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 Hugs<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 5.00, For Haskell 98.
31 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
32 \____/\/ /_/\____/|_| Type :? for help.
34 Loading package std ... linking ... done.
38 <para>There may be a short pause while GHCi loads the prelude and
39 standard libraries, after which the prompt is shown. If we follow
40 the instructions and type <literal>:?</literal> for help, we
44 Commands available from the prompt:
45 <stmt> evaluate/run <stmt>
46 :cd <dir> change directory to <dir>
47 :def <cmd> <expr> define a macro :<cmd>
48 :help, :? display this list of commands
49 :load <filename> load a module (and it dependents)
50 :module <mod> set the context for expression evaluation to <mod>
51 :reload reload the current module set
52 :set <option> ... set options
53 :type <expr> show the type of <expr>
54 :unset <option> ... unset options
56 :!<command> run the shell command <command>
57 Options for `:set' and `:unset':
58 +r revert top-level expressions after each evaluation
59 +s print timing/memory stats after each evaluation
60 +t print type after evaluation
61 -<flag> most GHC command line flags can also be set here
62 (eg. -v2, -fglasgow-exts, etc.)
65 <para>We'll explain most of these commands as we go along. For
66 Hugs users: many things work the same as in Hugs, so you should be
67 able to get going straight away.</para>
69 <para>Haskell expressions can be typed at the prompt:</para>
70 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
76 Prelude> let x = 42 in x / 9
81 <para>GHCi interprets the whole line as an expression to evaluate.
82 The expression may not span several lines - as soon as you press
83 enter, GHCi will attempt to evaluate it.</para>
87 <title>Loading source files</title>
89 <para>Suppose we have the following Haskell source code, which we
90 place in a file <filename>Main.hs</filename> in the current
100 <para>To load a Haskell source file into GHCi, use the
101 <literal>:load</literal> command:</para>
102 <indexterm><primary><literal>:load</literal></primary></indexterm>
106 Compiling Main ( Main.hs, interpreted )
107 Ok, modules loaded: Main.
111 <para>GHCi has loaded the <literal>Main</literal> module, and the
112 prompt has changed to “<literal>*Main></literal>” to
113 indicate that the current context for expressions typed at the
114 prompt is the <literal>Main</literal> module we just loaded (we'll
115 explain what the <literal>*</literal> means later in <xref
116 linkend="ghci-scope">). So we can now type expressions involving
117 the functions from <filename>Main.hs</filename>:</para>
124 <para>Loading a multi-module program is just as straightforward;
125 just give the name of the “topmost” module to the
126 <literal>:load</literal> command (hint: <literal>:load</literal>
127 can be abbreviated to <literal>:l</literal>). The topmost module
128 will normally be <literal>Main</literal>, but it doesn't have to
129 be. GHCi will discover which modules are required, directly or
130 indirectly, by the topmost module, and load them all in dependency
134 <title>Modules vs. filenames</title>
135 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
136 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
138 <para>Question: How does GHC find the filename which contains
139 module <replaceable>M</replaceable>? Answer: it looks for the
140 file <literal><replaceable>M</replaceable>.hs</literal>, or
141 <literal><replaceable>M</replaceable>.lhs</literal>. This means
142 that for most modules, the module name must match the filename.
143 If it doesn't, GHCi won't be able to find it.</para>
145 <para>There is one exception to this general rule: when you load
146 a program with <literal>:load</literal>, or specify it when you
147 invoke <literal>ghci</literal>, you can give a filename rather
148 than a module name. This filename is loaded if it exists, and
149 it may contain any module you like. This is particularly
150 convenient if you have several <literal>Main</literal> modules
151 in the same directory and you can't call them all
152 <filename>Main.hs</filename>.</para>
154 <para>The search path for finding source files is specified with
155 the <option>-i</option> option on the GHCi command line, like
157 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
159 <para>or it can be set using the <literal>:set</literal> command
160 from within GHCi (see <xref
161 linkend="ghci-cmd-line-options">)<footnote><para>Note that in
162 GHCi, and <option>––make</option> mode, the <option>-i</option>
163 option is used to specify the search path for
164 <emphasis>source</emphasis> files, whereas in standard
165 batch-compilation mode the <option>-i</option> option is used to
166 specify the search path for interface files, see <xref
167 linkend="options-finding-imports">.</para> </footnote></para>
169 <para>One consequence of the way that GHCi follows dependencies
170 to find modules to load is that every module must have a source
171 file. The only exception to the rule is modules that come from
172 a package, including the <literal>Prelude</literal> and standard
173 libraries such as <literal>IO</literal> and
174 <literal>Complex</literal>. If you attempt to load a module for
175 which GHCi can't find a source file, even if there are object
176 and interface files for the module, you'll get an error
179 <para>One final note: if you load a module called Main, it must
180 contain a <literal>main</literal> function, just like in
185 <title>Making changes and recompilation</title>
186 <indexterm><primary><literal>:reload</literal></primary></indexterm>
188 <para>If you make some changes to the source code and want GHCi
189 to recompile the program, give the <literal>:reload</literal>
190 command. The program will be recompiled as necessary, with GHCi
191 doing its best to avoid actually recompiling modules if their
192 external dependencies haven't changed. This is the same
193 mechanism we use to avoid re-compiling modules in the batch
194 compilation setting (see <xref linkend="recomp">).</para>
198 <sect1 id="ghci-compiled">
199 <title>Loading compiled code</title>
200 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
202 <para>When you load a Haskell source module into GHCi, it is
203 normally converted to byte-code and run using the interpreter.
