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.
18 <indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
19 <indexterm><primary>Foreign Function
20 Interface</primary><secondary>GHCi support</secondary></indexterm>.
21 GHCi also includes an interactive debugger (see <xref linkend="ghci-debugger"/>).</para>
23 <sect1 id="ghci-introduction">
24 <title>Introduction to GHCi</title>
26 <para>Let's start with an example GHCi session. You can fire up
27 GHCi with the command <literal>ghci</literal>:</para>
31 GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
32 Loading package base ... linking ... done.
36 <para>There may be a short pause while GHCi loads the prelude and
37 standard libraries, after which the prompt is shown. If we follow
38 the instructions and type <literal>:?</literal> for help, we
42 Commands available from the prompt:
44 <stmt> evaluate/run <stmt>
45 :add <filename> ... add module(s) to the current target set
46 :browse [*]<module> display the names defined by <module>
47 :cd <dir> change directory to <dir>
48 :def <cmd> <expr> define a command :<cmd>
49 :edit <file> edit file
50 :edit edit last module
51 :help, :? display this list of commands
52 :info [<name> ...] display information about the given names
53 :load <filename> ... load module(s) and their dependents
54 :module [+/-] [*]<mod> ... set the context for expression evaluation
55 :main [<arguments> ...] run the main function with the given arguments
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
61 :set prompt <prompt> set the prompt used in GHCi
62 :set editor <cmd> set the command used for :edit
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 (default: "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>
106 <sect1 id="loading-source-files">
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
363 <sect1 id="interactive-evaluation">
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: you can use <literal>import M</literal> as an
576 alternative to <literal>:module +M</literal>, and
577 <literal>:module</literal> can also be shortened to
578 <literal>:m</literal>). The full syntax of the
579 <literal>:module</literal> command is:</para>
582 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
585 <para>Using the <literal>+</literal> form of the
586 <literal>module</literal> commands adds modules to the current
587 scope, and <literal>-</literal> removes them. Without either
588 <literal>+</literal> or <literal>-</literal>, the current scope
589 is replaced by the set of modules specified. Note that if you
590 use this form and leave out <literal>Prelude</literal>, GHCi
591 will assume that you really wanted the
592 <literal>Prelude</literal> and add it in for you (if you don't
593 want the <literal>Prelude</literal>, then ask to remove it with
594 <literal>:m -Prelude</literal>).</para>
596 <para>The scope is automatically set after a
597 <literal>:load</literal> command, to the most recently loaded
598 "target" module, in a <literal>*</literal>-form if possible.
599 For example, if you say <literal>:load foo.hs bar.hs</literal>
600 and <filename>bar.hs</filename> contains module
601 <literal>Bar</literal>, then the scope will be set to
602 <literal>*Bar</literal> if <literal>Bar</literal> is
603 interpreted, or if <literal>Bar</literal> is compiled it will be
604 set to <literal>Prelude Bar</literal> (GHCi automatically adds
605 <literal>Prelude</literal> if it isn't present and there aren't
606 any <literal>*</literal>-form modules).</para>
608 <para>With multiple modules in scope, especially multiple
609 <literal>*</literal>-form modules, it is likely that name
610 clashes will occur. Haskell specifies that name clashes are
611 only reported when an ambiguous identifier is used, and GHCi
612 behaves in the same way for expressions typed at the
616 Hint: GHCi will tab-complete names that are in scope; for
617 example, if you run GHCi and type <literal>J<tab></literal>
618 then GHCi will expand it to <literal>Just </literal>.
622 <title>Qualified names</title>
624 <para>To make life slightly easier, the GHCi prompt also
625 behaves as if there is an implicit <literal>import
626 qualified</literal> declaration for every module in every
627 package, and every module currently loaded into GHCi.</para>
631 <title>The <literal>:main</literal> command</title>
634 When a program is compiled and executed, it can use the
635 <literal>getArgs</literal> function to access the
636 command-line arguments.
637 However, we cannot simply pass the arguments to the
638 <literal>main</literal> function while we are testing in ghci,
639 as the <literal>main</literal> function doesn't take its
644 Instead, we can use the <literal>:main</literal> command.
645 This runs whatever <literal>main</literal> is in scope, with
646 any arguments being treated the same as command-line arguments,
651 Prelude> let main = System.Environment.getArgs >>= print
652 Prelude> :main foo bar
661 <title>The <literal>it</literal> variable</title>
662 <indexterm><primary><literal>it</literal></primary>
665 <para>Whenever an expression (or a non-binding statement, to be
666 precise) is typed at the prompt, GHCi implicitly binds its value
667 to the variable <literal>it</literal>. For example:</para>
674 <para>What actually happens is that GHCi typechecks the
675 expression, and if it doesn't have an <literal>IO</literal> type,
676 then it transforms it as follows: an expression
677 <replaceable>e</replaceable> turns into
679 let it = <replaceable>e</replaceable>;
682 which is then run as an IO-action.</para>
684 <para>Hence, the original expression must have a type which is an
685 instance of the <literal>Show</literal> class, or GHCi will
691 <interactive>:1:0:
692 No instance for (Show (a -> a))
693 arising from use of `print' at <interactive>:1:0-1
694 Possible fix: add an instance declaration for (Show (a -> a))
695 In the expression: print it
696 In a 'do' expression: print it
699 <para>The error message contains some clues as to the
700 transformation happening internally.</para>
702 <para>If the expression was instead of type <literal>IO a</literal> for
703 some <literal>a</literal>, then <literal>it</literal> will be
704 bound to the result of the <literal>IO</literal> computation,
705 which is of type <literal>a</literal>. eg.:</para>
707 Prelude> Time.getClockTime
708 Wed Mar 14 12:23:13 GMT 2001
710 Wed Mar 14 12:23:13 GMT 2001
713 <para>The corresponding translation for an IO-typed
714 <replaceable>e</replaceable> is
716 it <- <replaceable>e</replaceable>
720 <para>Note that <literal>it</literal> is shadowed by the new
721 value each time you evaluate a new expression, and the old value
722 of <literal>it</literal> is lost.</para>
726 <sect2 id="extended-default-rules">
727 <title>Type defaulting in GHCi</title>
728 <indexterm><primary>Type default</primary></indexterm>
729 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
731 Consider this GHCi session:
735 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
736 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
737 on the type <literal>a</literal>. For example:
739 ghci> (reverse []) :: String
741 ghci> (reverse []) :: [Int]
744 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
745 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
746 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
747 a)</literal> for each type variable <literal>a</literal>, and defaults the
752 The type variable <literal>a</literal> appears in no
758 All the classes <literal>Ci</literal> are standard.
763 At least one of the classes <literal>Ci</literal> is
768 At the GHCi prompt, or with GHC if the
769 <literal>-fextended-default-rules</literal> flag is given,
770 the following additional differences apply:
774 Rule 2 above is relaxed thus:
775 <emphasis>All</emphasis> of the classes
776 <literal>Ci</literal> are single-parameter type classes.
781 Rule 3 above is relaxed this:
782 At least one of the classes <literal>Ci</literal> is
783 numeric, <emphasis>or is <literal>Show</literal>,
784 <literal>Eq</literal>, or
785 <literal>Ord</literal></emphasis>.
790 The unit type <literal>()</literal> is added to the
791 start of the standard list of types which are tried when
792 doing type defaulting.
796 The last point means that, for example, this program:
803 def :: (Num a, Enum a) => a
806 prints <literal>()</literal> rather than <literal>0</literal> as the
807 type is defaulted to <literal>()</literal> rather than
808 <literal>Integer</literal>.
811 The motivation for the change is that it means <literal>IO a</literal>
812 actions default to <literal>IO ()</literal>, which in turn means that
813 ghci won't try to print a result when running them. This is
814 particularly important for <literal>printf</literal>, which has an
815 instance that returns <literal>IO a</literal>.
