1 <?xml version="1.0" encoding="iso-8859-1"?>
3 <title>Using GHCi</title>
4 <indexterm><primary>GHCi</primary></indexterm>
5 <indexterm><primary>interpreter</primary><see>GHCi</see></indexterm>
6 <indexterm><primary>interactive</primary><see>GHCi</see></indexterm>
9 <para>The ‘i’ stands for “Interactive”</para>
11 is GHC's interactive environment, in which Haskell expressions can
12 be interactively evaluated and programs can be interpreted. If
13 you're familiar with <ulink url="http://www.haskell.org/hugs/">Hugs</ulink><indexterm><primary>Hugs</primary>
14 </indexterm>, then you'll be right at home with GHCi. However, GHCi
15 also has support for interactively loading compiled code, as well as
16 supporting all<footnote><para>except <literal>foreign export</literal>, at the moment</para>
17 </footnote> the language extensions that GHC provides.</para>
18 <indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
19 <indexterm><primary>Foreign Function Interface</primary><secondary>GHCi support</secondary></indexterm>
22 <title>Introduction to GHCi</title>
24 <para>Let's start with an example GHCi session. You can fire up
25 GHCi with the command <literal>ghci</literal>:</para>
31 / /_\// /_/ / / | | GHC Interactive, version 5.04, for Haskell 98.
32 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
33 \____/\/ /_/\____/|_| Type :? for help.
35 Loading package base ... linking ... done.
36 Loading package haskell98 ... linking ... done.
40 <para>There may be a short pause while GHCi loads the prelude and
41 standard libraries, after which the prompt is shown. If we follow
42 the instructions and type <literal>:?</literal> for help, we
46 Commands available from the prompt:
48 <stmt> evaluate/run <stmt>
49 :add <filename> ... add module(s) to the current target set
50 :browse [*]<module> display the names defined by <module>
51 :cd <dir> change directory to <dir>
52 :def <cmd> <expr> define a command :<cmd>
53 :help, :? display this list of commands
54 :info [<name> ...] display information about the given names
55 :load <filename> ... load module(s) and their dependents
56 :module [+/-] [*]<mod> ... set the context for expression evaluation
57 :reload reload the current module set
59 :set <option> ... set options
60 :set args <arg> ... set the arguments returned by System.getArgs
61 :set prog <progname> set the value returned by System.getProgName
63 :show modules show the currently loaded modules
64 :show bindings show the current bindings made at the prompt
66 :type <expr> show the type of <expr>
67 :kind <type> show the kind of <type>
68 :undef <cmd> undefine user-defined command :<cmd>
69 :unset <option> ... unset options
71 :!<command> run the shell command <command>
73 Options for `:set' and `:unset':
75 +r revert top-level expressions after each evaluation
76 +s print timing/memory stats after each evaluation
77 +t print type after evaluation
78 -<flags> most GHC command line flags can also be set here
79 (eg. -v2, -fglasgow-exts, etc.)
82 <para>We'll explain most of these commands as we go along. For
83 Hugs users: many things work the same as in Hugs, so you should be
84 able to get going straight away.</para>
86 <para>Haskell expressions can be typed at the prompt:</para>
87 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
93 Prelude> let x = 42 in x / 9
98 <para>GHCi interprets the whole line as an expression to evaluate.
99 The expression may not span several lines - as soon as you press
100 enter, GHCi will attempt to evaluate it.</para>
104 <title>Loading source files</title>
106 <para>Suppose we have the following Haskell source code, which we
107 place in a file <filename>Main.hs</filename>:</para>
110 main = print (fac 20)
113 fac n = n * fac (n-1)
116 <para>You can save <filename>Main.hs</filename> anywhere you like,
117 but if you save it somewhere other than the current
118 directory<footnote><para>If you started up GHCi from the command
119 line then GHCi's current directory is the same as the current
120 directory of the shell from which it was started. If you started
121 GHCi from the “Start” menu in Windows, then the
122 current directory is probably something like
123 <filename>C:\Documents and Settings\<replaceable>user
124 name</replaceable></filename>.</para> </footnote> then we will
125 need to change to the right directory in GHCi:</para>
128 Prelude> :cd <replaceable>dir</replaceable>
131 <para>where <replaceable>dir</replaceable> is the directory (or
132 folder) in which you saved <filename>Main.hs</filename>.</para>
134 <para>To load a Haskell source file into GHCi, use the
135 <literal>:load</literal> command:</para>
136 <indexterm><primary><literal>:load</literal></primary></indexterm>
140 Compiling Main ( Main.hs, interpreted )
141 Ok, modules loaded: Main.
145 <para>GHCi has loaded the <literal>Main</literal> module, and the
146 prompt has changed to “<literal>*Main></literal>” to
147 indicate that the current context for expressions typed at the
148 prompt is the <literal>Main</literal> module we just loaded (we'll
149 explain what the <literal>*</literal> means later in <xref
150 linkend="ghci-scope"/>). So we can now type expressions involving
151 the functions from <filename>Main.hs</filename>:</para>
158 <para>Loading a multi-module program is just as straightforward;
159 just give the name of the “topmost” module to the
160 <literal>:load</literal> command (hint: <literal>:load</literal>
161 can be abbreviated to <literal>:l</literal>). The topmost module
162 will normally be <literal>Main</literal>, but it doesn't have to
163 be. GHCi will discover which modules are required, directly or
164 indirectly, by the topmost module, and load them all in dependency
167 <sect2 id="ghci-modules-filenames">
168 <title>Modules vs. filenames</title>
169 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
170 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
172 <para>Question: How does GHC find the filename which contains
173 module <replaceable>M</replaceable>? Answer: it looks for the
174 file <literal><replaceable>M</replaceable>.hs</literal>, or
175 <literal><replaceable>M</replaceable>.lhs</literal>. This means
176 that for most modules, the module name must match the filename.
