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. As the banner
38 says, you can type <literal>:?</literal> to see the list of commands
39 available, and a half line description of each of them.</para>
41 <para>We'll explain most of these commands as we go along. For
42 Hugs users: many things work the same as in Hugs, so you should be
43 able to get going straight away.</para>
45 <para>Haskell expressions can be typed at the prompt:</para>
46 <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
52 Prelude> let x = 42 in x / 9
57 <para>GHCi interprets the whole line as an expression to evaluate.
58 The expression may not span several lines - as soon as you press
59 enter, GHCi will attempt to evaluate it.</para>
62 <sect1 id="loading-source-files">
63 <title>Loading source files</title>
65 <para>Suppose we have the following Haskell source code, which we
66 place in a file <filename>Main.hs</filename>:</para>
75 <para>You can save <filename>Main.hs</filename> anywhere you like,
76 but if you save it somewhere other than the current
77 directory<footnote><para>If you started up GHCi from the command
78 line then GHCi's current directory is the same as the current
79 directory of the shell from which it was started. If you started
80 GHCi from the “Start” menu in Windows, then the
81 current directory is probably something like
82 <filename>C:\Documents and Settings\<replaceable>user
83 name</replaceable></filename>.</para> </footnote> then we will
84 need to change to the right directory in GHCi:</para>
87 Prelude> :cd <replaceable>dir</replaceable>
90 <para>where <replaceable>dir</replaceable> is the directory (or
91 folder) in which you saved <filename>Main.hs</filename>.</para>
93 <para>To load a Haskell source file into GHCi, use the
94 <literal>:load</literal> command:</para>
95 <indexterm><primary><literal>:load</literal></primary></indexterm>
99 Compiling Main ( Main.hs, interpreted )
100 Ok, modules loaded: Main.
104 <para>GHCi has loaded the <literal>Main</literal> module, and the
105 prompt has changed to “<literal>*Main></literal>” to
106 indicate that the current context for expressions typed at the
107 prompt is the <literal>Main</literal> module we just loaded (we'll
108 explain what the <literal>*</literal> means later in <xref
109 linkend="ghci-scope"/>). So we can now type expressions involving
110 the functions from <filename>Main.hs</filename>:</para>
117 <para>Loading a multi-module program is just as straightforward;
118 just give the name of the “topmost” module to the
119 <literal>:load</literal> command (hint: <literal>:load</literal>
120 can be abbreviated to <literal>:l</literal>). The topmost module
121 will normally be <literal>Main</literal>, but it doesn't have to
122 be. GHCi will discover which modules are required, directly or
123 indirectly, by the topmost module, and load them all in dependency
126 <sect2 id="ghci-modules-filenames">
127 <title>Modules vs. filenames</title>
128 <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
129 <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
131 <para>Question: How does GHC find the filename which contains
132 module <replaceable>M</replaceable>? Answer: it looks for the
133 file <literal><replaceable>M</replaceable>.hs</literal>, or
134 <literal><replaceable>M</replaceable>.lhs</literal>. This means
135 that for most modules, the module name must match the filename.
136 If it doesn't, GHCi won't be able to find it.</para>
138 <para>There is one exception to this general rule: when you load
139 a program with <literal>:load</literal>, or specify it when you
140 invoke <literal>ghci</literal>, you can give a filename rather
141 than a module name. This filename is loaded if it exists, and
142 it may contain any module you like. This is particularly
143 convenient if you have several <literal>Main</literal> modules
144 in the same directory and you can't call them all
145 <filename>Main.hs</filename>.</para>
147 <para>The search path for finding source files is specified with
148 the <option>-i</option> option on the GHCi command line, like
150 <screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
152 <para>or it can be set using the <literal>:set</literal> command
153 from within GHCi (see <xref
154 linkend="ghci-cmd-line-options"/>)<footnote><para>Note that in
155 GHCi, and <option>––make</option> mode, the <option>-i</option>
156 option is used to specify the search path for
157 <emphasis>source</emphasis> files, whereas in standard
158 batch-compilation mode the <option>-i</option> option is used to
159 specify the search path for interface files, see <xref
160 linkend="search-path"/>.</para> </footnote></para>
162 <para>One consequence of the way that GHCi follows dependencies
163 to find modules to load is that every module must have a source
164 file. The only exception to the rule is modules that come from
165 a package, including the <literal>Prelude</literal> and standard
166 libraries such as <literal>IO</literal> and
167 <literal>Complex</literal>. If you attempt to load a module for
168 which GHCi can't find a source file, even if there are object
169 and interface files for the module, you'll get an error
174 <title>Making changes and recompilation</title>
175 <indexterm><primary><literal>:reload</literal></primary></indexterm>
177 <para>If you make some changes to the source code and want GHCi
178 to recompile the program, give the <literal>:reload</literal>
179 command. The program will be recompiled as necessary, with GHCi
180 doing its best to avoid actually recompiling modules if their
181 external dependencies haven't changed. This is the same
182 mechanism we use to avoid re-compiling modules in the batch
183 compilation setting (see <xref linkend="recomp"/>).</para>
187 <sect1 id="ghci-compiled">
188 <title>Loading compiled code</title>
189 <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
191 <para>When you load a Haskell source module into GHCi, it is
192 normally converted to byte-code and run using the interpreter.
193 However, interpreted code can also run alongside compiled code in
194 GHCi; indeed, normally when GHCi starts, it loads up a compiled
195 copy of the <literal>base</literal> package, which contains the
196 <literal>Prelude</literal>.</para>
198 <para>Why should we want to run compiled code? Well, compiled
199 code is roughly 10x faster than interpreted code, but takes about
200 2x longer to produce (perhaps longer if optimisation is on). So
201 it pays to compile the parts of a program that aren't changing
202 very often, and use the interpreter for the code being actively
205 <para>When loading up source files with <literal>:load</literal>,
206 GHCi looks for any corresponding compiled object files, and will
207 use one in preference to interpreting the source if possible. For
208 example, suppose we have a 4-module program consisting of modules
209 A, B, C, and D. Modules B and C both import D only,
210 and A imports both B & C:</para>
218 <para>We can compile D, then load the whole program, like this:</para>
220 Prelude> :! ghc -c D.hs
222 Compiling B ( B.hs, interpreted )
223 Compiling C ( C.hs, interpreted )
224 Compiling A ( A.hs, interpreted )
225 Ok, modules loaded: A, B, C, D.
229 <para>In the messages from the compiler, we see that there is no line
230 for <literal>D</literal>. This is because
231 it isn't necessary to compile <literal>D</literal>,
232 because the source and everything it depends on
233 is unchanged since the last compilation.</para>
235 <para>At any time you can use the command
236 <literal>:show modules</literal>
237 to get a list of the modules currently loaded
243 C ( C.hs, interpreted )
244 B ( B.hs, interpreted )
245 A ( A.hs, interpreted )
248 <para>If we now modify the source of D (or pretend to: using the Unix
249 command <literal>touch</literal> on the source file is handy for
250 this), the compiler will no longer be able to use the object file,
251 because it might be out of date:</para>
256 Compiling D ( D.hs, interpreted )
257 Ok, modules loaded: A, B, C, D.
261 <para>Note that module D was compiled, but in this instance
262 because its source hadn't really changed, its interface remained
263 the same, and the recompilation checker determined that A, B and C
264 didn't need to be recompiled.</para>
266 <para>So let's try compiling one of the other modules:</para>
269 *Main> :! ghc -c C.hs
271 Compiling D ( D.hs, interpreted )
272 Compiling B ( B.hs, interpreted )
273 Compiling C ( C.hs, interpreted )
274 Compiling A ( A.hs, interpreted )
275 Ok, modules loaded: A, B, C, D.
278 <para>We didn't get the compiled version of C! What happened?
279 Well, in GHCi a compiled module may only depend on other compiled
280 modules, and in this case C depends on D, which doesn't have an
281 object file, so GHCi also rejected C's object file. Ok, so let's
282 also compile D:</para>
285 *Main> :! ghc -c D.hs
287 Ok, modules loaded: A, B, C, D.
290 <para>Nothing happened! Here's another lesson: newly compiled
291 modules aren't picked up by <literal>:reload</literal>, only
292 <literal>:load</literal>:</para>
296 Compiling B ( B.hs, interpreted )
297 Compiling A ( A.hs, interpreted )
298 Ok, modules loaded: A, B, C, D.
301 <para>HINT: since GHCi will only use a compiled object file if it
302 can be sure that the compiled version is up-to-date, a good technique
303 when working on a large program is to occasionally run
304 <literal>ghc ––make</literal> to compile the whole project (say
305 before you go for lunch :-), then continue working in the
306 interpreter. As you modify code, the changed modules will be
307 interpreted, but the rest of the project will remain
312 <sect1 id="interactive-evaluation">
313 <title>Interactive evaluation at the prompt</title>
315 <para>When you type an expression at the prompt, GHCi immediately
316 evaluates and prints the result:
318 Prelude> reverse "hello"
325 <sect2><title>I/O actions at the prompt</title>
327 <para>GHCi does more than simple expression evaluation at the prompt.
328 If you type something of type <literal>IO a</literal> for some
329 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
330 as an IO-computation.
334 Prelude> putStrLn "hello"
337 Furthermore, GHCi will print the result of the I/O action if (and only
340 <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
341 <listitem><para>The result type is not
342 <literal>()</literal>.</para></listitem>
344 For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
346 Prelude> putStrLn "hello"
348 Prelude> do { putStrLn "hello"; return "yes" }
354 <sect2 id="ghci-stmts">
355 <title>Using <literal>do-</literal>notation at the prompt</title>
356 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
357 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
359 <para>GHCi actually accepts <firstterm>statements</firstterm>
360 rather than just expressions at the prompt. This means you can
361 bind values and functions to names, and use them in future
362 expressions or statements.</para>
364 <para>The syntax of a statement accepted at the GHCi prompt is
365 exactly the same as the syntax of a statement in a Haskell
366 <literal>do</literal> expression. However, there's no monad
367 overloading here: statements typed at the prompt must be in the
368 <literal>IO</literal> monad.
