2 <title>Using GHCi</title>
3 <indexterm><primary>GHCi</primary></indexterm>
4 <indexterm><primary>interpreter</primary><see>GHCi</see></indexterm>
5 <indexterm><primary>interactive</primary><see>GHCi</see></indexterm>
8 <para>The ‘i’ stands for “Interactive”</para>
10 is GHC's interactive environment, in which Haskell expressions can
11 be interactively evaluated and programs can be interpreted. If
12 you're famililar with <ulink url="http://www.haskell.org/hugs/">Hugs</ulink><indexterm><primary>Hugs</primary>
13 </indexterm>, then you'll be right at home with GHCi. However, GHCi
14 also has support for interactively loading compiled code, as well as
15 supporting all<footnote><para>except <literal>foreign export</literal>, at the moment</para>
16 </footnote> the language extensions that GHC provides.</para>
17 <indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
18 <indexterm><primary>Foreign Function Interface</primary><secondary>GHCi support</secondary></indexterm>
21 <title>Introduction to GHCi</title>
23 <para>Let's start with an example GHCi session. You can fire up
24 GHCi with the command <literal>ghci</literal>:</para>
30 / /_\// /_/ / / | | GHC Interactive, version 5.04, for Haskell 98.
31 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
32 \____/\/ /_/\____/|_| Type :? for help.
34 Loading package base ... linking ... done.
35 Loading package haskell98 ... linking ... done.
39 <para>There may be a short pause while GHCi loads the prelude and
40 standard libraries, after which the prompt is shown. If we follow
41 the instructions and type <literal>:?</literal> for help, we
45 Commands available from the prompt:
47 <stmt> evaluate/run <stmt>
48 :add <filename> ... add module(s) to the current target set
49 :browse [*]<module> display the names defined by <module>
50 :cd <dir> change directory to <dir>
51 :def <cmd> <expr> define a command :<cmd>
52 :help, :? display this list of commands
53 :info [<name> ...] display information about the given names
54 :load <filename> ... load module(s) and their dependents
55 :module [+/-] [*]<mod> ... set the context for expression evaluation
56 :reload reload the current module set
58 :set <option> ... set options
59 :set args <arg> ... set the arguments returned by System.getArgs
60 :set prog <progname> set the value returned by System.getProgName
62 :show modules show the currently loaded modules
63 :show bindings show the current bindings made at the prompt
65 :type <expr> show the type of <expr>
66 :undef <cmd> undefine user-defined command :<cmd>
67 :unset <option> ... unset options
69 :!<command> run the shell command <command>
71 Options for `:set' and `:unset':
73 +r revert top-level expressions after each evaluation
74 +s print timing/memory stats after each evaluation
75 +t print type after evaluation
76 -<flags> most GHC command line flags can also be set here
77 (eg. -v2, -fglasgow-exts, etc.)
80 <para>We'll explain most of these commands as we go along. For
81 Hugs users: many things work the same as in Hugs, so you should be
82 able to get going straight away.</para>
84 <para>Haskell expressions can be typed at the prompt:</para>
85 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
91 Prelude> let x = 42 in x / 9
96 <para>GHCi interprets the whole line as an expression to evaluate.
97 The expression may not span several lines - as soon as you press
98 enter, GHCi will attempt to evaluate it.</para>
102 <title>Loading source files</title>
104 <para>Suppose we have the following Haskell source code, which we
105 place in a file <filename>Main.hs</filename>:</para>
108 main = print (fac 20)
111 fac n = n * fac (n-1)
114 <para>You can save <filename>Main.hs</filename> anywhere you like,
115 but if you save it somewhere other than the current
116 directory<footnote><para>If you started up GHCi from the command
117 line then GHCi's current directory is the same as the current
118 directory of the shell from which it was started. If you started
119 GHCi from the “Start” menu in Windows, then the
120 current directory is probably something like
121 <filename>C:\Documents and Settings\<replaceable>user
122 name</replaceable></filename>.</para> </footnote> then we will
123 need to change to the right directory in GHCi:</para>
126 Prelude> :cd <replaceable>dir</replaceable>
129 <para>where <replaceable>dir</replaceable> is the directory (or
130 folder) in which you saved <filename>Main.hs</filename>.</para>
132 <para>To load a Haskell source file into GHCi, use the
133 <literal>:load</literal> command:</para>
134 <indexterm><primary><literal>:load</literal></primary></indexterm>
138 Compiling Main ( Main.hs, interpreted )
139 Ok, modules loaded: Main.
143 <para>GHCi has loaded the <literal>Main</literal> module, and the
144 prompt has changed to “<literal>*Main></literal>” to
145 indicate that the current context for expressions typed at the
146 prompt is the <literal>Main</literal> module we just loaded (we'll
147 explain what the <literal>*</literal> means later in <xref
148 linkend="ghci-scope">). So we can now type expressions involving
149 the functions from <filename>Main.hs</filename>:</para>
156 <para>Loading a multi-module program is just as straightforward;
157 just give the name of the “topmost” module to the
158 <literal>:load</literal> command (hint: <literal>:load</literal>
159 can be abbreviated to <literal>:l</literal>). The topmost module
160 will normally be <literal>Main</literal>, but it doesn't have to
161 be. GHCi will discover which modules are required, directly or
162 indirectly, by the topmost module, and load them all in dependency
165 <sect2 id="ghci-modules-filenames">
166 <title>Modules vs. filenames</title>
167 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
168 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
170 <para>Question: How does GHC find the filename which contains
171 module <replaceable>M</replaceable>? Answer: it looks for the
172 file <literal><replaceable>M</replaceable>.hs</literal>, or
173 <literal><replaceable>M</replaceable>.lhs</literal>. This means
174 that for most modules, the module name must match the filename.