177 If it doesn't, GHCi won't be able to find it.</para>
179 <para>There is one exception to this general rule: when you load
180 a program with <literal>:load</literal>, or specify it when you
181 invoke <literal>ghci</literal>, you can give a filename rather
182 than a module name. This filename is loaded if it exists, and
183 it may contain any module you like. This is particularly
184 convenient if you have several <literal>Main</literal> modules
185 in the same directory and you can't call them all
186 <filename>Main.hs</filename>.</para>
188 <para>The search path for finding source files is specified with
189 the <option>-i</option> option on the GHCi command line, like
191 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
193 <para>or it can be set using the <literal>:set</literal> command
194 from within GHCi (see <xref
195 linkend="ghci-cmd-line-options"/>)<footnote><para>Note that in
196 GHCi, and <option>––make</option> mode, the <option>-i</option>
197 option is used to specify the search path for
198 <emphasis>source</emphasis> files, whereas in standard
199 batch-compilation mode the <option>-i</option> option is used to
200 specify the search path for interface files, see <xref
201 linkend="search-path"/>.</para> </footnote></para>
203 <para>One consequence of the way that GHCi follows dependencies
204 to find modules to load is that every module must have a source
205 file. The only exception to the rule is modules that come from
206 a package, including the <literal>Prelude</literal> and standard
207 libraries such as <literal>IO</literal> and
208 <literal>Complex</literal>. If you attempt to load a module for
209 which GHCi can't find a source file, even if there are object
210 and interface files for the module, you'll get an error
215 <title>Making changes and recompilation</title>
216 <indexterm><primary><literal>:reload</literal></primary></indexterm>
218 <para>If you make some changes to the source code and want GHCi
219 to recompile the program, give the <literal>:reload</literal>
220 command. The program will be recompiled as necessary, with GHCi
221 doing its best to avoid actually recompiling modules if their
222 external dependencies haven't changed. This is the same
223 mechanism we use to avoid re-compiling modules in the batch
224 compilation setting (see <xref linkend="recomp"/>).</para>
228 <sect1 id="ghci-compiled">
229 <title>Loading compiled code</title>
230 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
232 <para>When you load a Haskell source module into GHCi, it is
233 normally converted to byte-code and run using the interpreter.
234 However, interpreted code can also run alongside compiled code in
235 GHCi; indeed, normally when GHCi starts, it loads up a compiled
236 copy of the <literal>base</literal> package, which contains the
237 <literal>Prelude</literal>.</para>
239 <para>Why should we want to run compiled code? Well, compiled
240 code is roughly 10x faster than interpreted code, but takes about
241 2x longer to produce (perhaps longer if optimisation is on). So
242 it pays to compile the parts of a program that aren't changing
243 very often, and use the interpreter for the code being actively
246 <para>When loading up source files with <literal>:load</literal>,
247 GHCi looks for any corresponding compiled object files, and will
248 use one in preference to interpreting the source if possible. For
249 example, suppose we have a 4-module program consisting of modules
250 A, B, C, and D. Modules B and C both import D only,
251 and A imports both B & C:</para>
259 <para>We can compile D, then load the whole program, like this:</para>
261 Prelude> :! ghc -c D.hs
263 Skipping D ( D.hs, D.o )
264 Compiling C ( C.hs, interpreted )
265 Compiling B ( B.hs, interpreted )
266 Compiling A ( A.hs, interpreted )
267 Ok, modules loaded: A, B, C, D.
271 <para>In the messages from the compiler, we see that it skipped D,
272 and used the object file <filename>D.o</filename>. The message
273 <literal>Skipping</literal> <replaceable>module</replaceable>
274 indicates that compilation for <replaceable>module</replaceable>
275 isn't necessary, because the source and everything it depends on
276 is unchanged since the last compilation.</para>
278 <para>At any time you can use the command
279 <literal>:show modules</literal>
280 to get a list of the modules currently loaded
286 C ( C.hs, interpreted )
287 B ( B.hs, interpreted )
288 A ( A.hs, interpreted )
291 <para>If we now modify the source of D (or pretend to: using Unix
292 command <literal>touch</literal> on the source file is handy for
293 this), the compiler will no longer be able to use the object file,
294 because it might be out of date:</para>
299 Compiling D ( D.hs, interpreted )
300 Skipping C ( C.hs, interpreted )
301 Skipping B ( B.hs, interpreted )
302 Skipping A ( A.hs, interpreted )
303 Ok, modules loaded: A, B, C, D.
307 <para>Note that module D was compiled, but in this instance
308 because its source hadn't really changed, its interface remained
309 the same, and the recompilation checker determined that A, B and C
310 didn't need to be recompiled.</para>
312 <para>So let's try compiling one of the other modules:</para>
315 *Main> :! ghc -c C.hs
317 Compiling D ( D.hs, interpreted )
318 Compiling C ( C.hs, interpreted )
319 Compiling B ( B.hs, interpreted )
320 Compiling A ( A.hs, interpreted )
321 Ok, modules loaded: A, B, C, D.
324 <para>We didn't get the compiled version of C! What happened?
