2 <title>Using GHCi</title>
3 <indexterm><primary>GHCi</primary></indexterm>
4 <indexterm><primary>interpreter</primary><see>GHCi</see></indexterm>
5 <indexterm><primary>interactive</primary><see>GHCi</see></indexterm>
8 <para>The ‘i’ stands for “Interactive”</para>
10 is GHC's interactive environment, in which Haskell expressions can
11 be interactively evaluated and programs can be interpreted. If
12 you're famililar with <ulink url="http://www.haskell.org/hugs/">Hugs</ulink><indexterm><primary>Hugs</primary>
13 </indexterm>, then you'll be right at home with GHCi. However, GHCi
14 also has support for interactively loading compiled code, as well as
15 supporting all<footnote><para>except <literal>foreign export</literal>, at the moment</para>
16 </footnote> the language extensions that GHC provides.</para>
17 <indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
18 <indexterm><primary>Foreign Function Interface</primary><secondary>GHCi support</secondary></indexterm>
21 <title>Introduction to GHCi</title>
23 <para>Let's start with an example GHCi session. You can fire up
24 GHCi with the command <literal>ghci</literal>:</para>
30 / /_\// /_/ / / | | GHC Interactive, version 5.04, for Haskell 98.
31 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
32 \____/\/ /_/\____/|_| Type :? for help.
34 Loading package base ... linking ... done.
35 Loading package haskell98 ... linking ... done.
39 <para>There may be a short pause while GHCi loads the prelude and
40 standard libraries, after which the prompt is shown. If we follow
41 the instructions and type <literal>:?</literal> for help, we
45 Commands available from the prompt:
47 <stmt> evaluate/run <stmt>
48 :add <filename> ... add module(s) to the current target set
49 :browse [*]<module> display the names defined by <module>
50 :cd <dir> change directory to <dir>
51 :def <cmd> <expr> define a command :<cmd>
52 :help, :? display this list of commands
53 :info [<name> ...] display information about the given names
54 :load <filename> ... load module(s) and their dependents
55 :module [+/-] [*]<mod> ... set the context for expression evaluation
56 :reload reload the current module set
58 :set <option> ... set options
59 :set args <arg> ... set the arguments returned by System.getArgs
60 :set prog <progname> set the value returned by System.getProgName
62 :show modules show the currently loaded modules
63 :show bindings show the current bindings made at the prompt
65 :type <expr> show the type of <expr>
66 :undef <cmd> undefine user-defined command :<cmd>
67 :unset <option> ... unset options
69 :!<command> run the shell command <command>
71 Options for `:set' and `:unset':
73 +r revert top-level expressions after each evaluation
74 +s print timing/memory stats after each evaluation
75 +t print type after evaluation
76 -<flags> most GHC command line flags can also be set here
77 (eg. -v2, -fglasgow-exts, etc.)
80 <para>We'll explain most of these commands as we go along. For
81 Hugs users: many things work the same as in Hugs, so you should be
82 able to get going straight away.</para>
84 <para>Haskell expressions can be typed at the prompt:</para>
85 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
91 Prelude> let x = 42 in x / 9
96 <para>GHCi interprets the whole line as an expression to evaluate.
97 The expression may not span several lines - as soon as you press
98 enter, GHCi will attempt to evaluate it.</para>
102 <title>Loading source files</title>
104 <para>Suppose we have the following Haskell source code, which we
105 place in a file <filename>Main.hs</filename>:</para>
108 main = print (fac 20)
111 fac n = n * fac (n-1)
114 <para>You can save <filename>Main.hs</filename> anywhere you like,
115 but if you save it somewhere other than the current
116 directory<footnote><para>If you started up GHCi from the command
117 line then GHCi's current directory is the same as the current
118 directory of the shell from which it was started. If you started
119 GHCi from the “Start” menu in Windows, then the
120 current directory is probably something like
121 <filename>C:\Documents and Settings\<replaceable>user
122 name</replaceable></filename>.</para> </footnote> then we will
123 need to change to the right directory in GHCi:</para>
126 Prelude> :cd <replaceable>dir</replaceable>
129 <para>where <replaceable>dir</replaceable> is the directory (or
130 folder) in which you saved <filename>Main.hs</filename>.</para>
132 <para>To load a Haskell source file into GHCi, use the
133 <literal>:load</literal> command:</para>
134 <indexterm><primary><literal>:load</literal></primary></indexterm>
138 Compiling Main ( Main.hs, interpreted )
139 Ok, modules loaded: Main.
143 <para>GHCi has loaded the <literal>Main</literal> module, and the
144 prompt has changed to “<literal>*Main></literal>” to
145 indicate that the current context for expressions typed at the
146 prompt is the <literal>Main</literal> module we just loaded (we'll
147 explain what the <literal>*</literal> means later in <xref
148 linkend="ghci-scope">). So we can now type expressions involving
149 the functions from <filename>Main.hs</filename>:</para>
156 <para>Loading a multi-module program is just as straightforward;
157 just give the name of the “topmost” module to the
158 <literal>:load</literal> command (hint: <literal>:load</literal>
159 can be abbreviated to <literal>:l</literal>). The topmost module
160 will normally be <literal>Main</literal>, but it doesn't have to
161 be. GHCi will discover which modules are required, directly or
162 indirectly, by the topmost module, and load them all in dependency
166 <title>Modules vs. filenames</title>
167 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
168 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
170 <para>Question: How does GHC find the filename which contains
171 module <replaceable>M</replaceable>? Answer: it looks for the
172 file <literal><replaceable>M</replaceable>.hs</literal>, or
173 <literal><replaceable>M</replaceable>.lhs</literal>. This means
174 that for most modules, the module name must match the filename.
