<chapter id="ghci">
<title>Using GHCi</title>
+ <indexterm><primary>GHCi</primary></indexterm>
+ <indexterm><primary>interpreter</primary><see>GHCi</see></indexterm>
+ <indexterm><primary>interactive</primary><see>GHCi</see></indexterm>
- <para>ToDo</para>
+ <para>GHCi<footnote>
+ <para>The ‘i’ stands for “Interactive”</para>
+ </footnote>
+ is GHC's interactive environment, in which Haskell expressions can
+ be interactively evaluated and programs can be interpreted. If
+ you're famililar with <ulink url="http://www.haskell.org/hugs/">Hugs</ulink><indexterm><primary>Hugs</primary>
+ </indexterm>, then you'll be right at home with GHCi. However, GHCi
+ also has support for interactively loading compiled code, as well as
+ supporting all<footnote><para>except <literal>foreign export</literal>, at the moment</para>
+ </footnote> the language extensions that GHC provides.</para>
+ <indexterm><primary>FFI</primary><secondary>GHCi support</secondary></indexterm>
+ <indexterm><primary>Foreign Function Interface</primary><secondary>GHCi support</secondary></indexterm>
+
+ <sect1>
+ <title>Introduction to GHCi</title>
+
+ <para>Let's start with an example GHCi session. You can fire up
+ GHCi with the command <literal>ghci</literal>:</para>
+
+<screen>
+$ ghci
+ ___ ___ _
+ / _ \ /\ /\/ __(_)
+ / /_\// /_/ / / | | GHC Interactive, version 5.04, for Haskell 98.
+/ /_\\/ __ / /___| | http://www.haskell.org/ghc/
+\____/\/ /_/\____/|_| Type :? for help.
+
+Loading package base ... linking ... done.
+Loading package haskell98 ... linking ... done.
+Prelude>
+</screen>
+
+ <para>There may be a short pause while GHCi loads the prelude and
+ standard libraries, after which the prompt is shown. If we follow
+ the instructions and type <literal>:?</literal> for help, we
+ get:</para>
+
+<screen>
+ Commands available from the prompt:
+
+ <stmt> evaluate/run <stmt>
+ :add <filename> ... add module(s) to the current target set
+ :browse [*]<module> display the names defined by <module>
+ :cd <dir> change directory to <dir>
+ :def <cmd> <expr> define a command :<cmd>
+ :help, :? display this list of commands
+ :info [<name> ...] display information about the given names
+ :load <filename> ... load module(s) and their dependents
+ :module [+/-] [*]<mod> ... set the context for expression evaluation
+ :reload reload the current module set
+
+ :set <option> ... set options
+ :set args <arg> ... set the arguments returned by System.getArgs
+ :set prog <progname> set the value returned by System.getProgName
+
+ :show modules show the currently loaded modules
+ :show bindings show the current bindings made at the prompt
+
+ :type <expr> show the type of <expr>
+ :undef <cmd> undefine user-defined command :<cmd>
+ :unset <option> ... unset options
+ :quit exit GHCi
+ :!<command> run the shell command <command>
+
+ Options for `:set' and `:unset':
+
+ +r revert top-level expressions after each evaluation
+ +s print timing/memory stats after each evaluation
+ +t print type after evaluation
+ -<flags> most GHC command line flags can also be set here
+ (eg. -v2, -fglasgow-exts, etc.)
+</screen>
+
+ <para>We'll explain most of these commands as we go along. For
+ Hugs users: many things work the same as in Hugs, so you should be
+ able to get going straight away.</para>
+
+ <para>Haskell expressions can be typed at the prompt:</para>
+ <indexterm><primary>prompt</primary><secondary>GHCi</secondary>
+ </indexterm>
+
+<screen>
+Prelude> 1+2
+3
+Prelude> let x = 42 in x / 9
+4.666666666666667
+Prelude>
+</screen>
+
+ <para>GHCi interprets the whole line as an expression to evaluate.
+ The expression may not span several lines - as soon as you press
+ enter, GHCi will attempt to evaluate it.</para>
+ </sect1>
+
+ <sect1>
+ <title>Loading source files</title>
+
+ <para>Suppose we have the following Haskell source code, which we
+ place in a file <filename>Main.hs</filename>:</para>
+
+<programlisting>
+main = print (fac 20)
+
+fac 0 = 1
+fac n = n * fac (n-1)
+</programlisting>
+
+ <para>You can save <filename>Main.hs</filename> anywhere you like,
+ but if you save it somewhere other than the current
+ directory<footnote><para>If you started up GHCi from the command
+ line then GHCi's current directory is the same as the current
+ directory of the shell from which it was started. If you started
+ GHCi from the “Start” menu in Windows, then the
+ current directory is probably something like
+ <filename>C:\Documents and Settings\<replaceable>user
+ name</replaceable></filename>.</para> </footnote> then we will
+ need to change to the right directory in GHCi:</para>
+
+<screen>
+Prelude> :cd <replaceable>dir</replaceable>
+</screen>
+
+ <para>where <replaceable>dir</replaceable> is the directory (or
+ folder) in which you saved <filename>Main.hs</filename>.</para>
+
+ <para>To load a Haskell source file into GHCi, use the
+ <literal>:load</literal> command:</para>
+ <indexterm><primary><literal>:load</literal></primary></indexterm>
+
+<screen>
+Prelude> :load Main
+Compiling Main ( Main.hs, interpreted )
+Ok, modules loaded: Main.
