1 <?xml version="1.0" encoding="iso-8859-1"?>
3 <title>Using GHCi</title>
4 <indexterm><primary>GHCi</primary></indexterm>
5 <indexterm><primary>interpreter</primary><see>GHCi</see></indexterm>
6 <indexterm><primary>interactive</primary><see>GHCi</see></indexterm>
9 <para>The ‘i’ stands for “Interactive”</para>
11 is GHC's interactive environment, in which Haskell expressions can
12 be interactively evaluated and programs can be interpreted. If
13 you're familiar with <ulink url="http://www.haskell.org/hugs/">Hugs</ulink><indexterm><primary>Hugs</primary>
14 </indexterm>, then you'll be right at home with GHCi. However, GHCi
15 also has support for interactively loading compiled code, as well as
16 supporting all<footnote><para>except <literal>foreign export</literal>, at the moment</para>
17 </footnote> the language extensions that GHC provides.</para>
18 <indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
19 <indexterm><primary>Foreign Function Interface</primary><secondary>GHCi support</secondary></indexterm>
22 <title>Introduction to GHCi</title>
24 <para>Let's start with an example GHCi session. You can fire up
25 GHCi with the command <literal>ghci</literal>:</para>
31 / /_\// /_/ / / | | GHC Interactive, version 5.04, for Haskell 98.
32 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
33 \____/\/ /_/\____/|_| Type :? for help.
35 Loading package base ... linking ... done.
36 Loading package haskell98 ... linking ... done.
40 <para>There may be a short pause while GHCi loads the prelude and
41 standard libraries, after which the prompt is shown. If we follow
42 the instructions and type <literal>:?</literal> for help, we
46 Commands available from the prompt:
48 <stmt> evaluate/run <stmt>
49 :add <filename> ... add module(s) to the current target set
50 :browse [*]<module> display the names defined by <module>
51 :cd <dir> change directory to <dir>
52 :def <cmd> <expr> define a command :<cmd>
53 :help, :? display this list of commands
54 :info [<name> ...] display information about the given names
55 :load <filename> ... load module(s) and their dependents
56 :module [+/-] [*]<mod> ... set the context for expression evaluation
57 :reload reload the current module set
59 :set <option> ... set options
60 :set args <arg> ... set the arguments returned by System.getArgs
61 :set prog <progname> set the value returned by System.getProgName
63 :show modules show the currently loaded modules
64 :show bindings show the current bindings made at the prompt
66 :type <expr> show the type of <expr>
67 :kind <type> show the kind of <type>
68 :undef <cmd> undefine user-defined command :<cmd>
69 :unset <option> ... unset options
71 :!<command> run the shell command <command>
73 Options for `:set' and `:unset':
75 +r revert top-level expressions after each evaluation
76 +s print timing/memory stats after each evaluation
77 +t print type after evaluation
78 -<flags> most GHC command line flags can also be set here
79 (eg. -v2, -fglasgow-exts, etc.)
82 <para>We'll explain most of these commands as we go along. For
83 Hugs users: many things work the same as in Hugs, so you should be
84 able to get going straight away.</para>
86 <para>Haskell expressions can be typed at the prompt:</para>
87 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
93 Prelude> let x = 42 in x / 9
98 <para>GHCi interprets the whole line as an expression to evaluate.
99 The expression may not span several lines - as soon as you press
100 enter, GHCi will attempt to evaluate it.</para>
104 <title>Loading source files</title>
106 <para>Suppose we have the following Haskell source code, which we
107 place in a file <filename>Main.hs</filename>:</para>
110 main = print (fac 20)
113 fac n = n * fac (n-1)
116 <para>You can save <filename>Main.hs</filename> anywhere you like,
117 but if you save it somewhere other than the current
118 directory<footnote><para>If you started up GHCi from the command
119 line then GHCi's current directory is the same as the current
120 directory of the shell from which it was started. If you started
121 GHCi from the “Start” menu in Windows, then the
122 current directory is probably something like
123 <filename>C:\Documents and Settings\<replaceable>user
124 name</replaceable></filename>.</para> </footnote> then we will
125 need to change to the right directory in GHCi:</para>
128 Prelude> :cd <replaceable>dir</replaceable>
131 <para>where <replaceable>dir</replaceable> is the directory (or
132 folder) in which you saved <filename>Main.hs</filename>.</para>
134 <para>To load a Haskell source file into GHCi, use the
135 <literal>:load</literal> command:</para>
136 <indexterm><primary><literal>:load</literal></primary></indexterm>
140 Compiling Main ( Main.hs, interpreted )
141 Ok, modules loaded: Main.
145 <para>GHCi has loaded the <literal>Main</literal> module, and the
146 prompt has changed to “<literal>*Main></literal>” to
147 indicate that the current context for expressions typed at the
148 prompt is the <literal>Main</literal> module we just loaded (we'll
149 explain what the <literal>*</literal> means later in <xref
150 linkend="ghci-scope"/>). So we can now type expressions involving
151 the functions from <filename>Main.hs</filename>:</para>
158 <para>Loading a multi-module program is just as straightforward;
159 just give the name of the “topmost” module to the
160 <literal>:load</literal> command (hint: <literal>:load</literal>
161 can be abbreviated to <literal>:l</literal>). The topmost module
162 will normally be <literal>Main</literal>, but it doesn't have to
163 be. GHCi will discover which modules are required, directly or
164 indirectly, by the topmost module, and load them all in dependency
167 <sect2 id="ghci-modules-filenames">
168 <title>Modules vs. filenames</title>
169 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
170 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
172 <para>Question: How does GHC find the filename which contains
173 module <replaceable>M</replaceable>? Answer: it looks for the
174 file <literal><replaceable>M</replaceable>.hs</literal>, or
175 <literal><replaceable>M</replaceable>.lhs</literal>. This means
176 that for most modules, the module name must match the filename.