175 If it doesn't, GHCi won't be able to find it.</para>
177 <para>There is one exception to this general rule: when you load
178 a program with <literal>:load</literal>, or specify it when you
179 invoke <literal>ghci</literal>, you can give a filename rather
180 than a module name. This filename is loaded if it exists, and
181 it may contain any module you like. This is particularly
182 convenient if you have several <literal>Main</literal> modules
183 in the same directory and you can't call them all
184 <filename>Main.hs</filename>.</para>
186 <para>The search path for finding source files is specified with
187 the <option>-i</option> option on the GHCi command line, like
189 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
191 <para>or it can be set using the <literal>:set</literal> command
192 from within GHCi (see <xref
193 linkend="ghci-cmd-line-options">)<footnote><para>Note that in
194 GHCi, and <option>––make</option> mode, the <option>-i</option>
195 option is used to specify the search path for
196 <emphasis>source</emphasis> files, whereas in standard
197 batch-compilation mode the <option>-i</option> option is used to
198 specify the search path for interface files, see <xref
199 linkend="options-finding-imports">.</para> </footnote></para>
201 <para>One consequence of the way that GHCi follows dependencies
202 to find modules to load is that every module must have a source
203 file. The only exception to the rule is modules that come from
204 a package, including the <literal>Prelude</literal> and standard
205 libraries such as <literal>IO</literal> and
206 <literal>Complex</literal>. If you attempt to load a module for
207 which GHCi can't find a source file, even if there are object
208 and interface files for the module, you'll get an error
211 <para>One final note: if you load a module called Main, it must
212 contain a <literal>main</literal> function, just like in
217 <title>Making changes and recompilation</title>
218 <indexterm><primary><literal>:reload</literal></primary></indexterm>
220 <para>If you make some changes to the source code and want GHCi
221 to recompile the program, give the <literal>:reload</literal>
222 command. The program will be recompiled as necessary, with GHCi
223 doing its best to avoid actually recompiling modules if their
224 external dependencies haven't changed. This is the same
225 mechanism we use to avoid re-compiling modules in the batch
226 compilation setting (see <xref linkend="recomp">).</para>
230 <sect1 id="ghci-compiled">
231 <title>Loading compiled code</title>
232 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
234 <para>When you load a Haskell source module into GHCi, it is
235 normally converted to byte-code and run using the interpreter.