+*Main>
+</screen>
+
+ <para>GHCi has loaded the <literal>Main</literal> module, and the
+ prompt has changed to “<literal>*Main></literal>” to
+ indicate that the current context for expressions typed at the
+ prompt is the <literal>Main</literal> module we just loaded (we'll
+ explain what the <literal>*</literal> means later in <xref
+ linkend="ghci-scope">). So we can now type expressions involving
+ the functions from <filename>Main.hs</filename>:</para>
+
+<screen>
+*Main> fac 17
+355687428096000
+</screen>
+
+ <para>Loading a multi-module program is just as straightforward;
+ just give the name of the “topmost” module to the
+ <literal>:load</literal> command (hint: <literal>:load</literal>
+ can be abbreviated to <literal>:l</literal>). The topmost module
+ will normally be <literal>Main</literal>, but it doesn't have to
+ be. GHCi will discover which modules are required, directly or
+ indirectly, by the topmost module, and load them all in dependency
+ order.</para>
+
+ <sect2 id="ghci-modules-filenames">
+ <title>Modules vs. filenames</title>
+ <indexterm><primary>modules</primary><secondary>and filenames</secondary></indexterm>
+ <indexterm><primary>filenames</primary><secondary>of modules</secondary></indexterm>
+
+ <para>Question: How does GHC find the filename which contains
+ module <replaceable>M</replaceable>? Answer: it looks for the
+ file <literal><replaceable>M</replaceable>.hs</literal>, or
+ <literal><replaceable>M</replaceable>.lhs</literal>. This means
+ that for most modules, the module name must match the filename.
+ If it doesn't, GHCi won't be able to find it.</para>
+
+ <para>There is one exception to this general rule: when you load
+ a program with <literal>:load</literal>, or specify it when you
+ invoke <literal>ghci</literal>, you can give a filename rather
+ than a module name. This filename is loaded if it exists, and
+ it may contain any module you like. This is particularly
+ convenient if you have several <literal>Main</literal> modules
+ in the same directory and you can't call them all
+ <filename>Main.hs</filename>.</para>
+
+ <para>The search path for finding source files is specified with
+ the <option>-i</option> option on the GHCi command line, like
+ so:</para>
+<screen>ghci -i<replaceable>dir<subscript>1</subscript></replaceable>:...:<replaceable>dir<subscript>n</subscript></replaceable></screen>
+
+ <para>or it can be set using the <literal>:set</literal> command
+ from within GHCi (see <xref
+ linkend="ghci-cmd-line-options">)<footnote><para>Note that in
+ GHCi, and <option>––make</option> mode, the <option>-i</option>
+ option is used to specify the search path for
+ <emphasis>source</emphasis> files, whereas in standard
+ batch-compilation mode the <option>-i</option> option is used to
+ specify the search path for interface files, see <xref
+ linkend="search-path">.</para> </footnote></para>
+
+ <para>One consequence of the way that GHCi follows dependencies
+ to find modules to load is that every module must have a source
+ file. The only exception to the rule is modules that come from
+ a package, including the <literal>Prelude</literal> and standard
+ libraries such as <literal>IO</literal> and
+ <literal>Complex</literal>. If you attempt to load a module for
+ which GHCi can't find a source file, even if there are object
+ and interface files for the module, you'll get an error
+ message.</para>
+ </sect2>
+
+ <sect2>
+ <title>Making changes and recompilation</title>
+ <indexterm><primary><literal>:reload</literal></primary></indexterm>
+
+ <para>If you make some changes to the source code and want GHCi
+ to recompile the program, give the <literal>:reload</literal>
+ command. The program will be recompiled as necessary, with GHCi
+ doing its best to avoid actually recompiling modules if their
+ external dependencies haven't changed. This is the same
+ mechanism we use to avoid re-compiling modules in the batch
+ compilation setting (see <xref linkend="recomp">).</para>
+ </sect2>
+ </sect1>
+
+ <sect1 id="ghci-compiled">
+ <title>Loading compiled code</title>
+ <indexterm><primary>compiled code</primary><secondary>in GHCi</secondary></indexterm>
+
+ <para>When you load a Haskell source module into GHCi, it is
+ normally converted to byte-code and run using the interpreter.
+ However, interpreted code can also run alongside compiled code in
+ GHCi; indeed, normally when GHCi starts, it loads up a compiled
+ copy of the <literal>base</literal> package, which contains the
+ <literal>Prelude</literal>.</para>
+
+ <para>Why should we want to run compiled code? Well, compiled
+ code is roughly 10x faster than interpreted code, but takes about
+ 2x longer to produce (perhaps longer if optimisation is on). So
+ it pays to compile the parts of a program that aren't changing
+ very often, and use the interpreter for the code being actively
+ developed.</para>
+
+ <para>When loading up source files with <literal>:load</literal>,
+ GHCi looks for any corresponding compiled object files, and will
+ use one in preference to interpreting the source if possible. For
+ example, suppose we have a 4-module program consisting of modules
+ A, B, C, and D. Modules B and C both import D only,
+ and A imports both B & C:</para>
+<screen>
+ A
+ / \
+ B C
+ \ /
+ D
+</screen>
+ <para>We can compile D, then load the whole program, like this:</para>
+<screen>
+Prelude> :! ghc -c D.hs
+Prelude> :load A
+Skipping D ( D.hs, D.o )
+Compiling C ( C.hs, interpreted )
+Compiling B ( B.hs, interpreted )
+Compiling A ( A.hs, interpreted )
+Ok, modules loaded: A, B, C, D.
+*Main>
+</screen>
+
+ <para>In the messages from the compiler, we see that it skipped D,
+ and used the object file <filename>D.o</filename>. The message
+ <literal>Skipping</literal> <replaceable>module</replaceable>
+ indicates that compilation for <replaceable>module</replaceable>
+ isn't necessary, because the source and everything it depends on
+ is unchanged since the last compilation.</para>
+
+ <para>At any time you can use the command
+ <literal>:show modules</literal>
+ to get a list of the modules currently loaded
+ into GHCi:</para>
+
+<screen>
+*Main> :show modules
+D ( D.hs, D.o )
+C ( C.hs, interpreted )
+B ( B.hs, interpreted )
+A ( A.hs, interpreted )
+*Main></screen>
+
+ <para>If we now modify the source of D (or pretend to: using Unix
+ command <literal>touch</literal> on the source file is handy for
+ this), the compiler will no longer be able to use the object file,
+ because it might be out of date:</para>
+
+<screen>
+*Main> :! touch D.hs
+*Main> :reload
+Compiling D ( D.hs, interpreted )
+Skipping C ( C.hs, interpreted )
+Skipping B ( B.hs, interpreted )
+Skipping A ( A.hs, interpreted )
+Ok, modules loaded: A, B, C, D.
