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" }
405 <sect2 id="ghci-stmts">
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>
447 The automatic printing of binding results can be supressed with
448 <option>:set -fno-print-bind-result</option> (this does not
449 supress printing the result of non-binding statements).
450 <indexterm><primary><option>-fno-print-bind-result</option></primary></indexterm><indexterm><primary><option>-fprint-bind-result</option></primary></indexterm>.
451 You might want to do this to prevent the result of binding
452 statements from being fully evaluated by the act of printing
453 them, for example.</para>
455 <para>Of course, you can also bind normal non-IO expressions
456 using the <literal>let</literal>-statement:</para>
463 <para>Another important difference between the two types of binding
464 is that the monadic bind (<literal>p <- e</literal>) is
465 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
466 whereas with the <literal>let</literal> form, the expression
467 isn't evaluated immediately:</para>
469 Prelude> let x = error "help!"
475 <para>Note that <literal>let</literal> bindings do not automatically
476 print the value bound, unlike monadic bindings.</para>
478 <para>Any exceptions raised during the evaluation or execution
479 of the statement are caught and printed by the GHCi command line
480 interface (for more information on exceptions, see the module
481 <literal>Control.Exception</literal> in the libraries
482 documentation).</para>
484 <para>Every new binding shadows any existing bindings of the
485 same name, including entities that are in scope in the current
486 module context.</para>
488 <para>WARNING: temporary bindings introduced at the prompt only
489 last until the next <literal>:load</literal> or
490 <literal>:reload</literal> command, at which time they will be
491 simply lost. However, they do survive a change of context with
492 <literal>:module</literal>: the temporary bindings just move to
493 the new location.</para>
495 <para>HINT: To get a list of the bindings currently in scope, use the
496 <literal>:show bindings</literal> command:</para>
499 Prelude> :show bindings
503 <para>HINT: if you turn on the <literal>+t</literal> option,
504 GHCi will show the type of each variable bound by a statement.
506 <indexterm><primary><literal>+t</literal></primary></indexterm>
509 Prelude> let (x:xs) = [1..]
516 <sect2 id="ghci-scope">
517 <title>What's really in scope at the prompt?</title>
519 <para>When you type an expression at the prompt, what
520 identifiers and types are in scope? GHCi provides a flexible
521 way to control exactly how the context for an expression is
522 constructed. Let's start with the simple cases; when you start
523 GHCi the prompt looks like this:</para>
525 <screen>Prelude></screen>
527 <para>Which indicates that everything from the module
528 <literal>Prelude</literal> is currently in scope. If we now
529 load a file into GHCi, the prompt will change:</para>
532 Prelude> :load Main.hs
533 Compiling Main ( Main.hs, interpreted )
537 <para>The new prompt is <literal>*Main</literal>, which
538 indicates that we are typing expressions in the context of the
539 top-level of the <literal>Main</literal> module. Everything
540 that is in scope at the top-level in the module
541 <literal>Main</literal> we just loaded is also in scope at the
542 prompt (probably including <literal>Prelude</literal>, as long
543 as <literal>Main</literal> doesn't explicitly hide it).</para>
546 <literal>*<replaceable>module</replaceable></literal> indicates
547 that it is the full top-level scope of
548 <replaceable>module</replaceable> that is contributing to the
549 scope for expressions typed at the prompt. Without the
550 <literal>*</literal>, just the exports of the module are
553 <para>We're not limited to a single module: GHCi can combine
554 scopes from multiple modules, in any mixture of
555 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
556 combines the scopes from all of these modules to form the scope
557 that is in effect at the prompt. For technical reasons, GHCi
558 can only support the <literal>*</literal>-form for modules which
559 are interpreted, so compiled modules and package modules can
560 only contribute their exports to the current scope.</para>
562 <para>The scope is manipulated using the
563 <literal>:module</literal> command. For example, if the current
564 scope is <literal>Prelude</literal>, then we can bring into
565 scope the exports from the module <literal>IO</literal> like
570 Prelude IO> hPutStrLn stdout "hello\n"
575 <para>(Note: <literal>:module</literal> can be shortened to
576 <literal>:m</literal>). The full syntax of the
577 <literal>:module</literal> command is:</para>
580 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
583 <para>Using the <literal>+</literal> form of the
584 <literal>module</literal> commands adds modules to the current
585 scope, and <literal>-</literal> removes them. Without either
586 <literal>+</literal> or <literal>-</literal>, the current scope
587 is replaced by the set of modules specified. Note that if you
588 use this form and leave out <literal>Prelude</literal>, GHCi
589 will assume that you really wanted the
590 <literal>Prelude</literal> and add it in for you (if you don't
591 want the <literal>Prelude</literal>, then ask to remove it with
592 <literal>:m -Prelude</literal>).</para>
594 <para>The scope is automatically set after a
595 <literal>:load</literal> command, to the most recently loaded
596 "target" module, in a <literal>*</literal>-form if possible.