236 However, interpreted code can also run alongside compiled code in
237 GHCi; indeed, normally when GHCi starts, it loads up a compiled
238 copy of the <literal>base</literal> package, which contains the
239 <literal>Prelude</literal>.</para>
241 <para>Why should we want to run compiled code? Well, compiled
242 code is roughly 10x faster than interpreted code, but takes about
243 2x longer to produce (perhaps longer if optimisation is on). So
244 it pays to compile the parts of a program that aren't changing
245 very often, and use the interpreter for the code being actively
248 <para>When loading up source files with <literal>:load</literal>,
249 GHCi looks for any corresponding compiled object files, and will
250 use one in preference to interpreting the source if possible. For
251 example, suppose we have a 4-module program consisting of modules
252 A, B, C, and D. Modules B and C both import D only,
253 and A imports both B & C:</para>
261 <para>We can compile D, then load the whole program, like this:</para>
263 Prelude> :! ghc -c D.hs
265 Skipping D ( D.hs, D.o )
266 Compiling C ( C.hs, interpreted )
267 Compiling B ( B.hs, interpreted )
268 Compiling A ( A.hs, interpreted )
269 Ok, modules loaded: A, B, C, D.
273 <para>In the messages from the compiler, we see that it skipped D,
274 and used the object file <filename>D.o</filename>. The message
275 <literal>Skipping</literal> <replaceable>module</replaceable>
276 indicates that compilation for <replaceable>module</replaceable>
277 isn't necessary, because the source and everything it depends on
278 is unchanged since the last compilation.</para>
280 <para>At any time you can use the command
281 <literal>:show modules</literal>
282 to get a list of the modules currently loaded
288 C ( C.hs, interpreted )
289 B ( B.hs, interpreted )
290 A ( A.hs, interpreted )
293 <para>If we now modify the source of D (or pretend to: using Unix
294 command <literal>touch</literal> on the source file is handy for
295 this), the compiler will no longer be able to use the object file,
296 because it might be out of date:</para>
301 Compiling D ( D.hs, interpreted )
302 Skipping C ( C.hs, interpreted )
303 Skipping B ( B.hs, interpreted )
304 Skipping A ( A.hs, interpreted )
305 Ok, modules loaded: A, B, C, D.
309 <para>Note that module D was compiled, but in this instance
310 because its source hadn't really changed, its interface remained
311 the same, and the recompilation checker determined that A, B and C
312 didn't need to be recompiled.</para>
314 <para>So let's try compiling one of the other modules:</para>
317 *Main> :! ghc -c C.hs
319 Compiling D ( D.hs, interpreted )
320 Compiling C ( C.hs, interpreted )
321 Compiling B ( B.hs, interpreted )
322 Compiling A ( A.hs, interpreted )
323 Ok, modules loaded: A, B, C, D.
326 <para>We didn't get the compiled version of C! What happened?
327 Well, in GHCi a compiled module may only depend on other compiled
328 modules, and in this case C depends on D, which doesn't have an
329 object file, so GHCi also rejected C's object file. Ok, so let's
330 also compile D:</para>
333 *Main> :! ghc -c D.hs
335 Ok, modules loaded: A, B, C, D.
338 <para>Nothing happened! Here's another lesson: newly compiled
339 modules aren't picked up by <literal>:reload</literal>, only
340 <literal>:load</literal>:</para>
344 Skipping D ( D.hs, D.o )
345 Skipping C ( C.hs, C.o )
346 Compiling B ( B.hs, interpreted )
347 Compiling A ( A.hs, interpreted )
348 Ok, modules loaded: A, B, C, D.
351 <para>HINT: since GHCi will only use a compiled object file if it
352 can sure that the compiled version is up-to-date, a good technique
353 when working on a large program is to occasionally run
354 <literal>ghc ––make</literal> to compile the whole project (say
355 before you go for lunch :-), then continue working in the
356 interpreter. As you modify code, the new modules will be
357 interpreted, but the rest of the project will remain
363 <title>Interactive evaluation at the prompt</title>
365 <para>When you type an expression at the prompt, GHCi immediately
366 evaluates and prints the result. But that's not the whole story:
367 if you type something of type <literal>IO a</literal> for some
368 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
369 as an IO-computation, and doesn't attempt to print the
375 Prelude> putStrLn "hello"
379 <para>What actually happens is that GHCi typechecks the
380 expression, and if it doesn't have an <literal>IO</literal> type,
381 then it transforms it as follows: an expression
382 <replaceable>e</replaceable> turns into
384 let it = <replaceable>e</replaceable>;
387 which is then run as an IO-action.</para>
389 <para>Hence, the original expression must have a type which is an
390 instance of the <literal>Show</literal> class, or GHCi will
395 No instance for `Show (a -> a)'
396 arising from use of `print'
397 in a `do' expression pattern binding: print it
400 <para>The error message contains some clues as to the
401 transformation happening internally.</para>
403 <sect2 id="ghci-scope">
404 <title>What's really in scope at the prompt?</title>
406 <para>When you type an expression at the prompt, what
407 identifiers and types are in scope? GHCi provides a flexible
408 way to control exactly how the context for an expression is
409 constructed. Let's start with the simple cases; when you start
410 GHCi the prompt looks like this:</para>
412 <screen>Prelude></screen>
414 <para>Which indicates that everything from the module
415 <literal>Prelude</literal> is currently in scope. If we now
416 load a file into GHCi, the prompt will change:</para>
419 Prelude> :load Main.hs
420 Compiling Main ( Main.hs, interpreted )
424 <para>The new prompt is <literal>*Main</literal>, which
425 indicates that we are typing expressions in the context of the
426 top-level of the <literal>Main</literal> module. Everything
427 that is in scope at the top-level in the module
428 <literal>Main</literal> we just loaded is also in scope at the
429 prompt (probably including <literal>Prelude</literal>, as long
430 as <literal>Main</literal> doesn't explicitly hide it).</para>
433 <literal>*<replaceable>module</replaceable></literal> indicates
434 that it is the full top-level scope of
435 <replaceable>module</replaceable> that is contributing to the
436 scope for expressions typed at the prompt. Without the
437 <literal>*</literal>, just the exports of the module are
440 <para>We're not limited to a single module: GHCi can combine
441 scopes from multiple modules, in any mixture of
442 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
443 combines the scopes from all of these modules to form the scope
444 that is in effect at the prompt. For technical reasons, GHCi
445 can only support the <literal>*</literal>-form for modules which
446 are interpreted, so compiled modules and package modules can
447 only contribute their exports to the current scope.</para>
449 <para>The scope is manipulated using the
450 <literal>:module</literal> command. For example, if the current
451 scope is <literal>Prelude</literal>, then we can bring into
452 scope the exports from the module <literal>IO</literal> like
457 Prelude,IO> hPutStrLn stdout "hello\n"
462 <para>(Note: <literal>:module</literal> can be shortened to
463 <literal>:m</literal>). The full syntax of the
464 <literal>:module</literal> command is:</para>
467 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
470 <para>Using the <literal>+</literal> form of the
471 <literal>module</literal> commands adds modules to the current
472 scope, and <literal>-</literal> removes them. Without either
473 <literal>+</literal> or <literal>-</literal>, the current scope
474 is replaced by the set of modules specified. Note that if you
475 use this form and leave out <literal>Prelude</literal>, GHCi
476 will assume that you really wanted the
477 <literal>Prelude</literal> and add it in for you (if you don't
478 want the <literal>Prelude</literal>, then ask to remove it with
479 <literal>:m -Prelude</literal>).</para>
481 <para>The scope is automatically set after a
482 <literal>:load</literal> command, to the most recently loaded
483 "target" module, in a <literal>*</literal>-form if possible.