204 However, interpreted code can also run alongside compiled code in
205 GHCi; indeed, normally when GHCi starts, it loads up a compiled
206 copy of package <literal>std</literal>, which contains the Prelude
207 and standard libraries.</para>
209 <para>Why should we want to run compiled code? Well, compiled
210 code is roughly 10x faster than interpreted code, but takes about
211 2x longer to produce (perhaps longer if optimisation is on). So
212 it pays to compile the parts of a program that aren't changing
213 very often, and use the interpreter for the code being actively
216 <para>When loading up source files with <literal>:load</literal>,
217 GHCi looks for any corresponding compiled object files, and will
218 use one in preference to interpreting the source if possible. For
219 example, suppose we have a 4-module program consisting of modules
220 A, B, C, and D. Modules B and C both import D only,
221 and A imports both B & C:</para>
229 <para>We can compile D, then load the whole program, like this:</para>
231 Prelude> :! ghc -c D.hs
233 Skipping D ( D.hs, D.o )
234 Compiling C ( C.hs, interpreted )
235 Compiling B ( B.hs, interpreted )
236 Compiling A ( A.hs, interpreted )
237 Ok, modules loaded: A, B, C, D.
241 <para>In the messages from the compiler, we see that it skipped D,
242 and used the object file <filename>D.o</filename>. The message
243 <literal>Skipping</literal> <replaceable>module</replaceable>
244 indicates that compilation for <replaceable>module</replaceable>
245 isn't necessary, because the source and everything it depends on
246 is unchanged since the last compilation.</para>
248 <para>At any time you can use the command
249 <literal>:show modules</literal>
250 to get a list of the modules currently loaded
256 C ( C.hs, interpreted )
257 B ( B.hs, interpreted )
258 A ( A.hs, interpreted )
261 <para>If we now modify the source of D (or pretend to: using Unix
262 command <literal>touch</literal> on the source file is handy for
263 this), the compiler will no longer be able to use the object file,
264 because it might be out of date:</para>
269 Compiling D ( D.hs, interpreted )
270 Skipping C ( C.hs, interpreted )
271 Skipping B ( B.hs, interpreted )
272 Skipping A ( A.hs, interpreted )
273 Ok, modules loaded: A, B, C, D.
277 <para>Note that module D was compiled, but in this instance
278 because its source hadn't really changed, its interface remained
279 the same, and the recompilation checker determined that A, B and C
280 didn't need to be recompiled.</para>
282 <para>So let's try compiling one of the other modules:</para>
285 *Main> :! ghc -c C.hs
287 Compiling D ( D.hs, interpreted )
288 Compiling C ( C.hs, interpreted )
289 Compiling B ( B.hs, interpreted )
290 Compiling A ( A.hs, interpreted )
291 Ok, modules loaded: A, B, C, D.
294 <para>We didn't get the compiled version of C! What happened?
