal>:module</literal> commands. </para>
2006 </listitem>
2007 </varlistentry>
2008
2009 <varlistentry>
2010 <term>
2011 <literal>:kind</literal> <replaceable>type</replaceable>
2012 <indexterm><primary><literal>:kind</literal></primary></indexterm>
2013 </term>
2014 <listitem>
2015 <para>Infers and prints the kind of
2016 <replaceable>type</replaceable>. The latter can be an arbitrary
2017 type expression, including a partial application of a type constructor,
2018 such as <literal>Either Int</literal>.</para>
2019 </listitem>
2020 </varlistentry>
2021
2022 <varlistentry>
2023 <term>
2024 <literal>:load</literal> <replaceable>module</replaceable> ...
2025 <indexterm><primary><literal>:load</literal></primary></indexterm>
2026 </term>
2027 <listitem>
2028 <para>Recursively loads the specified
2029 <replaceable>module</replaceable>s, and all the modules they
2030 depend on. Here, each <replaceable>module</replaceable>
2031 must be a module name or filename, but may not be the name
2032 of a module in a package.</para>
2033
2034 <para>All previously loaded modules, except package modules,
2035 are forgotten. The new set of modules is known as the
2036 <firstterm>target set</firstterm>. Note that
2037 <literal>:load</literal> can be used without any arguments
2038 to unload all the currently loaded modules and
2039 bindings.</para>
2040
2041 <para>After a <literal>:load</literal> command, the current
2042 context is set to:</para>
2043
2044 <itemizedlist>
2045 <listitem>
2046 <para><replaceable>module</replaceable>, if it was loaded
2047 successfully, or</para>
2048 </listitem>
2049 <listitem>
2050 <para>the most recently successfully loaded module, if
2051 any other modules were loaded as a result of the current
2052 <literal>:load</literal>, or</para>
2053 </listitem>
2054 <listitem>
2055 <para><literal>Prelude</literal> otherwise.</para>
2056 </listitem>
2057 </itemizedlist>
2058 </listitem>
2059 </varlistentry>
2060
2061 <varlistentry>
2062 <term>
2063 <literal>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
2064 <indexterm><primary><literal>:main</literal></primary></indexterm>
2065 </term>
2066 <listitem>
2067 <para>
2068 When a program is compiled and executed, it can use the
2069 <literal>getArgs</literal> function to access the
2070 command-line arguments.
2071 However, we cannot simply pass the arguments to the
2072 <literal>main</literal> function while we are testing in ghci,
2073 as the <literal>main</literal> function doesn't take its
2074 arguments directly.
2075 </para>
2076
2077 <para>
2078 Instead, we can use the <literal>:main</literal> command.
2079 This runs whatever <literal>main</literal> is in scope, with
2080 any arguments being treated the same as command-line arguments,
2081 e.g.:
2082 </para>
2083
2084 <screen>
2085 Prelude> let main = System.Environment.getArgs >>= print
2086 Prelude> :main foo bar
2087 ["foo","bar"]
2088 </screen>
2089
2090 </listitem>
2091 </varlistentry>
2092
2093 <varlistentry>
2094 <term>
2095 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
2096 <indexterm><primary><literal>:module</literal></primary></indexterm>
2097 </term>
2098 <term>
2099 <literal>import <replaceable>mod</replaceable></literal>
2100 </term>
2101 <listitem>
2102 <para>Sets or modifies the current context for statements
2103 typed at the prompt. The form <literal>import
2104 <replaceable>mod</replaceable></literal> is equivalent to
2105 <literal>:module +<replaceable>mod</replaceable></literal>.
2106 See <xref linkend="ghci-scope"/> for
2107 more details.</para>
2108 </listitem>
2109 </varlistentry>
2110
2111 <varlistentry>
2112 <term>
2113 <literal>:print </literal> <replaceable>names</replaceable> ...
2114 <indexterm><primary><literal>:print</literal></primary></indexterm>
2115 </term>
2116 <listitem>
2117 <para>Prints a value without forcing its evaluation.
