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
115 loaded. So we can now type expressions involving the functions
116 from <filename>Main.hs</filename>:</para>
123 <para>Loading a multi-module program is just as straightforward;
124 just give the name of the “topmost” module to the
125 <literal>:load</literal> command (hint: <literal>:load</literal>
126 can be abbreviated to <literal>:l</literal>). The topmost module
127 will normally be <literal>Main</literal>, but it doesn't have to
128 be. GHCi will discover which modules are required, directly or
129 indirectly, by the topmost module, and load them all in dependency
133 <title>Modules vs. filenames</title>
134 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
135 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
137 <para>Question: How does GHC find the filename which contains
138 module <replaceable>M</replaceable>? Answer: it looks for the
139 file <literal><replaceable>M</replaceable>.hs</literal>, or
140 <literal><replaceable>M</replaceable>.lhs</literal>. This means
141 that for most modules, the module name must match the filename.
142 If it doesn't, GHCi won't be able to find it.</para>
144 <para>There is one exception to this general rule: when you load
145 a program with <literal>:load</literal>, or specify it when you
146 invoke <literal>ghci</literal>, you can give a filename rather
147 than a module name. This filename is loaded if it exists, and
148 it may contain any module you like. This is particularly
149 convenient if you have several <literal>Main</literal> modules
150 in the same directory and you can't call them all
151 <filename>Main.hs</filename>.</para>
153 <para>One consequence of the way that GHCi follows dependencies
154 to find modules to load is that every module must have a source
155 file. The only exception to the rule is modules that come from
156 a package, including the <literal>Prelude</literal> and standard
157 libraries such as <literal>IO</literal> and
158 <literal>Complex</literal>. If you attempt to load a module for
159 which GHCi can't find a source file, even if there are object
160 and interface files for the module, you'll get an error
163 <para>One final note: if you load a module called Main, it must
164 contain a <literal>main</literal> function, just like in
169 <title>Making changes and recompilation</title>
170 <indexterm><primary><literal>:reload</literal></primary></indexterm>
172 <para>If you make some changes to the source code and want GHCi
173 to recompile the program, give the <literal>:reload</literal>
174 command. The program will be recompiled as necessary, with GHCi
175 doing its best to avoid actually recompiling modules if their
176 external dependencies haven't changed. This is the same
177 mechanism we use to avoid re-compiling modules in the batch
178 compilation setting (see <xref linkend="recomp">).</para>
182 <sect1 id="ghci-compiled">
183 <title>Loading compiled code</title>
184 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
186 <para>When you load a Haskell source module into GHCi, it is
187 normally converted to byte-code and run using the interpreter.
