2 <title>Using GHCi</title>
3 <indexterm><primary>GHCi</primary></indexterm>
4 <indexterm><primary>interpreter</primary><see>GHCi</see></indexterm>
5 <indexterm><primary>interactive</primary><see>GHCi</see></indexterm>
8 <para>The ‘i’ stands for “Interactive”</para>
10 is GHC's interactive environment, in which Haskell expressions can
11 be interactively evaluated and programs can be interpreted. If
12 you're famililar with <ulink url="http://www.haskell.org/hugs/">Hugs</ulink><indexterm><primary>Hugs</primary>
13 </indexterm>, then you'll be right at home with GHCi. However, GHCi
14 also has support for interactively loading compiled code, as well as
15 supporting all<footnote><para>except <literal>foreign export</literal>, at the moment</para>
16 </footnote> the language extensions that GHC provides.</para>
17 <indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
18 <indexterm><primary>Foreign Function Interface</primary><secondary>GHCi support</secondary></indexterm>
21 <title>Introduction to GHCi</title>
23 <para>Let's start with an example GHCi session. You can fire up
24 GHCi with the command <literal>ghci</literal>:</para>
30 / /_\// /_/ / / | | GHC Interactive, version 5.04, for Haskell 98.
31 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
32 \____/\/ /_/\____/|_| Type :? for help.
34 Loading package base ... linking ... done.
35 Loading package haskell98 ... linking ... done.
39 <para>There may be a short pause while GHCi loads the prelude and
40 standard libraries, after which the prompt is shown. If we follow
41 the instructions and type <literal>:?</literal> for help, we
45 Commands available from the prompt:
47 <stmt> evaluate/run <stmt>
48 :add <filename> ... add module(s) to the current target set
49 :browse [*]<module> display the names defined by <module>
50 :cd <dir> change directory to <dir>
51 :def <cmd> <expr> define a command :<cmd>
52 :help, :? display this list of commands
53 :info [<name> ...] display information about the given names
54 :load <filename> ... load module(s) and their dependents
55 :module [+/-] [*]<mod> ... set the context for expression evaluation
56 :reload reload the current module set
58 :set <option> ... set options
59 :set args <arg> ... set the arguments returned by System.getArgs
60 :set prog <progname> set the value returned by System.getProgName
62 :show modules show the currently loaded modules
63 :show bindings show the current bindings made at the prompt
65 :type <expr> show the type of <expr>
66 :kind <type> show the kind of <type>
67 :undef <cmd> undefine user-defined command :<cmd>
68 :unset <option> ... unset options
70 :!<command> run the shell command <command>
72 Options for `:set' and `:unset':
74 +r revert top-level expressions after each evaluation
75 +s print timing/memory stats after each evaluation
76 +t print type after evaluation
77 -<flags> most GHC command line flags can also be set here
78 (eg. -v2, -fglasgow-exts, etc.)
81 <para>We'll explain most of these commands as we go along. For
82 Hugs users: many things work the same as in Hugs, so you should be
83 able to get going straight away.</para>
85 <para>Haskell expressions can be typed at the prompt:</para>
86 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
92 Prelude> let x = 42 in x / 9
97 <para>GHCi interprets the whole line as an expression to evaluate.
98 The expression may not span several lines - as soon as you press
99 enter, GHCi will attempt to evaluate it.</para>
103 <title>Loading source files</title>
105 <para>Suppose we have the following Haskell source code, which we
106 place in a file <filename>Main.hs</filename>:</para>
109 main = print (fac 20)
112 fac n = n * fac (n-1)
115 <para>You can save <filename>Main.hs</filename> anywhere you like,
116 but if you save it somewhere other than the current
117 directory<footnote><para>If you started up GHCi from the command
118 line then GHCi's current directory is the same as the current
119 directory of the shell from which it was started. If you started
120 GHCi from the “Start” menu in Windows, then the
121 current directory is probably something like
122 <filename>C:\Documents and Settings\<replaceable>user
123 name</replaceable></filename>.</para> </footnote> then we will
124 need to change to the right directory in GHCi:</para>
127 Prelude> :cd <replaceable>dir</replaceable>
130 <para>where <replaceable>dir</replaceable> is the directory (or
131 folder) in which you saved <filename>Main.hs</filename>.</para>
133 <para>To load a Haskell source file into GHCi, use the
134 <literal>:load</literal> command:</para>
135 <indexterm><primary><literal>:load</literal></primary></indexterm>
139 Compiling Main ( Main.hs, interpreted )
140 Ok, modules loaded: Main.
144 <para>GHCi has loaded the <literal>Main</literal> module, and the
145 prompt has changed to “<literal>*Main></literal>” to
146 indicate that the current context for expressions typed at the
147 prompt is the <literal>Main</literal> module we just loaded (we'll
148 explain what the <literal>*</literal> means later in <xref
149 linkend="ghci-scope">). So we can now type expressions involving
150 the functions from <filename>Main.hs</filename>:</para>
157 <para>Loading a multi-module program is just as straightforward;
158 just give the name of the “topmost” module to the
159 <literal>:load</literal> command (hint: <literal>:load</literal>
160 can be abbreviated to <literal>:l</literal>). The topmost module
161 will normally be <literal>Main</literal>, but it doesn't have to
162 be. GHCi will discover which modules are required, directly or
163 indirectly, by the topmost module, and load them all in dependency
166 <sect2 id="ghci-modules-filenames">
167 <title>Modules vs. filenames</title>
168 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
169 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
171 <para>Question: How does GHC find the filename which contains
172 module <replaceable>M</replaceable>? Answer: it looks for the
173 file <literal><replaceable>M</replaceable>.hs</literal>, or
174 <literal><replaceable>M</replaceable>.lhs</literal>. This means
175 that for most modules, the module name must match the filename.