816 However, it is only able to return
817 <literal>undefined</literal>
818 (the reason for the instance having this type is to not require
819 extensions to the class system), so if the type defaults to
820 <literal>Integer</literal> then ghci gives an error when running a
826 <sect1 id="ghci-debugger">
827 <title>The GHCi Debugger</title>
828 <indexterm><primary>debugger</primary><secondary>in GHCi</secondary>
831 <para>GHCi contains a simple imperative-style debugger in which you can
832 stop a running computation in order to examine the values of
833 variables. The debugger is integrated into GHCi, and is turned on by
834 default: no flags are required to enable the debugging facilities. There
835 is one major restriction: breakpoints and single-stepping are only
836 available in <emphasis>interpreted</emphasis> modules; compiled code is
837 invisible to the debugger.</para>
839 <para>The debugger provides the following:
842 <para>The abilty to set a <firstterm>breakpoint</firstterm> on a
843 function definition or expression in the program. When the function
844 is called, or the expression evaluated, GHCi suspends
845 execution and returns to the prompt, where you can inspect the
846 values of local variables before continuing with the
850 <para>Execution can be <firstterm>single-stepped</firstterm>: the
851 evaluator will suspend execution approximately after every
852 reduction, allowing local variables to be inspected. This is
853 equivalent to setting a breakpoint at every point in the
857 <para>Execution can take place in <firstterm>tracing
858 mode</firstterm>, in which the evaluator remembers each
859 evaluation step as it happens, but doesn't suspend execution until
860 an actual breakpoint is reached. When this happens, the history of
861 evaluation steps can be inspected.</para>
864 <para>Exceptions (e.g. pattern matching failure and
865 <literal>error</literal>) can be treated as breakpoints, to help
866 locate the source of an exception in the program.</para>
871 <para>There is currently no support for obtaining a “stack
872 trace”, but the tracing and history features provide a useful
873 second-best, which will often be enough to establish the context of an
876 <sect2 id="breakpoints">
877 <title>Breakpoints and inspecting variables</title>
879 <para>Let's use quicksort as a running example. Here's the code:</para>
883 qsort (a:as) = qsort left ++ [a] ++ qsort right
884 where (left,right) = (filter (<=a) as, filter (>a) as)
886 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
889 <para>First, load the module into GHCi:</para>
893 [1 of 1] Compiling Main ( qsort.hs, interpreted )
894 Ok, modules loaded: Main.
898 <para>Now, let's set a breakpoint on the right-hand-side of the second
899 equation of qsort:</para>
903 Breakpoint 0 activated at qsort.hs:2:15-46
907 <para>The command <literal>:break 2</literal> sets a breakpoint on line
908 2 of the most recently-loaded module, in this case
909 <literal>qsort.hs</literal>. Specifically, it picks the
910 leftmost complete subexpression on that line on which to set the
911 breakpoint, which in this case is the expression
912 <literal>(qsort left ++ [a] ++ qsort right)</literal>.</para>
914 <para>Now, we run the program:</para>
918 Stopped at qsort.hs:2:15-46
923 [qsort.hs:2:15-46] *Main>
926 <para>Execution has stopped at the breakpoint. The prompt has changed to
927 indicate that we are currently stopped at a breakpoint, and the location:
928 <literal>[qsort.hs:2:15-46]</literal>. To further clarify the
929 location, we can use the <literal>:list</literal> command:</para>
932 [qsort.hs:2:15-46] *Main> :list
934 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
935 3 where (left,right) = (filter (<=a) as, filter (>a) as)
938 <para>The <literal>:list</literal> command lists the source code around
939 the current breakpoint. If your output device supports it, then GHCi
940 will highlight the active subexpression in bold.</para>
942 <para>GHCi has provided bindings for the free variables<footnote><para>We
943 originally provided bindings for all variables in scope, rather
945 the free variables of the expression, but found that this affected
946 performance considerably, hence the current restriction to just the
947 free variables.</para>
948 </footnote> of the expression
950 breakpoint was placed (<literal>a</literal>, <literal>left</literal>,
951 <literal>right</literal>), and additionally a binding for the result of
952 the expression (<literal>_result</literal>). These variables are just
953 like other variables that you might define in GHCi; you
954 can use them in expressions that you type at the prompt, you can ask
955 for their types with <literal>:type</literal>, and so on. There is one
956 important difference though: these variables may only have partial
957 types. For example, if we try to display the value of
958 <literal>left</literal>:</para>
961 [qsort.hs:2:15-46] *Main> left
963 <interactive>:1:0:
964 Ambiguous type variable `a' in the constraint:
965 `Show a' arising from a use of `print' at <interactive>:1:0-3
966 Cannot resolve unknown runtime types: a
967 Use :print or :force to determine these types
970 <para>This is because <literal>qsort</literal> is a polymorphic function,
971 and because GHCi does not carry type information at runtime, it cannot
972 determine the runtime types of free variables that involve type
973 variables. Hence, when you ask to display <literal>left</literal> at
974 the prompt, GHCi can't figure out which instance of
975 <literal>Show</literal> to use, so it emits the type error above.</para>
977 <para>Fortunately, the debugger includes a generic printing command,
978 <literal>:print</literal>, which can inspect the actual runtime value of a
979 variable and attempt to reconstruct its type. If we try it on
980 <literal>left</literal>:</para>
983 [qsort.hs:2:15-46] *Main> :print left
987 <para>This isn't particularly enlightening. What happened is that
988 <literal>left</literal> is bound to an unevaluated computation (a
989 suspension, or <firstterm>thunk</firstterm>), and
990 <literal>:print</literal> does not force any evaluation. The idea is
991 that <literal>:print</literal> can be used to inspect values at a
992 breakpoint without any unfortunate side effects. It won't force any
993 evaluation, which could cause the program to give a different answer
994 than it would normally, and hence it won't cause any exceptions to be
995 raised, infinite loops, or further breakpoints to be triggered (see
996 <xref linkend="nested-breakpoints" />).
997 Rather than forcing thunks, <literal>:print</literal>
998 binds each thunk to a fresh variable beginning with an
999 underscore, in this case
1000 <literal>_t1</literal>.</para>
1002 <para>If we aren't concerned about preserving the evaluatedness of a
1003 variable, we can use <literal>:force</literal> instead of
1004 <literal>:print</literal>. The <literal>:force</literal> command
1005 behaves exactly like <literal>:print</literal>, except that it forces
1006 the evaluation of any thunks it encounters:</para>
1009 [qsort.hs:2:15-46] *Main> :force left
1013 <para>Now, since <literal>:force</literal> has inspected the runtime
1014 value of <literal>left</literal>, it has reconstructed its type. We
1015 can see the results of this type reconstruction:</para>
1018 [qsort.hs:2:15-46] *Main> :show bindings
1019 _result :: [Integer]
1026 <para>Not only do we now know the type of <literal>left</literal>, but
1027 all the other partial types have also been resolved. So we can ask
1028 for the value of <literal>a</literal>, for example:</para>
1031 [qsort.hs:2:15-46] *Main> a
1035 <para>You might find it useful to use Haskell's
1036 <literal>seq</literal> function to evaluate individual thunks rather
1037 than evaluating the whole expression with <literal>:force</literal>.