370 Prelude> x <- return 42
376 The statement <literal>x <- return 42</literal> means
377 “execute <literal>return 42</literal> in the
378 <literal>IO</literal> monad, and bind the result to
379 <literal>x</literal>”. We can then use
380 <literal>x</literal> in future statements, for example to print
381 it as we did above.</para>
383 <para>GHCi will print the result of a statement if and only if:
386 <para>The statement is not a binding, or it is a monadic binding
387 (<literal>p <- e</literal>) that binds exactly one
391 <para>The variable's type is not polymorphic, is not
392 <literal>()</literal>, and is an instance of
393 <literal>Show</literal></para>
396 The automatic printing of binding results can be supressed with
397 <option>:set -fno-print-bind-result</option> (this does not
398 supress printing the result of non-binding statements).
399 <indexterm><primary><option>-fno-print-bind-result</option></primary></indexterm><indexterm><primary><option>-fprint-bind-result</option></primary></indexterm>.
400 You might want to do this to prevent the result of binding
401 statements from being fully evaluated by the act of printing
402 them, for example.</para>
404 <para>Of course, you can also bind normal non-IO expressions
405 using the <literal>let</literal>-statement:</para>
412 <para>Another important difference between the two types of binding
413 is that the monadic bind (<literal>p <- e</literal>) is
414 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
415 whereas with the <literal>let</literal> form, the expression
416 isn't evaluated immediately:</para>
418 Prelude> let x = error "help!"
424 <para>Note that <literal>let</literal> bindings do not automatically
425 print the value bound, unlike monadic bindings.</para>
427 <para>Any exceptions raised during the evaluation or execution
428 of the statement are caught and printed by the GHCi command line
429 interface (for more information on exceptions, see the module
430 <literal>Control.Exception</literal> in the libraries
431 documentation).</para>
433 <para>Every new binding shadows any existing bindings of the
434 same name, including entities that are in scope in the current
435 module context.</para>
437 <para>WARNING: temporary bindings introduced at the prompt only
438 last until the next <literal>:load</literal> or
439 <literal>:reload</literal> command, at which time they will be
440 simply lost. However, they do survive a change of context with
441 <literal>:module</literal>: the temporary bindings just move to
442 the new location.</para>
444 <para>HINT: To get a list of the bindings currently in scope, use the
445 <literal>:show bindings</literal> command:</para>
448 Prelude> :show bindings
452 <para>HINT: if you turn on the <literal>+t</literal> option,
453 GHCi will show the type of each variable bound by a statement.
455 <indexterm><primary><literal>+t</literal></primary></indexterm>
458 Prelude> let (x:xs) = [1..]
465 <sect2 id="ghci-scope">
466 <title>What's really in scope at the prompt?</title>
468 <para>When you type an expression at the prompt, what
469 identifiers and types are in scope? GHCi provides a flexible
470 way to control exactly how the context for an expression is
471 constructed. Let's start with the simple cases; when you start
472 GHCi the prompt looks like this:</para>
474 <screen>Prelude></screen>
476 <para>Which indicates that everything from the module
477 <literal>Prelude</literal> is currently in scope. If we now
478 load a file into GHCi, the prompt will change:</para>
481 Prelude> :load Main.hs
482 Compiling Main ( Main.hs, interpreted )
486 <para>The new prompt is <literal>*Main</literal>, which
487 indicates that we are typing expressions in the context of the
488 top-level of the <literal>Main</literal> module. Everything
489 that is in scope at the top-level in the module
490 <literal>Main</literal> we just loaded is also in scope at the
491 prompt (probably including <literal>Prelude</literal>, as long
492 as <literal>Main</literal> doesn't explicitly hide it).</para>
495 <literal>*<replaceable>module</replaceable></literal> indicates
496 that it is the full top-level scope of
497 <replaceable>module</replaceable> that is contributing to the
498 scope for expressions typed at the prompt. Without the
499 <literal>*</literal>, just the exports of the module are
502 <para>We're not limited to a single module: GHCi can combine
503 scopes from multiple modules, in any mixture of
504 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
505 combines the scopes from all of these modules to form the scope
506 that is in effect at the prompt. For technical reasons, GHCi
507 can only support the <literal>*</literal>-form for modules which
508 are interpreted, so compiled modules and package modules can
509 only contribute their exports to the current scope.</para>
511 <para>The scope is manipulated using the
512 <literal>:module</literal> command. For example, if the current
513 scope is <literal>Prelude</literal>, then we can bring into
514 scope the exports from the module <literal>IO</literal> like
519 Prelude IO> hPutStrLn stdout "hello\n"
524 <para>(Note: you can use <literal>import M</literal> as an
525 alternative to <literal>:module +M</literal>, and
526 <literal>:module</literal> can also be shortened to
527 <literal>:m</literal>). The full syntax of the
528 <literal>:module</literal> command is:</para>
531 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
534 <para>Using the <literal>+</literal> form of the
535 <literal>module</literal> commands adds modules to the current
536 scope, and <literal>-</literal> removes them. Without either
537 <literal>+</literal> or <literal>-</literal>, the current scope
538 is replaced by the set of modules specified. Note that if you
539 use this form and leave out <literal>Prelude</literal>, GHCi
540 will assume that you really wanted the
541 <literal>Prelude</literal> and add it in for you (if you don't
542 want the <literal>Prelude</literal>, then ask to remove it with
543 <literal>:m -Prelude</literal>).</para>
545 <para>The scope is automatically set after a
546 <literal>:load</literal> command, to the most recently loaded
547 "target" module, in a <literal>*</literal>-form if possible.
548 For example, if you say <literal>:load foo.hs bar.hs</literal>
549 and <filename>bar.hs</filename> contains module
550 <literal>Bar</literal>, then the scope will be set to
551 <literal>*Bar</literal> if <literal>Bar</literal> is
552 interpreted, or if <literal>Bar</literal> is compiled it will be
553 set to <literal>Prelude Bar</literal> (GHCi automatically adds
554 <literal>Prelude</literal> if it isn't present and there aren't
555 any <literal>*</literal>-form modules).</para>
557 <para>With multiple modules in scope, especially multiple
558 <literal>*</literal>-form modules, it is likely that name
559 clashes will occur. Haskell specifies that name clashes are
560 only reported when an ambiguous identifier is used, and GHCi
561 behaves in the same way for expressions typed at the
565 Hint: GHCi will tab-complete names that are in scope; for
566 example, if you run GHCi and type <literal>J<tab></literal>
567 then GHCi will expand it to “<literal>Just </literal>”.
571 <title>Qualified names</title>
573 <para>To make life slightly easier, the GHCi prompt also
574 behaves as if there is an implicit <literal>import
575 qualified</literal> declaration for every module in every
576 package, and every module currently loaded into GHCi.</para>
580 <title>The <literal>:main</literal> command</title>
583 When a program is compiled and executed, it can use the
584 <literal>getArgs</literal> function to access the
585 command-line arguments.
586 However, we cannot simply pass the arguments to the
587 <literal>main</literal> function while we are testing in ghci,
588 as the <literal>main</literal> function doesn't take its
593 Instead, we can use the <literal>:main</literal> command.
594 This runs whatever <literal>main</literal> is in scope, with
595 any arguments being treated the same as command-line arguments,
600 Prelude> let main = System.Environment.getArgs >>= print
601 Prelude> :main foo bar
610 <title>The <literal>it</literal> variable</title>
611 <indexterm><primary><literal>it</literal></primary>
614 <para>Whenever an expression (or a non-binding statement, to be
615 precise) is typed at the prompt, GHCi implicitly binds its value
616 to the variable <literal>it</literal>. For example:</para>
623 <para>What actually happens is that GHCi typechecks the
624 expression, and if it doesn't have an <literal>IO</literal> type,
625 then it transforms it as follows: an expression
626 <replaceable>e</replaceable> turns into
628 let it = <replaceable>e</replaceable>;
631 which is then run as an IO-action.</para>
633 <para>Hence, the original expression must have a type which is an
634 instance of the <literal>Show</literal> class, or GHCi will
640 <interactive>:1:0:
641 No instance for (Show (a -> a))
642 arising from use of `print' at <interactive>:1:0-1
643 Possible fix: add an instance declaration for (Show (a -> a))
644 In the expression: print it
645 In a 'do' expression: print it
648 <para>The error message contains some clues as to the
649 transformation happening internally.</para>
651 <para>If the expression was instead of type <literal>IO a</literal> for
652 some <literal>a</literal>, then <literal>it</literal> will be
653 bound to the result of the <literal>IO</literal> computation,
654 which is of type <literal>a</literal>. eg.:</para>
656 Prelude> Time.getClockTime
657 Wed Mar 14 12:23:13 GMT 2001
659 Wed Mar 14 12:23:13 GMT 2001
662 <para>The corresponding translation for an IO-typed
663 <replaceable>e</replaceable> is
665 it <- <replaceable>e</replaceable>
669 <para>Note that <literal>it</literal> is shadowed by the new
670 value each time you evaluate a new expression, and the old value
671 of <literal>it</literal> is lost.</para>
675 <sect2 id="extended-default-rules">
676 <title>Type defaulting in GHCi</title>
677 <indexterm><primary>Type default</primary></indexterm>
678 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
680 Consider this GHCi session:
684 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
685 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
686 on the type <literal>a</literal>. For example:
688 ghci> (reverse []) :: String
690 ghci> (reverse []) :: [Int]
693 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
694 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
695 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
696 a)</literal> for each type variable <literal>a</literal>, and defaults the
701 The type variable <literal>a</literal> appears in no
707 All the classes <literal>Ci</literal> are standard.
712 At least one of the classes <literal>Ci</literal> is
717 At the GHCi prompt, or with GHC if the
718 <literal>-XExtendedDefaultRules</literal> flag is given,
719 the following additional differences apply:
723 Rule 2 above is relaxed thus:
724 <emphasis>All</emphasis> of the classes
725 <literal>Ci</literal> are single-parameter type classes.
