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<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 familiar 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 6.6, for Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Loading package base ... 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;
   :add &lt;filename&gt; ...         add module(s) to the current target set
   :browse [*]&lt;module&gt;         display the names defined by &lt;module&gt;
   :cd &lt;dir&gt;                   change directory to &lt;dir&gt;
   :def &lt;cmd&gt; &lt;expr&gt;           define a command :&lt;cmd&gt;
   :edit &lt;file&gt;                edit file
   :edit                       edit last module
   :help, :?                   display this list of commands
   :info [&lt;name&gt; ...]          display information about the given names
   :load &lt;filename&gt; ...        load module(s) and their dependents
   :module [+/-] [*]&lt;mod&gt; ...  set the context for expression evaluation
   :main [&lt;arguments&gt; ...]     run the main function with the given arguments
   :reload                     reload the current module set

   :set &lt;option&gt; ...           set options
   :set args &lt;arg&gt; ...         set the arguments returned by System.getArgs
   :set prog &lt;progname&gt;        set the value returned by System.getProgName
   :set prompt &lt;prompt&gt;        set the prompt used in GHCi
   :set editor &lt;cmd&gt;        set the command used for :edit

   :show modules               show the currently loaded modules
   :show bindings              show the current bindings made at the prompt

   :ctags [&lt;file&gt;]             create tags file for Vi (default: "tags")
   :etags [&lt;file&gt;]             create tags file for Emacs (default: "TAGS")
   :type &lt;expr&gt;                show the type of &lt;expr&gt;
   :kind &lt;type&gt;                show the kind of &lt;type&gt;
   :undef &lt;cmd&gt;                undefine user-defined command :&lt;cmd&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;flags&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
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 &ldquo;Start&rdquo; 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 &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 (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 &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 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>&ndash;&ndash;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 &amp; 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 be sure that the compiled version is up-to-date, a good technique
    when working on a large program is to occasionally run
    <literal>ghc &ndash;&ndash;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:
<screen>
Prelude> reverse "hello"
"olleh"
Prelude> 5+5
10
</screen>
</para>

<sect2><title>I/O actions at the prompt</title>

<para>GHCi does more than simple expression evaluation at the prompt.
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.
<screen>
Prelude> "hello"
"hello"
Prelude> putStrLn "hello"
hello
</screen>
Furthermore, GHCi will print the result of the I/O action if (and only
if):
<itemizedlist>
  <listitem><para>The result type is an instance of <literal>Show</literal>.</para></listitem>
  <listitem><para>The result type is not
  <literal>()</literal>.</para></listitem>
</itemizedlist>
For example, remembering that <literal>putStrLn :: String -> IO ()</literal>:
<screen>
Prelude> putStrLn "hello"
hello
Prelude> do { putStrLn "hello"; return "yes" }
hello
"yes"
</screen>
</para></sect2>

    <sect2 id="ghci-stmts">
      <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.
<screen>
Prelude> x &lt;- return 42
42
Prelude> print x
42
Prelude>
</screen>
      The statement <literal>x &lt;- 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>GHCi will print the result of a statement if and only if: 
        <itemizedlist>
          <listitem>
            <para>The statement is not a binding, or it is a monadic binding 
              (<literal>p &lt;- e</literal>) that binds exactly one
              variable.</para>
          </listitem>
          <listitem>
            <para>The variable's type is not polymorphic, is not
              <literal>()</literal>, and is an instance of
              <literal>Show</literal></para>
          </listitem>
        </itemizedlist>
      The automatic printing of binding results can be supressed with
      <option>:set -fno-print-bind-result</option> (this does not
      supress printing the result of non-binding statements).
      <indexterm><primary><option>-fno-print-bind-result</option></primary></indexterm><indexterm><primary><option>-fprint-bind-result</option></primary></indexterm>.
      You might want to do this to prevent the result of binding
      statements from being fully evaluated by the act of printing
      them, for example.</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> x
42
Prelude>
</screen>
      <para>Another important difference between the two types of binding
      is that the monadic bind (<literal>p &lt;- 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>Note that <literal>let</literal> bindings do not automatically
        print the value bound, unlike monadic bindings.</para>