175 If it doesn't, GHCi won't be able to find it.</para>
177 <para>There is one exception to this general rule: when you load
178 a program with <literal>:load</literal>, or specify it when you
179 invoke <literal>ghci</literal>, you can give a filename rather
180 than a module name. This filename is loaded if it exists, and
181 it may contain any module you like. This is particularly
182 convenient if you have several <literal>Main</literal> modules
183 in the same directory and you can't call them all
184 <filename>Main.hs</filename>.</para>
186 <para>The search path for finding source files is specified with
187 the <option>-i</option> option on the GHCi command line, like
189 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
191 <para>or it can be set using the <literal>:set</literal> command
192 from within GHCi (see <xref
193 linkend="ghci-cmd-line-options">)<footnote><para>Note that in
194 GHCi, and <option>––make</option> mode, the <option>-i</option>
195 option is used to specify the search path for
196 <emphasis>source</emphasis> files, whereas in standard
197 batch-compilation mode the <option>-i</option> option is used to
198 specify the search path for interface files, see <xref
199 linkend="search-path">.</para> </footnote></para>
201 <para>One consequence of the way that GHCi follows dependencies
202 to find modules to load is that every module must have a source
203 file. The only exception to the rule is modules that come from
204 a package, including the <literal>Prelude</literal> and standard
205 libraries such as <literal>IO</literal> and
206 <literal>Complex</literal>. If you attempt to load a module for
207 which GHCi can't find a source file, even if there are object
208 and interface files for the module, you'll get an error
213 <title>Making changes and recompilation</title>
214 <indexterm><primary><literal>:reload</literal></primary></indexterm>
216 <para>If you make some changes to the source code and want GHCi
217 to recompile the program, give the <literal>:reload</literal>
218 command. The program will be recompiled as necessary, with GHCi
219 doing its best to avoid actually recompiling modules if their
220 external dependencies haven't changed. This is the same
221 mechanism we use to avoid re-compiling modules in the batch
222 compilation setting (see <xref linkend="recomp">).</para>
226 <sect1 id="ghci-compiled">
227 <title>Loading compiled code</title>
228 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
230 <para>When you load a Haskell source module into GHCi, it is
231 normally converted to byte-code and run using the interpreter.
232 However, interpreted code can also run alongside compiled code in
233 GHCi; indeed, normally when GHCi starts, it loads up a compiled
234 copy of the <literal>base</literal> package, which contains the
235 <literal>Prelude</literal>.</para>
237 <para>Why should we want to run compiled code? Well, compiled
238 code is roughly 10x faster than interpreted code, but takes about
239 2x longer to produce (perhaps longer if optimisation is on). So
240 it pays to compile the parts of a program that aren't changing
241 very often, and use the interpreter for the code being actively
244 <para>When loading up source files with <literal>:load</literal>,
245 GHCi looks for any corresponding compiled object files, and will
246 use one in preference to interpreting the source if possible. For
247 example, suppose we have a 4-module program consisting of modules
248 A, B, C, and D. Modules B and C both import D only,
249 and A imports both B & C:</para>
257 <para>We can compile D, then load the whole program, like this:</para>
259 Prelude> :! ghc -c D.hs
261 Skipping D ( D.hs, D.o )
262 Compiling C ( C.hs, interpreted )
263 Compiling B ( B.hs, interpreted )
264 Compiling A ( A.hs, interpreted )
265 Ok, modules loaded: A, B, C, D.
269 <para>In the messages from the compiler, we see that it skipped D,
270 and used the object file <filename>D.o</filename>. The message
271 <literal>Skipping</literal> <replaceable>module</replaceable>
272 indicates that compilation for <replaceable>module</replaceable>
273 isn't necessary, because the source and everything it depends on
274 is unchanged since the last compilation.</para>
276 <para>At any time you can use the command
277 <literal>:show modules</literal>
278 to get a list of the modules currently loaded
284 C ( C.hs, interpreted )
285 B ( B.hs, interpreted )
286 A ( A.hs, interpreted )
289 <para>If we now modify the source of D (or pretend to: using Unix
290 command <literal>touch</literal> on the source file is handy for
291 this), the compiler will no longer be able to use the object file,
292 because it might be out of date:</para>
297 Compiling D ( D.hs, interpreted )
298 Skipping C ( C.hs, interpreted )
299 Skipping B ( B.hs, interpreted )
300 Skipping A ( A.hs, interpreted )
301 Ok, modules loaded: A, B, C, D.
305 <para>Note that module D was compiled, but in this instance
306 because its source hadn't really changed, its interface remained
307 the same, and the recompilation checker determined that A, B and C
308 didn't need to be recompiled.</para>
310 <para>So let's try compiling one of the other modules:</para>
313 *Main> :! ghc -c C.hs
315 Compiling D ( D.hs, interpreted )
316 Compiling C ( C.hs, interpreted )
317 Compiling B ( B.hs, interpreted )
318 Compiling A ( A.hs, interpreted )
319 Ok, modules loaded: A, B, C, D.
322 <para>We didn't get the compiled version of C! What happened?