484 For example, if you say <literal>:load foo.hs bar.hs</literal>
485 and <filename>bar.hs</filename> contains module
486 <literal>Bar</literal>, then the scope will be set to
487 <literal>*Bar</literal> if <literal>Bar</literal> is
488 interpreted, or if <literal>Bar</literal> is compiled it will be
489 set to <literal>Prelude,Bar</literal> (GHCi automatically adds
490 <literal>Prelude</literal> if it isn't present and there aren't
491 any <literal>*</literal>-form modules).</para>
493 <para>With multiple modules in scope, especially multiple
494 <literal>*</literal>-form modules, it is likely that name
495 clashes will occur. Haskell specifies that name clashes are
496 only reported when an ambiguous identifier is used, and GHCi
497 behaves in the same way for expressions typed at the
501 <title>Qualified names</title>
503 <para>To make life slightly easier, the GHCi prompt also
504 behaves as if there is an implicit <literal>import
505 qualified</literal> declaration for every module in every
506 package, and every module currently loaded into GHCi.</para>
511 <title>Using <literal>do-</literal>notation at the prompt</title>
512 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
513 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
515 <para>GHCi actually accepts <firstterm>statements</firstterm>
516 rather than just expressions at the prompt. This means you can
517 bind values and functions to names, and use them in future
518 expressions or statements.</para>
520 <para>The syntax of a statement accepted at the GHCi prompt is
521 exactly the same as the syntax of a statement in a Haskell
522 <literal>do</literal> expression. However, there's no monad
523 overloading here: statements typed at the prompt must be in the
524 <literal>IO</literal> monad.</para>
526 <para>Here's an example:</para>
528 Prelude> x <- return 42
533 <para>The statement <literal>x <- return 42</literal> means
534 “execute <literal>return 42</literal> in the
535 <literal>IO</literal> monad, and bind the result to
536 <literal>x</literal>”. We can then use
537 <literal>x</literal> in future statements, for example to print
538 it as we did above.</para>
540 <para>Of course, you can also bind normal non-IO expressions
541 using the <literal>let</literal>-statement:</para>
548 <para>An important difference between the two types of binding
549 is that the monadic bind (<literal>p <- e</literal>) is
550 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
551 whereas with the <literal>let</literal> form, the expression
552 isn't evaluated immediately:</para>
554 Prelude> let x = error "help!"
559 <para>Any exceptions raised during the evaluation or execution
560 of the statement are caught and printed by the GHCi command line
561 interface (for more information on exceptions, see the module
562 <literal>Control.Exception</literal> in the libraries
563 documentation).</para>
565 <para>Every new binding shadows any existing bindings of the
566 same name, including entities that are in scope in the current
567 module context.</para>
569 <para>WARNING: temporary bindings introduced at the prompt only
570 last until the next <literal>:load</literal> or
571 <literal>:reload</literal> command, at which time they will be
572 simply lost. However, they do survive a change of context with
573 <literal>:module</literal>: the temporary bindings just move to
574 the new location.</para>
576 <para>HINT: To get a list of the bindings currently in scope, use the
577 <literal>:show bindings</literal> command:</para>
580 Prelude> :show bindings
584 <para>HINT: if you turn on the <literal>+t</literal> option,
585 GHCi will show the type of each variable bound by a statement.
587 <indexterm><primary><literal>+t</literal></primary></indexterm>
590 Prelude> let (x:xs) = [1..]
598 <title>The <literal>it</literal> variable</title>
599 <indexterm><primary><literal>it</literal></primary>
602 <para>Whenever an expression (or a non-binding statement, to be
603 precise) is typed at the prompt, GHCi implicitly binds its value
604 to the variable <literal>it</literal>. For example:</para>
612 <para>This is a result of the translation mentioned earlier,
613 namely that an expression <replaceable>e</replaceable> is
616 let it = <replaceable>e</replaceable>;
619 before execution, resulting in a binding for
620 <literal>it</literal>.</para>
622 <para>If the expression was of type <literal>IO a</literal> for
623 some <literal>a</literal>, then <literal>it</literal> will be
624 bound to the result of the <literal>IO</literal> computation,
625 which is of type <literal>a</literal>. eg.:</para>
627 Prelude> Time.getClockTime
629 Wed Mar 14 12:23:13 GMT 2001
632 <para>The corresponding translation for an IO-typed
633 <replaceable>e</replaceable> is
635 it <- <replaceable>e</replaceable>
639 <para>Note that <literal>it</literal> is shadowed by the new
