[ghc-hetmet.git] / ghc / docs / users_guide / ghci.sgml

<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>GHCi<footnote>
      <para>The &lsquo;i&rsquo; stands for &ldquo;Interactive&rdquo;</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 Hugs<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 the FFI, 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 4.11, For Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Loading package std ... 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:
   &lt;stmt&gt;              evaluate/run &lt;stmt&gt;
   :cd &lt;dir&gt;           change directory to &lt;dir&gt;
   :def &lt;cmd&gt; &lt;expr&gt;   define a macro :&lt;cmd&gt;
   :help, :?           display this list of commands
   :load &lt;filename&gt;    load a module (and it dependents)
   :module &lt;mod&gt;       set the context for expression evaluation to &lt;mod&gt;
   :reload             reload the current module set
   :set &lt;option&gt; ...   set options
   :type &lt;expr&gt;        show the type of &lt;expr&gt;
   :unset &lt;option&gt; ... unset options
   :quit               exit GHCi
   :!&lt;command&gt;         run the shell command &lt;command&gt;
 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
    -&lt;flag&gt;            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
PrePrelude> 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> in the current
    directory:</para>

<programlisting>
main = print (fac 20)

fac 0 = 1
fac n = n * fac (n-1)
</programlisting>

    <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 &ldquo;<literal>Main></literal>&rdquo; to
    indicate that the current context for expressions typed at the
    prompt is the <literal>Main</literal> module we just
    loaded.  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 &ldquo;topmost&rdquo; 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>
      <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>One final note: if you load a module called Main, it must
      contain a <literal>main</literal> function, just like in
      GHC.</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 package <literal>std</literal>, which contains the Prelude
    and standard libraries.</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>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 has a concept of a
       <firstterm>context</firstterm> module, which can be set using
       the <literal>:module</literal> command.</para>

      <para>The context module is shown in the prompt: for example,
      the prompt <literal>Prelude></literal> indicates that the
      current context for evaluating expressions is the Haskell
      <literal>Prelude</literal> module.  The Prelude is the default
      context when you start up GHCi.</para>
      <indexterm><primary><literal>Prelude</literal></primary></indexterm>

      <para>Exactly which entities are in scope in a given context
      depends on whether the context module is compiled or
      interpreted:</para>

      <itemizedlist>
        <listitem>
          <para>If the context module is interpreted, then everything
          that was in scope during compilation of that module is also
          in scope at the prompt, i.e. all the imports and any
          top-level functions, types and classes defined in that
          module.</para>
        </listitem>

        <listitem>
          <para>If the context module comes from a package, or is
          otherwise compiled, then only the exports of that module are
          in scope at the prompt.  So for example, when the current
          context module is <literal>Prelude</literal>, everything the
          <literal>Prelude</literal> exports is in scope, but if we
          switch context to eg. <literal>Time</literal>, then
          everything from the <literal>Prelude</literal> is now
          invisible.</para>
        </listitem>
      </itemizedlist>

      <para>The reason for this unfortunate distinction is boring: for
      a compiled module when the source isn't available, the compiler
      has no way of knowing what was in scope when the module was
      compiled (and we don't store this information in the interface
      file).  However, in practice it shouldn't be a problem: if you
      want both <literal>Time</literal> and <literal>Prelude</literal>
      in scope at the same time, just create a file containing the
      line <literal>import Time</literal> and load it into
      GHCi.</para>

      <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.  So in the above example where the
      <literal>Prelude</literal> was invisible, we can always get at
      <literal>Prelude</literal> identifiers by qualifying them, eg.
      <literal>Prelude.map</literal>.</para>
    </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
      &ldquo;execute <literal>return 42</literal> in the
      <literal>IO</literal> monad, and bind the result to
      <literal>x</literal>&rdquo;.  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 (see <xref linkend="sec-Exception"> for more
      information on GHC's Exception support).</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: 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>
  </sect1>

  <sect1>
    <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>.  A module or filename can
    also be specified on the command line; this instructs GHCi to load
    the that module or filename (and all the modules it depends on),
    just as if you had said <literal>:load
    <replaceable>module</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>Note: only <emphasis>one</emphasis> module name or filename
    may be given on the command line.</para>

