Fix Trac #1909: type of map in docs
[ghc.git] / docs / users_guide / ghci.xml
1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="ghci">
3 <title>Using GHCi</title>
4 <indexterm><primary>GHCi</primary></indexterm>
5 <indexterm><primary>interpreter</primary><see>GHCi</see></indexterm>
6 <indexterm><primary>interactive</primary><see>GHCi</see></indexterm>
7
8 <para>GHCi<footnote>
9 <para>The &lsquo;i&rsquo; stands for &ldquo;Interactive&rdquo;</para>
10 </footnote>
11 is GHC's interactive environment, in which Haskell expressions can
12 be interactively evaluated and programs can be interpreted. If
13 you're familiar with <ulink url="http://www.haskell.org/hugs/">Hugs</ulink><indexterm><primary>Hugs</primary>
14 </indexterm>, then you'll be right at home with GHCi. However, GHCi
15 also has support for interactively loading compiled code, as well as
16 supporting all<footnote><para>except <literal>foreign export</literal>, at the moment</para>
17 </footnote> the language extensions that GHC provides.
18 <indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
19 <indexterm><primary>Foreign Function
20 Interface</primary><secondary>GHCi support</secondary></indexterm>.
21 GHCi also includes an interactive debugger (see <xref linkend="ghci-debugger"/>).</para>
22
23 <sect1 id="ghci-introduction">
24 <title>Introduction to GHCi</title>
25
26 <para>Let's start with an example GHCi session. You can fire up
27 GHCi with the command <literal>ghci</literal>:</para>
28
29 <screen>
30 $ ghci
31 GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
32 Loading package base ... linking ... done.
33 Prelude>
34 </screen>
35
36 <para>There may be a short pause while GHCi loads the prelude and
37 standard libraries, after which the prompt is shown. As the banner
38 says, you can type <literal>:?</literal> to see the list of commands
39 available, and a half line description of each of them.</para>
40
41 <para>We'll explain most of these commands as we go along. For
42 Hugs users: many things work the same as in Hugs, so you should be
43 able to get going straight away.</para>
44
45 <para>Haskell expressions can be typed at the prompt:</para>
46 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
47 </indexterm>
48
49 <screen>
50 Prelude> 1+2
51 3
52 Prelude> let x = 42 in x / 9
53 4.666666666666667
54 Prelude>
55 </screen>
56
57 <para>GHCi interprets the whole line as an expression to evaluate.
58 The expression may not span several lines - as soon as you press
59 enter, GHCi will attempt to evaluate it.</para>
60 </sect1>
61
62 <sect1 id="loading-source-files">
63 <title>Loading source files</title>
64
65 <para>Suppose we have the following Haskell source code, which we
66 place in a file <filename>Main.hs</filename>:</para>
67
68 <programlisting>
69 main = print (fac 20)
70
71 fac 0 = 1
72 fac n = n * fac (n-1)
73 </programlisting>
74
75 <para>You can save <filename>Main.hs</filename> anywhere you like,
76 but if you save it somewhere other than the current
77 directory<footnote><para>If you started up GHCi from the command
78 line then GHCi's current directory is the same as the current
79 directory of the shell from which it was started. If you started
80 GHCi from the &ldquo;Start&rdquo; menu in Windows, then the
81 current directory is probably something like
82 <filename>C:\Documents and Settings\<replaceable>user
83 name</replaceable></filename>.</para> </footnote> then we will
84 need to change to the right directory in GHCi:</para>
85
86 <screen>
87 Prelude> :cd <replaceable>dir</replaceable>
88 </screen>
89
90 <para>where <replaceable>dir</replaceable> is the directory (or
91 folder) in which you saved <filename>Main.hs</filename>.</para>
92
93 <para>To load a Haskell source file into GHCi, use the
94 <literal>:load</literal> command:</para>
95 <indexterm><primary><literal>:load</literal></primary></indexterm>
96
97 <screen>
98 Prelude> :load Main
99 Compiling Main ( Main.hs, interpreted )
100 Ok, modules loaded: Main.
101 *Main>
102 </screen>
103
104 <para>GHCi has loaded the <literal>Main</literal> module, and the
105 prompt has changed to &ldquo;<literal>*Main></literal>&rdquo; to
106 indicate that the current context for expressions typed at the
107 prompt is the <literal>Main</literal> module we just loaded (we'll
108 explain what the <literal>*</literal> means later in <xref
109 linkend="ghci-scope"/>). So we can now type expressions involving
110 the functions from <filename>Main.hs</filename>:</para>
111
112 <screen>
113 *Main> fac 17
114 355687428096000
115 </screen>
116
117 <para>Loading a multi-module program is just as straightforward;
118 just give the name of the &ldquo;topmost&rdquo; module to the
119 <literal>:load</literal> command (hint: <literal>:load</literal>
120 can be abbreviated to <literal>:l</literal>). The topmost module
121 will normally be <literal>Main</literal>, but it doesn't have to
122 be. GHCi will discover which modules are required, directly or
123 indirectly, by the topmost module, and load them all in dependency
124 order.</para>
125
126 <sect2 id="ghci-modules-filenames">
127 <title>Modules vs. filenames</title>
128 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
129 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
130
131 <para>Question: How does GHC find the filename which contains
132 module <replaceable>M</replaceable>? Answer: it looks for the
133 file <literal><replaceable>M</replaceable>.hs</literal>, or
134 <literal><replaceable>M</replaceable>.lhs</literal>. This means
135 that for most modules, the module name must match the filename.
136 If it doesn't, GHCi won't be able to find it.</para>
137
138 <para>There is one exception to this general rule: when you load
139 a program with <literal>:load</literal>, or specify it when you
140 invoke <literal>ghci</literal>, you can give a filename rather
141 than a module name. This filename is loaded if it exists, and
142 it may contain any module you like. This is particularly
143 convenient if you have several <literal>Main</literal> modules
144 in the same directory and you can't call them all
145 <filename>Main.hs</filename>.</para>
146
147 <para>The search path for finding source files is specified with
148 the <option>-i</option> option on the GHCi command line, like
149 so:</para>
150 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
151
152 <para>or it can be set using the <literal>:set</literal> command
153 from within GHCi (see <xref
154 linkend="ghci-cmd-line-options"/>)<footnote><para>Note that in
155 GHCi, and <option>&ndash;&ndash;make</option> mode, the <option>-i</option>
156 option is used to specify the search path for
157 <emphasis>source</emphasis> files, whereas in standard
158 batch-compilation mode the <option>-i</option> option is used to
159 specify the search path for interface files, see <xref
160 linkend="search-path"/>.</para> </footnote></para>
161
162 <para>One consequence of the way that GHCi follows dependencies
163 to find modules to load is that every module must have a source
164 file. The only exception to the rule is modules that come from
165 a package, including the <literal>Prelude</literal> and standard
166 libraries such as <literal>IO</literal> and
167 <literal>Complex</literal>. If you attempt to load a module for
168 which GHCi can't find a source file, even if there are object
169 and interface files for the module, you'll get an error
170 message.</para>
171 </sect2>
172
173 <sect2>
174 <title>Making changes and recompilation</title>
175 <indexterm><primary><literal>:reload</literal></primary></indexterm>
176
177 <para>If you make some changes to the source code and want GHCi
178 to recompile the program, give the <literal>:reload</literal>
179 command. The program will be recompiled as necessary, with GHCi
180 doing its best to avoid actually recompiling modules if their
181 external dependencies haven't changed. This is the same
182 mechanism we use to avoid re-compiling modules in the batch
183 compilation setting (see <xref linkend="recomp"/>).</para>
184 </sect2>
185 </sect1>
186
187 <sect1 id="ghci-compiled">
188 <title>Loading compiled code</title>
189 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
190
191 <para>When you load a Haskell source module into GHCi, it is
192 normally converted to byte-code and run using the interpreter.
193 However, interpreted code can also run alongside compiled code in
194 GHCi; indeed, normally when GHCi starts, it loads up a compiled
195 copy of the <literal>base</literal> package, which contains the
196 <literal>Prelude</literal>.</para>
197
198 <para>Why should we want to run compiled code? Well, compiled
199 code is roughly 10x faster than interpreted code, but takes about
200 2x longer to produce (perhaps longer if optimisation is on). So
201 it pays to compile the parts of a program that aren't changing
202 very often, and use the interpreter for the code being actively
203 developed.</para>
204
205 <para>When loading up source files with <literal>:load</literal>,
206 GHCi looks for any corresponding compiled object files, and will
207 use one in preference to interpreting the source if possible. For
208 example, suppose we have a 4-module program consisting of modules
209 A, B, C, and D. Modules B and C both import D only,
210 and A imports both B &amp; C:</para>
211 <screen>
212 A
213 / \
214 B C
215 \ /
216 D
217 </screen>
218 <para>We can compile D, then load the whole program, like this:</para>
219 <screen>
220 Prelude> :! ghc -c D.hs
221 Prelude> :load A
222 Compiling B ( B.hs, interpreted )
223 Compiling C ( C.hs, interpreted )
224 Compiling A ( A.hs, interpreted )
225 Ok, modules loaded: A, B, C, D.
226 *Main>
227 </screen>
228
229 <para>In the messages from the compiler, we see that there is no line
230 for <literal>D</literal>. This is because
231 it isn't necessary to compile <literal>D</literal>,
232 because the source and everything it depends on
233 is unchanged since the last compilation.</para>
234
235 <para>At any time you can use the command
236 <literal>:show modules</literal>
237 to get a list of the modules currently loaded
238 into GHCi:</para>
239
240 <screen>
241 *Main> :show modules
242 D ( D.hs, D.o )
243 C ( C.hs, interpreted )
244 B ( B.hs, interpreted )
245 A ( A.hs, interpreted )
246 *Main></screen>
247
248 <para>If we now modify the source of D (or pretend to: using the Unix
249 command <literal>touch</literal> on the source file is handy for
250 this), the compiler will no longer be able to use the object file,
251 because it might be out of date:</para>
252
253 <screen>
254 *Main> :! touch D.hs
255 *Main> :reload
256 Compiling D ( D.hs, interpreted )
257 Ok, modules loaded: A, B, C, D.
258 *Main>
259 </screen>
260
261 <para>Note that module D was compiled, but in this instance
262 because its source hadn't really changed, its interface remained
263 the same, and the recompilation checker determined that A, B and C
264 didn't need to be recompiled.</para>
265
266 <para>So let's try compiling one of the other modules:</para>
267
268 <screen>
269 *Main> :! ghc -c C.hs
270 *Main> :load A
271 Compiling D ( D.hs, interpreted )
272 Compiling B ( B.hs, interpreted )
273 Compiling C ( C.hs, interpreted )
274 Compiling A ( A.hs, interpreted )
275 Ok, modules loaded: A, B, C, D.
276 </screen>
277
278 <para>We didn't get the compiled version of C! What happened?
279 Well, in GHCi a compiled module may only depend on other compiled
280 modules, and in this case C depends on D, which doesn't have an
281 object file, so GHCi also rejected C's object file. Ok, so let's
282 also compile D:</para>
283
284 <screen>
285 *Main> :! ghc -c D.hs
286 *Main> :reload
287 Ok, modules loaded: A, B, C, D.
288 </screen>
289
290 <para>Nothing happened! Here's another lesson: newly compiled
291 modules aren't picked up by <literal>:reload</literal>, only
292 <literal>:load</literal>:</para>
293
294 <screen>
295 *Main> :load A
296 Compiling B ( B.hs, interpreted )
297 Compiling A ( A.hs, interpreted )
298 Ok, modules loaded: A, B, C, D.
299 </screen>
300
301 <para>HINT: since GHCi will only use a compiled object file if it
302 can be sure that the compiled version is up-to-date, a good technique
303 when working on a large program is to occasionally run
304 <literal>ghc &ndash;&ndash;make</literal> to compile the whole project (say
305 before you go for lunch :-), then continue working in the
306 interpreter. As you modify code, the changed modules will be
307 interpreted, but the rest of the project will remain
308 compiled.</para>
309
310 </sect1>
311
312 <sect1 id="interactive-evaluation">
313 <title>Interactive evaluation at the prompt</title>
314
315 <para>When you type an expression at the prompt, GHCi immediately
316 evaluates and prints the result:
317 <screen>
318 Prelude> reverse "hello"
319 "olleh"
320 Prelude> 5+5
321 10
322 </screen>
323 </para>
324
325 <sect2><title>I/O actions at the prompt</title>
326
327 <para>GHCi does more than simple expression evaluation at the prompt.
328 If you type something of type <literal>IO a</literal> for some
329 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
330 as an IO-computation.
