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 Skipping D ( D.hs, D.o )
223 Compiling C ( C.hs, interpreted )
224 Compiling B ( B.hs, interpreted )
225 Compiling A ( A.hs, interpreted )
226 Ok, modules loaded: A, B, C, D.
230 <para>In the messages from the compiler, we see that it skipped D,
231 and used the object file <filename>D.o</filename>. The message
232 <literal>Skipping</literal> <replaceable>module</replaceable>
233 indicates that compilation for <replaceable>module</replaceable>
234 isn't necessary, because the source and everything it depends on
235 is unchanged since the last compilation.</para>
237 <para>At any time you can use the command
238 <literal>:show modules</literal>
239 to get a list of the modules currently loaded
245 C ( C.hs, interpreted )
246 B ( B.hs, interpreted )
247 A ( A.hs, interpreted )
250 <para>If we now modify the source of D (or pretend to: using Unix
251 command <literal>touch</literal> on the source file is handy for
252 this), the compiler will no longer be able to use the object file,
253 because it might be out of date:</para>
258 Compiling D ( D.hs, interpreted )
259 Skipping C ( C.hs, interpreted )
260 Skipping B ( B.hs, interpreted )
261 Skipping A ( A.hs, interpreted )
262 Ok, modules loaded: A, B, C, D.
266 <para>Note that module D was compiled, but in this instance
267 because its source hadn't really changed, its interface remained
268 the same, and the recompilation checker determined that A, B and C
269 didn't need to be recompiled.</para>
271 <para>So let's try compiling one of the other modules:</para>
274 *Main> :! ghc -c C.hs
276 Compiling D ( D.hs, interpreted )
277 Compiling C ( C.hs, interpreted )
278 Compiling B ( B.hs, interpreted )
279 Compiling A ( A.hs, interpreted )
280 Ok, modules loaded: A, B, C, D.
283 <para>We didn't get the compiled version of C! What happened?
284 Well, in GHCi a compiled module may only depend on other compiled
285 modules, and in this case C depends on D, which doesn't have an
286 object file, so GHCi also rejected C's object file. Ok, so let's
287 also compile D:</para>
290 *Main> :! ghc -c D.hs
292 Ok, modules loaded: A, B, C, D.
295 <para>Nothing happened! Here's another lesson: newly compiled
296 modules aren't picked up by <literal>:reload</literal>, only
297 <literal>:load</literal>:</para>
301 Skipping D ( D.hs, D.o )
302 Skipping C ( C.hs, C.o )
303 Compiling B ( B.hs, interpreted )
304 Compiling A ( A.hs, interpreted )
305 Ok, modules loaded: A, B, C, D.
308 <para>HINT: since GHCi will only use a compiled object file if it
309 can be sure that the compiled version is up-to-date, a good technique
310 when working on a large program is to occasionally run
311 <literal>ghc ––make</literal> to compile the whole project (say
312 before you go for lunch :-), then continue working in the
313 interpreter. As you modify code, the new modules will be
314 interpreted, but the rest of the project will remain
319 <sect1 id="interactive-evaluation">
320 <title>Interactive evaluation at the prompt</title>
322 <para>When you type an expression at the prompt, GHCi immediately
323 evaluates and prints the result:
325 Prelude> reverse "hello"
332 <sect2><title>I/O actions at the prompt</title>
334 <para>GHCi does more than simple expression evaluation at the prompt.
335 If you type something of type <literal>IO a</literal> for some
336 <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
337 as an IO-computation.
341 Prelude> putStrLn "hello"
344 Furthermore, GHCi will print the result of the I/O action if (and only
347 <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
348 <listitem><para>The result type is not
349 <literal>()</literal>.</para></listitem>
351 For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
353 Prelude> putStrLn "hello"
355 Prelude> do { putStrLn "hello"; return "yes" }
361 <sect2 id="ghci-stmts">
362 <title>Using <literal>do-</literal>notation at the prompt</title>
363 <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
364 <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
366 <para>GHCi actually accepts <firstterm>statements</firstterm>
367 rather than just expressions at the prompt. This means you can
368 bind values and functions to names, and use them in future
369 expressions or statements.</para>
371 <para>The syntax of a statement accepted at the GHCi prompt is
372 exactly the same as the syntax of a statement in a Haskell
373 <literal>do</literal> expression. However, there's no monad
374 overloading here: statements typed at the prompt must be in the
375 <literal>IO</literal> monad.
377 Prelude> x <- return 42
383 The statement <literal>x <- return 42</literal> means
384 “execute <literal>return 42</literal> in the
385 <literal>IO</literal> monad, and bind the result to
386 <literal>x</literal>”. We can then use
387 <literal>x</literal> in future statements, for example to print
388 it as we did above.</para>
390 <para>GHCi will print the result of a statement if and only if:
393 <para>The statement is not a binding, or it is a monadic binding
394 (<literal>p <- e</literal>) that binds exactly one
398 <para>The variable's type is not polymorphic, is not
399 <literal>()</literal>, and is an instance of
400 <literal>Show</literal></para>
403 The automatic printing of binding results can be supressed with
404 <option>:set -fno-print-bind-result</option> (this does not
405 supress printing the result of non-binding statements).
406 <indexterm><primary><option>-fno-print-bind-result</option></primary></indexterm><indexterm><primary><option>-fprint-bind-result</option></primary></indexterm>.
407 You might want to do this to prevent the result of binding
408 statements from being fully evaluated by the act of printing
409 them, for example.</para>
411 <para>Of course, you can also bind normal non-IO expressions
412 using the <literal>let</literal>-statement:</para>
419 <para>Another important difference between the two types of binding
420 is that the monadic bind (<literal>p <- e</literal>) is
421 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
422 whereas with the <literal>let</literal> form, the expression
423 isn't evaluated immediately:</para>
425 Prelude> let x = error "help!"
431 <para>Note that <literal>let</literal> bindings do not automatically
432 print the value bound, unlike monadic bindings.</para>
434 <para>Any exceptions raised during the evaluation or execution
435 of the statement are caught and printed by the GHCi command line
436 interface (for more information on exceptions, see the module
437 <literal>Control.Exception</literal> in the libraries
438 documentation).</para>
440 <para>Every new binding shadows any existing bindings of the
441 same name, including entities that are in scope in the current
442 module context.</para>
444 <para>WARNING: temporary bindings introduced at the prompt only
445 last until the next <literal>:load</literal> or
446 <literal>:reload</literal> command, at which time they will be
447 simply lost. However, they do survive a change of context with
448 <literal>:module</literal>: the temporary bindings just move to
449 the new location.</para>
451 <para>HINT: To get a list of the bindings currently in scope, use the
452 <literal>:show bindings</literal> command:</para>
455 Prelude> :show bindings
459 <para>HINT: if you turn on the <literal>+t</literal> option,
460 GHCi will show the type of each variable bound by a statement.
462 <indexterm><primary><literal>+t</literal></primary></indexterm>
465 Prelude> let (x:xs) = [1..]
472 <sect2 id="ghci-scope">
473 <title>What's really in scope at the prompt?</title>
475 <para>When you type an expression at the prompt, what
476 identifiers and types are in scope? GHCi provides a flexible
477 way to control exactly how the context for an expression is
478 constructed. Let's start with the simple cases; when you start
479 GHCi the prompt looks like this:</para>
481 <screen>Prelude></screen>
483 <para>Which indicates that everything from the module
484 <literal>Prelude</literal> is currently in scope. If we now
485 load a file into GHCi, the prompt will change:</para>
488 Prelude> :load Main.hs
489 Compiling Main ( Main.hs, interpreted )
493 <para>The new prompt is <literal>*Main</literal>, which
494 indicates that we are typing expressions in the context of the
495 top-level of the <literal>Main</literal> module. Everything
496 that is in scope at the top-level in the module
497 <literal>Main</literal> we just loaded is also in scope at the
498 prompt (probably including <literal>Prelude</literal>, as long
499 as <literal>Main</literal> doesn't explicitly hide it).</para>
502 <literal>*<replaceable>module</replaceable></literal> indicates
503 that it is the full top-level scope of
504 <replaceable>module</replaceable> that is contributing to the
505 scope for expressions typed at the prompt. Without the
506 <literal>*</literal>, just the exports of the module are
509 <para>We're not limited to a single module: GHCi can combine
510 scopes from multiple modules, in any mixture of
511 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
512 combines the scopes from all of these modules to form the scope
513 that is in effect at the prompt. For technical reasons, GHCi
514 can only support the <literal>*</literal>-form for modules which
515 are interpreted, so compiled modules and package modules can
516 only contribute their exports to the current scope.</para>
518 <para>The scope is manipulated using the
519 <literal>:module</literal> command. For example, if the current
520 scope is <literal>Prelude</literal>, then we can bring into
521 scope the exports from the module <literal>IO</literal> like
526 Prelude IO> hPutStrLn stdout "hello\n"
531 <para>(Note: you can use <literal>import M</literal> as an
532 alternative to <literal>:module +M</literal>, and
533 <literal>:module</literal> can also be shortened to
534 <literal>:m</literal>). The full syntax of the
535 <literal>:module</literal> command is:</para>
538 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
541 <para>Using the <literal>+</literal> form of the
542 <literal>module</literal> commands adds modules to the current
543 scope, and <literal>-</literal> removes them. Without either
544 <literal>+</literal> or <literal>-</literal>, the current scope
545 is replaced by the set of modules specified. Note that if you
546 use this form and leave out <literal>Prelude</literal>, GHCi
547 will assume that you really wanted the
548 <literal>Prelude</literal> and add it in for you (if you don't
549 want the <literal>Prelude</literal>, then ask to remove it with
550 <literal>:m -Prelude</literal>).</para>
552 <para>The scope is automatically set after a
553 <literal>:load</literal> command, to the most recently loaded
554 "target" module, in a <literal>*</literal>-form if possible.