597 For example, if you say <literal>:load foo.hs bar.hs</literal>
598 and <filename>bar.hs</filename> contains module
599 <literal>Bar</literal>, then the scope will be set to
600 <literal>*Bar</literal> if <literal>Bar</literal> is
601 interpreted, or if <literal>Bar</literal> is compiled it will be
602 set to <literal>Prelude Bar</literal> (GHCi automatically adds
603 <literal>Prelude</literal> if it isn't present and there aren't
604 any <literal>*</literal>-form modules).</para>
606 <para>With multiple modules in scope, especially multiple
607 <literal>*</literal>-form modules, it is likely that name
608 clashes will occur. Haskell specifies that name clashes are
609 only reported when an ambiguous identifier is used, and GHCi
610 behaves in the same way for expressions typed at the
614 Hint: GHCi will tab-complete names that are in scope; for
615 example, if you run GHCi and type <literal>J<tab></literal>
616 then GHCi will expand it to <literal>Just </literal>.
620 <title>Qualified names</title>
622 <para>To make life slightly easier, the GHCi prompt also
623 behaves as if there is an implicit <literal>import
624 qualified</literal> declaration for every module in every
625 package, and every module currently loaded into GHCi.</para>
629 <title>The <literal>:main</literal> command</title>
632 When a program is compiled and executed, it can use the
633 <literal>getArgs</literal> function to access the
634 command-line arguments.
635 However, we cannot simply pass the arguments to the
636 <literal>main</literal> function while we are testing in ghci,
637 as the <literal>main</literal> function doesn't take its
642 Instead, we can use the <literal>:main</literal> command.
643 This runs whatever <literal>main</literal> is in scope, with
644 any arguments being treated the same as command-line arguments,
649 Prelude> let main = System.Environment.getArgs >>= print
650 Prelude> :main foo bar
659 <title>The <literal>it</literal> variable</title>
660 <indexterm><primary><literal>it</literal></primary>
663 <para>Whenever an expression (or a non-binding statement, to be
664 precise) is typed at the prompt, GHCi implicitly binds its value
665 to the variable <literal>it</literal>. For example:</para>
672 <para>What actually happens is that GHCi typechecks the
673 expression, and if it doesn't have an <literal>IO</literal> type,
674 then it transforms it as follows: an expression
675 <replaceable>e</replaceable> turns into
677 let it = <replaceable>e</replaceable>;
680 which is then run as an IO-action.</para>
682 <para>Hence, the original expression must have a type which is an
683 instance of the <literal>Show</literal> class, or GHCi will
689 <interactive>:1:0:
690 No instance for (Show (a -> a))
691 arising from use of `print' at <interactive>:1:0-1
692 Possible fix: add an instance declaration for (Show (a -> a))
693 In the expression: print it
694 In a 'do' expression: print it
697 <para>The error message contains some clues as to the
698 transformation happening internally.</para>
700 <para>If the expression was instead of type <literal>IO a</literal> for
701 some <literal>a</literal>, then <literal>it</literal> will be
702 bound to the result of the <literal>IO</literal> computation,
703 which is of type <literal>a</literal>. eg.:</para>
705 Prelude> Time.getClockTime
706 Wed Mar 14 12:23:13 GMT 2001
708 Wed Mar 14 12:23:13 GMT 2001
711 <para>The corresponding translation for an IO-typed
712 <replaceable>e</replaceable> is
714 it <- <replaceable>e</replaceable>
