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 :tags -e|-c create tags file for Vi (-c) or Emacs (-e)
67 :type <expr> show the type of <expr>
68 :kind <type> show the kind of <type>
69 :undef <cmd> undefine user-defined command :<cmd>
70 :unset <option> ... unset options
72 :!<command> run the shell command <command>
74 Options for `:set' and `:unset':
76 +r revert top-level expressions after each evaluation
77 +s print timing/memory stats after each evaluation
78 +t print type after evaluation
79 -<flags> most GHC command line flags can also be set here
80 (eg. -v2, -fglasgow-exts, etc.)
83 <para>We'll explain most of these commands as we go along. For
84 Hugs users: many things work the same as in Hugs, so you should be
85 able to get going straight away.</para>
87 <para>Haskell expressions can be typed at the prompt:</para>
88 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
94 Prelude> let x = 42 in x / 9
99 <para>GHCi interprets the whole line as an expression to evaluate.
100 The expression may not span several lines - as soon as you press
101 enter, GHCi will attempt to evaluate it.</para>
105 <title>Loading source files</title>
107 <para>Suppose we have the following Haskell source code, which we
108 place in a file <filename>Main.hs</filename>:</para>
111 main = print (fac 20)
114 fac n = n * fac (n-1)
117 <para>You can save <filename>Main.hs</filename> anywhere you like,
118 but if you save it somewhere other than the current
119 directory<footnote><para>If you started up GHCi from the command
120 line then GHCi's current directory is the same as the current
121 directory of the shell from which it was started. If you started
122 GHCi from the “Start” menu in Windows, then the
123 current directory is probably something like
124 <filename>C:\Documents and Settings\<replaceable>user
125 name</replaceable></filename>.</para> </footnote> then we will
126 need to change to the right directory in GHCi:</para>
129 Prelude> :cd <replaceable>dir</replaceable>
132 <para>where <replaceable>dir</replaceable> is the directory (or
133 folder) in which you saved <filename>Main.hs</filename>.</para>
135 <para>To load a Haskell source file into GHCi, use the
136 <literal>:load</literal> command:</para>
137 <indexterm><primary><literal>:load</literal></primary></indexterm>
141 Compiling Main ( Main.hs, interpreted )
142 Ok, modules loaded: Main.
146 <para>GHCi has loaded the <literal>Main</literal> module, and the
147 prompt has changed to “<literal>*Main></literal>” to
148 indicate that the current context for expressions typed at the
149 prompt is the <literal>Main</literal> module we just loaded (we'll
150 explain what the <literal>*</literal> means later in <xref
151 linkend="ghci-scope"/>). So we can now type expressions involving
152 the functions from <filename>Main.hs</filename>:</para>
159 <para>Loading a multi-module program is just as straightforward;
160 just give the name of the “topmost” module to the
161 <literal>:load</literal> command (hint: <literal>:load</literal>
162 can be abbreviated to <literal>:l</literal>). The topmost module
163 will normally be <literal>Main</literal>, but it doesn't have to
164 be. GHCi will discover which modules are required, directly or
165 indirectly, by the topmost module, and load them all in dependency
168 <sect2 id="ghci-modules-filenames">
169 <title>Modules vs. filenames</title>
170 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
171 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
173 <para>Question: How does GHC find the filename which contains
174 module <replaceable>M</replaceable>? Answer: it looks for the
175 file <literal><replaceable>M</replaceable>.hs</literal>, or
176 <literal><replaceable>M</replaceable>.lhs</literal>. This means
177 that for most modules, the module name must match the filename.
178 If it doesn't, GHCi won't be able to find it.</para>
180 <para>There is one exception to this general rule: when you load
181 a program with <literal>:load</literal>, or specify it when you
182 invoke <literal>ghci</literal>, you can give a filename rather
183 than a module name. This filename is loaded if it exists, and
184 it may contain any module you like. This is particularly
185 convenient if you have several <literal>Main</literal> modules
186 in the same directory and you can't call them all
187 <filename>Main.hs</filename>.</para>
189 <para>The search path for finding source files is specified with
190 the <option>-i</option> option on the GHCi command line, like
192 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
194 <para>or it can be set using the <literal>:set</literal> command
195 from within GHCi (see <xref
196 linkend="ghci-cmd-line-options"/>)<footnote><para>Note that in
197 GHCi, and <option>––make</option> mode, the <option>-i</option>
198 option is used to specify the search path for
199 <emphasis>source</emphasis> files, whereas in standard
200 batch-compilation mode the <option>-i</option> option is used to
201 specify the search path for interface files, see <xref
202 linkend="search-path"/>.</para> </footnote></para>
204 <para>One consequence of the way that GHCi follows dependencies
205 to find modules to load is that every module must have a source
206 file. The only exception to the rule is modules that come from
207 a package, including the <literal>Prelude</literal> and standard
208 libraries such as <literal>IO</literal> and
209 <literal>Complex</literal>. If you attempt to load a module for
210 which GHCi can't find a source file, even if there are object
211 and interface files for the module, you'll get an error
216 <title>Making changes and recompilation</title>
217 <indexterm><primary><literal>:reload</literal></primary></indexterm>
219 <para>If you make some changes to the source code and want GHCi
220 to recompile the program, give the <literal>:reload</literal>
221 command. The program will be recompiled as necessary, with GHCi
222 doing its best to avoid actually recompiling modules if their
223 external dependencies haven't changed. This is the same
224 mechanism we use to avoid re-compiling modules in the batch
225 compilation setting (see <xref linkend="recomp"/>).</para>
229 <sect1 id="ghci-compiled">
230 <title>Loading compiled code</title>
231 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
233 <para>When you load a Haskell source module into GHCi, it is
234 normally converted to byte-code and run using the interpreter.