555 For example, if you say <literal>:load foo.hs bar.hs</literal>
556 and <filename>bar.hs</filename> contains module
557 <literal>Bar</literal>, then the scope will be set to
558 <literal>*Bar</literal> if <literal>Bar</literal> is
559 interpreted, or if <literal>Bar</literal> is compiled it will be
560 set to <literal>Prelude Bar</literal> (GHCi automatically adds
561 <literal>Prelude</literal> if it isn't present and there aren't
562 any <literal>*</literal>-form modules).</para>
564 <para>With multiple modules in scope, especially multiple
565 <literal>*</literal>-form modules, it is likely that name
566 clashes will occur. Haskell specifies that name clashes are
567 only reported when an ambiguous identifier is used, and GHCi
568 behaves in the same way for expressions typed at the
572 Hint: GHCi will tab-complete names that are in scope; for
573 example, if you run GHCi and type <literal>J<tab></literal>
574 then GHCi will expand it to <literal>Just </literal>.
578 <title>Qualified names</title>
580 <para>To make life slightly easier, the GHCi prompt also
581 behaves as if there is an implicit <literal>import
582 qualified</literal> declaration for every module in every
583 package, and every module currently loaded into GHCi.</para>
587 <title>The <literal>:main</literal> command</title>
590 When a program is compiled and executed, it can use the
591 <literal>getArgs</literal> function to access the
592 command-line arguments.
593 However, we cannot simply pass the arguments to the
594 <literal>main</literal> function while we are testing in ghci,
595 as the <literal>main</literal> function doesn't take its
600 Instead, we can use the <literal>:main</literal> command.
601 This runs whatever <literal>main</literal> is in scope, with
602 any arguments being treated the same as command-line arguments,
607 Prelude> let main = System.Environment.getArgs >>= print
608 Prelude> :main foo bar
617 <title>The <literal>it</literal> variable</title>
618 <indexterm><primary><literal>it</literal></primary>
621 <para>Whenever an expression (or a non-binding statement, to be
622 precise) is typed at the prompt, GHCi implicitly binds its value
623 to the variable <literal>it</literal>. For example:</para>
630 <para>What actually happens is that GHCi typechecks the
631 expression, and if it doesn't have an <literal>IO</literal> type,
632 then it transforms it as follows: an expression
633 <replaceable>e</replaceable> turns into
635 let it = <replaceable>e</replaceable>;
638 which is then run as an IO-action.</para>
640 <para>Hence, the original expression must have a type which is an
641 instance of the <literal>Show</literal> class, or GHCi will
647 <interactive>:1:0:
648 No instance for (Show (a -> a))
649 arising from use of `print' at <interactive>:1:0-1
650 Possible fix: add an instance declaration for (Show (a -> a))
651 In the expression: print it
652 In a 'do' expression: print it
655 <para>The error message contains some clues as to the
656 transformation happening internally.</para>
658 <para>If the expression was instead of type <literal>IO a</literal> for
659 some <literal>a</literal>, then <literal>it</literal> will be
660 bound to the result of the <literal>IO</literal> computation,
661 which is of type <literal>a</literal>. eg.:</para>
663 Prelude> Time.getClockTime
664 Wed Mar 14 12:23:13 GMT 2001
666 Wed Mar 14 12:23:13 GMT 2001
669 <para>The corresponding translation for an IO-typed
670 <replaceable>e</replaceable> is
672 it <- <replaceable>e</replaceable>
676 <para>Note that <literal>it</literal> is shadowed by the new
677 value each time you evaluate a new expression, and the old value
678 of <literal>it</literal> is lost.</para>
682 <sect2 id="extended-default-rules">
683 <title>Type defaulting in GHCi</title>
684 <indexterm><primary>Type default</primary></indexterm>
685 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
687 Consider this GHCi session:
691 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
692 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
693 on the type <literal>a</literal>. For example:
695 ghci> (reverse []) :: String
697 ghci> (reverse []) :: [Int]
700 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
701 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
702 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
703 a)</literal> for each type variable <literal>a</literal>, and defaults the
708 The type variable <literal>a</literal> appears in no
714 All the classes <literal>Ci</literal> are standard.
719 At least one of the classes <literal>Ci</literal> is
724 At the GHCi prompt, or with GHC if the
725 <literal>-fextended-default-rules</literal> flag is given,
726 the following additional differences apply:
730 Rule 2 above is relaxed thus:
731 <emphasis>All</emphasis> of the classes
732 <literal>Ci</literal> are single-parameter type classes.
737 Rule 3 above is relaxed this:
738 At least one of the classes <literal>Ci</literal> is
739 numeric, <emphasis>or is <literal>Show</literal>,
740 <literal>Eq</literal>, or
741 <literal>Ord</literal></emphasis>.
746 The unit type <literal>()</literal> is added to the
747 start of the standard list of types which are tried when
748 doing type defaulting.
752 The last point means that, for example, this program:
759 def :: (Num a, Enum a) => a
762 prints <literal>()</literal> rather than <literal>0</literal> as the
763 type is defaulted to <literal>()</literal> rather than
764 <literal>Integer</literal>.
767 The motivation for the change is that it means <literal>IO a</literal>
768 actions default to <literal>IO ()</literal>, which in turn means that
769 ghci won't try to print a result when running them. This is
770 particularly important for <literal>printf</literal>, which has an
771 instance that returns <literal>IO a</literal>.
772 However, it is only able to return
773 <literal>undefined</literal>
774 (the reason for the instance having this type is to not require
775 extensions to the class system), so if the type defaults to
776 <literal>Integer</literal> then ghci gives an error when running a
782 <sect1 id="ghci-debugger">
783 <title>The GHCi Debugger</title>
784 <indexterm><primary>debugger</primary><secondary>in GHCi</secondary>
787 <para>GHCi contains a simple imperative-style debugger in which you can
788 stop a running computation in order to examine the values of
789 variables. The debugger is integrated into GHCi, and is turned on by
790 default: no flags are required to enable the debugging facilities. There
791 is one major restriction: breakpoints and single-stepping are only
792 available in <emphasis>interpreted</emphasis> modules; compiled code is
793 invisible to the debugger.</para>
795 <para>The debugger provides the following:
798 <para>The abilty to set a <firstterm>breakpoint</firstterm> on a
799 function definition or expression in the program. When the function
800 is called, or the expression evaluated, GHCi suspends
801 execution and returns to the prompt, where you can inspect the
802 values of local variables before continuing with the
806 <para>Execution can be <firstterm>single-stepped</firstterm>: the
807 evaluator will suspend execution approximately after every
808 reduction, allowing local variables to be inspected. This is
809 equivalent to setting a breakpoint at every point in the
813 <para>Execution can take place in <firstterm>tracing
814 mode</firstterm>, in which the evaluator remembers each
815 evaluation step as it happens, but doesn't suspend execution until
816 an actual breakpoint is reached. When this happens, the history of
817 evaluation steps can be inspected.</para>
820 <para>Exceptions (e.g. pattern matching failure and
821 <literal>error</literal>) can be treated as breakpoints, to help
822 locate the source of an exception in the program.</para>
827 <para>There is currently no support for obtaining a “stack
828 trace”, but the tracing and history features provide a useful
829 second-best, which will often be enough to establish the context of an
832 <sect2 id="breakpoints">
833 <title>Breakpoints and inspecting variables</title>
835 <para>Let's use quicksort as a running example. Here's the code:</para>
839 qsort (a:as) = qsort left ++ [a] ++ qsort right
840 where (left,right) = (filter (<=a) as, filter (>a) as)
842 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
845 <para>First, load the module into GHCi:</para>
849 [1 of 1] Compiling Main ( qsort.hs, interpreted )