2118 <literal>:print</literal> may be used on values whose types are
2119 unknown or partially known, which might be the case for local
2120 variables with polymorphic types at a breakpoint. While inspecting
2121 the runtime value, <literal>:print</literal> attempts to
2122 reconstruct the type of the value, and will elaborate the type in
2123 GHCi's environment if possible. If any unevaluated components
2124 (thunks) are encountered, then <literal>:print</literal> binds
2125 a fresh variable with a name beginning with <literal>_t</literal>
2126 to each thunk. See <xref linkend="breakpoints" /> for more
2127 information. See also the <literal>:sprint</literal> command,
2128 which works like <literal>:print</literal> but does not bind new
2129 variables.</para>
2130 </listitem>
2131 </varlistentry>
2132
2133 <varlistentry>
2134 <term>
2135 <literal>:quit</literal>
2136 <indexterm><primary><literal>:quit</literal></primary></indexterm>
2137 </term>
2138 <listitem>
2139 <para>Quits GHCi. You can also quit by typing control-D
2140 at the prompt.</para>
2141 </listitem>
2142 </varlistentry>
2143
2144 <varlistentry>
2145 <term>
2146 <literal>:reload</literal>
2147 <indexterm><primary><literal>:reload</literal></primary></indexterm>
2148 </term>
2149 <listitem>
2150 <para>Attempts to reload the current target set (see
2151 <literal>:load</literal>) if any of the modules in the set,
2152 or any dependent module, has changed. Note that this may
2153 entail loading new modules, or dropping modules which are no
2154 longer indirectly required by the target.</para>
2155 </listitem>
2156 </varlistentry>
2157
2158 <varlistentry>
2159 <term>
2160 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
2161 <indexterm><primary><literal>:set</literal></primary></indexterm>
2162 </term>
2163 <listitem>
2164 <para>Sets various options. See <xref linkend="ghci-set"/> for a list of
2165 available options and <xref linkend="interactive-mode-options"/> for a
2166 list of GHCi-specific flags. The <literal>:set</literal> command by
2167 itself shows which options are currently set. It also lists the current
2168 dynamic flag settings, with GHCi-specific flags listed separately.</para>
2169 </listitem>
2170 </varlistentry>
2171
2172 <varlistentry>
2173 <term>
2174 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
2175 <indexterm><primary><literal>:set args</literal></primary></indexterm>
2176 </term>
2177 <listitem>
2178 <para>Sets the list of arguments which are returned when the
2179 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
2180 </indexterm>.</para>
2181 </listitem>
2182 </varlistentry>
2183
2184 <varlistentry>
2185 <term>
2186 <literal>:set</literal> <literal>editor</literal> <replaceable>cmd</replaceable>
2187 </term>
2188 <listitem>
2189 <para>Sets the command used by <literal>:edit</literal> to
2190 <replaceable>cmd</replaceable>.</para>
2191 </listitem>
2192 </varlistentry>
2193
2194 <varlistentry>
2195 <term>
2196 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
2197 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
2198 </term>
2199 <listitem>
2200 <para>Sets the string to be returned when the program calls
2201 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
2202 </indexterm>.</para>
2203 </listitem>
2204 </varlistentry>
2205
2206 <varlistentry>
2207 <term>
2208 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
2209 </term>
2210 <listitem>
2211 <para>Sets the string to be used as the prompt in GHCi.
2212 Inside <replaceable>prompt</replaceable>, the sequence
2213 <literal>%s</literal> is replaced by the names of the
2214 modules currently in scope, and <literal>%%</literal> is
2215 replaced by <literal>%</literal>.</para>
2216 </listitem>
2217 </varlistentry>
2218
2219 <varlistentry>
2220 <term>
2221 <literal>:set</literal> <literal>stop</literal>
2222 [<replaceable>num</replaceable>] <replaceable>cmd</replaceable>
2223 </term>
2224 <listitem>
2225 <para>Set a command to be executed when a breakpoint is hit, or a new
2226 item in the history is selected. The most common use of
2227 <literal>:set stop</literal> is to display the source code at the
2228 current location, e.g. <literal>:set stop :list</literal>.</para>
2229
2230 <para>If a number is given before the command, then the commands are
2231 run when the specified breakpoint (only) is hit. This can be quite
2232 useful: for example, <literal>:set stop 1 :continue</literal>
2233 effectively disables breakpoint 1, by running
2234 <literal>:continue</literal> whenever it is hit (although GHCi will
2235 still emit a message to say the breakpoint was hit). What's more,
2236 with cunning use of <literal>:def</literal> and
2237 <literal>:cmd</literal> you can use <literal>:set stop</literal> to