      <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 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>

      <para>
        Hint: GHCi will tab-complete names that are in scope; for
        example, if you run GHCi and type <literal>J&lt;tab&gt;</literal>
        then GHCi will expand it to <literal>Just </literal>.
      </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>

      <sect3>
        <title>The <literal>:main</literal> command</title>

        <para>
          When a program is compiled and executed, it can use the
          <literal>getArgs</literal> function to access the
          command-line arguments.
          However, we cannot simply pass the arguments to the
          <literal>main</literal> function while we are testing in ghci,
          as the <literal>main</literal> function doesn't take its
          directly.
        </para>

        <para>
          Instead, we can use the <literal>:main</literal> command.
          This runs whatever <literal>main</literal> is in scope, with
          any arguments being treated the same as command-line arguments,
          e.g.:
        </para>

<screen>
Prelude> let main = System.Environment.getArgs >>= print
Prelude> :main foo bar
["foo","bar"]
</screen>

      </sect3>
    </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>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&gt; id

&lt;interactive&gt;:1:0:
    No instance for (Show (a -&gt; a))
      arising from use of `print' at &lt;interactive&gt;:1:0-1
    Possible fix: add an instance declaration for (Show (a -> a))
    In the expression: print it
    In a 'do' expression: print it
</screen>

    <para>The error message contains some clues as to the
    transformation happening internally.</para>

      <para>If the expression was instead 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
Wed Mar 14 12:23:13 GMT 2001
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 &lt;- <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 id="extended-default-rules">
      <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.  The
    standard rules take each group of constraints <literal>(C1 a, C2 a, ..., Cn
    a)</literal> for each type variable <literal>a</literal>, and defaults the
    type variable if 
        <itemizedlist>
            <listitem><para> The type variable <literal>a</literal>
        appears in no other constraints </para></listitem>
            <listitem><para> All the classes <literal>Ci</literal> are standard.</para></listitem>
            <listitem><para> At least one of the classes <literal>Ci</literal> is
            numeric.</para></listitem>
      </itemizedlist>
   At the GHCi prompt, the second and third rules are relaxed as follows
   (differences italicised):
        <itemizedlist>
            <listitem><para> <emphasis>All</emphasis> of the classes
            <literal>Ci</literal> are single-parameter type classes.</para></listitem>
            <listitem><para> At least one of the classes <literal>Ci</literal> is
            numeric, <emphasis>or is <literal>Show</literal>, 
                <literal>Eq</literal>, or <literal>Ord</literal></emphasis>.</para></listitem>
      </itemizedlist>
   The same type-default behaviour can be enabled in an ordinary Haskell
   module, using the flag <literal>-fextended-default-rules</literal>. 
   </para>
    </sect2>
  </sect1>

  <sect1 id="ghci-invocation">
    <title>Invoking GHCi</title>
    <indexterm><primary>invoking</primary><secondary>GHCi</secondary></indexterm>
    <indexterm><primary><option>&ndash;&ndash;interactive</option></primary></indexterm>

    <para>GHCi is invoked with the command <literal>ghci</literal> or
    <literal>ghc &ndash;&ndash;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 readline
   ___         ___ _
  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 6.6, for Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Loading package base ... linking ... done.
Loading package readline-1.0 ... 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 &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>.  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 overridden 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>

      <para>Ordering of <option>-l</option> options matters: a library
      should be mentioned <emphasis>before</emphasis> the libraries it
      depends on (see <xref linkend="options-linker"/>).</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, with ambiguities being resolved in favour of the more
    commonly used commands.</para>