323 Well, in GHCi a compiled module may only depend on other compiled
324 modules, and in this case C depends on D, which doesn't have an
325 object file, so GHCi also rejected C's object file. Ok, so let's
326 also compile D:</para>
329 *Main> :! ghc -c D.hs
331 Ok, modules loaded: A, B, C, D.
334 <para>Nothing happened! Here's another lesson: newly compiled
335 modules aren't picked up by <literal>:reload</literal>, only
336 <literal>:load</literal>:</para>
340 Skipping D ( D.hs, D.o )
341 Skipping C ( C.hs, C.o )
342 Compiling B ( B.hs, interpreted )
343 Compiling A ( A.hs, interpreted )
344 Ok, modules loaded: A, B, C, D.
347 <para>HINT: since GHCi will only use a compiled object file if it
348 can sure that the compiled version is up-to-date, a good technique
349 when working on a large program is to occasionally run
350 <literal>ghc ––make</literal> to compile the whole project (say
351 before you go for lunch :-), then continue working in the
352 interpreter. As you modify code, the new modules will be
353 interpreted, but the rest of the project will remain
359 <title>Interactive evaluation at the prompt</title>
361 <para>When you type an expression at the prompt, GHCi immediately
362 evaluates and prints the result. But that's not the whole story:
363 if you type something of type <literal>IO a</literal> for some
364 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
365 as an IO-computation, and doesn't attempt to print the
371 Prelude> putStrLn "hello"
375 <para>What actually happens is that GHCi typechecks the
376 expression, and if it doesn't have an <literal>IO</literal> type,
377 then it transforms it as follows: an expression
378 <replaceable>e</replaceable> turns into
380 let it = <replaceable>e</replaceable>;
383 which is then run as an IO-action.</para>
385 <para>Hence, the original expression must have a type which is an
386 instance of the <literal>Show</literal> class, or GHCi will
391 No instance for `Show (a -> a)'
392 arising from use of `print'
393 in a `do' expression pattern binding: print it
396 <para>The error message contains some clues as to the
397 transformation happening internally.</para>
399 <sect2 id="ghci-scope">
400 <title>What's really in scope at the prompt?</title>
402 <para>When you type an expression at the prompt, what
403 identifiers and types are in scope? GHCi provides a flexible
404 way to control exactly how the context for an expression is
405 constructed. Let's start with the simple cases; when you start
406 GHCi the prompt looks like this:</para>
408 <screen>Prelude></screen>
410 <para>Which indicates that everything from the module
411 <literal>Prelude</literal> is currently in scope. If we now
412 load a file into GHCi, the prompt will change:</para>
415 Prelude> :load Main.hs
416 Compiling Main ( Main.hs, interpreted )
420 <para>The new prompt is <literal>*Main</literal>, which
421 indicates that we are typing expressions in the context of the
422 top-level of the <literal>Main</literal> module. Everything
423 that is in scope at the top-level in the module
424 <literal>Main</literal> we just loaded is also in scope at the
425 prompt (probably including <literal>Prelude</literal>, as long
426 as <literal>Main</literal> doesn't explicitly hide it).</para>
429 <literal>*<replaceable>module</replaceable></literal> indicates
430 that it is the full top-level scope of
431 <replaceable>module</replaceable> that is contributing to the
432 scope for expressions typed at the prompt. Without the
433 <literal>*</literal>, just the exports of the module are
436 <para>We're not limited to a single module: GHCi can combine
437 scopes from multiple modules, in any mixture of
438 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
439 combines the scopes from all of these modules to form the scope
440 that is in effect at the prompt. For technical reasons, GHCi
441 can only support the <literal>*</literal>-form for modules which
442 are interpreted, so compiled modules and package modules can
443 only contribute their exports to the current scope.</para>
445 <para>The scope is manipulated using the
446 <literal>:module</literal> command. For example, if the current
447 scope is <literal>Prelude</literal>, then we can bring into
448 scope the exports from the module <literal>IO</literal> like
453 Prelude,IO> hPutStrLn stdout "hello\n"
458 <para>(Note: <literal>:module</literal> can be shortened to
459 <literal>:m</literal>). The full syntax of the
460 <literal>:module</literal> command is:</para>
463 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
466 <para>Using the <literal>+</literal> form of the
467 <literal>module</literal> commands adds modules to the current
468 scope, and <literal>-</literal> removes them. Without either
469 <literal>+</literal> or <literal>-</literal>, the current scope
470 is replaced by the set of modules specified. Note that if you
471 use this form and leave out <literal>Prelude</literal>, GHCi
472 will assume that you really wanted the
473 <literal>Prelude</literal> and add it in for you (if you don't
474 want the <literal>Prelude</literal>, then ask to remove it with
475 <literal>:m -Prelude</literal>).</para>
477 <para>The scope is automatically set after a
478 <literal>:load</literal> command, to the most recently loaded
479 "target" module, in a <literal>*</literal>-form if possible.