295 Well, in GHCi a compiled module may only depend on other compiled
296 modules, and in this case C depends on D, which doesn't have an
297 object file, so GHCi also rejected C's object file. Ok, so let's
298 also compile D:</para>
301 *Main> :! ghc -c D.hs
303 Ok, modules loaded: A, B, C, D.
306 <para>Nothing happened! Here's another lesson: newly compiled
307 modules aren't picked up by <literal>:reload</literal>, only
308 <literal>:load</literal>:</para>
312 Skipping D ( D.hs, D.o )
313 Skipping C ( C.hs, C.o )
314 Compiling B ( B.hs, interpreted )
315 Compiling A ( A.hs, interpreted )
316 Ok, modules loaded: A, B, C, D.
319 <para>HINT: since GHCi will only use a compiled object file if it
320 can sure that the compiled version is up-to-date, a good technique
321 when working on a large program is to occasionally run
322 <literal>ghc ––make</literal> to compile the whole project (say
323 before you go for lunch :-), then continue working in the
324 interpreter. As you modify code, the new modules will be
325 interpreted, but the rest of the project will remain
331 <title>Interactive evaluation at the prompt</title>
333 <para>When you type an expression at the prompt, GHCi immediately
334 evaluates and prints the result. But that's not the whole story:
335 if you type something of type <literal>IO a</literal> for some
336 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
337 as an IO-computation, and doesn't attempt to print the
343 Prelude> putStrLn "hello"
347 <para>What actually happens is that GHCi typechecks the
348 expression, and if it doesn't have an <literal>IO</literal> type,
349 then it transforms it as follows: an expression
350 <replaceable>e</replaceable> turns into
352 let it = <replaceable>e</replaceable>;
355 which is then run as an IO-action.</para>
357 <para>Hence, the original expression must have a type which is an
358 instance of the <literal>Show</literal> class, or GHCi will
363 No instance for `Show (a -> a)'
364 arising from use of `print'
365 in a `do' expression pattern binding: print it
368 <para>The error message contains some clues as to the
369 transformation happening internally.</para>
371 <sect2 id="ghci-scope">
372 <title>What's really in scope at the prompt?</title>
374 <para>When you type an expression at the prompt, what
375 identifiers and types are in scope? GHCi provides a flexible
376 way to control exactly how the context for an expression is
377 constructed. Let's start with the simple cases; when you start
378 GHCi the prompt looks like this:</para>
380 <screen>Prelude></screen>
382 <para>Which indicates that everything from the module
383 <literal>Prelude</literal> is currently in scope. If we now
384 load a file into GHCi, the prompt will change:</para>
387 Prelude> :load Main.hs
388 Compiling Main ( Main.hs, interpreted )
392 <para>The new prompt is <literal>*Main</literal>, which
393 indicates that we are typing expressions in the context of the
394 top-level of the <literal>Main</literal> module. Everything
395 that is in scope at the top-level in the module
396 <literal>Main</literal> we just loaded is also in scope at the
397 prompt (probably including <literal>Prelude</literal>, as long
398 as <literal>Main</literal> doesn't explicitly hide it).</para>
401 <literal>*<replaceable>module</replaceable></literal> indicates
402 that it is the full top-level scope of
403 <replaceable>module</replaceable> that is contributing to the
404 scope for expressions typed at the prompt. Without the
405 <literal>*</literal>, just the exports of the module are
408 <para>We're not limited to a single module: GHCi can combine
409 scopes from multiple modules, in any mixture of
410 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
411 combines the scopes from all of these modules to form the scope
412 that is in effect at the prompt. For technical reasons, GHCi
413 can only support the <literal>*</literal>-form for modules which
414 are interpreted, so compiled modules and package modules can
415 only contribute their exports to the current scope.</para>
417 <para>The scope is manipulated using the
418 <literal>:module</literal> command. For example, if the current
419 scope is <literal>Prelude</literal>, then we can bring into
420 scope the exports from the module <literal>IO</literal> like
425 Prelude,IO> hPutStrLn stdout "hello\n"
430 <para>(Note: <literal>:module</literal> can be shortened to
431 <literal>:m</literal>). The full syntax of the
432 <literal>:module</literal> command is:</para>
435 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
438 <para>Using the <literal>+</literal> form of the
439 <literal>module</literal> commands adds modules to the current
440 scope, and <literal>-</literal> removes them. Without either
441 <literal>+</literal> or <literal>-</literal>, the current scope
442 is replaced by the set of modules specified. Note that if you
443 use this form and leave out <literal>Prelude</literal>, GHCi
444 will assume that you really wanted the
445 <literal>Prelude</literal> and add it in for you (if you don't
446 want the <literal>Prelude</literal>, then ask to remove it with
447 <literal>:m -Prelude</literal>).</para>
449 <para>The scope is automatically set after a
450 <literal>:load</literal> command, to the most recently loaded
451 "target" module, in a <literal>*</literal>-form if possible.