850 Ok, modules loaded: Main.
854 <para>Now, let's set a breakpoint on the right-hand-side of the second
855 equation of qsort:</para>
859 Breakpoint 0 activated at qsort.hs:2:15-46
863 <para>The command <literal>:break 2</literal> sets a breakpoint on line
864 2 of the most recently-loaded module, in this case
865 <literal>qsort.hs</literal>. Specifically, it picks the
866 leftmost complete subexpression on that line on which to set the
867 breakpoint, which in this case is the expression
868 <literal>(qsort left ++ [a] ++ qsort right)</literal>.</para>
870 <para>Now, we run the program:</para>
874 Stopped at qsort.hs:2:15-46
879 [qsort.hs:2:15-46] *Main>
882 <para>Execution has stopped at the breakpoint. The prompt has changed to
883 indicate that we are currently stopped at a breakpoint, and the location:
884 <literal>[qsort.hs:2:15-46]</literal>. To further clarify the
885 location, we can use the <literal>:list</literal> command:</para>
888 [qsort.hs:2:15-46] *Main> :list
890 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
891 3 where (left,right) = (filter (<=a) as, filter (>a) as)
894 <para>The <literal>:list</literal> command lists the source code around
895 the current breakpoint. If your output device supports it, then GHCi
896 will highlight the active subexpression in bold.</para>
898 <para>GHCi has provided bindings for the free variables<footnote><para>We
899 originally provided bindings for all variables in scope, rather
901 the free variables of the expression, but found that this affected
902 performance considerably, hence the current restriction to just the
903 free variables.</para>
904 </footnote> of the expression
906 breakpoint was placed (<literal>a</literal>, <literal>left</literal>,
907 <literal>right</literal>), and additionally a binding for the result of
908 the expression (<literal>_result</literal>). These variables are just
909 like other variables that you might define in GHCi; you
910 can use them in expressions that you type at the prompt, you can ask
911 for their types with <literal>:type</literal>, and so on. There is one
912 important difference though: these variables may only have partial
913 types. For example, if we try to display the value of
914 <literal>left</literal>:</para>
917 [qsort.hs:2:15-46] *Main> left
919 <interactive>:1:0:
920 Ambiguous type variable `a' in the constraint:
921 `Show a' arising from a use of `print' at <interactive>:1:0-3
922 Cannot resolve unknown runtime types: a
923 Use :print or :force to determine these types
926 <para>This is because <literal>qsort</literal> is a polymorphic function,
927 and because GHCi does not carry type information at runtime, it cannot
928 determine the runtime types of free variables that involve type
929 variables. Hence, when you ask to display <literal>left</literal> at
930 the prompt, GHCi can't figure out which instance of
931 <literal>Show</literal> to use, so it emits the type error above.</para>
933 <para>Fortunately, the debugger includes a generic printing command,
934 <literal>:print</literal>, which can inspect the actual runtime value of a
935 variable and attempt to reconstruct its type. If we try it on
936 <literal>left</literal>:</para>
939 [qsort.hs:2:15-46] *Main> :print left
943 <para>This isn't particularly enlightening. What happened is that
944 <literal>left</literal> is bound to an unevaluated computation (a
945 suspension, or <firstterm>thunk</firstterm>), and
946 <literal>:print</literal> does not force any evaluation. The idea is
947 that <literal>:print</literal> can be used to inspect values at a
948 breakpoint without any unfortunate side effects. It won't force any
949 evaluation, which could cause the program to give a different answer
950 than it would normally, and hence it won't cause any exceptions to be
951 raised, infinite loops, or further breakpoints to be triggered (see
952 <xref linkend="nested-breakpoints" />).
953 Rather than forcing thunks, <literal>:print</literal>
954 binds each thunk to a fresh variable beginning with an
955 underscore, in this case
956 <literal>_t1</literal>.</para>
958 <para>If we aren't concerned about preserving the evaluatedness of a
959 variable, we can use <literal>:force</literal> instead of
960 <literal>:print</literal>. The <literal>:force</literal> command
961 behaves exactly like <literal>:print</literal>, except that it forces
962 the evaluation of any thunks it encounters:</para>
965 [qsort.hs:2:15-46] *Main> :force left
969 <para>Now, since <literal>:force</literal> has inspected the runtime
970 value of <literal>left</literal>, it has reconstructed its type. We
971 can see the results of this type reconstruction:</para>
974 [qsort.hs:2:15-46] *Main> :show bindings
982 <para>Not only do we now know the type of <literal>left</literal>, but
983 all the other partial types have also been resolved. So we can ask
984 for the value of <literal>a</literal>, for example:</para>
987 [qsort.hs:2:15-46] *Main> a
991 <para>You might find it useful to use Haskell's
992 <literal>seq</literal> function to evaluate individual thunks rather
993 than evaluating the whole expression with <literal>:force</literal>.
997 [qsort.hs:2:15-46] *Main> :print right
998 right = (_t1::[Integer])
999 [qsort.hs:2:15-46] *Main> seq _t1 ()
1001 [qsort.hs:2:15-46] *Main> :print right
1002 right = 23 : (_t2::[Integer])
1005 <para>We evaluated only the <literal>_t1</literal> thunk, revealing the
1006 head of the list, and the tail is another thunk now bound to
1007 <literal>_t2</literal>. The <literal>seq</literal> function is a
1008 little inconvenient to use here, so you might want to use
1009 <literal>:def</literal> to make a nicer interface (left as an exercise
1010 for the reader!).</para>
1012 <para>Finally, we can continue the current execution:</para>
1015 [qsort.hs:2:15-46] *Main> :continue
1016 Stopped at qsort.hs:2:15-46
1021 [qsort.hs:2:15-46] *Main>
1024 <para>The execution continued at the point it previously stopped, and has
1025 now stopped at the breakpoint for a second time.</para>
1027 <sect3 id="setting-breakpoings">
1028 <title>Setting breakpoints</title>
1030 <para>Breakpoints can be set in various ways. Perhaps the easiest way to
1031 set a breakpoint is to name a top-level function:</para>
1034 :break <replaceable>identifier</replaceable>
1037 <para>Where <replaceable>identifier</replaceable> names any top-level
1038 function in an interpreted module currently loaded into GHCi (qualified
1039 names may be used). The breakpoint will be set on the body of the
1040 function, when it is fully applied but before any pattern matching has
1043 <para>Breakpoints can also be set by line (and optionally column)
1047 :break <replaceable>line</replaceable>
1048 :break <replaceable>line</replaceable> <replaceable>column</replaceable>
1049 :break <replaceable>module</replaceable> <replaceable>line</replaceable>
1050 :break <replaceable>module</replaceable> <replaceable>line</replaceable> <replaceable>column</replaceable>
1053 <para>When a breakpoint is set on a particular line, GHCi sets the
1055 leftmost subexpression that begins and ends on that line. If two
1056 complete subexpressions start at the same
1057 column, the longest one is picked. If there is no complete
1058 subexpression on the line, then the leftmost expression starting on
1059 the line is picked, and failing that the rightmost expression that
1060 partially or completely covers the line.</para>
1062 <para>When a breakpoint is set on a particular line and column, GHCi
1063 picks the smallest subexpression that encloses that location on which
1064 to set the breakpoint. Note: GHC considers the TAB character to have a
1065 width of 1, wherever it occurs; in other words it counts
1066 characters, rather than columns. This matches what some editors do,
1067 and doesn't match others. The best advice is to avoid tab
1068 characters in your source code altogether (see
1069 <option>-fwarn-tabs</option> in <xref linkend="options-sanity"
1072 <para>If the module is omitted, then the most recently-loaded module is
1075 <para>Not all subexpressions are potential breakpoint locations. Single
1076 variables are typically not considered to be breakpoint locations
1077 (unless the variable is the right-hand-side of a function definition,
1078 lambda, or case alternative). The rule of thumb is that all redexes
1079 are breakpoint locations, together with the bodies of functions,
1080 lambdas, case alternatives and binding statements. There is normally
1081 no breakpoint on a let expression, but there will always be a
1082 breakpoint on its body, because we are usually interested in inspecting
1083 the values of the variables bound by the let.</para>
1087 <title>Listing and deleting breakpoints</title>
1089 <para>The list of breakpoints currently enabled can be displayed using
1090 <literal>:show breaks</literal></para>:
1093 [0] Main qsort.hs:1:11-12
1094 [1] Main qsort.hs:2:15-46
1097 <para>To delete a breakpoint, use the <literal>:delete</literal>
1098 command with the number given in the output from <literal>:show breaks</literal>:</para>
1103 [1] Main qsort.hs:2:15-46
1106 <para>To delete all breakpoints at once, use <literal>:delete *</literal>.</para>
1111 <sect2 id="single-stepping">
1112 <title>Single-stepping</title>
1114 <para>Single-stepping is a great way to visualise the execution of your
1115 program, and it is also a useful tool for identifying the source of a
1116 bug. The concept is simple: single-stepping enables all the
1117 breakpoints in the program and executes until the next breakpoint is
1118 reached, at which point you can single-step again, or continue
1119 normally. For example:</para>
1123 Stopped at qsort.hs:5:7-47
1127 <para>The command <literal>:step
1128 <replaceable>expr</replaceable></literal> begins the evaluation of
1129 <replaceable>expr</replaceable> in single-stepping mode. If
1130 <replaceable>expr</replaceable> is ommitted, then it single-steps from
1131 the current breakpoint.</para>
1133 <para>The <literal>:list</literal> command is particularly useful when
1134 single-stepping, to see where you currently are:</para>
1137 [qsort.hs:5:7-47] *Main> :list
1139 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1141 [qsort.hs:5:7-47] *Main>
1144 <para>In fact, GHCi provides a way to run a command when a breakpoint is
1145 hit, so we can make it automatically do
1146 <literal>:list</literal>:</para>
1149 [qsort.hs:5:7-47] *Main> :set stop :list
1150 [qsort.hs:5:7-47] *Main> :step
1151 Stopped at qsort.hs:5:14-46
1152 _result :: [Integer]
1154 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1156 [qsort.hs:5:14-46] *Main>
1160 <sect2 id="nested-breakpoints">
1161 <title>Nested breakpoints</title>
1162 <para>When GHCi is stopped at a breakpoint, and an expression entered at
1163 the prompt triggers a
1164 second breakpoint, the new breakpoint becomes the “current”
1165 one, and the old one is saved on a stack. An arbitrary number of
1166 breakpoint contexts can be built up in this way. For example:</para>
1169 [qsort.hs:2:15-46] *Main> :st qsort [1,3]
1170 Stopped at qsort.hs:(1,0)-(3,55)
1172 ... [qsort.hs:(1,0)-(3,55)] *Main>
1175 <para>While stopped at the breakpoint on line 2 that we set earlier, we
1176 started a new evaluation with <literal>:step qsort [1,3]</literal>.