480 For example, if you say <literal>:load foo.hs bar.hs</literal>
481 and <filename>bar.hs</filename> contains module
482 <literal>Bar</literal>, then the scope will be set to
483 <literal>*Bar</literal> if <literal>Bar</literal> is
484 interpreted, or if <literal>Bar</literal> is compiled it will be
485 set to <literal>Prelude,Bar</literal> (GHCi automatically adds
486 <literal>Prelude</literal> if it isn't present and there aren't
487 any <literal>*</literal>-form modules).</para>
489 <para>With multiple modules in scope, especially multiple
490 <literal>*</literal>-form modules, it is likely that name
491 clashes will occur. Haskell specifies that name clashes are
492 only reported when an ambiguous identifier is used, and GHCi
493 behaves in the same way for expressions typed at the
497 <title>Qualified names</title>
499 <para>To make life slightly easier, the GHCi prompt also
500 behaves as if there is an implicit <literal>import
501 qualified</literal> declaration for every module in every
502 package, and every module currently loaded into GHCi.</para>
507 <title>Using <literal>do-</literal>notation at the prompt</title>
508 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
509 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
511 <para>GHCi actually accepts <firstterm>statements</firstterm>
512 rather than just expressions at the prompt. This means you can
513 bind values and functions to names, and use them in future
514 expressions or statements.</para>
516 <para>The syntax of a statement accepted at the GHCi prompt is
517 exactly the same as the syntax of a statement in a Haskell
518 <literal>do</literal> expression. However, there's no monad
519 overloading here: statements typed at the prompt must be in the
520 <literal>IO</literal> monad.</para>
522 <para>Here's an example:</para>
524 Prelude> x <- return 42
529 <para>The statement <literal>x <- return 42</literal> means
530 “execute <literal>return 42</literal> in the
531 <literal>IO</literal> monad, and bind the result to
532 <literal>x</literal>”. We can then use
533 <literal>x</literal> in future statements, for example to print
534 it as we did above.</para>
536 <para>Of course, you can also bind normal non-IO expressions
537 using the <literal>let</literal>-statement:</para>
544 <para>An important difference between the two types of binding
545 is that the monadic bind (<literal>p <- e</literal>) is
546 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
547 whereas with the <literal>let</literal> form, the expression
548 isn't evaluated immediately:</para>
550 Prelude> let x = error "help!"
555 <para>Any exceptions raised during the evaluation or execution
556 of the statement are caught and printed by the GHCi command line
557 interface (for more information on exceptions, see the module
558 <literal>Control.Exception</literal> in the libraries
559 documentation).</para>
561 <para>Every new binding shadows any existing bindings of the
562 same name, including entities that are in scope in the current
563 module context.</para>
565 <para>WARNING: temporary bindings introduced at the prompt only
566 last until the next <literal>:load</literal> or
567 <literal>:reload</literal> command, at which time they will be
568 simply lost. However, they do survive a change of context with
569 <literal>:module</literal>: the temporary bindings just move to
570 the new location.</para>
572 <para>HINT: To get a list of the bindings currently in scope, use the
573 <literal>:show bindings</literal> command:</para>
576 Prelude> :show bindings
580 <para>HINT: if you turn on the <literal>+t</literal> option,
581 GHCi will show the type of each variable bound by a statement.
583 <indexterm><primary><literal>+t</literal></primary></indexterm>
586 Prelude> let (x:xs) = [1..]
594 <title>The <literal>it</literal> variable</title>
595 <indexterm><primary><literal>it</literal></primary>
598 <para>Whenever an expression (or a non-binding statement, to be
599 precise) is typed at the prompt, GHCi implicitly binds its value
600 to the variable <literal>it</literal>. For example:</para>
608 <para>This is a result of the translation mentioned earlier,
609 namely that an expression <replaceable>e</replaceable> is
612 let it = <replaceable>e</replaceable>;
615 before execution, resulting in a binding for
616 <literal>it</literal>.</para>
618 <para>If the expression was of type <literal>IO a</literal> for
619 some <literal>a</literal>, then <literal>it</literal> will be
620 bound to the result of the <literal>IO</literal> computation,
621 which is of type <literal>a</literal>. eg.:</para>
623 Prelude> Time.getClockTime
625 Wed Mar 14 12:23:13 GMT 2001
628 <para>The corresponding translation for an IO-typed
629 <replaceable>e</replaceable> is
631 it <- <replaceable>e</replaceable>
635 <para>Note that <literal>it</literal> is shadowed by the new
636 value each time you evaluate a new expression, and the old value
637 of <literal>it</literal> is lost.</para>
642 <title>Type defaulting in GHCi</title>
643 <indexterm><primary>Type default</primary></indexterm>
644 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
646 Consider this GHCi session:
650 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
651 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
652 on the type <literal>a</literal>. For example:
654 ghci> (reverse []) :: String
656 ghci> (reverse []) :: [Int]
659 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
660 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. If the expression yields a set of
661 type constraints that are all from standard classes (<literal>Num</literal>, <literal>Eq</literal> etc.),
662 and at least one is either a numeric class <emphasis>or the <literal>Show</literal>,