452 For example, if you say <literal>:load foo.hs bar.hs</literal>
453 and <filename>bar.hs</filename> contains module
454 <literal>Bar</literal>, then the scope will be set to
455 <literal>*Bar</literal> if <literal>Bar</literal> is
456 interpreted, or if <literal>Bar</literal> is compiled it will be
457 set to <literal>Prelude,Bar</literal> (GHCi automatically adds
458 <literal>Prelude</literal> if it isn't present and there aren't
459 any <literal>*</literal>-form modules).</para>
461 <para>With multiple modules in scope, especially multiple
462 <literal>*</literal>-form modules, it is likely that name
463 clashes will occur. Haskell specifies that name clashes are
464 only reported when an ambiguous identifier is used, and GHCi
465 behaves in the same way for expressions typed at the
469 <title>Qualified names</title>
471 <para>To make life slightly easier, the GHCi prompt also
472 behaves as if there is an implicit <literal>import
473 qualified</literal> declaration for every module in every
474 package, and every module currently loaded into GHCi.</para>
479 <title>Using <literal>do-</literal>notation at the prompt</title>
480 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
481 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
483 <para>GHCi actually accepts <firstterm>statements</firstterm>
484 rather than just expressions at the prompt. This means you can
485 bind values and functions to names, and use them in future
486 expressions or statements.</para>
488 <para>The syntax of a statement accepted at the GHCi prompt is
489 exactly the same as the syntax of a statement in a Haskell
490 <literal>do</literal> expression. However, there's no monad
491 overloading here: statements typed at the prompt must be in the
492 <literal>IO</literal> monad.</para>
494 <para>Here's an example:</para>
496 Prelude> x <- return 42
501 <para>The statement <literal>x <- return 42</literal> means
502 “execute <literal>return 42</literal> in the
503 <literal>IO</literal> monad, and bind the result to
504 <literal>x</literal>”. We can then use
505 <literal>x</literal> in future statements, for example to print
506 it as we did above.</para>
508 <para>Of course, you can also bind normal non-IO expressions
509 using the <literal>let</literal>-statement:</para>
516 <para>An important difference between the two types of binding
517 is that the monadic bind (<literal>p <- e</literal>) is
518 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
519 whereas with the <literal>let</literal> form, the expression
520 isn't evaluated immediately:</para>
522 Prelude> let x = error "help!"
527 <para>Any exceptions raised during the evaluation or execution
528 of the statement are caught and printed by the GHCi command line
529 interface (see <xref linkend="sec-Exception"> for more
530 information on GHC's Exception support).</para>
532 <para>Every new binding shadows any existing bindings of the
533 same name, including entities that are in scope in the current
534 module context.</para>
536 <para>WARNING: temporary bindings introduced at the prompt only
537 last until the next <literal>:load</literal> or
538 <literal>:reload</literal> command, at which time they will be
539 simply lost. However, they do survive a change of context with
540 <literal>:module</literal>: the temporary bindings just move to
541 the new location.</para>
543 <para>HINT: To get a list of the bindings currently in scope, use the
544 <literal>:show bindings</literal> command:</para>
547 Prelude> :show bindings
551 <para>HINT: if you turn on the <literal>+t</literal> option,
552 GHCi will show the type of each variable bound by a statement.
554 <indexterm><primary><literal>+t</literal></primary></indexterm>
557 Prelude> let (x:xs) = [1..]
565 <title>The <literal>it</literal> variable</title>
566 <indexterm><primary><literal>it</literal></primary>
569 <para>Whenever an expression (or a non-binding statement, to be
570 precise) is typed at the prompt, GHCi implicitly binds its value
571 to the variable <literal>it</literal>. For example:</para>
579 <para>This is a result of the translation mentioned earlier,
580 namely that an expression <replaceable>e</replaceable> is
583 let it = <replaceable>e</replaceable>;
586 before execution, resulting in a binding for
587 <literal>it</literal>.</para>
589 <para>If the expression was of type <literal>IO a</literal> for
590 some <literal>a</literal>, then <literal>it</literal> will be
591 bound to the result of the <literal>IO</literal> computation,
592 which is of type <literal>a</literal>. eg.:</para>
594 Prelude> Time.getClockTime
596 Wed Mar 14 12:23:13 GMT 2001
599 <para>The corresponding translation for an IO-typed
600 <replaceable>e</replaceable> is
602 it <- <replaceable>e</replaceable>
606 <para>Note that <literal>it</literal> is shadowed by the new
607 value each time you evaluate a new expression, and the old value
608 of <literal>it</literal> is lost.</para>
613 <sect1 id="ghci-invokation">
614 <title>Invoking GHCi</title>
615 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
616 <indexterm><primary><option>––interactive</option></primary></indexterm>
618 <para>GHCi is invoked with the command <literal>ghci</literal> or
619 <literal>ghc ––interactive</literal>. One or more modules or
620 filenames can also be specified on the command line; this
621 instructs GHCi to load the specified modules or filenames (and all
622 the modules they depend on), just as if you had said
623 <literal>:load <replaceable>modules</replaceable></literal> at the
624 GHCi prompt (see <xref linkend="ghci-commands">). For example, to
625 start GHCi and load the program whose topmost module is in the
626 file <literal>Main.hs</literal>, we could say:</para>
632 <para>Most of the command-line options accepted by GHC (see <xref
633 linkend="using-ghc">) also make sense in interactive mode. The ones
634 that don't make sense are mostly obvious; for example, GHCi
635 doesn't generate interface files, so options related to interface
636 file generation won't have any effect.</para>
639 <title>Packages</title>
640 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
642 <para>GHCi can make use of all the packages that come with GHC,
643 For example, to start up GHCi with the <literal>text</literal>
644 package loaded:</para>
650 / /_\// /_/ / / | | GHC Interactive, version 5.00, For Haskell 98.
