</para>
</sect2>
+<!-- ===================== Recursive do-notation =================== -->
+
+<sect2 id="mdo-notation">
+<title>The recursive do-notation
+</title>
+
+<para> The recursive do-notation (also known as mdo-notation) is implemented as described in
+"A recursive do for Haskell",
+Levent Erkok, John Launchbury",
+Haskell Workshop 2002, pages: 29-37. Pittsburgh, Pennsylvania.
+</para>
+<para>
+The do-notation of Haskell does not allow <emphasis>recursive bindings</emphasis>,
+that is, the variables bound in a do-expression are visible only in the textually following
+code block. Compare this to a let-expression, where bound variables are visible in the entire binding
+group. It turns out that several applications can benefit from recursive bindings in
+the do-notation, and this extension provides the necessary syntactic support.
+</para>
+<para>
+Here is a simple (yet contrived) example:
+</para>
+<programlisting>
+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 MonadFix library introduces the <literal>MonadFix</literal> class. It's definition is:
+</para>
+<programlisting>
+class Monad m => MonadFix 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>MonadFix</literal> class.
+For details, see the above mentioned reference.
+</para>
+<para>
+The <literal>MonadFix</literal> library automatically declares List, Maybe, IO, and
+state monads (both lazy and strict) as instances of the <literal>MonadFix</literal> class.
+</para>
+<para>
+There are three important points in using the recursive-do notation:
+<itemizedlist>
+<listitem><para>
+The recursive version of the do-notation uses the keyword <literal>mdo</literal> (rather
+than <literal>do</literal>).
+</para></listitem>
+
+<listitem><para>
+If you want to declare an instance of the <literal>MonadFix</literal> class for one of
+your own monads, or you need to refer to the class name <literal>MonadFix</literal> in any other way (for instance in
+writing a type constraint), then your program should <literal>import Control.Monad.MonadFix</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>MonadFix</literal> should always be imported.)
+</para></listitem>
+
+<listitem><para>
+As with other extensions, ghc should be given the flag <literal>-fglasgow-exts</literal>
+</para></listitem>
+</itemizedlist>
+</para>
+
+<para>
+Historical note: The originial implementation of the mdo-notation, and most
+of the existing documents, use the names
+<literal>MonadRec</literal> for the class, and
+<literal>Control.Monad.MonadRec</literal> for the library. These names
+are no longer supported.
+</para>
+
+<para>
+The web page: <ulink url="http://www.cse.ogi.edu/PacSoft/projects/rmb">http://www.cse.ogi.edu/PacSoft/projects/rmb</ulink>
+contains up to date information on recursive monadic bindings.
+</para>
+
+</sect2>
+
<!-- ===================== PARALLEL LIST COMPREHENSIONS =================== -->
<sect2 id="parallel-list-comprehensions">
</para>
</sect2>
-<sect2 id="mdo-notation">
-<title>The recursive do-notation
-</title>
-
-<para> The recursive do-notation (a.k.a. mdo-notation) is implemented as described in
-"A recursive do for Haskell",
-Levent Erkok, John Launchbury",
-Haskell Workshop 2002, pages: 29-37. Pittsburgh, Pennsylvania.
-</para>
-<para>
-The do-notation of Haskell does not allow <emphasis>recursive bindings</emphasis>,
-that is, the variables bound in a do-expression are visible only in the textually following
-code block. Compare this to a let-expression, where bound variables are visible in the entire binding
-group. It turns out that several applications can benefit from recursive bindings in
-the do-notation, and this extension provides the necessary syntactic support.
-</para>
-<para>
-Here is a simple (yet contrived) example:
-</para>
-<programlisting>
-import Control.Monad.MonadRec
-
-justOnes = mdo xs <- Just (1:xs)
- return xs
-</programlisting>
-<para>
-There are three important points in using the recursive-do notation:
-<itemizedlist>
-<listitem><para>
-The recursive version of the do-notation uses the keyword <literal>mdo</literal> (rather
-than <literal>do</literal>).
-</para></listitem>
-
-<listitem><para>
-The scripts using <literal>mdo</literal> should <literal>import Control.Monad.MonadRec</literal>
-</para></listitem>
-
-<listitem><para>
-As with other extensions, ghc should be given the flag <literal>-fglasgow-exts</literal>
-</para></listitem>
-</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>
-contains up to date information on recursive monadic bindings.
-</para>
-
-</sect2>
</sect1>
projects / ghc-hetmet.git / blobdiff