718 <para>Note that <literal>it</literal> is shadowed by the new
719 value each time you evaluate a new expression, and the old value
720 of <literal>it</literal> is lost.</para>
724 <sect2 id="extended-default-rules">
725 <title>Type defaulting in GHCi</title>
726 <indexterm><primary>Type default</primary></indexterm>
727 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
729 Consider this GHCi session:
733 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
734 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
735 on the type <literal>a</literal>. For example:
737 ghci> (reverse []) :: String
739 ghci> (reverse []) :: [Int]
742 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
743 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
744 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
745 a)</literal> for each type variable <literal>a</literal>, and defaults the
748 <listitem><para> The type variable <literal>a</literal>
749 appears in no other constraints </para></listitem>
750 <listitem><para> All the classes <literal>Ci</literal> are standard.</para></listitem>
751 <listitem><para> At least one of the classes <literal>Ci</literal> is
752 numeric.</para></listitem>
754 At the GHCi prompt, the second and third rules are relaxed as follows
755 (differences italicised):
757 <listitem><para> <emphasis>All</emphasis> of the classes
758 <literal>Ci</literal> are single-parameter type classes.</para></listitem>
759 <listitem><para> At least one of the classes <literal>Ci</literal> is
760 numeric, <emphasis>or is <literal>Show</literal>,
761 <literal>Eq</literal>, or <literal>Ord</literal></emphasis>.</para></listitem>
763 The same type-default behaviour can be enabled in an ordinary Haskell
764 module, using the flag <literal>-fextended-default-rules</literal>.
769 <sect1 id="ghci-invocation">
770 <title>Invoking GHCi</title>
771 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
772 <indexterm><primary><option>––interactive</option></primary></indexterm>
774 <para>GHCi is invoked with the command <literal>ghci</literal> or
775 <literal>ghc ––interactive</literal>. One or more modules or
776 filenames can also be specified on the command line; this
777 instructs GHCi to load the specified modules or filenames (and all
778 the modules they depend on), just as if you had said
779 <literal>:load <replaceable>modules</replaceable></literal> at the
780 GHCi prompt (see <xref linkend="ghci-commands"/>). For example, to
781 start GHCi and load the program whose topmost module is in the
782 file <literal>Main.hs</literal>, we could say:</para>
788 <para>Most of the command-line options accepted by GHC (see <xref
789 linkend="using-ghc"/>) also make sense in interactive mode. The ones
790 that don't make sense are mostly obvious; for example, GHCi
791 doesn't generate interface files, so options related to interface
792 file generation won't have any effect.</para>
795 <title>Packages</title>
796 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
798 <para>Most packages (see <xref linkend="using-packages"/>) are
799 available without needing to specify any extra flags at all:
800 they will be automatically loaded the first time they are
803 <para>For non-auto packages, however, you need to request the
804 package be loaded by using the <literal>-package</literal> flag:</para>
807 $ ghci -package readline
810 / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98.
811 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
812 \____/\/ /_/\____/|_| Type :? for help.