176 If it doesn't, GHCi won't be able to find it.</para>
178 <para>There is one exception to this general rule: when you load
179 a program with <literal>:load</literal>, or specify it when you
180 invoke <literal>ghci</literal>, you can give a filename rather
181 than a module name. This filename is loaded if it exists, and
182 it may contain any module you like. This is particularly
183 convenient if you have several <literal>Main</literal> modules
184 in the same directory and you can't call them all
185 <filename>Main.hs</filename>.</para>
187 <para>The search path for finding source files is specified with
188 the <option>-i</option> option on the GHCi command line, like
190 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
192 <para>or it can be set using the <literal>:set</literal> command
193 from within GHCi (see <xref
194 linkend="ghci-cmd-line-options">)<footnote><para>Note that in
195 GHCi, and <option>––make</option> mode, the <option>-i</option>
196 option is used to specify the search path for
197 <emphasis>source</emphasis> files, whereas in standard
198 batch-compilation mode the <option>-i</option> option is used to
199 specify the search path for interface files, see <xref
200 linkend="search-path">.</para> </footnote></para>
202 <para>One consequence of the way that GHCi follows dependencies
203 to find modules to load is that every module must have a source
204 file. The only exception to the rule is modules that come from
205 a package, including the <literal>Prelude</literal> and standard
206 libraries such as <literal>IO</literal> and
207 <literal>Complex</literal>. If you attempt to load a module for
208 which GHCi can't find a source file, even if there are object
209 and interface files for the module, you'll get an error
214 <title>Making changes and recompilation</title>
215 <indexterm><primary><literal>:reload</literal></primary></indexterm>
217 <para>If you make some changes to the source code and want GHCi
218 to recompile the program, give the <literal>:reload</literal>
219 command. The program will be recompiled as necessary, with GHCi
220 doing its best to avoid actually recompiling modules if their
221 external dependencies haven't changed. This is the same
222 mechanism we use to avoid re-compiling modules in the batch
223 compilation setting (see <xref linkend="recomp">).</para>
227 <sect1 id="ghci-compiled">
228 <title>Loading compiled code</title>
229 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
231 <para>When you load a Haskell source module into GHCi, it is
232 normally converted to byte-code and run using the interpreter.
233 However, interpreted code can also run alongside compiled code in
234 GHCi; indeed, normally when GHCi starts, it loads up a compiled
235 copy of the <literal>base</literal> package, which contains the
236 <literal>Prelude</literal>.</para>
238 <para>Why should we want to run compiled code? Well, compiled
239 code is roughly 10x faster than interpreted code, but takes about
240 2x longer to produce (perhaps longer if optimisation is on). So
241 it pays to compile the parts of a program that aren't changing
242 very often, and use the interpreter for the code being actively
245 <para>When loading up source files with <literal>:load</literal>,
246 GHCi looks for any corresponding compiled object files, and will
247 use one in preference to interpreting the source if possible. For
248 example, suppose we have a 4-module program consisting of modules
249 A, B, C, and D. Modules B and C both import D only,
250 and A imports both B & C:</para>
258 <para>We can compile D, then load the whole program, like this:</para>
260 Prelude> :! ghc -c D.hs
262 Skipping D ( D.hs, D.o )
263 Compiling C ( C.hs, interpreted )
264 Compiling B ( B.hs, interpreted )
265 Compiling A ( A.hs, interpreted )
266 Ok, modules loaded: A, B, C, D.
270 <para>In the messages from the compiler, we see that it skipped D,
271 and used the object file <filename>D.o</filename>. The message
272 <literal>Skipping</literal> <replaceable>module</replaceable>
273 indicates that compilation for <replaceable>module</replaceable>
274 isn't necessary, because the source and everything it depends on
275 is unchanged since the last compilation.</para>
277 <para>At any time you can use the command
278 <literal>:show modules</literal>
279 to get a list of the modules currently loaded
285 C ( C.hs, interpreted )
286 B ( B.hs, interpreted )
287 A ( A.hs, interpreted )
290 <para>If we now modify the source of D (or pretend to: using Unix
291 command <literal>touch</literal> on the source file is handy for
292 this), the compiler will no longer be able to use the object file,
293 because it might be out of date:</para>
298 Compiling D ( D.hs, interpreted )
299 Skipping C ( C.hs, interpreted )
300 Skipping B ( B.hs, interpreted )
301 Skipping A ( A.hs, interpreted )
302 Ok, modules loaded: A, B, C, D.
306 <para>Note that module D was compiled, but in this instance
307 because its source hadn't really changed, its interface remained
308 the same, and the recompilation checker determined that A, B and C
309 didn't need to be recompiled.</para>
311 <para>So let's try compiling one of the other modules:</para>
314 *Main> :! ghc -c C.hs
316 Compiling D ( D.hs, interpreted )
317 Compiling C ( C.hs, interpreted )
318 Compiling B ( B.hs, interpreted )
319 Compiling A ( A.hs, interpreted )
320 Ok, modules loaded: A, B, C, D.
323 <para>We didn't get the compiled version of C! What happened?
