Improve documentation of INLINE pragmas
[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.</para>
18 <indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
19 <indexterm><primary>Foreign Function Interface</primary><secondary>GHCi support</secondary></indexterm>
20
21 <sect1>
22 <title>Introduction to GHCi</title>
23
24 <para>Let's start with an example GHCi session. You can fire up
25 GHCi with the command <literal>ghci</literal>:</para>
26
27 <screen>
28 $ ghci
29 ___ ___ _
30 / _ \ /\ /\/ __(_)
31 / /_\// /_/ / / | | GHC Interactive, version 5.04, for Haskell 98.
32 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
33 \____/\/ /_/\____/|_| Type :? for help.
34
35 Loading package base ... linking ... done.
36 Loading package haskell98 ... linking ... done.
37 Prelude>
38 </screen>
39
40 <para>There may be a short pause while GHCi loads the prelude and
41 standard libraries, after which the prompt is shown. If we follow
42 the instructions and type <literal>:?</literal> for help, we
43 get:</para>
44
45 <screen>
46 Commands available from the prompt:
47
48 &lt;stmt&gt; evaluate/run &lt;stmt&gt;
49 :add &lt;filename&gt; ... add module(s) to the current target set
50 :browse [*]&lt;module&gt; display the names defined by &lt;module&gt;
51 :cd &lt;dir&gt; change directory to &lt;dir&gt;
52 :def &lt;cmd&gt; &lt;expr&gt; define a command :&lt;cmd&gt;
53 :help, :? display this list of commands
54 :info [&lt;name&gt; ...] display information about the given names
55 :load &lt;filename&gt; ... load module(s) and their dependents
56 :module [+/-] [*]&lt;mod&gt; ... set the context for expression evaluation
57 :reload reload the current module set
58
59 :set &lt;option&gt; ... set options
60 :set args &lt;arg&gt; ... set the arguments returned by System.getArgs
61 :set prog &lt;progname&gt; set the value returned by System.getProgName
62 :set prompt &lt;prompt&gt; set the prompt used in GHCi
63
64 :show modules show the currently loaded modules
65 :show bindings show the current bindings made at the prompt
66
67 :ctags [&lt;file&gt;] create tags file for Vi (default: "tags")
68 :etags [&lt;file&gt;] create tags file for Emacs (defauilt: "TAGS")
69 :type &lt;expr&gt; show the type of &lt;expr&gt;
70 :kind &lt;type&gt; show the kind of &lt;type&gt;
71 :undef &lt;cmd&gt; undefine user-defined command :&lt;cmd&gt;
72 :unset &lt;option&gt; ... unset options
73 :quit exit GHCi
74 :!&lt;command&gt; run the shell command &lt;command&gt;
75
76 Options for `:set' and `:unset':
77
78 +r revert top-level expressions after each evaluation
79 +s print timing/memory stats after each evaluation
80 +t print type after evaluation
81 -&lt;flags&gt; most GHC command line flags can also be set here
82 (eg. -v2, -fglasgow-exts, etc.)
83 </screen>
84
85 <para>We'll explain most of these commands as we go along. For
86 Hugs users: many things work the same as in Hugs, so you should be
87 able to get going straight away.</para>
88
89 <para>Haskell expressions can be typed at the prompt:</para>
90 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
91 </indexterm>
92
93 <screen>
94 Prelude> 1+2
95 3
96 Prelude> let x = 42 in x / 9
97 4.666666666666667
98 Prelude>
99 </screen>
100
101 <para>GHCi interprets the whole line as an expression to evaluate.
102 The expression may not span several lines - as soon as you press
103 enter, GHCi will attempt to evaluate it.</para>
104 </sect1>
105
106 <sect1>
107 <title>Loading source files</title>
108
109 <para>Suppose we have the following Haskell source code, which we
110 place in a file <filename>Main.hs</filename>:</para>
111
112 <programlisting>
113 main = print (fac 20)
114
115 fac 0 = 1
116 fac n = n * fac (n-1)
117 </programlisting>
118
119 <para>You can save <filename>Main.hs</filename> anywhere you like,
120 but if you save it somewhere other than the current
121 directory<footnote><para>If you started up GHCi from the command
122 line then GHCi's current directory is the same as the current
123 directory of the shell from which it was started. If you started
124 GHCi from the &ldquo;Start&rdquo; menu in Windows, then the
125 current directory is probably something like
126 <filename>C:\Documents and Settings\<replaceable>user
127 name</replaceable></filename>.</para> </footnote> then we will
128 need to change to the right directory in GHCi:</para>
129
130 <screen>
131 Prelude> :cd <replaceable>dir</replaceable>
132 </screen>
133
134 <para>where <replaceable>dir</replaceable> is the directory (or
135 folder) in which you saved <filename>Main.hs</filename>.</para>
136
137 <para>To load a Haskell source file into GHCi, use the
138 <literal>:load</literal> command:</para>
139 <indexterm><primary><literal>:load</literal></primary></indexterm>
140
141 <screen>
142 Prelude> :load Main
143 Compiling Main ( Main.hs, interpreted )
144 Ok, modules loaded: Main.
145 *Main>
146 </screen>
147
148 <para>GHCi has loaded the <literal>Main</literal> module, and the
149 prompt has changed to &ldquo;<literal>*Main></literal>&rdquo; to
150 indicate that the current context for expressions typed at the
151 prompt is the <literal>Main</literal> module we just loaded (we'll
152 explain what the <literal>*</literal> means later in <xref
153 linkend="ghci-scope"/>). So we can now type expressions involving
154 the functions from <filename>Main.hs</filename>:</para>
155
156 <screen>
157 *Main> fac 17
158 355687428096000
159 </screen>
160
161 <para>Loading a multi-module program is just as straightforward;
162 just give the name of the &ldquo;topmost&rdquo; module to the
163 <literal>:load</literal> command (hint: <literal>:load</literal>
164 can be abbreviated to <literal>:l</literal>). The topmost module
165 will normally be <literal>Main</literal>, but it doesn't have to
166 be. GHCi will discover which modules are required, directly or
167 indirectly, by the topmost module, and load them all in dependency
168 order.</para>
169
170 <sect2 id="ghci-modules-filenames">
171 <title>Modules vs. filenames</title>
172 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
173 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
174
175 <para>Question: How does GHC find the filename which contains
176 module <replaceable>M</replaceable>? Answer: it looks for the
177 file <literal><replaceable>M</replaceable>.hs</literal>, or
178 <literal><replaceable>M</replaceable>.lhs</literal>. This means
179 that for most modules, the module name must match the filename.