663 <literal>Eq</literal>, or <literal>Ord</literal> class</emphasis>,
664 GHCi will try to use one of the <literal>default</literal> types, just as described in the Report.
669 <sect1 id="ghci-invokation">
670 <title>Invoking GHCi</title>
671 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
672 <indexterm><primary><option>––interactive</option></primary></indexterm>
674 <para>GHCi is invoked with the command <literal>ghci</literal> or
675 <literal>ghc ––interactive</literal>. One or more modules or
676 filenames can also be specified on the command line; this
677 instructs GHCi to load the specified modules or filenames (and all
678 the modules they depend on), just as if you had said
679 <literal>:load <replaceable>modules</replaceable></literal> at the
680 GHCi prompt (see <xref linkend="ghci-commands">). For example, to
681 start GHCi and load the program whose topmost module is in the
682 file <literal>Main.hs</literal>, we could say:</para>
688 <para>Most of the command-line options accepted by GHC (see <xref
689 linkend="using-ghc">) also make sense in interactive mode. The ones
690 that don't make sense are mostly obvious; for example, GHCi
691 doesn't generate interface files, so options related to interface
692 file generation won't have any effect.</para>
695 <title>Packages</title>
696 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
698 <para>Most packages (see <xref linkend="using-packages">) are
699 available without needing to specify any extra flags at all:
700 they will be automatically loaded the first time they are
703 <para>For non-auto packages, however, you need to request the
704 package be loaded by using the <literal>-package</literal> flag:</para>
710 / /_\// /_/ / / | | GHC Interactive, version 5.05, for Haskell 98.
711 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
712 \____/\/ /_/\____/|_| Type :? for help.
714 Loading package base ... linking ... done.
715 Loading package haskell98 ... linking ... done.
716 Loading package lang ... linking ... done.
717 Loading package concurrent ... linking ... done.
718 Loading package readline ... linking ... done.
719 Loading package unix ... linking ... done.
720 Loading package posix ... linking ... done.
721 Loading package util ... linking ... done.
722 Loading package data ... linking ... done.
726 <para>The following command works to load new packages into a
730 Prelude> :set -package <replaceable>name</replaceable>
733 <para>But note that doing this will cause all currently loaded
734 modules to be unloaded, and you'll be dumped back into the
735 <literal>Prelude</literal>.</para>
739 <title>Extra libraries</title>
740 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
742 <para>Extra libraries may be specified on the command line using
743 the normal <literal>-l<replaceable>lib</replaceable></literal>
744 option. (The term <emphasis>library</emphasis> here refers to
745 libraries of foreign object code; for using libraries of Haskell
746 source code, see <xref linkend="ghci-modules-filenames">.) For
747 example, to load the “m” library:</para>
753 <para>On systems with <literal>.so</literal>-style shared
754 libraries, the actual library loaded will the
755 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
756 searches the following places for libraries, in this order:</para>
760 <para>Paths specified using the
761 <literal>-L<replaceable>path</replaceable></literal>
762 command-line option,</para>
765 <para>the standard library search path for your system,
766 which on some systems may be overriden by setting the
767 <literal>LD_LIBRARY_PATH</literal> environment
772 <para>On systems with <literal>.dll</literal>-style shared
773 libraries, the actual library loaded will be
774 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
775 GHCi will signal an error if it can't find the library.</para>
777 <para>GHCi can also load plain object files
778 (<literal>.o</literal> or <literal>.obj</literal> depending on
779 your platform) from the command-line. Just add the name the
780 object file to the command line.</para>
785 <sect1 id="ghci-commands">
786 <title>GHCi commands</title>
788 <para>GHCi commands all begin with
789 ‘<literal>:</literal>’ and consist of a single command
790 name followed by zero or more parameters. The command name may be
791 abbreviated, as long as the abbreviation is not ambiguous. All of
792 the builtin commands, with the exception of
793 <literal>:unset</literal> and <literal>:undef</literal>, may be
794 abbreviated to a single letter.</para>
798 <term><literal>:add</literal>
799 <replaceable>module</replaceable> ...</term>
800 <indexterm><primary><literal>:add</literal></primary></indexterm>
802 <para>Add <replaceable>module</replaceable>(s) to the
803 current <firstterm>target set</firstterm>, and perform a
809 <term><literal>:browse</literal>
810 <optional><literal>*</literal></optional><replaceable>module</replaceable>
812 <indexterm><primary><literal>:browse</literal></primary>
815 <para>Displays the identifiers defined by the module
816 <replaceable>module</replaceable>, which must be either
817 loaded into GHCi or be a member of a package. If the