    <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>GHCi can make use of all the packages that come with GHC,
      but note: packages <emphasis>must</emphasis> be specified on the
      GHCi command line, you can't add extra packages after GHCi has
      started up.  For example, to start up GHCi with the
      <literal>text</literal> package loaded:</para>

<screen>
$ ghci -package text
   ___         ___ _
  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 4.11, For Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Loading package std ... linking ... done.
Loading package lang ... linking ... done.
Loading package text ... linking ... done.
Prelude> 
</screen>      

      <para>Note that GHCi also loaded the <literal>lang</literal>
      package even though we didn't ask for it: that's because the
      <literal>text</literal> package makes use of one or more of the
      modules in <literal>lang</literal>, and therefore has a
      dependency on it.</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.  For example, to load the &ldquo;m&rdquo; 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>.  If
      no such library exists on the standard library search path,
      including paths given using
      <literal>-L<replaceable>path</replaceable></literal>, then
      <literal>ghci</literal> will signal an error.</para>

      <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>
    </sect2>

  </sect1>

  <sect1 id="ghci-commands">
    <title>GHCi commands</title>

    <para>GHCi commands all begin with
    &lsquo;<literal>:</literal>&rsquo; 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>: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
          &lsquo;<literal>&tilde;</literal>&rsquo; symbol at the
          beginning of <replaceable>dir</replaceable> will be replaced
          by the contents of the environment variable
          <literal>HOME</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>:def</literal></term>
        <indexterm><primary><literal>:def</literal></primary></indexterm>
        <listitem>
          <para>ToDo.</para>
        </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>:load</literal> <replaceable>module</replaceable></term>
        <indexterm><primary><literal>:load</literal></primary></indexterm>
        <listitem>
          <para>Recursively loads <replaceable>module</replaceable>
          (which may be a module name or filename), and all the
          modules it depends on.  All previously loaded modules are
          forgotten.  The module <replaceable>module</replaceable> is
          known as the <firstterm>target</firstterm>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>:module</literal> <replaceable>module</replaceable></term>
        <indexterm><primary><literal>:module</literal></primary></indexterm>
        <listitem>
          <para>Sets the current context for statements typed at the
          prompt to <replaceable>module</replaceable>, which must be a
          module name which is already loaded or in a package.  See
          <xref linkend="ghci-scope"> for more information on what
          effect the context has on what entities are in scope at the
          prompt.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>:quit</literal> <replaceable>module</replaceable></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 (see
          <literal>:load</literal>) if it, or any module it depends
          on, has changed.  Note that this may entail loading new
          modules, or even 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>: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>: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
    &lsquo;<literal>+</literal>&rdquo; and &ldquo;command-line&rdquo;
    options, which begin with &lsquo;-&rsquo;.  Either type of option
    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 &lsquo;<literal>it</literal>&rsquo;.</para>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>In addition, any normal 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>.  Certain static options
    (<option>-I</option>, <option>-i</option>, and <option>-l</option>
    in particular) will also work, but may not take effect until the
    next reload.</para>
    <indexterm><primary>dynamic</primary><secondary>options</secondary></indexterm>
    <indexterm><primary>static</primary><secondary>options</secondary></indexterm>
  </sect1>

  <sect1>
    <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>
  </sect1>

  <sect1>
    <title>FAQ and Things To Watch Out For</title>
    
    <variablelist>
      <varlistentry>
        <term><literal>System.exit</literal> causes GHCi to exit!</term>
        <indexterm><primary><literal>System.exit</literal></primary><secondary>in
        GHCi</secondary></indexterm>
        <listitem>
          <para>Yes, it does.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><literal>System.getArgs</literal> returns GHCi's command
        line arguments!</term>
        <listitem>
          <para>Yes, it does.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>The interpreter can't load modules with FFI
        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>Hugs has a <literal>:add</literal> command for adding
        modules without throwing away any that are already loaded.
        Why doesn't this work in GHCi?</term>
        <listitem>
          <para>We haven't implemented it yet.  Sorry about that.</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>
    </variablelist>

  </sect1>

</chapter>

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