331 <screen>
332 Prelude> "hello"
333 "hello"
334 Prelude> putStrLn "hello"
335 hello
336 </screen>
337 Furthermore, GHCi will print the result of the I/O action if (and only
338 if):
339 <itemizedlist>
340 <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
341 <listitem><para>The result type is not
342 <literal>()</literal>.</para></listitem>
343 </itemizedlist>
344 For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
345 <screen>
346 Prelude> putStrLn "hello"
347 hello
348 Prelude> do { putStrLn "hello"; return "yes" }
349 hello
350 "yes"
351 </screen>
352 </para></sect2>
353
354 <sect2 id="ghci-stmts">
355 <title>Using <literal>do-</literal>notation at the prompt</title>
356 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
357 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
358
359 <para>GHCi actually accepts <firstterm>statements</firstterm>
360 rather than just expressions at the prompt. This means you can
361 bind values and functions to names, and use them in future
362 expressions or statements.</para>
363
364 <para>The syntax of a statement accepted at the GHCi prompt is
365 exactly the same as the syntax of a statement in a Haskell
366 <literal>do</literal> expression. However, there's no monad
367 overloading here: statements typed at the prompt must be in the
368 <literal>IO</literal> monad.
369 <screen>
370 Prelude> x &lt;- return 42
371 Prelude> print x
372 42
373 Prelude>
374 </screen>
375 The statement <literal>x &lt;- return 42</literal> means
376 &ldquo;execute <literal>return 42</literal> in the
377 <literal>IO</literal> monad, and bind the result to
378 <literal>x</literal>&rdquo;. We can then use
379 <literal>x</literal> in future statements, for example to print
380 it as we did above.</para>
381
382 <para>If <option>-fprint-bind-result</option> is set then
383 GHCi will print the result of a statement if and only if:
384 <itemizedlist>
385 <listitem>
386 <para>The statement is not a binding, or it is a monadic binding
387 (<literal>p &lt;- e</literal>) that binds exactly one
388 variable.</para>
389 </listitem>
390 <listitem>
391 <para>The variable's type is not polymorphic, is not
392 <literal>()</literal>, and is an instance of
393 <literal>Show</literal></para>
394 </listitem>
395 </itemizedlist>
396 <indexterm><primary><option>-fprint-bind-result</option></primary></indexterm><indexterm><primary><option>-fno-print-bind-result</option></primary></indexterm>.
397 </para>
398
399 <para>Of course, you can also bind normal non-IO expressions
400 using the <literal>let</literal>-statement:</para>
401 <screen>
402 Prelude> let x = 42
403 Prelude> x
404 42
405 Prelude>
406 </screen>
407 <para>Another important difference between the two types of binding
408 is that the monadic bind (<literal>p &lt;- e</literal>) is
409 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
410 whereas with the <literal>let</literal> form, the expression
411 isn't evaluated immediately:</para>
412 <screen>
413 Prelude> let x = error "help!"
414 Prelude> print x
415 *** Exception: help!
416 Prelude>
417 </screen>
418
419 <para>Note that <literal>let</literal> bindings do not automatically
420 print the value bound, unlike monadic bindings.</para>
421
422 <para>Hint: you can also use <literal>let</literal>-statements
423 to define functions at the prompt:</para>
424 <screen>
425 Prelude> let add a b = a + b
426 Prelude> add 1 2
427 3
428 Prelude>
429 </screen>
430 <para>However, this quickly gets tedious when defining functions
431 with multiple clauses, or groups of mutually recursive functions,
432 because the complete definition has to be given on a single line,
433 using explicit braces and semicolons instead of layout:</para>
434 <screen>
435 Prelude> let { f op n [] = n ; f op n (h:t) = h `op` f op n t }
436 Prelude> f (+) 0 [1..3]
437 6
438 Prelude>
439 </screen>
440 <para>To alleviate this issue, GHCi commands can be split over
441 multiple lines, by wrapping them in <literal>:{</literal> and
442 <literal>:}</literal> (each on a single line of its own):</para>
443 <screen>
444 Prelude> :{
445 Prelude| let { g op n [] = n
446 Prelude| ; g op n (h:t) = h `op` g op n t
447 Prelude| }
448 Prelude| :}
449 Prelude> g (*) 1 [1..3]
450 6
451 </screen>
452 <para>Such multiline commands can be used with any GHCi command,
453 and the lines between <literal>:{</literal> and
454 <literal>:}</literal> are simply merged into a single line for
455 interpretation. That implies that each such group must form a single
456 valid command when merged, and that no layout rule is used.
457 The main purpose of multiline commands is not to replace module
458 loading but to make definitions in .ghci-files (see <xref
459 linkend="ghci-dot-files"/>) more readable and maintainable.</para>
460
461 <para>Any exceptions raised during the evaluation or execution
462 of the statement are caught and printed by the GHCi command line
463 interface (for more information on exceptions, see the module
464 <literal>Control.Exception</literal> in the libraries
465 documentation).</para>
466
467 <para>Every new binding shadows any existing bindings of the
468 same name, including entities that are in scope in the current
469 module context.</para>
470
471 <para>WARNING: temporary bindings introduced at the prompt only
472 last until the next <literal>:load</literal> or
473 <literal>:reload</literal> command, at which time they will be
474 simply lost. However, they do survive a change of context with
475 <literal>:module</literal>: the temporary bindings just move to
476 the new location.</para>
477
478 <para>HINT: To get a list of the bindings currently in scope, use the
479 <literal>:show bindings</literal> command:</para>
480
481 <screen>
482 Prelude> :show bindings
483 x :: Int
484 Prelude></screen>
485
486 <para>HINT: if you turn on the <literal>+t</literal> option,
487 GHCi will show the type of each variable bound by a statement.
488 For example:</para>
489 <indexterm><primary><literal>+t</literal></primary></indexterm>
490 <screen>
491 Prelude> :set +t
492 Prelude> let (x:xs) = [1..]
493 x :: Integer
494 xs :: [Integer]
495 </screen>
496
497 </sect2>
498
499 <sect2 id="ghci-scope">
500 <title>What's really in scope at the prompt?</title>
501
502 <para>When you type an expression at the prompt, what
503 identifiers and types are in scope? GHCi provides a flexible
504 way to control exactly how the context for an expression is
505 constructed. Let's start with the simple cases; when you start
506 GHCi the prompt looks like this:</para>
507
508 <screen>Prelude></screen>
509
510 <para>Which indicates that everything from the module
511 <literal>Prelude</literal> is currently in scope. If we now
512 load a file into GHCi, the prompt will change:</para>
513
514 <screen>
515 Prelude> :load Main.hs
516 Compiling Main ( Main.hs, interpreted )
517 *Main>
518 </screen>
519
520 <para>The new prompt is <literal>*Main</literal>, which
521 indicates that we are typing expressions in the context of the
522 top-level of the <literal>Main</literal> module. Everything
523 that is in scope at the top-level in the module
524 <literal>Main</literal> we just loaded is also in scope at the
525 prompt (probably including <literal>Prelude</literal>, as long
526 as <literal>Main</literal> doesn't explicitly hide it).</para>
527
528 <para>The syntax
529 <literal>*<replaceable>module</replaceable></literal> indicates
530 that it is the full top-level scope of
531 <replaceable>module</replaceable> that is contributing to the
532 scope for expressions typed at the prompt. Without the
533 <literal>*</literal>, just the exports of the module are
534 visible.</para>
535
536 <para>We're not limited to a single module: GHCi can combine
537 scopes from multiple modules, in any mixture of
538 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
539 combines the scopes from all of these modules to form the scope
540 that is in effect at the prompt. For technical reasons, GHCi
541 can only support the <literal>*</literal>-form for modules which
542 are interpreted, so compiled modules and package modules can
543 only contribute their exports to the current scope.</para>
544
545 <para>The scope is manipulated using the
546 <literal>:module</literal> command. For example, if the current
547 scope is <literal>Prelude</literal>, then we can bring into
548 scope the exports from the module <literal>IO</literal> like
549 so:</para>
550
551 <screen>
552 Prelude> :module +IO
553 Prelude IO> hPutStrLn stdout "hello\n"
554 hello
555 Prelude IO>
556 </screen>
557
558 <para>(Note: you can use <literal>import M</literal> as an
559 alternative to <literal>:module +M</literal>, and
560 <literal>:module</literal> can also be shortened to
561 <literal>:m</literal>). The full syntax of the
562 <literal>:module</literal> command is:</para>
563
564 <screen>
565 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
566 </screen>
567
568 <para>Using the <literal>+</literal> form of the
569 <literal>module</literal> commands adds modules to the current
570 scope, and <literal>-</literal> removes them. Without either
571 <literal>+</literal> or <literal>-</literal>, the current scope
572 is replaced by the set of modules specified. Note that if you
573 use this form and leave out <literal>Prelude</literal>, GHCi
574 will assume that you really wanted the
575 <literal>Prelude</literal> and add it in for you (if you don't
576 want the <literal>Prelude</literal>, then ask to remove it with
577 <literal>:m -Prelude</literal>).</para>
578
579 <para>The scope is automatically set after a
580 <literal>:load</literal> command, to the most recently loaded
581 "target" module, in a <literal>*</literal>-form if possible.
582 For example, if you say <literal>:load foo.hs bar.hs</literal>
583 and <filename>bar.hs</filename> contains module
584 <literal>Bar</literal>, then the scope will be set to
585 <literal>*Bar</literal> if <literal>Bar</literal> is
586 interpreted, or if <literal>Bar</literal> is compiled it will be
587 set to <literal>Prelude Bar</literal> (GHCi automatically adds
588 <literal>Prelude</literal> if it isn't present and there aren't
589 any <literal>*</literal>-form modules).</para>
590
591 <para>With multiple modules in scope, especially multiple
592 <literal>*</literal>-form modules, it is likely that name
593 clashes will occur. Haskell specifies that name clashes are
594 only reported when an ambiguous identifier is used, and GHCi
595 behaves in the same way for expressions typed at the
596 prompt.</para>
597
598 <para>
599 Hint: GHCi will tab-complete names that are in scope; for
600 example, if you run GHCi and type <literal>J&lt;tab&gt;</literal>
601 then GHCi will expand it to &ldquo;<literal>Just </literal>&rdquo;.
602 </para>
603
604 <sect3>
605 <title>Qualified names</title>
606
607 <para>To make life slightly easier, the GHCi prompt also
608 behaves as if there is an implicit <literal>import
609 qualified</literal> declaration for every module in every
610 package, and every module currently loaded into GHCi.</para>
611 </sect3>
612
613 <sect3>
614 <title>The <literal>:main</literal> command</title>
615
616 <para>
617 When a program is compiled and executed, it can use the
618 <literal>getArgs</literal> function to access the
619 command-line arguments.
620 However, we cannot simply pass the arguments to the
621 <literal>main</literal> function while we are testing in ghci,
622 as the <literal>main</literal> function doesn't take its
623 directly.
624 </para>
625
626 <para>
627 Instead, we can use the <literal>:main</literal> command.
628 This runs whatever <literal>main</literal> is in scope, with
629 any arguments being treated the same as command-line arguments,
630 e.g.:
631 </para>
632
633 <screen>
634 Prelude> let main = System.Environment.getArgs >>= print
635 Prelude> :main foo bar
636 ["foo","bar"]
637 </screen>
638
639 </sect3>
640 </sect2>
641
642
643 <sect2>
644 <title>The <literal>it</literal> variable</title>
645 <indexterm><primary><literal>it</literal></primary>
646 </indexterm>
647
648 <para>Whenever an expression (or a non-binding statement, to be
649 precise) is typed at the prompt, GHCi implicitly binds its value
650 to the variable <literal>it</literal>. For example:</para>
651 <screen>
652 Prelude> 1+2
653 3
654 Prelude> it * 2
655 6
656 </screen>
657 <para>What actually happens is that GHCi typechecks the
658 expression, and if it doesn't have an <literal>IO</literal> type,
659 then it transforms it as follows: an expression
660 <replaceable>e</replaceable> turns into
661 <screen>
662 let it = <replaceable>e</replaceable>;
663 print it
664 </screen>
665 which is then run as an IO-action.</para>
666
667 <para>Hence, the original expression must have a type which is an
668 instance of the <literal>Show</literal> class, or GHCi will
669 complain:</para>
670
671 <screen>
672 Prelude&gt; id
673
674 &lt;interactive&gt;:1:0:
675 No instance for (Show (a -&gt; a))
676 arising from use of `print' at &lt;interactive&gt;:1:0-1
677 Possible fix: add an instance declaration for (Show (a -> a))
678 In the expression: print it
679 In a 'do' expression: print it
680 </screen>
681
682 <para>The error message contains some clues as to the
683 transformation happening internally.</para>
684
685 <para>If the expression was instead of type <literal>IO a</literal> for
686 some <literal>a</literal>, then <literal>it</literal> will be
687 bound to the result of the <literal>IO</literal> computation,
688 which is of type <literal>a</literal>. eg.:</para>
689 <screen>
690 Prelude> Time.getClockTime
691 Wed Mar 14 12:23:13 GMT 2001
692 Prelude> print it
693 Wed Mar 14 12:23:13 GMT 2001
694 </screen>
695
696 <para>The corresponding translation for an IO-typed
697 <replaceable>e</replaceable> is
698 <screen>
699 it &lt;- <replaceable>e</replaceable>
700 </screen>
701 </para>
702
703 <para>Note that <literal>it</literal> is shadowed by the new
704 value each time you evaluate a new expression, and the old value
705 of <literal>it</literal> is lost.</para>
706
707 </sect2>
708
709 <sect2 id="extended-default-rules">
710 <title>Type defaulting in GHCi</title>
711 <indexterm><primary>Type default</primary></indexterm>
712 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
713 <para>
714 Consider this GHCi session:
715 <programlisting>
716 ghci> reverse []
717 </programlisting>
718 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
719 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
720 on the type <literal>a</literal>. For example:
721 <programlisting>
722 ghci> (reverse []) :: String
723 ""
724 ghci> (reverse []) :: [Int]
725 []
726 </programlisting>
727 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
728 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
729 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
730 a)</literal> for each type variable <literal>a</literal>, and defaults the
731 type variable if
732 <orderedlist>
733 <listitem>
734 <para>
735 The type variable <literal>a</literal> appears in no
736 other constraints
737 </para>
738 </listitem>
739 <listitem>
740 <para>
741 All the classes <literal>Ci</literal> are standard.
742 </para>
743 </listitem>
744 <listitem>
745 <para>
746 At least one of the classes <literal>Ci</literal> is
747 numeric.