235 However, interpreted code can also run alongside compiled code in
236 GHCi; indeed, normally when GHCi starts, it loads up a compiled
237 copy of the <literal>base</literal> package, which contains the
238 <literal>Prelude</literal>.</para>
240 <para>Why should we want to run compiled code? Well, compiled
241 code is roughly 10x faster than interpreted code, but takes about
242 2x longer to produce (perhaps longer if optimisation is on). So
243 it pays to compile the parts of a program that aren't changing
244 very often, and use the interpreter for the code being actively
247 <para>When loading up source files with <literal>:load</literal>,
248 GHCi looks for any corresponding compiled object files, and will
249 use one in preference to interpreting the source if possible. For
250 example, suppose we have a 4-module program consisting of modules
251 A, B, C, and D. Modules B and C both import D only,
252 and A imports both B & C:</para>
260 <para>We can compile D, then load the whole program, like this:</para>
262 Prelude> :! ghc -c D.hs
264 Skipping D ( D.hs, D.o )
265 Compiling C ( C.hs, interpreted )
266 Compiling B ( B.hs, interpreted )
267 Compiling A ( A.hs, interpreted )
268 Ok, modules loaded: A, B, C, D.
272 <para>In the messages from the compiler, we see that it skipped D,
273 and used the object file <filename>D.o</filename>. The message
274 <literal>Skipping</literal> <replaceable>module</replaceable>
275 indicates that compilation for <replaceable>module</replaceable>
276 isn't necessary, because the source and everything it depends on
277 is unchanged since the last compilation.</para>
279 <para>At any time you can use the command
280 <literal>:show modules</literal>
281 to get a list of the modules currently loaded
287 C ( C.hs, interpreted )
288 B ( B.hs, interpreted )
289 A ( A.hs, interpreted )
292 <para>If we now modify the source of D (or pretend to: using Unix
293 command <literal>touch</literal> on the source file is handy for
294 this), the compiler will no longer be able to use the object file,
295 because it might be out of date:</para>
300 Compiling D ( D.hs, interpreted )
301 Skipping C ( C.hs, interpreted )
302 Skipping B ( B.hs, interpreted )
303 Skipping A ( A.hs, interpreted )
304 Ok, modules loaded: A, B, C, D.
308 <para>Note that module D was compiled, but in this instance
309 because its source hadn't really changed, its interface remained
310 the same, and the recompilation checker determined that A, B and C
311 didn't need to be recompiled.</para>
313 <para>So let's try compiling one of the other modules:</para>
316 *Main> :! ghc -c C.hs
318 Compiling D ( D.hs, interpreted )
319 Compiling C ( C.hs, interpreted )
320 Compiling B ( B.hs, interpreted )
321 Compiling A ( A.hs, interpreted )
322 Ok, modules loaded: A, B, C, D.
325 <para>We didn't get the compiled version of C! What happened?
326 Well, in GHCi a compiled module may only depend on other compiled
327 modules, and in this case C depends on D, which doesn't have an
328 object file, so GHCi also rejected C's object file. Ok, so let's
329 also compile D:</para>
332 *Main> :! ghc -c D.hs
334 Ok, modules loaded: A, B, C, D.
337 <para>Nothing happened! Here's another lesson: newly compiled
338 modules aren't picked up by <literal>:reload</literal>, only
339 <literal>:load</literal>:</para>
343 Skipping D ( D.hs, D.o )
344 Skipping C ( C.hs, C.o )
345 Compiling B ( B.hs, interpreted )
346 Compiling A ( A.hs, interpreted )
347 Ok, modules loaded: A, B, C, D.
350 <para>HINT: since GHCi will only use a compiled object file if it
351 can sure that the compiled version is up-to-date, a good technique
352 when working on a large program is to occasionally run
353 <literal>ghc ––make</literal> to compile the whole project (say
354 before you go for lunch :-), then continue working in the
355 interpreter. As you modify code, the new modules will be
356 interpreted, but the rest of the project will remain
362 <title>Interactive evaluation at the prompt</title>
364 <para>When you type an expression at the prompt, GHCi immediately
365 evaluates and prints the result:
367 Prelude> reverse "hello"
374 <sect2><title>I/O actions at the prompt</title>
376 <para>GHCi does more than simple expression evaluation at the prompt.
377 If you type something of type <literal>IO a</literal> for some
378 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
379 as an IO-computation.