177 If it doesn't, GHCi won't be able to find it.</para>
179 <para>There is one exception to this general rule: when you load
180 a program with <literal>:load</literal>, or specify it when you
181 invoke <literal>ghci</literal>, you can give a filename rather
182 than a module name. This filename is loaded if it exists, and
183 it may contain any module you like. This is particularly
184 convenient if you have several <literal>Main</literal> modules
185 in the same directory and you can't call them all
186 <filename>Main.hs</filename>.</para>
188 <para>The search path for finding source files is specified with
189 the <option>-i</option> option on the GHCi command line, like
191 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
193 <para>or it can be set using the <literal>:set</literal> command
194 from within GHCi (see <xref
195 linkend="ghci-cmd-line-options"/>)<footnote><para>Note that in
196 GHCi, and <option>––make</option> mode, the <option>-i</option>
197 option is used to specify the search path for
198 <emphasis>source</emphasis> files, whereas in standard
199 batch-compilation mode the <option>-i</option> option is used to
200 specify the search path for interface files, see <xref
201 linkend="search-path"/>.</para> </footnote></para>
203 <para>One consequence of the way that GHCi follows dependencies
204 to find modules to load is that every module must have a source
205 file. The only exception to the rule is modules that come from
206 a package, including the <literal>Prelude</literal> and standard
207 libraries such as <literal>IO</literal> and
208 <literal>Complex</literal>. If you attempt to load a module for
209 which GHCi can't find a source file, even if there are object
210 and interface files for the module, you'll get an error
215 <title>Making changes and recompilation</title>
216 <indexterm><primary><literal>:reload</literal></primary></indexterm>
218 <para>If you make some changes to the source code and want GHCi
219 to recompile the program, give the <literal>:reload</literal>
220 command. The program will be recompiled as necessary, with GHCi
221 doing its best to avoid actually recompiling modules if their
222 external dependencies haven't changed. This is the same
223 mechanism we use to avoid re-compiling modules in the batch
224 compilation setting (see <xref linkend="recomp"/>).</para>
228 <sect1 id="ghci-compiled">
229 <title>Loading compiled code</title>
230 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
232 <para>When you load a Haskell source module into GHCi, it is
233 normally converted to byte-code and run using the interpreter.
234 However, interpreted code can also run alongside compiled code in
235 GHCi; indeed, normally when GHCi starts, it loads up a compiled
236 copy of the <literal>base</literal> package, which contains the
237 <literal>Prelude</literal>.</para>
239 <para>Why should we want to run compiled code? Well, compiled
240 code is roughly 10x faster than interpreted code, but takes about
241 2x longer to produce (perhaps longer if optimisation is on). So
242 it pays to compile the parts of a program that aren't changing
243 very often, and use the interpreter for the code being actively
246 <para>When loading up source files with <literal>:load</literal>,
247 GHCi looks for any corresponding compiled object files, and will
248 use one in preference to interpreting the source if possible. For
249 example, suppose we have a 4-module program consisting of modules
250 A, B, C, and D. Modules B and C both import D only,
251 and A imports both B & C:</para>
259 <para>We can compile D, then load the whole program, like this:</para>
261 Prelude> :! ghc -c D.hs
263 Skipping D ( D.hs, D.o )
264 Compiling C ( C.hs, interpreted )
265 Compiling B ( B.hs, interpreted )
266 Compiling A ( A.hs, interpreted )
267 Ok, modules loaded: A, B, C, D.
271 <para>In the messages from the compiler, we see that it skipped D,
272 and used the object file <filename>D.o</filename>. The message
273 <literal>Skipping</literal> <replaceable>module</replaceable>
274 indicates that compilation for <replaceable>module</replaceable>
275 isn't necessary, because the source and everything it depends on
276 is unchanged since the last compilation.</para>
278 <para>At any time you can use the command
279 <literal>:show modules</literal>
280 to get a list of the modules currently loaded
286 C ( C.hs, interpreted )
287 B ( B.hs, interpreted )
288 A ( A.hs, interpreted )
291 <para>If we now modify the source of D (or pretend to: using Unix
292 command <literal>touch</literal> on the source file is handy for
293 this), the compiler will no longer be able to use the object file,
294 because it might be out of date:</para>
299 Compiling D ( D.hs, interpreted )
300 Skipping C ( C.hs, interpreted )
301 Skipping B ( B.hs, interpreted )
302 Skipping A ( A.hs, interpreted )
303 Ok, modules loaded: A, B, C, D.
307 <para>Note that module D was compiled, but in this instance
308 because its source hadn't really changed, its interface remained
309 the same, and the recompilation checker determined that A, B and C
310 didn't need to be recompiled.</para>
312 <para>So let's try compiling one of the other modules:</para>
315 *Main> :! ghc -c C.hs
317 Compiling D ( D.hs, interpreted )
318 Compiling C ( C.hs, interpreted )
319 Compiling B ( B.hs, interpreted )
320 Compiling A ( A.hs, interpreted )
321 Ok, modules loaded: A, B, C, D.
324 <para>We didn't get the compiled version of C! What happened?