748 </para>
749 </listitem>
750 </orderedlist>
751 At the GHCi prompt, or with GHC if the
752 <literal>-XExtendedDefaultRules</literal> flag is given,
753 the following additional differences apply:
754 <itemizedlist>
755 <listitem>
756 <para>
757 Rule 2 above is relaxed thus:
758 <emphasis>All</emphasis> of the classes
759 <literal>Ci</literal> are single-parameter type classes.
760 </para>
761 </listitem>
762 <listitem>
763 <para>
764 Rule 3 above is relaxed this:
765 At least one of the classes <literal>Ci</literal> is
766 numeric, <emphasis>or is <literal>Show</literal>,
767 <literal>Eq</literal>, or
768 <literal>Ord</literal></emphasis>.
769 </para>
770 </listitem>
771 <listitem>
772 <para>
773 The unit type <literal>()</literal> is added to the
774 start of the standard list of types which are tried when
775 doing type defaulting.
776 </para>
777 </listitem>
778 </itemizedlist>
779 The last point means that, for example, this program:
780 <programlisting>
781 main :: IO ()
782 main = print def
783
784 instance Num ()
785
786 def :: (Num a, Enum a) => a
787 def = toEnum 0
788 </programlisting>
789 prints <literal>()</literal> rather than <literal>0</literal> as the
790 type is defaulted to <literal>()</literal> rather than
791 <literal>Integer</literal>.
792 </para>
793 <para>
794 The motivation for the change is that it means <literal>IO a</literal>
795 actions default to <literal>IO ()</literal>, which in turn means that
796 ghci won't try to print a result when running them. This is
797 particularly important for <literal>printf</literal>, which has an
798 instance that returns <literal>IO a</literal>.
799 However, it is only able to return
800 <literal>undefined</literal>
801 (the reason for the instance having this type is so that printf
802 doesn't require extensions to the class system), so if the type defaults to
803 <literal>Integer</literal> then ghci gives an error when running a
804 printf.
805 </para>
806 </sect2>
807 </sect1>
808
809 <sect1 id="ghci-debugger">
810 <title>The GHCi Debugger</title>
811 <indexterm><primary>debugger</primary><secondary>in GHCi</secondary>
812 </indexterm>
813
814 <para>GHCi contains a simple imperative-style debugger in which you can
815 stop a running computation in order to examine the values of
816 variables. The debugger is integrated into GHCi, and is turned on by
817 default: no flags are required to enable the debugging facilities. There
818 is one major restriction: breakpoints and single-stepping are only
819 available in <emphasis>interpreted</emphasis> modules; compiled code is
820 invisible to the debugger.</para>
821
822 <para>The debugger provides the following:
823 <itemizedlist>
824 <listitem>
825 <para>The ability to set a <firstterm>breakpoint</firstterm> on a
826 function definition or expression in the program. When the function
827 is called, or the expression evaluated, GHCi suspends
828 execution and returns to the prompt, where you can inspect the
829 values of local variables before continuing with the
830 execution.</para>
831 </listitem>
832 <listitem>
833 <para>Execution can be <firstterm>single-stepped</firstterm>: the
834 evaluator will suspend execution approximately after every
835 reduction, allowing local variables to be inspected. This is
836 equivalent to setting a breakpoint at every point in the
837 program.</para>
838 </listitem>
839 <listitem>
840 <para>Execution can take place in <firstterm>tracing
841 mode</firstterm>, in which the evaluator remembers each
842 evaluation step as it happens, but doesn't suspend execution until
843 an actual breakpoint is reached. When this happens, the history of
844 evaluation steps can be inspected.</para>
845 </listitem>
846 <listitem>
847 <para>Exceptions (e.g. pattern matching failure and
848 <literal>error</literal>) can be treated as breakpoints, to help
849 locate the source of an exception in the program.</para>
850 </listitem>
851 </itemizedlist>
852 </para>
853
854 <para>There is currently no support for obtaining a &ldquo;stack
855 trace&rdquo;, but the tracing and history features provide a useful
856 second-best, which will often be enough to establish the context of an
857 error.</para>
858
859 <sect2 id="breakpoints">
860 <title>Breakpoints and inspecting variables</title>
861
862 <para>Let's use quicksort as a running example. Here's the code:</para>
863
864 <programlisting>
865 qsort [] = []
866 qsort (a:as) = qsort left ++ [a] ++ qsort right
867 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
868
869 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
870 </programlisting>
871
872 <para>First, load the module into GHCi:</para>
873
874 <screen>
875 Prelude> :l qsort.hs
876 [1 of 1] Compiling Main ( qsort.hs, interpreted )
877 Ok, modules loaded: Main.
878 *Main>
879 </screen>
880
881 <para>Now, let's set a breakpoint on the right-hand-side of the second
882 equation of qsort:</para>
883
884 <programlisting>
885 *Main> :break 2
886 Breakpoint 0 activated at qsort.hs:2:15-46
887 *Main>
888 </programlisting>
889
890 <para>The command <literal>:break 2</literal> sets a breakpoint on line
891 2 of the most recently-loaded module, in this case
892 <literal>qsort.hs</literal>. Specifically, it picks the
893 leftmost complete subexpression on that line on which to set the
894 breakpoint, which in this case is the expression
895 <literal>(qsort left ++ [a] ++ qsort right)</literal>.</para>
896
897 <para>Now, we run the program:</para>
898
899 <programlisting>
900 *Main> main
901 Stopped at qsort.hs:2:15-46
902 _result :: [a]
903 a :: a
904 left :: [a]
905 right :: [a]
906 [qsort.hs:2:15-46] *Main>
907 </programlisting>
908
909 <para>Execution has stopped at the breakpoint. The prompt has changed to
910 indicate that we are currently stopped at a breakpoint, and the location:
911 <literal>[qsort.hs:2:15-46]</literal>. To further clarify the
912 location, we can use the <literal>:list</literal> command:</para>
913
914 <programlisting>
915 [qsort.hs:2:15-46] *Main> :list
916 1 qsort [] = []
917 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
918 3 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
919 </programlisting>
920
921 <para>The <literal>:list</literal> command lists the source code around
922 the current breakpoint. If your output device supports it, then GHCi
923 will highlight the active subexpression in bold.</para>
924
925 <para>GHCi has provided bindings for the free variables<footnote><para>We
926 originally provided bindings for all variables in scope, rather
927 than just
928 the free variables of the expression, but found that this affected
929 performance considerably, hence the current restriction to just the
930 free variables.</para>
931 </footnote> of the expression
932 on which the
933 breakpoint was placed (<literal>a</literal>, <literal>left</literal>,
934 <literal>right</literal>), and additionally a binding for the result of
935 the expression (<literal>_result</literal>). These variables are just
936 like other variables that you might define in GHCi; you
937 can use them in expressions that you type at the prompt, you can ask
938 for their types with <literal>:type</literal>, and so on. There is one
939 important difference though: these variables may only have partial
940 types. For example, if we try to display the value of
941 <literal>left</literal>:</para>
942
943 <screen>
944 [qsort.hs:2:15-46] *Main> left
945
946 &lt;interactive&gt;:1:0:
947 Ambiguous type variable `a' in the constraint:
948 `Show a' arising from a use of `print' at &lt;interactive&gt;:1:0-3
949 Cannot resolve unknown runtime types: a
950 Use :print or :force to determine these types
951 </screen>
952
953 <para>This is because <literal>qsort</literal> is a polymorphic function,
954 and because GHCi does not carry type information at runtime, it cannot
955 determine the runtime types of free variables that involve type
956 variables. Hence, when you ask to display <literal>left</literal> at
957 the prompt, GHCi can't figure out which instance of
958 <literal>Show</literal> to use, so it emits the type error above.</para>
959
960 <para>Fortunately, the debugger includes a generic printing command,
961 <literal>:print</literal>, which can inspect the actual runtime value of a
962 variable and attempt to reconstruct its type. If we try it on
963 <literal>left</literal>:</para>
964
965 <screen>
966 [qsort.hs:2:15-46] *Main> :set -fprint-evld-with-show
967 [qsort.hs:2:15-46] *Main> :print left
968 left = (_t1::[a])
969 </screen>
970
971 <para>This isn't particularly enlightening. What happened is that
972 <literal>left</literal> is bound to an unevaluated computation (a
973 suspension, or <firstterm>thunk</firstterm>), and
974 <literal>:print</literal> does not force any evaluation. The idea is
975 that <literal>:print</literal> can be used to inspect values at a
976 breakpoint without any unfortunate side effects. It won't force any
977 evaluation, which could cause the program to give a different answer
978 than it would normally, and hence it won't cause any exceptions to be
979 raised, infinite loops, or further breakpoints to be triggered (see
980 <xref linkend="nested-breakpoints" />).
981 Rather than forcing thunks, <literal>:print</literal>
982 binds each thunk to a fresh variable beginning with an
983 underscore, in this case
984 <literal>_t1</literal>.</para>
985
986 <para>The flag <literal>-fprint-evld-with-show</literal> instructs
987 <literal>:print</literal> to reuse
988 available <literal>Show</literal> instances when possible. This happens
989 only when the contents of the variable being inspected
990 are completely evaluated.</para>
991
992
993 <para>If we aren't concerned about preserving the evaluatedness of a
994 variable, we can use <literal>:force</literal> instead of
995 <literal>:print</literal>. The <literal>:force</literal> command
996 behaves exactly like <literal>:print</literal>, except that it forces
997 the evaluation of any thunks it encounters:</para>
998
999 <screen>
1000 [qsort.hs:2:15-46] *Main> :force left
1001 left = [4,0,3,1]
1002 </screen>
1003
1004 <para>Now, since <literal>:force</literal> has inspected the runtime
1005 value of <literal>left</literal>, it has reconstructed its type. We
1006 can see the results of this type reconstruction:</para>
1007
1008 <screen>
1009 [qsort.hs:2:15-46] *Main> :show bindings
1010 _result :: [Integer]
1011 a :: Integer
1012 left :: [Integer]
1013 right :: [Integer]
1014 _t1 :: [Integer]
1015 </screen>
1016
1017 <para>Not only do we now know the type of <literal>left</literal>, but
1018 all the other partial types have also been resolved. So we can ask
1019 for the value of <literal>a</literal>, for example:</para>
1020
1021 <screen>
1022 [qsort.hs:2:15-46] *Main> a
1023 8
1024 </screen>
1025
1026 <para>You might find it useful to use Haskell's
1027 <literal>seq</literal> function to evaluate individual thunks rather
1028 than evaluating the whole expression with <literal>:force</literal>.