    <variablelist>
      <varlistentry>
        <term>
          <literal>:add</literal> <replaceable>module</replaceable> ...
          <indexterm><primary><literal>:add</literal></primary></indexterm>
        </term>
        <listitem>
          <para>Add <replaceable>module</replaceable>(s) to the
          current <firstterm>target set</firstterm>, and perform a
          reload.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <literal>:breakpoint</literal> <replaceable>list|add|continue|del|stop|step</replaceable> ...
          <indexterm><primary><literal>:breakpoint</literal></primary></indexterm>
        </term>
        <listitem>
          <para>Permits to add, delete or list the breakpoints in a debugging session.
          In order to make this command available, the 
          <literal>-fdebugging</literal> flag must be active. The easiest way is to launch
          GHCi with the <literal>-fdebugging</literal> option. For more
          details on how the debugger works, see <xref linkend="ghci-debugger"/>.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <literal>:browse</literal> <optional><literal>*</literal></optional><replaceable>module</replaceable> ...
          <indexterm><primary><literal>:browse</literal></primary></indexterm>
        </term>
        <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>
          <indexterm><primary><literal>:cd</literal></primary></indexterm>
        </term>
        <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>

          <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>:continue</literal> 
          <indexterm><primary><literal>:continue</literal></primary></indexterm>
        </term>
        <listitem><para>Shortcut to <literal>:breakpoint continue</literal></para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
          <indexterm><primary><literal>:def</literal></primary></indexterm>
        </term>
        <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
          &lsquo;<literal>\n</literal>&rsquo;.</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 &amp; 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
          &ldquo;<literal>ghc &ndash;&ndash;make Main</literal>&rdquo; in the
          current directory:</para>

<screen>
Prelude> :def make (\_ -> return ":! ghc &ndash;&ndash;make Main")
</screen>

          <para>We can define a command that reads GHCi input from a
          file.  This might be useful for creating a set of bindings
          that we want to repeatedly load into the GHCi session:</para>

<screen>
Prelude> :def . readFile
Prelude> :. cmds.ghci
</screen>

          <para>Notice that we named the command
          <literal>:.</literal>, by analogy with the
          &lsquo;<literal>.</literal>&rsquo; Unix shell command that
          does the same thing.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <literal>:edit <optional><replaceable>file</replaceable></optional></literal>
          <indexterm><primary><literal>:edit</literal></primary></indexterm>
        </term>
        <listitem>
          <para>Opens an editor to edit the file
          <replaceable>file</replaceable>, or the most recently loaded
          module if <replaceable>file</replaceable> is omitted.  The
          editor to invoke is taken from the <literal>EDITOR</literal>
          environment variable, or a default editor on your system if
          <literal>EDITOR</literal> is not set.  You can change the
          editor using <literal>:set editor</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <literal>:help</literal>
          <indexterm><primary><literal>:help</literal></primary></indexterm>
        </term>
        <term>
          <literal>:?</literal>
          <indexterm><primary><literal>:?</literal></primary></indexterm>
        </term>
        <listitem>
          <para>Displays a list of the available commands.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <literal>:info</literal> <replaceable>name</replaceable> ...
          <indexterm><primary><literal>:info</literal></primary></indexterm>
        </term>
        <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> ...
          <indexterm><primary><literal>:load</literal></primary></indexterm>
        </term>
        <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>:main <replaceable>arg<subscript>1</subscript></replaceable> ... <replaceable>arg<subscript>n</subscript></replaceable></literal>
          <indexterm><primary><literal>:main</literal></primary></indexterm>
        </term>
        <listitem>
          <para>
            When a program is compiled and executed, it can use the
            <literal>getArgs</literal> function to access the
            command-line arguments.
            However, we cannot simply pass the arguments to the
            <literal>main</literal> function while we are testing in ghci,
            as the <literal>main</literal> function doesn't take its
            directly.
          </para>

          <para>
            Instead, we can use the <literal>:main</literal> command.
            This runs whatever <literal>main</literal> is in scope, with
            any arguments being treated the same as command-line arguments,
            e.g.:
          </para>