730 Rule 3 above is relaxed this:
731 At least one of the classes <literal>Ci</literal> is
732 numeric, <emphasis>or is <literal>Show</literal>,
733 <literal>Eq</literal>, or
734 <literal>Ord</literal></emphasis>.
739 The unit type <literal>()</literal> is added to the
740 start of the standard list of types which are tried when
741 doing type defaulting.
745 The last point means that, for example, this program:
752 def :: (Num a, Enum a) => a
755 prints <literal>()</literal> rather than <literal>0</literal> as the
756 type is defaulted to <literal>()</literal> rather than
757 <literal>Integer</literal>.
760 The motivation for the change is that it means <literal>IO a</literal>
761 actions default to <literal>IO ()</literal>, which in turn means that
762 ghci won't try to print a result when running them. This is
763 particularly important for <literal>printf</literal>, which has an
764 instance that returns <literal>IO a</literal>.
765 However, it is only able to return
766 <literal>undefined</literal>
767 (the reason for the instance having this type is so that printf
768 doesn't require extensions to the class system), so if the type defaults to
769 <literal>Integer</literal> then ghci gives an error when running a
775 <sect1 id="ghci-debugger">
776 <title>The GHCi Debugger</title>
777 <indexterm><primary>debugger</primary><secondary>in GHCi</secondary>
780 <para>GHCi contains a simple imperative-style debugger in which you can
781 stop a running computation in order to examine the values of
782 variables. The debugger is integrated into GHCi, and is turned on by
783 default: no flags are required to enable the debugging facilities. There
784 is one major restriction: breakpoints and single-stepping are only
785 available in <emphasis>interpreted</emphasis> modules; compiled code is
786 invisible to the debugger.</para>
788 <para>The debugger provides the following:
791 <para>The ability to set a <firstterm>breakpoint</firstterm> on a
792 function definition or expression in the program. When the function
793 is called, or the expression evaluated, GHCi suspends
794 execution and returns to the prompt, where you can inspect the
795 values of local variables before continuing with the
799 <para>Execution can be <firstterm>single-stepped</firstterm>: the
800 evaluator will suspend execution approximately after every
801 reduction, allowing local variables to be inspected. This is
802 equivalent to setting a breakpoint at every point in the
806 <para>Execution can take place in <firstterm>tracing
807 mode</firstterm>, in which the evaluator remembers each
808 evaluation step as it happens, but doesn't suspend execution until
809 an actual breakpoint is reached. When this happens, the history of
810 evaluation steps can be inspected.</para>
813 <para>Exceptions (e.g. pattern matching failure and
814 <literal>error</literal>) can be treated as breakpoints, to help
815 locate the source of an exception in the program.</para>
820 <para>There is currently no support for obtaining a “stack
821 trace”, but the tracing and history features provide a useful
822 second-best, which will often be enough to establish the context of an
825 <sect2 id="breakpoints">
826 <title>Breakpoints and inspecting variables</title>
828 <para>Let's use quicksort as a running example. Here's the code:</para>
832 qsort (a:as) = qsort left ++ [a] ++ qsort right
833 where (left,right) = (filter (<=a) as, filter (>a) as)
835 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
838 <para>First, load the module into GHCi:</para>
842 [1 of 1] Compiling Main ( qsort.hs, interpreted )
843 Ok, modules loaded: Main.
847 <para>Now, let's set a breakpoint on the right-hand-side of the second
848 equation of qsort:</para>
852 Breakpoint 0 activated at qsort.hs:2:15-46
856 <para>The command <literal>:break 2</literal> sets a breakpoint on line
857 2 of the most recently-loaded module, in this case
858 <literal>qsort.hs</literal>. Specifically, it picks the
859 leftmost complete subexpression on that line on which to set the
860 breakpoint, which in this case is the expression
861 <literal>(qsort left ++ [a] ++ qsort right)</literal>.</para>
863 <para>Now, we run the program:</para>
867 Stopped at qsort.hs:2:15-46
872 [qsort.hs:2:15-46] *Main>
875 <para>Execution has stopped at the breakpoint. The prompt has changed to
876 indicate that we are currently stopped at a breakpoint, and the location:
877 <literal>[qsort.hs:2:15-46]</literal>. To further clarify the
878 location, we can use the <literal>:list</literal> command:</para>
881 [qsort.hs:2:15-46] *Main> :list
883 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
884 3 where (left,right) = (filter (<=a) as, filter (>a) as)
887 <para>The <literal>:list</literal> command lists the source code around
888 the current breakpoint. If your output device supports it, then GHCi
889 will highlight the active subexpression in bold.</para>
891 <para>GHCi has provided bindings for the free variables<footnote><para>We
892 originally provided bindings for all variables in scope, rather
894 the free variables of the expression, but found that this affected
895 performance considerably, hence the current restriction to just the
896 free variables.</para>
897 </footnote> of the expression
899 breakpoint was placed (<literal>a</literal>, <literal>left</literal>,
900 <literal>right</literal>), and additionally a binding for the result of
901 the expression (<literal>_result</literal>). These variables are just
902 like other variables that you might define in GHCi; you
903 can use them in expressions that you type at the prompt, you can ask
904 for their types with <literal>:type</literal>, and so on. There is one
905 important difference though: these variables may only have partial
906 types. For example, if we try to display the value of
907 <literal>left</literal>:</para>
910 [qsort.hs:2:15-46] *Main> left
912 <interactive>:1:0:
913 Ambiguous type variable `a' in the constraint:
914 `Show a' arising from a use of `print' at <interactive>:1:0-3
915 Cannot resolve unknown runtime types: a
916 Use :print or :force to determine these types
919 <para>This is because <literal>qsort</literal> is a polymorphic function,
920 and because GHCi does not carry type information at runtime, it cannot
921 determine the runtime types of free variables that involve type
922 variables. Hence, when you ask to display <literal>left</literal> at
923 the prompt, GHCi can't figure out which instance of
924 <literal>Show</literal> to use, so it emits the type error above.</para>
926 <para>Fortunately, the debugger includes a generic printing command,
927 <literal>:print</literal>, which can inspect the actual runtime value of a
928 variable and attempt to reconstruct its type. If we try it on
929 <literal>left</literal>:</para>
932 [qsort.hs:2:15-46] *Main> :print left
936 <para>This isn't particularly enlightening. What happened is that
937 <literal>left</literal> is bound to an unevaluated computation (a
938 suspension, or <firstterm>thunk</firstterm>), and
939 <literal>:print</literal> does not force any evaluation. The idea is
940 that <literal>:print</literal> can be used to inspect values at a
941 breakpoint without any unfortunate side effects. It won't force any
942 evaluation, which could cause the program to give a different answer
943 than it would normally, and hence it won't cause any exceptions to be
944 raised, infinite loops, or further breakpoints to be triggered (see
945 <xref linkend="nested-breakpoints" />).
946 Rather than forcing thunks, <literal>:print</literal>
947 binds each thunk to a fresh variable beginning with an
948 underscore, in this case
949 <literal>_t1</literal>.</para>
951 <para>If we aren't concerned about preserving the evaluatedness of a
952 variable, we can use <literal>:force</literal> instead of
953 <literal>:print</literal>. The <literal>:force</literal> command
954 behaves exactly like <literal>:print</literal>, except that it forces
955 the evaluation of any thunks it encounters:</para>
958 [qsort.hs:2:15-46] *Main> :force left
962 <para>Now, since <literal>:force</literal> has inspected the runtime
963 value of <literal>left</literal>, it has reconstructed its type. We
964 can see the results of this type reconstruction:</para>
967 [qsort.hs:2:15-46] *Main> :show bindings
975 <para>Not only do we now know the type of <literal>left</literal>, but
976 all the other partial types have also been resolved. So we can ask
977 for the value of <literal>a</literal>, for example:</para>
980 [qsort.hs:2:15-46] *Main> a
984 <para>You might find it useful to use Haskell's
985 <literal>seq</literal> function to evaluate individual thunks rather
986 than evaluating the whole expression with <literal>:force</literal>.