325 Well, in GHCi a compiled module may only depend on other compiled
326 modules, and in this case C depends on D, which doesn't have an
327 object file, so GHCi also rejected C's object file. Ok, so let's
328 also compile D:</para>
331 *Main> :! ghc -c D.hs
333 Ok, modules loaded: A, B, C, D.
336 <para>Nothing happened! Here's another lesson: newly compiled
337 modules aren't picked up by <literal>:reload</literal>, only
338 <literal>:load</literal>:</para>
342 Skipping D ( D.hs, D.o )
343 Skipping C ( C.hs, C.o )
344 Compiling B ( B.hs, interpreted )
345 Compiling A ( A.hs, interpreted )
346 Ok, modules loaded: A, B, C, D.
349 <para>HINT: since GHCi will only use a compiled object file if it
350 can sure that the compiled version is up-to-date, a good technique
351 when working on a large program is to occasionally run
352 <literal>ghc ––make</literal> to compile the whole project (say
353 before you go for lunch :-), then continue working in the
354 interpreter. As you modify code, the new modules will be
355 interpreted, but the rest of the project will remain
361 <title>Interactive evaluation at the prompt</title>
363 <para>When you type an expression at the prompt, GHCi immediately
364 evaluates and prints the result:
366 Prelude> reverse "hello"
373 <sect2><title>I/O actions at the prompt</title>
375 <para>GHCi does more than simple expression evaluation at the prompt.
376 If you type something of type <literal>IO a</literal> for some
377 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
378 as an IO-computation.
382 Prelude> putStrLn "hello"
385 Furthermore, GHCi will print the result of the I/O action if (and only
388 <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
389 <listitem><para>The result type is not
390 <literal>()</literal>.</para></listitem>
392 For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
394 Prelude> putStrLn "hello"
396 Prelude> do { putStrLn "hello"; return "yes" }
403 <title>Using <literal>do-</literal>notation at the prompt</title>
404 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
405 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
407 <para>GHCi actually accepts <firstterm>statements</firstterm>
408 rather than just expressions at the prompt. This means you can
409 bind values and functions to names, and use them in future
410 expressions or statements.</para>
412 <para>The syntax of a statement accepted at the GHCi prompt is
413 exactly the same as the syntax of a statement in a Haskell
414 <literal>do</literal> expression. However, there's no monad
415 overloading here: statements typed at the prompt must be in the
416 <literal>IO</literal> monad.
418 Prelude> x <- return 42
424 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>
440 <para>An important difference between the two types of binding
441 is that the monadic bind (<literal>p <- e</literal>) is
442 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
443 whereas with the <literal>let</literal> form, the expression
444 isn't evaluated immediately:</para>
446 Prelude> let x = error "help!"
451 <para>GHCi will print the result of a
452 <literal>x<-e</literal> statement if and only if:
454 <listitem><para>The statement binds exactly one variable</para></listitem>
455 <listitem><para>The variable's type is not polymorphic, is not
456 <literal>()</literal>,
457 and is an instance of <literal>Show</literal>
460 <para>Any exceptions raised during the evaluation or execution
461 of the statement are caught and printed by the GHCi command line
462 interface (for more information on exceptions, see the module
463 <literal>Control.Exception</literal> in the libraries
464 documentation).</para>
467 <para>Every new binding shadows any existing bindings of the
468 same name, including entities that are in scope in the current
469 module context.</para>
471 <para>WARNING: temporary bindings introduced at the prompt only
472 last until the next <literal>:load</literal> or
473 <literal>:reload</literal> command, at which time they will be
474 simply lost. However, they do survive a change of context with
475 <literal>:module</literal>: the temporary bindings just move to
476 the new location.</para>
478 <para>HINT: To get a list of the bindings currently in scope, use the
479 <literal>:show bindings</literal> command:</para>
482 Prelude> :show bindings
486 <para>HINT: if you turn on the <literal>+t</literal> option,
487 GHCi will show the type of each variable bound by a statement.
489 <indexterm><primary><literal>+t</literal></primary></indexterm>
492 Prelude> let (x:xs) = [1..]
499 <sect2 id="ghci-scope">
500 <title>What's really in scope at the prompt?</title>
502 <para>When you type an expression at the prompt, what
503 identifiers and types are in scope? GHCi provides a flexible
504 way to control exactly how the context for an expression is
505 constructed. Let's start with the simple cases; when you start
506 GHCi the prompt looks like this:</para>
508 <screen>Prelude></screen>
510 <para>Which indicates that everything from the module
511 <literal>Prelude</literal> is currently in scope. If we now
512 load a file into GHCi, the prompt will change:</para>
515 Prelude> :load Main.hs
516 Compiling Main ( Main.hs, interpreted )
520 <para>The new prompt is <literal>*Main</literal>, which
521 indicates that we are typing expressions in the context of the
522 top-level of the <literal>Main</literal> module. Everything
523 that is in scope at the top-level in the module
524 <literal>Main</literal> we just loaded is also in scope at the
525 prompt (probably including <literal>Prelude</literal>, as long
526 as <literal>Main</literal> doesn't explicitly hide it).</para>
529 <literal>*<replaceable>module</replaceable></literal> indicates
530 that it is the full top-level scope of
531 <replaceable>module</replaceable> that is contributing to the
532 scope for expressions typed at the prompt. Without the
533 <literal>*</literal>, just the exports of the module are
536 <para>We're not limited to a single module: GHCi can combine
537 scopes from multiple modules, in any mixture of
538 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
539 combines the scopes from all of these modules to form the scope
540 that is in effect at the prompt. For technical reasons, GHCi
541 can only support the <literal>*</literal>-form for modules which
542 are interpreted, so compiled modules and package modules can
543 only contribute their exports to the current scope.</para>
545 <para>The scope is manipulated using the
546 <literal>:module</literal> command. For example, if the current
547 scope is <literal>Prelude</literal>, then we can bring into
548 scope the exports from the module <literal>IO</literal> like
553 Prelude,IO> hPutStrLn stdout "hello\n"
558 <para>(Note: <literal>:module</literal> can be shortened to
559 <literal>:m</literal>). The full syntax of the
560 <literal>:module</literal> command is:</para>
563 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
566 <para>Using the <literal>+</literal> form of the
567 <literal>module</literal> commands adds modules to the current
568 scope, and <literal>-</literal> removes them. Without either
569 <literal>+</literal> or <literal>-</literal>, the current scope
570 is replaced by the set of modules specified. Note that if you
571 use this form and leave out <literal>Prelude</literal>, GHCi
572 will assume that you really wanted the
573 <literal>Prelude</literal> and add it in for you (if you don't
574 want the <literal>Prelude</literal>, then ask to remove it with
575 <literal>:m -Prelude</literal>).</para>
577 <para>The scope is automatically set after a
578 <literal>:load</literal> command, to the most recently loaded
579 "target" module, in a <literal>*</literal>-form if possible.
580 For example, if you say <literal>:load foo.hs bar.hs</literal>
581 and <filename>bar.hs</filename> contains module
582 <literal>Bar</literal>, then the scope will be set to
583 <literal>*Bar</literal> if <literal>Bar</literal> is
584 interpreted, or if <literal>Bar</literal> is compiled it will be
585 set to <literal>Prelude,Bar</literal> (GHCi automatically adds
586 <literal>Prelude</literal> if it isn't present and there aren't
587 any <literal>*</literal>-form modules).</para>
589 <para>With multiple modules in scope, especially multiple
590 <literal>*</literal>-form modules, it is likely that name
591 clashes will occur. Haskell specifies that name clashes are
592 only reported when an ambiguous identifier is used, and GHCi
593 behaves in the same way for expressions typed at the
597 <title>Qualified names</title>
599 <para>To make life slightly easier, the GHCi prompt also
600 behaves as if there is an implicit <literal>import
601 qualified</literal> declaration for every module in every
602 package, and every module currently loaded into GHCi.</para>
608 <title>The <literal>it</literal> variable</title>
609 <indexterm><primary><literal>it</literal></primary>
612 <para>Whenever an expression (or a non-binding statement, to be
613 precise) is typed at the prompt, GHCi implicitly binds its value
614 to the variable <literal>it</literal>. For example:</para>
621 <para>What actually happens is that GHCi typechecks the
622 expression, and if it doesn't have an <literal>IO</literal> type,
623 then it transforms it as follows: an expression
624 <replaceable>e</replaceable> turns into
626 let it = <replaceable>e</replaceable>;
629 which is then run as an IO-action.</para>
631 <para>Hence, the original expression must have a type which is an
632 instance of the <literal>Show</literal> class, or GHCi will
637 No instance for `Show (a -> a)'
638 arising from use of `print'
639 in a `do' expression pattern binding: print it
642 <para>The error message contains some clues as to the
643 transformation happening internally.</para>