+*Main>
+</screen>
+
+ <para>Note that module D was compiled, but in this instance
+ because its source hadn't really changed, its interface remained
+ the same, and the recompilation checker determined that A, B and C
+ didn't need to be recompiled.</para>
+
+ <para>So let's try compiling one of the other modules:</para>
+
+<screen>
+*Main> :! ghc -c C.hs
+*Main> :load A
+Compiling D ( D.hs, interpreted )
+Compiling C ( C.hs, interpreted )
+Compiling B ( B.hs, interpreted )
+Compiling A ( A.hs, interpreted )
+Ok, modules loaded: A, B, C, D.
+</screen>
+
+ <para>We didn't get the compiled version of C! What happened?
+ Well, in GHCi a compiled module may only depend on other compiled
+ modules, and in this case C depends on D, which doesn't have an
+ object file, so GHCi also rejected C's object file. Ok, so let's
+ also compile D:</para>
+
+<screen>
+*Main> :! ghc -c D.hs
+*Main> :reload
+Ok, modules loaded: A, B, C, D.
+</screen>
+
+ <para>Nothing happened! Here's another lesson: newly compiled
+ modules aren't picked up by <literal>:reload</literal>, only
+ <literal>:load</literal>:</para>
+
+<screen>
+*Main> :load A
+Skipping D ( D.hs, D.o )
+Skipping C ( C.hs, C.o )
+Compiling B ( B.hs, interpreted )
+Compiling A ( A.hs, interpreted )
+Ok, modules loaded: A, B, C, D.
+</screen>
+
+ <para>HINT: since GHCi will only use a compiled object file if it
+ can sure that the compiled version is up-to-date, a good technique
+ when working on a large program is to occasionally run
+ <literal>ghc ––make</literal> to compile the whole project (say
+ before you go for lunch :-), then continue working in the
+ interpreter. As you modify code, the new modules will be
+ interpreted, but the rest of the project will remain
+ compiled.</para>
+
+ </sect1>
+
+ <sect1>
+ <title>Interactive evaluation at the prompt</title>
+
+ <para>When you type an expression at the prompt, GHCi immediately
+ evaluates and prints the result. But that's not the whole story:
+ if you type something of type <literal>IO a</literal> for some
+ <literal>a</literal>, then GHCi <emphasis>executes</emphasis> it
+ as an IO-computation, and doesn't attempt to print the
+ result:.</para>
+
+<screen>
+Prelude> "hello"
+"hello"
+Prelude> putStrLn "hello"
+hello
+</screen>
+
+ <para>What actually happens is that GHCi typechecks the
+ expression, and if it doesn't have an <literal>IO</literal> type,
+ then it transforms it as follows: an expression
+ <replaceable>e</replaceable> turns into
+<screen>
+ let it = <replaceable>e</replaceable>;
+ print it
+</screen>
+ which is then run as an IO-action.</para>
+
+ <para>Hence, the original expression must have a type which is an
+ instance of the <literal>Show</literal> class, or GHCi will
+ complain:</para>
+
+<screen>
+Prelude> id
+No instance for `Show (a -> a)'
+arising from use of `print'
+in a `do' expression pattern binding: print it
+</screen>
+
+ <para>The error message contains some clues as to the
+ transformation happening internally.</para>
+
+ <sect2 id="ghci-scope">
+ <title>What's really in scope at the prompt?</title>
+
+ <para>When you type an expression at the prompt, what
+ identifiers and types are in scope? GHCi provides a flexible
+ way to control exactly how the context for an expression is
+ constructed. Let's start with the simple cases; when you start
+ GHCi the prompt looks like this:</para>
+
+<screen>Prelude></screen>
+
+ <para>Which indicates that everything from the module
+ <literal>Prelude</literal> is currently in scope. If we now
+ load a file into GHCi, the prompt will change:</para>
+
+<screen>
+Prelude> :load Main.hs
+Compiling Main ( Main.hs, interpreted )
+*Main>
+</screen>
+
+ <para>The new prompt is <literal>*Main</literal>, which
+ indicates that we are typing expressions in the context of the
+ top-level of the <literal>Main</literal> module. Everything
+ that is in scope at the top-level in the module
+ <literal>Main</literal> we just loaded is also in scope at the
+ prompt (probably including <literal>Prelude</literal>, as long
+ as <literal>Main</literal> doesn't explicitly hide it).</para>
+
+ <para>The syntax
+ <literal>*<replaceable>module</replaceable></literal> indicates
+ that it is the full top-level scope of
+ <replaceable>module</replaceable> that is contributing to the
+ scope for expressions typed at the prompt. Without the
+ <literal>*</literal>, just the exports of the module are
+ visible.</para>
+
+ <para>We're not limited to a single module: GHCi can combine
+ scopes from multiple modules, in any mixture of
+ <literal>*</literal> and non-<literal>*</literal> forms. GHCi
+ combines the scopes from all of these modules to form the scope
+ that is in effect at the prompt. For technical reasons, GHCi
+ can only support the <literal>*</literal>-form for modules which
+ are interpreted, so compiled modules and package modules can
+ only contribute their exports to the current scope.</para>
+
+ <para>The scope is manipulated using the
+ <literal>:module</literal> command. For example, if the current
+ scope is <literal>Prelude</literal>, then we can bring into
+ scope the exports from the module <literal>IO</literal> like
+ so:</para>
+
+<screen>
+Prelude> :module +IO
+Prelude,IO> hPutStrLn stdout "hello\n"
+hello
+Prelude,IO>
+</screen>
+
+ <para>(Note: <literal>:module</literal> can be shortened to
+ <literal>:m</literal>). The full syntax of the
+ <literal>:module</literal> command is:</para>
+
+<screen>
+:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable>
+</screen>
+
+ <para>Using the <literal>+</literal> form of the
+ <literal>module</literal> commands adds modules to the current
+ scope, and <literal>-</literal> removes them. Without either
+ <literal>+</literal> or <literal>-</literal>, the current scope
+ is replaced by the set of modules specified. Note that if you
+ use this form and leave out <literal>Prelude</literal>, GHCi
+ will assume that you really wanted the
+ <literal>Prelude</literal> and add it in for you (if you don't
+ want the <literal>Prelude</literal>, then ask to remove it with
+ <literal>:m -Prelude</literal>).</para>
+
+ <para>The scope is automatically set after a
+ <literal>:load</literal> command, to the most recently loaded
+ "target" module, in a <literal>*</literal>-form if possible.
