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
375 The statement <literal>x <- return 42</literal> means
376 “execute <literal>return 42</literal> in the
377 <literal>IO</literal> monad, and bind the result to
378 <literal>x</literal>”. We can then use
379 <literal>x</literal> in future statements, for example to print
380 it as we did above.</para>
382 <para>If <option>-fprint-bind-result</option> is set then
383 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 <indexterm><primary><option>-fprint-bind-result</option></primary></indexterm><indexterm><primary><option>-fno-print-bind-result</option></primary></indexterm>.
399 <para>Of course, you can also bind normal non-IO expressions
400 using the <literal>let</literal>-statement:</para>
407 <para>Another important difference between the two types of binding
408 is that the monadic bind (<literal>p <- e</literal>) is
409 <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
410 whereas with the <literal>let</literal> form, the expression
411 isn't evaluated immediately:</para>
413 Prelude> let x = error "help!"
419 <para>Note that <literal>let</literal> bindings do not automatically
420 print the value bound, unlike monadic bindings.</para>
422 <para>Hint: you can also use <literal>let</literal>-statements
423 to define functions at the prompt:</para>
425 Prelude> let add a b = a + b
430 <para>However, this quickly gets tedious when defining functions
431 with multiple clauses, or groups of mutually recursive functions,
432 because the complete definition has to be given on a single line,
433 using explicit braces and semicolons instead of layout:</para>
435 Prelude> let { f op n [] = n ; f op n (h:t) = h `op` f op n t }
436 Prelude> f (+) 0 [1..3]
440 <para>To alleviate this issue, GHCi commands can be split over
441 multiple lines, by wrapping them in <literal>:{</literal> and
442 <literal>:}</literal> (each on a single line of its own):</para>
445 Prelude| let { g op n [] = n
446 Prelude| ; g op n (h:t) = h `op` g op n t
449 Prelude> g (*) 1 [1..3]
452 <para>Such multiline commands can be used with any GHCi command,
453 and the lines between <literal>:{</literal> and
454 <literal>:}</literal> are simply merged into a single line for
455 interpretation. That implies that each such group must form a single
456 valid command when merged, and that no layout rule is used.
457 The main purpose of multiline commands is not to replace module
458 loading but to make definitions in .ghci-files (see <xref
459 linkend="ghci-dot-files"/>) more readable and maintainable.</para>
461 <para>Any exceptions raised during the evaluation or execution
462 of the statement are caught and printed by the GHCi command line
463 interface (for more information on exceptions, see the module
464 <literal>Control.Exception</literal> in the libraries
465 documentation).</para>
467 <para>Every new binding shadows any existing bindings of the
468 same name, including entities that are in scope in the current
469 module context.</para>
471 <para>WARNING: temporary bindings introduced at the prompt only
472 last until the next <literal>:load</literal> or
473 <literal>:reload</literal> command, at which time they will be
474 simply lost. However, they do survive a change of context with
475 <literal>:module</literal>: the temporary bindings just move to
476 the new location.</para>
478 <para>HINT: To get a list of the bindings currently in scope, use the
479 <literal>:show bindings</literal> command:</para>
482 Prelude> :show bindings
486 <para>HINT: if you turn on the <literal>+t</literal> option,
487 GHCi will show the type of each variable bound by a statement.
489 <indexterm><primary><literal>+t</literal></primary></indexterm>
492 Prelude> let (x:xs) = [1..]
499 <sect2 id="ghci-scope">
500 <title>What's really in scope at the prompt?</title>
502 <para>When you type an expression at the prompt, what
503 identifiers and types are in scope? GHCi provides a flexible
504 way to control exactly how the context for an expression is
505 constructed. Let's start with the simple cases; when you start
506 GHCi the prompt looks like this:</para>
508 <screen>Prelude></screen>
510 <para>Which indicates that everything from the module
511 <literal>Prelude</literal> is currently in scope. If we now
512 load a file into GHCi, the prompt will change:</para>
515 Prelude> :load Main.hs
516 Compiling Main ( Main.hs, interpreted )
520 <para>The new prompt is <literal>*Main</literal>, which
521 indicates that we are typing expressions in the context of the
522 top-level of the <literal>Main</literal> module. Everything
523 that is in scope at the top-level in the module
524 <literal>Main</literal> we just loaded is also in scope at the
525 prompt (probably including <literal>Prelude</literal>, as long
526 as <literal>Main</literal> doesn't explicitly hide it).</para>
529 <literal>*<replaceable>module</replaceable></literal> indicates
530 that it is the full top-level scope of
531 <replaceable>module</replaceable> that is contributing to the
532 scope for expressions typed at the prompt. Without the
533 <literal>*</literal>, just the exports of the module are
536 <para>We're not limited to a single module: GHCi can combine
537 scopes from multiple modules, in any mixture of
538 <literal>*</literal> and non-<literal>*</literal> forms. GHCi
539 combines the scopes from all of these modules to form the scope
540 that is in effect at the prompt. For technical reasons, GHCi
541 can only support the <literal>*</literal>-form for modules which
542 are interpreted, so compiled modules and package modules can
543 only contribute their exports to the current scope.</para>
545 <para>The scope is manipulated using the
546 <literal>:module</literal> command. For example, if the current
547 scope is <literal>Prelude</literal>, then we can bring into
548 scope the exports from the module <literal>IO</literal> like
553 Prelude IO> hPutStrLn stdout "hello\n"
558 <para>(Note: you can use <literal>import M</literal> as an
559 alternative to <literal>:module +M</literal>, and
560 <literal>:module</literal> can also be shortened to
561 <literal>:m</literal>). The full syntax of the
562 <literal>:module</literal> command is:</para>
565 :module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
568 <para>Using the <literal>+</literal> form of the
569 <literal>module</literal> commands adds modules to the current
570 scope, and <literal>-</literal> removes them. Without either
571 <literal>+</literal> or <literal>-</literal>, the current scope
572 is replaced by the set of modules specified. Note that if you
573 use this form and leave out <literal>Prelude</literal>, GHCi
574 will assume that you really wanted the
575 <literal>Prelude</literal> and add it in for you (if you don't
576 want the <literal>Prelude</literal>, then ask to remove it with
577 <literal>:m -Prelude</literal>).</para>
579 <para>The scope is automatically set after a
580 <literal>:load</literal> command, to the most recently loaded
581 "target" module, in a <literal>*</literal>-form if possible.
582 For example, if you say <literal>:load foo.hs bar.hs</literal>
583 and <filename>bar.hs</filename> contains module
584 <literal>Bar</literal>, then the scope will be set to
585 <literal>*Bar</literal> if <literal>Bar</literal> is
586 interpreted, or if <literal>Bar</literal> is compiled it will be
587 set to <literal>Prelude Bar</literal> (GHCi automatically adds
588 <literal>Prelude</literal> if it isn't present and there aren't
589 any <literal>*</literal>-form modules).</para>
591 <para>With multiple modules in scope, especially multiple
592 <literal>*</literal>-form modules, it is likely that name
593 clashes will occur. Haskell specifies that name clashes are
594 only reported when an ambiguous identifier is used, and GHCi
595 behaves in the same way for expressions typed at the
599 Hint: GHCi will tab-complete names that are in scope; for
600 example, if you run GHCi and type <literal>J<tab></literal>
601 then GHCi will expand it to “<literal>Just </literal>”.
605 <title>Qualified names</title>
607 <para>To make life slightly easier, the GHCi prompt also
608 behaves as if there is an implicit <literal>import
609 qualified</literal> declaration for every module in every
610 package, and every module currently loaded into GHCi.</para>
614 <title>The <literal>:main</literal> and <literal>:run</literal> commands</title>
617 When a program is compiled and executed, it can use the
618 <literal>getArgs</literal> function to access the
619 command-line arguments.