645 <para>If the expression was instead of type <literal>IO a</literal> for
646 some <literal>a</literal>, then <literal>it</literal> will be
647 bound to the result of the <literal>IO</literal> computation,
648 which is of type <literal>a</literal>. eg.:</para>
650 Prelude> Time.getClockTime
652 Wed Mar 14 12:23:13 GMT 2001
655 <para>The corresponding translation for an IO-typed
656 <replaceable>e</replaceable> is
658 it <- <replaceable>e</replaceable>
662 <para>Note that <literal>it</literal> is shadowed by the new
663 value each time you evaluate a new expression, and the old value
664 of <literal>it</literal> is lost.</para>
669 <title>Type defaulting in GHCi</title>
670 <indexterm><primary>Type default</primary></indexterm>
671 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
673 Consider this GHCi session:
677 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
678 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
679 on the type <literal>a</literal>. For example:
681 ghci> (reverse []) :: String
683 ghci> (reverse []) :: [Int]
686 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
687 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
688 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
689 a)</literal> for each type variable <literal>a</literal>, and defaults the
692 <listitem><para> The type variable <literal>a</literal>
693 appears in no other constraints </para></listitem>
694 <listitem><para> All the classes <literal>Ci</literal> are standard.</para></listitem>
695 <listitem><para> At least one of the classes <literal>Ci</literal> is
696 numeric.</para></listitem>
698 At the GHCi prompt, the second and third rules are relaxed as follows
699 (differences italicised):
701 <listitem><para> <emphasis>Any</emphasis> of the classes <literal>Ci</literal> is standard.</para></listitem>
702 <listitem><para> At least one of the classes <literal>Ci</literal> is
703 numeric, <emphasis>or is <literal>Show</literal>,
704 <literal>Eq</literal>, or <literal>Ord</literal></emphasis>.</para></listitem>
710 <sect1 id="ghci-invocation">
711 <title>Invoking GHCi</title>
712 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
713 <indexterm><primary><option>––interactive</option></primary></indexterm>
715 <para>GHCi is invoked with the command <literal>ghci</literal> or
716 <literal>ghc ––interactive</literal>. One or more modules or
717 filenames can also be specified on the command line; this
718 instructs GHCi to load the specified modules or filenames (and all
719 the modules they depend on), just as if you had said
720 <literal>:load <replaceable>modules</replaceable></literal> at the
721 GHCi prompt (see <xref linkend="ghci-commands"/>). For example, to
722 start GHCi and load the program whose topmost module is in the
723 file <literal>Main.hs</literal>, we could say:</para>
729 <para>Most of the command-line options accepted by GHC (see <xref
730 linkend="using-ghc"/>) also make sense in interactive mode. The ones
731 that don't make sense are mostly obvious; for example, GHCi
732 doesn't generate interface files, so options related to interface
733 file generation won't have any effect.</para>
736 <title>Packages</title>
737 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
739 <para>Most packages (see <xref linkend="using-packages"/>) are
740 available without needing to specify any extra flags at all:
741 they will be automatically loaded the first time they are
744 <para>For non-auto packages, however, you need to request the
745 package be loaded by using the <literal>-package</literal> flag:</para>
751 / /_\// /_/ / / | | GHC Interactive, version 5.05, for Haskell 98.
752 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
753 \____/\/ /_/\____/|_| Type :? for help.
755 Loading package base ... linking ... done.
756 Loading package haskell98 ... linking ... done.
757 Loading package lang ... linking ... done.
758 Loading package concurrent ... linking ... done.
759 Loading package readline ... linking ... done.
760 Loading package unix ... linking ... done.
761 Loading package posix ... linking ... done.
762 Loading package util ... linking ... done.
763 Loading package data ... linking ... done.
767 <para>The following command works to load new packages into a
771 Prelude> :set -package <replaceable>name</replaceable>
774 <para>But note that doing this will cause all currently loaded
775 modules to be unloaded, and you'll be dumped back into the
776 <literal>Prelude</literal>.</para>
780 <title>Extra libraries</title>
781 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
783 <para>Extra libraries may be specified on the command line using
784 the normal <literal>-l<replaceable>lib</replaceable></literal>
785 option. (The term <emphasis>library</emphasis> here refers to
786 libraries of foreign object code; for using libraries of Haskell
787 source code, see <xref linkend="ghci-modules-filenames"/>.) For
788 example, to load the “m” library:</para>
794 <para>On systems with <literal>.so</literal>-style shared
795 libraries, the actual library loaded will the
796 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
797 searches the following places for libraries, in this order:</para>
801 <para>Paths specified using the
802 <literal>-L<replaceable>path</replaceable></literal>
803 command-line option,</para>
806 <para>the standard library search path for your system,
807 which on some systems may be overridden by setting the
808 <literal>LD_LIBRARY_PATH</literal> environment
813 <para>On systems with <literal>.dll</literal>-style shared
814 libraries, the actual library loaded will be
815 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
816 GHCi will signal an error if it can't find the library.</para>
818 <para>GHCi can also load plain object files
819 (<literal>.o</literal> or <literal>.obj</literal> depending on
820 your platform) from the command-line. Just add the name the
821 object file to the command line.</para>
823 <para>Ordering of <option>-l</option> options matters: a library
824 should be mentioned <emphasis>before</emphasis> the libraries it
825 depends on (see <xref linkend="options-linker"/>).</para>
830 <sect1 id="ghci-commands">
831 <title>GHCi commands</title>
833 <para>GHCi commands all begin with
834 ‘<literal>:</literal>’ and consist of a single command
835 name followed by zero or more parameters. The command name may be
836 abbreviated, as long as the abbreviation is not ambiguous. All of
837 the builtin commands, with the exception of
838 <literal>:unset</literal> and <literal>:undef</literal>, may be
839 abbreviated to a single letter.</para>
844 <literal>:add</literal> <replaceable>module</replaceable> ...