1177 This new evaluation stopped after one step (at the definition of
1178 <literal>qsort</literal>). The prompt has changed, now prefixed with
1179 <literal>...</literal>, to indicate that there are saved breakpoints
1180 beyond the current one. To see the stack of contexts, use
1181 <literal>:show context</literal>:</para>
1184 ... [qsort.hs:(1,0)-(3,55)] *Main> :show context
1186 Stopped at qsort.hs:2:15-46
1188 Stopped at qsort.hs:(1,0)-(3,55)
1189 ... [qsort.hs:(1,0)-(3,55)] *Main>
1192 <para>To abandon the current evaluation, use
1193 <literal>:abandon</literal>:</para>
1196 ... [qsort.hs:(1,0)-(3,55)] *Main> :abandon
1197 [qsort.hs:2:15-46] *Main> :abandon
1202 <sect2 id="ghci-debugger-result">
1203 <title>The <literal>_result</literal> variable</title>
1204 <para>When stopped at a breakpoint or single-step, GHCi binds the
1205 variable <literal>_result</literal> to the value of the currently
1206 active expression. The value of <literal>_result</literal> is
1207 presumably not available yet, because we stopped its evaluation, but it
1208 can be forced: if the type is known and showable, then just entering
1209 <literal>_result</literal> at the prompt will show it. However,
1210 there's one caveat to doing this: evaluating <literal>_result</literal>
1211 will be likely to trigger further breakpoints, starting with the
1212 breakpoint we are currently stopped at (if we stopped at a real
1213 breakpoint, rather than due to <literal>:step</literal>). So it will
1214 probably be necessary to issue a <literal>:continue</literal>
1215 immediately when evaluating <literal>_result</literal>. Alternatively,
1216 you can use <literal>:force</literal> which ignores breakpoints.</para>
1219 <sect2 id="tracing">
1220 <title>Tracing and history</title>
1222 <para>A question that we often want to ask when debugging a program is
1223 “how did I get here?”. Traditional imperative debuggers
1224 usually provide some kind of stack-tracing feature that lets you see
1225 the stack of active function calls (sometimes called the “lexical
1226 call stack”), describing a path through the code
1227 to the current location. Unfortunately this is hard to provide in
1228 Haskell, because execution proceeds on a demand-driven basis, rather
1229 than a depth-first basis as in strict languages. The
1230 “stack“ in GHC's execution engine bears little
1231 resemblance to the lexical call stack. Ideally GHCi would maintain a
1232 separate lexical call stack in addition to the dynamic call stack, and
1233 in fact this is exactly
1234 what our profiling system does (<xref linkend="profiling" />), and what
1235 some other Haskell debuggers do. For the time being, however, GHCi
1236 doesn't maintain a lexical call stack (there are some technical
1237 challenges to be overcome). Instead, we provide a way to backtrack from a
1238 breakpoint to previous evaluation steps: essentially this is like
1239 single-stepping backwards, and should in many cases provide enough
1240 information to answer the “how did I get here?”
1243 <para>To use tracing, evaluate an expression with the
1244 <literal>:trace</literal> command. For example, if we set a breakpoint
1245 on the base case of <literal>qsort</literal>:</para>
1248 *Main> :list qsort
1250 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
1251 3 where (left,right) = (filter (<=a) as, filter (>a) as)
1254 Breakpoint 1 activated at qsort.hs:1:11-12
1258 <para>and then run a small <literal>qsort</literal> with
1262 *Main> :trace qsort [3,2,1]
1263 Stopped at qsort.hs:1:11-12
1265 [qsort.hs:1:11-12] *Main>
1268 <para>We can now inspect the history of evaluation steps:</para>
1271 [qsort.hs:1:11-12] *Main> :hist
1272 -1 : qsort.hs:3:24-38
1273 -2 : qsort.hs:3:23-55
1274 -3 : qsort.hs:(1,0)-(3,55)
1275 -4 : qsort.hs:2:15-24
1276 -5 : qsort.hs:2:15-46
1277 -6 : qsort.hs:3:24-38
1278 -7 : qsort.hs:3:23-55
1279 -8 : qsort.hs:(1,0)-(3,55)
1280 -9 : qsort.hs:2:15-24
1281 -10 : qsort.hs:2:15-46
1282 -11 : qsort.hs:3:24-38
1283 -12 : qsort.hs:3:23-55
1284 -13 : qsort.hs:(1,0)-(3,55)
1285 -14 : qsort.hs:2:15-24
1286 -15 : qsort.hs:2:15-46
1287 -16 : qsort.hs:(1,0)-(3,55)
1288 <end of history>
1291 <para>To examine one of the steps in the history, use
1292 <literal>:back</literal>:</para>
1295 [qsort.hs:1:11-12] *Main> :back
1296 Logged breakpoint at qsort.hs:3:24-38
1300 [-1: qsort.hs:3:24-38] *Main>
1303 <para>Note that the local variables at each step in the history have been
1304 preserved, and can be examined as usual. Also note that the prompt has
1305 changed to indicate that we're currently examining the first step in
1306 the history: <literal>-1</literal>. The command
1307 <literal>:forward</literal> can be used to traverse forward in the
1310 <para>The <literal>:trace</literal> command can be used with or without
1311 an expression. When used without an expression, tracing begins from
1312 the current breakpoint, just like <literal>:step</literal>.</para>
1314 <para>The history is only available when
1315 using <literal>:trace</literal>; the reason for this is we found that
1316 logging each breakpoint in the history cuts performance by a factor of
1317 2 or more. GHCi remembers the last 50 steps in the history (perhaps in
1318 the future we'll make this configurable).</para>
1321 <sect2 id="ghci-debugger-exceptions">
1322 <title>Debugging exceptions</title>
1323 <para>Another common question that comes up when debugging is
1324 “where did this exception come from?”. Exceptions such as
1325 those raised by <literal>error</literal> or <literal>head []</literal>
1326 have no context information attached to them. Finding which
1327 particular call to <literal>head</literal> in your program resulted in
1328 the error can be a painstaking process, usually involving
1329 <literal>Debug.Trace.trace</literal>, or compiling with
1330 profiling and using <literal>+RTS -xc</literal> (see <xref
1331 linkend="prof-time-options" />).</para>
1333 <para>The GHCi debugger offers a way to hopefully shed some light on
1334 these errors quickly and without modifying or recompiling the source
1335 code. One way would be to set a breakpoint on the location in the
1336 source code that throws the exception, and then use
1337 <literal>:trace</literal> and <literal>:history</literal> to establish
1338 the context. However, <literal>head</literal> is in a library and
1339 we can't set a breakpoint on it directly. For this reason, GHCi
1340 provides the flag <literal>-fbreak-on-exception</literal> which causes
1341 the evaluator to stop when an exception is thrown, just as it does when
1342 a breakpoint is hit. This is only really useful in conjunction with
1343 <literal>:trace</literal>, in order to log the steps leading up to the
1344 exception. For example:</para>
1347 *Main> :set -fbreak-on-exception
1348 *Main> :trace qsort ("abc" ++ undefined)
1349 "Stopped at <exception thrown>
1351 [<exception thrown>] *Main> :hist
1352 -1 : qsort.hs:3:24-38
1353 -2 : qsort.hs:3:23-55
1354 -3 : qsort.hs:(1,0)-(3,55)
1355 -4 : qsort.hs:2:15-24
1356 -5 : qsort.hs:2:15-46
1357 -6 : qsort.hs:(1,0)-(3,55)
1358 <end of history>
1359 [<exception thrown>] *Main> :back
1360 Logged breakpoint at qsort.hs:3:24-38
1364 [-1: qsort.hs:3:24-38] *Main> :force as
1365 *** Exception: Prelude.undefined
1366 [-1: qsort.hs:3:24-38] *Main> :print as
1367 as = 'b' : 'c' : (_t1::[Char])