188 However, interpreted code can also run alongside compiled code in
189 GHCi; indeed, normally when GHCi starts, it loads up a compiled
190 copy of package <literal>std</literal>, which contains the Prelude
191 and standard libraries.</para>
193 <para>Why should we want to run compiled code? Well, compiled
194 code is roughly 10x faster than interpreted code, but takes about
195 2x longer to produce (perhaps longer if optimisation is on). So
196 it pays to compile the parts of a program that aren't changing
197 very often, and use the interpreter for the code being actively
200 <para>When loading up source files with <literal>:load</literal>,
201 GHCi looks for any corresponding compiled object files, and will
202 use one in preference to interpreting the source if possible. For
203 example, suppose we have a 4-module program consisting of modules
204 A, B, C, and D. Modules B and C both import D only,
205 and A imports both B & C:</para>
213 <para>We can compile D, then load the whole program, like this:</para>
215 Prelude> :! ghc -c D.hs
217 Skipping D ( D.hs, D.o )
218 Compiling C ( C.hs, interpreted )
219 Compiling B ( B.hs, interpreted )
220 Compiling A ( A.hs, interpreted )
221 Ok, modules loaded: A, B, C, D.
225 <para>In the messages from the compiler, we see that it skipped D,
226 and used the object file <filename>D.o</filename>. The message
227 <literal>Skipping</literal> <replaceable>module</replaceable>
228 indicates that compilation for <replaceable>module</replaceable>
229 isn't necessary, because the source and everything it depends on
230 is unchanged since the last compilation.</para>
232 <para>If we now modify the source of D (or pretend to: using Unix
233 command <literal>touch</literal> on the source file is handy for
234 this), the compiler will no longer be able to use the object file,
235 because it might be out of date:</para>
240 Compiling D ( D.hs, interpreted )
241 Skipping C ( C.hs, interpreted )
242 Skipping B ( B.hs, interpreted )
243 Skipping A ( A.hs, interpreted )
244 Ok, modules loaded: A, B, C, D.
248 <para>Note that module D was compiled, but in this instance
249 because its source hadn't really changed, its interface remained
250 the same, and the recompilation checker determined that A, B and C
251 didn't need to be recompiled.</para>
253 <para>So let's try compiling one of the other modules:</para>
258 Compiling D ( D.hs, interpreted )
259 Compiling C ( C.hs, interpreted )
260 Compiling B ( B.hs, interpreted )
261 Compiling A ( A.hs, interpreted )
262 Ok, modules loaded: A, B, C, D.
265 <para>We didn't get the compiled version of C! What happened?
266 Well, in GHCi a compiled module may only depend on other compiled
267 modules, and in this case C depends on D, which doesn't have an
268 object file, so GHCi also rejected C's object file. Ok, so let's
269 also compile D:</para>
274 Ok, modules loaded: A, B, C, D.
277 <para>Nothing happened! Here's another lesson: newly compiled
278 modules aren't picked up by <literal>:reload</literal>, only
279 <literal>:load</literal>:</para>
283 Skipping D ( D.hs, D.o )
284 Skipping C ( C.hs, C.o )
285 Compiling B ( B.hs, interpreted )
286 Compiling A ( A.hs, interpreted )
287 Ok, modules loaded: A, B, C, D.
290 <para>HINT: since GHCi will only use a compiled object file if it
291 can sure that the compiled version is up-to-date, a good technique
292 when working on a large program is to occasionally run
293 <literal>ghc --make</literal> to compile the whole project (say
294 before you go for lunch :-), then continue working in the