383 Prelude> putStrLn "hello"
386 Furthermore, GHCi will print the result of the I/O action if (and only
389 <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
390 <listitem><para>The result type is not
391 <literal>()</literal>.</para></listitem>
393 For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
395 Prelude> putStrLn "hello"
397 Prelude> do { putStrLn "hello"; return "yes" }
404 <title>Using <literal>do-</literal>notation at the prompt</title>
405 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
406 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
408 <para>GHCi actually accepts <firstterm>statements</firstterm>
409 rather than just expressions at the prompt. This means you can
410 bind values and functions to names, and use them in future
411 expressions or statements.</para>
413 <para>The syntax of a statement accepted at the GHCi prompt is
414 exactly the same as the syntax of a statement in a Haskell
415 <literal>do</literal> expression. However, there's no monad
416 overloading here: statements typed at the prompt must be in the
417 <literal>IO</literal> monad.
419 Prelude> x <- return 42
425 The statement <literal>x <- return 42</literal> means
426 “execute <literal>return 42</literal> in the
427 <literal>IO</literal> monad, and bind the result to
428 <literal>x</literal>”. We can then use
429 <literal>x</literal> in future statements, for example to print
430 it as we did above.</para>
432 <para>GHCi will print the result of a statement if and only if:
435 <para>The statement is not a binding, or it is a monadic binding
436 (<literal>p <- e</literal>) that binds exactly one
440 <para>The variable's type is not polymorphic, is not
441 <literal>()</literal>, and is an instance of
442 <literal>Show</literal></para>
447 <para>Of course, you can also bind normal non-IO expressions
448 using the <literal>let</literal>-statement:</para>
455 <para>Another important difference between the two types of binding
456 is that the monadic bind (<literal>p <- e</literal>) is
457 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
458 whereas with the <literal>let</literal> form, the expression
459 isn't evaluated immediately:</para>
461 Prelude> let x = error "help!"
467 <para>Note that <literal>let</literal> bindings do not automatically
468 print the value bound, unlike monadic bindings.</para>
470 <para>Any exceptions raised during the evaluation or execution
471 of the statement are caught and printed by the GHCi command line
472 interface (for more information on exceptions, see the module
473 <literal>Control.Exception</literal> in the libraries
474 documentation).</para>
476 <para>Every new binding shadows any existing bindings of the
477 same name, including entities that are in scope in the current
478 module context.</para>
480 <para>WARNING: temporary bindings introduced at the prompt only
481 last until the next <literal>:load</literal> or
482 <literal>:reload</literal> command, at which time they will be
483 simply lost. However, they do survive a change of context with
484 <literal>:module</literal>: the temporary bindings just move to
485 the new location.</para>
487 <para>HINT: To get a list of the bindings currently in scope, use the
488 <literal>:show bindings</literal> command:</para>
491 Prelude> :show bindings
495 <para>HINT: if you turn on the <literal>+t</literal> option,
496 GHCi will show the type of each variable bound by a statement.
498 <indexterm><primary><literal>+t</literal></primary></indexterm>
501 Prelude> let (x:xs) = [1..]
508 <sect2 id="ghci-scope">
509 <title>What's really in scope at the prompt?</title>
511 <para>When you type an expression at the prompt, what
512 identifiers and types are in scope? GHCi provides a flexible
513 way to control exactly how the context for an expression is
514 constructed. Let's start with the simple cases; when you start
515 GHCi the prompt looks like this:</para>
517 <screen>Prelude></screen>
519 <para>Which indicates that everything from the module
520 <literal>Prelude</literal> is currently in scope. If we now
521 load a file into GHCi, the prompt will change:</para>
524 Prelude> :load Main.hs
525 Compiling Main ( Main.hs, interpreted )
529 <para>The new prompt is <literal>*Main</literal>, which
530 indicates that we are typing expressions in the context of the
531 top-level of the <literal>Main</literal> module. Everything
532 that is in scope at the top-level in the module
533 <literal>Main</literal> we just loaded is also in scope at the
534 prompt (probably including <literal>Prelude</literal>, as long
535 as <literal>Main</literal> doesn't explicitly hide it).</para>
538 <literal>*<replaceable>module</replaceable></literal> indicates
539 that it is the full top-level scope of
540 <replaceable>module</replaceable> that is contributing to the
541 scope for expressions typed at the prompt. Without the
542 <literal>*</literal>, just the exports of the module are
545 <para>We're not limited to a single module: GHCi can combine
546 scopes from multiple modules, in any mixture of
547 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
548 combines the scopes from all of these modules to form the scope
549 that is in effect at the prompt. For technical reasons, GHCi
550 can only support the <literal>*</literal>-form for modules which
551 are interpreted, so compiled modules and package modules can
552 only contribute their exports to the current scope.</para>
554 <para>The scope is manipulated using the
555 <literal>:module</literal> command. For example, if the current
556 scope is <literal>Prelude</literal>, then we can bring into
557 scope the exports from the module <literal>IO</literal> like
562 Prelude,IO> hPutStrLn stdout "hello\n"
567 <para>(Note: <literal>:module</literal> can be shortened to
568 <literal>:m</literal>). The full syntax of the
569 <literal>:module</literal> command is:</para>
572 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
575 <para>Using the <literal>+</literal> form of the
576 <literal>module</literal> commands adds modules to the current
577 scope, and <literal>-</literal> removes them. Without either
578 <literal>+</literal> or <literal>-</literal>, the current scope
579 is replaced by the set of modules specified. Note that if you
580 use this form and leave out <literal>Prelude</literal>, GHCi
581 will assume that you really wanted the
582 <literal>Prelude</literal> and add it in for you (if you don't
583 want the <literal>Prelude</literal>, then ask to remove it with
584 <literal>:m -Prelude</literal>).</para>
586 <para>The scope is automatically set after a
587 <literal>:load</literal> command, to the most recently loaded
588 "target" module, in a <literal>*</literal>-form if possible.