814 Loading package base ... linking ... done.
815 Loading package readline-1.0 ... linking ... done.
819 <para>The following command works to load new packages into a
823 Prelude> :set -package <replaceable>name</replaceable>
826 <para>But note that doing this will cause all currently loaded
827 modules to be unloaded, and you'll be dumped back into the
828 <literal>Prelude</literal>.</para>
832 <title>Extra libraries</title>
833 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
835 <para>Extra libraries may be specified on the command line using
836 the normal <literal>-l<replaceable>lib</replaceable></literal>
837 option. (The term <emphasis>library</emphasis> here refers to
838 libraries of foreign object code; for using libraries of Haskell
839 source code, see <xref linkend="ghci-modules-filenames"/>.) For
840 example, to load the “m” library:</para>
846 <para>On systems with <literal>.so</literal>-style shared
847 libraries, the actual library loaded will the
848 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
849 searches the following places for libraries, in this order:</para>
853 <para>Paths specified using the
854 <literal>-L<replaceable>path</replaceable></literal>
855 command-line option,</para>
858 <para>the standard library search path for your system,
859 which on some systems may be overridden by setting the
860 <literal>LD_LIBRARY_PATH</literal> environment
865 <para>On systems with <literal>.dll</literal>-style shared
866 libraries, the actual library loaded will be
867 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
868 GHCi will signal an error if it can't find the library.</para>
870 <para>GHCi can also load plain object files
871 (<literal>.o</literal> or <literal>.obj</literal> depending on
872 your platform) from the command-line. Just add the name the
873 object file to the command line.</para>
875 <para>Ordering of <option>-l</option> options matters: a library
876 should be mentioned <emphasis>before</emphasis> the libraries it
877 depends on (see <xref linkend="options-linker"/>).</para>
882 <sect1 id="ghci-commands">
883 <title>GHCi commands</title>
885 <para>GHCi commands all begin with
886 ‘<literal>:</literal>’ and consist of a single command
887 name followed by zero or more parameters. The command name may be
888 abbreviated, with ambiguities being resolved in favour of the more
889 commonly used commands.</para>
894 <literal>:add</literal> <replaceable>module</replaceable> ...
895 <indexterm><primary><literal>:add</literal></primary></indexterm>
898 <para>Add <replaceable>module</replaceable>(s) to the
899 current <firstterm>target set</firstterm>, and perform a
906 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
907 <indexterm><primary><literal>:browse</literal></primary></indexterm>
910 <para>Displays the identifiers defined by the module
911 <replaceable>module</replaceable>, which must be either
912 loaded into GHCi or be a member of a package. If the
913 <literal>*</literal> symbol is placed before the module
914 name, then <emphasis>all</emphasis> the identifiers defined
915 in <replaceable>module</replaceable> are shown; otherwise
916 the list is limited to the exports of
917 <replaceable>module</replaceable>. The
918 <literal>*</literal>-form is only available for modules
919 which are interpreted; for compiled modules (including
920 modules from packages) only the non-<literal>*</literal>
921 form of <literal>:browse</literal> is available.</para>
927 <literal>:cd</literal> <replaceable>dir</replaceable>
928 <indexterm><primary><literal>:cd</literal></primary></indexterm>
931 <para>Changes the current working directory to
932 <replaceable>dir</replaceable>. A
933 ‘<literal>˜</literal>’ symbol at the
934 beginning of <replaceable>dir</replaceable> will be replaced
935 by the contents of the environment variable
936 <literal>HOME</literal>.</para>
938 <para>NOTE: changing directories causes all currently loaded
939 modules to be unloaded. This is because the search path is
940 usually expressed using relative directories, and changing
941 the search path in the middle of a session is not
948 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
949 <indexterm><primary><literal>:def</literal></primary></indexterm>
952 <para>The command <literal>:def</literal>
953 <replaceable>name</replaceable>
954 <replaceable>expr</replaceable> defines a new GHCi command
955 <literal>:<replaceable>name</replaceable></literal>,
956 implemented by the Haskell expression
957 <replaceable>expr</replaceable>, which must have type
958 <literal>String -> IO String</literal>. When
959 <literal>:<replaceable>name</replaceable>
960 <replaceable>args</replaceable></literal> is typed at the
961 prompt, GHCi will run the expression
962 <literal>(<replaceable>name</replaceable>
963 <replaceable>args</replaceable>)</literal>, take the
964 resulting <literal>String</literal>, and feed it back into
965 GHCi as a new sequence of commands. Separate commands in
966 the result must be separated by
967 ‘<literal>\n</literal>’.</para>
969 <para>That's all a little confusing, so here's a few
970 examples. To start with, here's a new GHCi command which
971 doesn't take any arguments or produce any results, it just
972 outputs the current date & time:</para>
975 Prelude> let date _ = Time.getClockTime >>= print >> return ""
976 Prelude> :def date date
978 Fri Mar 23 15:16:40 GMT 2001
981 <para>Here's an example of a command that takes an argument.