620 However, we cannot simply pass the arguments to the
621 <literal>main</literal> function while we are testing in ghci,
622 as the <literal>main</literal> function doesn't take its
627 Instead, we can use the <literal>:main</literal> command.
628 This runs whatever <literal>main</literal> is in scope, with
629 any arguments being treated the same as command-line arguments,
634 Prelude> let main = System.Environment.getArgs >>= print
635 Prelude> :main foo bar
640 We can also quote arguments which contains characters like
641 spaces, and they are treated like Haskell strings, or we can
642 just use Haskell list syntax:
646 Prelude> :main foo "bar baz"
648 Prelude> :main ["foo", "bar baz"]
653 Finally, other functions can be called, either with the
654 <literal>-main-is</literal> flag or the <literal>:run</literal>
659 Prelude> let foo = putStrLn "foo" >> System.Environment.getArgs >>= print
660 Prelude> let bar = putStrLn "bar" >> System.Environment.getArgs >>= print
661 Prelude> :set -main-is foo
662 Prelude> :main foo "bar baz"
665 Prelude> :run bar ["foo", "bar baz"]
675 <title>The <literal>it</literal> variable</title>
676 <indexterm><primary><literal>it</literal></primary>
679 <para>Whenever an expression (or a non-binding statement, to be
680 precise) is typed at the prompt, GHCi implicitly binds its value
681 to the variable <literal>it</literal>. For example:</para>
688 <para>What actually happens is that GHCi typechecks the
689 expression, and if it doesn't have an <literal>IO</literal> type,
690 then it transforms it as follows: an expression
691 <replaceable>e</replaceable> turns into
693 let it = <replaceable>e</replaceable>;
696 which is then run as an IO-action.</para>
698 <para>Hence, the original expression must have a type which is an
699 instance of the <literal>Show</literal> class, or GHCi will
705 <interactive>:1:0:
706 No instance for (Show (a -> a))
707 arising from use of `print' at <interactive>:1:0-1
708 Possible fix: add an instance declaration for (Show (a -> a))
709 In the expression: print it
710 In a 'do' expression: print it
713 <para>The error message contains some clues as to the
714 transformation happening internally.</para>
716 <para>If the expression was instead of type <literal>IO a</literal> for
717 some <literal>a</literal>, then <literal>it</literal> will be
718 bound to the result of the <literal>IO</literal> computation,
719 which is of type <literal>a</literal>. eg.:</para>
721 Prelude> Time.getClockTime
722 Wed Mar 14 12:23:13 GMT 2001
724 Wed Mar 14 12:23:13 GMT 2001
727 <para>The corresponding translation for an IO-typed
728 <replaceable>e</replaceable> is
730 it <- <replaceable>e</replaceable>
734 <para>Note that <literal>it</literal> is shadowed by the new
735 value each time you evaluate a new expression, and the old value
736 of <literal>it</literal> is lost.</para>
740 <sect2 id="extended-default-rules">
741 <title>Type defaulting in GHCi</title>
742 <indexterm><primary>Type default</primary></indexterm>
743 <indexterm><primary><literal>Show</literal> class</primary></indexterm>
745 Consider this GHCi session:
749 What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
750 (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
751 on the type <literal>a</literal>. For example:
753 ghci> (reverse []) :: String
755 ghci> (reverse []) :: [Int]
758 However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
759 rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The
760 standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
761 a)</literal> for each type variable <literal>a</literal>, and defaults the
766 The type variable <literal>a</literal> appears in no
772 All the classes <literal>Ci</literal> are standard.
777 At least one of the classes <literal>Ci</literal> is
782 At the GHCi prompt, or with GHC if the
783 <literal>-XExtendedDefaultRules</literal> flag is given,
784 the following additional differences apply:
788 Rule 2 above is relaxed thus:
789 <emphasis>All</emphasis> of the classes
790 <literal>Ci</literal> are single-parameter type classes.
795 Rule 3 above is relaxed this:
796 At least one of the classes <literal>Ci</literal> is
797 numeric, <emphasis>or is <literal>Show</literal>,
798 <literal>Eq</literal>, or
799 <literal>Ord</literal></emphasis>.
804 The unit type <literal>()</literal> is added to the
805 start of the standard list of types which are tried when
806 doing type defaulting.
810 The last point means that, for example, this program:
817 def :: (Num a, Enum a) => a
820 prints <literal>()</literal> rather than <literal>0</literal> as the
821 type is defaulted to <literal>()</literal> rather than
822 <literal>Integer</literal>.
825 The motivation for the change is that it means <literal>IO a</literal>
826 actions default to <literal>IO ()</literal>, which in turn means that
827 ghci won't try to print a result when running them. This is
828 particularly important for <literal>printf</literal>, which has an
829 instance that returns <literal>IO a</literal>.
830 However, it is only able to return
831 <literal>undefined</literal>
832 (the reason for the instance having this type is so that printf
833 doesn't require extensions to the class system), so if the type defaults to
834 <literal>Integer</literal> then ghci gives an error when running a
840 <sect1 id="ghci-debugger">
841 <title>The GHCi Debugger</title>
842 <indexterm><primary>debugger</primary><secondary>in GHCi</secondary>
845 <para>GHCi contains a simple imperative-style debugger in which you can
846 stop a running computation in order to examine the values of
847 variables. The debugger is integrated into GHCi, and is turned on by
848 default: no flags are required to enable the debugging
849 facilities. There is one major restriction: breakpoints and
850 single-stepping are only available in interpreted modules;
851 compiled code is invisible to the debugger<footnote><para>Note that packages
852 only contain compiled code, so debugging a package requires
853 finding its source and loading that directly.</para></footnote>.</para>
855 <para>The debugger provides the following:
858 <para>The ability to set a <firstterm>breakpoint</firstterm> on a
859 function definition or expression in the program. When the function
860 is called, or the expression evaluated, GHCi suspends
861 execution and returns to the prompt, where you can inspect the
862 values of local variables before continuing with the
866 <para>Execution can be <firstterm>single-stepped</firstterm>: the
867 evaluator will suspend execution approximately after every
868 reduction, allowing local variables to be inspected. This is
869 equivalent to setting a breakpoint at every point in the
873 <para>Execution can take place in <firstterm>tracing
874 mode</firstterm>, in which the evaluator remembers each
875 evaluation step as it happens, but doesn't suspend execution until
876 an actual breakpoint is reached. When this happens, the history of
877 evaluation steps can be inspected.</para>
880 <para>Exceptions (e.g. pattern matching failure and
881 <literal>error</literal>) can be treated as breakpoints, to help
882 locate the source of an exception in the program.</para>
887 <para>There is currently no support for obtaining a “stack
888 trace”, but the tracing and history features provide a
889 useful second-best, which will often be enough to establish the
890 context of an error. For instance, it is possible to break
891 automatically when an exception is thrown, even if it is thrown
892 from within compiled code (see <xref
893 linkend="ghci-debugger-exceptions" />).</para>
895 <sect2 id="breakpoints">
896 <title>Breakpoints and inspecting variables</title>
898 <para>Let's use quicksort as a running example. Here's the code:</para>
902 qsort (a:as) = qsort left ++ [a] ++ qsort right
903 where (left,right) = (filter (<=a) as, filter (>a) as)
905 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
908 <para>First, load the module into GHCi:</para>
912 [1 of 1] Compiling Main ( qsort.hs, interpreted )
913 Ok, modules loaded: Main.
917 <para>Now, let's set a breakpoint on the right-hand-side of the second
918 equation of qsort:</para>
922 Breakpoint 0 activated at qsort.hs:2:15-46
926 <para>The command <literal>:break 2</literal> sets a breakpoint on line
927 2 of the most recently-loaded module, in this case
928 <literal>qsort.hs</literal>. Specifically, it picks the
929 leftmost complete subexpression on that line on which to set the
930 breakpoint, which in this case is the expression
931 <literal>(qsort left ++ [a] ++ qsort right)</literal>.</para>
933 <para>Now, we run the program:</para>
937 Stopped at qsort.hs:2:15-46
942 [qsort.hs:2:15-46] *Main>
945 <para>Execution has stopped at the breakpoint. The prompt has changed to
946 indicate that we are currently stopped at a breakpoint, and the location:
947 <literal>[qsort.hs:2:15-46]</literal>. To further clarify the
948 location, we can use the <literal>:list</literal> command:</para>
951 [qsort.hs:2:15-46] *Main> :list
953 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
954 3 where (left,right) = (filter (<=a) as, filter (>a) as)
957 <para>The <literal>:list</literal> command lists the source code around
958 the current breakpoint. If your output device supports it, then GHCi
959 will highlight the active subexpression in bold.</para>
961 <para>GHCi has provided bindings for the free variables<footnote><para>We
962 originally provided bindings for all variables in scope, rather
964 the free variables of the expression, but found that this affected
965 performance considerably, hence the current restriction to just the
966 free variables.</para>
967 </footnote> of the expression
969 breakpoint was placed (<literal>a</literal>, <literal>left</literal>,
970 <literal>right</literal>), and additionally a binding for the result of
971 the expression (<literal>_result</literal>). These variables are just
972 like other variables that you might define in GHCi; you
973 can use them in expressions that you type at the prompt, you can ask
974 for their types with <literal>:type</literal>, and so on. There is one
975 important difference though: these variables may only have partial
976 types. For example, if we try to display the value of
977 <literal>left</literal>:</para>
980 [qsort.hs:2:15-46] *Main> left
982 <interactive>:1:0:
983 Ambiguous type variable `a' in the constraint:
984 `Show a' arising from a use of `print' at <interactive>:1:0-3
985 Cannot resolve unknown runtime types: a
986 Use :print or :force to determine these types
989 <para>This is because <literal>qsort</literal> is a polymorphic function,
990 and because GHCi does not carry type information at runtime, it cannot
991 determine the runtime types of free variables that involve type
992 variables. Hence, when you ask to display <literal>left</literal> at
993 the prompt, GHCi can't figure out which instance of
994 <literal>Show</literal> to use, so it emits the type error above.</para>
996 <para>Fortunately, the debugger includes a generic printing command,
997 <literal>:print</literal>, which can inspect the actual runtime value of a
998 variable and attempt to reconstruct its type. If we try it on
999 <literal>left</literal>:</para>
1002 [qsort.hs:2:15-46] *Main> :set -fprint-evld-with-show
1003 [qsort.hs:2:15-46] *Main> :print left
1007 <para>This isn't particularly enlightening. What happened is that
1008 <literal>left</literal> is bound to an unevaluated computation (a
1009 suspension, or <firstterm>thunk</firstterm>), and
1010 <literal>:print</literal> does not force any evaluation. The idea is
1011 that <literal>:print</literal> can be used to inspect values at a
1012 breakpoint without any unfortunate side effects. It won't force any
1013 evaluation, which could cause the program to give a different answer
1014 than it would normally, and hence it won't cause any exceptions to be
1015 raised, infinite loops, or further breakpoints to be triggered (see
1016 <xref linkend="nested-breakpoints" />).