589 For example, if you say <literal>:load foo.hs bar.hs</literal>
590 and <filename>bar.hs</filename> contains module
591 <literal>Bar</literal>, then the scope will be set to
592 <literal>*Bar</literal> if <literal>Bar</literal> is
593 interpreted, or if <literal>Bar</literal> is compiled it will be
594 set to <literal>Prelude,Bar</literal> (GHCi automatically adds
595 <literal>Prelude</literal> if it isn't present and there aren't
596 any <literal>*</literal>-form modules).</para>
598 <para>With multiple modules in scope, especially multiple
599 <literal>*</literal>-form modules, it is likely that name
600 clashes will occur. Haskell specifies that name clashes are
601 only reported when an ambiguous identifier is used, and GHCi
602 behaves in the same way for expressions typed at the
606 <title>Qualified names</title>
608 <para>To make life slightly easier, the GHCi prompt also
609 behaves as if there is an implicit <literal>import
610 qualified</literal> declaration for every module in every
611 package, and every module currently loaded into GHCi.</para>
617 <title>The <literal>it</literal> variable</title>
618 <indexterm><primary><literal>it</literal></primary>
621 <para>Whenever an expression (or a non-binding statement, to be
622 precise) is typed at the prompt, GHCi implicitly binds its value
623 to the variable <literal>it</literal>. For example:</para>
630 <para>What actually happens is that GHCi typechecks the
631 expression, and if it doesn't have an <literal>IO</literal> type,
632 then it transforms it as follows: an expression
633 <replaceable>e</replaceable> turns into
635 let it = <replaceable>e</replaceable>;
638 which is then run as an IO-action.</para>
640 <para>Hence, the original expression must have a type which is an
641 instance of the <literal>Show</literal> class, or GHCi will
646 No instance for `Show (a -> a)'
647 arising from use of `print'
648 in a `do' expression pattern binding: print it
651 <para>The error message contains some clues as to the
652 transformation happening internally.</para>
654 <para>If the expression was instead of type <literal>IO a</literal> for
655 some <literal>a</literal>, then <literal>it</literal> will be
656 bound to the result of the <literal>IO</literal> computation,
657 which is of type <literal>a</literal>. eg.:</para>
659 Prelude> Time.getClockTime
661 Wed Mar 14 12:23:13 GMT 2001
664 <para>The corresponding translation for an IO-typed
665 <replaceable>e</replaceable> is
667 it <- <replaceable>e</replaceable>
671 <para>Note that <literal>it</literal> is shadowed by the new
672 value each time you evaluate a new expression, and the old value
673 of <literal>it</literal> is lost.</para>
678 <title>Type defaulting in GHCi</title>
679 <indexterm><primary>Type default</primary></indexterm>
680 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
682 Consider this GHCi session:
686 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
687 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
688 on the type <literal>a</literal>. For example:
690 ghci> (reverse []) :: String
692 ghci> (reverse []) :: [Int]
695 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
696 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
697 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
698 a)</literal> for each type variable <literal>a</literal>, and defaults the
701 <listitem><para> The type variable <literal>a</literal>
702 appears in no other constraints </para></listitem>
703 <listitem><para> All the classes <literal>Ci</literal> are standard.</para></listitem>
704 <listitem><para> At least one of the classes <literal>Ci</literal> is
705 numeric.</para></listitem>
707 At the GHCi prompt, the second and third rules are relaxed as follows
708 (differences italicised):
710 <listitem><para> <emphasis>Any</emphasis> of the classes <literal>Ci</literal> is standard.</para></listitem>
711 <listitem><para> At least one of the classes <literal>Ci</literal> is
712 numeric, <emphasis>or is <literal>Show</literal>,
713 <literal>Eq</literal>, or <literal>Ord</literal></emphasis>.</para></listitem>
719 <sect1 id="ghci-invocation">
720 <title>Invoking GHCi</title>
721 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
722 <indexterm><primary><option>––interactive</option></primary></indexterm>
724 <para>GHCi is invoked with the command <literal>ghci</literal> or
725 <literal>ghc ––interactive</literal>. One or more modules or
726 filenames can also be specified on the command line; this
727 instructs GHCi to load the specified modules or filenames (and all
728 the modules they depend on), just as if you had said
729 <literal>:load <replaceable>modules</replaceable></literal> at the
730 GHCi prompt (see <xref linkend="ghci-commands"/>). For example, to
731 start GHCi and load the program whose topmost module is in the
732 file <literal>Main.hs</literal>, we could say:</para>
738 <para>Most of the command-line options accepted by GHC (see <xref
739 linkend="using-ghc"/>) also make sense in interactive mode. The ones
740 that don't make sense are mostly obvious; for example, GHCi
741 doesn't generate interface files, so options related to interface
742 file generation won't have any effect.</para>
745 <title>Packages</title>
746 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
748 <para>Most packages (see <xref linkend="using-packages"/>) are
749 available without needing to specify any extra flags at all:
750 they will be automatically loaded the first time they are
753 <para>For non-auto packages, however, you need to request the
754 package be loaded by using the <literal>-package</literal> flag:</para>
760 / /_\// /_/ / / | | GHC Interactive, version 5.05, for Haskell 98.