982 It's a re-implementation of <literal>:cd</literal>:</para>
985 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
986 Prelude> :def mycd mycd
990 <para>Or I could define a simple way to invoke
991 “<literal>ghc ––make Main</literal>” in the
992 current directory:</para>
995 Prelude> :def make (\_ -> return ":! ghc ––make Main")
998 <para>We can define a command that reads GHCi input from a
999 file. This might be useful for creating a set of bindings
1000 that we want to repeatedly load into the GHCi session:</para>
1003 Prelude> :def . readFile
1004 Prelude> :. cmds.ghci
1007 <para>Notice that we named the command
1008 <literal>:.</literal>, by analogy with the
1009 ‘<literal>.</literal>’ Unix shell command that
1010 does the same thing.</para>
1016 <literal>:edit <optional><replaceable>file</replaceable></optional></literal>
1017 <indexterm><primary><literal>:edit</literal></primary></indexterm>
1020 <para>Opens an editor to edit the file
1021 <replaceable>file</replaceable>, or the most recently loaded
1022 module if <replaceable>file</replaceable> is omitted. The
1023 editor to invoke is taken from the <literal>EDITOR</literal>
1024 environment variable, or a default editor on your system if
1025 <literal>EDITOR</literal> is not set.</para>
1031 <literal>:help</literal>
1032 <indexterm><primary><literal>:help</literal></primary></indexterm>
1035 <literal>:?</literal>
1036 <indexterm><primary><literal>:?</literal></primary></indexterm>
1039 <para>Displays a list of the available commands.</para>
1045 <literal>:info</literal> <replaceable>name</replaceable> ...
1046 <indexterm><primary><literal>:info</literal></primary></indexterm>
1049 <para>Displays information about the given name(s). For
1050 example, if <replaceable>name</replaceable> is a class, then
1051 the class methods and their types will be printed; if
1052 <replaceable>name</replaceable> is a type constructor, then
1053 its definition will be printed; if
1054 <replaceable>name</replaceable> is a function, then its type
1055 will be printed. If <replaceable>name</replaceable> has
1056 been loaded from a source file, then GHCi will also display
1057 the location of its definition in the source.</para>
1063 <literal>:load</literal> <replaceable>module</replaceable> ...
1064 <indexterm><primary><literal>:load</literal></primary></indexterm>
1067 <para>Recursively loads the specified
1068 <replaceable>module</replaceable>s, and all the modules they
1069 depend on. Here, each <replaceable>module</replaceable>
1070 must be a module name or filename, but may not be the name
1071 of a module in a package.</para>
1073 <para>All previously loaded modules, except package modules,
1074 are forgotten. The new set of modules is known as the
1075 <firstterm>target set</firstterm>. Note that
1076 <literal>:load</literal> can be used without any arguments
1077 to unload all the currently loaded modules and
1080 <para>After a <literal>:load</literal> command, the current
1081 context is set to:</para>
1085 <para><replaceable>module</replaceable>, if it was loaded
1086 successfully, or</para>
1089 <para>the most recently successfully loaded module, if
1090 any other modules were loaded as a result of the current
1091 <literal>:load</literal>, or</para>
1094 <para><literal>Prelude</literal> otherwise.</para>
1102 <literal>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
1103 <indexterm><primary><literal>:main</literal></primary></indexterm>
1107 When a program is compiled and executed, it can use the
1108 <literal>getArgs</literal> function to access the
1109 command-line arguments.