1017 Rather than forcing thunks, <literal>:print</literal>
1018 binds each thunk to a fresh variable beginning with an
1019 underscore, in this case
1020 <literal>_t1</literal>.</para>
1022 <para>The flag <literal>-fprint-evld-with-show</literal> instructs
1023 <literal>:print</literal> to reuse
1024 available <literal>Show</literal> instances when possible. This happens
1025 only when the contents of the variable being inspected
1026 are completely evaluated.</para>
1029 <para>If we aren't concerned about preserving the evaluatedness of a
1030 variable, we can use <literal>:force</literal> instead of
1031 <literal>:print</literal>. The <literal>:force</literal> command
1032 behaves exactly like <literal>:print</literal>, except that it forces
1033 the evaluation of any thunks it encounters:</para>
1036 [qsort.hs:2:15-46] *Main> :force left
1040 <para>Now, since <literal>:force</literal> has inspected the runtime
1041 value of <literal>left</literal>, it has reconstructed its type. We
1042 can see the results of this type reconstruction:</para>
1045 [qsort.hs:2:15-46] *Main> :show bindings
1046 _result :: [Integer]
1053 <para>Not only do we now know the type of <literal>left</literal>, but
1054 all the other partial types have also been resolved. So we can ask
1055 for the value of <literal>a</literal>, for example:</para>
1058 [qsort.hs:2:15-46] *Main> a
1062 <para>You might find it useful to use Haskell's
1063 <literal>seq</literal> function to evaluate individual thunks rather
1064 than evaluating the whole expression with <literal>:force</literal>.
1068 [qsort.hs:2:15-46] *Main> :print right
1069 right = (_t1::[Integer])
1070 [qsort.hs:2:15-46] *Main> seq _t1 ()
1072 [qsort.hs:2:15-46] *Main> :print right
1073 right = 23 : (_t2::[Integer])
1076 <para>We evaluated only the <literal>_t1</literal> thunk, revealing the
1077 head of the list, and the tail is another thunk now bound to
1078 <literal>_t2</literal>. The <literal>seq</literal> function is a
1079 little inconvenient to use here, so you might want to use
1080 <literal>:def</literal> to make a nicer interface (left as an exercise
1081 for the reader!).</para>
1083 <para>Finally, we can continue the current execution:</para>
1086 [qsort.hs:2:15-46] *Main> :continue
1087 Stopped at qsort.hs:2:15-46
1092 [qsort.hs:2:15-46] *Main>
1095 <para>The execution continued at the point it previously stopped, and has
1096 now stopped at the breakpoint for a second time.</para>
1099 <sect3 id="setting-breakpoints">
1100 <title>Setting breakpoints</title>
1102 <para>Breakpoints can be set in various ways. Perhaps the easiest way to
1103 set a breakpoint is to name a top-level function:</para>
1106 :break <replaceable>identifier</replaceable>
1109 <para>Where <replaceable>identifier</replaceable> names any top-level
1110 function in an interpreted module currently loaded into GHCi (qualified
1111 names may be used). The breakpoint will be set on the body of the
1112 function, when it is fully applied but before any pattern matching has
1115 <para>Breakpoints can also be set by line (and optionally column)
1119 :break <replaceable>line</replaceable>
1120 :break <replaceable>line</replaceable> <replaceable>column</replaceable>
1121 :break <replaceable>module</replaceable> <replaceable>line</replaceable>
1122 :break <replaceable>module</replaceable> <replaceable>line</replaceable> <replaceable>column</replaceable>
1125 <para>When a breakpoint is set on a particular line, GHCi sets the
1127 leftmost subexpression that begins and ends on that line. If two
1128 complete subexpressions start at the same
1129 column, the longest one is picked. If there is no complete
1130 subexpression on the line, then the leftmost expression starting on
1131 the line is picked, and failing that the rightmost expression that
1132 partially or completely covers the line.</para>
1134 <para>When a breakpoint is set on a particular line and column, GHCi
1135 picks the smallest subexpression that encloses that location on which
1136 to set the breakpoint. Note: GHC considers the TAB character to have a
1137 width of 1, wherever it occurs; in other words it counts
1138 characters, rather than columns. This matches what some editors do,
1139 and doesn't match others. The best advice is to avoid tab
1140 characters in your source code altogether (see
1141 <option>-fwarn-tabs</option> in <xref linkend="options-sanity"
1144 <para>If the module is omitted, then the most recently-loaded module is
1147 <para>Not all subexpressions are potential breakpoint locations. Single
1148 variables are typically not considered to be breakpoint locations
1149 (unless the variable is the right-hand-side of a function definition,
1150 lambda, or case alternative). The rule of thumb is that all redexes
1151 are breakpoint locations, together with the bodies of functions,
1152 lambdas, case alternatives and binding statements. There is normally
1153 no breakpoint on a let expression, but there will always be a
1154 breakpoint on its body, because we are usually interested in inspecting
1155 the values of the variables bound by the let.</para>
1159 <title>Listing and deleting breakpoints</title>
1161 <para>The list of breakpoints currently enabled can be displayed using
1162 <literal>:show breaks</literal>:</para>
1165 [0] Main qsort.hs:1:11-12
1166 [1] Main qsort.hs:2:15-46
1169 <para>To delete a breakpoint, use the <literal>:delete</literal>
1170 command with the number given in the output from <literal>:show breaks</literal>:</para>
1175 [1] Main qsort.hs:2:15-46
1178 <para>To delete all breakpoints at once, use <literal>:delete *</literal>.</para>
1183 <sect2 id="single-stepping">
1184 <title>Single-stepping</title>
1186 <para>Single-stepping is a great way to visualise the execution of your
1187 program, and it is also a useful tool for identifying the source of a
1188 bug. GHCi offers two variants of stepping. Use
1189 <literal>:step</literal> to enable all the
1190 breakpoints in the program, and execute until the next breakpoint is
1191 reached. Use <literal>:steplocal</literal> to limit the set
1192 of enabled breakpoints to those in the current top level function.
1193 Similarly, use <literal>:stepmodule</literal> to single step only on
1194 breakpoints contained in the current module.
1199 Stopped at qsort.hs:5:7-47
1203 <para>The command <literal>:step
1204 <replaceable>expr</replaceable></literal> begins the evaluation of
1205 <replaceable>expr</replaceable> in single-stepping mode. If
1206 <replaceable>expr</replaceable> is omitted, then it single-steps from
1207 the current breakpoint. <literal>:stepover</literal>
1208 works similarly.</para>
1210 <para>The <literal>:list</literal> command is particularly useful when
1211 single-stepping, to see where you currently are:</para>
1214 [qsort.hs:5:7-47] *Main> :list
1216 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1218 [qsort.hs:5:7-47] *Main>
1221 <para>In fact, GHCi provides a way to run a command when a breakpoint is
1222 hit, so we can make it automatically do
1223 <literal>:list</literal>:</para>
1226 [qsort.hs:5:7-47] *Main> :set stop :list
1227 [qsort.hs:5:7-47] *Main> :step
1228 Stopped at qsort.hs:5:14-46
1229 _result :: [Integer]
1231 5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
1233 [qsort.hs:5:14-46] *Main>
1237 <sect2 id="nested-breakpoints">
1238 <title>Nested breakpoints</title>
1239 <para>When GHCi is stopped at a breakpoint, and an expression entered at
1240 the prompt triggers a
1241 second breakpoint, the new breakpoint becomes the “current”
1242 one, and the old one is saved on a stack. An arbitrary number of
1243 breakpoint contexts can be built up in this way. For example:</para>
1246 [qsort.hs:2:15-46] *Main> :st qsort [1,3]
1247 Stopped at qsort.hs:(1,0)-(3,55)
1249 ... [qsort.hs:(1,0)-(3,55)] *Main>
1252 <para>While stopped at the breakpoint on line 2 that we set earlier, we
1253 started a new evaluation with <literal>:step qsort [1,3]</literal>.