+ For example, if you say <literal>:load foo.hs bar.hs</literal>
+ and <filename>bar.hs</filename> contains module
+ <literal>Bar</literal>, then the scope will be set to
+ <literal>*Bar</literal> if <literal>Bar</literal> is
+ interpreted, or if <literal>Bar</literal> is compiled it will be
+ set to <literal>Prelude,Bar</literal> (GHCi automatically adds
+ <literal>Prelude</literal> if it isn't present and there aren't
+ any <literal>*</literal>-form modules).</para>
+
+ <para>With multiple modules in scope, especially multiple
+ <literal>*</literal>-form modules, it is likely that name
+ clashes will occur. Haskell specifies that name clashes are
+ only reported when an ambiguous identifier is used, and GHCi
+ behaves in the same way for expressions typed at the
+ prompt.</para>
+
+ <sect3>
+ <title>Qualified names</title>
+
+ <para>To make life slightly easier, the GHCi prompt also
+ behaves as if there is an implicit <literal>import
+ qualified</literal> declaration for every module in every
+ package, and every module currently loaded into GHCi.</para>
+ </sect3>
+ </sect2>
+
+ <sect2>
+ <title>Using <literal>do-</literal>notation at the prompt</title>
+ <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
+ <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
+
+ <para>GHCi actually accepts <firstterm>statements</firstterm>
+ rather than just expressions at the prompt. This means you can
+ bind values and functions to names, and use them in future
+ expressions or statements.</para>
+
+ <para>The syntax of a statement accepted at the GHCi prompt is
+ exactly the same as the syntax of a statement in a Haskell
+ <literal>do</literal> expression. However, there's no monad
+ overloading here: statements typed at the prompt must be in the
+ <literal>IO</literal> monad.</para>
+
+ <para>Here's an example:</para>
+<screen>
+Prelude> x <- return 42
+Prelude> print x
+42
+Prelude>
+</screen>
+ <para>The statement <literal>x <- return 42</literal> means
+ “execute <literal>return 42</literal> in the
+ <literal>IO</literal> monad, and bind the result to
+ <literal>x</literal>”. We can then use
+ <literal>x</literal> in future statements, for example to print
+ it as we did above.</para>
+
+ <para>Of course, you can also bind normal non-IO expressions
+ using the <literal>let</literal>-statement:</para>
+<screen>
+Prelude> let x = 42
+Prelude> print x
+42
+Prelude>
+</screen>
+ <para>An important difference between the two types of binding
+ is that the monadic bind (<literal>p <- e</literal>) is
+ <emphasis>strict</emphasis> (it evaluates <literal>e</literal>),
+ whereas with the <literal>let</literal> form, the expression
+ isn't evaluated immediately:</para>
+<screen>
+Prelude> let x = error "help!"
+Prelude> print x
+*** Exception: help!
+Prelude>
+</screen>
+ <para>Any exceptions raised during the evaluation or execution
+ of the statement are caught and printed by the GHCi command line
+ interface (for more information on exceptions, see the module
+ <literal>Control.Exception</literal> in the libraries
+ documentation).</para>
+
+ <para>Every new binding shadows any existing bindings of the
+ same name, including entities that are in scope in the current
+ module context.</para>
+
+ <para>WARNING: temporary bindings introduced at the prompt only
+ last until the next <literal>:load</literal> or
+ <literal>:reload</literal> command, at which time they will be
+ simply lost. However, they do survive a change of context with
+ <literal>:module</literal>: the temporary bindings just move to
+ the new location.</para>
+
+ <para>HINT: To get a list of the bindings currently in scope, use the
+ <literal>:show bindings</literal> command:</para>
+
+<screen>
+Prelude> :show bindings
+x :: Int
+Prelude></screen>
+
+ <para>HINT: if you turn on the <literal>+t</literal> option,
+ GHCi will show the type of each variable bound by a statement.
+ For example:</para>
+ <indexterm><primary><literal>+t</literal></primary></indexterm>
+<screen>
+Prelude> :set +t
+Prelude> let (x:xs) = [1..]