1370 <para>The exception itself is bound to a new variable,
1371 <literal>_exception</literal>.</para>
1373 <para>Breaking on exceptions is particularly useful for finding out what
1374 your program was doing when it was in an infinite loop. Just hit
1375 Control-C, and examine the history to find out what was going
1379 <sect2><title>Example: inspecting functions</title>
1381 It is possible to use the debugger to examine function values.
1382 When we are at a breakpoint and a function is in scope, the debugger
1384 you the source code for it; however, it is possible to get some
1385 information by applying it to some arguments and observing the result.
1389 The process is slightly complicated when the binding is polymorphic.
1390 We show the process by means of an example.
1391 To keep things simple, we will use the well known <literal>map</literal> function:
1393 import Prelude hiding (map)
1395 map :: (a->b) -> a -> b
1397 map f (x:xs) = f x : map f xs
1402 We set a breakpoint on <literal>map</literal>, and call it.
1405 Breakpoint 0 activated at map.hs:5:15-28
1406 *Main> map Just [1..5]
1407 Stopped at map.hs:(4,0)-(5,12)
1413 GHCi tells us that, among other bindings, <literal>f</literal> is in scope.
1414 However, its type is not fully known yet,
1415 and thus it is not possible to apply it to any
1416 arguments. Nevertheless, observe that the type of its first argument is the
1417 same as the type of <literal>x</literal>, and its result type is shared
1418 with <literal>_result</literal>.
1422 As we demonstrated earlier (<xref linkend="breakpoints" />), the
1423 debugger has some intelligence built-in to update the type of
1424 <literal>f</literal> whenever the types of <literal>x</literal> or
1425 <literal>_result</literal> are discovered. So what we do in this
1427 force <literal>x</literal> a bit, in order to recover both its type
1428 and the argument part of <literal>f</literal>.
1436 We can check now that as expected, the type of <literal>x</literal>
1437 has been reconstructed, and with it the
1438 type of <literal>f</literal> has been too:</para>
1446 From here, we can apply f to any argument of type Integer and observe
1454 Ambiguous type variable `b' in the constraint:
1455 `Show b' arising from a use of `print' at <interactive>:1:0
1467 f :: Integer -> Maybe Integer
1471 [Just 1, Just 2, Just 3, Just 4, Just 5]
1473 In the first application of <literal>f</literal>, we had to do
1474 some more type reconstruction
1475 in order to recover the result type of <literal>f</literal>.
1476 But after that, we are free to use
1477 <literal>f</literal> normally.
1481 <sect2><title>Limitations</title>
1484 <para>When stopped at a breakpoint, if you try to evaluate a variable
1485 that is already under evaluation, the second evaluation will hang.
1487 that GHC knows the variable is under evaluation, so the new
1488 evaluation just waits for the result before continuing, but of
1489 course this isn't going to happen because the first evaluation is
1490 stopped at a breakpoint. Control-C can interrupt the hung
1491 evaluation and return to the prompt.</para>
1492 <para>The most common way this can happen is when you're evaluating a
1493 CAF (e.g. main), stop at a breakpoint, and ask for the value of the
1494 CAF at the prompt again.</para>
1497 Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
1498 at the scope of a breakpoint if there is a explicit type signature.
1505 <sect1 id="ghci-invocation">
1506 <title>Invoking GHCi</title>
1507 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
1508 <indexterm><primary><option>––interactive</option></primary></indexterm>
1510 <para>GHCi is invoked with the command <literal>ghci</literal> or
1511 <literal>ghc ––interactive</literal>. One or more modules or
1512 filenames can also be specified on the command line; this
1513 instructs GHCi to load the specified modules or filenames (and all
1514 the modules they depend on), just as if you had said
1515 <literal>:load <replaceable>modules</replaceable></literal> at the
1516 GHCi prompt (see <xref linkend="ghci-commands" />). For example, to
1517 start GHCi and load the program whose topmost module is in the
1518 file <literal>Main.hs</literal>, we could say:</para>
1524 <para>Most of the command-line options accepted by GHC (see <xref
1525 linkend="using-ghc"/>) also make sense in interactive mode. The ones
1526 that don't make sense are mostly obvious; for example, GHCi
1527 doesn't generate interface files, so options related to interface
1528 file generation won't have any effect.</para>
1531 <title>Packages</title>
1532 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
1534 <para>Most packages (see <xref linkend="using-packages"/>) are
1535 available without needing to specify any extra flags at all:
1536 they will be automatically loaded the first time they are
1539 <para>For hidden packages, however, you need to request the
1540 package be loaded by using the <literal>-package</literal> flag:</para>
1543 $ ghci -package readline
1546 / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98.
1547 / /_\\/ __ / /___| | http://www.haskell.org/ghc/
1548 \____/\/ /_/\____/|_| Type :? for help.
1550 Loading package base ... linking ... done.
1551 Loading package readline-1.0 ... linking ... done.
1555 <para>The following command works to load new packages into a
1556 running GHCi:</para>
1559 Prelude> :set -package <replaceable>name</replaceable>
1562 <para>But note that doing this will cause all currently loaded
1563 modules to be unloaded, and you'll be dumped back into the
1564 <literal>Prelude</literal>.</para>
1568 <title>Extra libraries</title>
1569 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
1571 <para>Extra libraries may be specified on the command line using
1572 the normal <literal>-l<replaceable>lib</replaceable></literal>
1573 option. (The term <emphasis>library</emphasis> here refers to
1574 libraries of foreign object code; for using libraries of Haskell
1575 source code, see <xref linkend="ghci-modules-filenames"/>.) For
1576 example, to load the “m” library:</para>
1582 <para>On systems with <literal>.so</literal>-style shared
1583 libraries, the actual library loaded will the
1584 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
1585 searches the following places for libraries, in this order:</para>
1589 <para>Paths specified using the
1590 <literal>-L<replaceable>path</replaceable></literal>
1591 command-line option,</para>
1594 <para>the standard library search path for your system,
1595 which on some systems may be overridden by setting the
1596 <literal>LD_LIBRARY_PATH</literal> environment
1601 <para>On systems with <literal>.dll</literal>-style shared
1602 libraries, the actual library loaded will be
1603 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
1604 GHCi will signal an error if it can't find the library.</para>
1606 <para>GHCi can also load plain object files
1607 (<literal>.o</literal> or <literal>.obj</literal> depending on
1608 your platform) from the command-line. Just add the name the
1609 object file to the command line.</para>
1611 <para>Ordering of <option>-l</option> options matters: a library
1612 should be mentioned <emphasis>before</emphasis> the libraries it
1613 depends on (see <xref linkend="options-linker"/>).</para>
1618 <sect1 id="ghci-commands">
1619 <title>GHCi commands</title>
1621 <para>GHCi commands all begin with
1622 ‘<literal>:</literal>’ and consist of a single command
1623 name followed by zero or more parameters. The command name may be
1624 abbreviated, with ambiguities being resolved in favour of the more
1625 commonly used commands.</para>
1630 <literal>:abandon</literal>
1631 <indexterm><primary><literal>:abandon</literal></primary></indexterm>
1634 <para>Abandons the current evaluation (only available when stopped at
1635 a breakpoint).</para>
1641 <literal>:add</literal> <replaceable>module</replaceable> ...