1254 This new evaluation stopped after one step (at the definition of
1255 <literal>qsort</literal>). The prompt has changed, now prefixed with
1256 <literal>...</literal>, to indicate that there are saved breakpoints
1257 beyond the current one. To see the stack of contexts, use
1258 <literal>:show context</literal>:</para>
1261 ... [qsort.hs:(1,0)-(3,55)] *Main> :show context
1263 Stopped at qsort.hs:2:15-46
1265 Stopped at qsort.hs:(1,0)-(3,55)
1266 ... [qsort.hs:(1,0)-(3,55)] *Main>
1269 <para>To abandon the current evaluation, use
1270 <literal>:abandon</literal>:</para>
1273 ... [qsort.hs:(1,0)-(3,55)] *Main> :abandon
1274 [qsort.hs:2:15-46] *Main> :abandon
1279 <sect2 id="ghci-debugger-result">
1280 <title>The <literal>_result</literal> variable</title>
1281 <para>When stopped at a breakpoint or single-step, GHCi binds the
1282 variable <literal>_result</literal> to the value of the currently
1283 active expression. The value of <literal>_result</literal> is
1284 presumably not available yet, because we stopped its evaluation, but it
1285 can be forced: if the type is known and showable, then just entering
1286 <literal>_result</literal> at the prompt will show it. However,
1287 there's one caveat to doing this: evaluating <literal>_result</literal>
1288 will be likely to trigger further breakpoints, starting with the
1289 breakpoint we are currently stopped at (if we stopped at a real
1290 breakpoint, rather than due to <literal>:step</literal>). So it will
1291 probably be necessary to issue a <literal>:continue</literal>
1292 immediately when evaluating <literal>_result</literal>. Alternatively,
1293 you can use <literal>:force</literal> which ignores breakpoints.</para>
1296 <sect2 id="tracing">
1297 <title>Tracing and history</title>
1299 <para>A question that we often want to ask when debugging a program is
1300 “how did I get here?”. Traditional imperative debuggers
1301 usually provide some kind of stack-tracing feature that lets you see
1302 the stack of active function calls (sometimes called the “lexical
1303 call stack”), describing a path through the code
1304 to the current location. Unfortunately this is hard to provide in
1305 Haskell, because execution proceeds on a demand-driven basis, rather
1306 than a depth-first basis as in strict languages. The
1307 “stack“ in GHC's execution engine bears little
1308 resemblance to the lexical call stack. Ideally GHCi would maintain a
1309 separate lexical call stack in addition to the dynamic call stack, and
1310 in fact this is exactly
1311 what our profiling system does (<xref linkend="profiling" />), and what
1312 some other Haskell debuggers do. For the time being, however, GHCi
1313 doesn't maintain a lexical call stack (there are some technical
1314 challenges to be overcome). Instead, we provide a way to backtrack from a
1315 breakpoint to previous evaluation steps: essentially this is like
1316 single-stepping backwards, and should in many cases provide enough
1317 information to answer the “how did I get here?”
1320 <para>To use tracing, evaluate an expression with the
1321 <literal>:trace</literal> command. For example, if we set a breakpoint
1322 on the base case of <literal>qsort</literal>:</para>
1325 *Main> :list qsort
1327 2 qsort (a:as) = qsort left ++ [a] ++ qsort right
1328 3 where (left,right) = (filter (<=a) as, filter (>a) as)
1331 Breakpoint 1 activated at qsort.hs:1:11-12
1335 <para>and then run a small <literal>qsort</literal> with
1339 *Main> :trace qsort [3,2,1]
1340 Stopped at qsort.hs:1:11-12
1342 [qsort.hs:1:11-12] *Main>
1345 <para>We can now inspect the history of evaluation steps:</para>
1348 [qsort.hs:1:11-12] *Main> :hist
1349 -1 : qsort.hs:3:24-38
1350 -2 : qsort.hs:3:23-55
1351 -3 : qsort.hs:(1,0)-(3,55)
1352 -4 : qsort.hs:2:15-24
1353 -5 : qsort.hs:2:15-46
1354 -6 : qsort.hs:3:24-38
1355 -7 : qsort.hs:3:23-55
1356 -8 : qsort.hs:(1,0)-(3,55)
1357 -9 : qsort.hs:2:15-24
1358 -10 : qsort.hs:2:15-46
1359 -11 : qsort.hs:3:24-38
1360 -12 : qsort.hs:3:23-55
1361 -13 : qsort.hs:(1,0)-(3,55)
1362 -14 : qsort.hs:2:15-24
1363 -15 : qsort.hs:2:15-46
1364 -16 : qsort.hs:(1,0)-(3,55)
1365 <end of history>
1368 <para>To examine one of the steps in the history, use
1369 <literal>:back</literal>:</para>
1372 [qsort.hs:1:11-12] *Main> :back
1373 Logged breakpoint at qsort.hs:3:24-38
1377 [-1: qsort.hs:3:24-38] *Main>
1380 <para>Note that the local variables at each step in the history have been
1381 preserved, and can be examined as usual. Also note that the prompt has
1382 changed to indicate that we're currently examining the first step in
1383 the history: <literal>-1</literal>. The command
1384 <literal>:forward</literal> can be used to traverse forward in the
1387 <para>The <literal>:trace</literal> command can be used with or without
1388 an expression. When used without an expression, tracing begins from
1389 the current breakpoint, just like <literal>:step</literal>.</para>
1391 <para>The history is only available when
1392 using <literal>:trace</literal>; the reason for this is we found that
1393 logging each breakpoint in the history cuts performance by a factor of
1394 2 or more. GHCi remembers the last 50 steps in the history (perhaps in
1395 the future we'll make this configurable).</para>
1398 <sect2 id="ghci-debugger-exceptions">
1399 <title>Debugging exceptions</title>
1400 <para>Another common question that comes up when debugging is
1401 “where did this exception come from?”. Exceptions such as
1402 those raised by <literal>error</literal> or <literal>head []</literal>
1403 have no context information attached to them. Finding which
1404 particular call to <literal>head</literal> in your program resulted in
1405 the error can be a painstaking process, usually involving
1406 <literal>Debug.Trace.trace</literal>, or compiling with
1407 profiling and using <literal>+RTS -xc</literal> (see <xref
1408 linkend="prof-time-options" />).</para>
1410 <para>The GHCi debugger offers a way to hopefully shed some light on
1411 these errors quickly and without modifying or recompiling the source
1412 code. One way would be to set a breakpoint on the location in the
1413 source code that throws the exception, and then use
1414 <literal>:trace</literal> and <literal>:history</literal> to establish
1415 the context. However, <literal>head</literal> is in a library and
1416 we can't set a breakpoint on it directly. For this reason, GHCi
1417 provides the flags <literal>-fbreak-on-exception</literal> which causes
1418 the evaluator to stop when an exception is thrown, and <literal>
1419 -fbreak-on-error</literal>, which works similarly but stops only on
1420 uncaught exceptions. When stopping at an exception, GHCi will act
1421 just as it does when a breakpoint is hit, with the deviation that it
1422 will not show you any source code location. Due to this, these
1423 commands are only really useful in conjunction with
1424 <literal>:trace</literal>, in order to log the steps leading up to the
1425 exception. For example:</para>
1428 *Main> :set -fbreak-on-exception
1429 *Main> :trace qsort ("abc" ++ undefined)
1430 "Stopped at <exception thrown>
1432 [<exception thrown>] *Main> :hist
1433 -1 : qsort.hs:3:24-38
1434 -2 : qsort.hs:3:23-55
1435 -3 : qsort.hs:(1,0)-(3,55)
1436 -4 : qsort.hs:2:15-24
1437 -5 : qsort.hs:2:15-46
1438 -6 : qsort.hs:(1,0)-(3,55)
1439 <end of history>
1440 [<exception thrown>] *Main> :back
1441 Logged breakpoint at qsort.hs:3:24-38
1445 [-1: qsort.hs:3:24-38] *Main> :force as
1446 *** Exception: Prelude.undefined
1447 [-1: qsort.hs:3:24-38] *Main> :print as
1448 as = 'b' : 'c' : (_t1::[Char])
1451 <para>The exception itself is bound to a new variable,
1452 <literal>_exception</literal>.</para>
1454 <para>Breaking on exceptions is particularly useful for finding out what
1455 your program was doing when it was in an infinite loop. Just hit
1456 Control-C, and examine the history to find out what was going
1460 <sect2><title>Example: inspecting functions</title>
1462 It is possible to use the debugger to examine function values.
1463 When we are at a breakpoint and a function is in scope, the debugger
1465 you the source code for it; however, it is possible to get some
1466 information by applying it to some arguments and observing the result.
1470 The process is slightly complicated when the binding is polymorphic.
1471 We show the process by means of an example.
1472 To keep things simple, we will use the well known <literal>map</literal> function:
1474 import Prelude hiding (map)
1476 map :: (a->b) -> [a] -> [b]
1478 map f (x:xs) = f x : map f xs
1483 We set a breakpoint on <literal>map</literal>, and call it.
1486 Breakpoint 0 activated at map.hs:5:15-28
1487 *Main> map Just [1..5]
1488 Stopped at map.hs:(4,0)-(5,12)
1494 GHCi tells us that, among other bindings, <literal>f</literal> is in scope.
1495 However, its type is not fully known yet,
1496 and thus it is not possible to apply it to any
1497 arguments. Nevertheless, observe that the type of its first argument is the
1498 same as the type of <literal>x</literal>, and its result type is shared
1499 with <literal>_result</literal>.
1503 As we demonstrated earlier (<xref linkend="breakpoints" />), the
1504 debugger has some intelligence built-in to update the type of
1505 <literal>f</literal> whenever the types of <literal>x</literal> or
1506 <literal>_result</literal> are discovered. So what we do in this
1508 force <literal>x</literal> a bit, in order to recover both its type
1509 and the argument part of <literal>f</literal>.
1517 We can check now that as expected, the type of <literal>x</literal>
1518 has been reconstructed, and with it the
1519 type of <literal>f</literal> has been too:</para>
1527 From here, we can apply f to any argument of type Integer and observe
1535 Ambiguous type variable `b' in the constraint:
1536 `Show b' arising from a use of `print' at <interactive>:1:0
1548 f :: Integer -> Maybe Integer
1552 [Just 1, Just 2, Just 3, Just 4, Just 5]
1554 In the first application of <literal>f</literal>, we had to do
1555 some more type reconstruction
1556 in order to recover the result type of <literal>f</literal>.