761 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
762 \____/\/ /_/\____/|_| Type :? for help.
764 Loading package base ... linking ... done.
765 Loading package haskell98 ... linking ... done.
766 Loading package lang ... linking ... done.
767 Loading package concurrent ... linking ... done.
768 Loading package readline ... linking ... done.
769 Loading package unix ... linking ... done.
770 Loading package posix ... linking ... done.
771 Loading package util ... linking ... done.
772 Loading package data ... linking ... done.
776 <para>The following command works to load new packages into a
780 Prelude> :set -package <replaceable>name</replaceable>
783 <para>But note that doing this will cause all currently loaded
784 modules to be unloaded, and you'll be dumped back into the
785 <literal>Prelude</literal>.</para>
789 <title>Extra libraries</title>
790 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
792 <para>Extra libraries may be specified on the command line using
793 the normal <literal>-l<replaceable>lib</replaceable></literal>
794 option. (The term <emphasis>library</emphasis> here refers to
795 libraries of foreign object code; for using libraries of Haskell
796 source code, see <xref linkend="ghci-modules-filenames"/>.) For
797 example, to load the “m” library:</para>
803 <para>On systems with <literal>.so</literal>-style shared
804 libraries, the actual library loaded will the
805 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
806 searches the following places for libraries, in this order:</para>
810 <para>Paths specified using the
811 <literal>-L<replaceable>path</replaceable></literal>
812 command-line option,</para>
815 <para>the standard library search path for your system,
816 which on some systems may be overridden by setting the
817 <literal>LD_LIBRARY_PATH</literal> environment
822 <para>On systems with <literal>.dll</literal>-style shared
823 libraries, the actual library loaded will be
824 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
825 GHCi will signal an error if it can't find the library.</para>
827 <para>GHCi can also load plain object files
828 (<literal>.o</literal> or <literal>.obj</literal> depending on
829 your platform) from the command-line. Just add the name the
830 object file to the command line.</para>
832 <para>Ordering of <option>-l</option> options matters: a library
833 should be mentioned <emphasis>before</emphasis> the libraries it
834 depends on (see <xref linkend="options-linker"/>).</para>
839 <sect1 id="ghci-commands">
840 <title>GHCi commands</title>
842 <para>GHCi commands all begin with
843 ‘<literal>:</literal>’ and consist of a single command
844 name followed by zero or more parameters. The command name may be
845 abbreviated, as long as the abbreviation is not ambiguous. All of
846 the builtin commands, with the exception of
847 <literal>:unset</literal> and <literal>:undef</literal>, may be
848 abbreviated to a single letter.</para>
853 <literal>:add</literal> <replaceable>module</replaceable> ...