1642 <indexterm><primary><literal>:add</literal></primary></indexterm>
1645 <para>Add <replaceable>module</replaceable>(s) to the
1646 current <firstterm>target set</firstterm>, and perform a
1653 <literal>:back</literal>
1654 <indexterm><primary><literal>:back</literal></primary></indexterm>
1657 <para>Travel back one step in the history. See <xref
1658 linkend="tracing" />. See also:
1659 <literal>:trace</literal>, <literal>:history</literal>,
1660 <literal>:forward</literal>.</para>
1666 <literal>:break [<replaceable>identifier</replaceable> |
1667 [<replaceable>module</replaceable>] <replaceable>line</replaceable>
1668 [<replaceable>column</replaceable>]]</literal>
1670 <indexterm><primary><literal>:break</literal></primary></indexterm>
1672 <para>Set a breakpoint on the specified function or line and
1673 column. See <xref linkend="setting-breakpoints" />.</para>
1679 <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
1680 <indexterm><primary><literal>:browse</literal></primary></indexterm>
1683 <para>Displays the identifiers defined by the module
1684 <replaceable>module</replaceable>, which must be either
1685 loaded into GHCi or be a member of a package. If the
1686 <literal>*</literal> symbol is placed before the module
1687 name, then <emphasis>all</emphasis> the identifiers defined
1688 in <replaceable>module</replaceable> are shown; otherwise
1689 the list is limited to the exports of
1690 <replaceable>module</replaceable>. The
1691 <literal>*</literal>-form is only available for modules
1692 which are interpreted; for compiled modules (including
1693 modules from packages) only the non-<literal>*</literal>
1694 form of <literal>:browse</literal> is available.</para>
1700 <literal>:cd</literal> <replaceable>dir</replaceable>
1701 <indexterm><primary><literal>:cd</literal></primary></indexterm>
1704 <para>Changes the current working directory to
1705 <replaceable>dir</replaceable>. A
1706 ‘<literal>˜</literal>’ symbol at the
1707 beginning of <replaceable>dir</replaceable> will be replaced
1708 by the contents of the environment variable
1709 <literal>HOME</literal>.</para>
1711 <para>NOTE: changing directories causes all currently loaded
1712 modules to be unloaded. This is because the search path is
1713 usually expressed using relative directories, and changing
1714 the search path in the middle of a session is not
1721 <literal>:continue</literal>
1722 <indexterm><primary><literal>:continue</literal></primary></indexterm>
1724 <listitem><para>Continue the current evaluation, when stopped at a
1731 <literal>:cmd</literal> <replaceable>expr</replaceable>
1732 <indexterm><primary><literal>:cmd</literal></primary></indexterm>
1735 <para>Executes <replaceable>expr</replaceable> as a computation of
1736 type <literal>IO String</literal>, and then executes the resulting
1737 string as a list of GHCi commands. Multiple commands are separated
1738 by newlines. The <literal>:cmd</literal> command is useful with
1739 <literal>:def</literal> and <literal>:set stop</literal>.</para>
1745 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1746 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1747 <indexterm><primary><literal>:etags</literal></primary>
1749 <indexterm><primary><literal>:etags</literal></primary>
1753 <para>Generates a “tags” file for Vi-style editors
1754 (<literal>:ctags</literal>) or Emacs-style editors (<literal>etags</literal>). If
1755 no filename is specified, the defaulit <filename>tags</filename> or
1756 <filename>TAGS</filename> is
1757 used, respectively. Tags for all the functions, constructors and
1758 types in the currently loaded modules are created. All modules must
1759 be interpreted for these commands to work.</para>
1760 <para>See also <xref linkend="hasktags" />.</para>
1766 <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
1767 <indexterm><primary><literal>:def</literal></primary></indexterm>
1770 <para>The command <literal>:def</literal>
1771 <replaceable>name</replaceable>
1772 <replaceable>expr</replaceable> defines a new GHCi command
1773 <literal>:<replaceable>name</replaceable></literal>,
1774 implemented by the Haskell expression
1775 <replaceable>expr</replaceable>, which must have type
1776 <literal>String -> IO String</literal>. When
1777 <literal>:<replaceable>name</replaceable>
1778 <replaceable>args</replaceable></literal> is typed at the
1779 prompt, GHCi will run the expression
1780 <literal>(<replaceable>name</replaceable>
1781 <replaceable>args</replaceable>)</literal>, take the
1782 resulting <literal>String</literal>, and feed it back into
1783 GHCi as a new sequence of commands. Separate commands in
1784 the result must be separated by
1785 ‘<literal>\n</literal>’.</para>
1787 <para>That's all a little confusing, so here's a few
1788 examples. To start with, here's a new GHCi command which
1789 doesn't take any arguments or produce any results, it just
1790 outputs the current date & time:</para>
1793 Prelude> let date _ = Time.getClockTime >>= print >> return ""
1794 Prelude> :def date date
1796 Fri Mar 23 15:16:40 GMT 2001
1799 <para>Here's an example of a command that takes an argument.
1800 It's a re-implementation of <literal>:cd</literal>:</para>
1803 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
1804 Prelude> :def mycd mycd
1808 <para>Or I could define a simple way to invoke
1809 “<literal>ghc ––make Main</literal>” in the
1810 current directory:</para>
1813 Prelude> :def make (\_ -> return ":! ghc ––make Main")
1816 <para>We can define a command that reads GHCi input from a
1817 file. This might be useful for creating a set of bindings
1818 that we want to repeatedly load into the GHCi session:</para>
1821 Prelude> :def . readFile
1822 Prelude> :. cmds.ghci
1825 <para>Notice that we named the command
1826 <literal>:.</literal>, by analogy with the
1827 ‘<literal>.</literal>’ Unix shell command that
1828 does the same thing.</para>
1834 <literal>:delete * | <replaceable>num</replaceable> ...</literal>
1835 <indexterm><primary><literal>:delete</literal></primary></indexterm>
1838 <para>Delete one or more breakpoints by number (use <literal>:show
1839 breaks</literal> to see the number of each breakpoint). The
1840 <literal>*</literal> form deletes all the breakpoints.</para>
1846 <literal>:edit <optional><replaceable>file</replaceable></optional></literal>
1847 <indexterm><primary><literal>:edit</literal></primary></indexterm>
1850 <para>Opens an editor to edit the file
1851 <replaceable>file</replaceable>, or the most recently loaded
1852 module if <replaceable>file</replaceable> is omitted. The
1853 editor to invoke is taken from the <literal>EDITOR</literal>
1854 environment variable, or a default editor on your system if
1855 <literal>EDITOR</literal> is not set. You can change the
1856 editor using <literal>:set editor</literal>.</para>
1862 <literal>:force <replaceable>identifier</replaceable> ...</literal>
1863 <indexterm><primary><literal>:force</literal></primary></indexterm>
1866 <para>Prints the value of <replaceable>identifier</replaceable> in
1867 the same way as <literal>:print</literal>. Unlike
1868 <literal>:print</literal>, <literal>:force</literal> evaluates each
1869 thunk that it encounters while traversing the value. This may
1870 cause exceptions or infinite loops, or further breakpoints (which
1871 are ignored, but displayed).</para>
1877 <literal>:forward</literal>
1878 <indexterm><primary><literal>:forward</literal></primary></indexterm>
1881 <para>Move forward in the history. See <xref
1882 linkend="tracing" />. See also:
1883 <literal>:trace</literal>, <literal>:history</literal>,
1884 <literal>:back</literal>.</para>
1890 <literal>:help</literal>
1891 <indexterm><primary><literal>:help</literal></primary></indexterm>
1894 <literal>:?</literal>
1895 <indexterm><primary><literal>:?</literal></primary></indexterm>
1898 <para>Displays a list of the available commands.</para>
1904 <literal>:history [<replaceable>num</replaceable>]</literal>
1905 <indexterm><primary><literal>:history</literal></primary></indexterm>
1908 <para>Display the history of evaluation steps. With a number,
1909 displays that many steps (default: 20). For use with
1910 <literal>:trace</literal>; see <xref
1911 linkend="tracing" />.</para>
1917 <literal>:info</literal> <replaceable>name</replaceable> ...
1918 <indexterm><primary><literal>:info</literal></primary></indexterm>
1921 <para>Displays information about the given name(s). For
1922 example, if <replaceable>name</replaceable> is a class, then
1923 the class methods and their types will be printed; if
1924 <replaceable>name</replaceable> is a type constructor, then
1925 its definition will be printed; if
1926 <replaceable>name</replaceable> is a function, then its type
1927 will be printed. If <replaceable>name</replaceable> has
1928 been loaded from a source file, then GHCi will also display
1929 the location of its definition in the source.</para>
1935 <literal>:kind</literal> <replaceable>type</replaceable>
1936 <indexterm><primary><literal>:kind</literal></primary></indexterm>
1939 <para>Infers and prints the kind of
1940 <replaceable>type</replaceable>. The latter can be an arbitrary
1941 type expression, including a partial application of a type constructor,
1942 such as <literal>Either Int</literal>.</para>
1948 <literal>:load</literal> <replaceable>module</replaceable> ...