1557 But after that, we are free to use
1558 <literal>f</literal> normally.
1562 <sect2><title>Limitations</title>
1565 <para>When stopped at a breakpoint, if you try to evaluate a variable
1566 that is already under evaluation, the second evaluation will hang.
1568 that GHC knows the variable is under evaluation, so the new
1569 evaluation just waits for the result before continuing, but of
1570 course this isn't going to happen because the first evaluation is
1571 stopped at a breakpoint. Control-C can interrupt the hung
1572 evaluation and return to the prompt.</para>
1573 <para>The most common way this can happen is when you're evaluating a
1574 CAF (e.g. main), stop at a breakpoint, and ask for the value of the
1575 CAF at the prompt again.</para>
1578 Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
1579 at the scope of a breakpoint if there is an explicit type signature.
1586 <sect1 id="ghci-invocation">
1587 <title>Invoking GHCi</title>
1588 <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
1589 <indexterm><primary><option>––interactive</option></primary></indexterm>
1591 <para>GHCi is invoked with the command <literal>ghci</literal> or
1592 <literal>ghc ––interactive</literal>. One or more modules or
1593 filenames can also be specified on the command line; this
1594 instructs GHCi to load the specified modules or filenames (and all
1595 the modules they depend on), just as if you had said
1596 <literal>:load <replaceable>modules</replaceable></literal> at the
1597 GHCi prompt (see <xref linkend="ghci-commands" />). For example, to
1598 start GHCi and load the program whose topmost module is in the
1599 file <literal>Main.hs</literal>, we could say:</para>
1605 <para>Most of the command-line options accepted by GHC (see <xref
1606 linkend="using-ghc"/>) also make sense in interactive mode. The ones
1607 that don't make sense are mostly obvious.</para>
1610 <title>Packages</title>
1611 <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
1613 <para>Most packages (see <xref linkend="using-packages"/>) are
1614 available without needing to specify any extra flags at all:
1615 they will be automatically loaded the first time they are
1618 <para>For hidden packages, however, you need to request the
1619 package be loaded by using the <literal>-package</literal> flag:</para>
1622 $ ghci -package readline
1623 GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
1624 Loading package base ... linking ... done.
1625 Loading package readline-1.0 ... linking ... done.
1629 <para>The following command works to load new packages into a
1630 running GHCi:</para>
1633 Prelude> :set -package <replaceable>name</replaceable>
1636 <para>But note that doing this will cause all currently loaded
1637 modules to be unloaded, and you'll be dumped back into the
1638 <literal>Prelude</literal>.</para>
1642 <title>Extra libraries</title>
1643 <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
1645 <para>Extra libraries may be specified on the command line using
1646 the normal <literal>-l<replaceable>lib</replaceable></literal>
1647 option. (The term <emphasis>library</emphasis> here refers to
1648 libraries of foreign object code; for using libraries of Haskell
1649 source code, see <xref linkend="ghci-modules-filenames"/>.) For
1650 example, to load the “m” library:</para>
1656 <para>On systems with <literal>.so</literal>-style shared
1657 libraries, the actual library loaded will the
1658 <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
1659 searches the following places for libraries, in this order:</para>
1663 <para>Paths specified using the
1664 <literal>-L<replaceable>path</replaceable></literal>
1665 command-line option,</para>
1668 <para>the standard library search path for your system,
1669 which on some systems may be overridden by setting the
1670 <literal>LD_LIBRARY_PATH</literal> environment
1675 <para>On systems with <literal>.dll</literal>-style shared
1676 libraries, the actual library loaded will be
1677 <filename><replaceable>lib</replaceable>.dll</filename>. Again,
1678 GHCi will signal an error if it can't find the library.</para>
1680 <para>GHCi can also load plain object files
1681 (<literal>.o</literal> or <literal>.obj</literal> depending on
1682 your platform) from the command-line. Just add the name the
1683 object file to the command line.</para>
1685 <para>Ordering of <option>-l</option> options matters: a library
1686 should be mentioned <emphasis>before</emphasis> the libraries it
1687 depends on (see <xref linkend="options-linker"/>).</para>
1692 <sect1 id="ghci-commands">
1693 <title>GHCi commands</title>
1695 <para>GHCi commands all begin with
1696 ‘<literal>:</literal>’ and consist of a single command
1697 name followed by zero or more parameters. The command name may be
1698 abbreviated, with ambiguities being resolved in favour of the more
1699 commonly used commands.</para>
1704 <literal>:abandon</literal>
1705 <indexterm><primary><literal>:abandon</literal></primary></indexterm>
1708 <para>Abandons the current evaluation (only available when stopped at
1709 a breakpoint).</para>
1715 <literal>:add</literal> <replaceable>module</replaceable> ...
1716 <indexterm><primary><literal>:add</literal></primary></indexterm>
1719 <para>Add <replaceable>module</replaceable>(s) to the
1720 current <firstterm>target set</firstterm>, and perform a
1727 <literal>:back</literal>
1728 <indexterm><primary><literal>:back</literal></primary></indexterm>
1731 <para>Travel back one step in the history. See <xref
1732 linkend="tracing" />. See also:
1733 <literal>:trace</literal>, <literal>:history</literal>,
1734 <literal>:forward</literal>.</para>
1740 <literal>:break [<replaceable>identifier</replaceable> |
1741 [<replaceable>module</replaceable>] <replaceable>line</replaceable>
1742 [<replaceable>column</replaceable>]]</literal>
1744 <indexterm><primary><literal>:break</literal></primary></indexterm>
1746 <para>Set a breakpoint on the specified function or line and
1747 column. See <xref linkend="setting-breakpoints" />.</para>
1753 <literal>:browse</literal><optional><literal>!</literal></optional> <optional><optional><literal>*</literal></optional><replaceable>module</replaceable></optional> ...
1754 <indexterm><primary><literal>:browse</literal></primary></indexterm>
1757 <para>Displays the identifiers defined by the module
1758 <replaceable>module</replaceable>, which must be either
1759 loaded into GHCi or be a member of a package. If
1760 <replaceable>module</replaceable> is omitted, the most
1761 recently-loaded module is used.</para>
1763 <para>If the <literal>*</literal> symbol is placed before
1764 the module name, then <emphasis>all</emphasis> the
1765 identifiers in scope in <replaceable>module</replaceable> are
1766 shown; otherwise the list is limited to the exports of
1767 <replaceable>module</replaceable>. The
1768 <literal>*</literal>-form is only available for modules
1769 which are interpreted; for compiled modules (including
1770 modules from packages) only the non-<literal>*</literal>
1771 form of <literal>:browse</literal> is available.
1772 If the <literal>!</literal> symbol is appended to the
1773 command, data constructors and class methods will be
1774 listed individually, otherwise, they will only be listed
1775 in the context of their data type or class declaration.
1776 The <literal>!</literal>-form also annotates the listing
1777 with comments giving possible imports for each group of
1780 Prelude> :browse! Data.Maybe
1781 -- not currently imported
1782 Data.Maybe.catMaybes :: [Maybe a] -> [a]
1783 Data.Maybe.fromJust :: Maybe a -> a
1784 Data.Maybe.fromMaybe :: a -> Maybe a -> a
1785 Data.Maybe.isJust :: Maybe a -> Bool
1786 Data.Maybe.isNothing :: Maybe a -> Bool
1787 Data.Maybe.listToMaybe :: [a] -> Maybe a
1788 Data.Maybe.mapMaybe :: (a -> Maybe b) -> [a] -> [b]
1789 Data.Maybe.maybeToList :: Maybe a -> [a]
1790 -- imported via Prelude
1791 Just :: a -> Maybe a
1792 data Maybe a = Nothing | Just a
1794 maybe :: b -> (a -> b) -> Maybe a -> b
1797 This output shows that, in the context of the current session, in the scope
1798 of <literal>Prelude</literal>, the first group of items from
1799 <literal>Data.Maybe</literal> have not been imported (but are available in
1800 fully qualified form in the GHCi session - see <xref
1801 linkend="ghci-scope"/>), whereas the second group of items have been
1802 imported via <literal>Prelude</literal> and are therefore available either
1803 unqualified, or with a <literal>Prelude.</literal> qualifier.