640 value each time you evaluate a new expression, and the old value
641 of <literal>it</literal> is lost.</para>
646 <sect1 id="ghci-invokation">
647 <title>Invoking GHCi</title>
648 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
649 <indexterm><primary><option>––interactive</option></primary></indexterm>
651 <para>GHCi is invoked with the command <literal>ghci</literal> or
652 <literal>ghc ––interactive</literal>. One or more modules or
653 filenames can also be specified on the command line; this
654 instructs GHCi to load the specified modules or filenames (and all
655 the modules they depend on), just as if you had said
656 <literal>:load <replaceable>modules</replaceable></literal> at the
657 GHCi prompt (see <xref linkend="ghci-commands">). For example, to
658 start GHCi and load the program whose topmost module is in the
659 file <literal>Main.hs</literal>, we could say:</para>
665 <para>Most of the command-line options accepted by GHC (see <xref
666 linkend="using-ghc">) also make sense in interactive mode. The ones
667 that don't make sense are mostly obvious; for example, GHCi
668 doesn't generate interface files, so options related to interface
669 file generation won't have any effect.</para>
672 <title>Packages</title>
673 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
675 <para>GHCi can make use of all the packages that come with GHC,
676 For example, to start up GHCi with the <literal>network</literal>
677 package loaded:</para>
680 $ ghci -package network
683 / /_\// /_/ / / | | GHC Interactive, version 5.04, for Haskell 98.
684 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
685 \____/\/ /_/\____/|_| Type :? for help.
687 Loading package base ... linking ... done.
688 Loading package haskell98 ... linking ... done.
689 Loading package network ... linking ... done.
693 <para>Note that GHCi will also automatically load any packages
694 on which the requested package depends.</para>
696 <para>The following command works to load new packages into a
700 Prelude> :set -package <replaceable>name</replaceable>
703 <para>But note that doing this will cause all currently loaded
704 modules to be unloaded, and you'll be dumped back into the
709 <title>Extra libraries</title>
710 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
712 <para>Extra libraries may be specified on the command line using
713 the normal <literal>-l<replaceable>lib</replaceable></literal>
714 option. For example, to load the “m” library:</para>
720 <para>On systems with <literal>.so</literal>-style shared
721 libraries, the actual library loaded will the
722 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
723 searches the following places for libraries, in this order:</para>
727 <para>Paths specified using the
728 <literal>-L<replaceable>path</replaceable></literal>
729 command-line option,</para>
732 <para>the standard library search path for your system,
733 which on some systems may be overriden by setting the
734 <literal>LD_LIBRARY_PATH</literal> environment
739 <para>On systems with <literal>.dll</literal>-style shared
740 libraries, the actual library loaded will be
741 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
742 GHCi will signal an error if it can't find the library.</para>
744 <para>GHCi can also load plain object files
745 (<literal>.o</literal> or <literal>.obj</literal> depending on
746 your platform) from the command-line. Just add the name the
747 object file to the command line.</para>
752 <sect1 id="ghci-commands">
753 <title>GHCi commands</title>
755 <para>GHCi commands all begin with
756 ‘<literal>:</literal>’ and consist of a single command
757 name followed by zero or more parameters. The command name may be
758 abbreviated, as long as the abbreviation is not ambiguous. All of
759 the builtin commands, with the exception of
760 <literal>:unset</literal> and <literal>:undef</literal>, may be
761 abbreviated to a single letter.</para>
765 <term><literal>:add</literal>
766 <replaceable>module</replaceable> ...</term>
767 <indexterm><primary><literal>:add</literal></primary></indexterm>
769 <para>Add <replaceable>module</replaceable>(s) to the
770 current <firstterm>target set</firstterm>, and perform a
776 <term><literal>:browse</literal>
777 <optional><literal>*</literal></optional><replaceable>module</replaceable>
779 <indexterm><primary><literal>:browse</literal></primary>
782 <para>Displays the identifiers defined by the module
783 <replaceable>module</replaceable>, which must be either
784 loaded into GHCi or be a member of a package. If the
785 <literal>*</literal> symbol is placed before the module
786 name, then <emphasis>all</emphasis> the identifiers defined
787 in <replaceable>module</replaceable> are shown; otherwise
788 the list is limited to the exports of
789 <replaceable>module</replaceable>. The