1029 For example:</para>
1030
1031 <screen>
1032 [qsort.hs:2:15-46] *Main> :print right
1033 right = (_t1::[Integer])
1034 [qsort.hs:2:15-46] *Main> seq _t1 ()
1035 ()
1036 [qsort.hs:2:15-46] *Main> :print right
1037 right = 23 : (_t2::[Integer])
1038 </screen>
1039
1040 <para>We evaluated only the <literal>_t1</literal> thunk, revealing the
1041 head of the list, and the tail is another thunk now bound to
1042 <literal>_t2</literal>. The <literal>seq</literal> function is a
1043 little inconvenient to use here, so you might want to use
1044 <literal>:def</literal> to make a nicer interface (left as an exercise
1045 for the reader!).</para>
1046
1047 <para>Finally, we can continue the current execution:</para>
1048
1049 <screen>
1050 [qsort.hs:2:15-46] *Main> :continue
1051 Stopped at qsort.hs:2:15-46
1052 _result :: [a]
1053 a :: a
1054 left :: [a]
1055 right :: [a]
1056 [qsort.hs:2:15-46] *Main>
1057 </screen>
1058
1059 <para>The execution continued at the point it previously stopped, and has
1060 now stopped at the breakpoint for a second time.</para>
1061
1062
1063 <sect3 id="setting-breakpoints">
1064 <title>Setting breakpoints</title>
1065
1066 <para>Breakpoints can be set in various ways. Perhaps the easiest way to
1067 set a breakpoint is to name a top-level function:</para>
1068
1069 <screen>
1070 :break <replaceable>identifier</replaceable>
1071 </screen>
1072
1073 <para>Where <replaceable>identifier</replaceable> names any top-level
1074 function in an interpreted module currently loaded into GHCi (qualified
1075 names may be used). The breakpoint will be set on the body of the
1076 function, when it is fully applied but before any pattern matching has
1077 taken place.</para>
1078
1079 <para>Breakpoints can also be set by line (and optionally column)
1080 number:</para>
1081
1082 <screen>
1083 :break <replaceable>line</replaceable>
1084 :break <replaceable>line</replaceable> <replaceable>column</replaceable>
1085 :break <replaceable>module</replaceable> <replaceable>line</replaceable>
1086 :break <replaceable>module</replaceable> <replaceable>line</replaceable> <replaceable>column</replaceable>
1087 </screen>
1088
1089 <para>When a breakpoint is set on a particular line, GHCi sets the
1090 breakpoint on the
1091 leftmost subexpression that begins and ends on that line. If two
1092 complete subexpressions start at the same
1093 column, the longest one is picked. If there is no complete
1094 subexpression on the line, then the leftmost expression starting on
1095 the line is picked, and failing that the rightmost expression that
1096 partially or completely covers the line.</para>
1097
1098 <para>When a breakpoint is set on a particular line and column, GHCi
1099 picks the smallest subexpression that encloses that location on which
1100 to set the breakpoint. Note: GHC considers the TAB character to have a
1101 width of 1, wherever it occurs; in other words it counts
1102 characters, rather than columns. This matches what some editors do,
1103 and doesn't match others. The best advice is to avoid tab
1104 characters in your source code altogether (see
1105 <option>-fwarn-tabs</option> in <xref linkend="options-sanity"
1106 />).</para>
1107
1108 <para>If the module is omitted, then the most recently-loaded module is
1109 used.</para>
1110
1111 <para>Not all subexpressions are potential breakpoint locations. Single
1112 variables are typically not considered to be breakpoint locations
1113 (unless the variable is the right-hand-side of a function definition,
1114 lambda, or case alternative). The rule of thumb is that all redexes
1115 are breakpoint locations, together with the bodies of functions,
1116 lambdas, case alternatives and binding statements. There is normally
1117 no breakpoint on a let expression, but there will always be a
1118 breakpoint on its body, because we are usually interested in inspecting
1119 the values of the variables bound by the let.</para>
1120
1121 </sect3>
1122 <sect3>
1123 <title>Listing and deleting breakpoints</title>
1124
1125 <para>The list of breakpoints currently enabled can be displayed using
1126 <literal>:show&nbsp;breaks</literal>:</para>
1127 <screen>
1128 *Main> :show breaks
1129 [0] Main qsort.hs:1:11-12
1130 [1] Main qsort.hs:2:15-46
1131 </screen>
1132
1133 <para>To delete a breakpoint, use the <literal>:delete</literal>
1134 command with the number given in the output from <literal>:show&nbsp;breaks</literal>:</para>
1135
1136 <screen>
1137 *Main> :delete 0
1138 *Main> :show breaks
1139 [1] Main qsort.hs:2:15-46
1140 </screen>
1141
1142 <para>To delete all breakpoints at once, use <literal>:delete *</literal>.</para>
1143
1144 </sect3>
1145 </sect2>
1146
1147 <sect2 id="single-stepping">
1148 <title>Single-stepping</title>
1149
1150 <para>Single-stepping is a great way to visualise the execution of your
1151 program, and it is also a useful tool for identifying the source of a
1152 bug. GHCi offers two variants of stepping. Use
1153 <literal>:step</literal> to enable all the
1154 breakpoints in the program, and execute until the next breakpoint is
1155 reached. Use <literal>:steplocal</literal> to limit the set
1156 of enabled breakpoints to those in the current top level function.
1157 Similarly, use <literal>:stepmodule</literal> to single step only on
1158 breakpoints contained in the current module.
1159 For example:</para>
1160
1161 <screen>
1162 *Main> :step main
1163 Stopped at qsort.hs:5:7-47
1164 _result :: IO ()
1165 </screen>
1166
1167 <para>The command <literal>:step
1168 <replaceable>expr</replaceable></literal> begins the evaluation of
1169 <replaceable>expr</replaceable> in single-stepping mode. If
1170 <replaceable>expr</replaceable> is omitted, then it single-steps from
1171 the current breakpoint. <literal>:stepover</literal>
1172 works similarly.</para>
1173
1174 <para>The <literal>:list</literal> command is particularly useful when
1175 single-stepping, to see where you currently are:</para>
1176
1177 <screen>
1178 [qsort.hs:5:7-47] *Main> :list
1179 4
1180 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1181 6
1182 [qsort.hs:5:7-47] *Main>
1183 </screen>
1184
1185 <para>In fact, GHCi provides a way to run a command when a breakpoint is
1186 hit, so we can make it automatically do
1187 <literal>:list</literal>:</para>
1188
1189 <screen>
1190 [qsort.hs:5:7-47] *Main> :set stop :list
1191 [qsort.hs:5:7-47] *Main> :step
1192 Stopped at qsort.hs:5:14-46
1193 _result :: [Integer]
1194 4
1195 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1196 6
1197 [qsort.hs:5:14-46] *Main>
1198 </screen>
1199 </sect2>
1200
1201 <sect2 id="nested-breakpoints">
1202 <title>Nested breakpoints</title>
1203 <para>When GHCi is stopped at a breakpoint, and an expression entered at
1204 the prompt triggers a
1205 second breakpoint, the new breakpoint becomes the &ldquo;current&rdquo;
1206 one, and the old one is saved on a stack. An arbitrary number of
1207 breakpoint contexts can be built up in this way. For example:</para>
1208
1209 <screen>
1210 [qsort.hs:2:15-46] *Main> :st qsort [1,3]
1211 Stopped at qsort.hs:(1,0)-(3,55)
1212 _result :: [a]
1213 ... [qsort.hs:(1,0)-(3,55)] *Main>
1214 </screen>
1215
1216 <para>While stopped at the breakpoint on line 2 that we set earlier, we
1217 started a new evaluation with <literal>:step qsort [1,3]</literal>.
1218 This new evaluation stopped after one step (at the definition of
1219 <literal>qsort</literal>). The prompt has changed, now prefixed with
1220 <literal>...</literal>, to indicate that there are saved breakpoints
1221 beyond the current one. To see the stack of contexts, use
1222 <literal>:show context</literal>:</para>
1223
1224 <screen>
1225 ... [qsort.hs:(1,0)-(3,55)] *Main> :show context
1226 --> main
1227 Stopped at qsort.hs:2:15-46
1228 --> qsort [1,3]
1229 Stopped at qsort.hs:(1,0)-(3,55)
1230 ... [qsort.hs:(1,0)-(3,55)] *Main>
1231 </screen>
1232
1233 <para>To abandon the current evaluation, use
1234 <literal>:abandon</literal>:</para>
1235
1236 <screen>
1237 ... [qsort.hs:(1,0)-(3,55)] *Main> :abandon
1238 [qsort.hs:2:15-46] *Main> :abandon
1239 *Main>
1240 </screen>
1241 </sect2>
1242
1243 <sect2 id="ghci-debugger-result">
1244 <title>The <literal>_result</literal> variable</title>
1245 <para>When stopped at a breakpoint or single-step, GHCi binds the
1246 variable <literal>_result</literal> to the value of the currently
1247 active expression. The value of <literal>_result</literal> is
1248 presumably not available yet, because we stopped its evaluation, but it
1249 can be forced: if the type is known and showable, then just entering
1250 <literal>_result</literal> at the prompt will show it. However,
1251 there's one caveat to doing this: evaluating <literal>_result</literal>
1252 will be likely to trigger further breakpoints, starting with the
1253 breakpoint we are currently stopped at (if we stopped at a real
1254 breakpoint, rather than due to <literal>:step</literal>). So it will
1255 probably be necessary to issue a <literal>:continue</literal>
1256 immediately when evaluating <literal>_result</literal>. Alternatively,
1257 you can use <literal>:force</literal> which ignores breakpoints.</para>
1258 </sect2>
1259
1260 <sect2 id="tracing">
1261 <title>Tracing and history</title>
1262
1263 <para>A question that we often want to ask when debugging a program is
1264 &ldquo;how did I get here?&rdquo;. Traditional imperative debuggers
1265 usually provide some kind of stack-tracing feature that lets you see
1266 the stack of active function calls (sometimes called the &ldquo;lexical
1267 call stack&rdquo;), describing a path through the code
1268 to the current location. Unfortunately this is hard to provide in
1269 Haskell, because execution proceeds on a demand-driven basis, rather
1270 than a depth-first basis as in strict languages. The
1271 &ldquo;stack&ldquo; in GHC's execution engine bears little
1272 resemblance to the lexical call stack. Ideally GHCi would maintain a
1273 separate lexical call stack in addition to the dynamic call stack, and
1274 in fact this is exactly
1275 what our profiling system does (<xref linkend="profiling" />), and what
1276 some other Haskell debuggers do. For the time being, however, GHCi
1277 doesn't maintain a lexical call stack (there are some technical
1278 challenges to be overcome). Instead, we provide a way to backtrack from a
1279 breakpoint to previous evaluation steps: essentially this is like
1280 single-stepping backwards, and should in many cases provide enough
1281 information to answer the &ldquo;how did I get here?&rdquo;
1282 question.</para>
1283
1284 <para>To use tracing, evaluate an expression with the
1285 <literal>:trace</literal> command. For example, if we set a breakpoint
1286 on the base case of <literal>qsort</literal>:</para>
1287
1288 <screen>
1289 *Main&gt; :list qsort
1290 1 qsort [] = []
1291 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
1292 3 where (left,right) = (filter (&lt;=a) as, filter (&gt;a) as)
1293 4
1294 *Main&gt; :b 1
1295 Breakpoint 1 activated at qsort.hs:1:11-12
1296 *Main&gt;
1297 </screen>
1298
1299 <para>and then run a small <literal>qsort</literal> with
1300 tracing:</para>
1301
1302 <screen>
1303 *Main> :trace qsort [3,2,1]
1304 Stopped at qsort.hs:1:11-12
1305 _result :: [a]
1306 [qsort.hs:1:11-12] *Main>
1307 </screen>
1308
1309 <para>We can now inspect the history of evaluation steps:</para>
1310
1311 <screen>
1312 [qsort.hs:1:11-12] *Main> :hist
1313 -1 : qsort.hs:3:24-38
1314 -2 : qsort.hs:3:23-55
1315 -3 : qsort.hs:(1,0)-(3,55)
1316 -4 : qsort.hs:2:15-24
1317 -5 : qsort.hs:2:15-46
1318 -6 : qsort.hs:3:24-38
1319 -7 : qsort.hs:3:23-55
1320 -8 : qsort.hs:(1,0)-(3,55)
1321 -9 : qsort.hs:2:15-24
1322 -10 : qsort.hs:2:15-46
1323 -11 : qsort.hs:3:24-38
1324 -12 : qsort.hs:3:23-55
1325 -13 : qsort.hs:(1,0)-(3,55)
1326 -14 : qsort.hs:2:15-24
1327 -15 : qsort.hs:2:15-46
1328 -16 : qsort.hs:(1,0)-(3,55)
1329 &lt;end of history&gt;
1330 </screen>
1331
1332 <para>To examine one of the steps in the history, use
1333 <literal>:back</literal>:</para>
1334
1335 <screen>
1336 [qsort.hs:1:11-12] *Main> :back
1337 Logged breakpoint at qsort.hs:3:24-38
1338 _result :: [a]
1339 as :: [a]
1340 a :: a
1341 [-1: qsort.hs:3:24-38] *Main>
1342 </screen>
1343
1344 <para>Note that the local variables at each step in the history have been
1345 preserved, and can be examined as usual. Also note that the prompt has
1346 changed to indicate that we're currently examining the first step in
1347 the history: <literal>-1</literal>. The command
1348 <literal>:forward</literal> can be used to traverse forward in the
1349 history.</para>
1350
1351 <para>The <literal>:trace</literal> command can be used with or without
1352 an expression. When used without an expression, tracing begins from
1353 the current breakpoint, just like <literal>:step</literal>.</para>
1354
1355 <para>The history is only available when
1356 using <literal>:trace</literal>; the reason for this is we found that
1357 logging each breakpoint in the history cuts performance by a factor of
1358 2 or more. GHCi remembers the last 50 steps in the history (perhaps in
1359 the future we'll make this configurable).</para>
1360 </sect2>
1361
1362 <sect2 id="ghci-debugger-exceptions">
1363 <title>Debugging exceptions</title>
1364 <para>Another common question that comes up when debugging is
1365 &ldquo;where did this exception come from?&rdquo;. Exceptions such as
1366 those raised by <literal>error</literal> or <literal>head []</literal>
1367 have no context information attached to them. Finding which
1368 particular call to <literal>head</literal> in your program resulted in
1369 the error can be a painstaking process, usually involving
1370 <literal>Debug.Trace.trace</literal>, or compiling with
1371 profiling and using <literal>+RTS -xc</literal> (see <xref
1372 linkend="prof-time-options" />).</para>
1373
1374 <para>The GHCi debugger offers a way to hopefully shed some light on
1375 these errors quickly and without modifying or recompiling the source
1376 code. One way would be to set a breakpoint on the location in the
1377 source code that throws the exception, and then use
1378 <literal>:trace</literal> and <literal>:history</literal> to establish
1379 the context. However, <literal>head</literal> is in a library and
1380 we can't set a breakpoint on it directly. For this reason, GHCi
1381 provides the flags <literal>-fbreak-on-exception</literal> which causes
1382 the evaluator to stop when an exception is thrown, and <literal>
1383 -fbreak-on-error</literal>, which works similarly but stops only on
1384 uncaught exceptions. When stopping at an exception, GHCi will act
1385 just as it does when a breakpoint is hit, with the deviation that it
1386 will not show you any source code location. Due to this, these
1387 commands are only really useful in conjunction with
1388 <literal>:trace</literal>, in order to log the steps leading up to the
1389 exception. For example:</para>
1390
1391 <screen>
1392 *Main> :set -fbreak-on-exception
1393 *Main> :trace qsort ("abc" ++ undefined)
1394 "Stopped at &lt;exception thrown&gt;
1395 _exception :: e
1396 [&lt;exception thrown&gt;] *Main&gt; :hist
1397 -1 : qsort.hs:3:24-38
1398 -2 : qsort.hs:3:23-55
1399 -3 : qsort.hs:(1,0)-(3,55)
1400 -4 : qsort.hs:2:15-24
1401 -5 : qsort.hs:2:15-46
1402 -6 : qsort.hs:(1,0)-(3,55)
1403 &lt;end of history&gt;
1404 [&lt;exception thrown&gt;] *Main&gt; :back
1405 Logged breakpoint at qsort.hs:3:24-38
1406 _result :: [a]
1407 as :: [a]
1408 a :: a
1409 [-1: qsort.hs:3:24-38] *Main&gt; :force as
1410 *** Exception: Prelude.undefined
1411 [-1: qsort.hs:3:24-38] *Main&gt; :print as
1412 as = 'b' : 'c' : (_t1::[Char])