2238 implement conditional breakpoints:</para>
2239 <screen>
2240 *Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
2241 *Main> :set stop 0 :cond (x &lt; 3)
2242 </screen>
2243 <para>Ignoring breakpoints for a specified number of iterations is
2244 also possible using similar techniques.</para>
2245 </listitem>
2246 </varlistentry>
2247
2248 <varlistentry>
2249 <term>
2250 <literal>:show bindings</literal>
2251 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
2252 </term>
2253 <listitem>
2254 <para>Show the bindings made at the prompt and their
2255 types.</para>
2256 </listitem>
2257 </varlistentry>
2258
2259 <varlistentry>
2260 <term>
2261 <literal>:show breaks</literal>
2262 <indexterm><primary><literal>:show breaks</literal></primary></indexterm>
2263 </term>
2264 <listitem>
2265 <para>List the active breakpoints.</para>
2266 </listitem>
2267 </varlistentry>
2268
2269 <varlistentry>
2270 <term>
2271 <literal>:show context</literal>
2272 <indexterm><primary><literal>:show context</literal></primary></indexterm>
2273 </term>
2274 <listitem>
2275 <para>List the active evaluations that are stopped at breakpoints.</para>
2276 </listitem>
2277 </varlistentry>
2278
2279 <varlistentry>
2280 <term>
2281 <literal>:show modules</literal>
2282 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
2283 </term>
2284 <listitem>
2285 <para>Show the list of modules currently loaded.</para>
2286 </listitem>
2287 </varlistentry>
2288
2289 <varlistentry>
2290 <term>
2291 <literal>:show packages</literal>
2292 <indexterm><primary><literal>:show packages</literal></primary></indexterm>
2293 </term>
2294 <listitem>
2295 <para>Show the currently active package flags, as well as the list of
2296 packages currently loaded.</para>
2297 </listitem>
2298 </varlistentry>
2299
2300 <varlistentry>
2301 <term>
2302 <literal>:show languages</literal>
2303 <indexterm><primary><literal>:show languages</literal></primary></indexterm>
2304 </term>
2305 <listitem>
2306 <para>Show the currently active language flags.</para>
2307 </listitem>
2308 </varlistentry>
2309
2310
2311 <varlistentry>
2312 <term>
2313 <literal>:show [args|prog|prompt|editor|stop]</literal>
2314 <indexterm><primary><literal>:show</literal></primary></indexterm>
2315 </term>
2316 <listitem>
2317 <para>Displays the specified setting (see
2318 <literal>:set</literal>).</para>
2319 </listitem>
2320 </varlistentry>
2321
2322 <varlistentry>
2323 <term>
2324 <literal>:sprint</literal>
2325 <indexterm><primary><literal>:sprint</literal></primary></indexterm>
2326 </term>
2327 <listitem>
2328 <para>Prints a value without forcing its evaluation.
2329 <literal>:sprint</literal> is similar to <literal>:print</literal>,
2330 with the difference that unevaluated subterms are not bound to new
2331 variables, they are simply denoted by &lsquo;_&rsquo;.</para>
2332 </listitem>
2333 </varlistentry>
2334
2335 <varlistentry>
2336 <term>
2337 <literal>:step [<replaceable>expr</replaceable>]</literal>
2338 <indexterm><primary><literal>:step</literal></primary></indexterm>
2339 </term>
2340 <listitem>
2341 <para>Single-step from the last breakpoint. With an expression
2342 argument, begins evaluation of the expression with a
2343 single-step.</para>
2344 </listitem>
2345 </varlistentry>
2346
2347 <varlistentry>
2348 <term>
2349 <literal>:trace [<replaceable>expr</replaceable>]</literal>
2350 <indexterm><primary><literal>:trace</literal></primary></indexterm>
2351 </term>
2352 <listitem>
2353 <para>Evaluates the given expression (or from the last breakpoint if
2354 no expression is given), and additionally logs the evaluation
2355 steps for later inspection using <literal>:history</literal>. See
2356 <xref linkend="tracing" />.</para>
2357 </listitem>
2358 </varlistentry>
2359
2360 <varlistentry>
2361 <term>
2362 <literal>:type</literal> <replaceable>expression</replaceable>
2363 <indexterm><primary><literal>:type</literal></primary></indexterm>
2364 </term>
2365 <listitem>
2366 <para>Infers and prints the type of
2367 <replaceable>expression</replaceable>, including explicit
2368 forall quantifiers for polymorphic types. The monomorphism
2369 restriction is <emphasis>not</emphasis> applied to the
2370 expression during type inference.</para>
2371 </listitem>
2372 </varlistentry>
2373
2374 <varlistentry>
2375 <term>
2376 <literal>:undef</literal> <replaceable>name</replaceable>
2377 <indexterm><primary><literal>:undef</literal></primary></indexterm>
2378 </term>
2379 <listitem>
2380 <para>Undefines the user-defined command
2381 <replaceable>name</replaceable> (see <literal>:def</literal>
2382 above).</para>
2383 </listitem>
2384 </varlistentry>
2385
2386 <varlistentry>
2387 <term>
2388 <literal>:unset</literal> <replaceable>option</replaceable>...
