Here is a simple (yet contrived) example:
</para>
<programlisting>
-import Control.Monad.MonadRec
-
justOnes = mdo xs <- Just (1:xs)
return xs
</programlisting>
<para>
+As you can guess <literal>justOnes</literal> will evaluate to <literal>Just [1,1,1,...</literal>.
+</para>
+
+<para>
+The MonadRec library introduces the <literal>MonadRec</literal> class. It's definition is:
+</para>
+<programlisting>
+class Monad m => MonadRec m where
+ mfix :: (a -> m a) -> m a
+</programlisting>
+<para>
+The function <literal>mfix</literal>
+dictates how the required recursion operation should be performed. If recursive bindings are required for a monad,
+then that monad must be declared an instance of the <literal>MonadRec</literal> class.
+For details, see the above mentioned reference.
+</para>
+<para>
+The <literal>MonadRec</literal> library automatically declares List, Maybe, IO, and
+state monads (both lazy and strict) as instances of the <literal>MonadRec</literal> class.
+</para>
+<para>
There are three important points in using the recursive-do notation:
<itemizedlist>
<listitem><para>
</para></listitem>
<listitem><para>
-The scripts using <literal>mdo</literal> should <literal>import Control.Monad.MonadRec</literal>
+If you want to declare an instance of the <literal>MonadRec</literal> class for one of
+your own monads, or you need to refer to the class name <literal>MonadRec</literal> in any other way (for instance in
+writing a type constraint), then your program should <literal>import Control.Monad.MonadRec</literal>.
+Otherwise, you don't need to import any special libraries to use the mdo-notation. That is,
+as long as you only use the predefined instances mentioned above, the mdo-notation will
+be automatically available. (Note: This differs from the Hugs implementation, where
+<literal>MonadRec</literal> should always be imported.)
</para></listitem>
<listitem><para>
</itemizedlist>
</para>
-<para>
-The MonadRec library introduces the <literal>MonadRec</literal> class. It's definition is:
-</para>
-<programlisting>
-class Monad m => MonadRec m where
- mfix :: (a -> m a) -> m a
-</programlisting>
-<para>
-The <literal>MonadRec</literal> class declares the function <literal>mfix</literal>,
-which dictates how the recursion should behave. If recursive bindings are required for a monad,
-then that monad must be declared an instance of the <literal>MonadRec</literal> class.
-For details, see the above mentione reference.
-</para>
-<para>
-The MonadRec library automatically declares List, Maybe, IO, and
-state monads (both lazy and strict) as instances of the <literal>MonadRec</literal> class.
-</para>
<para>
The web page: <ulink url="http://www.cse.ogi.edu/PacSoft/projects/rmb">http://www.cse.ogi.edu/PacSoft/projects/rmb</ulink>