845 <indexterm><primary><literal>:add</literal></primary></indexterm>
848 <para>Add <replaceable>module</replaceable>(s) to the
849 current <firstterm>target set</firstterm>, and perform a
856 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
857 <indexterm><primary><literal>:browse</literal></primary></indexterm>
860 <para>Displays the identifiers defined by the module
861 <replaceable>module</replaceable>, which must be either
862 loaded into GHCi or be a member of a package. If the
863 <literal>*</literal> symbol is placed before the module
864 name, then <emphasis>all</emphasis> the identifiers defined
865 in <replaceable>module</replaceable> are shown; otherwise
866 the list is limited to the exports of
867 <replaceable>module</replaceable>. The
868 <literal>*</literal>-form is only available for modules
869 which are interpreted; for compiled modules (including
870 modules from packages) only the non-<literal>*</literal>
871 form of <literal>:browse</literal> is available.</para>
877 <literal>:cd</literal> <replaceable>dir</replaceable>
878 <indexterm><primary><literal>:cd</literal></primary></indexterm>
881 <para>Changes the current working directory to
882 <replaceable>dir</replaceable>. A
883 ‘<literal>˜</literal>’ symbol at the
884 beginning of <replaceable>dir</replaceable> will be replaced
885 by the contents of the environment variable
886 <literal>HOME</literal>.</para>
888 <para>NOTE: changing directories causes all currently loaded
889 modules to be unloaded. This is because the search path is
890 usually expressed using relative directories, and changing
891 the search path in the middle of a session is not
898 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
899 <indexterm><primary><literal>:def</literal></primary></indexterm>
902 <para>The command <literal>:def</literal>
903 <replaceable>name</replaceable>
904 <replaceable>expr</replaceable> defines a new GHCi command
905 <literal>:<replaceable>name</replaceable></literal>,
906 implemented by the Haskell expression
907 <replaceable>expr</replaceable>, which must have type
908 <literal>String -> IO String</literal>. When
909 <literal>:<replaceable>name</replaceable>
910 <replaceable>args</replaceable></literal> is typed at the
911 prompt, GHCi will run the expression
912 <literal>(<replaceable>name</replaceable>
913 <replaceable>args</replaceable>)</literal>, take the
914 resulting <literal>String</literal>, and feed it back into
915 GHCi as a new sequence of commands. Separate commands in
916 the result must be separated by
917 ‘<literal>\n</literal>’.</para>
919 <para>That's all a little confusing, so here's a few
920 examples. To start with, here's a new GHCi command which
921 doesn't take any arguments or produce any results, it just
922 outputs the current date & time:</para>
925 Prelude> let date _ = Time.getClockTime >>= print >> return ""
926 Prelude> :def date date
928 Fri Mar 23 15:16:40 GMT 2001
931 <para>Here's an example of a command that takes an argument.
932 It's a re-implementation of <literal>:cd</literal>:</para>
935 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
936 Prelude> :def mycd mycd
940 <para>Or I could define a simple way to invoke
941 “<literal>ghc ––make Main</literal>” in the
942 current directory:</para>
945 Prelude> :def make (\_ -> return ":! ghc ––make Main")
948 <para>We can define a command that reads GHCi input from a
949 file. This might be useful for creating a set of bindings
950 that we want to repeatedly load into the GHCi session:</para>
953 Prelude> :def . readFile
954 Prelude> :. cmds.ghci
957 <para>Notice that we named the command
958 <literal>:.</literal>, by analogy with the
959 ‘<literal>.</literal>’ Unix shell command that
960 does the same thing.</para>
966 <literal>:help</literal>
967 <indexterm><primary><literal>:help</literal></primary></indexterm>
970 <literal>:?</literal>
971 <indexterm><primary><literal>:?</literal></primary></indexterm>
974 <para>Displays a list of the available commands.</para>
980 <literal>:info</literal> <replaceable>name</replaceable> ...
981 <indexterm><primary><literal>:info</literal></primary></indexterm>
984 <para>Displays information about the given name(s). For
985 example, if <replaceable>name</replaceable> is a class, then
986 the class methods and their types will be printed; if
987 <replaceable>name</replaceable> is a type constructor, then
988 its definition will be printed; if
989 <replaceable>name</replaceable> is a function, then its type
990 will be printed. If <replaceable>name</replaceable> has
991 been loaded from a source file, then GHCi will also display
992 the location of its definition in the source.</para>
998 <literal>:load</literal> <replaceable>module</replaceable> ...