324 Well, in GHCi a compiled module may only depend on other compiled
325 modules, and in this case C depends on D, which doesn't have an
326 object file, so GHCi also rejected C's object file. Ok, so let's
327 also compile D:</para>
330 *Main> :! ghc -c D.hs
332 Ok, modules loaded: A, B, C, D.
335 <para>Nothing happened! Here's another lesson: newly compiled
336 modules aren't picked up by <literal>:reload</literal>, only
337 <literal>:load</literal>:</para>
341 Skipping D ( D.hs, D.o )
342 Skipping C ( C.hs, C.o )
343 Compiling B ( B.hs, interpreted )
344 Compiling A ( A.hs, interpreted )
345 Ok, modules loaded: A, B, C, D.
348 <para>HINT: since GHCi will only use a compiled object file if it
349 can sure that the compiled version is up-to-date, a good technique
350 when working on a large program is to occasionally run
351 <literal>ghc ––make</literal> to compile the whole project (say
352 before you go for lunch :-), then continue working in the
353 interpreter. As you modify code, the new modules will be
354 interpreted, but the rest of the project will remain
360 <title>Interactive evaluation at the prompt</title>
362 <para>When you type an expression at the prompt, GHCi immediately
363 evaluates and prints the result. But that's not the whole story:
364 if you type something of type <literal>IO a</literal> for some
365 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
366 as an IO-computation, and doesn't attempt to print the
372 Prelude> putStrLn "hello"
376 <para>What actually happens is that GHCi typechecks the
377 expression, and if it doesn't have an <literal>IO</literal> type,
378 then it transforms it as follows: an expression
379 <replaceable>e</replaceable> turns into
381 let it = <replaceable>e</replaceable>;
384 which is then run as an IO-action.</para>
386 <para>Hence, the original expression must have a type which is an
387 instance of the <literal>Show</literal> class, or GHCi will
392 No instance for `Show (a -> a)'
393 arising from use of `print'
394 in a `do' expression pattern binding: print it
397 <para>The error message contains some clues as to the
398 transformation happening internally.</para>
400 <sect2 id="ghci-scope">
401 <title>What's really in scope at the prompt?</title>
403 <para>When you type an expression at the prompt, what
404 identifiers and types are in scope? GHCi provides a flexible
405 way to control exactly how the context for an expression is
406 constructed. Let's start with the simple cases; when you start
407 GHCi the prompt looks like this:</para>
409 <screen>Prelude></screen>
411 <para>Which indicates that everything from the module
412 <literal>Prelude</literal> is currently in scope. If we now
413 load a file into GHCi, the prompt will change:</para>
416 Prelude> :load Main.hs
417 Compiling Main ( Main.hs, interpreted )
421 <para>The new prompt is <literal>*Main</literal>, which
422 indicates that we are typing expressions in the context of the
423 top-level of the <literal>Main</literal> module. Everything
424 that is in scope at the top-level in the module
425 <literal>Main</literal> we just loaded is also in scope at the
426 prompt (probably including <literal>Prelude</literal>, as long
427 as <literal>Main</literal> doesn't explicitly hide it).</para>
430 <literal>*<replaceable>module</replaceable></literal> indicates
431 that it is the full top-level scope of
432 <replaceable>module</replaceable> that is contributing to the
433 scope for expressions typed at the prompt. Without the
434 <literal>*</literal>, just the exports of the module are
437 <para>We're not limited to a single module: GHCi can combine
438 scopes from multiple modules, in any mixture of
439 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
440 combines the scopes from all of these modules to form the scope
441 that is in effect at the prompt. For technical reasons, GHCi
442 can only support the <literal>*</literal>-form for modules which
443 are interpreted, so compiled modules and package modules can
444 only contribute their exports to the current scope.</para>
446 <para>The scope is manipulated using the
447 <literal>:module</literal> command. For example, if the current
448 scope is <literal>Prelude</literal>, then we can bring into
449 scope the exports from the module <literal>IO</literal> like
454 Prelude,IO> hPutStrLn stdout "hello\n"
459 <para>(Note: <literal>:module</literal> can be shortened to
460 <literal>:m</literal>). The full syntax of the
461 <literal>:module</literal> command is:</para>
464 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
467 <para>Using the <literal>+</literal> form of the
468 <literal>module</literal> commands adds modules to the current
469 scope, and <literal>-</literal> removes them. Without either
470 <literal>+</literal> or <literal>-</literal>, the current scope
471 is replaced by the set of modules specified. Note that if you
472 use this form and leave out <literal>Prelude</literal>, GHCi
473 will assume that you really wanted the
474 <literal>Prelude</literal> and add it in for you (if you don't
475 want the <literal>Prelude</literal>, then ask to remove it with
476 <literal>:m -Prelude</literal>).</para>
478 <para>The scope is automatically set after a
479 <literal>:load</literal> command, to the most recently loaded
480 "target" module, in a <literal>*</literal>-form if possible.