1041 [qsort.hs:2:15-46] *Main> :print right
1042 right = (_t1::[Integer])
1043 [qsort.hs:2:15-46] *Main> seq _t1 ()
1045 [qsort.hs:2:15-46] *Main> :print right
1046 right = 23 : (_t2::[Integer])
1049 <para>We evaluated only the <literal>_t1</literal> thunk, revealing the
1050 head of the list, and the tail is another thunk now bound to
1051 <literal>_t2</literal>. The <literal>seq</literal> function is a
1052 little inconvenient to use here, so you might want to use
1053 <literal>:def</literal> to make a nicer interface (left as an exercise
1054 for the reader!).</para>
1056 <para>Finally, we can continue the current execution:</para>
1059 [qsort.hs:2:15-46] *Main> :continue
1060 Stopped at qsort.hs:2:15-46
1065 [qsort.hs:2:15-46] *Main>
1068 <para>The execution continued at the point it previously stopped, and has
1069 now stopped at the breakpoint for a second time.</para>
1071 <sect3 id="setting-breakpoings">
1072 <title>Setting breakpoints</title>
1074 <para>Breakpoints can be set in various ways. Perhaps the easiest way to
1075 set a breakpoint is to name a top-level function:</para>
1078 :break <replaceable>identifier</replaceable>
1081 <para>Where <replaceable>identifier</replaceable> names any top-level
1082 function in an interpreted module currently loaded into GHCi (qualified
1083 names may be used). The breakpoint will be set on the body of the
1084 function, when it is fully applied but before any pattern matching has
1087 <para>Breakpoints can also be set by line (and optionally column)
1091 :break <replaceable>line</replaceable>
1092 :break <replaceable>line</replaceable> <replaceable>column</replaceable>
1093 :break <replaceable>module</replaceable> <replaceable>line</replaceable>
1094 :break <replaceable>module</replaceable> <replaceable>line</replaceable> <replaceable>column</replaceable>
1097 <para>When a breakpoint is set on a particular line, GHCi sets the
1099 leftmost subexpression that begins and ends on that line. If two
1100 complete subexpressions start at the same
1101 column, the longest one is picked. If there is no complete
1102 subexpression on the line, then the leftmost expression starting on
1103 the line is picked, and failing that the rightmost expression that
1104 partially or completely covers the line.</para>
1106 <para>When a breakpoint is set on a particular line and column, GHCi
1107 picks the smallest subexpression that encloses that location on which
1108 to set the breakpoint. Note: GHC considers the TAB character to have a
1109 width of 1, wherever it occurs; in other words it counts
1110 characters, rather than columns. This matches what some editors do,
1111 and doesn't match others. The best advice is to avoid tab
1112 characters in your source code altogether (see
1113 <option>-fwarn-tabs</option> in <xref linkend="options-sanity"
1116 <para>If the module is omitted, then the most recently-loaded module is
1119 <para>Not all subexpressions are potential breakpoint locations. Single
1120 variables are typically not considered to be breakpoint locations
1121 (unless the variable is the right-hand-side of a function definition,
1122 lambda, or case alternative). The rule of thumb is that all redexes
1123 are breakpoint locations, together with the bodies of functions,
1124 lambdas, case alternatives and binding statements. There is normally
1125 no breakpoint on a let expression, but there will always be a
1126 breakpoint on its body, because we are usually interested in inspecting
1127 the values of the variables bound by the let.</para>
1131 <title>Listing and deleting breakpoints</title>
1133 <para>The list of breakpoints currently enabled can be displayed using
1134 <literal>:show breaks</literal></para>:
1137 [0] Main qsort.hs:1:11-12
1138 [1] Main qsort.hs:2:15-46
1141 <para>To delete a breakpoint, use the <literal>:delete</literal>
1142 command with the number given in the output from <literal>:show breaks</literal>:</para>
1147 [1] Main qsort.hs:2:15-46
1150 <para>To delete all breakpoints at once, use <literal>:delete *</literal>.</para>
1155 <sect2 id="single-stepping">
1156 <title>Single-stepping</title>
1158 <para>Single-stepping is a great way to visualise the execution of your
1159 program, and it is also a useful tool for identifying the source of a
1160 bug. The concept is simple: single-stepping enables all the
1161 breakpoints in the program and executes until the next breakpoint is
1162 reached, at which point you can single-step again, or continue
1163 normally. For example:</para>
1167 Stopped at qsort.hs:5:7-47
1171 <para>The command <literal>:step
1172 <replaceable>expr</replaceable></literal> begins the evaluation of
1173 <replaceable>expr</replaceable> in single-stepping mode. If
1174 <replaceable>expr</replaceable> is ommitted, then it single-steps from
1175 the current breakpoint.</para>
1177 <para>The <literal>:list</literal> command is particularly useful when
1178 single-stepping, to see where you currently are:</para>
1181 [qsort.hs:5:7-47] *Main> :list
1183 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1185 [qsort.hs:5:7-47] *Main>
1188 <para>In fact, GHCi provides a way to run a command when a breakpoint is
1189 hit, so we can make it automatically do
1190 <literal>:list</literal>:</para>
1193 [qsort.hs:5:7-47] *Main> :set stop :list
1194 [qsort.hs:5:7-47] *Main> :step
1195 Stopped at qsort.hs:5:14-46
1196 _result :: [Integer]
1198 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1200 [qsort.hs:5:14-46] *Main>
1204 <sect2 id="nested-breakpoints">
1205 <title>Nested breakpoints</title>
1206 <para>When GHCi is stopped at a breakpoint, and an expression entered at
1207 the prompt triggers a
1208 second breakpoint, the new breakpoint becomes the “current”
1209 one, and the old one is saved on a stack. An arbitrary number of
1210 breakpoint contexts can be built up in this way. For example:</para>
1213 [qsort.hs:2:15-46] *Main> :st qsort [1,3]
1214 Stopped at qsort.hs:(1,0)-(3,55)
1216 ... [qsort.hs:(1,0)-(3,55)] *Main>
1219 <para>While stopped at the breakpoint on line 2 that we set earlier, we
1220 started a new evaluation with <literal>:step qsort [1,3]</literal>.
1221 This new evaluation stopped after one step (at the definition of
1222 <literal>qsort</literal>). The prompt has changed, now prefixed with
1223 <literal>...</literal>, to indicate that there are saved breakpoints
1224 beyond the current one. To see the stack of contexts, use
1225 <literal>:show context</literal>:</para>
1228 ... [qsort.hs:(1,0)-(3,55)] *Main> :show context
1230 Stopped at qsort.hs:2:15-46
1232 Stopped at qsort.hs:(1,0)-(3,55)
1233 ... [qsort.hs:(1,0)-(3,55)] *Main>
1236 <para>To abandon the current evaluation, use
1237 <literal>:abandon</literal>:</para>
1240 ... [qsort.hs:(1,0)-(3,55)] *Main> :abandon
1241 [qsort.hs:2:15-46] *Main> :abandon
1246 <sect2 id="ghci-debugger-result">
1247 <title>The <literal>_result</literal> variable</title>
1248 <para>When stopped at a breakpoint or single-step, GHCi binds the
1249 variable <literal>_result</literal> to the value of the currently
1250 active expression. The value of <literal>_result</literal> is
1251 presumably not available yet, because we stopped its evaluation, but it
1252 can be forced: if the type is known and showable, then just entering
1253 <literal>_result</literal> at the prompt will show it. However,
1254 there's one caveat to doing this: evaluating <literal>_result</literal>
1255 will be likely to trigger further breakpoints, starting with the
1256 breakpoint we are currently stopped at (if we stopped at a real
1257 breakpoint, rather than due to <literal>:step</literal>). So it will
1258 probably be necessary to issue a <literal>:continue</literal>
1259 immediately when evaluating <literal>_result</literal>. Alternatively,
1260 you can use <literal>:force</literal> which ignores breakpoints.</para>
1263 <sect2 id="tracing">
1264 <title>Tracing and history</title>
1266 <para>A question that we often want to ask when debugging a program is
1267 “how did I get here?”. Traditional imperative debuggers
1268 usually provide some kind of stack-tracing feature that lets you see
1269 the stack of active function calls (sometimes called the “lexical
1270 call stack”), describing a path through the code
1271 to the current location. Unfortunately this is hard to provide in
1272 Haskell, because execution proceeds on a demand-driven basis, rather
1273 than a depth-first basis as in strict languages. The
1274 “stack“ in GHC's execution engine bears little
1275 resemblance to the lexical call stack. Ideally GHCi would maintain a
1276 separate lexical call stack in addition to the dynamic call stack, and
1277 in fact this is exactly
1278 what our profiling system does (<xref linkend="profiling" />), and what
1279 some other Haskell debuggers do. For the time being, however, GHCi
1280 doesn't maintain a lexical call stack (there are some technical
1281 challenges to be overcome). Instead, we provide a way to backtrack from a
1282 breakpoint to previous evaluation steps: essentially this is like
1283 single-stepping backwards, and should in many cases provide enough
1284 information to answer the “how did I get here?”