+x :: Integer
+xs :: [Integer]
+</screen>
+
+ </sect2>
+
+ <sect2>
+ <title>The <literal>it</literal> variable</title>
+ <indexterm><primary><literal>it</literal></primary>
+ </indexterm>
+
+ <para>Whenever an expression (or a non-binding statement, to be
+ precise) is typed at the prompt, GHCi implicitly binds its value
+ to the variable <literal>it</literal>. For example:</para>
+<screen>
+Prelude> 1+2
+3
+Prelude> it * 2
+6
+</screen>
+
+ <para>This is a result of the translation mentioned earlier,
+ namely that an expression <replaceable>e</replaceable> is
+ translated to
+<screen>
+ let it = <replaceable>e</replaceable>;
+ print it
+</screen>
+ before execution, resulting in a binding for
+ <literal>it</literal>.</para>
+
+ <para>If the expression was of type <literal>IO a</literal> for
+ some <literal>a</literal>, then <literal>it</literal> will be
+ bound to the result of the <literal>IO</literal> computation,
+ which is of type <literal>a</literal>. eg.:</para>
+<screen>
+Prelude> Time.getClockTime
+Prelude> print it
+Wed Mar 14 12:23:13 GMT 2001
+</screen>
+
+ <para>The corresponding translation for an IO-typed
+ <replaceable>e</replaceable> is
+<screen>
+ it <- <replaceable>e</replaceable>
+</screen>
+ </para>
+
+ <para>Note that <literal>it</literal> is shadowed by the new
+ value each time you evaluate a new expression, and the old value
+ of <literal>it</literal> is lost.</para>
+
+ </sect2>
+
+ <sect2>
+ <title>Type defaulting in GHCi</title>
+ <indexterm><primary>Type default</primary></indexterm>
+ <indexterm><primary><literal>Show</literal> class</primary></indexterm>
+ <para>
+ Consider this GHCi session:
+<programlisting>
+ ghci> reverse []
+</programlisting>
+ What should GHCi do? Strictly speaking, the program is ambiguous. <literal>show (reverse [])</literal>
+ (which is what GHCi computes here) has type <literal>Show a => a</literal> and how that displays depends
+ on the type <literal>a</literal>. For example:
+<programlisting>
+ ghci> (reverse []) :: String
+ ""
+ ghci> (reverse []) :: [Int]
+ []
+</programlisting>
+ However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell's type-defaulting
+ rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. If the expression yields a set of
+ type constraints that are all from standard classes (<literal>Num</literal>, <literal>Eq</literal> etc.),
+ and at least one is either a numeric class <emphasis>or the <literal>Show</literal>,
+ <literal>Eq</literal>, or <literal>Ord</literal> class</emphasis>,
+ GHCi will try to use one of the <literal>default</literal> types, just as described in the Report.
+ </para>
+ </sect2>
+ </sect1>
+
+ <sect1 id="ghci-invokation">
+ <title>Invoking GHCi</title>
+ <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
+ <indexterm><primary><option>––interactive</option></primary></indexterm>
+
+ <para>GHCi is invoked with the command <literal>ghci</literal> or
+ <literal>ghc ––interactive</literal>. One or more modules or
+ filenames can also be specified on the command line; this
+ instructs GHCi to load the specified modules or filenames (and all
+ the modules they depend on), just as if you had said
+ <literal>:load <replaceable>modules</replaceable></literal> at the
+ GHCi prompt (see <xref linkend="ghci-commands">). For example, to
+ start GHCi and load the program whose topmost module is in the
+ file <literal>Main.hs</literal>, we could say:</para>
+
+<screen>
+$ ghci Main.hs
+</screen>
+
+ <para>Most of the command-line options accepted by GHC (see <xref
+ linkend="using-ghc">) also make sense in interactive mode. The ones
+ that don't make sense are mostly obvious; for example, GHCi
+ doesn't generate interface files, so options related to interface
+ file generation won't have any effect.</para>
+
+ <sect2>
+ <title>Packages</title>
+ <indexterm><primary>packages</primary><secondary>with GHCi</secondary></indexterm>
+
+ <para>Most packages (see <xref linkend="using-packages">) are
+ available without needing to specify any extra flags at all:
+ they will be automatically loaded the first time they are
+ needed.</para>
+
+ <para>For non-auto packages, however, you need to request the
+ package be loaded by using the <literal>-package</literal> flag:</para>
+
+<screen>
+$ ghci -package data
+ ___ ___ _
+ / _ \ /\ /\/ __(_)
+ / /_\// /_/ / / | | GHC Interactive, version 5.05, for Haskell 98.
+/ /_\\/ __ / /___| | http://www.haskell.org/ghc/
+\____/\/ /_/\____/|_| Type :? for help.
+
+Loading package base ... linking ... done.
+Loading package haskell98 ... linking ... done.
+Loading package lang ... linking ... done.
+Loading package concurrent ... linking ... done.
+Loading package readline ... linking ... done.
+Loading package unix ... linking ... done.
+Loading package posix ... linking ... done.
+Loading package util ... linking ... done.
+Loading package data ... linking ... done.
+Prelude>
+</screen>
+
+ <para>The following command works to load new packages into a
+ running GHCi:</para>
+
+<screen>
+Prelude> :set -package <replaceable>name</replaceable>
+</screen>
+
+ <para>But note that doing this will cause all currently loaded
+ modules to be unloaded, and you'll be dumped back into the
+ <literal>Prelude</literal>.</para>
+ </sect2>
+
+ <sect2>
+ <title>Extra libraries</title>
+ <indexterm><primary>libraries</primary><secondary>with GHCi</secondary></indexterm>
+
+ <para>Extra libraries may be specified on the command line using
+ the normal <literal>-l<replaceable>lib</replaceable></literal>
+ option. (The term <emphasis>library</emphasis> here refers to
+ libraries of foreign object code; for using libraries of Haskell
+ source code, see <xref linkend="ghci-modules-filenames">.) For
+ example, to load the “m” library:</para>
+
+<screen>
+$ ghci -lm
+</screen>
+
+ <para>On systems with <literal>.so</literal>-style shared
+ libraries, the actual library loaded will the
+ <filename>lib<replaceable>lib</replaceable>.so</filename>. GHCi
+ searches the following places for libraries, in this order:</para>
+
+ <itemizedlist>
+ <listitem>
+ <para>Paths specified using the
+ <literal>-L<replaceable>path</replaceable></literal>
+ command-line option,</para>
+ </listitem>
+ <listitem>
+ <para>the standard library search path for your system,
+ which on some systems may be overriden by setting the
+ <literal>LD_LIBRARY_PATH</literal> environment
+ variable.</para>
+ </listitem>
+ </itemizedlist>
+
+ <para>On systems with <literal>.dll</literal>-style shared