<screen>
Prelude> let main = System.Environment.getArgs >>= print
Prelude> :main foo bar
["foo","bar"]
</screen>

        </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>
          <indexterm><primary><literal>:module</literal></primary></indexterm>
        </term>
        <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>:print </literal> <replaceable>names</replaceable> ...
          <indexterm><primary><literal>:print</literal></primary></indexterm>
        </term>
        <listitem>
          <para> Prints a semievaluated value without forcing its evaluation. 
          <literal>:print </literal> works just like <literal>:sprint</literal> but additionally, 
           <literal>:print</literal> binds the unevaluated parts -called 
           <quote>suspensions</quote>-
          to names which you can play with. For example:
<screen>
Prelude> let li = map Just [1..5]
Prelude> :sp li
li - _
Prelude> :p li
li - (_t1::[Maybe Integer])
Prelude> head li
Just 1
Prelude> :sp li
li - [Just 1 | _]
Prelude> :p li
li - [Just 1 | (_t2::[Maybe Integer])]
Prelude> last li
Just 5
Prelude> :sp li
li - [Just 1,_,_,_,Just 5]
Prelude> :p li
li - [Just 1,(_t3::Maybe Integer),(_t4::Maybe Integer),(_t5::Maybe Integer),Just 4]
Prelude> _t4
Just 3
Prelude> :p li
li - [Just 1,(_t6::Maybe Integer),Just 3,(_t7::Maybe Integer),Just 4]
</screen>
         The example uses <literal>:print</literal> and  <literal>:sprint</literal> 
         to help us observe how the <literal>li</literal> variable is evaluated progressively as we operate
         with it. Note for instance how <quote>last</quote> traverses all the elements of
         the list to compute its result, but without evaluating the individual elements.</para>
           <para>Finally note that the Prolog convention of [head | tail] is used by 
         <literal>:sprint</literal> to display unevaluated lists.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <literal>:quit</literal>
          <indexterm><primary><literal>:quit</literal></primary></indexterm>
        </term>
        <listitem>
          <para>Quits GHCi.  You can also quit by typing a control-D
          at the prompt.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <literal>:reload</literal>
          <indexterm><primary><literal>:reload</literal></primary></indexterm>
        </term>
        <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>
          <indexterm><primary><literal>:set</literal></primary></indexterm>
        </term>
        <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> ...
          <indexterm><primary><literal>:set args</literal></primary></indexterm>
        </term>
        <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>editor</literal> <replaceable>cmd</replaceable>
        </term>
        <listitem>
          <para>Sets the command used by <literal>:edit</literal> to
          <replaceable>cmd</replaceable>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
           <literal>:set</literal> <literal>prog</literal> <replaceable>prog</replaceable>
           <indexterm><primary><literal>:set prog</literal></primary></indexterm>
        </term>
        <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>:set</literal> <literal>prompt</literal> <replaceable>prompt</replaceable>
        </term>
        <listitem>
          <para>Sets the string to be used as the prompt in GHCi.
          Inside <replaceable>prompt</replaceable>, the sequence
          <literal>%s</literal> is replaced by the names of the
          modules currently in scope, and <literal>%%</literal> is
          replaced by <literal>%</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <literal>:show bindings</literal>
          <indexterm><primary><literal>:show bindings</literal></primary></indexterm>
        </term>
        <listitem>
          <para>Show the bindings made at the prompt and their
          types.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <literal>:show modules</literal>
          <indexterm><primary><literal>:show modules</literal></primary></indexterm>
        </term>
        <listitem>
          <para>Show the list of modules currently load.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <literal>:sprint</literal>
          <indexterm><primary><literal>:sprint</literal></primary></indexterm>
        </term>
        <listitem>
          <para>Prints a semievaluated value without forcing its evaluation. 
          <literal>:sprint</literal> and its sibling <literal>:print</literal> 
          are very useful to observe how lazy evaluation works in your code. For example:
<screen>
Prelude> let li = map Just [1..5]
Prelude> :sp li
li - _
Prelude> head li
Just 1
Prelude> :sp li
li - [Just 1 | _]
Prelude> last li
Just 5
Prelude> :sp li