1287 <para>To use tracing, evaluate an expression with the
1288 <literal>:trace</literal> command. For example, if we set a breakpoint
1289 on the base case of <literal>qsort</literal>:</para>
1292 *Main> :list qsort
1294 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
1295 3 where (left,right) = (filter (<=a) as, filter (>a) as)
1298 Breakpoint 1 activated at qsort.hs:1:11-12
1302 <para>and then run a small <literal>qsort</literal> with
1306 *Main> :trace qsort [3,2,1]
1307 Stopped at qsort.hs:1:11-12
1309 [qsort.hs:1:11-12] *Main>
1312 <para>We can now inspect the history of evaluation steps:</para>
1315 [qsort.hs:1:11-12] *Main> :hist
1316 -1 : qsort.hs:3:24-38
1317 -2 : qsort.hs:3:23-55
1318 -3 : qsort.hs:(1,0)-(3,55)
1319 -4 : qsort.hs:2:15-24
1320 -5 : qsort.hs:2:15-46
1321 -6 : qsort.hs:3:24-38
1322 -7 : qsort.hs:3:23-55
1323 -8 : qsort.hs:(1,0)-(3,55)
1324 -9 : qsort.hs:2:15-24
1325 -10 : qsort.hs:2:15-46
1326 -11 : qsort.hs:3:24-38
1327 -12 : qsort.hs:3:23-55
1328 -13 : qsort.hs:(1,0)-(3,55)
1329 -14 : qsort.hs:2:15-24
1330 -15 : qsort.hs:2:15-46
1331 -16 : qsort.hs:(1,0)-(3,55)
1332 <end of history>
1335 <para>To examine one of the steps in the history, use
1336 <literal>:back</literal>:</para>
1339 [qsort.hs:1:11-12] *Main> :back
1340 Logged breakpoint at qsort.hs:3:24-38
1344 [-1: qsort.hs:3:24-38] *Main>
1347 <para>Note that the local variables at each step in the history have been
1348 preserved, and can be examined as usual. Also note that the prompt has
1349 changed to indicate that we're currently examining the first step in
1350 the history: <literal>-1</literal>. The command
1351 <literal>:forward</literal> can be used to traverse forward in the
1354 <para>The <literal>:trace</literal> command can be used with or without
1355 an expression. When used without an expression, tracing begins from
1356 the current breakpoint, just like <literal>:step</literal>.</para>
1358 <para>The history is only available when
1359 using <literal>:trace</literal>; the reason for this is we found that
1360 logging each breakpoint in the history cuts performance by a factor of
1361 2 or more. GHCi remembers the last 50 steps in the history (perhaps in
1362 the future we'll make this configurable).</para>
1365 <sect2 id="ghci-debugger-exceptions">
1366 <title>Debugging exceptions</title>
1367 <para>Another common question that comes up when debugging is
1368 “where did this exception come from?”. Exceptions such as
1369 those raised by <literal>error</literal> or <literal>head []</literal>
1370 have no context information attached to them. Finding which
1371 particular call to <literal>head</literal> in your program resulted in
1372 the error can be a painstaking process, usually involving
1373 <literal>Debug.Trace.trace</literal>, or compiling with
1374 profiling and using <literal>+RTS -xc</literal> (see <xref
1375 linkend="prof-time-options" />).</para>
1377 <para>The GHCi debugger offers a way to hopefully shed some light on
1378 these errors quickly and without modifying or recompiling the source
1379 code. One way would be to set a breakpoint on the location in the
1380 source code that throws the exception, and then use
1381 <literal>:trace</literal> and <literal>:history</literal> to establish
1382 the context. However, <literal>head</literal> is in a library and
1383 we can't set a breakpoint on it directly. For this reason, GHCi
1384 provides the flag <literal>-fbreak-on-exception</literal> which causes
1385 the evaluator to stop when an exception is thrown, just as it does when
1386 a breakpoint is hit. This is only really useful in conjunction with
1387 <literal>:trace</literal>, in order to log the steps leading up to the
1388 exception. For example:</para>
1391 *Main> :set -fbreak-on-exception
1392 *Main> :trace qsort ("abc" ++ undefined)
1393 "Stopped at <exception thrown>
1395 [<exception thrown>] *Main> :hist
1396 -1 : qsort.hs:3:24-38
1397 -2 : qsort.hs:3:23-55
1398 -3 : qsort.hs:(1,0)-(3,55)
1399 -4 : qsort.hs:2:15-24
1400 -5 : qsort.hs:2:15-46
1401 -6 : qsort.hs:(1,0)-(3,55)
1402 <end of history>
1403 [<exception thrown>] *Main> :back
1404 Logged breakpoint at qsort.hs:3:24-38
1408 [-1: qsort.hs:3:24-38] *Main> :force as
1409 *** Exception: Prelude.undefined
1410 [-1: qsort.hs:3:24-38] *Main> :print as
1411 as = 'b' : 'c' : (_t1::[Char])
1414 <para>The exception itself is bound to a new variable,
1415 <literal>_exception</literal>.</para>
1417 <para>Breaking on exceptions is particularly useful for finding out what
1418 your program was doing when it was in an infinite loop. Just hit
1419 Control-C, and examine the history to find out what was going
1423 <sect2><title>Example: inspecting functions</title>
1425 It is possible to use the debugger to examine function values.
1426 When we are at a breakpoint and a function is in scope, the debugger
1428 you the source code for it; however, it is possible to get some
1429 information by applying it to some arguments and observing the result.
1433 The process is slightly complicated when the binding is polymorphic.
1434 We show the process by means of an example.
1435 To keep things simple, we will use the well known <literal>map</literal> function:
1437 import Prelude hiding (map)
1439 map :: (a->b) -> a -> b
1441 map f (x:xs) = f x : map f xs
1446 We set a breakpoint on <literal>map</literal>, and call it.
1449 Breakpoint 0 activated at map.hs:5:15-28
1450 *Main> map Just [1..5]
1451 Stopped at map.hs:(4,0)-(5,12)
1457 GHCi tells us that, among other bindings, <literal>f</literal> is in scope.
1458 However, its type is not fully known yet,
1459 and thus it is not possible to apply it to any
1460 arguments. Nevertheless, observe that the type of its first argument is the
1461 same as the type of <literal>x</literal>, and its result type is shared
1462 with <literal>_result</literal>.
1466 As we demonstrated earlier (<xref linkend="breakpoints" />), the
1467 debugger has some intelligence built-in to update the type of
1468 <literal>f</literal> whenever the types of <literal>x</literal> or
1469 <literal>_result</literal> are discovered. So what we do in this
1471 force <literal>x</literal> a bit, in order to recover both its type
1472 and the argument part of <literal>f</literal>.
1480 We can check now that as expected, the type of <literal>x</literal>
1481 has been reconstructed, and with it the
1482 type of <literal>f</literal> has been too:</para>
1490 From here, we can apply f to any argument of type Integer and observe
1498 Ambiguous type variable `b' in the constraint:
1499 `Show b' arising from a use of `print' at <interactive>:1:0
1511 f :: Integer -> Maybe Integer
1515 [Just 1, Just 2, Just 3, Just 4, Just 5]
1517 In the first application of <literal>f</literal>, we had to do
1518 some more type reconstruction
1519 in order to recover the result type of <literal>f</literal>.