1810 <literal>:cd</literal> <replaceable>dir</replaceable>
1811 <indexterm><primary><literal>:cd</literal></primary></indexterm>
1814 <para>Changes the current working directory to
1815 <replaceable>dir</replaceable>. A
1816 ‘<literal>˜</literal>’ symbol at the
1817 beginning of <replaceable>dir</replaceable> will be replaced
1818 by the contents of the environment variable
1819 <literal>HOME</literal>.</para>
1821 <para>NOTE: changing directories causes all currently loaded
1822 modules to be unloaded. This is because the search path is
1823 usually expressed using relative directories, and changing
1824 the search path in the middle of a session is not
1831 <literal>:cmd</literal> <replaceable>expr</replaceable>
1832 <indexterm><primary><literal>:cmd</literal></primary></indexterm>
1835 <para>Executes <replaceable>expr</replaceable> as a computation of
1836 type <literal>IO String</literal>, and then executes the resulting
1837 string as a list of GHCi commands. Multiple commands are separated
1838 by newlines. The <literal>:cmd</literal> command is useful with
1839 <literal>:def</literal> and <literal>:set stop</literal>.</para>
1845 <literal>:continue</literal>
1846 <indexterm><primary><literal>:continue</literal></primary></indexterm>
1848 <listitem><para>Continue the current evaluation, when stopped at a
1855 <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
1856 <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
1857 <indexterm><primary><literal>:etags</literal></primary>
1859 <indexterm><primary><literal>:etags</literal></primary>
1863 <para>Generates a “tags” file for Vi-style editors
1864 (<literal>:ctags</literal>) or
1865 Emacs-style editors (<literal>:etags</literal>). If
1866 no filename is specified, the default <filename>tags</filename> or
1867 <filename>TAGS</filename> is
1868 used, respectively. Tags for all the functions, constructors and
1869 types in the currently loaded modules are created. All modules must
1870 be interpreted for these commands to work.</para>
1871 <para>See also <xref linkend="hasktags" />.</para>
1877 <literal>:def<optional>!</optional> <optional><replaceable>name</replaceable> <replaceable>expr</replaceable></optional></literal>
1878 <indexterm><primary><literal>:def</literal></primary></indexterm>
1881 <para><literal>:def</literal> is used to define new
1882 commands, or macros, in GHCi. The command
1883 <literal>:def</literal> <replaceable>name</replaceable>
1884 <replaceable>expr</replaceable> defines a new GHCi command
1885 <literal>:<replaceable>name</replaceable></literal>,
1886 implemented by the Haskell expression
1887 <replaceable>expr</replaceable>, which must have type
1888 <literal>String -> IO String</literal>. When
1889 <literal>:<replaceable>name</replaceable>
1890 <replaceable>args</replaceable></literal> is typed at the
1891 prompt, GHCi will run the expression
1892 <literal>(<replaceable>name</replaceable>
1893 <replaceable>args</replaceable>)</literal>, take the
1894 resulting <literal>String</literal>, and feed it back into
1895 GHCi as a new sequence of commands. Separate commands in
1896 the result must be separated by
1897 ‘<literal>\n</literal>’.</para>
1899 <para>That's all a little confusing, so here's a few
1900 examples. To start with, here's a new GHCi command which
1901 doesn't take any arguments or produce any results, it just
1902 outputs the current date & time:</para>
1905 Prelude> let date _ = Time.getClockTime >>= print >> return ""
1906 Prelude> :def date date
1908 Fri Mar 23 15:16:40 GMT 2001
1911 <para>Here's an example of a command that takes an argument.
1912 It's a re-implementation of <literal>:cd</literal>:</para>
1915 Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
1916 Prelude> :def mycd mycd
1920 <para>Or I could define a simple way to invoke
1921 “<literal>ghc ––make Main</literal>” in the
1922 current directory:</para>
1925 Prelude> :def make (\_ -> return ":! ghc ––make Main")
1928 <para>We can define a command that reads GHCi input from a
1929 file. This might be useful for creating a set of bindings
1930 that we want to repeatedly load into the GHCi session:</para>
1933 Prelude> :def . readFile
1934 Prelude> :. cmds.ghci
1937 <para>Notice that we named the command
1938 <literal>:.</literal>, by analogy with the
1939 ‘<literal>.</literal>’ Unix shell command that
1940 does the same thing.</para>
1942 <para>Typing <literal>:def</literal> on its own lists the
1943 currently-defined macros. Attempting to redefine an
1944 existing command name results in an error unless the
1945 <literal>:def!</literal> form is used, in which case the old
1946 command with that name is silently overwritten.</para>
1952 <literal>:delete * | <replaceable>num</replaceable> ...</literal>
1953 <indexterm><primary><literal>:delete</literal></primary></indexterm>
1956 <para>Delete one or more breakpoints by number (use <literal>:show
1957 breaks</literal> to see the number of each breakpoint). The
1958 <literal>*</literal> form deletes all the breakpoints.</para>
1964 <literal>:edit <optional><replaceable>file</replaceable></optional></literal>
1965 <indexterm><primary><literal>:edit</literal></primary></indexterm>
1968 <para>Opens an editor to edit the file
1969 <replaceable>file</replaceable>, or the most recently loaded
1970 module if <replaceable>file</replaceable> is omitted. The
1971 editor to invoke is taken from the <literal>EDITOR</literal>
1972 environment variable, or a default editor on your system if
1973 <literal>EDITOR</literal> is not set. You can change the
1974 editor using <literal>:set editor</literal>.</para>
1980 <literal>:etags</literal>
1983 <para>See <literal>:ctags</literal>.</para>
1989 <literal>:force <replaceable>identifier</replaceable> ...</literal>
1990 <indexterm><primary><literal>:force</literal></primary></indexterm>
1993 <para>Prints the value of <replaceable>identifier</replaceable> in
1994 the same way as <literal>:print</literal>. Unlike
1995 <literal>:print</literal>, <literal>:force</literal> evaluates each
1996 thunk that it encounters while traversing the value. This may
1997 cause exceptions or infinite loops, or further breakpoints (which
1998 are ignored, but displayed).</para>
2004 <literal>:forward</literal>
2005 <indexterm><primary><literal>:forward</literal></primary></indexterm>
2008 <para>Move forward in the history. See <xref
2009 linkend="tracing" />. See also:
2010 <literal>:trace</literal>, <literal>:history</literal>,
2011 <literal>:back</literal>.</para>
2017 <literal>:help</literal>
2018 <indexterm><primary><literal>:help</literal></primary></indexterm>
2021 <literal>:?</literal>
2022 <indexterm><primary><literal>:?</literal></primary></indexterm>
2025 <para>Displays a list of the available commands.</para>
2031 <literal>:</literal>
2032 <indexterm><primary><literal>:</literal></primary></indexterm>
2035 <para>Repeat the previous command.</para>
2042 <literal>:history [<replaceable>num</replaceable>]</literal>
2043 <indexterm><primary><literal>:history</literal></primary></indexterm>
2046 <para>Display the history of evaluation steps. With a number,
2047 displays that many steps (default: 20). For use with
2048 <literal>:trace</literal>; see <xref
2049 linkend="tracing" />.</para>
2055 <literal>:info</literal> <replaceable>name</replaceable> ...
2056 <indexterm><primary><literal>:info</literal></primary></indexterm>
2059 <para>Displays information about the given name(s). For
2060 example, if <replaceable>name</replaceable> is a class, then
2061 the class methods and their types will be printed; if
2062 <replaceable>name</replaceable> is a type constructor, then
2063 its definition will be printed; if
2064 <replaceable>name</replaceable> is a function, then its type
2065 will be printed. If <replaceable>name</replaceable> has
2066 been loaded from a source file, then GHCi will also display
2067 the location of its definition in the source.</para>
2068 <para>For types and classes, GHCi also summarises instances that
2069 mention them. To avoid showing irrelevant information, an instance
2070 is shown only if (a) its head mentions <replaceable>name</replaceable>,
2071 and (b) all the other things mentioned in the instance
2072 are in scope (either qualified or otherwise) as a result of
2073 a <literal>:load</literal> or <literal>:module</literal> commands. </para>
2079 <literal>:kind</literal> <replaceable>type</replaceable>
2080 <indexterm><primary><literal>:kind</literal></primary></indexterm>
2083 <para>Infers and prints the kind of
2084 <replaceable>type</replaceable>. The latter can be an arbitrary
2085 type expression, including a partial application of a type constructor,
2086 such as <literal>Either Int</literal>.</para>
2092 <literal>:load</literal> <replaceable>module</replaceable> ...
2093 <indexterm><primary><literal>:load</literal></primary></indexterm>
2096 <para>Recursively loads the specified
2097 <replaceable>module</replaceable>s, and all the modules they
2098 depend on. Here, each <replaceable>module</replaceable>
2099 must be a module name or filename, but may not be the name
2100 of a module in a package.</para>
2102 <para>All previously loaded modules, except package modules,
2103 are forgotten. The new set of modules is known as the
2104 <firstterm>target set</firstterm>. Note that
2105 <literal>:load</literal> can be used without any arguments
2106 to unload all the currently loaded modules and
2109 <para>After a <literal>:load</literal> command, the current
2110 context is set to:</para>
2114 <para><replaceable>module</replaceable>, if it was loaded
2115 successfully, or</para>
2118 <para>the most recently successfully loaded module, if
2119 any other modules were loaded as a result of the current
2120 <literal>:load</literal>, or</para>
2123 <para><literal>Prelude</literal> otherwise.</para>
2131 <literal>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
2132 <indexterm><primary><literal>:main</literal></primary></indexterm>
2136 When a program is compiled and executed, it can use the
2137 <literal>getArgs</literal> function to access the
2138 command-line arguments.