180 If it doesn't, GHCi won't be able to find it.</para>
181
182 <para>There is one exception to this general rule: when you load
183 a program with <literal>:load</literal>, or specify it when you
184 invoke <literal>ghci</literal>, you can give a filename rather
185 than a module name. This filename is loaded if it exists, and
186 it may contain any module you like. This is particularly
187 convenient if you have several <literal>Main</literal> modules
188 in the same directory and you can't call them all
189 <filename>Main.hs</filename>.</para>
190
191 <para>The search path for finding source files is specified with
192 the <option>-i</option> option on the GHCi command line, like
193 so:</para>
194 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
195
196 <para>or it can be set using the <literal>:set</literal> command
197 from within GHCi (see <xref
198 linkend="ghci-cmd-line-options"/>)<footnote><para>Note that in
199 GHCi, and <option>&ndash;&ndash;make</option> mode, the <option>-i</option>
200 option is used to specify the search path for
201 <emphasis>source</emphasis> files, whereas in standard
202 batch-compilation mode the <option>-i</option> option is used to
203 specify the search path for interface files, see <xref
204 linkend="search-path"/>.</para> </footnote></para>
205
206 <para>One consequence of the way that GHCi follows dependencies
207 to find modules to load is that every module must have a source
208 file. The only exception to the rule is modules that come from
209 a package, including the <literal>Prelude</literal> and standard
210 libraries such as <literal>IO</literal> and
211 <literal>Complex</literal>. If you attempt to load a module for
212 which GHCi can't find a source file, even if there are object
213 and interface files for the module, you'll get an error
214 message.</para>
215 </sect2>
216
217 <sect2>
218 <title>Making changes and recompilation</title>
219 <indexterm><primary><literal>:reload</literal></primary></indexterm>
220
221 <para>If you make some changes to the source code and want GHCi
222 to recompile the program, give the <literal>:reload</literal>
223 command. The program will be recompiled as necessary, with GHCi
224 doing its best to avoid actually recompiling modules if their
225 external dependencies haven't changed. This is the same
226 mechanism we use to avoid re-compiling modules in the batch
227 compilation setting (see <xref linkend="recomp"/>).</para>
228 </sect2>
229 </sect1>
230
231 <sect1 id="ghci-compiled">
232 <title>Loading compiled code</title>
233 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
234
235 <para>When you load a Haskell source module into GHCi, it is
236 normally converted to byte-code and run using the interpreter.
237 However, interpreted code can also run alongside compiled code in
238 GHCi; indeed, normally when GHCi starts, it loads up a compiled
239 copy of the <literal>base</literal> package, which contains the
240 <literal>Prelude</literal>.</para>
241
242 <para>Why should we want to run compiled code? Well, compiled
243 code is roughly 10x faster than interpreted code, but takes about
244 2x longer to produce (perhaps longer if optimisation is on). So
245 it pays to compile the parts of a program that aren't changing
246 very often, and use the interpreter for the code being actively
247 developed.</para>
248
249 <para>When loading up source files with <literal>:load</literal>,
250 GHCi looks for any corresponding compiled object files, and will
251 use one in preference to interpreting the source if possible. For
252 example, suppose we have a 4-module program consisting of modules
253 A, B, C, and D. Modules B and C both import D only,
254 and A imports both B &amp; C:</para>
255 <screen>
256 A
257 / \
258 B C
259 \ /
260 D
261 </screen>
262 <para>We can compile D, then load the whole program, like this:</para>
263 <screen>
264 Prelude> :! ghc -c D.hs
265 Prelude> :load A
266 Skipping D ( D.hs, D.o )
267 Compiling C ( C.hs, interpreted )
268 Compiling B ( B.hs, interpreted )
269 Compiling A ( A.hs, interpreted )
270 Ok, modules loaded: A, B, C, D.
271 *Main>
272 </screen>
273
274 <para>In the messages from the compiler, we see that it skipped D,
275 and used the object file <filename>D.o</filename>. The message
276 <literal>Skipping</literal> <replaceable>module</replaceable>
277 indicates that compilation for <replaceable>module</replaceable>
278 isn't necessary, because the source and everything it depends on
279 is unchanged since the last compilation.</para>
280
281 <para>At any time you can use the command
282 <literal>:show modules</literal>
283 to get a list of the modules currently loaded
284 into GHCi:</para>
285
286 <screen>
287 *Main> :show modules
288 D ( D.hs, D.o )
289 C ( C.hs, interpreted )
290 B ( B.hs, interpreted )
291 A ( A.hs, interpreted )
292 *Main></screen>
293
294 <para>If we now modify the source of D (or pretend to: using Unix
295 command <literal>touch</literal> on the source file is handy for
296 this), the compiler will no longer be able to use the object file,
297 because it might be out of date:</para>
298
299 <screen>
300 *Main> :! touch D.hs
301 *Main> :reload
302 Compiling D ( D.hs, interpreted )
303 Skipping C ( C.hs, interpreted )
304 Skipping B ( B.hs, interpreted )
305 Skipping A ( A.hs, interpreted )
306 Ok, modules loaded: A, B, C, D.
307 *Main>
308 </screen>
309
310 <para>Note that module D was compiled, but in this instance
311 because its source hadn't really changed, its interface remained
312 the same, and the recompilation checker determined that A, B and C
313 didn't need to be recompiled.</para>
314
315 <para>So let's try compiling one of the other modules:</para>
316
317 <screen>
318 *Main> :! ghc -c C.hs
319 *Main> :load A
320 Compiling D ( D.hs, interpreted )
321 Compiling C ( C.hs, interpreted )
322 Compiling B ( B.hs, interpreted )
323 Compiling A ( A.hs, interpreted )
324 Ok, modules loaded: A, B, C, D.
325 </screen>
326
327 <para>We didn't get the compiled version of C! What happened?
328 Well, in GHCi a compiled module may only depend on other compiled
329 modules, and in this case C depends on D, which doesn't have an
330 object file, so GHCi also rejected C's object file. Ok, so let's
331 also compile D:</para>
332
333 <screen>
334 *Main> :! ghc -c D.hs
335 *Main> :reload
336 Ok, modules loaded: A, B, C, D.
337 </screen>
338
339 <para>Nothing happened! Here's another lesson: newly compiled
340 modules aren't picked up by <literal>:reload</literal>, only
341 <literal>:load</literal>:</para>
342
343 <screen>
344 *Main> :load A
345 Skipping D ( D.hs, D.o )
346 Skipping C ( C.hs, C.o )
347 Compiling B ( B.hs, interpreted )
348 Compiling A ( A.hs, interpreted )
349 Ok, modules loaded: A, B, C, D.
350 </screen>
351
352 <para>HINT: since GHCi will only use a compiled object file if it
353 can sure that the compiled version is up-to-date, a good technique
354 when working on a large program is to occasionally run
355 <literal>ghc &ndash;&ndash;make</literal> to compile the whole project (say
356 before you go for lunch :-), then continue working in the
357 interpreter. As you modify code, the new modules will be
358 interpreted, but the rest of the project will remain
359 compiled.</para>
360
361 </sect1>
362
363 <sect1>
364 <title>Interactive evaluation at the prompt</title>
365
366 <para>When you type an expression at the prompt, GHCi immediately
367 evaluates and prints the result:
368 <screen>
369 Prelude> reverse "hello"
370 "olleh"
371 Prelude> 5+5
372 10
373 </screen>
374 </para>
375
376 <sect2><title>I/O actions at the prompt</title>
377
378 <para>GHCi does more than simple expression evaluation at the prompt.