2389 <indexterm><primary><literal>:unset</literal></primary></indexterm>
2390 </term>
2391 <listitem>
2392 <para>Unsets certain options. See <xref linkend="ghci-set"/>
2393 for a list of available options.</para>
2394 </listitem>
2395 </varlistentry>
2396
2397 <varlistentry>
2398 <term>
2399 <literal>:!</literal> <replaceable>command</replaceable>...
2400 <indexterm><primary><literal>:!</literal></primary></indexterm>
2401 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
2402 </term>
2403 <listitem>
2404 <para>Executes the shell command
2405 <replaceable>command</replaceable>.</para>
2406 </listitem>
2407 </varlistentry>
2408
2409 </variablelist>
2410 </sect1>
2411
2412 <sect1 id="ghci-set">
2413 <title>The <literal>:set</literal> command</title>
2414 <indexterm><primary><literal>:set</literal></primary></indexterm>
2415
2416 <para>The <literal>:set</literal> command sets two types of
2417 options: GHCi options, which begin with
2418 &lsquo;<literal>+</literal>&rsquo;, and &ldquo;command-line&rdquo;
2419 options, which begin with &lsquo;-&rsquo;. </para>
2420
2421 <para>NOTE: at the moment, the <literal>:set</literal> command
2422 doesn't support any kind of quoting in its arguments: quotes will
2423 not be removed and cannot be used to group words together. For
2424 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
2425 you expect.</para>
2426
2427 <sect2>
2428 <title>GHCi options</title>
2429 <indexterm><primary>options</primary><secondary>GHCi</secondary>
2430 </indexterm>
2431
2432 <para>GHCi options may be set using <literal>:set</literal> and
2433 unset using <literal>:unset</literal>.</para>
2434
2435 <para>The available GHCi options are:</para>
2436
2437 <variablelist>
2438 <varlistentry>
2439 <term>
2440 <literal>+r</literal>
2441 <indexterm><primary><literal>+r</literal></primary></indexterm>
2442 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
2443 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
2444 </term>
2445 <listitem>
2446 <para>Normally, any evaluation of top-level expressions
2447 (otherwise known as CAFs or Constant Applicative Forms) in
2448 loaded modules is retained between evaluations. Turning
2449 on <literal>+r</literal> causes all evaluation of
2450 top-level expressions to be discarded after each
2451 evaluation (they are still retained
2452 <emphasis>during</emphasis> a single evaluation).</para>
2453
2454 <para>This option may help if the evaluated top-level
2455 expressions are consuming large amounts of space, or if
2456 you need repeatable performance measurements.</para>
2457 </listitem>
2458 </varlistentry>
2459
2460 <varlistentry>
2461 <term>
2462 <literal>+s</literal>
2463 <indexterm><primary><literal>+s</literal></primary></indexterm>
2464 </term>
2465 <listitem>
2466 <para>Display some stats after evaluating each expression,
2467 including the elapsed time and number of bytes allocated.