295 interpreter. As you modify code, the new modules will be
296 interpreted, but the rest of the project will remain
302 <title>Interactive evaluation at the prompt</title>
304 <para>When you type an expression at the prompt, GHCi immediately
305 evaluates and prints the result. But that's not the whole story:
306 if you type something of type <literal>IO a</literal> for some
307 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
308 as an IO-computation, and doesn't attempt to print the
314 Prelude> putStrLn "hello"
318 <para>What actually happens is that GHCi typechecks the
319 expression, and if it doesn't have an <literal>IO</literal> type,
320 then it transforms it as follows: an expression
321 <replaceable>e</replaceable> turns into
323 let it = <replaceable>e</replaceable>;
326 which is then run as an IO-action.</para>
328 <para>Hence, the original expression must have a type which is an
329 instance of the <literal>Show</literal> class, or GHCi will
334 No instance for `Show (a -> a)'
335 arising from use of `print'
336 in a `do' expression pattern binding: print it
339 <para>The error message contains some clues as to the
340 transformation happening internally.</para>
342 <sect2 id="ghci-scope">
343 <title>What's really in scope at the prompt?</title>
345 <para>When you type an expression at the prompt, what
346 identifiers and types are in scope? GHCi has a concept of a
347 <firstterm>context</firstterm> module, which can be set using
348 the <literal>:module</literal> command.</para>
350 <para>The context module is shown in the prompt: for example,
351 the prompt <literal>Prelude></literal> indicates that the
352 current context for evaluating expressions is the Haskell
353 <literal>Prelude</literal> module. The Prelude is the default
354 context when you start up GHCi.</para>
355 <indexterm><primary><literal>Prelude</literal></primary></indexterm>
357 <para>Exactly which entities are in scope in a given context
358 depends on whether the context module is compiled or
363 <para>If the context module is interpreted, then everything
364 that was in scope during compilation of that module is also
365 in scope at the prompt, i.e. all the imports and any
366 top-level functions, types and classes defined in that
371 <para>If the context module comes from a package, or is
372 otherwise compiled, then only the exports of that module are
373 in scope at the prompt. So for example, when the current
374 context module is <literal>Prelude</literal>, everything the
375 <literal>Prelude</literal> exports is in scope, but if we
376 switch context to eg. <literal>Time</literal>, then
377 everything from the <literal>Prelude</literal> is now
382 <para>The reason for this unfortunate distinction is boring: for
383 a compiled module when the source isn't available, the compiler
384 has no way of knowing what was in scope when the module was
385 compiled (and we don't store this information in the interface
386 file). However, in practice it shouldn't be a problem: if you
387 want both <literal>Time</literal> and <literal>Prelude</literal>
388 in scope at the same time, just create a file containing the
389 line <literal>import Time</literal> and load it into
392 <para>To make life slightly easier, the GHCi prompt also behaves
393 as if there is an implicit <literal>import qualified</literal>
394 declaration for every module in every package, and every module
395 currently loaded into GHCi. So in the above example where the
396 <literal>Prelude</literal> was invisible, we can always get at
397 <literal>Prelude</literal> identifiers by qualifying them, eg.