379 If you type something of type <literal>IO a</literal> for some
380 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
381 as an IO-computation.
382 <screen>
383 Prelude> "hello"
384 "hello"
385 Prelude> putStrLn "hello"
386 hello
387 </screen>
388 Furthermore, GHCi will print the result of the I/O action if (and only
389 if):
390 <itemizedlist>
391 <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
392 <listitem><para>The result type is not
393 <literal>()</literal>.</para></listitem>
394 </itemizedlist>
395 For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
396 <screen>
397 Prelude> putStrLn "hello"
398 hello
399 Prelude> do { putStrLn "hello"; return "yes" }
400 hello
401 "yes"
402 </screen>
403 </para></sect2>
404
405 <sect2>
406 <title>Using <literal>do-</literal>notation at the prompt</title>
407 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
408 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
409
410 <para>GHCi actually accepts <firstterm>statements</firstterm>
411 rather than just expressions at the prompt. This means you can
412 bind values and functions to names, and use them in future
413 expressions or statements.</para>
414
415 <para>The syntax of a statement accepted at the GHCi prompt is
416 exactly the same as the syntax of a statement in a Haskell
417 <literal>do</literal> expression. However, there's no monad
418 overloading here: statements typed at the prompt must be in the
419 <literal>IO</literal> monad.
420 <screen>
421 Prelude> x &lt;- return 42
422 42
423 Prelude> print x
424 42
425 Prelude>
426 </screen>
427 The statement <literal>x &lt;- return 42</literal> means
428 &ldquo;execute <literal>return 42</literal> in the
429 <literal>IO</literal> monad, and bind the result to
430 <literal>x</literal>&rdquo;. We can then use
431 <literal>x</literal> in future statements, for example to print
432 it as we did above.</para>
433
434 <para>GHCi will print the result of a statement if and only if:
435 <itemizedlist>
436 <listitem>
437 <para>The statement is not a binding, or it is a monadic binding
438 (<literal>p &lt;- e</literal>) that binds exactly one
439 variable.</para>
440 </listitem>
441 <listitem>
442 <para>The variable's type is not polymorphic, is not
443 <literal>()</literal>, and is an instance of
444 <literal>Show</literal></para>
445 </listitem>
446 </itemizedlist>
447 </para>
448
449 <para>Of course, you can also bind normal non-IO expressions
450 using the <literal>let</literal>-statement:</para>
451 <screen>
452 Prelude> let x = 42
453 Prelude> x
454 42
455 Prelude>
456 </screen>
457 <para>Another important difference between the two types of binding
458 is that the monadic bind (<literal>p &lt;- e</literal>) is
459 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
460 whereas with the <literal>let</literal> form, the expression
461 isn't evaluated immediately:</para>
462 <screen>
463 Prelude> let x = error "help!"
464 Prelude> print x
465 *** Exception: help!
466 Prelude>
467 </screen>
468
469 <para>Note that <literal>let</literal> bindings do not automatically
470 print the value bound, unlike monadic bindings.</para>
471
472 <para>Any exceptions raised during the evaluation or execution
473 of the statement are caught and printed by the GHCi command line
474 interface (for more information on exceptions, see the module
475 <literal>Control.Exception</literal> in the libraries
476 documentation).</para>
477
478 <para>Every new binding shadows any existing bindings of the
479 same name, including entities that are in scope in the current
480 module context.</para>
481
482 <para>WARNING: temporary bindings introduced at the prompt only
483 last until the next <literal>:load</literal> or
484 <literal>:reload</literal> command, at which time they will be
485 simply lost. However, they do survive a change of context with
486 <literal>:module</literal>: the temporary bindings just move to
487 the new location.</para>
488
489 <para>HINT: To get a list of the bindings currently in scope, use the
490 <literal>:show bindings</literal> command:</para>
491
492 <screen>
493 Prelude> :show bindings
494 x :: Int
495 Prelude></screen>
496
497 <para>HINT: if you turn on the <literal>+t</literal> option,
498 GHCi will show the type of each variable bound by a statement.
499 For example:</para>
500 <indexterm><primary><literal>+t</literal></primary></indexterm>
501 <screen>
502 Prelude> :set +t
503 Prelude> let (x:xs) = [1..]
504 x :: Integer
505 xs :: [Integer]
506 </screen>
507
508 </sect2>
509
510 <sect2 id="ghci-scope">
511 <title>What's really in scope at the prompt?</title>
512
513 <para>When you type an expression at the prompt, what
514 identifiers and types are in scope? GHCi provides a flexible
515 way to control exactly how the context for an expression is
516 constructed. Let's start with the simple cases; when you start
517 GHCi the prompt looks like this:</para>
518
519 <screen>Prelude></screen>
520
521 <para>Which indicates that everything from the module
522 <literal>Prelude</literal> is currently in scope. If we now
523 load a file into GHCi, the prompt will change:</para>
524
525 <screen>
526 Prelude> :load Main.hs
527 Compiling Main ( Main.hs, interpreted )
528 *Main>
529 </screen>
530
531 <para>The new prompt is <literal>*Main</literal>, which
532 indicates that we are typing expressions in the context of the
533 top-level of the <literal>Main</literal> module. Everything
534 that is in scope at the top-level in the module
535 <literal>Main</literal> we just loaded is also in scope at the
536 prompt (probably including <literal>Prelude</literal>, as long
537 as <literal>Main</literal> doesn't explicitly hide it).</para>
538
539 <para>The syntax
540 <literal>*<replaceable>module</replaceable></literal> indicates
541 that it is the full top-level scope of
542 <replaceable>module</replaceable> that is contributing to the
543 scope for expressions typed at the prompt. Without the
544 <literal>*</literal>, just the exports of the module are
545 visible.</para>
546
547 <para>We're not limited to a single module: GHCi can combine
548 scopes from multiple modules, in any mixture of
549 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
550 combines the scopes from all of these modules to form the scope
551 that is in effect at the prompt. For technical reasons, GHCi
552 can only support the <literal>*</literal>-form for modules which
553 are interpreted, so compiled modules and package modules can
554 only contribute their exports to the current scope.</para>
555
556 <para>The scope is manipulated using the
557 <literal>:module</literal> command. For example, if the current
558 scope is <literal>Prelude</literal>, then we can bring into
559 scope the exports from the module <literal>IO</literal> like
560 so:</para>
561
562 <screen>
563 Prelude> :module +IO
564 Prelude,IO> hPutStrLn stdout "hello\n"
565 hello
566 Prelude,IO>
567 </screen>
568
569 <para>(Note: <literal>:module</literal> can be shortened to
570 <literal>:m</literal>). The full syntax of the
571 <literal>:module</literal> command is:</para>
572
573 <screen>
574 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
575 </screen>
576
577 <para>Using the <literal>+</literal> form of the
578 <literal>module</literal> commands adds modules to the current
579 scope, and <literal>-</literal> removes them. Without either
580 <literal>+</literal> or <literal>-</literal>, the current scope
581 is replaced by the set of modules specified. Note that if you
582 use this form and leave out <literal>Prelude</literal>, GHCi
583 will assume that you really wanted the
584 <literal>Prelude</literal> and add it in for you (if you don't
585 want the <literal>Prelude</literal>, then ask to remove it with
586 <literal>:m -Prelude</literal>).</para>
587
588 <para>The scope is automatically set after a
589 <literal>:load</literal> command, to the most recently loaded
590 "target" module, in a <literal>*</literal>-form if possible.