651 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
652 \____/\/ /_/\____/|_| Type :? for help.
654 Loading package std ... linking ... done.
655 Loading package lang ... linking ... done.
656 Loading package text ... linking ... done.
660 <para>Note that GHCi also loaded the <literal>lang</literal>
661 package even though we didn't ask for it: that's because the
662 <literal>text</literal> package makes use of one or more of the
663 modules in <literal>lang</literal>, and therefore has a
664 dependency on it.</para>
666 <para>The following command works to load new packages into a
670 Prelude> :set -package <replaceable>name</replaceable>
673 <para>But note that doing this will cause all currently loaded
674 modules to be unloaded, and you'll be dumped back into the
679 <title>Extra libraries</title>
680 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
682 <para>Extra libraries may be specified on the command line using
683 the normal <literal>-l<replaceable>lib</replaceable></literal>
684 option. For example, to load the “m” library:</para>
690 <para>On systems with <literal>.so</literal>-style shared
691 libraries, the actual library loaded will the
692 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
693 searches the following places for libraries, in this order:</para>
697 <para>Paths specified using the
698 <literal>-L<replaceable>path</replaceable></literal>
699 command-line option,</para>
702 <para>the standard library search path for your system,
703 which on some systems may be overriden by setting the
704 <literal>LD_LIBRARY_PATH</literal> environment
709 <para>On systems with <literal>.dll</literal>-style shared
710 libraries, the actual library loaded will be
711 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
712 GHCi will signal an error if it can't find the library.</para>
714 <para>GHCi can also load plain object files
715 (<literal>.o</literal> or <literal>.obj</literal> depending on
716 your platform) from the command-line. Just add the name the
717 object file to the command line.</para>
722 <sect1 id="ghci-commands">
723 <title>GHCi commands</title>
725 <para>GHCi commands all begin with
726 ‘<literal>:</literal>’ and consist of a single command
727 name followed by zero or more parameters. The command name may be
728 abbreviated, as long as the abbreviation is not ambiguous. All of
729 the builtin commands, with the exception of
730 <literal>:unset</literal> and <literal>:undef</literal>, may be
731 abbreviated to a single letter.</para>
735 <term><literal>:add</literal>
736 <replaceable>module</replaceable> ...</term>
737 <indexterm><primary><literal>:add</literal></primary></indexterm>
739 <para>Add <replaceable>module</replaceable>(s) to the
740 current <firstterm>target set</firstterm>, and perform a
746 <term><literal>:browse</literal>
747 <optional><literal>*</literal></optional><replaceable>module</replaceable>
749 <indexterm><primary><literal>:browse</literal></primary>
752 <para>Displays the identifiers defined by the module
753 <replaceable>module</replaceable>, which must be either
754 loaded into GHCi or be a member of a package. If the
755 <literal>*</literal> symbol is placed before the module
756 name, then <emphasis>all</emphasis> the identifiers defined
757 in <replaceable>module</replaceable> are shown; otherwise
758 the list is limited to the exports of
759 <replaceable>module</replaceable>. The
760 <literal>*</literal>-form is only available for modules
761 which are interpreted; for compiled modules (including
762 modules from packages) only the non-<literal>*</literal>
763 form of <literal>:browse</literal> is available.</para>
768 <term><literal>:cd</literal> <replaceable>dir</replaceable></term>
769 <indexterm><primary><literal>:cd</literal></primary></indexterm>
771 <para>Changes the current working directory to
772 <replaceable>dir</replaceable>. A
773 ‘<literal>˜</literal>’ symbol at the
774 beginning of <replaceable>dir</replaceable> will be replaced
775 by the contents of the environment variable
776 <literal>HOME</literal>.</para>
781 <term><literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable></term>
782 <indexterm><primary><literal>:def</literal></primary></indexterm>
784 <para>The command <literal>:def</literal>
785 <replaceable>name</replaceable>
786 <replaceable>expr</replaceable> defines a new GHCi command
787 <literal>:<replaceable>name</replaceable></literal>,
788 implemented by the Haskell expression
789 <replaceable>expr</replaceable>, which must have type
790 <literal>String -> IO String</literal>. When
791 <literal>:<replaceable>name</replaceable>
792 <replaceable>args</replaceable></literal> is typed at the
793 prompt, GHCi will run the expression
794 <literal>(<replaceable>name</replaceable>
795 <replaceable>args</replaceable>)</literal>, take the
796 resulting <literal>String</literal>, and feed it back into
797 GHCi as a new sequence of commands. Separate commands in
798 the result must be separated by
799 ‘<literal>\n</literal>’.</para>
801 <para>That's all a little confusing, so here's a few
802 examples. To start with, here's a new GHCi command which
803 doesn't take any arguments or produce any results, it just
804 outputs the current date & time:</para>
807 Prelude> let date _ = Time.getClockTime >>= print >> return ""
808 Prelude> :def date date
810 Fri Mar 23 15:16:40 GMT 2001
813 <para>Here's an example of a command that takes an argument.
