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 6.6, for Haskell 98.
32 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
33 \____/\/ /_/\____/|_| Type :? for help.
35 Loading package base ... 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 :main [<arguments> ...] run the main function with the given arguments
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
62 :set prompt <prompt> set the prompt used in GHCi
64 :show modules show the currently loaded modules
65 :show bindings show the current bindings made at the prompt
67 :ctags [<file>] create tags file for Vi (default: "tags")
68 :etags [<file>] create tags file for Emacs (defauilt: "TAGS")
69 :type <expr> show the type of <expr>
70 :kind <type> show the kind of <type>
71 :undef <cmd> undefine user-defined command :<cmd>
72 :unset <option> ... unset options
74 :!<command> run the shell command <command>
76 Options for ':set' and ':unset':
78 +r revert top-level expressions after each evaluation
79 +s print timing/memory stats after each evaluation
80 +t print type after evaluation
81 -<flags> most GHC command line flags can also be set here
82 (eg. -v2, -fglasgow-exts, etc.)
85 <para>We'll explain most of these commands as we go along. For
86 Hugs users: many things work the same as in Hugs, so you should be
87 able to get going straight away.</para>
89 <para>Haskell expressions can be typed at the prompt:</para>
90 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
96 Prelude> let x = 42 in x / 9
101 <para>GHCi interprets the whole line as an expression to evaluate.
102 The expression may not span several lines - as soon as you press
103 enter, GHCi will attempt to evaluate it.</para>
107 <title>Loading source files</title>
109 <para>Suppose we have the following Haskell source code, which we
110 place in a file <filename>Main.hs</filename>:</para>
113 main = print (fac 20)
116 fac n = n * fac (n-1)
119 <para>You can save <filename>Main.hs</filename> anywhere you like,
120 but if you save it somewhere other than the current
121 directory<footnote><para>If you started up GHCi from the command
122 line then GHCi's current directory is the same as the current
123 directory of the shell from which it was started. If you started
124 GHCi from the “Start” menu in Windows, then the
125 current directory is probably something like
126 <filename>C:\Documents and Settings\<replaceable>user
127 name</replaceable></filename>.</para> </footnote> then we will
128 need to change to the right directory in GHCi:</para>
131 Prelude> :cd <replaceable>dir</replaceable>
134 <para>where <replaceable>dir</replaceable> is the directory (or
135 folder) in which you saved <filename>Main.hs</filename>.</para>
137 <para>To load a Haskell source file into GHCi, use the
138 <literal>:load</literal> command:</para>
139 <indexterm><primary><literal>:load</literal></primary></indexterm>
143 Compiling Main ( Main.hs, interpreted )
144 Ok, modules loaded: Main.
148 <para>GHCi has loaded the <literal>Main</literal> module, and the
149 prompt has changed to “<literal>*Main></literal>” to
150 indicate that the current context for expressions typed at the
151 prompt is the <literal>Main</literal> module we just loaded (we'll
152 explain what the <literal>*</literal> means later in <xref
153 linkend="ghci-scope"/>). So we can now type expressions involving
154 the functions from <filename>Main.hs</filename>:</para>
161 <para>Loading a multi-module program is just as straightforward;
162 just give the name of the “topmost” module to the
163 <literal>:load</literal> command (hint: <literal>:load</literal>
164 can be abbreviated to <literal>:l</literal>). The topmost module
165 will normally be <literal>Main</literal>, but it doesn't have to
166 be. GHCi will discover which modules are required, directly or
167 indirectly, by the topmost module, and load them all in dependency
170 <sect2 id="ghci-modules-filenames">
171 <title>Modules vs. filenames</title>
172 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
173 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
175 <para>Question: How does GHC find the filename which contains
176 module <replaceable>M</replaceable>? Answer: it looks for the
177 file <literal><replaceable>M</replaceable>.hs</literal>, or
178 <literal><replaceable>M</replaceable>.lhs</literal>. This means
179 that for most modules, the module name must match the filename.
180 If it doesn't, GHCi won't be able to find it.</para>
182 <para>There is one exception to this general rule: when you load
183 a program with <literal>:load</literal>, or specify it when you
184 invoke <literal>ghci</literal>, you can give a filename rather
185 than a module name. This filename is loaded if it exists, and
186 it may contain any module you like. This is particularly
187 convenient if you have several <literal>Main</literal> modules
188 in the same directory and you can't call them all
189 <filename>Main.hs</filename>.</para>
191 <para>The search path for finding source files is specified with
192 the <option>-i</option> option on the GHCi command line, like
194 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
196 <para>or it can be set using the <literal>:set</literal> command
197 from within GHCi (see <xref
198 linkend="ghci-cmd-line-options"/>)<footnote><para>Note that in
199 GHCi, and <option>––make</option> mode, the <option>-i</option>
200 option is used to specify the search path for
201 <emphasis>source</emphasis> files, whereas in standard
202 batch-compilation mode the <option>-i</option> option is used to
203 specify the search path for interface files, see <xref
204 linkend="search-path"/>.</para> </footnote></para>
206 <para>One consequence of the way that GHCi follows dependencies
207 to find modules to load is that every module must have a source
208 file. The only exception to the rule is modules that come from
209 a package, including the <literal>Prelude</literal> and standard
210 libraries such as <literal>IO</literal> and
211 <literal>Complex</literal>. If you attempt to load a module for
212 which GHCi can't find a source file, even if there are object
213 and interface files for the module, you'll get an error
218 <title>Making changes and recompilation</title>
219 <indexterm><primary><literal>:reload</literal></primary></indexterm>
221 <para>If you make some changes to the source code and want GHCi
222 to recompile the program, give the <literal>:reload</literal>
223 command. The program will be recompiled as necessary, with GHCi
224 doing its best to avoid actually recompiling modules if their
225 external dependencies haven't changed. This is the same
226 mechanism we use to avoid re-compiling modules in the batch
227 compilation setting (see <xref linkend="recomp"/>).</para>
231 <sect1 id="ghci-compiled">
232 <title>Loading compiled code</title>
233 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
235 <para>When you load a Haskell source module into GHCi, it is
236 normally converted to byte-code and run using the interpreter.
