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 :ctags [<file>] create tags file for Vi (default: "tags")
67 :etags [<file>] create tags file for Emacs (defauilt: "TAGS")
68 :type <expr> show the type of <expr>
69 :kind <type> show the kind of <type>
70 :undef <cmd> undefine user-defined command :<cmd>
71 :unset <option> ... unset options
73 :!<command> run the shell command <command>
75 Options for `:set' and `:unset':
77 +r revert top-level expressions after each evaluation
78 +s print timing/memory stats after each evaluation
79 +t print type after evaluation
80 -<flags> most GHC command line flags can also be set here
81 (eg. -v2, -fglasgow-exts, etc.)
84 <para>We'll explain most of these commands as we go along. For
85 Hugs users: many things work the same as in Hugs, so you should be
86 able to get going straight away.</para>
88 <para>Haskell expressions can be typed at the prompt:</para>
89 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
95 Prelude> let x = 42 in x / 9
100 <para>GHCi interprets the whole line as an expression to evaluate.
101 The expression may not span several lines - as soon as you press
102 enter, GHCi will attempt to evaluate it.</para>
106 <title>Loading source files</title>
108 <para>Suppose we have the following Haskell source code, which we
109 place in a file <filename>Main.hs</filename>:</para>
112 main = print (fac 20)
115 fac n = n * fac (n-1)
118 <para>You can save <filename>Main.hs</filename> anywhere you like,
119 but if you save it somewhere other than the current
120 directory<footnote><para>If you started up GHCi from the command
121 line then GHCi's current directory is the same as the current
122 directory of the shell from which it was started. If you started
123 GHCi from the “Start” menu in Windows, then the
124 current directory is probably something like
125 <filename>C:\Documents and Settings\<replaceable>user
126 name</replaceable></filename>.</para> </footnote> then we will
127 need to change to the right directory in GHCi:</para>
130 Prelude> :cd <replaceable>dir</replaceable>
133 <para>where <replaceable>dir</replaceable> is the directory (or
134 folder) in which you saved <filename>Main.hs</filename>.</para>
136 <para>To load a Haskell source file into GHCi, use the
137 <literal>:load</literal> command:</para>
138 <indexterm><primary><literal>:load</literal></primary></indexterm>
142 Compiling Main ( Main.hs, interpreted )
143 Ok, modules loaded: Main.
147 <para>GHCi has loaded the <literal>Main</literal> module, and the
148 prompt has changed to “<literal>*Main></literal>” to
149 indicate that the current context for expressions typed at the
150 prompt is the <literal>Main</literal> module we just loaded (we'll
151 explain what the <literal>*</literal> means later in <xref
152 linkend="ghci-scope"/>). So we can now type expressions involving
153 the functions from <filename>Main.hs</filename>:</para>
160 <para>Loading a multi-module program is just as straightforward;
161 just give the name of the “topmost” module to the
162 <literal>:load</literal> command (hint: <literal>:load</literal>
163 can be abbreviated to <literal>:l</literal>). The topmost module
164 will normally be <literal>Main</literal>, but it doesn't have to
165 be. GHCi will discover which modules are required, directly or
166 indirectly, by the topmost module, and load them all in dependency
169 <sect2 id="ghci-modules-filenames">
170 <title>Modules vs. filenames</title>
171 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
172 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
174 <para>Question: How does GHC find the filename which contains
175 module <replaceable>M</replaceable>? Answer: it looks for the
176 file <literal><replaceable>M</replaceable>.hs</literal>, or
177 <literal><replaceable>M</replaceable>.lhs</literal>. This means
178 that for most modules, the module name must match the filename.
179 If it doesn't, GHCi won't be able to find it.</para>
181 <para>There is one exception to this general rule: when you load
182 a program with <literal>:load</literal>, or specify it when you
183 invoke <literal>ghci</literal>, you can give a filename rather
184 than a module name. This filename is loaded if it exists, and
185 it may contain any module you like. This is particularly
186 convenient if you have several <literal>Main</literal> modules
187 in the same directory and you can't call them all
188 <filename>Main.hs</filename>.</para>
190 <para>The search path for finding source files is specified with
191 the <option>-i</option> option on the GHCi command line, like
193 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
195 <para>or it can be set using the <literal>:set</literal> command
196 from within GHCi (see <xref
197 linkend="ghci-cmd-line-options"/>)<footnote><para>Note that in
198 GHCi, and <option>––make</option> mode, the <option>-i</option>
199 option is used to specify the search path for
200 <emphasis>source</emphasis> files, whereas in standard
201 batch-compilation mode the <option>-i</option> option is used to
202 specify the search path for interface files, see <xref
203 linkend="search-path"/>.</para> </footnote></para>
205 <para>One consequence of the way that GHCi follows dependencies
206 to find modules to load is that every module must have a source
207 file. The only exception to the rule is modules that come from
208 a package, including the <literal>Prelude</literal> and standard
209 libraries such as <literal>IO</literal> and
210 <literal>Complex</literal>. If you attempt to load a module for
211 which GHCi can't find a source file, even if there are object
212 and interface files for the module, you'll get an error
217 <title>Making changes and recompilation</title>
218 <indexterm><primary><literal>:reload</literal></primary></indexterm>
220 <para>If you make some changes to the source code and want GHCi
221 to recompile the program, give the <literal>:reload</literal>
222 command. The program will be recompiled as necessary, with GHCi
223 doing its best to avoid actually recompiling modules if their
224 external dependencies haven't changed. This is the same
225 mechanism we use to avoid re-compiling modules in the batch
226 compilation setting (see <xref linkend="recomp"/>).</para>
230 <sect1 id="ghci-compiled">
231 <title>Loading compiled code</title>
232 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
234 <para>When you load a Haskell source module into GHCi, it is
235 normally converted to byte-code and run using the interpreter.