481 For example, if you say <literal>:load foo.hs bar.hs</literal>
482 and <filename>bar.hs</filename> contains module
483 <literal>Bar</literal>, then the scope will be set to
484 <literal>*Bar</literal> if <literal>Bar</literal> is
485 interpreted, or if <literal>Bar</literal> is compiled it will be
486 set to <literal>Prelude,Bar</literal> (GHCi automatically adds
487 <literal>Prelude</literal> if it isn't present and there aren't
488 any <literal>*</literal>-form modules).</para>
490 <para>With multiple modules in scope, especially multiple
491 <literal>*</literal>-form modules, it is likely that name
492 clashes will occur. Haskell specifies that name clashes are
493 only reported when an ambiguous identifier is used, and GHCi
494 behaves in the same way for expressions typed at the
498 <title>Qualified names</title>
500 <para>To make life slightly easier, the GHCi prompt also
501 behaves as if there is an implicit <literal>import
502 qualified</literal> declaration for every module in every
503 package, and every module currently loaded into GHCi.</para>
508 <title>Using <literal>do-</literal>notation at the prompt</title>
509 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
510 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
512 <para>GHCi actually accepts <firstterm>statements</firstterm>
513 rather than just expressions at the prompt. This means you can
514 bind values and functions to names, and use them in future
515 expressions or statements.</para>
517 <para>The syntax of a statement accepted at the GHCi prompt is
518 exactly the same as the syntax of a statement in a Haskell
519 <literal>do</literal> expression. However, there's no monad
520 overloading here: statements typed at the prompt must be in the
521 <literal>IO</literal> monad.</para>
523 <para>Here's an example:</para>
525 Prelude> x <- return 42
530 <para>The statement <literal>x <- return 42</literal> means
531 “execute <literal>return 42</literal> in the
532 <literal>IO</literal> monad, and bind the result to
533 <literal>x</literal>”. We can then use
534 <literal>x</literal> in future statements, for example to print
535 it as we did above.</para>
537 <para>Of course, you can also bind normal non-IO expressions
538 using the <literal>let</literal>-statement:</para>
545 <para>An important difference between the two types of binding
546 is that the monadic bind (<literal>p <- e</literal>) is
547 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
548 whereas with the <literal>let</literal> form, the expression
549 isn't evaluated immediately:</para>
551 Prelude> let x = error "help!"
556 <para>Any exceptions raised during the evaluation or execution
557 of the statement are caught and printed by the GHCi command line
558 interface (for more information on exceptions, see the module
559 <literal>Control.Exception</literal> in the libraries
560 documentation).</para>
562 <para>Every new binding shadows any existing bindings of the
563 same name, including entities that are in scope in the current
564 module context.</para>
566 <para>WARNING: temporary bindings introduced at the prompt only
567 last until the next <literal>:load</literal> or
568 <literal>:reload</literal> command, at which time they will be
569 simply lost. However, they do survive a change of context with
570 <literal>:module</literal>: the temporary bindings just move to
571 the new location.</para>
573 <para>HINT: To get a list of the bindings currently in scope, use the
574 <literal>:show bindings</literal> command:</para>
577 Prelude> :show bindings
581 <para>HINT: if you turn on the <literal>+t</literal> option,
582 GHCi will show the type of each variable bound by a statement.
584 <indexterm><primary><literal>+t</literal></primary></indexterm>
587 Prelude> let (x:xs) = [1..]
595 <title>The <literal>it</literal> variable</title>
596 <indexterm><primary><literal>it</literal></primary>
599 <para>Whenever an expression (or a non-binding statement, to be
600 precise) is typed at the prompt, GHCi implicitly binds its value
601 to the variable <literal>it</literal>. For example:</para>
609 <para>This is a result of the translation mentioned earlier,
610 namely that an expression <replaceable>e</replaceable> is
613 let it = <replaceable>e</replaceable>;
616 before execution, resulting in a binding for
617 <literal>it</literal>.</para>
619 <para>If the expression was of type <literal>IO a</literal> for
620 some <literal>a</literal>, then <literal>it</literal> will be
621 bound to the result of the <literal>IO</literal> computation,
622 which is of type <literal>a</literal>. eg.:</para>
624 Prelude> Time.getClockTime
626 Wed Mar 14 12:23:13 GMT 2001
629 <para>The corresponding translation for an IO-typed
630 <replaceable>e</replaceable> is
632 it <- <replaceable>e</replaceable>
636 <para>Note that <literal>it</literal> is shadowed by the new
637 value each time you evaluate a new expression, and the old value
638 of <literal>it</literal> is lost.</para>
643 <title>Type defaulting in GHCi</title>
644 <indexterm><primary>Type default</primary></indexterm>
645 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
647 Consider this GHCi session:
651 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
652 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
653 on the type <literal>a</literal>. For example:
655 ghci> (reverse []) :: String
657 ghci> (reverse []) :: [Int]
660 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
661 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. If the expression yields a set of
662 type constraints that are all from standard classes (<literal>Num</literal>, <literal>Eq</literal> etc.),
663 and at least one is either a numeric class <emphasis>or the <literal>Show</literal>,