237 However, interpreted code can also run alongside compiled code in
238 GHCi; indeed, normally when GHCi starts, it loads up a compiled
239 copy of the <literal>base</literal> package, which contains the
240 <literal>Prelude</literal>.</para>
242 <para>Why should we want to run compiled code? Well, compiled
243 code is roughly 10x faster than interpreted code, but takes about
244 2x longer to produce (perhaps longer if optimisation is on). So
245 it pays to compile the parts of a program that aren't changing
246 very often, and use the interpreter for the code being actively
249 <para>When loading up source files with <literal>:load</literal>,
250 GHCi looks for any corresponding compiled object files, and will
251 use one in preference to interpreting the source if possible. For
252 example, suppose we have a 4-module program consisting of modules
253 A, B, C, and D. Modules B and C both import D only,
254 and A imports both B & C:</para>
262 <para>We can compile D, then load the whole program, like this:</para>
264 Prelude> :! ghc -c D.hs
266 Skipping D ( D.hs, D.o )
267 Compiling C ( C.hs, interpreted )
268 Compiling B ( B.hs, interpreted )
269 Compiling A ( A.hs, interpreted )
270 Ok, modules loaded: A, B, C, D.
274 <para>In the messages from the compiler, we see that it skipped D,
275 and used the object file <filename>D.o</filename>. The message
276 <literal>Skipping</literal> <replaceable>module</replaceable>
277 indicates that compilation for <replaceable>module</replaceable>
278 isn't necessary, because the source and everything it depends on
279 is unchanged since the last compilation.</para>
281 <para>At any time you can use the command
282 <literal>:show modules</literal>
283 to get a list of the modules currently loaded
289 C ( C.hs, interpreted )
290 B ( B.hs, interpreted )
291 A ( A.hs, interpreted )
294 <para>If we now modify the source of D (or pretend to: using Unix
295 command <literal>touch</literal> on the source file is handy for
296 this), the compiler will no longer be able to use the object file,
297 because it might be out of date:</para>
302 Compiling D ( D.hs, interpreted )
303 Skipping C ( C.hs, interpreted )
304 Skipping B ( B.hs, interpreted )
305 Skipping A ( A.hs, interpreted )
306 Ok, modules loaded: A, B, C, D.
310 <para>Note that module D was compiled, but in this instance
311 because its source hadn't really changed, its interface remained
312 the same, and the recompilation checker determined that A, B and C
313 didn't need to be recompiled.</para>
315 <para>So let's try compiling one of the other modules:</para>
318 *Main> :! ghc -c C.hs
320 Compiling D ( D.hs, interpreted )
321 Compiling C ( C.hs, interpreted )
322 Compiling B ( B.hs, interpreted )
323 Compiling A ( A.hs, interpreted )
324 Ok, modules loaded: A, B, C, D.
327 <para>We didn't get the compiled version of C! What happened?
328 Well, in GHCi a compiled module may only depend on other compiled
329 modules, and in this case C depends on D, which doesn't have an
330 object file, so GHCi also rejected C's object file. Ok, so let's
331 also compile D:</para>
334 *Main> :! ghc -c D.hs
336 Ok, modules loaded: A, B, C, D.
339 <para>Nothing happened! Here's another lesson: newly compiled
340 modules aren't picked up by <literal>:reload</literal>, only
341 <literal>:load</literal>:</para>
345 Skipping D ( D.hs, D.o )
346 Skipping C ( C.hs, C.o )
347 Compiling B ( B.hs, interpreted )
348 Compiling A ( A.hs, interpreted )
349 Ok, modules loaded: A, B, C, D.
352 <para>HINT: since GHCi will only use a compiled object file if it
353 can be sure that the compiled version is up-to-date, a good technique
354 when working on a large program is to occasionally run
355 <literal>ghc ––make</literal> to compile the whole project (say
356 before you go for lunch :-), then continue working in the
357 interpreter. As you modify code, the new modules will be
358 interpreted, but the rest of the project will remain
364 <title>Interactive evaluation at the prompt</title>
366 <para>When you type an expression at the prompt, GHCi immediately
367 evaluates and prints the result:
369 Prelude> reverse "hello"
376 <sect2><title>I/O actions at the prompt</title>
378 <para>GHCi does more than simple expression evaluation at the prompt.
379 If you type something of type <literal>IO a</literal> for some
380 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
381 as an IO-computation.