2468 NOTE: the allocation figure is only accurate to the size
2469 of the storage manager's allocation area, because it is
2470 calculated at every GC. Hence, you might see values of
2471 zero if no GC has occurred.</para>
2472 </listitem>
2473 </varlistentry>
2474
2475 <varlistentry>
2476 <term>
2477 <literal>+t</literal>
2478 <indexterm><primary><literal>+t</literal></primary></indexterm>
2479 </term>
2480 <listitem>
2481 <para>Display the type of each variable bound after a
2482 statement is entered at the prompt. If the statement is a
2483 single expression, then the only variable binding will be
2484 for the variable
2485 &lsquo;<literal>it</literal>&rsquo;.</para>
2486 </listitem>
2487 </varlistentry>
2488 </variablelist>
2489 </sect2>
2490
2491 <sect2 id="ghci-cmd-line-options">
2492 <title>Setting GHC command-line options in GHCi</title>
2493
2494 <para>Normal GHC command-line options may also be set using
2495 <literal>:set</literal>. For example, to turn on
2496 <option>-fglasgow-exts</option>, you would say:</para>
2497
2498 <screen>
2499 Prelude> :set -fglasgow-exts
2500 </screen>
2501
2502 <para>Any GHC command-line option that is designated as
2503 <firstterm>dynamic</firstterm> (see the table in <xref
2504 linkend="flag-reference"/>), may be set using
2505 <literal>:set</literal>. To unset an option, you can set the
2506 reverse option:</para>
2507 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
2508
2509 <screen>
2510 Prelude> :set -fno-glasgow-exts
2511 </screen>
2512
2513 <para><xref linkend="flag-reference"/> lists the reverse for each
2514 option where applicable.</para>
2515
2516 <para>Certain static options (<option>-package</option>,
2517 <option>-I</option>, <option>-i</option>, and
2518 <option>-l</option> in particular) will also work, but some may
2519 not take effect until the next reload.</para>
2520 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
2521 </sect2>
2522 </sect1>
2523 <sect1 id="ghci-dot-files">
2524 <title>The <filename>.ghci</filename> file</title>
2525 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
2526 </indexterm>
2527 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
2528 </indexterm>
2529
2530 <para>When it starts, unless the <literal>-ignore-dot-ghci</literal>
2531 flag is given, GHCi reads and executes commands from
2532 <filename>./.ghci</filename>, followed by
2533 <filename>$HOME/.ghci</filename>.</para>
2534
2535 <para>The <filename>.ghci</filename> in your home directory is
2536 most useful for turning on favourite options (eg. <literal>:set
2537 +s</literal>), and defining useful macros. Placing a
2538 <filename>.ghci</filename> file in a directory with a Haskell
2539 project is a useful way to set certain project-wide options so you
2540 don't have to type them everytime you start GHCi: eg. if your
2541 project uses GHC extensions and CPP, and has source files in three
2542 subdirectories A, B and C, you might put the following lines in
2543 <filename>.ghci</filename>:</para>
2544
2545 <screen>
2546 :set -fglasgow-exts -cpp
2547 :set -iA:B:C
2548 </screen>
2549
2550 <para>(Note that strictly speaking the <option>-i</option> flag is
2551 a static one, but in fact it works to set it using
2552 <literal>:set</literal> like this. The changes won't take effect
2553 until the next <literal>:load</literal>, though.)</para>
2554
2555 <para>Two command-line options control whether the
2556 <filename>.ghci</filename> files are read:</para>
2557
2558 <variablelist>
2559 <varlistentry>
2560 <term>
2561 <option>-ignore-dot-ghci</option>
2562 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
2563 </term>
2564 <listitem>
2565 <para>Don't read either <filename>./.ghci</filename> or
2566 <filename>$HOME/.ghci</filename> when starting up.</para>
2567 </listitem>
2568 </varlistentry>
2569 <varlistentry>
2570 <term>
2571 <option>-read-dot-ghci</option>
2572 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
2573 </term>
2574 <listitem>
2575 <para>Read <filename>.ghci</filename> and
2576 <filename>$HOME/.ghci</filename>. This is normally the
2577 default, but the <option>-read-dot-ghci</option> option may
2578 be used to override a previous
2579 <option>-ignore-dot-ghci</option> option.</para>
2580 </listitem>
2581 </varlistentry>
2582 </variablelist>
2583
2584 </sect1>
2585
2586 <sect1 id="ghci-obj">
2587 <title>Compiling to object code inside GHCi</title>
2588
2589 <para>By default, GHCi compiles Haskell source code into byte-code
2590 that is interpreted by the runtime system. GHCi can also compile
2591 Haskell code to object code: to turn on this feature, use the
2592 <option>-fobject-code</option> flag either on the command line or
2593 with <literal>:set</literal> (the option
2594 <option>-fbyte-code</option> restores byte-code compilation
2595 again). Compiling to object code takes longer, but typically the
2596 code will execute 10-20 times faster than byte-code.</para>
2597
2598 <para>Compiling to object code inside GHCi is particularly useful
2599 if you are developing a compiled application, because the
2600 <literal>:reload</literal> command typically runs much faster than
2601 restarting GHC with <option>--make</option> from the command-line,
2602 because all the interface files are already cached in
2603 memory.</para>
2604
2605 <para>There are disadvantages to compiling to object-code: you
2606 can't set breakpoints in object-code modules, for example. Only
2607 the exports of an object-code module will be visible in GHCi,
2608 rather than all top-level bindings as in interpreted
2609 modules.</para>
2610 </sect1>
2611
2612 <sect1 id="ghci-faq">
2613 <title>FAQ and Things To Watch Out For</title>
2614
2615 <variablelist>
2616 <varlistentry>
2617 <term>The interpreter can't load modules with foreign export
2618 declarations!</term>
2619 <listitem>
2620 <para>Unfortunately not. We haven't implemented it yet.