1413 </screen>
1414
1415 <para>The exception itself is bound to a new variable,
1416 <literal>_exception</literal>.</para>
1417
1418 <para>Breaking on exceptions is particularly useful for finding out what
1419 your program was doing when it was in an infinite loop. Just hit
1420 Control-C, and examine the history to find out what was going
1421 on.</para>
1422 </sect2>
1423
1424 <sect2><title>Example: inspecting functions</title>
1425 <para>
1426 It is possible to use the debugger to examine function values.
1427 When we are at a breakpoint and a function is in scope, the debugger
1428 cannot show
1429 you the source code for it; however, it is possible to get some
1430 information by applying it to some arguments and observing the result.
1431 </para>
1432
1433 <para>
1434 The process is slightly complicated when the binding is polymorphic.
1435 We show the process by means of an example.
1436 To keep things simple, we will use the well known <literal>map</literal> function:
1437 <programlisting>
1438 import Prelude hiding (map)
1439
1440 map :: (a->b) -> [a] -> [b]
1441 map f [] = []
1442 map f (x:xs) = f x : map f xs
1443 </programlisting>
1444 </para>
1445
1446 <para>
1447 We set a breakpoint on <literal>map</literal>, and call it.
1448 <screen>
1449 *Main> :break 5
1450 Breakpoint 0 activated at map.hs:5:15-28
1451 *Main> map Just [1..5]
1452 Stopped at map.hs:(4,0)-(5,12)
1453 _result :: [b]
1454 x :: a
1455 f :: a -> b
1456 xs :: [a]
1457 </screen>
1458 GHCi tells us that, among other bindings, <literal>f</literal> is in scope.
1459 However, its type is not fully known yet,
1460 and thus it is not possible to apply it to any
1461 arguments. Nevertheless, observe that the type of its first argument is the
1462 same as the type of <literal>x</literal>, and its result type is shared
1463 with <literal>_result</literal>.
1464 </para>
1465
1466 <para>
1467 As we demonstrated earlier (<xref linkend="breakpoints" />), the
1468 debugger has some intelligence built-in to update the type of
1469 <literal>f</literal> whenever the types of <literal>x</literal> or
1470 <literal>_result</literal> are discovered. So what we do in this
1471 scenario is
1472 force <literal>x</literal> a bit, in order to recover both its type
1473 and the argument part of <literal>f</literal>.
1474 <screen>
1475 *Main> seq x ()
1476 *Main> :print x
1477 x = 1
1478 </screen>
1479 </para>
1480 <para>
1481 We can check now that as expected, the type of <literal>x</literal>
1482 has been reconstructed, and with it the
1483 type of <literal>f</literal> has been too:</para>
1484 <screen>
1485 *Main> :t x
1486 x :: Integer
1487 *Main> :t f
1488 f :: Integer -> b
1489 </screen>
1490 <para>
1491 From here, we can apply f to any argument of type Integer and observe
1492 the results.
1493 <screen><![CDATA[
1494 *Main> let b = f 10
1495 *Main> :t b
1496 b :: b
1497 *Main> b
1498 <interactive>:1:0:
1499 Ambiguous type variable `b' in the constraint:
1500 `Show b' arising from a use of `print' at <interactive>:1:0
1501 *Main> :p b
1502 b = (_t2::a)
1503 *Main> seq b ()
1504 ()
1505 *Main> :t b
1506 b :: a
1507 *Main> :p b
1508 b = Just 10
1509 *Main> :t b
1510 b :: Maybe Integer
1511 *Main> :t f
1512 f :: Integer -> Maybe Integer
1513 *Main> f 20
1514 Just 20
1515 *Main> map f [1..5]
1516 [Just 1, Just 2, Just 3, Just 4, Just 5]
1517 ]]></screen>
1518 In the first application of <literal>f</literal>, we had to do
1519 some more type reconstruction
1520 in order to recover the result type of <literal>f</literal>.
1521 But after that, we are free to use
1522 <literal>f</literal> normally.
1523 </para>
1524 </sect2>
1525
1526 <sect2><title>Limitations</title>
1527 <itemizedlist>
1528 <listitem>
1529 <para>When stopped at a breakpoint, if you try to evaluate a variable
1530 that is already under evaluation, the second evaluation will hang.
1531 The reason is
1532 that GHC knows the variable is under evaluation, so the new
1533 evaluation just waits for the result before continuing, but of
1534 course this isn't going to happen because the first evaluation is
1535 stopped at a breakpoint. Control-C can interrupt the hung
1536 evaluation and return to the prompt.</para>
1537 <para>The most common way this can happen is when you're evaluating a
1538 CAF (e.g. main), stop at a breakpoint, and ask for the value of the
1539 CAF at the prompt again.</para>
1540 </listitem>
1541 <listitem><para>
1542 Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
1543 at the scope of a breakpoint if there is an explicit type signature.
1544 </para>
1545 </listitem>
1546 </itemizedlist>
1547 </sect2>
1548 </sect1>
1549
1550 <sect1 id="ghci-invocation">
1551 <title>Invoking GHCi</title>
1552 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
1553 <indexterm><primary><option>&ndash;&ndash;interactive</option></primary></indexterm>
1554
1555 <para>GHCi is invoked with the command <literal>ghci</literal> or
1556 <literal>ghc &ndash;&ndash;interactive</literal>. One or more modules or
1557 filenames can also be specified on the command line; this
1558 instructs GHCi to load the specified modules or filenames (and all
1559 the modules they depend on), just as if you had said
1560 <literal>:load <replaceable>modules</replaceable></literal> at the
1561 GHCi prompt (see <xref linkend="ghci-commands" />). For example, to
1562 start GHCi and load the program whose topmost module is in the
1563 file <literal>Main.hs</literal>, we could say:</para>
1564
1565 <screen>
1566 $ ghci Main.hs
1567 </screen>
1568
1569 <para>Most of the command-line options accepted by GHC (see <xref
1570 linkend="using-ghc"/>) also make sense in interactive mode. The ones
1571 that don't make sense are mostly obvious.</para>
1572
1573 <sect2>
1574 <title>Packages</title>
1575 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
1576
1577 <para>Most packages (see <xref linkend="using-packages"/>) are
1578 available without needing to specify any extra flags at all:
1579 they will be automatically loaded the first time they are
1580 needed.</para>
1581
1582 <para>For hidden packages, however, you need to request the
1583 package be loaded by using the <literal>-package</literal> flag:</para>
1584
1585 <screen>
1586 $ ghci -package readline
1587 GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
1588 Loading package base ... linking ... done.
1589 Loading package readline-1.0 ... linking ... done.
1590 Prelude>
1591 </screen>
1592
1593 <para>The following command works to load new packages into a
1594 running GHCi:</para>
1595
1596 <screen>
1597 Prelude> :set -package <replaceable>name</replaceable>
1598 </screen>
1599
1600 <para>But note that doing this will cause all currently loaded
1601 modules to be unloaded, and you'll be dumped back into the
1602 <literal>Prelude</literal>.</para>
1603 </sect2>
1604
1605 <sect2>
1606 <title>Extra libraries</title>
1607 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
1608
1609 <para>Extra libraries may be specified on the command line using
1610 the normal <literal>-l<replaceable>lib</replaceable></literal>
1611 option. (The term <emphasis>library</emphasis> here refers to
1612 libraries of foreign object code; for using libraries of Haskell
1613 source code, see <xref linkend="ghci-modules-filenames"/>.) For
1614 example, to load the &ldquo;m&rdquo; library:</para>
1615
1616 <screen>
1617 $ ghci -lm
1618 </screen>
1619
1620 <para>On systems with <literal>.so</literal>-style shared
1621 libraries, the actual library loaded will the
1622 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
1623 searches the following places for libraries, in this order:</para>
1624
1625 <itemizedlist>
1626 <listitem>
1627 <para>Paths specified using the
1628 <literal>-L<replaceable>path</replaceable></literal>
1629 command-line option,</para>
1630 </listitem>
1631 <listitem>
1632 <para>the standard library search path for your system,
1633 which on some systems may be overridden by setting the
1634 <literal>LD_LIBRARY_PATH</literal> environment
1635 variable.</para>
1636 </listitem>
1637 </itemizedlist>
1638
1639 <para>On systems with <literal>.dll</literal>-style shared
1640 libraries, the actual library loaded will be
1641 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
1642 GHCi will signal an error if it can't find the library.</para>
1643
1644 <para>GHCi can also load plain object files
1645 (<literal>.o</literal> or <literal>.obj</literal> depending on
1646 your platform) from the command-line. Just add the name the
1647 object file to the command line.</para>
1648
1649 <para>Ordering of <option>-l</option> options matters: a library
1650 should be mentioned <emphasis>before</emphasis> the libraries it
1651 depends on (see <xref linkend="options-linker"/>).</para>
1652 </sect2>
1653
1654 </sect1>
1655
1656 <sect1 id="ghci-commands">
1657 <title>GHCi commands</title>
1658
1659 <para>GHCi commands all begin with
1660 &lsquo;<literal>:</literal>&rsquo; and consist of a single command
1661 name followed by zero or more parameters. The command name may be
1662 abbreviated, with ambiguities being resolved in favour of the more
1663 commonly used commands.</para>
1664
1665 <variablelist>
1666 <varlistentry>
1667 <term>
1668 <literal>:abandon</literal>
1669 <indexterm><primary><literal>:abandon</literal></primary></indexterm>
1670 </term>
1671 <listitem>
1672 <para>Abandons the current evaluation (only available when stopped at
1673 a breakpoint).</para>
1674 </listitem>
1675 </varlistentry>
1676
1677 <varlistentry>
1678 <term>
1679 <literal>:add</literal> <replaceable>module</replaceable> ...
1680 <indexterm><primary><literal>:add</literal></primary></indexterm>
1681 </term>
1682 <listitem>
1683 <para>Add <replaceable>module</replaceable>(s) to the
1684 current <firstterm>target set</firstterm>, and perform a
1685 reload.</para>
1686 </listitem>
1687 </varlistentry>
1688
1689 <varlistentry>
1690 <term>
1691 <literal>:back</literal>
1692 <indexterm><primary><literal>:back</literal></primary></indexterm>
1693 </term>
1694 <listitem>
1695 <para>Travel back one step in the history. See <xref
1696 linkend="tracing" />. See also:
1697 <literal>:trace</literal>, <literal>:history</literal>,
1698 <literal>:forward</literal>.</para>
1699 </listitem>
1700 </varlistentry>
1701
1702 <varlistentry>
1703 <term>
1704 <literal>:break [<replaceable>identifier</replaceable> |
1705 [<replaceable>module</replaceable>] <replaceable>line</replaceable>
1706 [<replaceable>column</replaceable>]]</literal>
1707 </term>
1708 <indexterm><primary><literal>:break</literal></primary></indexterm>
1709 <listitem>
1710 <para>Set a breakpoint on the specified function or line and
1711 column. See <xref linkend="setting-breakpoints" />.</para>
1712 </listitem>
1713 </varlistentry>
1714
1715 <varlistentry>
1716 <term>
1717 <literal>:browse</literal><optional><literal>!</literal></optional> <optional><optional><literal>*</literal></optional><replaceable>module</replaceable></optional> ...