1520 But after that, we are free to use
1521 <literal>f</literal> normally.
1525 <sect2><title>Limitations</title>
1528 <para>When stopped at a breakpoint, if you try to evaluate a variable
1529 that is already under evaluation, the second evaluation will hang.
1531 that GHC knows the variable is under evaluation, so the new
1532 evaluation just waits for the result before continuing, but of
1533 course this isn't going to happen because the first evaluation is
1534 stopped at a breakpoint. Control-C can interrupt the hung
1535 evaluation and return to the prompt.</para>
1536 <para>The most common way this can happen is when you're evaluating a
1537 CAF (e.g. main), stop at a breakpoint, and ask for the value of the
1538 CAF at the prompt again.</para>
1541 Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
1542 at the scope of a breakpoint if there is a explicit type signature.
1549 <sect1 id="ghci-invocation">
1550 <title>Invoking GHCi</title>
1551 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
1552 <indexterm><primary><option>––interactive</option></primary></indexterm>
1554 <para>GHCi is invoked with the command <literal>ghci</literal> or
1555 <literal>ghc ––interactive</literal>. One or more modules or
1556 filenames can also be specified on the command line; this
1557 instructs GHCi to load the specified modules or filenames (and all
1558 the modules they depend on), just as if you had said
1559 <literal>:load <replaceable>modules</replaceable></literal> at the
1560 GHCi prompt (see <xref linkend="ghci-commands" />). For example, to
1561 start GHCi and load the program whose topmost module is in the
1562 file <literal>Main.hs</literal>, we could say:</para>
1568 <para>Most of the command-line options accepted by GHC (see <xref
1569 linkend="using-ghc"/>) also make sense in interactive mode. The ones
1570 that don't make sense are mostly obvious; for example, GHCi
1571 doesn't generate interface files, so options related to interface
1572 file generation won't have any effect.</para>
1575 <title>Packages</title>
1576 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
1578 <para>Most packages (see <xref linkend="using-packages"/>) are
1579 available without needing to specify any extra flags at all:
1580 they will be automatically loaded the first time they are
1583 <para>For hidden packages, however, you need to request the
1584 package be loaded by using the <literal>-package</literal> flag:</para>
1587 $ ghci -package readline
1590 / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98.
1591 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
1592 \____/\/ /_/\____/|_| Type :? for help.
1594 Loading package base ... linking ... done.
1595 Loading package readline-1.0 ... linking ... done.
1599 <para>The following command works to load new packages into a
1600 running GHCi:</para>
1603 Prelude> :set -package <replaceable>name</replaceable>
1606 <para>But note that doing this will cause all currently loaded
1607 modules to be unloaded, and you'll be dumped back into the
1608 <literal>Prelude</literal>.</para>
1612 <title>Extra libraries</title>
1613 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
1615 <para>Extra libraries may be specified on the command line using
1616 the normal <literal>-l<replaceable>lib</replaceable></literal>
1617 option. (The term <emphasis>library</emphasis> here refers to
1618 libraries of foreign object code; for using libraries of Haskell
1619 source code, see <xref linkend="ghci-modules-filenames"/>.) For
1620 example, to load the “m” library:</para>
1626 <para>On systems with <literal>.so</literal>-style shared
1627 libraries, the actual library loaded will the
1628 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
1629 searches the following places for libraries, in this order:</para>
1633 <para>Paths specified using the
1634 <literal>-L<replaceable>path</replaceable></literal>
1635 command-line option,</para>
1638 <para>the standard library search path for your system,
1639 which on some systems may be overridden by setting the
1640 <literal>LD_LIBRARY_PATH</literal> environment
1645 <para>On systems with <literal>.dll</literal>-style shared
1646 libraries, the actual library loaded will be
1647 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
1648 GHCi will signal an error if it can't find the library.</para>
1650 <para>GHCi can also load plain object files
1651 (<literal>.o</literal> or <literal>.obj</literal> depending on
1652 your platform) from the command-line. Just add the name the
1653 object file to the command line.</para>
1655 <para>Ordering of <option>-l</option> options matters: a library
1656 should be mentioned <emphasis>before</emphasis> the libraries it
1657 depends on (see <xref linkend="options-linker"/>).</para>
1662 <sect1 id="ghci-commands">
1663 <title>GHCi commands</title>
1665 <para>GHCi commands all begin with
1666 ‘<literal>:</literal>’ and consist of a single command
1667 name followed by zero or more parameters. The command name may be
1668 abbreviated, with ambiguities being resolved in favour of the more
1669 commonly used commands.</para>
1674 <literal>:abandon</literal>
1675 <indexterm><primary><literal>:abandon</literal></primary></indexterm>
1678 <para>Abandons the current evaluation (only available when stopped at
1679 a breakpoint).</para>
1685 <literal>:add</literal> <replaceable>module</replaceable> ...
1686 <indexterm><primary><literal>:add</literal></primary></indexterm>
1689 <para>Add <replaceable>module</replaceable>(s) to the
1690 current <firstterm>target set</firstterm>, and perform a
1697 <literal>:back</literal>
1698 <indexterm><primary><literal>:back</literal></primary></indexterm>
1701 <para>Travel back one step in the history. See <xref
1702 linkend="tracing" />. See also:
1703 <literal>:trace</literal>, <literal>:history</literal>,
1704 <literal>:forward</literal>.</para>
1710 <literal>:break [<replaceable>identifier</replaceable> |
1711 [<replaceable>module</replaceable>] <replaceable>line</replaceable>
1712 [<replaceable>column</replaceable>]]</literal>
1714 <indexterm><primary><literal>:break</literal></primary></indexterm>
1716 <para>Set a breakpoint on the specified function or line and
1717 column. See <xref linkend="setting-breakpoints" />.</para>
1723 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
1724 <indexterm><primary><literal>:browse</literal></primary></indexterm>
1727 <para>Displays the identifiers defined by the module
1728 <replaceable>module</replaceable>, which must be either
1729 loaded into GHCi or be a member of a package. If the
1730 <literal>*</literal> symbol is placed before the module
1731 name, then <emphasis>all</emphasis> the identifiers defined
1732 in <replaceable>module</replaceable> are shown; otherwise
1733 the list is limited to the exports of
1734 <replaceable>module</replaceable>. The
1735 <literal>*</literal>-form is only available for modules
1736 which are interpreted; for compiled modules (including
1737 modules from packages) only the non-<literal>*</literal>
1738 form of <literal>:browse</literal> is available.</para>
1744 <literal>:cd</literal> <replaceable>dir</replaceable>
1745 <indexterm><primary><literal>:cd</literal></primary></indexterm>
1748 <para>Changes the current working directory to
1749 <replaceable>dir</replaceable>. A
1750 ‘<literal>˜</literal>’ symbol at the
1751 beginning of <replaceable>dir</replaceable> will be replaced
1752 by the contents of the environment variable
1753 <literal>HOME</literal>.</para>
1755 <para>NOTE: changing directories causes all currently loaded
1756 modules to be unloaded. This is because the search path is
1757 usually expressed using relative directories, and changing
1758 the search path in the middle of a session is not
1765 <literal>:continue</literal>
1766 <indexterm><primary><literal>:continue</literal></primary></indexterm>
1768 <listitem><para>Continue the current evaluation, when stopped at a
1775 <literal>:cmd</literal> <replaceable>expr</replaceable>
1776 <indexterm><primary><literal>:cmd</literal></primary></indexterm>
1779 <para>Executes <replaceable>expr</replaceable> as a computation of
1780 type <literal>IO String</literal>, and then executes the resulting
1781 string as a list of GHCi commands. Multiple commands are separated
1782 by newlines. The <literal>:cmd</literal> command is useful with
1783 <literal>:def</literal> and <literal>:set stop</literal>.</para>
1789 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1790 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1791 <indexterm><primary><literal>:etags</literal></primary>
1793 <indexterm><primary><literal>:etags</literal></primary>
1797 <para>Generates a “tags” file for Vi-style editors
1798 (<literal>:ctags</literal>) or Emacs-style editors (<literal>etags</literal>). If
1799 no filename is specified, the defaulit <filename>tags</filename> or
1800 <filename>TAGS</filename> is
1801 used, respectively. Tags for all the functions, constructors and
1802 types in the currently loaded modules are created. All modules must
1803 be interpreted for these commands to work.</para>
1804 <para>See also <xref linkend="hasktags" />.</para>
1810 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
1811 <indexterm><primary><literal>:def</literal></primary></indexterm>
1814 <para>The command <literal>:def</literal>
1815 <replaceable>name</replaceable>
1816 <replaceable>expr</replaceable> defines a new GHCi command
1817 <literal>:<replaceable>name</replaceable></literal>,
1818 implemented by the Haskell expression
1819 <replaceable>expr</replaceable>, which must have type
1820 <literal>String -> IO String</literal>. When
1821 <literal>:<replaceable>name</replaceable>
1822 <replaceable>args</replaceable></literal> is typed at the
1823 prompt, GHCi will run the expression
1824 <literal>(<replaceable>name</replaceable>
1825 <replaceable>args</replaceable>)</literal>, take the
1826 resulting <literal>String</literal>, and feed it back into
1827 GHCi as a new sequence of commands. Separate commands in
1828 the result must be separated by
1829 ‘<literal>\n</literal>’.</para>
1831 <para>That's all a little confusing, so here's a few
1832 examples. To start with, here's a new GHCi command which
1833 doesn't take any arguments or produce any results, it just
1834 outputs the current date & time:</para>
1837 Prelude> let date _ = Time.getClockTime >>= print >> return ""
1838 Prelude> :def date date
1840 Fri Mar 23 15:16:40 GMT 2001
1843 <para>Here's an example of a command that takes an argument.