814 It's a re-implementation of <literal>:cd</literal>:</para>
817 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
818 Prelude> :def mycd mycd
822 <para>Or I could define a simple way to invoke
823 “<literal>ghc ––make Main</literal>” in the
824 current directory:</para>
827 Prelude> :def make (\_ -> return ":! ghc ––make Main")
834 <term><literal>:help</literal></term>
835 <indexterm><primary><literal>:help</literal></primary></indexterm>
836 <term><literal>:?</literal></term>
837 <indexterm><primary><literal>:?</literal></primary></indexterm>
839 <para>Displays a list of the available commands.</para>
844 <term><literal>:info</literal> <replaceable>name</replaceable>
846 <indexterm><primary><literal>:info</literal></primary>
849 <para>Displays information about the given name(s). For
850 example, if <replaceable>name</replaceable> is a class, then
851 the class methods and their types will be printed; if
852 <replaceable>name</replaceable> is a type constructor, then
853 its definition will be printed; if
854 <replaceable>name</replaceable> is a function, then its type
855 will be printed. If <replaceable>name</replaceable> has
856 been loaded from a source file, then GHCi will also display
857 the location of its definition in the source.</para>
862 <term><literal>:load</literal>
863 <replaceable>module</replaceable> ...</term>
864 <indexterm><primary><literal>:load</literal></primary></indexterm>
866 <para>Recursively loads the specified
867 <replaceable>module</replaceable>s, and all the modules they
868 depend on. Here, each <replaceable>module</replaceable>
869 must be a module name or filename, but may not be the name
870 of a module in a package.</para>
872 <para>All previously loaded modules, except package modules,
873 are forgotten. The new set of modules is known as the
874 <firstterm>target set</firstterm>. Note that
875 <literal>:load</literal> can be used without any arguments
876 to unload all the currently loaded modules and
879 <para>After a <literal>:load</literal> command, the current
880 context is set to:</para>
884 <para><replaceable>module</replaceable>, if it was loaded
885 successfully, or</para>
888 <para>the most recently successfully loaded module, if
889 any other modules were loaded as a result of the current
890 <literal>:load</literal>, or</para>
893 <para><literal>Prelude</literal> otherwise.</para>
900 <term><literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal></term>
901 <indexterm><primary><literal>:module</literal></primary></indexterm>
903 <para>Sets or modifies the current context for statements
904 typed at the prompt. See <xref linkend="ghci-scope"> for
910 <term><literal>:quit</literal></term>
911 <indexterm><primary><literal>:quit</literal></primary></indexterm>
913 <para>Quits GHCi. You can also quit by typing a control-D
914 at the prompt.</para>
919 <term><literal>:reload</literal></term>
920 <indexterm><primary><literal>:reload</literal></primary></indexterm>
922 <para>Attempts to reload the current target set (see
923 <literal>:load</literal>) if any of the modules in the set,
924 or any dependent module, has changed. Note that this may
925 entail loading new modules, or dropping modules which are no
926 longer indirectly required by the target.</para>
931 <term><literal>:set</literal> <optional><replaceable>option</replaceable>...</optional></term>
932 <indexterm><primary><literal>:set</literal></primary></indexterm>
934 <para>Sets various options. See <xref linkend="ghci-set">
935 for a list of available options. The
936 <literal>:set</literal> command by itself shows which
937 options are currently set.</para>
942 <term><literal>:set</literal> <literal>args</literal>
943 <replaceable>arg</replaceable> ...</term>
944 <indexterm><primary><literal>:set</literal></primary></indexterm>
946 <para>Sets the list of arguments which are returned when the
947 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
953 <term><literal>:set</literal> <literal>prog</literal>
954 <replaceable>prog</replaceable></term>
955 <indexterm><primary><literal>:set</literal></primary></indexterm>
957 <para>Sets the string to be returned when the program calls
958 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
964 <term><literal>:show bindings</literal></term>
965 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
967 <para>Show the bindings made at the prompt and their
973 <term><literal>:show modules</literal></term>
974 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
976 <para>Show the list of modules currently load.</para>
981 <term><literal>:type</literal> <replaceable>expression</replaceable></term>
982 <indexterm><primary><literal>:type</literal></primary></indexterm>
984 <para>Infers and prints the type of
985 <replaceable>expression</replaceable>, including explicit