385 Prelude> putStrLn "hello"
388 Furthermore, GHCi will print the result of the I/O action if (and only
391 <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
392 <listitem><para>The result type is not
393 <literal>()</literal>.</para></listitem>
395 For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
397 Prelude> putStrLn "hello"
399 Prelude> do { putStrLn "hello"; return "yes" }
406 <title>Using <literal>do-</literal>notation at the prompt</title>
407 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
408 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
410 <para>GHCi actually accepts <firstterm>statements</firstterm>
411 rather than just expressions at the prompt. This means you can
412 bind values and functions to names, and use them in future
413 expressions or statements.</para>
415 <para>The syntax of a statement accepted at the GHCi prompt is
416 exactly the same as the syntax of a statement in a Haskell
417 <literal>do</literal> expression. However, there's no monad
418 overloading here: statements typed at the prompt must be in the
419 <literal>IO</literal> monad.
421 Prelude> x <- return 42
427 The statement <literal>x <- return 42</literal> means
428 “execute <literal>return 42</literal> in the
429 <literal>IO</literal> monad, and bind the result to
430 <literal>x</literal>”. We can then use
431 <literal>x</literal> in future statements, for example to print
432 it as we did above.</para>
434 <para>GHCi will print the result of a statement if and only if:
437 <para>The statement is not a binding, or it is a monadic binding
438 (<literal>p <- e</literal>) that binds exactly one
442 <para>The variable's type is not polymorphic, is not
443 <literal>()</literal>, and is an instance of
444 <literal>Show</literal></para>
449 <para>Of course, you can also bind normal non-IO expressions
450 using the <literal>let</literal>-statement:</para>
457 <para>Another important difference between the two types of binding
458 is that the monadic bind (<literal>p <- e</literal>) is
459 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
460 whereas with the <literal>let</literal> form, the expression
461 isn't evaluated immediately:</para>
463 Prelude> let x = error "help!"
469 <para>Note that <literal>let</literal> bindings do not automatically
470 print the value bound, unlike monadic bindings.</para>
472 <para>Any exceptions raised during the evaluation or execution
473 of the statement are caught and printed by the GHCi command line
474 interface (for more information on exceptions, see the module
475 <literal>Control.Exception</literal> in the libraries
476 documentation).</para>
478 <para>Every new binding shadows any existing bindings of the
479 same name, including entities that are in scope in the current
480 module context.</para>
482 <para>WARNING: temporary bindings introduced at the prompt only
483 last until the next <literal>:load</literal> or
484 <literal>:reload</literal> command, at which time they will be
485 simply lost. However, they do survive a change of context with
486 <literal>:module</literal>: the temporary bindings just move to
487 the new location.</para>
489 <para>HINT: To get a list of the bindings currently in scope, use the
490 <literal>:show bindings</literal> command:</para>
493 Prelude> :show bindings
497 <para>HINT: if you turn on the <literal>+t</literal> option,
498 GHCi will show the type of each variable bound by a statement.
500 <indexterm><primary><literal>+t</literal></primary></indexterm>
503 Prelude> let (x:xs) = [1..]
510 <sect2 id="ghci-scope">
511 <title>What's really in scope at the prompt?</title>
513 <para>When you type an expression at the prompt, what
514 identifiers and types are in scope? GHCi provides a flexible
515 way to control exactly how the context for an expression is
516 constructed. Let's start with the simple cases; when you start
517 GHCi the prompt looks like this:</para>
519 <screen>Prelude></screen>
521 <para>Which indicates that everything from the module
522 <literal>Prelude</literal> is currently in scope. If we now
523 load a file into GHCi, the prompt will change:</para>
526 Prelude> :load Main.hs
527 Compiling Main ( Main.hs, interpreted )
531 <para>The new prompt is <literal>*Main</literal>, which
532 indicates that we are typing expressions in the context of the
533 top-level of the <literal>Main</literal> module. Everything
534 that is in scope at the top-level in the module
535 <literal>Main</literal> we just loaded is also in scope at the
536 prompt (probably including <literal>Prelude</literal>, as long
537 as <literal>Main</literal> doesn't explicitly hide it).</para>
540 <literal>*<replaceable>module</replaceable></literal> indicates
541 that it is the full top-level scope of
542 <replaceable>module</replaceable> that is contributing to the
543 scope for expressions typed at the prompt. Without the
544 <literal>*</literal>, just the exports of the module are
547 <para>We're not limited to a single module: GHCi can combine
548 scopes from multiple modules, in any mixture of
549 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
550 combines the scopes from all of these modules to form the scope
551 that is in effect at the prompt. For technical reasons, GHCi
552 can only support the <literal>*</literal>-form for modules which
553 are interpreted, so compiled modules and package modules can
554 only contribute their exports to the current scope.</para>
556 <para>The scope is manipulated using the
557 <literal>:module</literal> command. For example, if the current
558 scope is <literal>Prelude</literal>, then we can bring into
559 scope the exports from the module <literal>IO</literal> like
564 Prelude IO> hPutStrLn stdout "hello\n"
569 <para>(Note: <literal>:module</literal> can be shortened to
570 <literal>:m</literal>). The full syntax of the
571 <literal>:module</literal> command is:</para>
574 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
577 <para>Using the <literal>+</literal> form of the
578 <literal>module</literal> commands adds modules to the current
579 scope, and <literal>-</literal> removes them. Without either
580 <literal>+</literal> or <literal>-</literal>, the current scope
581 is replaced by the set of modules specified. Note that if you
582 use this form and leave out <literal>Prelude</literal>, GHCi
583 will assume that you really wanted the
584 <literal>Prelude</literal> and add it in for you (if you don't
585 want the <literal>Prelude</literal>, then ask to remove it with
586 <literal>:m -Prelude</literal>).</para>
588 <para>The scope is automatically set after a
589 <literal>:load</literal> command, to the most recently loaded
590 "target" module, in a <literal>*</literal>-form if possible.