li - [Just 1,_,_,_,Just 5]
</screen>
         The example uses <literal>:sprint</literal> to help us observe how the <literal>li</literal> variable is evaluated progressively as we operate
         with it. Note for instance how <quote>last</quote> traverses all the elements of
         the list to compute its result, but without evaluating the individual elements.</para>
           <para>Finally note that the Prolog convention of [head | tail] is used by 
         <literal>:sprint</literal> to display unevaluated lists.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <literal>:ctags</literal> <optional><replaceable>filename</replaceable></optional>
          <literal>:etags</literal> <optional><replaceable>filename</replaceable></optional>
          <indexterm><primary><literal>:etags</literal></primary>
          </indexterm>
          <indexterm><primary><literal>:etags</literal></primary>
          </indexterm>
        </term>
        <listitem>
          <para>Generates a &ldquo;tags&rdquo; file for Vi-style editors
            (<literal>:ctags</literal>) or Emacs-style editors (<literal>etags</literal>).  If
            no filename is specified, the defaulit <filename>tags</filename> or
            <filename>TAGS</filename> is
            used, respectively.  Tags for all the functions, constructors and
            types in the currently loaded modules are created.  All modules must
            be interpreted for these commands to work.</para>
          <para>See also <xref linkend="hasktags" />.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
         <literal>:type</literal> <replaceable>expression</replaceable>
         <indexterm><primary><literal>:type</literal></primary></indexterm>
        </term>
        <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>:kind</literal> <replaceable>type</replaceable>
          <indexterm><primary><literal>:kind</literal></primary></indexterm>
        </term>
        <listitem>
          <para>Infers and prints the kind of
          <replaceable>type</replaceable>. The latter can be an arbitrary
            type expression, including a partial application of a type constructor,
            such as <literal>Either Int</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <literal>:undef</literal> <replaceable>name</replaceable>
          <indexterm><primary><literal>:undef</literal></primary></indexterm>
        </term>
        <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>...
          <indexterm><primary><literal>:unset</literal></primary></indexterm>
        </term>
        <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>...
          <indexterm><primary><literal>:!</literal></primary></indexterm>
          <indexterm><primary>shell commands</primary><secondary>in GHCi</secondary></indexterm>
        </term>
        <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;.  </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>
            <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>
          </term>
          <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>
            <indexterm><primary><literal>+s</literal></primary></indexterm>
          </term>
          <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>
            <indexterm><primary><literal>+t</literal></primary></indexterm>
          </term>
          <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>
    </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-debugger">
    <title>The GHCi debugger</title>
    <indexterm><primary>debugger</primary></indexterm>
    <para>GHCi embeds an utility debugger with a very basic set of operations. The debugger
          is always available in ghci, you do not need to do anything to activate it. </para>
    <para>The following conditions must hold before a module can be debugged in GHCi:
      <itemizedlist>
         <listitem>
           <para>The module must have been loaded interpreted, i.e. not loaded from an <filename>.o</filename> file compiled by ghc </para>
         </listitem>
         <listitem>
           <para>The module must have been loaded with the <literal>-fdebugging</literal> flag
           </para></listitem>
       </itemizedlist></para>
    <sect2><title>Using the debugger</title>
    <para>The debugger allows the insertion of breakpoints at specific locations in the source code. These locations are governed by event sites, and not by line as in traditional debuggers such as gdb. </para> <para>
      Once a breakpointed event is hit, the debugger stops the execution and you can examine the local variables in scope
      in the context of the event, as well as evaluate arbitrary Haskell expressions in
      a special interactive prompt. </para><para>
      