664 <literal>Eq</literal>, or <literal>Ord</literal> class</emphasis>,
665 GHCi will try to use one of the <literal>default</literal> types, just as described in the Report.
670 <sect1 id="ghci-invokation">
671 <title>Invoking GHCi</title>
672 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
673 <indexterm><primary><option>––interactive</option></primary></indexterm>
675 <para>GHCi is invoked with the command <literal>ghci</literal> or
676 <literal>ghc ––interactive</literal>. One or more modules or
677 filenames can also be specified on the command line; this
678 instructs GHCi to load the specified modules or filenames (and all
679 the modules they depend on), just as if you had said
680 <literal>:load <replaceable>modules</replaceable></literal> at the
681 GHCi prompt (see <xref linkend="ghci-commands">). For example, to
682 start GHCi and load the program whose topmost module is in the
683 file <literal>Main.hs</literal>, we could say:</para>
689 <para>Most of the command-line options accepted by GHC (see <xref
690 linkend="using-ghc">) also make sense in interactive mode. The ones
691 that don't make sense are mostly obvious; for example, GHCi
692 doesn't generate interface files, so options related to interface
693 file generation won't have any effect.</para>
696 <title>Packages</title>
697 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
699 <para>Most packages (see <xref linkend="using-packages">) are
700 available without needing to specify any extra flags at all:
701 they will be automatically loaded the first time they are
704 <para>For non-auto packages, however, you need to request the
705 package be loaded by using the <literal>-package</literal> flag:</para>
711 / /_\// /_/ / / | | GHC Interactive, version 5.05, for Haskell 98.
712 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
713 \____/\/ /_/\____/|_| Type :? for help.
715 Loading package base ... linking ... done.
716 Loading package haskell98 ... linking ... done.
717 Loading package lang ... linking ... done.
718 Loading package concurrent ... linking ... done.
719 Loading package readline ... linking ... done.
720 Loading package unix ... linking ... done.
721 Loading package posix ... linking ... done.
722 Loading package util ... linking ... done.
723 Loading package data ... linking ... done.
727 <para>The following command works to load new packages into a
731 Prelude> :set -package <replaceable>name</replaceable>
734 <para>But note that doing this will cause all currently loaded
735 modules to be unloaded, and you'll be dumped back into the
736 <literal>Prelude</literal>.</para>
740 <title>Extra libraries</title>
741 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
743 <para>Extra libraries may be specified on the command line using
744 the normal <literal>-l<replaceable>lib</replaceable></literal>
745 option. (The term <emphasis>library</emphasis> here refers to
746 libraries of foreign object code; for using libraries of Haskell
747 source code, see <xref linkend="ghci-modules-filenames">.) For
748 example, to load the “m” library:</para>
754 <para>On systems with <literal>.so</literal>-style shared
755 libraries, the actual library loaded will the
756 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
757 searches the following places for libraries, in this order:</para>
761 <para>Paths specified using the
762 <literal>-L<replaceable>path</replaceable></literal>
763 command-line option,</para>
766 <para>the standard library search path for your system,
767 which on some systems may be overriden by setting the
768 <literal>LD_LIBRARY_PATH</literal> environment
773 <para>On systems with <literal>.dll</literal>-style shared
774 libraries, the actual library loaded will be
775 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
776 GHCi will signal an error if it can't find the library.</para>
778 <para>GHCi can also load plain object files
779 (<literal>.o</literal> or <literal>.obj</literal> depending on
780 your platform) from the command-line. Just add the name the
781 object file to the command line.</para>
786 <sect1 id="ghci-commands">
787 <title>GHCi commands</title>
789 <para>GHCi commands all begin with
790 ‘<literal>:</literal>’ and consist of a single command
791 name followed by zero or more parameters. The command name may be
792 abbreviated, as long as the abbreviation is not ambiguous. All of
793 the builtin commands, with the exception of
794 <literal>:unset</literal> and <literal>:undef</literal>, may be
795 abbreviated to a single letter.</para>
799 <term><literal>:add</literal>
800 <replaceable>module</replaceable> ...</term>
801 <indexterm><primary><literal>:add</literal></primary></indexterm>
803 <para>Add <replaceable>module</replaceable>(s) to the
804 current <firstterm>target set</firstterm>, and perform a
810 <term><literal>:browse</literal>
811 <optional><literal>*</literal></optional><replaceable>module</replaceable>
813 <indexterm><primary><literal>:browse</literal></primary>
816 <para>Displays the identifiers defined by the module
817 <replaceable>module</replaceable>, which must be either
818 loaded into GHCi or be a member of a package. If the
819 <literal>*</literal> symbol is placed before the module
820 name, then <emphasis>all</emphasis> the identifiers defined