986 forall quantifiers for polymorphic types. The monomorphism
987 restriction is <emphasis>not</emphasis> applied to the
988 expression during type inference.</para>
993 <term><literal>:undef</literal> <replaceable>name</replaceable></term>
994 <indexterm><primary><literal>:undef</literal></primary></indexterm>
996 <para>Undefines the user-defined command
997 <replaceable>name</replaceable> (see <literal>:def</literal>
1003 <term><literal>:unset</literal> <replaceable>option</replaceable>...</term>
1004 <indexterm><primary><literal>:unset</literal></primary></indexterm>
1006 <para>Unsets certain options. See <xref linkend="ghci-set">
1007 for a list of available options.</para>
1012 <term><literal>:!</literal> <replaceable>command</replaceable>...</term>
1013 <indexterm><primary><literal>:!</literal></primary></indexterm>
1014 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
1016 <para>Executes the shell command
1017 <replaceable>command</replaceable>.</para>
1024 <sect1 id="ghci-set">
1025 <title>The <literal>:set</literal> command</title>
1026 <indexterm><primary><literal>:set</literal></primary></indexterm>
1028 <para>The <literal>:set</literal> command sets two types of
1029 options: GHCi options, which begin with
1030 ‘<literal>+</literal>” and “command-line”
1031 options, which begin with ‘-’. </para>
1033 <para>NOTE: at the moment, the <literal>:set</literal> command
1034 doesn't support any kind of quoting in its arguments: quotes will
1035 not be removed and cannot be used to group words together. For
1036 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
1040 <title>GHCi options</title>
1041 <indexterm><primary>options</primary><secondary>GHCi</secondary>
1044 <para>GHCi options may be set using <literal>:set</literal> and
1045 unset using <literal>:unset</literal>.</para>
1047 <para>The available GHCi options are:</para>
1051 <term><literal>+r</literal></term>
1052 <indexterm><primary><literal>+r</literal></primary></indexterm>
1053 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
1054 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
1056 <para>Normally, any evaluation of top-level expressions
1057 (otherwise known as CAFs or Constant Applicative Forms) in
1058 loaded modules is retained between evaluations. Turning
1059 on <literal>+r</literal> causes all evaluation of
1060 top-level expressions to be discarded after each
1061 evaluation (they are still retained
1062 <emphasis>during</emphasis> a single evaluation).</para>
1064 <para>This option may help if the evaluated top-level
1065 expressions are consuming large amounts of space, or if
1066 you need repeatable performance measurements.</para>
1071 <term><literal>+s</literal></term>
1072 <indexterm><primary><literal>+s</literal></primary></indexterm>
1074 <para>Display some stats after evaluating each expression,
1075 including the elapsed time and number of bytes allocated.
1076 NOTE: the allocation figure is only accurate to the size
1077 of the storage manager's allocation area, because it is
1078 calculated at every GC. Hence, you might see values of
1079 zero if no GC has occurred.</para>
1084 <term><literal>+t</literal></term>
1085 <indexterm><primary><literal>+t</literal></primary></indexterm>
1087 <para>Display the type of each variable bound after a
1088 statement is entered at the prompt. If the statement is a
1089 single expression, then the only variable binding will be
1091 ‘<literal>it</literal>’.</para>
1097 <sect2 id="ghci-cmd-line-options">
1098 <title>Setting GHC command-line options in GHCi</title>
1100 <para>Normal GHC command-line options may also be set using
1101 <literal>:set</literal>. For example, to turn on
1102 <option>-fglasgow-exts</option>, you would say:</para>
1105 Prelude> :set -fglasgow-exts
1108 <para>Any GHC command-line option that is designated as
1109 <firstterm>dynamic</firstterm> (see the table in <xref
1110 linkend="flag-reference">), may be set using
1111 <literal>:set</literal>. To unset an option, you can set the
1112 reverse option:</para>
1113 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
1116 Prelude> :set -fno-glasgow-exts
1119 <para><xref linkend="flag-reference"> lists the reverse for each
1120 option where applicable.</para>
1122 <para>Certain static options (<option>-package</option>,
1123 <option>-I</option>, <option>-i</option>, and
1124 <option>-l</option> in particular) will also work, but some may
1125 not take effect until the next reload.</para>
1126 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
1130 <sect1 id="ghci-dot-files">
1131 <title>The <filename>.ghci</filename> file</title>
1132 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
1134 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
1137 <para>When it starts, GHCi always reads and executes commands from