398 <literal>Prelude.map</literal>.</para>
402 <title>Using <literal>do-</literal>notation at the prompt</title>
403 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
404 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
406 <para>GHCi actually accepts <firstterm>statements</firstterm>
407 rather than just expressions at the prompt. This means you can
408 bind values and functions to names, and use them in future
409 expressions or statements.</para>
411 <para>The syntax of a statement accepted at the GHCi prompt is
412 exactly the same as the syntax of a statement in a Haskell
413 <literal>do</literal> expression. However, there's no monad
414 overloading here: statements typed at the prompt must be in the
415 <literal>IO</literal> monad.</para>
417 <para>Here's an example:</para>
419 Prelude> x <- return 42
424 <para>The statement <literal>x <- return 42</literal> means
425 “execute <literal>return 42</literal> in the
426 <literal>IO</literal> monad, and bind the result to
427 <literal>x</literal>”. We can then use
428 <literal>x</literal> in future statements, for example to print
429 it as we did above.</para>
431 <para>Of course, you can also bind normal non-IO expressions
432 using the <literal>let</literal>-statement:</para>
439 <para>An important difference between the two types of binding
440 is that the monadic bind (<literal>p <- e</literal>) is
441 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
442 whereas with the <literal>let</literal> form, the expression
443 isn't evaluated immediately:</para>
445 Prelude> let x = error "help!"
450 <para>Any exceptions raised during the evaluation or execution
451 of the statement are caught and printed by the GHCi command line
452 interface (see <xref linkend="sec-Exception"> for more
453 information on GHC's Exception support).</para>
455 <para>Every new binding shadows any existing bindings of the
456 same name, including entities that are in scope in the current
457 module context.</para>
459 <para>WARNING: temporary bindings introduced at the prompt only
460 last until the next <literal>:load</literal> or
461 <literal>:reload</literal> command, at which time they will be
462 simply lost. However, they do survive a change of context with
463 <literal>:module</literal>: the temporary bindings just move to
464 the new location.</para>
466 <para>HINT: if you turn on the <literal>+t</literal> option,
467 GHCi will show the type of each variable bound by a statement.
469 <indexterm><primary><literal>+t</literal></primary></indexterm>
472 Prelude> let (x:xs) = [1..]
480 <title>The <literal>it</literal> variable</title>
481 <indexterm><primary><literal>it</literal></primary>
484 <para>Whenever an expression (or a non-binding statement, to be
485 precise) is typed at the prompt, GHCi implicitly binds its value
486 to the variable <literal>it</literal>. For example:</para>
494 <para>This is a result of the translation mentioned earlier,
495 namely that an expression <replaceable>e</replaceable> is
498 let it = <replaceable>e</replaceable>;
501 before execution, resulting in a binding for
502 <literal>it</literal>.</para>
504 <para>If the expression was of type <literal>IO a</literal> for
505 some <literal>a</literal>, then <literal>it</literal> will be
506 bound to the result of the <literal>IO</literal> computation,
507 which is of type <literal>a</literal>. eg.:</para>
509 Prelude> Time.getClockTime
511 Wed Mar 14 12:23:13 GMT 2001
514 <para>The corresponding translation for an IO-typed
515 <replaceable>e</replaceable> is
517 it <- <replaceable>e</replaceable>
521 <para>Note that <literal>it</literal> is shadowed by the new
522 value each time you evaluate a new expression, and the old value
523 of <literal>it</literal> is lost.</para>
528 <sect1 id="ghci-invokation">
529 <title>Invoking GHCi</title>
530 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
531 <indexterm><primary><option>--interactive</option></primary></indexterm>
533 <para>GHCi is invoked with the command <literal>ghci</literal> or
534 <literal>ghc --interactive</literal>. One or more modules or
535 filenames can also be specified on the command line; this
536 instructs GHCi to load the specified modules or filenames (and all
537 the modules they depend on), just as if you had said
538 <literal>:load <replaceable>modules</replaceable></literal> at the
539 GHCi prompt (see <xref linkend="ghci-commands">). For example, to
540 start GHCi and load the program whose topmost module is in the
541 file <literal>Main.hs</literal>, we could say:</para>
547 <para>Most of the command-line options accepted by GHC (see <xref
548 linkend="using-ghc">) also make sense in interactive mode. The ones
549 that don't make sense are mostly obvious; for example, GHCi
550 doesn't generate interface files, so options related to interface
551 file generation won't have any effect.</para>