591 For example, if you say <literal>:load foo.hs bar.hs</literal>
592 and <filename>bar.hs</filename> contains module
593 <literal>Bar</literal>, then the scope will be set to
594 <literal>*Bar</literal> if <literal>Bar</literal> is
595 interpreted, or if <literal>Bar</literal> is compiled it will be
596 set to <literal>Prelude Bar</literal> (GHCi automatically adds
597 <literal>Prelude</literal> if it isn't present and there aren't
598 any <literal>*</literal>-form modules).</para>
600 <para>With multiple modules in scope, especially multiple
601 <literal>*</literal>-form modules, it is likely that name
602 clashes will occur. Haskell specifies that name clashes are
603 only reported when an ambiguous identifier is used, and GHCi
604 behaves in the same way for expressions typed at the
608 Hint: GHCi will tab-complete names that are in scope; for
609 example, if you run GHCi and type <literal>J<tab></literal>
610 then GHCi will expand it to <literal>Just </literal>.
614 <title>Qualified names</title>
616 <para>To make life slightly easier, the GHCi prompt also
617 behaves as if there is an implicit <literal>import
618 qualified</literal> declaration for every module in every
619 package, and every module currently loaded into GHCi.</para>
623 <title>The <literal>:main</literal> command</title>
626 When a program is compiled and executed, it can use the
627 <literal>getArgs</literal> function to access the
628 command-line arguments.
629 However, we cannot simply pass the arguments to the
630 <literal>main</literal> function while we are testing in ghci,
631 as the <literal>main</literal> function doesn't take its
636 Instead, we can use the <literal>:main</literal> command.
637 This runs whatever <literal>main</literal> is in scope, with
638 any arguments being treated the same as command-line arguments,
643 Prelude> let main = System.Environment.getArgs >>= print
644 Prelude> :main foo bar
653 <title>The <literal>it</literal> variable</title>
654 <indexterm><primary><literal>it</literal></primary>
657 <para>Whenever an expression (or a non-binding statement, to be
658 precise) is typed at the prompt, GHCi implicitly binds its value
659 to the variable <literal>it</literal>. For example:</para>
666 <para>What actually happens is that GHCi typechecks the
667 expression, and if it doesn't have an <literal>IO</literal> type,
668 then it transforms it as follows: an expression
669 <replaceable>e</replaceable> turns into
671 let it = <replaceable>e</replaceable>;
674 which is then run as an IO-action.</para>
676 <para>Hence, the original expression must have a type which is an
677 instance of the <literal>Show</literal> class, or GHCi will
683 <interactive>:1:0:
684 No instance for (Show (a -> a))
685 arising from use of `print' at <interactive>:1:0-1
686 Possible fix: add an instance declaration for (Show (a -> a))
687 In the expression: print it
688 In a 'do' expression: print it
691 <para>The error message contains some clues as to the
692 transformation happening internally.</para>
694 <para>If the expression was instead of type <literal>IO a</literal> for
695 some <literal>a</literal>, then <literal>it</literal> will be
696 bound to the result of the <literal>IO</literal> computation,
697 which is of type <literal>a</literal>. eg.:</para>
699 Prelude> Time.getClockTime
700 Wed Mar 14 12:23:13 GMT 2001
702 Wed Mar 14 12:23:13 GMT 2001
705 <para>The corresponding translation for an IO-typed
706 <replaceable>e</replaceable> is
708 it <- <replaceable>e</replaceable>
712 <para>Note that <literal>it</literal> is shadowed by the new
713 value each time you evaluate a new expression, and the old value
714 of <literal>it</literal> is lost.</para>
718 <sect2 id="extended-default-rules">
719 <title>Type defaulting in GHCi</title>
720 <indexterm><primary>Type default</primary></indexterm>
721 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
723 Consider this GHCi session:
727 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
728 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
729 on the type <literal>a</literal>. For example:
731 ghci> (reverse []) :: String
733 ghci> (reverse []) :: [Int]
736 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
737 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
738 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
739 a)</literal> for each type variable <literal>a</literal>, and defaults the
742 <listitem><para> The type variable <literal>a</literal>
743 appears in no other constraints </para></listitem>
744 <listitem><para> All the classes <literal>Ci</literal> are standard.</para></listitem>
745 <listitem><para> At least one of the classes <literal>Ci</literal> is
746 numeric.</para></listitem>
748 At the GHCi prompt, the second and third rules are relaxed as follows
749 (differences italicised):
751 <listitem><para> <emphasis>All</emphasis> of the classes
752 <literal>Ci</literal> are single-parameter type classes.</para></listitem>
753 <listitem><para> At least one of the classes <literal>Ci</literal> is
754 numeric, <emphasis>or is <literal>Show</literal>,
755 <literal>Eq</literal>, or <literal>Ord</literal></emphasis>.</para></listitem>
757 The same type-default behaviour can be enabled in an ordinary Haskell
758 module, using the flag <literal>-fextended-default-rules</literal>.