1110 However, we cannot simply pass the arguments to the
1111 <literal>main</literal> function while we are testing in ghci,
1112 as the <literal>main</literal> function doesn't take its
1117 Instead, we can use the <literal>:main</literal> command.
1118 This runs whatever <literal>main</literal> is in scope, with
1119 any arguments being treated the same as command-line arguments,
1124 Prelude> let main = System.Environment.getArgs >>= print
1125 Prelude> :main foo bar
1134 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
1135 <indexterm><primary><literal>:module</literal></primary></indexterm>
1138 <para>Sets or modifies the current context for statements
1139 typed at the prompt. See <xref linkend="ghci-scope"/> for
1140 more details.</para>
1146 <literal>:quit</literal>
1147 <indexterm><primary><literal>:quit</literal></primary></indexterm>
1150 <para>Quits GHCi. You can also quit by typing a control-D
1151 at the prompt.</para>
1157 <literal>:reload</literal>
1158 <indexterm><primary><literal>:reload</literal></primary></indexterm>
1161 <para>Attempts to reload the current target set (see
1162 <literal>:load</literal>) if any of the modules in the set,
1163 or any dependent module, has changed. Note that this may
1164 entail loading new modules, or dropping modules which are no
1165 longer indirectly required by the target.</para>
1171 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
1172 <indexterm><primary><literal>:set</literal></primary></indexterm>
1175 <para>Sets various options. See <xref linkend="ghci-set"/>
1176 for a list of available options. The
1177 <literal>:set</literal> command by itself shows which
1178 options are currently set.</para>
1184 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
1185 <indexterm><primary><literal>:set args</literal></primary></indexterm>
1188 <para>Sets the list of arguments which are returned when the
1189 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
1190 </indexterm>.</para>
1196 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
1197 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
1200 <para>Sets the string to be returned when the program calls
1201 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
1202 </indexterm>.</para>
1208 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
1211 <para>Sets the string to be used as the prompt in GHCi.
1212 Inside <replaceable>prompt</replaceable>, the sequence
1213 <literal>%s</literal> is replaced by the names of the
1214 modules currently in scope, and <literal>%%</literal> is
1215 replaced by <literal>%</literal>.</para>
1221 <literal>:show bindings</literal>
1222 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
1225 <para>Show the bindings made at the prompt and their
1232 <literal>:show modules</literal>
1233 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
1236 <para>Show the list of modules currently load.</para>
1242 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1243 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1244 <indexterm><primary><literal>:etags</literal></primary>
1246 <indexterm><primary><literal>:etags</literal></primary>
1250 <para>Generates a “tags” file for Vi-style editors
1251 (<literal>:ctags</literal>) or Emacs-style editors (<literal>etags</literal>). If
1252 no filename is specified, the defaulit <filename>tags</filename> or
1253 <filename>TAGS</filename> is
1254 used, respectively. Tags for all the functions, constructors and
1255 types in the currently loaded modules are created. All modules must
1256 be interpreted for these commands to work.</para>
1257 <para>See also <xref linkend="hasktags" />.</para>
1263 <literal>:type</literal> <replaceable>expression</replaceable>
1264 <indexterm><primary><literal>:type</literal></primary></indexterm>
1267 <para>Infers and prints the type of
1268 <replaceable>expression</replaceable>, including explicit
1269 forall quantifiers for polymorphic types. The monomorphism
1270 restriction is <emphasis>not</emphasis> applied to the
1271 expression during type inference.</para>
1277 <literal>:kind</literal> <replaceable>type</replaceable>
1278 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1281 <para>Infers and prints the kind of
1282 <replaceable>type</replaceable>. The latter can be an arbitrary
1283 type expression, including a partial application of a type constructor,
1284 such as <literal>Either Int</literal>.</para>
1290 <literal>:undef</literal> <replaceable>name</replaceable>
1291 <indexterm><primary><literal>:undef</literal></primary></indexterm>
1294 <para>Undefines the user-defined command
1295 <replaceable>name</replaceable> (see <literal>:def</literal>
1302 <literal>:unset</literal> <replaceable>option</replaceable>...