591 For example, if you say <literal>:load foo.hs bar.hs</literal>
592 and <filename>bar.hs</filename> contains module
593 <literal>Bar</literal>, then the scope will be set to
594 <literal>*Bar</literal> if <literal>Bar</literal> is
595 interpreted, or if <literal>Bar</literal> is compiled it will be
596 set to <literal>Prelude,Bar</literal> (GHCi automatically adds
597 <literal>Prelude</literal> if it isn't present and there aren't
598 any <literal>*</literal>-form modules).</para>
599
600 <para>With multiple modules in scope, especially multiple
601 <literal>*</literal>-form modules, it is likely that name
602 clashes will occur. Haskell specifies that name clashes are
603 only reported when an ambiguous identifier is used, and GHCi
604 behaves in the same way for expressions typed at the
605 prompt.</para>
606
607 <sect3>
608 <title>Qualified names</title>
609
610 <para>To make life slightly easier, the GHCi prompt also
611 behaves as if there is an implicit <literal>import
612 qualified</literal> declaration for every module in every
613 package, and every module currently loaded into GHCi.</para>
614 </sect3>
615 </sect2>
616
617
618 <sect2>
619 <title>The <literal>it</literal> variable</title>
620 <indexterm><primary><literal>it</literal></primary>
621 </indexterm>
622
623 <para>Whenever an expression (or a non-binding statement, to be
624 precise) is typed at the prompt, GHCi implicitly binds its value
625 to the variable <literal>it</literal>. For example:</para>
626 <screen>
627 Prelude> 1+2
628 3
629 Prelude> it * 2
630 6
631 </screen>
632 <para>What actually happens is that GHCi typechecks the
633 expression, and if it doesn't have an <literal>IO</literal> type,
634 then it transforms it as follows: an expression
635 <replaceable>e</replaceable> turns into
636 <screen>
637 let it = <replaceable>e</replaceable>;
638 print it
639 </screen>
640 which is then run as an IO-action.</para>
641
642 <para>Hence, the original expression must have a type which is an
643 instance of the <literal>Show</literal> class, or GHCi will
644 complain:</para>
645
646 <screen>
647 Prelude> id
648 No instance for `Show (a -> a)'
649 arising from use of `print'
650 in a `do' expression pattern binding: print it
651 </screen>
652
653 <para>The error message contains some clues as to the
654 transformation happening internally.</para>
655
656 <para>If the expression was instead of type <literal>IO a</literal> for
657 some <literal>a</literal>, then <literal>it</literal> will be
658 bound to the result of the <literal>IO</literal> computation,
659 which is of type <literal>a</literal>. eg.:</para>
660 <screen>
661 Prelude> Time.getClockTime
662 Prelude> print it
663 Wed Mar 14 12:23:13 GMT 2001
664 </screen>
665
666 <para>The corresponding translation for an IO-typed
667 <replaceable>e</replaceable> is
668 <screen>
669 it &lt;- <replaceable>e</replaceable>
670 </screen>
671 </para>
672
673 <para>Note that <literal>it</literal> is shadowed by the new
674 value each time you evaluate a new expression, and the old value
675 of <literal>it</literal> is lost.</para>
676
677 </sect2>
678
679 <sect2>
680 <title>Type defaulting in GHCi</title>
681 <indexterm><primary>Type default</primary></indexterm>
682 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
683 <para>
684 Consider this GHCi session:
685 <programlisting>
686 ghci> reverse []
687 </programlisting>
688 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
689 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
690 on the type <literal>a</literal>. For example:
691 <programlisting>
692 ghci> (reverse []) :: String
693 ""
694 ghci> (reverse []) :: [Int]
695 []
696 </programlisting>
697 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
698 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
699 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
700 a)</literal> for each type variable <literal>a</literal>, and defaults the
701 type variable if
702 <itemizedlist>
703 <listitem><para> The type variable <literal>a</literal>
704 appears in no other constraints </para></listitem>
705 <listitem><para> All the classes <literal>Ci</literal> are standard.</para></listitem>
706 <listitem><para> At least one of the classes <literal>Ci</literal> is
707 numeric.</para></listitem>
708 </itemizedlist>
709 At the GHCi prompt, the second and third rules are relaxed as follows
710 (differences italicised):
711 <itemizedlist>
712 <listitem><para> <emphasis>All</emphasis> of the classes
713 <literal>Ci</literal> are single-parameter type classes.</para></listitem>
714 <listitem><para> At least one of the classes <literal>Ci</literal> is
715 numeric, <emphasis>or is <literal>Show</literal>,
716 <literal>Eq</literal>, or <literal>Ord</literal></emphasis>.</para></listitem>
717 </itemizedlist>
718 </para>
719 </sect2>
720 </sect1>
721
722 <sect1 id="ghci-invocation">
723 <title>Invoking GHCi</title>
724 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
725 <indexterm><primary><option>&ndash;&ndash;interactive</option></primary></indexterm>
726
727 <para>GHCi is invoked with the command <literal>ghci</literal> or
728 <literal>ghc &ndash;&ndash;interactive</literal>. One or more modules or
729 filenames can also be specified on the command line; this
730 instructs GHCi to load the specified modules or filenames (and all
731 the modules they depend on), just as if you had said
732 <literal>:load <replaceable>modules</replaceable></literal> at the
733 GHCi prompt (see <xref linkend="ghci-commands"/>). For example, to
734 start GHCi and load the program whose topmost module is in the
735 file <literal>Main.hs</literal>, we could say:</para>
736
737 <screen>
738 $ ghci Main.hs
739 </screen>
740
741 <para>Most of the command-line options accepted by GHC (see <xref
742 linkend="using-ghc"/>) also make sense in interactive mode. The ones
743 that don't make sense are mostly obvious; for example, GHCi
744 doesn't generate interface files, so options related to interface
745 file generation won't have any effect.</para>
746
747 <sect2>
748 <title>Packages</title>
749 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
750
751 <para>Most packages (see <xref linkend="using-packages"/>) are
752 available without needing to specify any extra flags at all:
753 they will be automatically loaded the first time they are
754 needed.</para>
755
756 <para>For non-auto packages, however, you need to request the
757 package be loaded by using the <literal>-package</literal> flag:</para>
758
759 <screen>
760 $ ghci -package data
761 ___ ___ _
762 / _ \ /\ /\/ __(_)
763 / /_\// /_/ / / | | GHC Interactive, version 5.05, for Haskell 98.