763 <sect1 id="ghci-invocation">
764 <title>Invoking GHCi</title>
765 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
766 <indexterm><primary><option>––interactive</option></primary></indexterm>
768 <para>GHCi is invoked with the command <literal>ghci</literal> or
769 <literal>ghc ––interactive</literal>. One or more modules or
770 filenames can also be specified on the command line; this
771 instructs GHCi to load the specified modules or filenames (and all
772 the modules they depend on), just as if you had said
773 <literal>:load <replaceable>modules</replaceable></literal> at the
774 GHCi prompt (see <xref linkend="ghci-commands"/>). For example, to
775 start GHCi and load the program whose topmost module is in the
776 file <literal>Main.hs</literal>, we could say:</para>
782 <para>Most of the command-line options accepted by GHC (see <xref
783 linkend="using-ghc"/>) also make sense in interactive mode. The ones
784 that don't make sense are mostly obvious; for example, GHCi
785 doesn't generate interface files, so options related to interface
786 file generation won't have any effect.</para>
789 <title>Packages</title>
790 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
792 <para>Most packages (see <xref linkend="using-packages"/>) are
793 available without needing to specify any extra flags at all:
794 they will be automatically loaded the first time they are
797 <para>For non-auto packages, however, you need to request the
798 package be loaded by using the <literal>-package</literal> flag:</para>
801 $ ghci -package readline
804 / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98.
805 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
806 \____/\/ /_/\____/|_| Type :? for help.
808 Loading package base ... linking ... done.
809 Loading package readline-1.0 ... linking ... done.
813 <para>The following command works to load new packages into a
817 Prelude> :set -package <replaceable>name</replaceable>
820 <para>But note that doing this will cause all currently loaded
821 modules to be unloaded, and you'll be dumped back into the
822 <literal>Prelude</literal>.</para>
826 <title>Extra libraries</title>
827 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
829 <para>Extra libraries may be specified on the command line using
830 the normal <literal>-l<replaceable>lib</replaceable></literal>
831 option. (The term <emphasis>library</emphasis> here refers to
832 libraries of foreign object code; for using libraries of Haskell
833 source code, see <xref linkend="ghci-modules-filenames"/>.) For
834 example, to load the “m” library:</para>
840 <para>On systems with <literal>.so</literal>-style shared
841 libraries, the actual library loaded will the
842 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
843 searches the following places for libraries, in this order:</para>
847 <para>Paths specified using the
848 <literal>-L<replaceable>path</replaceable></literal>
849 command-line option,</para>
852 <para>the standard library search path for your system,
853 which on some systems may be overridden by setting the
854 <literal>LD_LIBRARY_PATH</literal> environment
859 <para>On systems with <literal>.dll</literal>-style shared
860 libraries, the actual library loaded will be
861 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
862 GHCi will signal an error if it can't find the library.</para>
864 <para>GHCi can also load plain object files
865 (<literal>.o</literal> or <literal>.obj</literal> depending on
866 your platform) from the command-line. Just add the name the
867 object file to the command line.</para>
869 <para>Ordering of <option>-l</option> options matters: a library
870 should be mentioned <emphasis>before</emphasis> the libraries it
871 depends on (see <xref linkend="options-linker"/>).</para>
876 <sect1 id="ghci-commands">
877 <title>GHCi commands</title>
879 <para>GHCi commands all begin with
880 ‘<literal>:</literal>’ and consist of a single command
881 name followed by zero or more parameters. The command name may be
882 abbreviated, with ambiguities being resolved in favour of the more
883 commonly used commands.</para>
888 <literal>:add</literal> <replaceable>module</replaceable> ...
889 <indexterm><primary><literal>:add</literal></primary></indexterm>
892 <para>Add <replaceable>module</replaceable>(s) to the
893 current <firstterm>target set</firstterm>, and perform a
900 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
901 <indexterm><primary><literal>:browse</literal></primary></indexterm>
904 <para>Displays the identifiers defined by the module
905 <replaceable>module</replaceable>, which must be either
906 loaded into GHCi or be a member of a package. If the
907 <literal>*</literal> symbol is placed before the module
908 name, then <emphasis>all</emphasis> the identifiers defined
909 in <replaceable>module</replaceable> are shown; otherwise
910 the list is limited to the exports of
911 <replaceable>module</replaceable>. The
912 <literal>*</literal>-form is only available for modules
913 which are interpreted; for compiled modules (including
914 modules from packages) only the non-<literal>*</literal>
915 form of <literal>:browse</literal> is available.</para>
921 <literal>:cd</literal> <replaceable>dir</replaceable>
922 <indexterm><primary><literal>:cd</literal></primary></indexterm>
925 <para>Changes the current working directory to
926 <replaceable>dir</replaceable>. A
927 ‘<literal>˜</literal>’ symbol at the
928 beginning of <replaceable>dir</replaceable> will be replaced
929 by the contents of the environment variable
930 <literal>HOME</literal>.</para>
932 <para>NOTE: changing directories causes all currently loaded
933 modules to be unloaded. This is because the search path is
934 usually expressed using relative directories, and changing
935 the search path in the middle of a session is not
942 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
943 <indexterm><primary><literal>:def</literal></primary></indexterm>
946 <para>The command <literal>:def</literal>
947 <replaceable>name</replaceable>
948 <replaceable>expr</replaceable> defines a new GHCi command
949 <literal>:<replaceable>name</replaceable></literal>,
950 implemented by the Haskell expression
951 <replaceable>expr</replaceable>, which must have type
952 <literal>String -> IO String</literal>. When
953 <literal>:<replaceable>name</replaceable>
954 <replaceable>args</replaceable></literal> is typed at the
955 prompt, GHCi will run the expression
956 <literal>(<replaceable>name</replaceable>
957 <replaceable>args</replaceable>)</literal>, take the
958 resulting <literal>String</literal>, and feed it back into
959 GHCi as a new sequence of commands. Separate commands in
960 the result must be separated by
961 ‘<literal>\n</literal>’.</para>
963 <para>That's all a little confusing, so here's a few
964 examples. To start with, here's a new GHCi command which
965 doesn't take any arguments or produce any results, it just
966 outputs the current date & time:</para>
969 Prelude> let date _ = Time.getClockTime >>= print >> return ""
970 Prelude> :def date date
972 Fri Mar 23 15:16:40 GMT 2001
975 <para>Here's an example of a command that takes an argument.