1949 <indexterm><primary><literal>:load</literal></primary></indexterm>
1952 <para>Recursively loads the specified
1953 <replaceable>module</replaceable>s, and all the modules they
1954 depend on. Here, each <replaceable>module</replaceable>
1955 must be a module name or filename, but may not be the name
1956 of a module in a package.</para>
1958 <para>All previously loaded modules, except package modules,
1959 are forgotten. The new set of modules is known as the
1960 <firstterm>target set</firstterm>. Note that
1961 <literal>:load</literal> can be used without any arguments
1962 to unload all the currently loaded modules and
1965 <para>After a <literal>:load</literal> command, the current
1966 context is set to:</para>
1970 <para><replaceable>module</replaceable>, if it was loaded
1971 successfully, or</para>
1974 <para>the most recently successfully loaded module, if
1975 any other modules were loaded as a result of the current
1976 <literal>:load</literal>, or</para>
1979 <para><literal>Prelude</literal> otherwise.</para>
1987 <literal>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
1988 <indexterm><primary><literal>:main</literal></primary></indexterm>
1992 When a program is compiled and executed, it can use the
1993 <literal>getArgs</literal> function to access the
1994 command-line arguments.
1995 However, we cannot simply pass the arguments to the
1996 <literal>main</literal> function while we are testing in ghci,
1997 as the <literal>main</literal> function doesn't take its
2002 Instead, we can use the <literal>:main</literal> command.
2003 This runs whatever <literal>main</literal> is in scope, with
2004 any arguments being treated the same as command-line arguments,
2009 Prelude> let main = System.Environment.getArgs >>= print
2010 Prelude> :main foo bar
2019 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
2020 <indexterm><primary><literal>:module</literal></primary></indexterm>
2023 <literal>import <replaceable>mod</replaceable></literal>
2026 <para>Sets or modifies the current context for statements
2027 typed at the prompt. The form <literal>import
2028 <replaceable>mod</replaceable></literal> is equivalent to
2029 <literal>:module +<replaceable>mod</replaceable></literal>.
2030 See <xref linkend="ghci-scope"/> for
2031 more details.</para>
2037 <literal>:print </literal> <replaceable>names</replaceable> ...
2038 <indexterm><primary><literal>:print</literal></primary></indexterm>
2041 <para>Prints a value without forcing its evaluation.
2042 <literal>:print</literal> may be used on values whose types are
2043 unkonwn or partially known, which might be the case for local
2044 variables with polymorphic types at a breakpoint. While inspecting
2045 the runtime value, <literal>:print</literal> attempts to
2046 reconstruct the type of the value, and will elaborate the type in
2047 GHCi's environment if possible. If any unevaluated components
2048 (thunks) are encountered, then <literal>:print</literal> binds
2049 a fresh variable with a name beginning with <literal>_t</literal>
2050 to each thunk. See <xref linkend="breakpoints" /> for more
2051 information. See also the <literal>:sprint</literal> command,
2052 which works like <literal>:print</literal> but does not bind new
2059 <literal>:quit</literal>
2060 <indexterm><primary><literal>:quit</literal></primary></indexterm>
2063 <para>Quits GHCi. You can also quit by typing a control-D
2064 at the prompt.</para>
2070 <literal>:reload</literal>
2071 <indexterm><primary><literal>:reload</literal></primary></indexterm>
2074 <para>Attempts to reload the current target set (see
2075 <literal>:load</literal>) if any of the modules in the set,
2076 or any dependent module, has changed. Note that this may
2077 entail loading new modules, or dropping modules which are no
2078 longer indirectly required by the target.</para>
2084 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
2085 <indexterm><primary><literal>:set</literal></primary></indexterm>
2088 <para>Sets various options. See <xref linkend="ghci-set"/>
2089 for a list of available options. The
2090 <literal>:set</literal> command by itself shows which
2091 options are currently set.</para>
2097 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
2098 <indexterm><primary><literal>:set args</literal></primary></indexterm>
2101 <para>Sets the list of arguments which are returned when the
2102 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
2103 </indexterm>.</para>
2109 <literal>:set</literal> <literal>editor</literal> <replaceable>cmd</replaceable>
2112 <para>Sets the command used by <literal>:edit</literal> to
2113 <replaceable>cmd</replaceable>.</para>
2119 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
2120 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
2123 <para>Sets the string to be returned when the program calls
2124 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
2125 </indexterm>.</para>
2131 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
2134 <para>Sets the string to be used as the prompt in GHCi.
2135 Inside <replaceable>prompt</replaceable>, the sequence
2136 <literal>%s</literal> is replaced by the names of the
2137 modules currently in scope, and <literal>%%</literal> is
2138 replaced by <literal>%</literal>.</para>
2144 <literal>:set</literal> <literal>stop</literal>
2145 [<replaceable>num</replaceable>] <replaceable>cmd</replaceable>
2148 <para>Set a command to be executed when a breakpoint is hit, or a new
2149 item in the history is selected. The most common use of
2150 <literal>:set stop</literal> is to display the source code at the
2151 current location, e.g. <literal>:set stop :list</literal>.</para>
2153 <para>If a number is given before the command, then the commands are
2154 run when the specified breakpoint (only) is hit. This can be quite
2155 useful: for example, <literal>:set stop 1 :continue</literal>
2156 effectively disables breakpoint 1, by running
2157 <literal>:continue</literal> whenever it is hit (although GHCi will
2158 still emit a message to say the breakpoint was hit). What's more,
2159 with cunning use of <literal>:def</literal> and
2160 <literal>:cmd</literal> you can use <literal>:set stop</literal> to
2161 implement conditional breakpoints:</para>
2163 *Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
2164 *Main> :set stop 0 :cond (x < 3)
2166 <para>Ignoring breakpoints for a specified number of iterations is
2167 also possible using similar techniques.</para>
2173 <literal>:show bindings</literal>
2174 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
2177 <para>Show the bindings made at the prompt and their
2184 <literal>:show breaks</literal>
2185 <indexterm><primary><literal>:show breaks</literal></primary></indexterm>
2188 <para>List the active breakpoints.</para>
2194 <literal>:show context</literal>
2195 <indexterm><primary><literal>:show context</literal></primary></indexterm>
2198 <para>List the active evaluations that are stopped at breakpoints.</para>
2204 <literal>:show modules</literal>
2205 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
2208 <para>Show the list of modules currently load.</para>
2214 <literal>:show [args|prog|prompt|editor|stop]</literal>
2215 <indexterm><primary><literal>:show</literal></primary></indexterm>
2218 <para>Displays the specified setting (see
2219 <literal>:set</literal>).</para>
2225 <literal>:sprint</literal>
2226 <indexterm><primary><literal>:sprint</literal></primary></indexterm>
2229 <para>Prints a value without forcing its evaluation.
2230 <literal>:sprint</literal> is similar to <literal>:print</literal>,
2231 with the difference that unevaluated subterms are not bound to new
2232 variables, they are simply denoted by ‘_’.</para>
2238 <literal>:step [<replaceable>expr</replaceable>]</literal>
2239 <indexterm><primary><literal>:step</literal></primary></indexterm>
2242 <para>Single-step from the last breakpoint. With an expression
2243 argument, begins evaluation of the expression with a
2250 <literal>:trace [<replaceable>expr</replaceable>]</literal>
2251 <indexterm><primary><literal>:trace</literal></primary></indexterm>
2254 <para>Evaluates the given expression (or from the last breakpoint if
2255 no expression is given), and additionally logs the evaluation
2256 steps for later inspection using <literal>:history</literal>. See
2257 <xref linkend="tracing" />.</para>
2263 <literal>:type</literal> <replaceable>expression</replaceable>
2264 <indexterm><primary><literal>:type</literal></primary></indexterm>
2267 <para>Infers and prints the type of
2268 <replaceable>expression</replaceable>, including explicit
2269 forall quantifiers for polymorphic types. The monomorphism
2270 restriction is <emphasis>not</emphasis> applied to the
2271 expression during type inference.</para>
2277 <literal>:undef</literal> <replaceable>name</replaceable>
2278 <indexterm><primary><literal>:undef</literal></primary></indexterm>
2281 <para>Undefines the user-defined command
2282 <replaceable>name</replaceable> (see <literal>:def</literal>
2289 <literal>:unset</literal> <replaceable>option</replaceable>...
2290 <indexterm><primary><literal>:unset</literal></primary></indexterm>
2293 <para>Unsets certain options. See <xref linkend="ghci-set"/>
2294 for a list of available options.</para>
2300 <literal>:!</literal> <replaceable>command</replaceable>...