1844 It's a re-implementation of <literal>:cd</literal>:</para>
1847 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
1848 Prelude> :def mycd mycd
1852 <para>Or I could define a simple way to invoke
1853 “<literal>ghc ––make Main</literal>” in the
1854 current directory:</para>
1857 Prelude> :def make (\_ -> return ":! ghc ––make Main")
1860 <para>We can define a command that reads GHCi input from a
1861 file. This might be useful for creating a set of bindings
1862 that we want to repeatedly load into the GHCi session:</para>
1865 Prelude> :def . readFile
1866 Prelude> :. cmds.ghci
1869 <para>Notice that we named the command
1870 <literal>:.</literal>, by analogy with the
1871 ‘<literal>.</literal>’ Unix shell command that
1872 does the same thing.</para>
1878 <literal>:delete * | <replaceable>num</replaceable> ...</literal>
1879 <indexterm><primary><literal>:delete</literal></primary></indexterm>
1882 <para>Delete one or more breakpoints by number (use <literal>:show
1883 breaks</literal> to see the number of each breakpoint). The
1884 <literal>*</literal> form deletes all the breakpoints.</para>
1890 <literal>:edit <optional><replaceable>file</replaceable></optional></literal>
1891 <indexterm><primary><literal>:edit</literal></primary></indexterm>
1894 <para>Opens an editor to edit the file
1895 <replaceable>file</replaceable>, or the most recently loaded
1896 module if <replaceable>file</replaceable> is omitted. The
1897 editor to invoke is taken from the <literal>EDITOR</literal>
1898 environment variable, or a default editor on your system if
1899 <literal>EDITOR</literal> is not set. You can change the
1900 editor using <literal>:set editor</literal>.</para>
1906 <literal>:force <replaceable>identifier</replaceable> ...</literal>
1907 <indexterm><primary><literal>:force</literal></primary></indexterm>
1910 <para>Prints the value of <replaceable>identifier</replaceable> in
1911 the same way as <literal>:print</literal>. Unlike
1912 <literal>:print</literal>, <literal>:force</literal> evaluates each
1913 thunk that it encounters while traversing the value. This may
1914 cause exceptions or infinite loops, or further breakpoints (which
1915 are ignored, but displayed).</para>
1921 <literal>:forward</literal>
1922 <indexterm><primary><literal>:forward</literal></primary></indexterm>
1925 <para>Move forward in the history. See <xref
1926 linkend="tracing" />. See also:
1927 <literal>:trace</literal>, <literal>:history</literal>,
1928 <literal>:back</literal>.</para>
1934 <literal>:help</literal>
1935 <indexterm><primary><literal>:help</literal></primary></indexterm>
1938 <literal>:?</literal>
1939 <indexterm><primary><literal>:?</literal></primary></indexterm>
1942 <para>Displays a list of the available commands.</para>
1948 <literal>:history [<replaceable>num</replaceable>]</literal>
1949 <indexterm><primary><literal>:history</literal></primary></indexterm>
1952 <para>Display the history of evaluation steps. With a number,
1953 displays that many steps (default: 20). For use with
1954 <literal>:trace</literal>; see <xref
1955 linkend="tracing" />.</para>
1961 <literal>:info</literal> <replaceable>name</replaceable> ...
1962 <indexterm><primary><literal>:info</literal></primary></indexterm>
1965 <para>Displays information about the given name(s). For
1966 example, if <replaceable>name</replaceable> is a class, then
1967 the class methods and their types will be printed; if
1968 <replaceable>name</replaceable> is a type constructor, then
1969 its definition will be printed; if
1970 <replaceable>name</replaceable> is a function, then its type
1971 will be printed. If <replaceable>name</replaceable> has
1972 been loaded from a source file, then GHCi will also display
1973 the location of its definition in the source.</para>
1979 <literal>:kind</literal> <replaceable>type</replaceable>
1980 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1983 <para>Infers and prints the kind of
1984 <replaceable>type</replaceable>. The latter can be an arbitrary
1985 type expression, including a partial application of a type constructor,
1986 such as <literal>Either Int</literal>.</para>
1992 <literal>:load</literal> <replaceable>module</replaceable> ...
1993 <indexterm><primary><literal>:load</literal></primary></indexterm>
1996 <para>Recursively loads the specified
1997 <replaceable>module</replaceable>s, and all the modules they
1998 depend on. Here, each <replaceable>module</replaceable>
1999 must be a module name or filename, but may not be the name
2000 of a module in a package.</para>
2002 <para>All previously loaded modules, except package modules,
2003 are forgotten. The new set of modules is known as the
2004 <firstterm>target set</firstterm>. Note that
2005 <literal>:load</literal> can be used without any arguments
2006 to unload all the currently loaded modules and
2009 <para>After a <literal>:load</literal> command, the current
2010 context is set to:</para>
2014 <para><replaceable>module</replaceable>, if it was loaded
2015 successfully, or</para>
2018 <para>the most recently successfully loaded module, if
2019 any other modules were loaded as a result of the current
2020 <literal>:load</literal>, or</para>
2023 <para><literal>Prelude</literal> otherwise.</para>
2031 <literal>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
2032 <indexterm><primary><literal>:main</literal></primary></indexterm>
2036 When a program is compiled and executed, it can use the
2037 <literal>getArgs</literal> function to access the
2038 command-line arguments.
2039 However, we cannot simply pass the arguments to the
2040 <literal>main</literal> function while we are testing in ghci,
2041 as the <literal>main</literal> function doesn't take its
2046 Instead, we can use the <literal>:main</literal> command.
2047 This runs whatever <literal>main</literal> is in scope, with
2048 any arguments being treated the same as command-line arguments,
2053 Prelude> let main = System.Environment.getArgs >>= print
2054 Prelude> :main foo bar
2063 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
2064 <indexterm><primary><literal>:module</literal></primary></indexterm>
2067 <literal>import <replaceable>mod</replaceable></literal>
2070 <para>Sets or modifies the current context for statements
2071 typed at the prompt. The form <literal>import
2072 <replaceable>mod</replaceable></literal> is equivalent to
2073 <literal>:module +<replaceable>mod</replaceable></literal>.