976 It's a re-implementation of <literal>:cd</literal>:</para>
979 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
980 Prelude> :def mycd mycd
984 <para>Or I could define a simple way to invoke
985 “<literal>ghc ––make Main</literal>” in the
986 current directory:</para>
989 Prelude> :def make (\_ -> return ":! ghc ––make Main")
992 <para>We can define a command that reads GHCi input from a
993 file. This might be useful for creating a set of bindings
994 that we want to repeatedly load into the GHCi session:</para>
997 Prelude> :def . readFile
998 Prelude> :. cmds.ghci
1001 <para>Notice that we named the command
1002 <literal>:.</literal>, by analogy with the
1003 ‘<literal>.</literal>’ Unix shell command that
1004 does the same thing.</para>
1010 <literal>:help</literal>
1011 <indexterm><primary><literal>:help</literal></primary></indexterm>
1014 <literal>:?</literal>
1015 <indexterm><primary><literal>:?</literal></primary></indexterm>
1018 <para>Displays a list of the available commands.</para>
1024 <literal>:info</literal> <replaceable>name</replaceable> ...
1025 <indexterm><primary><literal>:info</literal></primary></indexterm>
1028 <para>Displays information about the given name(s). For
1029 example, if <replaceable>name</replaceable> is a class, then
1030 the class methods and their types will be printed; if
1031 <replaceable>name</replaceable> is a type constructor, then
1032 its definition will be printed; if
1033 <replaceable>name</replaceable> is a function, then its type
1034 will be printed. If <replaceable>name</replaceable> has
1035 been loaded from a source file, then GHCi will also display
1036 the location of its definition in the source.</para>
1042 <literal>:load</literal> <replaceable>module</replaceable> ...
1043 <indexterm><primary><literal>:load</literal></primary></indexterm>
1046 <para>Recursively loads the specified
1047 <replaceable>module</replaceable>s, and all the modules they
1048 depend on. Here, each <replaceable>module</replaceable>
1049 must be a module name or filename, but may not be the name
1050 of a module in a package.</para>
1052 <para>All previously loaded modules, except package modules,
1053 are forgotten. The new set of modules is known as the
1054 <firstterm>target set</firstterm>. Note that
1055 <literal>:load</literal> can be used without any arguments
1056 to unload all the currently loaded modules and
1059 <para>After a <literal>:load</literal> command, the current
1060 context is set to:</para>
1064 <para><replaceable>module</replaceable>, if it was loaded
1065 successfully, or</para>
1068 <para>the most recently successfully loaded module, if
1069 any other modules were loaded as a result of the current
1070 <literal>:load</literal>, or</para>
1073 <para><literal>Prelude</literal> otherwise.</para>
1081 <literal>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
1082 <indexterm><primary><literal>:main</literal></primary></indexterm>
1086 When a program is compiled and executed, it can use the
1087 <literal>getArgs</literal> function to access the
1088 command-line arguments.
1089 However, we cannot simply pass the arguments to the
1090 <literal>main</literal> function while we are testing in ghci,
1091 as the <literal>main</literal> function doesn't take its
1096 Instead, we can use the <literal>:main</literal> command.
1097 This runs whatever <literal>main</literal> is in scope, with
1098 any arguments being treated the same as command-line arguments,
1103 Prelude> let main = System.Environment.getArgs >>= print
1104 Prelude> :main foo bar
1113 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
1114 <indexterm><primary><literal>:module</literal></primary></indexterm>
1117 <para>Sets or modifies the current context for statements
1118 typed at the prompt. See <xref linkend="ghci-scope"/> for
1119 more details.</para>
1125 <literal>:quit</literal>
1126 <indexterm><primary><literal>:quit</literal></primary></indexterm>
1129 <para>Quits GHCi. You can also quit by typing a control-D
1130 at the prompt.</para>
1136 <literal>:reload</literal>
1137 <indexterm><primary><literal>:reload</literal></primary></indexterm>
1140 <para>Attempts to reload the current target set (see
1141 <literal>:load</literal>) if any of the modules in the set,
1142 or any dependent module, has changed. Note that this may
1143 entail loading new modules, or dropping modules which are no
1144 longer indirectly required by the target.</para>
1150 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
1151 <indexterm><primary><literal>:set</literal></primary></indexterm>
1154 <para>Sets various options. See <xref linkend="ghci-set"/>
1155 for a list of available options. The
1156 <literal>:set</literal> command by itself shows which
1157 options are currently set.</para>
1163 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
1164 <indexterm><primary><literal>:set args</literal></primary></indexterm>
1167 <para>Sets the list of arguments which are returned when the
1168 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
1169 </indexterm>.</para>
1175 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
1176 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
1179 <para>Sets the string to be returned when the program calls
1180 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
1181 </indexterm>.</para>
1187 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
1190 <para>Sets the string to be used as the prompt in GHCi.