999 <indexterm><primary><literal>:load</literal></primary></indexterm>
1002 <para>Recursively loads the specified
1003 <replaceable>module</replaceable>s, and all the modules they
1004 depend on. Here, each <replaceable>module</replaceable>
1005 must be a module name or filename, but may not be the name
1006 of a module in a package.</para>
1008 <para>All previously loaded modules, except package modules,
1009 are forgotten. The new set of modules is known as the
1010 <firstterm>target set</firstterm>. Note that
1011 <literal>:load</literal> can be used without any arguments
1012 to unload all the currently loaded modules and
1015 <para>After a <literal>:load</literal> command, the current
1016 context is set to:</para>
1020 <para><replaceable>module</replaceable>, if it was loaded
1021 successfully, or</para>
1024 <para>the most recently successfully loaded module, if
1025 any other modules were loaded as a result of the current
1026 <literal>:load</literal>, or</para>
1029 <para><literal>Prelude</literal> otherwise.</para>
1037 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
1038 <indexterm><primary><literal>:module</literal></primary></indexterm>
1041 <para>Sets or modifies the current context for statements
1042 typed at the prompt. See <xref linkend="ghci-scope"/> for
1043 more details.</para>
1049 <literal>:quit</literal>
1050 <indexterm><primary><literal>:quit</literal></primary></indexterm>
1053 <para>Quits GHCi. You can also quit by typing a control-D
1054 at the prompt.</para>
1060 <literal>:reload</literal>
1061 <indexterm><primary><literal>:reload</literal></primary></indexterm>
1064 <para>Attempts to reload the current target set (see
1065 <literal>:load</literal>) if any of the modules in the set,
1066 or any dependent module, has changed. Note that this may
1067 entail loading new modules, or dropping modules which are no
1068 longer indirectly required by the target.</para>
1074 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
1075 <indexterm><primary><literal>:set</literal></primary></indexterm>
1078 <para>Sets various options. See <xref linkend="ghci-set"/>
1079 for a list of available options. The
1080 <literal>:set</literal> command by itself shows which
1081 options are currently set.</para>
1087 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
1088 <indexterm><primary><literal>:set</literal></primary></indexterm>
1091 <para>Sets the list of arguments which are returned when the
1092 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
1093 </indexterm>.</para>
1099 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
1100 <indexterm><primary><literal>:set</literal></primary></indexterm>
1103 <para>Sets the string to be returned when the program calls
1104 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
1105 </indexterm>.</para>
1111 <literal>:show bindings</literal>
1112 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
1115 <para>Show the bindings made at the prompt and their
1122 <literal>:show modules</literal>
1123 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
1126 <para>Show the list of modules currently load.</para>
1132 <literal>:type</literal> <replaceable>expression</replaceable>
1133 <indexterm><primary><literal>:type</literal></primary></indexterm>
1136 <para>Infers and prints the type of
1137 <replaceable>expression</replaceable>, including explicit
1138 forall quantifiers for polymorphic types. The monomorphism
1139 restriction is <emphasis>not</emphasis> applied to the
1140 expression during type inference.</para>
1146 <literal>:kind</literal> <replaceable>type</replaceable>
1147 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1150 <para>Infers and prints the kind of
1151 <replaceable>type</replaceable>. The latter can be an arbitrary
1152 type expression, including a partial application of a type constructor,
1153 such as <literal>Either Int</literal>.</para>
1159 <literal>:undef</literal> <replaceable>name</replaceable>
1160 <indexterm><primary><literal>:undef</literal></primary></indexterm>
1163 <para>Undefines the user-defined command
1164 <replaceable>name</replaceable> (see <literal>:def</literal>
1171 <literal>:unset</literal> <replaceable>option</replaceable>...
1172 <indexterm><primary><literal>:unset</literal></primary></indexterm>
1175 <para>Unsets certain options. See <xref linkend="ghci-set"/>
1176 for a list of available options.</para>
1182 <literal>:!</literal> <replaceable>command</replaceable>...
1183 <indexterm><primary><literal>:!</literal></primary></indexterm>
1184 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
1187 <para>Executes the shell command
1188 <replaceable>command</replaceable>.</para>
1195 <sect1 id="ghci-set">
1196 <title>The <literal>:set</literal> command</title>
1197 <indexterm><primary><literal>:set</literal></primary></indexterm>
1199 <para>The <literal>:set</literal> command sets two types of
1200 options: GHCi options, which begin with
1201 ‘<literal>+</literal>” and “command-line”
1202 options, which begin with ‘-’. </para>
1204 <para>NOTE: at the moment, the <literal>:set</literal> command
1205 doesn't support any kind of quoting in its arguments: quotes will
1206 not be removed and cannot be used to group words together. For
1207 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
1211 <title>GHCi options</title>
1212 <indexterm><primary>options</primary><secondary>GHCi</secondary>
1215 <para>GHCi options may be set using <literal>:set</literal> and
1216 unset using <literal>:unset</literal>.</para>
1218 <para>The available GHCi options are:</para>
1223 <literal>+r</literal>
1224 <indexterm><primary><literal>+r</literal></primary></indexterm>
1225 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
1226 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
1229 <para>Normally, any evaluation of top-level expressions
1230 (otherwise known as CAFs or Constant Applicative Forms) in
1231 loaded modules is retained between evaluations. Turning
1232 on <literal>+r</literal> causes all evaluation of
1233 top-level expressions to be discarded after each
1234 evaluation (they are still retained
1235 <emphasis>during</emphasis> a single evaluation).</para>
1237 <para>This option may help if the evaluated top-level
1238 expressions are consuming large amounts of space, or if
1239 you need repeatable performance measurements.</para>
1245 <literal>+s</literal>
1246 <indexterm><primary><literal>+s</literal></primary></indexterm>
1249 <para>Display some stats after evaluating each expression,
1250 including the elapsed time and number of bytes allocated.