764 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
765 \____/\/ /_/\____/|_| Type :? for help.
766
767 Loading package base ... linking ... done.
768 Loading package haskell98 ... linking ... done.
769 Loading package lang ... linking ... done.
770 Loading package concurrent ... linking ... done.
771 Loading package readline ... linking ... done.
772 Loading package unix ... linking ... done.
773 Loading package posix ... linking ... done.
774 Loading package util ... linking ... done.
775 Loading package data ... linking ... done.
776 Prelude>
777 </screen>
778
779 <para>The following command works to load new packages into a
780 running GHCi:</para>
781
782 <screen>
783 Prelude> :set -package <replaceable>name</replaceable>
784 </screen>
785
786 <para>But note that doing this will cause all currently loaded
787 modules to be unloaded, and you'll be dumped back into the
788 <literal>Prelude</literal>.</para>
789 </sect2>
790
791 <sect2>
792 <title>Extra libraries</title>
793 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
794
795 <para>Extra libraries may be specified on the command line using
796 the normal <literal>-l<replaceable>lib</replaceable></literal>
797 option. (The term <emphasis>library</emphasis> here refers to
798 libraries of foreign object code; for using libraries of Haskell
799 source code, see <xref linkend="ghci-modules-filenames"/>.) For
800 example, to load the &ldquo;m&rdquo; library:</para>
801
802 <screen>
803 $ ghci -lm
804 </screen>
805
806 <para>On systems with <literal>.so</literal>-style shared
807 libraries, the actual library loaded will the
808 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
809 searches the following places for libraries, in this order:</para>
810
811 <itemizedlist>
812 <listitem>
813 <para>Paths specified using the
814 <literal>-L<replaceable>path</replaceable></literal>
815 command-line option,</para>
816 </listitem>
817 <listitem>
818 <para>the standard library search path for your system,
819 which on some systems may be overridden by setting the
820 <literal>LD_LIBRARY_PATH</literal> environment
821 variable.</para>
822 </listitem>
823 </itemizedlist>
824
825 <para>On systems with <literal>.dll</literal>-style shared
826 libraries, the actual library loaded will be
827 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
828 GHCi will signal an error if it can't find the library.</para>
829
830 <para>GHCi can also load plain object files
831 (<literal>.o</literal> or <literal>.obj</literal> depending on
832 your platform) from the command-line. Just add the name the
833 object file to the command line.</para>
834
835 <para>Ordering of <option>-l</option> options matters: a library
836 should be mentioned <emphasis>before</emphasis> the libraries it
837 depends on (see <xref linkend="options-linker"/>).</para>
838 </sect2>
839
840 </sect1>
841
842 <sect1 id="ghci-commands">
843 <title>GHCi commands</title>
844
845 <para>GHCi commands all begin with
846 &lsquo;<literal>:</literal>&rsquo; and consist of a single command
847 name followed by zero or more parameters. The command name may be
848 abbreviated, as long as the abbreviation is not ambiguous. All of
849 the builtin commands, with the exception of
850 <literal>:unset</literal> and <literal>:undef</literal>, may be
851 abbreviated to a single letter.</para>
852
853 <variablelist>
854 <varlistentry>
855 <term>
856 <literal>:add</literal> <replaceable>module</replaceable> ...
857 <indexterm><primary><literal>:add</literal></primary></indexterm>
858 </term>
859 <listitem>
860 <para>Add <replaceable>module</replaceable>(s) to the
861 current <firstterm>target set</firstterm>, and perform a
862 reload.</para>
863 </listitem>
864 </varlistentry>
865
866 <varlistentry>
867 <term>
868 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
869 <indexterm><primary><literal>:browse</literal></primary></indexterm>
870 </term>
871 <listitem>
872 <para>Displays the identifiers defined by the module
873 <replaceable>module</replaceable>, which must be either
874 loaded into GHCi or be a member of a package. If the
875 <literal>*</literal> symbol is placed before the module
876 name, then <emphasis>all</emphasis> the identifiers defined
877 in <replaceable>module</replaceable> are shown; otherwise
878 the list is limited to the exports of
879 <replaceable>module</replaceable>. The
880 <literal>*</literal>-form is only available for modules
881 which are interpreted; for compiled modules (including
882 modules from packages) only the non-<literal>*</literal>
883 form of <literal>:browse</literal> is available.</para>
884 </listitem>
885 </varlistentry>
886
887 <varlistentry>
888 <term>
889 <literal>:cd</literal> <replaceable>dir</replaceable>
890 <indexterm><primary><literal>:cd</literal></primary></indexterm>
891 </term>
892 <listitem>
893 <para>Changes the current working directory to
894 <replaceable>dir</replaceable>. A
895 &lsquo;<literal>&tilde;</literal>&rsquo; symbol at the
896 beginning of <replaceable>dir</replaceable> will be replaced
897 by the contents of the environment variable
898 <literal>HOME</literal>.</para>
899
900 <para>NOTE: changing directories causes all currently loaded
901 modules to be unloaded. This is because the search path is
902 usually expressed using relative directories, and changing
903 the search path in the middle of a session is not
904 supported.</para>
905 </listitem>
906 </varlistentry>
907
908 <varlistentry>
909 <term>
910 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
911 <indexterm><primary><literal>:def</literal></primary></indexterm>
912 </term>
913 <listitem>
914 <para>The command <literal>:def</literal>
915 <replaceable>name</replaceable>
916 <replaceable>expr</replaceable> defines a new GHCi command
917 <literal>:<replaceable>name</replaceable></literal>,
918 implemented by the Haskell expression
919 <replaceable>expr</replaceable>, which must have type
920 <literal>String -> IO String</literal>. When
921 <literal>:<replaceable>name</replaceable>
922 <replaceable>args</replaceable></literal> is typed at the
923 prompt, GHCi will run the expression
924 <literal>(<replaceable>name</replaceable>
925 <replaceable>args</replaceable>)</literal>, take the
926 resulting <literal>String</literal>, and feed it back into
927 GHCi as a new sequence of commands. Separate commands in
928 the result must be separated by
929 &lsquo;<literal>\n</literal>&rsquo;.</para>
930
931 <para>That's all a little confusing, so here's a few
932 examples. To start with, here's a new GHCi command which
933 doesn't take any arguments or produce any results, it just
934 outputs the current date &amp; time:</para>
935
936 <screen>
937 Prelude> let date _ = Time.getClockTime >>= print >> return ""
938 Prelude> :def date date
939 Prelude> :date
940 Fri Mar 23 15:16:40 GMT 2001
941 </screen>
942
943 <para>Here's an example of a command that takes an argument.