236 However, interpreted code can also run alongside compiled code in
237 GHCi; indeed, normally when GHCi starts, it loads up a compiled
238 copy of the <literal>base</literal> package, which contains the
239 <literal>Prelude</literal>.</para>
241 <para>Why should we want to run compiled code? Well, compiled
242 code is roughly 10x faster than interpreted code, but takes about
243 2x longer to produce (perhaps longer if optimisation is on). So
244 it pays to compile the parts of a program that aren't changing
245 very often, and use the interpreter for the code being actively
248 <para>When loading up source files with <literal>:load</literal>,
249 GHCi looks for any corresponding compiled object files, and will
250 use one in preference to interpreting the source if possible. For
251 example, suppose we have a 4-module program consisting of modules
252 A, B, C, and D. Modules B and C both import D only,
253 and A imports both B & C:</para>
261 <para>We can compile D, then load the whole program, like this:</para>
263 Prelude> :! ghc -c D.hs
265 Skipping D ( D.hs, D.o )
266 Compiling C ( C.hs, interpreted )
267 Compiling B ( B.hs, interpreted )
268 Compiling A ( A.hs, interpreted )
269 Ok, modules loaded: A, B, C, D.
273 <para>In the messages from the compiler, we see that it skipped D,
274 and used the object file <filename>D.o</filename>. The message
275 <literal>Skipping</literal> <replaceable>module</replaceable>
276 indicates that compilation for <replaceable>module</replaceable>
277 isn't necessary, because the source and everything it depends on
278 is unchanged since the last compilation.</para>
280 <para>At any time you can use the command
281 <literal>:show modules</literal>
282 to get a list of the modules currently loaded
288 C ( C.hs, interpreted )
289 B ( B.hs, interpreted )
290 A ( A.hs, interpreted )
293 <para>If we now modify the source of D (or pretend to: using Unix
294 command <literal>touch</literal> on the source file is handy for
295 this), the compiler will no longer be able to use the object file,
296 because it might be out of date:</para>
301 Compiling D ( D.hs, interpreted )
302 Skipping C ( C.hs, interpreted )
303 Skipping B ( B.hs, interpreted )
304 Skipping A ( A.hs, interpreted )
305 Ok, modules loaded: A, B, C, D.
309 <para>Note that module D was compiled, but in this instance
310 because its source hadn't really changed, its interface remained
311 the same, and the recompilation checker determined that A, B and C
312 didn't need to be recompiled.</para>
314 <para>So let's try compiling one of the other modules:</para>
317 *Main> :! ghc -c C.hs
319 Compiling D ( D.hs, interpreted )
320 Compiling C ( C.hs, interpreted )
321 Compiling B ( B.hs, interpreted )
322 Compiling A ( A.hs, interpreted )
323 Ok, modules loaded: A, B, C, D.
326 <para>We didn't get the compiled version of C! What happened?
327 Well, in GHCi a compiled module may only depend on other compiled
328 modules, and in this case C depends on D, which doesn't have an
329 object file, so GHCi also rejected C's object file. Ok, so let's
330 also compile D:</para>
333 *Main> :! ghc -c D.hs
335 Ok, modules loaded: A, B, C, D.
338 <para>Nothing happened! Here's another lesson: newly compiled
339 modules aren't picked up by <literal>:reload</literal>, only
340 <literal>:load</literal>:</para>
344 Skipping D ( D.hs, D.o )
345 Skipping C ( C.hs, C.o )
346 Compiling B ( B.hs, interpreted )
347 Compiling A ( A.hs, interpreted )
348 Ok, modules loaded: A, B, C, D.
351 <para>HINT: since GHCi will only use a compiled object file if it
352 can sure that the compiled version is up-to-date, a good technique
353 when working on a large program is to occasionally run
354 <literal>ghc ––make</literal> to compile the whole project (say
355 before you go for lunch :-), then continue working in the
356 interpreter. As you modify code, the new modules will be
357 interpreted, but the rest of the project will remain
363 <title>Interactive evaluation at the prompt</title>
365 <para>When you type an expression at the prompt, GHCi immediately
366 evaluates and prints the result:
368 Prelude> reverse "hello"
375 <sect2><title>I/O actions at the prompt</title>
377 <para>GHCi does more than simple expression evaluation at the prompt.
378 If you type something of type <literal>IO a</literal> for some
379 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
380 as an IO-computation.
