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