     When you are done you issue the <literal>:continue</literal> 
      command to leave the breakpoint and let the execution go on. 
     Note that not all the GHCi commands are supported in a breakpoint. 

    </para>
    <sidebar><title>Events</title><?dbfo float-type="left"?>
    <para> Events are the places in source code where you can set a breakpoint.
<programlisting>
qsort [] = <co id="name-binding-co"/> []
qsort (x:xs) = 
   <coref linkend="name-binding-co"/> let left  = <coref linkend="name-binding-co"/> filter (\y -> <co id="lambda-co"/> y &lt; x) xs
           right = <coref linkend="name-binding-co"/> case filter (\y -> <coref linkend="lambda-co"/> y &gt; x) xs of 
                              right_val -> <co id="case-co"/> right_val
    in <co id="let-co"/> qsort left ++ [x] ++ qsort right
main = <coref linkend="name-binding-co"/> do { 
   arg &lt;- <coref linkend="name-binding-co"/> getLine ;
   let num = <coref linkend="name-binding-co"/> read arg :: [Int] ;
 <co id="stmts-co"/> print (qsort num) ;
 <coref linkend="stmts-co"/> putStrLn "GoodBye!" }
</programlisting>
     The GHCi debugger recognizes the following event types:
    <calloutlist>
      <callout arearefs="name-binding-co" id="name-binding">
        <para>Function definition and local bindings in let/where</para>
    </callout>
    <callout arearefs="lambda-co" id="lambda">
        <para>Lambda expression entry point</para>
    </callout>
    <callout arearefs="let-co" id="let">
      <para>Let expression body</para>
    </callout>
    <callout arearefs="case-co" id="case">
      <para>Case alternative body</para>
    </callout>
    <callout arearefs="stmts-co" id="stmts">
      <para>do notation statements</para>
    </callout>
    </calloutlist></para>
    <para>In reality however, ghci eliminates some redundant event sites. 
    For instance, sites with two co-located breakpoint events are coalesced into a single one,
    and sites with no bindings in scope are assumed to be uninteresting and no breakpoint can be set in them.</para>
    </sidebar>

<para>
      You don't need to do anything special in order to start the debugging session.
      Simply use ghci to evaluate your Haskell expressions and whenever a breakpoint
      is hit, the debugger will enter the stage:
<programlisting>
*main:Main> :break add Main 2
Breakpoint set at (2,15)

*main:Main> qsort [10,9..1]
Local bindings in scope:
  x :: a, xs :: [a], left :: [a], right :: [a]

qsort2.hs:2:15-46>   
</programlisting>
      What is happening here is that GHCi has interrupted the evaluation of 
      <literal>qsort</literal> at the breakpoint set in line 2, as the prompt indicates.
      At this point you can freely explore the contents of the bindings in scope,
      but with two catches. </para><para>
      First, take into account that due to the lazy nature of Haskell, some of
      these bindings may be unevaluated, and that exploring their contents may 
      trigger a computation. </para><para>
      Second: look at the types of the things in scope.
      GHCi has left its types parameterised by a variable!
      Look at the type of <literal>qsort</literal>, which is 
      polymorphic on the type of its argument. It does not 
      tell us really what the types of <literal>x</literal> and <literal>xs</literal> can be. 
      In general, polymorphic programs deal with polymorphic values,
      and this means that some of the bindings available in a breakpoint site
      will be parametrically typed.
      </para><para>
      So, what can we do with a value without concrete type? Very few interesting
      things. The <literal>:print</literal> command in ghci allows you to 
      explore its contents and see if it is evaluated or not. 
      This is useful because you cannot just type <literal>x</literal> in the 
      prompt and expect GHCi to return you its value. Perhaps you know for 
      sure that 
      <literal>x</literal> is of type <literal>Int</literal>, which is an instance of 
      <literal>Show</literal>, but GHCi does not have this information. 
      <literal>:print</literal> however is fine, because it does not need to know the 
      type to do its work. </para>
      <para> Let's go on with the debugging session of the <literal>qsort</literal>
      example:
<example id="debuggingEx"><title>A short debugging session</title>
<programlisting>
qsort2.hs:2:15-46> x
This is an untyped, unevaluated computation. You can use seq to 
force its evaluation and then :print to recover its type <co id="seq1"/>
qsort2.hs:2:15-46> seq x ()  <co id="seq2"/>
() 
qsort2.hs:2:15-46> x <co id="seq3"/>
This is an untyped, unevaluated computation. You can use seq to 
force its evaluation and then :print to recover its type