384 Prelude> putStrLn "hello"
387 Furthermore, GHCi will print the result of the I/O action if (and only
390 <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
391 <listitem><para>The result type is not
392 <literal>()</literal>.</para></listitem>
394 For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
396 Prelude> putStrLn "hello"
398 Prelude> do { putStrLn "hello"; return "yes" }
405 <title>Using <literal>do-</literal>notation at the prompt</title>
406 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
407 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
409 <para>GHCi actually accepts <firstterm>statements</firstterm>
410 rather than just expressions at the prompt. This means you can
411 bind values and functions to names, and use them in future
412 expressions or statements.</para>
414 <para>The syntax of a statement accepted at the GHCi prompt is
415 exactly the same as the syntax of a statement in a Haskell
416 <literal>do</literal> expression. However, there's no monad
417 overloading here: statements typed at the prompt must be in the
418 <literal>IO</literal> monad.
420 Prelude> x <- return 42
426 The statement <literal>x <- return 42</literal> means
427 “execute <literal>return 42</literal> in the
428 <literal>IO</literal> monad, and bind the result to
429 <literal>x</literal>”. We can then use
430 <literal>x</literal> in future statements, for example to print
431 it as we did above.</para>
433 <para>GHCi will print the result of a statement if and only if:
436 <para>The statement is not a binding, or it is a monadic binding
437 (<literal>p <- e</literal>) that binds exactly one
441 <para>The variable's type is not polymorphic, is not
442 <literal>()</literal>, and is an instance of
443 <literal>Show</literal></para>
448 <para>Of course, you can also bind normal non-IO expressions
449 using the <literal>let</literal>-statement:</para>
456 <para>Another important difference between the two types of binding
457 is that the monadic bind (<literal>p <- e</literal>) is
458 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
459 whereas with the <literal>let</literal> form, the expression
460 isn't evaluated immediately:</para>
462 Prelude> let x = error "help!"
468 <para>Note that <literal>let</literal> bindings do not automatically
469 print the value bound, unlike monadic bindings.</para>
471 <para>Any exceptions raised during the evaluation or execution
472 of the statement are caught and printed by the GHCi command line
473 interface (for more information on exceptions, see the module
474 <literal>Control.Exception</literal> in the libraries
475 documentation).</para>
477 <para>Every new binding shadows any existing bindings of the
478 same name, including entities that are in scope in the current
479 module context.</para>
481 <para>WARNING: temporary bindings introduced at the prompt only
482 last until the next <literal>:load</literal> or
483 <literal>:reload</literal> command, at which time they will be
484 simply lost. However, they do survive a change of context with
485 <literal>:module</literal>: the temporary bindings just move to
486 the new location.</para>
488 <para>HINT: To get a list of the bindings currently in scope, use the
489 <literal>:show bindings</literal> command:</para>
492 Prelude> :show bindings
496 <para>HINT: if you turn on the <literal>+t</literal> option,
497 GHCi will show the type of each variable bound by a statement.
499 <indexterm><primary><literal>+t</literal></primary></indexterm>
502 Prelude> let (x:xs) = [1..]
509 <sect2 id="ghci-scope">
510 <title>What's really in scope at the prompt?</title>
512 <para>When you type an expression at the prompt, what
513 identifiers and types are in scope? GHCi provides a flexible
514 way to control exactly how the context for an expression is
515 constructed. Let's start with the simple cases; when you start
516 GHCi the prompt looks like this:</para>
518 <screen>Prelude></screen>
520 <para>Which indicates that everything from the module
521 <literal>Prelude</literal> is currently in scope. If we now
522 load a file into GHCi, the prompt will change:</para>
525 Prelude> :load Main.hs
526 Compiling Main ( Main.hs, interpreted )
530 <para>The new prompt is <literal>*Main</literal>, which
531 indicates that we are typing expressions in the context of the
532 top-level of the <literal>Main</literal> module. Everything
533 that is in scope at the top-level in the module
534 <literal>Main</literal> we just loaded is also in scope at the
535 prompt (probably including <literal>Prelude</literal>, as long
536 as <literal>Main</literal> doesn't explicitly hide it).</para>
539 <literal>*<replaceable>module</replaceable></literal> indicates
540 that it is the full top-level scope of
541 <replaceable>module</replaceable> that is contributing to the
542 scope for expressions typed at the prompt. Without the
543 <literal>*</literal>, just the exports of the module are
546 <para>We're not limited to a single module: GHCi can combine
547 scopes from multiple modules, in any mixture of
548 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
549 combines the scopes from all of these modules to form the scope
550 that is in effect at the prompt. For technical reasons, GHCi
551 can only support the <literal>*</literal>-form for modules which
552 are interpreted, so compiled modules and package modules can
553 only contribute their exports to the current scope.</para>
555 <para>The scope is manipulated using the
556 <literal>:module</literal> command. For example, if the current
557 scope is <literal>Prelude</literal>, then we can bring into
558 scope the exports from the module <literal>IO</literal> like
563 Prelude,IO> hPutStrLn stdout "hello\n"
568 <para>(Note: <literal>:module</literal> can be shortened to
569 <literal>:m</literal>). The full syntax of the
570 <literal>:module</literal> command is:</para>
573 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
576 <para>Using the <literal>+</literal> form of the
577 <literal>module</literal> commands adds modules to the current
578 scope, and <literal>-</literal> removes them. Without either
579 <literal>+</literal> or <literal>-</literal>, the current scope
580 is replaced by the set of modules specified. Note that if you
581 use this form and leave out <literal>Prelude</literal>, GHCi
582 will assume that you really wanted the
583 <literal>Prelude</literal> and add it in for you (if you don't
584 want the <literal>Prelude</literal>, then ask to remove it with
585 <literal>:m -Prelude</literal>).</para>
587 <para>The scope is automatically set after a
588 <literal>:load</literal> command, to the most recently loaded
589 "target" module, in a <literal>*</literal>-form if possible.