854 <indexterm><primary><literal>:add</literal></primary></indexterm>
857 <para>Add <replaceable>module</replaceable>(s) to the
858 current <firstterm>target set</firstterm>, and perform a
865 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
866 <indexterm><primary><literal>:browse</literal></primary></indexterm>
869 <para>Displays the identifiers defined by the module
870 <replaceable>module</replaceable>, which must be either
871 loaded into GHCi or be a member of a package. If the
872 <literal>*</literal> symbol is placed before the module
873 name, then <emphasis>all</emphasis> the identifiers defined
874 in <replaceable>module</replaceable> are shown; otherwise
875 the list is limited to the exports of
876 <replaceable>module</replaceable>. The
877 <literal>*</literal>-form is only available for modules
878 which are interpreted; for compiled modules (including
879 modules from packages) only the non-<literal>*</literal>
880 form of <literal>:browse</literal> is available.</para>
886 <literal>:cd</literal> <replaceable>dir</replaceable>
887 <indexterm><primary><literal>:cd</literal></primary></indexterm>
890 <para>Changes the current working directory to
891 <replaceable>dir</replaceable>. A
892 ‘<literal>˜</literal>’ symbol at the
893 beginning of <replaceable>dir</replaceable> will be replaced
894 by the contents of the environment variable
895 <literal>HOME</literal>.</para>
897 <para>NOTE: changing directories causes all currently loaded
898 modules to be unloaded. This is because the search path is
899 usually expressed using relative directories, and changing
900 the search path in the middle of a session is not
907 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
908 <indexterm><primary><literal>:def</literal></primary></indexterm>
911 <para>The command <literal>:def</literal>
912 <replaceable>name</replaceable>
913 <replaceable>expr</replaceable> defines a new GHCi command
914 <literal>:<replaceable>name</replaceable></literal>,
915 implemented by the Haskell expression
916 <replaceable>expr</replaceable>, which must have type
917 <literal>String -> IO String</literal>. When
918 <literal>:<replaceable>name</replaceable>
919 <replaceable>args</replaceable></literal> is typed at the
920 prompt, GHCi will run the expression
921 <literal>(<replaceable>name</replaceable>
922 <replaceable>args</replaceable>)</literal>, take the
923 resulting <literal>String</literal>, and feed it back into
924 GHCi as a new sequence of commands. Separate commands in
925 the result must be separated by
926 ‘<literal>\n</literal>’.</para>
928 <para>That's all a little confusing, so here's a few
929 examples. To start with, here's a new GHCi command which
930 doesn't take any arguments or produce any results, it just
931 outputs the current date & time:</para>
934 Prelude> let date _ = Time.getClockTime >>= print >> return ""
935 Prelude> :def date date
937 Fri Mar 23 15:16:40 GMT 2001
940 <para>Here's an example of a command that takes an argument.
941 It's a re-implementation of <literal>:cd</literal>:</para>
944 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
945 Prelude> :def mycd mycd
949 <para>Or I could define a simple way to invoke
950 “<literal>ghc ––make Main</literal>” in the
951 current directory:</para>
954 Prelude> :def make (\_ -> return ":! ghc ––make Main")
957 <para>We can define a command that reads GHCi input from a
958 file. This might be useful for creating a set of bindings
959 that we want to repeatedly load into the GHCi session:</para>
962 Prelude> :def . readFile
963 Prelude> :. cmds.ghci
966 <para>Notice that we named the command
967 <literal>:.</literal>, by analogy with the
968 ‘<literal>.</literal>’ Unix shell command that
969 does the same thing.</para>
975 <literal>:help</literal>
976 <indexterm><primary><literal>:help</literal></primary></indexterm>
979 <literal>:?</literal>
980 <indexterm><primary><literal>:?</literal></primary></indexterm>
983 <para>Displays a list of the available commands.</para>
989 <literal>:info</literal> <replaceable>name</replaceable> ...
990 <indexterm><primary><literal>:info</literal></primary></indexterm>
993 <para>Displays information about the given name(s). For
994 example, if <replaceable>name</replaceable> is a class, then
995 the class methods and their types will be printed; if
996 <replaceable>name</replaceable> is a type constructor, then
997 its definition will be printed; if
998 <replaceable>name</replaceable> is a function, then its type
999 will be printed. If <replaceable>name</replaceable> has
1000 been loaded from a source file, then GHCi will also display
1001 the location of its definition in the source.</para>
1007 <literal>:load</literal> <replaceable>module</replaceable> ...
1008 <indexterm><primary><literal>:load</literal></primary></indexterm>
1011 <para>Recursively loads the specified
1012 <replaceable>module</replaceable>s, and all the modules they
1013 depend on. Here, each <replaceable>module</replaceable>
1014 must be a module name or filename, but may not be the name
1015 of a module in a package.</para>
1017 <para>All previously loaded modules, except package modules,
1018 are forgotten. The new set of modules is known as the
1019 <firstterm>target set</firstterm>. Note that
1020 <literal>:load</literal> can be used without any arguments
1021 to unload all the currently loaded modules and
1024 <para>After a <literal>:load</literal> command, the current
1025 context is set to:</para>
1029 <para><replaceable>module</replaceable>, if it was loaded
1030 successfully, or</para>
1033 <para>the most recently successfully loaded module, if
1034 any other modules were loaded as a result of the current
1035 <literal>:load</literal>, or</para>
1038 <para><literal>Prelude</literal> otherwise.</para>
1046 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