818 <literal>*</literal> symbol is placed before the module
819 name, then <emphasis>all</emphasis> the identifiers defined
820 in <replaceable>module</replaceable> are shown; otherwise
821 the list is limited to the exports of
822 <replaceable>module</replaceable>. The
823 <literal>*</literal>-form is only available for modules
824 which are interpreted; for compiled modules (including
825 modules from packages) only the non-<literal>*</literal>
826 form of <literal>:browse</literal> is available.</para>
831 <term><literal>:cd</literal> <replaceable>dir</replaceable></term>
832 <indexterm><primary><literal>:cd</literal></primary></indexterm>
834 <para>Changes the current working directory to
835 <replaceable>dir</replaceable>. A
836 ‘<literal>˜</literal>’ symbol at the
837 beginning of <replaceable>dir</replaceable> will be replaced
838 by the contents of the environment variable
839 <literal>HOME</literal>.</para>
841 <para>NOTE: changing directories causes all currently loaded
842 modules to be unloaded. This is because the search path is
843 usually expressed using relative directories, and changing
844 the search path in the middle of a session is not
850 <term><literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable></term>
851 <indexterm><primary><literal>:def</literal></primary></indexterm>
853 <para>The command <literal>:def</literal>
854 <replaceable>name</replaceable>
855 <replaceable>expr</replaceable> defines a new GHCi command
856 <literal>:<replaceable>name</replaceable></literal>,
857 implemented by the Haskell expression
858 <replaceable>expr</replaceable>, which must have type
859 <literal>String -> IO String</literal>. When
860 <literal>:<replaceable>name</replaceable>
861 <replaceable>args</replaceable></literal> is typed at the
862 prompt, GHCi will run the expression
863 <literal>(<replaceable>name</replaceable>
864 <replaceable>args</replaceable>)</literal>, take the
865 resulting <literal>String</literal>, and feed it back into
866 GHCi as a new sequence of commands. Separate commands in
867 the result must be separated by
868 ‘<literal>\n</literal>’.</para>
870 <para>That's all a little confusing, so here's a few
871 examples. To start with, here's a new GHCi command which
872 doesn't take any arguments or produce any results, it just
873 outputs the current date & time:</para>
876 Prelude> let date _ = Time.getClockTime >>= print >> return ""
877 Prelude> :def date date
879 Fri Mar 23 15:16:40 GMT 2001
882 <para>Here's an example of a command that takes an argument.
883 It's a re-implementation of <literal>:cd</literal>:</para>
886 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
887 Prelude> :def mycd mycd
891 <para>Or I could define a simple way to invoke
892 “<literal>ghc ––make Main</literal>” in the
893 current directory:</para>
896 Prelude> :def make (\_ -> return ":! ghc ––make Main")
903 <term><literal>:help</literal></term>
904 <indexterm><primary><literal>:help</literal></primary></indexterm>
905 <term><literal>:?</literal></term>
906 <indexterm><primary><literal>:?</literal></primary></indexterm>
908 <para>Displays a list of the available commands.</para>
913 <term><literal>:info</literal> <replaceable>name</replaceable>
915 <indexterm><primary><literal>:info</literal></primary>
918 <para>Displays information about the given name(s). For
919 example, if <replaceable>name</replaceable> is a class, then
920 the class methods and their types will be printed; if
921 <replaceable>name</replaceable> is a type constructor, then
922 its definition will be printed; if
923 <replaceable>name</replaceable> is a function, then its type
924 will be printed. If <replaceable>name</replaceable> has
925 been loaded from a source file, then GHCi will also display
926 the location of its definition in the source.</para>
931 <term><literal>:load</literal>
932 <replaceable>module</replaceable> ...</term>
933 <indexterm><primary><literal>:load</literal></primary></indexterm>
935 <para>Recursively loads the specified
936 <replaceable>module</replaceable>s, and all the modules they
937 depend on. Here, each <replaceable>module</replaceable>
938 must be a module name or filename, but may not be the name
939 of a module in a package.</para>
941 <para>All previously loaded modules, except package modules,
942 are forgotten. The new set of modules is known as the
943 <firstterm>target set</firstterm>. Note that
944 <literal>:load</literal> can be used without any arguments
945 to unload all the currently loaded modules and
948 <para>After a <literal>:load</literal> command, the current
949 context is set to:</para>
953 <para><replaceable>module</replaceable>, if it was loaded
954 successfully, or</para>
957 <para>the most recently successfully loaded module, if
958 any other modules were loaded as a result of the current
959 <literal>:load</literal>, or</para>
962 <para><literal>Prelude</literal> otherwise.</para>
969 <term><literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal></term>
970 <indexterm><primary><literal>:module</literal></primary></indexterm>
972 <para>Sets or modifies the current context for statements
973 typed at the prompt. See <xref linkend="ghci-scope"> for