1718 <indexterm><primary><literal>:browse</literal></primary></indexterm>
1719 </term>
1720 <listitem>
1721 <para>Displays the identifiers defined by the module
1722 <replaceable>module</replaceable>, which must be either
1723 loaded into GHCi or be a member of a package. If
1724 <replaceable>module</replaceable> is omitted, the most
1725 recently-loaded module is used.</para>
1726
1727 <para>If the <literal>*</literal> symbol is placed before
1728 the module name, then <emphasis>all</emphasis> the
1729 identifiers in scope in <replaceable>module</replaceable> are
1730 shown; otherwise the list is limited to the exports of
1731 <replaceable>module</replaceable>. The
1732 <literal>*</literal>-form is only available for modules
1733 which are interpreted; for compiled modules (including
1734 modules from packages) only the non-<literal>*</literal>
1735 form of <literal>:browse</literal> is available.
1736 If the <literal>!</literal> symbol is appended to the
1737 command, data constructors and class methods will be
1738 listed individually, otherwise, they will only be listed
1739 in the context of their data type or class declaration.
1740 The <literal>!</literal>-form also annotates the listing
1741 with comments giving possible imports for each group of
1742 entries.</para>
1743 <screen>
1744 Prelude> :browse! Data.Maybe
1745 -- not currently imported
1746 Data.Maybe.catMaybes :: [Maybe a] -> [a]
1747 Data.Maybe.fromJust :: Maybe a -> a
1748 Data.Maybe.fromMaybe :: a -> Maybe a -> a
1749 Data.Maybe.isJust :: Maybe a -> Bool
1750 Data.Maybe.isNothing :: Maybe a -> Bool
1751 Data.Maybe.listToMaybe :: [a] -> Maybe a
1752 Data.Maybe.mapMaybe :: (a -> Maybe b) -> [a] -> [b]
1753 Data.Maybe.maybeToList :: Maybe a -> [a]
1754 -- imported via Prelude
1755 Just :: a -> Maybe a
1756 data Maybe a = Nothing | Just a
1757 Nothing :: Maybe a
1758 maybe :: b -> (a -> b) -> Maybe a -> b
1759 </screen>
1760 <para>
1761 This output shows that, in the context of the current session, in the scope
1762 of <literal>Prelude</literal>, the first group of items from
1763 <literal>Data.Maybe</literal> have not been imported (but are available in
1764 fully qualified form in the GHCi session - see <xref
1765 linkend="ghci-scope"/>), whereas the second group of items have been
1766 imported via <literal>Prelude</literal> and are therefore available either
1767 unqualified, or with a <literal>Prelude.</literal> qualifier.
1768 </para>
1769 </listitem>
1770 </varlistentry>
1771
1772 <varlistentry>
1773 <term>
1774 <literal>:cd</literal> <replaceable>dir</replaceable>
1775 <indexterm><primary><literal>:cd</literal></primary></indexterm>
1776 </term>
1777 <listitem>
1778 <para>Changes the current working directory to
1779 <replaceable>dir</replaceable>. A
1780 &lsquo;<literal>&tilde;</literal>&rsquo; symbol at the
1781 beginning of <replaceable>dir</replaceable> will be replaced
1782 by the contents of the environment variable
1783 <literal>HOME</literal>.</para>
1784
1785 <para>NOTE: changing directories causes all currently loaded
1786 modules to be unloaded. This is because the search path is
1787 usually expressed using relative directories, and changing
1788 the search path in the middle of a session is not
1789 supported.</para>
1790 </listitem>
1791 </varlistentry>
1792
1793 <varlistentry>
1794 <term>
1795 <literal>:cmd</literal> <replaceable>expr</replaceable>
1796 <indexterm><primary><literal>:cmd</literal></primary></indexterm>
1797 </term>
1798 <listitem>
1799 <para>Executes <replaceable>expr</replaceable> as a computation of
1800 type <literal>IO String</literal>, and then executes the resulting
1801 string as a list of GHCi commands. Multiple commands are separated
1802 by newlines. The <literal>:cmd</literal> command is useful with
1803 <literal>:def</literal> and <literal>:set stop</literal>.</para>
1804 </listitem>
1805 </varlistentry>
1806
1807 <varlistentry>
1808 <term>
1809 <literal>:continue</literal>
1810 <indexterm><primary><literal>:continue</literal></primary></indexterm>
1811 </term>
1812 <listitem><para>Continue the current evaluation, when stopped at a
1813 breakpoint.</para>
1814 </listitem>
1815 </varlistentry>
1816
1817 <varlistentry>
1818 <term>
1819 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1820 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1821 <indexterm><primary><literal>:etags</literal></primary>
1822 </indexterm>
1823 <indexterm><primary><literal>:etags</literal></primary>
1824 </indexterm>
1825 </term>
1826 <listitem>
1827 <para>Generates a &ldquo;tags&rdquo; file for Vi-style editors
1828 (<literal>:ctags</literal>) or
1829 Emacs-style editors (<literal>:etags</literal>). If
1830 no filename is specified, the default <filename>tags</filename> or
1831 <filename>TAGS</filename> is
1832 used, respectively. Tags for all the functions, constructors and
1833 types in the currently loaded modules are created. All modules must
1834 be interpreted for these commands to work.</para>
1835 <para>See also <xref linkend="hasktags" />.</para>
1836 </listitem>
1837 </varlistentry>
1838
1839 <varlistentry>
1840 <term>
1841 <literal>:def<optional>!</optional> <optional><replaceable>name</replaceable> <replaceable>expr</replaceable></optional></literal>
1842 <indexterm><primary><literal>:def</literal></primary></indexterm>
1843 </term>
1844 <listitem>
1845 <para><literal>:def</literal> is used to define new
1846 commands, or macros, in GHCi. The command
1847 <literal>:def</literal> <replaceable>name</replaceable>
1848 <replaceable>expr</replaceable> defines a new GHCi command
1849 <literal>:<replaceable>name</replaceable></literal>,
1850 implemented by the Haskell expression
1851 <replaceable>expr</replaceable>, which must have type
1852 <literal>String -> IO String</literal>. When
1853 <literal>:<replaceable>name</replaceable>
1854 <replaceable>args</replaceable></literal> is typed at the
1855 prompt, GHCi will run the expression
1856 <literal>(<replaceable>name</replaceable>
1857 <replaceable>args</replaceable>)</literal>, take the
1858 resulting <literal>String</literal>, and feed it back into
1859 GHCi as a new sequence of commands. Separate commands in
1860 the result must be separated by
1861 &lsquo;<literal>\n</literal>&rsquo;.</para>
1862
1863 <para>That's all a little confusing, so here's a few
1864 examples. To start with, here's a new GHCi command which
1865 doesn't take any arguments or produce any results, it just
1866 outputs the current date &amp; time:</para>
1867
1868 <screen>
1869 Prelude> let date _ = Time.getClockTime >>= print >> return ""
1870 Prelude> :def date date
1871 Prelude> :date
1872 Fri Mar 23 15:16:40 GMT 2001
1873 </screen>
1874
1875 <para>Here's an example of a command that takes an argument.
1876 It's a re-implementation of <literal>:cd</literal>:</para>
1877
1878 <screen>
1879 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
1880 Prelude> :def mycd mycd
1881 Prelude> :mycd ..
1882 </screen>
1883
1884 <para>Or I could define a simple way to invoke
1885 &ldquo;<literal>ghc &ndash;&ndash;make Main</literal>&rdquo; in the
1886 current directory:</para>
1887
1888 <screen>
1889 Prelude> :def make (\_ -> return ":! ghc &ndash;&ndash;make Main")
1890 </screen>
1891
1892 <para>We can define a command that reads GHCi input from a
1893 file. This might be useful for creating a set of bindings
1894 that we want to repeatedly load into the GHCi session:</para>
1895
1896 <screen>
1897 Prelude> :def . readFile
1898 Prelude> :. cmds.ghci
1899 </screen>
1900
1901 <para>Notice that we named the command
1902 <literal>:.</literal>, by analogy with the
1903 &lsquo;<literal>.</literal>&rsquo; Unix shell command that
1904 does the same thing.</para>
1905
1906 <para>Typing <literal>:def</literal> on its own lists the
1907 currently-defined macros. Attempting to redefine an
1908 existing command name results in an error unless the
1909 <literal>:def!</literal> form is used, in which case the old
1910 command with that name is silently overwritten.</para>
1911 </listitem>
1912 </varlistentry>
1913
1914 <varlistentry>
1915 <term>
1916 <literal>:delete * | <replaceable>num</replaceable> ...</literal>
1917 <indexterm><primary><literal>:delete</literal></primary></indexterm>
1918 </term>
1919 <listitem>
1920 <para>Delete one or more breakpoints by number (use <literal>:show
1921 breaks</literal> to see the number of each breakpoint). The
1922 <literal>*</literal> form deletes all the breakpoints.</para>
1923 </listitem>
1924 </varlistentry>
1925
1926 <varlistentry>
1927 <term>
1928 <literal>:edit <optional><replaceable>file</replaceable></optional></literal>
1929 <indexterm><primary><literal>:edit</literal></primary></indexterm>
1930 </term>
1931 <listitem>
1932 <para>Opens an editor to edit the file
1933 <replaceable>file</replaceable>, or the most recently loaded
1934 module if <replaceable>file</replaceable> is omitted. The
1935 editor to invoke is taken from the <literal>EDITOR</literal>
1936 environment variable, or a default editor on your system if
1937 <literal>EDITOR</literal> is not set. You can change the
1938 editor using <literal>:set editor</literal>.</para>
1939 </listitem>
1940 </varlistentry>
1941
1942 <varlistentry>
1943 <term>
1944 <literal>:etags</literal>
1945 </term>
1946 <listitem>
1947 <para>See <literal>:ctags</literal>.</para>
1948 </listitem>
1949 </varlistentry>
1950
1951 <varlistentry>
1952 <term>
1953 <literal>:force <replaceable>identifier</replaceable> ...</literal>
1954 <indexterm><primary><literal>:force</literal></primary></indexterm>
1955 </term>
1956 <listitem>
1957 <para>Prints the value of <replaceable>identifier</replaceable> in
1958 the same way as <literal>:print</literal>. Unlike
1959 <literal>:print</literal>, <literal>:force</literal> evaluates each
1960 thunk that it encounters while traversing the value. This may
1961 cause exceptions or infinite loops, or further breakpoints (which
1962 are ignored, but displayed).</para>
1963 </listitem>
1964 </varlistentry>
1965
1966 <varlistentry>
1967 <term>
1968 <literal>:forward</literal>
1969 <indexterm><primary><literal>:forward</literal></primary></indexterm>
1970 </term>
1971 <listitem>
1972 <para>Move forward in the history. See <xref
1973 linkend="tracing" />. See also:
1974 <literal>:trace</literal>, <literal>:history</literal>,
1975 <literal>:back</literal>.</para>
1976 </listitem>
1977 </varlistentry>
1978
1979 <varlistentry>
1980 <term>
1981 <literal>:help</literal>
1982 <indexterm><primary><literal>:help</literal></primary></indexterm>
1983 </term>
1984 <term>
1985 <literal>:?</literal>
1986 <indexterm><primary><literal>:?</literal></primary></indexterm>
1987 </term>
1988 <listitem>
1989 <para>Displays a list of the available commands.</para>
1990 </listitem>
1991 </varlistentry>
1992
1993 <varlistentry>
1994 <term>
1995 <literal>:history [<replaceable>num</replaceable>]</literal>
1996 <indexterm><primary><literal>:history</literal></primary></indexterm>
1997 </term>
1998 <listitem>
1999 <para>Display the history of evaluation steps. With a number,
2000 displays that many steps (default: 20). For use with
2001 <literal>:trace</literal>; see <xref
2002 linkend="tracing" />.</para>
2003 </listitem>
2004 </varlistentry>
2005
2006 <varlistentry>
2007 <term>
2008 <literal>:info</literal> <replaceable>name</replaceable> ...