2139 However, we cannot simply pass the arguments to the
2140 <literal>main</literal> function while we are testing in ghci,
2141 as the <literal>main</literal> function doesn't take its
2146 Instead, we can use the <literal>:main</literal> command.
2147 This runs whatever <literal>main</literal> is in scope, with
2148 any arguments being treated the same as command-line arguments,
2153 Prelude> let main = System.Environment.getArgs >>= print
2154 Prelude> :main foo bar
2159 We can also quote arguments which contains characters like
2160 spaces, and they are treated like Haskell strings, or we can
2161 just use Haskell list syntax:
2165 Prelude> :main foo "bar baz"
2167 Prelude> :main ["foo", "bar baz"]
2172 Finally, other functions can be called, either with the
2173 <literal>-main-is</literal> flag or the <literal>:run</literal>
2178 Prelude> let foo = putStrLn "foo" >> System.Environment.getArgs >>= print
2179 Prelude> let bar = putStrLn "bar" >> System.Environment.getArgs >>= print
2180 Prelude> :set -main-is foo
2181 Prelude> :main foo "bar baz"
2184 Prelude> :run bar ["foo", "bar baz"]
2194 <literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal>
2195 <indexterm><primary><literal>:module</literal></primary></indexterm>
2198 <literal>import <replaceable>mod</replaceable></literal>
2201 <para>Sets or modifies the current context for statements
2202 typed at the prompt. The form <literal>import
2203 <replaceable>mod</replaceable></literal> is equivalent to
2204 <literal>:module +<replaceable>mod</replaceable></literal>.
2205 See <xref linkend="ghci-scope"/> for
2206 more details.</para>
2212 <literal>:print </literal> <replaceable>names</replaceable> ...
2213 <indexterm><primary><literal>:print</literal></primary></indexterm>
2216 <para>Prints a value without forcing its evaluation.
2217 <literal>:print</literal> may be used on values whose types are
2218 unknown or partially known, which might be the case for local
2219 variables with polymorphic types at a breakpoint. While inspecting
2220 the runtime value, <literal>:print</literal> attempts to
2221 reconstruct the type of the value, and will elaborate the type in
2222 GHCi's environment if possible. If any unevaluated components
2223 (thunks) are encountered, then <literal>:print</literal> binds
2224 a fresh variable with a name beginning with <literal>_t</literal>
2225 to each thunk. See <xref linkend="breakpoints" /> for more
2226 information. See also the <literal>:sprint</literal> command,
2227 which works like <literal>:print</literal> but does not bind new
2234 <literal>:quit</literal>
2235 <indexterm><primary><literal>:quit</literal></primary></indexterm>
2238 <para>Quits GHCi. You can also quit by typing control-D
2239 at the prompt.</para>
2245 <literal>:reload</literal>
2246 <indexterm><primary><literal>:reload</literal></primary></indexterm>
2249 <para>Attempts to reload the current target set (see
2250 <literal>:load</literal>) if any of the modules in the set,
2251 or any dependent module, has changed. Note that this may
2252 entail loading new modules, or dropping modules which are no
2253 longer indirectly required by the target.</para>
2259 <literal>:set</literal> <optional><replaceable>option</replaceable>...</optional>
2260 <indexterm><primary><literal>:set</literal></primary></indexterm>
2263 <para>Sets various options. See <xref linkend="ghci-set"/> for a list of
2264 available options and <xref linkend="interactive-mode-options"/> for a
2265 list of GHCi-specific flags. The <literal>:set</literal> command by
2266 itself shows which options are currently set. It also lists the current
2267 dynamic flag settings, with GHCi-specific flags listed separately.</para>
2273 <literal>:set</literal> <literal>args</literal> <replaceable>arg</replaceable> ...
2274 <indexterm><primary><literal>:set args</literal></primary></indexterm>
2277 <para>Sets the list of arguments which are returned when the
2278 program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
2279 </indexterm>.</para>
2285 <literal>:set</literal> <literal>editor</literal> <replaceable>cmd</replaceable>
2288 <para>Sets the command used by <literal>:edit</literal> to
2289 <replaceable>cmd</replaceable>.</para>
2295 <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
2296 <indexterm><primary><literal>:set prog</literal></primary></indexterm>
2299 <para>Sets the string to be returned when the program calls
2300 <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
2301 </indexterm>.</para>
2307 <literal>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
2310 <para>Sets the string to be used as the prompt in GHCi.
2311 Inside <replaceable>prompt</replaceable>, the sequence
2312 <literal>%s</literal> is replaced by the names of the
2313 modules currently in scope, and <literal>%%</literal> is
2314 replaced by <literal>%</literal>.</para>
2320 <literal>:set</literal> <literal>stop</literal>
2321 [<replaceable>num</replaceable>] <replaceable>cmd</replaceable>
2324 <para>Set a command to be executed when a breakpoint is hit, or a new
2325 item in the history is selected. The most common use of
2326 <literal>:set stop</literal> is to display the source code at the
2327 current location, e.g. <literal>:set stop :list</literal>.</para>
2329 <para>If a number is given before the command, then the commands are
2330 run when the specified breakpoint (only) is hit. This can be quite
2331 useful: for example, <literal>:set stop 1 :continue</literal>
2332 effectively disables breakpoint 1, by running
2333 <literal>:continue</literal> whenever it is hit (although GHCi will
2334 still emit a message to say the breakpoint was hit). What's more,
2335 with cunning use of <literal>:def</literal> and
2336 <literal>:cmd</literal> you can use <literal>:set stop</literal> to
2337 implement conditional breakpoints:</para>
2339 *Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
2340 *Main> :set stop 0 :cond (x < 3)
2342 <para>Ignoring breakpoints for a specified number of iterations is
2343 also possible using similar techniques.</para>
2349 <literal>:show bindings</literal>
2350 <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
2353 <para>Show the bindings made at the prompt and their
2360 <literal>:show breaks</literal>
2361 <indexterm><primary><literal>:show breaks</literal></primary></indexterm>
2364 <para>List the active breakpoints.</para>
2370 <literal>:show context</literal>
2371 <indexterm><primary><literal>:show context</literal></primary></indexterm>
2374 <para>List the active evaluations that are stopped at breakpoints.</para>
2380 <literal>:show modules</literal>
2381 <indexterm><primary><literal>:show modules</literal></primary></indexterm>
2384 <para>Show the list of modules currently loaded.</para>
2390 <literal>:show packages</literal>
2391 <indexterm><primary><literal>:show packages</literal></primary></indexterm>
2394 <para>Show the currently active package flags, as well as the list of
2395 packages currently loaded.</para>
2401 <literal>:show languages</literal>
2402 <indexterm><primary><literal>:show languages</literal></primary></indexterm>
2405 <para>Show the currently active language flags.</para>
2412 <literal>:show [args|prog|prompt|editor|stop]</literal>
2413 <indexterm><primary><literal>:show</literal></primary></indexterm>
2416 <para>Displays the specified setting (see
2417 <literal>:set</literal>).</para>
2423 <literal>:sprint</literal>
2424 <indexterm><primary><literal>:sprint</literal></primary></indexterm>
2427 <para>Prints a value without forcing its evaluation.
2428 <literal>:sprint</literal> is similar to <literal>:print</literal>,
2429 with the difference that unevaluated subterms are not bound to new
2430 variables, they are simply denoted by ‘_’.</para>
2436 <literal>:step [<replaceable>expr</replaceable>]</literal>
2437 <indexterm><primary><literal>:step</literal></primary></indexterm>
2440 <para>Single-step from the last breakpoint. With an expression
2441 argument, begins evaluation of the expression with a
2448 <literal>:trace [<replaceable>expr</replaceable>]</literal>
2449 <indexterm><primary><literal>:trace</literal></primary></indexterm>
2452 <para>Evaluates the given expression (or from the last breakpoint if
2453 no expression is given), and additionally logs the evaluation
2454 steps for later inspection using <literal>:history</literal>. See
2455 <xref linkend="tracing" />.</para>
2461 <literal>:type</literal> <replaceable>expression</replaceable>
2462 <indexterm><primary><literal>:type</literal></primary></indexterm>
2465 <para>Infers and prints the type of
2466 <replaceable>expression</replaceable>, including explicit
2467 forall quantifiers for polymorphic types. The monomorphism
2468 restriction is <emphasis>not</emphasis> applied to the
2469 expression during type inference.</para>
2475 <literal>:undef</literal> <replaceable>name</replaceable>
2476 <indexterm><primary><literal>:undef</literal></primary></indexterm>
2479 <para>Undefines the user-defined command
2480 <replaceable>name</replaceable> (see <literal>:def</literal>
2487 <literal>:unset</literal> <replaceable>option</replaceable>...
2488 <indexterm><primary><literal>:unset</literal></primary></indexterm>
2491 <para>Unsets certain options. See <xref linkend="ghci-set"/>
2492 for a list of available options.</para>
2498 <literal>:!</literal> <replaceable>command</replaceable>...