590 For example, if you say <literal>:load foo.hs bar.hs</literal>
591 and <filename>bar.hs</filename> contains module
592 <literal>Bar</literal>, then the scope will be set to
593 <literal>*Bar</literal> if <literal>Bar</literal> is
594 interpreted, or if <literal>Bar</literal> is compiled it will be
595 set to <literal>Prelude,Bar</literal> (GHCi automatically adds
596 <literal>Prelude</literal> if it isn't present and there aren't
597 any <literal>*</literal>-form modules).</para>
599 <para>With multiple modules in scope, especially multiple
600 <literal>*</literal>-form modules, it is likely that name
601 clashes will occur. Haskell specifies that name clashes are
602 only reported when an ambiguous identifier is used, and GHCi
603 behaves in the same way for expressions typed at the
607 <title>Qualified names</title>
609 <para>To make life slightly easier, the GHCi prompt also
610 behaves as if there is an implicit <literal>import
611 qualified</literal> declaration for every module in every
612 package, and every module currently loaded into GHCi.</para>
618 <title>The <literal>it</literal> variable</title>
619 <indexterm><primary><literal>it</literal></primary>
622 <para>Whenever an expression (or a non-binding statement, to be
623 precise) is typed at the prompt, GHCi implicitly binds its value
624 to the variable <literal>it</literal>. For example:</para>
631 <para>What actually happens is that GHCi typechecks the
632 expression, and if it doesn't have an <literal>IO</literal> type,
633 then it transforms it as follows: an expression
634 <replaceable>e</replaceable> turns into
636 let it = <replaceable>e</replaceable>;
639 which is then run as an IO-action.</para>
641 <para>Hence, the original expression must have a type which is an
642 instance of the <literal>Show</literal> class, or GHCi will
647 No instance for `Show (a -> a)'
648 arising from use of `print'
649 in a `do' expression pattern binding: print it
652 <para>The error message contains some clues as to the
653 transformation happening internally.</para>
655 <para>If the expression was instead of type <literal>IO a</literal> for
656 some <literal>a</literal>, then <literal>it</literal> will be
657 bound to the result of the <literal>IO</literal> computation,
658 which is of type <literal>a</literal>. eg.:</para>
660 Prelude> Time.getClockTime
662 Wed Mar 14 12:23:13 GMT 2001
665 <para>The corresponding translation for an IO-typed
666 <replaceable>e</replaceable> is
668 it <- <replaceable>e</replaceable>
672 <para>Note that <literal>it</literal> is shadowed by the new
673 value each time you evaluate a new expression, and the old value
674 of <literal>it</literal> is lost.</para>
679 <title>Type defaulting in GHCi</title>
680 <indexterm><primary>Type default</primary></indexterm>
681 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
683 Consider this GHCi session:
687 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
688 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
689 on the type <literal>a</literal>. For example:
691 ghci> (reverse []) :: String
693 ghci> (reverse []) :: [Int]
696 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
697 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
698 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
699 a)</literal> for each type variable <literal>a</literal>, and defaults the
702 <listitem><para> The type variable <literal>a</literal>
703 appears in no other constraints </para></listitem>
704 <listitem><para> All the classes <literal>Ci</literal> are standard.</para></listitem>
705 <listitem><para> At least one of the classes <literal>Ci</literal> is
706 numeric.</para></listitem>
708 At the GHCi prompt, the second and third rules are relaxed as follows
709 (differences italicised):
711 <listitem><para> <emphasis>All</emphasis> of the classes
712 <literal>Ci</literal> are single-parameter type classes.</para></listitem>
713 <listitem><para> At least one of the classes <literal>Ci</literal> is
714 numeric, <emphasis>or is <literal>Show</literal>,
715 <literal>Eq</literal>, or <literal>Ord</literal></emphasis>.</para></listitem>
721 <sect1 id="ghci-invocation">
722 <title>Invoking GHCi</title>
723 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
724 <indexterm><primary><option>––interactive</option></primary></indexterm>
726 <para>GHCi is invoked with the command <literal>ghci</literal> or
727 <literal>ghc ––interactive</literal>. One or more modules or
728 filenames can also be specified on the command line; this
729 instructs GHCi to load the specified modules or filenames (and all
730 the modules they depend on), just as if you had said
731 <literal>:load <replaceable>modules</replaceable></literal> at the
732 GHCi prompt (see <xref linkend="ghci-commands"/>). For example, to
733 start GHCi and load the program whose topmost module is in the
734 file <literal>Main.hs</literal>, we could say:</para>
740 <para>Most of the command-line options accepted by GHC (see <xref
741 linkend="using-ghc"/>) also make sense in interactive mode. The ones
742 that don't make sense are mostly obvious; for example, GHCi
743 doesn't generate interface files, so options related to interface
744 file generation won't have any effect.</para>
747 <title>Packages</title>
748 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
750 <para>Most packages (see <xref linkend="using-packages"/>) are
751 available without needing to specify any extra flags at all:
752 they will be automatically loaded the first time they are
755 <para>For non-auto packages, however, you need to request the
756 package be loaded by using the <literal>-package</literal> flag:</para>
762 / /_\// /_/ / / | | GHC Interactive, version 5.05, for Haskell 98.