554 <title>Packages</title>
555 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
557 <para>GHCi can make use of all the packages that come with GHC,
558 For example, to start up GHCi with the <literal>text</literal>
559 package loaded:</para>
565 / /_\// /_/ / / | | GHC Interactive, version 5.00, For Haskell 98.
566 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
567 \____/\/ /_/\____/|_| Type :? for help.
569 Loading package std ... linking ... done.
570 Loading package lang ... linking ... done.
571 Loading package text ... linking ... done.
575 <para>Note that GHCi also loaded the <literal>lang</literal>
576 package even though we didn't ask for it: that's because the
577 <literal>text</literal> package makes use of one or more of the
578 modules in <literal>lang</literal>, and therefore has a
579 dependency on it.</para>
581 <para>The following command works to load new packages into a
585 Prelude> :set -package <replaceable>name</replaceable>
588 <para>But note that doing this will cause all currently loaded
589 modules to be unloaded, and you'll be dumped back into the
594 <title>Extra libraries</title>
595 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
597 <para>Extra libraries may be specified on the command line using
598 the normal <literal>-l<replaceable>lib</replaceable></literal>
599 option. For example, to load the “m” library:</para>
605 <para>On systems with <literal>.so</literal>-style shared
606 libraries, the actual library loaded will the
607 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
608 searches the following places for libraries, in this order:</para>
612 <para>Paths specified using the
613 <literal>-L<replaceable>path</replaceable></literal>
614 command-line option,</para>
617 <para>the standard library search path for your system,
618 which on some systems may be overriden by setting the
619 <literal>LD_LIBRARY_PATH</literal> environment
624 <para>On systems with <literal>.dll</literal>-style shared
625 libraries, the actual library loaded will be
626 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
627 GHCi will signal an error if it can't find the library.</para>
629 <para>GHCi can also load plain object files
630 (<literal>.o</literal> or <literal>.obj</literal> depending on
631 your platform) from the command-line. Just add the name the
632 object file to the command line.</para>
637 <sect1 id="ghci-commands">
638 <title>GHCi commands</title>
640 <para>GHCi commands all begin with
641 ‘<literal>:</literal>’ and consist of a single command
642 name followed by zero or more parameters. The command name may be
643 abbreviated, as long as the abbreviation is not ambiguous. All of
644 the builtin commands, with the exception of
645 <literal>:unset</literal> and <literal>:undef</literal>, may be
646 abbreviated to a single letter.</para>
650 <term><literal>:add</literal>
651 <replaceable>module</replaceable> ...</term>
652 <indexterm><primary><literal>:add</literal></primary></indexterm>
654 <para>Add <replaceable>module</replaceable>(s) to the
655 current <firstterm>target set</firstterm>, and perform a
661 <term><literal>:cd</literal> <replaceable>dir</replaceable></term>
662 <indexterm><primary><literal>:cd</literal></primary></indexterm>
664 <para>Changes the current working directory to
665 <replaceable>dir</replaceable>. A
666 ‘<literal>˜</literal>’ symbol at the
667 beginning of <replaceable>dir</replaceable> will be replaced
668 by the contents of the environment variable
669 <literal>HOME</literal>.</para>
674 <term><literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable></term>
675 <indexterm><primary><literal>:def</literal></primary></indexterm>
677 <para>The command <literal>:def</literal>
678 <replaceable>name</replaceable>
679 <replaceable>expr</replaceable> defines a new GHCi command
680 <literal>:<replaceable>name</replaceable></literal>,
681 implemented by the Haskell expression
682 <replaceable>expr</replaceable>, which must have type
683 <literal>String -> IO String</literal>. When
684 <literal>:<replaceable>name</replaceable>
685 <replaceable>args</replaceable></literal> is typed at the
686 prompt, GHCi will run the expression
687 <literal>(<replaceable>name</replaceable>
688 <replaceable>args</replaceable>)</literal>, take the
689 resulting <literal>String</literal>, and feed it back into
690 GHCi as a new sequence of commands. Separate commands in
691 the result must be separated by
692 ‘<literal>\n</literal>’.</para>
694 <para>That's all a little confusing, so here's a few
695 examples. To start with, here's a new GHCi command which
696 doesn't take any arguments or produce any results, it just
697 outputs the current date & time:</para>
700 Prelude> let date _ = Time.getClockTime >>= print >> return ""
701 Prelude> :def date date
703 Fri Mar 23 15:16:40 GMT 2001
706 <para>Here's an example of a command that takes an argument.