qsort2.hs:2:15-46> :t x
x :: GHC.Base.Unknown
qsort2.hs:2:15-46> :p x <co id="seq4"/>
x - 10
qsort2.hs:2:15-46> :t x <co id="seq5"/>
x :: Int
</programlisting>
</example>
      <calloutlist>
        <callout arearefs="seq1">
          <para>GHCi reminds us that this value is untyped, and instructs us to force its evaluation </para>
        </callout>
        <callout arearefs="seq2">
          <para>This line forces the evaluation of <literal>x</literal> </para>
        </callout>
        <callout arearefs="seq3">
          <para>Even though x has been evaluated, we cannot simply use its name to see its value! 
          This is a bit counterintuitive, but currently in GHCi the type of a binding
          cannot be a type variable <literal>a</literal>. 
          Thus, the binding <literal>x</literal> gets assigned the concrete type Unknown.</para>
        </callout>
        <callout arearefs="seq4">
          <para>We can explore <literal>x</literal> using the <literal>:print</literal> 
          command, which does find out that <literal>x</literal> is of type Int and prints
          its value accordingly.</para>
        </callout>
        <callout arearefs="seq5">
           <para><literal>:print</literal> also updates the type of <literal>x</literal> with
           the most concrete type information available.</para>
        </callout>
      </calloutlist>
      The example shows the standard way to proceeed with polymorphic values in a breakpoint. 
      </para>
    </sect2>
    <sect2><title>Commands</title>
    <para>Breakpoints can be set in several ways using the <literal>:breakpoint</literal> command. Note that you can take advantage of the command abbreviation feature of GHCi and use simply <literal>:bre</literal> to save quite a few keystrokes.
<variablelist>
<varlistentry>
  <term>
    <literal>:breakpoint add <replaceable>module</replaceable> <replaceable>line</replaceable></literal>
  </term>
  <listitem><para>
    Adds a breakpoint at the first event found at line <literal>line</literal> in <literal>module</literal>, if any.
  </para></listitem>
</varlistentry>
<varlistentry>
  <term>
    <literal>:breakpoint add <replaceable>module</replaceable> <replaceable>line</replaceable> <replaceable>column</replaceable></literal>
  </term>
  <listitem><para>
    Adds a breakpoint at the first event found after column <literal>column</literal>
    at line <literal>line</literal> in <literal>module</literal>, if any.
  </para></listitem>
</varlistentry>

<varlistentry>
  <term>
    <literal>:breakpoint continue</literal>
  </term>
  <listitem><para>
   When at a breakpoint, continue execution up to the next breakpoint
   or end of evaluation.
  </para></listitem>
</varlistentry>

<varlistentry>
  <term>
    <literal>:continue</literal>
  </term>
  <listitem><para>
      Shortcut for <literal>:breakpoint continue</literal>
  </para></listitem>
</varlistentry>