2499 <indexterm><primary><literal>:!</literal></primary></indexterm>
2500 <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
2503 <para>Executes the shell command
2504 <replaceable>command</replaceable>.</para>
2511 <sect1 id="ghci-set">
2512 <title>The <literal>:set</literal> command</title>
2513 <indexterm><primary><literal>:set</literal></primary></indexterm>
2515 <para>The <literal>:set</literal> command sets two types of
2516 options: GHCi options, which begin with
2517 ‘<literal>+</literal>’, and “command-line”
2518 options, which begin with ‘-’. </para>
2520 <para>NOTE: at the moment, the <literal>:set</literal> command
2521 doesn't support any kind of quoting in its arguments: quotes will
2522 not be removed and cannot be used to group words together. For
2523 example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
2527 <title>GHCi options</title>
2528 <indexterm><primary>options</primary><secondary>GHCi</secondary>
2531 <para>GHCi options may be set using <literal>:set</literal> and
2532 unset using <literal>:unset</literal>.</para>
2534 <para>The available GHCi options are:</para>
2539 <literal>+r</literal>
2540 <indexterm><primary><literal>+r</literal></primary></indexterm>
2541 <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
2542 <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
2545 <para>Normally, any evaluation of top-level expressions
2546 (otherwise known as CAFs or Constant Applicative Forms) in
2547 loaded modules is retained between evaluations. Turning
2548 on <literal>+r</literal> causes all evaluation of
2549 top-level expressions to be discarded after each
2550 evaluation (they are still retained
2551 <emphasis>during</emphasis> a single evaluation).</para>
2553 <para>This option may help if the evaluated top-level
2554 expressions are consuming large amounts of space, or if
2555 you need repeatable performance measurements.</para>
2561 <literal>+s</literal>
2562 <indexterm><primary><literal>+s</literal></primary></indexterm>
2565 <para>Display some stats after evaluating each expression,
2566 including the elapsed time and number of bytes allocated.
2567 NOTE: the allocation figure is only accurate to the size
2568 of the storage manager's allocation area, because it is
2569 calculated at every GC. Hence, you might see values of
2570 zero if no GC has occurred.</para>
2576 <literal>+t</literal>
2577 <indexterm><primary><literal>+t</literal></primary></indexterm>
2580 <para>Display the type of each variable bound after a
2581 statement is entered at the prompt. If the statement is a
2582 single expression, then the only variable binding will be
2584 ‘<literal>it</literal>’.</para>
2590 <sect2 id="ghci-cmd-line-options">
2591 <title>Setting GHC command-line options in GHCi</title>
2593 <para>Normal GHC command-line options may also be set using
2594 <literal>:set</literal>. For example, to turn on
2595 <option>-fglasgow-exts</option>, you would say:</para>
2598 Prelude> :set -fglasgow-exts
2601 <para>Any GHC command-line option that is designated as
2602 <firstterm>dynamic</firstterm> (see the table in <xref
2603 linkend="flag-reference"/>), may be set using
2604 <literal>:set</literal>. To unset an option, you can set the
2605 reverse option:</para>
2606 <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
2609 Prelude> :set -fno-glasgow-exts
2612 <para><xref linkend="flag-reference"/> lists the reverse for each
2613 option where applicable.</para>
2615 <para>Certain static options (<option>-package</option>,
2616 <option>-I</option>, <option>-i</option>, and
2617 <option>-l</option> in particular) will also work, but some may
2618 not take effect until the next reload.</para>
2619 <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
2622 <sect1 id="ghci-dot-files">
2623 <title>The <filename>.ghci</filename> file</title>
2624 <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
2626 <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
2629 <para>When it starts, unless the <literal>-ignore-dot-ghci</literal>
2630 flag is given, GHCi reads and executes commands from the following
2631 files, in this order, if they exist:</para>
2635 <para><filename>./.ghci</filename></para>
2638 <para><literal><replaceable>appdata</replaceable>/ghc/ghci.conf</literal>,
2639 where <replaceable>appdata</replaceable> depends on your system,
2640 but is usually something like <literal>C:/Documents and Settings/<replaceable>user</replaceable>/Application Data</literal></para>
2643 <para>On Unix: <literal>$HOME/.ghc/ghci.conf</literal></para>
2646 <para><literal>$HOME/.ghci</literal></para>
2650 <para>The <filename>ghci.conf</filename> file is most useful for
2651 turning on favourite options (eg. <literal>:set +s</literal>), and
2652 defining useful macros. Placing a <filename>.ghci</filename> file
2653 in a directory with a Haskell project is a useful way to set
2654 certain project-wide options so you don't have to type them
2655 everytime you start GHCi: eg. if your project uses GHC extensions
2656 and CPP, and has source files in three subdirectories A, B and C,
2657 you might put the following lines in
2658 <filename>.ghci</filename>:</para>
2661 :set -fglasgow-exts -cpp
2665 <para>(Note that strictly speaking the <option>-i</option> flag is
2666 a static one, but in fact it works to set it using
2667 <literal>:set</literal> like this. The changes won't take effect
2668 until the next <literal>:load</literal>, though.)</para>
2670 <para>Once you have a library of GHCi macros, you may want
2671 to source them from separate files, or you may want to source
2672 your <filename>.ghci</filename> file into your running GHCi
2673 session while debugging it</para>
2676 :def source readFile
2679 <para>With this macro defined in your <filename>.ghci</filename>
2680 file, you can use <literal>:source file</literal> to read GHCi
2681 commands from <literal>file</literal>. You can find (and contribute!-)
2682 other suggestions for <filename>.ghci</filename> files on this Haskell
2684 url="http://haskell.org/haskellwiki/GHC/GHCi">GHC/GHCi</ulink></para>
2686 <para>Two command-line options control whether the
2687 startup files files are read:</para>
2692 <option>-ignore-dot-ghci</option>
2693 <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
2696 <para>Don't read either <filename>./.ghci</filename> or the
2697 other startup files when starting up.</para>
2702 <option>-read-dot-ghci</option>
2703 <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
2706 <para>Read <filename>./.ghci</filename> and the other
2707 startup files (see above). This is normally the
2708 default, but the <option>-read-dot-ghci</option> option may
2709 be used to override a previous
2710 <option>-ignore-dot-ghci</option> option.</para>
2717 <sect1 id="ghci-obj">
2718 <title>Compiling to object code inside GHCi</title>
2720 <para>By default, GHCi compiles Haskell source code into byte-code
2721 that is interpreted by the runtime system. GHCi can also compile
2722 Haskell code to object code: to turn on this feature, use the
2723 <option>-fobject-code</option> flag either on the command line or
2724 with <literal>:set</literal> (the option
2725 <option>-fbyte-code</option> restores byte-code compilation
2726 again). Compiling to object code takes longer, but typically the
2727 code will execute 10-20 times faster than byte-code.</para>
2729 <para>Compiling to object code inside GHCi is particularly useful
2730 if you are developing a compiled application, because the
2731 <literal>:reload</literal> command typically runs much faster than
2732 restarting GHC with <option>--make</option> from the command-line,
2733 because all the interface files are already cached in
2736 <para>There are disadvantages to compiling to object-code: you
2737 can't set breakpoints in object-code modules, for example. Only
2738 the exports of an object-code module will be visible in GHCi,
2739 rather than all top-level bindings as in interpreted
2743 <sect1 id="ghci-faq">
2744 <title>FAQ and Things To Watch Out For</title>
2748 <term>The interpreter can't load modules with foreign export
2749 declarations!</term>
2751 <para>Unfortunately not. We haven't implemented it yet.
2752 Please compile any offending modules by hand before loading
2753 them into GHCi.</para>
2759 <literal>-O</literal> doesn't work with GHCi!
2760 <indexterm><primary><option>-O</option></primary></indexterm>
2763 <para>For technical reasons, the bytecode compiler doesn't
2764 interact well with one of the optimisation passes, so we
2765 have disabled optimisation when using the interpreter. This
2766 isn't a great loss: you'll get a much bigger win by
2767 compiling the bits of your code that need to go fast, rather
2768 than interpreting them with optimisation turned on.</para>
2773 <term>Unboxed tuples don't work with GHCi</term>
2775 <para>That's right. You can always compile a module that
2776 uses unboxed tuples and load it into GHCi, however.
2777 (Incidentally the previous point, namely that
2778 <literal>-O</literal> is incompatible with GHCi, is because
2779 the bytecode compiler can't deal with unboxed
2785 <term>Concurrent threads don't carry on running when GHCi is
2786 waiting for input.</term>
2788 <para>This should work, as long as your GHCi was built with
2789 the <option>-threaded</option> switch, which is the default.
2790 Consult whoever supplied your GHCi installation.</para>
2795 <term>After using <literal>getContents</literal>, I can't use
2796 <literal>stdin</literal> again until I do
2797 <literal>:load</literal> or <literal>:reload</literal>.</term>
2800 <para>This is the defined behaviour of
2801 <literal>getContents</literal>: it puts the stdin Handle in
2802 a state known as <firstterm>semi-closed</firstterm>, wherein
2803 any further I/O operations on it are forbidden. Because I/O
2804 state is retained between computations, the semi-closed
2805 state persists until the next <literal>:load</literal> or
2806 <literal>:reload</literal> command.</para>
2808 <para>You can make <literal>stdin</literal> reset itself
2809 after every evaluation by giving GHCi the command
2810 <literal>:set +r</literal>. This works because
2811 <literal>stdin</literal> is just a top-level expression that
2812 can be reverted to its unevaluated state in the same way as
2813 any other top-level expression (CAF).</para>
2818 <term>I can't use Control-C to interrupt computations in
2819 GHCi on Windows.</term>
2821 <para>See <xref linkend="ghci-windows"/>.</para>
2826 <term>The default buffering mode is different in GHCi to GHC.</term>
2829 In GHC, the stdout handle is line-buffered by default.
2830 However, in GHCi we turn off the buffering on stdout,
2831 because this is normally what you want in an interpreter:
2832 output appears as it is generated.
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