707 It's a re-implementation of <literal>:cd</literal>:</para>
710 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
711 Prelude> :def mycd mycd
715 <para>Or I could define a simple way to invoke
716 “<literal>ghc --make Main</literal>” in the
717 current directory:</para>
720 Prelude> :def make (\_ -> return ":! ghc --make Main")
727 <term><literal>:help</literal></term>
728 <indexterm><primary><literal>:help</literal></primary></indexterm>
729 <term><literal>:?</literal></term>
730 <indexterm><primary><literal>:?</literal></primary></indexterm>
732 <para>Displays a list of the available commands.</para>
737 <term><literal>:info</literal> <replaceable>name</replaceable>
739 <indexterm><primary><literal>:info</literal></primary>
742 <para>Displays information about the given name(s). For
743 example, if <replaceable>name</replaceable> is a class, then
744 the class methods and their types will be printed; if
745 <replaceable>name</replaceable> is a type constructor, then
746 its definition will be printed; if
747 <replaceable>name</replaceable> is a function, then its type
748 will be printed. If <replaceable>name</replaceable> has
749 been loaded from a source file, then GHCi will also display
750 the location of its definition in the source.</para>
755 <term><literal>:load</literal>
756 <replaceable>module</replaceable> ...</term>
757 <indexterm><primary><literal>:load</literal></primary></indexterm>
759 <para>Recursively loads the specified
760 <replaceable>module</replaceable>s, and all the modules they
761 depend on. Here, each <replaceable>module</replaceable>
762 must be a module name or filename, but may not be the name
763 of a module in a package.</para>
765 <para>All previously loaded modules, except package modules,
766 are forgotten. The new set of modules is known as the
767 <firstterm>target set</firstterm>.</para>
769 <para>After a <literal>:load</literal> command, the current
770 context is set to:</para>
774 <para><replaceable>module</replaceable>, if it was loaded
775 successfully, or</para>
778 <para>the most recently successfully loaded module, if
779 any other modules were loaded as a result of the current
780 <literal>:load</literal>, or</para>
783 <para><literal>Prelude</literal> otherwise.</para>
790 <term><literal>:module</literal> <replaceable>module</replaceable></term>
791 <indexterm><primary><literal>:module</literal></primary></indexterm>
793 <para>Sets the current context for statements typed at the
794 prompt to <replaceable>module</replaceable>, which must be a
795 module name which is already loaded or in a package. See
796 <xref linkend="ghci-scope"> for more information on what
797 effect the context has on what entities are in scope at the
803 <term><literal>:quit</literal></term>
804 <indexterm><primary><literal>:quit</literal></primary></indexterm>
806 <para>Quits GHCi. You can also quit by typing a control-D
807 at the prompt.</para>
812 <term><literal>:reload</literal></term>
813 <indexterm><primary><literal>:reload</literal></primary></indexterm>
815 <para>Attempts to reload the current target set (see
816 <literal>:load</literal>) if any of the modules in the set,
817 or any dependent module, has changed. Note that this may
818 entail loading new modules, or dropping modules which are no
819 longer indirectly required by the target.</para>
824 <term><literal>:set</literal> <optional><replaceable>option</replaceable>...</optional></term>
825 <indexterm><primary><literal>:set</literal></primary></indexterm>
827 <para>Sets various options. See <xref linkend="ghci-set">
828 for a list of available options. The
829 <literal>:set</literal> command by itself shows which
830 options are currently set.</para>
835 <term><literal>:type</literal> <replaceable>expression</replaceable></term>
836 <indexterm><primary><literal>:type</literal></primary></indexterm>
838 <para>Infers and prints the type of
839 <replaceable>expression</replaceable>, including explicit
840 forall quantifiers for polymorphic types. The monomorphism
841 restriction is <emphasis>not</emphasis> applied to the
842 expression during type inference.</para>
847 <term><literal>:undef</literal> <replaceable>name</replaceable></term>
848 <indexterm><primary><literal>:undef</literal></primary></indexterm>
850 <para>Undefines the user-defined command
851 <replaceable>name</replaceable> (see <literal>:def</literal>
857 <term><literal>:unset</literal> <replaceable>option</replaceable>...</term>
858 <indexterm><primary><literal>:unset</literal></primary></indexterm>
860 <para>Unsets certain options. See <xref linkend="ghci-set">
861 for a list of available options.</para>
866 <term><literal>:!</literal> <replaceable>command</replaceable>...</term>
867 <indexterm><primary><literal>:!</literal></primary></indexterm>
868 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
870 <para>Executes the shell command
871 <replaceable>command</replaceable>.</para>
878 <sect1 id="ghci-set">
879 <title>The <literal>:set</literal> command</title>
880 <indexterm><primary><literal>:set</literal></primary></indexterm>
882 <para>The <literal>:set</literal> command sets two types of
883 options: GHCi options, which begin with
884 ‘<literal>+</literal>” and “command-line”
885 options, which begin with ‘-’. </para>
888 <title>GHCi options</title>
889 <indexterm><primary>options</primary><secondary>GHCi</secondary>
892 <para>GHCi options may be set using <literal>:set</literal> and
893 unset using <literal>:unset</literal>.</para>
895 <para>The available GHCi options are:</para>
899 <term><literal>+r</literal></term>
900 <indexterm><primary><literal>+r</literal></primary></indexterm>
901 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
902 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
904 <para>Normally, any evaluation of top-level expressions
905 (otherwise known as CAFs or Constant Applicative Forms) in
906 loaded modules is retained between evaluations. Turning
907 on <literal>+r</literal> causes all evaluation of
908 top-level expressions to be discarded after each
909 evaluation (they are still retained
910 <emphasis>during</emphasis> a single evaluation).</para>
912 <para>This option may help if the evaluated top-level
913 expressions are consuming large amounts of space, or if
914 you need repeatable performance measurements.</para>
919 <term><literal>+s</literal></term>
920 <indexterm><primary><literal>+s</literal></primary></indexterm>
922 <para>Display some stats after evaluating each expression,
923 including the elapsed time and number of bytes allocated.
