<literal>IO</literal> monad.
<screen>
Prelude> x <- return 42
-42
Prelude> print x
42
Prelude>
<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:
+ <para>If <option>-fprint-bind-result</option> is set then
+ 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>Show</literal></para>
</listitem>
</itemizedlist>
- The automatic printing of binding results can be suppressed with
- <option>:set -fno-print-bind-result</option> (this does not
- suppress 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>
+ <indexterm><primary><option>-fprint-bind-result</option></primary></indexterm><indexterm><primary><option>-fno-print-bind-result</option></primary></indexterm>.
+ </para>
<para>Of course, you can also bind normal non-IO expressions
using the <literal>let</literal>-statement:</para>
<programlisting>
import Prelude hiding (map)
-map :: (a->b) -> a -> b
+map :: (a->b) -> [a] -> [b]
map f [] = []
map f (x:xs) = f x : map f xs
</programlisting>
The <literal>!</literal>-form also annotates the listing
with comments giving possible imports for each group of
entries.</para>
+<screen>
+Prelude> :browse! Data.Maybe
+-- not currently imported
+Data.Maybe.catMaybes :: [Maybe a] -> [a]
+Data.Maybe.fromJust :: Maybe a -> a
+Data.Maybe.fromMaybe :: a -> Maybe a -> a
+Data.Maybe.isJust :: Maybe a -> Bool
+Data.Maybe.isNothing :: Maybe a -> Bool
+Data.Maybe.listToMaybe :: [a] -> Maybe a
+Data.Maybe.mapMaybe :: (a -> Maybe b) -> [a] -> [b]
+Data.Maybe.maybeToList :: Maybe a -> [a]
+-- imported via Prelude
+Just :: a -> Maybe a
+data Maybe a = Nothing | Just a
+Nothing :: Maybe a
+maybe :: b -> (a -> b) -> Maybe a -> b
+</screen>
+ <para>
+ This output shows that, in the context of the current session, in the scope
+ of <literal>Prelude</literal>, the first group of items from
+ <literal>Data.Maybe</literal> have not been imported (but are available in
+ fully qualified form in the GHCi session - see <xref
+ linkend="ghci-scope"/>), whereas the second group of items have been
+ imported via <literal>Prelude</literal> and are therefore available either
+ unqualified, or with a <literal>Prelude.</literal> qualifier.
+ </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
- <literal>:def</literal> <replaceable>name</replaceable> <replaceable>expr</replaceable>
+ <literal>:def<optional>!</optional> <optional><replaceable>name</replaceable> <replaceable>expr</replaceable></optional></literal>
<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
- ‘<literal>\n</literal>’.</para>
+ <para><literal>:def</literal> is used to define new
+ commands, or macros, in GHCi. The command
+ <literal>:def</literal> <replaceable>name</replaceable>
+ <replaceable>expr</replaceable> defines a new GHCi command
+ <literal>:<replaceable>name</replaceable></literal>,
+ implemented by the Haskell expression
+ <replaceable>expr</replaceable>, which must have type
+ <literal>String -> IO String</literal>. When
+ <literal>:<replaceable>name</replaceable>
+ <replaceable>args</replaceable></literal> is typed at the
+ prompt, GHCi will run the expression
+ <literal>(<replaceable>name</replaceable>
+ <replaceable>args</replaceable>)</literal>, take the
+ resulting <literal>String</literal>, and feed it back into
+ GHCi as a new sequence of commands. Separate commands in
+ the result must be separated by
+ ‘<literal>\n</literal>’.</para>
<para>That's all a little confusing, so here's a few
examples. To start with, here's a new GHCi command which
<literal>:.</literal>, by analogy with the
‘<literal>.</literal>’ Unix shell command that
does the same thing.</para>
+
+ <para>Typing <literal>:def</literal> on its own lists the
+ currently-defined macros. Attempting to redefine an
+ existing command name results in an error unless the
+ <literal>:def!</literal> form is used, in which case the old
+ command with that name is silently overwritten.</para>
</listitem>
</varlistentry>
</varlistentry>
<varlistentry>
+ <term>
+ <literal>:</literal>
+ <indexterm><primary><literal>:</literal></primary></indexterm>
+ </term>
+ <listitem>
+ <para>Repeat the previous command.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+
<term>
<literal>:history [<replaceable>num</replaceable>]</literal>
<indexterm><primary><literal>:history</literal></primary></indexterm>