1191 Inside <replaceable>prompt</replaceable>, the sequence
1192 <literal>%s</literal> is replaced by the names of the
1193 modules currently in scope, and <literal>%%</literal> is
1194 replaced by <literal>%</literal>.</para>
1200 <literal>:show bindings</literal>
1201 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
1204 <para>Show the bindings made at the prompt and their
1211 <literal>:show modules</literal>
1212 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
1215 <para>Show the list of modules currently load.</para>
1221 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1222 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1223 <indexterm><primary><literal>:etags</literal></primary>
1225 <indexterm><primary><literal>:etags</literal></primary>
1229 <para>Generates a “tags” file for Vi-style editors
1230 (<literal>:ctags</literal>) or Emacs-style editors (<literal>etags</literal>). If
1231 no filename is specified, the defaulit <filename>tags</filename> or
1232 <filename>TAGS</filename> is
1233 used, respectively. Tags for all the functions, constructors and
1234 types in the currently loaded modules are created. All modules must
1235 be interpreted for these commands to work.</para>
1236 <para>See also <xref linkend="hasktags" />.</para>
1242 <literal>:type</literal> <replaceable>expression</replaceable>
1243 <indexterm><primary><literal>:type</literal></primary></indexterm>
1246 <para>Infers and prints the type of
1247 <replaceable>expression</replaceable>, including explicit
1248 forall quantifiers for polymorphic types. The monomorphism
1249 restriction is <emphasis>not</emphasis> applied to the
1250 expression during type inference.</para>
1256 <literal>:kind</literal> <replaceable>type</replaceable>
1257 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1260 <para>Infers and prints the kind of
1261 <replaceable>type</replaceable>. The latter can be an arbitrary
1262 type expression, including a partial application of a type constructor,
1263 such as <literal>Either Int</literal>.</para>
1269 <literal>:undef</literal> <replaceable>name</replaceable>
1270 <indexterm><primary><literal>:undef</literal></primary></indexterm>
1273 <para>Undefines the user-defined command
1274 <replaceable>name</replaceable> (see <literal>:def</literal>
1281 <literal>:unset</literal> <replaceable>option</replaceable>...
1282 <indexterm><primary><literal>:unset</literal></primary></indexterm>
1285 <para>Unsets certain options. See <xref linkend="ghci-set"/>
1286 for a list of available options.</para>
1292 <literal>:!</literal> <replaceable>command</replaceable>...
1293 <indexterm><primary><literal>:!</literal></primary></indexterm>
1294 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
1297 <para>Executes the shell command
1298 <replaceable>command</replaceable>.</para>
1305 <sect1 id="ghci-set">
1306 <title>The <literal>:set</literal> command</title>
1307 <indexterm><primary><literal>:set</literal></primary></indexterm>
1309 <para>The <literal>:set</literal> command sets two types of
1310 options: GHCi options, which begin with
1311 ‘<literal>+</literal>” and “command-line”
1312 options, which begin with ‘-’. </para>
1314 <para>NOTE: at the moment, the <literal>:set</literal> command
1315 doesn't support any kind of quoting in its arguments: quotes will
1316 not be removed and cannot be used to group words together. For
1317 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
1321 <title>GHCi options</title>
1322 <indexterm><primary>options</primary><secondary>GHCi</secondary>
1325 <para>GHCi options may be set using <literal>:set</literal> and
1326 unset using <literal>:unset</literal>.</para>
1328 <para>The available GHCi options are:</para>
1333 <literal>+r</literal>
1334 <indexterm><primary><literal>+r</literal></primary></indexterm>
1335 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
1336 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
1339 <para>Normally, any evaluation of top-level expressions
1340 (otherwise known as CAFs or Constant Applicative Forms) in
1341 loaded modules is retained between evaluations. Turning
1342 on <literal>+r</literal> causes all evaluation of
1343 top-level expressions to be discarded after each
1344 evaluation (they are still retained
1345 <emphasis>during</emphasis> a single evaluation).</para>
1347 <para>This option may help if the evaluated top-level
1348 expressions are consuming large amounts of space, or if
1349 you need repeatable performance measurements.</para>
1355 <literal>+s</literal>
1356 <indexterm><primary><literal>+s</literal></primary></indexterm>
1359 <para>Display some stats after evaluating each expression,
1360 including the elapsed time and number of bytes allocated.