944 It's a re-implementation of <literal>:cd</literal>:</para>
945
946 <screen>
947 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
948 Prelude> :def mycd mycd
949 Prelude> :mycd ..
950 </screen>
951
952 <para>Or I could define a simple way to invoke
953 &ldquo;<literal>ghc &ndash;&ndash;make Main</literal>&rdquo; in the
954 current directory:</para>
955
956 <screen>
957 Prelude> :def make (\_ -> return ":! ghc &ndash;&ndash;make Main")
958 </screen>
959
960 <para>We can define a command that reads GHCi input from a
961 file. This might be useful for creating a set of bindings
962 that we want to repeatedly load into the GHCi session:</para>
963
964 <screen>
965 Prelude> :def . readFile
966 Prelude> :. cmds.ghci
967 </screen>
968
969 <para>Notice that we named the command
970 <literal>:.</literal>, by analogy with the
971 &lsquo;<literal>.</literal>&rsquo; Unix shell command that
972 does the same thing.</para>
973 </listitem>
974 </varlistentry>
975
976 <varlistentry>
977 <term>
978 <literal>:help</literal>
979 <indexterm><primary><literal>:help</literal></primary></indexterm>
980 </term>
981 <term>
982 <literal>:?</literal>
983 <indexterm><primary><literal>:?</literal></primary></indexterm>
984 </term>
985 <listitem>
986 <para>Displays a list of the available commands.</para>
987 </listitem>
988 </varlistentry>
989
990 <varlistentry>
991 <term>
992 <literal>:info</literal> <replaceable>name</replaceable> ...
993 <indexterm><primary><literal>:info</literal></primary></indexterm>
994 </term>
995 <listitem>
996 <para>Displays information about the given name(s). For
997 example, if <replaceable>name</replaceable> is a class, then
998 the class methods and their types will be printed; if
999 <replaceable>name</replaceable> is a type constructor, then
1000 its definition will be printed; if
1001 <replaceable>name</replaceable> is a function, then its type
1002 will be printed. If <replaceable>name</replaceable> has
1003 been loaded from a source file, then GHCi will also display
1004 the location of its definition in the source.</para>
1005 </listitem>
1006 </varlistentry>
1007
1008 <varlistentry>
1009 <term>
1010 <literal>:load</literal> <replaceable>module</replaceable> ...
1011 <indexterm><primary><literal>:load</literal></primary></indexterm>
1012 </term>
1013 <listitem>
1014 <para>Recursively loads the specified
1015 <replaceable>module</replaceable>s, and all the modules they
1016 depend on. Here, each <replaceable>module</replaceable>
1017 must be a module name or filename, but may not be the name
1018 of a module in a package.</para>
1019
1020 <para>All previously loaded modules, except package modules,
1021 are forgotten. The new set of modules is known as the
1022 <firstterm>target set</firstterm>. Note that
1023 <literal>:load</literal> can be used without any arguments
1024 to unload all the currently loaded modules and
1025 bindings.</para>
1026
1027 <para>After a <literal>:load</literal> command, the current
1028 context is set to:</para>
1029
1030 <itemizedlist>
1031 <listitem>
1032 <para><replaceable>module</replaceable>, if it was loaded
1033 successfully, or</para>
1034 </listitem>
1035 <listitem>
1036 <para>the most recently successfully loaded module, if
1037 any other modules were loaded as a result of the current
1038 <literal>:load</literal>, or</para>
1039 </listitem>
1040 <listitem>
1041 <para><literal>Prelude</literal> otherwise.</para>
1042 </listitem>
1043 </itemizedlist>
1044 </listitem>
1045 </varlistentry>
1046
1047 <varlistentry>
1048 <term>
1049 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
1050 <indexterm><primary><literal>:module</literal></primary></indexterm>
1051 </term>
1052 <listitem>
1053 <para>Sets or modifies the current context for statements
1054 typed at the prompt. See <xref linkend="ghci-scope"/> for
1055 more details.</para>
1056 </listitem>
1057 </varlistentry>
1058
1059 <varlistentry>
1060 <term>
1061 <literal>:quit</literal>
1062 <indexterm><primary><literal>:quit</literal></primary></indexterm>
1063 </term>
1064 <listitem>
1065 <para>Quits GHCi. You can also quit by typing a control-D
1066 at the prompt.</para>
1067 </listitem>
1068 </varlistentry>
1069
1070 <varlistentry>
1071 <term>
1072 <literal>:reload</literal>
1073 <indexterm><primary><literal>:reload</literal></primary></indexterm>
1074 </term>
1075 <listitem>
1076 <para>Attempts to reload the current target set (see
1077 <literal>:load</literal>) if any of the modules in the set,
1078 or any dependent module, has changed. Note that this may
1079 entail loading new modules, or dropping modules which are no
1080 longer indirectly required by the target.</para>
1081 </listitem>
1082 </varlistentry>
1083
1084 <varlistentry>
1085 <term>
1086 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
1087 <indexterm><primary><literal>:set</literal></primary></indexterm>
1088 </term>
1089 <listitem>
1090 <para>Sets various options. See <xref linkend="ghci-set"/>
1091 for a list of available options. The
1092 <literal>:set</literal> command by itself shows which
1093 options are currently set.</para>
1094 </listitem>
1095 </varlistentry>
1096
1097 <varlistentry>
1098 <term>
1099 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
1100 <indexterm><primary><literal>:set args</literal></primary></indexterm>
1101 </term>
1102 <listitem>
1103 <para>Sets the list of arguments which are returned when the
1104 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
1105 </indexterm>.</para>
1106 </listitem>
1107 </varlistentry>
1108
1109 <varlistentry>
1110 <term>
1111 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
1112 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
1113 </term>
1114 <listitem>
1115 <para>Sets the string to be returned when the program calls
1116 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
1117 </indexterm>.</para>
1118 </listitem>
1119 </varlistentry>
1120
1121 <varlistentry>
1122 <term>
1123 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
1124 </term>
1125 <listitem>
1126 <para>Sets the string to be used as the prompt in GHCi.