<varlistentry>
  <term>
    <literal>:breakpoint list</literal>
  </term>
  <listitem><para>
    Lists the currently set up breakpoints.
  </para></listitem>
</varlistentry>
<varlistentry>
  <term>
    <literal>:breakpoint del <replaceable>num</replaceable></literal>
  </term>
  <listitem><para>
    Deletes the breakpoint at position <literal>num</literal> in the list of
    breakpoints shown by <literal>:breakpoint list</literal>.
  </para></listitem>
</varlistentry>
<varlistentry>
  <term>
    <literal>:breakpoint del <replaceable>module</replaceable> <replaceable>line</replaceable></literal>
  </term>
  <listitem><para>
  Dels the breakpoint at line <literal>line</literal> in <literal>module</literal>, if any.
  </para></listitem>
</varlistentry>
<varlistentry>
  <term>
    <literal>:breakpoint del <replaceable>module</replaceable> <replaceable>line</replaceable><replaceable>col</replaceable> </literal>
  </term>
  <listitem><para>
    Deletes the first breakpoint found after column <literal>column</literal>
    at line <literal>line</literal> in <literal>module</literal>, if any.
  </para></listitem>
</varlistentry>
<varlistentry>
  <term>
    <literal>:breakpoint stop </literal>
  </term>
  <listitem><para>
    Stop the program being executed. This interrupts a debugging session
    and returns to the top level.
  </para></listitem>
</varlistentry>
</variablelist></para>
    </sect2>
    <sect2><title>Limitations</title>
     <para>
      <itemizedlist>
        <listitem><para>
          Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available 
          at the scope of a breakpoint if there is a explicit type signature.
        </para></listitem>
      </itemizedlist>
      <itemizedlist>
        <listitem><para>
          Modules compiled by GHCi under the <literal>-fdebugging
        </literal> flag  will perform slower: the debugging mode introduces some overhead.
      Modules compiled to object code by ghc are not affected.
        </para></listitem>
      </itemizedlist>      
     </para>
    </sect2>
    <sect2><title>Tips</title>
      <variablelist>
        <varlistentry><term>* Use PRAGMAs to fine tune which modules are loaded under debugging mode</term>
          <listitem>
            <programlisting>{-# OPTIONS_GHC -fdebugging #-}</programlisting>
          </listitem>
        </varlistentry>
        <varlistentry> <term>* Repeated use of <literal>seq</literal> and 
            <literal>:print</literal> may be necessary to observe unevaluated
            untyped bindings</term> 
          <listitem><para>see <xref linkend="debuggingEx"/> 
          </para></listitem>
        </varlistentry>
        <varlistentry> <term> * <literal>GHC.Exts.unsafeCoerce</literal> can help if you are positive about the type of a binding</term> 
          <listitem><para><programlisting>
type MyLongType a = [Maybe [Maybe a]]

main:Main> :m +GHC.Exts
main:Main> main
Local bindings in scope:
  x :: a
Main.hs:15> let x' = unsafeCoerce x :: MyLongType Bool
Main.hs:15> x'
[Just [Just False, Just True]]
          </programlisting>
           Note that a wrong coercion will likely result in your debugging session being interrupted by a segmentation fault 
          </para></listitem>
        </varlistentry>
        <varlistentry> <term> * The undocumented (and unsupported) &colon;force command </term>
          <listitem><para> 
              equivalent to <literal> :print</literal> with automatic 
              <literal>seq</literal> forcing, 
              may prove useful to replace sequences of <literal>seq</literal> and 
              <literal>&colon;print</literal> in some situations. 
          </para></listitem>
        </varlistentry> 
    </variablelist>
    </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>
          <indexterm><primary><option>-ignore-dot-ghci</option></primary></indexterm>
        </term>
        <listitem>
          <para>Don't read either <filename>./.ghci</filename> or
          <filename>$HOME/.ghci</filename> when starting up.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>-read-dot-ghci</option>
          <indexterm><primary><option>-read-dot-ghci</option></primary></indexterm>
        </term>
        <listitem>
          <para>Read <filename>.ghci</filename> and
          <filename>$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 id="ghci-faq">
    <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!
          <indexterm><primary><option>-O</option></primary></indexterm>
         </term>
        <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>This should work, as long as your GHCi was built with
          the <option>-threaded</option> switch, which is the default.
          Consult whoever supplied your GHCi installation.</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>

      <varlistentry>
        <term>I can't use Control-C to interrupt computations in
          GHCi on Windows.</term>
        <listitem>
          <para>See <xref linkend="ghci-windows"/></para>
        </listitem>
      </varlistentry>
    </variablelist>
  </sect1>

</chapter>

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