2074 See <xref linkend="ghci-scope"/> for
2075 more details.</para>
2081 <literal>:print </literal> <replaceable>names</replaceable> ...
2082 <indexterm><primary><literal>:print</literal></primary></indexterm>
2085 <para>Prints a value without forcing its evaluation.
2086 <literal>:print</literal> may be used on values whose types are
2087 unkonwn or partially known, which might be the case for local
2088 variables with polymorphic types at a breakpoint. While inspecting
2089 the runtime value, <literal>:print</literal> attempts to
2090 reconstruct the type of the value, and will elaborate the type in
2091 GHCi's environment if possible. If any unevaluated components
2092 (thunks) are encountered, then <literal>:print</literal> binds
2093 a fresh variable with a name beginning with <literal>_t</literal>
2094 to each thunk. See <xref linkend="breakpoints" /> for more
2095 information. See also the <literal>:sprint</literal> command,
2096 which works like <literal>:print</literal> but does not bind new
2103 <literal>:quit</literal>
2104 <indexterm><primary><literal>:quit</literal></primary></indexterm>
2107 <para>Quits GHCi. You can also quit by typing a control-D
2108 at the prompt.</para>
2114 <literal>:reload</literal>
2115 <indexterm><primary><literal>:reload</literal></primary></indexterm>
2118 <para>Attempts to reload the current target set (see
2119 <literal>:load</literal>) if any of the modules in the set,
2120 or any dependent module, has changed. Note that this may
2121 entail loading new modules, or dropping modules which are no
2122 longer indirectly required by the target.</para>
2128 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
2129 <indexterm><primary><literal>:set</literal></primary></indexterm>
2132 <para>Sets various options. See <xref linkend="ghci-set"/>
2133 for a list of available options. The
2134 <literal>:set</literal> command by itself shows which
2135 options are currently set.</para>
2141 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
2142 <indexterm><primary><literal>:set args</literal></primary></indexterm>
2145 <para>Sets the list of arguments which are returned when the
2146 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
2147 </indexterm>.</para>
2153 <literal>:set</literal> <literal>editor</literal> <replaceable>cmd</replaceable>
2156 <para>Sets the command used by <literal>:edit</literal> to
2157 <replaceable>cmd</replaceable>.</para>
2163 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
2164 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
2167 <para>Sets the string to be returned when the program calls
2168 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
2169 </indexterm>.</para>
2175 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
2178 <para>Sets the string to be used as the prompt in GHCi.
2179 Inside <replaceable>prompt</replaceable>, the sequence
2180 <literal>%s</literal> is replaced by the names of the
2181 modules currently in scope, and <literal>%%</literal> is
2182 replaced by <literal>%</literal>.</para>
2188 <literal>:set</literal> <literal>stop</literal>
2189 [<replaceable>num</replaceable>] <replaceable>cmd</replaceable>
2192 <para>Set a command to be executed when a breakpoint is hit, or a new
2193 item in the history is selected. The most common use of
2194 <literal>:set stop</literal> is to display the source code at the
2195 current location, e.g. <literal>:set stop :list</literal>.</para>
2197 <para>If a number is given before the command, then the commands are
2198 run when the specified breakpoint (only) is hit. This can be quite
2199 useful: for example, <literal>:set stop 1 :continue</literal>
2200 effectively disables breakpoint 1, by running
2201 <literal>:continue</literal> whenever it is hit (although GHCi will
2202 still emit a message to say the breakpoint was hit). What's more,
2203 with cunning use of <literal>:def</literal> and
2204 <literal>:cmd</literal> you can use <literal>:set stop</literal> to
2205 implement conditional breakpoints:</para>
2207 *Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
2208 *Main> :set stop 0 :cond (x < 3)
2210 <para>Ignoring breakpoints for a specified number of iterations is
2211 also possible using similar techniques.</para>
2217 <literal>:show bindings</literal>
2218 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
2221 <para>Show the bindings made at the prompt and their
2228 <literal>:show breaks</literal>
2229 <indexterm><primary><literal>:show breaks</literal></primary></indexterm>
2232 <para>List the active breakpoints.</para>
2238 <literal>:show context</literal>
2239 <indexterm><primary><literal>:show context</literal></primary></indexterm>
2242 <para>List the active evaluations that are stopped at breakpoints.</para>
2248 <literal>:show modules</literal>
2249 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
2252 <para>Show the list of modules currently load.</para>
2258 <literal>:show [args|prog|prompt|editor|stop]</literal>
2259 <indexterm><primary><literal>:show</literal></primary></indexterm>
2262 <para>Displays the specified setting (see
2263 <literal>:set</literal>).</para>
2269 <literal>:sprint</literal>
2270 <indexterm><primary><literal>:sprint</literal></primary></indexterm>
2273 <para>Prints a value without forcing its evaluation.
2274 <literal>:sprint</literal> is similar to <literal>:print</literal>,
2275 with the difference that unevaluated subterms are not bound to new
2276 variables, they are simply denoted by ‘_’.</para>
2282 <literal>:step [<replaceable>expr</replaceable>]</literal>
2283 <indexterm><primary><literal>:step</literal></primary></indexterm>
2286 <para>Single-step from the last breakpoint. With an expression
2287 argument, begins evaluation of the expression with a
2294 <literal>:trace [<replaceable>expr</replaceable>]</literal>
2295 <indexterm><primary><literal>:trace</literal></primary></indexterm>
2298 <para>Evaluates the given expression (or from the last breakpoint if
2299 no expression is given), and additionally logs the evaluation
2300 steps for later inspection using <literal>:history</literal>. See
2301 <xref linkend="tracing" />.</para>
2307 <literal>:type</literal> <replaceable>expression</replaceable>
2308 <indexterm><primary><literal>:type</literal></primary></indexterm>
2311 <para>Infers and prints the type of
2312 <replaceable>expression</replaceable>, including explicit
2313 forall quantifiers for polymorphic types. The monomorphism
2314 restriction is <emphasis>not</emphasis> applied to the
2315 expression during type inference.</para>
2321 <literal>:undef</literal> <replaceable>name</replaceable>
2322 <indexterm><primary><literal>:undef</literal></primary></indexterm>
2325 <para>Undefines the user-defined command
2326 <replaceable>name</replaceable> (see <literal>:def</literal>
2333 <literal>:unset</literal> <replaceable>option</replaceable>...
2334 <indexterm><primary><literal>:unset</literal></primary></indexterm>
2337 <para>Unsets certain options. See <xref linkend="ghci-set"/>
2338 for a list of available options.</para>
2344 <literal>:!</literal> <replaceable>command</replaceable>...