+ libraries, the actual library loaded will be
+ <filename><replaceable>lib</replaceable>.dll</filename>. Again,
+ GHCi will signal an error if it can't find the library.</para>
+
+ <para>GHCi can also load plain object files
+ (<literal>.o</literal> or <literal>.obj</literal> depending on
+ your platform) from the command-line. Just add the name the
+ object file to the command line.</para>
+ </sect2>
+
+ </sect1>
+
+ <sect1 id="ghci-commands">
+ <title>GHCi commands</title>
+
+ <para>GHCi commands all begin with
+ ‘<literal>:</literal>’ and consist of a single command
+ name followed by zero or more parameters. The command name may be
+ abbreviated, as long as the abbreviation is not ambiguous. All of
+ the builtin commands, with the exception of
+ <literal>:unset</literal> and <literal>:undef</literal>, may be
+ abbreviated to a single letter.</para>
+
+ <variablelist>
+ <varlistentry>
+ <term><literal>:add</literal>
+ <replaceable>module</replaceable> ...</term>
+ <indexterm><primary><literal>:add</literal></primary></indexterm>
+ <listitem>
+ <para>Add <replaceable>module</replaceable>(s) to the
+ current <firstterm>target set</firstterm>, and perform a
+ reload.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:browse</literal>
+ <optional><literal>*</literal></optional><replaceable>module</replaceable>
+ ...</term>
+ <indexterm><primary><literal>:browse</literal></primary>
+ </indexterm>
+ <listitem>
+ <para>Displays the identifiers defined by the module
+ <replaceable>module</replaceable>, which must be either
+ loaded into GHCi or be a member of a package. If the
+ <literal>*</literal> symbol is placed before the module
+ name, then <emphasis>all</emphasis> the identifiers defined
+ in <replaceable>module</replaceable> are shown; otherwise
+ the list is limited to the exports of
+ <replaceable>module</replaceable>. The
+ <literal>*</literal>-form is only available for modules
+ which are interpreted; for compiled modules (including
+ modules from packages) only the non-<literal>*</literal>
+ form of <literal>:browse</literal> is available.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:cd</literal> <replaceable>dir</replaceable></term>
+ <indexterm><primary><literal>:cd</literal></primary></indexterm>
+ <listitem>
+ <para>Changes the current working directory to
+ <replaceable>dir</replaceable>. A
+ ‘<literal>˜</literal>’ symbol at the
+ beginning of <replaceable>dir</replaceable> will be replaced
+ by the contents of the environment variable
+ <literal>HOME</literal>.</para>
+
+ <para>NOTE: changing directories causes all currently loaded
+ modules to be unloaded. This is because the search path is
+ usually expressed using relative directories, and changing
+ the search path in the middle of a session is not
+ supported.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable></term>
+ <indexterm><primary><literal>:def</literal></primary></indexterm>
+ <listitem>
+ <para>The command <literal>:def</literal>
+ <replaceable>name</replaceable>
+ <replaceable>expr</replaceable> defines a new GHCi command
+ <literal>:<replaceable>name</replaceable></literal>,
+ implemented by the Haskell expression
+ <replaceable>expr</replaceable>, which must have type
+ <literal>String -> IO String</literal>. When
+ <literal>:<replaceable>name</replaceable>
+ <replaceable>args</replaceable></literal> is typed at the
+ prompt, GHCi will run the expression
+ <literal>(<replaceable>name</replaceable>
+ <replaceable>args</replaceable>)</literal>, take the
+ resulting <literal>String</literal>, and feed it back into
+ GHCi as a new sequence of commands. Separate commands in
+ the result must be separated by
+ ‘<literal>\n</literal>’.</para>
+
+ <para>That's all a little confusing, so here's a few
+ examples. To start with, here's a new GHCi command which
+ doesn't take any arguments or produce any results, it just
+ outputs the current date & time:</para>
+
+<screen>
+Prelude> let date _ = Time.getClockTime >>= print >> return ""
+Prelude> :def date date
+Prelude> :date
+Fri Mar 23 15:16:40 GMT 2001
+</screen>
+
+ <para>Here's an example of a command that takes an argument.
+ It's a re-implementation of <literal>:cd</literal>:</para>
+
+<screen>
+Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
+Prelude> :def mycd mycd
+Prelude> :mycd ..
+</screen>
+
+ <para>Or I could define a simple way to invoke
+ “<literal>ghc ––make Main</literal>” in the
+ current directory:</para>
+
+<screen>
+Prelude> :def make (\_ -> return ":! ghc ––make Main")
+</screen>
+
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:help</literal></term>
+ <indexterm><primary><literal>:help</literal></primary></indexterm>
+ <term><literal>:?</literal></term>
+ <indexterm><primary><literal>:?</literal></primary></indexterm>
+ <listitem>
+ <para>Displays a list of the available commands.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:info</literal> <replaceable>name</replaceable>
+ ...</term>
+ <indexterm><primary><literal>:info</literal></primary>
+ </indexterm>
+ <listitem>
+ <para>Displays information about the given name(s). For
+ example, if <replaceable>name</replaceable> is a class, then
+ the class methods and their types will be printed; if
+ <replaceable>name</replaceable> is a type constructor, then
+ its definition will be printed; if
+ <replaceable>name</replaceable> is a function, then its type
+ will be printed. If <replaceable>name</replaceable> has
+ been loaded from a source file, then GHCi will also display
+ the location of its definition in the source.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:load</literal>
+ <replaceable>module</replaceable> ...</term>
+ <indexterm><primary><literal>:load</literal></primary></indexterm>
+ <listitem>
+ <para>Recursively loads the specified
+ <replaceable>module</replaceable>s, and all the modules they
+ depend on. Here, each <replaceable>module</replaceable>
+ must be a module name or filename, but may not be the name
+ of a module in a package.</para>
+
+ <para>All previously loaded modules, except package modules,
+ are forgotten. The new set of modules is known as the
+ <firstterm>target set</firstterm>. Note that
+ <literal>:load</literal> can be used without any arguments
+ to unload all the currently loaded modules and
+ bindings.</para>
+
+ <para>After a <literal>:load</literal> command, the current
+ context is set to:</para>
+
+ <itemizedlist>
+ <listitem>
+ <para><replaceable>module</replaceable>, if it was loaded
+ successfully, or</para>
+ </listitem>