1127 Inside <replaceable>prompt</replaceable>, the sequence
1128 <literal>%s</literal> is replaced by the names of the
1129 modules currently in scope, and <literal>%%</literal> is
1130 replaced by <literal>%</literal>.</para>
1131 </listitem>
1132 </varlistentry>
1133
1134 <varlistentry>
1135 <term>
1136 <literal>:show bindings</literal>
1137 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
1138 </term>
1139 <listitem>
1140 <para>Show the bindings made at the prompt and their
1141 types.</para>
1142 </listitem>
1143 </varlistentry>
1144
1145 <varlistentry>
1146 <term>
1147 <literal>:show modules</literal>
1148 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
1149 </term>
1150 <listitem>
1151 <para>Show the list of modules currently load.</para>
1152 </listitem>
1153 </varlistentry>
1154
1155 <varlistentry>
1156 <term>
1157 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1158 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1159 <indexterm><primary><literal>:etags</literal></primary>
1160 </indexterm>
1161 <indexterm><primary><literal>:etags</literal></primary>
1162 </indexterm>
1163 </term>
1164 <listitem>
1165 <para>Generates a &ldquo;tags&rdquo; file for Vi-style editors
1166 (<literal>:ctags</literal>) or Emacs-style editors (<literal>etags</literal>). If
1167 no filename is specified, the defaulit <filename>tags</filename> or
1168 <filename>TAGS</filename> is
1169 used, respectively. Tags for all the functions, constructors and
1170 types in the currently loaded modules are created. All modules must
1171 be interpreted for these commands to work.</para>
1172 <para>See also <xref linkend="hasktags" />.</para>
1173 </listitem>
1174 </varlistentry>
1175
1176 <varlistentry>
1177 <term>
1178 <literal>:type</literal> <replaceable>expression</replaceable>
1179 <indexterm><primary><literal>:type</literal></primary></indexterm>
1180 </term>
1181 <listitem>
1182 <para>Infers and prints the type of
1183 <replaceable>expression</replaceable>, including explicit
1184 forall quantifiers for polymorphic types. The monomorphism
1185 restriction is <emphasis>not</emphasis> applied to the
1186 expression during type inference.</para>
1187 </listitem>
1188 </varlistentry>
1189
1190 <varlistentry>
1191 <term>
1192 <literal>:kind</literal> <replaceable>type</replaceable>
1193 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1194 </term>
1195 <listitem>
1196 <para>Infers and prints the kind of
1197 <replaceable>type</replaceable>. The latter can be an arbitrary
1198 type expression, including a partial application of a type constructor,
1199 such as <literal>Either Int</literal>.</para>
1200 </listitem>
1201 </varlistentry>
1202
1203 <varlistentry>
1204 <term>
1205 <literal>:undef</literal> <replaceable>name</replaceable>
1206 <indexterm><primary><literal>:undef</literal></primary></indexterm>
1207 </term>
1208 <listitem>
1209 <para>Undefines the user-defined command
1210 <replaceable>name</replaceable> (see <literal>:def</literal>
1211 above).</para>
1212 </listitem>
1213 </varlistentry>
1214
1215 <varlistentry>
1216 <term>
1217 <literal>:unset</literal> <replaceable>option</replaceable>...
1218 <indexterm><primary><literal>:unset</literal></primary></indexterm>
1219 </term>
1220 <listitem>
1221 <para>Unsets certain options. See <xref linkend="ghci-set"/>
1222 for a list of available options.</para>
1223 </listitem>
1224 </varlistentry>
1225
1226 <varlistentry>
1227 <term>
1228 <literal>:!</literal> <replaceable>command</replaceable>...
1229 <indexterm><primary><literal>:!</literal></primary></indexterm>
1230 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
1231 </term>
1232 <listitem>
1233 <para>Executes the shell command
1234 <replaceable>command</replaceable>.</para>
1235 </listitem>
1236 </varlistentry>
1237
1238 </variablelist>
1239 </sect1>
1240
1241 <sect1 id="ghci-set">
1242 <title>The <literal>:set</literal> command</title>
1243 <indexterm><primary><literal>:set</literal></primary></indexterm>
1244
1245 <para>The <literal>:set</literal> command sets two types of
1246 options: GHCi options, which begin with
1247 &lsquo;<literal>+</literal>&rdquo; and &ldquo;command-line&rdquo;
1248 options, which begin with &lsquo;-&rsquo;. </para>
1249
1250 <para>NOTE: at the moment, the <literal>:set</literal> command
1251 doesn't support any kind of quoting in its arguments: quotes will
1252 not be removed and cannot be used to group words together. For
1253 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
1254 you expect.</para>
1255
1256 <sect2>
1257 <title>GHCi options</title>
1258 <indexterm><primary>options</primary><secondary>GHCi</secondary>
1259 </indexterm>
1260
1261 <para>GHCi options may be set using <literal>:set</literal> and
1262 unset using <literal>:unset</literal>.</para>
1263
1264 <para>The available GHCi options are:</para>
1265
1266 <variablelist>
1267 <varlistentry>
1268 <term>
1269 <literal>+r</literal>
1270 <indexterm><primary><literal>+r</literal></primary></indexterm>
1271 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
1272 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
1273 </term>
1274 <listitem>
1275 <para>Normally, any evaluation of top-level expressions
1276 (otherwise known as CAFs or Constant Applicative Forms) in
1277 loaded modules is retained between evaluations. Turning
1278 on <literal>+r</literal> causes all evaluation of
1279 top-level expressions to be discarded after each
1280 evaluation (they are still retained
1281 <emphasis>during</emphasis> a single evaluation).</para>
1282
1283 <para>This option may help if the evaluated top-level
1284 expressions are consuming large amounts of space, or if
1285 you need repeatable performance measurements.</para>
1286 </listitem>
1287 </varlistentry>
1288
1289 <varlistentry>
1290 <term>
1291 <literal>+s</literal>
1292 <indexterm><primary><literal>+s</literal></primary></indexterm>
1293 </term>
1294 <listitem>
1295 <para>Display some stats after evaluating each expression,
1296 including the elapsed time and number of bytes allocated.