790 <literal>*</literal>-form is only available for modules
791 which are interpreted; for compiled modules (including
792 modules from packages) only the non-<literal>*</literal>
793 form of <literal>:browse</literal> is available.</para>
798 <term><literal>:cd</literal> <replaceable>dir</replaceable></term>
799 <indexterm><primary><literal>:cd</literal></primary></indexterm>
801 <para>Changes the current working directory to
802 <replaceable>dir</replaceable>. A
803 ‘<literal>˜</literal>’ symbol at the
804 beginning of <replaceable>dir</replaceable> will be replaced
805 by the contents of the environment variable
806 <literal>HOME</literal>.</para>
811 <term><literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable></term>
812 <indexterm><primary><literal>:def</literal></primary></indexterm>
814 <para>The command <literal>:def</literal>
815 <replaceable>name</replaceable>
816 <replaceable>expr</replaceable> defines a new GHCi command
817 <literal>:<replaceable>name</replaceable></literal>,
818 implemented by the Haskell expression
819 <replaceable>expr</replaceable>, which must have type
820 <literal>String -> IO String</literal>. When
821 <literal>:<replaceable>name</replaceable>
822 <replaceable>args</replaceable></literal> is typed at the
823 prompt, GHCi will run the expression
824 <literal>(<replaceable>name</replaceable>
825 <replaceable>args</replaceable>)</literal>, take the
826 resulting <literal>String</literal>, and feed it back into
827 GHCi as a new sequence of commands. Separate commands in
828 the result must be separated by
829 ‘<literal>\n</literal>’.</para>
831 <para>That's all a little confusing, so here's a few
832 examples. To start with, here's a new GHCi command which
833 doesn't take any arguments or produce any results, it just
834 outputs the current date & time:</para>
837 Prelude> let date _ = Time.getClockTime >>= print >> return ""
838 Prelude> :def date date
840 Fri Mar 23 15:16:40 GMT 2001
843 <para>Here's an example of a command that takes an argument.
844 It's a re-implementation of <literal>:cd</literal>:</para>
847 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
848 Prelude> :def mycd mycd
852 <para>Or I could define a simple way to invoke
853 “<literal>ghc ––make Main</literal>” in the
854 current directory:</para>
857 Prelude> :def make (\_ -> return ":! ghc ––make Main")
864 <term><literal>:help</literal></term>
865 <indexterm><primary><literal>:help</literal></primary></indexterm>
866 <term><literal>:?</literal></term>
867 <indexterm><primary><literal>:?</literal></primary></indexterm>
869 <para>Displays a list of the available commands.</para>
874 <term><literal>:info</literal> <replaceable>name</replaceable>
876 <indexterm><primary><literal>:info</literal></primary>
879 <para>Displays information about the given name(s). For
880 example, if <replaceable>name</replaceable> is a class, then
881 the class methods and their types will be printed; if
882 <replaceable>name</replaceable> is a type constructor, then
883 its definition will be printed; if
884 <replaceable>name</replaceable> is a function, then its type
885 will be printed. If <replaceable>name</replaceable> has
886 been loaded from a source file, then GHCi will also display
887 the location of its definition in the source.</para>
892 <term><literal>:load</literal>
893 <replaceable>module</replaceable> ...</term>
894 <indexterm><primary><literal>:load</literal></primary></indexterm>
896 <para>Recursively loads the specified
897 <replaceable>module</replaceable>s, and all the modules they
898 depend on. Here, each <replaceable>module</replaceable>
899 must be a module name or filename, but may not be the name
900 of a module in a package.</para>
902 <para>All previously loaded modules, except package modules,
903 are forgotten. The new set of modules is known as the
904 <firstterm>target set</firstterm>. Note that
905 <literal>:load</literal> can be used without any arguments
906 to unload all the currently loaded modules and
909 <para>After a <literal>:load</literal> command, the current
910 context is set to:</para>
914 <para><replaceable>module</replaceable>, if it was loaded
915 successfully, or</para>
918 <para>the most recently successfully loaded module, if
919 any other modules were loaded as a result of the current
920 <literal>:load</literal>, or</para>
923 <para><literal>Prelude</literal> otherwise.</para>
930 <term><literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal></term>
931 <indexterm><primary><literal>:module</literal></primary></indexterm>
933 <para>Sets or modifies the current context for statements
934 typed at the prompt. See <xref linkend="ghci-scope"> for
940 <term><literal>:quit</literal></term>
941 <indexterm><primary><literal>:quit</literal></primary></indexterm>
943 <para>Quits GHCi. You can also quit by typing a control-D
944 at the prompt.</para>