763 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
764 \____/\/ /_/\____/|_| Type :? for help.
766 Loading package base ... linking ... done.
767 Loading package haskell98 ... linking ... done.
768 Loading package lang ... linking ... done.
769 Loading package concurrent ... linking ... done.
770 Loading package readline ... linking ... done.
771 Loading package unix ... linking ... done.
772 Loading package posix ... linking ... done.
773 Loading package util ... linking ... done.
774 Loading package data ... linking ... done.
778 <para>The following command works to load new packages into a
782 Prelude> :set -package <replaceable>name</replaceable>
785 <para>But note that doing this will cause all currently loaded
786 modules to be unloaded, and you'll be dumped back into the
787 <literal>Prelude</literal>.</para>
791 <title>Extra libraries</title>
792 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
794 <para>Extra libraries may be specified on the command line using
795 the normal <literal>-l<replaceable>lib</replaceable></literal>
796 option. (The term <emphasis>library</emphasis> here refers to
797 libraries of foreign object code; for using libraries of Haskell
798 source code, see <xref linkend="ghci-modules-filenames"/>.) For
799 example, to load the “m” library:</para>
805 <para>On systems with <literal>.so</literal>-style shared
806 libraries, the actual library loaded will the
807 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
808 searches the following places for libraries, in this order:</para>
812 <para>Paths specified using the
813 <literal>-L<replaceable>path</replaceable></literal>
814 command-line option,</para>
817 <para>the standard library search path for your system,
818 which on some systems may be overridden by setting the
819 <literal>LD_LIBRARY_PATH</literal> environment
824 <para>On systems with <literal>.dll</literal>-style shared
825 libraries, the actual library loaded will be
826 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
827 GHCi will signal an error if it can't find the library.</para>
829 <para>GHCi can also load plain object files
830 (<literal>.o</literal> or <literal>.obj</literal> depending on
831 your platform) from the command-line. Just add the name the
832 object file to the command line.</para>
834 <para>Ordering of <option>-l</option> options matters: a library
835 should be mentioned <emphasis>before</emphasis> the libraries it
836 depends on (see <xref linkend="options-linker"/>).</para>
841 <sect1 id="ghci-commands">
842 <title>GHCi commands</title>
844 <para>GHCi commands all begin with
845 ‘<literal>:</literal>’ and consist of a single command
846 name followed by zero or more parameters. The command name may be
847 abbreviated, as long as the abbreviation is not ambiguous. All of
848 the builtin commands, with the exception of
849 <literal>:unset</literal> and <literal>:undef</literal>, may be
850 abbreviated to a single letter.</para>
855 <literal>:add</literal> <replaceable>module</replaceable> ...