325 Well, in GHCi a compiled module may only depend on other compiled
326 modules, and in this case C depends on D, which doesn't have an
327 object file, so GHCi also rejected C's object file. Ok, so let's
328 also compile D:</para>
331 *Main> :! ghc -c D.hs
333 Ok, modules loaded: A, B, C, D.
336 <para>Nothing happened! Here's another lesson: newly compiled
337 modules aren't picked up by <literal>:reload</literal>, only
338 <literal>:load</literal>:</para>
342 Skipping D ( D.hs, D.o )
343 Skipping C ( C.hs, C.o )
344 Compiling B ( B.hs, interpreted )
345 Compiling A ( A.hs, interpreted )
346 Ok, modules loaded: A, B, C, D.
349 <para>HINT: since GHCi will only use a compiled object file if it
350 can sure that the compiled version is up-to-date, a good technique
351 when working on a large program is to occasionally run
352 <literal>ghc ––make</literal> to compile the whole project (say
353 before you go for lunch :-), then continue working in the
354 interpreter. As you modify code, the new modules will be
355 interpreted, but the rest of the project will remain
361 <title>Interactive evaluation at the prompt</title>
363 <para>When you type an expression at the prompt, GHCi immediately
364 evaluates and prints the result. But that's not the whole story:
365 if you type something of type <literal>IO a</literal> for some
366 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
367 as an IO-computation, and doesn't attempt to print the
373 Prelude> putStrLn "hello"
377 <para>What actually happens is that GHCi typechecks the
378 expression, and if it doesn't have an <literal>IO</literal> type,
379 then it transforms it as follows: an expression
380 <replaceable>e</replaceable> turns into
382 let it = <replaceable>e</replaceable>;
385 which is then run as an IO-action.</para>
387 <para>Hence, the original expression must have a type which is an
388 instance of the <literal>Show</literal> class, or GHCi will
393 No instance for `Show (a -> a)'
394 arising from use of `print'
395 in a `do' expression pattern binding: print it
398 <para>The error message contains some clues as to the
399 transformation happening internally.</para>
401 <sect2 id="ghci-scope">
402 <title>What's really in scope at the prompt?</title>
404 <para>When you type an expression at the prompt, what
405 identifiers and types are in scope? GHCi provides a flexible
406 way to control exactly how the context for an expression is
407 constructed. Let's start with the simple cases; when you start
408 GHCi the prompt looks like this:</para>
410 <screen>Prelude></screen>
412 <para>Which indicates that everything from the module
413 <literal>Prelude</literal> is currently in scope. If we now
414 load a file into GHCi, the prompt will change:</para>
417 Prelude> :load Main.hs
418 Compiling Main ( Main.hs, interpreted )
422 <para>The new prompt is <literal>*Main</literal>, which
423 indicates that we are typing expressions in the context of the
424 top-level of the <literal>Main</literal> module. Everything
425 that is in scope at the top-level in the module
426 <literal>Main</literal> we just loaded is also in scope at the
427 prompt (probably including <literal>Prelude</literal>, as long
428 as <literal>Main</literal> doesn't explicitly hide it).</para>
431 <literal>*<replaceable>module</replaceable></literal> indicates
432 that it is the full top-level scope of
433 <replaceable>module</replaceable> that is contributing to the
434 scope for expressions typed at the prompt. Without the
435 <literal>*</literal>, just the exports of the module are
438 <para>We're not limited to a single module: GHCi can combine
439 scopes from multiple modules, in any mixture of
440 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
441 combines the scopes from all of these modules to form the scope
442 that is in effect at the prompt. For technical reasons, GHCi
443 can only support the <literal>*</literal>-form for modules which
444 are interpreted, so compiled modules and package modules can
445 only contribute their exports to the current scope.</para>
447 <para>The scope is manipulated using the
448 <literal>:module</literal> command. For example, if the current
449 scope is <literal>Prelude</literal>, then we can bring into
450 scope the exports from the module <literal>IO</literal> like
455 Prelude,IO> hPutStrLn stdout "hello\n"
460 <para>(Note: <literal>:module</literal> can be shortened to
461 <literal>:m</literal>). The full syntax of the
462 <literal>:module</literal> command is:</para>
465 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
468 <para>Using the <literal>+</literal> form of the
469 <literal>module</literal> commands adds modules to the current
470 scope, and <literal>-</literal> removes them. Without either
471 <literal>+</literal> or <literal>-</literal>, the current scope
472 is replaced by the set of modules specified. Note that if you
473 use this form and leave out <literal>Prelude</literal>, GHCi
474 will assume that you really wanted the
475 <literal>Prelude</literal> and add it in for you (if you don't
476 want the <literal>Prelude</literal>, then ask to remove it with
477 <literal>:m -Prelude</literal>).</para>
479 <para>The scope is automatically set after a
480 <literal>:load</literal> command, to the most recently loaded
481 "target" module, in a <literal>*</literal>-form if possible.