821 in <replaceable>module</replaceable> are shown; otherwise
822 the list is limited to the exports of
823 <replaceable>module</replaceable>. The
824 <literal>*</literal>-form is only available for modules
825 which are interpreted; for compiled modules (including
826 modules from packages) only the non-<literal>*</literal>
827 form of <literal>:browse</literal> is available.</para>
832 <term><literal>:cd</literal> <replaceable>dir</replaceable></term>
833 <indexterm><primary><literal>:cd</literal></primary></indexterm>
835 <para>Changes the current working directory to
836 <replaceable>dir</replaceable>. A
837 ‘<literal>˜</literal>’ symbol at the
838 beginning of <replaceable>dir</replaceable> will be replaced
839 by the contents of the environment variable
840 <literal>HOME</literal>.</para>
842 <para>NOTE: changing directories causes all currently loaded
843 modules to be unloaded. This is because the search path is
844 usually expressed using relative directories, and changing
845 the search path in the middle of a session is not
851 <term><literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable></term>
852 <indexterm><primary><literal>:def</literal></primary></indexterm>
854 <para>The command <literal>:def</literal>
855 <replaceable>name</replaceable>
856 <replaceable>expr</replaceable> defines a new GHCi command
857 <literal>:<replaceable>name</replaceable></literal>,
858 implemented by the Haskell expression
859 <replaceable>expr</replaceable>, which must have type
860 <literal>String -> IO String</literal>. When
861 <literal>:<replaceable>name</replaceable>
862 <replaceable>args</replaceable></literal> is typed at the
863 prompt, GHCi will run the expression
864 <literal>(<replaceable>name</replaceable>
865 <replaceable>args</replaceable>)</literal>, take the
866 resulting <literal>String</literal>, and feed it back into
867 GHCi as a new sequence of commands. Separate commands in
868 the result must be separated by
869 ‘<literal>\n</literal>’.</para>
871 <para>That's all a little confusing, so here's a few
872 examples. To start with, here's a new GHCi command which
873 doesn't take any arguments or produce any results, it just
874 outputs the current date & time:</para>
877 Prelude> let date _ = Time.getClockTime >>= print >> return ""
878 Prelude> :def date date
880 Fri Mar 23 15:16:40 GMT 2001
883 <para>Here's an example of a command that takes an argument.
884 It's a re-implementation of <literal>:cd</literal>:</para>
887 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
888 Prelude> :def mycd mycd
892 <para>Or I could define a simple way to invoke
893 “<literal>ghc ––make Main</literal>” in the
894 current directory:</para>
897 Prelude> :def make (\_ -> return ":! ghc ––make Main")
904 <term><literal>:help</literal></term>
905 <indexterm><primary><literal>:help</literal></primary></indexterm>
906 <term><literal>:?</literal></term>
907 <indexterm><primary><literal>:?</literal></primary></indexterm>
909 <para>Displays a list of the available commands.</para>
914 <term><literal>:info</literal> <replaceable>name</replaceable>
916 <indexterm><primary><literal>:info</literal></primary>
919 <para>Displays information about the given name(s). For
920 example, if <replaceable>name</replaceable> is a class, then
921 the class methods and their types will be printed; if
922 <replaceable>name</replaceable> is a type constructor, then
923 its definition will be printed; if
924 <replaceable>name</replaceable> is a function, then its type
925 will be printed. If <replaceable>name</replaceable> has
926 been loaded from a source file, then GHCi will also display
927 the location of its definition in the source.</para>
932 <term><literal>:load</literal>
933 <replaceable>module</replaceable> ...</term>
934 <indexterm><primary><literal>:load</literal></primary></indexterm>
936 <para>Recursively loads the specified
937 <replaceable>module</replaceable>s, and all the modules they
938 depend on. Here, each <replaceable>module</replaceable>
939 must be a module name or filename, but may not be the name
940 of a module in a package.</para>
942 <para>All previously loaded modules, except package modules,
943 are forgotten. The new set of modules is known as the
944 <firstterm>target set</firstterm>. Note that
945 <literal>:load</literal> can be used without any arguments
946 to unload all the currently loaded modules and
949 <para>After a <literal>:load</literal> command, the current
950 context is set to:</para>
954 <para><replaceable>module</replaceable>, if it was loaded
955 successfully, or</para>
958 <para>the most recently successfully loaded module, if
959 any other modules were loaded as a result of the current
960 <literal>:load</literal>, or</para>
963 <para><literal>Prelude</literal> otherwise.</para>
970 <term><literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal></term>
971 <indexterm><primary><literal>:module</literal></primary></indexterm>
973 <para>Sets or modifies the current context for statements
974 typed at the prompt. See <xref linkend="ghci-scope"> for
980 <term><literal>:quit</literal></term>
981 <indexterm><primary><literal>:quit</literal></primary></indexterm>
983 <para>Quits GHCi. You can also quit by typing a control-D
984 at the prompt.</para>
989 <term><literal>:reload</literal></term>