+ <listitem>
+ <para>the most recently successfully loaded module, if
+ any other modules were loaded as a result of the current
+ <literal>:load</literal>, or</para>
+ </listitem>
+ <listitem>
+ <para><literal>Prelude</literal> otherwise.</para>
+ </listitem>
+ </itemizedlist>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:module <optional>+|-</optional> <optional>*</optional><replaceable>mod<subscript>1</subscript></replaceable> ... <optional>*</optional><replaceable>mod<subscript>n</subscript></replaceable></literal></term>
+ <indexterm><primary><literal>:module</literal></primary></indexterm>
+ <listitem>
+ <para>Sets or modifies the current context for statements
+ typed at the prompt. See <xref linkend="ghci-scope"> for
+ more details.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:quit</literal></term>
+ <indexterm><primary><literal>:quit</literal></primary></indexterm>
+ <listitem>
+ <para>Quits GHCi. You can also quit by typing a control-D
+ at the prompt.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:reload</literal></term>
+ <indexterm><primary><literal>:reload</literal></primary></indexterm>
+ <listitem>
+ <para>Attempts to reload the current target set (see
+ <literal>:load</literal>) if any of the modules in the set,
+ or any dependent module, has changed. Note that this may
+ entail loading new modules, or dropping modules which are no
+ longer indirectly required by the target.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:set</literal> <optional><replaceable>option</replaceable>...</optional></term>
+ <indexterm><primary><literal>:set</literal></primary></indexterm>
+ <listitem>
+ <para>Sets various options. See <xref linkend="ghci-set">
+ for a list of available options. The
+ <literal>:set</literal> command by itself shows which
+ options are currently set.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:set</literal> <literal>args</literal>
+ <replaceable>arg</replaceable> ...</term>
+ <indexterm><primary><literal>:set</literal></primary></indexterm>
+ <listitem>
+ <para>Sets the list of arguments which are returned when the
+ program calls <literal>System.getArgs</literal><indexterm><primary>getArgs</primary>
+ </indexterm>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:set</literal> <literal>prog</literal>
+ <replaceable>prog</replaceable></term>
+ <indexterm><primary><literal>:set</literal></primary></indexterm>
+ <listitem>
+ <para>Sets the string to be returned when the program calls
+ <literal>System.getProgName</literal><indexterm><primary>getProgName</primary>
+ </indexterm>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:show bindings</literal></term>
+ <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
+ <listitem>
+ <para>Show the bindings made at the prompt and their
+ types.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:show modules</literal></term>
+ <indexterm><primary><literal>:show modules</literal></primary></indexterm>
+ <listitem>
+ <para>Show the list of modules currently load.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:type</literal> <replaceable>expression</replaceable></term>
+ <indexterm><primary><literal>:type</literal></primary></indexterm>
+ <listitem>
+ <para>Infers and prints the type of
+ <replaceable>expression</replaceable>, including explicit
+ forall quantifiers for polymorphic types. The monomorphism
+ restriction is <emphasis>not</emphasis> applied to the
+ expression during type inference.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:undef</literal> <replaceable>name</replaceable></term>
+ <indexterm><primary><literal>:undef</literal></primary></indexterm>
+ <listitem>
+ <para>Undefines the user-defined command
+ <replaceable>name</replaceable> (see <literal>:def</literal>
+ above).</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:unset</literal> <replaceable>option</replaceable>...</term>
+ <indexterm><primary><literal>:unset</literal></primary></indexterm>
+ <listitem>
+ <para>Unsets certain options. See <xref linkend="ghci-set">
+ for a list of available options.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>:!</literal> <replaceable>command</replaceable>...</term>
+ <indexterm><primary><literal>:!</literal></primary></indexterm>
+ <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
+ <listitem>
+ <para>Executes the shell command
+ <replaceable>command</replaceable>.</para>
+ </listitem>
+ </varlistentry>
+
+ </variablelist>
+ </sect1>
+
+ <sect1 id="ghci-set">
+ <title>The <literal>:set</literal> command</title>
+ <indexterm><primary><literal>:set</literal></primary></indexterm>
+
+ <para>The <literal>:set</literal> command sets two types of
+ options: GHCi options, which begin with
+ ‘<literal>+</literal>” and “command-line”
+ options, which begin with ‘-’. </para>
+
+ <para>NOTE: at the moment, the <literal>:set</literal> command
+ doesn't support any kind of quoting in its arguments: quotes will
+ not be removed and cannot be used to group words together. For
+ example, <literal>:set -DFOO='BAR BAZ'</literal> will not do what
+ you expect.</para>
+
+ <sect2>
+ <title>GHCi options</title>
+ <indexterm><primary>options</primary><secondary>GHCi</secondary>
+ </indexterm>
+
+ <para>GHCi options may be set using <literal>:set</literal> and
+ unset using <literal>:unset</literal>.</para>
+
+ <para>The available GHCi options are:</para>
+
+ <variablelist>
+ <varlistentry>
+ <term><literal>+r</literal></term>
+ <indexterm><primary><literal>+r</literal></primary></indexterm>
+ <indexterm><primary>CAFs</primary><secondary>in GHCi</secondary></indexterm>
+ <indexterm><primary>Constant Applicative Form</primary><see>CAFs</see></indexterm>
+ <listitem>
+ <para>Normally, any evaluation of top-level expressions
+ (otherwise known as CAFs or Constant Applicative Forms) in
+ loaded modules is retained between evaluations. Turning
+ on <literal>+r</literal> causes all evaluation of
+ top-level expressions to be discarded after each
+ evaluation (they are still retained
+ <emphasis>during</emphasis> a single evaluation).</para>
+
+ <para>This option may help if the evaluated top-level
+ expressions are consuming large amounts of space, or if
+ you need repeatable performance measurements.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>+s</literal></term>
+ <indexterm><primary><literal>+s</literal></primary></indexterm>
+ <listitem>
+ <para>Display some stats after evaluating each expression,
+ including the elapsed time and number of bytes allocated.