979 <term><literal>:quit</literal></term>
980 <indexterm><primary><literal>:quit</literal></primary></indexterm>
982 <para>Quits GHCi. You can also quit by typing a control-D
983 at the prompt.</para>
988 <term><literal>:reload</literal></term>
989 <indexterm><primary><literal>:reload</literal></primary></indexterm>
991 <para>Attempts to reload the current target set (see
992 <literal>:load</literal>) if any of the modules in the set,
993 or any dependent module, has changed. Note that this may
994 entail loading new modules, or dropping modules which are no
995 longer indirectly required by the target.</para>
1000 <term><literal>:set</literal> <optional><replaceable>option</replaceable>...</optional></term>
1001 <indexterm><primary><literal>:set</literal></primary></indexterm>
1003 <para>Sets various options. See <xref linkend="ghci-set">
1004 for a list of available options. The
1005 <literal>:set</literal> command by itself shows which
1006 options are currently set.</para>
1011 <term><literal>:set</literal> <literal>args</literal>
1012 <replaceable>arg</replaceable> ...</term>
1013 <indexterm><primary><literal>:set</literal></primary></indexterm>
1015 <para>Sets the list of arguments which are returned when the
1016 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
1017 </indexterm>.</para>
1022 <term><literal>:set</literal> <literal>prog</literal>
1023 <replaceable>prog</replaceable></term>
1024 <indexterm><primary><literal>:set</literal></primary></indexterm>
1026 <para>Sets the string to be returned when the program calls
1027 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
1028 </indexterm>.</para>
1033 <term><literal>:show bindings</literal></term>
1034 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
1036 <para>Show the bindings made at the prompt and their
1042 <term><literal>:show modules</literal></term>
1043 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
1045 <para>Show the list of modules currently load.</para>
1050 <term><literal>:type</literal> <replaceable>expression</replaceable></term>
1051 <indexterm><primary><literal>:type</literal></primary></indexterm>
1053 <para>Infers and prints the type of
1054 <replaceable>expression</replaceable>, including explicit
1055 forall quantifiers for polymorphic types. The monomorphism
1056 restriction is <emphasis>not</emphasis> applied to the
1057 expression during type inference.</para>
1062 <term><literal>:undef</literal> <replaceable>name</replaceable></term>
1063 <indexterm><primary><literal>:undef</literal></primary></indexterm>
1065 <para>Undefines the user-defined command
1066 <replaceable>name</replaceable> (see <literal>:def</literal>
1072 <term><literal>:unset</literal> <replaceable>option</replaceable>...</term>
1073 <indexterm><primary><literal>:unset</literal></primary></indexterm>
1075 <para>Unsets certain options. See <xref linkend="ghci-set">
1076 for a list of available options.</para>
1081 <term><literal>:!</literal> <replaceable>command</replaceable>...</term>
1082 <indexterm><primary><literal>:!</literal></primary></indexterm>
1083 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
1085 <para>Executes the shell command
1086 <replaceable>command</replaceable>.</para>
1093 <sect1 id="ghci-set">
1094 <title>The <literal>:set</literal> command</title>
1095 <indexterm><primary><literal>:set</literal></primary></indexterm>
1097 <para>The <literal>:set</literal> command sets two types of
1098 options: GHCi options, which begin with
1099 ‘<literal>+</literal>” and “command-line”
1100 options, which begin with ‘-’. </para>
1102 <para>NOTE: at the moment, the <literal>:set</literal> command
1103 doesn't support any kind of quoting in its arguments: quotes will
1104 not be removed and cannot be used to group words together. For
1105 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
1109 <title>GHCi options</title>
1110 <indexterm><primary>options</primary><secondary>GHCi</secondary>
1113 <para>GHCi options may be set using <literal>:set</literal> and
1114 unset using <literal>:unset</literal>.</para>
1116 <para>The available GHCi options are:</para>
1120 <term><literal>+r</literal></term>
1121 <indexterm><primary><literal>+r</literal></primary></indexterm>
1122 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
1123 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
1125 <para>Normally, any evaluation of top-level expressions
1126 (otherwise known as CAFs or Constant Applicative Forms) in
1127 loaded modules is retained between evaluations. Turning
1128 on <literal>+r</literal> causes all evaluation of
1129 top-level expressions to be discarded after each
1130 evaluation (they are still retained
1131 <emphasis>during</emphasis> a single evaluation).</para>
1133 <para>This option may help if the evaluated top-level
1134 expressions are consuming large amounts of space, or if
1135 you need repeatable performance measurements.</para>
1140 <term><literal>+s</literal></term>
1141 <indexterm><primary><literal>+s</literal></primary></indexterm>
1143 <para>Display some stats after evaluating each expression,
1144 including the elapsed time and number of bytes allocated.