2345 <indexterm><primary><literal>:!</literal></primary></indexterm>
2346 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
2349 <para>Executes the shell command
2350 <replaceable>command</replaceable>.</para>
2357 <sect1 id="ghci-set">
2358 <title>The <literal>:set</literal> command</title>
2359 <indexterm><primary><literal>:set</literal></primary></indexterm>
2361 <para>The <literal>:set</literal> command sets two types of
2362 options: GHCi options, which begin with
2363 ‘<literal>+</literal>” and “command-line”
2364 options, which begin with ‘-’. </para>
2366 <para>NOTE: at the moment, the <literal>:set</literal> command
2367 doesn't support any kind of quoting in its arguments: quotes will
2368 not be removed and cannot be used to group words together. For
2369 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
2373 <title>GHCi options</title>
2374 <indexterm><primary>options</primary><secondary>GHCi</secondary>
2377 <para>GHCi options may be set using <literal>:set</literal> and
2378 unset using <literal>:unset</literal>.</para>
2380 <para>The available GHCi options are:</para>
2385 <literal>+r</literal>
2386 <indexterm><primary><literal>+r</literal></primary></indexterm>
2387 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
2388 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
2391 <para>Normally, any evaluation of top-level expressions
2392 (otherwise known as CAFs or Constant Applicative Forms) in
2393 loaded modules is retained between evaluations. Turning
2394 on <literal>+r</literal> causes all evaluation of
2395 top-level expressions to be discarded after each
2396 evaluation (they are still retained
2397 <emphasis>during</emphasis> a single evaluation).</para>
2399 <para>This option may help if the evaluated top-level
2400 expressions are consuming large amounts of space, or if
2401 you need repeatable performance measurements.</para>
2407 <literal>+s</literal>
2408 <indexterm><primary><literal>+s</literal></primary></indexterm>
2411 <para>Display some stats after evaluating each expression,
2412 including the elapsed time and number of bytes allocated.
2413 NOTE: the allocation figure is only accurate to the size
2414 of the storage manager's allocation area, because it is
2415 calculated at every GC. Hence, you might see values of
2416 zero if no GC has occurred.</para>
2422 <literal>+t</literal>
2423 <indexterm><primary><literal>+t</literal></primary></indexterm>
2426 <para>Display the type of each variable bound after a
2427 statement is entered at the prompt. If the statement is a
2428 single expression, then the only variable binding will be
2430 ‘<literal>it</literal>’.</para>
2436 <sect2 id="ghci-cmd-line-options">
2437 <title>Setting GHC command-line options in GHCi</title>
2439 <para>Normal GHC command-line options may also be set using
2440 <literal>:set</literal>. For example, to turn on
2441 <option>-fglasgow-exts</option>, you would say:</para>
2444 Prelude> :set -fglasgow-exts
2447 <para>Any GHC command-line option that is designated as
2448 <firstterm>dynamic</firstterm> (see the table in <xref
2449 linkend="flag-reference"/>), may be set using
2450 <literal>:set</literal>. To unset an option, you can set the
2451 reverse option:</para>
2452 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
2455 Prelude> :set -fno-glasgow-exts
2458 <para><xref linkend="flag-reference"/> lists the reverse for each
2459 option where applicable.</para>
2461 <para>Certain static options (<option>-package</option>,
2462 <option>-I</option>, <option>-i</option>, and
2463 <option>-l</option> in particular) will also work, but some may
2464 not take effect until the next reload.</para>
2465 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
2468 <sect1 id="ghci-dot-files">
2469 <title>The <filename>.ghci</filename> file</title>
2470 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
2472 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
2475 <para>When it starts, GHCi always reads and executes commands from
2476 <filename>$HOME/.ghci</filename>, followed by
2477 <filename>./.ghci</filename>.</para>
2479 <para>The <filename>.ghci</filename> in your home directory is
2480 most useful for turning on favourite options (eg. <literal>:set
2481 +s</literal>), and defining useful macros. Placing a
2482 <filename>.ghci</filename> file in a directory with a Haskell
2483 project is a useful way to set certain project-wide options so you
2484 don't have to type them everytime you start GHCi: eg. if your
2485 project uses GHC extensions and CPP, and has source files in three
2486 subdirectories A B and C, you might put the following lines in
2487 <filename>.ghci</filename>:</para>
2490 :set -fglasgow-exts -cpp
2494 <para>(Note that strictly speaking the <option>-i</option> flag is
2495 a static one, but in fact it works to set it using
2496 <literal>:set</literal> like this. The changes won't take effect
2497 until the next <literal>:load</literal>, though.)</para>
2499 <para>Two command-line options control whether the
2500 <filename>.ghci</filename> files are read:</para>
2505 <option>-ignore-dot-ghci</option>
2506 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
2509 <para>Don't read either <filename>./.ghci</filename> or
2510 <filename>$HOME/.ghci</filename> when starting up.</para>
2515 <option>-read-dot-ghci</option>
2516 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
2519 <para>Read <filename>.ghci</filename> and
2520 <filename>$HOME/.ghci</filename>. This is normally the
2521 default, but the <option>-read-dot-ghci</option> option may
2522 be used to override a previous
2523 <option>-ignore-dot-ghci</option> option.</para>
2530 <sect1 id="ghci-obj">
2531 <title>Compiling to object code inside GHCi</title>
2533 <para>By default, GHCi compiles Haskell source code into byte-code
2534 that is interpreted by the runtime system. GHCi can also compile
2535 Haskell code to object code: to turn on this feature, use the
2536 <option>-fobject-code</option> flag either on the command line or
2537 with <literal>:set</literal> (the option
2538 <option>-fbyte-code</option> restores byte-code compilation
2539 again). Compiling to object code takes longer, but typically the
2540 code will execute 10-20 times faster than byte-code.</para>
2542 <para>Compiling to object code inside GHCi is particularly useful
2543 if you are developing a compiled application, because the
2544 <literal>:reload</literal> command typically runs much faster than
2545 restarting GHC with <option>--make</option> from the command-line,
2546 because all the interface files are already cached in
2549 <para>There are disadvantages to compiling to object-code: you
2550 can't set breakpoints in object-code modules, for example. Only
2551 the exports of an object-code module will be visible in GHCi,
2552 rather than all top-level bindings as in interpreted
2556 <sect1 id="ghci-faq">
2557 <title>FAQ and Things To Watch Out For</title>
2561 <term>The interpreter can't load modules with foreign export
2562 declarations!</term>
2564 <para>Unfortunately not. We haven't implemented it yet.
2565 Please compile any offending modules by hand before loading
2566 them into GHCi.</para>
2572 <literal>-O</literal> doesn't work with GHCi!
2573 <indexterm><primary><option>-O</option></primary></indexterm>
2576 <para>For technical reasons, the bytecode compiler doesn't
2577 interact well with one of the optimisation passes, so we
2578 have disabled optimisation when using the interpreter. This
2579 isn't a great loss: you'll get a much bigger win by
2580 compiling the bits of your code that need to go fast, rather
2581 than interpreting them with optimisation turned on.</para>
2586 <term>Unboxed tuples don't work with GHCi</term>
2588 <para>That's right. You can always compile a module that
2589 uses unboxed tuples and load it into GHCi, however.
2590 (Incidentally the previous point, namely that
2591 <literal>-O</literal> is incompatible with GHCi, is because
2592 the bytecode compiler can't deal with unboxed
2598 <term>Concurrent threads don't carry on running when GHCi is
2599 waiting for input.</term>
2601 <para>This should work, as long as your GHCi was built with
2602 the <option>-threaded</option> switch, which is the default.
2603 Consult whoever supplied your GHCi installation.</para>
2608 <term>After using <literal>getContents</literal>, I can't use
2609 <literal>stdin</literal> again until I do
2610 <literal>:load</literal> or <literal>:reload</literal>.</term>
2613 <para>This is the defined behaviour of
2614 <literal>getContents</literal>: it puts the stdin Handle in
2615 a state known as <firstterm>semi-closed</firstterm>, wherein
2616 any further I/O operations on it are forbidden. Because I/O
2617 state is retained between computations, the semi-closed
2618 state persists until the next <literal>:load</literal> or
2619 <literal>:reload</literal> command.</para>
2621 <para>You can make <literal>stdin</literal> reset itself
2622 after every evaluation by giving GHCi the command
2623 <literal>:set +r</literal>. This works because
2624 <literal>stdin</literal> is just a top-level expression that
2625 can be reverted to its unevaluated state in the same way as
2626 any other top-level expression (CAF).</para>
2631 <term>I can't use Control-C to interrupt computations in
2632 GHCi on Windows.</term>
2634 <para>See <xref linkend="ghci-windows"/></para>
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