+ NOTE: the allocation figure is only accurate to the size
+ of the storage manager's allocation area, because it is
+ calculated at every GC. Hence, you might see values of
+ zero if no GC has occurred.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>+t</literal></term>
+ <indexterm><primary><literal>+t</literal></primary></indexterm>
+ <listitem>
+ <para>Display the type of each variable bound after a
+ statement is entered at the prompt. If the statement is a
+ single expression, then the only variable binding will be
+ for the variable
+ ‘<literal>it</literal>’.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </sect2>
+
+ <sect2 id="ghci-cmd-line-options">
+ <title>Setting GHC command-line options in GHCi</title>
+
+ <para>Normal GHC command-line options may also be set using
+ <literal>:set</literal>. For example, to turn on
+ <option>-fglasgow-exts</option>, you would say:</para>
+
+<screen>
+Prelude> :set -fglasgow-exts
+</screen>
+
+ <para>Any GHC command-line option that is designated as
+ <firstterm>dynamic</firstterm> (see the table in <xref
+ linkend="flag-reference">), may be set using
+ <literal>:set</literal>. To unset an option, you can set the
+ reverse option:</para>
+ <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
+
+<screen>
+Prelude> :set -fno-glasgow-exts
+</screen>
+
+ <para><xref linkend="flag-reference"> lists the reverse for each
+ option where applicable.</para>
+
+ <para>Certain static options (<option>-package</option>,
+ <option>-I</option>, <option>-i</option>, and
+ <option>-l</option> in particular) will also work, but some may
+ not take effect until the next reload.</para>
+ <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
+ </sect2>
+ </sect1>
+
+ <sect1 id="ghci-dot-files">
+ <title>The <filename>.ghci</filename> file</title>
+ <indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
+ </indexterm>
+ <indexterm><primary>startup</primary><secondary>files, GHCi</secondary>
+ </indexterm>
+
+ <para>When it starts, GHCi always reads and executes commands from
+ <filename>$HOME/.ghci</filename>, followed by
+ <filename>./.ghci</filename>.</para>
+
+ <para>The <filename>.ghci</filename> in your home directory is
+ most useful for turning on favourite options (eg. <literal>:set
+ +s</literal>), and defining useful macros. Placing a
+ <filename>.ghci</filename> file in a directory with a Haskell
+ project is a useful way to set certain project-wide options so you
+ don't have to type them everytime you start GHCi: eg. if your
+ project uses GHC extensions and CPP, and has source files in three
+ subdirectories A B and C, you might put the following lines in
+ <filename>.ghci</filename>:</para>
+
+<screen>
+:set -fglasgow-exts -cpp
+:set -iA:B:C
+</screen>
+
+ <para>(Note that strictly speaking the <option>-i</option> flag is
+ a static one, but in fact it works to set it using
+ <literal>:set</literal> like this. The changes won't take effect
+ until the next <literal>:load</literal>, though.)</para>
+
+ <para>Two command-line options control whether the
+ <filename>.ghci</filename> files are read:</para>
+
+ <variablelist>
+ <varlistentry>
+ <term><option>-ignore-dot-ghci</option></term>
+ <indexterm><primary><option>-ignore-dot-ghci</option></primary>
+ </indexterm>
+ <listitem>
+ <para>Don't read either <filename>./.ghci</filename> or
+ <filename>$HOME/.ghci</filename> when starting up.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term><option>-read-dot-ghci</option></term>
+ <indexterm><primary><option>-read-dot-ghci</option></primary>
+ </indexterm>
+ <listitem>
+ <para>Read <filename>.ghci</filename> and
+ <filename>$HOME/.ghci</filename>. This is normally the
+ default, but the <option>-read-dot-ghci</option> option may
+ be used to override a previous
+ <option>-ignore-dot-ghci</option> option.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+
+ </sect1>
+
+ <sect1>
+ <title>FAQ and Things To Watch Out For</title>
+
+ <variablelist>
+ <varlistentry>
+ <term>The interpreter can't load modules with foreign export
+ declarations!</term>
+ <listitem>
+ <para>Unfortunately not. We haven't implemented it yet.
+ Please compile any offending modules by hand before loading
+ them into GHCi.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>-O</literal> doesn't work with GHCi!</term>
+ <indexterm><primary><option>-O</option></primary>
+ </indexterm>
+ <listitem>
+ <para>For technical reasons, the bytecode compiler doesn't
+ interact well with one of the optimisation passes, so we
+ have disabled optimisation when using the interpreter. This
+ isn't a great loss: you'll get a much bigger win by
+ compiling the bits of your code that need to go fast, rather
+ than interpreting them with optimisation turned on.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>Unboxed tuples don't work with GHCi</term>
+ <listitem>
+ <para>That's right. You can always compile a module that
+ uses unboxed tuples and load it into GHCi, however.
+ (Incidentally the previous point, namely that
+ <literal>-O</literal> is incompatible with GHCi, is because
+ the bytecode compiler can't deal with unboxed
+ tuples).</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>Concurrent threads don't carry on running when GHCi is
+ waiting for input.</term>
+ <listitem>
+ <para>No, they don't. This is because the Haskell binding
+ to the GNU readline library doesn't support reading from the
+ terminal in a non-blocking way, which is required to work
+ properly with GHC's concurrency model.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>After using <literal>getContents</literal>, I can't use
+ <literal>stdin</literal> again until I do
+ <literal>:load</literal> or <literal>:reload</literal>.</term>
+
+ <listitem>
+ <para>This is the defined behaviour of
+ <literal>getContents</literal>: it puts the stdin Handle in
+ a state known as <firstterm>semi-closed</firstterm>, wherein
+ any further I/O operations on it are forbidden. Because I/O
+ state is retained between computations, the semi-closed
+ state persists until the next <literal>:load</literal> or
+ <literal>:reload</literal> command.</para>
+
+ <para>You can make <literal>stdin</literal> reset itself
+ after every evaluation by giving GHCi the command
+ <literal>:set +r</literal>. This works because
+ <literal>stdin</literal> is just a top-level expression that
+ can be reverted to its unevaluated state in the same way as
+ any other top-level expression (CAF).</para>
+ </listitem>
+ </varlistentry>
+
+ </variablelist>
+ </sect1>
+
</chapter>
<!-- Emacs stuff:
projects / ghc-hetmet.git / blobdiff