990 <indexterm><primary><literal>:reload</literal></primary></indexterm>
992 <para>Attempts to reload the current target set (see
993 <literal>:load</literal>) if any of the modules in the set,
994 or any dependent module, has changed. Note that this may
995 entail loading new modules, or dropping modules which are no
996 longer indirectly required by the target.</para>
1001 <term><literal>:set</literal> <optional><replaceable>option</replaceable>...</optional></term>
1002 <indexterm><primary><literal>:set</literal></primary></indexterm>
1004 <para>Sets various options. See <xref linkend="ghci-set">
1005 for a list of available options. The
1006 <literal>:set</literal> command by itself shows which
1007 options are currently set.</para>
1012 <term><literal>:set</literal> <literal>args</literal>
1013 <replaceable>arg</replaceable> ...</term>
1014 <indexterm><primary><literal>:set</literal></primary></indexterm>
1016 <para>Sets the list of arguments which are returned when the
1017 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
1018 </indexterm>.</para>
1023 <term><literal>:set</literal> <literal>prog</literal>
1024 <replaceable>prog</replaceable></term>
1025 <indexterm><primary><literal>:set</literal></primary></indexterm>
1027 <para>Sets the string to be returned when the program calls
1028 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
1029 </indexterm>.</para>
1034 <term><literal>:show bindings</literal></term>
1035 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
1037 <para>Show the bindings made at the prompt and their
1043 <term><literal>:show modules</literal></term>
1044 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
1046 <para>Show the list of modules currently load.</para>
1051 <term><literal>:type</literal> <replaceable>expression</replaceable></term>
1052 <indexterm><primary><literal>:type</literal></primary></indexterm>
1054 <para>Infers and prints the type of
1055 <replaceable>expression</replaceable>, including explicit
1056 forall quantifiers for polymorphic types. The monomorphism
1057 restriction is <emphasis>not</emphasis> applied to the
1058 expression during type inference.</para>
1063 <term><literal>:kind</literal> <replaceable>type</replaceable></term>
1064 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1066 <para>Infers and prints the kind of
1067 <replaceable>type</replaceable>. The latter can be an arbitrary
1068 type expression, including a partial application of a type constructor,
1069 such as <literal>Either Int</literal>.</para>
1074 <term><literal>:undef</literal> <replaceable>name</replaceable></term>
1075 <indexterm><primary><literal>:undef</literal></primary></indexterm>
1077 <para>Undefines the user-defined command
1078 <replaceable>name</replaceable> (see <literal>:def</literal>
1084 <term><literal>:unset</literal> <replaceable>option</replaceable>...</term>
1085 <indexterm><primary><literal>:unset</literal></primary></indexterm>
1087 <para>Unsets certain options. See <xref linkend="ghci-set">
1088 for a list of available options.</para>
1093 <term><literal>:!</literal> <replaceable>command</replaceable>...</term>
1094 <indexterm><primary><literal>:!</literal></primary></indexterm>
1095 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
1097 <para>Executes the shell command
1098 <replaceable>command</replaceable>.</para>
1105 <sect1 id="ghci-set">
1106 <title>The <literal>:set</literal> command</title>
1107 <indexterm><primary><literal>:set</literal></primary></indexterm>
1109 <para>The <literal>:set</literal> command sets two types of
1110 options: GHCi options, which begin with
1111 ‘<literal>+</literal>” and “command-line”
1112 options, which begin with ‘-’. </para>
1114 <para>NOTE: at the moment, the <literal>:set</literal> command
1115 doesn't support any kind of quoting in its arguments: quotes will
1116 not be removed and cannot be used to group words together. For
1117 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
1121 <title>GHCi options</title>
1122 <indexterm><primary>options</primary><secondary>GHCi</secondary>
1125 <para>GHCi options may be set using <literal>:set</literal> and
1126 unset using <literal>:unset</literal>.</para>
1128 <para>The available GHCi options are:</para>
1132 <term><literal>+r</literal></term>
1133 <indexterm><primary><literal>+r</literal></primary></indexterm>
1134 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
1135 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
1137 <para>Normally, any evaluation of top-level expressions
1138 (otherwise known as CAFs or Constant Applicative Forms) in
1139 loaded modules is retained between evaluations. Turning
1140 on <literal>+r</literal> causes all evaluation of
1141 top-level expressions to be discarded after each
1142 evaluation (they are still retained
1143 <emphasis>during</emphasis> a single evaluation).</para>
1145 <para>This option may help if the evaluated top-level
1146 expressions are consuming large amounts of space, or if
1147 you need repeatable performance measurements.</para>
1152 <term><literal>+s</literal></term>
1153 <indexterm><primary><literal>+s</literal></primary></indexterm>
1155 <para>Display some stats after evaluating each expression,
1156 including the elapsed time and number of bytes allocated.