924 NOTE: the allocation figure is only accurate to the size
925 of the storage manager's allocation area, because it is
926 calculated at every GC. Hence, you might see values of
927 zero if no GC has occurred.</para>
932 <term><literal>+t</literal></term>
933 <indexterm><primary><literal>+t</literal></primary></indexterm>
935 <para>Display the type of each variable bound after a
936 statement is entered at the prompt. If the statement is a
937 single expression, then the only variable binding will be
939 ‘<literal>it</literal>’.</para>
946 <title>Setting GHC command-line options in GHCi</title>
948 <para>Normal GHC command-line options may also be set using
949 <literal>:set</literal>. For example, to turn on
950 <option>-fglasgow-exts</option>, you would say:</para>
953 Prelude> :set -fglasgow-exts
956 <para>Any GHC command-line option that is designated as
957 <firstterm>dynamic</firstterm> (see the table in <xref
958 linkend="flag-reference">), may be set using
959 <literal>:set</literal>. To unset an option, you can set the
960 reverse option:</para>
961 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
964 Prelude> :set -fno-glasgow-exts
967 <para><xref linkend="flag-reference"> lists the reverse for each
968 option where applicable.</para>
970 <para>Certain static options (<option>-package</option>,
971 <option>-I</option>, <option>-i</option>, and
972 <option>-l</option> in particular) will also work, but some may
973 not take effect until the next reload.</para>
974 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
978 <sect1 id="ghci-dot-files">
979 <title>The <filename>.ghci</filename> file</title>
980 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
982 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
985 <para>When it starts, GHCi always reads and executes commands from
986 <filename>$HOME/.ghci</filename>, followed by
987 <filename>./.ghci</filename>.</para>
989 <para>The <filename>.ghci</filename> in your home directory is
990 most useful for turning on favourite options (eg. <literal>:set
991 +s</literal>), and defining useful macros. Placing a
992 <filename>.ghci</filename> file in a directory with a Haskell
993 project is a useful way to set certain project-wide options so you
994 don't have to type them everytime you start GHCi: eg. if your
995 project uses GHC extensions and CPP, and has source files in three
996 subdirectories A B and C, you might put the following lines in
997 <filename>.ghci</filename>:</para>
1000 :set -fglasgow-exts -cpp
1004 <para>(Note that strictly speaking the <option>-i</option> flag is
1005 a static one, but in fact it works to set it using
1006 <literal>:set</literal> like this. The changes won't take effect
1007 until the next <literal>:load</literal>, though.)</para>
1009 <para>Two command-line options control whether the
1010 <filename>.ghci</filename> files are read:</para>
1014 <term><option>-ignore-dot-ghci</option></term>
1015 <indexterm><primary><option>-ignore-dot-ghci</option></primary>
1018 <para>Don't read either <filename>./.ghci</filename> or
1019 <filename>$HOME/.ghci</filename> when starting up.</para>
1023 <term><option>-read-dot-ghci</option></term>
1024 <indexterm><primary><option>-read-dot-ghci</option></primary>
1027 <para>Read <filename>.ghci</filename> and
1028 <filename>$HOME/.ghci</filename>. This is normally the
1029 default, but the <option>-read-dot-ghci</option> option may
1030 be used to override a previous
1031 <option>-ignore-dot-ghci</option> option.</para>
1039 <title>FAQ and Things To Watch Out For</title>
1043 <term>GHCi complains about <function>main</function> not being
1044 in scope when I load a module.</term>
1045 <indexterm><primary><function>main</function></primary><secondary>with GHCi</secondary>
1048 <para>You probably omitted the <literal>module</literal>
1049 declaration at the top of the module, which causes the
1050 module name to default to <literal>Main</literal>. In
1051 Haskell, the <literal>Main</literal> module must define a
1052 function called <function>main</function>. Admittedly this
1053 doesn't make a great deal of sense for an interpreter, but
1054 the rule was kept for compatibility with GHC.</para>
1059 <term><literal>System.getArgs</literal> returns GHCi's command
1060 line arguments!</term>
1062 <para>Yes, it does.</para>
1067 <term>The interpreter can't load modules with foreign export
1068 declarations!</term>
1070 <para>Unfortunately not. We haven't implemented it yet.
1071 Please compile any offending modules by hand before loading
1072 them into GHCi.</para>
1077 <term><literal>-O</literal> doesn't work with GHCi!</term>
1078 <indexterm><primary><option>-O</option></primary>
1081 <para>For technical reasons, the bytecode compiler doesn't
1082 interact well with one of the optimisation passes, so we
1083 have disabled optimisation when using the interpreter. This
1084 isn't a great loss: you'll get a much bigger win by
1085 compiling the bits of your code that need to go fast, rather
1086 than interpreting them with optimisation turned on.</para>
1091 <term>Unboxed tuples don't work with GHCi</term>
1093 <para>That's right. You can always compile a module that
1094 uses unboxed tuples and load it into GHCi, however.
1095 (Incidentally the previous point, namely that
1096 <literal>-O</literal> is incompatible with GHCi, is because
1097 the bytecode compiler can't deal with unboxed
1103 <term>Concurrent threads don't carry on running when GHCi is
1104 waiting for input.</term>
1106 <para>No, they don't. This is because the Haskell binding
1107 to the GNU readline library doesn't support reading from the
1108 terminal in a non-blocking way, which is required to work
1109 properly with GHC's concurrency model.</para>
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