1047 <indexterm><primary><literal>:module</literal></primary></indexterm>
1050 <para>Sets or modifies the current context for statements
1051 typed at the prompt. See <xref linkend="ghci-scope"/> for
1052 more details.</para>
1058 <literal>:quit</literal>
1059 <indexterm><primary><literal>:quit</literal></primary></indexterm>
1062 <para>Quits GHCi. You can also quit by typing a control-D
1063 at the prompt.</para>
1069 <literal>:reload</literal>
1070 <indexterm><primary><literal>:reload</literal></primary></indexterm>
1073 <para>Attempts to reload the current target set (see
1074 <literal>:load</literal>) if any of the modules in the set,
1075 or any dependent module, has changed. Note that this may
1076 entail loading new modules, or dropping modules which are no
1077 longer indirectly required by the target.</para>
1083 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
1084 <indexterm><primary><literal>:set</literal></primary></indexterm>
1087 <para>Sets various options. See <xref linkend="ghci-set"/>
1088 for a list of available options. The
1089 <literal>:set</literal> command by itself shows which
1090 options are currently set.</para>
1096 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
1097 <indexterm><primary><literal>:set</literal></primary></indexterm>
1100 <para>Sets the list of arguments which are returned when the
1101 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
1102 </indexterm>.</para>
1108 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
1109 <indexterm><primary><literal>:set</literal></primary></indexterm>
1112 <para>Sets the string to be returned when the program calls
1113 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
1114 </indexterm>.</para>
1120 <literal>:show bindings</literal>
1121 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
1124 <para>Show the bindings made at the prompt and their
1131 <literal>:show modules</literal>
1132 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
1135 <para>Show the list of modules currently load.</para>
1141 <literal>:tags -c|-e</literal>
1142 <indexterm><primary><literal>:tags</literal></primary>
1146 <para>Generates a “tags” file for either Vi-style editors
1147 (<literal>-c</literal>) or Emacs-style editors
1148 (<literal>-e</literal>). The file generated will be called
1149 <filename>tags</filename> or <filename>TAGS</filename>
1150 respectively.</para>
1152 <para>See also <xref linkend="hasktags" />.</para>
1158 <literal>:type</literal> <replaceable>expression</replaceable>
1159 <indexterm><primary><literal>:type</literal></primary></indexterm>
1162 <para>Infers and prints the type of
1163 <replaceable>expression</replaceable>, including explicit
1164 forall quantifiers for polymorphic types. The monomorphism
1165 restriction is <emphasis>not</emphasis> applied to the
1166 expression during type inference.</para>
1172 <literal>:kind</literal> <replaceable>type</replaceable>
1173 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1176 <para>Infers and prints the kind of
1177 <replaceable>type</replaceable>. The latter can be an arbitrary
1178 type expression, including a partial application of a type constructor,
1179 such as <literal>Either Int</literal>.</para>
1185 <literal>:undef</literal> <replaceable>name</replaceable>
1186 <indexterm><primary><literal>:undef</literal></primary></indexterm>
1189 <para>Undefines the user-defined command
1190 <replaceable>name</replaceable> (see <literal>:def</literal>
1197 <literal>:unset</literal> <replaceable>option</replaceable>...
1198 <indexterm><primary><literal>:unset</literal></primary></indexterm>
1201 <para>Unsets certain options. See <xref linkend="ghci-set"/>
1202 for a list of available options.</para>
1208 <literal>:!</literal> <replaceable>command</replaceable>...
1209 <indexterm><primary><literal>:!</literal></primary></indexterm>
1210 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
1213 <para>Executes the shell command
1214 <replaceable>command</replaceable>.</para>
1221 <sect1 id="ghci-set">
1222 <title>The <literal>:set</literal> command</title>
1223 <indexterm><primary><literal>:set</literal></primary></indexterm>
1225 <para>The <literal>:set</literal> command sets two types of
1226 options: GHCi options, which begin with
1227 ‘<literal>+</literal>” and “command-line”
1228 options, which begin with ‘-’. </para>
1230 <para>NOTE: at the moment, the <literal>:set</literal> command
1231 doesn't support any kind of quoting in its arguments: quotes will
1232 not be removed and cannot be used to group words together. For
1233 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
1237 <title>GHCi options</title>
1238 <indexterm><primary>options</primary><secondary>GHCi</secondary>
1241 <para>GHCi options may be set using <literal>:set</literal> and
1242 unset using <literal>:unset</literal>.</para>
1244 <para>The available GHCi options are:</para>
1249 <literal>+r</literal>
1250 <indexterm><primary><literal>+r</literal></primary></indexterm>
1251 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
1252 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
1255 <para>Normally, any evaluation of top-level expressions
1256 (otherwise known as CAFs or Constant Applicative Forms) in
1257 loaded modules is retained between evaluations. Turning
1258 on <literal>+r</literal> causes all evaluation of
1259 top-level expressions to be discarded after each
1260 evaluation (they are still retained
1261 <emphasis>during</emphasis> a single evaluation).</para>
1263 <para>This option may help if the evaluated top-level
1264 expressions are consuming large amounts of space, or if
1265 you need repeatable performance measurements.</para>
1271 <literal>+s</literal>
1272 <indexterm><primary><literal>+s</literal></primary></indexterm>
1275 <para>Display some stats after evaluating each expression,
1276 including the elapsed time and number of bytes allocated.