1157 NOTE: the allocation figure is only accurate to the size
1158 of the storage manager's allocation area, because it is
1159 calculated at every GC. Hence, you might see values of
1160 zero if no GC has occurred.</para>
1165 <term><literal>+t</literal></term>
1166 <indexterm><primary><literal>+t</literal></primary></indexterm>
1168 <para>Display the type of each variable bound after a
1169 statement is entered at the prompt. If the statement is a
1170 single expression, then the only variable binding will be
1172 ‘<literal>it</literal>’.</para>
1178 <sect2 id="ghci-cmd-line-options">
1179 <title>Setting GHC command-line options in GHCi</title>
1181 <para>Normal GHC command-line options may also be set using
1182 <literal>:set</literal>. For example, to turn on
1183 <option>-fglasgow-exts</option>, you would say:</para>
1186 Prelude> :set -fglasgow-exts
1189 <para>Any GHC command-line option that is designated as
1190 <firstterm>dynamic</firstterm> (see the table in <xref
1191 linkend="flag-reference">), may be set using
1192 <literal>:set</literal>. To unset an option, you can set the
1193 reverse option:</para>
1194 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
1197 Prelude> :set -fno-glasgow-exts
1200 <para><xref linkend="flag-reference"> lists the reverse for each
1201 option where applicable.</para>
1203 <para>Certain static options (<option>-package</option>,
1204 <option>-I</option>, <option>-i</option>, and
1205 <option>-l</option> in particular) will also work, but some may
1206 not take effect until the next reload.</para>
1207 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
1211 <sect1 id="ghci-dot-files">
1212 <title>The <filename>.ghci</filename> file</title>
1213 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
1215 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
1218 <para>When it starts, GHCi always reads and executes commands from
1219 <filename>$HOME/.ghci</filename>, followed by
1220 <filename>./.ghci</filename>.</para>
1222 <para>The <filename>.ghci</filename> in your home directory is
1223 most useful for turning on favourite options (eg. <literal>:set
1224 +s</literal>), and defining useful macros. Placing a
1225 <filename>.ghci</filename> file in a directory with a Haskell
1226 project is a useful way to set certain project-wide options so you
1227 don't have to type them everytime you start GHCi: eg. if your
1228 project uses GHC extensions and CPP, and has source files in three
1229 subdirectories A B and C, you might put the following lines in
1230 <filename>.ghci</filename>:</para>
1233 :set -fglasgow-exts -cpp
1237 <para>(Note that strictly speaking the <option>-i</option> flag is
1238 a static one, but in fact it works to set it using
1239 <literal>:set</literal> like this. The changes won't take effect
1240 until the next <literal>:load</literal>, though.)</para>
1242 <para>Two command-line options control whether the
1243 <filename>.ghci</filename> files are read:</para>
1247 <term><option>-ignore-dot-ghci</option></term>
1248 <indexterm><primary><option>-ignore-dot-ghci</option></primary>
1251 <para>Don't read either <filename>./.ghci</filename> or
1252 <filename>$HOME/.ghci</filename> when starting up.</para>
1256 <term><option>-read-dot-ghci</option></term>
1257 <indexterm><primary><option>-read-dot-ghci</option></primary>
1260 <para>Read <filename>.ghci</filename> and
1261 <filename>$HOME/.ghci</filename>. This is normally the
1262 default, but the <option>-read-dot-ghci</option> option may
1263 be used to override a previous
1264 <option>-ignore-dot-ghci</option> option.</para>
1272 <title>FAQ and Things To Watch Out For</title>
1276 <term>The interpreter can't load modules with foreign export
1277 declarations!</term>
1279 <para>Unfortunately not. We haven't implemented it yet.
1280 Please compile any offending modules by hand before loading
1281 them into GHCi.</para>
1286 <term><literal>-O</literal> doesn't work with GHCi!</term>
1287 <indexterm><primary><option>-O</option></primary>
1290 <para>For technical reasons, the bytecode compiler doesn't
1291 interact well with one of the optimisation passes, so we
1292 have disabled optimisation when using the interpreter. This
1293 isn't a great loss: you'll get a much bigger win by
1294 compiling the bits of your code that need to go fast, rather
1295 than interpreting them with optimisation turned on.</para>
1300 <term>Unboxed tuples don't work with GHCi</term>
1302 <para>That's right. You can always compile a module that
1303 uses unboxed tuples and load it into GHCi, however.
1304 (Incidentally the previous point, namely that
1305 <literal>-O</literal> is incompatible with GHCi, is because
1306 the bytecode compiler can't deal with unboxed
1312 <term>Concurrent threads don't carry on running when GHCi is
1313 waiting for input.</term>
1315 <para>No, they don't. This is because the Haskell binding
1316 to the GNU readline library doesn't support reading from the
1317 terminal in a non-blocking way, which is required to work
1318 properly with GHC's concurrency model.</para>
1323 <term>After using <literal>getContents</literal>, I can't use
1324 <literal>stdin</literal> again until I do
1325 <literal>:load</literal> or <literal>:reload</literal>.</term>
1328 <para>This is the defined behaviour of
1329 <literal>getContents</literal>: it puts the stdin Handle in
1330 a state known as <firstterm>semi-closed</firstterm>, wherein
1331 any further I/O operations on it are forbidden. Because I/O
1332 state is retained between computations, the semi-closed
1333 state persists until the next <literal>:load</literal> or
1334 <literal>:reload</literal> command.</para>
1336 <para>You can make <literal>stdin</literal> reset itself
1337 after every evaluation by giving GHCi the command
1338 <literal>:set +r</literal>. This works because
1339 <literal>stdin</literal> is just a top-level expression that
1340 can be reverted to its unevaluated state in the same way as
1341 any other top-level expression (CAF).</para>
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