482 For example, if you say <literal>:load foo.hs bar.hs</literal>
483 and <filename>bar.hs</filename> contains module
484 <literal>Bar</literal>, then the scope will be set to
485 <literal>*Bar</literal> if <literal>Bar</literal> is
486 interpreted, or if <literal>Bar</literal> is compiled it will be
487 set to <literal>Prelude,Bar</literal> (GHCi automatically adds
488 <literal>Prelude</literal> if it isn't present and there aren't
489 any <literal>*</literal>-form modules).</para>
491 <para>With multiple modules in scope, especially multiple
492 <literal>*</literal>-form modules, it is likely that name
493 clashes will occur. Haskell specifies that name clashes are
494 only reported when an ambiguous identifier is used, and GHCi
495 behaves in the same way for expressions typed at the
499 <title>Qualified names</title>
501 <para>To make life slightly easier, the GHCi prompt also
502 behaves as if there is an implicit <literal>import
503 qualified</literal> declaration for every module in every
504 package, and every module currently loaded into GHCi.</para>
509 <title>Using <literal>do-</literal>notation at the prompt</title>
510 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
511 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
513 <para>GHCi actually accepts <firstterm>statements</firstterm>
514 rather than just expressions at the prompt. This means you can
515 bind values and functions to names, and use them in future
516 expressions or statements.</para>
518 <para>The syntax of a statement accepted at the GHCi prompt is
519 exactly the same as the syntax of a statement in a Haskell
520 <literal>do</literal> expression. However, there's no monad
521 overloading here: statements typed at the prompt must be in the
522 <literal>IO</literal> monad.</para>
524 <para>Here's an example:</para>
526 Prelude> x <- return 42
531 <para>The statement <literal>x <- return 42</literal> means
532 “execute <literal>return 42</literal> in the
533 <literal>IO</literal> monad, and bind the result to
534 <literal>x</literal>”. We can then use
535 <literal>x</literal> in future statements, for example to print
536 it as we did above.</para>
538 <para>Of course, you can also bind normal non-IO expressions
539 using the <literal>let</literal>-statement:</para>
546 <para>An important difference between the two types of binding
547 is that the monadic bind (<literal>p <- e</literal>) is
548 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
549 whereas with the <literal>let</literal> form, the expression
550 isn't evaluated immediately:</para>
552 Prelude> let x = error "help!"
557 <para>Any exceptions raised during the evaluation or execution
558 of the statement are caught and printed by the GHCi command line
559 interface (for more information on exceptions, see the module
560 <literal>Control.Exception</literal> in the libraries
561 documentation).</para>
563 <para>Every new binding shadows any existing bindings of the
564 same name, including entities that are in scope in the current
565 module context.</para>
567 <para>WARNING: temporary bindings introduced at the prompt only
568 last until the next <literal>:load</literal> or
569 <literal>:reload</literal> command, at which time they will be
570 simply lost. However, they do survive a change of context with
571 <literal>:module</literal>: the temporary bindings just move to
572 the new location.</para>
574 <para>HINT: To get a list of the bindings currently in scope, use the
575 <literal>:show bindings</literal> command:</para>
578 Prelude> :show bindings
582 <para>HINT: if you turn on the <literal>+t</literal> option,
583 GHCi will show the type of each variable bound by a statement.
585 <indexterm><primary><literal>+t</literal></primary></indexterm>
588 Prelude> let (x:xs) = [1..]
596 <title>The <literal>it</literal> variable</title>
597 <indexterm><primary><literal>it</literal></primary>
600 <para>Whenever an expression (or a non-binding statement, to be
601 precise) is typed at the prompt, GHCi implicitly binds its value
602 to the variable <literal>it</literal>. For example:</para>
610 <para>This is a result of the translation mentioned earlier,
611 namely that an expression <replaceable>e</replaceable> is
614 let it = <replaceable>e</replaceable>;
617 before execution, resulting in a binding for
618 <literal>it</literal>.</para>
620 <para>If the expression was of type <literal>IO a</literal> for
621 some <literal>a</literal>, then <literal>it</literal> will be
622 bound to the result of the <literal>IO</literal> computation,
623 which is of type <literal>a</literal>. eg.:</para>
625 Prelude> Time.getClockTime
627 Wed Mar 14 12:23:13 GMT 2001
630 <para>The corresponding translation for an IO-typed
631 <replaceable>e</replaceable> is
633 it <- <replaceable>e</replaceable>
637 <para>Note that <literal>it</literal> is shadowed by the new
638 value each time you evaluate a new expression, and the old value
639 of <literal>it</literal> is lost.</para>
644 <title>Type defaulting in GHCi</title>
645 <indexterm><primary>Type default</primary></indexterm>
646 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
648 Consider this GHCi session:
652 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
653 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
654 on the type <literal>a</literal>. For example:
656 ghci> (reverse []) :: String
658 ghci> (reverse []) :: [Int]
661 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
662 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. If the expression yields a set of
663 type constraints that are all from standard classes (<literal>Num</literal>, <literal>Eq</literal> etc.),
664 and at least one is either a numeric class <emphasis>or the <literal>Show</literal>,