1303 <indexterm><primary><literal>:unset</literal></primary></indexterm>
1306 <para>Unsets certain options. See <xref linkend="ghci-set"/>
1307 for a list of available options.</para>
1313 <literal>:!</literal> <replaceable>command</replaceable>...
1314 <indexterm><primary><literal>:!</literal></primary></indexterm>
1315 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
1318 <para>Executes the shell command
1319 <replaceable>command</replaceable>.</para>
1326 <sect1 id="ghci-set">
1327 <title>The <literal>:set</literal> command</title>
1328 <indexterm><primary><literal>:set</literal></primary></indexterm>
1330 <para>The <literal>:set</literal> command sets two types of
1331 options: GHCi options, which begin with
1332 ‘<literal>+</literal>” and “command-line”
1333 options, which begin with ‘-’. </para>
1335 <para>NOTE: at the moment, the <literal>:set</literal> command
1336 doesn't support any kind of quoting in its arguments: quotes will
1337 not be removed and cannot be used to group words together. For
1338 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
1342 <title>GHCi options</title>
1343 <indexterm><primary>options</primary><secondary>GHCi</secondary>
1346 <para>GHCi options may be set using <literal>:set</literal> and
1347 unset using <literal>:unset</literal>.</para>
1349 <para>The available GHCi options are:</para>
1354 <literal>+r</literal>
1355 <indexterm><primary><literal>+r</literal></primary></indexterm>
1356 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
1357 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
1360 <para>Normally, any evaluation of top-level expressions
1361 (otherwise known as CAFs or Constant Applicative Forms) in
1362 loaded modules is retained between evaluations. Turning
1363 on <literal>+r</literal> causes all evaluation of
1364 top-level expressions to be discarded after each
1365 evaluation (they are still retained
1366 <emphasis>during</emphasis> a single evaluation).</para>
1368 <para>This option may help if the evaluated top-level
1369 expressions are consuming large amounts of space, or if
1370 you need repeatable performance measurements.</para>
1376 <literal>+s</literal>
1377 <indexterm><primary><literal>+s</literal></primary></indexterm>
1380 <para>Display some stats after evaluating each expression,
1381 including the elapsed time and number of bytes allocated.
1382 NOTE: the allocation figure is only accurate to the size
1383 of the storage manager's allocation area, because it is
1384 calculated at every GC. Hence, you might see values of
1385 zero if no GC has occurred.</para>
1391 <literal>+t</literal>
1392 <indexterm><primary><literal>+t</literal></primary></indexterm>
1395 <para>Display the type of each variable bound after a
1396 statement is entered at the prompt. If the statement is a
1397 single expression, then the only variable binding will be
1399 ‘<literal>it</literal>’.</para>
1405 <sect2 id="ghci-cmd-line-options">
1406 <title>Setting GHC command-line options in GHCi</title>
1408 <para>Normal GHC command-line options may also be set using
1409 <literal>:set</literal>. For example, to turn on
1410 <option>-fglasgow-exts</option>, you would say:</para>
1413 Prelude> :set -fglasgow-exts
1416 <para>Any GHC command-line option that is designated as
1417 <firstterm>dynamic</firstterm> (see the table in <xref
1418 linkend="flag-reference"/>), may be set using
1419 <literal>:set</literal>. To unset an option, you can set the
1420 reverse option:</para>
1421 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
1424 Prelude> :set -fno-glasgow-exts
1427 <para><xref linkend="flag-reference"/> lists the reverse for each
1428 option where applicable.</para>
1430 <para>Certain static options (<option>-package</option>,
1431 <option>-I</option>, <option>-i</option>, and
1432 <option>-l</option> in particular) will also work, but some may
1433 not take effect until the next reload.</para>
1434 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
1438 <sect1 id="ghci-dot-files">
1439 <title>The <filename>.ghci</filename> file</title>
1440 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
1442 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