1361 NOTE: the allocation figure is only accurate to the size
1362 of the storage manager's allocation area, because it is
1363 calculated at every GC. Hence, you might see values of
1364 zero if no GC has occurred.</para>
1370 <literal>+t</literal>
1371 <indexterm><primary><literal>+t</literal></primary></indexterm>
1374 <para>Display the type of each variable bound after a
1375 statement is entered at the prompt. If the statement is a
1376 single expression, then the only variable binding will be
1378 ‘<literal>it</literal>’.</para>
1384 <sect2 id="ghci-cmd-line-options">
1385 <title>Setting GHC command-line options in GHCi</title>
1387 <para>Normal GHC command-line options may also be set using
1388 <literal>:set</literal>. For example, to turn on
1389 <option>-fglasgow-exts</option>, you would say:</para>
1392 Prelude> :set -fglasgow-exts
1395 <para>Any GHC command-line option that is designated as
1396 <firstterm>dynamic</firstterm> (see the table in <xref
1397 linkend="flag-reference"/>), may be set using
1398 <literal>:set</literal>. To unset an option, you can set the
1399 reverse option:</para>
1400 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
1403 Prelude> :set -fno-glasgow-exts
1406 <para><xref linkend="flag-reference"/> lists the reverse for each
1407 option where applicable.</para>
1409 <para>Certain static options (<option>-package</option>,
1410 <option>-I</option>, <option>-i</option>, and
1411 <option>-l</option> in particular) will also work, but some may
1412 not take effect until the next reload.</para>
1413 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
1417 <sect1 id="ghci-dot-files">
1418 <title>The <filename>.ghci</filename> file</title>
1419 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
1421 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
1424 <para>When it starts, GHCi always reads and executes commands from
1425 <filename>$HOME/.ghci</filename>, followed by
1426 <filename>./.ghci</filename>.</para>
1428 <para>The <filename>.ghci</filename> in your home directory is
1429 most useful for turning on favourite options (eg. <literal>:set
1430 +s</literal>), and defining useful macros. Placing a
1431 <filename>.ghci</filename> file in a directory with a Haskell
1432 project is a useful way to set certain project-wide options so you
1433 don't have to type them everytime you start GHCi: eg. if your
1434 project uses GHC extensions and CPP, and has source files in three
1435 subdirectories A B and C, you might put the following lines in
1436 <filename>.ghci</filename>:</para>
1439 :set -fglasgow-exts -cpp
1443 <para>(Note that strictly speaking the <option>-i</option> flag is
1444 a static one, but in fact it works to set it using
1445 <literal>:set</literal> like this. The changes won't take effect
1446 until the next <literal>:load</literal>, though.)</para>
1448 <para>Two command-line options control whether the
1449 <filename>.ghci</filename> files are read:</para>
1454 <option>-ignore-dot-ghci</option>
1455 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
1458 <para>Don't read either <filename>./.ghci</filename> or
1459 <filename>$HOME/.ghci</filename> when starting up.</para>
1464 <option>-read-dot-ghci</option>
1465 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
1468 <para>Read <filename>.ghci</filename> and
1469 <filename>$HOME/.ghci</filename>. This is normally the
1470 default, but the <option>-read-dot-ghci</option> option may
1471 be used to override a previous
1472 <option>-ignore-dot-ghci</option> option.</para>
1480 <title>FAQ and Things To Watch Out For</title>
1484 <term>The interpreter can't load modules with foreign export
1485 declarations!</term>
1487 <para>Unfortunately not. We haven't implemented it yet.
1488 Please compile any offending modules by hand before loading
1489 them into GHCi.</para>
1495 <literal>-O</literal> doesn't work with GHCi!
1496 <indexterm><primary><option>-O</option></primary></indexterm>
1499 <para>For technical reasons, the bytecode compiler doesn't
1500 interact well with one of the optimisation passes, so we
1501 have disabled optimisation when using the interpreter. This
1502 isn't a great loss: you'll get a much bigger win by
1503 compiling the bits of your code that need to go fast, rather
1504 than interpreting them with optimisation turned on.</para>
1509 <term>Unboxed tuples don't work with GHCi</term>
1511 <para>That's right. You can always compile a module that
1512 uses unboxed tuples and load it into GHCi, however.
1513 (Incidentally the previous point, namely that
1514 <literal>-O</literal> is incompatible with GHCi, is because
1515 the bytecode compiler can't deal with unboxed
1521 <term>Concurrent threads don't carry on running when GHCi is
1522 waiting for input.</term>
1524 <para>No, they don't. This is because the Haskell binding
1525 to the GNU readline library doesn't support reading from the
1526 terminal in a non-blocking way, which is required to work
1527 properly with GHC's concurrency model.</para>
1532 <term>After using <literal>getContents</literal>, I can't use
1533 <literal>stdin</literal> again until I do
1534 <literal>:load</literal> or <literal>:reload</literal>.</term>
1537 <para>This is the defined behaviour of
1538 <literal>getContents</literal>: it puts the stdin Handle in
1539 a state known as <firstterm>semi-closed</firstterm>, wherein
1540 any further I/O operations on it are forbidden. Because I/O
1541 state is retained between computations, the semi-closed
1542 state persists until the next <literal>:load</literal> or
1543 <literal>:reload</literal> command.</para>
1545 <para>You can make <literal>stdin</literal> reset itself
1546 after every evaluation by giving GHCi the command
1547 <literal>:set +r</literal>. This works because
1548 <literal>stdin</literal> is just a top-level expression that
1549 can be reverted to its unevaluated state in the same way as
1550 any other top-level expression (CAF).</para>
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