665 <literal>Eq</literal>, or <literal>Ord</literal> class</emphasis>,
666 GHCi will try to use one of the <literal>default</literal> types, just as described in the Report.
671 <sect1 id="ghci-invocation">
672 <title>Invoking GHCi</title>
673 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
674 <indexterm><primary><option>––interactive</option></primary></indexterm>
676 <para>GHCi is invoked with the command <literal>ghci</literal> or
677 <literal>ghc ––interactive</literal>. One or more modules or
678 filenames can also be specified on the command line; this
679 instructs GHCi to load the specified modules or filenames (and all
680 the modules they depend on), just as if you had said
681 <literal>:load <replaceable>modules</replaceable></literal> at the
682 GHCi prompt (see <xref linkend="ghci-commands"/>). For example, to
683 start GHCi and load the program whose topmost module is in the
684 file <literal>Main.hs</literal>, we could say:</para>
690 <para>Most of the command-line options accepted by GHC (see <xref
691 linkend="using-ghc"/>) also make sense in interactive mode. The ones
692 that don't make sense are mostly obvious; for example, GHCi
693 doesn't generate interface files, so options related to interface
694 file generation won't have any effect.</para>
697 <title>Packages</title>
698 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
700 <para>Most packages (see <xref linkend="using-packages"/>) are
701 available without needing to specify any extra flags at all:
702 they will be automatically loaded the first time they are
705 <para>For non-auto packages, however, you need to request the
706 package be loaded by using the <literal>-package</literal> flag:</para>
712 / /_\// /_/ / / | | GHC Interactive, version 5.05, for Haskell 98.
713 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
714 \____/\/ /_/\____/|_| Type :? for help.
716 Loading package base ... linking ... done.
717 Loading package haskell98 ... linking ... done.
718 Loading package lang ... linking ... done.
719 Loading package concurrent ... linking ... done.
720 Loading package readline ... linking ... done.
721 Loading package unix ... linking ... done.
722 Loading package posix ... linking ... done.
723 Loading package util ... linking ... done.
724 Loading package data ... linking ... done.
728 <para>The following command works to load new packages into a
732 Prelude> :set -package <replaceable>name</replaceable>
735 <para>But note that doing this will cause all currently loaded
736 modules to be unloaded, and you'll be dumped back into the
737 <literal>Prelude</literal>.</para>
741 <title>Extra libraries</title>
742 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
744 <para>Extra libraries may be specified on the command line using
745 the normal <literal>-l<replaceable>lib</replaceable></literal>
746 option. (The term <emphasis>library</emphasis> here refers to
747 libraries of foreign object code; for using libraries of Haskell
748 source code, see <xref linkend="ghci-modules-filenames"/>.) For
749 example, to load the “m” library:</para>
755 <para>On systems with <literal>.so</literal>-style shared
756 libraries, the actual library loaded will the
757 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
758 searches the following places for libraries, in this order:</para>
762 <para>Paths specified using the
763 <literal>-L<replaceable>path</replaceable></literal>
764 command-line option,</para>
767 <para>the standard library search path for your system,
768 which on some systems may be overridden by setting the
769 <literal>LD_LIBRARY_PATH</literal> environment
774 <para>On systems with <literal>.dll</literal>-style shared
775 libraries, the actual library loaded will be
776 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
777 GHCi will signal an error if it can't find the library.</para>
779 <para>GHCi can also load plain object files
780 (<literal>.o</literal> or <literal>.obj</literal> depending on
781 your platform) from the command-line. Just add the name the
782 object file to the command line.</para>
784 <para>Ordering of <option>-l</option> options matters: a library
785 should be mentioned <emphasis>before</emphasis> the libraries it
786 depends on (see <xref linkend="options-linker"/>).</para>
791 <sect1 id="ghci-commands">
792 <title>GHCi commands</title>
794 <para>GHCi commands all begin with
795 ‘<literal>:</literal>’ and consist of a single command
796 name followed by zero or more parameters. The command name may be
797 abbreviated, as long as the abbreviation is not ambiguous. All of
798 the builtin commands, with the exception of
799 <literal>:unset</literal> and <literal>:undef</literal>, may be
800 abbreviated to a single letter.</para>
805 <literal>:add</literal> <replaceable>module</replaceable> ...