1445 <para>When it starts, GHCi always reads and executes commands from
1446 <filename>$HOME/.ghci</filename>, followed by
1447 <filename>./.ghci</filename>.</para>
1449 <para>The <filename>.ghci</filename> in your home directory is
1450 most useful for turning on favourite options (eg. <literal>:set
1451 +s</literal>), and defining useful macros. Placing a
1452 <filename>.ghci</filename> file in a directory with a Haskell
1453 project is a useful way to set certain project-wide options so you
1454 don't have to type them everytime you start GHCi: eg. if your
1455 project uses GHC extensions and CPP, and has source files in three
1456 subdirectories A B and C, you might put the following lines in
1457 <filename>.ghci</filename>:</para>
1460 :set -fglasgow-exts -cpp
1464 <para>(Note that strictly speaking the <option>-i</option> flag is
1465 a static one, but in fact it works to set it using
1466 <literal>:set</literal> like this. The changes won't take effect
1467 until the next <literal>:load</literal>, though.)</para>
1469 <para>Two command-line options control whether the
1470 <filename>.ghci</filename> files are read:</para>
1475 <option>-ignore-dot-ghci</option>
1476 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
1479 <para>Don't read either <filename>./.ghci</filename> or
1480 <filename>$HOME/.ghci</filename> when starting up.</para>
1485 <option>-read-dot-ghci</option>
1486 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
1489 <para>Read <filename>.ghci</filename> and
1490 <filename>$HOME/.ghci</filename>. This is normally the
1491 default, but the <option>-read-dot-ghci</option> option may
1492 be used to override a previous
1493 <option>-ignore-dot-ghci</option> option.</para>
1501 <title>FAQ and Things To Watch Out For</title>
1505 <term>The interpreter can't load modules with foreign export
1506 declarations!</term>
1508 <para>Unfortunately not. We haven't implemented it yet.
1509 Please compile any offending modules by hand before loading
1510 them into GHCi.</para>
1516 <literal>-O</literal> doesn't work with GHCi!
1517 <indexterm><primary><option>-O</option></primary></indexterm>
1520 <para>For technical reasons, the bytecode compiler doesn't
1521 interact well with one of the optimisation passes, so we
1522 have disabled optimisation when using the interpreter. This
1523 isn't a great loss: you'll get a much bigger win by
1524 compiling the bits of your code that need to go fast, rather
1525 than interpreting them with optimisation turned on.</para>
1530 <term>Unboxed tuples don't work with GHCi</term>
1532 <para>That's right. You can always compile a module that
1533 uses unboxed tuples and load it into GHCi, however.
1534 (Incidentally the previous point, namely that
1535 <literal>-O</literal> is incompatible with GHCi, is because
1536 the bytecode compiler can't deal with unboxed
1542 <term>Concurrent threads don't carry on running when GHCi is
1543 waiting for input.</term>
1545 <para>No, they don't. This is because the Haskell binding
1546 to the GNU readline library doesn't support reading from the
1547 terminal in a non-blocking way, which is required to work
1548 properly with GHC's concurrency model.</para>
1553 <term>After using <literal>getContents</literal>, I can't use
1554 <literal>stdin</literal> again until I do
1555 <literal>:load</literal> or <literal>:reload</literal>.</term>
1558 <para>This is the defined behaviour of
1559 <literal>getContents</literal>: it puts the stdin Handle in
1560 a state known as <firstterm>semi-closed</firstterm>, wherein
1561 any further I/O operations on it are forbidden. Because I/O
1562 state is retained between computations, the semi-closed
1563 state persists until the next <literal>:load</literal> or
1564 <literal>:reload</literal> command.</para>
1566 <para>You can make <literal>stdin</literal> reset itself
1567 after every evaluation by giving GHCi the command
1568 <literal>:set +r</literal>. This works because
1569 <literal>stdin</literal> is just a top-level expression that
1570 can be reverted to its unevaluated state in the same way as
1571 any other top-level expression (CAF).</para>
1581 ;;; Local Variables: ***
1583 ;;; sgml-parent-document: ("users_guide.xml" "book" "chapter") ***