1251 NOTE: the allocation figure is only accurate to the size
1252 of the storage manager's allocation area, because it is
1253 calculated at every GC. Hence, you might see values of
1254 zero if no GC has occurred.</para>
1260 <literal>+t</literal>
1261 <indexterm><primary><literal>+t</literal></primary></indexterm>
1264 <para>Display the type of each variable bound after a
1265 statement is entered at the prompt. If the statement is a
1266 single expression, then the only variable binding will be
1268 ‘<literal>it</literal>’.</para>
1274 <sect2 id="ghci-cmd-line-options">
1275 <title>Setting GHC command-line options in GHCi</title>
1277 <para>Normal GHC command-line options may also be set using
1278 <literal>:set</literal>. For example, to turn on
1279 <option>-fglasgow-exts</option>, you would say:</para>
1282 Prelude> :set -fglasgow-exts
1285 <para>Any GHC command-line option that is designated as
1286 <firstterm>dynamic</firstterm> (see the table in <xref
1287 linkend="flag-reference"/>), may be set using
1288 <literal>:set</literal>. To unset an option, you can set the
1289 reverse option:</para>
1290 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
1293 Prelude> :set -fno-glasgow-exts
1296 <para><xref linkend="flag-reference"/> lists the reverse for each
1297 option where applicable.</para>
1299 <para>Certain static options (<option>-package</option>,
1300 <option>-I</option>, <option>-i</option>, and
1301 <option>-l</option> in particular) will also work, but some may
1302 not take effect until the next reload.</para>
1303 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
1307 <sect1 id="ghci-dot-files">
1308 <title>The <filename>.ghci</filename> file</title>
1309 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
1311 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
1314 <para>When it starts, GHCi always reads and executes commands from
1315 <filename>$HOME/.ghci</filename>, followed by
1316 <filename>./.ghci</filename>.</para>
1318 <para>The <filename>.ghci</filename> in your home directory is
1319 most useful for turning on favourite options (eg. <literal>:set
1320 +s</literal>), and defining useful macros. Placing a
1321 <filename>.ghci</filename> file in a directory with a Haskell
1322 project is a useful way to set certain project-wide options so you
1323 don't have to type them everytime you start GHCi: eg. if your
1324 project uses GHC extensions and CPP, and has source files in three
1325 subdirectories A B and C, you might put the following lines in
1326 <filename>.ghci</filename>:</para>
1329 :set -fglasgow-exts -cpp
1333 <para>(Note that strictly speaking the <option>-i</option> flag is
1334 a static one, but in fact it works to set it using
1335 <literal>:set</literal> like this. The changes won't take effect
1336 until the next <literal>:load</literal>, though.)</para>
1338 <para>Two command-line options control whether the
1339 <filename>.ghci</filename> files are read:</para>
1344 <option>-ignore-dot-ghci</option>
1345 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
1348 <para>Don't read either <filename>./.ghci</filename> or
1349 <filename>$HOME/.ghci</filename> when starting up.</para>
1354 <option>-read-dot-ghci</option>
1355 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
1358 <para>Read <filename>.ghci</filename> and
1359 <filename>$HOME/.ghci</filename>. This is normally the
1360 default, but the <option>-read-dot-ghci</option> option may
1361 be used to override a previous
1362 <option>-ignore-dot-ghci</option> option.</para>
1370 <title>FAQ and Things To Watch Out For</title>
1374 <term>The interpreter can't load modules with foreign export
1375 declarations!</term>
1377 <para>Unfortunately not. We haven't implemented it yet.
1378 Please compile any offending modules by hand before loading
1379 them into GHCi.</para>
1385 <literal>-O</literal> doesn't work with GHCi!
1386 <indexterm><primary><option>-O</option></primary></indexterm>
1389 <para>For technical reasons, the bytecode compiler doesn't
1390 interact well with one of the optimisation passes, so we
1391 have disabled optimisation when using the interpreter. This
1392 isn't a great loss: you'll get a much bigger win by
1393 compiling the bits of your code that need to go fast, rather
1394 than interpreting them with optimisation turned on.</para>
1399 <term>Unboxed tuples don't work with GHCi</term>
1401 <para>That's right. You can always compile a module that
1402 uses unboxed tuples and load it into GHCi, however.
1403 (Incidentally the previous point, namely that
1404 <literal>-O</literal> is incompatible with GHCi, is because
1405 the bytecode compiler can't deal with unboxed
1411 <term>Concurrent threads don't carry on running when GHCi is
1412 waiting for input.</term>
1414 <para>No, they don't. This is because the Haskell binding
1415 to the GNU readline library doesn't support reading from the
1416 terminal in a non-blocking way, which is required to work
1417 properly with GHC's concurrency model.</para>
1422 <term>After using <literal>getContents</literal>, I can't use
1423 <literal>stdin</literal> again until I do
1424 <literal>:load</literal> or <literal>:reload</literal>.</term>
1427 <para>This is the defined behaviour of
1428 <literal>getContents</literal>: it puts the stdin Handle in
1429 a state known as <firstterm>semi-closed</firstterm>, wherein
1430 any further I/O operations on it are forbidden. Because I/O
1431 state is retained between computations, the semi-closed
1432 state persists until the next <literal>:load</literal> or
1433 <literal>:reload</literal> command.</para>
1435 <para>You can make <literal>stdin</literal> reset itself
1436 after every evaluation by giving GHCi the command
1437 <literal>:set +r</literal>. This works because
1438 <literal>stdin</literal> is just a top-level expression that
1439 can be reverted to its unevaluated state in the same way as
1440 any other top-level expression (CAF).</para>
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