1277 NOTE: the allocation figure is only accurate to the size
1278 of the storage manager's allocation area, because it is
1279 calculated at every GC. Hence, you might see values of
1280 zero if no GC has occurred.</para>
1286 <literal>+t</literal>
1287 <indexterm><primary><literal>+t</literal></primary></indexterm>
1290 <para>Display the type of each variable bound after a
1291 statement is entered at the prompt. If the statement is a
1292 single expression, then the only variable binding will be
1294 ‘<literal>it</literal>’.</para>
1300 <sect2 id="ghci-cmd-line-options">
1301 <title>Setting GHC command-line options in GHCi</title>
1303 <para>Normal GHC command-line options may also be set using
1304 <literal>:set</literal>. For example, to turn on
1305 <option>-fglasgow-exts</option>, you would say:</para>
1308 Prelude> :set -fglasgow-exts
1311 <para>Any GHC command-line option that is designated as
1312 <firstterm>dynamic</firstterm> (see the table in <xref
1313 linkend="flag-reference"/>), may be set using
1314 <literal>:set</literal>. To unset an option, you can set the
1315 reverse option:</para>
1316 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
1319 Prelude> :set -fno-glasgow-exts
1322 <para><xref linkend="flag-reference"/> lists the reverse for each
1323 option where applicable.</para>
1325 <para>Certain static options (<option>-package</option>,
1326 <option>-I</option>, <option>-i</option>, and
1327 <option>-l</option> in particular) will also work, but some may
1328 not take effect until the next reload.</para>
1329 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
1333 <sect1 id="ghci-dot-files">
1334 <title>The <filename>.ghci</filename> file</title>
1335 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
1337 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
1340 <para>When it starts, GHCi always reads and executes commands from
1341 <filename>$HOME/.ghci</filename>, followed by
1342 <filename>./.ghci</filename>.</para>
1344 <para>The <filename>.ghci</filename> in your home directory is
1345 most useful for turning on favourite options (eg. <literal>:set
1346 +s</literal>), and defining useful macros. Placing a
1347 <filename>.ghci</filename> file in a directory with a Haskell
1348 project is a useful way to set certain project-wide options so you
1349 don't have to type them everytime you start GHCi: eg. if your
1350 project uses GHC extensions and CPP, and has source files in three
1351 subdirectories A B and C, you might put the following lines in
1352 <filename>.ghci</filename>:</para>
1355 :set -fglasgow-exts -cpp
1359 <para>(Note that strictly speaking the <option>-i</option> flag is
1360 a static one, but in fact it works to set it using
1361 <literal>:set</literal> like this. The changes won't take effect
1362 until the next <literal>:load</literal>, though.)</para>
1364 <para>Two command-line options control whether the
1365 <filename>.ghci</filename> files are read:</para>
1370 <option>-ignore-dot-ghci</option>
1371 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
1374 <para>Don't read either <filename>./.ghci</filename> or
1375 <filename>$HOME/.ghci</filename> when starting up.</para>
1380 <option>-read-dot-ghci</option>
1381 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
1384 <para>Read <filename>.ghci</filename> and
1385 <filename>$HOME/.ghci</filename>. This is normally the
1386 default, but the <option>-read-dot-ghci</option> option may
1387 be used to override a previous
1388 <option>-ignore-dot-ghci</option> option.</para>
1396 <title>FAQ and Things To Watch Out For</title>
1400 <term>The interpreter can't load modules with foreign export
1401 declarations!</term>
1403 <para>Unfortunately not. We haven't implemented it yet.
1404 Please compile any offending modules by hand before loading
1405 them into GHCi.</para>
1411 <literal>-O</literal> doesn't work with GHCi!
1412 <indexterm><primary><option>-O</option></primary></indexterm>
1415 <para>For technical reasons, the bytecode compiler doesn't
1416 interact well with one of the optimisation passes, so we
1417 have disabled optimisation when using the interpreter. This
1418 isn't a great loss: you'll get a much bigger win by
1419 compiling the bits of your code that need to go fast, rather
1420 than interpreting them with optimisation turned on.</para>
1425 <term>Unboxed tuples don't work with GHCi</term>
1427 <para>That's right. You can always compile a module that
1428 uses unboxed tuples and load it into GHCi, however.
1429 (Incidentally the previous point, namely that
1430 <literal>-O</literal> is incompatible with GHCi, is because
1431 the bytecode compiler can't deal with unboxed
1437 <term>Concurrent threads don't carry on running when GHCi is
1438 waiting for input.</term>
1440 <para>No, they don't. This is because the Haskell binding
1441 to the GNU readline library doesn't support reading from the
1442 terminal in a non-blocking way, which is required to work
1443 properly with GHC's concurrency model.</para>
1448 <term>After using <literal>getContents</literal>, I can't use
1449 <literal>stdin</literal> again until I do
1450 <literal>:load</literal> or <literal>:reload</literal>.</term>
1453 <para>This is the defined behaviour of
1454 <literal>getContents</literal>: it puts the stdin Handle in
1455 a state known as <firstterm>semi-closed</firstterm>, wherein
1456 any further I/O operations on it are forbidden. Because I/O
1457 state is retained between computations, the semi-closed
1458 state persists until the next <literal>:load</literal> or
1459 <literal>:reload</literal> command.</para>
1461 <para>You can make <literal>stdin</literal> reset itself
1462 after every evaluation by giving GHCi the command
1463 <literal>:set +r</literal>. This works because
1464 <literal>stdin</literal> is just a top-level expression that
1465 can be reverted to its unevaluated state in the same way as
1466 any other top-level expression (CAF).</para>
1476 ;;; Local Variables: ***
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