1145 NOTE: the allocation figure is only accurate to the size
1146 of the storage manager's allocation area, because it is
1147 calculated at every GC. Hence, you might see values of
1148 zero if no GC has occurred.</para>
1153 <term><literal>+t</literal></term>
1154 <indexterm><primary><literal>+t</literal></primary></indexterm>
1156 <para>Display the type of each variable bound after a
1157 statement is entered at the prompt. If the statement is a
1158 single expression, then the only variable binding will be
1160 ‘<literal>it</literal>’.</para>
1166 <sect2 id="ghci-cmd-line-options">
1167 <title>Setting GHC command-line options in GHCi</title>
1169 <para>Normal GHC command-line options may also be set using
1170 <literal>:set</literal>. For example, to turn on
1171 <option>-fglasgow-exts</option>, you would say:</para>
1174 Prelude> :set -fglasgow-exts
1177 <para>Any GHC command-line option that is designated as
1178 <firstterm>dynamic</firstterm> (see the table in <xref
1179 linkend="flag-reference">), may be set using
1180 <literal>:set</literal>. To unset an option, you can set the
1181 reverse option:</para>
1182 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
1185 Prelude> :set -fno-glasgow-exts
1188 <para><xref linkend="flag-reference"> lists the reverse for each
1189 option where applicable.</para>
1191 <para>Certain static options (<option>-package</option>,
1192 <option>-I</option>, <option>-i</option>, and
1193 <option>-l</option> in particular) will also work, but some may
1194 not take effect until the next reload.</para>
1195 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
1199 <sect1 id="ghci-dot-files">
1200 <title>The <filename>.ghci</filename> file</title>
1201 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
1203 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
1206 <para>When it starts, GHCi always reads and executes commands from
1207 <filename>$HOME/.ghci</filename>, followed by
1208 <filename>./.ghci</filename>.</para>
1210 <para>The <filename>.ghci</filename> in your home directory is
1211 most useful for turning on favourite options (eg. <literal>:set
1212 +s</literal>), and defining useful macros. Placing a
1213 <filename>.ghci</filename> file in a directory with a Haskell
1214 project is a useful way to set certain project-wide options so you
1215 don't have to type them everytime you start GHCi: eg. if your
1216 project uses GHC extensions and CPP, and has source files in three
1217 subdirectories A B and C, you might put the following lines in
1218 <filename>.ghci</filename>:</para>
1221 :set -fglasgow-exts -cpp
1225 <para>(Note that strictly speaking the <option>-i</option> flag is
1226 a static one, but in fact it works to set it using
1227 <literal>:set</literal> like this. The changes won't take effect
1228 until the next <literal>:load</literal>, though.)</para>
1230 <para>Two command-line options control whether the
1231 <filename>.ghci</filename> files are read:</para>
1235 <term><option>-ignore-dot-ghci</option></term>
1236 <indexterm><primary><option>-ignore-dot-ghci</option></primary>
1239 <para>Don't read either <filename>./.ghci</filename> or
1240 <filename>$HOME/.ghci</filename> when starting up.</para>
1244 <term><option>-read-dot-ghci</option></term>
1245 <indexterm><primary><option>-read-dot-ghci</option></primary>
1248 <para>Read <filename>.ghci</filename> and
1249 <filename>$HOME/.ghci</filename>. This is normally the
1250 default, but the <option>-read-dot-ghci</option> option may
1251 be used to override a previous
1252 <option>-ignore-dot-ghci</option> option.</para>
1260 <title>FAQ and Things To Watch Out For</title>
1264 <term>The interpreter can't load modules with foreign export
1265 declarations!</term>
1267 <para>Unfortunately not. We haven't implemented it yet.
1268 Please compile any offending modules by hand before loading
1269 them into GHCi.</para>
1274 <term><literal>-O</literal> doesn't work with GHCi!</term>
1275 <indexterm><primary><option>-O</option></primary>
1278 <para>For technical reasons, the bytecode compiler doesn't
1279 interact well with one of the optimisation passes, so we
1280 have disabled optimisation when using the interpreter. This
1281 isn't a great loss: you'll get a much bigger win by
1282 compiling the bits of your code that need to go fast, rather
1283 than interpreting them with optimisation turned on.</para>
1288 <term>Unboxed tuples don't work with GHCi</term>
1290 <para>That's right. You can always compile a module that
1291 uses unboxed tuples and load it into GHCi, however.
1292 (Incidentally the previous point, namely that
1293 <literal>-O</literal> is incompatible with GHCi, is because
1294 the bytecode compiler can't deal with unboxed
1300 <term>Concurrent threads don't carry on running when GHCi is
1301 waiting for input.</term>
1303 <para>No, they don't. This is because the Haskell binding
1304 to the GNU readline library doesn't support reading from the
1305 terminal in a non-blocking way, which is required to work
1306 properly with GHC's concurrency model.</para>
1311 <term>After using <literal>getContents</literal>, I can't use
1312 <literal>stdin</literal> again until I do
1313 <literal>:load</literal> or <literal>:reload</literal>.</term>
1316 <para>This is the defined behaviour of
1317 <literal>getContents</literal>: it puts the stdin Handle in
1318 a state known as <firstterm>semi-closed</firstterm>, wherein
1319 any further I/O operations on it are forbidden. Because I/O
1320 state is retained between computations, the semi-closed
1321 state persists until the next <literal>:load</literal> or
1322 <literal>:reload</literal> command.</para>
1324 <para>You can make <literal>stdin</literal> reset itself
1325 after every evaluation by giving GHCi the command
1326 <literal>:set +r</literal>. This works because
1327 <literal>stdin</literal> is just a top-level expression that
1328 can be reverted to its unevaluated state in the same way as
1329 any other top-level expression (CAF).</para>
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