2009 <indexterm><primary><literal>:info</literal></primary></indexterm>
2010 </term>
2011 <listitem>
2012 <para>Displays information about the given name(s). For
2013 example, if <replaceable>name</replaceable> is a class, then
2014 the class methods and their types will be printed; if
2015 <replaceable>name</replaceable> is a type constructor, then
2016 its definition will be printed; if
2017 <replaceable>name</replaceable> is a function, then its type
2018 will be printed. If <replaceable>name</replaceable> has
2019 been loaded from a source file, then GHCi will also display
2020 the location of its definition in the source.</para>
2021 <para>For types and classes, GHCi also summarises instances that
2022 mention them. To avoid showing irrelevant information, an instance
2023 is shown only if (a) its head mentions <replaceable>name</replaceable>,
2024 and (b) all the other things mentioned in the instance
2025 are in scope (either qualified or otherwise) as a result of
2026 a <literal>:load</literal> or <literal>:module</literal> commands. </para>
2027 </listitem>
2028 </varlistentry>
2029
2030 <varlistentry>
2031 <term>
2032 <literal>:kind</literal> <replaceable>type</replaceable>
2033 <indexterm><primary><literal>:kind</literal></primary></indexterm>
2034 </term>
2035 <listitem>
2036 <para>Infers and prints the kind of
2037 <replaceable>type</replaceable>. The latter can be an arbitrary
2038 type expression, including a partial application of a type constructor,
2039 such as <literal>Either Int</literal>.</para>
2040 </listitem>
2041 </varlistentry>
2042
2043 <varlistentry>
2044 <term>
2045 <literal>:load</literal> <replaceable>module</replaceable> ...
2046 <indexterm><primary><literal>:load</literal></primary></indexterm>
2047 </term>
2048 <listitem>
2049 <para>Recursively loads the specified
2050 <replaceable>module</replaceable>s, and all the modules they
2051 depend on. Here, each <replaceable>module</replaceable>
2052 must be a module name or filename, but may not be the name
2053 of a module in a package.</para>
2054
2055 <para>All previously loaded modules, except package modules,
2056 are forgotten. The new set of modules is known as the
2057 <firstterm>target set</firstterm>. Note that
2058 <literal>:load</literal> can be used without any arguments
2059 to unload all the currently loaded modules and
2060 bindings.</para>
2061
2062 <para>After a <literal>:load</literal> command, the current
2063 context is set to:</para>
2064
2065 <itemizedlist>
2066 <listitem>
2067 <para><replaceable>module</replaceable>, if it was loaded
2068 successfully, or</para>
2069 </listitem>
2070 <listitem>
2071 <para>the most recently successfully loaded module, if
2072 any other modules were loaded as a result of the current
2073 <literal>:load</literal>, or</para>
2074 </listitem>
2075 <listitem>
2076 <para><literal>Prelude</literal> otherwise.</para>
2077 </listitem>
2078 </itemizedlist>
2079 </listitem>
2080 </varlistentry>
2081
2082 <varlistentry>
2083 <term>
2084 <literal>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
2085 <indexterm><primary><literal>:main</literal></primary></indexterm>
2086 </term>
2087 <listitem>
2088 <para>
2089 When a program is compiled and executed, it can use the
2090 <literal>getArgs</literal> function to access the
2091 command-line arguments.
2092 However, we cannot simply pass the arguments to the
2093 <literal>main</literal> function while we are testing in ghci,
2094 as the <literal>main</literal> function doesn't take its
2095 arguments directly.
2096 </para>
2097
2098 <para>
2099 Instead, we can use the <literal>:main</literal> command.
2100 This runs whatever <literal>main</literal> is in scope, with
2101 any arguments being treated the same as command-line arguments,
2102 e.g.:
2103 </para>
2104
2105 <screen>
2106 Prelude> let main = System.Environment.getArgs >>= print
2107 Prelude> :main foo bar
2108 ["foo","bar"]
2109 </screen>
2110
2111 </listitem>
2112 </varlistentry>
2113
2114 <varlistentry>
2115 <term>
2116 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
2117 <indexterm><primary><literal>:module</literal></primary></indexterm>
2118 </term>
2119 <term>
2120 <literal>import <replaceable>mod</replaceable></literal>
2121 </term>
2122 <listitem>
2123 <para>Sets or modifies the current context for statements
2124 typed at the prompt. The form <literal>import
2125 <replaceable>mod</replaceable></literal> is equivalent to
2126 <literal>:module +<replaceable>mod</replaceable></literal>.
2127 See <xref linkend="ghci-scope"/> for
2128 more details.</para>
2129 </listitem>
2130 </varlistentry>
2131
2132 <varlistentry>
2133 <term>
2134 <literal>:print </literal> <replaceable>names</replaceable> ...
2135 <indexterm><primary><literal>:print</literal></primary></indexterm>
2136 </term>
2137 <listitem>
2138 <para>Prints a value without forcing its evaluation.
2139 <literal>:print</literal> may be used on values whose types are
2140 unknown or partially known, which might be the case for local
2141 variables with polymorphic types at a breakpoint. While inspecting
2142 the runtime value, <literal>:print</literal> attempts to
2143 reconstruct the type of the value, and will elaborate the type in
2144 GHCi's environment if possible. If any unevaluated components
2145 (thunks) are encountered, then <literal>:print</literal> binds
2146 a fresh variable with a name beginning with <literal>_t</literal>
2147 to each thunk. See <xref linkend="breakpoints" /> for more
2148 information. See also the <literal>:sprint</literal> command,
2149 which works like <literal>:print</literal> but does not bind new
2150 variables.</para>
2151 </listitem>
2152 </varlistentry>
2153
2154 <varlistentry>
2155 <term>
2156 <literal>:quit</literal>
2157 <indexterm><primary><literal>:quit</literal></primary></indexterm>
2158 </term>
2159 <listitem>
2160 <para>Quits GHCi. You can also quit by typing control-D
2161 at the prompt.</para>
2162 </listitem>
2163 </varlistentry>
2164
2165 <varlistentry>
2166 <term>
2167 <literal>:reload</literal>
2168 <indexterm><primary><literal>:reload</literal></primary></indexterm>
2169 </term>
2170 <listitem>
2171 <para>Attempts to reload the current target set (see
2172 <literal>:load</literal>) if any of the modules in the set,
2173 or any dependent module, has changed. Note that this may
2174 entail loading new modules, or dropping modules which are no
2175 longer indirectly required by the target.</para>
2176 </listitem>
2177 </varlistentry>
2178
2179 <varlistentry>
2180 <term>
2181 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
2182 <indexterm><primary><literal>:set</literal></primary></indexterm>
2183 </term>
2184 <listitem>
2185 <para>Sets various options. See <xref linkend="ghci-set"/> for a list of
2186 available options and <xref linkend="interactive-mode-options"/> for a
2187 list of GHCi-specific flags. The <literal>:set</literal> command by
2188 itself shows which options are currently set. It also lists the current
2189 dynamic flag settings, with GHCi-specific flags listed separately.</para>
2190 </listitem>
2191 </varlistentry>
2192
2193 <varlistentry>
2194 <term>
2195 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
2196 <indexterm><primary><literal>:set args</literal></primary></indexterm>
2197 </term>
2198 <listitem>
2199 <para>Sets the list of arguments which are returned when the
2200 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
2201 </indexterm>.</para>
2202 </listitem>
2203 </varlistentry>
2204
2205 <varlistentry>
2206 <term>
2207 <literal>:set</literal> <literal>editor</literal> <replaceable>cmd</replaceable>
2208 </term>
2209 <listitem>
2210 <para>Sets the command used by <literal>:edit</literal> to
2211 <replaceable>cmd</replaceable>.</para>
2212 </listitem>
2213 </varlistentry>
2214
2215 <varlistentry>
2216 <term>
2217 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
2218 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
2219 </term>
2220 <listitem>
2221 <para>Sets the string to be returned when the program calls
2222 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
2223 </indexterm>.</para>
2224 </listitem>
2225 </varlistentry>
2226
2227 <varlistentry>
2228 <term>
2229 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
2230 </term>
2231 <listitem>
2232 <para>Sets the string to be used as the prompt in GHCi.
2233 Inside <replaceable>prompt</replaceable>, the sequence
2234 <literal>%s</literal> is replaced by the names of the
2235 modules currently in scope, and <literal>%%</literal> is
2236 replaced by <literal>%</literal>.</para>
2237 </listitem>
2238 </varlistentry>
2239
2240 <varlistentry>
2241 <term>
2242 <literal>:set</literal> <literal>stop</literal>
2243 [<replaceable>num</replaceable>] <replaceable>cmd</replaceable>
2244 </term>
2245 <listitem>
2246 <para>Set a command to be executed when a breakpoint is hit, or a new
2247 item in the history is selected. The most common use of
2248 <literal>:set stop</literal> is to display the source code at the
2249 current location, e.g. <literal>:set stop :list</literal>.</para>
2250
2251 <para>If a number is given before the command, then the commands are
2252 run when the specified breakpoint (only) is hit. This can be quite
2253 useful: for example, <literal>:set stop 1 :continue</literal>
2254 effectively disables breakpoint 1, by running
2255 <literal>:continue</literal> whenever it is hit (although GHCi will
2256 still emit a message to say the breakpoint was hit). What's more,
2257 with cunning use of <literal>:def</literal> and
2258 <literal>:cmd</literal> you can use <literal>:set stop</literal> to
2259 implement conditional breakpoints:</para>
2260 <screen>
2261 *Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
2262 *Main> :set stop 0 :cond (x &lt; 3)
2263 </screen>
2264 <para>Ignoring breakpoints for a specified number of iterations is
2265 also possible using similar techniques.</para>
2266 </listitem>
2267 </varlistentry>
2268
2269 <varlistentry>
2270 <term>
2271 <literal>:show bindings</literal>
2272 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
2273 </term>
2274 <listitem>
2275 <para>Show the bindings made at the prompt and their
2276 types.</para>
2277 </listitem>
2278 </varlistentry>
2279
2280 <varlistentry>
2281 <term>
2282 <literal>:show breaks</literal>
2283 <indexterm><primary><literal>:show breaks</literal></primary></indexterm>
2284 </term>
2285 <listitem>
2286 <para>List the active breakpoints.</para>
2287 </listitem>
2288 </varlistentry>
2289
2290 <varlistentry>
2291 <term>
2292 <literal>:show context</literal>
2293 <indexterm><primary><literal>:show context</literal></primary></indexterm>
2294 </term>
2295 <listitem>
2296 <para>List the active evaluations that are stopped at breakpoints.</para>
2297 </listitem>
2298 </varlistentry>
2299
2300 <varlistentry>
2301 <term>
2302 <literal>:show modules</literal>
2303 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
2304 </term>
2305 <listitem>
2306 <para>Show the list of modules currently loaded.</para>
2307 </listitem>
2308 </varlistentry>
2309
2310 <varlistentry>
2311 <term>
2312 <literal>:show packages</literal>
2313 <indexterm><primary><literal>:show packages</literal></primary></indexterm>
2314 </term>
2315 <listitem>
2316 <para>Show the currently active package flags, as well as the list of
2317 packages currently loaded.</para>
2318 </listitem>
2319 </varlistentry>
2320
2321 <varlistentry>
2322 <term>
2323 <literal>:show languages</literal>
2324 <indexterm><primary><literal>:show languages</literal></primary></indexterm>
2325 </term>
2326 <listitem>
2327 <para>Show the currently active language flags.</para>
2328 </listitem>
2329 </varlistentry>
2330
2331
2332 <varlistentry>
2333 <term>
2334 <literal>:show [args|prog|prompt|editor|stop]</literal>
2335 <indexterm><primary><literal>:show</literal></primary></indexterm>
2336 </term>
2337 <listitem>
2338 <para>Displays the specified setting (see
2339 <literal>:set</literal>).</para>
2340 </listitem>
2341 </varlistentry>
2342
2343 <varlistentry>
2344 <term>
2345 <literal>:sprint</literal>
2346 <indexterm><primary><literal>:sprint</literal></primary></indexterm>
2347 </term>
2348 <listitem>
2349 <para>Prints a value without forcing its evaluation.
2350 <literal>:sprint</literal> is similar to <literal>:print</literal>,
2351 with the difference that unevaluated subterms are not bound to new
2352 variables, they are simply denoted by &lsquo;_&rsquo;.</para>
2353 </listitem>
2354 </varlistentry>
2355
2356 <varlistentry>
2357 <term>
2358 <literal>:step [<replaceable>expr</replaceable>]</literal>
2359 <indexterm><primary><literal>:step</literal></primary></indexterm>
2360 </term>
2361 <listitem>
2362 <para>Single-step from the last breakpoint. With an expression
2363 argument, begins evaluation of the expression with a
2364 single-step.</para>
2365 </listitem>
2366 </varlistentry>
2367
2368 <varlistentry>
2369 <term>
2370 <literal>:trace [<replaceable>expr</replaceable>]</literal>
2371 <indexterm><primary><literal>:trace</literal></primary></indexterm>
2372 </term>
2373 <listitem>
2374 <para>Evaluates the given expression (or from the last breakpoint if
2375 no expression is given), and additionally logs the evaluation
2376 steps for later inspection using <literal>:history</literal>. See
2377 <xref linkend="tracing" />.</para>
2378 </listitem>
2379 </varlistentry>
2380
2381 <varlistentry>
2382 <term>
2383 <literal>:type</literal> <replaceable>expression</replaceable>
2384 <indexterm><primary><literal>:type</literal></primary></indexterm>
2385 </term>
2386 <listitem>
2387 <para>Infers and prints the type of
2388 <replaceable>expression</replaceable>, including explicit
2389 forall quantifiers for polymorphic types. The monomorphism
2390 restriction is <emphasis>not</emphasis> applied to the
2391 expression during type inference.</para>
2392 </listitem>
2393 </varlistentry>
2394
2395 <varlistentry>
2396 <term>
2397 <literal>:undef</literal> <replaceable>name</replaceable>
2398 <indexterm><primary><literal>:undef</literal></primary></indexterm>
2399 </term>
2400 <listitem>
2401 <para>Undefines the user-defined command
2402 <replaceable>name</replaceable> (see <literal>:def</literal>
2403 above).</para>
2404 </listitem>
2405 </varlistentry>
2406
2407 <varlistentry>
2408 <term>
2409 <literal>:unset</literal> <replaceable>option</replaceable>...