1297 NOTE: the allocation figure is only accurate to the size
1298 of the storage manager's allocation area, because it is
1299 calculated at every GC. Hence, you might see values of
1300 zero if no GC has occurred.</para>
1301 </listitem>
1302 </varlistentry>
1303
1304 <varlistentry>
1305 <term>
1306 <literal>+t</literal>
1307 <indexterm><primary><literal>+t</literal></primary></indexterm>
1308 </term>
1309 <listitem>
1310 <para>Display the type of each variable bound after a
1311 statement is entered at the prompt. If the statement is a
1312 single expression, then the only variable binding will be
1313 for the variable
1314 &lsquo;<literal>it</literal>&rsquo;.</para>
1315 </listitem>
1316 </varlistentry>
1317 </variablelist>
1318 </sect2>
1319
1320 <sect2 id="ghci-cmd-line-options">
1321 <title>Setting GHC command-line options in GHCi</title>
1322
1323 <para>Normal GHC command-line options may also be set using
1324 <literal>:set</literal>. For example, to turn on
1325 <option>-fglasgow-exts</option>, you would say:</para>
1326
1327 <screen>
1328 Prelude> :set -fglasgow-exts
1329 </screen>
1330
1331 <para>Any GHC command-line option that is designated as
1332 <firstterm>dynamic</firstterm> (see the table in <xref
1333 linkend="flag-reference"/>), may be set using
1334 <literal>:set</literal>. To unset an option, you can set the
1335 reverse option:</para>
1336 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
1337
1338 <screen>
1339 Prelude> :set -fno-glasgow-exts
1340 </screen>
1341
1342 <para><xref linkend="flag-reference"/> lists the reverse for each
1343 option where applicable.</para>
1344
1345 <para>Certain static options (<option>-package</option>,
1346 <option>-I</option>, <option>-i</option>, and
1347 <option>-l</option> in particular) will also work, but some may
1348 not take effect until the next reload.</para>
1349 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
1350 </sect2>
1351 </sect1>
1352
1353 <sect1 id="ghci-dot-files">
1354 <title>The <filename>.ghci</filename> file</title>
1355 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
1356 </indexterm>
1357 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
1358 </indexterm>
1359
1360 <para>When it starts, GHCi always reads and executes commands from
1361 <filename>$HOME/.ghci</filename>, followed by
1362 <filename>./.ghci</filename>.</para>
1363
1364 <para>The <filename>.ghci</filename> in your home directory is
1365 most useful for turning on favourite options (eg. <literal>:set
1366 +s</literal>), and defining useful macros. Placing a
1367 <filename>.ghci</filename> file in a directory with a Haskell
1368 project is a useful way to set certain project-wide options so you
1369 don't have to type them everytime you start GHCi: eg. if your
1370 project uses GHC extensions and CPP, and has source files in three
1371 subdirectories A B and C, you might put the following lines in
1372 <filename>.ghci</filename>:</para>
1373
1374 <screen>
1375 :set -fglasgow-exts -cpp
1376 :set -iA:B:C
1377 </screen>
1378
1379 <para>(Note that strictly speaking the <option>-i</option> flag is
1380 a static one, but in fact it works to set it using
1381 <literal>:set</literal> like this. The changes won't take effect
1382 until the next <literal>:load</literal>, though.)</para>
1383
1384 <para>Two command-line options control whether the
1385 <filename>.ghci</filename> files are read:</para>
1386
1387 <variablelist>
1388 <varlistentry>
1389 <term>
1390 <option>-ignore-dot-ghci</option>
1391 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
1392 </term>
1393 <listitem>
1394 <para>Don't read either <filename>./.ghci</filename> or
1395 <filename>$HOME/.ghci</filename> when starting up.</para>
1396 </listitem>
1397 </varlistentry>
1398 <varlistentry>
1399 <term>
1400 <option>-read-dot-ghci</option>
1401 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
1402 </term>
1403 <listitem>
1404 <para>Read <filename>.ghci</filename> and
1405 <filename>$HOME/.ghci</filename>. This is normally the
1406 default, but the <option>-read-dot-ghci</option> option may
1407 be used to override a previous
1408 <option>-ignore-dot-ghci</option> option.</para>
1409 </listitem>
1410 </varlistentry>
1411 </variablelist>
1412
1413 </sect1>
1414
1415 <sect1>
1416 <title>FAQ and Things To Watch Out For</title>
1417
1418 <variablelist>
1419 <varlistentry>
1420 <term>The interpreter can't load modules with foreign export
1421 declarations!</term>
1422 <listitem>
1423 <para>Unfortunately not. We haven't implemented it yet.
1424 Please compile any offending modules by hand before loading
1425 them into GHCi.</para>
1426 </listitem>
1427 </varlistentry>
1428
1429 <varlistentry>
1430 <term>
1431 <literal>-O</literal> doesn't work with GHCi!
1432 <indexterm><primary><option>-O</option></primary></indexterm>
1433 </term>
1434 <listitem>
1435 <para>For technical reasons, the bytecode compiler doesn't
1436 interact well with one of the optimisation passes, so we
1437 have disabled optimisation when using the interpreter. This
1438 isn't a great loss: you'll get a much bigger win by
1439 compiling the bits of your code that need to go fast, rather
1440 than interpreting them with optimisation turned on.</para>
1441 </listitem>
1442 </varlistentry>
1443
1444 <varlistentry>
1445 <term>Unboxed tuples don't work with GHCi</term>
1446 <listitem>
1447 <para>That's right. You can always compile a module that
1448 uses unboxed tuples and load it into GHCi, however.
1449 (Incidentally the previous point, namely that
1450 <literal>-O</literal> is incompatible with GHCi, is because
1451 the bytecode compiler can't deal with unboxed
1452 tuples).</para>
1453 </listitem>
1454 </varlistentry>
1455
1456 <varlistentry>
1457 <term>Concurrent threads don't carry on running when GHCi is
1458 waiting for input.</term>
1459 <listitem>
1460 <para>No, they don't. This is because the Haskell binding
1461 to the GNU readline library doesn't support reading from the
1462 terminal in a non-blocking way, which is required to work
1463 properly with GHC's concurrency model.</para>
1464 </listitem>
1465 </varlistentry>
1466
1467 <varlistentry>
1468 <term>After using <literal>getContents</literal>, I can't use
1469 <literal>stdin</literal> again until I do
1470 <literal>:load</literal> or <literal>:reload</literal>.</term>
1471
1472 <listitem>
1473 <para>This is the defined behaviour of
1474 <literal>getContents</literal>: it puts the stdin Handle in
1475 a state known as <firstterm>semi-closed</firstterm>, wherein
1476 any further I/O operations on it are forbidden. Because I/O
1477 state is retained between computations, the semi-closed
1478 state persists until the next <literal>:load</literal> or
1479 <literal>:reload</literal> command.</para>
1480
1481 <para>You can make <literal>stdin</literal> reset itself
1482 after every evaluation by giving GHCi the command
1483 <literal>:set +r</literal>. This works because
1484 <literal>stdin</literal> is just a top-level expression that
1485 can be reverted to its unevaluated state in the same way as
1486 any other top-level expression (CAF).</para>
1487 </listitem>
1488 </varlistentry>
1489
1490 </variablelist>
1491 </sect1>
1492
1493 </chapter>
1494
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