806 <indexterm><primary><literal>:add</literal></primary></indexterm>
809 <para>Add <replaceable>module</replaceable>(s) to the
810 current <firstterm>target set</firstterm>, and perform a
817 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
818 <indexterm><primary><literal>:browse</literal></primary></indexterm>
821 <para>Displays the identifiers defined by the module
822 <replaceable>module</replaceable>, which must be either
823 loaded into GHCi or be a member of a package. If the
824 <literal>*</literal> symbol is placed before the module
825 name, then <emphasis>all</emphasis> the identifiers defined
826 in <replaceable>module</replaceable> are shown; otherwise
827 the list is limited to the exports of
828 <replaceable>module</replaceable>. The
829 <literal>*</literal>-form is only available for modules
830 which are interpreted; for compiled modules (including
831 modules from packages) only the non-<literal>*</literal>
832 form of <literal>:browse</literal> is available.</para>
838 <literal>:cd</literal> <replaceable>dir</replaceable>
839 <indexterm><primary><literal>:cd</literal></primary></indexterm>
842 <para>Changes the current working directory to
843 <replaceable>dir</replaceable>. A
844 ‘<literal>˜</literal>’ symbol at the
845 beginning of <replaceable>dir</replaceable> will be replaced
846 by the contents of the environment variable
847 <literal>HOME</literal>.</para>
849 <para>NOTE: changing directories causes all currently loaded
850 modules to be unloaded. This is because the search path is
851 usually expressed using relative directories, and changing
852 the search path in the middle of a session is not
859 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
860 <indexterm><primary><literal>:def</literal></primary></indexterm>
863 <para>The command <literal>:def</literal>
864 <replaceable>name</replaceable>
865 <replaceable>expr</replaceable> defines a new GHCi command
866 <literal>:<replaceable>name</replaceable></literal>,
867 implemented by the Haskell expression
868 <replaceable>expr</replaceable>, which must have type
869 <literal>String -> IO String</literal>. When
870 <literal>:<replaceable>name</replaceable>
871 <replaceable>args</replaceable></literal> is typed at the
872 prompt, GHCi will run the expression
873 <literal>(<replaceable>name</replaceable>
874 <replaceable>args</replaceable>)</literal>, take the
875 resulting <literal>String</literal>, and feed it back into
876 GHCi as a new sequence of commands. Separate commands in
877 the result must be separated by
878 ‘<literal>\n</literal>’.</para>
880 <para>That's all a little confusing, so here's a few
881 examples. To start with, here's a new GHCi command which
882 doesn't take any arguments or produce any results, it just
883 outputs the current date & time:</para>
886 Prelude> let date _ = Time.getClockTime >>= print >> return ""
887 Prelude> :def date date
889 Fri Mar 23 15:16:40 GMT 2001
892 <para>Here's an example of a command that takes an argument.
893 It's a re-implementation of <literal>:cd</literal>:</para>
896 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
897 Prelude> :def mycd mycd
901 <para>Or I could define a simple way to invoke
902 “<literal>ghc ––make Main</literal>” in the
903 current directory:</para>
906 Prelude> :def make (\_ -> return ":! ghc ––make Main")
909 <para>We can define a command that reads GHCi input from a
910 file. This might be useful for creating a set of bindings
911 that we want to repeatedly load into the GHCi session:</para>
914 Prelude> :def . readFile
915 Prelude> :. cmds.ghci
918 <para>Notice that we named the command
919 <literal>:.</literal>, by analogy with the
920 ‘<literal>.</literal>’ Unix shell command that
921 does the same thing.</para>
927 <literal>:help</literal>
928 <indexterm><primary><literal>:help</literal></primary></indexterm>
931 <literal>:?</literal>
932 <indexterm><primary><literal>:?</literal></primary></indexterm>
935 <para>Displays a list of the available commands.</para>
941 <literal>:info</literal> <replaceable>name</replaceable> ...
942 <indexterm><primary><literal>:info</literal></primary></indexterm>
945 <para>Displays information about the given name(s). For
946 example, if <replaceable>name</replaceable> is a class, then
947 the class methods and their types will be printed; if
948 <replaceable>name</replaceable> is a type constructor, then
949 its definition will be printed; if
950 <replaceable>name</replaceable> is a function, then its type
951 will be printed. If <replaceable>name</replaceable> has
952 been loaded from a source file, then GHCi will also display
953 the location of its definition in the source.</para>
959 <literal>:load</literal> <replaceable>module</replaceable> ...