990 [qsort.hs:2:15-46] *Main> :print right
991 right = (_t1::[Integer])
992 [qsort.hs:2:15-46] *Main> seq _t1 ()
994 [qsort.hs:2:15-46] *Main> :print right
995 right = 23 : (_t2::[Integer])
998 <para>We evaluated only the <literal>_t1</literal> thunk, revealing the
999 head of the list, and the tail is another thunk now bound to
1000 <literal>_t2</literal>. The <literal>seq</literal> function is a
1001 little inconvenient to use here, so you might want to use
1002 <literal>:def</literal> to make a nicer interface (left as an exercise
1003 for the reader!).</para>
1005 <para>Finally, we can continue the current execution:</para>
1008 [qsort.hs:2:15-46] *Main> :continue
1009 Stopped at qsort.hs:2:15-46
1014 [qsort.hs:2:15-46] *Main>
1017 <para>The execution continued at the point it previously stopped, and has
1018 now stopped at the breakpoint for a second time.</para>
1020 <sect3 id="setting-breakpoints">
1021 <title>Setting breakpoints</title>
1023 <para>Breakpoints can be set in various ways. Perhaps the easiest way to
1024 set a breakpoint is to name a top-level function:</para>
1027 :break <replaceable>identifier</replaceable>
1030 <para>Where <replaceable>identifier</replaceable> names any top-level
1031 function in an interpreted module currently loaded into GHCi (qualified
1032 names may be used). The breakpoint will be set on the body of the
1033 function, when it is fully applied but before any pattern matching has
1036 <para>Breakpoints can also be set by line (and optionally column)
1040 :break <replaceable>line</replaceable>
1041 :break <replaceable>line</replaceable> <replaceable>column</replaceable>
1042 :break <replaceable>module</replaceable> <replaceable>line</replaceable>
1043 :break <replaceable>module</replaceable> <replaceable>line</replaceable> <replaceable>column</replaceable>
1046 <para>When a breakpoint is set on a particular line, GHCi sets the
1048 leftmost subexpression that begins and ends on that line. If two
1049 complete subexpressions start at the same
1050 column, the longest one is picked. If there is no complete
1051 subexpression on the line, then the leftmost expression starting on
1052 the line is picked, and failing that the rightmost expression that
1053 partially or completely covers the line.</para>
1055 <para>When a breakpoint is set on a particular line and column, GHCi
1056 picks the smallest subexpression that encloses that location on which
1057 to set the breakpoint. Note: GHC considers the TAB character to have a
1058 width of 1, wherever it occurs; in other words it counts
1059 characters, rather than columns. This matches what some editors do,
1060 and doesn't match others. The best advice is to avoid tab
1061 characters in your source code altogether (see
1062 <option>-fwarn-tabs</option> in <xref linkend="options-sanity"
1065 <para>If the module is omitted, then the most recently-loaded module is
1068 <para>Not all subexpressions are potential breakpoint locations. Single
1069 variables are typically not considered to be breakpoint locations
1070 (unless the variable is the right-hand-side of a function definition,
1071 lambda, or case alternative). The rule of thumb is that all redexes
1072 are breakpoint locations, together with the bodies of functions,
1073 lambdas, case alternatives and binding statements. There is normally
1074 no breakpoint on a let expression, but there will always be a
1075 breakpoint on its body, because we are usually interested in inspecting
1076 the values of the variables bound by the let.</para>
1080 <title>Listing and deleting breakpoints</title>
1082 <para>The list of breakpoints currently enabled can be displayed using
1083 <literal>:show breaks</literal>:</para>
1086 [0] Main qsort.hs:1:11-12
1087 [1] Main qsort.hs:2:15-46
1090 <para>To delete a breakpoint, use the <literal>:delete</literal>
1091 command with the number given in the output from <literal>:show breaks</literal>:</para>
1096 [1] Main qsort.hs:2:15-46
1099 <para>To delete all breakpoints at once, use <literal>:delete *</literal>.</para>
1104 <sect2 id="single-stepping">
1105 <title>Single-stepping</title>
1107 <para>Single-stepping is a great way to visualise the execution of your
1108 program, and it is also a useful tool for identifying the source of a
1109 bug. GHCi offers two variants of stepping. Use
1110 <literal>:step</literal> to enable all the
1111 breakpoints in the program, and execute until the next breakpoint is
1112 reached. Use <literal>:stepover</literal> to step over function
1113 applications, which of course are executed all the same.
1118 Stopped at qsort.hs:5:7-47
1122 <para>The command <literal>:step
1123 <replaceable>expr</replaceable></literal> begins the evaluation of
1124 <replaceable>expr</replaceable> in single-stepping mode. If
1125 <replaceable>expr</replaceable> is ommitted, then it single-steps from
1126 the current breakpoint. <literal>:stepover</literal>
1127 works similarly.</para>
1129 <para>In the current version of the debugger, <literal>:stepover</literal>
1130 is limited to work locally in the lexical sense, that is in the context
1131 of the current expression body.
1132 When you run to the end of the expression, <literal>:stepover</literal>
1133 stops working and switches to behave like regular <literal>:step</literal>.
1135 that it will fail to step over the last function application. As a result,
1136 currently <literal>:stepover</literal> works great for monadic code, but
1137 interacts less perfectly with pure code. For example, if stopped at the
1138 line 2, on the entire expression
1139 <literal>qsort left ++ [a] ++ qsort right</literal>:</para>
1141 ... [qsort2.hs:2:15-46] *Main> :step
1142 Stopped at qsort2.hs:2:15-46
1144 ... [qsort2.hs:2:15-46] *Main> :list
1145 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
1148 <para> The first <literal>:stepover</literal> will step over the first
1149 <literal>qsort</literal> recursive call, as expected. The second one
1150 will step over the evaluation of <literal>[a]</literal>, again as
1151 expected. However, the third one has lexically <quote>run out</quote>
1152 of the current expression, and will behave like regular
1153 <literal>:step</literal>, performing one step of lazy evaluation and
1154 stopping at the next breakpoint. In this case it is indeed the second
1155 recursive application of <literal>qsort</literal>.</para>
1157 [qsort2.hs:2:36-46] *Main> :stepover
1158 Warning: no more breakpoints in this function body, switching to :step
1159 Stopped at qsort2.hs:(1,0)-(3,55)
1161 [qsort2.hs:2:36-46] *Main> :list
1164 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
1165 3 where (left,right) = (filter (<=a) as, filter (>a) as)
1167 <para>The <literal>:list</literal> command is particularly useful when
1168 single-stepping, to see where you currently are, as just shown
1169 in the above example.</para>
1172 [qsort.hs:5:7-47] *Main> :list
1174 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1176 [qsort.hs:5:7-47] *Main>
1179 <para>In fact, GHCi provides a way to run a command when a breakpoint is
1180 hit, so we can make it automatically do
1181 <literal>:list</literal>:</para>
1184 [qsort.hs:5:7-47] *Main> :set stop :list
1185 [qsort.hs:5:7-47] *Main> :step
1186 Stopped at qsort.hs:5:14-46
1187 _result :: [Integer]
1189 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1191 [qsort.hs:5:14-46] *Main>
1195 <sect2 id="nested-breakpoints">
1196 <title>Nested breakpoints</title>
1197 <para>When GHCi is stopped at a breakpoint, and an expression entered at
1198 the prompt triggers a
1199 second breakpoint, the new breakpoint becomes the “current”
1200 one, and the old one is saved on a stack. An arbitrary number of
1201 breakpoint contexts can be built up in this way. For example:</para>
1204 [qsort.hs:2:15-46] *Main> :st qsort [1,3]
1205 Stopped at qsort.hs:(1,0)-(3,55)
1207 ... [qsort.hs:(1,0)-(3,55)] *Main>
1210 <para>While stopped at the breakpoint on line 2 that we set earlier, we
1211 started a new evaluation with <literal>:step qsort [1,3]</literal>.
1212 This new evaluation stopped after one step (at the definition of
1213 <literal>qsort</literal>). The prompt has changed, now prefixed with
1214 <literal>...</literal>, to indicate that there are saved breakpoints
1215 beyond the current one. To see the stack of contexts, use
1216 <literal>:show context</literal>:</para>
1219 ... [qsort.hs:(1,0)-(3,55)] *Main> :show context
1221 Stopped at qsort.hs:2:15-46
1223 Stopped at qsort.hs:(1,0)-(3,55)
1224 ... [qsort.hs:(1,0)-(3,55)] *Main>
1227 <para>To abandon the current evaluation, use
1228 <literal>:abandon</literal>:</para>
1231 ... [qsort.hs:(1,0)-(3,55)] *Main> :abandon
1232 [qsort.hs:2:15-46] *Main> :abandon
1237 <sect2 id="ghci-debugger-result">
1238 <title>The <literal>_result</literal> variable</title>
1239 <para>When stopped at a breakpoint or single-step, GHCi binds the
1240 variable <literal>_result</literal> to the value of the currently
1241 active expression. The value of <literal>_result</literal> is
1242 presumably not available yet, because we stopped its evaluation, but it
1243 can be forced: if the type is known and showable, then just entering
1244 <literal>_result</literal> at the prompt will show it. However,
1245 there's one caveat to doing this: evaluating <literal>_result</literal>
1246 will be likely to trigger further breakpoints, starting with the
1247 breakpoint we are currently stopped at (if we stopped at a real
1248 breakpoint, rather than due to <literal>:step</literal>). So it will
1249 probably be necessary to issue a <literal>:continue</literal>
1250 immediately when evaluating <literal>_result</literal>. Alternatively,
1251 you can use <literal>:force</literal> which ignores breakpoints.</para>
1254 <sect2 id="tracing">
1255 <title>Tracing and history</title>
1257 <para>A question that we often want to ask when debugging a program is
1258 “how did I get here?”. Traditional imperative debuggers
1259 usually provide some kind of stack-tracing feature that lets you see
1260 the stack of active function calls (sometimes called the “lexical
1261 call stack”), describing a path through the code
1262 to the current location. Unfortunately this is hard to provide in
1263 Haskell, because execution proceeds on a demand-driven basis, rather
1264 than a depth-first basis as in strict languages. The
1265 “stack“ in GHC's execution engine bears little
1266 resemblance to the lexical call stack. Ideally GHCi would maintain a
1267 separate lexical call stack in addition to the dynamic call stack, and
1268 in fact this is exactly
1269 what our profiling system does (<xref linkend="profiling" />), and what
1270 some other Haskell debuggers do. For the time being, however, GHCi
1271 doesn't maintain a lexical call stack (there are some technical
1272 challenges to be overcome). Instead, we provide a way to backtrack from a
1273 breakpoint to previous evaluation steps: essentially this is like
1274 single-stepping backwards, and should in many cases provide enough
1275 information to answer the “how did I get here?”