949 <term><literal>:reload</literal></term>
950 <indexterm><primary><literal>:reload</literal></primary></indexterm>
952 <para>Attempts to reload the current target set (see
953 <literal>:load</literal>) if any of the modules in the set,
954 or any dependent module, has changed. Note that this may
955 entail loading new modules, or dropping modules which are no
956 longer indirectly required by the target.</para>
961 <term><literal>:set</literal> <optional><replaceable>option</replaceable>...</optional></term>
962 <indexterm><primary><literal>:set</literal></primary></indexterm>
964 <para>Sets various options. See <xref linkend="ghci-set">
965 for a list of available options. The
966 <literal>:set</literal> command by itself shows which
967 options are currently set.</para>
972 <term><literal>:set</literal> <literal>args</literal>
973 <replaceable>arg</replaceable> ...</term>
974 <indexterm><primary><literal>:set</literal></primary></indexterm>
976 <para>Sets the list of arguments which are returned when the
977 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
983 <term><literal>:set</literal> <literal>prog</literal>
984 <replaceable>prog</replaceable></term>
985 <indexterm><primary><literal>:set</literal></primary></indexterm>
987 <para>Sets the string to be returned when the program calls
988 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
994 <term><literal>:show bindings</literal></term>
995 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
997 <para>Show the bindings made at the prompt and their
1003 <term><literal>:show modules</literal></term>
1004 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
1006 <para>Show the list of modules currently load.</para>
1011 <term><literal>:type</literal> <replaceable>expression</replaceable></term>
1012 <indexterm><primary><literal>:type</literal></primary></indexterm>
1014 <para>Infers and prints the type of
1015 <replaceable>expression</replaceable>, including explicit
1016 forall quantifiers for polymorphic types. The monomorphism
1017 restriction is <emphasis>not</emphasis> applied to the
1018 expression during type inference.</para>
1023 <term><literal>:undef</literal> <replaceable>name</replaceable></term>
1024 <indexterm><primary><literal>:undef</literal></primary></indexterm>
1026 <para>Undefines the user-defined command
1027 <replaceable>name</replaceable> (see <literal>:def</literal>
1033 <term><literal>:unset</literal> <replaceable>option</replaceable>...</term>
1034 <indexterm><primary><literal>:unset</literal></primary></indexterm>
1036 <para>Unsets certain options. See <xref linkend="ghci-set">
1037 for a list of available options.</para>
1042 <term><literal>:!</literal> <replaceable>command</replaceable>...</term>
1043 <indexterm><primary><literal>:!</literal></primary></indexterm>
1044 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
1046 <para>Executes the shell command
1047 <replaceable>command</replaceable>.</para>
1054 <sect1 id="ghci-set">
1055 <title>The <literal>:set</literal> command</title>
1056 <indexterm><primary><literal>:set</literal></primary></indexterm>
1058 <para>The <literal>:set</literal> command sets two types of
1059 options: GHCi options, which begin with
1060 ‘<literal>+</literal>” and “command-line”
1061 options, which begin with ‘-’. </para>
1063 <para>NOTE: at the moment, the <literal>:set</literal> command
1064 doesn't support any kind of quoting in its arguments: quotes will
1065 not be removed and cannot be used to group words together. For
1066 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
1070 <title>GHCi options</title>
1071 <indexterm><primary>options</primary><secondary>GHCi</secondary>
1074 <para>GHCi options may be set using <literal>:set</literal> and
1075 unset using <literal>:unset</literal>.</para>
1077 <para>The available GHCi options are:</para>
1081 <term><literal>+r</literal></term>
1082 <indexterm><primary><literal>+r</literal></primary></indexterm>
1083 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
1084 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
1086 <para>Normally, any evaluation of top-level expressions
1087 (otherwise known as CAFs or Constant Applicative Forms) in
1088 loaded modules is retained between evaluations. Turning
1089 on <literal>+r</literal> causes all evaluation of
1090 top-level expressions to be discarded after each
1091 evaluation (they are still retained
1092 <emphasis>during</emphasis> a single evaluation).</para>
1094 <para>This option may help if the evaluated top-level
1095 expressions are consuming large amounts of space, or if
1096 you need repeatable performance measurements.</para>
1101 <term><literal>+s</literal></term>
1102 <indexterm><primary><literal>+s</literal></primary></indexterm>
1104 <para>Display some stats after evaluating each expression,
1105 including the elapsed time and number of bytes allocated.