960 <indexterm><primary><literal>:load</literal></primary></indexterm>
963 <para>Recursively loads the specified
964 <replaceable>module</replaceable>s, and all the modules they
965 depend on. Here, each <replaceable>module</replaceable>
966 must be a module name or filename, but may not be the name
967 of a module in a package.</para>
969 <para>All previously loaded modules, except package modules,
970 are forgotten. The new set of modules is known as the
971 <firstterm>target set</firstterm>. Note that
972 <literal>:load</literal> can be used without any arguments
973 to unload all the currently loaded modules and
976 <para>After a <literal>:load</literal> command, the current
977 context is set to:</para>
981 <para><replaceable>module</replaceable>, if it was loaded
982 successfully, or</para>
985 <para>the most recently successfully loaded module, if
986 any other modules were loaded as a result of the current
987 <literal>:load</literal>, or</para>
990 <para><literal>Prelude</literal> otherwise.</para>
998 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
999 <indexterm><primary><literal>:module</literal></primary></indexterm>
1002 <para>Sets or modifies the current context for statements
1003 typed at the prompt. See <xref linkend="ghci-scope"/> for
1004 more details.</para>
1010 <literal>:quit</literal>
1011 <indexterm><primary><literal>:quit</literal></primary></indexterm>
1014 <para>Quits GHCi. You can also quit by typing a control-D
1015 at the prompt.</para>
1021 <literal>:reload</literal>
1022 <indexterm><primary><literal>:reload</literal></primary></indexterm>
1025 <para>Attempts to reload the current target set (see
1026 <literal>:load</literal>) if any of the modules in the set,
1027 or any dependent module, has changed. Note that this may
1028 entail loading new modules, or dropping modules which are no
1029 longer indirectly required by the target.</para>
1035 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
1036 <indexterm><primary><literal>:set</literal></primary></indexterm>
1039 <para>Sets various options. See <xref linkend="ghci-set"/>
1040 for a list of available options. The
1041 <literal>:set</literal> command by itself shows which
1042 options are currently set.</para>
1048 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
1049 <indexterm><primary><literal>:set</literal></primary></indexterm>
1052 <para>Sets the list of arguments which are returned when the
1053 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
1054 </indexterm>.</para>
1060 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
1061 <indexterm><primary><literal>:set</literal></primary></indexterm>
1064 <para>Sets the string to be returned when the program calls
1065 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
1066 </indexterm>.</para>
1072 <literal>:show bindings</literal>
1073 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
1076 <para>Show the bindings made at the prompt and their
1083 <literal>:show modules</literal>
1084 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
1087 <para>Show the list of modules currently load.</para>
1093 <literal>:type</literal> <replaceable>expression</replaceable>
1094 <indexterm><primary><literal>:type</literal></primary></indexterm>
1097 <para>Infers and prints the type of
1098 <replaceable>expression</replaceable>, including explicit
1099 forall quantifiers for polymorphic types. The monomorphism
1100 restriction is <emphasis>not</emphasis> applied to the
1101 expression during type inference.</para>
1107 <literal>:kind</literal> <replaceable>type</replaceable>
1108 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1111 <para>Infers and prints the kind of
1112 <replaceable>type</replaceable>. The latter can be an arbitrary
1113 type expression, including a partial application of a type constructor,
1114 such as <literal>Either Int</literal>.</para>
1120 <literal>:undef</literal> <replaceable>name</replaceable>
1121 <indexterm><primary><literal>:undef</literal></primary></indexterm>
1124 <para>Undefines the user-defined command
1125 <replaceable>name</replaceable> (see <literal>:def</literal>
1132 <literal>:unset</literal> <replaceable>option</replaceable>...
1133 <indexterm><primary><literal>:unset</literal></primary></indexterm>
1136 <para>Unsets certain options. See <xref linkend="ghci-set"/>
1137 for a list of available options.</para>
1143 <literal>:!</literal> <replaceable>command</replaceable>...
1144 <indexterm><primary><literal>:!</literal></primary></indexterm>
1145 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
1148 <para>Executes the shell command
1149 <replaceable>command</replaceable>.</para>
1156 <sect1 id="ghci-set">
1157 <title>The <literal>:set</literal> command</title>
1158 <indexterm><primary><literal>:set</literal></primary></indexterm>
1160 <para>The <literal>:set</literal> command sets two types of
1161 options: GHCi options, which begin with
1162 ‘<literal>+</literal>” and “command-line”
1163 options, which begin with ‘-’. </para>
1165 <para>NOTE: at the moment, the <literal>:set</literal> command
1166 doesn't support any kind of quoting in its arguments: quotes will
1167 not be removed and cannot be used to group words together. For
1168 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
1172 <title>GHCi options</title>
1173 <indexterm><primary>options</primary><secondary>GHCi</secondary>
1176 <para>GHCi options may be set using <literal>:set</literal> and
1177 unset using <literal>:unset</literal>.</para>
1179 <para>The available GHCi options are:</para>
1184 <literal>+r</literal>
1185 <indexterm><primary><literal>+r</literal></primary></indexterm>
1186 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
1187 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
1190 <para>Normally, any evaluation of top-level expressions
1191 (otherwise known as CAFs or Constant Applicative Forms) in
1192 loaded modules is retained between evaluations. Turning
1193 on <literal>+r</literal> causes all evaluation of
1194 top-level expressions to be discarded after each
1195 evaluation (they are still retained
1196 <emphasis>during</emphasis> a single evaluation).</para>
1198 <para>This option may help if the evaluated top-level
1199 expressions are consuming large amounts of space, or if
1200 you need repeatable performance measurements.</para>
1206 <literal>+s</literal>
1207 <indexterm><primary><literal>+s</literal></primary></indexterm>
1210 <para>Display some stats after evaluating each expression,
1211 including the elapsed time and number of bytes allocated.