856 <indexterm><primary><literal>:add</literal></primary></indexterm>
859 <para>Add <replaceable>module</replaceable>(s) to the
860 current <firstterm>target set</firstterm>, and perform a
867 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
868 <indexterm><primary><literal>:browse</literal></primary></indexterm>
871 <para>Displays the identifiers defined by the module
872 <replaceable>module</replaceable>, which must be either
873 loaded into GHCi or be a member of a package. If the
874 <literal>*</literal> symbol is placed before the module
875 name, then <emphasis>all</emphasis> the identifiers defined
876 in <replaceable>module</replaceable> are shown; otherwise
877 the list is limited to the exports of
878 <replaceable>module</replaceable>. The
879 <literal>*</literal>-form is only available for modules
880 which are interpreted; for compiled modules (including
881 modules from packages) only the non-<literal>*</literal>
882 form of <literal>:browse</literal> is available.</para>
888 <literal>:cd</literal> <replaceable>dir</replaceable>
889 <indexterm><primary><literal>:cd</literal></primary></indexterm>
892 <para>Changes the current working directory to
893 <replaceable>dir</replaceable>. A
894 ‘<literal>˜</literal>’ symbol at the
895 beginning of <replaceable>dir</replaceable> will be replaced
896 by the contents of the environment variable
897 <literal>HOME</literal>.</para>
899 <para>NOTE: changing directories causes all currently loaded
900 modules to be unloaded. This is because the search path is
901 usually expressed using relative directories, and changing
902 the search path in the middle of a session is not
909 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
910 <indexterm><primary><literal>:def</literal></primary></indexterm>
913 <para>The command <literal>:def</literal>
914 <replaceable>name</replaceable>
915 <replaceable>expr</replaceable> defines a new GHCi command
916 <literal>:<replaceable>name</replaceable></literal>,
917 implemented by the Haskell expression
918 <replaceable>expr</replaceable>, which must have type
919 <literal>String -> IO String</literal>. When
920 <literal>:<replaceable>name</replaceable>
921 <replaceable>args</replaceable></literal> is typed at the
922 prompt, GHCi will run the expression
923 <literal>(<replaceable>name</replaceable>
924 <replaceable>args</replaceable>)</literal>, take the
925 resulting <literal>String</literal>, and feed it back into
926 GHCi as a new sequence of commands. Separate commands in
927 the result must be separated by
928 ‘<literal>\n</literal>’.</para>
930 <para>That's all a little confusing, so here's a few
931 examples. To start with, here's a new GHCi command which
932 doesn't take any arguments or produce any results, it just
933 outputs the current date & time:</para>
936 Prelude> let date _ = Time.getClockTime >>= print >> return ""
937 Prelude> :def date date
939 Fri Mar 23 15:16:40 GMT 2001
942 <para>Here's an example of a command that takes an argument.
943 It's a re-implementation of <literal>:cd</literal>:</para>
946 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
947 Prelude> :def mycd mycd
951 <para>Or I could define a simple way to invoke
952 “<literal>ghc ––make Main</literal>” in the
953 current directory:</para>
956 Prelude> :def make (\_ -> return ":! ghc ––make Main")
959 <para>We can define a command that reads GHCi input from a
960 file. This might be useful for creating a set of bindings
961 that we want to repeatedly load into the GHCi session:</para>
964 Prelude> :def . readFile
965 Prelude> :. cmds.ghci
968 <para>Notice that we named the command
969 <literal>:.</literal>, by analogy with the
970 ‘<literal>.</literal>’ Unix shell command that
971 does the same thing.</para>
977 <literal>:help</literal>
978 <indexterm><primary><literal>:help</literal></primary></indexterm>
981 <literal>:?</literal>
982 <indexterm><primary><literal>:?</literal></primary></indexterm>
985 <para>Displays a list of the available commands.</para>
991 <literal>:info</literal> <replaceable>name</replaceable> ...
992 <indexterm><primary><literal>:info</literal></primary></indexterm>
995 <para>Displays information about the given name(s). For
996 example, if <replaceable>name</replaceable> is a class, then
997 the class methods and their types will be printed; if
998 <replaceable>name</replaceable> is a type constructor, then
999 its definition will be printed; if
1000 <replaceable>name</replaceable> is a function, then its type
1001 will be printed. If <replaceable>name</replaceable> has
1002 been loaded from a source file, then GHCi will also display
1003 the location of its definition in the source.</para>
1009 <literal>:load</literal> <replaceable>module</replaceable> ...
1010 <indexterm><primary><literal>:load</literal></primary></indexterm>
1013 <para>Recursively loads the specified
1014 <replaceable>module</replaceable>s, and all the modules they
1015 depend on. Here, each <replaceable>module</replaceable>
1016 must be a module name or filename, but may not be the name
1017 of a module in a package.</para>
1019 <para>All previously loaded modules, except package modules,
1020 are forgotten. The new set of modules is known as the
1021 <firstterm>target set</firstterm>. Note that
1022 <literal>:load</literal> can be used without any arguments
1023 to unload all the currently loaded modules and
1026 <para>After a <literal>:load</literal> command, the current
1027 context is set to:</para>
1031 <para><replaceable>module</replaceable>, if it was loaded
1032 successfully, or</para>
1035 <para>the most recently successfully loaded module, if
1036 any other modules were loaded as a result of the current
1037 <literal>:load</literal>, or</para>
1040 <para><literal>Prelude</literal> otherwise.</para>
1048 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
1049 <indexterm><primary><literal>:module</literal></primary></indexterm>
1052 <para>Sets or modifies the current context for statements
1053 typed at the prompt. See <xref linkend="ghci-scope"/> for