2410 <indexterm><primary><literal>:unset</literal></primary></indexterm>
2411 </term>
2412 <listitem>
2413 <para>Unsets certain options. See <xref linkend="ghci-set"/>
2414 for a list of available options.</para>
2415 </listitem>
2416 </varlistentry>
2417
2418 <varlistentry>
2419 <term>
2420 <literal>:!</literal> <replaceable>command</replaceable>...
2421 <indexterm><primary><literal>:!</literal></primary></indexterm>
2422 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
2423 </term>
2424 <listitem>
2425 <para>Executes the shell command
2426 <replaceable>command</replaceable>.</para>
2427 </listitem>
2428 </varlistentry>
2429
2430 </variablelist>
2431 </sect1>
2432
2433 <sect1 id="ghci-set">
2434 <title>The <literal>:set</literal> command</title>
2435 <indexterm><primary><literal>:set</literal></primary></indexterm>
2436
2437 <para>The <literal>:set</literal> command sets two types of
2438 options: GHCi options, which begin with
2439 &lsquo;<literal>+</literal>&rsquo;, and &ldquo;command-line&rdquo;
2440 options, which begin with &lsquo;-&rsquo;. </para>
2441
2442 <para>NOTE: at the moment, the <literal>:set</literal> command
2443 doesn't support any kind of quoting in its arguments: quotes will
2444 not be removed and cannot be used to group words together. For
2445 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
2446 you expect.</para>
2447
2448 <sect2>
2449 <title>GHCi options</title>
2450 <indexterm><primary>options</primary><secondary>GHCi</secondary>
2451 </indexterm>
2452
2453 <para>GHCi options may be set using <literal>:set</literal> and
2454 unset using <literal>:unset</literal>.</para>
2455
2456 <para>The available GHCi options are:</para>
2457
2458 <variablelist>
2459 <varlistentry>
2460 <term>
2461 <literal>+r</literal>
2462 <indexterm><primary><literal>+r</literal></primary></indexterm>
2463 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
2464 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
2465 </term>
2466 <listitem>
2467 <para>Normally, any evaluation of top-level expressions
2468 (otherwise known as CAFs or Constant Applicative Forms) in
2469 loaded modules is retained between evaluations. Turning
2470 on <literal>+r</literal> causes all evaluation of
2471 top-level expressions to be discarded after each
2472 evaluation (they are still retained
2473 <emphasis>during</emphasis> a single evaluation).</para>
2474
2475 <para>This option may help if the evaluated top-level
2476 expressions are consuming large amounts of space, or if
2477 you need repeatable performance measurements.</para>
2478 </listitem>
2479 </varlistentry>
2480
2481 <varlistentry>
2482 <term>
2483 <literal>+s</literal>
2484 <indexterm><primary><literal>+s</literal></primary></indexterm>
2485 </term>
2486 <listitem>
2487 <para>Display some stats after evaluating each expression,
2488 including the elapsed time and number of bytes allocated.
2489 NOTE: the allocation figure is only accurate to the size
2490 of the storage manager's allocation area, because it is
2491 calculated at every GC. Hence, you might see values of
2492 zero if no GC has occurred.</para>
2493 </listitem>
2494 </varlistentry>
2495
2496 <varlistentry>
2497 <term>
2498 <literal>+t</literal>
2499 <indexterm><primary><literal>+t</literal></primary></indexterm>
2500 </term>
2501 <listitem>
2502 <para>Display the type of each variable bound after a
2503 statement is entered at the prompt. If the statement is a
2504 single expression, then the only variable binding will be
2505 for the variable
2506 &lsquo;<literal>it</literal>&rsquo;.</para>
2507 </listitem>
2508 </varlistentry>
2509 </variablelist>
2510 </sect2>
2511
2512 <sect2 id="ghci-cmd-line-options">
2513 <title>Setting GHC command-line options in GHCi</title>
2514
2515 <para>Normal GHC command-line options may also be set using
2516 <literal>:set</literal>. For example, to turn on
2517 <option>-fglasgow-exts</option>, you would say:</para>
2518
2519 <screen>
2520 Prelude> :set -fglasgow-exts
2521 </screen>
2522
2523 <para>Any GHC command-line option that is designated as
2524 <firstterm>dynamic</firstterm> (see the table in <xref
2525 linkend="flag-reference"/>), may be set using
2526 <literal>:set</literal>. To unset an option, you can set the
2527 reverse option:</para>
2528 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
2529
2530 <screen>
2531 Prelude> :set -fno-glasgow-exts
2532 </screen>
2533
2534 <para><xref linkend="flag-reference"/> lists the reverse for each
2535 option where applicable.</para>
2536
2537 <para>Certain static options (<option>-package</option>,
2538 <option>-I</option>, <option>-i</option>, and
2539 <option>-l</option> in particular) will also work, but some may
2540 not take effect until the next reload.</para>
2541 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
2542 </sect2>
2543 </sect1>
2544 <sect1 id="ghci-dot-files">
2545 <title>The <filename>.ghci</filename> file</title>
2546 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
2547 </indexterm>
2548 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
2549 </indexterm>
2550
2551 <para>When it starts, unless the <literal>-ignore-dot-ghci</literal>
2552 flag is given, GHCi reads and executes commands from
2553 <filename>./.ghci</filename>, followed by
2554 <filename>$HOME/.ghci</filename>.</para>
2555
2556 <para>The <filename>.ghci</filename> in your home directory is
2557 most useful for turning on favourite options (eg. <literal>:set
2558 +s</literal>), and defining useful macros. Placing a
2559 <filename>.ghci</filename> file in a directory with a Haskell
2560 project is a useful way to set certain project-wide options so you
2561 don't have to type them everytime you start GHCi: eg. if your
2562 project uses GHC extensions and CPP, and has source files in three
2563 subdirectories A, B and C, you might put the following lines in
2564 <filename>.ghci</filename>:</para>
2565
2566 <screen>
2567 :set -fglasgow-exts -cpp
2568 :set -iA:B:C
2569 </screen>
2570
2571 <para>(Note that strictly speaking the <option>-i</option> flag is
2572 a static one, but in fact it works to set it using
2573 <literal>:set</literal> like this. The changes won't take effect
2574 until the next <literal>:load</literal>, though.)</para>
2575
2576 <para>Two command-line options control whether the
2577 <filename>.ghci</filename> files are read:</para>
2578
2579 <variablelist>
2580 <varlistentry>
2581 <term>
2582 <option>-ignore-dot-ghci</option>
2583 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
2584 </term>
2585 <listitem>
2586 <para>Don't read either <filename>./.ghci</filename> or
2587 <filename>$HOME/.ghci</filename> when starting up.</para>
2588 </listitem>
2589 </varlistentry>
2590 <varlistentry>
2591 <term>
2592 <option>-read-dot-ghci</option>
2593 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
2594 </term>
2595 <listitem>
2596 <para>Read <filename>.ghci</filename> and
2597 <filename>$HOME/.ghci</filename>. This is normally the
2598 default, but the <option>-read-dot-ghci</option> option may
2599 be used to override a previous
2600 <option>-ignore-dot-ghci</option> option.</para>
2601 </listitem>
2602 </varlistentry>
2603 </variablelist>
2604
2605 </sect1>
2606
2607 <sect1 id="ghci-obj">
2608 <title>Compiling to object code inside GHCi</title>
2609
2610 <para>By default, GHCi compiles Haskell source code into byte-code
2611 that is interpreted by the runtime system. GHCi can also compile
2612 Haskell code to object code: to turn on this feature, use the
2613 <option>-fobject-code</option> flag either on the command line or
2614 with <literal>:set</literal> (the option
2615 <option>-fbyte-code</option> restores byte-code compilation
2616 again). Compiling to object code takes longer, but typically the
2617 code will execute 10-20 times faster than byte-code.</para>
2618
2619 <para>Compiling to object code inside GHCi is particularly useful
2620 if you are developing a compiled application, because the
2621 <literal>:reload</literal> command typically runs much faster than
2622 restarting GHC with <option>--make</option> from the command-line,
2623 because all the interface files are already cached in
2624 memory.</para>
2625
2626 <para>There are disadvantages to compiling to object-code: you
2627 can't set breakpoints in object-code modules, for example. Only
2628 the exports of an object-code module will be visible in GHCi,
2629 rather than all top-level bindings as in interpreted
2630 modules.</para>
2631 </sect1>
2632
2633 <sect1 id="ghci-faq">
2634 <title>FAQ and Things To Watch Out For</title>
2635
2636 <variablelist>
2637 <varlistentry>
2638 <term>The interpreter can't load modules with foreign export
2639 declarations!</term>
2640 <listitem>
2641 <para>Unfortunately not. We haven't implemented it yet.
2642 Please compile any offending modules by hand before loading
2643 them into GHCi.</para>
2644 </listitem>
2645 </varlistentry>
2646
2647 <varlistentry>
2648 <term>
2649 <literal>-O</literal> doesn't work with GHCi!
2650 <indexterm><primary><option>-O</option></primary></indexterm>
2651 </term>
2652 <listitem>
2653 <para>For technical reasons, the bytecode compiler doesn't
2654 interact well with one of the optimisation passes, so we
2655 have disabled optimisation when using the interpreter. This
2656 isn't a great loss: you'll get a much bigger win by
2657 compiling the bits of your code that need to go fast, rather
2658 than interpreting them with optimisation turned on.</para>
2659 </listitem>
2660 </varlistentry>
2661
2662 <varlistentry>
2663 <term>Unboxed tuples don't work with GHCi</term>
2664 <listitem>
2665 <para>That's right. You can always compile a module that
2666 uses unboxed tuples and load it into GHCi, however.
2667 (Incidentally the previous point, namely that
2668 <literal>-O</literal> is incompatible with GHCi, is because
2669 the bytecode compiler can't deal with unboxed
2670 tuples).</para>
2671 </listitem>
2672 </varlistentry>
2673
2674 <varlistentry>
2675 <term>Concurrent threads don't carry on running when GHCi is
2676 waiting for input.</term>
2677 <listitem>
2678 <para>This should work, as long as your GHCi was built with
2679 the <option>-threaded</option> switch, which is the default.
2680 Consult whoever supplied your GHCi installation.</para>
2681 </listitem>
2682 </varlistentry>
2683
2684 <varlistentry>
2685 <term>After using <literal>getContents</literal>, I can't use
2686 <literal>stdin</literal> again until I do
2687 <literal>:load</literal> or <literal>:reload</literal>.</term>
2688
2689 <listitem>
2690 <para>This is the defined behaviour of
2691 <literal>getContents</literal>: it puts the stdin Handle in
2692 a state known as <firstterm>semi-closed</firstterm>, wherein
2693 any further I/O operations on it are forbidden. Because I/O
2694 state is retained between computations, the semi-closed
2695 state persists until the next <literal>:load</literal> or
2696 <literal>:reload</literal> command.</para>
2697
2698 <para>You can make <literal>stdin</literal> reset itself
2699 after every evaluation by giving GHCi the command
2700 <literal>:set +r</literal>. This works because
2701 <literal>stdin</literal> is just a top-level expression that
2702 can be reverted to its unevaluated state in the same way as
2703 any other top-level expression (CAF).</para>
2704 </listitem>
2705 </varlistentry>
2706
2707 <varlistentry>
2708 <term>I can't use Control-C to interrupt computations in
2709 GHCi on Windows.</term>
2710 <listitem>
2711 <para>See <xref linkend="ghci-windows"/>.</para>
2712 </listitem>
2713 </varlistentry>
2714
2715 <varlistentry>
2716 <term>The default buffering mode is different in GHCi to GHC.</term>
2717 <listitem>
2718 <para>
2719 In GHC, the stdout handle is line-buffered by default.
2720 However, in GHCi we turn off the buffering on stdout,
2721 because this is normally what you want in an interpreter:
2722 output appears as it is generated.
2723 </para>
2724 </listitem>
2725 </varlistentry>
2726 </variablelist>
2727 </sect1>
2728
2729 </chapter>
2730
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