1138 <filename>$HOME/.ghci</filename>, followed by
1139 <filename>./.ghci</filename>.</para>
1141 <para>The <filename>.ghci</filename> in your home directory is
1142 most useful for turning on favourite options (eg. <literal>:set
1143 +s</literal>), and defining useful macros. Placing a
1144 <filename>.ghci</filename> file in a directory with a Haskell
1145 project is a useful way to set certain project-wide options so you
1146 don't have to type them everytime you start GHCi: eg. if your
1147 project uses GHC extensions and CPP, and has source files in three
1148 subdirectories A B and C, you might put the following lines in
1149 <filename>.ghci</filename>:</para>
1152 :set -fglasgow-exts -cpp
1156 <para>(Note that strictly speaking the <option>-i</option> flag is
1157 a static one, but in fact it works to set it using
1158 <literal>:set</literal> like this. The changes won't take effect
1159 until the next <literal>:load</literal>, though.)</para>
1161 <para>Two command-line options control whether the
1162 <filename>.ghci</filename> files are read:</para>
1166 <term><option>-ignore-dot-ghci</option></term>
1167 <indexterm><primary><option>-ignore-dot-ghci</option></primary>
1170 <para>Don't read either <filename>./.ghci</filename> or
1171 <filename>$HOME/.ghci</filename> when starting up.</para>
1175 <term><option>-read-dot-ghci</option></term>
1176 <indexterm><primary><option>-read-dot-ghci</option></primary>
1179 <para>Read <filename>.ghci</filename> and
1180 <filename>$HOME/.ghci</filename>. This is normally the
1181 default, but the <option>-read-dot-ghci</option> option may
1182 be used to override a previous
1183 <option>-ignore-dot-ghci</option> option.</para>
1191 <title>FAQ and Things To Watch Out For</title>
1195 <term>GHCi complains about <function>main</function> not being
1196 in scope when I load a module.</term>
1197 <indexterm><primary><function>main</function></primary><secondary>with GHCi</secondary>
1200 <para>You probably omitted the <literal>module</literal>
1201 declaration at the top of the module, which causes the
1202 module name to default to <literal>Main</literal>. In
1203 Haskell, the <literal>Main</literal> module must define a
1204 function called <function>main</function>. Admittedly this
1205 doesn't make a great deal of sense for an interpreter, but
1206 the rule was kept for compatibility with GHC.</para>
1211 <term>The interpreter can't load modules with foreign export
1212 declarations!</term>
1214 <para>Unfortunately not. We haven't implemented it yet.
1215 Please compile any offending modules by hand before loading
1216 them into GHCi.</para>
1221 <term><literal>-O</literal> doesn't work with GHCi!</term>
1222 <indexterm><primary><option>-O</option></primary>
1225 <para>For technical reasons, the bytecode compiler doesn't
1226 interact well with one of the optimisation passes, so we
1227 have disabled optimisation when using the interpreter. This
1228 isn't a great loss: you'll get a much bigger win by
1229 compiling the bits of your code that need to go fast, rather
1230 than interpreting them with optimisation turned on.</para>
1235 <term>Unboxed tuples don't work with GHCi</term>
1237 <para>That's right. You can always compile a module that
1238 uses unboxed tuples and load it into GHCi, however.
1239 (Incidentally the previous point, namely that
1240 <literal>-O</literal> is incompatible with GHCi, is because
1241 the bytecode compiler can't deal with unboxed
1247 <term>Concurrent threads don't carry on running when GHCi is
1248 waiting for input.</term>
1250 <para>No, they don't. This is because the Haskell binding
1251 to the GNU readline library doesn't support reading from the
1252 terminal in a non-blocking way, which is required to work
1253 properly with GHC's concurrency model.</para>
1258 <term>After using <literal>getContents</literal>, I can't use
1259 <literal>stdin</literal> again until I do
1260 <literal>:load</literal> or <literal>:reload</literal>.</term>
1263 <para>This is the defined behaviour of
1264 <literal>getContents</literal>: it puts the stdin Handle in
1265 a state known as <firstterm>semi-closed</firstterm>, wherein
1266 any further I/O operations on it are forbidden. Because I/O
1267 state is retained between computations, the semi-closed
1268 state persists until the next <literal>:load</literal> or
1269 <literal>:reload</literal> command.</para>
1271 <para>You can make <literal>stdin</literal> reset itself
1272 after every evaluation by giving GHCi the command
1273 <literal>:set +r</literal>. This works because
1274 <literal>stdin</literal> is just a top-level expression that
1275 can be reverted to its unevaluated state in the same way as
1276 any other top-level expression (CAF).</para>
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