1212 NOTE: the allocation figure is only accurate to the size
1213 of the storage manager's allocation area, because it is
1214 calculated at every GC. Hence, you might see values of
1215 zero if no GC has occurred.</para>
1221 <literal>+t</literal>
1222 <indexterm><primary><literal>+t</literal></primary></indexterm>
1225 <para>Display the type of each variable bound after a
1226 statement is entered at the prompt. If the statement is a
1227 single expression, then the only variable binding will be
1229 ‘<literal>it</literal>’.</para>
1235 <sect2 id="ghci-cmd-line-options">
1236 <title>Setting GHC command-line options in GHCi</title>
1238 <para>Normal GHC command-line options may also be set using
1239 <literal>:set</literal>. For example, to turn on
1240 <option>-fglasgow-exts</option>, you would say:</para>
1243 Prelude> :set -fglasgow-exts
1246 <para>Any GHC command-line option that is designated as
1247 <firstterm>dynamic</firstterm> (see the table in <xref
1248 linkend="flag-reference"/>), may be set using
1249 <literal>:set</literal>. To unset an option, you can set the
1250 reverse option:</para>
1251 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
1254 Prelude> :set -fno-glasgow-exts
1257 <para><xref linkend="flag-reference"/> lists the reverse for each
1258 option where applicable.</para>
1260 <para>Certain static options (<option>-package</option>,
1261 <option>-I</option>, <option>-i</option>, and
1262 <option>-l</option> in particular) will also work, but some may
1263 not take effect until the next reload.</para>
1264 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
1268 <sect1 id="ghci-dot-files">
1269 <title>The <filename>.ghci</filename> file</title>
1270 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
1272 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
1275 <para>When it starts, GHCi always reads and executes commands from
1276 <filename>$HOME/.ghci</filename>, followed by
1277 <filename>./.ghci</filename>.</para>
1279 <para>The <filename>.ghci</filename> in your home directory is
1280 most useful for turning on favourite options (eg. <literal>:set
1281 +s</literal>), and defining useful macros. Placing a
1282 <filename>.ghci</filename> file in a directory with a Haskell
1283 project is a useful way to set certain project-wide options so you
1284 don't have to type them everytime you start GHCi: eg. if your
1285 project uses GHC extensions and CPP, and has source files in three
1286 subdirectories A B and C, you might put the following lines in
1287 <filename>.ghci</filename>:</para>
1290 :set -fglasgow-exts -cpp
1294 <para>(Note that strictly speaking the <option>-i</option> flag is
1295 a static one, but in fact it works to set it using
1296 <literal>:set</literal> like this. The changes won't take effect
1297 until the next <literal>:load</literal>, though.)</para>
1299 <para>Two command-line options control whether the
1300 <filename>.ghci</filename> files are read:</para>
1305 <option>-ignore-dot-ghci</option>
1306 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
1309 <para>Don't read either <filename>./.ghci</filename> or
1310 <filename>$HOME/.ghci</filename> when starting up.</para>
1315 <option>-read-dot-ghci</option>
1316 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
1319 <para>Read <filename>.ghci</filename> and
1320 <filename>$HOME/.ghci</filename>. This is normally the
1321 default, but the <option>-read-dot-ghci</option> option may
1322 be used to override a previous
1323 <option>-ignore-dot-ghci</option> option.</para>
1331 <title>FAQ and Things To Watch Out For</title>
1335 <term>The interpreter can't load modules with foreign export
1336 declarations!</term>
1338 <para>Unfortunately not. We haven't implemented it yet.
1339 Please compile any offending modules by hand before loading
1340 them into GHCi.</para>
1346 <literal>-O</literal> doesn't work with GHCi!
1347 <indexterm><primary><option>-O</option></primary></indexterm>
1350 <para>For technical reasons, the bytecode compiler doesn't
1351 interact well with one of the optimisation passes, so we
1352 have disabled optimisation when using the interpreter. This
1353 isn't a great loss: you'll get a much bigger win by
1354 compiling the bits of your code that need to go fast, rather
1355 than interpreting them with optimisation turned on.</para>
1360 <term>Unboxed tuples don't work with GHCi</term>
1362 <para>That's right. You can always compile a module that
1363 uses unboxed tuples and load it into GHCi, however.
1364 (Incidentally the previous point, namely that
1365 <literal>-O</literal> is incompatible with GHCi, is because
1366 the bytecode compiler can't deal with unboxed
1372 <term>Concurrent threads don't carry on running when GHCi is
1373 waiting for input.</term>
1375 <para>No, they don't. This is because the Haskell binding
1376 to the GNU readline library doesn't support reading from the
1377 terminal in a non-blocking way, which is required to work
1378 properly with GHC's concurrency model.</para>
1383 <term>After using <literal>getContents</literal>, I can't use
1384 <literal>stdin</literal> again until I do
1385 <literal>:load</literal> or <literal>:reload</literal>.</term>
1388 <para>This is the defined behaviour of
1389 <literal>getContents</literal>: it puts the stdin Handle in
1390 a state known as <firstterm>semi-closed</firstterm>, wherein
1391 any further I/O operations on it are forbidden. Because I/O
1392 state is retained between computations, the semi-closed
1393 state persists until the next <literal>:load</literal> or
1394 <literal>:reload</literal> command.</para>
1396 <para>You can make <literal>stdin</literal> reset itself
1397 after every evaluation by giving GHCi the command
1398 <literal>:set +r</literal>. This works because
1399 <literal>stdin</literal> is just a top-level expression that
1400 can be reverted to its unevaluated state in the same way as
1401 any other top-level expression (CAF).</para>
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