2621 Please compile any offending modules by hand before loading
2622 them into GHCi.</para>
2623 </listitem>
2624 </varlistentry>
2625
2626 <varlistentry>
2627 <term>
2628 <literal>-O</literal> doesn't work with GHCi!
2629 <indexterm><primary><option>-O</option></primary></indexterm>
2630 </term>
2631 <listitem>
2632 <para>For technical reasons, the bytecode compiler doesn't
2633 interact well with one of the optimisation passes, so we
2634 have disabled optimisation when using the interpreter. This
2635 isn't a great loss: you'll get a much bigger win by
2636 compiling the bits of your code that need to go fast, rather
2637 than interpreting them with optimisation turned on.</para>
2638 </listitem>
2639 </varlistentry>
2640
2641 <varlistentry>
2642 <term>Unboxed tuples don't work with GHCi</term>
2643 <listitem>
2644 <para>That's right. You can always compile a module that
2645 uses unboxed tuples and load it into GHCi, however.
2646 (Incidentally the previous point, namely that
2647 <literal>-O</literal> is incompatible with GHCi, is because
2648 the bytecode compiler can't deal with unboxed
2649 tuples).</para>
2650 </listitem>
2651 </varlistentry>
2652
2653 <varlistentry>
2654 <term>Concurrent threads don't carry on running when GHCi is
2655 waiting for input.</term>
2656 <listitem>
2657 <para>This should work, as long as your GHCi was built with
2658 the <option>-threaded</option> switch, which is the default.
2659 Consult whoever supplied your GHCi installation.</para>
2660 </listitem>
2661 </varlistentry>
2662
2663 <varlistentry>
2664 <term>After using <literal>getContents</literal>, I can't use
2665 <literal>stdin</literal> again until I do
2666 <literal>:load</literal> or <literal>:reload</literal>.</term>
2667
2668 <listitem>
2669 <para>This is the defined behaviour of
2670 <literal>getContents</literal>: it puts the stdin Handle in
2671 a state known as <firstterm>semi-closed</firstterm>, wherein
2672 any further I/O operations on it are forbidden. Because I/O
2673 state is retained between computations, the semi-closed
2674 state persists until the next <literal>:load</literal> or
2675 <literal>:reload</literal> command.</para>
2676
2677 <para>You can make <literal>stdin</literal> reset itself
2678 after every evaluation by giving GHCi the command
2679 <literal>:set +r</literal>. This works because
2680 <literal>stdin</literal> is just a top-level expression that
2681 can be reverted to its unevaluated state in the same way as
2682 any other top-level expression (CAF).</para>
2683 </listitem>
2684 </varlistentry>
2685
2686 <varlistentry>
2687 <term>I can't use Control-C to interrupt computations in
2688 GHCi on Windows.</term>
2689 <listitem>
2690 <para>See <xref linkend="ghci-windows"/>.</para>
2691 </listitem>
2692 </varlistentry>
2693
2694 <varlistentry>
2695 <term>The default buffering mode is different in GHCi to GHC.</term>
2696 <listitem>
2697 <para>
2698 In GHC, the stdout handle is line-buffered by default.
2699 However, in GHCi we turn off the buffering on stdout,
2700 because this is normally what you want in an interpreter:
2701 output appears as it is generated.
2702 </para>
2703 </listitem>
2704 </varlistentry>
2705 </variablelist>
2706 </sect1>
2707
2708 </chapter>
2709
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