1278 <para>To use tracing, evaluate an expression with the
1279 <literal>:trace</literal> command. For example, if we set a breakpoint
1280 on the base case of <literal>qsort</literal>:</para>
1283 *Main> :list qsort
1285 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
1286 3 where (left,right) = (filter (<=a) as, filter (>a) as)
1289 Breakpoint 1 activated at qsort.hs:1:11-12
1293 <para>and then run a small <literal>qsort</literal> with
1297 *Main> :trace qsort [3,2,1]
1298 Stopped at qsort.hs:1:11-12
1300 [qsort.hs:1:11-12] *Main>
1303 <para>We can now inspect the history of evaluation steps:</para>
1306 [qsort.hs:1:11-12] *Main> :hist
1307 -1 : qsort.hs:3:24-38
1308 -2 : qsort.hs:3:23-55
1309 -3 : qsort.hs:(1,0)-(3,55)
1310 -4 : qsort.hs:2:15-24
1311 -5 : qsort.hs:2:15-46
1312 -6 : qsort.hs:3:24-38
1313 -7 : qsort.hs:3:23-55
1314 -8 : qsort.hs:(1,0)-(3,55)
1315 -9 : qsort.hs:2:15-24
1316 -10 : qsort.hs:2:15-46
1317 -11 : qsort.hs:3:24-38
1318 -12 : qsort.hs:3:23-55
1319 -13 : qsort.hs:(1,0)-(3,55)
1320 -14 : qsort.hs:2:15-24
1321 -15 : qsort.hs:2:15-46
1322 -16 : qsort.hs:(1,0)-(3,55)
1323 <end of history>
1326 <para>To examine one of the steps in the history, use
1327 <literal>:back</literal>:</para>
1330 [qsort.hs:1:11-12] *Main> :back
1331 Logged breakpoint at qsort.hs:3:24-38
1335 [-1: qsort.hs:3:24-38] *Main>
1338 <para>Note that the local variables at each step in the history have been
1339 preserved, and can be examined as usual. Also note that the prompt has
1340 changed to indicate that we're currently examining the first step in
1341 the history: <literal>-1</literal>. The command
1342 <literal>:forward</literal> can be used to traverse forward in the
1345 <para>The <literal>:trace</literal> command can be used with or without
1346 an expression. When used without an expression, tracing begins from
1347 the current breakpoint, just like <literal>:step</literal>.</para>
1349 <para>The history is only available when
1350 using <literal>:trace</literal>; the reason for this is we found that
1351 logging each breakpoint in the history cuts performance by a factor of
1352 2 or more. GHCi remembers the last 50 steps in the history (perhaps in
1353 the future we'll make this configurable).</para>
1356 <sect2 id="ghci-debugger-exceptions">
1357 <title>Debugging exceptions</title>
1358 <para>Another common question that comes up when debugging is
1359 “where did this exception come from?”. Exceptions such as
1360 those raised by <literal>error</literal> or <literal>head []</literal>
1361 have no context information attached to them. Finding which
1362 particular call to <literal>head</literal> in your program resulted in
1363 the error can be a painstaking process, usually involving
1364 <literal>Debug.Trace.trace</literal>, or compiling with
1365 profiling and using <literal>+RTS -xc</literal> (see <xref
1366 linkend="prof-time-options" />).</para>
1368 <para>The GHCi debugger offers a way to hopefully shed some light on
1369 these errors quickly and without modifying or recompiling the source
1370 code. One way would be to set a breakpoint on the location in the
1371 source code that throws the exception, and then use
1372 <literal>:trace</literal> and <literal>:history</literal> to establish
1373 the context. However, <literal>head</literal> is in a library and
1374 we can't set a breakpoint on it directly. For this reason, GHCi
1375 provides the flag <literal>-fbreak-on-exception</literal> which causes
1376 the evaluator to stop when an exception is thrown, just as it does when
1377 a breakpoint is hit. This is only really useful in conjunction with
1378 <literal>:trace</literal>, in order to log the steps leading up to the
1379 exception. For example:</para>
1382 *Main> :set -fbreak-on-exception
1383 *Main> :trace qsort ("abc" ++ undefined)
1384 "Stopped at <exception thrown>
1386 [<exception thrown>] *Main> :hist
1387 -1 : qsort.hs:3:24-38
1388 -2 : qsort.hs:3:23-55
1389 -3 : qsort.hs:(1,0)-(3,55)
1390 -4 : qsort.hs:2:15-24
1391 -5 : qsort.hs:2:15-46
1392 -6 : qsort.hs:(1,0)-(3,55)
1393 <end of history>
1394 [<exception thrown>] *Main> :back
1395 Logged breakpoint at qsort.hs:3:24-38
1399 [-1: qsort.hs:3:24-38] *Main> :force as
1400 *** Exception: Prelude.undefined
1401 [-1: qsort.hs:3:24-38] *Main> :print as
1402 as = 'b' : 'c' : (_t1::[Char])
1405 <para>The exception itself is bound to a new variable,
1406 <literal>_exception</literal>.</para>
1408 <para>Breaking on exceptions is particularly useful for finding out what
1409 your program was doing when it was in an infinite loop. Just hit
1410 Control-C, and examine the history to find out what was going
1414 <sect2><title>Example: inspecting functions</title>
1416 It is possible to use the debugger to examine function values.
1417 When we are at a breakpoint and a function is in scope, the debugger
1419 you the source code for it; however, it is possible to get some
1420 information by applying it to some arguments and observing the result.
1424 The process is slightly complicated when the binding is polymorphic.
1425 We show the process by means of an example.
1426 To keep things simple, we will use the well known <literal>map</literal> function:
1428 import Prelude hiding (map)
1430 map :: (a->b) -> a -> b
1432 map f (x:xs) = f x : map f xs
1437 We set a breakpoint on <literal>map</literal>, and call it.
1440 Breakpoint 0 activated at map.hs:5:15-28
1441 *Main> map Just [1..5]
1442 Stopped at map.hs:(4,0)-(5,12)
1448 GHCi tells us that, among other bindings, <literal>f</literal> is in scope.
1449 However, its type is not fully known yet,
1450 and thus it is not possible to apply it to any
1451 arguments. Nevertheless, observe that the type of its first argument is the
1452 same as the type of <literal>x</literal>, and its result type is shared
1453 with <literal>_result</literal>.
1457 As we demonstrated earlier (<xref linkend="breakpoints" />), the
1458 debugger has some intelligence built-in to update the type of
1459 <literal>f</literal> whenever the types of <literal>x</literal> or
1460 <literal>_result</literal> are discovered. So what we do in this
1462 force <literal>x</literal> a bit, in order to recover both its type
1463 and the argument part of <literal>f</literal>.
1471 We can check now that as expected, the type of <literal>x</literal>
1472 has been reconstructed, and with it the
1473 type of <literal>f</literal> has been too:</para>
1481 From here, we can apply f to any argument of type Integer and observe
1489 Ambiguous type variable `b' in the constraint:
1490 `Show b' arising from a use of `print' at <interactive>:1:0
1502 f :: Integer -> Maybe Integer
1506 [Just 1, Just 2, Just 3, Just 4, Just 5]
1508 In the first application of <literal>f</literal>, we had to do
1509 some more type reconstruction
1510 in order to recover the result type of <literal>f</literal>.
1511 But after that, we are free to use
1512 <literal>f</literal> normally.
1516 <sect2><title>Limitations</title>
1519 <para>When stopped at a breakpoint, if you try to evaluate a variable
1520 that is already under evaluation, the second evaluation will hang.
1522 that GHC knows the variable is under evaluation, so the new
1523 evaluation just waits for the result before continuing, but of
1524 course this isn't going to happen because the first evaluation is
1525 stopped at a breakpoint. Control-C can interrupt the hung
1526 evaluation and return to the prompt.</para>
1527 <para>The most common way this can happen is when you're evaluating a
1528 CAF (e.g. main), stop at a breakpoint, and ask for the value of the
1529 CAF at the prompt again.</para>
1531 <listitem> <literal>:stepover</literal> only works lexically locally, in the
1532 body of the current expression. As a result, it can be rather impredictable
1533 when used in pure functional code, as opposed to monadic code.
1536 Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
1537 at the scope of a breakpoint if there is an explicit type signature.
1544 <sect1 id="ghci-invocation">
1545 <title>Invoking GHCi</title>
1546 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
1547 <indexterm><primary><option>––interactive</option></primary></indexterm>
1549 <para>GHCi is invoked with the command <literal>ghci</literal> or
1550 <literal>ghc ––interactive</literal>. One or more modules or
1551 filenames can also be specified on the command line; this
1552 instructs GHCi to load the specified modules or filenames (and all
1553 the modules they depend on), just as if you had said
1554 <literal>:load <replaceable>modules</replaceable></literal> at the
1555 GHCi prompt (see <xref linkend="ghci-commands" />). For example, to
1556 start GHCi and load the program whose topmost module is in the
1557 file <literal>Main.hs</literal>, we could say:</para>
1563 <para>Most of the command-line options accepted by GHC (see <xref
1564 linkend="using-ghc"/>) also make sense in interactive mode. The ones
1565 that don't make sense are mostly obvious.</para>
1568 <title>Packages</title>
1569 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
1571 <para>Most packages (see <xref linkend="using-packages"/>) are
1572 available without needing to specify any extra flags at all:
1573 they will be automatically loaded the first time they are
1576 <para>For hidden packages, however, you need to request the
1577 package be loaded by using the <literal>-package</literal> flag:</para>
1580 $ ghci -package readline
1581 GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
1582 Loading package base ... linking ... done.
1583 Loading package readline-1.0 ... linking ... done.
1587 <para>The following command works to load new packages into a
1588 running GHCi:</para>
1591 Prelude> :set -package <replaceable>name</replaceable>
1594 <para>But note that doing this will cause all currently loaded
1595 modules to be unloaded, and you'll be dumped back into the
1596 <literal>Prelude</literal>.</para>
1600 <title>Extra libraries</title>
1601 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
1603 <para>Extra libraries may be specified on the command line using
1604 the normal <literal>-l<replaceable>lib</replaceable></literal>
1605 option. (The term <emphasis>library</emphasis> here refers to
1606 libraries of foreign object code; for using libraries of Haskell
1607 source code, see <xref linkend="ghci-modules-filenames"/>.) For
1608 example, to load the “m” library:</para>
1614 <para>On systems with <literal>.so</literal>-style shared
1615 libraries, the actual library loaded will the
1616 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
1617 searches the following places for libraries, in this order:</para>
1621 <para>Paths specified using the
1622 <literal>-L<replaceable>path</replaceable></literal>
1623 command-line option,</para>
1626 <para>the standard library search path for your system,
1627 which on some systems may be overridden by setting the
1628 <literal>LD_LIBRARY_PATH</literal> environment
1633 <para>On systems with <literal>.dll</literal>-style shared
1634 libraries, the actual library loaded will be
1635 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
1636 GHCi will signal an error if it can't find the library.</para>
1638 <para>GHCi can also load plain object files
1639 (<literal>.o</literal> or <literal>.obj</literal> depending on
1640 your platform) from the command-line. Just add the name the
1641 object file to the command line.</para>
1643 <para>Ordering of <option>-l</option> options matters: a library
1644 should be mentioned <emphasis>before</emphasis> the libraries it
1645 depends on (see <xref linkend="options-linker"/>).</para>
1650 <sect1 id="ghci-commands">
1651 <title>GHCi commands</title>
1653 <para>GHCi commands all begin with
1654 ‘<literal>:</literal>’ and consist of a single command
1655 name followed by zero or more parameters. The command name may be
1656 abbreviated, with ambiguities being resolved in favour of the more
1657 commonly used commands.</para>
1662 <literal>:abandon</literal>
1663 <indexterm><primary><literal>:abandon</literal></primary></indexterm>
1666 <para>Abandons the current evaluation (only available when stopped at
1667 a breakpoint).</para>
1673 <literal>:add</literal> <replaceable>module</replaceable> ...
1674 <indexterm><primary><literal>:add</literal></primary></indexterm>
1677 <para>Add <replaceable>module</replaceable>(s) to the
1678 current <firstterm>target set</firstterm>, and perform a
1685 <literal>:back</literal>
1686 <indexterm><primary><literal>:back</literal></primary></indexterm>
1689 <para>Travel back one step in the history. See <xref
1690 linkend="tracing" />. See also:
1691 <literal>:trace</literal>, <literal>:history</literal>,
1692 <literal>:forward</literal>.</para>
1698 <literal>:break [<replaceable>identifier</replaceable> |
1699 [<replaceable>module</replaceable>] <replaceable>line</replaceable>
1700 [<replaceable>column</replaceable>]]</literal>
1702 <indexterm><primary><literal>:break</literal></primary></indexterm>
1704 <para>Set a breakpoint on the specified function or line and
1705 column. See <xref linkend="setting-breakpoints" />.</para>
1711 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
1712 <indexterm><primary><literal>:browse</literal></primary></indexterm>
1715 <para>Displays the identifiers defined by the module
1716 <replaceable>module</replaceable>, which must be either
1717 loaded into GHCi or be a member of a package. If the
1718 <literal>*</literal> symbol is placed before the module
1719 name, then <emphasis>all</emphasis> the identifiers defined
1720 in <replaceable>module</replaceable> are shown; otherwise
1721 the list is limited to the exports of
1722 <replaceable>module</replaceable>. The
1723 <literal>*</literal>-form is only available for modules
1724 which are interpreted; for compiled modules (including
1725 modules from packages) only the non-<literal>*</literal>
1726 form of <literal>:browse</literal> is available.</para>
1732 <literal>:cd</literal> <replaceable>dir</replaceable>
1733 <indexterm><primary><literal>:cd</literal></primary></indexterm>
1736 <para>Changes the current working directory to
1737 <replaceable>dir</replaceable>. A
1738 ‘<literal>˜</literal>’ symbol at the
1739 beginning of <replaceable>dir</replaceable> will be replaced
1740 by the contents of the environment variable
1741 <literal>HOME</literal>.</para>
1743 <para>NOTE: changing directories causes all currently loaded
1744 modules to be unloaded. This is because the search path is
1745 usually expressed using relative directories, and changing
1746 the search path in the middle of a session is not
1753 <literal>:cmd</literal> <replaceable>expr</replaceable>
1754 <indexterm><primary><literal>:cmd</literal></primary></indexterm>
1757 <para>Executes <replaceable>expr</replaceable> as a computation of
1758 type <literal>IO String</literal>, and then executes the resulting
1759 string as a list of GHCi commands. Multiple commands are separated
1760 by newlines. The <literal>:cmd</literal> command is useful with
1761 <literal>:def</literal> and <literal>:set stop</literal>.</para>
1767 <literal>:continue</literal>
1768 <indexterm><primary><literal>:continue</literal></primary></indexterm>
1770 <listitem><para>Continue the current evaluation, when stopped at a
1777 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1778 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1779 <indexterm><primary><literal>:etags</literal></primary>
1781 <indexterm><primary><literal>:etags</literal></primary>
1785 <para>Generates a “tags” file for Vi-style editors
1786 (<literal>:ctags</literal>) or
1787 Emacs-style editors (<literal>:etags</literal>). If
1788 no filename is specified, the defaulit <filename>tags</filename> or
1789 <filename>TAGS</filename> is
1790 used, respectively. Tags for all the functions, constructors and
1791 types in the currently loaded modules are created. All modules must
1792 be interpreted for these commands to work.</para>
1793 <para>See also <xref linkend="hasktags" />.</para>
1799 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
1800 <indexterm><primary><literal>:def</literal></primary></indexterm>
1803 <para>The command <literal>:def</literal>
1804 <replaceable>name</replaceable>
1805 <replaceable>expr</replaceable> defines a new GHCi command
1806 <literal>:<replaceable>name</replaceable></literal>,
1807 implemented by the Haskell expression
1808 <replaceable>expr</replaceable>, which must have type
1809 <literal>String -> IO String</literal>. When
1810 <literal>:<replaceable>name</replaceable>
1811 <replaceable>args</replaceable></literal> is typed at the
1812 prompt, GHCi will run the expression
1813 <literal>(<replaceable>name</replaceable>
1814 <replaceable>args</replaceable>)</literal>, take the
1815 resulting <literal>String</literal>, and feed it back into
1816 GHCi as a new sequence of commands. Separate commands in
1817 the result must be separated by
1818 ‘<literal>\n</literal>’.</para>
1820 <para>That's all a little confusing, so here's a few
1821 examples. To start with, here's a new GHCi command which
1822 doesn't take any arguments or produce any results, it just
1823 outputs the current date & time:</para>
1826 Prelude> let date _ = Time.getClockTime >>= print >> return ""
1827 Prelude> :def date date
1829 Fri Mar 23 15:16:40 GMT 2001
1832 <para>Here's an example of a command that takes an argument.
1833 It's a re-implementation of <literal>:cd</literal>:</para>
1836 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
1837 Prelude> :def mycd mycd
1841 <para>Or I could define a simple way to invoke
1842 “<literal>ghc ––make Main</literal>” in the
1843 current directory:</para>
1846 Prelude> :def make (\_ -> return ":! ghc ––make Main")
1849 <para>We can define a command that reads GHCi input from a
1850 file. This might be useful for creating a set of bindings
1851 that we want to repeatedly load into the GHCi session:</para>
1854 Prelude> :def . readFile
1855 Prelude> :. cmds.ghci
1858 <para>Notice that we named the command
1859 <literal>:.</literal>, by analogy with the
1860 ‘<literal>.</literal>’ Unix shell command that
1861 does the same thing.</para>
1867 <literal>:delete * | <replaceable>num</replaceable> ...</literal>
1868 <indexterm><primary><literal>:delete</literal></primary></indexterm>
1871 <para>Delete one or more breakpoints by number (use <literal>:show
1872 breaks</literal> to see the number of each breakpoint). The
1873 <literal>*</literal> form deletes all the breakpoints.</para>
1879 <literal>:edit <optional><replaceable>file</replaceable></optional></literal>
1880 <indexterm><primary><literal>:edit</literal></primary></indexterm>
1883 <para>Opens an editor to edit the file
1884 <replaceable>file</replaceable>, or the most recently loaded
1885 module if <replaceable>file</replaceable> is omitted. The
1886 editor to invoke is taken from the <literal>EDITOR</literal>
1887 environment variable, or a default editor on your system if
1888 <literal>EDITOR</literal> is not set. You can change the
1889 editor using <literal>:set editor</literal>.</para>
1895 <literal>:etags</literal>
1898 <para>See <literal>:ctags</literal>.</para>
1904 <literal>:force <replaceable>identifier</replaceable> ...</literal>
1905 <indexterm><primary><literal>:force</literal></primary></indexterm>
1908 <para>Prints the value of <replaceable>identifier</replaceable> in
1909 the same way as <literal>:print</literal>. Unlike
1910 <literal>:print</literal>, <literal>:force</literal> evaluates each
1911 thunk that it encounters while traversing the value. This may
1912 cause exceptions or infinite loops, or further breakpoints (which
1913 are ignored, but displayed).</para>
1919 <literal>:forward</literal>
1920 <indexterm><primary><literal>:forward</literal></primary></indexterm>
1923 <para>Move forward in the history. See <xref
1924 linkend="tracing" />. See also:
1925 <literal>:trace</literal>, <literal>:history</literal>,
1926 <literal>:back</literal>.</para>
1932 <literal>:help</literal>
1933 <indexterm><primary><literal>:help</literal></primary></indexterm>
1936 <literal>:?</literal>
1937 <indexterm><primary><literal>:?</literal></primary></indexterm>
1940 <para>Displays a list of the available commands.</para>
1946 <literal>:history [<replaceable>num</replaceable>]</literal>
1947 <indexterm><primary><literal>:history</literal></primary></indexterm>
1950 <para>Display the history of evaluation steps. With a number,
1951 displays that many steps (default: 20). For use with
1952 <literal>:trace</literal>; see <xref
1953 linkend="tracing" />.</para>
1959 <literal>:info</literal> <replaceable>name</replaceable> ...