2301 <indexterm><primary><literal>:!</literal></primary></indexterm>
2302 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
2305 <para>Executes the shell command
2306 <replaceable>command</replaceable>.</para>
2313 <sect1 id="ghci-set">
2314 <title>The <literal>:set</literal> command</title>
2315 <indexterm><primary><literal>:set</literal></primary></indexterm>
2317 <para>The <literal>:set</literal> command sets two types of
2318 options: GHCi options, which begin with
2319 ‘<literal>+</literal>” and “command-line”
2320 options, which begin with ‘-’. </para>
2322 <para>NOTE: at the moment, the <literal>:set</literal> command
2323 doesn't support any kind of quoting in its arguments: quotes will
2324 not be removed and cannot be used to group words together. For
2325 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
2329 <title>GHCi options</title>
2330 <indexterm><primary>options</primary><secondary>GHCi</secondary>
2333 <para>GHCi options may be set using <literal>:set</literal> and
2334 unset using <literal>:unset</literal>.</para>
2336 <para>The available GHCi options are:</para>
2341 <literal>+r</literal>
2342 <indexterm><primary><literal>+r</literal></primary></indexterm>
2343 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
2344 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
2347 <para>Normally, any evaluation of top-level expressions
2348 (otherwise known as CAFs or Constant Applicative Forms) in
2349 loaded modules is retained between evaluations. Turning
2350 on <literal>+r</literal> causes all evaluation of
2351 top-level expressions to be discarded after each
2352 evaluation (they are still retained
2353 <emphasis>during</emphasis> a single evaluation).</para>
2355 <para>This option may help if the evaluated top-level
2356 expressions are consuming large amounts of space, or if
2357 you need repeatable performance measurements.</para>
2363 <literal>+s</literal>
2364 <indexterm><primary><literal>+s</literal></primary></indexterm>
2367 <para>Display some stats after evaluating each expression,
2368 including the elapsed time and number of bytes allocated.
2369 NOTE: the allocation figure is only accurate to the size
2370 of the storage manager's allocation area, because it is
2371 calculated at every GC. Hence, you might see values of
2372 zero if no GC has occurred.</para>
2378 <literal>+t</literal>
2379 <indexterm><primary><literal>+t</literal></primary></indexterm>
2382 <para>Display the type of each variable bound after a
2383 statement is entered at the prompt. If the statement is a
2384 single expression, then the only variable binding will be
2386 ‘<literal>it</literal>’.</para>
2392 <sect2 id="ghci-cmd-line-options">
2393 <title>Setting GHC command-line options in GHCi</title>
2395 <para>Normal GHC command-line options may also be set using
2396 <literal>:set</literal>. For example, to turn on
2397 <option>-fglasgow-exts</option>, you would say:</para>
2400 Prelude> :set -fglasgow-exts
2403 <para>Any GHC command-line option that is designated as
2404 <firstterm>dynamic</firstterm> (see the table in <xref
2405 linkend="flag-reference"/>), may be set using
2406 <literal>:set</literal>. To unset an option, you can set the
2407 reverse option:</para>
2408 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
2411 Prelude> :set -fno-glasgow-exts
2414 <para><xref linkend="flag-reference"/> lists the reverse for each
2415 option where applicable.</para>
2417 <para>Certain static options (<option>-package</option>,
2418 <option>-I</option>, <option>-i</option>, and
2419 <option>-l</option> in particular) will also work, but some may
2420 not take effect until the next reload.</para>
2421 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
2424 <sect1 id="ghci-dot-files">
2425 <title>The <filename>.ghci</filename> file</title>
2426 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
2428 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
2431 <para>When it starts, GHCi always reads and executes commands from
2432 <filename>$HOME/.ghci</filename>, followed by
2433 <filename>./.ghci</filename>.</para>
2435 <para>The <filename>.ghci</filename> in your home directory is
2436 most useful for turning on favourite options (eg. <literal>:set
2437 +s</literal>), and defining useful macros. Placing a
2438 <filename>.ghci</filename> file in a directory with a Haskell
2439 project is a useful way to set certain project-wide options so you
2440 don't have to type them everytime you start GHCi: eg. if your
2441 project uses GHC extensions and CPP, and has source files in three
2442 subdirectories A B and C, you might put the following lines in
2443 <filename>.ghci</filename>:</para>
2446 :set -fglasgow-exts -cpp
2450 <para>(Note that strictly speaking the <option>-i</option> flag is
2451 a static one, but in fact it works to set it using
2452 <literal>:set</literal> like this. The changes won't take effect
2453 until the next <literal>:load</literal>, though.)</para>
2455 <para>Two command-line options control whether the
2456 <filename>.ghci</filename> files are read:</para>
2461 <option>-ignore-dot-ghci</option>
2462 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
2465 <para>Don't read either <filename>./.ghci</filename> or
2466 <filename>$HOME/.ghci</filename> when starting up.</para>
2471 <option>-read-dot-ghci</option>
2472 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
2475 <para>Read <filename>.ghci</filename> and
2476 <filename>$HOME/.ghci</filename>. This is normally the
2477 default, but the <option>-read-dot-ghci</option> option may
2478 be used to override a previous
2479 <option>-ignore-dot-ghci</option> option.</para>
2486 <sect1 id="ghci-obj">
2487 <title>Compiling to object code inside GHCi</title>
2489 <para>By default, GHCi compiles Haskell source code into byte-code
2490 that is interpreted by the runtime system. GHCi can also compile
2491 Haskell code to object code: to turn on this feature, use the
2492 <option>-fobject-code</option> flag either on the command line or
2493 with <literal>:set</literal> (the option
2494 <option>-fbyte-code</option> restores byte-code compilation
2495 again). Compiling to object code takes longer, but typically the
2496 code will execute 10-20 times faster than byte-code.</para>
2498 <para>Compiling to object code inside GHCi is particularly useful
2499 if you are developing a compiled application, because the
2500 <literal>:reload</literal> command typically runs much faster than
2501 restarting GHC with <option>--make</option> from the command-line,
2502 because all the interface files are already cached in
2505 <para>There are disadvantages to compiling to object-code: you
2506 can't set breakpoints in object-code modules, for example. Only
2507 the exports of an object-code module will be visible in GHCi,
2508 rather than all top-level bindings as in interpreted
2512 <sect1 id="ghci-faq">
2513 <title>FAQ and Things To Watch Out For</title>
2517 <term>The interpreter can't load modules with foreign export
2518 declarations!</term>
2520 <para>Unfortunately not. We haven't implemented it yet.
2521 Please compile any offending modules by hand before loading
2522 them into GHCi.</para>
2528 <literal>-O</literal> doesn't work with GHCi!
2529 <indexterm><primary><option>-O</option></primary></indexterm>
2532 <para>For technical reasons, the bytecode compiler doesn't
2533 interact well with one of the optimisation passes, so we
2534 have disabled optimisation when using the interpreter. This
2535 isn't a great loss: you'll get a much bigger win by
2536 compiling the bits of your code that need to go fast, rather
2537 than interpreting them with optimisation turned on.</para>
2542 <term>Unboxed tuples don't work with GHCi</term>
2544 <para>That's right. You can always compile a module that
2545 uses unboxed tuples and load it into GHCi, however.
2546 (Incidentally the previous point, namely that
2547 <literal>-O</literal> is incompatible with GHCi, is because
2548 the bytecode compiler can't deal with unboxed
2554 <term>Concurrent threads don't carry on running when GHCi is
2555 waiting for input.</term>
2557 <para>This should work, as long as your GHCi was built with
2558 the <option>-threaded</option> switch, which is the default.
2559 Consult whoever supplied your GHCi installation.</para>
2564 <term>After using <literal>getContents</literal>, I can't use
2565 <literal>stdin</literal> again until I do
2566 <literal>:load</literal> or <literal>:reload</literal>.</term>
2569 <para>This is the defined behaviour of
2570 <literal>getContents</literal>: it puts the stdin Handle in
2571 a state known as <firstterm>semi-closed</firstterm>, wherein
2572 any further I/O operations on it are forbidden. Because I/O
2573 state is retained between computations, the semi-closed
2574 state persists until the next <literal>:load</literal> or
2575 <literal>:reload</literal> command.</para>
2577 <para>You can make <literal>stdin</literal> reset itself
2578 after every evaluation by giving GHCi the command
2579 <literal>:set +r</literal>. This works because
2580 <literal>stdin</literal> is just a top-level expression that
2581 can be reverted to its unevaluated state in the same way as
2582 any other top-level expression (CAF).</para>
2587 <term>I can't use Control-C to interrupt computations in
2588 GHCi on Windows.</term>
2590 <para>See <xref linkend="ghci-windows"/></para>
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