1054 more details.</para>
1060 <literal>:quit</literal>
1061 <indexterm><primary><literal>:quit</literal></primary></indexterm>
1064 <para>Quits GHCi. You can also quit by typing a control-D
1065 at the prompt.</para>
1071 <literal>:reload</literal>
1072 <indexterm><primary><literal>:reload</literal></primary></indexterm>
1075 <para>Attempts to reload the current target set (see
1076 <literal>:load</literal>) if any of the modules in the set,
1077 or any dependent module, has changed. Note that this may
1078 entail loading new modules, or dropping modules which are no
1079 longer indirectly required by the target.</para>
1085 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
1086 <indexterm><primary><literal>:set</literal></primary></indexterm>
1089 <para>Sets various options. See <xref linkend="ghci-set"/>
1090 for a list of available options. The
1091 <literal>:set</literal> command by itself shows which
1092 options are currently set.</para>
1098 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
1099 <indexterm><primary><literal>:set</literal></primary></indexterm>
1102 <para>Sets the list of arguments which are returned when the
1103 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
1104 </indexterm>.</para>
1110 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
1111 <indexterm><primary><literal>:set</literal></primary></indexterm>
1114 <para>Sets the string to be returned when the program calls
1115 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
1116 </indexterm>.</para>
1122 <literal>:show bindings</literal>
1123 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
1126 <para>Show the bindings made at the prompt and their
1133 <literal>:show modules</literal>
1134 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
1137 <para>Show the list of modules currently load.</para>
1143 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1144 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1145 <indexterm><primary><literal>:etags</literal></primary>
1147 <indexterm><primary><literal>:etags</literal></primary>
1151 <para>Generates a “tags” file for Vi-style editors
1152 (<literal>:ctags</literal>) or Emacs-style editors (<literal>etags</literal>). If
1153 no filename is specified, the defaulit <filename>tags</filename> or
1154 <filename>TAGS</filename> is
1155 used, respectively. Tags for all the functions, constructors and
1156 types in the currently loaded modules are created. All modules must
1157 be interpreted for these commands to work.</para>
1158 <para>See also <xref linkend="hasktags" />.</para>
1164 <literal>:type</literal> <replaceable>expression</replaceable>
1165 <indexterm><primary><literal>:type</literal></primary></indexterm>
1168 <para>Infers and prints the type of
1169 <replaceable>expression</replaceable>, including explicit
1170 forall quantifiers for polymorphic types. The monomorphism
1171 restriction is <emphasis>not</emphasis> applied to the
1172 expression during type inference.</para>
1178 <literal>:kind</literal> <replaceable>type</replaceable>
1179 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1182 <para>Infers and prints the kind of
1183 <replaceable>type</replaceable>. The latter can be an arbitrary
1184 type expression, including a partial application of a type constructor,
1185 such as <literal>Either Int</literal>.</para>
1191 <literal>:undef</literal> <replaceable>name</replaceable>
1192 <indexterm><primary><literal>:undef</literal></primary></indexterm>
1195 <para>Undefines the user-defined command
1196 <replaceable>name</replaceable> (see <literal>:def</literal>
1203 <literal>:unset</literal> <replaceable>option</replaceable>...
1204 <indexterm><primary><literal>:unset</literal></primary></indexterm>
1207 <para>Unsets certain options. See <xref linkend="ghci-set"/>
1208 for a list of available options.</para>
1214 <literal>:!</literal> <replaceable>command</replaceable>...
1215 <indexterm><primary><literal>:!</literal></primary></indexterm>
1216 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
1219 <para>Executes the shell command
1220 <replaceable>command</replaceable>.</para>
1227 <sect1 id="ghci-set">
1228 <title>The <literal>:set</literal> command</title>
1229 <indexterm><primary><literal>:set</literal></primary></indexterm>
1231 <para>The <literal>:set</literal> command sets two types of
1232 options: GHCi options, which begin with
1233 ‘<literal>+</literal>” and “command-line”
1234 options, which begin with ‘-’. </para>
1236 <para>NOTE: at the moment, the <literal>:set</literal> command
1237 doesn't support any kind of quoting in its arguments: quotes will
1238 not be removed and cannot be used to group words together. For
1239 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
1243 <title>GHCi options</title>
1244 <indexterm><primary>options</primary><secondary>GHCi</secondary>
1247 <para>GHCi options may be set using <literal>:set</literal> and
1248 unset using <literal>:unset</literal>.</para>
1250 <para>The available GHCi options are:</para>
1255 <literal>+r</literal>
1256 <indexterm><primary><literal>+r</literal></primary></indexterm>
1257 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
1258 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
1261 <para>Normally, any evaluation of top-level expressions
1262 (otherwise known as CAFs or Constant Applicative Forms) in
1263 loaded modules is retained between evaluations. Turning
1264 on <literal>+r</literal> causes all evaluation of
1265 top-level expressions to be discarded after each
1266 evaluation (they are still retained
1267 <emphasis>during</emphasis> a single evaluation).</para>
1269 <para>This option may help if the evaluated top-level
1270 expressions are consuming large amounts of space, or if
1271 you need repeatable performance measurements.</para>
1277 <literal>+s</literal>
1278 <indexterm><primary><literal>+s</literal></primary></indexterm>
1281 <para>Display some stats after evaluating each expression,
1282 including the elapsed time and number of bytes allocated.