1960 <indexterm><primary><literal>:info</literal></primary></indexterm>
1963 <para>Displays information about the given name(s). For
1964 example, if <replaceable>name</replaceable> is a class, then
1965 the class methods and their types will be printed; if
1966 <replaceable>name</replaceable> is a type constructor, then
1967 its definition will be printed; if
1968 <replaceable>name</replaceable> is a function, then its type
1969 will be printed. If <replaceable>name</replaceable> has
1970 been loaded from a source file, then GHCi will also display
1971 the location of its definition in the source.</para>
1972 <para>For types and classes, GHCi also summarises instances that
1973 mention them. To avoid showing irrelevant information, an instance
1974 is shown only if (a) its head mentions <replaceable>name</replaceable>,
1975 and (b) all the other things mentioned in the instance
1976 are in scope (either qualified or otherwise) as a result of
1977 a <literal>:load</literal> or <literal>:module</literal> commands. </para>
1983 <literal>:kind</literal> <replaceable>type</replaceable>
1984 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1987 <para>Infers and prints the kind of
1988 <replaceable>type</replaceable>. The latter can be an arbitrary
1989 type expression, including a partial application of a type constructor,
1990 such as <literal>Either Int</literal>.</para>
1996 <literal>:load</literal> <replaceable>module</replaceable> ...
1997 <indexterm><primary><literal>:load</literal></primary></indexterm>
2000 <para>Recursively loads the specified
2001 <replaceable>module</replaceable>s, and all the modules they
2002 depend on. Here, each <replaceable>module</replaceable>
2003 must be a module name or filename, but may not be the name
2004 of a module in a package.</para>
2006 <para>All previously loaded modules, except package modules,
2007 are forgotten. The new set of modules is known as the
2008 <firstterm>target set</firstterm>. Note that
2009 <literal>:load</literal> can be used without any arguments
2010 to unload all the currently loaded modules and
2013 <para>After a <literal>:load</literal> command, the current
2014 context is set to:</para>
2018 <para><replaceable>module</replaceable>, if it was loaded
2019 successfully, or</para>
2022 <para>the most recently successfully loaded module, if
2023 any other modules were loaded as a result of the current
2024 <literal>:load</literal>, or</para>
2027 <para><literal>Prelude</literal> otherwise.</para>
2035 <literal>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
2036 <indexterm><primary><literal>:main</literal></primary></indexterm>
2040 When a program is compiled and executed, it can use the
2041 <literal>getArgs</literal> function to access the
2042 command-line arguments.
2043 However, we cannot simply pass the arguments to the
2044 <literal>main</literal> function while we are testing in ghci,
2045 as the <literal>main</literal> function doesn't take its
2050 Instead, we can use the <literal>:main</literal> command.
2051 This runs whatever <literal>main</literal> is in scope, with
2052 any arguments being treated the same as command-line arguments,
2057 Prelude> let main = System.Environment.getArgs >>= print
2058 Prelude> :main foo bar
2067 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
2068 <indexterm><primary><literal>:module</literal></primary></indexterm>
2071 <literal>import <replaceable>mod</replaceable></literal>
2074 <para>Sets or modifies the current context for statements
2075 typed at the prompt. The form <literal>import
2076 <replaceable>mod</replaceable></literal> is equivalent to
2077 <literal>:module +<replaceable>mod</replaceable></literal>.
2078 See <xref linkend="ghci-scope"/> for
2079 more details.</para>
2085 <literal>:print </literal> <replaceable>names</replaceable> ...
2086 <indexterm><primary><literal>:print</literal></primary></indexterm>
2089 <para>Prints a value without forcing its evaluation.
2090 <literal>:print</literal> may be used on values whose types are
2091 unknown or partially known, which might be the case for local
2092 variables with polymorphic types at a breakpoint. While inspecting
2093 the runtime value, <literal>:print</literal> attempts to
2094 reconstruct the type of the value, and will elaborate the type in
2095 GHCi's environment if possible. If any unevaluated components
2096 (thunks) are encountered, then <literal>:print</literal> binds
2097 a fresh variable with a name beginning with <literal>_t</literal>
2098 to each thunk. See <xref linkend="breakpoints" /> for more
2099 information. See also the <literal>:sprint</literal> command,
2100 which works like <literal>:print</literal> but does not bind new
2107 <literal>:quit</literal>
2108 <indexterm><primary><literal>:quit</literal></primary></indexterm>
2111 <para>Quits GHCi. You can also quit by typing control-D
2112 at the prompt.</para>
2118 <literal>:reload</literal>
2119 <indexterm><primary><literal>:reload</literal></primary></indexterm>
2122 <para>Attempts to reload the current target set (see
2123 <literal>:load</literal>) if any of the modules in the set,
2124 or any dependent module, has changed. Note that this may
2125 entail loading new modules, or dropping modules which are no
2126 longer indirectly required by the target.</para>
2132 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
2133 <indexterm><primary><literal>:set</literal></primary></indexterm>
2136 <para>Sets various options. See <xref linkend="ghci-set"/>
2137 for a list of available options. The
2138 <literal>:set</literal> command by itself shows which
2139 options are currently set.</para>
2145 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
2146 <indexterm><primary><literal>:set args</literal></primary></indexterm>
2149 <para>Sets the list of arguments which are returned when the
2150 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
2151 </indexterm>.</para>
2157 <literal>:set</literal> <literal>editor</literal> <replaceable>cmd</replaceable>
2160 <para>Sets the command used by <literal>:edit</literal> to
2161 <replaceable>cmd</replaceable>.</para>
2167 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
2168 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
2171 <para>Sets the string to be returned when the program calls
2172 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
2173 </indexterm>.</para>
2179 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
2182 <para>Sets the string to be used as the prompt in GHCi.
2183 Inside <replaceable>prompt</replaceable>, the sequence
2184 <literal>%s</literal> is replaced by the names of the
2185 modules currently in scope, and <literal>%%</literal> is
2186 replaced by <literal>%</literal>.</para>
2192 <literal>:set</literal> <literal>stop</literal>
2193 [<replaceable>num</replaceable>] <replaceable>cmd</replaceable>
2196 <para>Set a command to be executed when a breakpoint is hit, or a new
2197 item in the history is selected. The most common use of
2198 <literal>:set stop</literal> is to display the source code at the
2199 current location, e.g. <literal>:set stop :list</literal>.</para>
2201 <para>If a number is given before the command, then the commands are
2202 run when the specified breakpoint (only) is hit. This can be quite
2203 useful: for example, <literal>:set stop 1 :continue</literal>
2204 effectively disables breakpoint 1, by running
2205 <literal>:continue</literal> whenever it is hit (although GHCi will
2206 still emit a message to say the breakpoint was hit). What's more,
2207 with cunning use of <literal>:def</literal> and
2208 <literal>:cmd</literal> you can use <literal>:set stop</literal> to
2209 implement conditional breakpoints:</para>
2211 *Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
2212 *Main> :set stop 0 :cond (x < 3)
2214 <para>Ignoring breakpoints for a specified number of iterations is
2215 also possible using similar techniques.</para>
2221 <literal>:show bindings</literal>
2222 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
2225 <para>Show the bindings made at the prompt and their
2232 <literal>:show breaks</literal>
2233 <indexterm><primary><literal>:show breaks</literal></primary></indexterm>
2236 <para>List the active breakpoints.</para>
2242 <literal>:show context</literal>
2243 <indexterm><primary><literal>:show context</literal></primary></indexterm>
2246 <para>List the active evaluations that are stopped at breakpoints.</para>
2252 <literal>:show modules</literal>
2253 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
2256 <para>Show the list of modules currently loaded.</para>
2262 <literal>:show [args|prog|prompt|editor|stop]</literal>
2263 <indexterm><primary><literal>:show</literal></primary></indexterm>
2266 <para>Displays the specified setting (see
2267 <literal>:set</literal>).</para>
2273 <literal>:sprint</literal>
2274 <indexterm><primary><literal>:sprint</literal></primary></indexterm>
2277 <para>Prints a value without forcing its evaluation.
2278 <literal>:sprint</literal> is similar to <literal>:print</literal>,
2279 with the difference that unevaluated subterms are not bound to new
2280 variables, they are simply denoted by ‘_’.</para>
2286 <literal>:step [<replaceable>expr</replaceable>]</literal>
2287 <indexterm><primary><literal>:step</literal></primary></indexterm>
2290 <para>Single-step from the last breakpoint. With an expression
2291 argument, begins evaluation of the expression with a
2298 <literal>:trace [<replaceable>expr</replaceable>]</literal>
2299 <indexterm><primary><literal>:trace</literal></primary></indexterm>
2302 <para>Evaluates the given expression (or from the last breakpoint if
2303 no expression is given), and additionally logs the evaluation
2304 steps for later inspection using <literal>:history</literal>. See
2305 <xref linkend="tracing" />.</para>
2311 <literal>:type</literal> <replaceable>expression</replaceable>
2312 <indexterm><primary><literal>:type</literal></primary></indexterm>
2315 <para>Infers and prints the type of
2316 <replaceable>expression</replaceable>, including explicit
2317 forall quantifiers for polymorphic types. The monomorphism
2318 restriction is <emphasis>not</emphasis> applied to the
2319 expression during type inference.</para>
2325 <literal>:undef</literal> <replaceable>name</replaceable>
2326 <indexterm><primary><literal>:undef</literal></primary></indexterm>
2329 <para>Undefines the user-defined command
2330 <replaceable>name</replaceable> (see <literal>:def</literal>
2337 <literal>:unset</literal> <replaceable>option</replaceable>...