1106 NOTE: the allocation figure is only accurate to the size
1107 of the storage manager's allocation area, because it is
1108 calculated at every GC. Hence, you might see values of
1109 zero if no GC has occurred.</para>
1114 <term><literal>+t</literal></term>
1115 <indexterm><primary><literal>+t</literal></primary></indexterm>
1117 <para>Display the type of each variable bound after a
1118 statement is entered at the prompt. If the statement is a
1119 single expression, then the only variable binding will be
1121 ‘<literal>it</literal>’.</para>
1127 <sect2 id="ghci-cmd-line-options">
1128 <title>Setting GHC command-line options in GHCi</title>
1130 <para>Normal GHC command-line options may also be set using
1131 <literal>:set</literal>. For example, to turn on
1132 <option>-fglasgow-exts</option>, you would say:</para>
1135 Prelude> :set -fglasgow-exts
1138 <para>Any GHC command-line option that is designated as
1139 <firstterm>dynamic</firstterm> (see the table in <xref
1140 linkend="flag-reference">), may be set using
1141 <literal>:set</literal>. To unset an option, you can set the
1142 reverse option:</para>
1143 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
1146 Prelude> :set -fno-glasgow-exts
1149 <para><xref linkend="flag-reference"> lists the reverse for each
1150 option where applicable.</para>
1152 <para>Certain static options (<option>-package</option>,
1153 <option>-I</option>, <option>-i</option>, and
1154 <option>-l</option> in particular) will also work, but some may
1155 not take effect until the next reload.</para>
1156 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
1160 <sect1 id="ghci-dot-files">
1161 <title>The <filename>.ghci</filename> file</title>
1162 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
1164 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
1167 <para>When it starts, GHCi always reads and executes commands from
1168 <filename>$HOME/.ghci</filename>, followed by
1169 <filename>./.ghci</filename>.</para>
1171 <para>The <filename>.ghci</filename> in your home directory is
1172 most useful for turning on favourite options (eg. <literal>:set
1173 +s</literal>), and defining useful macros. Placing a
1174 <filename>.ghci</filename> file in a directory with a Haskell
1175 project is a useful way to set certain project-wide options so you
1176 don't have to type them everytime you start GHCi: eg. if your
1177 project uses GHC extensions and CPP, and has source files in three
1178 subdirectories A B and C, you might put the following lines in
1179 <filename>.ghci</filename>:</para>
1182 :set -fglasgow-exts -cpp
1186 <para>(Note that strictly speaking the <option>-i</option> flag is
1187 a static one, but in fact it works to set it using
1188 <literal>:set</literal> like this. The changes won't take effect
1189 until the next <literal>:load</literal>, though.)</para>
1191 <para>Two command-line options control whether the
1192 <filename>.ghci</filename> files are read:</para>
1196 <term><option>-ignore-dot-ghci</option></term>
1197 <indexterm><primary><option>-ignore-dot-ghci</option></primary>
1200 <para>Don't read either <filename>./.ghci</filename> or
1201 <filename>$HOME/.ghci</filename> when starting up.</para>
1205 <term><option>-read-dot-ghci</option></term>
1206 <indexterm><primary><option>-read-dot-ghci</option></primary>
1209 <para>Read <filename>.ghci</filename> and
1210 <filename>$HOME/.ghci</filename>. This is normally the
1211 default, but the <option>-read-dot-ghci</option> option may
1212 be used to override a previous
1213 <option>-ignore-dot-ghci</option> option.</para>
1221 <title>FAQ and Things To Watch Out For</title>
1225 <term>GHCi complains about <function>main</function> not being
1226 in scope when I load a module.</term>
1227 <indexterm><primary><function>main</function></primary><secondary>with GHCi</secondary>
1230 <para>You probably omitted the <literal>module</literal>
1231 declaration at the top of the module, which causes the
1232 module name to default to <literal>Main</literal>. In
1233 Haskell, the <literal>Main</literal> module must define a
1234 function called <function>main</function>. Admittedly this
1235 doesn't make a great deal of sense for an interpreter, but
1236 the rule was kept for compatibility with GHC.</para>
1241 <term>The interpreter can't load modules with foreign export
1242 declarations!</term>
1244 <para>Unfortunately not. We haven't implemented it yet.
1245 Please compile any offending modules by hand before loading
1246 them into GHCi.</para>
1251 <term><literal>-O</literal> doesn't work with GHCi!</term>
1252 <indexterm><primary><option>-O</option></primary>
1255 <para>For technical reasons, the bytecode compiler doesn't
1256 interact well with one of the optimisation passes, so we
1257 have disabled optimisation when using the interpreter. This
1258 isn't a great loss: you'll get a much bigger win by
1259 compiling the bits of your code that need to go fast, rather
1260 than interpreting them with optimisation turned on.</para>
1265 <term>Unboxed tuples don't work with GHCi</term>
1267 <para>That's right. You can always compile a module that
1268 uses unboxed tuples and load it into GHCi, however.
1269 (Incidentally the previous point, namely that
1270 <literal>-O</literal> is incompatible with GHCi, is because
1271 the bytecode compiler can't deal with unboxed
1277 <term>Concurrent threads don't carry on running when GHCi is
1278 waiting for input.</term>
1280 <para>No, they don't. This is because the Haskell binding
1281 to the GNU readline library doesn't support reading from the
1282 terminal in a non-blocking way, which is required to work
1283 properly with GHC's concurrency model.</para>
1288 <term>After using <literal>getContents</literal>, I can't use
1289 <literal>stdin</literal> again until I do
1290 <literal>:load</literal> or <literal>:reload</literal>.</term>
1293 <para>This is the defined behaviour of
1294 <literal>getContents</literal>: it puts the stdin Handle in
1295 a state known as <firstterm>semi-closed</firstterm>, wherein
1296 any further I/O operations on it are forbidden. Because I/O
1297 state is retained between computations, the semi-closed
1298 state persists until the next <literal>:load</literal> or
1299 <literal>:reload</literal> command.</para>
1301 <para>You can make <literal>stdin</literal> reset itself
1302 after every evaluation by giving GHCi the command
1303 <literal>:set +r</literal>. This works because
1304 <literal>stdin</literal> is just a top-level expression that
1305 can be reverted to its unevaluated state in the same way as
1306 any other top-level expression (CAF).</para>
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