1283 NOTE: the allocation figure is only accurate to the size
1284 of the storage manager's allocation area, because it is
1285 calculated at every GC. Hence, you might see values of
1286 zero if no GC has occurred.</para>
1292 <literal>+t</literal>
1293 <indexterm><primary><literal>+t</literal></primary></indexterm>
1296 <para>Display the type of each variable bound after a
1297 statement is entered at the prompt. If the statement is a
1298 single expression, then the only variable binding will be
1300 ‘<literal>it</literal>’.</para>
1306 <sect2 id="ghci-cmd-line-options">
1307 <title>Setting GHC command-line options in GHCi</title>
1309 <para>Normal GHC command-line options may also be set using
1310 <literal>:set</literal>. For example, to turn on
1311 <option>-fglasgow-exts</option>, you would say:</para>
1314 Prelude> :set -fglasgow-exts
1317 <para>Any GHC command-line option that is designated as
1318 <firstterm>dynamic</firstterm> (see the table in <xref
1319 linkend="flag-reference"/>), may be set using
1320 <literal>:set</literal>. To unset an option, you can set the
1321 reverse option:</para>
1322 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
1325 Prelude> :set -fno-glasgow-exts
1328 <para><xref linkend="flag-reference"/> lists the reverse for each
1329 option where applicable.</para>
1331 <para>Certain static options (<option>-package</option>,
1332 <option>-I</option>, <option>-i</option>, and
1333 <option>-l</option> in particular) will also work, but some may
1334 not take effect until the next reload.</para>
1335 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
1339 <sect1 id="ghci-dot-files">
1340 <title>The <filename>.ghci</filename> file</title>
1341 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
1343 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
1346 <para>When it starts, GHCi always reads and executes commands from
1347 <filename>$HOME/.ghci</filename>, followed by
1348 <filename>./.ghci</filename>.</para>
1350 <para>The <filename>.ghci</filename> in your home directory is
1351 most useful for turning on favourite options (eg. <literal>:set
1352 +s</literal>), and defining useful macros. Placing a
1353 <filename>.ghci</filename> file in a directory with a Haskell
1354 project is a useful way to set certain project-wide options so you
1355 don't have to type them everytime you start GHCi: eg. if your
1356 project uses GHC extensions and CPP, and has source files in three
1357 subdirectories A B and C, you might put the following lines in
1358 <filename>.ghci</filename>:</para>
1361 :set -fglasgow-exts -cpp
1365 <para>(Note that strictly speaking the <option>-i</option> flag is
1366 a static one, but in fact it works to set it using
1367 <literal>:set</literal> like this. The changes won't take effect
1368 until the next <literal>:load</literal>, though.)</para>
1370 <para>Two command-line options control whether the
1371 <filename>.ghci</filename> files are read:</para>
1376 <option>-ignore-dot-ghci</option>
1377 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
1380 <para>Don't read either <filename>./.ghci</filename> or
1381 <filename>$HOME/.ghci</filename> when starting up.</para>
1386 <option>-read-dot-ghci</option>
1387 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
1390 <para>Read <filename>.ghci</filename> and
1391 <filename>$HOME/.ghci</filename>. This is normally the
1392 default, but the <option>-read-dot-ghci</option> option may
1393 be used to override a previous
1394 <option>-ignore-dot-ghci</option> option.</para>
1402 <title>FAQ and Things To Watch Out For</title>
1406 <term>The interpreter can't load modules with foreign export
1407 declarations!</term>
1409 <para>Unfortunately not. We haven't implemented it yet.
1410 Please compile any offending modules by hand before loading
1411 them into GHCi.</para>
1417 <literal>-O</literal> doesn't work with GHCi!
1418 <indexterm><primary><option>-O</option></primary></indexterm>
1421 <para>For technical reasons, the bytecode compiler doesn't
1422 interact well with one of the optimisation passes, so we
1423 have disabled optimisation when using the interpreter. This
1424 isn't a great loss: you'll get a much bigger win by
1425 compiling the bits of your code that need to go fast, rather
1426 than interpreting them with optimisation turned on.</para>
1431 <term>Unboxed tuples don't work with GHCi</term>
1433 <para>That's right. You can always compile a module that
1434 uses unboxed tuples and load it into GHCi, however.
1435 (Incidentally the previous point, namely that
1436 <literal>-O</literal> is incompatible with GHCi, is because
1437 the bytecode compiler can't deal with unboxed
1443 <term>Concurrent threads don't carry on running when GHCi is
1444 waiting for input.</term>
1446 <para>No, they don't. This is because the Haskell binding
1447 to the GNU readline library doesn't support reading from the
1448 terminal in a non-blocking way, which is required to work
1449 properly with GHC's concurrency model.</para>
1454 <term>After using <literal>getContents</literal>, I can't use
1455 <literal>stdin</literal> again until I do
1456 <literal>:load</literal> or <literal>:reload</literal>.</term>
1459 <para>This is the defined behaviour of
1460 <literal>getContents</literal>: it puts the stdin Handle in
1461 a state known as <firstterm>semi-closed</firstterm>, wherein
1462 any further I/O operations on it are forbidden. Because I/O
1463 state is retained between computations, the semi-closed
1464 state persists until the next <literal>:load</literal> or
1465 <literal>:reload</literal> command.</para>
1467 <para>You can make <literal>stdin</literal> reset itself
1468 after every evaluation by giving GHCi the command
1469 <literal>:set +r</literal>. This works because
1470 <literal>stdin</literal> is just a top-level expression that
1471 can be reverted to its unevaluated state in the same way as
1472 any other top-level expression (CAF).</para>
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