2338 <indexterm><primary><literal>:unset</literal></primary></indexterm>
2341 <para>Unsets certain options. See <xref linkend="ghci-set"/>
2342 for a list of available options.</para>
2348 <literal>:!</literal> <replaceable>command</replaceable>...
2349 <indexterm><primary><literal>:!</literal></primary></indexterm>
2350 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
2353 <para>Executes the shell command
2354 <replaceable>command</replaceable>.</para>
2361 <sect1 id="ghci-set">
2362 <title>The <literal>:set</literal> command</title>
2363 <indexterm><primary><literal>:set</literal></primary></indexterm>
2365 <para>The <literal>:set</literal> command sets two types of
2366 options: GHCi options, which begin with
2367 ‘<literal>+</literal>’, and “command-line”
2368 options, which begin with ‘-’. </para>
2370 <para>NOTE: at the moment, the <literal>:set</literal> command
2371 doesn't support any kind of quoting in its arguments: quotes will
2372 not be removed and cannot be used to group words together. For
2373 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
2377 <title>GHCi options</title>
2378 <indexterm><primary>options</primary><secondary>GHCi</secondary>
2381 <para>GHCi options may be set using <literal>:set</literal> and
2382 unset using <literal>:unset</literal>.</para>
2384 <para>The available GHCi options are:</para>
2389 <literal>+r</literal>
2390 <indexterm><primary><literal>+r</literal></primary></indexterm>
2391 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
2392 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
2395 <para>Normally, any evaluation of top-level expressions
2396 (otherwise known as CAFs or Constant Applicative Forms) in
2397 loaded modules is retained between evaluations. Turning
2398 on <literal>+r</literal> causes all evaluation of
2399 top-level expressions to be discarded after each
2400 evaluation (they are still retained
2401 <emphasis>during</emphasis> a single evaluation).</para>
2403 <para>This option may help if the evaluated top-level
2404 expressions are consuming large amounts of space, or if
2405 you need repeatable performance measurements.</para>
2411 <literal>+s</literal>
2412 <indexterm><primary><literal>+s</literal></primary></indexterm>
2415 <para>Display some stats after evaluating each expression,
2416 including the elapsed time and number of bytes allocated.
2417 NOTE: the allocation figure is only accurate to the size
2418 of the storage manager's allocation area, because it is
2419 calculated at every GC. Hence, you might see values of
2420 zero if no GC has occurred.</para>
2426 <literal>+t</literal>
2427 <indexterm><primary><literal>+t</literal></primary></indexterm>
2430 <para>Display the type of each variable bound after a
2431 statement is entered at the prompt. If the statement is a
2432 single expression, then the only variable binding will be
2434 ‘<literal>it</literal>’.</para>
2440 <sect2 id="ghci-cmd-line-options">
2441 <title>Setting GHC command-line options in GHCi</title>
2443 <para>Normal GHC command-line options may also be set using
2444 <literal>:set</literal>. For example, to turn on
2445 <option>-fglasgow-exts</option>, you would say:</para>
2448 Prelude> :set -fglasgow-exts
2451 <para>Any GHC command-line option that is designated as
2452 <firstterm>dynamic</firstterm> (see the table in <xref
2453 linkend="flag-reference"/>), may be set using
2454 <literal>:set</literal>. To unset an option, you can set the
2455 reverse option:</para>
2456 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
2459 Prelude> :set -fno-glasgow-exts
2462 <para><xref linkend="flag-reference"/> lists the reverse for each
2463 option where applicable.</para>
2465 <para>Certain static options (<option>-package</option>,
2466 <option>-I</option>, <option>-i</option>, and
2467 <option>-l</option> in particular) will also work, but some may
2468 not take effect until the next reload.</para>
2469 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
2472 <sect1 id="ghci-dot-files">
2473 <title>The <filename>.ghci</filename> file</title>
2474 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
2476 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
2479 <para>When it starts, unless the <literal>-ignore-dot-ghci</literal>
2480 flag is given, GHCi reads and executes commands from
2481 <filename>./.ghci</filename>, followed by
2482 <filename>$HOME/.ghci</filename>.</para>
2484 <para>The <filename>.ghci</filename> in your home directory is
2485 most useful for turning on favourite options (eg. <literal>:set
2486 +s</literal>), and defining useful macros. Placing a
2487 <filename>.ghci</filename> file in a directory with a Haskell
2488 project is a useful way to set certain project-wide options so you
2489 don't have to type them everytime you start GHCi: eg. if your
2490 project uses GHC extensions and CPP, and has source files in three
2491 subdirectories A, B and C, you might put the following lines in
2492 <filename>.ghci</filename>:</para>
2495 :set -fglasgow-exts -cpp
2499 <para>(Note that strictly speaking the <option>-i</option> flag is
2500 a static one, but in fact it works to set it using
2501 <literal>:set</literal> like this. The changes won't take effect
2502 until the next <literal>:load</literal>, though.)</para>
2504 <para>Two command-line options control whether the
2505 <filename>.ghci</filename> files are read:</para>
2510 <option>-ignore-dot-ghci</option>
2511 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
2514 <para>Don't read either <filename>./.ghci</filename> or
2515 <filename>$HOME/.ghci</filename> when starting up.</para>
2520 <option>-read-dot-ghci</option>
2521 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
2524 <para>Read <filename>.ghci</filename> and
2525 <filename>$HOME/.ghci</filename>. This is normally the
2526 default, but the <option>-read-dot-ghci</option> option may
2527 be used to override a previous
2528 <option>-ignore-dot-ghci</option> option.</para>
2535 <sect1 id="ghci-obj">
2536 <title>Compiling to object code inside GHCi</title>
2538 <para>By default, GHCi compiles Haskell source code into byte-code
2539 that is interpreted by the runtime system. GHCi can also compile
2540 Haskell code to object code: to turn on this feature, use the
2541 <option>-fobject-code</option> flag either on the command line or
2542 with <literal>:set</literal> (the option
2543 <option>-fbyte-code</option> restores byte-code compilation
2544 again). Compiling to object code takes longer, but typically the
2545 code will execute 10-20 times faster than byte-code.</para>
2547 <para>Compiling to object code inside GHCi is particularly useful
2548 if you are developing a compiled application, because the
2549 <literal>:reload</literal> command typically runs much faster than
2550 restarting GHC with <option>--make</option> from the command-line,
2551 because all the interface files are already cached in
2554 <para>There are disadvantages to compiling to object-code: you
2555 can't set breakpoints in object-code modules, for example. Only
2556 the exports of an object-code module will be visible in GHCi,
2557 rather than all top-level bindings as in interpreted
2561 <sect1 id="ghci-faq">
2562 <title>FAQ and Things To Watch Out For</title>
2566 <term>The interpreter can't load modules with foreign export
2567 declarations!</term>
2569 <para>Unfortunately not. We haven't implemented it yet.
2570 Please compile any offending modules by hand before loading
2571 them into GHCi.</para>
2577 <literal>-O</literal> doesn't work with GHCi!
2578 <indexterm><primary><option>-O</option></primary></indexterm>
2581 <para>For technical reasons, the bytecode compiler doesn't
2582 interact well with one of the optimisation passes, so we
2583 have disabled optimisation when using the interpreter. This
2584 isn't a great loss: you'll get a much bigger win by
2585 compiling the bits of your code that need to go fast, rather
2586 than interpreting them with optimisation turned on.</para>
2591 <term>Unboxed tuples don't work with GHCi</term>
2593 <para>That's right. You can always compile a module that
2594 uses unboxed tuples and load it into GHCi, however.
2595 (Incidentally the previous point, namely that
2596 <literal>-O</literal> is incompatible with GHCi, is because
2597 the bytecode compiler can't deal with unboxed
2603 <term>Concurrent threads don't carry on running when GHCi is
2604 waiting for input.</term>
2606 <para>This should work, as long as your GHCi was built with
2607 the <option>-threaded</option> switch, which is the default.
2608 Consult whoever supplied your GHCi installation.</para>
2613 <term>After using <literal>getContents</literal>, I can't use
2614 <literal>stdin</literal> again until I do
2615 <literal>:load</literal> or <literal>:reload</literal>.</term>
2618 <para>This is the defined behaviour of
2619 <literal>getContents</literal>: it puts the stdin Handle in
2620 a state known as <firstterm>semi-closed</firstterm>, wherein
2621 any further I/O operations on it are forbidden. Because I/O
2622 state is retained between computations, the semi-closed
2623 state persists until the next <literal>:load</literal> or
2624 <literal>:reload</literal> command.</para>
2626 <para>You can make <literal>stdin</literal> reset itself
2627 after every evaluation by giving GHCi the command
2628 <literal>:set +r</literal>. This works because
2629 <literal>stdin</literal> is just a top-level expression that
2630 can be reverted to its unevaluated state in the same way as
2631 any other top-level expression (CAF).</para>
2636 <term>I can't use Control-C to interrupt computations in
2637 GHCi on Windows.</term>
2639 <para>See <xref linkend="ghci-windows"/>.</para>
2644 <term>The default buffering mode is different in GHCi to GHC.</term>
2647 In GHC, the stdout handle is line-buffered by default.
2648 However, in GHCi we turn off the buffering on stdout,
2649 because this is normally what you want in an interpreter:
2650 output appears as it is generated.
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